Ablation of pigment epithelium‐derived factor receptor, PEDF‐R, causes photoreceptor degeneration

Abstract

Photoreceptor cells produce a receptor protein for pigment epithelium‐derived factor (PEDF‐R), which is encoded by the PNPLA2 gene of the patatin‐like phospholipase domain‐containing 2 family. PEDF‐R is a membrane‐linked lipase detected in the inner segments of photoreceptors. We aim to reveal the physiological role of PEDF‐R in photoreceptors by characterizing CRISPR Pnpla2 knock‐out mouse lines.CRISPR‐derived Pnpla2 knock‐out mouse lines were generated in a C57BL/6 and rd8 free background. RT‐PCR was performed from dissected retinas. Mice were evaluated for fundoscopy, electroretinography and angiography. Organs of the thorax were harvested and imaged using a LEICA microscope. Plasma was collected for lipid analyses. Eyes were enucleated and processed for histology, immunofluorescence, confocal microscopy, and transmission electron microscopy. Pnpla2‐/‐ mice had undetectable retinal Pnpla2 expression levels, enlarged whiter hearts, and lower plasma free fatty acids and triglycerides levels than their Pnpla2+/+ littermate controls. Their retinal pigment epithelial cells accumulated lipid droplets and their photoreceptors were deformed. Fundi of Pnpla2+/‐ eyes had white spots. The optic nerve area of Pnpla2‐/‐ and Pnpla2+/‐ mice had an excess of pigment. The a‐wave in Pnpla2‐/‐and Pnpla2+/‐ mice, and b‐wave in Pnpla2+/‐ mice were attenuated relative to controls. Retinal rhodopsin and opsin gene expression of Pnpla2‐/‐and Pnpla2+/‐ were lower than in controls. The immunofluorescent intensities of rhodopsin and opsin antibodies decreased in Pnpla2+/‐and Pnpla2‐/‐ photoreceptors relative to controls. The immunofluorescence of PKCα and synaptophysin antibodies (markers for bipolar cells and ribbon synapse) in the Pnpla2+/‐ retinas were lower than in controls. Ablation of Pnpla2 in mice causes malformation of photoreceptors and affects photoreceptor performance, identifying PEDF‐R as an important component for photoreceptor structure and function. These findings suggest that the absence of PEDF‐R in photoreceptor cells interrupts PEDF neurotrophic action, thereby heightening degeneration in the mice.