ArPIKfyve-PIKfyve interaction and role in insulin-regulated GLUT4 translocation and glucose transport in 3T3-L1 adipocytes

Regulation of Macronutrients in Insulin Resistance and Glucose Homeostasis during Type 2 Diabetes Mellitus

Insulin resistance is an important feature of metabolic syndrome and a precursor of type 2 diabetes mellitus (T2DM). Overnutrition-induced obesity is a major risk factor for the development of insulin resistance and T2DM. The intake of macronutrients plays a key role in maintaining energy balance. The components of macronutrients distinctly regulate insulin sensitivity and glucose homeostasis. Precisely adjusting the beneficial food compound intake is important for the prevention of insulin resistance and T2DM. Here, we reviewed the effects of different components of macronutrients on insulin sensitivity and their underlying mechanisms, including fructose, dietary fiber, saturated and unsaturated fatty acids, and amino acids. Understanding the diet-gene interaction will help us to better uncover the molecular mechanisms of T2DM and promote the application of precision nutrition in practice by integrating multi-omics analysis.

A plant virus hijacks phosphatidylinositol-3,5-bisphosphate to escape autophagic degradation in its insect vector

Hosts can initiate macroautophagy/autophagy as an antiviral defense response, while viruses have developed multiple ways to evade the host autophagic degradation. However, little is known as to whether viruses can target lipids to subvert autophagic degradation. Here, we show that a low abundant signaling lipid, phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2), is required for rice black-streaked dwarf virus (RBSDV) to evade the autophagic degradation in the insect vector Laodelphax striatellus. RBSDV binds to PtdIns(3,5)P2 and elevates its level through its main capsid protein P10, leading to inhibited autophagy and promoted virus propagation. Furthermore, we show that PtdIns(3,5)P2 inhibits the autophagy pathway by preventing the fusion of autophagosomes and lysosomes through activation of Trpml (transient receptor potential cation channel, mucolipin), an effector of PtdIns(3,5)P2. These findings uncover a strategy whereby a plant virus hijacks PtdIns(3,5)P2 via its viral capsid protein to evade autophagic degradation and promote its survival in insects.

Proximity Interactome Map of the Vac14-Fig4 Complex Using BioID

Conversion between phosphatidylinositol-3-phosphate and phosphatidylinositol-3,5-bisphosphate on endosomal membranes is critical for the maturation of early endosomes to late endosomes/lysosomes and is regulated by the PIKfyve-Vac14-Fig4 complex. Despite the importance of this complex for endosomal homeostasis and vesicular trafficking, there is little known about how its activity is regulated or how it interacts with other cellular proteins. Here, we screened for the cellular interactome of Vac14 and Fig4 using proximity-dependent biotin labeling (BioID). After independently screening the interactomes of Vac14 and Fig4, we identified 89 high-confidence protein hits shared by both proteins. Network analysis of these hits revealed pathways with known involvement of the PIKfyve-Vac14-Fig4 complex, including vesicular organization and PI3K/Akt signaling, as well as novel pathways including cell cycle and mitochondrial regulation. We also identified subunits of coatomer complex I (COPI), a Golgi-associated complex with an emerging role in endosomal dynamics. Using proximity ligation assays, we validated the interaction between Vac14 and COPI subunit COPB1 and between Vac14 and Arf1, a GTPase required for COPI assembly. In summary, this study used BioID to comprehensively map the Vac14-Fig4 interactome, revealing potential roles for these proteins in diverse cellular processes and pathways, including preliminary evidence of an interaction between Vac14 and COPI. Data are available via ProteomeXchange with the identifier PXD027917.

Genetic factors affect the susceptibility to bacterial infections in diabetes

Diabetes increases the risk of bacterial infections. We investigated whether common genetic variants associate with infection susceptibility in Finnish diabetic individuals. We performed genome-wide association studies and pathway analysis for bacterial infection frequency in Finnish adult diabetic individuals (FinnDiane Study; N = 5092, Diabetes Registry Vaasa; N = 4247) using national register data on antibiotic prescription purchases. Replication analyses were performed in a Swedish diabetic population (ANDIS; N = 9602) and in a Finnish non-diabetic population (FinnGen; N = 159,166). Genome-wide data indicated moderate but significant narrow-sense heritability for infection susceptibility (h² = 16%, P = 0.02). Variants on chromosome 2 were associated with reduced infection susceptibility (rs62192851, P = 2.23 × 10^–7). Homozygotic carriers of the rs62192851 effect allele (N = 44) had a 37% lower median annual antibiotic purchase rate, compared to homozygotic carriers of the reference allele (N = 4231): 0.38 [IQR 0.22–0.90] and 0.60 [0.30–1.20] respectively, P = 0.01). Variants rs6727834 and rs10188087, in linkage disequilibrium with rs62192851, replicated in the FinnGen-cohort (P < 0.05), but no variants replicated in the ANDIS-cohort. Pathway analysis suggested the IRAK1 mediated NF-κB activation through IKK complex recruitment-pathway to be a mediator of the phenotype. Common genetic variants on chromosome 2 may associate with reduced risk of bacterial infections in Finnish individuals with diabetes.

Small molecule PIKfyve inhibitors as cancer therapeutics: Translational promises and limitations

Through synthesis of two rare phosphoinositides, PtdIns(3,5)P₂ and PtdIns5P, the ubiquitously expressed phosphoinositide kinase PIKfyve is implicated in pleiotropic cellular functions. Small molecules specifically inhibiting PIKfyve activity cause cytoplasmic vacuolation in all dividing cells in culture yet trigger non-apoptotic death through excessive vacuolation only in cancer cells. Intriguingly, cancer cell toxicity appears to be inhibitor-specific suggesting that additional targets beyond PIKfyve are affected. One PIKfyve inhibitor - apilimod - is already in clinical trials for treatment of B-cell malignancies. However, apilimod is inactivated in cultured cells and exhibits unexpectedly low plasma levels in patients treated with maximum oral dosage. Thus, the potential widespread use of PIKfyve inhibitors as cancer therapeutics requires progress on multiple fronts: (i) advances in methods for isolating relevant cancer cells from individual patients; (ii) delineation of the molecular mechanisms potentiating the vacuolation induced by PIKfyve inhibitors in sensitive cancer cells; (iii) design of PIKfyve inhibitors with favorable pharmacokinetics; and (iv) development of effective drug combinations.

