Increased insulin action in SKIP heterozygous knockout mice

Knockdown of INPP5K compromises the differentiation of N2A cells

Inositol polyphosphate 5-phosphatase K (INPP5K), also known as SKIP (skeletal muscle and kidney-enriched inositol phosphatase), is a cytoplasmic enzyme with 5-phosphatase activity toward phosphoinositides (PIs). Mutations in INPP5K are associated with autosomal recessive congenital muscular dystrophy with cataracts and intellectual disability (MDCCAID). Notably, muscular dystrophy is characterized by the hypoglycosylation of dystroglycan. Thus, far, the underlying mechanisms are only partially understood. In this study, we show that INPP5K expression increases during brain development. Knockdown of INPP5K in the neuroblastoma-derived cell line N2A impaired their neuronal-like differentiation and interfered with protein glycosylation.

Phosphoinositides in New Spaces

Phosphoinositides (PIs) are phospholipids derived from phosphatidylinositol. PIs are regulated via reversible phosphorylation, which is directed by the opposing actions of PI kinases and phosphatases. PIs constitute a minor fraction of the total cellular lipid pool but play pleiotropic roles in multiple aspects of cell biology. Genetic mutations of PI regulatory enzymes have been identified in rare congenital developmental syndromes, including ciliopathies, and in numerous human diseases, such as cancer and metabolic and neurological disorders. Accordingly, PI regulatory enzymes have been targeted in the design of potential therapeutic interventions for human diseases. Recent advances place PIs as central regulators of membrane dynamics within functionally distinct subcellular compartments. This brief review focuses on the emerging role PIs play in regulating cell signaling within the primary cilium and in directing transfer of molecules at interorganelle membrane contact sites and identifies new roles for PIs in subcellular spaces.

Increased body mass index is linked to systemic inflammation through altered chromatin co-accessibility in human preadipocytes

Obesity-induced adipose tissue dysfunction can cause low-grade inflammation and downstream obesity comorbidities. Although preadipocytes may contribute to this pro-inflammatory environment, the underlying mechanisms are unclear. We used human primary preadipocytes from body mass index (BMI) -discordant monozygotic (MZ) twin pairs to generate epigenetic (ATAC-sequence) and transcriptomic (RNA-sequence) data for testing whether increased BMI alters the subnuclear compartmentalization of open chromatin in the twins' preadipocytes, causing downstream inflammation. Here we show that the co-accessibility of open chromatin, i.e. compartmentalization of chromatin activity, is altered in the higher vs lower BMI MZ siblings for a large subset ( ~ 88.5 Mb) of the active subnuclear compartments. Using the UK Biobank we show that variants within these regions contribute to systemic inflammation through interactions with BMI on C-reactive protein. In summary, open chromatin co-accessibility in human preadipocytes is disrupted among the higher BMI siblings, suggesting a mechanism how obesity may lead to inflammation via gene-environment interactions.

PI(4,5)P2-dependent regulation of endothelial tip cell specification contributes to angiogenesis

Dynamic positioning of endothelial tip and stalk cells, via the interplay between VEGFR2 and NOTCH signaling, is essential for angiogenesis. VEGFR2 activates PI3K, which phosphorylates PI(4,5)P₂ to PI(3,4,5)P₃, activating AKT; however, PI3K/AKT does not direct tip cell specification. We report that PI(4,5)P₂ hydrolysis by the phosphoinositide-5-phosphatase, INPP5K, contributes to angiogenesis. INPP5K ablation disrupted tip cell specification and impaired embryonic angiogenesis associated with enhanced DLL4/NOTCH signaling. INPP5K degraded a pool of PI(4,5)P₂ generated by PIP5K1C phosphorylation of PI(4)P in endothelial cells. INPP5K ablation increased PI(4,5)P₂, thereby releasing β-catenin from the plasma membrane, and concurrently increased PI(3,4,5)P₃-dependent AKT activation, conditions that licensed DLL4/NOTCH transcription. Suppression of PI(4,5)P₂ in INPP5K-siRNA cells by PIP5K1C-siRNA, restored β-catenin membrane localization and normalized AKT signaling. Pharmacological NOTCH or AKT inhibition in vivo or genetic β-catenin attenuation rescued angiogenesis defects in INPP5K-null mice. Therefore, PI(4,5)P₂ is critical for β-catenin/DLL4/NOTCH signaling, which governs tip cell specification during angiogenesis.

Genes controlling skeletal muscle glucose uptake and their regulation by endurance and resistance exercise

Exercise improves the insulin sensitivity of glucose uptake in skeletal muscle. Due to that, exercise has become a cornerstone treatment for type 2 diabetes mellitus (T2DM). The mechanisms by which exercise improves skeletal muscle insulin sensitivity are, however, incompletely understood. We conducted a systematic review to identify all genes whose gain or loss of function alters skeletal muscle glucose uptake. We subsequently cross-referenced these genes with recently generated data sets on exercise-induced gene expression and signaling. Our search revealed 176 muscle glucose-uptake genes, meaning that their genetic manipulation altered glucose uptake in skeletal muscle. Notably, exercise regulates the expression or phosphorylation of more than 50% of the glucose-uptake genes or their protein products. This included many genes that previously have not been associated with exercise-induced insulin sensitivity. Interestingly, endurance and resistance exercise triggered some common but mostly unique changes in expression and phosphorylation of glucose-uptake genes or their protein products. Collectively, our work provides a resource of potentially new molecular effectors that play a role in the incompletely understood regulation of muscle insulin sensitivity by exercise.

