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94 trends in CELL BIOLOGY (Vol. 9) March 1999

Progression of the cell cycle and con-trol of apoptosis (programmed celldeath) are thought to be intimatelylinked processes, acting to preservehomeostasis and drive developmentalmorphogenesis. When cells are treatedwith the microtubule (MT)-stabilizingagent taxol, they die because of cell-cycle arrest. Li et al.1 show that over-expression of the 16.5-kDa ‘survivin’protein protects cells from undergo-ing taxol-induced apoptosis.

Survivin expression is under tran-scriptional control, 40-fold upregu-lated at G2/M phase of the cell cycle,and survivin colocalizes with spindleMTs throughout mitosis. The authorsshow that survivin binds to MTs inHeLa cell extracts, and the dissoci-ation constant from MTs of purerecombinant survivin is 5–7 mM. Thisis 10- to 50-fold higher than the Kdof canonical microtubule-associatedproteins (MAPs). Interestingly, the

cytoprotective function of survivinand the interaction with MTs wasabolished when the C-terminalcoil–coil region was deleted. Takentogether, the cytoprotective activityof survivin requires interaction withMTs, but its binding to MTs in vivocould be indirect.

Survivin contains an IAP repeat (BIR),which, in other proteins, is requiredfor inactivation of ‘caspases’ (key proteases in the apoptotic pathway).

Apoptosis and spindle-assembly united

Most animals are bilaterally asym-metric, but how is this left–right asymmetry established? In mammals,genetic defects affecting cilia oftencause defects in left–right asymmetry,implying that cilia are somehowinvolved in establishing this asymme-try. The ciliated node cells of the mouseembryo are known to be important fordevelopment of left–right asymmetry,but, as their cilia were thought to benon-motile, their function was unclear.

Nonaka et al.1 investigated ciliaryfunction by knocking out the geneencoding KIF3B in mice. KIF3B is acomponent of the KIF 3 motor com-plex (KIF3A–KIF3B heterodimer and

KAP3), which is essential for assemblyof cilia in other organisms. As pre-dicted, cilia do not assemble in thenode cells of homozygous-knockoutmice. Remarkably, these embryos alsofail to establish left–right asymmetry. In KIF3B-knockout embryos, heart-looping asymmetry is randomized, and the gene lefty-2, which is onlyexpressed on the left side of normalembryos, is now expressed on bothsides or not expressed at all. Thus, itappears that ciliary defects in the nodecells cause defects in left–right asym-metry of the whole mouse.

This prompted a re-evaluation of the motility of nodal cilia. Video

microscopy revealed that the nodalcilia are motile after all, and theirwhip-like motion produces a highlydirectional right-to-left flow of thesurrounding fluid across the node.Nodal cilia might use this directionalflow to set up left–right asymmetry byconcentrating a secreted factor at theleft edge of the node.

Cilia whip up left–right asymmetry

1 Nonaka, S. et al. (1998)Randomization of left–rightasymmetry due to loss of nodal ciliagenerating leftward flow ofextraembryonic fluid in mice lackingKIF3B motor protein, Cell 95, 829–837

This month’sheadlines werecontributed by

Søren Andersen,Wallace Marshall,Kirsten Sadler andPeter Thomason.

The centriole is a cylinder of ninetriplet microtubules found at the coreof the centrosome. The location ofcentrioles within the centrosome hasalways seemed to suggest a role incentrosome function, but proof ofthis has remained elusive. In part, thisis due to the lack of convenient meth-ods to remove centrioles specifically.For example, microsurgery or laserablation will remove or damage notjust the centriole but the entire cen-trosome as well.

Bobinnec et al.1 have come up withan elegant way to test the role of cen-trioles in centrosome function, usingantibodies to polyglutamylated tubu-lin, a tubulin isoform that, in HeLacells, is found only in the centriole.

Bobinnec et al. introduced mono-clonal antibodies to polyglutamylatedtubulin into HeLa cells by bothmicroinjection and electroporation,and the effect was dramatic.Centrioles disappeared completelyfrom antibody-treated cells.

Remarkably, when centrioles disap-peared, centrosomes were no longerdetectable either. No microtubule-organizing centres remained, and thepericentriolar material (e.g. g-tubulin)became dispersed throughout thecytoplasm. Because presumably theantibodies acted directly on centri-oles, this result implies that centriolesare necessary to maintain centrosomeorganization. As the antibodiesdegraded, centrioles reappeared,

and, as soon as they did, functionalcentrosomes reassembled aroundthem, suggesting that centrioles aresufficient to direct the assembly of acentrosome.

This study breathes new life into the old idea that centrioles are impor-tant for centrosome function andought to spark a fresh round ofinquiry into these mysterious littleorganelles.

No centriole, no centrosome

1 Bobinnec, Y., Khodjakov, A., Mir, L. M.,Rieder, C. L., Edde, B. and Bornens, M.(1998) Centriole disassembly in vivoand its effect on centrosome structureand function in vertebrate cells, J. CellBiol. 143, 1575–1589

Interestingly, a cysteine-to-alaninemutation in the IAP repeat of survivinabolished its cytoprotective function,led to cellular activation of caspase3, but had no effect on its interactionwith MTs. Moreover, expression of antisense survivin resulted inincreased caspase 3 activity, and sur-vivin is overexpressed in some cancercells.

