Phosphatidylinositol 3,5-bisphosphate: metabolism and cellular functions

Phosphatidylinositol-3,5-bisphosphate: no longer the poor PIP2

Phosphoinositides play an important role in organelle identity by recruiting effector proteins to the host membrane organelle, thus decorating that organelle with molecular identity. Phosphatidylinositol-3,5-bisphos- phate [PtdIns(3,5)P(2) ] is a low-abundance phosphoinositide that predominates in endolysosomes in higher eukaryotes and in the yeast vacuole. Compared to other phosphoinositides such as PtdIns(4,5)P(2) , our understanding of the regulation and function of PtdIns(3,5)P(2) remained rudimentary until more recently. Here, we review many of the recent developments in PtdIns(3,5)P(2) function and regulation. PtdIns(3,5)P(2) is now known to espouse functions, not only in the regulation of endolysosome morphology, trafficking and acidification, but also in autophagy, signaling mediation in response to stresses and hormonal cues and control of membrane and ion transport. In fact, PtdIns(3,5)P(2) misregulation is now linked with several human neuropathologies including Charcot-Marie-Tooth disease and amyotrophic lateral sclerosis. Given the functional versatility of PtdIns(3,5)P(2) , it is not surprising that regulation of PtdIns(3,5)P(2) metabolism is proving rather elaborate. PtdIns(3,5)P(2) synthesis and turnover are tightly coupled via a protein complex that includes the Fab1/PIKfyve lipid kinase and its antagonistic Fig4/Sac3 lipid phosphatase. Most interestingly, many PtdIns(3,5)P(2) regulators play simultaneous roles in its synthesis and turnover.

The nucleophosmin-anaplastic lymphoma kinase oncogene interacts, activates, and uses the kinase PIKfyve to increase invasiveness

NPM-ALK is a chimeric tyrosine kinase detected in most anaplastic large cell lymphomas that results from the reciprocal translocation t(2,5)(p23;q35) that fuses the N-terminal domain of nucleophosmin (NPM) to the catalytic domain of the anaplastic lymphoma kinase (ALK) receptor. The constitutive activity of the kinase is responsible for its oncogenicity through the stimulation of several downstream signaling pathways, leading to cell proliferation, migration, and survival. We demonstrated previously that the high level of phosphatidylinositol 5-phosphate measured in NPM-ALK-expressing cells is controlled by the phosphoinositide kinase PIKfyve, a lipid kinase known for its role in vesicular trafficking. Here, we show that PIKfyve associates with NPM-ALK and that the interaction involves the 181-300 region of the oncogene. Moreover, we demonstrate that the tyrosine kinase activity of the oncogene controls PIKfyve lipid kinase activity but is dispensable for the formation of the complex. Silencing or inhibition of PIKfyve using siRNA or the PIKfyve inhibitor YM201636 have no effect on NPM-ALK-mediated proliferation and migration but strongly reduce invasive capacities of NPM-ALK-expressing cells and their capacity to degrade the extracellular matrix. Accordingly, immunofluorescence studies confirm a perturbation of matrix metalloproteinase 9 localization at the cell surface and defect in maturation. Altogether, these results suggest a role for PIKfyve in NPM-ALK-mediated invasion.

Phosphoinositide [PI(3,5)P2] lipid-dependent regulation of the general transcriptional regulator Tup1

Transcriptional activity of a gene is governed by transcriptional regulatory complexes that assemble/disassemble on the gene and control the chromatin architecture. How cytoplasmic components influence the assembly/disassembly of transcriptional regulatory complexes is poorly understood. Here we report that the budding yeast Saccharomyces cerevisiae has a chromatin architecture-modulating mechanism that is dependent on the endosomal lipid PI(3,5)P(2). We identified Tup1 and Cti6 as new, highly specific PI(3,5)P(2) interactors. Tup1--which associates with multiple transcriptional regulators, including the HDAC (histone deacetylase) and SAGA complexes--plays a crucial role in determining an activated or repressed chromatin state of numerous genes, including GAL1. We show that, in the context that the Gal4 activation pathway is compromised, PI(3,5)P(2) plays an essential role in converting the Tup1-driven repressed chromatin structure into a SAGA-containing activated chromatin structure at the GAL1 promoter. Biochemical and cell biological experiments suggest that PI(3,5)P(2) recruits Cti6 and the Cyc8-Tup1 corepressor complex to the late endosomal/vacuolar membrane and mediates the assembly of a Cti6-Cyc8-Tup1 coactivator complex that functions to recruit the SAGA complex to the GAL1 promoter. Our findings provide important insights toward understanding how the chromatin architecture and epigenetic status of a gene are regulated by cytoplasmic components.

The phosphoinositide phosphatase MTM-1 regulates apoptotic cell corpse clearance through CED-5–CED-12 in C. elegans

Multicellular organisms use programmed cell death to eliminate unwanted or potentially harmful cells. Improper cell corpse removal can lead to autoimmune diseases. The development of interventional therapies that increase engulfment activity could represent an attractive approach to treat such diseases. Here, we describe mtm-1, the Caenorhabditis elegans homolog of human myotubularin 1, as a potential negative regulator of apoptotic cell corpse clearance. Loss of mtm-1 function leads to substantially reduced numbers of persistent cell corpses in engulfment mutants, which is a result of a restoration of engulfment function rather than of impaired or delayed programmed cell death. Epistatic analyses place mtm-1 upstream of the ternary GEF complex, which consists of ced-2, ced-5 and ced-12, and parallel to mig-2. Over-activation of engulfment results in the removal of viable cells that have been brought to the verge of death under limiting caspase activity. In addition, mtm-1 also promotes phagosome maturation in the hermaphrodite gonad, potentially through CED-1 receptor recycling. Finally, we show that the CED-12 PH domain can bind to PtdIns(3,5)P2 (one target of MTM-1 phosphatase activity), suggesting that MTM-1 might regulate CED-12 recruitment to the plasma membrane.

