Cloning and characterization of a wortmannin-sensitive human phosphatidylinositol 4-kinase

Phosphatidylinositol 4-phosphate; A minor lipid with multiple personalities

Phosphorylated products of phosphatidylinositol (PI), named Diphosphoinositide (DPI) and triphosphoinositide (TPI) were identified long time ago and found to exhibit high turnover rates based on their rapid 32P-phosphate labeling. The PI kinase activities that were responsible for their production were subsequently identified and found to be associated with different organelle membranes, including the plasma membrane. These activities were then linked with a certain group of cell surface receptors that activated phospholipase C enzymes to hydrolyze PI and used calcium or cGMP as a second messenger. This visionary concept was introduced in the seminal BBA review written by Robert Michell, exactly 50 years ago. The enzymology and functional diversity of PI 4-phosphate (PI4P) (the term that has replaced DPI) has since underwent an expansion that could not have been foreseen. In this review I will attempt to revisit this expansion with some historical reflections celebrating the 50th anniversary of the Michell review.

A genome wide CRISPR screen reveals novel determinants of long-lived plasma cell secretory capacity

Plasma cell subsets vary in their lifespans and ability to sustain humoral immunity. We conducted a genome-wide CRISPR-Cas9 screen in a myeloma cell line for factors that promote surface expression of CD98, a marker of longevity in primary mouse plasma cells. A large fraction of genes found to promote CD98 expression in this screen are involved in secretory and other vesicles, including many subunits of the V-type ATPase complex. Chemical inhibition or genetic ablation of V-type ATPases in myeloma cells reduced antibody secretion. Primary mouse and human long-lived plasma cells had greater numbers of acidified vesicles than did their short-lived counterparts, and this correlated with increased secretory capacity of IgM, IgG, and IgA. The screen also identified PI4KB, which promoted acidified vesicle numbers and secretory capacity, and DDX3X, an ATP-dependent RNA helicase, the deletion of which reduced immunoglobulin secretion independently of vesicular acidification. Finally, we report a plasma-cell intrinsic function of the signaling adapter MYD88 in both antibody secretion and plasma cell survival in vivo. These data reveal novel regulators of plasma cell secretory capacity, including those that also promote lifespan.

Orpinolide disrupts a leukemic dependency on cholesterol transport by inhibiting OSBP

Metabolic alterations in cancer precipitate in associated dependencies that can be therapeutically exploited. To meet this goal, natural product-inspired small molecules can provide a resource of invaluable chemotypes. Here, we identify orpinolide, a synthetic withanolide analog with pronounced antileukemic properties, via orthogonal chemical screening. Through multiomics profiling and genome-scale CRISPR-Cas9 screens, we identify that orpinolide disrupts Golgi homeostasis via a mechanism that requires active phosphatidylinositol 4-phosphate signaling at the endoplasmic reticulum-Golgi membrane interface. Thermal proteome profiling and genetic validation studies reveal the oxysterol-binding protein OSBP as the direct and phenotypically relevant target of orpinolide. Collectively, these data reaffirm sterol transport as a therapeutically actionable dependency in leukemia and motivate ensuing translational investigation via the probe-like compound orpinolide.

Beyond PI3Ks: targeting phosphoinositide kinases in disease

Lipid phosphoinositides are master regulators of almost all aspects of a cell's life and death and are generated by the tightly regulated activity of phosphoinositide kinases. Although extensive efforts have focused on drugging class I phosphoinositide 3-kinases (PI3Ks), recent years have revealed opportunities for targeting almost all phosphoinositide kinases in human diseases, including cancer, immunodeficiencies, viral infection and neurodegenerative disease. This has led to widespread efforts in the clinical development of potent and selective inhibitors of phosphoinositide kinases. This Review summarizes our current understanding of the molecular basis for the involvement of phosphoinositide kinases in disease and assesses the preclinical and clinical development of phosphoinositide kinase inhibitors.

Molecular mechanisms of PI4K regulation and their involvement in viral replication

Lipid phosphoinositides are master signaling molecules in eukaryotic cells and key markers of organelle identity. Because of these important roles, the kinases and phosphatases that generate phosphoinositides must be tightly regulated. Viruses can manipulate this regulation, with the Type III phosphatidylinositol 4-kinases (PI4KA and PI4KB) being hijacked by many RNA viruses to mediate their intracellular replication through the formation of phosphatidylinositol 4-phosphate (PI4P)-enriched replication organelles (ROs). Different viruses have evolved unique approaches toward activating PI4K enzymes to form ROs, through both direct binding of PI4Ks and modulation of PI4K accessory proteins. This review will focus on PI4KA and PI4KB and discuss their roles in signaling, functions in membrane trafficking and manipulation by viruses. Our focus will be the molecular basis for how PI4KA and PI4KB are activated by both protein-binding partners and post-translational modifications, with an emphasis on understanding the different molecular mechanisms viruses have evolved to usurp PI4Ks. We will also discuss the chemical tools available to study the role of PI4Ks in viral infection.

Advances in chemical proteomic evaluation of lipid kinases-DAG kinases as a case study

Advancements in chemical proteomics and mass spectrometry lipidomics are providing new opportunities to understand lipid kinase activity, specificity, and regulation on a global cellular scale. Here, we describe recent developments in chemical biology of lipid kinases with a focus on those members that phosphorylate diacylglycerols. We further discuss future implications of how these mass spectrometry-based approaches can be adapted for studies of additional lipid kinase members with the aim of bridging the gap between protein and lipid kinase-focused investigations.

Heterologous Expression, Purification, and Functional Analysis of the Plasmodium falciparum Phosphatidylinositol 4-Kinase IIIβ

Recently, we heterologously expressed, purified, and analyzed the function of the sole Plasmodium falciparum phosphatidylinositol 3-kinase (PI3K), found that the enzyme is a "class III" or "Vps34" PI3K, and found that it is irreversibly inhibited by Fe²⁺-mediated covalent, nonspecific interactions with the leading antimalarial drug, dihydroartemisinin [Hassett, M. R., et al. (2017) Biochemistry 56, 4335-4345]. One of several P. falciparum phosphatidylinositol 4-kinases [putative IIIβ isoform (PfPI4KIIIβ)] has generated similar interest as a druggable target; however, no validation of the mechanism of action for putative PfPI4K inhibitors has yet been possible due to the lack of purified PfPI4KIIIβ. We therefore codon optimized the pfpi4kIIIβ gene, successfully expressed the protein in yeast, and purified an N-lobe catalytic domain PfPI4KIIIβ protein. Using an enzyme-linked immunosorbent assay strategy previously perfected for analysis of PfPI3K (PfVps34), we measured the apparent initial rate, K_m,app(ATP), and other enzyme characteristics and found full activity for the construct and that PfPI4KIIIβ activity is most consistent with the class IIIβ designation. Because several novel antimalarial drug candidates with different chemical scaffolds have been proposed to target PfPI4KIIIβ, we titrated enzyme inhibition for these candidates versus purified PfPI4KIIIβ and PfVps34. We also analyzed the activity versus purified PfPI4KIIIβ mutants previously expressed in P. falciparum selected for resistance to these drugs. Interestingly, we found that a putative PfPI4KIIIβ inhibitor currently in advanced trials (MMV390048; MMV '0048) is a potent inhibitor of both PfVps34 and PfPI4KIIIβ. These data are helpful for further preclinical optimization of an exciting new class of P. falciparum PI kinase inhibitor ("PfPIKi") antimalarial drugs.

