Regulation of the self-renewal vs. differentiation decision in the C.elegans germline

Adult stem cells are the basis of our reproductive potential, of tissue growth and maintenance, and of wound healing and regeneration. Ageing phenotypes such as hair loss or brittle bones in fact reflect the gradual decline of the proliferative capacities of adult stem cells with time. On the other hand, stem cells that proliferate too fast to provoque diseases such as cancer. Understanding the molecular players regulating the decision of stem cells to divide or differentiate is therefore of prime importance to treat conditions caused by stem cell dysfunction. Many pathways that regulate the maintenance and differentiation of stem cells, as well as features distinguishing stem cells from their more committed progeny, are known today, among them the conserved Notch signaling pathway. How these molecules instruct stem cell identity is however largely unknown.
C.elegans germline stem cells, which are maintained by Notch signaling, constitute an excellent model to study the regulation of stem cell identity. C.elegans is amenable to genetic manipulation, to gene knockdown by RNAi, and to transgenesis. Additionally, the C.elegans germline can be readily monitored in live animals through the transparent cuticle as well as dissected for gene expression studies or immunofluorescence.
Here, we use these tools to identify genes that respond to Notch signaling in germline stem cells. We find that many Notch responsive genes reside on the X chromosome. In the following, we uncover a crosstalk between Notch signaling and the C.elegans Polycomb proteins MES-2,-3,-4, and -6, which are known to repress X linked genes in the germline.
Additionally, we identify a role of the CIP/KIP cell cycle inhibitor CKI-2 in the self-renewal versus differentiation decision of C.elegans germline stem cells. We find that CKI-2 is a key target of two conserved stem cell regulators, the RNA binding proteins FBF/Pumilio and MEX-3. Particularly FBF/Pumilio proteins have been implied in stem cell regulation in many organisms, and in C.elegans are required to maintain stem cells during adulthood. Key targets among the mRNAs targeted by FBF/Pumilio proteins have however remained elusive. We demonstrate that both MEX-3 and FBF/Pumilio proteins associate with the cki-2 3’UTR, that FBF/Pumilio binding elements are required for 3’UTR mediated regulation in vivo, and that regulation of CKI-2 by FBF/Pumilio contributes to the maintenance of germline stem cells.