Phosphatidylinositol 3,5-bisphosphate increases intracellular free Ca2+ in arterial smooth muscle cells and elicits vasocontraction

Phosphoinositide (3,5)-bisphosphate [PI(3,5)P(2)] is a newly identified phosphoinositide that modulates intracellular Ca(2+) by activating ryanodine receptors (RyRs). Since the contractile state of arterial smooth muscle depends on the concentration of intracellular Ca(2+), we hypothesized that by mobilizing sarcoplasmic reticulum (SR) Ca(2+) stores PI(3,5)P(2) would increase intracellular Ca(2+) in arterial smooth muscle cells and cause vasocontraction. Using immunohistochemistry, we found that PI(3,5)P(2) was present in the mouse aorta and that exogenously applied PI(3,5)P(2) readily entered aortic smooth muscle cells. In isolated aortic smooth muscle cells, exogenous PI(3,5)P(2) elevated intracellular Ca(2+), and it also contracted aortic rings. Both the rise in intracellular Ca(2+) and the contraction caused by PI(3,5)P(2) were prevented by antagonizing RyRs, while the majority of the PI(3,5)P(2) response was intact after blockade of inositol (1,4,5)-trisphosphate receptors. Depletion of SR Ca(2+) stores with thapsigargin or caffeine and/or ryanodine blunted the Ca(2+) response and greatly attenuated the contraction elicited by PI(3,5)P(2). The removal of extracellular Ca(2+) or addition of verapamil to inhibit voltage-dependent Ca(2+) channels reduced but did not eliminate the Ca(2+) or contractile responses to PI(3,5)P(2). We also found that PI(3,5)P(2) depolarized aortic smooth muscle cells and that LaCl(3) inhibited those aspects of the PI(3,5)P(2) response attributable to extracellular Ca(2+). Thus, full and sustained aortic contractions to PI(3,5)P(2) required the release of SR Ca(2+), probably via the activation of RyR, and also extracellular Ca(2+) entry via voltage-dependent Ca(2+) channels.

The phosphoinositide kinase PIKfyve is vital in early embryonic development: preimplantation lethality of PIKfyve-/- embryos but normality of PIKfyve+/- mice

Gene mutations in the phosphoinositide-metabolizing enzymes are linked to various human diseases. In mammals, PIKfyve synthesizes PtdIns(3,5)P(2) and PtdIns5P lipids that regulate endosomal trafficking and responses to extracellular stimuli. The consequence of pikfyve gene ablation in mammals is unknown. To clarify the importance of PIKfyve and PIKfyve lipid products, in this study, we have characterized the first mouse model with global deletion of the pikfyve gene using the Cre-loxP approach. We report that nearly all PIKfyve(KO/KO) mutant embryos died before the 32-64-cell stage. Cultured fibroblasts derived from PIKfyve(flox/flox) embryos and rendered pikfyve-null by Cre recombinase expression displayed severely reduced DNA synthesis, consistent with impaired cell division causing early embryo lethality. The heterozygous PIKfyve(WT/KO) mice were born at the expected Mendelian ratio and developed into adulthood. PIKfyve(WT/KO) mice were ostensibly normal by several other in vivo, ex vivo, and in vitro criteria despite the fact that their levels of the PIKfyve protein and in vitro enzymatic activity in cells and tissues were 50-55% lower than those of wild-type mice. Consistently, steady-state levels of the PIKfyve products PtdIns(3,5)P(2) and PtdIns5P selectively decreased, but this reduction (35-40%) was 10-15% less than that expected based on PIKfyve protein reduction. The nonlinear decrease of the PIKfyve protein versus PIKfyve lipid products, the potential mechanism(s) discussed herein, may explain how one functional allele in PIKfyve(WT/KO) mice is able to support the demands for PtdIns(3,5)P(2)/PtdIns5P synthesis during life. Our data also shed light on the known human disorder linked to PIKFYVE mutations.

WIPI1 coordinates melanogenic gene transcription and melanosome formation via TORC1 inhibition

Recent studies implicate a role for WD repeat domain, phosphoinositide-interacting 1 (WIPI1) in the biogenesis of melanosomes, cell type-specific lysosome-related organelles. In this study, we determined that WIPI1, an ATG18 homologue that is shown to localize to both autophagosomes and early endosomes, inhibited mammalian target of rapamycin (MTOR) signaling, leading to increased transcription of melanogenic enzymes and the formation of mature melanosomes. WIPI1 suppressed the target of rapamycin complex 1 (TORC1) activity, resulting in glycogen synthase kinase 3β inhibition, β-Catenin stabilization, and increased transcription of microphthalmia transcription factor and its target genes. WIPI1-depleted cells accumulated stage I melanosomes but lacked stage III-IV melanosomes. Inhibition of TORC1 by rapamycin treatment resulted in the accumulation of stage IV melanosomes but not autophagosomes, whereas starvation resulted in the formation of autophagosomes but not melanin accumulation. Taken together, our studies define a distinct role for WIPI1 and TORC1 signaling in controlling the transcription of melanogenic enzymes and melanosome maturation, a process that is distinct from starvation-induced autophagy.

The myotubularin family of lipid phosphatases in disease and in spermatogenesis

The MTM (myotubularin)/MTMR (myotubularin-related) protein family is comprised of 15 lipid phosphatases, of which nine members are catalytically active. MTMs are known to play a fundamental role in human physiology as gene mutations can give rise to X-linked myotubular myopathy or Charcot-Marie-Tooth disease, which manifest in skeletal muscle or in peripheral neurons respectively. Interestingly, studies have shown MTMR2 and MTMR5, two MTM family members, to be highly expressed in the testis, particularly in Sertoli and germ cells, and knockout of either gene resulted in spermatogenic defects. Other studies have shown that MTMR2 functions in endocytosis and membrane trafficking. In the testis, MTMR2 interacts and co-localizes with c-Src/phospho-Src-(Tyr⁴¹⁶), a non-receptor protein tyrosine kinase that regulates the phosphorylation state of proteins at the apical ES (ectoplasmic specialization), a unique type of cell junction found between Sertoli cells and elongating/elongated spermatids. In the present review, we highlight recent findings that have made a significant impact on our understanding of this protein family in normal cell function and in disease, with the emphasis on the role of MTMs and MTMRs in spermatogenesis. We also describe a working model to explain how MTMR2 interacts with other proteins such as c-Src, dynamin 2, EPS8 (growth factor receptor pathway substrate 8) and ARP2/3 (actin-related protein 2/3) at the apical ES and the apical TBC (tubulobulbar complex; tubular-like invaginations that function in the disassembly of the apical ES and in the recycling of its components) to regulate spermiation at late stage VIII of the seminiferous epithelial cycle.

Lipid signaling in T-cell development and function

Second messenger molecules relay, amplify, and diversify cell surface receptor signals. Two important examples are phosphorylated D-myo-inositol derivatives, such as phosphoinositide lipids within cellular membranes, and soluble inositol phosphates. Here, we review how phosphoinositide metabolism generates multiple second messengers with important roles in T-cell development and function. They include soluble inositol(1,4,5)trisphosphate, long known for its Ca(2+)-mobilizing function, and phosphatidylinositol(3,4,5)trisphosphate, whose generation by phosphoinositide 3-kinase and turnover by the phosphatases PTEN and SHIP control a key "hub" of TCR signaling. More recent studies unveiled important second messenger functions for diacylglycerol, phosphatidic acid, and soluble inositol(1,3,4,5)tetrakisphosphate (IP(4)) in immune cells. Inositol(1,3,4,5)tetrakisphosphate acts as a soluble phosphatidylinositol(3,4,5)trisphosphate analog to control protein membrane recruitment. We propose that phosphoinositide lipids and soluble inositol phosphates (IPs) can act as complementary partners whose interplay could have broadly important roles in cellular signaling.

