Comparative proximity biotinylation implicates the small GTPase RAB18 in sterol mobilization and biosynthesis

Loss of functional RAB18 causes the autosomal recessive condition Warburg Micro syndrome. To better understand this disease, we used proximity biotinylation to generate an inventory of potential RAB18 effectors. A restricted set of 28 RAB18 interactions were dependent on the binary RAB3GAP1-RAB3GAP2 RAB18-guanine nucleotide exchange factor complex. Twelve of these 28 interactions are supported by prior reports, and we have directly validated novel interactions with SEC22A, TMCO4, and INPP5B. Consistent with a role for RAB18 in regulating membrane contact sites, interactors included groups of microtubule/membrane-remodeling proteins, membrane-tethering and docking proteins, and lipid-modifying/transporting proteins. Two of the putative interactors, EBP and OSBPL2/ORP2, have sterol substrates. EBP is a Δ8-Δ7 sterol isomerase, and ORP2 is a lipid transport protein. This prompted us to investigate a role for RAB18 in cholesterol biosynthesis. We found that the cholesterol precursor and EBP-product lathosterol accumulates in both RAB18-null HeLa cells and RAB3GAP1-null fibroblasts derived from an affected individual. Furthermore, de novo cholesterol biosynthesis is impaired in cells in which RAB18 is absent or dysregulated or in which ORP2 expression is disrupted. Our data demonstrate that guanine nucleotide exchange factor-dependent Rab interactions are highly amenable to interrogation by proximity biotinylation and may suggest that Micro syndrome is a cholesterol biosynthesis disorder.

The Arf family GTPases: Regulation of vesicle biogenesis and beyond

The Arf family proteins are best known for their roles in the vesicle biogenesis. However, they also play fundamental roles in a wide range of cellular regulation besides vesicular trafficking, such as modulation of lipid metabolic enzymes, cytoskeleton remodeling, ciliogenesis, lysosomal, and mitochondrial morphology and functions. Growing studies continue to expand the downstream effector landscape of Arf proteins, especially for the less-studied members, revealing new biological functions, such as amino acid sensing. Experiments with cutting-edge technologies and in vivo functional studies in the last decade help to provide a more comprehensive view of Arf family functions. In this review, we summarize the cellular functions that are regulated by at least two different Arf members with an emphasis on those beyond vesicle biogenesis.

Oculocerebrorenal syndrome of Lowe (OCRL) controls leukemic T-cell survival by preventing excessive PI(4,5)P2 hydrolysis in the plasma membrane

T-cell acute lymphoblastic leukemia (T-ALL) is one of the deadliest and most aggressive hematological malignancies, but its pathological mechanism in controlling cell survival is not fully understood. Oculocerebrorenal syndrome (also called Lowe syndrome) is a rare X-linked recessive disorder characterized by cataracts, intellectual disability, and proteinuria. This disease has been shown to be caused by mutation of Oculocerebrorenal syndrome of Lowe 1 (OCRL1; OCRL), encoding a phosphatidylinositol 4,5-diphosphate [PI(4,5)P₂] 5-phosphatase involved in regulating membrane trafficking, however, its function in cancer cells is unclear. Here, we uncovered that OCRL1 is overexpressed in T-ALL cells and knockdown of OCRL1 results in cell death, indicating the essential role of OCRL in controlling T-ALL cell survival. We show OCRL is primarily localized in the Golgi, and can translocate to plasma membrane (PM) upon ligand stimulation. We found OCRL interacts with OSBP-related protein 4L (ORP4L), which facilitates OCRL translocation from the Golgi to the PM upon cluster of differentiation 3 (CD3) stimulation. Thus, OCRL represses the activity of ORP4L to prevent excessive PI(4,5)P₂ hydrolysis by phosphoinositide phospholipase C β3 (PLCβ3) and uncontrolled Ca²⁺ release from the endoplasmic reticulum (ER). We propose OCRL1 deletion leads to accumulation of PI(4,5)P₂ in the PM, disrupting the normal Ca²⁺ oscillation pattern in the cytosol and leading to mitochondrial Ca²⁺ overloading, ultimately causing T-ALL cell mitochondrial dysfunction and cell death. These results highlight a critical role for OCRL in maintaining moderate PI(4,5)P₂ availability in T-ALL cells. Our findings also raise the possibility of targeting OCRL1 to treat T-ALL disease.

The inositol 5-phosphatase INPP5B regulates B cell receptor clustering and signaling

Upon antigen binding, the B cell receptor (BCR) undergoes clustering to form a signalosome that propagates downstream signaling required for normal B cell development and physiology. BCR clustering is dependent on remodeling of the cortical actin network, but the mechanisms that regulate actin remodeling in this context remain poorly defined. In this study, we identify the inositol 5-phosphatase INPP5B as a key regulator of actin remodeling, BCR clustering, and downstream signaling in antigen-stimulated B cells. INPP5B acts via dephosphorylation of the inositol lipid PI(4,5)P2 that in turn is necessary for actin disassembly, BCR mobilization, and cell spreading on immobilized surface antigen. These effects can be explained by increased actin severing by cofilin and loss of actin linking to the plasma membrane by ezrin, both of which are sensitive to INPP5B-dependent PI(4,5)P2 hydrolysis. INPP5B is therefore a new player in BCR signaling and may represent an attractive target for treatment of B cell malignancies caused by aberrant BCR signaling.

Defining the proximal interaction networks of Arf GTPases reveals a mechanism for the regulation of PLD1 and PI4KB

The Arf GTPase family is involved in a wide range of cellular regulation including membrane trafficking and organelle-structure assembly. Here, we have generated a proximity interaction network for the Arf family using the miniTurboID approach combined with TMT-based quantitative mass spectrometry. Our interactome confirmed known interactions and identified many novel interactors that provide leads for defining Arf pathway cell biological functions. We explored the unexpected finding that phospholipase D1 (PLD1) preferentially interacts with two closely related but poorly studied Arf family GTPases, ARL11 and ARL14, showing that PLD1 is activated by ARL11/14 and may recruit these GTPases to membrane vesicles, and that PLD1 and ARL11 collaborate to promote macrophage phagocytosis. Moreover, ARL5A and ARL5B were found to interact with and recruit phosphatidylinositol 4-kinase beta (PI4KB) at trans-Golgi, thus promoting PI4KB's function in PI4P synthesis and protein secretion.

Decreased INPP5B expression predicts poor prognosis in lung adenocarcinoma

Background

Inositol Polyphosphate-5-Phosphatase B (INPP5B), a inositol 5-phosphatase, plays an important role in many biological processes through phosphorylating PI(4,5)P₂ and/or PI(3,4,5)P₃ at the 5-position. Nevertheless, little is known about its function and cellular pathways in tumors. This study aims to investigate the potential role of INPP5B as a diagnostic and prognostic biomarker for lung adenocarcinoma (LUAD), as well as its biological functions and molecular mechanisms in LUAD.