Computational analysis of data from a genome-wide screening identifies new PARP1 functional interactors as potential therapeutic targets

Knowledge of interaction network between different proteins can be a useful tool in cancer therapy. To develop new therapeutic treatments, understanding how these proteins contribute to dysregulated cellular pathways is an important task. PARP1 inhibitors are drugs used in cancer therapy, in particular where DNA repair is defective. It is crucial to find new candidate interactors of PARP1 as new therapeutic targets in order to increase efficacy of PARP1 inhibitors and expand their clinical utility. By a yeast-based genome wide screening, we previously discovered 90 candidate deletion genes that suppress growth-inhibition phenotype conferred by PARP1 in yeast. Here, we performed an integrated and computational analysis to deeply study these genes. First, we identified which pathways these genes are involved in and putative relations with PARP1 through g:Profiler. Then, we studied mutation pattern and their relation to cancer by interrogating COSMIC and DisGeNET database; finally, we evaluated expression and alteration in several cancers with cBioPortal, and the interaction network with GeneMANIA. We identified 12 genes belonging to PARP1-related pathways. We decided to further validate RIT1, INCENP and PSTA1 in MCF7 breast cancer cells. We found that RIT1 and INCENP affected PARylation and PARP1 protein level more significantly in PARP1 inhibited cells. Furthermore, downregulation of RIT1, INCENP and PSAT1 affected olaparib sensitivity of MCF7 cells. Our study identified candidate genes that could have an effect on PARP inhibition therapy. Moreover, we also confirm that yeast-based screenings could be very helpful to identify novel potential therapy factors.

Reorganization of Ternary Lipid Mixtures of Nonphosphorylated Phosphatidylinositol Interacting with Angiomotin

Phosphatidylinositol (PI) lipids are necessary for many cellular signaling pathways of membrane associated proteins, such as angiomotin (Amot). The Amot family regulates cellular polarity, growth, and migration. Given the low concentration of PI lipids in these membranes, it is likely that such protein-membrane interactions are stabilized by lipid domains or small lipid clusters. By small-angle X-ray scattering, we show that nonphosphorylated PI lipids induce lipid demixing in ternary mixtures of phosphatidylcholine (PC) and phosphatidylethanolamine (PE), likely because of preferential interactions between the head groups of PE and PI. These results were obtained in the presence of buffer containing tris(hydroxymethyl)aminomethane, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, NaCl, ethylenediaminetetraacetic acid, dithiothreitol, and benzamidine at pH 8.0 that in previous work showed an ability to cause PC to phase separate but are necessary to stabilize Amot for in vitro experimentation. Collectively, this provided a framework for determining the effect of Amot on lipid organization. Using fluorescence spectroscopy, we were able to show that the association of Amot with this lipid platform causes significant reorganization of the lipid into a more homogenous structure. This reorganization mechanism could be the basis for Amot membrane association and fusogenic activity previously described in the literature and should be taken into consideration in future protein-membrane interaction studies.

Myotubularin related protein-2 and its phospholipid substrate PIP2 control Piezo2-mediated mechanotransduction in peripheral sensory neurons

Piezo2 ion channels are critical determinants of the sense of light touch in vertebrates. Yet, their regulation is only incompletely understood. We recently identified myotubularin related protein-2 (Mtmr2), a phosphoinositide (PI) phosphatase, in the native Piezo2 interactome of murine dorsal root ganglia (DRG). Here, we demonstrate that Mtmr2 attenuates Piezo2-mediated rapidly adapting mechanically activated (RA-MA) currents. Interestingly, heterologous Piezo1 and other known MA current subtypes in DRG appeared largely unaffected by Mtmr2. Experiments with catalytically inactive Mtmr2, pharmacological blockers of PI(3,5)P₂ synthesis, and osmotic stress suggest that Mtmr2-dependent Piezo2 inhibition involves depletion of PI(3,5)P₂. Further, we identified a PI(3,5)P₂ binding region in Piezo2, but not Piezo1, that confers sensitivity to Mtmr2 as indicated by functional analysis of a domain-swapped Piezo2 mutant. Altogether, our results propose local PI(3,5)P₂ modulation via Mtmr2 in the vicinity of Piezo2 as a novel mechanism to dynamically control Piezo2-dependent mechanotransduction in peripheral sensory neurons.

We often take our sense of touch for granted. Yet, our every-day life greatly depends on the ability to perceive our environment to alert us of danger or to further social interactions, such as mother-child bonding. Our sense of touch relies on the conversion of mechanical stimuli to electrical signals (this is known as mechanotransduction), which then travel to brain to be processed. This task is fulfilled by specific ion channels called Piezo2, which are activated when cells are exposed to pressure and other mechanical forces. These channels can be found in sensory nerves and specialized structures in the skin, where they help to detect physical contact, roughness of surfaces and the position of our body parts.

It is still not clear how Piezo2 channels are regulated but previous research by several laboratories suggests that they work in conjunction with other proteins. One of these proteins is the myotubularin related protein-2, or Mtmr2 for short. Now, Narayanan et al. – including some of the researchers involved in the previous research – set out to advance our understanding of the molecular basis of touch and looked more closely at Mtmr2.

To test if Mtmr2 played a role in mechanotransduction, Narayanan et al. both increased and reduced the levels of this protein in sensory neurons of mice grown in the laboratory. When Mtmr2 levels were low, the activity of Piezo2 channels increased. However, when the protein levels were high, Piezo2 channels were inhibited. These results suggest that Mtmr2 can control the activity of Piezo2. Further experiments, in which Mtmr2 was genetically modified or sensory neurons were treated with chemicals, revealed that Mtmr2 reduces a specific fatty acid in the membrane of nerve cells, which in turn attenuates the activity of Piezo2.

