C elegans cell cycle machinery

Components of the conserved cell cycle machinery are rapidly being identified in C. elegans and their roles are being tested in the context of the developing worm. For example, a number of Cdc2-related protein kinases were identified from the genomic sequence, and the gene encoding one of these, ncc-1 was found by RNA-mediated interference (RNAi) experiments, and by mutant phenotypes, to be required for mitosis in embryonic and postembryonic cell divisions (Boxem et al 1999). ncc-1 is not required for S phase, and this M-specific role indicates that ncc-1 encodes an orthologue of Cdk1/Cdc2. Other members of the C. elegans Cdc2-related class of kinases do not seem to have critical roles, as judged by gene inactivation experiments, suggesting that they may act redundantly.

C. elegans cyclin A and B have been identified, and functional characterization of them suggests roles in oogenesis and early embryogenesis (Kreutzer et al 1995). The C. elegans cyd-1 and cdk-4 genes encode cyclin D and a cyclin kinase related to Cdk4/ Cdk6, respectively (Park & Krause 1999). Both cyd-1 and cdk-4 are expressed during embryonic development, and during many postembryonic cell lineages with a temporal pattern coincident with postembryonic G1/S progression. Loss-of-function phenotypes of cyd-1 and cdk-4 indicate that cyclin D and CDK-4 are required for postembryonic development, but not for embryonic cell cycles (Park & Krause 1999), perhaps consistent with an apparent lack of G1 phases in the worm embryonic cell cycles. It is not known what developmental regulators govern the timing of cyd-1 and cdk-4 expression during the larval stages and to what extent cyclin D and CDK-4 are limiting factors controlling timing of cell cycle progression. In contrast to cyclin D, which may be specific to postembryonic development, the C. elegans cyclin E seems to be required for both embryonic and postembryonic cell cycle progression (Fay & Han 2000).

Cullins were identified in C. elegans by mutations of cul-1 and cul-2 that cause an over-proliferation of postembryonic cell divisions (Kipreos et al 1996). They define a protein family that is widely employed for targeting proteins for ubiquitin-mediated proteolysis. The CUL-1 and CUL-2 proteins appear to control the number of cell divisions in numerous C. elegans larval cell lineages by targeting cyclins and other cell cycle components (Kipreos et al 1996, Feng et al 1999). The developmental regulation of cul-1 and cul-2 activity is likely to underlie at least some of the strict control of cell division timing and cell cycle number in

C. elegans.

Cyclin kinase inhibitors (CKIs) of the p21/p27 class (Sherr & Roberts 1999) are represented in C. elegans by the products of the cki-1 and cki-2 genes (Hong et al 1998, Feng et al 1999). cki-1 is the best characterized of these two. Inactivation of cki-1 results in extensive postembryonic over-proliferation phenotypes, suggesting that cki-1 is widely employed throughout C. elegans embryonic and postembryonic development to limit the number of cell cycles (Hong et al 1998).

cki-1 is expressed in differentiating cells, and in numerous resting progenitor cells at various stages of C. elegans development, and also in larvae developmentally-arrested as dauer larvae or starved hatchlings (Hong et al 1998; Table 1). This developmental pattern of expression is displayed by transgenes with the cki-1 promoter fused to gfp, indicating that cki-1 transcription is developmentally regulated. The cki-1 expression patterns in resting progenitor cells are consistent with the cki-1 over-proliferation phenotypes, and suggest that cki-1 inhibits G1 cyclin/Cdk complex(es).

In some cases, such as the hermaphrodite vulval lineage, cki-1 loss-of-function defects include an extra round of precursor cell division, without any appreciable change in the timing of subsequent vulval differentiation. Although the timing of vulva differentiation is normal in cki-1 loss-of-function animals, the vulva is often non-functional, suggesting that cki-1 is required for the completion of proper vulval differentiation. cki-1 is expressed in differentiating neurons, muscle and hypodermal cells, and the cki-1 loss-of-function phenotype includes uncoordinated behaviour (Hong et al 1998), which may result from defects in neuronal or muscle differentiation.

C. elegans lin-35 encodes a protein closely related to retinoblastoma (Rb) tumour suppressor (Lu & Horvitz 1998). The lin-35 gene product, together with the product of lin-53, which is similar to the RbAp48 Rb-binding protein, antagonizes the Ras signalling involved in vulval cell fate determination. lin-53 may also have essential roles as well (Lu & Horvitz 1998), but it is not clear if C. elegans Rb and its partners are involved in controlling cell cycle progression, as in vertebrates.

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