PIKfyve is an essential component of the endolysosomal pathway within photoreceptors and the retinal pigment epithelium

Phosphoinositides (PIs) are a family of seven low abundance membrane lipids, each with distinct signaling functions. The phosphoinositide kinase PIKfyve generates phosphoinositide-3,5-bisphosphate (PI(3,5)P2) and PI5P. Emerging evidence implicates PIKfyve in key cellular processes, including autophagy, phagocytosis, endosomal trafficking, lysosomal maintenance, and melanosome formation. Complete loss of PIKfyve function is embryonic lethal in model organisms. In humans, heterozygous mutations in PIKFYVE are associated with Fleck corneal dystrophy and congenital cataracts. In this study, we investigate the role of PIKfyve in photoreceptors and the adjacent retinal pigment epithelium (RPE), host to dynamic endolysosomal pathways required for enduring the high oxidative stress environment, transporting metabolites and phototransduction components, and the breakdown of outer segment discs. To assess PIKfyve function in the retina and RPE in our zebrafish model, we employed CRISPR/Cas9-mediated gene editing and pharmacological inhibition using the specific PIKfyve inhibitor apilimod. Loss of PIKfyve activity leads to RPE expansion characterized by the accumulation of LC3- and LAMP1-positive vacuoles, along with defects in phagosome degradation and minor changes to melanosome biogenesis. Photoreceptors deprived of PIKfyve function develop a single large vacuole in the inner segment, while the OS remains largely intact over the timespan analyzed. Electroretinography (ERG) recordings revealed complete visual impairment in pikfyve crispant larvae, and significantly reduced visual function in larvae treated with apilimod post embryogenesis. These findings highlight the critical role of PIKfyve in the development and homeostasis of the RPE and retina.