Characterization of the c10orf76-PI4KB complex and its necessity for Golgi PI4P levels and enterovirus replication

The lipid kinase PI4KB, which generates phosphatidylinositol 4-phosphate (PI4P), is a key enzyme in regulating membrane transport and is also hijacked by multiple picornaviruses to mediate viral replication. PI4KB can interact with multiple protein binding partners, which are differentially manipulated by picornaviruses to facilitate replication. The protein c10orf76 is a PI4KB-associated protein that increases PI4P levels at the Golgi and is essential for the viral replication of specific enteroviruses. We used hydrogen-deuterium exchange mass spectrometry to characterize the c10orf76-PI4KB complex and reveal that binding is mediated by the kinase linker of PI4KB, with formation of the heterodimeric complex modulated by PKA-dependent phosphorylation. Complex-disrupting mutations demonstrate that PI4KB is required for membrane recruitment of c10orf76 to the Golgi, and that an intact c10orf76-PI4KB complex is required for the replication of c10orf76-dependent enteroviruses. Intriguingly, c10orf76 also contributed to proper Arf1 activation at the Golgi, providing a putative mechanism for the c10orf76-dependent increase in PI4P levels at the Golgi.

Understanding phosphoinositides: rare, dynamic, and essential membrane phospholipids

Polyphosphoinositides (PPIs) are essential phospholipids located in the cytoplasmic leaflet of eukaryotic cell membranes. Despite contributing only a small fraction to the bulk of cellular phospholipids, they make remarkable contributions to practically all aspects of a cell's life and death. They do so by recruiting cytoplasmic proteins/effectors or by interacting with cytoplasmic domains of membrane proteins at the membrane-cytoplasm interface to organize and mold organelle identity. The present study summarizes aspects of our current understanding concerning the metabolism, manipulation, measurement, and intimate roles these lipids play in regulating membrane homeostasis and vital cell signaling reactions in health and disease.

PI3K class II α regulates δ-opioid receptor export from the trans-Golgi network

The interplay between signaling and trafficking by G protein-coupled receptors (GPCRs) has focused mainly on endocytic trafficking. Whether and how surface delivery of newly synthesized GPCRs is regulated by extracellular signals is less understood. Here we define a signaling-regulated checkpoint at the trans-Golgi network (TGN) that controls the surface delivery of the delta opioid receptor (δR). In PC12 cells, inhibition of phosphoinositide-3 kinase (PI3K) activity blocked export of newly synthesized δR from the Golgi and delivery to the cell surface, similar to treatment with nerve growth factor (NGF). Depletion of class II phosphoinositide-3 kinase α (PI3K C2A), but not inhibition of class I PI3K, blocked δR export to comparable levels and attenuated δR-mediated cAMP inhibition. NGF treatment displaced PI3K C2A from the Golgi and optogenetic recruitment of the PI3K C2A kinase domain to the TGN-induced δR export downstream of NGF. Of importance, PI3K C2A expression promotes export of endogenous δR in primary trigeminal ganglion neurons. Taken together, our results identify PI3K C2A as being required and sufficient for δR export and surface delivery in neuronal cells and suggest that it could be a key modulator of a novel Golgi export checkpoint that coordinates GPCR delivery to the surface.

Phosphatidylinositol 4,5-bisphosphate (PIP2 ) modulates afterhyperpolarizations in oxytocin neurons of the supraoptic nucleus

KEY POINTS

Afterhyperpolarizations (AHPs) generated by repetitive action potentials in supraoptic magnocellular neurons regulate repetitive firing and spike frequency adaptation but relatively little is known about PIP₂ 's control of these AHPs. We examined how changes in PIP₂ levels affected AHPs, somatic [Ca²⁺ ]_i , and whole cell Ca²⁺ currents. Manipulations of PIP₂ levels affected both medium and slow AHP currents in oxytocin (OT) neurons of the supraoptic nucleus. Manipulations of PIP₂ levels did not modulate AHPs by influencing Ca²⁺ release from IP₃ -triggered Ca²⁺ stores, suggesting more direct modulation of channels by PIP₂ . PIP₂ depletion reduced spike-evoked Ca²⁺ entry and voltage-gated Ca²⁺ currents. PIP₂ appears to influence AHPs in OT neurons by reducing Ca²⁺ influx during spiking.

ABSTRACT

Oxytocin (OT)- and vasopressin (VP)-secreting magnocellular neurons of the supraoptic nucleus (SON) display calcium-dependent afterhyperpolarizations (AHPs) following a train of action potentials that are critical to shaping the firing patterns of these cells. Previous work demonstrated that the lipid phosphatidylinositol 4,5-bisphosphate (PIP₂ ) enabled the slow AHP component (sAHP) in cortical pyramidal neurons. We investigated whether this phenomenon occurred in OT and VP neurons of the SON. Using whole cell recordings in coronal hypothalamic slices from adult female rats, we demonstrated that inhibition of PIP₂ synthesis with wortmannin robustly blocked both the medium and slow AHP currents (I_mAHP and I_sAHP ) of OT, but not VP neurons with high affinity. We further tested this by introducing a water-soluble PIP₂ analogue (diC₈ -PIP₂ ) into neurons, which in OT neurons not only prevented wortmannin's inhibitory effect, but slowed rundown of the I_mAHP and I_sAHP . Inhibition of phospholipase C (PLC) with U73122 did not inhibit either I_mAHP or I_sAHP in OT neurons, consistent with wortmannin's effects not being due to reducing diacylglycerol (DAG) or IP₃ availability, i.e. PIP₂ modulation of AHPs is not likely to involve downstream Ca²⁺ release from inositol 1,4,5-trisphosphate (IP₃ )-triggered Ca²⁺ -store release, or channel modulation via DAG and protein kinase C (PKC). We found that wortmannin reduced [Ca²⁺ ]_i increase induced by spike trains in OT neurons, but had no effect on AHPs evoked by uncaging intracellular Ca²⁺ . Finally, wortmannin selectively reduced whole cell Ca²⁺ currents in OT neurons while leaving VP neurons unaffected. The results indicate that PIP₂ modulates both the I_mAHP and I_sAHP in OT neurons, most likely by controlling Ca²⁺ entry through voltage-gated Ca²⁺ channels opened during spike trains.

