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. 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

膜卷曲相关蛋白（Membrane frizzled-related protein, MFRP）是一类定位于视网膜色素上皮（retinal pigment epithelium, RPE）的整合膜蛋白，对眼部发育与视网膜生理功能具有关键作用。MFRP基因突变与常染色体隐性遗传性非综合征性小眼球症密切相关，可引发重度远视及早发性视网膜色素变性。尽管多项临床前基因补充与基因编辑试验为未来阻滞视网膜退行性病变、恢复视网膜功能的治疗方案带来了潜在可能，但MFRP相关生物学过程的分子机制仍未得到充分阐明。本研究借助小鼠模型，探究了MFRP的生化特性及其缺失所引发的分子效应。通过转录组学与脂质组学分析手段，我们发现二十二碳六烯酸（DHA）的异常蓄积是MFRP缺陷型RPE细胞的核心代谢缺陷。生化实验结果显示，MFRP可发生广泛的糖基化修饰，并能优先结合多种脂质类别，包括磷脂酰丝氨酸与磷脂酰肌醇4-磷酸，同时还可与多种其他跨膜蛋白产生相互作用，其中尤以脂联素受体1（adiponectin receptor 1, ADIPOR1）及内向整流钾通道13（inward rectifier potassium channel 13, KCNJ13）最为显著。此外，MFRP可在体内调控RPE细胞中ADIPOR1与KCNJ13的亚细胞定位，该调控功能在MFRP缺失时会受损，且可通过基因疗法得以恢复。综上，本研究结果表明，MFRP是RPE细胞顶端膜上的重要蛋白互作枢纽，对RPE细胞内的蛋白质转运与亚细胞定位，以及全视网膜的脂质稳态维持均不可或缺。研究整体设计：对1月龄野生型与视网膜变性6型（retinal degeneration 6, rd6）小鼠的RPE细胞进行RNA测序转录组分析。