Loss of the lipid kinase PIKfyve may disrupt endolysosomal function in photoreceptors and retinal pigment epithelial cells, leading to progressive retinal degeneration in preclinical models.
Researchers generated mouse models with rod photoreceptor-specific or retinal pigment epithelial cell–specific deletion of PIKfyve to examine its role in retinal maintenance. They assessed retinal morphology, electroretinography, immunohistochemistry, transmission electron microscopy, metabolomics, lysosomal and autophagic activity, rhodopsin localization, and retinal pigment epithelial (RPE) phagocytosis. To determine whether reduced PIKfyve accelerated inherited retinal degeneration, they also crossed PIKfyve-deficient mice with a P23H rhodopsin–mutated mice in a model involving autosomal dominant retinitis pigmentosa.
Deletion of PIKfyve in rod photoreceptors reduced retinal phosphatidylinositol 3,5-bisphosphate levels by approximately 80% while increasing phosphatidylinositol 3-phosphate about 2.5-fold, confirming disruption of PIKfyve signaling. At 4 weeks, retinal structure and electroretinogram responses remained intact despite increased expression of the lysosomal proteins LAMP1 and LAMP2, indicating that lysosomal dysfunction preceded detectable retinal degeneration.
By 16 weeks, mice lacking PIKfyve in rod photoreceptors exhibited thinning of the outer nuclear layer, diminished rod- and cone-mediated electroretinogram responses, and mislocalization of rhodopsin from the outer segments into the inner retina. Transmission electron microscopy revealed marked accumulation of enlarged intracellular vacuoles within photoreceptor inner segments, increasing from approximately 2 to 3 vacuoles per field in control mice to 36 to 40 vacuoles per field in knockout mice. Mean vacuole size increased from 0.0555 μm² to 0.673 μm². Complementary cell studies demonstrated impaired endolysosomal maturation and defective autophagic processing.
The researchers found that reducing PIKfyve expression worsened retinal degeneration in the P23H model. Compared with mice carrying the P23H mutation alone, mice with both the P23H mutation and reduced PIKfyve expression had greater photoreceptor loss, lower rhodopsin expression, and more severe retinal degeneration.
Deletion of PIKfyve in RPE cells also impaired degradative pathways. Knockout mice accumulated rhodopsin, lysosomal proteins, and lipid droplets, indicating impaired phagocytosis, lysosomal function, autophagy, and lipid clearance. Metabolomic analyses identified alterations in amino acid, lipid, nucleotide, glycolytic, and tricarboxylic acid cycle pathways, suggesting broader metabolic consequences of disrupted lysosomal homeostasis.
Pharmacologic inhibition of PIKfyve is under investigation for autoimmune, neurodegenerative, and infectious diseases. However, photoreceptors and RPE cells appeared particularly dependent on intact PIKfyve-mediated endolysosomal function, underscoring the importance of considering the potential retinal effects of these therapeutics.
The study was limited by its reliance on genetically engineered mouse models and complementary cell culture experiments, which may not fully reflect human retinal disease.
“Together, our results demonstrate that PIKfyve-generated [phosphatidylinositol 3,5-bisphosphate] is crucial for regulating the endolysosomal pathway in photoreceptors and RPE cells, suggesting that enhancing PIKfyve function may provide neuroprotection in retinal diseases,” wrote lead study author Ammaji Rajala, of the Department of Ophthalmology at the University of Oklahoma Health Sciences Center, and colleagues.
The study authors reported no conflicts of interest.
Source: Cell Death & Disease