Phosphoinositide-derived messengers in endocrine signaling

Mitochondrial metabolic flexibility is critical for CD8+ T cell antitumor immunity

Mitochondria use different substrates for energy production and intermediatory metabolism according to the availability of nutrients and oxygen levels. The role of mitochondrial metabolic flexibility for CD8+ T cell immune response is poorly understood. Here, we report that the deletion or pharmacological inhibition of protein tyrosine phosphatase, mitochondrial 1 (PTPMT1) significantly decreased CD8+ effector T cell development and clonal expansion. In addition, PTPMT1 deletion impaired stem-like CD8+ T cell maintenance and accelerated CD8+ T cell exhaustion/dysfunction, leading to aggravated tumor growth. Mechanistically, the loss of PTPMT1 critically altered mitochondrial fuel selection-the utilization of pyruvate, a major mitochondrial substrate derived from glucose-was inhibited, whereas fatty acid utilization was enhanced. Persistent mitochondrial substrate shift and metabolic inflexibility induced oxidative stress, DNA damage, and apoptosis in PTPMT1 knockout cells. Collectively, this study reveals an important role of PTPMT1 in facilitating mitochondrial utilization of carbohydrates and that mitochondrial flexibility in energy source selection is critical for CD8+ T cell antitumor immunity.

Loss of Ptpmt1 limits mitochondrial utilization of carbohydrates and leads to muscle atrophy and heart failure in tissue-specific knockout mice

While mitochondria in different tissues have distinct preferences for energy sources, they are flexible in utilizing competing substrates for metabolism according to physiological and nutritional circumstances. However, the regulatory mechanisms and significance of metabolic flexibility are not completely understood. Here, we report that the deletion of Ptpmt1, a mitochondria-based phosphatase, critically alters mitochondrial fuel selection - the utilization of pyruvate, a key mitochondrial substrate derived from glucose (the major simple carbohydrate), is inhibited, whereas the fatty acid utilization is enhanced. Ptpmt1 knockout does not impact the development of the skeletal muscle or heart. However, the metabolic inflexibility ultimately leads to muscular atrophy, heart failure, and sudden death. Mechanistic analyses reveal that the prolonged substrate shift from carbohydrates to lipids causes oxidative stress and mitochondrial destruction, which in turn results in marked accumulation of lipids and profound damage in the knockout muscle cells and cardiomyocytes. Interestingly, Ptpmt1 deletion from the liver or adipose tissue does not generate any local or systemic defects. These findings suggest that Ptpmt1 plays an important role in maintaining mitochondrial flexibility and that their balanced utilization of carbohydrates and lipids is essential for both the skeletal muscle and the heart despite the two tissues having different preferred energy sources.

Regulation of Phosphoinositide Signaling by Scaffolds at Cytoplasmic Membranes

Cytoplasmic phosphoinositides (PI) are critical regulators of the membrane-cytosol interface that control a myriad of cellular functions despite their low abundance among phospholipids. The metabolic cycle that generates different PI species is crucial to their regulatory role, controlling membrane dynamics, vesicular trafficking, signal transduction, and other key cellular events. The synthesis of phosphatidylinositol (3,4,5)-triphosphate (PI3,4,5P3) in the cytoplamic PI3K/Akt pathway is central to the life and death of a cell. This review will focus on the emerging evidence that scaffold proteins regulate the PI3K/Akt pathway in distinct membrane structures in response to diverse stimuli, challenging the belief that the plasma membrane is the predominant site for PI3k/Akt signaling. In addition, we will discuss how PIs regulate the recruitment of specific scaffolding complexes to membrane structures to coordinate vesicle formation, fusion, and reformation during autophagy as well as a novel lysosome repair pathway.

Divergence in interspecific and intersubspecific gene expression between two closely related horseshoe bats (Rhinolophus)

Closely related species have been used as representative systems to investigate the genetic mechanisms involved in the early stages of species differentiation. Previous studies have indicated that variation in gene expression might be a sensitive indicator of initial species divergence, although the role of expression divergence, and especially that associated with phenotypic variation remained relatively undefined. For three organs (cochlea, brain, and liver) from two closely related bat species (Rhinolophus siamensis and R. episcopus), the interspecific and intersubspecific gene expression profiles were compared using transcriptomics in this study. Striking organ specificity of expression was observed, and expression profiles exhibited similarities between cochlea and brain tissues. Numerous differentially expressed genes (DEGs) were identified for each organ in the interspecific comparison (cochlea/brain/liver: 1,069/647/692) and intersubspecific comparison (608/528/368). Functional enrichment analysis indicated vital variation in expression related to the immune system, ion activities, neuronal function, and multisensory system regulation in both comparisons. DEGs relevant to the variation in echolocation calls (RF) were found, and some of them were involved in the pivotal patterns of expression variation. The regulation of immune, ion channel, neural activity, and sophisticated sensory functions at the expression level might be key mechanisms in the early species divergence of bats, and the expression variation related to acoustical signal could have played a crucial part. This study expands our knowledge of gene expression and patterns of variation for three key organs to echolocation at both the interspecific and intersubspecific levels. Further, the framework described here provides insight into the genetic basis of phenotypic variation during the incipient stage of species differentiation.

Myosin light chain kinase regulates intestinal permeability of mucosal homeostasis in Crohn's disease

Introduction: Researchers have investigated the potential role of intestinal permeability in Crohn's disease pathogenesis. Intestinal permeability is usually mediated by cytoskeleton and intercellular junctions. The myosin light chain kinase (MLCK) is an enzyme that activates the myosin light chain to exert its function related to cytoskeleton contraction and tight junction regulation. The correlation between MLCK and Crohn's disease pathogenesis has been consistently proven. Areas covered: This study aims to expand the understanding of the regulation and function of MLCK in Crohn's disease. An extensive literature search in the MEDLINE database (via PubMed) has been performed up to Oct. 2020. The roles of MLCK in tight junction activation, intestinal permeability enhancement, and cell signal regulation are comprehensively discussed. Expert opinion: Targeting the MLCK-related pathways such as TNF-α in CD treatment has been put into clinical use. More accurate targeting such as MLCK and TNFR2 has been proposed to reduce side effects. MLCK may also have the potential to become biomarkers in fields like CD activity. With the application of cutting age research methods and tools, the MLCK research could be accelerated.