Methods

TCGA, GEO, CTPAC, and HPA datasets were used for differential expression analysis and pathological stratification comparison. The prognostic and diagnostic role of INPP5B was determined by Kaplan–Meier curves, univariate and multivariate Cox regression analysis, and receiver operating characteristics (ROC) curve analyses. The potential mechanism of INPP5B was explored through GO, KEGG, and GSEA enrichment analysis, as well as GeneMANIA and STRING protein–protein interaction (PPI) network. PicTar, PITA, and miRmap databases were used for exploring miRNA targeting INPP5B. In molecular biology experiments, immunohistochemical analyses and Western blot analyses were used to determine protein expression. Co-immunoprecipitation assay was used to detect protein–protein interactions. CCK8 assays and colony formation assays were used for the measurement of cell proliferation. Cell cycle was assessed by PI staining with flow cytometry. Cell migration was performed by Transwell assays and wound healing assays.

Result

INPP5B was decreased in LUAD tissues compared with normal adjacent tissues. And the low expression of INPP5B was associated with late-stage pathological features. In addition, INPP5B was found to be a significant independent prognostic and diagnostic factor for LUAD patients. Hsa-miR-582-5p was predicted as a negative regulator of INPP5B mRNA expression. INPP5B was significantly correlated with the expression of PTEN and the activity of PI3K/AKT signaling pathways, as determined by enrichment analysis and PPI network. In vitro experiments partially confirmed the aforementioned findings. INPP5B could interact directly with PTEN. INPP5B overexpression inhibited LUAD cell proliferation and migration while downregulating the AKT pathway.

Conclusion

Our results demonstrated that INPP5B could inhibit the proliferation and metastasis of LUAD cells. It could serve as a novel diagnostic and prognostic biomarker for LUAD patients.

Trial registration LUAD tissues and corresponding para-cancerous tissues were collected from 10 different LUAD patients at Hangzhou First People’s Hospital. The Ethics Committee of Hangzhou First People’s Hospital has approved this study. (registration number: IIT-20210907-0031-01; registration date: 2021.09.13)

Supplementary Information

The online version contains supplementary material available at 10.1186/s12935-022-02609-8.

Rab5 regulates macropinocytosis by recruiting the inositol 5-phosphatases OCRL and Inpp5b that hydrolyse PtdIns(4,5)P2

Rab5 is required for macropinosome formation, but its site and mode of action remain unknown. We report that Rab5 acts at the plasma membrane, downstream of ruffling, to promote macropinosome sealing and scission. Dominant-negative Rab5, which obliterates macropinocytosis, had no effect on the development of membrane ruffles. However, Rab5-containing vesicles were recruited to circular membrane ruffles, and soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-dependent endomembrane fusion was necessary for the completion of macropinocytosis. This fusion event coincided with the disappearance of PtdIns(4,5)P2 that accompanies macropinosome closure. Counteracting the depletion of PtdIns(4,5)P2 by expression of phosphatidylinositol-4-phosphate 5-kinase impaired macropinosome formation. Importantly, we found that the removal of PtdIns(4,5)P2 is dependent on Rab5, through the Rab5-mediated recruitment of the inositol 5-phosphatases OCRL and Inpp5b, via APPL1. Knockdown of OCRL and Inpp5b, or APPL1, prevented macropinosome closure without affecting ruffling. We therefore propose that Rab5 is essential for the clearance of PtdIns(4,5)P2 needed to complete the scission of macropinosomes or to prevent their back-fusion with the plasmalemma.

Induced Dimerization Tools to Deplete Specific Phosphatidylinositol Phosphates

Chemical dimerization systems have been used to drive acute depletion of polyphosphoinsitides (PPIns). They do so by inducing subcellular localization of enzymes that catabolize PPIns. By using this approach, all seven PPIns can be depleted in living cells and in real time. The rapid permeation of dimerizer agents and the specific expression of recruiter proteins confer great spatial and temporal resolution with minimal cell perturbation. In this chapter, we provide detailed instructions to monitor and induce depletion of PPIns in live cells.

Phosphoinositide lipids in primary cilia biology

Primary cilia are solitary signalling organelles projecting from the surface of most cell types. Although the ciliary membrane is continuous with the plasma membrane it exhibits a unique phospholipid composition, a feature essential for normal cilia formation and function. Recent studies have illustrated that distinct phosphoinositide lipid species localise to specific cilia subdomains, and have begun to build a 'phosphoinositide map' of the cilium. The abundance and localisation of phosphoinositides are tightly regulated by the opposing actions of lipid kinases and lipid phosphatases that have also been recently discovered at cilia. The critical role of phosphoinositides in cilia biology is highlighted by the devastating consequences of genetic defects in cilia-associated phosphoinositide regulatory enzymes leading to ciliopathy phenotypes in humans and experimental mouse and zebrafish models. Here we provide a general introduction to primary cilia and the roles phosphoinositides play in cilia biology. In addition to increasing our understanding of fundamental cilia biology, this rapidly expanding field may inform novel approaches to treat ciliopathy syndromes caused by deregulated phosphoinositide metabolism.

Altered expression of microRNA-223 in the plasma of patients with first-episode schizophrenia and its possible relation to neuronal migration-related genes

Recent studies have shown that microRNAs (miRNAs) play a role as regulators of neurodevelopment by modulating gene expression. Altered miRNA expression has been reported in various psychiatric disorders, including schizophrenia. However, the changes in the miRNA expression profile that occur during the initial stage of schizophrenia have not been fully investigated. To explore the global alterations in miRNA expression profiles that may be associated with the onset of schizophrenia, we first profiled miRNA expression in plasma from 17 patients with first-episode schizophrenia and 17 healthy controls using microarray analysis. Among the miRNAs that showed robust changes, the elevated expression of has-miR-223-3p (miR-223) was validated via quantitative reverse transcription-polymerase chain reaction (qRT-PCR) using another independent sample set of 21 schizophrenia patients and 21 controls. To identify the putative targets of miR-223, we conducted a genome-wide gene expression analysis in neuronally differentiated SK-N-SH cells with stable miR-223 overexpression and an in silico analysis. We found that the mRNA expression levels of four genes related to the cytoskeleton or cell migration were significantly downregulated in miR-223-overexpressing cells, possibly due to interactions with miR-223. The in silico analysis suggested the presence of miR-223 target sites in these four genes. Lastly, a luciferase assay confirmed that miR-223 directly interacted with the 3' untranslated regions (UTRs) of all four genes. Our results reveal an increase in miR-223 in plasma during both the first episode and the later stage of schizophrenia, which may affect the expression of cell migration-related genes targeted by miR-223.

The Great Escape: how phosphatidylinositol 4-kinases and PI4P promote vesicle exit from the Golgi (and drive cancer)

Phosphatidylinositol 4-phosphate (PI4P) is a membrane glycerophospholipid and a major regulator of the characteristic appearance of the Golgi complex as well as its vesicular trafficking, signalling and metabolic functions. Phosphatidylinositol 4-kinases, and in particular the PI4KIIIβ isoform, act in concert with PI4P to recruit macromolecular complexes to initiate the biogenesis of trafficking vesicles for several Golgi exit routes. Dysregulation of Golgi PI4P metabolism and the PI4P protein interactome features in many cancers and is often associated with tumour progression and a poor prognosis. Increased expression of PI4P-binding proteins, such as GOLPH3 or PITPNC1, induces a malignant secretory phenotype and the release of proteins that can remodel the extracellular matrix, promote angiogenesis and enhance cell motility. Aberrant Golgi PI4P metabolism can also result in the impaired post-translational modification of proteins required for focal adhesion formation and cell–matrix interactions, thereby potentiating the development of aggressive metastatic and invasive tumours. Altered expression of the Golgi-targeted PI 4-kinases, PI4KIIIβ, PI4KIIα and PI4KIIβ, or the PI4P phosphate Sac1, can also modulate oncogenic signalling through effects on TGN-endosomal trafficking. A Golgi trafficking role for a PIP 5-kinase has been recently described, which indicates that PI4P is not the only functionally important phosphoinositide at this subcellular location. This review charts new developments in our understanding of phosphatidylinositol 4-kinase function at the Golgi and how PI4P-dependent trafficking can be deregulated in malignant disease.

