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ScienceWeek
SCIENCEWEEK
ScienceWeek
May 23, 2003
Vol. 7 Number 21A
An Online Digest of Research in the Sciences
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There are living systems, there is no "living matter".
-- Jacques Lucien Monod (1910-1976)
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Section 1
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Symposium: Circadian Rhythms in Biology
1. Introduction
2. Prevalence of Circadian Rhythms
3. Genes and Circadian Rhythms
4. Circadian Rhythms and the Mammalian Nervous System
5. Photoperiodism in Plants
6. Computational Models of Circadian Rhythms
Notices and Subscription Information
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Section 2
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SYMPOSIUM: CIRCADIAN RHYTHMS IN BIOLOGY
1. INTRODUCTION
In biology, a "circadian rhythm" is a daily cyclical process, be
it biochemical, or physiological, or behavioral. The human sleep-
wake cycle is the most familiar example. Circadian rhythms are
often described in terms of endogenous "biological clocks", with
the thrust of research to reduce some particular behavioral or
physiological circadian rhythm to biochemical events. These
clocks are usually set by environmental cues such as the light-
dark cycle, and what is characteristic of an endogenous clock is
that if one removes the environmental cue, keeps the organism in
constant light, for example, the endogenous rhythm will continue,
but will tend to drift out of phase with the outdoors
environmental light-dark cycle. Restoring the external light-dark
cue will reset the clock to its normal intrinsic rhythm.
ON CIRCADIAN RHYTHMS
It might be thought that the obvious relations of many plant and
animal activities to day and night would have invited close study
even in ancient times. However, it appears that the facts were
simply too familiar. It was not until 1729 that the French
geologist J. de Mairan (1678-1771) did the simple experiment of
placing a plant under constant temperature and illumination, and
observed that its normal daily period of fluctuations still
persisted. This showed that periodic behavior could be a function
of the organism itself. Although this finding aroused some
interest, people still did not quite know what to make of it;
there were suspicions that the experiment might be affected by
some undetectable rays or forces.
It was not until the 1930s that biologists began to make the
connection between photoperiodicity (the changing illumination
during the day) and bodily rhythms. A breakthrough came in 1950,
in the work of two German biologists, Gustav Kramer and Karl von
Frisch (1886-1982). Kramer showed that birds could use the sun as
a compass by virtue of the fact that they have an "internal
clock" which, in effect, tells them the time of day and how much
to correct for the position (azimuth) of the sun in the sky. Von
Frisch came to a similar conclusion for bees. A number of
biologists then initiated the search for the cellular basis of
the "internal clock", a search that has continued to the present.
The importance of circadian rhythms has grown in parallel with
the increasing understanding of their mechanisms, and the
increasing realization of how pervasive they are in the life of
the organism.
Adapted from: G.M. Shepherd: Neurobiology. 2nd Edition. Oxford
University Press 1988, p.507.
ON BIOLOGICAL CLOCKS
Virtually all plants and animals adjust their physiology and
behavior to the 24-hour day-night cycle under the governance of
circadian clocks. Molecular biological studies have now indicated
much about the genes and proteins that make up the machinery of
these clocks, a story that began nearly 30 years ago.
In the early 1970s, Ron Konopka and Seymour Benzer, working at
the California Institute of Technology, discovered three mutant
strains of fruit flies whose circadian rhythms were abnormal.
Further analysis showed the mutants to be alleles of a single
locus, which Konopka and Benzer called the *period* or *per*
gene. In the absence of normal environmental cues (that is, in
constant light or dark), wild-type flies have periods of activity
geared to a 24-hour cycle; *per-S* mutants have 19 hour rhythms,
*per-1* mutants have 29-hour rhythms, and *per-0* mutants have no
apparent rhythm. About 10 years later, Michael Young at
Rockefeller University and Jeffrey Hall and Michael Rosbash at
Brandeis University independently cloned the first of the three
*per* genes. Cloning a gene does not necessarily reveal its
function, however, and so it was in this case. Nonetheless, the
gene product PER, a nuclear protein, is found in many Drosophila
cells pertinent to the production of the fly's circadian rhythms.
Moreover, normal flies show a circadian variation in the amount
of *per* mRNA and PER protein, whereas *per-0* flies, which lack
a circadian rhythm, do not show this circadian rhythmicity of
gene expression.
Many of the genes and proteins responsible for circadian rhythms
in fruit flies have now been discovered in mammals. In mice, the
circadian clock arises from the temporally regulated activity of
proteins (in capital letters) and genes [between asterisks],
including CRY (*cryptochrome*), CLOCK (C) (*Circadian locomotor
output cycles kaput, Clk*), BMAL1 (B) {*brain and muscle, ARNT-
like*), PERI (*Period1*), PER2 (*Period2*), PER3 (*Period3*), and
vasopressin prepropressophysin (VP) (*clock controlled genes;
ccg*). These genes and their proteins give rise to
transcription/translation autoregulatory feedback loops with both
excitatory and inhibitory components. The key points to
understanding this system are: (1) the concentrations of BMAL1
(B) and the three PER proteins cycle in counterpoint; (2) PER2 is
a positive regulator of the *Bmall* loop; and (3) that CRY is a
negative regulator of the *period* and *cryptochrome* loops. The
two positive components of this loop are influenced, albeit
indirectly, by light or temperature.
At the start of the day, the transcription of *Clk* and *Bmall*
commences, and the proteins CLOCK (C) and BMAL1 (B) are
synthesized in tandem. When the concentrations of C and B
increase sufficiently, they associate as dimers and bind to
regulatory DNA sequences (E-boxes ) that act as a circadian
transcriptional enhancers of the genes *Cry*, *Per 1*,*Per2*, and
*Per3* and *CCG*. [E boxes contain the sequence CANNTG (where N
is any nucleotide)]. As a result, the proteins PERI, 2, and 3,
CRY, and proteins such as VP are produced. These proteins then
diffuse from the nucleus into the cytoplasm, where they are
modified.
Although the functions of PERI and PER3 remain to be elucidated,
when the cytoplasmic concentrations of PER2 and CRY increase,
they associate as CRY-PER2, and diffuse back into the nucleus.
Here, PER2 stimulates the synthesis of C, and B, and CRY binds to
C-B dimers, inhibiting their ability to stimulate the synthesis
of the other genes. The complete time course of these feedback
loops is 24 hours.
Adapted from: D. Purves et al (Eds.): Neuroscience. 2nd Edition.
Sinauer Associates 2001, p.608.
ON CIRCADIAN RHYTHMS AND THE NERVOUS SYSTEM
Of particular importance in the life of an animal are the
circadian rhythms that control the day-night sleep-wakefulness
cycle. In the absence of all external cues, 24-hour rhythmical
cycles are maintained by an internal clock for prolonged periods,
weeks or months, in invertebrates as well as vertebrates, and
even in explants or neurons in culture. The internal timing
mechanism can be altered (or "entrained") by providing regularly
spaced light and dark stimuli. Autonomic functions are strongly
influenced by biological clocks that act on the pineal gland and
the secretion of melatonin.
Information about cellular and molecular mechanisms that allow
neurons to produce regular night and day cycles has been obtained
in both invertebrates and vertebrates. For example, there is an
aggregate of secretory nerve cells in the visual pathway of
crustaceans known as the eyestalk. In this structure it has been
possible to maintain rhythms of metabolic activity, secretion,
and impulse firing while the isolated organ is kept in culture.
Electrical recordings have been made from pacemaker cells, the
peptides that they release have been characterized, and their
mechanisms of action have been analyzed. Moreover, in culture the
activity of the pacemaker neurons can be entrained by the
imposition of alternating light and dark cycles.
In mammals, a key structure in the hypothalamus for generating
the rhythm of the internal clock is the suprachiasmatic nucleus
(SCN). An important input to this nucleus is from the eye. After
destruction of the suprachiasmatic nucleus in rats, light and
dark entrainment of endogenous rhythms becomes lost. Locomotor
activity, drinking, and sleep-waking cycles, as well as rhythms
of hormone secretion, become disrupted. If fetal hypothalamic
tissue containing the SCN is transplanted to a host previously
rendered arrhythmic by a complete lesion of the SCN, then
rhythmicity is restored with a free running period corresponding
to the donor genotype. In neurons of the suprachiasmatic nucleus,
the frequency of spontaneous action potentials increases during
the day and decreases at night.
Adapted from: J.G. Nicholls et al: From Neuron to Brain. 4th
Edition. Sinauer Associates 2001, p.328.
ON THE MAMMALIAN CLOCK-EYE
T. Roenneberg and M. Merrow (University of Munich, DE) discuss
mammalian clocks, the authors making the following points:
1) Even without time cues from the environment, physiological
events, from gene expression to behavior, recur with a high
regularity but not necessarily in a precise 24 hour rhythm --
hence the term "circadian" ("about one day"). Circadian rhythms
are controlled by endogenous "clocks" which are synchronized, or
"entrained", to the 24-hour day predominantly by light [1]. The
central circadian pacemaker in mammals resides above the optic
chiasm in the suprachiasmatic nuclei (SCN). It has long been
known that light entrainment in mammals requires the eyes, but it
was unclear through which photoreceptor the signal was processed.
It came as a surprise that the circadian clock remains perfectly
entrainable by light in mutant mice devoid of rods and cones [2].
2) Researchers are racing to identify the novel receptor in the
mammalian retina. Its spectral characteristics have been defined
in mice and, more recently, in humans [3,4]. In addition to its
role in entrainment, the novel photoreceptor is responsible for
several other non-visual light responses, such as melatonin
suppression, pupillary constriction and direct effects of light
on motor-activity ("masking"), or for many other "vegetative"
light effects, for example on cortisol levels or heart rate.
Hankins and Lucas (5) have taken our understanding of this novel
light input pathway a step further, showing that its influence is
already apparent in the primary steps of intra-retinal signal
processing.
3) The authors discuss vision vs. irradiation detection. Vision
capitalizes on photons, using rods or cones as "pixels" to create
a retinal image that is processed in the thalamus and the cortex.
