Cell Lines from Kidney Proximal Tubules of a Patient with Lowe Syndrome Lack OCRL Inositol Polyphosphate 5-Phosphatase and Accumulate Phosphatidylinositol 4,5-Bisphosphate

Abstract

The protein product of the gene that when mutated is responsible for Lowe syndrome, or oculocerebrorenal syndrome (OCRL), is an inositol polyphosphate 5-phosphatase. It has a marked preference for phosphatidylinositol 4,5-bisphosphate although it hydrolyzes all four of the known inositol polyphosphate 5-phosphatase substrates: inositol 1,4,5-trisphosphate, inositol 1,3,4,5-tetrakisphosphate, phosphatidylinositol 4,5-bisphosphate, and phosphatidylinositol 3,4,5-trisphosphate. The enzyme activity of this protein is determined by a region of 672 out of a total of 970 amino acids that is homologous to inositol polyphosphate 5-phosphatase II. Cell lines from kidney proximal tubules of a patient with Lowe syndrome and a normal individual were used to study the function of OCRL. The cells from the Lowe syndrome patient lack OCRL protein. OCRL is the major phosphatidylinositol 4,5-bisphosphate 5-phosphatase in these cells. As a result, these cells accumulate phosphatidylinositol 4,5-bisphosphate even though at least four other inositol polyphosphate 5-phosphatase isozymes are present in these cells. OCRL is associated with lysosomal membranes in control proximal tubule cell lines suggesting that OCRL may function in lysosomal membrane trafficking by regulating the specific pool of phosphatidylinositol 4,5-bisphosphate that is associated with lysosomes.Lowe syndrome, or oculocerebrorenal syndrome, is a rare X chromosome-linked disorder that is characterized by severe mental retardation, congenital cataracts, and renal Fanconi syndrome (1). The renal Fanconi syndrome develops in the neonatal period with impaired renal proximal tubular function including acidosis, amino aciduria, phosphaturia, and proteinuria (1, 2). The gene responsible for Lowe syndrome was identified by positional cloning of X chromosome breakpoints in two affected females (3). The predicted protein, designated OCRL, 1 is comprised of 970 amino acids and is 51% identical to inositol polyphosphate 5-phosphatase type II (5-phosphatase II) over a span of 672 amino acids. The amino-terminal one-third has no homology to 5-phosphatase II or any other sequences in GenBank (3). The striking homology between OCRL and 5-phosphatase II suggested that OCRL belongs to the 5-phosphatase gene family and that Lowe syndrome represents an inborn error of inositol phosphate metabolism.Inositol polyphosphate 5-phosphatases (5-phosphatases) are a group of enzymes containing 5-phosphatase homology domains, and two conserved signature motifs within these domains define proteins that have 5-phosphatase activity (for review, see Refs. 4 -6). The substrates of 5-phosphatases include two soluble inositol polyphosphates inositol 1,4,5-trisphosphate (Ins 1,4,5-P 3 ) and inositol 1,3,4,5-tetrakisphosphate (Ins 1,3,4,5-P 4 ), and two inositol lipids phosphatidylinositol 4,5-bisphosphate (PtdIns 4,5-P 2 ) and phosphatidylinositol 3,4,5-trisphosphate (PtdIns 3,4,5-P 3 ). The 5-phosphatase isozymes have varying substrate specificities. Seven mammalian 5-phosphatases have been cloned and...

The inositol polyphosphate 5-phosphatase Ocrl associates with endosomes that are partially coated with clathrin

“…Ocrl has been localized to lysosomes in kidney proximal tubule cells and to the trans-Golgi network (TGN) in fibroblasts (4,5). This localization is consistent with a role for Ocrl in lysosomal enzyme trafficking from the TGN to lysosomes.…”

“…We also confirm the previously published Golgi localization of Ocrl (14). Ocrl has been localized to lysosomes in kidney epithelial cells (4). This study relied partially on sucrose-treated cells to identify mature lysosomes.…”

“…Other members of this family include type II 5-phosphatase, synaptojanin-1, and synaptojanin-2 (3). The preferred substrate of Ocrl is phosphatidylinositol-4,5-bisphosphate, and this lipid accumulates in kidney proximal tubule cells from Lowe syndrome patients (4). In addition to the 5-phosphatase domain, Ocrl also contains a RhoGap domain and an N-terminal region of unknown function.…”

The Yeast Inositol Polyphosphate 5-Phosphatase Inp54p Localizes to the Endoplasmic Reticulum via a C-terminal Hydrophobic Anchoring Tail

“…The localization of Inp54p to the ER is the first evidence of the targeting of a yeast 5-phosphatase to a specific subcellular compartment in the resting cell. To date no mammalian 5-phosphatase has been localized to the ER, although the Lowe's protein has been identified in both the Golgi and the lysosomal compartment (56,57), and a novel 72-kDa 5-phosphatase has been localized to the cytoplasmic surface of the Golgi, where the enzyme is proposed to regulate phosphoinositide-mediated vesicular trafficking (58).…”

Abnormal bradykinin signalling in fibroblasts deficient in the PIP2 5‐phosphatase, ocrl1

“…The deficiency of ocrl1 leads to elevated cellular levels of PIP 2 (Wenk et al 2003;Zhang et al 1998). However, it is unknown at present why a deficiency of this widely expressed protein primarily affects the lens, kidney and brain in Lowe syndrome.…”