Lowe syndrome-linked endocytic adaptors direct membrane cycling kinetics with OCRL in Dictyostelium discoideum

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.

In vivo identification of GTPase interactors by mitochondrial relocalization and proximity biotinylation

The GTPases of the Ras superfamily regulate cell growth, membrane traffic and the cytoskeleton, and a wide range of diseases are caused by mutations in particular members. They function as switchable landmarks with the active GTP-bound form recruiting to the membrane a specific set of effector proteins. The GTPases are precisely controlled by regulators that promote acquisition of GTP (GEFs) or its hydrolysis to GDP (GAPs). We report here MitoID, a method for identifying effectors and regulators by performing in vivo proximity biotinylation with mitochondrially-localized forms of the GTPases. Applying this to 11 human Rab GTPases identified many known effectors and GAPs, as well as putative novel effectors, with examples of the latter validated for Rab2, Rab5, Rab9 and Rab11. MitoID can also efficiently identify effectors and GAPs of Rho and Ras family GTPases such as Cdc42, RhoA, Rheb, and N-Ras, and can identify GEFs by use of GDP-bound forms.

Lipid Modifications in Cilia Biology

Cilia are specialized cellular structures with distinctive roles in various signaling cascades. Ciliary proteins need to be trafficked to the cilium to function properly; however, it is not completely understood how these proteins are delivered to their final localization. In this review, we will focus on how different lipid modifications are important in ciliary protein trafficking and, consequently, regulation of signaling pathways. Lipid modifications can play a variety of roles, including tethering proteins to the membrane, aiding trafficking through facilitating interactions with transporter proteins, and regulating protein stability and abundance. Future studies focusing on the role of lipid modifications of ciliary proteins will help our understanding of how cilia maintain specific protein pools strictly connected to their functions.

Phosphatidylinositol Kinases and Phosphatases in Entamoeba histolytica

Phosphatidylinositol (PtdIns) metabolism is indispensable in eukaryotes. Phosphoinositides (PIs) are phosphorylated derivatives of PtdIns and consist of seven species generated by reversible phosphorylation of the inositol moieties at the positions 3, 4, and 5. Each of the seven PIs has a unique subcellular and membrane domain distribution. In the enteric protozoan parasite Entamoeba histolytica, it has been previously shown that the PIs phosphatidylinositol 3-phosphate (PtdIns3P), PtdIns(4,5)P₂, and PtdIns(3,4,5)P₃ are localized to phagosomes/phagocytic cups, plasma membrane, and phagocytic cups, respectively. The localization of these PIs in E. histolytica is similar to that in mammalian cells, suggesting that PIs have orthologous functions in E. histolytica. In contrast, the conservation of the enzymes that metabolize PIs in this organism has not been well-documented. In this review, we summarized the full repertoire of the PI kinases and PI phosphatases found in E. histolytica via a genome-wide survey of the current genomic information. E. histolytica appears to have 10 PI kinases and 23 PI phosphatases. It has a panel of evolutionarily conserved enzymes that generate all the seven PI species. However, class II PI 3-kinases, type II PI 4-kinases, type III PI 5-phosphatases, and PI 4P-specific phosphatases are not present. Additionally, regulatory subunits of class I PI 3-kinases and type III PI 4-kinases have not been identified. Instead, homologs of class I PI 3-kinases and PTEN, a PI 3-phosphatase, exist as multiple isoforms, which likely reflects that elaborate signaling cascades mediated by PtdIns(3,4,5)P₃ are present in this organism. There are several enzymes that have the nuclear localization signal: one phosphatidylinositol phosphate (PIP) kinase, two PI 3-phosphatases, and one PI 5-phosphatase; this suggests that PI metabolism also has conserved roles related to nuclear functions in E. histolytica, as it does in model organisms.

The 5-phosphatase OCRL in Lowe syndrome and Dent disease 2

Lowe syndrome is an X-linked disease that is characterized by congenital cataracts, central hypotonia, intellectual disability and renal Fanconi syndrome. The disease is caused by mutations in OCRL, which encodes an inositol polyphosphate 5-phosphatase (OCRL) that acts on phosphoinositides - quantitatively minor constituents of cell membranes that are nonetheless pivotal regulators of intracellular trafficking. In this Review we summarize the considerable progress made over the past decade in understanding the cellular roles of OCRL in regulating phosphoinositide balance along the endolysosomal pathway, a fundamental system for the reabsorption of proteins and solutes by proximal tubular cells. We discuss how studies of OCRL have led to important discoveries about the basic mechanisms of membrane trafficking and describe the key features and limitations of the currently available animal models of Lowe syndrome. Mutations in OCRL can also give rise to a milder pathology, Dent disease 2, which is characterized by renal Fanconi syndrome in the absence of extrarenal pathologies. Understanding how mutations in OCRL give rise to two clinical entities with differing extrarenal manifestations represents an opportunity to identify molecular pathways that could be targeted to develop treatments for these conditions.

Crystal Structures of Type-II Inositol Polyphosphate 5-Phosphatase INPP5B with Synthetic Inositol Polyphosphate Surrogates Reveal New Mechanistic Insights for the Inositol 5-Phosphatase Family

The inositol polyphosphate 5-phosphatase INPP5B hydrolyzes the 5-phosphate group from water- and lipid-soluble signaling messengers. Two synthetic benzene and biphenyl polyphosphates (BzP/BiPhPs), simplified surrogates of inositol phosphates and phospholipid headgroups, were identified by thermodynamic studies as potent INPP5B ligands. The X-ray structure of the complex between INPP5B and biphenyl 3,3′,4,4′,5,5′-hexakisphosphate [BiPh(3,3′,4,4′,5,5′)P₆, IC₅₀ 5.5 μM] was determined at 2.89 Å resolution. One inhibitor pole locates in the phospholipid headgroup binding site and the second solvent-exposed ring binds to the His-Tag of another INPP5B molecule, while a molecule of inorganic phosphate is also present in the active site. Benzene 1,2,3-trisphosphate [Bz(1,2,3)P₃] [one ring of BiPh(3,3′,4,4′,5,5′)P₆] inhibits INPP5B ca. 6-fold less potently. Co-crystallization with benzene 1,2,4,5-tetrakisphosphate [Bz(1,2,4,5)P₄, IC₅₀ = 6.3 μM] yielded a structure refined at 2.9 Å resolution. Conserved residues among the 5-phosphatase family mediate interactions with Bz(1,2,4,5)P₄ and BiPh(3,3′,4,4′,5,5′)P₆ similar to those with the polar groups present in positions 1, 4, 5, and 6 on the inositol ring of the substrate. 5-Phosphatase specificity most likely resides in the variable zone located close to the 2- and 3-positions of the inositol ring, offering insights to inhibitor design. We propose that the inorganic phosphate present in the INPP5B–BiPh(3,3′,4,4′,5,5′)P₆ complex mimics the postcleavage substrate 5-phosphate released by INPP5B in the catalytic site, allowing elucidation of two new key features in the catalytic mechanism proposed for the family of phosphoinositide 5-phosphatases: first, the involvement of the conserved Arg-451 in the interaction with the 5-phosphate and second, identification of the water molecule that initiates 5-phosphate hydrolysis. Our model also has implications for the proposed “moving metal” mechanism.