These observations are consistentwith survivin functioning in the regu-lation of apoptosis. Why MT bindingis required for activity, how it binds toMTs and how it regulates caspaseactivities are open questions. Thisstudy suggests that a constitutiveapoptotic pathway, monitoringproper spindle MT assembly, is nega-tively regulated by survivin. The paper

paves the way for the study of anexciting pathway that links spindleMT assembly to the regulation ofapoptotic events.

headlines

trends in CELL BIOLOGY (Vol. 9) March 1999 95

Sequencing of the yeast Saccharo-myces cerevisiae genome finished just one year ago, but post-genomicanalyses are in full swing. The firstmajor operation has been to surveythe 6000 or so genes to see which of them show cell-cycle-regulatedexpression. The answer is at least 400and perhaps as many as 800. To arriveat these numbers, mRNA wasextracted from synchronized yeastcells throughout the cell cycle andanalysed by DNA microarray technol-ogy. The two studies to date userather different methods and on theface of it seem to give very differentresults; the first study1 failed to findalmost 500 genes identified in thesecond, more sensitive, analysis2 (andfound instead 100 others), but,

looking on the bright side, there isagreement on 300 genes. Most of thegenes show up for obvious reasons,being involved in controlling the cellcycle itself, in DNA replication andrepair, and in mitosis, for example.However, there are surprises too, withgenes involved in regulating nutritioncropping up, for instance. This sug-gests that both ‘cause’ and ‘effect’messages are within the reach ofthese genome chips.

Given the genome sequence, sub-stantial promoter analysis is possible.Most of the identified genes do, hap-pily, have target sites for known cell-cycle transcription factors. Indeed,through this analysis, it has beenpossible to refine the binding sitesfor certain of these factors. Other

identified genes do not have recog-nizable sites; thus, it is not so easypresently to understand their oscilla-tory behaviour. However, the datagenerated by these studies provide avery valuable resource for furtheranalyses, and also might act as aguide for the behaviour of humanhomologues of many of these genes.

800 oscillations per cycle

1 Cho, R. J. et al. (1998) A genome-widetranscriptional analysis of the mitoticcell cycle, Mol. Cell 2, 65–73

2 Spellman, P. T. et al. (1998)Comprehensive identification of cellcycle-regulated genes of the yeastSaccharomyces cerevisiae by microarrayhybridization, Mol. Biol. Cell 9,3273–3297

1 Li, F. et al. (1998) Control of apoptosisand mitotic spindle checkpoint bysurvivin, Nature 396, 580–584

The confusing nomenclature of theAurora/ Ipl1 family of proteins is paral-leled by the enigmatic function ofthese kinases. The members of thisgrowing family, including humanAIK-1, mouse IAK-1 and IAK-3, ratAIM-1, Xenopus Eg2 and the recentlyreported Caenorhabditis elegans AIR-1and AIR-21, are all presumed kinases.Disrupting their function results in vari-ous cell-cycle defects, usually affectingthe spindle, centrosomes or cytokine-sis. Although much of the work onthese proteins has focused on somaticcells, there is evidence that a few ofthe family members have a role duringmeiosis. Schumacher et al.2 addressedsystematically both the localizationand the function of AIR-2 during themeiotic and mitotic cell cycles ofC. elegans.

Using RNA-mediated interference,the authors abolished all oocyte andembryonic AIR-2 transcripts. The

resulting oocytes underwent abnormalmeiotic divisions, forming internalpolar bodies, which then went on todivide. When fertilized, AIR-2-depletedeggs developed into polyploid, unicel-lular embryos owing to multiplenuclear divisions without cytokinesis.Their spindles appeared normal butwere associated with abnormally dis-tributed chromatin that underwentaberrant karyokinesis. Live observationof AIR-2-depleted embryos reveals theyare able to initiate, but unable to com-plete, cytokinesis.

The interpretation of these results isaided by the immunolocalizationstudies of air-2 in wild-type animals.Air-2 localizes to oocyte chromo-somes just prior to or during meioticmaturation and remains in the polarbodies. Interestingly, without spermwaiting in the spermatheca for releaseof the mature eggs, air-2 does notlocalize properly. During embryogen-

esis, air-2 is found on metaphasechromosomes, at the mid-body fol-lowing anaphase, and a small amountis detected on the membrane follow-ing cytokinesis. These studies providea strong basis for the hypothesis thatAIR-2 is involved in cytokinesis,although what its targets are, or whatis responsible for its discrete andimportant localization, is still literallyup in the air.

Clearing the air on an Aurora/Ipl2 family member

1 Schumacher, J. M., Ashcroft, N.,Donovan, P. J. and Golden, A. (1998)Development 125, 4391–4402

2 Schumacher, J. M., Golden, A. andDonovan, P. J. (1998) AIR-2: AnAurora/ Ipl1-related protein kinaseassociated with chromosomes andmidbody microtubules is required forpolar body extrusion and cytokinesisin Caenorhabditis elegans embryos, J. Cell Biol. 143, 1635–1646