Cdhr1a and pcdh15b link photoreceptor outer segments with inner segment calyceal processes revealing a potential mechanism for cone-rod dystrophy

Cone rod dystrophy (CRD) is a macular degeneration disorder characterized by initial cone cell photoreceptor degeneration and subsequently of rod photoreceptors. Mutations in CDHR1, a photoreceptor specific cadherin have been found to be associated with the incidence of cone-rod dystrophy and recapitulated in mouse CDHR1 knockouts. However, the molecular function of CDHR1 remains unknown. CDHR1 has been shown to localize at the leading edge of murine rod nascent outer segment (OS) making junctions to an unknown partner in the inner segment. Using Structured Illumination Microscopy (SIM), we observed that the localization of zebrafish cdhr1a extends from basal nascent OS discs above the periciliary ridge of the inner segment to a considerable length along the OS, akin to calyceal process (CPs). When labeling the CPs using pcdh15b, a CP specific cadherin, we observed that cdhr1a at the leading edge of OS juxtaposes with pcdh15b in the CP. Similar localization patterns were detected in human, macaque, xenopus, ducks, and various rodent PRCs indicating conservation. Importantly, using immunoprecipitation and K652 cell aggregation assays we demonstrate that pcdh15b and cdhr1a can interact and potentially link the OS and CP. To analyze the consequences of OS-CP interactions in CRD, we established a zebrafish cdhr1a mutant line ( cdhr1a ^fs*146 ) and analyzed CRD progression at high temporal resolution. Homozygous cdhr1a ^fs*146 mutants begin to exhibit minor cone OS morphology defects starting at 15 dpf (days post fertilization) and severe OS disruption and cell loss by 3 months. Rod OS defects were delayed until 3-6 months. Furthermore, we show that loss of cdhr1a function leads to disorganization and shortening of CPs coinciding with cone outer OS defects which is significantly exacerbated when combined with the loss of pcdh15b. In conclusion, we propose that cdhr1a and pcdh15b function to link cone OSs with CPs to maintain proper OS homeostasis thus revealing a potential novel mechanism for CRD.