Role of Ocrl1 in primary cilia assembly

Lowe syndrome is a lethal X-linked genetic disorder characterized by congenital cataracts, mental retardation, and kidney dysfunction. It is caused by mutations in the OCRL1 (oculocerebrorenal syndrome of Lowe) gene that encodes a phosphatidylinositol 5-phosphatase (EC 3.1.3.36). The gene product Ocrl1 has been linked to a multitude of functions due to the central role played by phosphoinositides in signaling. Moreover, this protein also has the ability to bind Rho GTPases, the master regulators of the actin cytoskeleton, and to interact with elements of the vesicle trafficking machinery. It is currently under investigation how deficiencies in Ocrl1 affect these different processes and contribute to patient symptoms. This chapter outlines the known physiological roles of Ocrl1 which might be relevant to the mechanism underlying Lowe syndrome.

OCRL-mutated fibroblasts from patients with Dent-2 disease exhibit INPP5B-independent phenotypic variability relatively to Lowe syndrome cells

OCRL mutations are associated with both Lowe syndrome and Dent-2 disease, two rare X-linked conditions. Lowe syndrome is an oculo-cerebro-renal disorder, whereas Dent-2 patients mainly present renal proximal tubulopathy. Loss of OCRL-1, a phosphoinositide-5-phosphatase, leads in Lowe patients' fibroblasts to phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2) accumulation, with defects in F-actin network, α-actinin distribution and ciliogenesis, whereas fibroblasts of Dent-2 patients are still uncharacterized. To search for mechanisms linked to clinical variability observed between these two OCRL mutation-associated pathologies, we compared dermal fibroblasts from independent patients, four affected by Dent-2 disease and six with Lowe syndrome. For the first time, we describe that Dent-2 fibroblasts with OCRL loss-of-function (LOF) mutations exhibit decrease in actin stress fibers, appearance of punctate α-actinin signals and alteration in primary cilia formation. Interestingly, we quantified these phenotypes as clearly intermediate between Lowe and control fibroblasts, thus suggesting that levels of these defects correlate with clinical variations observed between patients with OCRL mutations. In addition, we show that Lowe and Dent-2 fibroblasts display similar PI(4,5)P2 accumulation levels. Finally, we analyzed INPP5B, a paralogous gene already reported to exhibit functional redundancy with OCRL, and report neither differences in its expression at RNA or protein levels, nor specific allelic variations between fibroblasts of patients. Altogether, we describe here differential phenotypes between fibroblasts from Lowe and Dent-2 patients, both associated with OCRL LOF mutations, we exclude direct roles of PI(4,5)P2 and INPP5B in this phenotypic variability and we underline potential key alterations leading to ocular and neurological clinical features in Lowe syndrome.

Inositol lipid phosphatases in membrane trafficking and human disease

The specific interaction of phosphoinositides with proteins is critical for a plethora of cellular processes, including cytoskeleton remodelling, mitogenic signalling, ion channel regulation and membrane traffic. The spatiotemporal restriction of different phosphoinositide species helps to define compartments within the cell, and this is particularly important for membrane trafficking within both the secretory and endocytic pathways. Phosphoinositide homoeostasis is tightly regulated by a large number of inositol kinases and phosphatases, which respectively phosphorylate and dephosphorylate distinct phosphoinositide species. Many of these enzymes have been implicated in regulating membrane trafficking and, accordingly, their dysregulation has been linked to a number of human diseases. In the present review, we focus on the inositol phosphatases, concentrating on their roles in membrane trafficking and the human diseases with which they have been associated.

A role of OCRL in clathrin-coated pit dynamics and uncoating revealed by studies of Lowe syndrome cells

Mutations in the inositol 5-phosphatase OCRL cause Lowe syndrome and Dent's disease. Although OCRL, a direct clathrin interactor, is recruited to late-stage clathrin-coated pits, clinical manifestations have been primarily attributed to intracellular sorting defects. Here we show that OCRL loss in Lowe syndrome patient fibroblasts impacts clathrin-mediated endocytosis and results in an endocytic defect. These cells exhibit an accumulation of clathrin-coated vesicles and an increase in U-shaped clathrin-coated pits, which may result from sequestration of coat components on uncoated vesicles. Endocytic vesicles that fail to lose their coat nucleate the majority of the numerous actin comets present in patient cells. SNX9, an adaptor that couples late-stage endocytic coated pits to actin polymerization and which we found to bind OCRL directly, remains associated with such vesicles. These results indicate that OCRL acts as an uncoating factor and that defects in clathrin-mediated endocytosis likely contribute to pathology in patients with OCRL mutations.

DOI:http://dx.doi.org/10.7554/eLife.02975.001

Oculo-Cerebro-Renal syndrome of Lowe (Lowe syndrome) is a rare genetic disorder that can cause cataracts, mental disabilities and kidney dysfunction. It is caused by mutations in the gene encoding OCRL, a protein that modifies a membrane lipid and that is found on membranes transporting molecules (cargo) into cells by a process known as endocytosis.

During endocytosis, the cell outer membrane is deformed into a pit that engulfs the cargo to be taken up by the cell. The pit then pinches off from the outer membrane to form a vesicle—a bubble-like compartment—inside the cell that transports the cargo to its destination. In one type of endocytosis, this process is mediated by a basket-like coat primarily made up from the protein clathrin that assembles at the membrane patch to be internalized. After the vesicle is released from the cell membrane, the clathrin coat is broken apart and its components are shed and recycled for use by new budding endocytic vesicles.

The OCRL protein had previously been observed associated to newly forming clathrin-coated vesicles, but the significance of this was not known. Now, Nández et al. have used a range of imaging and analytical techniques to further investigate the properties of OCRL, taking advantage of cells from patients with Lowe syndrome. These cells lack OCRL, and so allow the effect of OCRL's absence on cell function to be deduced. OCRL destroys the membrane lipid that helps to connect the clathrin coat to the membrane, and Nández et al. show that without OCRL the newly formed vesicle moves into the cell but fails to efficiently shed its clathrin coat. Thus, a large fraction of clathrin coat components remain trapped on the vesicles, reducing the amount of such components available to help new pits develop into vesicles. As a consequence, the cell has difficulty internalizing molecules.

Collectively, the findings of Nández et al. outline that OCRL plays a role in the regulation of endocytosis in addition to its previously reported actions in the control of intracellular membrane traffic. The results also help to explain some of the symptoms seen in Lowe syndrome patients.