The insulin-sensitizing mechanism of myo-inositol is associated with AMPK activation and GLUT-4 expression in human endometrial cells exposed to a PCOS environment

Polycystic ovary syndrome (PCOS) is an endocrine-metabolic disorder characterized by hyperandrogenism and ovulatory dysfunction but also obesity and hyperinsulinemia. These characteristics induce an insulin-resistant state in tissues such as the endometrium, affecting its reproductive functions. Myo-inositol (MYO) is an insulin-sensitizing compound used in PCOS patients; however, its insulin-sensitizing mechanism is unclear. To understand the relationship of MYO with insulin action in endometrial cells, sodium/myo-inositol transporter 1 (SMIT-1) (MYO-transporter), and MYO effects on protein levels related to the insulin pathway were evaluated. SMIT-1 was assessed in endometrial tissue from women with normal weight, obesity, insulin resistance, and PCOS; additionally, using an in vitro model of human endometrial cells exposed to an environment resembling hyperinsulinemic-obese-PCOS, MYO effect was evaluated on p-AMPK and GLUT-4 levels and glucose uptake by Western blot, immunocytochemistry, and confocal microscopy, respectively. SMIT-1 was detected in endometrial tissue from all groups and decreased in PCOS and obesity (P < 0.05 vs. normal weight). In the in vitro model, PCOS conditions decreased p-AMPK levels, while they were restored with MYO (P < 0.05). The diminished GLUT-4 protein levels promoted by PCOS environment were restored by MYO through SMIT-1 and p-AMPK-dependent mechanism (P < 0.05). Also, MYO restored glucose uptake in cells under PCOS condition through a p-AMPK-dependent mechanism. Finally, these results were similar to those obtained with metformin treatment in the same in vitro conditions. Consequently, MYO could be a potential insulin sensitizer through its positive effects on insulin-resistant tissues as PCOS-endometrium, acting through SMIT-1, provoking AMPK activation and elevated GLUT-4 levels and, consequently, increase glucose uptake by human endometrial cells. Therefore, MYO may be used as an effective treatment option in insulin-resistant PCOS women.

The HCV Replicase Complex and Viral RNA Synthesis

Replication of hepatitis C virus (HCV) is tightly linked to membrane alterations designated the membranous web, harboring the viral replicase complex. In this chapter we describe the morphology and 3D architecture of the HCV-induced replication organelles, mainly consisting of double membrane vesicles, which are generated by a concerted action of the nonstructural proteins NS3 to NS5B. Recent studies have furthermore identified a number of host cell proteins and lipids contributing to the biogenesis of the membranous web, which are discussed in this chapter. Viral RNA synthesis is tightly associated with these membrane alterations and mainly driven by the viral RNA dependent RNA polymerase NS5B. We summarize our current knowledge of the structure and function of NS5B, the role of cis-acting replication elements at the termini of the genome in regulating RNA synthesis and the contribution of additional viral and host factors to viral RNA synthesis, which is still ill defined.

Plant phosphoinositide signaling - dynamics on demand

Eukaryotic membranes contain small amounts of lipids with regulatory roles. An important class of such regulatory lipids are phosphoinositides (PIs). Within membranes, PIs serve as recruitment signals, as regulators of membrane protein function or as precursors for second messenger production, thereby influencing a multitude of cellular processes with key importance for plant function and development. Plant PIs occur locally and transiently within membrane microdomains, and their abundance is strictly controlled. To understand the functions of the plant PI-network it is important to understand not only downstream PI-effects, but also to identify and characterize factors contributing to dynamic PI formation. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

Role of PI4K and PI3K-AKT in ERK1/2 activation by GnRH in the pituitary gonadotropes

The role of PI4K and PI3K-AKT in ERK1/2 activation by GnRH was examined. A relatively long preincubation (60 min) with wortmannin (10 nM and 10 μM), and LY294002 (10 μM and 100 μM) (doses known to inhibit PI3K and PI4K, respectively), were required to inhibit GnRH-and PMA-stimulated ERK1/2 activity in αT3-1 and LβT2 gonadotrope cells. A similar preincubation protocol was required to demonstrate inhibition of IGF-1-stimulated AKT activation lending support for the need of prolonged incubation (60 min) with wortmannin in contrast to other cellular systems. To rule out that the inhibitors acted upon PI(4,5)P2 levels, we followed the [Ca(2+)]i response to GnRH and found that wortmannin has no significant effect on GnRH-induced [Ca(2+)]i responses. Surprisingly, GnRH and PMA reduced, while IGF-1 increased AKT phosphorylation. We suggest that PI3K inhibits GnRH-stimulated αGSU activity, has no effect upon GnRH-stimulated LHβ activity and enhanced the GnRH-stimulated FSHβ transcription. Hence, PI4K and PI3K-AKT play a role in GnRH to ERK1/2 signaling, while PI3K may regulate also GnRH-induced gonadotropin gene expression.

Regulation of PLCβ2 by the electrostatic and mechanical properties of lipid bilayers

Phosphoinositide-specific phospholipase C (PLC) is an important family of enzymes constituting a junction between phosphoinositide lipid signaling and the trans-membrane signal transduction processes that are crucial to many living cells. However, the regulatory mechanism of PLC is not yet understood in detail. To address this issue, activity studies were carried out using lipid vesicles in a model system that was specifically designed to study protein-protein and lipid-protein interactions in concert. Evidence was found for a direct interaction between PLC and the GTPases that mediate phospholipase activation. Furthermore, for the first time, the relationships between PLC activity and substrate presentation in lipid vesicles of various sizes, as well as lipid composition and membrane mechanical properties, were analyzed. PLC activity was found to depend upon the electrostatic potential and the stored curvature elastic stress of the lipid membranes.

Plant phosphoinositides-complex networks controlling growth and adaptation

Plants differ in many ways from mammals or yeast. However, plants employ phosphoinositides for the regulation of essential cellular functions as do all other eukaryotes. In recent years the plant phosphoinositide system has been linked to the control of cell polarity. Phosphoinositides are also implicated in plant adaptive responses to changing environmental conditions. The current understanding is that plant phosphoinositides control membrane trafficking, ion channels and the cytoskeleton in similar ways as in other eukaryotic systems, but adapted to meet plant cellular requirements and with some plant-specific features. In addition, the formation of soluble inositol polyphosphates from phosphoinositides is important for the perception of important phytohormones, as the relevant receptor proteins contain such molecules as structural cofactors. Overall, the essential nature of phosphoinositides in plants has been established. Still, the complexity of the phosphoinositide networks in plant cells is only emerging and invites further study of its molecular details. This article is part of a special issue entitled Phosphoinositides.

