The Lowe Syndrome Protein OCRL1 Is Required for Endocytosis in the Zebrafish Pronephric Tubule

Abstract

Lowe syndrome and Dent-2 disease are caused by mutation of the inositol 5-phosphatase OCRL1. Despite our increased understanding of the cellular functions of OCRL1, the underlying basis for the renal tubulopathy seen in both human disorders, of which a hallmark is low molecular weight proteinuria, is currently unknown. Here, we show that deficiency in OCRL1 causes a defect in endocytosis in the zebrafish pronephric tubule, a model for the mammalian renal tubule. This coincides with a reduction in levels of the scavenger receptor megalin and its accumulation in endocytic compartments, consistent with reduced recycling within the endocytic pathway. We also observe reduced numbers of early endocytic compartments and enlarged vacuolar endosomes in the sub-apical region of pronephric cells. Cell polarity within the pronephric tubule is unaffected in mutant embryos. The OCRL1-deficient embryos exhibit a mild ciliogenesis defect, but this cannot account for the observed impairment of endocytosis. Catalytic activity of OCRL1 is required for renal tubular endocytosis and the endocytic defect can be rescued by suppression of PIP5K. These results indicate for the first time that OCRL1 is required for endocytic trafficking in vivo, and strongly support the hypothesis that endocytic defects are responsible for the renal tubulopathy in Lowe syndrome and Dent-2 disease. Moreover, our results reveal PIP5K as a potential therapeutic target for Lowe syndrome and Dent-2 disease.

Receptor-Mediated Endocytosis in the Proximal Tubule

“…Additionally, an intriguing recent study in zebrafish found dramatically reduced levels of megalin in the pronephric kidney of OCRL1 knockout transgenic zebrafish. This was accompanied by the loss of early endosomes and the appearance of enlarged vacuoles that accumulated residual megalin (148). The reduction in megalin expression is similar to that reported in the Dent disease mouse model and consistent with the reported reduction in levels of shed megalin in the urine of Lowe syndrome and Dent disease patients.…”

Endocytic Control of Cellular Signaling at the Primary Cilium

“…Conversely, signaling by primary cilia may also regulate endocytosis remotely; for example, in the kidney proximal tubules, where flow-induced ciliary Ca 2+ and purinergic signaling was reported to promote endocytic clearance of low molecular weight proteins and other ligands from the glomerular filtrate via remote activation of LRP2 located at the base of microvilli [98]. Interestingly, a recent study in zebrafish revealed that recycling of LRP2-positive endosomes to the plasma membrane of pronephric cells requires the enzymatic activity of the Lowe syndrome protein OCRL1 [99], which hydrolyzes phosphatidylinositol (PI) (4,5)P 2 to PI(4)P [100]. Notably, OCRL1 localizes to the primary cilium via binding to Rab8 and has been implicated in ciliary membrane biogenesis [101][102][103][104].…”

“…Notably, OCRL1 localizes to the primary cilium via binding to Rab8 and has been implicated in ciliary membrane biogenesis [101][102][103][104]. However, the role of OCRL1 in mediating endocytic recycling of LRP2 in the zebrafish kidney pronephros appears to be independent of its ciliogenic function [99]. By contrast, in cells of the trabecular meshwork (TM) in the eye, OCRL1 was implicated in regulation of cilia-mediated intraocular pressure sensation via interaction with the ciliary mechanosensory channel TRP vanilloid 4 (TRPV4), which in turn was found to impinge on transcription of the genes encoding TNF-/, TGF-b, and GLI1 [105].…”

Role of Ocrl1 and Inpp5E in primary cilia assembly and maintenance: a phosphatidylinositol phosphatase relay system?

“…A recent study showed in vivo evidence in Ocrl1 -/-zebrafish of defective megalin recycling and hence reduced uptake of its ligand RAP. 58 However, another study reported no defect in megalin recycling or megalin ligand uptake upon Ocrl1 KD. 55 Absence of Ocrl1 function also triggers the accumulation of PI(4,5)P 2 on endosomes that in turn leads to enhanced, N-WASP-dependent actin polymerization producing the so-called "actin comet tails".…”