The Caenorhabditis elegans vulva is an excellent model to study how cell proliferation is regulated during animal development. During the first larval stage, six equivalent vulval precursor cells (VPCs) are born. These cells remain arrested in the G1 phase of the cell cycle until the beginning of the third larval stage, when their fates are specified and the three proximal VPCs proliferate to generate exactly 22 vulval cells. It is yet unknown how the exit of the 22 terminally differentiated vulval cells from the cell cycle is established and maintained throughout adulthood. The hox gene
lin-39is necessary to maintain the VPCs proliferating by regulating the expression of the two cell-cycle regulators
cye-1(cyclin E) and
cdk-4 (Roiz et al. 2016). Accordingly, the VPCs cease to divide as soon as the levels of LIN-39 drop below a certain threshold level when the animals reach the fourth larval (L4) stage. We have used the CRISPR-Cas9 system to generate a conditional knock-down allele of
lin-39 that allows us to induce LIN-39 protein degradation at different stages during vulval cell proliferation. We observed that secondary (2 deg ) VPCs lacking LIN-39 prematurely exit from the cell cycle as they undergo only two instead of three rounds of cell division. Furthermore, using VPC-specific
lin-39 over-expression we observed that Pn.p cells expressing LIN-39 enter the cell cycle prematurely during the L2 stage, generating a hyperplastic phenotype with an excess number of Pn.p cells. These results indicate the LIN-39 is necessary and sufficient to promote VPC proliferation in larvae. Nevertheless, vulval cells overexpressing LIN-39 exit the cell cycle after the L4 stage, indicating that LIN-39 is not sufficient to keep somatic cells proliferating in adults. Exploiting the hyperplastic phenotype of the animals over-expressing LIN-39 as a genetic background we are searching for factors that are required to maintain a prolonged proliferating state in adult somatic cells. References: Roiz, Daniel, et al. "The C. elegans hox gene
lin-39 controls cell cycle progression during vulval development." Developmental biology 418.1 (2016): 124-134.