While a memory of these "pictures" may be stored in the brain,
the retinal picture itself has to be renewable within
milliseconds for instant detection of any changes. Visual
processing thus requires both fast kinetics and high spatial
resolution. In contrast, a detector for the assessment of day and
night should not care about a flash of lightning or the shadow of
a flying object. Its task is to integrate photons over a long
time. This integration mechanism is partially responsible for the
difficulties that shift workers have in adjusting their
biological clocks to socially enforced schedules -- the
competition between indoor and outdoor light cannot be won. A
worker who is exposed to 500 lux over an eight-hour night shift
collects a similar quantity of photons waiting 15 minutes for the
bus, even on a cloudy day. As a result, the circadian system
remains entrained to the "real" day -- it cannot adjust to the
implemented night shift, so workers try to be active and alert
when their physiology is tuned to sleep. In fact, workers on
night shifts, with most of the rest of the day free to spend
outdoors, may collect more day light than their non-shifting
colleagues. The invention of artificial light has ironically
created a biological shadow world because we spend more time
indoors.
4) In summary: Light is the most reliable environmental signal
for adjusting biological clocks to the 24-hour day. Mammals
receive this signal exclusively through the eyes, but not just
via rods and cones. New evidence has been uncovered for a novel
photoreceptor that may be responsible for more than just
adjusting the clock.
References (abridged):
1. Roenneberg T. and Foster R.G. (1997) Twilight Times-light and
the circadian system. Photochem. Photobiol., 66:549-561
2. Freedman M.S., Lucas R.J., Soni B., von Schantz M., Munoz M.,
David-Gray Z.K. and Foster R. (1999) Non-rod, non-cone ocular
photoreceptors regulate the mammalian circadian behavior.
Science, 284:502-504
3. Brainard G.C., Hanifin J.P., Greeson J.M., Byrne B., Glickman
G., Gerner E. and Rolag M.D. (2001) Action spectrum for melatonin
regulation in humans: evidence for a novel circadian
photoreceptor. J. Neurosci., 21:6405-6412
4. Thapan K., Arendt J. and Skene D.J. (2001) An action spectrum
for melatonin suppression: evidence for a novel non-rod, non-cone
photoreceptor system in humans. J. Physiol., 535:261-267
5. Hankins, M.W. and Lucas, R.J. (2002). A novel photopigment in
the human retina regulates the activity of primary visual
pathways according to long-term light exposure. (in press)
Current Biology 2002 12:R163
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2. PREVALENCE OF CIRCADIAN RHYTHMS
CIRCADIAN RHYTHMS FROM FLIES TO HUMAN
The following points are made by S. Panda et al (Scripps Research
Institute, US):
1) In this era of jet travel, our body "remembers" the previous
time zone, such that when we travel, our sleep wake pattern,
mental alertness, eating habits and many other physiological
processes temporarily suffer the consequences of time
displacement until we adjust to the new time zone. Although the
existence of a circadian clock in humans had been postulated for
decades, an understanding of the molecular mechanisms has
required the full complement of research tools. To gain the
initial insights into circadian mechanisms, researchers turned to
genetically tractable model organisms such as the fruit fly
Drosophila.
2) The rotation of the Earth causes predictable changes in light
and temperature in our natural environment. Accordingly, natural
selection has favored the evolution of circadian (from the Latin
"circa", meaning about, and "dies", meaning day) clocks or
biological clocks -- endogenous cellular mechanisms for keeping
track of time. These clocks impart a survival advantage by
enabling an organism to anticipate daily environmental changes
and thus tailor its behavior and physiology to the appropriate
time of the day. The clock is synchronized by the day night
cycle, allowing the organism to accommodate not only the daily
cycles of light and dark attributable to the Earth's rotation,
but also the alteration in relative span of day and night caused
by the tilting of the Earth's axis relative to the Sun. Thus, a
circadian timing mechanism that undergoes daily adjustment is
useful as a seasonal timer as well.
3) As several aspects of Drosophila physiology and behavior are
restricted to particular times of day, the organism became a
natural model system for molecular investigation of circadian
regulation. Adult flies emerge from their pupal cases (eclose)
when it is cool and moist during the early morning, so minimizing
the risk of desiccation as the emerging fly expands its folded
wings and hardens its cuticle. Pupae exposed to a 12-h light:12-h
dark cycle and subsequently kept in constant darkness also time
their eclosion to when they expect dawn (subjective dawn),
indicating the presence of an internal pacemaker(1). Once
emerged, adult flies restrict flight, foraging and mating
activities to the day (or subjective day), while they tend to
"sleep" (that is, they are relatively unresponsive to sensory
stimuli and exhibit rest homeostasis(2,3) during the night.
4) Circadian regulation of such physiology and behavior results
from coordination of the activities of multiple tissues and cell
types. An example is the consolidation of feeding behavior to the
day phase, which involves regulation of the sensitivity of
chemosensory organs to locate food, activity of the wing muscles
to move towards the food, and the action of the digestive system
to assimilate nutrients. This locomotor activity rhythm is
relatively refractory to acute or minor changes in light levels,
such as during lightning and full moons, but is exquisitely
sensitive to the timing of dawn and dusk to adapt to the
seasonally changing day length.(4,5)
References (abridged):
1. Pittendrigh, C. S. Circadian systems. I. The driving
oscillation and its assay in Drosophila pseudoobscura. Proc. Natl
Acad. Sci. USA 58, 1762-1767 (1967)
2. Shaw, P. J., Cirelli, C., Greenspan, R. J. & Tononi, G.
Correlates of sleep and waking in Drosophila melanogaster.
Science 287, 1834-1837 (2000)
3. Hendricks, J. C. et al. Rest in Drosophila is a sleep-like
state. Neuron 25, 129-138 (2000)
4. Konopka, R. J. & Benzer, S. Clock mutants of Drosophila
melanogaster. Proc. Natl Acad. Sci. USA 68, 2112-2116 (1971)
5. Handler, A. M. & Konopka, R. J. Transplantation of a circadian
pacemaker in Drosophila. Nature 279, 236-238 (1979)
Nature 2002 417:329
Related Background:
ON CIRCADIAN CLOCKS
The following points are made by T. Roenneberg and M. Merrow
(University of Munich, DE):
1) To cope with the regular daily changes in their environment,
organisms, from cyanobacteria to humans, have evolved an
endogenous clock that anticipates the changes and programs their
physiology accordingly. One of the most conspicuous circadian
features is the ability to maintain approximately one-day
oscillations in constant conditions, known as a free-running
clock. Depending on the organism and the nature of the constant
artificial environment it is exposed to -- the light intensity or
color, temperature, nutrient composition and so on -- the free-
running period can range from approximately 19 to 29 hours.
Although these periodicities are extremely precise from cycle to
cycle, they do not accurately represent the 24-hour day. In
nature, therefore, circadian clocks have to be synchronized on a
daily basis. The environmental signals used for this
"entrainment" are called "zeitgebers", and light appears to be
the most dominant zeitgeber.
2) Experiments that were designed to identify the circadian
photoreceptor -- for example, analyses of action spectra or of
mutants with altered circadian behavior -- show that the clock
can recruit light information from several redundant receptors
and pathways [1 3]. To complicate matters, light entrainment
pathways turn out to be themselves under clock control [1,4,5],
and genes that are essential for clock function turn out to be
closely linked to light transduction mechanisms. Recent research
investigating the circadian system in the higher plant
Arabidopsis take this complexity even further. The Arabidopsis
clock remains entrainable by light even when four known
photoreceptor genes -- phyA, phyB, cry1 and cry2 -- are "knocked
out" in a quadruple mutant, and likely candidates for additional
light inputs to the clock have been identified. An Arabidopsis
mutant (toc1-1) shows reduced light responses -- in this case to
the photoperiod -- even though the gene and its product are not
even light responsive. Finally, unlike in animals, the different
cellular clocks in different parts of the plant body appear to
function with complete autonomy.
3) Circadian clocks can be reset by a single light pulse. This
means that some variable of the mechanism that generates the
endogenous rhythmicity must directly or indirectly be affected by
light -- for example, by a decrease in its concentration.
Depending on the phase of the oscillation -- the "circadian time"
-- a light pulse will either delay or advance the oscillator.
These phase-specific light responses are the bases for
entrainment and can be represented by what is known as a "phase
response curve".
References (abridged):
1. Roenneberg, T, Merrow, M, and Eisensamer, B (1998). Cellular
mechanisms of circadian systems. Zoology 100, 273-286.
2. Freedman, MS, Lucas, RJ, Soni, B, von Schantz, M, Munoz, M,
David-Gray, ZK, and Foster, R (1999). Regulation of mammalian
circadian behavior by non-rod, non-cone, ocular photoreceptors.
Science 284, 502-504
3. Somers, DE, Devlin, PF, and Kay, SA (1998). Phytochromes and
cryptochromes in the entrainment of the Arabidopsis circadian
clock. Science 282, 1488-1490
4. Emery, P, So, WV, Kaneko, M, Hall, JC, and Rosbash, M (1998).
CRY, a Drosophila clock and light-regulated cryptochrome, is a
major contributor to circadian rhythm resetting and
photosensitivity. Cell 95, 669-679
5. Bognar, LK, Adam, AH, Thain, SC, Nagy, F, and Millar, AJ
(1999). The circadian clock controls the expression pattern of
the circadian input photoreceptor, phytochrome B. Proc Natl Acad
Sci USA 96, 14652-14657
Current Biology 2000 10:R742
Related Background:
RESETTING THE CIRCADIAN CLOCK BY SOCIAL EXPERIENCE IN DROSOPHILA
MELANOGASTER
The following points are made by J.D. Levine et al (Brandeis
University, US):
1) Circadian clocks in animals regulate the timing of molecular,
physiological, and behavioral rhythms. Environmental features
such as photoperiod and temperature cycles reset these biological
oscillators, enabling anticipation of dawn, dusk, and season (1-
5). Other kinds of cues ("nonphotic") also influence clock time.
For example, studies on humans, rodents, fish, and bees have
demonstrated social influences on rhythmicity, but underlying
sensory mechanisms remain unexplained. It is nonetheless clear
that multiple sensory pathways transmit ambient temporal
information from the periphery to clock cells in the brain.