INPP5K and Atlastin-1 maintain the nonuniform distribution of ER-plasma membrane contacts in neurons

In neurons, the ER extends throughout all cellular processes, forming multiple contacts with the plasma membrane (PM) to fine-tune neuronal physiology. However, the mechanisms that regulate the distribution of neuronal ER-PM contacts are not known. Here, we used the Caenorhabditis elegans DA9 motor neuron as our model system and found that neuronal ER-PM contacts are enriched in soma and dendrite and mostly absent in axons. Using forward genetic screen, we identified that the inositol 5-phosphatase, CIL-1 (human INPP5K), and the dynamin-like GTPase, ATLN-1 (human Atlastin-1), help to maintain the non-uniform, somatodendritic enrichment of neuronal ER-PM contacts. Mechanistically, CIL-1 acts upstream of ATLN-1 to maintain the balance between ER tubules and sheets. In mutants of CIL-1 or ATLN-1, ER sheets expand and invade into the axon. This is accompanied by the ectopic formation of axonal ER-PM contacts and defects in axon regeneration following laser-induced axotomy. As INPP5K and Atlastin-1 have been linked to neurological disorders, the unique distribution of neuronal ER-PM contacts maintained by these proteins may support neuronal resilience during the onset and progression of these diseases.

Defective lysosome reformation during autophagy causes skeletal muscle disease

The regulation of autophagy-dependent lysosome homeostasis in vivo is unclear. We showed that the inositol polyphosphate 5-phosphatase INPP5K regulates autophagic lysosome reformation (ALR), a lysosome recycling pathway, in muscle. INPP5K hydrolyzes phosphatidylinositol-4,5-bisphosphate [PI(4,5)P₂] to phosphatidylinositol 4-phosphate [PI(4)P], and INPP5K mutations cause muscular dystrophy by unknown mechanisms. We report that loss of INPP5K in muscle caused severe disease, autophagy inhibition, and lysosome depletion. Reduced PI(4,5)P₂ turnover on autolysosomes in Inpp5k^–/– muscle suppressed autophagy and lysosome repopulation via ALR inhibition. Defective ALR in Inpp5k^–/– myoblasts was characterized by enlarged autolysosomes and the persistence of hyperextended reformation tubules, structures that participate in membrane recycling to form lysosomes. Reduced disengagement of the PI(4,5)P₂ effector clathrin was observed on reformation tubules, which we propose interfered with ALR completion. Inhibition of PI(4,5)P₂ synthesis or expression of WT INPP5K but not INPP5K disease mutants in INPP5K-depleted myoblasts restored lysosomal homeostasis. Therefore, bidirectional interconversion of PI(4)P/PI(4,5)P₂ on autolysosomes was integral to lysosome replenishment and autophagy function in muscle. Activation of TFEB-dependent de novo lysosome biogenesis did not compensate for loss of ALR in Inpp5k^–/– muscle, revealing a dependence on this lysosome recycling pathway. Therefore, in muscle, ALR is indispensable for lysosome homeostasis during autophagy and when defective is associated with muscular dystrophy.

Signaling roles of phosphoinositides in the retina

The field of phosphoinositide signaling has expanded significantly in recent years. Phosphoinositides (also known as phosphatidylinositol phosphates or PIPs) are universal signaling molecules that directly interact with membrane proteins or with cytosolic proteins containing domains that directly bind phosphoinositides and are recruited to cell membranes. Through the activities of phosphoinositide kinases and phosphoinositide phosphatases, seven distinct phosphoinositide lipid molecules are formed from the parent molecule, phosphatidylinositol. PIP signals regulate a wide range of cellular functions, including cytoskeletal assembly, membrane budding and fusion, ciliogenesis, vesicular transport, and signal transduction. Given the many excellent reviews on phosphoinositide kinases, phosphoinositide phosphatases, and PIPs in general, in this review, we discuss recent studies and advances in PIP lipid signaling in the retina. We specifically focus on PIP lipids from vertebrate (e.g., bovine, rat, mouse, toad, and zebrafish) and invertebrate (e.g., Drosophila, horseshoe crab, and squid) retinas. We also discuss the importance of PIPs revealed from animal models and human diseases, and methods to study PIP levels both in vitro and in vivo. We propose that future studies should investigate the function and mechanism of activation of PIP-modifying enzymes/phosphatases and further unravel PIP regulation and function in the different cell types of the retina.

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.

The phosphoinositide 5-phosphatase INPP5K: From gene structure to in vivo functions

INPP5K (Inositol Polyphosphate 5-Phosphatase K, or SKIP (for Skeletal muscle and Kidney enriched Inositol Phosphatase) is a member of the phosphoinositide 5-phosphatases family. Its protein structure is comprised of a N-terminal catalytic domain which hydrolyses both PtdIns(4,5)P2 and PtdIns(3,4,5)P3, followed by a SKICH domain at the C-terminus which is responsible for protein-protein interactions and subcellular localization of INPP5K. Strikingly, INPP5K is mostly concentrated in the endoplasmic reticulum, although it is also detected at the plasma membrane, in the cytosol and the nucleus. Recently, mutations in INPP5K have been detected in patients with a rare form of autosomal recessive congenital muscular dystrophy with cataract, short stature and intellectual disability. INPP5K functions extend from control of insulin signaling, endoplasmic reticulum stress response and structural integrity, myoblast differentiation, cytoskeleton organization, cell adhesion and migration, renal osmoregulation, to cancer. The goal of this review is thus to summarize and comment recent and less recent data in the literature on INPP5K, in particular on the structure, expression, intracellular localization, interactions and functions of this specific member of the 5-phosphatases family.