Low and high dietary protein:carbohydrate ratios during pregnancy affect materno-fetal glucose metabolism in pigs

Inadequate dietary protein during pregnancy causes intrauterine growth retardation. Whether this is related to altered maternal and fetal glucose metabolism was examined in pregnant sows comparing a high-protein:low-carbohydrate diet (HP-LC; 30% protein, 39% carbohydrates) with a moderately low-protein:high-carbohydrate diet (LP-HC; 6.5% protein, 68% carbohydrates) and the isoenergetic standard diet (ST; 12.1% protein, 60% carbohydrates). During late pregnancy, maternal and umbilical glucose metabolism and fetal hepatic mRNA expression of gluconeogenic enzymes were examined. During an i.v. glucose tolerance test (IVGTT), the LP-HC-fed sows had lower insulin concentrations and area under the curve (AUC), and higher glucose:insulin ratios than the ST- and the HP-LC-fed sows (P < 0.05). Insulin sensitivity and glucose clearance were higher in the LP-HC sows compared with ST sows (P < 0.05). Glucagon concentrations during postabsorptive conditions and IVGTT, and glucose AUC during IVGTT, were higher in the HP-LC group compared with the other groups (P < 0.001). (13)C glucose oxidation was lower in the HP-LC sows than in the ST and LP-HC sows (P < 0.05). The HP-LC fetuses were lighter and had a higher brain:liver ratio than the ST group (P < 0.05). The umbilical arterial inositol concentration was greater in the HP-LC group (P < 0.05) and overall small fetuses (230-572 g) had higher values than medium and heavy fetuses (≥573 g) (P < 0.05). Placental lactate release was lower in the LP-HC group than in the ST group (P < 0.05). Fetal glucose extraction tended to be lower in the LP-HC group than in the ST group (P = 0.07). In the HP-LC and LP-HC fetuses, hepatic mRNA expression of cytosolic phosphoenolpyruvate carboxykinase (PCK1) and glucose-6-phosphatase (G6PC) was higher than in the ST fetuses (P < 0.05). In conclusion, the HP-LC and LP-HC sows adapted by reducing glucose turnover and oxidation and having higher glucose utilization, respectively. The HP-LC and LP-HC fetuses adapted via prematurely expressed hepatic gluconeogenic enzymes.

Phosphatidylinositol 4,5-bisphosphate influences PIN polarization by controlling clathrin-mediated membrane trafficking in Arabidopsis

The functions of the minor phospholipid phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2] during vegetative plant growth remain obscure. Here, we targeted two related phosphatidylinositol 4-phosphate 5-kinases (PI4P 5-kinases) PIP5K1 and PIP5K2, which are expressed ubiquitously in Arabidopsis thaliana. A pip5k1 pip5k2 double mutant with reduced PtdIns(4,5)P2 levels showed dwarf stature and phenotypes suggesting defects in auxin distribution. The roots of the pip5k1 pip5k2 double mutant had normal auxin levels but reduced auxin transport and altered distribution. Fluorescence-tagged auxin efflux carriers PIN-FORMED (PIN1)-green fluorescent protein (GFP) and PIN2-GFP displayed abnormal, partially apolar distribution. Furthermore, fewer brefeldin A-induced endosomal bodies decorated by PIN1-GFP or PIN2-GFP formed in pip5k1 pip5k2 mutants. Inducible overexpressor lines for PIP5K1 or PIP5K2 also exhibited phenotypes indicating misregulation of auxin-dependent processes, and immunolocalization showed reduced membrane association of PIN1 and PIN2. PIN cycling and polarization require clathrin-mediated endocytosis and labeled clathrin light chain also displayed altered localization patterns in the pip5k1 pip5k2 double mutant, consistent with a role for PtdIns(4,5)P2 in the regulation of clathrin-mediated endocytosis. Further biochemical tests on subcellular fractions enriched for clathrin-coated vesicles (CCVs) indicated that pip5k1 and pip5k2 mutants have reduced CCV-associated PI4P 5-kinase activity. Together, the data indicate an important role for PtdIns(4,5)P2 in the control of clathrin dynamics and in auxin distribution in Arabidopsis.

Combined treatment with Myo-inositol and selenium ensures euthyroidism in subclinical hypothyroidism patients with autoimmune thyroiditis

Background. Hashimoto's thyroiditis (HT), also known as chronic lymphocytic thyroiditis or chronic autoimmune thyroiditis, is the most common form of thyroiditis affecting more than 10% of females and 2% of males. The present study aims to evaluate the beneficial effect of a combined treatment, Myo-Inositol plus selenomethionine, on subclinical hypothyroidism. Methods. The study was designed as a double-blind randomized controlled trial. Eligible patients were women diagnosed with subclinical hypothyroidism having Tg antibodies (TgAb) titer higher than 350 IU/mL. Outcome measures were Thyroid Stimulating Hormone (TSH) levels, thyroid peroxidase antibodies (TPOAb) and TgAb titer, selenium, and Myo-Inositol plasma concentration. Results. In the present paper, we demonstrated that the beneficial effects obtained by selenomethionine treatment on patients affected by subclinical hypothyroidism, likely due to the presence of autoantibody (TPOAb and TgAb), are further improved by cotreatment with Myo-Inositol. Conclusions. Indeed, due to its action as TSH second messenger, Myo-Inositol treatment reduces TSH levels closer to physiological concentrations.

Arranged marriage in lipid signalling? The limited choices of PtdIns(4,5)P2 in finding the right partner

Inositol-containing phospholipids (phosphoinositides, PIs) control numerous cellular processes in eukaryotic cells. For plants, a key involvement of PIs has been demonstrated in the regulation of membrane trafficking, cytoskeletal dynamics and in processes mediating the adaptation to changing environmental conditions. Phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P(2)) mediates its cellular functions via binding to various alternative target proteins. Such downstream targets of PtdIns(4,5)P(2) are characterised by the possession of specific lipid-binding domains, and binding of the PtdIns(4,5)P(2) ligand exerts effects on their activity or localisation. The large number of potential alternative binding partners - and associated cellular processes - raises the question how alternative or even contrapuntal effects of PtdIns(4,5)P(2) are orchestrated to enable cellular function. This article aims to provide an overview of recent insights and new views on how distinct functional pools of PtdIns(4,5)P(2) are generated and maintained. The emerging picture suggests that PtdIns(4,5)P(2) species containing different fatty acids influence the lateral mobility of the lipids in the membrane, possibly enabling specific interactions of PtdIns(4,5)P(2) pools with certain downstream targets. PtdIns(4,5)P(2) pools with certain functions might also be defined by protein-protein interactions of PI4P 5-kinases, which pass PtdIns(4,5)P(2) only to certain downstream partners. Individually or in combination, PtdIns(4,5)P(2) species and specific protein-protein interactions of PI4P 5-kinases might contribute to the channelling of PtdIns(4,5)P(2) signals towards specific functional effects. The dynamic nature of PI-dependent signalling complexes with specific functions is an added challenge for future studies of plant PI signalling.

Vps34 deficiency reveals the importance of endocytosis for podocyte homeostasis

The molecular mechanisms that maintain podocytes and consequently, the integrity of the glomerular filtration barrier are incompletely understood. Here, we show that the class III phosphoinositide 3-kinase vacuolar protein sorting 34 (Vps34) plays a central role in modulating endocytic pathways, maintaining podocyte homeostasis. In mice, podocyte-specific conditional knockout of Vps34 led to early proteinuria, glomerular scarring, and death within 3-9 weeks of age. Vps34-deficient podocytes exhibited substantial vacuolization and foot process effacement. Although the formation of autophagosomes and autophagic flux were impaired, comparisons between podocyte-specific Vps34-deficient mice, autophagy-deficient mice, and doubly deficient mice suggested that defective autophagy was not primarily responsible for the severe phenotype caused by the loss of Vps34. In fact, Rab5-positive endosomal compartments, endocytosis, and fluid-phase uptake were severely disrupted in Vps34-deficient podocytes. Vps34 deficiency in nephrocytes, the podocyte-like cells of Drosophila melanogaster, resulted in a block between Rab5- and Rab7-positive endosomal compartments. In summary, these data identify Vps34 as a major regulator of endolysosomal pathways in podocytes and underline the fundamental roles of endocytosis and fluid-phase uptake for the maintenance of the glomerular filtration barrier.

