Inpp5e is a Critical Regulator of Protein Transport to Photoreceptor Outer Segments

In humans, inositol polyphosphate-5-phosphatase e (INPP5E) mutations cause retinal degeneration as part of Joubert and MORM syndromes and can also cause non-syndromic blindness. In mice, mutations cause a spectrum of brain, kidney, and other anomalies and prevent the formation of photoreceptor outer segments. To further explore the function of Inpp5e in photoreceptors, we generated conditional and inducible knockouts of mouse Inpp5e where the gene was deleted either during outer segment formation or after outer segments were fully formed. In both cases, the loss of Inpp5e led to severe defects in photoreceptor outer segment morphology and ultimately photoreceptor cell loss. The primary morphological defect consisted of outer segment shortening and reduction in the number of newly forming discs at the outer segment base. This was accompanied by structural abnormalities of the Golgi apparatus, mislocalized rhodopsin, and an accumulation of extracellular vesicles. In addition, knockout cells showed a reduction in the size and prevalence of the actin network at the site of new disc morphogenesis and the occasional formation of membrane whorls instead of discs in a subset of cells. Together, these data demonstrate that Inpp5e plays a critical role in protein trafficking to the outer segment and the normal process of outer segment renewal depends on the activity of this enzyme.