2) The authors investigated social influence on circadian timing
in the fruit fly Drosophila melanogaster. The authors initially
hypothesized that the circadian phases [marked by the peak of
locomotor activity in DD (constant darkness)] would be more
coherent for Drosophila living together (group-housed) than those
of isolates, because groups of flies might agree about the time
of day even without photic cues. Locomotor activity rhythms from
group-housed wild-type individuals were compared to those of
sibling isolates. After an initial 5 days in 12 hours of light
and 12 hours of dark (LD 12:12), isolates and group-housed
subjects were maintained for 2 weeks in DD. Isolates were then
placed in activity monitors, whereas the group-housed flies were
separated and monitored in DD to assess the effects on individual
rhythmicity.
3) In summary: Circadian clocks are influenced by social
interactions in a variety of species, but little is known about
the sensory mechanisms underlying these effects. The authors
investigated whether social cues could reset circadian rhythms in
Drosophila melanogaster by addressing two questions: Is there a
social influence on circadian timing? If so, how is that
influence communicated? The authors suggest their experiments
indicate that in a social context Drosophila transmit and receive
cues that influence circadian time and that these cues are likely
olfactory.
References (abridged):
1. J. C. Dunlap, Cell 96, 271 (1999)
2. J. C. Hall, Adv. Genet. 38, 135 (1998)
3. M. W. Young, Annu. Rev. Biochem. 67, 135 (1998)
4. J. A. Williams and A. Sehgal, Annu. Rev. Physiol. 63, 729
(2001)
5. M. Moore-Ede, F. Sulzman, C. Fuller, The Clocks That Time Us
(Harvard Univ. Press, Cambridge, MA, 1982)
Science 2002 298:2010
Related Background Brief:
PHOTOPIGMENTS AND CIRCADIAN SYSTEMS OF VERTEBRATES. In the
retinal degeneration (rd) mouse the absence of rod cells and the
progressive loss of cones does not result in a decrease in
circadian phase shifting responses to light. By contrast, rd/rd
mice are unable to perform simple visual tasks. In addition,
rodless transgenic mice, and mice homozygous for the retinal
degeneration slow (rds) mutation, show unattenuated circadian
responses to light. Collectively these data suggest that cone
cells lacking outer segments are sufficient to maintain normal
circadian responses to light, or some unidentified photoreceptor
within the retina. An action spectrum for circadian responses to
light in rd/rd mice, and molecular analysis of retinally
degenerate mice and blind mole rat eyes, suggests the involvement
of a mid-to-long wavelength sensitive cone opsin in
photoentrainment. Extraocular photoreceptors of non-mammalian
vertebrates are currently being analyzed in order to identify
functional and evolutionary similarities between visual and non-
visual photoreceptor systems. S.M. Argamaso et al: Biophys Chem
1995 56:3
Related Background:
ON THE VERTEBRATE CIRCADIAN CLOCK SYSTEM
M.P. Pando et al (Louis Pasteur University Strasbourg, FR)
discuss the vertebrate circadian clock system, the authors making
the following points:
1) In recent years a new dimension has been added to our
knowledge of the vertebrate circadian clock system. The classical
view of the circadian system describes it as diverse
physiological rhythms regulated by a centralized clock structure.
However, during the past few years, the idea that the clock
consists exclusively of a few centralized structures has been
challenged. Data coming from both vertebrate and invertebrate
systems have demonstrated that the circadian timing system is
dispersed throughout the animal, and that possibly every cell
contains a functional circadian clock.
2) In these studies, it has been revealed that a variety of
tissues and cells contain functional autonomous clocks, and these
clocks are able to maintain an oscillation when placed in vitro
and removed from any external cues or signals that originate from
the classical clock structures and/or the environment. The
discovery of a number of genes involved in the generation and
maintenance of circadian oscillations, and the recent realization
that the circadian system consists of a complex network of
independent clocks that are somehow synchronized to properly
regulate all physiological rhythms has necessitated the
development of new tools and methodologies for deciphering the
circadian system. An ideal tool is an in vitro cell-based system
that displays robust circadian rhythms: cultured cells may be
used to fully understand the complex molecular mechanisms, signal
coupling, and regulatory feedback loops that are responsible for
the proper timing of a circadian oscillation.
Proc. Nat. Acad. Sci. 2001 98:10178
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3. GENES AND CIRCADIAN RHYTHMS
ORIGIN AND EVOLUTION OF CIRCADIAN CLOCK GENES IN PROKARYOTES
The following points are made by V. Dvornyk et al (University of
Haifa, IL):
1) Circadian clock genes are a vital and essential feature of
eukaryotes (1). Cyanobacteria were the first prokaryotes reported
to have the circadian clock regulated by a cluster of three
genes: kaiA, kaiB, and kaiC (2). Cyanobacteria are among the
oldest organisms on the earth, and they are among the most
successful in terms of ecological plasticity and adaptability
(3). In adaptation strategy of cyanobacteria, circadian clock
genes are of particular importance, because they underlie
fundamental physiological processes such as the regulation of
nitrogen fixation, cell division, and photosynthesis (4). The
clock genes are ubiquitous in cyanobacteria (5). In most
cyanobacteria, these genes were reported as a single copy (2,5),
although some of them or even a whole cluster may be duplicated.
They were shown to operate as a single unit and to follow a
feedback model of regulation: kaiA positively affects kaiBC
promoter, whereas overexpression of kaiC represses it (2). Among
these genes, kaiC is a crucial component of clock precession in
cyanobacteria (9-11).
2) Although the kai genes are under comprehensive study with
regard to the mechanism of action, their evolution has yet to be
resolved completely. The kaiC gene has a double-domain structure,
and each of the domains has an ATP/GTP-binding site, or Walker's
motif (2). Based on its structure and sequence homology, the kaiC
genes were classified as a family related to the RecA gene family
of ATP-dependent recombinases. In addition to the kaiC genes with
the typical double-domain structure, there are many single-domain
homologous genes in Archaea and Proteobacteria. It was assumed
that an ancestral single-domain kaiC gene was horizontally
transferred from Bacteria to Archaea and then the double-domain
kaiC evolved through duplication and subsequent fusion in Archaea
(12). Although the evolution of the kaiC genes has been
hypothesized, no data or hypotheses are available regarding the
evolution of two other circadian clock genes, kaiA and kaiB. The
evidence about the key role of kaiC in cyanobacterial clock
regulation, along with its homology to archaeal RecA genes,
suggests that this gene is evolutionarily the oldest among the
three.
3) In summary: Regulation of physiological functions with
approximate daily periodicity, or circadian rhythms, is a
characteristic feature of eukaryotes. Until recently,
cyanobacteria were the only prokaryotes reported to possess
circadian rhythmicity. It is controlled by a cluster of three
genes: kaiA, kaiB, and kaiC. Using sequence data of 70 complete
prokaryotic genomes from the various public depositories, the
authors show here that the kai genes and their homologs have
quite a different evolutionary history and occur in Archaea and
Proteobacteria as well. Among the three genes, kaiC is
evolutionarily the oldest, and kaiA is the youngest and likely
evolved only in cyanobacteria. The authors suggest their data
indicates that the prokaryotic circadian pacemakers have evolved
in parallel with the geological history of the earth, and that
natural selection, multiple lateral transfers, and gene
duplications and losses have been the major factors shaping their
evolution.
References (abridged):
1. Pittendrigh, C. S. (1993) Annu. Rev. Physiol. 55, 16-54
2. Ishiura, M. , Kutsuna, S. , Aoki, S. , Iwasaki, H. ,
Andersson, C. R. , Tanabe, A. , Golden, S. S. , Johnson, C. H. &
Kondo, T. (1998) Science 281, 1519-1523
3. Whitton, B. A. (1987) in Survival and Dormancy of
Microorganisms, ed. Hennis, Y. (Wiley, New York), pp. 109-167
4. Johnson, C. H. & Golden, S. S. (1999) Annu. Rev. Microbiol.
53, 389-409
5. Lorne, J. , Scheffer, J. , Lee, A. , Painter, M. & Miao, V. P.
(2000) FEMS Microbiol. Lett. 189, 129-133
Proc. Nat. Acad. Sci. 2003 100:2495
Related Background:
ON THE MOLECULAR CIRCADIAN SYSTEM
The following points are made by T. Roenneberg and M. Merrow
(University of Munich, DE):
1) The circadian system, is found in organisms of all phyla. The
term circadian, literally "about one day", refers to the
observation that the endogenous day is generally slightly shorter
or longer than 24 hours when the biological clock "free-runs" in
constant conditions, shielded from all environmental time cues
(zeitgebers). In free-run conditions, the temporal sequence of
endogenous events proceeds essentially unchanged; those events
that are normally scheduled to the light period occur in the
"subjective day", and those that normally take place in darkness
occur in the "subjective night".
2) Different centers that control circadian physiology have been
localized in the nervous systems of many animals, from
cockroaches to mammals. In humans, this center resides a couple
of centimeters behind the bridge of the nose, in a pair of nuclei
above the crossing of the optic nerves. Each of these
"suprachiasmatic nuclei" (SCN) is only about the size of a grain
of rice, but their qualities are remarkable. Individual rat SCN
cells in culture exhibit a circadian rhythm in spontaneous firing
rate that appears to be sustained indefinitely [1] . Through
their coupling, these cellular clocks acquire stunning functional
properties, such as the ability to activate or silence genes
throughout the body at the appropriate times, or to modulate our
senses and behavior. When SCN tissue is cross-transplanted
between two animals, circadian qualities are carried along [2];
for example, the activity rest cycle of the recipient reflects
the period of the donor.
3) The SCN thus appears to be responsible for organizing
endogenous daily programs throughout the body. When it became
clear that isolated body parts of insects are able to produce
circadian rhythms [3 5] , however, researchers looked at cultured
mammalian cells, such as rat fibroblasts, and found that they
also exhibit circadian gene expression. In tissues as different
as brain, heart, muscle or lung, a similar set of "clock genes"
undergo oscillatory changes in expression level. The SCN
"pacemaker" and these organ clocks have different qualities,
however, forming a hierarchy within the circadian system. The
same genes whose expression levels reach a maximum in the early
morning in the SCN do so several hours later in the periphery.
