Accumulation and maintenance of stem cells is key to proper organogenesis during development and to tissue homeostasis in adulthood. However, our understanding of signals regulating these cells remains incomplete. The C. elegans germ line provides a simple model for studying cellular and molecular regulation of the accumulation and maintenance of germline stem/progenitor cells. Previously, our lab established that the Target of Rapamycin (TOR) pathway: let-363/TOR, daf-15/RAPTOR, rsks-1/p-70-S6-Kinase (S6K) and ife-1/eukaryotic initiation factor-4E (eIF4E), and ample food, promote the accumulation of proliferative germ cells during larval development.
Additionally, and unexpectedly, we found that S6K acts in concert with GLP-1/Notch to maintain undifferentiated germ cells. GLP-1/Notch signaling prevents differentiation and/or maintains proliferative germ cells. We found that loss of S6K enhances and suppresses specific phenotypes associated with reduced and elevated glp-1 activity, respectively. More recently we found that, similar to its role in promoting germ cell accumulation, S6K acts germline-autonomously and requires a conserved TOR phosphorylation site to prevent germ cell differentiation. However, neither TOR nor eIF4E shared the same genetic interaction with glp-1. These results suggest that the role of S6K in preventing differentiation may be a special feature of the S6K branch of the TOR pathway.
S6K is highly conserved. While its role as translational regulator is well characterized, examples of S6K-mediated cell fate regulation are limited. Therefore, we are using our genetics-based functional assays to undertake both candidate and unbiased approaches to determine the molecular mechanism by which S6K prevents differentiation. Others have shown that, like loss of S6K, reduced cye-1/Cyclin-E or MAPK activity can enhance glp-1. However, we found that neither acts in a simple linear fashion with S6K to regulate cell fate. We are currently testing additional candidates, including translational regulators, for their roles in germ cell fate maintenance and functional interaction with S6K. In parallel, we are conducting genome-wide genetic and proteomic screens to identify candidates that will be tested for their role in cell fate regulation.