Phosphoinositide phosphatases in cell biology and disease

Phosphoinositides are essential signaling molecules linked to a diverse array of cellular processes in eukaryotic cells. The metabolic interconversions of these phospholipids are subject to exquisite spatial and temporal regulation executed by arrays of phosphatidylinositol (PtdIns) and phosphoinositide-metabolizing enzymes. These include PtdIns- and phosphoinositide-kinases that drive phosphoinositide synthesis, and phospholipases and phosphatases that regulate phosphoinositide degradation. In the past decade, phosphoinositide phosphatases have emerged as topics of particular interest. This interest is driven by the recent appreciation that these enzymes represent primary mechanisms for phosphoinositide degradation, and because of their ever-increasing connections with human diseases. Herein, we review the biochemical properties of six major phosphoinositide phosphatases, the functional involvements of these enzymes in regulating phosphoinositide metabolism, the pathologies that arise from functional derangements of individual phosphatases, and recent ideas concerning the involvements of phosphoinositide phosphatases in membrane traffic control.

New methods for capturing the mystery lipid, PtdIns5P

The enormous versatility of phosphatidylinositol as a mediator of intracellular signalling is due to its variable phosphorylation on every combination of the 3', 4' and 5' positions, as well as an even more complex range of phosphorylated products when inositol phosphate is released by phospholipase C activity. The phosphoinositides are produced by distinct enzymes in distinct intracellular membranes, and recruit and regulate downstream signalling proteins containing binding domains [PH (pleckstrin homology), PX (Phox homology), FYVE etc.] that are relatively specific for these lipids. Specific recruitment of downstream proteins presumably involves a coincidence detection mechanism, in which a combination of lipid-protein and protein-protein interactions define specificity. Of the seven intracellular phosphoinositide, quantification of PtdIns5P levels in intact cells has remained difficult. In this issue of the Biochemical Journal, Sarkes and Rameh describe a novel HPLC-based approach which makes possible an analysis of the subcellular distribution of PtdIns5P and other phosphoinositides.

Mutation in archain 1, a subunit of COPI coatomer complex, causes diluted coat color and Purkinje cell degeneration

Intracellular trafficking is critical for delivering molecules and organelles to their proper destinations to carry out normal cellular functions. Disruption of intracellular trafficking has been implicated in the pathogenesis of various neurodegenerative disorders. In addition, a number of genes involved in vesicle/organelle trafficking are also essential for pigmentation, and loss of those genes is often associated with mouse coat-color dilution and human hypopigmentary disorders. Hence, we postulated that screening for mouse mutants with both neurological defects and coat-color dilution will help identify additional factors associated with intracellular trafficking in neuronal cells. In this study, we characterized a mouse mutant with a unique N-ethyl-N-nitrosourea (ENU)-induced mutation, named nur17. nur17 mutant mice exhibit both coat-color dilution and ataxia due to Purkinje cell degeneration in the cerebellum. By positional cloning, we identified that the nur17 mouse carries a T-to-C missense mutation in archain 1 (Arcn1) gene which encodes the delta subunit of the coat protein I (COPI) complex required for intracellular trafficking. Consistent with this function, we found that intracellular trafficking is disrupted in nur17 melanocytes. Moreover, the nur17 mutation leads to common characteristics of neurodegenerative disorders such as abnormal protein accumulation, ER stress, and neurofibrillary tangles. Our study documents for the first time the physiological consequences of the impairment of the ARCN1 function in the whole animal and demonstrates a direct association between ARCN1 and neurodegeneration.

Phosphoinositide signalling in cancer: beyond PI3K and PTEN

There are numerous studies that suggest multiple links between the cellular phosphoinositide system and cancer. As key roles in cancer have been established for PI3K and PTEN - enzymes that regulate the levels of phosphatidylinositol-3,4,5-trisphosphate - compounds targeting this pathway are entering the clinic at a rapid pace. Several other phosphoinositide-modifying enzymes, including phosphoinositide kinases, phosphatases and phospholipase C enzymes, have been implicated in the generation and progression of tumours. Studies of these enzymes are providing new insights into the mechanisms and the extent of their involvement in cancer, highlighting new potential targets for therapeutic intervention.

Spatial and temporal integration of signalling networks regulating pollen tube growth

The overall function of a cell is determined by its contingent of active signal transduction cascades interacting on multiple levels with metabolic pathways, cytoskeletal organization, and regulation of gene expression. Much work has been devoted to analysis of individual signalling cascades interacting with unique cellular targets. However, little is known about how cells integrate information across hierarchical signalling networks. Recent work on pollen tube growth indicates that several key signalling cascades respond to changes in cell hydrodynamics and apical volume. Combined with known effects on cytoarchitecture and signalling from other cell systems, hydrodynamics has the potential to integrate and synchronize the function of the broader signalling network in pollen tubes. This review will explore recent work on cell hydrodynamics in a variety of systems including pollen, and discuss hydrodynamic regulation of cell signalling and function including exocytosis and endocytosis, actin cytoskeleton reorganization, cell wall deposition and assembly, phospholipid and inositol polyphosphate signalling, ion flux, small G-proteins, fertilization, and self-incompatibility. The combined data support a newly emerging model of pollen tube growth.

Regulation of immune cell development through soluble inositol-1,3,4,5-tetrakisphosphate

The membrane lipid phosphatidylinositol-3,4,5-trisphosphate (PtdInsP(3)) regulates membrane receptor signalling in many cells, including immunoreceptor signalling. Here, we review recent data that have indicated essential roles for the soluble PtdInsP(3) analogue inositol-1,3,4,5-tetrakisphosphate (InsP(4)) in T cell, B cell and neutrophil development and function. Decreased InsP(4) production in leukocytes causes immunodeficiency in mice and might contribute to inflammatory vasculitis in Kawasaki disease in humans. InsP(4)-producing kinases could therefore provide attractive drug targets for inflammatory and infectious diseases.