DOI:http://dx.doi.org/10.7554/eLife.02975.002

The cellular and physiological functions of the Lowe syndrome protein OCRL1

Phosphoinositide lipids play a key role in cellular physiology, participating in a wide array of cellular processes. Consequently, mutation of phosphoinositide-metabolizing enzymes is responsible for a growing number of diseases in humans. Two related disorders, oculocerebrorenal syndrome of Lowe (OCRL) and Dent-2 disease, are caused by mutation of the inositol 5-phosphatase OCRL1. Here, we review recent advances in our understanding of OCRL1 function. OCRL1 appears to regulate many processes within the cell, most of which depend upon coordination of membrane dynamics with remodeling of the actin cytoskeleton. Recently developed animal models have managed to recapitulate features of Lowe syndrome and Dent-2 disease, and revealed new insights into the underlying mechanisms of these disorders. The continued use of both cell-based approaches and animal models will be key to fully unraveling OCRL1 function, how its loss leads to disease and, importantly, the development of therapeutics to treat patients.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Inositol polyphosphate phosphatases in human disease

Phosphoinositide signalling molecules interact with a plethora of effector proteins to regulate cell proliferation and survival, vesicular trafficking, metabolism, actin dynamics and many other cellular functions. The generation of specific phosphoinositide species is achieved by the activity of phosphoinositide kinases and phosphatases, which phosphorylate and dephosphorylate, respectively, the inositol headgroup of phosphoinositide molecules. The phosphoinositide phosphatases can be classified as 3-, 4- and 5-phosphatases based on their specificity for dephosphorylating phosphates from specific positions on the inositol head group. The SAC phosphatases show less specificity for the position of the phosphate on the inositol ring. The phosphoinositide phosphatases regulate PI3K/Akt signalling, insulin signalling, endocytosis, vesicle trafficking, cell migration, proliferation and apoptosis. Mouse knockout models of several of the phosphoinositide phosphatases have revealed significant physiological roles for these enzymes, including the regulation of embryonic development, fertility, neurological function, the immune system and insulin sensitivity. Importantly, several phosphoinositide phosphatases have been directly associated with a range of human diseases. Genetic mutations in the 5-phosphatase INPP5E are causative of the ciliopathy syndromes Joubert and MORM, and mutations in the 5-phosphatase OCRL result in Lowe's syndrome and Dent 2 disease. Additionally, polymorphisms in the 5-phosphatase SHIP2 confer diabetes susceptibility in specific populations, whereas reduced protein expression of SHIP1 is reported in several human leukaemias. The 4-phosphatase, INPP4B, has recently been identified as a tumour suppressor in human breast and prostate cancer. Mutations in one SAC phosphatase, SAC3/FIG4, results in the degenerative neuropathy, Charcot-Marie-Tooth disease. Indeed, an understanding of the precise functions of phosphoinositide phosphatases is not only important in the context of normal human physiology, but to reveal the mechanisms by which these enzyme families are implicated in an increasing repertoire of human diseases.

The 5-phosphatase OCRL mediates retrograde transport of the mannose 6-phosphate receptor by regulating a Rac1-cofilin signalling module

Mutations in the OCRL gene encoding the phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) 5-phosphatase OCRL cause Lowe syndrome (LS), which is characterized by intellectual disability, cataracts and selective proximal tubulopathy. OCRL localizes membrane-bound compartments and is implicated in intracellular transport. Comprehensive analysis of clathrin-mediated endocytosis in fibroblasts of patients with LS did not reveal any difference in trafficking of epidermal growth factor, low density lipoprotein or transferrin, compared with normal fibroblasts. However, LS fibroblasts displayed reduced mannose 6-phosphate receptor (MPR)-mediated re-uptake of the lysosomal enzyme arylsulfatase B. In addition, endosome-to-trans Golgi network (TGN) transport of MPRs was decreased significantly, leading to higher levels of cell surface MPRs and their enrichment in enlarged, retromer-positive endosomes in OCRL-depleted HeLa cells. In line with the higher steady-state concentration of MPRs in the endosomal compartment in equilibrium with the cell surface, anterograde transport of the lysosomal enzyme, cathepsin D was impaired. Wild-type OCRL counteracted accumulation of MPR in endosomes in an activity-dependent manner, suggesting that PI(4,5)P(2) modulates the activity state of proteins regulated by this phosphoinositide. Indeed, we detected an increased amount of the inactive, phosphorylated form of cofilin and lower levels of the active form of PAK3 upon OCRL depletion. Levels of active Rac1 and RhoA were reduced or enhanced, respectively. Overexpression of Rac1 rescued both enhanced levels of phosphorylated cofilin and MPR accumulation in enlarged endosomes. Our data suggest that PI(4,5)P(2) dephosphorylation through OCRL regulates a Rac1-cofilin signalling cascade implicated in MPR trafficking from endosomes to the TGN.

Phosphoinositide phosphatases: just as important as the kinases

Phosphoinositide phosphatases comprise several large enzyme families with over 35 mammalian enzymes identified to date that degrade many phosphoinositide signals. Growth factor or insulin stimulation activates the phosphoinositide 3-kinase that phosphorylates phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P(2)] to form phosphatidylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P(3)], which is rapidly dephosphorylated either by PTEN (phosphatase and tensin homologue deleted on chromosome 10) to PtdIns(4,5)P(2), or by the 5-phosphatases (inositol polyphosphate 5-phosphatases), generating PtdIns(3,4)P(2). 5-phosphatases also hydrolyze PtdIns(4,5)P(2) forming PtdIns(4)P. Ten mammalian 5-phosphatases have been identified, which regulate hematopoietic cell proliferation, synaptic vesicle recycling, insulin signaling, and embryonic development. Two 5-phosphatase genes, OCRL and INPP5E are mutated in Lowe and Joubert syndrome respectively. SHIP [SH2 (Src homology 2)-domain inositol phosphatase] 2, and SKIP (skeletal muscle- and kidney-enriched inositol phosphatase) negatively regulate insulin signaling and glucose homeostasis. SHIP2 polymorphisms are associated with a predisposition to insulin resistance. SHIP1 controls hematopoietic cell proliferation and is mutated in some leukemias. The inositol polyphosphate 4-phosphatases, INPP4A and INPP4B degrade PtdIns(3,4)P(2) to PtdIns(3)P and regulate neuroexcitatory cell death, or act as a tumor suppressor in breast cancer respectively. The Sac phosphatases degrade multiple phosphoinositides, such as PtdIns(3)P, PtdIns(4)P, PtdIns(5)P and PtdIns(3,5)P(2) to form PtdIns. Mutation in the Sac phosphatase gene, FIG4, leads to a degenerative neuropathy. Therefore the phosphatases, like the lipid kinases, play major roles in regulating cellular functions and their mutation or altered expression leads to many human diseases.

Phosphoinositides in Golgi complex function

The Golgi complex is a ribbon-like organelle composed of stacks of flat cisternae interconnected by tubular junctions. It occupies a central position in the endomembrane system as proteins and lipids that are synthesized in the endoplasmic reticulum (ER) pass through the Golgi complex to undergo biosynthetic modification (mainly glycosylation) and to be sorted to their final destinations. In addition the Golgi complex possesses a number of activities, apparently not directly connected with its main role in trafficking and sorting, which have been recently reviewed in Wilson et al. 2011. In spite of the constant massive flux of material the Golgi complex maintains its identity and phosphoinositides (PIs), among other factors, play a central role in this process. The active metabolism of PIs at the Golgi is necessary for the proper functioning of the organelle both in terms of membrane trafficking/sorting and its manifold metabolic and signalling activities. Phosphatidylinositol 4-phosphate (PtdIns4P), in particular, is responsible for the recruitment of numerous cytosolic proteins that recognise and bind PtdIns4P via specific lipid-binding domains. In this chapter we will summarize the findings that have contributed to our current understanding of the role of PIs in the biology of the Golgi complex in terms of the regulation of PI metabolism and the functional roles and regulation of PtdIns4P effectors.