This study identified Mtmr2 and distinct fatty acids in the cell membrane as new components of the complex setup required for the sense of touch. A next step will be to test if these molecules also influence the activity of Piezo2 when the skin has become injured or upon inflammation.

PI5P and PI(3,5)P2: Minor, but Essential Phosphoinositides

In most eukaryotes, phosphoinositides (PIs) have crucial roles in multiple cellular functions. Although the cellular levels of phosphatidylinositol 5-phosphate (PI5P) and phosphatidylinositol 3,5-bisphosphate (PI(3,5)P₂) are extremely low relative to some other PIs, emerging evidence demonstrates that both lipids are crucial for the endocytic pathway, intracellular signaling, and adaptation to stress. Mutations that causes defects in the biosynthesis of PI5P and PI(3,5)P₂ are linked to human diseases including neurodegenerative disorders. Here, we review recent findings on cellular roles of PI5P and PI(3,5)P₂, as well as the pathophysiological importance of these lipids.Key words: Phosphoinositides, Membrane trafficking, Endocytosis, Vacuoles/Lysosomes, Fab1/PIKfyve.

Metabolic Reprogramming by the PI3K-Akt-mTOR Pathway in Cancer

In the past decade, there has been a resurgence of interest in elucidating how metabolism is altered in cancer cells and how such dependencies can be targeted for therapeutic gain. At the core of this research is the concept that metabolic pathways are reprogrammed in cancer cells to divert nutrients toward anabolic processes to facilitate enhanced growth and proliferation. Importantly, physiological cellular signaling mechanisms normally tightly regulate the ability of cells to gain access to and utilize nutrients, posing a fundamental barrier to transformation. This barrier is often overcome by aberrations in cellular signaling that drive tumor pathogenesis by enabling cancer cells to make critical cellular decisions in a cell-autonomous manner. One of the most frequently altered pathways in human cancer is the PI3K-Akt-mTOR signaling pathway. Here, we describe mechanisms by which this signaling network is responsible for controlling cellular metabolism. Through both the post-translational regulation and the induction of transcriptional programs, the PI3K-Akt-mTOR pathway coordinates the uptake and utilization of multiple nutrients, including glucose, glutamine, nucleotides, and lipids, in a manner best suited for supporting the enhanced growth and proliferation of cancer cells. These regulatory mechanisms illustrate how metabolic changes in cancer are closely intertwined with oncogenic signaling pathways that drive tumor initiation and progression.

Unexpected severe consequences of Pikfyve deletion by aP2- or Aq-promoter-driven Cre expression for glucose homeostasis and mammary gland development

Systemic deficiency of PIKfyve, the evolutionarily conserved phosphoinositide kinase synthesizing cellular PtdIns5P and PtdIns(3,5)P₂ and implicated in insulin signaling, causes early embryonic death in mice. In contrast, mice with muscle‐specific Pikfyve disruption have normal lifespan but exhibit early‐age whole‐body glucose intolerance and muscle insulin resistance, thus establishing the key role of muscle PIKfyve in glucose homeostasis. Fat and muscle tissues control postprandial glucose clearance through different mechanisms, raising questions as to whether adipose Pikfyve disruption will also trigger whole‐body metabolic abnormalities, and if so, what the mechanism might be. To clarify these issues, here we have characterized two new mouse models with adipose tissue disruption of Pikfyve through Cre recombinase expression driven by adipose‐specific aP2‐ or adiponectin (Aq) promoters. Whereas both mouse lines were ostensibly normal until adulthood, their glucose homeostasis and systemic insulin sensitivity were severely dysregulated. These abnormalities stemmed in part from accelerated fat‐cell lipolysis and elevated serum FFA. Intriguingly, aP2‐Cre‐PIKfyve^fl/fl but not Aq‐Cre‐PIKfyve^fl/fl females had severely impaired pregnancy‐induced mammary gland differentiation and lactogenesis, consistent with aP2‐Cre‐mediated Pikfyve excision in nonadipogenic tissues underlying this defect. Intriguingly, whereas mammary glands from postpartum control and Aq‐Cre‐PIKfyve^fl/fl mice or ex vivo mammary gland explants showed profound upregulation of PIKfyve protein levels subsequent to prolactin receptor activation, such increases were not apparent in aP2‐Cre‐PIKfyve^fl/fl females. Collectively, our data identify for the first time that adipose tissue Pikfyve plays a key role in the mechanisms regulating glucose homeostasis and that the PIKfyve pathway is critical in mammary epithelial differentiation during pregnancy and lactogenesis downstream of prolactin receptor signaling.

The Protein Complex of Neurodegeneration-related Phosphoinositide Phosphatase Sac3 and ArPIKfyve Binds the Lewy Body-associated Synphilin-1, Preventing Its Aggregation

The 5-phosphoinositide phosphatase Sac3, in which loss-of-function mutations are linked to neurodegenerative disorders, forms a stable cytosolic complex with the scaffolding protein ArPIKfyve. The ArPIKfyve-Sac3 heterodimer interacts with the phosphoinositide 5-kinase PIKfyve in a ubiquitous ternary complex that couples PtdIns(3,5)P2 synthesis with turnover at endosomal membranes, thereby regulating the housekeeping endocytic transport in eukaryotes. Neuron-specific associations of the ArPIKfyve-Sac3 heterodimer, which may shed light on the neuropathological mechanisms triggered by Sac3 dysfunction, are unknown. Here we conducted mass spectrometry analysis for brain-derived interactors of ArPIKfyve-Sac3 and unraveled the α-synuclein-interacting protein Synphilin-1 (Sph1) as a new component of the ArPIKfyve-Sac3 complex. Sph1, a predominantly neuronal protein that facilitates aggregation of α-synuclein, is a major component of Lewy body inclusions in neurodegenerative α-synucleinopathies. Modulations in ArPIKfyve/Sac3 protein levels by RNA silencing or overexpression in several mammalian cell lines, including human neuronal SH-SY5Y or primary mouse cortical neurons, revealed that the ArPIKfyve-Sac3 complex specifically altered the aggregation properties of Sph1-GFP. This effect required an active Sac3 phosphatase and proceeded through mechanisms that involved increased Sph1-GFP partitioning into the cytosol and removal of Sph1-GFP aggregates by basal autophagy but not by the proteasomal system. If uncoupled from ArPIKfyve elevation, overexpressed Sac3 readily aggregated, markedly enhancing the aggregation potential of Sph1-GFP. These data identify a novel role of the ArPIKfyve-Sac3 complex in the mechanisms controlling aggregate formation of Sph1 and suggest that Sac3 protein deficiency or overproduction may facilitate aggregation of aggregation-prone proteins, thereby precipitating the onset of multiple neuronal disorders.