A novel fluorescence-based biosynthetic trafficking method provides pharmacologic evidence that PI4-kinase IIIα is important for protein trafficking from the endoplasmic reticulum to the plasma membrane

Background

Biosynthetic trafficking of receptors and other membrane-associated proteins from the endoplasmic reticulum (ER) to the plasma membrane (PM) underlies the capacity of these proteins to participate in crucial cellular roles. Phosphoinositides have been shown to mediate distinct biological functions in cells, and phosphatidylinositol 4-phosphate (PI4P), in particular, has emerged as a key regulator of biosynthetic trafficking.

Results

To investigate the source of PI4P that orchestrates trafficking events, we developed a novel flow cytometry based method to monitor biosynthetic trafficking of transiently transfected proteins. We demonstrated that our method can be used to assess the trafficking of both type-1 transmembrane and GPI-linked proteins, and that it can accurately monitor the pharmacological disruption of biosynthetic trafficking with brefeldin A, a well-documented inhibitor of early biosynthetic trafficking. Furthermore, utilizing our newly developed method, we applied pharmacological inhibition of different isoforms of PI 4-kinase to reveal a role for a distinct pool of PI4P, synthesized by PI4KIIIα, in ER-to-PM trafficking.

Conclusions

Taken together, these findings provide evidence that a specific pool of PI4P plays a role in biosynthetic trafficking of two different classes of proteins from the ER to the Golgi complex. Furthermore, our simple, flow cytometry-based biosynthetic trafficking assay can be widely applied to the study of multiple classes of proteins and varied pharmacological and genetic perturbations.

Detection and manipulation of phosphoinositides

Phosphoinositides (PIs) are minor components of cell membranes, but play key roles in cell function. Recent refinements in techniques for their detection, together with imaging methods to study their distribution and changes, have greatly facilitated the study of these lipids. Such methods have been complemented by the parallel development of techniques for the acute manipulation of their levels, which in turn allow bypassing the long-term adaptive changes implicit in genetic perturbations. Collectively, these advancements have helped elucidate the role of PIs in physiology and the impact of the dysfunction of their metabolism in disease. Combining methods for detection and manipulation enables the identification of specific roles played by each of the PIs and may eventually lead to the complete deconstruction of the PI signaling network. Here, we review current techniques used for the study and manipulation of cellular PIs and also discuss advantages and disadvantages associated with the various methods. This article is part of a Special Issue entitled Phosphoinositides.

The Concise Guide to PHARMACOLOGY 2013/14: enzymes

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Enzymes are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, nuclear hormone receptors, catalytic receptors and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

PtdIns(4)P signalling and recognition systems

The Golgi apparatus is a sorting platform that exchanges extensively with the endoplasmic reticulum (ER), endosomes (Es) and plasma membrane (PM) compartments. The last compartment of the Golgi, the trans-Golgi Network (TGN) is a large complex of highly deformed membranes from which vesicles depart to their targeted organelles but also are harbored from retrograde pathways. The phosphoinositide (PI) composition of the TGN is marked by an important contingent of phosphatidylinositol-4-phosphate (PtdIns(4)P). Although this PI is present throughout the Golgi, its proportion grows along the successive cisternae and peaks at the TGN. The levels of this phospholipid are controlled by a set of kinases and phosphatases that regulate its concentrations in the Golgi and maintain a dynamic gradient that determines the cellular localization of several interacting proteins. Though not exclusive to the Golgi, the synthesis of PtdIns(4)P in other membranes is relatively marginal and has unclear consequences. The significance of PtdIns(4)P within the TGN has been demonstrated for numerous cellular events such as vesicle formation, lipid metabolism, and membrane trafficking.

Phosphoinositides: tiny lipids with giant impact on cell regulation

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

Mammalian phosphatidylinositol 4-kinases as modulators of membrane trafficking and lipid signaling networks

The four mammalian phosphatidylinositol 4-kinases modulate inter-organelle lipid trafficking, phosphoinositide signalling and intracellular vesicle trafficking. In addition to catalytic domains required for the synthesis of PI4P, the phosphatidylinositol 4-kinases also contain isoform-specific structural motifs that mediate interactions with proteins such as AP-3 and the E3 ubiquitin ligase Itch, and such structural differences determine isoform-specific roles in membrane trafficking. Moreover, different permutations of phosphatidylinositol 4-kinase isozymes may be required for a single cellular function such as occurs during distinct stages of GPCR signalling and in Golgi to lysosome trafficking. Phosphatidylinositol 4-kinases have recently been implicated in human disease. Emerging paradigms include increased phosphatidylinositol 4-kinase expression in some cancers, impaired functioning associated with neurological pathologies, the subversion of PI4P trafficking functions in bacterial infection and the activation of lipid kinase activity in viral disease. We discuss how the diverse and sometimes overlapping functions of the phosphatidylinositol 4-kinases present challenges for the design of isoform-specific inhibitors in a therapeutic context.

[Exploration for anti-enterovirus compounds and analysis on the mechanism of its inhibitory effect on virus infection]

Poliovirus (PV) is a small non-enveloped virus belonging to the family Picornaviridae, and is the causative agent of poliomyelitis. With established vaccines, the global eradication program for poliomyelitis is ongoing by the World Health Organization since 1988. In the eradication program, antivirals are anticipated to have some roles in the endgame and post-eradication era of PV. During our search for potent anti-PV compounds, we identified candidate compounds that are associated with a common resistance mutation in viral protein 3A similar to enviroxime (designated as enviroxime-like compounds). Recently, PIK93, an inhibitor of host phosphatidylinositol 4-kinase III beta (PI4KB), was identified as a potent anti-enterovirus compound (Hsu et al., Cell 141:799-811). We found that PIK93 is an enviroxime-like compound, and showed that T-00127-HEV1, which is a novel enviroxime-like compound identified in high-throughput screening, is a specific PI4KB inhibitor. We also showed that PI4KB is an enterovirus-specific host factor required for its viral RNA replication. Analysis of anti-enterovirus compounds would unravel novel host factors that could serve as promising antiviral targets of prophylaxis and therapy of the infection.