Phosphoinositides in insulin action and diabetes

Phosphoinositides play an essential role in insulin signaling, serving as a localization signal for a variety of proteins that participate in the regulation of cellular growth and metabolism. This chapter will examine the regulation and localization of phosphoinositide species, and will explore the roles of these lipids in insulin action. We will also discuss the changes in phosphoinositide metabolism that occur in various pathophysiological states such as insulin resistance and diabetes.

Impaired neural development in a zebrafish model for Lowe syndrome

Lowe syndrome, which is characterized by defects in the central nervous system, eyes and kidneys, is caused by mutation of the phosphoinositide 5-phosphatase OCRL1. The mechanisms by which loss of OCRL1 leads to the phenotypic manifestations of Lowe syndrome are currently unclear, in part, owing to the lack of an animal model that recapitulates the disease phenotype. Here, we describe a zebrafish model for Lowe syndrome using stable and transient suppression of OCRL1 expression. Deficiency of OCRL1, which is enriched in the brain, leads to neurological defects similar to those reported in Lowe syndrome patients, namely increased susceptibility to heat-induced seizures and cystic brain lesions. In OCRL1-deficient embryos, Akt signalling is reduced and there is both increased apoptosis and reduced proliferation, most strikingly in the neural tissue. Rescue experiments indicate that catalytic activity and binding to the vesicle coat protein clathrin are essential for OCRL1 function in these processes. Our results indicate a novel role for OCRL1 in neural development, and support a model whereby dysregulation of phosphoinositide metabolism and clathrin-mediated membrane traffic leads to the neurological symptoms of Lowe syndrome.

Lowe Syndrome protein OCRL1 supports maturation of polarized epithelial cells

Mutations in the inositol polyphosphate 5-phosphatase OCRL1 cause Lowe Syndrome, leading to cataracts, mental retardation and renal failure. We noted that cell types affected in Lowe Syndrome are highly polarized, and therefore we studied OCRL1 in epithelial cells as they mature from isolated individual cells into polarized sheets and cysts with extensive communication between neighbouring cells. We show that a proportion of OCRL1 targets intercellular junctions at the early stages of their formation, co-localizing both with adherens junctional components and with tight junctional components. Correlating with this distribution, OCRL1 forms complexes with junctional components α-catenin and zonula occludens (ZO)-1/2/3. Depletion of OCRL1 in epithelial cells growing as a sheet inhibits maturation; cells remain flat, fail to polarize apical markers and also show reduced proliferation. The effect on shape is reverted by re-expressed OCRL1 and requires the 5'-phosphatase domain, indicating that down-regulation of 5-phosphorylated inositides is necessary for epithelial development. The effect of OCRL1 in epithelial maturation is seen more strongly in 3-dimensional cultures, where epithelial cells lacking OCRL1 not only fail to form a central lumen, but also do not have the correct intracellular distribution of ZO-1, suggesting that OCRL1 functions early in the maturation of intercellular junctions when cells grow as cysts. A role of OCRL1 in junctions of polarized cells may explain the pattern of organs affected in Lowe Syndrome.

Role of Rab GTPases in membrane traffic and cell physiology

Intracellular membrane traffic defines a complex network of pathways that connects many of the membrane-bound organelles of eukaryotic cells. Although each pathway is governed by its own set of factors, they all contain Rab GTPases that serve as master regulators. In this review, we discuss how Rabs can regulate virtually all steps of membrane traffic from the formation of the transport vesicle at the donor membrane to its fusion at the target membrane. Some of the many regulatory functions performed by Rabs include interacting with diverse effector proteins that select cargo, promoting vesicle movement, and verifying the correct site of fusion. We describe cascade mechanisms that may define directionality in traffic and ensure that different Rabs do not overlap in the pathways that they regulate. Throughout this review we highlight how Rab dysfunction leads to a variety of disease states ranging from infectious diseases to cancer.

The PH domain proteins IPIP27A and B link OCRL1 to receptor recycling in the endocytic pathway

We identify two new binding partners for the OCRL1 protein that is mutated in Lowe syndrome and type 2 Dent disease, which we call IPIP27A and B. The IPIPs are required for receptor recycling in the endocytic pathway, suggesting that defects in this process lead to the aforementioned disorders.

Mutation of the inositol polyphosphate 5-phosphatase OCRL1 results in two disorders in humans, namely Lowe syndrome (characterized by ocular, nervous system, and renal defects) and type 2 Dent disease (in which only the renal symptoms are evident). The disease mechanisms of these syndromes are poorly understood. Here we identify two novel OCRL1-binding proteins, termed inositol polyphosphate phosphatase interacting protein of 27 kDa (IPIP27)A and B (also known as Ses1 and 2), that also bind the related 5-phosphatase Inpp5b. The IPIPs bind to the C-terminal region of these phosphatases via a conserved motif similar to that found in the signaling protein APPL1. IPIP27A and B, which form homo- and heterodimers, localize to early and recycling endosomes and the trans-Golgi network (TGN). The IPIPs are required for receptor recycling from endosomes, both to the TGN and to the plasma membrane. Our results identify IPIP27A and B as key players in endocytic trafficking and strongly suggest that defects in this process are responsible for the pathology of Lowe syndrome and Dent disease.

Species-specific difference in expression and splice-site choice in Inpp5b, an inositol polyphosphate 5-phosphatase paralogous to the enzyme deficient in Lowe Syndrome

The oculocerebrorenal syndrome of Lowe (OCRL; MIM #309000) is an X-linked human disorder characterized by congenital cataracts, mental retardation, and renal proximal tubular dysfunction caused by loss-of-function mutations in the OCRL gene that encodes Ocrl, a type II phosphatidylinositol bisphosphate (PtdIns4,5P₂) 5-phosphatase. In contrast, mice with complete loss-of-function of the highly homologous ortholog Ocrl have no detectable renal, ophthalmological, or central nervous system abnormalities. We inferred that the disparate phenotype between Ocrl-deficient humans and mice was likely due to differences in how the two species compensate for loss of the Ocrl enzyme. We therefore turned our attention to Inpp5b, another type II PtdIns4,5P₂ 5-phosphatase encoded by Inpp5b in mice and INPP5B in humans, as potential compensating genes in the two species, because Inpp5b/INPP5B are the most highly conserved paralogs to Ocrl/OCRL in the respective genomes of both species and Inpp5b demonstrates functional overlap with Ocrl in mice in vivo. We used in silico sequence analysis, reverse-transcription PCR, quantitative PCR, and transient transfection assays of promoter function to define splice-site usage and the function of an internal promoter in mouse Inpp5b versus human INPP5B. We found mouse Inpp5b and human INPP5B differ in their transcription, splicing, and primary amino acid sequence. These observations form the foundation for analyzing the functional basis for the difference in how Inpp5b and INPP5B compensate for loss of Ocrl function and, by providing insight into the cellular roles of Ocrl and Inpp5b, aid in the development of a model system in which to study Lowe syndrome.

