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.