The Fab1/PIKfyve phosphoinositide phosphate kinase is not necessary to maintain the pH of lysosomes and of the yeast vacuole

Lysosomes and the yeast vacuole are degradative and acidic organelles. Phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2), a master architect of endolysosome and vacuole identity, is thought to be necessary for vacuolar acidification in yeast. There is also evidence that PtdIns(3,5)P2 may play a role in lysosomal acidification in higher eukaryotes. Nevertheless, these conclusions rely on qualitative assays of lysosome/vacuole pH. For example, quinacrine, an acidotropic fluorescent base, does not accumulate in the vacuoles of fab1Δ yeast. Fab1, along with its mammalian ortholog PIKfyve, is the lipid kinase responsible for synthesizing PtdIns(3,5)P2. In this study, we employed several assays that quantitatively assessed the lysosomal and vacuolar pH in PtdIns(3,5)P2-depleted cells. Using ratiometric imaging, we conclude that lysosomes retain a pH < 5 in PIKfyve-inhibited mammalian cells. In addition, quantitative fluorescence microscopy of vacuole-targeted pHluorin, a pH-sensitive GFP variant, indicates that fab1Δ vacuoles are as acidic as wild-type yeast. Importantly, we also employed fluorimetry of vacuoles loaded with cDCFDA, a pH-sensitive dye, to show that both wild-type and fab1Δ vacuoles have a pH < 5.0. In comparison, the vacuolar pH of the V-ATPase mutant vph1Δ or vph1Δ fab1Δ double mutant was 6.1. Although the steady-state vacuolar pH is not affected by PtdIns(3,5)P2 depletion, it may have a role in stabilizing the vacuolar pH during salt shock. Overall, we propose a model in which PtdIns(3,5)P2 does not govern the steady-state pH of vacuoles or lysosomes.

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.

Plentiful PtdIns5P from scanty PtdIns(3,5)P2 or from ample PtdIns? PIKfyve-dependent models: Evidence and speculation (response to: DOI 10.1002/bies.201300012)

Recently, we have presented data supporting the notion that PIKfyve not only produces the majority of constitutive phosphatidylinositol 5-phosphate (PtdIns5P) in mammalian cells but that it does so through direct synthesis from PtdIns. Another group, albeit obtaining similar data, suggests an alternative pathway whereby the low-abundance PtdIns(3,5)P2 undergoes hydrolysis by unidentified 3-phosphatases, thereby serving as a precursor for most of PtdIns5P. Here, we review the experimental evidence supporting constitutive synthesis of PtdIns5P from PtdIns by PIKfyve. We further emphasize that the experiments presented in support of the alternative pathway are also compatible with a direct mechanism for PIKfyve-catalyzed synthesis of PtdIns5P. While agreeing with the authors that constitutive PtdIns5P could theoretically be produced from PtdIns(3,5)P2 by 3-dephosphorylation, we argue that until direct evidence for such an alternative pathway is obtained, we should adhere to the existing experimental evidence and quantitative considerations, which favor direct PIKfyve-catalyzed synthesis for most constitutive PtdIns5P.

Small molecule adenosine 5'-monophosphate activated protein kinase (AMPK) modulators and human diseases

Adenosine 5'-monophosphate activated protein kinase (AMPK) is a master sensor of cellular energy status that plays a key role in the regulation of whole-body energy homeostasis. AMPK is a serine/threonine kinase that is activated by upstream kinases LKB1, CaMKKβ, and Tak1, among others. AMPK exists as αβγ trimeric complexes that are allosterically regulated by AMP, ADP, and ATP. Dysregulation of AMPK has been implicated in a number of metabolic diseases including type 2 diabetes mellitus and obesity. Recent studies have associated roles of AMPK with the development of cancer and neurological disorders, making it a potential therapeutic target to treat human diseases. This review focuses on the structure and function of AMPK, its role in human diseases, and its direct substrates and provides a brief synopsis of key AMPK modulators and their relevance in human diseases.

Cellular regulation of glucose uptake by glucose transporter GLUT4

GLUT4 is regulated by its intracellular localization. In the absence of insulin, GLUT4 is efficiently retained intracellularly within storage compartments in muscle and fat cells. Upon insulin stimulation (and contraction in muscle), GLUT4 translocates from these compartments to the cell surface where it transports glucose from the extracellular milieu into the cell. Its implication in insulin-regulated glucose uptake makes GLUT4 not only a key player in normal glucose homeostasis but also an important element in insulin resistance and type 2 diabetes. Nevertheless, how GLUT4 is retained intracellularly and how insulin acts on this retention mechanism is largely unclear. In this review, the current knowledge regarding the various molecular processes that govern GLUT4 physiology is discussed as well as the questions that remain.