Neurotrophic actions initiated by proNGF in adult sensory neurons may require peri-somatic glia to drive local cleavage to NGF

The nerve growth factor (NGF) precursor, proNGF, is implicated in various neuropathological states. ProNGF signals apoptosis by forming a complex with the receptors p75 and sortilin, however, it can also induce neurite growth, proposed to be mediated by the receptor of mature NGF, tyrosine kinase receptor A (TrkA). The way in which these dual effects occur in adult neurons is unclear. We investigated the neurotrophic effects of proNGF on peptidergic sensory neurons isolated from adult mouse dorsal root ganglia and found that proNGF stimulated neurite extension and branching, requiring p75, sortilin and TrkA. Neurite growth rarely occurred in sortilin-expressing neurons but was commonly observed in TrkA-positive, sortilin-negative neurons that associated closely with sortilin-positive glia. ProNGF was unable to induce local trophic effects at growth cones where sortilin-positive glia was absent. We propose that in adult sensory neurons the neurotrophic response to proNGF is mediated by NGF and TrkA, and that peri-somatic glia may participate in sortilin- and p-75 dependent cleavage of proNGF. The potential ability of local glial cells to provide a targeted supply of NGF may provide an important way to promote trophic (rather than apoptotic) outcomes under conditions where regeneration or sprouting is required.

Essential role for phosphatidylinositol 4,5-bisphosphate in the expression, regulation, and gating of the slow afterhyperpolarization current in the cerebral cortex

Many neurons of the CNS and peripheral nervous system express a slow afterhyperpolarization that is mediated by a slow calcium-activated potassium current. Previous work has shown that this aftercurrent regulates repetitive firing and is an important target for neuromodulators signaling through receptors coupled to G-proteins of the Gα(q-11) and Gα(s) subtypes. Yet, despite considerable effort, a molecular-level understanding of the potassium current underlying the slow afterhyperpolarization and its modulation has proven elusive. Here, we use a combination of pharmacological and molecular biological approaches in cortical brain slices to show that the functional expression of the slow calcium-activated afterhyperpolarizing current in pyramidal cells is critically dependent on membrane phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] and that this dependence accounts for its inhibition by 5-HT(2A) receptors. Furthermore, we show that PtdIns(4,5)P(2) regulates the calcium sensitivity of I(sAHP) in a manner that suggests it acts downstream from the rise in intracellular calcium. These results clarify key functional aspects of the slow afterhyperpolarization current and its modulation by 5-HT(2A) receptors and point to a key role for PtdIns(4,5)P(2) in the gating of this current.

The phosphatidylinositol 4-kinases: don't call it a comeback

Phosphatidylinositol 4-phosphate (PtdIns4P) is a quantitatively minor membrane phospholipid which is the precursor of PtdIns(4,5)P (2) in the classical agonist-regulated phospholipase C signalling pathway. However, PtdIns4P also governs the recruitment and function of numerous trafficking molecules, principally in the Golgi complex. The majority of phosphoinositides (PIs) phosphorylated at the D4 position of the inositol headgroup are derived from PtdIns4P and play roles in a diverse array of fundamental cellular processes including secretion, cell migration, apoptosis and mitogenesis; therefore, PtdIns4P biosynthesis can be regarded as key point of regulation in many PI-dependent processes.Two structurally distinct sequence families, the type II and type III PtdIns 4-kinases, are responsible for PtdIns4P synthesis in eukaryotic organisms. These important proteins are differentially expressed, localised and regulated by distinct mechanisms, indicating that the enzymes perform non-redundant roles in trafficking and signalling. In recent years, major advances have been made in our understanding of PtdIns4K biology and here we summarise current knowledge of PtdIns4K structure, function and regulation.

Fibrinolytic activation promoted by the cyclopentapeptide malformin: involvement of cytoskeletal reorganization

Malformin A₁, a cyclopentapeptide of fungal origin, enhances cellular fibrinolytic activity depending on the existence of a cofactor in blood plasma. However, the nature of this cofactor remains unknown. Here, we report that vitronectin acts as a plasma cofactor of malformin A₁. We purified the cofactor from bovine plasma by activity-based fractionation, and confirmed that vitronectin in conjunction with plasminogen supports the activity of malformin A₁ to promote the fibrinolytic activity of U937 cells. Malformin A₁ action was abolished by Arg-Gly-Asp peptide (a competitor of vitronectin-integrin binding), wortmannin (an inhibitor of signaling kinases), and cytochalasin B (an inhibitor of actin polymerization). Changes in actin organization and a decrease in filopodia were observed in cells treated with malformin A₁ and plasma. A focal localization of plasminogen on the cell surface was augmented by malformin A₁, whereas the amount of cell-surface-bound plasminogen was minimally altered by the treatment. Our results suggest the involvement of cytoskeletal reorganization via vitronectin signaling in the cellular fibrinolytic activity-enhancing action of malformin A₁.

A homogeneous and nonisotopic assay for phosphatidylinositol 4-kinases

Phosphatidylinositol 4-kinases (PI 4-kinases) catalyze the conversion of phosphatidylinositol to phosphatidylinositol 4-phosphate (PtdIns4P). The four known mammalian PI 4-kinases, PI4KA, PI4KB, PI4K2A, and PI4K2B have roles in intracellular lipid and protein trafficking. PI4KA and PI4KB also assist in the replication of several positive-sense RNA viruses. The identification of selective inhibitors of these kinases would be facilitated by assays suitable for high-throughput screening. We describe a homogeneous and nonisotopic assay for PI 4-kinase activity based on the bioluminescent detection of the ADP produced by kinase reactions. We have evaluated this assay with known nonselective inhibitors of PI 4-kinases and show that it performs similar to radiometric assay formats previously described in the literature. In addition, this assay generates Z-factor values of >0.7 for PI4KA in a 384-well format, demonstrating its suitability for high-throughput screening applications.

Genetic and functional studies of phosphatidyl-inositol 4-kinase type IIIα

Phosphatidylinositol 4-kinase type IIIa (PI4KIIIα) is one of four mammalian PI 4-kinases that catalyzes the first committed step in polyphosphoinositide synthesis. PI4KIIIα has been linked to regulation of ER exit sites and to the synthesis of plasma membrane phosphoinositides and recent studies have also revealed its importance in replication of the Hepatitis C virus in liver. Two isoforms of the mammalian PI4KIIIα have been described and annotated in GenBank: a larger, ~230kDa (isoform 2) and a shorter splice variant containing only the ~97kDa C-terminus that includes the catalytic domain (isoform 1). However, Northern analysis of human tissues and cancer cells showed only a single transcript of ~7.5kb with the exception of the proerythroleukemia line K562, which contained significantly higher level of the 7.5kb transcript along with smaller ones of 2.4, 3.5 and 4.2kb size. Bioinformatic analysis also confirmed the high copy number of PI4KIIIα transcript in K562 cells along with several genes located in the same region in Chr22, including two pseudogenes that cover most exons coding for isoform 1, consistent with chromosome amplification. A panel of polyclonal antibodies raised against peptides within the C-terminal half of PI4KIIIα failed to detect the shorter isoform 1 either in COS-7 cells or K562 cells. Moreover, expression of a cDNA encoding isoform 1 yielded a protein of ~97kDa that showed no catalytic activity and failed to rescue hepatitis C virus replication. These data draw attention to PI4KIIIα as one of the genes found in Chr22q11, a region affected by chromosomal instability, but do not substantiate the existence of a functionally relevant short form of PI4KIIIα.