Regulation of the actin cytoskeleton-plasma membrane interplay by phosphoinositides

The plasma membrane and the underlying cortical actin cytoskeleton undergo continuous dynamic interplay that is responsible for many essential aspects of cell physiology. Polymerization of actin filaments against cellular membranes provides the force for a number of cellular processes such as migration, morphogenesis, and endocytosis. Plasma membrane phosphoinositides (especially phosphatidylinositol bis- and trisphosphates) play a central role in regulating the organization and dynamics of the actin cytoskeleton by acting as platforms for protein recruitment, by triggering signaling cascades, and by directly regulating the activities of actin-binding proteins. Furthermore, a number of actin-associated proteins, such as BAR domain proteins, are capable of directly deforming phosphoinositide-rich membranes to induce plasma membrane protrusions or invaginations. Recent studies have also provided evidence that the actin cytoskeleton-plasma membrane interactions are misregulated in a number of pathological conditions such as cancer and during pathogen invasion. Here, we summarize the wealth of knowledge on how the cortical actin cytoskeleton is regulated by phosphoinositides during various cell biological processes. We also discuss the mechanisms by which interplay between actin dynamics and certain membrane deforming proteins regulate the morphology of the plasma membrane.

Changing phosphoinositides "on the fly": how trafficking vesicles avoid an identity crisis

Joining an antagonistic phosphoinositide (PtdInsP) kinase and phosphatase into a single protein complex may regulate rapid and local PtdInsP changes. This may be important for processes such as membrane fission that require a specific PtdInsP and that are innately local and rapid. Such a complex could couple vesicle formation, with erasing of the identity of the donor organelle from the vesicle prior to its fusion with target organelles, thus preventing organelle identity intermixing. Coordinating signals are postulated to switch the relative activities of the kinase and phosphatase in a spatio-temporal manner that matches membrane fission events. The discovery of two such complexes supports this hypothesis. One regulates the interconversion of phosphatidylinositol and PtdIns(3)P by joining the Vps34 PtdIns 3-kinase and the myotubularin 3-phosphatases. The other regulates the interconversion between PtdIns(3)P and PtdIns(3,5)P(2) through the Fab1/PIKfyve kinase and the Fig4/mFig4 phosphatase. These lipids are essential components of the endosomal identity code.

PIKfyve regulates CaV1.2 degradation and prevents excitotoxic cell death

Voltage-gated Ca(2+) channels (VGCCs) play a key role in neuronal signaling but can also contribute to cellular dysfunction and death under pathological conditions such as stroke and neurodegenerative diseases. We report that activation of N-methyl-D-aspartic acid receptors causes internalization and degradation of Ca(V)1.2 channels, resulting in decreased Ca(2+) entry and reduced toxicity. Ca(V)1.2 internalization and degradation requires binding to phosphatidylinositol 3-phosphate 5-kinase (PIKfyve), a lipid kinase which generates phosphatidylinositol (3,5)-bisphosphate (PtdIns(3,5)P(2)) and regulates endosome and lysosome function. Sustained activation of glutamate receptors recruits PIKfyve to Ca(V)1.2 channels, increases cellular levels of PtdIns(3,5)P(2), and promotes targeting of Ca(V)1.2 to lysosomes. Knockdown of PIKfyve prevents Ca(V)1.2 degradation and increases neuronal susceptibility to excitotoxicity. These experiments identify a novel mechanism by which neurons are protected from excitotoxicity and provide a possible explanation for neuronal death in diseases caused by mutations that affect PtdIns(3,5)P(2) regulation.

Rethinking phosphatidylinositol 3-monophosphate

A generally accepted view considers phosphatidylinositol 3-monophosphate (PtdIns3P) as a lipid confined to the endosomal compartment where it regulates trafficking pathways and is produced constitutively and exclusively by class III phosphoinositide 3-kinase (PI3K). Recent evidence suggests that this phosphoinositide has a more complex role as a second messenger involved in different physiological and pathological events and that specific intracellular localization of kinases and/or phosphatases is critical for PtdIns3P synthesis and PtdIns3P-dependent intracellular functions. Here, we review the current knowledge of the regulation and function of PtdIns3P and discuss how the view of PtdIns3P changed in the last few years.

PIKfyve regulation of endosome-linked pathways

The phosphoinositide 5-kinase (PIKfyve) is a critical enzyme for the synthesis of PtdIns(3,5)P2, that has been implicated in various trafficking events associated with the endocytic pathway. We have now directly compared the effects of siRNA-mediated knockdown of PIKfyve in HeLa cells with a specific pharmacological inhibitor of enzyme activity. Both approaches induce changes in the distribution of CI-M6PR and trans-Golgi network (TGN)-46 proteins, which cycles between endosomes and TGN, leading to their accumulation in dispersed punctae, whilst the TGN marker golgin-245 retains a perinuclear disposition. Trafficking of CD8-CI-M6PR (retromer-dependent) and CD8-Furin (retromer-independent) chimeras from the cell surface to the TGN is delayed following drug administration, as is the transport of the Shiga toxin B-subunit. siRNA knockdown of PIKfyve produced no defect in epidermal growth factor receptor (EGFR) degradation, unless combined with knockdown of its activator molecule Vac14, suggesting that a low threshold of PtdIns(3,5)P2 is necessary and sufficient for this pathway. Accordingly pharmacological inhibition of PIKfyve results in a profound block to the lysosomal degradation of activated epidermal growth factor (EGF) and Met receptors. Immunofluorescence revealed EGF receptors to be trapped in the interior of a swollen endosomal compartment. In cells starved of amino acids, PIKfyve inhibition leads to the accumulation of the lipidated form of GFP-LC3, a marker of autophagosomal structures, which can be visualized as fluorescent punctae. We suggest that PIKfyve inhibition may render the late endosome/lysosome compartment refractory to fusion with both autophagosomes and with EGFR-containing multivesicular bodies.

Cellular consequences of inositol depletion

The inositol-depletion hypothesis was suggested to explain the therapeutic mechanism of mood-stabilizing drugs. Focus was previously on the phosphatidylinositol signalling pathway and on the regulatory roles of Ins(3,4,5)P3 and DAG (diacylglycerol). Recent findings indicate that inositol and inositol-containing molecules, including phosphoinositides and inositol phosphates, have signalling and regulatory roles in many cellular processes. This suggests that depleting inositol may lead to perturbation of a wide range of cellular functions, at least some of which may be associated with bipolar disorder.

