Primary cilia are signaling centers present on the majority of vertebrate cells in diverse tissues, particularly on progenitor cells, consistent with role in regulating proliferation and differentiation of sensory and other signaling cells. Cilia ar important in regulating mesenchymal lineage decisions (e .g, by Sonic Hedgehog signaling). We find that cilia are required for adipogenesis and that CD34+ pre-adipocytes isolated from adult mice are uniformly ciliated. Specifically, primary cilia stimulate insulin signaling by organizing sensitized IGF1 receptors. We have identified the asymmetric cell division regulator AGS3 as a new centriolar/ciliary player in adipogenesis: AGS3-null mice are resistant to high fat diet-induced adipose tissue expansion leading to obesity, and AGS3-null CD34+ pre-adipocytes have a ciliation and adipogenesis defect. We show that AGS3 localizes to centrosomes and directly binds insulin substrate receptor 2 (IRS2 - a central insulin/IGF1 effector) to cause a striking translocation of IRS2 and AKT to centrioles upon insulin treatment. Translocation is required for deciliation and stimulation of pre-adipocyte mitoses critical for adipocyte differentiation. We hypothesize that ciliated CD34+ mesenchymal stem cells play a role in other mesenchymal lineages and parenchymal organs, and that the primary cilium and centriolar AKT broadly function in ciliated progenitor cells to sense differentiation signals and to couple deciliation to mitosis and differentiation. The proposed aims are directed toward (1) understanding the Ags3-Irs2-Akt ciliary signaling network for activating Akt at centrosomes, including looking for ciliary GPCRs important in adipocytes; (2) understanding how the Akt mitotic pathway triggers differentiation, including finding new mitotic and epigenetic factors; and (3) further validating the role of ciliated stem cells by histological analysis of various tissues nd by examining a mouse model of liver regeneration and an in vitro model of osteogenesis. From new genes we find implicated in ciliary stem cell function, we will collaborate with human and cancer genetics labs to look for disease alleles of our new genes. Our study has strong translational potential to define new diagnostics and therapeutic targets for regenerative diseases, obesity and cancer.
The goal of this project is to define the mechanism of ciliary insulin/IGF1 via the Ags3/IRS2 pathway and to validate the importance of the mechanism in mouse models of tissue regeneration. In vitro studies, cultured preadipocytes, and mice will be the model system. The approaches include biochemistry, proteomics/mass spectrometry, molecular biology, time-lapse microscopy, histology and human genetic screening.