Dual roles for the Drosophila PI 4-kinase four wheel drive in localizing Rab11 during cytokinesis

Successful completion of cytokinesis relies on addition of new membrane, and requires the recycling endosome regulator Rab11, which localizes to the midzone. Despite the critical role of Rab11 in this process, little is known about the formation and composition of Rab11-containing organelles. Here, we identify the phosphatidylinositol (PI) 4-kinase III beta four wheel drive (Fwd) as a key regulator of Rab11 during cytokinesis in Drosophila melanogaster spermatocytes. We show Fwd is required for synthesis of PI 4-phosphate (PI4P) on Golgi membranes and for formation of PI4P-containing secretory organelles that localize to the midzone. Fwd binds and colocalizes with Rab11 on Golgi membranes, and is required for localization of Rab11 in dividing cells. A kinase-dead version of Fwd also binds Rab11 and partially restores cytokinesis to fwd mutant flies. Moreover, activated Rab11 partially suppresses loss of fwd. Our data suggest Fwd plays catalytic and noncatalytic roles in regulating Rab11 during cytokinesis.

Phosphatidylinositol (4,5)-bisphosphate regulation of N-methyl-D-aspartate receptor channels in cortical neurons

The membrane phospholipid phosphatidylinositol (4,5)-bisphosphate (PIP(2)) has been implicated in the regulation of several ion channels and transporters. In this study, we examined the impact of PIP(2) on N-methyl-D-aspartate receptors (NMDARs) in cortical neurons. Blocking PIP(2) synthesis by inhibiting phosphoinositide-4 kinase, or stimulating PIP(2) hydrolysis via activation of phospholipase C (PLC), or blocking PIP(2) function with an antibody caused a significant reduction of NMDAR-mediated currents. On the other hand, inhibition of PLC or application of PIP(2) caused an enhancement of NMDAR currents. These electrophysiological effects were accompanied by changes in NMDAR surface clusters induced by agents that manipulate PIP(2) levels. The PIP(2) regulation of NMDAR currents was abolished by the dynamin inhibitory peptide, which blocks receptor internalization. Agents perturbing actin stability prevented PIP(2) regulation of NMDAR currents, suggesting the actin-dependence of this effect of PIP(2). Cofilin, a major actin depolymerizing factor, which has a common binding sequence for actin and PIP(2), was required for PIP(2) regulation of NMDAR currents. It is noteworthy that the PIP(2) regulation of NMDAR channels was impaired in a transgenic mouse model of Alzheimer's disease, probably because of the amyloid-beta disruption of PIP(2) metabolism. Taken together, our data suggest that continuous synthesis of PIP(2) at the membrane might be important for the maintenance of NMDARs at the cell surface. When PIP(2) is lost, cofilin is released from the PIP(2) complex and is rendered free to depolymerize actin. With the actin cytoskeleton no longer intact, NMDARs are internalized via a dynamin/clathrin-dependent mechanism, leading to reduced NMDAR currents.

TRAPPII is required for cleavage furrow ingression and localization of Rab11 in dividing male meiotic cells of Drosophila

Although membrane addition is crucial for cytokinesis in many animal cell types, the specific mechanisms supporting cleavage furrow ingression are not yet understood. Mutations in the gene brunelleschi (bru), which encodes the Drosophila ortholog of the yeast Trs120p subunit of TRAPPII, cause failure of furrow ingression in male meiotic cells. In non-dividing cells, Brunelleschi protein fused to GFP is dispersed throughout the cytoplasm and enriched at Golgi organelles, similarly to another Drosophila TRAPPII subunit, dBet3. Localization of the membrane-trafficking GTPase Rab11 to the cleavage furrow requires wild-type function of bru, and genetic interactions between bru and Rab11 increase the failure of meiotic cytokinesis and cause synthetic lethality. bru also genetically interacts with four wheel drive (fwd), which encodes a PI4Kbeta, such that double mutants exhibit enhanced failure of male meiotic cytokinesis. These results suggest that Bru cooperates with Rab11 and PI4Kbeta to regulate the efficiency of membrane addition to the cleavage furrow, thus promoting cytokinesis in Drosophila male meiotic cells.

Contribution of natural inhibitors to the understanding of the PI3K/PDK1/PKB pathway in the insulin-mediated intracellular signaling cascade

The critical initial steps in insulin action include phosphorylation of adapter proteins and activation of phosphatidylinositol 3-kinase (PI3K). One of important components in this process is a protein called Akt/protein kinase B (PKB). The work of numerous different researchers indicates a role of PKB in regulating insulin-stimulated glucose uptake. The crucial role of lipid second messengers in PKB activation has been dissected through the use of the PI3K-specific inhibitors wortmannin and LY294002. Receptor-activated PI3K synthesizes the lipid second messenger PtdIns[3,4,5]-trisphosphate, leading to the recruitment of PKB to the membrane. Membrane attachment of PKB is mediated by its pleckstrin homology domain binding to PtdIns[3,4,5]-trisphosphate or PtdIns[3,4]-bisphosphate with high affinity. Activation of PKB alpha is then achieved at the plasma membrane by phosphorylation of Thr308 in the activation-loop of the kinase domain and Ser473 in the carboxy-terminal regulatory region, respectively. 3-Phosphoinositide-dependent protein kinase-1 (PDK1) is responsible for T308 phosphorylation. The usage of specific inhibitors and natural compound has significantly contributed to investigate the molecular mechanism of PI3K/PDK1/PKB signaling pathway, leading to the putative therapeutics benefits of patients. This review focuses on the contribution of natural inhibitor or compound in our understanding of the mechanism by which insulin induces, especially in PI3K/PDK1/PKB signaling.

The multiple roles of phosphoinositide 3-kinase in mast cell biology

Mast cells play a central role in the initiation of inflammatory responses associated with asthma and other allergic disorders. Receptor-mediated mast cell growth, differentiation, homing to their target tissues, survival and activation are all controlled, to varying degrees, by phosphoinositide-3-kinase (PI3K)-driven pathways. It is not fully understood how such diverse responses can be differentially regulated by PI3K. However, recent studies have provided greater insight into the mechanisms that control, and those that are controlled by, different PI3K subunit isoforms in mast cells. In this review, we discuss how PI3K influences the mast cell processes described above. Furthermore, we describe how different mast cell receptors use alternative isoforms of PI3K for these functions and discuss potential downstream targets of these isoforms.

