Deletion of the phosphatase INPP5E in the murine retina impairs photoreceptor axoneme formation and prevents disc morphogenesis.

INPP5E, also known as pharbin, is a ubiquitously expressed phosphatidylinositol polyphosphate 5-phosphatase that is typically located in the primary cilia and modulates the phosphoinositide composition of membranes. Mutations to or loss of INPP5E is associated with ciliary dysfunction. INPP5E missense mutations of the phosphatase catalytic domain cause Joubert syndrome in humans-a syndromic ciliopathy affecting multiple tissues including the brain, liver, kidney, and retina. In contrast to other primary cilia, photoreceptor INPP5E is prominently expressed in the inner segment and connecting cilium and absent in the outer segment, which is a modified primary cilium dedicated to phototransduction. To investigate how loss of INPP5e causes retina degeneration, we generated mice with a retina-specific KO (Inpp5eF/F;Six3Cre, abbreviated as retInpp5e-/-). These mice exhibit a rapidly progressing rod-cone degeneration resembling Leber congenital amaurosis that is nearly completed by postnatal day 21 (P21) in the central retina. Mutant cone outer segments contain vesicles instead of discs as early as P8. Although P10 mutant outer segments contain structural and phototransduction proteins, axonemal structure and disc membranes fail to form. Connecting cilia of retInpp5e-/- rods display accumulation of intraflagellar transport particles A and B at their distal ends, suggesting disrupted intraflagellar transport. Although INPP5E ablation may not prevent delivery of outer segment-specific proteins by means of the photoreceptor secretory pathway, its absence prevents the assembly of axonemal and disc components. Herein, we suggest a model for INPP5E-Leber congenital amaurosis, proposing how deletion of INPP5E may interrupt axoneme extension and disc membrane elaboration.

C8ORF37 Is Required for Photoreceptor Outer Segment Disc Morphogenesis by Maintaining Outer Segment Membrane Protein Homeostasis.

C8ORF37 is a causative gene for three different clinical forms of incurable retinal degeneration. However, the completely unknown function of C8ORF37 limits our understanding of the pathogenicity of C8ORF37 mutations. Here, we performed a comprehensive phenotypic characterization of a C8orf37 KO mouse line, generated using CRISPR/Cas9 technology. Both C8orf37 KO male and female mice exhibited progressive and simultaneous degeneration of rod and cone photoreceptors but no non-ocular phenotypes. The major ultrastructural feature of C8orf37 KO photoreceptors was massive disorganization of the outer segment (OS) membrane discs starting from the onset of disc morphogenesis during development. At the molecular level, the amounts of multiple OS-specific membrane proteins, including proteins involved in membrane disc organization, were reduced, although these proteins were targeted normally to the OS. Considering the distribution of C8ORF37 throughout the photoreceptor cell body, the normal structure of the KO photoreceptor connecting cilium, and the absence of defects in other ciliary organs of the KO mice, our findings do not support the previous notion that C8ORF37 was a ciliary protein. Because C8ORF37 is absent in the photoreceptor OS, C8ORF37 may participate in the secretory pathway of OS membrane proteins in the photoreceptor cell body and thus maintain the homeostasis of these proteins. This study established a valid animal model for future therapeutic studies of C8ORF37-associated retinal degeneration. This study also shed new light on the role of C8ORF37 in photoreceptors and on the pathogenic mechanism underlying retinal degeneration caused by C8ORF37 mutations.SIGNIFICANCE STATEMENT Inherited retinal degeneration is a group of incurable conditions with poorly understood underlying molecular mechanisms. We investigated C8ORF37, a causative gene for three retinal degenerative conditions: retinitis pigmentosa, cone-rod dystrophy, and Bardet-Biedl syndrome. C8ORF37 encodes a protein with no known functional domains and thus its biological function is unpredictable. We knocked out the C8ORF37 ortholog in mice, which resulted in a retinal phenotype similar to that observed in patients. We further demonstrated that C8ORF37 is required for photoreceptor outer segment disc formation and alignment, a process that is critical for photoreceptor function and survival. This study advances our understanding of the pathogenesis of retinal degeneration and establishes a valuable mouse model for future therapeutic development.