Characterization of the regulation of APC/CFZR-1 in the germline of Caenorhabditis elegans

Abstract

[EN] Cell cycle regulation in dividing and differentiating cells is usually studied in vitro or in unicellular organism models. However, the C. elegans germline represents an excellent in vivo model for studying stem cell dynamics and differentiation. The germline is composed of a pool of stem cells arranged spatiotemporally, supported by a niche cell called Distal Tip Cell (DTC). As the stem cells leave the vicinity of the niche, they enter meiosis. This unique physical organization enables its study by microscopy, which is enhanced by the easiness of genetic manipulations. The maintenance of the germline features requires the essential MES-3 and MES-4 chromatin regulators, which previous research in our group showed to be downregulated by the conserved E3 ubiquitin ligase APC/CFZR-1 . This cell cycle regulatory complex marks these proteins for degradation during the transition to meiosis. In this work, we showed that APC/CFZR-1 is tightly downregulated and upregulated at several different levels to allow the timely down-regulation of MES-3 and MES-4 proteins. It was intriguing how the niche could coordinate the signaling to inactivate APC/CFZR-1 in the mitotic zone. However, as cells differentiate into meiosis, the negative regulation was timely lifted, and the chromatin regulators were degraded. In the mitotic zone, the Notch Pathway, through FBF RNA-binding proteins, represses fzr-1 translation. Indirectly, FBF proteins also activate the translation of CYE-1 cyclin, resulting in high CDK levels that inhibit APC/CFZR-1 activity. In contrast, upon the cells differentiate to meiosis, APC/CFZR-1 activity is upregulated through two main pathways: polyA polymerase GLD-2 and another E3 ubiquitin ligase complex, SCFPROM-1. SCFPROM-1 acts by initiating the degradation of CYE-1, while GLD-2 promotes new fzr-1 translation and acts upstream of CKI-2, a conserved CDK inhibitor. We discovered that after upregulation, APC/CFZR-1 could also recognize and mark for degradation CYE-1, acting as a backup of SCFPROM-1 . It also recognizes CYA-1, a cyclin highly expressed at the end of the mitotic zone, and finally, can catalyze the degradation of its own coactivator, FZR-1.