Eukaryotic elongation factor 1A2 cooperates with phosphatidylinositol-4 kinase III beta to stimulate production of filopodia through increased phosphatidylinositol-4,5 bisphosphate generation

Eukaryotic elongation factor 1 alpha 2 (eEF1A2) is a transforming gene product that is highly expressed in human tumors of the ovary, lung, and breast. eEF1A2 also stimulates actin remodeling, and the expression of this factor is sufficient to induce the formation of filopodia, long cellular processes composed of bundles of parallel actin filaments. Here, we find that eEF1A2 stimulates formation of filopodia by increasing the cellular abundance of cytosolic and plasma membrane-bound phosphatidylinositol-4,5 bisphosphate [PI(4,5)P(2)]. We have previously reported that the eEF1A2 protein binds and activates phosphatidylinositol-4 kinase III beta (PI4KIIIbeta), and we find that production of eEF1A2-dependent PI(4,5)P(2) and generation of filopodia require PI4KIIIbeta. Furthermore, PI4KIIIbeta is itself capable of activating both the production of PI(4,5)P(2) and the creation of filopodia. We propose a model for extrusion of filopodia in which eEF1A2 activates PI4KIIIbeta, and activated PI4KIIIbeta stimulates production of PI(4,5)P(2) and filopodia by increasing PI4P abundance. Our work suggests an important role for both eEF1A2 and PI4KIIIbeta in the control of PI(4,5)P(2) signaling and actin remodeling.

Differential regulation of cytokine production by PI3Kdelta in human monocytes upon acute and chronic inflammatory conditions

Deregulated production of cytokines, including IL-1beta, IL-6 and TNF plays an important role in chronic inflammation. Relevant to this condition, direct cellular contact with stimulated T cells is a potent inducer of cytokine production in human monocytes/macrophages. We previously demonstrated that PI3Ks regulate differential production of IL-1beta and its specific inhibitor secreted IL-1 receptor antagonist (sIL-1Ra) by human monocytes. Here we show that in contrast with PI3Kalpha, beta and gamma, PI3Kdelta accounts for most of the PI3K-dependent signaling ruling the production of IL-1beta, IL-6, TNF and sIL-1Ra in monocytes activated by cellular contact with stimulated T cells (mimicked by CHAPS-solubilized membranes of stimulated T cells, CE sHUT) and lipopolysaccharides (LPS); the latter stimuli being relevant to chronic/sterile and acute/infectious inflammation, respectively. Interestingly, PI3Kdelta activity dampened the production of pro-inflammatory cytokines in LPS-activated monocytes, but induced it in CE sHUT-activated cells. In both CE sHUT- and LPS-activated monocytes PI3Kdelta regulated cytokine transcript expression through the phosphorylation/inactivation of glycogen synthase kinase-3beta (GSK3beta). The blockade of GSK3beta displayed inverse effects to those of PI3Kdelta blockade. Thus, by displaying opposite functions in conditions mimicking chronic/sterile and acute/infectious inflammation, i.e., by repressing pro-inflammatory cytokine expression in LPS-activated monocytes but inducing such mediators in T cell contact-activated monocytes, PI3Kdelta represents a potential therapeutic target specific to chronic/sterile inflammatory conditions.

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.

Leptin stimulates alpha1(I) collagen expression in human hepatic stellate cells via the phosphatidylinositol 3-kinase/Akt signalling pathway

BACKGROUND/AIMS

Leptin has been recognized as a profibrogenic hormone in the liver and is involved in collagen type I formation by activated hepatic stellate cells (HSCs) in response to fibrogenic substances, but the molecular signal mechanisms by which leptin promotes liver fibrogenesis through upregulation of collagen type I expression is not clear. We investigated whether leptin-induced collagen type I is mediated by the Janus kinase-phosphatidylinositol 3-kinase-Akt (JAKs-PI3K-Akt) pathway in a human HSC cell line, LX-2.

METHODS

LX-2 cells were treated with or without various inhibitors in the presence of leptin.

RESULTS

Leptin increased alpha1(I) collagen mRNA and protein. JAK1, PI3K and Akt were activated after leptin stimulation. AG490, a JAK inhibitor, blocked JAK1 phosphorylation accompanied by inhibition of PI3K and Akt activation as well as alpha1(I) collagen mRNA expression, indicating a JAK1-dependent mechanism. Wortmannin, a PI3K inhibitor, prevented PI3K and Akt activation and resulted in suppression of alpha1(I) collagen mRNA expression, suggesting a PI3K-mediated process. These changes were reproduced by overexpression of the dominant-negative p85alpha mutant. A443654.3, an Akt inhibitor, opposed Akt activation, leading to downregulation of alpha1(I) collagen mRNA. Overexpression of the dominant-negative Akt mutant led to similar alterations.

CONCLUSION

Leptin has a direct action on liver fibrogenesis by stimulating alpha1(I) collagen production in activated HSC. The process appears to be mediated by the PI3K/Akt pathway through activated JAK1.

Targeting phosphoinositide 3-kinase: moving towards therapy

Phosphoinositide 3-kinases (PI3K) orchestrate cell responses including mitogenic signaling, cell survival and growth, metabolic control, vesicular trafficking, degranulation, cytoskeletal rearrangement and migration. Deregulation of the PI3K pathway occurs by activating mutations in growth factor receptors or the PIK3CA locus coding for PI3Kalpha, by loss of function of the lipid phosphatase and tensin homolog deleted in chromosome ten (PTEN/MMAC/TEP1), by the up-regulation of protein kinase B (PKB/Akt), or the impairment of the tuberous sclerosis complex (TSC1/2). All these events are linked to growth and proliferation, and have thus prompted a significant interest in the pharmaceutical targeting of the PI3K pathway in cancer. Genetic targeting of PI3Kgamma (p110gamma) and PI3Kdelta (p110delta) in mice has underlined a central role of these PI3K isoforms in inflammation and allergy, as they modulate chemotaxis of leukocytes and degranulation in mast cells. Proof-of-concept molecules selective for PI3Kgamma have already successfully alleviated disease progress in murine models of rheumatoid arthritis and lupus erythematosus. As targeting PI3K moves forward to therapy of chronic, non-fatal disease, safety concerns for PI3K inhibitors increase. Many of the present inhibitor series interfere with target of rapamycin (TOR), DNA-dependent protein kinase (DNA-PK(cs)) and activity of the ataxia telangiectasia mutated gene product (ATM). Here we review the current disease-relevant knowledge for isoform-specific PI3K function in the above mentioned diseases, and review the progress of >400 recent patents covering pharmaceutical targeting of PI3K. Currently, several drugs targeting the PI3K pathway have entered clinical trials (phase I) for solid tumors and suppression of tissue damage after myocardial infarction (phases I,II).