PTEN and Other PtdIns(3,4,5)P3 Lipid Phosphatases in Breast Cancer

The phosphoinositide 3-kinase (PI3K)/AKT signalling pathway is hyperactivated in ~70% of breast cancers. Class I PI3K generates PtdIns(3,4,5)P₃ at the plasma membrane in response to growth factor stimulation, leading to AKT activation to drive cell proliferation, survival and migration. PTEN negatively regulates PI3K/AKT signalling by dephosphorylating PtdIns(3,4,5)P₃ to form PtdIns(4,5)P₂. PtdIns(3,4,5)P₃ can also be hydrolysed by the inositol polyphosphate 5-phosphatases (5-phosphatases) to produce PtdIns(3,4)P₂. Interestingly, while PTEN is a bona fide tumour suppressor and is frequently mutated/lost in breast cancer, 5-phosphatases such as PIPP, SHIP2 and SYNJ2, have demonstrated more diverse roles in regulating mammary tumourigenesis. Reduced PIPP expression is associated with triple negative breast cancers and reduced relapse-free and overall survival. Although PIPP depletion enhances AKT phosphorylation and supports tumour growth, this also inhibits cell migration and metastasis in vivo, in a breast cancer oncogene-driven murine model. Paradoxically, SHIP2 and SYNJ2 are increased in primary breast tumours, which correlates with invasive disease and reduced survival. SHIP2 or SYNJ2 overexpression promotes breast tumourigenesis via AKT-dependent and independent mechanisms. This review will discuss how PTEN, PIPP, SHIP2 and SYNJ2 distinctly regulate multiple functional targets, and the mechanisms by which dysregulation of these distinct phosphoinositide phosphatases differentially affect breast cancer progression.

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.

A Recurrent Pathogenic Variant of INPP5K Underlies Autosomal Recessive Congenital Muscular Dystrophy With Cataracts and Intellectual Disability: Evidence for a Founder Effect in Southern Italy

Inositol polyphosphate-5-phosphatase K [INPP5K (MIM: 607875)] acts as a PIP₃ 5-phosphatase and regulates actin cytoskeleton, insulin, and cell migration. Biallelic pathogenic variants in INPP5K have recently been reported in patients affected by a form of muscular dystrophy with childhood onset. Affected patients have limb girdle muscle weakness, often associated with bilateral cataracts, short stature, and intellectual disability. Here we report four patients affected by INPP5K-related muscle dystrophy, who were apparently unrelated but originated from the same geographical area in South Italy. These patients manifest a recognizable phenotype characterized by early onset muscular dystrophy associated with short stature and intellectual disability. All affected subjects were homozygous or compound heterozygous for the c.67G > A (p.Val23Met) missense change and shared a common haplotype, indicating the occurrence of a founder effect.

The Prediction and Prognostic Significance of INPP5K Expression in Patients with Liver Cancer

Liver cancer is a devastating disease for humans with poor prognosis. Although the survival rate of patients with liver cancer has improved in the past decades, the recurrence and metastasis of liver cancer are still obstacles for us. Inositol polyphosphate-5-phosphatase K (INPP5K) belongs to the family of phosphoinositide 5-phosphatases (PI 5-phosphatases), which have been reported to be associated with cell migration, polarity, adhesion, and cell invasion, especially in cancers. However, there have been few studies on the correlation of INPP5K and liver cancer. In this study, we explored the prognostic significance of INPP5K in liver cancer through bioinformatics analysis of data collected from The Cancer Genome Atlas (TCGA) database. Chi-square and Fisher exact tests were used to evaluate the relationship between INPP5K expression and clinical characteristics. Our results showed that low INPP5K expression was correlated with poor outcomes in liver cancer patients. Univariate and multivariate Cox analyses demonstrated that low INPP5K mRNA expression played a significant role in shortening overall survival (OS) and relapse-free survival (RFS), which might serve as the useful biomarker and prognostic factor for liver cancer. In conclusion, low INPP5K mRNA expression is an independent risk factor for poor prognosis in liver cancer.

PI(3,4)P2 Signaling in Cancer and Metabolism

The phosphatidylinositide 3 kinases (PI3Ks) and their downstream mediators AKT and mammalian target of rapamycin (mTOR) are central regulators of glycolysis, cancer metabolism, and cancer cell proliferation. At the molecular level, PI3K signaling involves the generation of the second messenger lipids phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3] and phosphatidylinositol 3,4-bisphosphate [PI(3,4)P2]. There is increasing evidence that PI(3,4)P2 is not only the waste product for the removal of PI(3,4,5)P3 but can also act as a signaling molecule. The selective cellular functions for PI(3,4)P2 independent of PI(3,4,5)P3 have been recently described, including clathrin-mediated endocytosis and mTOR regulation. However, the specific spatiotemporal dynamics and signaling role of PI3K minor lipid messenger PI(3,4)P2 are not well-understood. This review aims at highlighting the biological functions of this lipid downstream of phosphoinositide kinases and phosphatases and its implication in cancer metabolism.