While the SCN rhythms continue indefinitely, the organ clocks
appear to dampen within a few days. When rats are subjected to a
"jetlag" experiment, in which the light:dark cycle is shifted by
several hours, rhythms shift with different speeds in different
organs. While the SCN apparently adjusts within one cycle, the
liver can take more than six days to synchronize with the new
light:dark cycle. This was a surprising observation, because the
mammalian activity rest rhythm is an output of the SCN and takes
several cycles to adjust to a shifted light regime.
References (abridged):
1. Welsh D.K., Logothetis D.E., Meister M. and Reppert S.M.
(1995) Individual neurons dissociated from rat suprachiasmatic
nucleus express independently phased circadian firing rhythms.
Neuron, 14:697-706
2. Ralph M.R., Foster R.G., Davis F.C. and Menaker M. (1990)
Transplanted suprachiasmatic nucleus determines circadian period.
Science, 247:975-978
3. Plautz J.D., Kaneko M., Hall J.C. and Kay S.A. (1997)
Independent photoreceptive circadian clocks throughout Drosophila
Science, 278:1632-1635
4. Brandes C., Plautz J.D., Stanewsky R., Jamison C.F., Straume
M., Wood K.V., Kay S.A. and Hall J.C. (1996) Novel features of
Drosophila period transcription revealed by real-time luciferase
reporting. Neuron, 16:687-692
5. Hege D.M., Stanewsky R., Hall J.C. and Giebultowicz J.M.
(1997) Rhythmic expression of a PER reporter in the Malpighian
tubules of decapitated Drosophila: evidence for a brain-
independent circadian clock. J. Biol. Rhythms, 12:300-308
Current Biology 2003 13:R198
Related Background:
ON CIRCADIAN CLOCKS
The following points are made by U. Schibler et al (University of
Geneva, CH):
1) In mammals, physiology and behavior are subject to daily
oscillations that are driven by an endogenous clock. The master
clock (circadian pacemaker) resides in the suprachiasmatic
nucleus (SCN) of the brain's hypothalamus. In the absence of
external time cues, the SCN master clock generates cycles of
approximately but not exactly 24 hours, and its phase must
therefore be readjusted every day. This task depends on the
retina, which detects changes in light intensity during the day's
light-dark cycle (the photoperiod) and transmits this information
to the SCN neurons.
2) Although the SCN is essential for circadian rhythmicity,
circadian oscillators composed of many of the same proteins as
the master clock seem to operate in most cells of the body. Under
normal conditions, the SCN pacemaker synchronizes these
peripheral clocks through neuronal and humoral signals. It is
intriguing that if feeding is restricted in mammals, the phase of
the oscillators in liver, kidney, heart, and pancreas becomes
completely uncoupled from the phase of the SCN timekeeper. These
findings clearly point toward an intricate interplay between
metabolism and the circadian timing system (1).
3) Rutter et al (3) report that the binding of two highly related
master clock proteins, the transcription factors NPAS2 and Clock,
to their DNA recognition sequences depends on the ratio of
oxidized to reduced nicotinamide adenine dinucleotide (NAD and
NADH, respectively). These molecules are essential components of
the respiratory enzyme chain, and the ratio between them
fluctuates according to changes in cellular metabolism. Reick et
al (2) screened a neuroblastoma cell line for target genes of the
NPAS2:BMAL1 transcription complex. They identified mRNA
transcripts for several essential clock components, including
PER1, PER2, CRY1, and BMAL1.
4) Circadian clocks are thought to operate through interlocking
feedback loops of gene expression. The first mammalian clock gene
identified, dubbed "Clock", was isolated by positional cloning in
a mutant mouse with altered circadian locomotor activity (4).
Clock is a basic helix-loop-helix (bHLH) transcription factor
that binds to E-box (CACGTG) DNA motifs when partnered with
BMAL1, another bHLH protein required for circadian clock activity
(5). The Clock:BMAL1 heterodimer stimulates the expression of
other essential pacemaker components, such as the period proteins
PER1 and PER2 and the cryptochromes CRY1 and CRY2. The CRYs
repress the transcription of target genes switched on by
Clock:BMAL1, and thereby establish a negative feedback loop in
which Per and Cry gene expression switches off Clock:BMAL1
transcriptional activity. Cryptochromes carry a flavin adenine
dinucleotide (FAD) cofactor and serve as circadian photoreceptors
in plants and the fruit fly Drosophila.
References (abridged):
1. J. A. Ripperger, U. Schibler, Curr. Opin. Cell Biol. 13, 357
(2001) [Medline].
2. M. Reick, J. A. Garcia, C. Dudley, S. L. McKnight, Science
293, 506 (2001); published online 5 July 2001
(10.1126/science.1060699).
3. J. Rutter, M. Reick, L. C. Wu, S. L. McKnight, Science 293,
510 (2001); 5 July 2001 (10.1126/science.1060698).
4. D. P. King et al., Cell 89, 641 (1997) [Medline].
5. M. K. Bunger et al., Cell 103, 1009 (2000) [Medline].
Science 2001 293:437
Related Background:
ENTRAINMENT OF THE CIRCADIAN CLOCK IN THE LIVER BY FEEDING
The following points are made by K-A. Stokkan et al (University
of Virginia, US):
1) The light-dark (LD) cycle is the most reliable and strongest
external signal that synchronizes (entrains) biological rhythms
with the environment. In mammals, LD information is perceived by
specialized retinal photoreceptors and conveyed directly to the
SCN of the hypothalamus, where it entrains circadian oscillators
in what is regarded as the master clock of the organism (1,2). In
addition, other cyclic inputs, such as temperature, noise, social
cues, or rhythmic access to food, may also act as entraining
agents, although the effects of these rhythmic signals on
behavior are often weak.
2) When food is available only for a limited time each day, rats
increase their locomotor activity 2 to 4 hours before the onset
of food availability (3). Such anticipatory behavior also occurs
in other mammals and in birds and is often paralleled by
increases in body temperature, adrenal secretion of
corticosterone, gastrointestinal motility, and activity of
digestive enzymes (4-5). Entrainment of anticipatory locomotion
by restricted feeding (RF) occurs independently of the LD cycle,
in constant light, and in SCN-lesioned animals, suggesting that
the circadian oscillators entrained by RF are distinct from those
entrained by light.
3) In summary: Circadian rhythms of behavior are driven by
oscillators in the brain that are coupled to the environmental
light cycle. Circadian rhythms of gene expression occur widely in
peripheral organs. It is unclear how these multiple rhythms are
coupled together to form a coherent system. To study such
coupling, the authors investigated the effects of cycles of food
availability (which exert powerful entraining effects on
behavior) on the rhythms of gene expression in the liver, lung,
and suprachiasmatic nucleus (SCN). The authors used a transgenic
rat model whose tissues express luciferase in vitro. Although
rhythmicity in the SCN remained phase-locked to the light-dark
cycle, restricted feeding rapidly entrained the liver, shifting
its rhythm by 10 hours within 2 days. The authors suggest their
results demonstrate that feeding cycles can entrain the liver
independently of the SCN and the light cycle, and they suggest
the need to reexamine the mammalian circadian hierarchy. The
results also raise the possibility that peripheral circadian
oscillators like those in the liver may be coupled to the SCN
primarily through rhythmic behavior, such as feeding.
References (abridged):
1. M. S. Freedman, et al., Science 284, 502 (1999)
2. D. C. Klein, R. Y. Moore, S. M. Reppert, Suprachiasmatic
Nucleus: The Mind's Clock (Oxford Univ. Press, New York, 1991)
3. C. P. Richter, Comp. Psychol. Monogr. 1, 1 (1922)
4. D. T. Krieger, Endocrinology 95, 1195 (1974)
5. C. A. Comperatore and F. K. Stephan, J. Biol. Rhythms 2, 227
(1987)
Science 2001 291:490
ScienceWeek http://www.scienceweek.com
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4. CIRCADIAN RHYTHMS AND THE MAMMALIAN NERVOUS SYSTEM
COORDINATION OF CIRCADIAN TIMING IN MAMMALS
The following points are made by S.M. Reppert and D.R. Weaver
(University of Massachusetts):
1) Circadian rhythms, as exemplified by the sleep/wake cycle, are
the outward manifestation of an internal timing system. The full
force of genetic, molecular and biochemical approaches,
complemented by precise behavioral observations, has rapidly
advanced our knowledge of circadian timing in mammals. The focal
point of this system is a master clock, located in the
suprachiasmatic nuclei (SCN) of the anterior hypothalamus, which
orchestrates the circadian program(1). Principal advances in
understanding the molecular and biochemical basis of circadian
timing have provided a rapidly evolving model of the underlying
"clockwork". Recent developments have also revolutionized our
view of SCN input and output mechanisms. These include the
discovery of a new visual pathway from retina to the SCN that
entrains (synchronizes) circadian rhythms to the solar day, and
the elucidation of ways in which the SCN clock ultimately
generates output rhythms in physiology and behavior.
2) Defining the molecular basis of circadian timing in mammals
has profound implications. In terms of fundamental brain
mechanisms, the circadian system is among the most tractable
models for providing a complete understanding of the cellular and
molecular events connecting genes to behavior. Thorough
dissection of the genetic basis of circadian behavior may help to
decipher this connection for more complex behaviors.
Understanding the molecular clock could increase our knowledge of
how gene mutations of the molecular clock contribute to
psychopathology (for example, major depression and seasonal
affective disorder)(2). Similarly, such understanding should lead
to new strategies for pharmacological manipulation of the human
clock to improve the treatment of jet lag and ailments affecting
shift workers, and of clock-related sleep and psychiatric
disorders.
3) Circadian timing in mammals is organized in a hierarchy of
multiple circadian oscillators. The oscillatory machinery of the
master clock is contained within single neurons(3), and it is
possible that most of the approximately 20,000 neurons that
comprise the bilateral SCN are "clock cells". Molecular evidence
is beginning to emerge for functionally distinct populations of
clock cells within the SCN(4-5).