Specificity, promiscuity and localization of ARF protein interactions with NCS-1 and phosphatidylinositol-4 kinase-III beta

ADP-ribosylation factor (ARF) proteins are involved in multiple intracellular vesicular transport pathways. Most studies have focused on the functions of ARF1 or ARF6 and little is known about the remaining ARF isoforms. Although the mammalian ARF proteins share a high degree of sequence identity, recent evidence has indicated that they may control distinct trafficking steps within cells. A unanswered issue is the degree of specificity of ARF family members for different interacting proteins. To investigate potential functional differences between the human ARF proteins, we have examined the localization of all human ARF isoforms and their interactions with two ARF1 binding proteins, neuronal calcium sensor-1 (NCS-1) and phosphatidylinositol-4 kinase-IIIbeta (PI4Kbeta). Use of a fluorescent protein fragment complementation method showed direct interactions between ARFs 1, 3, 5 and 6 with NCS-1 but at different intracellular locations in live HeLa cells. Photobleaching experiments indicated that complementation did not detect dynamic changes in protein interactions over short-time scales. A more specific interaction between ARFs 1/3 and PI4Kbeta was observed. Consistent with these latter findings ARF1 but not ARF5 or 6 enhanced the stimulatory effect of PI4Kbeta on regulated exocytosis, suggesting a specific role for class-I ARFs in the regulation of PI4Kbeta.

Modulation of cyclic nucleotide-regulated HCN channels by PIP(2) and receptors coupled to phospholipase C

Recent results indicate that phosphoinositides, including phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)), directly enhance the opening of hyperpolarization-activated, cyclic nucleotide-regulated (HCN) channels by shifting their activation gating to more positive voltages. This contrasts with the action of phosphoinositides to inhibit the opening of the related cyclic nucleotide-gated (CNG) channels involved in sensory signaling. We both review previous studies and present new experiments that investigate whether HCN channels may be regulated by dynamic changes in PI(4,5)P(2) levels caused by the receptor-mediated activation of phospholipase C (PLC). We coexpressed HCN1 or HCN2 channels in Xenopus oocytes with the PLC-coupled bradykinin BK(2) receptor, the muscarinic M1 receptor, or the TrkA receptor. Activation of all three receptors produced a positive shift in HCN channel voltage gating, the opposite of the effect expected for PI(4,5)P(2) depletion. This action was not caused by alterations in cAMP as the effect was preserved in HCN mutant channels that fail to bind cAMP. The receptor effects were mediated by PLC activity, but did not depend on signaling through the downstream products of PI(4,5)P(2) hydrolysis: IP(3) or diacylglycerol (DAG). Importantly, the modulatory effects on gating were blocked by inhibitors of phosphatidylinositol (PI) kinases, suggesting a role for increased PI(4,5)P(2) synthesis. Finally, we found that bradykinin exerted a similar PI kinase-dependent effect on the gating of native HCN channels in cardiac sinoatrial node cells, suggesting that this pathway may represent a novel, physiologically relevant mechanism for enhancing HCN channel function.

Exploring the specificity of the PI3K family inhibitor LY294002

The PI3Ks (phosphatidylinositol 3-kinases) regulate cellular signalling networks that are involved in processes linked to the survival, growth, proliferation, metabolism and specialized differentiated functions of cells. The subversion of this network is common in cancer and has also been linked to disorders of inflammation. The elucidation of the physiological function of PI3K has come from pharmacological studies, which use the enzyme inhibitors Wortmannin and LY294002, and from PI3K genetic knockout models of the effects of loss of PI3K function. Several reports have shown that LY294002 is not exclusively selective for the PI3Ks, and could in fact act on other lipid kinases and additional apparently unrelated proteins. Since this inhibitor still remains a drug of choice in numerous PI3K studies (over 500 in the last year), it is important to establish the precise specificity of this compound. We report here the use of a chemical proteomic strategy in which an analogue of LY294002, PI828, was immobilized onto epoxy-activated Sepharose beads. This affinity material was then used as a bait to fish-out potential protein targets from cellular extracts. Proteins with high affinity for immobilized PI828 were separated by one-dimensional gel electrophoresis and identified by liquid chromatography-tandem MS. The present study reveals that LY294002 not only binds to class I PI3Ks and other PI3K-related kinases, but also to novel targets seemingly unrelated to the PI3K family.

Synthesis and function of membrane phosphoinositides in budding yeast, Saccharomyces cerevisiae

It is now well appreciated that derivatives of phosphatidylinositol (PtdIns) are key regulators of many cellular processes in eukaryotes. Of particular interest are phosphoinositides (mono- and polyphosphorylated adducts to the inositol ring in PtdIns), which are located at the cytoplasmic face of cellular membranes. Phosphoinositides serve both a structural and a signaling role via their recruitment of proteins that contain phosphoinositide-binding domains. Phosphoinositides also have a role as precursors of several types of second messengers for certain intracellular signaling pathways. Realization of the importance of phosphoinositides has brought increased attention to characterization of the enzymes that regulate their synthesis, interconversion, and turnover. Here we review the current state of our knowledge about the properties and regulation of the ATP-dependent lipid kinases responsible for synthesis of phosphoinositides and also the additional temporal and spatial controls exerted by the phosphatases and a phospholipase that act on phosphoinositides in yeast.

Requirement for Phosphoinositide 3-Kinase p110δ Signaling in B Cell Antigen Receptor-Mediated Antigen Presentation

The BCR serves to both signal cellular activation and enhance uptake and presentation of Ags by B cells; however, the intracellular signaling mechanisms linking the BCR to Ag presentation functions have been controversial. PI3Ks are critical signaling enzymes controlling many cellular processes, with the p110delta isoform playing a critical role in BCR signaling. In this study, we used pharmacological and genetic approaches to evaluate the role of p110delta signaling in Ag presentation by primary B lymphocytes. It was found that activation of allogeneic T cells is significantly reduced when B cells are pretreated with global PI3K inhibitors, but was intact when p110delta signaling was specifically inactivated. In contrast, inactivation of p110delta significantly impaired the ability of B cells to activate T cells in a BCR-mediated Ag uptake and presentation model. Prestimulation of p110delta-inactivated B cells with anti-CD40 or LPS could not rescue their BCR-mediated Ag presentation ability to normal levels. p110delta signaling was required for efficient presentation of either anti-Ig or protein Ag via a lysozyme-specific BCR. p110delta-inactivated B cells were able to internalize Ag normally, and no defects in association of Ag with lysosome-associated membrane protein 1(+) late endosomes were observed; however, these cells were less effective in forming polarized conjugates with Ag-specific T cells. Our data demonstrate a role for p110delta signaling in B cell Ag presentation function, implicating 3-phosphoinositides and their targets in the latter stages of this process.

