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.