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CELL BIOLOGY: ON CENTRIOLES

The following points are made by J.L. Feldman and W.F. Marshall (Current Biology 2004 14:R659):

1) The centriole is a cylindrical structure resembling an old fashioned lawn mower composed of a rotationally symmetric array of nine triplet-microtubule blades. These blades are arranged around a nine-spoked central hub called the cartwheel. Recently, Matsuura et al[1] identified Bld10p, a novel coiled-coil protein that localizes to the cartwheel. This study has breathed new life into a century-long debate concerning the function of centrioles.

2) Centrioles, together with so-called pericentriolar material, comprise the centrosome, the microtubule-organizing center of a eukaryotic cell. The centriole also serves as a basal body to nucleate the assembly of a cilium or flagellum. Recently, genes have been identified in Drosophila, Caenorhabditis elegans, Paramecium, and the biflagellate unicellular green alga Chlamydomonas reinhardtii that are involved in centriolar assembly. In particular, d-tubulin and e-tubulin, members of the tubulin superfamily, are crucial for proper centriolar assembly in Chlamydomonas[2,3]. Loss of d-tubulin results in centrioles with doublet, instead of triplet, microtubules [2]. Reduction of e-tubulin by a hypomorphic mutation, bld2-1, results in cells with shortened centrioles composed of singlet microtubules [3]. Complete deletion of e-tubulin is lethal [4], suggesting that the centriole may be essential in Chlamydomonas.

3) Contrary to this idea, Matsuura et al.[1] have now reported that Bld10p is essential for centriolar assembly, but not for cell survival. They found that Chlamydomonas cells lacking Bld10p have no identifiable centrioles. These mutant cells are aflagellate -- the so-called "bald" phenotype -- with aberrantly placed mitotic spindles and generally disorganized cytoskeletal structures, a phenotype reminiscent of bld2-1 cells [3,5]. Yet unlike e-tubulin, Bld10p does not appear to be essential for cell survival; bld10 deletion mutants divide, albeit slowly, despite their defective mitotic spindles.

4) So are centrioles essential or not? If centrioles are essential, then mutations in genes required for centriolar assembly should be lethal. Why then is the bld10 mutant viable? One plausible explanation is that bld10 cells still have centrioles. Matsuura et al.[1] reportedly see no morphologically distinguishable basal bodies in more than a thousand thin sections. This negative result may be misleading, however, given the difficulty in observing small centriolar structures. For example, Goodenough and St. Clair (1975) reported finding only 15 sections among several thousand thin sections of bld2-1 cells that contained singlet-microtubule centriolar structures. As Bld10p staining is not perturbed in bld2 cells [1], Bld10p may act earlier than e-tubulin. If so, bld10 cells might have even smaller centrioles than those in bld2 cells, which would be even harder to detect.

References (abridged):

1 Matsuura, K.X Lefebvre, P.A.X Kamiya, R. and Hirono, M. (2004). Bld10p, a novel protein essential for basal body assembly in Chlamydomonas: localization to the cartwheel, the first ninefold symmetrical structure appearing during assembly. J. Cell Biol. 165, 663-671

2 Dutcher, S.K. and Trabuco, E.C. (1998). The UNI3 gene is required for assembly of basal bodies of Chlamydomonas reinhardtii and encodes d-tubulin, a new member of the tubulin superfamily. Mol. Biol. Cell. 9, 1293-1308

3 Dutcher, S.K.X Morrissette, N.S.X Preble, A.M.X Rackley, C. and Stanga, J. (2002). e-Tubulin is an essential component of the centriole. Mol. Biol. Cell. 13, 3859-3869

4 Preble, A.M.X Giddings, T.H. and Dutcher, S.K. (2001). Extragenic bypass suppressors of mutations in the essential gene BLD2 promote assembly of basal bodies with abnormal microtubules in Chlamydomonas reinhardtii. Genetics 157, 163-181

5 Ehler, L.L.X Holmes, J.A. and Dutcher, S.K. (1995). Loss of spatial control of the mitotic spindle apparatus in a Chlamydomonas reinhardtii mutant strain lacking basal bodies. Genetics 141, 945-960

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CELL BIOLOGY: ON CENTROSOME-NUCLEUS ATTACHMENT

The following points are made by Pierre Goenczy (Current Biology 2004 14:R268):

1) The centrosome plays a key role in cellular architecture by determining the position of several associated organelles, including the nucleus. The attachment of centrosome and nucleus is robust, withstanding cell lysis and homogenization, as well as solubilization of the nuclear envelope [1,2]. Microtubules appear to play a role in this attachment, as centrosomes and nuclei can be separated by treatment of sea urchin embryos with the microtubule destabilizing drug colcemid [3]. Dynein has been implicated as well, because attachment defects are apparent in a fraction of Drosophila or Caenorhabditis elegans embryos with compromised dynein function [4,5].