4) In summary: Time in the biological sense is measured by cycles
that range from milliseconds to years. Circadian rhythms, which
measure time on a scale of 24 h, are generated by one of the most
ubiquitous and well-studied timing systems. At the core of this
timing mechanism is an intricate molecular mechanism that ticks
away in many different tissues throughout the body. However,
these independent rhythms are tamed by a master clock in the
brain, which coordinates tissue-specific rhythms according to
light input it receives from the outside world.
References (abridged):
1. Reppert, S. M. & Weaver, D. R. Molecular analysis of mammalian
circadian rhythms. Ann. Rev. Physiol. 63, 647-676 (2001)
2. Bunney, W. E. & Bunney, B. G. Molecular clock genes in man and
lower animals: possible implications for circadian abnormalities
in depression. Neuropsychopharmacology 22, 335-345 (2000)
3. Welsh, D. K., Logothetis, D. E., Meister, M. & Reppert, S. M.
Individual neurons dissociated from rat suprachiasmatic nucleus
express independently phase circadian firing rhythms. Neuron 14,
697-706 (1995)
4. Jagota, A., de la Iglesia, H. O. & Schwartz, W. J. Morning and
evening circadian oscillations in the suprachiasmatic nucleus in
vitro. Nature Neurosci. 3, 372-376 (2000)
5. Low-Zeddies, S. S. & Takahashi, J. S. Chimera analysis of the
Clock mutation in mice shows that complex cellular integration
determines circadian behavior. Cell 105, 25-42 (2001)
Nature 2002 418:935
Related Background:
ON CIRCADIAN RHYTHMS AND THE SUPRACHIASMATIC NUCLEUS
The following points are made by M.H. Hastings (MRC Centre
Cambridge, UK):
1) The body's neural and physiological preparations for
wakefulness are not just a response to the world outside, but are
strictly controlled by an internal clock -- a region in the
hypothalamus of the brain called the suprachiasmatic nucleus
(SCN)1. Neurons in the SCN have the remarkable ability to
autonomously generate a cycle of electrical activity with a
period very close to 24 hours.
2) A compelling picture of the core SCN clockwork has been
established by molecular genetics. In essence, the canonical
clock genes Period and Cryptochrome make up a self-sustaining
circadian oscillator(1) in which the critical mechanism is
delayed negative feedback. These genes are switched on by the
proteins Clock and Bmal, and are periodically switched off by a
complex of their own encoded proteins, Per and Cry. So gene turn-
off inevitably follows gene turn-on, in an inexorable daily loop,
and mutations that affect the loop's stability are tightly linked
to inherited human sleep disorders3.
3) A complementary molecular-genetics approach to understanding
sleep comes from the study of the gene encoding the
hypocretin/orexin peptides. Mutations in this gene, or in that
for the receptor protein that enables neurons to detect the
peptides, are linked to the sleep disorder narcolepsy(4), in
which people uncontrollably fall into brief periods of deep
sleep. The cells that make hypocretins/orexins lie in the dorsal
hypothalamus and have a powerful excitatory effect on neural
systems that sustain wakefulness. But what sits between the
oscillatory molecular cycle of the SCN and the neural machinery
that directly causes sleep and wakefulness? In other words, what
is the messenger of circadian time?
4) SCN neurons "fire" in a circadian pattern, suggesting that
they signal time by means of conventional electrochemical
communication with other neurons. Yet their physical connections
with other neurons are sparse and principally local. In fact,
intracerebral transplant studies have shown that physical contact
between SCN and target neurons is not necessary for them to
communicate(5). Rather, some molecule, presumably secreted from
the SCN under the influence of electrical firing, confers
circadian control of sleep and wakefulness. Cheng et al.(2) now
reveal that a small protein, prokineticin 2, may well be time's
messenger. Consisting of 81 amino acids and identified previously
as a regulator of gastrointestinal movements6, this protein
fulfils many of the criteria expected of the missing temporal
link.
References (abridged):
1. Reppert, S. M. & Weaver, D. R. Annu. Rev. Physiol . 63, 647-
676 (2001)
2. Cheng, M. Y. et al. Nature 417, 405-410 (2002)
3. Toh, K. L. et al. Science 291, 1040-1043 (2001)
4. Mignot, E. Neuropsychopharmacology 25, S5-S13 (2001)
5. Silver, R. et al. Nature 382, 810-813 (1996)
Nature 2002 417:391
Related Background:
PROKINETICIN 2 TRANSMITS THE BEHAVIORAL CIRCADIAN RHYTHM OF THE
SUPRACHIASMATIC NUCLEUS
The following points are made by M.Y. Cheng et al (University of
California Irvine, US):
1) The organization of physiology and behaviour with recurring
daily environmental conditions is an adaptation that occurs in
essentially all living organisms. Circadian rhythms are regulated
by three components: the circadian pacemaker, an input mechanism
and an output mechanism. In mammals, the pacemaker that drives
circadian rhythms resides in the suprachiasmatic nuclei (SCN) of
the anterior hypothalamus(1). Environmental light/dark cycles
entrain the SCN clock to the 24-h day by means of retinal
hypothalamic projections(2). Synchronization of SCN neurons leads
to coordinated circadian outputs that regulate expressed rhythms.
2) A clear understanding of the molecular clock mechanisms within
the SCN has emerged(3-5). The molecular clockwork consists of
autoregulatory transcriptional and translational feedback loops
that have both positive and negative elements. Positive
transcriptional elements include two basic helix loop helix, PAS-
domain-containing transcription factors, Clock and Bmal1, which
form heterodimers and drive the transcription of three mammalian
period genes (Per1, Per2 and Per3) and two cryptochrome genes
(Cry1 and Cry2) by binding to their respective CACGTG E-box
enhancers. The Per and Cry proteins act as negative components of
the feedback loop, with Cry proteins having dominant inhibitory
roles. Evidence also supports the existence of an interacting
molecular loop such that Per2 positively regulates Bmal1
transcription11.
3) Much of our understanding of the molecular clock mechanisms
results from genetic studies with mutant animals. Mice deficient
in Cry1, Cry2 or Per3 show subtle changes in circadian cycle
length, but retain rhythmicity. Mutations in clock, Per1 or Per2
genes result in a more severe circadian phenotype that includes
arrhythmicity after long-term housing in constant darkness. Mice
with disrupted Bmal1, or that are deficient in both Per1 and
Per2, exhibit the most severe phenotype: they are arrhythmic
immediately after placement in constant darkness. Similar
arrhythmicity also occurs in mice deficient in both Cry1 and Cry2
genes. The cloning of the Tau gene, which codes for a casein
kinase and whose mutation causes a shortened circadian cycle in
a strain of hamster, has revealed the importance of post-
transcriptional mechanisms for SCN clockwork.
4) In summary: The suprachiasmatic nucleus (SCN) controls the
circadian rhythm of physiological and behavioral processes in
mammals. The authors demonstrate that prokineticin 2 (PK2), a
cysteine-rich secreted protein, functions as an output molecule
from the SCN circadian clock. PK2 messenger RNA is rhythmically
expressed in the SCN, and the phase of PK2 rhythm is responsive
to light entrainment. Molecular and genetic studies have revealed
that PK2 is a gene that is controlled by a circadian clock
(clock-controlled). The receptor for PK2 (PKR2) is abundantly
expressed in major target nuclei of the SCN output pathway.
Inhibition of nocturnal locomotor activity in rats by
intracerebroventricular delivery of recombinant PK2 during
subjective night, when the endogenous PK2 mRNA level is low,
further supports the hypothesis that PK2 is an output molecule
that transmits behavioral circadian rhythm. The high expression
of PKR2 mRNA within the SCN and the positive feedback of PK2 on
its own transcription through activation of PKR2 suggest that PK2
may also function locally within the SCN to synchronize output.
References (abridged):
1. Klein, D. C., Moore, R. Y. & Reppert, S. M. (eds)
Suprachiasmatic Nucleus: The Mind's Clock 467 (Oxford Univ.
Press, New York, 1991)
2. Moore, R. Y. Circadian rhythms: basic neurobiology and
clinical applications. Ann. Rev. Med. 48, 253-266 (1997)
3. Dunlap, J. C. Molecular bases for circadian clocks. Cell 96,
271-290 (1999)
4. Gillette, M. U. & Tischkau, S. A. Suprachiasmatic nucleus: the
brain's clock. Rec. Prog. Hormone Res. 54, 33-59 (1999)
5. Hastings, M. H. Circadian clockwork: two loops are better than
one. Nature Rev. Neurosci. 1, 143-146 (2000)
Nature 2002 417:405
Related Background:
DIURNAL MODULATION OF PACEMAKER POTENTIALS AND CALCIUM CURRENT IN
THE MAMMALIAN CIRCADIAN CLOCK
The following points are made by C.M. Pennartz et al (Netherlands
Institute for Brain Research, NL):
1) It has long been known that neurons in the suprachiasmatic
nucleus (SCN) transmit circadian output to other brain areas by
diurnal modulation of their spontaneous discharge frequency(1-5).
When firing activity of SCN neurons is monitored in cultures of
dissociated cells, many of them express a circadian rhythm that
is asynchronous with that of other cells, indicating the cell-
autonomous nature of the oscillator(4). The molecular loops
operating within clock cells involve transcription, translation
and negative protein-mediated feedback on gene expression(5).
2) Much less is known about the signals by which the core loop
communicates with ionic channels and transporters in the plasma
membrane, which directly regulate membrane excitability. Ionic
channels may be under direct transcriptional control of the core
loop or, alternatively, may be regulated by as-yet unidentified
clock-controlled genes(5). A principal step in the elucidation of
this problem is to dissect the ionic mechanisms by which the
circadian message from the molecular clock is transduced into
bioelectric output. This, however, is not a simple problem.