Phosphatidylinositol- and phosphatidylcholine-transfer activity of PITPbeta is essential for COPI-mediated retrograde transport from the Golgi to the endoplasmic reticulum

Vesicles formed by the COPI complex function in retrograde transport from the Golgi to the endoplasmic reticulum (ER). Phosphatidylinositol transfer protein beta (PITPbeta), an essential protein that possesses phosphatidylinositol (PtdIns) and phosphatidylcholine (PtdCho) lipid transfer activity is known to localise to the Golgi and ER but its role in these membrane systems is not clear. To examine the function of PITPbeta at the Golgi-ER interface, RNA interference (RNAi) was used to knockdown PITPbeta protein expression in HeLa cells. Depletion of PITPbeta leads to a decrease in PtdIns(4)P levels, compaction of the Golgi complex and protection from brefeldin-A-mediated dispersal to the ER. Using specific transport assays, we show that anterograde traffic is unaffected but that KDEL-receptor-dependent retrograde traffic is inhibited. This phenotype can be rescued by expression of wild-type PITPbeta but not by mutants defective in docking, PtdIns transfer and PtdCho transfer. These data demonstrate that the PtdIns and PtdCho exchange activity of PITPbeta is essential for COPI-mediated retrograde transport from the Golgi to the ER.

Comprehensive screening for novel rab-binding proteins by GST pull-down assay using 60 different mammalian Rabs

The Rab family belongs to the Ras-like small GTPase superfamily and is implicated in membrane trafficking through interaction with specific effector molecules. Because of the large number of Rab isoforms in mammals, however, the effectors of most of the mammalian Rabs are yet to be identified. In this study, we systematically screened five different cell or tissue lysates for novel Rab effectors by a combination of glutathione S-transferase (GST) pull-down assay with 60 different mammalian Rabs and mass spectroscopic analysis. Three of the 21 Rab-binding proteins we identified, mKIAA1055/TBC1D2B (Rab22-binding protein), GAPCenA/TBC1D11 (Rab36-binding protein) and centaurin beta2/ACAP2 (Rab35-binding protein), are GTPase-activating proteins (GAPs) for Rab or Arf. Although it has recently been proposed that the Rab-GAP (Tre-2 /Bub2/Cdc16) domain physically interacts with its substrate Rab, these three GAPs interacted with specific Rabs via a domain other than a GAP domain, e.g. centaurin beta2 binds GTP-Rab35 via the ankyrin repeat (ANKR) domain. Although centaurin beta2 did not exhibit any Rab35-GAP activity in vitro, the Rab35-binding ANKR domain of centaurin beta2 was found to be required for its plasma membrane localization and regulation of Rab35-dependent neurite outgrowth of PC12 cells through inactivation of Arf6. These findings suggest a novel mode of interaction between Rab and GAP.

OCRL1 function in renal epithelial membrane traffic

The X-linked disorder Lowe syndrome arises from mutations in OCRL1, a lipid phosphatase that hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP(2)). Most patients with Lowe syndrome develop proteinuria very early in life. PIP(2) dynamics are known to modulate numerous steps in membrane trafficking, and it has been proposed that OCRL1 activity regulates the biogenesis or trafficking of the multiligand receptor megalin. To examine this possibility, we investigated the effects of siRNA-mediated OCRL1 knockdown on biosynthetic and postendocytic membrane traffic in canine and human renal epithelial cells. Cells depleted of OCRL1 did not have significantly elevated levels of cellular PIP(2) but displayed an increase in actin comets, as previously observed in cultured cells derived from Lowe patients. Using assays to independently quantitate the endocytic trafficking of megalin and of megalin ligands, we could observe no defect in the trafficking or function of megalin upon OCRL1 knockdown. Moreover, apical delivery of a newly synthesized marker protein was unaffected. OCRL1 knockdown did result in a significant increase in secretion of the lysosomal hydrolase cathepsin D, consistent with a role for OCRL1 in membrane trafficking between the trans-Golgi network and endosomes. Together, our studies suggest that OCRL1 does not directly modulate endocytosis or postendocytic membrane traffic and that the renal manifestations observed in Lowe syndrome patients are downstream consequences of the loss of OCRL1 function.

Regulation of Golgi function via phosphoinositide lipids

Phosphoinositides play important roles in Golgi traffic and structural integrity. Specific lipid kinases and phosphatases associate with the Golgi complex and regulate the multiplicity of trafficking routes from this organelle. Work in different model systems showed that the basic elements that regulate lipid signaling at the Golgi are conserved from yeast to humans. Many of the enzymes involved in Golgi phosphoinositide metabolism are essential for viability or cause severe human disease when malfunctioning. Phosphoinositide effectors at the Golgi control both non-vesicular transfer of lipids and sorting of secretory and membrane proteins. In addition, Golgi phosphoinositides were recently implicated in the metabolic and cell growth-dependent regulation of the secretory pathway.

Rab GTPase function in Golgi trafficking

The Rab, ARF, and Arl members of the Ras superfamily of small GTPases work together to control specific intracellular trafficking pathways. Here we focus on their roles in protein transport to and within the Golgi apparatus.

Lowe syndrome patient fibroblasts display Ocrl1-specific cell migration defects that cannot be rescued by the homologous Inpp5b phosphatase

The Lowe syndrome (LS) is a life-threatening, developmental disease characterized by mental retardation, cataracts and renal failure. Although this human illness has been linked to defective function of the phosphatidylinositol 5-phosphatase, Ocrl1 (Oculo-Cerebro-Renal syndrome ofLowe protein1), the mechanism by which this enzyme deficiency triggers the disease is not clear. Ocrl1 is known to localize mainly to the Golgi apparatus and endosomes, however it translocates to plasma membrane ruffles upon cell stimulation with growth factors. The functional implications of this inducible translocation to the plasma membrane are presently unknown. Here we show that Ocrl1 is required for proper cell migration, spreading and fluid-phase uptake in both established cell lines and human dermal fibroblasts. We found that primary fibroblasts from two patients diagnosed with LS displayed defects in these cellular processes. Importantly, these abnormalities were suppressed by expressing wild-type Ocrl1 but not by a phosphatase-deficient mutant. Interestingly, the homologous human PI-5-phosphatase, Inpp5b, was unable to complement the Ocrl1-dependent cell migration defect. Further, Ocrl1 variants that cannot bind the endocytic adaptor AP2 or clathrin, like Inpp5b, were less apt to rescue the migration phenotype. However, no defect in membrane recruitment of AP2/clathrin or in transferrin endocytosis by patient cells was detected. Collectively, our results suggest that Ocrl1, but not Inpp5b, is involved in ruffle-mediated membrane remodeling. Our results provide new elements for understanding how Ocrl1 deficiency leads to the abnormalities associated with the LS.

Mammalian phosphoinositide kinases and phosphatases

Phosphoinositides are lipids that are present in the cytoplasmic leaflet of a cell's plasma and internal membranes and play pivotal roles in the regulation of a wide variety of cellular processes. Phosphoinositides are molecularly diverse due to variable phosphorylation of the hydroxyl groups of their inositol rings. The rapid and reversible configuration of the seven known phosphoinositide species is controlled by a battery of phosphoinositide kinases and phosphoinositide phosphatases, which are thus critical for phosphoinositide isomer-specific localization and functions. Significantly, a given phosphoinositide generated by different isozymes of these phosphoinositide kinases and phosphatases can have different biological effects. In mammals, close to 50 genes encode the phosphoinositide kinases and phosphoinositide phosphatases that regulate phosphoinositide metabolism and thus allow cells to respond rapidly and effectively to ever-changing environmental cues. Understanding the distinct and overlapping functions of these phosphoinositide-metabolizing enzymes is important for our knowledge of both normal human physiology and the growing list of human diseases whose etiologies involve these proteins. This review summarizes the structural and biological properties of all the known mammalian phosphoinositide kinases and phosphoinositide phosphatases, as well as their associations with human disorders.