Phosphoinositide phosphatases in cancer cell dynamics-Beyond PI3K and PTEN

Phosphoinositides are a group of lipids that regulate intracellular signaling and subcellular biological events. The signaling by phosphatidylinositol-3,4,5-trisphosphate and Akt mediates the action of growth factors that are essential for cell proliferation, gene transcription, cell migration, and polarity. The hyperactivation of this signaling has been identified in different cancer cells; and, it has been implicated in oncogenic transformation and cancer cell malignancy. Recent studies have argued the role of phosphoinositides in cancer cell dynamics, including actin cytoskeletal rearrangement at the plasma membrane and the organization of intracellular compartments. The focus of this review is to summarize the impact of the activities of phosphoinositide phosphatases on intracellular signaling related to cancer cell dynamics and to discuss how the abnormalities in the activities of the enzymes alter the levels of phosphoinositides in cancer cells.

The inositol 5-phosphatase INPP5K participates in the fine control of ER organization

Endoplasmic reticulum (ER) morphology is dynamic and key to its function during different cellular processes. Dong et al. now show in convergent studies in human cells and Caenorhabditis elegans neurons that a phosphoinositide phosphatase (INPP5K) is localized on the surface of the ER network and helps control the shape of the ER.

INPP5K (SKIP) is an inositol 5-phosphatase that localizes in part to the endoplasmic reticulum (ER). We show that recruitment of INPP5K to the ER is mediated by ARL6IP1, which shares features of ER-shaping proteins. Like ARL6IP1, INPP5K is preferentially localized in ER tubules and enriched, relative to other ER resident proteins (Sec61β, VAPB, and Sac1), in newly formed tubules that grow along microtubule tracks. Depletion of either INPP5K or ARL6IP1 results in the increase of ER sheets. In a convergent but independent study, a screen for mutations affecting the distribution of the ER network in dendrites of the PVD neurons of Caenorhabditis elegans led to the isolation of mutants in CIL-1, which encodes the INPP5K worm orthologue. The mutant phenotype was rescued by expression of wild type, but not of catalytically inactive CIL-1. Our results reveal an unexpected role of an ER localized polyphosphoinositide phosphatase in the fine control of ER network organization.

Mutations in INPP5K, Encoding a Phosphoinositide 5-Phosphatase, Cause Congenital Muscular Dystrophy with Cataracts and Mild Cognitive Impairment

Phosphoinositides are small phospholipids that control diverse cellular downstream signaling events. Their spatial and temporal availability is tightly regulated by a set of specific lipid kinases and phosphatases. Congenital muscular dystrophies are hereditary disorders characterized by hypotonia and weakness from birth with variable eye and central nervous system involvement. In individuals exhibiting congenital muscular dystrophy, early-onset cataracts, and mild intellectual disability but normal cranial magnetic resonance imaging, we identified bi-allelic mutations in INPP5K, encoding inositol polyphosphate-5-phosphatase K. Mutations impaired phosphatase activity toward the phosphoinositide phosphatidylinositol (4,5)-bisphosphate or altered the subcellular localization of INPP5K. Downregulation of INPP5K orthologs in zebrafish embryos disrupted muscle fiber morphology and resulted in abnormal eye development. These data link congenital muscular dystrophies to defective phosphoinositide 5-phosphatase activity that is becoming increasingly recognized for its role in mediating pivotal cellular mechanisms contributing to disease.

Lowered Expression of Tumor Suppressor Candidate MYO1C Stimulates Cell Proliferation, Suppresses Cell Adhesion and Activates AKT

Myosin-1C (MYO1C) is a tumor suppressor candidate located in a region of recurrent losses distal to TP53. Myo1c can tightly and specifically bind to PIP2, the substrate of Phosphoinositide 3-kinase (PI3K), and to Rictor, suggesting a role for MYO1C in the PI3K pathway. This study was designed to examine MYO1C expression status in a panel of well-stratified endometrial carcinomas as well as to assess the biological significance of MYO1C as a tumor suppressor in vitro. We found a significant correlation between the tumor stage and lowered expression of MYO1C in endometrial carcinoma samples. In cell transfection experiments, we found a negative correlation between MYO1C expression and cell proliferation, and MYO1C silencing resulted in diminished cell migration and adhesion. Cells expressing excess of MYO1C had low basal level of phosphorylated protein kinase B (PKB, a.k.a. AKT) and cells with knocked down MYO1C expression showed a quicker phosphorylated AKT (pAKT) response in reaction to serum stimulation. Taken together the present study gives further evidence for tumor suppressor activity of MYO1C and suggests MYO1C mediates its tumor suppressor function through inhibition of PI3K pathway and its involvement in loss of contact inhibition.

Inpp5e suppresses polycystic kidney disease via inhibition of PI3K/Akt-dependent mTORC1 signaling

Polycystic kidney disease (PKD) is a common cause of renal failure with few effective treatments. INPP5E is an inositol polyphosphate 5-phosphatase that dephosphorylates phosphoinositide 3-kinase (PI3K)-generated PI(3,4,5)P₃ and is mutated in ciliopathy syndromes. Germline Inpp5e deletion is embryonically lethal, attributed to cilia stability defects, and is associated with polycystic kidneys. However, the molecular mechanisms responsible for PKD development upon Inpp5e loss remain unknown. Here, we show conditional inactivation of Inpp5e in mouse kidney epithelium results in severe PKD and renal failure, associated with a partial reduction in cilia number and hyperactivation of PI3K/Akt and downstream mammalian target of rapamycin complex 1 (mTORC1) signaling. Treatment with an mTORC1 inhibitor improved kidney morphology and function, but did not affect cilia number or length. Therefore, we identify Inpp5e as an essential inhibitor of the PI3K/Akt/mTORC1 signaling axis in renal epithelial cells, and demonstrate a critical role for Inpp5e-dependent mTORC1 regulation in PKD suppression.