First, there are many ionic currents in the SCN that may be
targeted by the molecular clock, yet their contribution to
spontaneous firing is largely unknown. Second, ionic currents in
SCN neurons can be monitored by patch-clamp recordings for
several hours, which is, however, too short to study circadian
modulation within a single neuron. This technical limitation
necessitates groupwise comparisons between neurons recorded
during different circadian phases. This consideration, combined
with the finding that clock cells become desynchronized in
dissociated-cell cultures(4), led the authors to use acutely
prepared brain slices in studying diurnal modulation of ionic
conductances.
3) In summary: The central biological clock of the mammalian
brain is located in the suprachiasmatic nucleus. This
hypothalamic region contains neurons that generate a circadian
rhythm on a single-cell basis. Clock cells transmit their
circadian timing signals to other brain areas by diurnal
modulation of their spontaneous firing rate. The intracellular
mechanism underlying rhythm generation is thought to consist of
one or more self-regulating molecular loops, but it is unknown
how these loops interact with the plasma membrane to modulate the
ionic conductances that regulate firing behavior. The authors
demonstrate a diurnal modulation of Ca2+ current in
suprachiasmatic neurons. This current strongly contributes to the
generation of spontaneous oscillations in membrane potential,
which occur selectively during daytime and are tightly coupled to
spike generation. Thus, day night modulation of Ca2+ current is a
central step in transducing the intracellular cycling of
molecular clocks to the rhythm in spontaneous firing rate.
References (abridged):
1. Inouye, S.-I. T. & Kawamura, H. Persistence of circadian
rhythmicity in a mammalian hypothalamic "island" containing the
suprachiasmatic nucleus. Proc. Natl Acad. Sci. USA 76, 5962-5966
(1979)
2. Green, D. J. & Gillette, R. Circadian rhythm of firing rate
recorded from single cells in the rat suprachiasmatic brain
slice. Brain Res. 245, 198-200 (1982)
3. Schwartz, W. J., Gross, R. A. & Morton, M. T. The
suprachiasmatic nuclei contain a tetrodotoxin-resistant circadian
pacemaker. Proc. Natl Acad. Sci. USA 84, 1694-1698 (1987)
4. Welsh, D. K., Logothetis, D. E., Meister, M. & Reppert, S. M.
Individual neurons dissociated from rat suprachiasmatic nucleus
express independently phased circadian firing rhythms. Neuron 14,
697-706 (1995)
5. Reppert, S. M. & Weaver, D. R. Molecular analysis of mammalian
circadian rhythms. Annu. Rev. Physiol. 63, 547-676 (2001)
Nature 2002 416:286
Related Background:
PHOTOTRANSDUCTION BY RETINAL GANGLION CELLS THAT SET THE
CIRCADIAN CLOCK
The following points are made by D.M. Berson et al (Brown
University, US):
1) The SCN is the circadian pacemaker of the mammalian brain,
driving daily cycles in activity, hormonal levels, and other
physiological variables. Light can phase-shift the endogenous
oscillator in the SCN, synchronizing it with the environmental
day-night cycle. This process, the photic entrainment of
circadian rhythms, originates in the eye and involves a direct
axonal pathway from a small fraction of retinal ganglion cells to
the SCN (1-3). A striking feature of this neural circuit is its
apparent independence from conventional retinal
phototransduction. In functionally blind transgenic mice lacking
virtually all known photoreceptors (rods and cones), photic
entrainment persists with undiminished sensitivity (4). Candidate
photoreceptors for this system are non-rod, non-cone retinal
neurons, including some ganglion cells, that contain novel opsins
or cryptochromes (5).
2) To determine whether retinal ganglion cells innervating the
SCN are capable of phototransduction, the authors labeled them in
the rat retina by retrograde transport of fluorescent
microspheres injected into the hypothalamus. In isolated retinas,
whole-cell recordings were made of the responses of labeled
ganglion cells to light. In most of these cells (n = 150), light
evoked large depolarizations with superimposed fast action
potentials. The light response persisted during bath application
of 2 mM cobalt chloride, which blocks calcium-mediated synaptic
release from rods, cones, and other retinal neurons. In contrast,
other ganglion cells prepared and recorded under identical
conditions but not selectively labeled from the SCN (control
cells) lacked detectable response to light even without synaptic
blockade. This is presumably because rod and cone photopigments
were extensively bleached. A few control cells (3/50) exhibited
weak, evanescent responses to light, but these were abolished by
bath-applied cobalt.
3) In summary: Light synchronizes mammalian circadian rhythms
with environmental time by modulating retinal input to the
circadian pacemaker -- the suprachiasmatic nucleus (SCN) of the
hypothalamus. Such photic entrainment requires neither rods nor
cones, the only known retinal photoreceptors. The authors
demonstrate that retinal ganglion cells innervating the SCN are
intrinsically photosensitive. Unlike other ganglion cells, they
depolarized in response to light even when all synaptic input
from rods and cones was blocked. The sensitivity, spectral
tuning, and slow kinetics of this light response matched those of
the photic entrainment mechanism, suggesting that these ganglion
cells may be the primary photoreceptors for this system.
References (abridged):
1. R. Y. Moore, J. C. Speh, J. P. Card, J. Comp. Neurol. 352, 351
(1995)
2. T. Roenneberg and R. G. Foster, Photochem. Photobiol. 66, 549
(1997)
3. M. von Schantz, I. Provencio, R. G. Foster, Invest.
Ophthalmol. Vis. Sci. 41, 1605 (2000)
4. M. S. Freedman et al., Science 284, 502 (1999)
5. I. Provencio, G. Jiang, W. J. De Grip, W. P. Hayes, M. D.
Rollag, Proc. Natl. Acad. Sci. U.S.A. 95, 340 (1998)
Science 2002 295:1070
Related Background Brief:
ACTION SPECTRUM FOR MELATONIN REGULATION IN HUMANS: EVIDENCE FOR
A NOVEL CIRCADIAN PHOTORECEPTOR. The photopigment in the human
eye that transduces light for circadian and neuroendocrine
regulation is unknown. The aim of this study was to establish an
action spectrum for light-induced melatonin suppression that
could help elucidate the ocular photoreceptor system for
regulating the human pineal gland. Subjects (37 females, 35
males, mean age of 24.5 +/- 0.3 years) were healthy and had
normal color vision. Full-field, monochromatic light exposures
took place between 2:00 and 3:30 A.M. while subjects' pupils were
dilated. Blood samples collected before and after light exposures
were quantified for melatonin. Each subject was tested with at
least seven different irradiances of one wavelength with a
minimum of 1 week between each nighttime exposure. Nighttime
melatonin suppression tests (n = 627) were completed with
wavelengths from 420 to 600 nm. The data were fit to eight
univariant, sigmoidal fluence-response curves (R(2) = 0.81-0.95).
The action spectrum constructed from these data fit an opsin
template (R(2) = 0.91), which identifies 446-477 nm as the most
potent wavelength region providing circadian input for regulating
melatonin secretion. The results suggest that, in humans, a
single photopigment may be primarily responsible for melatonin
suppression, and its peak absorbance appears to be distinct from
that of rod and cone cell photopigments for vision. The data also
suggest that this new photopigment is retinaldehyde based. These
findings suggest that there is a novel opsin photopigment in the
human eye that mediates circadian photoreception. G.C. Brainard
et al: J. Neurosci 2001 21:6405
Related Background:
ENTRAINMENT OF FREE-RUNNING CIRCADIAN RHYTHMS BY MELATONIN IN
BLIND PEOPLE
In humans, the endogenous circadian pacemaker oscillates with a
period that is slightly longer than 24 hours, and the pacemaker
therefore requires synchronization ("entrainment") to the 24-hour
day.
In mammals, including humans, the biological clock apparently
resides in a group of neuron clusters in a part of the brain
called the hypothalamus, a region that responds to many chemical
inputs, including the hormone melatonin, an indole derived from
the metabolism of serotonin. Melatonin is secreted by another
hypothalamic brain structure, the pineal gland, which in turn is
stimulated by neurons in a nearby cluster (the suprachiasmatic
nucleus) that receives input from the retina of the eye. So this
is the apparent pathway of light-induced secretion of melatonin
and action in mammals: light on the retina, electrical activity
in the retino-hypothalamic tract, activity in a hypothalamic
region called the suprachiasmatic nucleus, electrical signals to
the pineal gland, secretion of the hormone melatonin, action of
melatonin on other neural structures in the hypothalamus and
elsewhere.
In totally blind people, light cues are unavailable, and
disturbances of circadian rhythms are common.
R.L. Sack et al (4 authors at Oregon Health Sciences University
Portland, US) report a study of the effects of melatonin
administration on the circadian rhythms of totally blind people,
the authors making the following points:
1) Most totally blind people have circadian rhythms that are
"free-running", i.e., that are not synchronized to environmental
time cues and that oscillate on a cycle slightly longer than 24
hours. This condition causes recurrent insomnia and daytime
sleepiness when the rhythms drift out of phase with the normal
24-hour cycle. The authors investigated whether a daily dose of
melatonin could entrain the circadian rhythms of totally blind
people to a normal 24-hour cycle.
2) A crossover study was performed on 7 totally blind subjects
who had free-running circadian rhythms. The subjects were given
10 milligrams of melatonin or a placebo daily, one hour before
their preferred bedtime, for 3 to 9 weeks. They were then given
the other treatment. The timing of the production of endogenous
melatonin was measured as a marker of the circadian time (phase),
and sleep was monitored by *polysomnography.
3) At base line, the subjects had free-running circadian rhythms
with distinct and predictable cycles averaging 24.5 hours (range:
24.2 to 24.9). These rhythms were unaffected by the
administration of placebo. In 6 of the 7 subjects, the rhythm was
entrained to a 24.0 hour cycle during melatonin treatment.
4) The authors point out that there are approximately 1 million
blind people in the US, of whom approximately 20 percent are
totally blind, and it is estimated that at least half of this 20
percent (approximately 100,000 people) probably have free-running
circadian rhythms, with a high proportion having circadian sleep-
wake disorders. The authors suggest melatonin may prove to be a
safe and effective treatment for many of these people.