Towards understanding the function of stress-inducible PtdIns(4,5)P(2) in plants

The phosphoinositide (PI) system has been conserved in evolution between all eukaryotic kingdoms. Studies on mammalian and yeast cells indicate that cellular functions regulated by PIs include the production of soluble inositolpolyphosphates with signaling functions as well as recruitment of proteins required for endo- or exocytosis. In contrast to other models, knowledge on PI functions in plants is limited and, despite of reports of transient PI-increases upon stress-treatments, plant cellular processes involving changes in PI-levels have remained unclear. In previous studies various groups have proposed that PI-increases upon hyperosmotic stress support the generation of soluble second messengers with possible roles in stress adaptation. Based on a combination of biochemical analysis and imaging of fluorescent reporters we have now demonstrated that intact phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P(2)) associates with clathrin-coated vesicles (CCVs) in a stress-inducible manner in plant cells. In analogy to previous studies on other models, association with CCVs suggests a role for PtdIns(4,5)P(2) in the recruitment of vesicle coat proteins and in membrane internalization that is alternative to functions in second messenger production. The determination of subcellular sites of PtdIns(4,5)P(2) increases, thus, opens new avenues of investigation in the plant PI-field and allows development of testable hypotheses to delineate PI-functions in plants.

Leptin triggers Ca(2+) imbalance in monocytes of overweight subjects

Obesity is a major risk factor in numerous diseases, in which elevated intracellular Ca(2+) plays a major role in increased adiposity. We examined the difference between Ca(2+) signals in monocytes of lean and overweight subjects and the relationship between leptin induced NADPH oxidase activation and intracellular calcium concentration [Ca(2+)](i) homeostasis. Our results are as follows: (1) The basal level of [Ca(2+)](i) in resting monocytes of overweight subjects (OW monocytes) was higher than that in control cells, whereas the leptin-induced peak of the Ca(2+) signal was lower and the return to basal level was delayed. (2) Ca(2+) signals were more pronounced in OW monocytes than in control cells. (3) Using different inhibitors of cellular signaling, we found that in control cells the Ca(2+) signals originated from intracellular pools, whereas in OW cells they were generated predominantly by Ca(2+)-influx from medium. Finally, we found correlation between leptin induced superoxide anion generation and Ca(2+) signals. The disturbed [Ca(2+)](i) homeostasis in OW monocytes was fully restored in the presence of fluvastatin. Statins have pleiotropic effects involving the inhibition of free radical generation that may account for its beneficial effect on elevated [Ca(2+)](i) and consequently on the pathomechanism of obesity.

Modulation of synaptic function by VAC14, a protein that regulates the phosphoinositides PI(3,5)P₂ and PI(5)P

Mice deficient for VAC14, a scaffolding protein required for PIP2 biosynthesis and linked to human neuropathies, show increased postsynaptic function due to altered AMPA receptor trafficking.

Normal steady-state levels of the signalling lipids PI(3,5)P₂ and PI(5)P require the lipid kinase FAB1/PIKfyve and its regulators, VAC14 and FIG4. Mutations in the PIKfyve/VAC14/FIG4 pathway are associated with Charcot-Marie-Tooth syndrome and amyotrophic lateral sclerosis in humans, and profound neurodegeneration in mice. Hence, tight regulation of this pathway is critical for neural function. Here, we examine the localization and physiological role of VAC14 in neurons. We report that endogenous VAC14 localizes to endocytic organelles in fibroblasts and neurons. Unexpectedly, VAC14 exhibits a pronounced synaptic localization in hippocampal neurons, suggesting a role in regulating synaptic function. Indeed, the amplitude of miniature excitatory postsynaptic currents is enhanced in both Vac14^−/− and Fig4^−/− neurons. Re-introduction of VAC14 in postsynaptic Vac14^−/− cells reverses this effect. These changes in synaptic strength in Vac14^−/− neurons are associated with enhanced surface levels of the AMPA-type glutamate receptor subunit GluA2, an effect that is due to diminished regulated endocytosis of AMPA receptors. Thus, VAC14, PI(3,5)P₂ and/or PI(5)P play a role in controlling postsynaptic function via regulation of endocytic cycling of AMPA receptors.

Lipids of plant membrane rafts

Lipids tend to organize in mono or bilayer phases in a hydrophilic environment. While they have long been thought to be incapable of coherent lateral segregation, it is now clear that spontaneous assembly of these compounds can confer microdomain organization beyond spontaneous fluidity. Membrane raft microdomains have the ability to influence spatiotemporal organization of protein complexes, thereby allowing regulation of cellular processes. In this review, we aim at summarizing briefly: (i) the history of raft discovery in animals and plants, (ii) the main findings about structural and signalling plant lipids involved in raft segregation, (iii) imaging of plant membrane domains, and their biochemical purification through detergent-insoluble membranes, as well as the existing debate on the topic. We also discuss the potential involvement of rafts in the regulation of plant physiological processes, and further discuss the prospects of future research into plant membrane rafts.

Evolution of the voltage sensor domain of the voltage-sensitive phosphoinositide phosphatase VSP/TPTE suggests a role as a proton channel in eutherian mammals

The voltage-sensitive phosphoinositide phosphatases provide a mechanism to couple changes in the transmembrane electrical potential to intracellular signal transduction pathways. These proteins share a domain architecture that is conserved in deuterostomes. However, gene duplication events in primates, including humans, give rise to the paralogs TPTE and TPTE2 that retain protein domain organization but, in the case of TPTE, have lost catalytic activity. Here, we present evidence that these human proteins contain a functional voltage sensor, similar to that in nonmammalian orthologs. However, domains of these human proteins can also generate a noninactivating outward current that is not observed in zebra fish or tunicate orthologs. This outward current has the anticipated characteristics of a voltage-sensitive proton current and is due to the appearance of a single histidine residue in the S4 transmembrane segment of the voltage sensor. Histidine is observed at this position only during the eutherian radiation. Domains from both human paralogs generate proton currents. This apparent gain of proton channel function during the evolution of the TPTE protein family may account for the conservation of voltage sensor domains despite the loss of phosphatase activity in some human paralogs.

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.

