EGFR and Polycystin-2 in Renal and Dental Epithelial Ciliary Signaling

Craniofacial/tooth disorders and polycystic kidney disease frequently present together in humans and are likely linked to common defects in primary cilium protein components.� The primary cilium is crucial in organ development based on the mechanosensory and signaling functions of cilia in regulating cell growth and differentiation. Key cilial signaling proteins include the polycystin-2 (PC2) calcium channel, which is regulated by EGF-mediated signaling and interaction with epidermal growth factor receptor (EGFR). Altered EGFR expression is a complicating factor in polycystic kidney disease, as well as in craniofacial/tooth disorders. Therefore, our hypothesis is that PC2 and EGFR are targeted to the primary cilium of tooth and renal epithelial cells using a conserved trafficking mechanism where they are organized in specific membrane domains and function coordinately to modulate cell signaling. Objective: To elucidate the commonalities between cilia of renal epithelia and tooth-derived odontoblasts and determine if there is a shared mechanism for ciliary targeting and organization of EGFR and associated polycystins. Methods: Immunofluorescence and co-immunoprecipitation experiments to elucidate protein localization and interactions in renal epithelia and an odontoblast cell line MO6-G3. Results: EGFR and both polycystins colocalize to the primary cilium of renal epithelial and odontoblasts and are assembled in a co-complex recovered by coprecipitation. Furthermore, the polycystins and EGFR associate in cilia with specialized membrane domains organized by cholesterol binding flotillin-1 and -2 proteins. A conserved VxPx motif was identified to be shared by the polycystins and EGFR and was shown by our lab to be essential for Arf4 GTPase-mediated ciliary trafficking of PC1. Purified, active Arf4 also interacts with EGFR. �Conclusion: The data suggest a conserved ciliary trafficking mechanism and protein complex consisting of flotillins, EGFR and polycystins in renal and odontoblast cells, defects in which may explain overlapping pathologies of kidney and craniofacial/tooth disorders.