5) The authors also suggest that the phase-shifting effects of
melatonin observed in their study of circadian rhythms in blind
people may be relevant to the treatment of sighted people as
well. "People who fly across multiple time zones or who work
nighttime or early-morning shifts routinely have symptoms of
disordered sleep as a result of circadian disturbances. Similar
pathophysiologic mechanisms have been proposed for advanced and
delayed sleep-phase syndromes as well as for winter depression."
6) The authors conclude: "Free-running circadian rhythms in blind
people can be entrained to a 24-hour cycle with a daily dose of
melatonin, thereby preventing a burdensome sleep disorder."
New Engl. J. Med. 2000 343:1070
Notes:
*polysomnography: This technique involves synchronized recordings
of electrical activity in the brain, muscles, and eyes, as well
as other physiological measures (e.g., electrocardiogram) during
sleep.
Related Background:
ON MELATONIN
Melatonin is a hormone secreted by the human pineal gland during
night-time darkness, and it is now being marketed in the US as a
nutritional supplement. The hormone is an indoleamine compound
derived from the amino acid *tryptophan, with *serotonin as an
intermediate precursor.
R.L. Sack reviews the neurobiology and medical aspects of
melatonin, and makes the following points:
1) The most important role of melatonin in all species is to
provide a hormonal signal of night-time darkness. The secretion
of the hormone is tightly controlled by the *circadian pacemaker.
2) Melatonin is a phylogenetically ancient hormone, found even in
some single-cell organisms and in some plants. In lower
vertebrates (e.g., reptiles), the pineal body lies close to the
skin and is directly photosensitive: sunlight falling on the
overlying skin inhibits melatonin production. In these species,
the pineal body has been called a "third eye". In mammals, the
pineal gland is deep within the skull and is not photosensitive.
The timing of melatonin secretion in mammals is controlled by
neural pathways: tracts from the retina of the eye to the
*hypothalamus (retino-hypothalamic tract) and from the
hypothalamus (suprachiasmatic *nucleus) to the pineal gland. The
suprachiasmatic nucleus of the hypothalamus is the master
circadian pacemaker in mammals, controlling the timing of most
circadian rhythms, including core body temperature, *cortisol
secretion, sleepiness, and melatonin secretion.
3) At the cellular level, melatonin receptors are members of the
superfamily of *G protein-coupled receptors, which
characteristically have 7 *transmembrane domains. Activation of
these receptors inhibits *cyclic AMP production by the enzyme
adenylyl cyclase.
4) The author suggests that as a therapeutic agent, melatonin can
be useful in the treatment of certain sleep and mood disorders.
The author suggests the basis for this is circadian phase-
shifting and the release of accumulated sleep drive.
5) Concerning its use as a nutritional supplement, the author
says, "Melatonin appears to be remarkably safe, at least for
short-term use... The effects of long-term administration are not
defined." Concerns have been raised about possible reproductive
effects, but most studies have shown little or no effect on
reproductive hormone levels. There are reports that melatonin
modifies the *vasoconstriction response in rat arteries.
Science & Medicine 1998 Sep/Oct
Notes:
*tryptophan: A nutritionally essential amino acid that serves as
a precursor for many molecular entities of importance in the
nervous system.
*serotonin: (5-hydroxytryptamine, 5-HT) Synthesized from
tryptophan. Acts as both a peripheral neurotransmitter in the gut
and a central neurotransmitter in the brain.
*circadian pacemaker: Many organisms exhibit daily (circadian)
rhythms, cyclical variations in various bodily functions,
metabolisms, etc., even in constant light or constant darkness.
In simple organisms, the pacemakers are biochemical reaction
loops; in higher organisms, complex signaling structures are
involved in the rhythms.
*hypothalamus: A deep brain structure with various clusters of
nerve cells controlling several important homeostatic functions
such as temperature regulation and food intake, and in addition
the sex drive, aggressive emotions, psychosomatic effects, etc.
The hypothalamus essentially integrates the activity of the
autonomic nervous system, and it acts as an intermediary between
the endocrine (hormone) system and the nervous system, with
various hypothalamic neuron types secreting hormones themselves.
In general, the term "hormones" refers to chemical messengers
which are distributed systemically via the bloodstream.
*nucleus: In this context, the term "nucleus" refers to a cluster
of nerve cells involved in a particular neurological function.
*cortisol: A corticosteroid hormone secreted by the adrenal
gland.
*G protein: G-proteins are a family of signal-coupling proteins
that act as intermediaries between activated cell receptors and
effectors, for example, the transduction of hormonal signals from
the cell surface to the cell interior, and certain G-proteins are
known to interact with adenylyl cyclase. The G-protein is
apparently embedded in the cell membrane with parts exposed on
the outside surface and inside surface. The outside moiety is
activated by the first messenger, and the inside moiety activates
the second messenger, the G-protein thus acting as a trans-
membrane signal transducer.
*transmembrane domains: A transmembrane domain is a segment of
protein anchored in the plasma membrane bilayer. If one
visualizes the protein as a long linear polymer, the polymer can
be looped back and forth across the plasma membrane with
different segments of the protein anchored in the membrane
according to lipid solubility characteristics of the segments of
the polymer chain.
*cyclic AMP: ATP (adenosine triphosphate) is the most important
chemical energy source in all living cells, intimately involved
in various cell functions and cell metabolism, and an entity in
numerous cyclic chemical pathways involved in the synthesis of
components. One of the reaction products of ATP is cAMP (cyclic
AMP, or adenosine 3,5-monophosphate), which acts as an
intracellular hormone (i.e., a chemical messenger). Cyclic AMP is
derived from ATP in a reaction catalyzed by the enzyme adenylyl
cyclase (also called adenyl cyclase and adenylate cyclase).
Cyclic AMP is called the second messenger; the first messenger is
the hormone that interacts with its receptor on the cell surface.
*vasoconstriction response: In general, the term vasoconstriction
refers to a narrowing of blood vessels, which in higher organisms
is under physiological control via various signaling systems.
Vasoconstriction produces an increase in blood pressure, systemic
or local, depending on the distribution of signals.
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5. PHOTOPERIODISM IN PLANTS
REGULATION OF PHOTOPERIODIC FLOWERING BY ARABIDOPSIS
PHOTORECEPTORS
The following points are made by T. Mockler et al (University of
California Los Angeles, US):
1) Photoperiodic flowering in plants was the first photoperiodism
phenomenon documented (1). The flowering of long-day (LD) or
short-day (SD) plants occurs or is accelerated in the LD or SD
condition, respectively. Arabidopsis is a facultative LD plant
for which flowering-time regulation has been extensively studied
(2-5). Although the detailed mechanism underlying photoperiodism
is not well understood, extensive plant physiological studies
support a hypothesis referred to as the "external coincidence
model". According to this hypothesis, the light signal must
interact at the appropriate time of the day (or "coincide") with
the photoperiodic response rhythm (PRR) of a cellular activity to
confer photoperiodic responsiveness. It has been found that mRNA
expression of flowering-time genes in Arabidopsis, including CO,
GI, and FT, exhibited circadian rhythms, which have different
phase shapes in plants grown in LD compared with plants grown in
SD (9-12). Therefore, the day-length-dependent circadian
expression of one or more flowering-time genes may represent the
PRR.
2) Arabidopsis relies on at least nine photosensory receptors,
including five phytochromes (phyA-phyE), two cryptochromes (cry1
and cry2), and two phototropins (phot1 and phot2), to regulate
most of its light responses. Among these photoreceptors,
phytochromes and cryptochromes are known to regulate flowering
time (5). It has also been found that phyA and cry2 protein
abundance is regulated by light and that cry2 expression changes
in response to photoperiod. These studies indicate that cry2 and
phyA may act as major day-length sensors. Indeed, it has been
found that the coincidence of light perception by cry2 and phyA
with the peak circadian expression of the CO gene is critical for
the induction of the expression of the flowering-time gene FT and
photoperiodic flowering.
3) In summary: Photoperiodism is a day-length-dependent seasonal
change of physiological or developmental activities that is
widely found in plants and animals. Photoperiodic flowering in
plants is regulated by photosensory receptors including the
red/far-red light-receptor phytochromes and the blue/UV-A light-
receptor cryptochromes. However, the molecular mechanisms
underlying the specific roles of individual photoreceptors have
remained poorly understood. The authors report a study of the
day-length-dependent response of cryptochrome 2 (cry2) and
phytochrome A (phyA) and their role as day-length sensors in
Arabidopsis. The protein abundance of cry2 and phyA showed a
diurnal rhythm in plants grown in short-day but not in plants
grown in long-day. The short-day-specific diurnal rhythm of cry2
is determined primarily by blue light-dependent cry2 turnover.
Consistent with a proposition that cry2 and phyA are the major
day-length sensors in Arabidopsis, the authors demonstrate that
phyA mediates far-red light promotion of flowering with modes of
action similar to that of cry2. Based on these results and a
finding that the photoperiodic responsiveness of plants depends
on light quality, the authors propose a model to explain how
individual phytochromes and cryptochromes work together to confer
photoperiodic responsiveness in Arabidopsis.
References (abridged):
1. Garner, W. W. & Allard, H. A. (1920) J. Agric. Res. 18, 553-
606
2. Koornneef, M. , Alonso-Blanco, C. , Peeters, A. J. M. & Soppe,
W. (1998) Annu. Rev. Plant Physiol. Plant Mol. Biol. 49, 345-370
3. Levy, Y. Y. & Dean, C. (1998) Plant Cell 10, 1973-1990
4. Pineiro, M. & Coupland, G. (1998) Plant Physiol. 117, 1-8
5. Lin, C. (2000) Plant Physiol. 123, 39-50
Proc. Nat. Acad. Sci. 2003 100:2140
Related Background:
PHOTOPERIODISM: THE COINCIDENTAL PERCEPTION OF THE SEASON
The following points are made by Seth J. Davis (Max Planck
Institute for Plant Breeding Research Cologne, DE):
1) Day-length sensing, or photoperiodism, has fascinated
biologists for greater than a century. Work on plants in
particular has provided insight into the physiological, and now
molecular, mechanisms that underlie this dynamic process [1].
