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

Phosphoinositides (PIs) are a family of seven membrane lipids, each playing distinct roles as signaling molecules. PIKfyve is a phosphoinositide kinase responsible for generating two low-abundance lipids: phosphoinositide-3,5-bisphosphate (PI(3,5)P₂) and phosphoinositide-5-phosphate (PI5P), whose functions remain incompletely understood. Emerging evidence implicates PIKfyve in key cellular processes, including melanosome formation, phagocytosis, endosomal trafficking, lysosomal maintenance, and autophagy. Complete loss of PIKfyve function is embryonically 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). Photoreceptors detect light via the outer segment (OS), a specialized sensory structure composed of stacked membranous discs rich in photopigment. The OS is continuously renewed, and a primary function of the RPE is to phagocytose OS tips. 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 mostly intact over the timespan analyzed. Electroretinogram (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.