Phosphatidylinositol 3-phosphate 5-kinase (PIKfyve) is an AMPK target participating in contraction-stimulated glucose uptake in skeletal muscle

PIKfyve (FYVE domain-containing phosphatidylinositol 3-phosphate 5-kinase), the lipid kinase that phosphorylates PtdIns3P to PtdIns(3,5)P2, has been implicated in insulin-stimulated glucose uptake. We investigated whether PIKfyve could also be involved in contraction/AMPK (AMP-activated protein kinase)-stimulated glucose uptake in skeletal muscle. Incubation of rat epitrochlearis muscles with YM201636, a selective PIKfyve inhibitor, reduced contraction- and AICAriboside (5-amino-4-imidazolecarboxamide riboside)-stimulated glucose uptake. Consistently, PIKfyve knockdown in C2C12 myotubes reduced AICAriboside-stimulated glucose transport. Furthermore, muscle contraction increased PtdIns(3,5)P2 levels and PIKfyve phosphorylation. AMPK phosphorylated PIKfyve at Ser307 both in vitro and in intact cells. Following subcellular fractionation, PIKfyve recovery in a crude intracellular membrane fraction was increased in contracting versus resting muscles. Also in opossum kidney cells, wild-type, but not S307A mutant, PIKfyve was recruited to endosomal vesicles in response to AMPK activation. We propose that PIKfyve activity is required for the stimulation of skeletal muscle glucose uptake by contraction/AMPK activation. PIKfyve is a new AMPK substrate whose phosphorylation at Ser307 could promote PIKfyve translocation to endosomes for PtdIns(3,5)P2 synthesis to facilitate GLUT4 (glucose transporter 4) translocation.

The PIKfyve-ArPIKfyve-Sac3 triad in human breast cancer: Functional link between elevated Sac3 phosphatase and enhanced proliferation of triple negative cell lines

The phosphoinositide 5-kinase PIKfyve and 5-phosphatase Sac3 are scaffolded by ArPIKfyve in the PIKfyve-ArPIKfyve-Sac3 (PAS) regulatory complex to trigger a unique loop of PtdIns3P-PtdIns(3,5)P2 synthesis and turnover. Whereas the metabolizing enzymes of the other 3-phosphoinositides have already been implicated in breast cancer, the role of the PAS proteins and the PtdIns3P-PtdIns(3,5)P2 conversion is unknown. To begin elucidating their roles, in this study we monitored the endogenous levels of the PAS complex proteins in cell lines derived from hormone-receptor positive (MCF7 and T47D) or triple-negative breast cancers (TNBC) (BT20, BT549 and MDA-MB-231) as well as in MCF10A cells derived from non-tumorigenic mastectomy. We report profound upregulation of Sac3 and ArPIKfyve in the triple negative vs. hormone-sensitive breast cancer or non-tumorigenic cells, with BT cell lines showing the highest levels. siRNA-mediated knockdown of Sac3, but not that of PIKfyve, significantly inhibited proliferation of BT20 and BT549 cells. In these cells, knockdown of ArPIKfyve had only a minor effect, consistent with a primary role for Sac3 in TNBC cell proliferation. Intriguingly, steady-state levels of PtdIns(3,5)P2 in BT20 and T47D cells were similar despite the 6-fold difference in Sac3 levels between these cell lines. However, steady-state levels of PtdIns3P and PtdIns5P, both regulated by the PAS complex, were significantly reduced in BT20 vs. T47D or MCF10A cell lines, consistent with elevated Sac3 affecting directly or indirectly the homeostasis of these lipids in TNBC. Together, our results uncover an unexpected role for Sac3 phosphatase in TNBC cell proliferation. Database analyses, discussed herein, reinforce the involvement of Sac3 in breast cancer pathogenesis.

PtdIns5P: news and views of its appearance, disappearance and deeds

Accumulated evidence indicates that PtdIns5P, one of the seven phosphoinositides, found now to be constitutively present in yeast, plants and metazoa, serves as a signaling molecule to modulate pleiotropic cellular functions in both the nucleus and the cytoplasm. The enzymatic routes in biogenesis of basal PtdIns5P have remained incompletely understood. The role for candidate kinase PIKfyve that is principally involved in PtdIns(3,5)P2 production, has been questioned. In this review article we scrutinize the past obstacles that prevented the definitive implication of PIKfyve in PtdIns5P biosynthesis from PtdIns and focus on the recent pharmacological and genetic advancements that now make this conclusion well supported. We further summarize our current knowledge of the diverse stimuli modulating PtdIns5P levels, binding partners and regulated cellular process, with particular reference to the available mechanistic insights for the relevant signaling pathways.

Muscle-specific Pikfyve gene disruption causes glucose intolerance, insulin resistance, adiposity, and hyperinsulinemia but not muscle fiber-type switching

The evolutionarily conserved kinase PIKfyve that synthesizes PtdIns5P and PtdIns(3,5)P₂ has been implicated in insulin-regulated GLUT4 translocation/glucose entry in 3T3-L1 adipocytes. To decipher PIKfyve's role in muscle and systemic glucose metabolism, here we have developed a novel mouse model with Pikfyve gene disruption in striated muscle (MPIfKO). These mice exhibited systemic glucose intolerance and insulin resistance at an early age but had unaltered muscle mass or proportion of slow/fast-twitch muscle fibers. Insulin stimulation of in vivo or ex vivo glucose uptake and GLUT4 surface translocation was severely blunted in skeletal muscle. These changes were associated with premature attenuation of Akt phosphorylation in response to in vivo insulin, as tested in young mice. Starting at 10-11 wk of age, MPIfKO mice progressively accumulated greater body weight and fat mass. Despite increased adiposity, serum free fatty acid and triglyceride levels were normal until adulthood. Together with the undetectable lipid accumulation in liver, these data suggest that lipotoxicity and muscle fiber switching do not contribute to muscle insulin resistance in MPIfKO mice. Furthermore, the 80% increase in total fat mass resulted from increased fat cell size rather than altered fat cell number. The observed profound hyperinsulinemia combined with the documented increases in constitutive Akt activation, in vivo glucose uptake, and gene expression of key enzymes for fatty acid biosynthesis in MPIfKO fat tissue suggest that the latter is being sensitized for de novo lipid anabolism. Our data provide the first in vivo evidence that PIKfyve is essential for systemic glucose homeostasis and insulin-regulated glucose uptake/GLUT4 translocation in skeletal muscle.