Glucose-regulated protein 78 (GRP78) binds directly to PIP3 phosphatase SKIP and determines its localization

Skeletal muscle and kidney-enriched inositol polyphosphate phosphatase (SKIP), a PIP3 phosphatase, has been implicated in the regulation of insulin signaling in skeletal muscle. SKIP interacts with Pak1 and glucose-regulated protein 78 (GRP78), both of which are necessary for the regulation of insulin signaling. In this study, we showed that GRP78 directly binds to the SKIP C-terminal homology (SKICH) domain of SKIP and that this binding is necessary for the localization of SKIP at the ER. In addition, in vitro binding analysis showed that GRP78 and Pak1 competitively bind to SKIP. Taken together, these findings suggest a model by which GRP78 regulates intracellular localization of SKIP and how SKIP binds to Pak1 on insulin stimulation.

Regulation of PtdIns(3,4,5)P3/Akt signalling by inositol polyphosphate 5-phosphatases

The phosphoinositide 3-kinase (PI3K) generated lipid signals, PtdIns(3,4,5)P3 and PtdIns(3,4)P2, are both required for the maximal activation of the serine/threonine kinase proto-oncogene Akt. The inositol polyphosphate 5-phosphatases (5-phosphatases) hydrolyse the 5-position phosphate from the inositol head group of PtdIns(3,4,5)P3 to yield PtdIns(3,4)P2. Extensive work has revealed several 5-phosphatases inhibit PI3K-driven Akt signalling, by decreasing PtdIns(3,4,5)P3 despite increasing cellular levels of PtdIns(3,4)P2. The roles that 5-phosphatases play in suppressing cell proliferation and transformation are slow to emerge; however, the 5-phosphatase PIPP [proline-rich inositol polyphosphate 5-phosphatase; inositol polyphosphate 5-phosphatase (INPP5J)] has recently been identified as a putative tumour suppressor in melanoma and breast cancer and SHIP1 [SH2 (Src homology 2)-containing inositol phosphatase 1] inhibits haematopoietic cell proliferation. INPP5E regulates cilia stability and INPP5E mutations have been implicated ciliopathy syndromes. This review will examine 5-phosphatase regulation of PI3K/Akt signalling, focussing on the role PtdIns(3,4,5)P3 5-phosphatases play in developmental diseases and cancer.

Phosphatidylinositol 3,4,5-Trisphosphate Phosphatase SKIP Links Endoplasmic Reticulum Stress in Skeletal Muscle to Insulin Resistance

Insulin resistance is critical in the pathogenesis of type 2 diabetes. Endoplasmic reticulum (ER) stress in liver and adipose tissues plays an important role in the development of insulin resistance. Although skeletal muscle is a primary site for insulin-dependent glucose disposal, it is unclear if ER stress in those tissues contributes to insulin resistance. In this study, we show that skeletal muscle kidney-enriched inositol polyphosphate phosphatase (SKIP), a PIP3 (phosphatidylinositol-3,4,5-trisphosphate) phosphatase, links ER stress to insulin resistance in skeletal muscle. SKIP expression was increased due to ER stress and was higher in the skeletal muscle isolated from high-fat-diet-fed mice and db/db mice than in that from wild-type mice. Mechanistically, ER stress promotes activating transcription factor 6 (ATF6) and X-box binding protein 1 (XBP1)-dependent expression of SKIP. These findings underscore the specific and prominent role of SKIP in the development of insulin resistance in skeletal muscle.

Phosphatidylinositolphosphate phosphatase activities and cancer

Signaling through the phosphoinositide 3-kinase pathways mediates the actions of a plethora of hormones, growth factors, cytokines, and neurotransmitters upon their target cells following receptor occupation. Overactivation of these pathways has been implicated in a number of pathologies, in particular a range of malignancies. The tight regulation of signaling pathways necessitates the involvement of both stimulatory and terminating enzymes; inappropriate activation of a pathway can thus result from activation or inhibition of the two signaling arms. The focus of this review is to discuss, in detail, the activities of the identified families of phosphoinositide phosphatase expressed in humans, and how they regulate the levels of phosphoinositides implicated in promoting malignancy.

Phosphoinositides: Key modulators of energy metabolism

Phosphoinositides are key players in many trafficking and signaling pathways. Recent advances regarding the synthesis, location and functions of these lipids have dramatically improved our understanding of how and when these lipids are generated and what their roles are in animal physiology. In particular, phosphoinositides play a central role in insulin signaling, and manipulation of PtdIns(3,4,5)P₃levels in particular, may be an important potential therapeutic target for the alleviation of insulin resistance associated with obesity and the metabolic syndrome. In this article we review the metabolism, regulation and functional roles of phosphoinositides in insulin signaling and the regulation of energy metabolism. This article is part of a Special Issue entitled Phosphoinositides.