Photoperiodic perception is one of the most significant
environmental interactions a plant has -- so much so that the
change of photoperiod throughout the year is perceived as a cue
for seasonal change. One of the most profound, and often
beautiful, hallmarks of a plant's response to seasonal change is
the developmentally timed generation of flowers. The molecular
events that constitute floral timing in response to inductive
photoperiods are starting to be resolved.
2) Molecular genetic analyses have identified photoreceptors and
light-signaling components, and components of the circadian
system, which are essential for a plant to make a proper
photoperiodic response [2]. Independent studies by the Carre [3]
and Kay [4] groups have now provided a molecular foundation for
understanding how light perception is integrated with the
circadian system in the generation of a photoperiodic response.
Their work supports the classical "external coincidence" model,
which functions through the circadian expression of the CONSTANS
gene and light activation of the encoded protein [3,4].
3) In the external-coincidence model, a physiological response,
such as flowering, is triggered when light perception coincides
with the time when expression of a circadian-regulated gene
exceeds a required threshold. For example, under long days (LD),
light is perceived both at dawn and dusk of the expression phase,
whereas under short days (SD), threshold expression is restricted
to the dark. In the internal-coincidence model, the effect of
light is simply to entrain two distinct circadian oscillators.
For example, long days cause the two rhythms to be entrained with
similar phases; this could generate two regulatory molecules that
require each other's activity for physiological function. Under
short days, the phases of the two entrained rhythms are further
apart; this could restrict the simultaneous expression of two
factors, thus inhibiting their co-action.[5]
References (abridged):
1. Thomas B. and Vince-Prue D. Photoperiodism in Plants. (1997)
(2nd Edition): Academic Press
2. Mouradov A., Cremer F. and Coupland G. (2002) Control of
flowering time: interacting pathways as a basis for diversity.
Plant Cell, 14:suppl:S111-S130
3. Roden L.C., Song H.R., Jackson S., Morris K. and Carre I.A.
(2002) Floral responses to photoperiod are correlated with the
timing of rhythmic expression relative to dawn and dusk in
Arabidopsis Proc. Natl. Acad. Sci. USA, 99:13313-13318
4. Yanovsky M.J. and Kay S.A. (2002) Molecular basis of seasonal
time measurement in Arabidopsis Nature, 419:308-312
5. Mockler T.C., Guo H., Yang H., Duong H. and Lin C. (1999)
Antagonistic actions of Arabidopsis cryptochromes and phytochrome
B in the regulation of floral induction. Development, 126:2073-
2082
Current Biology 200212:R841
Related Background:
MOLECULAR BASIS OF SEASONAL TIME MEASUREMENT IN ARABIDOPSIS
The following points are made by M.J. Yanovsky and S.A. Kay
(Scripps Research Institute, US):
1) Several organisms have evolved the ability to measure day-
length, or photoperiod, allowing them to adjust their development
in anticipation of annual seasonal changes. Day-length
measurement requires the integration of temporal information,
provided by the circadian system, with light/dark discrimination,
initiated by specific photoreceptors.
2) The molecular genetic dissection of flowering time in
Arabidopsis has identified several photoreceptors and light
signaling proteins, as well as some putative clock components,
that are essential for an appropriate photoperiodic response.
However, we still lack an adequate understanding of how temporal
and light environment information are integrated at the molecular
level to allow daylength regulation of flowering time.
3) The authors demonstrate that in Arabidopsis this integration
takes place at the level of CONSTANS (CO)(1) function. CO is a
transcriptional activator that accelerates flowering time in long
days, at least in part by inducing the expression of FLOWERING
LOCUS T (FT)(2-5). First, the authors show that precise clock
control of the timing of CO expression, such that it is high
during daytime only in long days, is critical for day-length
discrimination. The authors then provide evidence that CO
activation of FT expression requires the presence of light
perceived through cryptochrome 2 (cry2) or phytochrome A (phyA).
The authors conclude that an external coincidence mechanism,
based on the endogenous circadian control of CO messenger RNA
levels, and the modulation of CO function by light, constitutes
the molecular basis for the regulation of flowering time by
daylength in Arabidopsis.
References (abridged):
1. Putterill, J., Robson, F., Lee, K., Simon, R. & Coupland, G.
The CONSTANS gene of Arabidopsis promotes flowering and encodes a
protein showing similarities to zinc finger transcription
factors. Cell 80, 847-857 (1995)
2. Samach, A. et al. Distinct role of CONSTANS target genes in
reproductive development of Arabidopsis. Science 288, 1613-1616
(2000)
3. Kardailsky, I. et al. Activation tagging of the floral inducer
FT. Science 286, 1962-1965 (1999)
4. Onouchi, H., Igeno, M. I., Perilleux, C., Graves, K. &
Coupland, G. Mutagenesis of plants overexpressing CONSTANS
demonstrates novel interactions among Arabidopsis flowering-time
genes. Plant Cell 12, 885-900 (2000)
5. Kobayashi, Y., Kaya, H., Goto, K., Iwabuchi, M. & Araki, T. A
pair of related genes with antagonistic roles in mediating
flowering signals. Science 286, 1960-1962 (1999)
Nature 2002 419:308
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6. COMPUTATIONAL MODELS OF CIRCADIAN RHYTHMS
COMPUTATIONAL APPROACHES TO CELLULAR RHYTHMS
The following points are made by A. Goldbeter (Universit‚ Libre
de Bruxelles, BE):
1) Rhythmic phenomena represent one of the most striking
manifestations of dynamic behavior in biological systems. In
1936, Fessard(1) published a book on the Rhythmic Properties of
Living Matter. This book was devoted solely to the oscillatory
properties of nerve cells, but it is now clear that rhythms are
encountered at all levels of biological organization, with
periods ranging from a fraction of a second to years(2,3). These
rhythms find their roots in the many regulatory mechanisms that
control the dynamics of living systems. Thus, at the cellular
level, neural and cardiac rhythms are associated with the
regulation of voltage-dependent ion channels, metabolic
oscillations originate from the regulation of enzyme activity,
pulsatile intercellular signals and intracellular calcium
oscillations involve the control of receptor activity or
transport processes, while regulation of gene expression
underlies circadian rhythms.
2) Understanding the molecular and cellular mechanisms
responsible for oscillations is crucial for unraveling the
dynamics of life. When based firmly on experiments, computational
biology provides an essential tool for studying these mechanisms
which, because of their complexity, cannot be comprehended by
sheer intuition alone.
3) Theoretical models for biological rhythms were first used in
ecology to study the oscillations resulting from interactions
between populations of predators and prey(4). Neural rhythms
represent another field where such models were used at an early
stage: the formalism developed by Hodgkin and Huxley(5) still
forms the core of most models for oscillations of the membrane
potential in nerve and cardiac cells. Of more recent vintage are
models for oscillations of non-electrical nature that arise at
the cellular level from regulation of enzyme, receptor or gene
activity (see ref. 3 for a detailed list of references).
4) In summary: Oscillations arise in genetic and metabolic
networks as a result of various modes of cellular regulation. In
view of the large number of variables involved and of the
complexity of feedback processes that generate oscillations,
mathematical models and numerical simulations are needed to fully
grasp the molecular mechanisms and functions of biological
rhythms. Models are also necessary to comprehend the transition
from simple to complex oscillatory behavior and to delineate the
conditions under which they arise. Examples ranging from calcium
oscillations to pulsatile intercellular communication and
circadian rhythms illustrate how computational biology
contributes to clarify the molecular and dynamical bases of
cellular rhythms.
References (abridged):
1. Fessard, A. Propri‚t‚s Rythmiques de la MatiŠre Vivante
(Hermann, Paris, 1936)
2. Winfree, A. T. The Geometry of Biological Time 2nd edn
(Springer, New York, 2001)
3. Goldbeter, A. Biochemical Oscillations and Cellular Rhythms.
The Molecular Bases of Periodic and Chaotic Behaviour (Cambridge
Univ. Press, Cambridge, 1996)
4. Volterra, V. Fluctuations in the abundance of a species
considered mathematically. Nature 118, 558-560 (1926)
5. Hodgkin, A. L. & Huxley, A. F. A quantitative description of
membrane currents and its application to conduction and
excitation in nerve. J. Physiol. (Lond.) 117, 500-544 (1952)
Nature 2002 420:238
Related Background:
ROBUSTNESS OF CIRCADIAN RHYTHMS WITH RESPECT TO MOLECULAR NOISE
The following points are made by D. Gonze et al (Universit‚ Libre
de Bruxelles, BE):
1) Circadian rhythms characterized by a period close to 24 h are
observed in nearly all living organisms from cyanobacteria to
Neurospora, plants, insects such as Drosophila, and mammals. The
molecular mechanism of these rhythms relies on negative
autoregulatory feedback on gene expression (1-4). Theoretical
models for circadian rhythms based on such control mechanisms
have been proposed (5). The question arises as to the biological
validity of these models when the numbers of mRNA and protein
molecules involved in the regulatory mechanism are small, as may
occur in cellular conditions.
2) The authors use a core molecular model proposed for circadian
rhythms in Drosophila to assess its robustness with respect to
molecular noise. By means of stochastic simulations the authors
demonstrate that robust circadian oscillations already can be
produced by the autoregulatory mechanism when the maximum numbers
of mRNA and protein molecules are in the order of tens and
hundreds, respectively. The robustness of circadian oscillations
increases with both the number of molecules and the degree of
cooperativity of the repression process, and entrainment by
light/dark (LD) cycles stabilizes the phase of the oscillations
with respect to molecular noise.
References (abridged):
1. Dunlap, J. C. (1999) Cell 96, 271-290
2. Young, M. W. & Kay, S. A. (2001) Nat. Rev. Genet. 2, 702-715
3. Williams, J. A. & Sehgal, A. (2001) Annu. Rev. Physiol. 63,
729-755
4. Reppert, S. M. & Weaver, D. R. (2001) Annu. Rev. Physiol. 63,
647-676
5. Goldbeter, A. (1995) Proc. R. Soc. London Ser. B 261, 319-324
Proc. Nat. Acad. Sci. 2002 99:673
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