Dlg1, Sec8, and Mtmr2 regulate membrane homeostasis in Schwann cell myelination

How membrane biosynthesis and homeostasis is achieved in myelinating glia is mostly unknown. We previously reported that loss of myotubularin-related protein 2 (MTMR2) provokes autosomal recessive demyelinating Charcot-Marie-Tooth type 4B1 neuropathy, characterized by excessive redundant myelin, also known as myelin outfoldings. We generated a Mtmr2-null mouse that models the human neuropathy. We also found that, in Schwann cells, Mtmr2 interacts with Discs large 1 (Dlg1), a scaffold involved in polarized trafficking and membrane addition, whose localization in Mtmr2-null nerves is altered. We here report that, in Schwann cells, Dlg1 also interacts with kinesin 13B (kif13B) and Sec8, which are involved in vesicle transport and membrane tethering in polarized cells, respectively. Taking advantage of the Mtmr2-null mouse as a model of impaired membrane formation, we provide here the first evidence for a machinery that titrates membrane formation during myelination. We established Schwann cell/DRG neuron cocultures from Mtmr2-null mice, in which myelin outfoldings were reproduced and almost completely rescued by Mtmr2 replacement. By exploiting this in vitro model, we propose a mechanism whereby kif13B kinesin transports Dlg1 to sites of membrane remodeling where it coordinates a homeostatic control of myelination. The interaction of Dlg1 with the Sec8 exocyst component promotes membrane addition, whereas with Mtmr2, negatively regulates membrane formation. Myelin outfoldings thus arise as a consequence of the loss of negative control on the amount of membrane, which is produced during myelination.

Cinnamic acid, from the bark of Cinnamomum cassia, regulates glucose transport via activation of GLUT4 on L6 myotubes in a phosphatidylinositol 3-kinase-independent manner

BACKGROUND

Cinnamomum cassia (Family: Lauraceae) is an Ayurvedic medicinal plant used traditionally for the treatment of a number of diseases, including diabetes. The hypoglycemic effect of this plant has been established in vivo. However, the effects of cinnamic acid, isolated from C. cassia, on the insulin signaling cascade in an in vitro model have not been elucidated. Hence, the aim of the present study was to evaluate the anti-diabetic effect of cinnamic acid on glucose transport by L6 myotubes.

METHODS

The mechanism of action of cinnamic acid was determined using specific targets in the insulin signaling pathway, including protein tyrosine phosphatase (PTP) 1B, phosphatidylinositol 3-kinase (PI3-K) and the glucose transporter GLUT4. After differentiation of myoblast to myotubes, the cells were serum deprived for 5 h and then treated with 1 ng/mL cinnamic acid and 50 μmol/L rosiglitazone for 18 h and 100 nmol/L insulin for 20 min for gene expression studies.

RESULTS

Expression of GLUT4 mRNA was increased following treatment of L6 myotubes with 1 ng/mL cinnamic acid. Furthermore, cinnamic acid inhibited PTP1B activity (by 96.5%), but had no significant effect on PI3-K activity.

CONCLUSION

On the basis of the results of the present study, we postulate that cinnamic acid isolated from the hydro-alcoholic extract of Cinnamomum cassia activates glucose transport by a PI3-K-independent pathway. However, the detailed mechanism of action requires further analysis.

Deficiency of MIP/MTMR14 phosphatase induces a muscle disorder by disrupting Ca(2+) homeostasis

The intracellular Ca(2+) concentration ([Ca(2+)](i)) in skeletal muscles must be rapidly regulated during the excitation-contraction-relaxation process. However, the signalling components involved in such rapid Ca(2+) movement are not fully understood. Here we report that mice deficient in the newly identified PtdInsP (phosphatidylinositol phosphate) phosphatase MIP/MTMR14 (muscle-specific inositol phosphatase) show muscle weakness and fatigue. Muscles isolated from MIP/MTMR14(-/-) mice produced less contractile force, had markedly prolonged relaxation and showed exacerbated fatigue relative to normal muscles. Further analyses revealed that MIP/MTMR14 deficiency resulted in spontaneous Ca(2+) leakage from the internal store - the sarcoplasmic reticulum. This was attributed to decreased metabolism (dephosphorylation) and the subsequent accumulation of MIP/MTMR14 substrates, especially PtdIns(3,5)P(2) and PtdIns (3,4)P(2). Furthermore, we found that PtdIns(3,5)P(2) and PtdIns(3,4)P(2) bound to, and directly activated, the Ca(2+) release channel (ryanodine receptor 1, RyR1) of the sarcoplasmic reticulum. These studies provide the first evidence that finely controlled PtdInsP levels in muscle cells are essential for maintaining Ca(2+) homeostasis and muscle performance.

Mutations in phosphoinositide metabolizing enzymes and human disease

Phosphoinositides are implicated in the regulation of a wide variety of cellular functions. Their importance in cellular and organismal physiology is underscored by the growing number of human diseases linked to perturbation of kinases and phosphatases that catalyze interconversion from one phosphoinositide to another. Many such enzymes are attractive targets for therapeutic interventions. Here, we review diseases linked to inheritable or somatic mutations of these enzymes.

Phosphatidylinositol 3,5-bisphosphate and Fab1p/PIKfyve underPPIn endo-lysosome function

PtdIns(3,5)P(2) is one of the seven regulatory PPIn (polyphosphoinositides) that are ubiquitous in eukaryotes. It controls membrane trafficking at multiple points in the endosomal/lysosomal system and consequently regulates the size, shape and acidity of at least one endo-lysosomal compartment. PtdIns(3,5)P(2) appears to exert this control via multiple effector proteins, with each effector specific for a subset of the various PtdIns(3,5)P(2)-dependent processes. Some putative PtdIns(3,5)P(2) effectors have been identified, including Atg18p-related PROPPIN [beta-propeller(s) that bind PPIn] proteins and the epsin-like proteins Ent3p and Ent5p, whereas others remain to be defined. One of the principal functions of PtdIns(3,5)P(2) is to regulate the fission/fragmentation of endo-lysosomal sub-compartments. PtdIns(3,5)P(2) is required for vesicle formation during protein trafficking between endo-lysosomes and also for fragmentation of endo-lysosomes into smaller compartments. In yeast, hyperosmotic stress accelerates the latter process. In the present review we highlight and discuss recent studies that reveal the role of the HOPS-CORVET complex and the vacuolar H(+)-ATPase in the process of endo-lysosome fission, and speculate on connections between these machineries and the Fab1p pathway. We also discuss new evidence linking PtdIns(3,5)P(2) and PtdIns5P to the regulation of exocytosis.