GLUT4 traffic through an ESCRT-III-dependent sorting compartment in adipocytes

In insulin target tissues, GLUT4 is known to traffic through multiple compartments that may involve ubiquitin- and/or SUMO-dependent targeting. During these trafficking steps, GLUT4 is sorted into a storage reservoir compartment that is acutely released by insulin signalling processes that are downstream of PI 3-kinase associated changes in inositol phospholipids. As ESCRT components have recently been found to influence cellular sorting processes that are related to changes in both ubiquitination and inositol phospholipids, we have examined whether GLUT4 traffic is routed through ESCRT dependent sorting steps. Introduction of the dominant negative inhibitory constructs of the ESCRT-III components CHMP3 (CHMP3(1–179)) and Vps4 (GFP-Vps4^E235Q) into rat adipocytes leads to the accumulation of GLUT4 in large, coalesced and extended vesicles structures that co-localise with the inhibitory constructs over large parts of the extended structure. A new swollen hybrid and extensively ubiquitinated compartment is produced in which GLUT4 co-localises more extensively with the endosomal markers including EEA1 and transferrin receptors but also with the TGN marker syntaxin6. These perturbations are associated with failure of insulin action on GLUT4 traffic to the cell surface and suggest impairment in an ESCRT-dependent sorting step used for GLUT4 traffic to its specialised reservoir compartment.

Functional dissociation between PIKfyve-synthesized PtdIns5P and PtdIns(3,5)P2 by means of the PIKfyve inhibitor YM201636

PIKfyve is an essential mammalian lipid kinase with pleiotropic cellular functions whose genetic knockout in mice leads to preimplantation lethality. Despite several reports for PIKfyve-catalyzed synthesis of phosphatidylinositol 5-phosphate (PtdIns5P) along with phosphatidylinositol-3,5-biphosphate [PtdIns(3,5)P(2)] in vitro and in vivo, the role of the PIKfyve pathway in intracellular PtdIns5P production remains underappreciated and the function of the PIKfyve-synthesized PtdIns5P pool poorly characterized. Hence, the recently discovered potent PIKfyve-selective inhibitor, the YM201636 compound, has been solely tested for inhibiting PtdIns(3,5)P(2) synthesis. Here, we have compared the in vitro and in vivo inhibitory potency of YM201636 toward PtdIns5P and PtdIns(3,5)P(2). Unexpectedly, we observed that at low doses (10-25 nM), YM201636 inhibited preferentially PtdIns5P rather than PtdIns(3,5)P(2) production in vitro, whereas at higher doses, the two products were similarly inhibited. In cellular contexts, YM201636 at 160 nM inhibited PtdIns5P synthesis twice more effectively compared with PtdIns(3,5)P(2) synthesis. In 3T3L1 adipocytes, human embryonic kidney 293 and Chinese hamster ovary (CHO-T) cells, levels of PtdIns5P dropped by 62-71% of the corresponding untreated controls, whereas those of PtdIns(3,5)P(2) fell by only 28-46%. The preferential inhibition of PtdIns5P versus PtdIns(3,5)P(2) at low doses of YM201636 was explored to probe contributions of the PIKfyve-catalyzed PtdIns5P pool to insulin-induced actin stress fiber disassembly in CHO-T cells, GLUT4 translocation in 3T3L1 adipocytes, and induction of aberrant cellular vacuolation in these or other cell types. The results provide the first experimental evidence that the principal pathway for PtdIns5P intracellular production is through PIKfyve and that insulin effect on actin stress fiber disassembly is mediated entirely by the PIKfyve-produced PtdIns5P pool.

Phosphoinositides in neuroexocytosis and neuronal diseases

Phosphoinositides (PIs) are a family of phospholipids derived from phosphatidylinositol (PtdIns), whose location, synthesis, and degradation depend on specific PI kinases and phosphatases. PIs have emerged as fundamental regulators of secretory processes, such as neurotransmitter release, hormone secretion, and histamine release in allergic responses. In neurons and neuroendocrine cells, regulated secretion requires the calcium-dependent fusion of transmitter-containing vesicles with the plasma membrane. The role played by PIs in exocytosis is best exemplified by the Ca²⁺-dependent binding of vesicular Synaptotagmin1 to the plasma membrane PtdIns(4,5)P₂, and the recently demonstrated role of PtdIns(4,5)P₂ in the mobilization of secretory vesicles to the plasma membrane. New evidence has also recently emerged of an alternative PI pathway that can control exocytosis positively (via PtdIn3P) or negatively (via PtdIns(3,5)P₂). However, the positive or negative effectors for these pathways remain to be established. Reducing PtdIns(3,5)P₂ potentiates neuroexocytosis but leads to neuronal degeneration and has been linked to certain forms of Charcot-Marie-Tooth disease and amyotrophic lateral sclerosis. The goal of this review is to describe the role of PIs in neuroexocytosis and explore the current hypotheses linking these effects to 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.

PIKfyve and its Lipid products in health and in sickness

PIKfyve, a phosphoinositide 5-kinase synthesizing PtdIns(3,5)P₂ and PtdIns5P in a cellular context, belongs to an evolutionarily ancient gene family of PtdIns(3,5)P₂-synthesizing enzymes that, except for plants, are products of a single-copy gene across species. In the dozen years after its discovery, enormous progress has been made in characterizing the numerous PIKfyve cellular functions and the regulatory mechanisms that govern these functions. It became clear that PIKfyve does not act alone but, rather, it engages the scaffolding regulator ArPIKfyve and the phosphatase Sac3 to make a multiprotein "PAS" complex, so called for the first letters of the protein names. This complex relays antagonistic signals, one for synthesis, another for turnover of PtdIns(3,5)P₂, whose dysregulated coordination is linked to several human diseases. The physiological significance for each protein in the PAS complex is underscored by the early lethality of the mouse models with disruption in any of the three genes. This chapter summarizes our current knowledge of the diverse and complex functionality of PIKfyve and PtdIns(3,5)P₂/PtdIns5P products with particular highlights on recent discoveries of inherited or somatic mutations in PIKfyve and Sac3 linked to human disorders.

