The cyclin-dependent kinase inhibitors, cki-1 and cki-2, act in overlapping but distinct pathways to control cell-cycle quiescence duringC. elegansdevelopment

Abstract

Cyclin-dependent kinase inhibitors (CKIs) are major contributors to the decision to enter or exit the cell cycle. The Caenorhabditis elegans genome encodes two CKIs belonging to the Cip/Kip family, cki-1 and cki-2. cki-1 has been shown to act as a canonical negative regulator of cell-cycle entry, while the role of cki-2 remains unclear. We identified cki-2 in a genome-wide RNAi screen to reveal genes essential for developmental cell-cycle quiescence. Examination of cki-2 knockout animals revealed extra rounds of cell divisions, verifying a role in establishing or maintaining the temporary cell-cycle arrest. Despite the overlapping defects, the pathways mediated by cki-1 and cki-2 are discrete since the extra cell phenotype conferred by a putative cki-2(null) mutation is enhanced upon additional loss of cki-1 activity. Moreover, the extra cell division defect of cki-2 is not increased with the additional loss of lin-35 Rb, as is seen with cki-1. Thus, both cki-1 and cki-2 mediate cell-cycle quiescence, but our genetic and phenotypic analyses demonstrate that they act within distinct pathways to exert control over the cell-cycle machinery.

Analysis of centriole elimination duringC. elegansoogenesis

“…We found also that SAS-4 foci are rarely present in diakinesis GCs carrying the null allele cki-2(ok2105), and always disappear thereafter, suggesting that cki-2 contributes only in a minor fashion to timely elimination (Fig. 6D) (Buck et al, 2009).…”

“…However, more recent findings using a cki-2 deletion allele instead led to the suggestion that CKI-2 is not needed for centriole elimination, raising the possibility that the earlier observation resulted from pleiotropic effects of cosuppression (Buck et al, 2009). Apart from the possible role of CKI-2 in this process, other components and mechanisms that contribute to elimination are not known.…”

“…The following genotypes were used: sas-6(ok2554) (Kitagawa et al, 2011), zyg-1(it25) (Kemp et al, 2007), spd-2(oj29) (O'Connell et al, 1998), sas-5(t2033) , sas-5(gk400) (http://www.wormbase.org), sas-4(tm3951) (Kitagawa et al, 2011), cki-2(ok2105) (Buck et al, 2009), fog-1(q253) (Barton and Kimble, 1990) (shifted to the restrictive temperature of 25°C as L1/L2), fog-2(q71) (Schedl and Kimble, 1988), fog-3(q470) (Ellis and Kimble, 1994), tra-1(e1076) (Schedl et al, 1989), her-1(hv1y101) unc-42 and her-1(hv1y101) unc-42 dpy-11;lon-2 (Francis et al, 1995b), analyzed as described (Bean et al, 2004), mog-1(q233) (Graham and Kimble, 1993), fem-3(q96gf) (Barton et al, 1987), cgh-1(ok492) (Audhya et al, 2005), glo-1(zu391) (Hermann et al, 2005), rrf-1(pk1417) (Sijen et al, 2001), GFP-SAS-4 (Kirkham et al, 2003), GFP-SAS-5 , GFP-SAS-6 , GFP-SPD-2 (Kemp et al, 2004), GFP-TBG-1 (Oegema et al, 2001), GFP -TAC-1 (Bellanger and Gönczy, 2003), VIT-2-GFP (Grant and Hirsh, 1999), NMY-2-GFP (Nance et al, 2003).…”

pRB/CKI pathways at the interface of cell cycle and development

“…This L1-L3 VPC quiescence is maintained with amazing precision in wild type animals, suggesting a robust system of redundant controls that are coupled to developmental signals. Genetic studies of mutants displaying precocious VPC cell cycle entry identified regulators of VPC quiescence [4][5][6][7][8][9][10] that include prB/CKi pathways at the interface of cell cycle and development the evolutionarily conserved retinoblastoma protein (pRB, the product of the lin-35 gene in C. elegans 10,11 ) and cyclin-kinase inhibitors (CKIs) of the Cip/Kip family (of which there are two family members in C. elegans, encoded by the cki-1 and cki-2 genes 5,6,[8][9][10] ). Recent work from the Saito lab, reported on page XXX of this issue of Cell Cycle, 5 adds new understanding of the complex functional relationships among pRB and CKI pathways that ensure the precise developmental control of VPC quiescence and cell cycle reentry.…”

“…7 Moreover, since the screen demands that precocious VPC cell divisions produce cells with the lin-12-responsiveness of normal VPCs, purely cell cycle phenotypes can be distinguished from more general developmental timing defects characteristic of heterochronic genes. 4 Buck et al 5 identified cki-2 as a regulator of VPC quiescence, and placed cki-2 in a pathway regulating cyclin E (and hence likely as an inhibitor of CDK-2/ cyclin E 12 ). They then employed a series of genetic interaction experiments to characterize the functional relationships between cki-2 and its other family member, cki-1, and with the another key quiescence factor, pRB/LIN-35.…”