A critical role of mitochondrial phosphatase Ptpmt1 in embryogenesis reveals a mitochondrial metabolic stress-induced differentiation checkpoint in embryonic stem cells

Mitochondria are highly dynamic organelles that play multiple roles in cells. How mitochondria cooperatively modulate embryonic stem (ES) cell function during development is not fully understood. Global disruption of Ptpmt1, a mitochondrial Pten-like phosphatidylinositol phosphate (PIP) phosphatase, resulted in developmental arrest and postimplantation lethality. Ptpmt1(-/-) blastocysts failed to outgrow, and inner-cell-mass cells failed to thrive. Depletion of Ptpmt1 in conditional knockout ES cells decreased proliferation without affecting energy homeostasis or cell survival. Differentiation of Ptpmt1-depleted ES cells was essentially blocked. This was accompanied by upregulation of cyclin-dependent kinase inhibitors and a significant cell cycle delay. Reintroduction of wild-type but not of catalytically deficient Ptpmt1 C132S or truncated Ptpmt1 lacking the mitochondrial localization signal restored the differentiation capabilities of Ptpmt1 knockout ES cells. Intriguingly, Ptpmt1 is specifically important for stem cells, as ablation of Ptpmt1 in differentiated embryonic fibroblasts did not disturb cellular function. Further analyses demonstrated that oxygen consumption of Ptpmt1-depleted cells was decreased, while glycolysis was concomitantly enhanced. In addition, mitochondrial fusion/dynamics were compromised in Ptpmt1 knockout cells due to accumulation of PIPs. These studies, while establishing a crucial role for Ptpmt1 phosphatase in embryogenesis, reveal a mitochondrial metabolic stress-activated checkpoint in the control of ES cell differentiation.

Differential involvement of phosphoinositide 3-kinase in gonadotrophin-releasing hormone actions in gonadotrophs and somatotrophs of goldfish, Carassius auratus

In goldfish, two endogenous gonadotrophin-releasing hormones (GnRHs) [salmon (s)GnRH and chicken (c)GnRH-II] control maturational gonadotrophin-II [lutenising hormone (LH)] and growth hormone (GH) secretion via Ca(2+)-dependent intracellular signalling pathways. We investigated the involvement of phosphoinositide 3-kinase (PI3K) in GnRH-evoked LH and GH release and associated intracellular Ca(2+) increases ([Ca(2+)](i) ) in goldfish gonadotrophs and somatotrophs. Immunoreactive PI3K p85α, the predominant regulatory subunit for class IA PI3Ks, was detected in goldfish pituitary tissue extracts and both endogenous GnRH isoforms increased phosphorylation of PI3K p85α in excised pituitary fragments. sGnRH- and cGnRH-II-elicited LH release responses from primary cultures of pituitary cells and [Ca(2+)](i) increases in identified gonadotrophs were significantly reduced in the presence of PI3K inhibitors wortmannin (100 nm) and LY294002 (10 μm). Unexpectedly, wortmannin and LY294002 inhibited GnRH-evoked GH release but only attenuated the [Ca(2+)](i) response in identified somatotrophs to cGnRH-II, and not sGnRH. On the other hand, Ca(2+) ionophore-evoked LH and GH secretion remained unaltered in the presence of the PI3K inhibitors, suggesting that general decreases in the releasable hormone pool or sensitivity to [Ca(2+)](i) changes did not underlie the ability of wortmannin and LY294002 to reduce the actions of GnRH. These results provide the first evidence for the presence and involvement of PI3K in GnRH-induced LH and GH release in any primary pituitary cell system. In gonadotrophs, the inhibitory action of PI3K on both sGnRH and cGnRH-II involves the attenuation of their evoked [Ca(2+)](i); in contrast, GnRH isoform-specific effects occur in somatotrophs.

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

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

Muscle-specific inositide phosphatase (MIP/MTMR14) is reduced with age and its loss accelerates skeletal muscle aging process by altering calcium homeostasis

We have recently reported that a novel muscle-specific inositide phosphatase (MIP/MTMR14) plays a critical role in [Ca2+]i homeostasis through dephosphorylation of sn-1-stearoyl-2-arachidonoyl phosphatidylinositol (3,5) bisphosphate (PI(3,5)P2). Loss of function mutations in MIP have been identified in human centronuclear myopathy. We developed a MIP knockout (MIPKO) animal model and found that MIPKO mice were more susceptible to exercise-induced muscle damage, a trademark of muscle functional changes in older subjects. We used wild-type (Wt) mice and MIPKO mice to elucidate the roles of MIP in muscle function during aging. We found MIP mRNA expression, MIP protein levels, and MIP phosphatase activity significantly decreased in old Wt mice. The mature MIPKO mice displayed phenotypes that closely resembled those seen in old Wt mice: i) decreased walking speed, ii) decreased treadmill activity, iii) decreased contractile force, and iv) decreased power generation, classical features of sarcopenia in rodents and humans. Defective Ca2+ homeostasis is also present in mature MIPKO and old Wt mice, suggesting a putative role of MIP in the decline of muscle function during aging. Our studies offer a new avenue for the investigation of MIP roles in skeletal muscle function and as a potential therapeutic target to treat aging sarcopenia.

Phosphatidylinositol 4,5-bisphosphate directs spermatid cell polarity and exocyst localization in Drosophila

During spermiogenesis, Drosophila melanogaster spermatids coordinate their elongation in interconnected cysts that become highly polarized, with nuclei localizing to one end and sperm tail growth occurring at the other. Remarkably little is known about the signals that drive spermatid polarity and elongation. Here we identify phosphoinositides as critical regulators of these processes. Reduction of plasma membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)) by low-level expression of the PIP(2) phosphatase SigD or mutation of the PIP(2) biosynthetic enzyme Skittles (Sktl) results in dramatic defects in spermatid cysts, which become bipolar and fail to fully elongate. Defects in polarity are evident from the earliest stages of elongation, indicating that phosphoinositides are required for establishment of polarity. Sktl and PIP(2) localize to the growing end of the cysts together with the exocyst complex. Strikingly, the exocyst becomes completely delocalized when PIP(2) levels are reduced, and overexpression of Sktl restores exocyst localization and spermatid cyst polarity. Moreover, the exocyst is required for polarity, as partial loss of function of the exocyst subunit Sec8 results in bipolar cysts. Our data are consistent with a mechanism in which localized synthesis of PIP(2) recruits the exocyst to promote targeted membrane delivery and polarization of the elongating cysts.

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.

Phosphoinositides, exocytosis and polarity in yeast: all about actin?

Cell polarity is necessary for cell division, morphogenesis and motility in eukaryotes, and is determined by dynamic control of the cytoskeleton and secretory pathway to promote directional growth. In yeast, three essential and tightly-regulated processes orchestrate polarization and facilitate bud growth. These processes include phosphoinositide (PI) signaling, Rho GTPase regulation of the actin cytoskeleton, and exocytosis. As yet, the interplay between these different processes is unclear, and two main models (Spatial Landmark and Allosteric Local Activation) have been proposed for Rho GTPase control of polarization in yeast. Here, we summarize the inter-relationship between these growth processes and present a more unified model, the Exocytic Signal model, which proposes that exocytosis and actin regulation are fully integrated events mediated by PI signaling.