Visualization of cellular phosphoinositide pools with GFP-fused protein-domains

This unit describes the method of following phosphoinositide dynamics in live cells. Inositol phospholipids have emerged as universal signaling molecules present in virtually every membrane of eukaryotic cells. Phosphoinositides are present in only tiny amounts as compared to structural lipids, but they are metabolically very active as they are produced and degraded by the numerous inositide kinase and phosphatase enzymes. Phosphoinositides control the membrane recruitment and activity of many membrane protein signaling complexes in specific membrane compartments, and they have been implicated in the regulation of a variety of signaling and trafficking pathways. It has been a challenge to develop methods that allow detection of phosphoinositides at the single-cell level. The only available technique in live cell applications is based on the use of the same protein domains selected by evolution to recognize cellular phosphoinositides. Some of these isolated protein modules, when fused to fluorescent proteins, can follow dynamic changes in phosphoinositides. While this technique can provide information on phosphoinositide dynamics in live cells with subcellular localization, and it has rapidly gained popularity, it also has several limitations that must be taken into account when interpreting the data. This unit summarizes the design and practical use of these constructs and also reviews important considerations for interpretation of the data obtained by this technique.

Melanoregulin (MREG) modulates lysosome function in pigment epithelial cells

Melanoregulin (MREG), the product of the Mreg(dsu) gene, is a small highly charged protein, hypothesized to play a role in organelle biogenesis due to its effect on pigmentation in dilute, ashen, and leaden mutant mice. Here we provide evidence that MREG is required in lysosome-dependent phagosome degradation. In the Mreg(-/-) mouse, we show that loss of MREG function results in phagosome accumulation due to delayed degradation of engulfed material. Over time, the Mreg(-/-) mouse retinal pigment epithelial cells accumulate the lipofuscin component, A2E. MREG-deficient human and mouse retinal pigment epithelial cells exhibit diminished activity of the lysosomal hydrolase, cathepsin D, due to defective processing. Moreover, MREG localizes to small intracellular vesicles and associates with the endosomal phosphoinositide, phosphatidylinositol 3,5-biphosphate. Collectively, these studies suggest that MREG is required for lysosome maturation and support a role for MREG in intracellular trafficking.

A protein complex that regulates PtdIns(3,5)P2 levels

Phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2) is needed for retrograde membrane trafficking from lysosomal and late endosomal compartments and its synthesis is tightly regulated. But how cells regulate PtdIns(3,5)P2 synthesis--for example, in response to hyperosmotic shock--remains unexplained. A paper from the Weisman group gives the most complete picture so far of a multiprotein complex that controls PtdIns(3,5)P2 synthesis and explains how a VAC14 mutation functionally impairs the scaffold protein at the heart of the complex and causes a neurodegenerative condition in mice.

Kinesin adapter JLP links PIKfyve to microtubule-based endosome-to-trans-Golgi network traffic of furin

JIPs (c-Jun N-terminal kinase interacting proteins), which scaffold JNK/p38 MAP kinase signaling modules, also bind conventional kinesins and are implicated in microtubule-based membrane trafficking in neuronal cells. Here we have identified a novel splice variant of the Jip4 gene product JLP(L) (JNK-interacting leucine zipper protein) in yeast-two hybrid screens with the phosphoinositide kinase PIKfyve. The interaction was confirmed by pulldown and coimmunoprecipitation assays in native cells. It engages the PIKfyve cpn60_TCP1 consensus sequence and the last 75 residues of the JLP C terminus. Subpopulations of both proteins cofractionated and populated similar structures at the cell perinuclear region. Because PIKfyve is essential in endosome-to-trans-Golgi network (TGN) cargo transport, we tested whether JLP is a PIKfyve functional partner in this trafficking pathway. Short interfering RNA (siRNA)-mediated depletion of endogenous JLP or PIKfyve profoundly delayed the microtubule-based transport of chimeric furin (Tac-furin) from endosomes to the TGN in a CHO cell line, which was rescued upon ectopic expression of siRNA-resistant JLP or PIKfyve constructs. Peptides from the contact sites in PIKfyve and JLP, or a dominant-negative PIKfyve mutant introduced into cells by ectopic expression or microinjection, induced a similar defect. Because Tac-TGN38 delivery from endosomes to the TGN, unlike that of Tac-furin, does not require intact microtubules, we monitored the effect of JLP and PIKfyve depletion or the interacting peptides administration on Tac-TGN38 trafficking. Remarkably, neither maneuver altered the Tac-TGN38 delivery to the TGN. Our data indicate that JLP interacts with PIKfyve and that both proteins and their association are required in microtubule-based, but not in microtubule-independent, endosome-to-TGN cargo transport.

Inositol lipid-dependent functions in Saccharomyces cerevisiae: analysis of phosphatidylinositol phosphates

Inositol phospholipids regulate many cellular processes, including cell survival, membrane trafficking, and actin polymerization. Quantification of inositol lipids is one of the essential techniques needed for studies that aim to decipher inositol lipid-dependent cellular functions. The study of phosphoinositides in most organisms is hampered by a lack of facile genetic tools. However, the essential elements of most inositol lipid signaling pathways appear to be conserved across eukaryote phylogeny. They can therefore readily be elucidated (both genetically and biochemically) in the budding yeast Saccharomyces cerevisiae. Because of the low abundance of polyphosphoinositides in cells, many analytical methods start by radioactively labeling intact cells and then extracting the lipids with chloroform/methanol/ water mixtures based on those first devised half a century ago. Yeast present special extraction problems because the cell wall must be broken in order to facilitate solvent access and maximize lipid yield. Once lipids have been extracted, fatty acids are removed and the resulting water-soluble glycerophosphoinositol phosphates are analysed by anion-exchange HPLC. This chapter describes how to extract and quantify the inositol lipids of S. cerevisiae cells that have been radiolabeled to isotopic equilibrium with [3H]myo-inositol.

Phosphoinositides in plants: novel functions in membrane trafficking

Tight regulation of membrane trafficking is crucial to the proper maintenance of the endomembrane trafficking system of eukaryotic cells. Distinct organelles must maintain their identities while at the same time continuously accepting, sorting, and exchanging membrane and luminal cargo constituents. Additionally, many of these organelles differentiate specialized subdomains containing distinct sets of lipids and proteins and restrict certain aspects of membrane trafficking to these regions of the organelle. Phosphoinositides (PIs) are a class of membrane lipids that have emerged as key components in some of these membrane trafficking events. The ability of these lipids to be rapidly produced, modified, and hydrolyzed by distinct classes of phosphatidylinositol (PtdIns) kinases, phosphatases, and phospholipases, allows for their use as finely tuned spatial and temporal landmarks for organelle and sub-organelle domains. In this review we will attempt to highlight some of the recent studies of the roles of this class of lipids in plant membrane trafficking, particularly on their important roles in polarized membrane trafficking in plants.