The structure of phosphoinositide phosphatases: Insights into substrate specificity and catalysis

Phosphoinositides (PIs) are a group of key signaling and structural lipid molecules involved in a myriad of cellular processes. PI phosphatases, together with PI kinases, are responsible for the conversion of PIs between distinctive phosphorylation states. PI phosphatases are a large collection of enzymes that are evolved from at least two disparate ancestors. One group is distantly related to endonucleases, which apply divalent metal ions for phosphoryl transfer. The other group is related to protein tyrosine phosphatases, which contain a highly conserved active site motif Cys-X5-Arg (CX5R). In this review, we focus on structural insights to illustrate current understandings of the molecular mechanisms of each PI phosphatase family, with emphasis on their structural basis for substrate specificity determinants and catalytic mechanisms. This article is part of a Special Issue entitled Phosphoinositides.

Skeletal muscle hypertrophy and regeneration: interplay between the myogenic regulatory factors (MRFs) and insulin-like growth factors (IGFs) pathways

Adult skeletal muscle can regenerate in response to muscle damage. This ability is conferred by the presence of myogenic stem cells called satellite cells. In response to stimuli such as injury or exercise, these cells become activated and express myogenic regulatory factors (MRFs), i.e., transcription factors of the myogenic lineage including Myf5, MyoD, myogenin, and Mrf4 to proliferate and differentiate into myofibers. The MRF family of proteins controls the transcription of important muscle-specific proteins such as myosin heavy chain and muscle creatine kinase. Different growth factors are secreted during muscle repair among which insulin-like growth factors (IGFs) are the only ones that promote both muscle cell proliferation and differentiation and that play a key role in muscle regeneration and hypertrophy. Different isoforms of IGFs are expressed during muscle repair: IGF-IEa, IGF-IEb, or IGF-IEc (also known as mechano growth factor, MGF) and IGF-II. MGF is expressed first and is observed in satellite cells and in proliferating myoblasts whereas IGF-Ia and IGF-II expression occurs at the state of muscle fiber formation. Interestingly, several studies report the induction of MRFs in response to IGFs stimulation. Inversely, IGFs expression may also be regulated by MRFs. Various mechanisms are proposed to support these interactions. In this review, we describe the general process of muscle hypertrophy and regeneration and decipher the interactions between the two groups of factors involved in the process.

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.

Inositol polyphosphate 5-phosphatases; new players in the regulation of cilia and ciliopathies

Phosphoinositides regulate numerous cellular events via the recruitment and activation of multiple lipid-binding effector proteins. The precise temporal and spatial regulation of phosphoinositide signals by the co-ordinated activities of phosphoinositide kinases and phosphatases is essential for homeostasis and development. Mutations in two inositol polyphosphate 5-phosphatases, INPP5E and OCRL, cause the cerebrorenal syndromes of Joubert and Lowe's, respectively. INPP5E and OCRL exhibit overlapping phosphoinositide substrate specificity and subcellular localisation, including an association with the primary cilia. Here, we review recent studies that identify a new role for these enzymes in the regulation of primary cilia function. Joubert syndrome has been extensively linked to primary cilia defects, and Lowe's may represent a new class of 'ciliopathy associated' syndromes.

Role of phosphatidylinositol 3,4,5-trisphosphate (PIP3) 5-phosphatase skeletal muscle- and kidney-enriched inositol polyphosphate phosphatase (SKIP) in myoblast differentiation

Insulin-like growth factors (IGFs) are essential for the development, regeneration, and hypertrophy of skeletal muscles. IGF-II promotes myoblast differentiation through phosphatidylinositol 3-kinase (PI 3-kinase), Akt, and mTOR signaling. Here, we report that skeletal muscle- and kidney-enriched inositol polyphosphate phosphatase (SKIP) negatively regulates myogenesis through inhibition of IGF-II production and attenuation of the IGF-II-Akt-mTOR signaling pathway. We also demonstrate that SKIP expression, which was markedly elevated during differentiation, was controlled by MyoD in C2C12 cells. Expression of SKIP inhibited IGF-II at the transcription level. These results indicate that SKIP regulates MyoD-mediated muscle differentiation. Silencing of SKIP increased IGF-II transcription and myoblast differentiation. Furthermore, knockdown of SKIP resulted in thick myotubes with a larger number of nuclei than that in control C2C12 cells. Taken together, these data indicate that SKIP controls the IGF-II-PI 3-kinase-Akt-mTOR auto-regulation loop during myogenesis. Our findings identify SKIP as a key regulator of muscle cell differentiation.

Regulation of insulin signaling by the phosphatidylinositol 3,4,5-triphosphate phosphatase SKIP through the scaffolding function of Pak1

Skeletal muscle and kidney-enriched inositol polyphosphate phosphatase (SKIP) has previously been implicated in the regulation of insulin signaling in skeletal muscle. Here, we present the first report of the mechanisms by which SKIP specifically suppresses insulin signaling and the subsequent glucose uptake. Upon insulin stimulation, SKIP is translocated to the membrane ruffles, where it binds to the active form of Pak1, which mediates multiple protein complex formation with phosphatidylinositol 3,4,5-triphosphate (PIP(3)) effectors such as Akt2, PDK1, and Rac1; this leads to inactivation of these proteins. SKIP also promotes the inhibition of Rac1-dependent kinase activity and the scaffolding function of Pak1, which results in the dissociation of Akt2 and PDK1 from Pak1. Thus, specific suppression of insulin signaling is achieved via the spatiotemporal regulation of SKIP through the scaffolding function of Pak1. These interactions are the foundation of the specific and prominent role of SKIP in the regulation of insulin signaling.

