MFRP in the retina is a molecular hub that organizes the apical membrane of RPE cells by engaging in interactions with specific protein and lipid molecules
Membrane frizzled-related protein (MFRP) is an integral membrane protein present in the retinal pigment epithelium (RPE) and is essential for ocular development and the physiology of the retina. Mutations in MFRP are associated with autosomal recessive non-syndromic nanophthalmos, leading to severe hyperopia and early-onset retinitis pigmentosa. While several preclinical gene augmentation and gene editing trials hold promise for future therapies aimed at stopping degeneration and restoring retinal function, the molecular mechanisms underlying MFRP biology remain largely undefined. Here, we studied the biochemical properties of MFRP and the molecular consequences of its loss in a mouse model. Using transcriptomic and lipidomic approaches, we observed that DHA accumulation constitutes a primary defect in the MFRP-deficient RPE. In biochemical assays, we showed that MFRP undergoes extensive glycosylation and preferentially binds several classes of lipids, including phosphatidylserine and phosphatidylinositol 4-phosphate, as well as several other transmembrane proteins, notably adiponectin receptor 1 (ADIPOR1) and inward rectifier potassium channel 13 (KCNJ13). Moreover, MFRP determines ADIPOR1 and KCNJ13 subcellular localization in the RPE in vivo. This feature is affected upon MFRP deficiency and can be restored by gene therapy approaches. Overall, our observations suggest that MFRP constitutes an important interaction hub within the apical membrane of RPE cells, essential for protein trafficking and subcellular localization within the RPE and lipid homeostasis within the entire retina. Overall design: RNA-seq profiling of RPE cells from 1 mo wt and retinal degeneration 6 (rd6) mice