ER Aggregation Causes Synaptic Protein Imbalance in Retinitis Pigmentosa Mutant Photoreceptor Neurons

Rod photoreceptor neurons in the retina detect scotopic light by packaging large quantities of the visual pigment protein rhodopsin (Rho) into stacked membrane discs within their outer segments (OS). Efficient Rho trafficking to the OS through the inner rod compartments is critical for long-term rod health since diseases like retinitis pigmentosa (RP) induce Rho mislocalization in these inner compartments, including in the rod presynaptic terminals (i.e., “spherules”). Given the importance of protein trafficking to the OS, less is known about the trafficking of rod synaptic proteins that maintain critical synapses between rods and inner retina neurons. Furthermore, the subcellular impact of Rho mislocalization on rod spherules has not been investigated. In this study we used super-resolution and electron microscopies, along with proteomic measurements of rod synaptic proteins, to perform an intensive subcellular analysis of Rho synaptic mislocalization in P23H-Rho-RFP RP mutant mice of both sexes. We discovered mutant P23H-Rho-RFP protein mislocalized in distinct ER aggregations within the spherule cytoplasm which we confirmed in wild type (WT) rods overexpressing P23H-Rho-RFP. Additionally, we found significant protein abundance differences in Dystrophin, BASSOON, ELFN1 and other synaptic proteins in P23H-Rho-RFP mice. By comparison, Rho mislocalized along the spherule plasma membrane in WT rods and in rd10 RP mutant rods, in which there was no synaptic protein disruption. Throughout the study, we also identified a network of ER membranes within WT rod presynaptic spherules. Together, our findings establish a previously uncharacterized ER-based secretory system that mediates normal trafficking and turnover at mouse rod synapses.