Reconstruction of signaling networks regulating fungal morphogenesis by transcriptomics

Coordinated control of hyphal elongation and branching is essential for sustaining mycelial growth of filamentous fungi. In order to study the molecular machinery ensuring polarity control in the industrial fungus Aspergillus niger, we took advantage of the temperature-sensitive (ts) apical-branching ramosa-1 mutant. We show here that this strain serves as an excellent model system to study critical steps of polar growth control during mycelial development and report for the first time a transcriptomic fingerprint of apical branching for a filamentous fungus. This fingerprint indicates that several signal transduction pathways, including TORC2, phospholipid, calcium, and cell wall integrity signaling, concertedly act to control apical branching. We furthermore identified the genetic locus affected in the ramosa-1 mutant by complementation of the ts phenotype. Sequence analyses demonstrated that a single amino acid exchange in the RmsA protein is responsible for induced apical branching of the ramosa-1 mutant. Deletion experiments showed that the corresponding rmsA gene is essential for the growth of A. niger, and complementation analyses with Saccharomyces cerevisiae evidenced that RmsA serves as a functional equivalent of the TORC2 component Avo1p. TORC2 signaling is required for actin polarization and cell wall integrity in S. cerevisiae. Congruently, our microscopic investigations showed that polarized actin organization and chitin deposition are disturbed in the ramosa-1 mutant. The integration of the transcriptomic, genetic, and phenotypic data obtained in this study allowed us to reconstruct a model for cellular events involved in apical branching.

Human eosinophil cationic protein enhances stress fiber formation in Balb/c 3T3 fibroblasts and differentiation of rat neonatal cardiomyocytes

We found that eosinophil cationic protein (ECP) stimulated the growth of mouse Balb/c 3T3 fibroblasts. ECP-treated 3T3 cells were more flattened and exhibited enhanced stress fiber formation. The enhancement of cytoskeleton after addition of recombinant ECP appeared stable and was able to inhibit disassembly of actin filaments that was induced by fibroblast growth factor-2. The ROCK inhibitor, Y-27632, abrogated this enhancement on stress fiber formation that was induced by ECP indicating the involvement of Rho/ROCK signaling pathway. The effect of ECP was assessed on the differentiation of primary cardiomyocytes derived from rat neonatal heart since the development of actin filaments is significantly related with organization of stress fibers. As the result, both beating rate and the expression of cardiac muscle specific markers such as atrial natriuretic factor were enhanced in the presence of ECP. Thus ECP may also function as a cardiomyocyte differentiation factor.

Immunocytochemical techniques reveal multiple, distinct cellular pools of PtdIns4P and PtdIns(4,5)P(2)

PtdIns4P is the major precursor for the synthesis of the multifunctional plasma membrane lipid, PtdIns(4,5)P₂. Yet PtdIns4P also functions as a regulatory lipid in its own right, particularly at the Golgi apparatus. In the present study we define specific conditions that enable preservation of several organellar membranes for the immunocytochemical detection of PtdIns4P. We report distinct pools of this lipid in both Golgi and plasma membranes, which are synthesized by different PI4K (phosphatidylinositol 4-kinase) activities, and also the presence of PtdIns4P in cytoplasmic vesicles, which are not readily identifiable as PI4K containing trafficking intermediates. In addition, we present evidence that the majority of PtdIns4P resides in the plasma membrane, where it is metabolically distinct from the steady-state plasma membrane pool of PtdIns(4,5)P₂.

Using genetic tools to understand plant phosphoinositide signalling

Phosphoinositides (PIs) are regulatory lipids that control various physiological processes in eukaryotic organisms. As in other eukaryotes, the plant PI system is a central regulator of metabolism. The analysis of mutant plants that lack certain PI species has revealed their physiological relevance; however, knowledge of the factors controlling the distribution of PIs and the effects on their target proteins is still limited. To understand PI functions better, genetic approaches should be combined with biochemical analyses and cell biology, as has been done in several recent publications. Here, I highlight plant-specific physiological processes that are controlled by PIs and suggest future avenues of research. A detailed understanding of the functions and effects of PIs might offer new opportunities for modulating plant growth and hardiness against environmental influences.

Reversible binding and rapid diffusion of proteins in complex with inositol lipids serves to coordinate free movement with spatial information

Polyphosphoinositol lipids convey spatial information partly by their interactions with cellular proteins within defined domains. However, these interactions are prevented when the lipids' head groups are masked by the recruitment of cytosolic effector proteins, whereas these effectors must also have sufficient mobility to maximize functional interactions. To investigate quantitatively how these conflicting functional needs are optimized, we used different fluorescence recovery after photobleaching techniques to investigate inositol lipid–effector protein kinetics in terms of the real-time dissociation from, and diffusion within, the plasma membrane. We find that the protein–lipid complexes retain a relatively rapid (∼0.1–1 µm²/s) diffusion coefficient in the membrane, likely dominated by protein–protein interactions, but the limited time scale (seconds) of these complexes, dictated principally by lipid–protein interactions, limits their range of action to a few microns. Moreover, our data reveal that GAP1^IP4BP, a protein that binds PtdIns(4,5)P₂ and PtdIns(3,4,5)P₃ in vitro with similar affinity, is able to “read” PtdIns(3,4,5)P₃ signals in terms of an elongated residence time at the membrane.

Type B phosphatidylinositol-4-phosphate 5-kinases mediate Arabidopsis and Nicotiana tabacum pollen tube growth by regulating apical pectin secretion

Phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P(2)] occurs in the apical plasma membrane of growing pollen tubes. Because enzymes responsible for PtdIns(4,5)P(2) production at that location are uncharacterized, functions of PtdIns(4,5)P(2) in pollen tube tip growth are unresolved. Two candidate genes encoding pollen-expressed Arabidopsis thaliana phosphatidylinositol-4-phosphate 5-kinases (PI4P 5-kinases) of Arabidopsis subfamily B were identified (PIP5K4 and PIP5K5), and their recombinant proteins were characterized as being PI4P 5-kinases. Pollen of T-DNA insertion lines deficient in both PIP5K4 and PIP5K5 exhibited reduced pollen germination and defects in pollen tube elongation. Fluorescence-tagged PIP5K4 and PIP5K5 localized to an apical plasma membrane microdomain in Arabidopsis and tobacco (Nicotiana tabacum) pollen tubes, and overexpression of either PIP5K4 or PIP5K5 triggered multiple tip branching events. Further studies using the tobacco system revealed that overexpression caused massive apical pectin deposition accompanied by plasma membrane invaginations. By contrast, callose deposition and cytoskeletal structures were unaltered in the overexpressors. Morphological effects depended on PtdIns(4,5)P(2) production, as an inactive enzyme variant did not produce any effects. The data indicate that excessive PtdIns(4,5)P(2) production by type B PI4P 5-kinases disturbs the balance of membrane trafficking and apical pectin deposition. Polar tip growth of pollen tubes may thus be modulated by PtdIns(4,5)P(2) via regulatory effects on membrane trafficking and/or apical pectin deposition.

