Rod photoreceptor specific expression and function of Frmpd1 (FERM and PDZ domain containing 1)

Vision loss from retinal degeneration is largely associated with the death or dysfunction of photoreceptor cells (rods and cones). Many therapeutic approaches are thus targeted toward protecting against cell death or replacing photoreceptors, as no new rods or cones are formed after maturation. It is not completely understood why rods seem to be more vulnerable to disease than cones, yet a precocious degeneration of rods often leads to a secondary loss of cone function and severe visual defects. A more thorough understanding of the molecular mechanisms contributing to proper rod photoreceptor development and function are therefore necessary for advancing the development of more effective treatments for vision-related diseases. We hypothesised that genes dramatically increasing in expression during rod photoreceptor development and regulated by key rod transcription factors would play a substantial role in the functional maturation and homeostasis of these cells. Using temporal RNA-seq transcriptome data from purified photoreceptors, we identified Frmpd1 (FERM and PDZ domain containing 1) as a novel gene associated with rod development. An investigation of the regulatory mechanisms of Frmpd1 revealed that transcription of this gene is initiated from a unique retina-specific alternative promoter, which is modulated by key rod transcription factors Nrl (neural retina leucine zipper) and Crx (cone-rod homeobox). Interestingly, a CRISPR/Cas9-mediated genomic deletion of this alternative promoter resulted in a retina-specific deletion of Frmpd1 at both the RNA and protein levels. Subsequent analyses of Frmpd1 demonstrate that it localises to the inner segments and synapses of rod photoreceptors, where it interacts with the key phototransduction component transducin and its effector Gpsm2 (G-protein signalling modulator 2). Frmpd1-/- mice exhibit a delayed return of transducin to outer segments following light adaptation, and a delay in the recovery of rod photoresponse after a moderate visual pigment bleach. Frmpd1 thus appears to play a role in the maintenance of rod homeostasis through mediating the adaptive response of rods following light exposure, making it an interesting potential therapeutic target for better preserving both natural and stem-cell derived photoreceptors to restore vision. Moreover, these studies demonstrate the novel approach of targeting tissue-specific promoters by CRISPR-Cas9 to generate tissue-specific knockdown mouse lines.