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.

ArPIKfyve regulates Sac3 protein abundance and turnover: disruption of the mechanism by Sac3I41T mutation causing Charcot-Marie-Tooth 4J disorder

The mammalian phosphatidylinositol (3,5)-bisphosphate (PtdIns(3,5)P(2)) phosphatase Sac3 and ArPIKfyve, the associated regulator of the PtdIns3P-5 kinase PIKfyve, form a stable binary complex that associates with PIKfyve in a ternary complex to increase PtdIns(3,5)P(2) production. Whether the ArPIKfyve-Sac3 subcomplex functions outside the PIKfyve context is unknown. Here we show that stable or transient expression of ArPIKfyve(WT) in mammalian cells elevates steady-state protein levels and the PtdIns(3,5)P(2)-hydrolyzing activity of Sac3, whereas knockdown of ArPIKfyve has the opposite effect. These manipulations do not alter the Sac3 mRNA levels, suggesting that ArPIKfyve might control Sac3 protein degradation. Inhibition of protein synthesis in COS cells by cycloheximide reveals remarkably rapid turnover of expressed Sac3(WT) (t((1/2)) = 18.8 min), resulting from a proteasome-dependent clearance as evidenced by the extended Sac3(WT) half-life upon inhibiting proteasome activity. Coexpression of ArPIKfyve(WT), but not the N- or C-terminal halves, prolongs the Sac3(WT) half-life consistent with enhanced Sac3 protein stability through association with full-length ArPIKfyve. We further demonstrate that mutant Sac3, harboring the pathogenic Ile-to-Thr substitution at position 41 found in patients with CMT4J disorder, is similar to Sac3(WT) with regard to PtdIns(3,5)P(2)-hydrolyzing activity, association with ArPIKfyve, or rapid proteasome-dependent clearance. Remarkably, however, neither is the steady-state Sac3(I41T) elevated nor is the Sac3(I41T) half-life extended by coexpressed ArPIKfyve(WT), indicating that unlike with Sac3(WT), ArPIKfyve fails to prevent Sac3(I41T) rapid loss. Together, our data indentify a novel regulatory mechanism whereby ArPIKfyve enhances Sac3 abundance by attenuating Sac3 proteasome-dependent degradation and suggest that a failure of this mechanism could be the primary molecular defect in the pathogenesis of CMT4J.

PIKfyve-ArPIKfyve-Sac3 core complex: contact sites and their consequence for Sac3 phosphatase activity and endocytic membrane homeostasis

The phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P(2)) metabolizing enzymes, the kinase PIKfyve and the phosphatase Sac3, constitute a single multiprotein complex organized by the PIKfyve regulator ArPIKfyve and its ability to homodimerize. We previously established that PIKfyve is activated within the triple PIKfyve-ArPIKfyve-Sac3 (PAS) core. These data assign an atypical function for the phosphatase in PtdIns(3,5)P(2) biosynthesis, thus raising the question of whether Sac3 retains its PtdIns(3,5)P(2) hydrolyzing activity within the PAS complex. Herein, we address the issue of Sac3 functionality by a combination of biochemical and morphological assays in triple-transfected COS cells using a battery of truncated or point mutants of the three proteins. We identified the Cpn60_TCP1 domain of PIKfyve as a major determinant for associating the ArPIKfyve-Sac3 subcomplex. Neither Sac3 nor PIKfyve enzymatic activities affected the PAS complex formation or stability. Using the well established formation of aberrant cell vacuoles as a sensitive functional measure of localized PtdIns(3,5)P(2) reduction, we observed a mitigated vacuolar phenotype by kinase-deficient PIKfyve(K1831E) if its ArPIKfyve-Sac3 binding region was deleted, suggesting reduced Sac3 access to, and turnover of PtdIns(3,5)P(2). In contrast, PIKfyve(K1831E), which displays intact ArPIKfyve-Sac3 binding, triggered a more severe vacuolar phenotype if coexpressed with ArPIKfyve(WT)-Sac3(WT) but minimal defects when coexpressed with ArPIKfyve(WT) and phosphatase-deficient Sac3(D488A). These data indicate that Sac3 assembled in the PAS regulatory core complex is an active PtdIns(3,5)P(2) phosphatase. Based on these and other data, presented herein, we propose a model of domain interactions within the PAS core and their role in regulating the enzymatic activities.

Arabidopsis FAB1/PIKfyve proteins are essential for development of viable pollen

Phosphatidylinositol 3,5-bisphosphate [PtdIns(3,5)P(2)] is a phospholipid that has a role in controlling membrane trafficking events in yeast and animal cells. The function of this lipid in plants is unknown, although its synthesis has been shown to be up-regulated upon osmotic stress in plant cells. PtdIns(3,5)P(2) is synthesized by the PIKfyve/Fab1 family of proteins, with two orthologs, FAB1A and FAB1B, being present in Arabidopsis (Arabidopsis thaliana). In this study, we attempt to address the role of this lipid by analyzing the phenotypes of plants mutated in FAB1A and FAB1B. It was not possible to generate plants homozygous for mutations in both genes, although single mutants were isolated. Both homozygous single mutant plant lines exhibited a leaf curl phenotype that was more marked in FAB1B mutants. Genetic transmission analysis revealed that failure to generate double mutant lines was entirely due to inviability of pollen carrying mutant alleles of both FAB1A and FAB1B. This pollen displayed severe defects in vacuolar reorganization following the first mitotic division of development. The presence of abnormally large vacuoles in pollen at the tricellular stage resulted in the collapse of the majority of grains carrying both mutant alleles. This demonstrates a crucial role for PtdIns(3,5)P(2) in modulating the dynamics of vacuolar rearrangement essential for successful pollen development. Taken together, our results are consistent with PtdIns(3,5)P(2) production being central to cellular responses to changes in osmotic conditions.

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.

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.

