Dynamics of Notch signaling in a self-renewing tissue, the C. elegans germline

Notch signaling was in many stem cell systems shown to play not only an important role in self-renewal but also in differentiation. It is not surprising that perturbations within the pathway are associated with a variety of diseases. These range from loss- of-function mutations to activating mutations within the Notch signaling pathway and are mostly located within the Notch receptor itself. Hyperactivating mutations of Notch are often associated with auto-activation of the receptor or a failure of the activated receptor to be degraded. In the C. elegans stem cell system - the germline - the Notch receptor GLP-1 was shown to be absolutely essential for the maintenance of germline stem cells. Similar to the described hyperactivation found in cancer, hyperactivation of GLP-1 in the C. elegans germline is associated with the formation of a tumor. As in other stem cell systems the output of the signaling pathway has to be tightly controlled, which can generally happen on the various levels that assure receptor availability and its activation.
To study GLP-1 dynamics in the C. elegans germline we constructed a functional GFP knock-in within the intracellular domain of GLP-1, which is the form of Notch that translocates into the nucleus to activate gene expression. We found considerable differences in the receptor activation between the larval and adult germline and could show that this is likely due to differential turnover of the NICD by the ubiquitin- proteasome-system. In Notch signaling contexts in other organisms but also in Notch cell fate decisions in C. elegans, which are distinct from the germline stem cell fate decision, nuclear turnover of the activated Notch receptor was shown to be mediated by FBXW7/SEL-10. In contrast to the mechanism proposed for other Notch signaling events, we could show that activated GLP-1 is likely not turned over by SEL-10 but by a mechanism involving the U-box-containing E3 ligase PRP-19.