The role of the inositol polyphosphate 5-phosphatases in cellular function and human disease

Phosphoinositides are membrane-bound signalling molecules that regulate cell proliferation and survival, cytoskeletal reorganization and vesicular trafficking by recruiting effector proteins to cellular membranes. Growth factor or insulin stimulation induces a canonical cascade resulting in the transient phosphorylation of PtdIns(4,5)P(2) by PI3K (phosphoinositide 3-kinase) to form PtdIns(3,4,5)P(3), which is rapidly dephosphorylated either by PTEN (phosphatase and tensin homologue deleted on chromosome 10) back to PtdIns(4,5)P(2), or by the 5-ptases (inositol polyphosphate 5-phosphatases), generating PtdIns(3,4)P(2). The 5-ptases also hydrolyse PtdIns(4,5)P(2), forming PtdIns4P. Ten mammalian 5-ptases have been identified, which share a catalytic mechanism similar to that of the apurinic/apyrimidinic endonucleases. Gene-targeted deletion of 5-ptases in mice has revealed that these enzymes regulate haemopoietic cell proliferation, synaptic vesicle recycling, insulin signalling, endocytosis, vesicular trafficking and actin polymerization. Several studies have revealed that the molecular basis of Lowe's syndrome is due to mutations in the 5-ptase OCRL (oculocerebrorenal syndrome of Lowe). Futhermore, the 5-ptases SHIP [SH2 (Src homology 2)-domain-containing inositol phosphatase] 2, SKIP (skeletal muscle- and kidney-enriched inositol phosphatase) and 72-5ptase (72 kDa 5-ptase)/Type IV/Inpp5e (inositol polyphosphate 5-phosphatase E) are implicated in negatively regulating insulin signalling and glucose homoeostasis in specific tissues. SHIP2 polymorphisms are associated with a predisposition to insulin resistance. Gene profiling studies have identified changes in the expression of various 5-ptases in specific cancers. In addition, 5-ptases such as SHIP1, SHIP2 and 72-5ptase/Type IV/Inpp5e regulate macrophage phagocytosis, and SHIP1 also controls haemopoietic cell proliferation. Therefore the 5-ptases are a significant family of signal-modulating enzymes that govern a plethora of cellular functions by regulating the levels of specific phosphoinositides. Emerging studies have implicated their loss or gain of function in human disease.

Differential clathrin binding and subcellular localization of OCRL1 splice isoforms

Mutation of the inositol polyphosphate 5-phosphatase OCRL1 causes the X-linked disorder oculocerebrorenal syndrome of Lowe, characterized by defects in the brain, kidneys, and eyes. OCRL1 exists as two splice isoforms that differ by a single exon encoding 8 amino acids. The longer protein, termed isoform a, is the only form in brain, whereas both isoforms are present in all other tissues. The significance of OCRL1 splicing is currently unclear. Given its proximity to a clathrin-binding site, we hypothesized that splicing may alter the clathrin binding properties of OCRL1. Here we show that this is indeed the case. OCRL1 isoform a binds clathrin with higher affinity than isoform b and is significantly more enriched in clathrin-coated trafficking intermediates. We also identify a second clathrin-binding site in OCRL1 that contributes to clathrin binding of both isoforms. Association of OCRL1 with clathrin-coated intermediates requires membrane association through interaction with Rab GTPases but not binding to the clathrin adaptor AP2. Expression of OCRL1 isoform a lacking the 5-phosphatase domain impairs transferrin endocytosis, whereas an equivalent version of isoform b does not. Our results suggest that OCRL1 exists as two functional pools, one participating in clathrin-mediated trafficking events such as endocytosis and another that is much less or not involved in this process.

The inositol polyphosphate 5-phosphatase OCRL1 restricts intracellular growth of Legionella, localizes to the replicative vacuole and binds to the bacterial effector LpnE

Legionella pneumophila, the causative agent of Legionnaires' disease, replicates within a specific vacuole in amoebae and macrophages. To form these 'Legionella-containing vacuoles' (LCVs), the bacteria employ the Icm/Dot type IV secretion system and effector proteins, some of which anchor to the LCV membrane via the host glycolipid phosphatidylinositol 4-phosphate [PtdIns(4)P]. Here we analysed the role of inositol polyphosphate 5-phosphatases (IP5Ps) during L. pneumophila infections. Bacterial replication and LCV formation occurred more efficiently in Dictyostelium discoideum amoebae lacking the IP5P Dd5P4, a homologue of human OCRL1 (Oculocerebrorenal syndrome of Lowe), implicated in retrograde endosome to Golgi trafficking. The phenotype was complemented by Dd5P4 but not the catalytically inactive 5-phosphatase. Ectopically expressed Dd5P4 or OCRL1 localized to LCVs in D. discoideum via an N-terminal domain previously not implicated in membrane targeting, and OCRL1 was also identified on LCVs in macrophages. Dd5P4 was catalytically active on LCVs and accumulated on LCVs harbouring wild-type but not DeltaicmT mutant L. pneumophila. The N-terminal domain of OCRL1 bound L. pneumophila LpnE, a Sel1-like repeat protein involved in LCV formation, which localizes to LCVs and selectively binds PtdIns(3)P. Our results indicate that OCRL1 restricts intracellular growth of L. pneumophila and binds to LCVs in association with LpnE.

Large scale screening for novel rab effectors reveals unexpected broad Rab binding specificity

Small GTPase Rab is generally thought to control intracellular membrane trafficking through interaction with specific effector molecules. Because of the large number of Rab isoforms in mammals, however, the effectors of most of the mammalian Rabs have never been identified, and the Rab binding specificity of the Rab effectors previously reported has never been thoroughly investigated. In this study we systematically screened for novel Rab effectors by a yeast two-hybrid assay with 28 different mouse or human Rabs (Rab1-30) as bait and identified 27 Rab-binding proteins, including 19 novel ones. We further investigated their Rab binding specificity by a yeast two-hybrid assay with a panel of 60 different GTP-locked mouse or human Rabs. Unexpectedly most (17 of 27) of the Rab-binding proteins we identified exhibited broad Rab binding specificity and bound multiple Rab isoforms. As an example, inositol-polyphosphate 5-phosphatase OCRL (oculocerebrorenal syndrome of Lowe) bound the greatest number of Rabs (i.e. 16 distinct Rabs). Others, however, specifically recognized only a single Rab isoform or only two closely related Rab isoforms. The interaction of eight of the novel Rab-binding proteins identified (e.g. INPP5E and Cog4) with a specific Rab isoform was confirmed by co-immunoprecipitation assay and/or colocalization analysis in mammalian cell cultures, and the novel Rab2B-binding domain of Golgi-associated Rab2B interactor (GARI) and GARI-like proteins was identified by deletion and homology search analyses. The findings suggest that most Rab effectors (or Rab-binding proteins) regulate intracellular membrane trafficking through interaction with several Rab isoforms rather than through a single Rab isoform.