YM201636, an inhibitor of retroviral budding and PIKfyve-catalyzed PtdIns(3,5)P2 synthesis, halts glucose entry by insulin in adipocytes

Silencing of PIKfyve, the sole enzyme for PtdIns(3,5)P(2) biosynthesis that controls proper endosome dynamics, inhibits retroviral replication. A novel PIKfyve-specific inhibitor YM201636 disrupts retroviral budding at 800 nM, suggesting its potential use as an antiretroviral therapeutic. Because PIKfyve is also required for optimal insulin activation of GLUT4 surface translocation and glucose influx, we tested the outcome of YM201636 application on insulin responsiveness in 3T3L1 adipocytes. YM201636 almost completely inhibited basal and insulin-activated 2-deoxyglucose uptake at doses as low as 160 nM, with IC(50)=54+/-4 nM for the net insulin response. Insulin-induced GLUT4 translocation was partially inhibited at substantially higher doses, comparable to those required for inhibition of insulin-induced phosphorylation of Akt/PKB. In addition to PIKfyve, YM201636 also completely inhibited insulin-dependent activation of class IA PI 3-kinase. We suggest that apart from PIKfyve, there are at least two additional targets for YM201636 in the context of insulin signaling to GLUT4 and glucose uptake: the insulin-activated class IA PI 3-kinase and a here-unidentified high-affinity target responsible for the greater inhibition of glucose entry vs. GLUT4 translocation. The profound inhibition of the net insulin effect on glucose influx at YM201636 doses markedly lower than those required for efficient retroviral budding disruption warns of severe perturbations in glucose homeostasis associated with potential YM201636 use in antiretroviral therapy.

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.

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.

Phosphoinositides in insulin action on GLUT4 dynamics: not just PtdIns(3,4,5)P3

Accumulated evidence over the last several years indicates that insulin regulates multiple steps in the overall translocation of GLUT4 vesicles to the fat/muscle cell surface, including formation of an intracellular storage pool of GLUT4 vesicles, its movement to the proximity of the cell surface, and the subsequent docking/fusion with the plasma membrane. Insulin-stimulated formation of phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3); and in some cases, of its catabolite PtdIns(3,4)P(2)] plays a pivotal role in this process. PtdIns(3,4,5)P(3) is synthesized by the activated wortmannin-sensitive class IA phosphoinositide (PI) 3-kinase and controls the rate-limiting cell surface terminal stages of the GLUT4 journey. However, recent research is consistent with the conclusion that signals by each of the remaining five PIs, i.e., PtdIns(3)P, PtdIns(4)P, PtdIns(5)P, PtdIns(3,5)P(2), and PtdIns(4,5)P(2), may act in concert with that of PtdIns(3,4,5)P(3) in integrating the insulin receptor-issued signals with GLUT4 surface translocation and glucose transport activation. This review summarizes the experimental evidence supporting the complementary function of these PIs in insulin responsiveness of fat and muscle cells, with particular reference to mechanistic insights and functional significance in the regulation of overall GLUT4 vesicle dynamics.

Insulin action on glucose transporters through molecular switches, tracks and tethers

Glucose entry into muscle cells is precisely regulated by insulin, through recruitment of GLUT4 (glucose transporter-4) to the membrane of muscle and fat cells. Work done over more than two decades has contributed to mapping the insulin signalling and GLUT4 vesicle trafficking events underpinning this response. In spite of this intensive scientific research, there are outstanding questions that continue to challenge us today. The present review summarizes the knowledge in the field, with emphasis on the latest breakthroughs in insulin signalling at the level of AS160 (Akt substrate of 160 kDa), TBC1D1 (tre-2/USP6, BUB2, cdc16 domain family member 1) and their target Rab proteins; in vesicle trafficking at the level of vesicle mobilization, tethering, docking and fusion with the membrane; and in the participation of the cytoskeleton to achieve optimal temporal and spatial location of insulin-derived signals and GLUT4 vesicles.

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.

Core protein machinery for mammalian phosphatidylinositol 3,5-bisphosphate synthesis and turnover that regulates the progression of endosomal transport. Novel Sac phosphatase joins the ArPIKfyve-PIKfyve complex

Perturbations in phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2)-synthesizing enzymes result in enlarged endocytic organelles from yeast to humans, indicating evolutionarily conserved function of PtdIns(3,5)P2 in endosome-related events. This is reinforced by the structural and functional homology of yeast Vac14 and human Vac14 (ArPIKfyve), which activate yeast and mammalian PtdIns(3,5)P2-producing enzymes, Fab1 and PIKfyve, respectively. In yeast, PtdIns(3,5)P2-specific phosphatase, Fig4, in association with Vac14, turns over PtdIns(3,5)P2, but whether such a mechanism operates in mammalian cells and what the identity of mammalian Fig4 may be are unknown. Here we have identified and characterized Sac3, a Sac domain phosphatase, as the Fig4 mammalian counterpart. Endogenous Sac3, a widespread 97-kDa protein, formed a stable ternary complex with ArPIKfyve and PIKfyve. Concordantly, Sac3 cofractionated and colocalized with ArPIKfyve and PIKfyve. The intrinsic Sac3(WT) phosphatase activity preferably hydrolyzed PtdIns(3,5)P2 in vitro, although the other D5-phosphorylated polyphosphoinositides were also substrates. Ablation of endogenous Sac3 by short interfering RNAs elevated PtdIns(3,5)P2 in (32)P-labeled HEK293 cells. Ectopically expressed Sac3(WT) in COS cells colocalized with and dilated EEA1-positive endosomes, consistent with the PtdIns(3,5)P2 requirement in early endosome dynamics. In vitro reconstitution of carrier vesicle formation from donor early endosomes revealed a gain of function upon Sac3 loss, whereas PIKfyve or ArPIKfyve protein depletion produced a loss of function. These data demonstrate a coupling between the machinery for PtdIns(3,5)P2 synthesis and turnover achieved through a physical assembly of PIKfyve, ArPIKfyve, and Sac3. We suggest that the tight regulation in PtdIns(3,5)P2 homeostasis is mechanistically linked to early endosome dynamics in the course of cargo transport.