Lowe syndrome–linked endocytic adaptors direct membrane cycling kinetics with OCRL inDictyostelium discoideum

Abstract

Mutations of the inositol 5-phosphatase OCRL cause Lowe syndrome (LS), characterized by congenital cataract, low IQ, and defective kidney proximal tubule resorption. A key subset of LS mutants abolishes OCRL’s interactions with endocytic adaptors containing F&H peptide motifs. Converging unbiased methods examining human peptides and the unicellular phagocytic organism Dictyostelium discoideum reveal that, like OCRL, the Dictyostelium OCRL orthologue Dd5P4 binds two proteins closely related to the F&H proteins APPL1 and Ses1/2 (also referred to as IPIP27A/B). In addition, a novel conserved F&H interactor was identified, GxcU (in Dictyostelium) and the Cdc42-GEF FGD1-related F-actin binding protein (Frabin) (in human cells). Examining these proteins in D. discoideum, we find that, like OCRL, Dd5P4 acts at well-conserved and physically distinct endocytic stations. Dd5P4 functions in coordination with F&H proteins to control membrane deformation at multiple stages of endocytosis and suppresses GxcU-mediated activity during fluid-phase micropinocytosis. We also reveal that OCRL/Dd5P4 acts at the contractile vacuole, an exocytic osmoregulatory organelle. We propose F&H peptide-containing proteins may be key modifiers of LS phenotypes.

The endocytic pathways of Dictyostelium discoideum

“…Both macropinosome and phagosome membranes initially contain high levels of PI(3,4)P 2 . PI(3,4)P 2 is generated by the rapid de-phosphorylation of PI(3,4,5)P 3 by the PI-5 phosphatase called Oculocerebrorenal syndrome of Lowe (OCRL), during and immediately following cup closure (Dormann et al, 2004;Li et al, 2018;Loovers et al, 2007;Luscher et al, 2019). On phagosomal membranes PI(3,4)P 2 content peaks immediately following cup closure and is gradually lost over time (Dormann et al, 2004).…”

The Polarized Redistribution of the Contractile Vacuole to the Rear of the Cell is Critical for Streaming and is Regulated by PI(4,5)P2-Mediated Exocytosis

“…OCRL is an inositol polyphosphate 5-phosphatase that hydrolyzes the 5-phosphate of PI(4,5)P2 into PI4P ( Prosseda et al, 2017 ). Interestingly, a recent study in D. discoideum has shown that OCRL-like protein of D. discoideum , Dd5P4, is recruited to the CV membrane upon the kiss-and-run exocytic event ( Luscher et al, 2019 ). Therefore, it is possible that the exocytic function of ORCL contributes to the pathological process of Lowe syndrome.…”

“…SecA is therefore necessary for CV fusion to the PM and water discharge. A recent study suggested that PI(4,5)P2 regulates the trafficking of the CV to the fusion site and cells with a defect in Dd5P4, the enzyme which uses the 5-phosphates of PI(4,5)P2 as substrate, displayed inefficient CV fusion ( Luscher et al, 2019 ).…”