Live cell imaging of phosphoinositides with expressed inositide binding protein domains

Inositol lipids and calcium signaling has been inseparable twins during the 1980s when the molecular details of phospholipase C-mediated generation of inositol 1,4,5-trisphosphate (InsP3) and its Ca2+ mobilizing action were discovered. Since then, both the Ca2+ and inositol lipid signaling fields have hugely expanded and the tools allowing dissection of the finest details of their molecular organization also followed closely. Although phosphoinositides regulate many cell functions unrelated to Ca2+ signaling there are still many open questions even in the Ca2+ field that would benefit from single cell monitoring of PtdIns(4,5)P2 or InsP3 changes during agonist stimulation. This chapter is designed to provide practical guidance as well as some theoretical background on measurements of phosphoinositides in live cells using protein domain-GFP chimeras that could be also useful for people working on calcium signaling.

ArPIKfyve homomeric and heteromeric interactions scaffold PIKfyve and Sac3 in a complex to promote PIKfyve activity and functionality

PtdIns(3,5)P(2) (with PtdIns indicating phosphatidylinositol) is vital in the differentiation and development of multicellular organisms because knockout of the PtdIns(3,5)P(2)-synthesizing enzyme PIKfyve (phosphoinositide kinase for position 5 containing a FYVE finger domain) or its associated regulator ArPIKfyve is lethal. In previous work with endogenous proteins, we identified that Sac3, a phosphatase that turns over PtdIns(3,5)P(2), associates with the PIKfyve-ArPIKfyve biosynthetic complex. However, whether the three proteins suffice for the organization/maintenance of this complex [referred to as the PAS (PIKfyve-ArPIKfyve-Sac3) complex], how they interact with one another, and what the functional relevance of this ternary association would be remained unresolved. Using co-immunoprecipitation analyses in transfected mammalian cells with increased or decreased levels of the three proteins, singly or in double versus triple combinations, herein we report that the triad is sufficient to form and maintain the PAS complex. ArPIKfyve is the principal organizer interacting with both Sac3 and PIKfyve, whereas Sac3 is permissive for maximal PIKfyve-ArPIKfyve association in the PAS complex. We further identified that ArPIKfyve scaffolds the PAS complex through homomeric interactions, mediated via its conserved C-terminal domain. Introduction of the C-terminal peptide fragment of the ArPIKfyve-ArPIKfyve contact sites effectively disassembled the PAS complex and reduced the in vitro PIKfyve lipid kinase activity. Exploring insulin-regulated GLUT4 translocation in 3T3L1 adipocytes as a functional readout, a process that is positively regulated by PIKfyve activity and ArPIKfyve levels, we determined that ectopic expression of the ArPIKfyve C-terminal peptide inhibits GLUT4 surface accumulation. Our data indicate that the PAS complex is organized to provide optimal PIKfyve functionality and is maintained via ArPIKfyve homomeric and heteromeric interactions.

PtdIns5P regulation through evolution: roles in membrane trafficking?

Phosphoinositides are lipid second messengers that are essential for many cellular processes, including signal transduction and cell compartmentalization. Among them, phosphatidylinositol 5-phosphate (PtdIns5P) is the least characterized, although several proteins involved in its regulation are implicated in human diseases. We studied the distribution of 32 PtdIns5P-metabolizing proteins in 39 eukaryotic genomes. Phylogenetic profiles identify four groups of co-evolving proteins, confirming known protein complexes and revealing new ones. The complexes comprise a phosphatase, a kinase and a regulator; this indicates that physical interactions between the three partners are necessary for the acute spatial regulation of PtdIns5P turnover. By examining PtdIns5P metabolism in this new perspective, we propose a role for PtdIns5P in membrane trafficking from late endosomal compartments to the plasma membrane.

Toxoplasma gondii Hsp20 is a stripe-arranged chaperone-like protein associated with the outer leaflet of the inner membrane complex

BACKGROUND INFORMATION

Toxoplasma gondii is among the most successful parasites, with nearly half of the human population chronically infected. T. gondii has five sHsps [small Hsps (heat-shock proteins)] located in different subcellular compartments. Among them, Hsp20 showed to be localized at the periphery of the parasite body. sHsps are widespread, constituting the most poorly conserved family of molecular chaperones. The presence of sHsps in membrane structures is unusual.

RESULTS

The localization of Hsp20 was further analysed using high-resolution fluorescent light microscopy as well as electron microscopy, which revealed that Hsp20 is associated with the outer surface of the IMC (inner membrane complex), in a set of discontinuous stripes following the same spiralling trajectories as the subpellicular microtubules. The detergent extraction profile of Hsp20 was similar to that of GAP45 [45 kDa GAP (gliding-associated protein)], a glideosome protein associated with the IMC, but was different from that of IMC1 protein. Although we were unable to detect interacting protein partners of Hsp20 either in normal or stressed tachyzoites, an interaction of Hsp20 with phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate phospholipids could be observed.

CONCLUSIONS

Hsp20 was shown to be associated with a specialized membranous structure of the parasite, the IMC. This discontinuous striped-arrangement is unique in T. gondii, indicating that the topology of the outer leaflet of the IMC is not homogeneous.

Assembly of a Fab1 phosphoinositide kinase signaling complex requires the Fig4 phosphoinositide phosphatase

Phosphatidylinositol-3,5-bisphosphate [PtdIns(3,5)P(2)] regulates several vacuolar functions, including acidification, morphology, and membrane traffic. The lipid kinase Fab1 converts phosphatidylinositol-3-phosphate [PtdIns(3)P] to PtdIns(3,5)P(2). PtdIns(3,5)P(2) levels are controlled by the adaptor-like protein Vac14 and the Fig4 PtdIns(3,5)P(2)-specific 5-phosphatase. Interestingly, Vac14 and Fig4 serve a dual function: they are both implicated in the synthesis and turnover of PtdIns(3,5)P(2) by an unknown mechanism. We now show that Fab1, through its chaperonin-like domain, binds to Vac14 and Fig4 and forms a vacuole-associated signaling complex. The Fab1 complex is tethered to the vacuole via an interaction between the FYVE domain in Fab1 and PtdIns(3)P on the vacuole. Moreover, Vac14 and Fig4 bind to each other directly and are mutually dependent for interaction with the Fab1 kinase. Our observations identify a protein complex that incorporates the antagonizing Fab1 lipid kinase and Fig4 lipid phosphatase into a common functional unit. We propose a model explaining the dual roles of Vac14 and Fig4 in the synthesis and turnover of PtdIns(3,5)P(2).