Salt-stress-induced association of phosphatidylinositol 4,5-bisphosphate with clathrin-coated vesicles in plants

Plants exposed to hyperosmotic stress undergo changes in membrane dynamics and lipid composition to maintain cellular integrity and avoid membrane leakage. Various plant species respond to hyperosmotic stress with transient increases in PtdIns(4,5)P(2); however, the physiological role of such increases is unresolved. The plasma membrane represents the outermost barrier between the symplast of plant cells and its apoplastic surroundings. In the present study, the spatio-temporal dynamics of stress-induced changes in phosphoinositides were analysed in subcellular fractions of Arabidopsis leaves to delineate possible physiological roles. Unlabelled lipids were separated by TLC and quantified by gas-chromatographic detection of associated fatty acids. Transient PtdIns(4,5)P(2) increases upon exposure to hyperosmotic stress were detected first in enriched plasmamembrane fractions, however, at later time points, PtdIns(4,5)P(2) was increased in the endomembrane fractions of the corresponding two-phase systems. When major endomembranes were enriched from rosette leaves prior to hyperosmotic stress and during stimulation for 60 min, no stress-induced increases in the levels of PtdIns(4,5)P(2) were found in fractions enriched for endoplasmic reticulum, nuclei or plastidial membranes. Instead, increased PtdIns(4,5)P(2) was found in CCVs (clathrin-coated vesicles), which proliferated several-fold in mass within 60 min of hyperosmotic stress, according to the abundance of CCV-associated proteins and lipids. Monitoring the subcellular distribution of fluorescence-tagged reporters for clathrin and PtdIns(4,5)P(2) during transient co-expression in onion epidermal cells indicates rapid stress-induced co-localization of clathrin with PtdIns(4,5)P(2) at the plasma membrane. The results indicate that PtdIns(4,5)P(2) may act in stress-induced formation of CCVs in plant cells, highlighting the evolutionary conservation of the phosphoinositide system between organismic kingdoms.

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.

Tails wagging the dogs: On phosphoinositides and their fatty acyl moieties

Phosphoinositides (PIs) control various cellular functions of eukaryotic cells. PIs are derived from phosphatidylinositol (PtdIns) by phosphorylation of the inositol-ring in the lipid-head group; the action of specific lipid kinases gives rise to a family of structurally-related PIs, in plants representing PtdIns-mono-, and -bisphosphates. Specific PIs, such as phosphatidylinositol4,5-bisphosphate (PtdIns(4,5)P(2)), can influence more than one physiological process, raising the question as to how interactions with alternative protein partners are coordinated. Previous studies have proposed that PIs are organized by spatiotemporal compartmentation into distinct functional pools, however, mechanisms for the generation and maintenance of such pools have not been presented. Several recent studies now indicate that not only the distinctive inositolpolyphosphate head groups may be relevant for PI function but also the associated fatty acyl-moieties, which may be involved in sorting of PI precursors into distinct pools. This mini-review aims at highlighting recent evidence that PI acylgroups exert relevant effects on signaling.

Determination of content and fatty acid composition of unlabeled phosphoinositide species by thin-layer chromatography and gas chromatography

Recent advances in research on the physiological roles of phosphoinositides in eukaryotic organisms indicate a need to distinguish molecular phosphoinositide species on the basis of their characteristic head groups as well as their glycerolipid moieties. Accurate identification of phosphoinositide species in biological samples poses an analytical challenge, because structurally similar inositol phosphate head groups must be resolved, as must lipid-associated fatty acids. Although intact phosphoinositide species have been successfully analyzed, such analyses employ state-of-the-art liquid chromatography/mass spectrometry and require expensive equipment not accessible to many researchers. Described here is a cost-efficient and reliable alternative developed by adaptation of a combination of classic methods for lipid analysis, thin-layer chromatography and gas chromatography.

Alternative metabolic fates of phosphatidylinositol produced by phosphatidylinositol synthase isoforms in Arabidopsis thaliana

PtdIns is an important precursor for inositol-containing lipids, including polyphosphoinositides, which have multiple essential functions in eukaryotic cells. It was previously proposed that different regulatory functions of inositol-containing lipids may be performed by independent lipid pools; however, it remains unclear how such subcellular pools are established and maintained. In the present paper, a previously uncharacterized Arabidopsis gene product with similarity to the known Arabidopsis PIS (PtdIns synthase), PIS1, is shown to be an active enzyme, PIS2, capable of producing PtdIns in vitro. PIS1 and PIS2 diverged slightly in substrate preferences for CDP-DAG [cytidinediphospho-DAG (diacylglycerol)] species differing in fatty acid composition, PIS2 preferring unsaturated substrates in vitro. Transient expression of fluorescently tagged PIS1 or PIS2 in onion epidermal cells indicates localization of both enzymes in the ER (endoplasmic reticulum) and, possibly, Golgi, as was reported previously for fungal and mammalian homologues. Constitutive ectopic overexpression of PIS1 or PIS2 in Arabidopsis plants resulted in elevated levels of PtdIns in leaves. PIS2-overexpressors additionally exhibited significantly elevated levels of PtdIns(4)P and PtdIns(4,5)P(2), whereas polyphosphoinositides were not elevated in plants overexpressing PIS1. In contrast, PIS1-overexpressors contained significantly elevated levels of DAG and PtdEtn (phosphatidylethanolamine), an effect not observed in plants overexpressing PIS2. Biochemical analysis of transgenic plants with regards to fatty acids associated with relevant lipids indicates that lipids increasing with PIS1 overexpression were enriched in saturated or monounsaturated fatty acids, whereas lipids increasing with PIS2 overexpression, including polyphosphoinositides, contained more unsaturated fatty acids. The results indicate that PtdIns populations originating from different PIS isoforms may enter alternative routes of metabolic conversion, possibly based on specificity and immediate metabolic context of the biosynthetic enzymes.

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.

PI3K signaling effects in hypothalamic neurons mediated by estrogen

Multiple mechanisms mediate the effects of estrogen in the central nervous system, including signal transduction pathways such as protein kinase A, protein kinase C, and phosphatidylinositol 3-kinase (PI3K) pathways. Previously we demonstrated that estrogen regulates a number of PI3K-related genes in the hypothalamus, including the PI3K p55gamma regulatory subunit. We hypothesized that PI3K activation is critical for the effects of estrogen and that the p55gamma subunit may be more prevalent than the p85alpha regulatory subunit in the hypothalamus. Therefore, in the present study, we compared the mRNA distribution of the p55gamma and p85alpha regulatory subunits by using in situ hybridization in guinea pig. Expression level of p55gamma mRNA was greater than p85alpha in most hypothalamic nuclei. Twenty-four hours of estrogen treatment increased p55gamma mRNA expression in the paraventricular, suprachiasmatic, arcuate, and ventromedial nuclei, and little or no change was observed for p85alpha mRNA. Quantitative real-time PCR confirmed the in situ hybridization results. Next, we investigated the general role of PI3K signaling in the estrogen-mediated changes of arcuate proopiomelanocortin (POMC) neuronal excitability by using whole-cell recording. One cellular mechanism by which estrogen increases neuronal excitability is to desensitize (uncouple) gamma-aminobutyric acid type B (GABA(B)) receptors from their G-protein-gated inwardly rectifying K(+) channels in hypothalamic neurons. We found that the PI3K inhibitors wortmannin and LY294002 significantly reduced the estrogen-mediated GABA(B) receptor desensitization in POMC arcuate neurons, suggesting that PI3K signaling is a critical downstream mediator of the estrogen-mediated rapid effects. Collectively, these data suggest that the interplay between estrogen and PI3K occurs at multiple levels, including transcriptional and membrane-initiated signaling events that ultimately lead to changes in homeostatic function.