2) Dynein anchored on the outer nuclear envelope may thus contribute to attachment with the centrosome by transporting the nuclear cargo towards the minus-end of microtubules [5], but other components must be involved. A recent study has shown that the Hook protein ZYG-12 is indispensable for attachment of centrosome and nucleus in C. elegans; the results suggest a two-step mechanism for retention of the centrosome-nucleus connection.

3) The sperm contributes the sole pair of centrioles to the newly fertilized wild-type C. elegans embryo. The first centrosome in the zygote is associated with the male pronucleus, in close proximity to the posterior cortex. After centrosome duplication, the two centrosomes separate along the nuclear envelope to opposite sides of the male pronucleus. Concomitantly, astral microtubules are thought to push the centrosomes and associated male pronucleus away from the posterior cortex, and to mediate movement of the female pronucleus towards the centrosomes through minus-end-directed motor activity of dynein anchored on the female pronucleus. Thereafter, centrosomes are juxtaposed with the two joined pronuclei, such that chromosomes are placed between the two centrosomes after nuclear envelope breakdown.

4) Malone et al.(2004) have reported that centrosomes fail to localize next to the male pronucleus in all C. elegans embryos lacking zyg-12 function, and that analogous defects occur in sub-sequent cell cycles. The two existing zyg-12 alleles are temperature-sensitive, and centrosome detachment is observed to occur within one minute after shifting to the restrictive temperature. This indicates that ZYG-12 is required, not only for establishing attachment, but also for maintaining it.

References (abridged):

1 Bornens, M. (1977). Is the centriole bound to the nuclear membrane?. Nature 270, 80-82

2 Nadezhdina, E.S., Fais, D., and Chentsov, Y.S. (1979). On the association of centrioles with the interphase nucleus. Eur. J. Cell Biol. 19, 109-115

3 Aronson, J.F. (1971). Demonstration of a colcemid-sensitive attractive force acting between the nucleus and a center. J. Cell Biol. 51, 579-583

4 Robinson, J.T., Wojcik, E.J., Sanders, M.A., McGrail, M., and Hays, T.S. (1999). Cytoplasmic dynein is required for the nuclear attachment and migration of centrosomes during mitosis in Drosophila. J. Cell Biol. 146, 597-608

5 Goenczy, P., Pichler, S., Kirkham, M., and Hyman, A.A. (1999). Cytoplasmic dynein is required for distinct aspects of MTOC positioning, including centrosome separation, in the one cell stage Caenorhabditis elegans embryo. J. Cell Biol. 147, 135-150

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CELL BIOLOGY: ON THE CENTROSOME

The following points are made by Edward H. Hinchcliffe (Current Biology 2003 13:R646):

1) The key to a successful division of a eukaryotic cell is the assembly of a bipolar mitotic spindle: this allows all chromosomes to attach their kinetochores to opposite poles, and segregate quickly and equally into the two daughter cells. The bipolar spindle axis is ultimately determined by the duplication of the centrosome, an organelle that consists of a pair of centrioles -- mother and daughter -- surrounded by a matrix of pericentriolar material (1). If the centrosome duplicates more than once per cell cycle, the possibility that the cell will assemble a multipolar spindle and become aneuploid increases(2).

2) While the major function of the centrosome is to organize interphase microtubules, this network will self-organize in the absence of centrosomes(3). Of interest is that in the absence of centrosomes, the cell will also assemble a bipolar mitotic spindle, induced in the region of the chromosomes (4). While such findings have challenged the importance of the centrosome, the centrosome still appears to be the dominant microtubule-organizing center and spindle pole when present in a cell (5).

3) In addition to its role as microtubule-organizing center and spindle pole organizer, the centrosome is also required for the completion of cytokinesis, and cell-cycle progression in interphase. Removal of the centrosome, either by laser ablation or micro-surgery, does not prevent the cell from forming a bipolar spindle, but these cells -- termed karyoplasts -- exhibit defects in cytokinesis and cannot progress to the next S phase. Furthermore, mitosis in certain cells (BSC-1 karyoplasts) is significantly delayed compared to controls, with timing defects observed before and after anaphase onset, suggesting that the presence of a centrosome and/or astral microtubules is necessary to ensure timely mitotic progression. The results of these studies have revealed a role for the centrosome in cell cycle regulation, but not the molecular mechanisms underlying these functions.

References (abridged):

1. Steams, T. (2001). Centrosome duplication. A centriolar pas de deux. Cell 2001, 705,417-420

2. Brinkley, B.R. (2001). Managing the centrosome numbers game: from chaos to stability in cancer cell division. Trends Cell Biol. 2001, 77,18-21

3. McNiven, M.A., Wang, M., and Porter, K.R. (1984). Microtubule polarity and the direction of pigment transport reverse simultaneously in surgically severed melanophore arms. Cell 37, 753-765

4. Heald, R., and Weis, K. (2000). Spindles get the ran around. Trends Cell Biol. 70,1-4

5. Raff, J.W. (2001). Centrosomes: Central no more? Curr. Biol. 77, R159-R161

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