Regulation of insulin signaling and glucose transporter 4 (GLUT4) exocytosis by phosphatidylinositol 3,4,5-trisphosphate (PIP3) phosphatase, skeletal muscle, and kidney enriched inositol polyphosphate phosphatase (SKIP)

The glucose transporter 4 (GLUT4) is responsible for glucose uptake in the skeletal muscle. Insulin-induced translocation of GLUT4 to the plasma membrane requires phosphatidylinositol 3-kinase activation-mediated generation of phosphatidylinositol 3,4,5-trisphosphate PIP(3) and subsequent activation of Akt. Previous studies suggested that skeletal muscle and kidney enriched inositol polyphosphate phosphatase (SKIP) has negative effects on the regulation of insulin signaling in the skeletal muscle cells. Here, we compared its effects on insulin signaling by selective inhibition of SKIP, SHIP2, and phosphatase and tensin homologue on chromosome 10 (PTEN) by short interfering RNA in the C2C12 myoblast cells. Suppression of SKIP significantly increased the insulin-stimulated phosphatidylinositol 3,4,5-trisphosphate levels and Akt phosphorylation. Furthermore, silencing of SKIP, but not of PTEN, increased the insulin-dependent recruitment of GLUT4 vesicles to the plasma membrane. Taken together, these results imply that SKIP negatively regulates insulin signaling and glucose uptake by inhibiting GLUT4 docking and/or fusion to the plasma membrane.

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 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.

An introduction to phosphoinositides

Phosphoinositides (PIs) are minor components of cellular membranes that play critical regulatory roles in several intracellular functions. This chapter describes the main enzymes regulating the turnover of each of the seven PIs in mammalian cells and introduces to some of their intracellular functions and to some evidences of their involvement in human diseases. Due to the complex interrelation between the distinct PIs and the plethora of functions that they can regulate inside a cell, this chapter is not meant to be a comprehensive coverage of all aspects of PI signalling but rather an introduction to this complex signalling field. For more details of their regulation/functions and extensive description of their intracellular roles, more detailed reviews are suggested on each single topic.

The effect of high-intensity intermittent swimming on post-exercise glycogen supercompensation in rat skeletal muscle

A single bout of prolonged endurance exercise stimulates glucose transport in skeletal muscles, leading to post-exercise muscle glycogen supercompensation if sufficient carbohydrate is provided after the cessation of exercise. Although we recently found that short-term sprint interval exercise also stimulates muscle glucose transport, the effect of this type of exercise on glycogen supercompensation is uncertain. Therefore, we compared the extent of muscle glycogen accumulation in response to carbohydrate feeding following sprint interval exercise with that following endurance exercise. In this study, 16-h-fasted rats underwent a bout of high-intensity intermittent swimming (HIS) as a model of sprint interval exercise or low-intensity prolonged swimming (LIS) as a model of endurance exercise. During HIS, the rats swam for eight 20-s sessions while burdened with a weight equal to 18% of their body weight. The LIS rats swam with no load for 3 h. The exercised rats were then refed for 4, 8, 12, or 16 h. Glycogen levels were almost depleted in the epitrochlearis muscles of HIS- or LIS-exercised rats immediately after the cessation of exercise. A rapid increase in muscle glycogen levels occurred during 4 h of refeeding, and glycogen levels had peaked at the end of 8 h of refeeding in each group of exercised refed rats. The peak glycogen levels during refeeding were not different between HIS- and LIS-exercised refed rats. Furthermore, although a large accumulation of muscle glycogen in response to carbohydrate refeeding is known to be associated with decreased insulin responsiveness of glucose transport, and despite the fact that muscle glycogen supercompensation was observed in the muscles of our exercised rats at the end of 4 h of refeeding, insulin responsiveness was not decreased in the muscles of either HIS- or LIS-exercised refed rats compared with non-exercised fasted control rats at this time point. These results suggest that sprint interval exercise enhances muscle glycogen supercompensation in response to carbohydrate refeeding as well as prolonged endurance exercise does. Furthermore, in this study, both HIS and LIS exercise prevented insulin resistance of glucose transport in glycogen supercompensated muscle during the early phase of carbohydrate refeeding. This probably led to the enhanced muscle glycogen supercompensation after exercise.

Silencer of death domains (SODD) inhibits skeletal muscle and kidney enriched inositol 5-phosphatase (SKIP) and regulates phosphoinositide 3-kinase (PI3K)/Akt signaling to the actin cytoskeleton