Design of drug-resistant alleles of type-III phosphatidylinositol 4-kinases using mutagenesis and molecular modeling

Molecular modeling and site directed mutagenesis were used to analyze the structural features determining the unique inhibitor sensitivities of type-III phosphatidylinositol 4-kinase enzymes (PI4Ks). Mutation of a highly conserved Tyr residue that provides the bottom of the hydrophobic pocket for ATP yielded a PI4KIIIbeta enzyme that showed greatly reduced wortmannin sensitivity and was catalytically still active. Similar substitutions were not tolerated in the type-IIIalpha enzyme rendering it catalytically inactive. Two conserved Cys residues located in the active site of PI4KIIIalpha were found responsible for the high sensitivity of this enzyme to the oxidizing agent, phenylarsine oxide. Mutation of one of these Cys residues reduced the phenylarsine oxide sensitivity of the enzyme to the same level observed with the PI4KIIIbeta protein. In search of inhibitors that would discriminate between the closely related PI4KIIIalpha and -IIIbeta enzymes, the PI3Kgamma inhibitor, PIK93, was found to inhibit PI4KIIIbeta with significantly greater potency than PI4KIIIalpha. These studies should aid development of subtype-specific inhibitors of type-III PI4Ks and help to better understand the significance of localized PtdIns4P production by the various PI4Ks isoforms in specific cellular compartments.

The type B phosphatidylinositol-4-phosphate 5-kinase 3 is essential for root hair formation in Arabidopsis thaliana

Root hairs are extensions of root epidermal cells and a model system for directional tip growth of plant cells. A previously uncharacterized Arabidopsis thaliana phosphatidylinositol-4-phosphate 5-kinase gene (PIP5K3) was identified and found to be expressed in the root cortex, epidermal cells, and root hairs. Recombinant PIP5K3 protein was catalytically active and converted phosphatidylinositol-4-phosphate to phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2]. Arabidopsis mutant plants homozygous for T-DNA-disrupted PIP5K3 alleles were compromised in root hair formation, a phenotype complemented by expression of wild-type PIP5K3 cDNA under the control of a 1500-bp PIP5K3 promoter fragment. Root hair-specific PIP5K3 overexpression resulted in root hair deformation and loss of cell polarity with increasing accumulation of PIP5K3 transcript. Using reestablishment of root hair formation in T-DNA mutants as a bioassay for physiological functionality of engineered PIP5K3 variants, catalytic activity was found to be essential for physiological function, indicating that PtdIns(4,5)P2 formation is required for root hair development. An N-terminal domain containing membrane occupation and recognition nexus repeats, which is not required for catalytic activity, was found to be essential for the establishment of root hair growth. Fluorescence-tagged PIP5K3 localized to the periphery of the apical region of root hair cells, possibly associating with the plasma membrane and/or exocytotic vesicles. Transient heterologous expression of full-length PIP5K3 in tobacco (Nicotiana tabacum) pollen tubes increased plasma membrane association of a PtdIns(4,5)P2-specific reporter in these tip-growing cells. The data demonstrate that root hair development requires PIP5K3-dependent PtdIns(4,5)P2 production in the apical region of root hair cells.

Necessity of inositol (1,4,5)-trisphosphate receptor 1 and mu-calpain in NO-induced osteoclast motility

In skeletal remodeling, osteoclasts degrade bone, detach and move to new locations. Mechanical stretch and estrogen regulate osteoclast motility via nitric oxide (NO). We have found previously that NO stimulates guanylyl cyclase, activating the cGMP-dependent protein kinase 1 (PKG1), reversibly terminating osteoclast matrix degradation and attachment, and initiating motility. The PKG1 substrate vasodilator-stimulated protein (VASP), a membrane-attachment-related protein found in complexes with the integrin alphavbeta3 in adherent osteoclasts, was also required for motility. Here, we studied downstream mechanisms by which the NO-dependent pathway mediates osteoclast relocation. We found that NO-stimulated motility is dependent on activation of the Ca(2+)-activated proteinase mu-calpain. RNA interference (RNAi) showed that NO-dependent activation of mu-calpain also requires PKG1 and VASP. Inhibition of Src kinases, which are involved in the regulation of adhesion complexes, also abolished NO-stimulated calpain activity. Pharmacological inhibition and RNAi showed that calpain activation in this process is mediated by the inositol (1,4,5)-trisphosphate receptor 1 [Ins(1,4,5)P(3)R1] Ca(2+) channel. We conclude that NO-induced motility in osteoclasts requires regulated Ca(2+) release, which activates mu-calpain. This occurs via the Ins(1,4,5)P(3)R1.

A role for diacylglycerol in annexin A7-mediated fusion of lung lamellar bodies

Lung surfactant secretion in alveolar type II cells occurs following lamellar body fusion with plasma membrane. Annexin A7 is a Ca2+-dependent membrane-binding protein that is postulated to promote membrane fusion during exocytosis in some cell types including type II cells. Since annexin A7 preferably binds to lamellar body membranes, we postulated that specific lipids could modify the mode of annexin A7 interaction with membranes and its membrane fusion activity. Initial studies with phospholipid vesicles containing phosphatidylserine and other lipids showed that certain lipids affected protein interaction with vesicle membranes as determined by change in protein tryptophan fluorescence, protein interaction with trans membranes, and by protein sensitivity to limited proteolysis. The presence of signaling lipids, diacylglycerol or phosphatidylinositol-4,5-bisphosphate, as minor components also modified the lipid vesicle effect on these characteristics and membrane fusion activity of annexin A7. In vitro incubation of lamellar bodies with diacylglycerol or phosphatidylinositol-4,5-bisphosphate caused their enrichment with either lipid, and increased the annexin A7 and Ca2+-mediated fusion of lamellar bodies. Treatment of isolated lung lamellar bodies with phosphatidylinositol- or phosphatidylcholine phospholipase C to increase diacylglycerol, without or with preincubation with phosphatidylinositol-4,5-bisphosphate, augmented the fusion activity of annexin A7. Thus, increased diacylglycerol in lamellar bodies following cell stimulation with secretagogues may enhance membrane fusion activity of annexin A7.

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.