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Abstract

WNT signaling is an evolutionarily conserved pathway that plays essential roles in development and disease. A recently described branch of WNT signaling, known as WNT/STOP (stabilization of proteins), promotes cell growth and division by stabilizing target proteins from glycogen synthase 3 (GSK3)-mediated degradation. Key regulators of WNT/STOP are Cyclin Y (CCNY) and Cyclin Y like 1 (CCNYL1), conserved cyclins that, together with their associated cyclin dependent kinase 14 (CDK14) and 16 (CDK16), phosphorylate and prime the WNT co-receptor LRP6, thereby inhibiting GSK3. Most of the biological functions of WNT/STOP signaling have been described in vitro, or in post-transcriptional germ cells. However, whether or not WNT/STOP signaling plays essential roles in dividing somatic cells is incompletely understood. In order to study the in vivo functions of WNT/STOP signaling in somatic cells, Ccny/Ccnyl1 double knockout (DKO) mice were generated and analyzed. Strikingly, mutant embryos displayed severe defects in the neocortex characterized by a thinner lateral cortex and reduced basal progenitors and post mitotic neurons. Mechanistically, WNT/STOP is shown to promote asymmetric cell division by regulating the levels of apical-basal astral microtubules, and the differentiation of neural progenitors by stabilizing Sox4 and Sox11, two neurogenic transcription factors that are characterized as direct GSK3 substrates in this thesis. Apart from the neurogenesis defects, Ccny/l1 deficiency also led to defects in primary cilia formation in apical progenitors. Primary cilia are microtubule-based organelles involved in transducing cell signaling pathways and defects in cilia formation are associated with human disease. A detailed phenotypic analysis of DKO embryos and cell lines deficient or mutant for Ccny/l1 revealed that the regulation of ciliogenesis by WNT/STOP signaling was also extended to developing kidneys, 293T cells and adult mouse preadipocytes. Mechanistically, CCNY and LRP6 concentrate in primary cilia and LRP6 becomes phosphorylated following WNT stimulation, suggesting that cilia act as WNT-responsive organelles. WNT/STOP signaling activates the Protein phosphatase 1 regulatory subunit PPP1R2 in the cilia, which then inhibits the negative ciliary regulator protein phosphatase 1 (PP1). In summary, the findings in this thesis unveil crucial in vivo roles for WNT/STOP signaling in neocortex development and primary cilia formation, with important implications for embryonic development and disease.