Phosphoinositide 3-kinase (PI3K) regulates cell polarity and migration by generating phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P(3)) at the leading edge of migrating cells. The serine-threonine protein kinase Akt binds to PI(3,4,5)P(3), resulting in its activation. Active Akt promotes spatially regulated actin cytoskeletal remodeling and thereby directed cell migration. The inositol polyphosphate 5-phosphatases (5-ptases) degrade PI(3,4,5)P(3) to form PI(3,4)P(2), which leads to diminished Akt activation. Several 5-ptases, including SKIP and SHIP2, inhibit actin cytoskeletal reorganization by opposing PI3K/Akt signaling. In this current study, we identify a molecular co-chaperone termed silencer of death domains (SODD/BAG4) that forms a complex with several 5-ptase family members, including SKIP, SHIP1, and SHIP2. The interaction between SODD and SKIP exerts an inhibitory effect on SKIP PI(3,4,5)P(3) 5-ptase catalytic activity and consequently enhances the recruitment of PI(3,4,5)P(3)-effectors to the plasma membrane. In contrast, SODD(-/-) mouse embryonic fibroblasts exhibit reduced Akt-Ser(473) and -Thr(308) phosphorylation following EGF stimulation, associated with increased SKIP PI(3,4,5)P(3)-5-ptase activity. SODD(-/-) mouse embryonic fibroblasts exhibit decreased EGF-stimulated F-actin stress fibers, lamellipodia, and focal adhesion complexity, a phenotype that is rescued by the expression of constitutively active Akt1. Furthermore, reduced cell migration was observed in SODD(-/-) macrophages, which express the three 5-ptases shown to interact with SODD (SKIP, SHIP1, and SHIP2). Therefore, this study identifies SODD as a novel regulator of PI3K/Akt signaling to the actin cytoskeleton.

MICER targeting vectors for manipulating the mouse genome

The mouse has become an important model for understanding human development, physiology and disease because of its genetic and biological similarity to humans. Desired mouse mutants with precise genetic alterations can now be generated through gene targeting in mouse embryonic stem cells. The rate-limiting factor in a gene-targeting experiment is the time needed for cloning to construct targeting vectors. The establishment of the Mutagenic Insertion and Chromosome Engineering Resource has made available targeting vectors for the insertional mutagenesis of a large number of mouse genes as well as for chromosome engineering throughout the mouse genome. This unique resource has enriched the repertoire of the genetic reagents for targeted manipulation of the mouse genome.

MyoD control of SKIP expression during pig skeletal muscle development

Skeletal muscle and kidney enriched inositol phosphatase (SKIP) was identified as a 5'-inositol phosphatase that hydrolyzes PI(3,4,5)P3 to PI(3,4)P2 that negatively regulates insulin-induced phosphatidylinositol 3-kinase signaling in skeletal muscle. In this study, we obtained a 1575-bp mRNA sequence of porcine SKIP that included the full coding region encoding a protein of 450 amino acids. With the use of comparative mapping, we mapped this gene to SSC12 q1.3, where many QTLs affect Backfat thickness at 10th rib, carcass yield, the number of muscle fibers, and ham weight traits. As a candidate gene for growth and carcass traits, a novel single nucleotide polymorphism in exon 12 (G>A) was detected by PCR-RFLP. The results showed that the GG genotype had higher skin percentage (SP), carcass length to first spondyle (CL1), carcass length to first rib (CL2), but lower intramuscular fat (IMF) as compared with genotype AG (P<0.05), and allele G seemed to be associated with an increase in the growth trait. Porcine SKIP was expressed abundantly in skeletal muscle tissue and was transcriptionally upregulated during skeletal muscle differentiation. Analysis of the porcine SKIP promoter sequence demonstrated that MyoD was involved in regulating SKIP mRNA expression in myotubes, partly via the cis-acting elements in SKIP promoter. In summary, we suggested that SKIP might play a role in the regulation of skeletal muscle development in pigs.

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.

Sac3 is an insulin-regulated phosphatidylinositol 3,5-bisphosphate phosphatase: gain in insulin responsiveness through Sac3 down-regulation in adipocytes

Insulin-regulated stimulation of glucose entry and mobilization of fat/muscle-specific glucose transporter GLUT4 onto the cell surface require the phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P(2)) pathway for optimal performance. The reduced insulin responsiveness observed under ablation of the PtdIns(3,5)P(2)-synthesizing PIKfyve and its associated activator ArPIKfyve in 3T3L1 adipocytes suggests that dysfunction of the PtdIns(3,5)P(2)-specific phosphatase Sac3 may yield the opposite effect. Paradoxically, as uncovered recently, in addition to turnover Sac3 also supports PtdIns(3,5)P(2) biosynthesis by allowing optimal PIKfyve-ArPIKfyve association. These opposing inputs raise the key question as to whether reduced Sac3 protein levels and/or hydrolyzing activity will produce gain in insulin responsiveness. Here we report that small interfering RNA-mediated knockdown of endogenous Sac3 by approximately 60%, which resulted in a slight but significant elevation of PtdIns(3,5)P(2) in 3T3L1 adipocytes, increased GLUT4 translocation and glucose entry in response to insulin. In contrast, ectopic expression of Sac3(WT), but not phosphatase-deficient Sac3(D488A), reduced GLUT4 surface abundance in the presence of insulin. Endogenous Sac3 physically assembled with PIKfyve and ArPIKfyve in both membrane and soluble fractions of 3T3L1 adipocytes, but this remained insulin-insensitive. Importantly, acute insulin markedly reduced the in vitro C8-PtdIns(3,5)P(2) hydrolyzing activity of Sac3. The insulin-sensitive Sac3 pool likely controls a discrete PtdIns(3,5)P(2) subfraction as the high pressure liquid chromatography-measurable insulin-dependent elevation in total [(3)H]inositol-PtdIns(3,5)P(2) was minor. Together, our data identify Sac3 as an insulin-sensitive phosphatase whose down-regulation increases insulin responsiveness, thus implicating Sac3 as a novel drug target in insulin resistance.

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.