Transport of phosphatidylinositol 3-phosphate into the vacuole via autophagic membranes in Saccharomyces cerevisiae

Vps34, the sole PtdIns 3-kinase in yeast, is essential for autophagy. Here, we show that the lipid-kinase activity of Vps34 is required for autophagy, implying an essential role of its product PtdIns(3)P. The protein-kinase activity of Vps15, a regulatory subunit of the PtdIns 3-kinase complex, is also required for efficient autophagy. We monitored the distribution of PtdIns(3)P in living cells using a specific indicator, the 2xFYVE domain derived from mammalian Hrs. PtdIns(3)P was abundant at endosomes and on the vacuolar membrane during logarithmic growth phase. Under starvation conditions, we observed massive transport of PtdIns(3)P into the vacuole. This accumulation was dependent on the membrane dynamics of autophagy. Notably, PtdIns(3)P was highly enriched and delivered into the vacuole as a component of autophagosome membranes but not as a cargo enclosed within them, implying direct involvement of this phosphoinositide in autophagosome formation. We also found a possible enrichment of PtdIns(3)P on the inner autophagosomal membrane compared to the outer membrane. Based on these results we discuss the function of PtdIns(3)P in autophagy.

PIKfyve: Partners, significance, debates and paradoxes

Key components of membrane trafficking and signaling machinery in eukaryotic cells are proteins that bind or synthesize phosphoinositides. PIKfyve, a product of an evolutionarily conserved single-copy gene has both these features. It binds to membrane phosphatidylinositol (PtdIns)3P and synthesizes PtdIns(3,5)P2 and PtdIns5P. Molecular functions of PIKfyve are elusive but recent advances are consistent with a key role in the course of endosomal transport. PIKfyve dysfunction induces endosome enlargement and profound cytoplasmic vacuolation, likely as a result of impaired normal endosome processing and membrane exit out of endosomes. Multicellular organisms with genetically impaired function of PIKfyve or that of the PIKfyve protein partners regulating PtdIns(3,5)P2 homeostasis display severe disorders, including embryonic/perinatal death. This review describes recent advances on PIKfyve functionality in higher eukaryotes, with particular reference to biochemical and genetic insights in PIKfyve protein partners.

Sorting nexin-1 defines an early phase of Salmonella-containing vacuole-remodeling during Salmonella infection

Salmonella enterica serovar Typhimurium replicate within host cells in a specialized membrane-bound compartment, the Salmonella-containing vacuole (SCV). Interaction of SCVs with the host endocytic network is modulated by bacterial effectors, some of which, such as SigD/SopB, manipulate the level of endosomal phosphoinositides. Here, we establish that at early stages of Salmonella infection, sorting nexin-1 (SNX1) - a host phosphoinositide-binding protein that normally associates with early endosomes and regulates transport to the trans-Golgi network (TGN) - undergoes a rapid and transient translocation to bacterial entry sites, an event promoted by SigD/SopB. Recruitment of SNX1 to SCVs results in the formation of extensive, long-range tubules that we have termed ;spacious vacuole-associated tubules'. Formation of these tubules is coupled with size reduction of vacuoles and the removal of TGN-resident cargo. SNX1 suppression perturbs intracellular progress of bacteria, resulting in a delayed replication. We propose that SNX1 is important in tubular-based re-modeling of nascent SCVs and, in doing so, regulates intracellular bacterial progression and replication.

Elevated levels of PtdIns5P in NPM-ALK transformed cells: implication of PIKfyve

Phosphatidylinositol 5-monophosphate (PtdIns5P), one of the latest phosphoinositides discovered, has been suggested to play important cellular functions. Here, we report the presence of higher levels of this lipid in cells expressing the oncogenic tyrosine kinase nucleophosmin anaplastic lymphoma kinase (NPM-ALK), a chimeric protein found in the large majority of anaplastic large cell lymphomas (ALCLs). In addition, we describe that a pool of PtdIns5P is located in the membrane extensions characteristic of NPM-ALK-transformed cells. Finally, we show that the increase of PtdIns5P is controlled by the kinase PIKfyve, which is known for its role in vesicular trafficking. These data suggest for the first time a role of PtdIns5P and PIKfyve in oncogenesis, potentially linking intracellular trafficking to cancer.

Endosomal phosphoinositides and human diseases

Phosphoinositides (PIs) are lipid second messengers implicated in signal transduction and membrane trafficking. Seven distinct PIs can be synthesized by phosphorylation of the inositol ring of phosphatidylinositol (PtdIns), and their metabolism is accurately regulated by PI kinases and phosphatases. Two of the PIs, PtdIns3P and PtdIns(3,5)P₂, are present on intracellular endosomal compartments, and several studies suggest that they have a role in membrane remodeling and trafficking. We refer to them as ‘endosomal PIs’. An increasing number of human genetic diseases including myopathy and neuropathies are associated to mutations in enzymes regulating the turnover of these endosomal PIs. The PtdIns3P and PtdIns(3,5)P₂ 3-phosphatase myotubularin gene is mutated in X-linked centronuclear myopathy, whereas its homologs MTMR2 and MTMR13 and the PtdIns(3,5)P₂ 5-phosphatase SAC3/FIG4 are implicated in Charcot–Marie–Tooth peripheral neuropathies. Mutations in the gene encoding the PtdIns3P5-kinase PIP5K3/PIKfyve have been found in patients affected with François–Neetens fleck corneal dystrophy. This review presents the roles of the endosomal PIs and their regulators and proposes defects of membrane remodeling as a common pathological mechanism for the corresponding diseases.

Glycerophospholipid identification and quantitation by electrospray ionization mass spectrometry

Glycerophospholipids are the structural building blocks of the cellular membrane. In addition to creating a protective barrier around the cell, lipids are precursors of intracellular signaling molecules that modulate membrane trafficking and are involved in transmembrane signal transduction. Phospholipids are also increasingly recognized as important participants in the regulation and control of cellular function and disease. Analysis and characterization of lipid species by mass spectrometry (MS) have evolved and advanced with improvements in instrumentation and technology. Key advances, including the development of "soft" ionization techniques for MS such as electrospray ionization (ESI), matrix-assisted laser desorption/ionization (MALDI), and tandem mass spectrometry (MS/MS), have facilitated the analysis of complex lipid mixtures by overcoming the earlier limitations. ESI-MS has become the technique of choice for the analysis of multi-component mixtures of lipids from biological samples due to its exceptional sensitivity and capacity for high throughput. This chapter covers qualitative and quantitative MS methods used for the elucidation of glycerophospholipid identity and quantity in cell or tissue extracts. Sections are included on the extraction, MS analysis, and data analysis of glycerophospholipids and polyphosphoinositides.