A membrane capture assay for lipid kinase activity

Phosphoinositide kinases such as PI3-kinase synthesize lipid second messengers that control diverse cellular processes. Recently, these enzymes have emerged as an important class of drug targets, and there is significant interest in discovering new lipid kinase inhibitors. We describe here a procedure for the high-throughput determination of lipid kinase inhibitor IC50 values. This assay exploits the fact that phosphoinositides, but not nucleotides such as ATP, bind irreversibly to nitrocellulose membranes. As a result, the radiolabeled lipids from a kinase assay can be isolated by spotting the crude reaction on a nitrocellulose membrane and then washing. We show that diverse phosphoinositide kinases can be assayed using this approach and outline how to perform the assay in 96-well plates. We also describe a MATLAB script that automates the data analysis. The complete procedure requires 3-4 h.

Ethanol inhibits cold-menthol receptor TRPM8 by modulating its interaction with membrane phosphatidylinositol 4,5-bisphosphate

Ethanol has opposite effects on two members of the transient receptor potential (TRP) family of ion channels: it inhibits the cold-menthol receptor TRPM8, whereas it potentiates the activity of the heat- and capsaicin-gated vanilloid receptor TRPV1. Both thermosensitive cation channels are critically regulated by the membrane lipid, phosphatidylinositol 4,5-bisphosphate (PIP(2)). The effects of this phospholipid on TRPM8 and TRPV1 are also functionally opposite: PIP(2) is necessary for the activation of TRPM8 but it constitutively inhibits TRPV1. This parallel led us to investigate the possible role of PIP(2) in the ethanol-induced modulation of rat TRPM8, heterologously expressed in HEK293T cells. In this study, we characterize the effects of ethanol (0.1-10%) on whole-cell currents produced by menthol and by low temperature (< 17 degrees C). We show that the inclusion of PIP(2) in the intracellular solution results in a strong reduction in the ethanol-induced inhibition of menthol-evoked responses. Conversely, intracellular dialysis with anti-PIP(2) antibody or with the PIP(2) scavenger, poly L-lysine, enhanced the ethanol-induced inhibition of TRPM8. A 20 min pre-incubation with wortmannin caused a modest decrease in inhibition produced by 1% ethanol, indicating that the ethanol-induced inhibition is not mediated by lipid kinases. These findings suggest that ethanol inhibits TRPM8 by weakening the PIP(2)-TRPM8 channel interaction; a similar mechanism may contribute to the ethanol-mediated modulation of some other PIP(2)-sensitive TRP channels.

Functional Implication of Neuronal Calcium Sensor-1 and Phosphoinositol 4-Kinase-β Interaction in Regulated Exocytosis of PC12 Cells

Several studies have shown that the neuronal calcium sensor (NCS-1) and phosphoinositol 4-kinase-beta (PI4K-beta) regulate the exocytotic process of nerve and neuroendocrine cells. The aim of our study was to investigate their possible interaction at rest and during stimulation in living cells and to decipher the role of this interaction in the secretory process. In PC12 cells, we observed a stimulation-induced recruitment of NCS-1 and PI4K-beta from the intracellular compartment toward the plasma membrane. This recruitment was highly correlated to the intracellular Ca(2+) rise induced by secretagogues. Using fluorescence resonance energy transfer between PI4K-beta-ECFP and NCS-1-EYFP, we show that both proteins are interacting in resting cells and that this interaction increases with stimulation. It appears that the membrane insertion of NCS-1 is necessary for the interaction with PI4K-beta, since a mutation that prevented the membrane insertion of NCS-1 abolished NCS-1-PI4K-beta interaction, as revealed by fluorescence resonance energy transfer analysis. Additionally, the overexpression of mutated NCS-1 prevents the stimulatory effect on secretion induced by PI4K-beta, suggesting that the interaction of the two proteins on a membrane compartment is necessary for the secretory function. Moreover, extinction of endogenous PI4K-beta by small interfering RNA inhibits secretion and completely prevents the stimulatory effect of NCS-1 on calcium-evoked exocytosis from permeabilized PC12 cells, showing directly for the first time the functional implication of a NCS-1.PI4K-beta complex in regulated exocytosis.

ATP and PIP2 dependence of the magnesium-inhibited, TRPM7-like cation channel in cardiac myocytes

The Mg(2+)-inhibited cation (MIC) current (I(MIC)) in cardiac myocytes biophysically resembles currents of heterologously expressed transient receptor potential (TRP) channels, particularly TRPM6 and TRPM7, known to be important in Mg(2+) homeostasis. To understand the regulation of MIC channels in cardiac cells, we used the whole cell voltage-clamp technique to investigate the role of intracellular ATP in pig, rat, and guinea pig isolated ventricular myocytes. I(MIC), studied in the presence or absence of extracellular divalent cations, was sustained for >or=50 min after patch rupture in ATP-dialyzed cells, whereas in ATP-depleted cells I(MIC) exhibited complete rundown. Equimolar substitution of internal ATP by its nonhydrolyzable analog adenosine 5'-(beta,gamma-imido)triphosphate failed to prevent rundown. In ATP-depleted cells, inhibition of lipid phosphatases by fluoride + vanadate + pyrophosphate prevented I(MIC) rundown. In contrast, under similar conditions neither the inhibition of protein phosphatases 1, 2A, 2B or of protein tyrosine phosphatase nor the activation of protein kinase A (forskolin, 20 microM) or protein kinase C (phorbol myristate acetate, 100 nM) could prevent rundown. In ATP-loaded cells, depletion of phosphatidylinositol 4,5-bisphosphate (PIP(2)) by prevention of its resynthesis (10 microM wortmannin or 15 microM phenylarsine oxide) induced rundown of I(MIC). Finally, loading ATP-depleted cells with exogenous PIP(2) (10 microM) prevented rundown. These results suggest that PIP(2), likely generated by ATP-utilizing lipid kinases, is necessary for maintaining cardiac MIC channel activity.

Gene silencing reveals a specific function of hVps34 phosphatidylinositol 3-kinase in late versus early endosomes

The human type III phosphatidylinositol 3-kinase, hVps34, converts phosphatidylinositol (PtdIns) to phosphatidylinositol 3-phosphate [PtdIns(3)P]. Studies using inhibitors of phosphatidylinositide 3-kinases have indicated that production of PtdIns(3)P is important for a variety of vesicle-mediated trafficking events, including endocytosis, sorting of receptors in multivesicular endosomes, and transport of lysosomal enzymes from the trans-Golgi network (TGN) to the endosomes and lysosomes. This study utilizes small interfering (si)RNA-mediated gene silencing to define the specific trafficking pathways in which hVps34 functions in human U-251 glioblastoma cells. Suppression of hVps34 expression reduced the cellular growth rate and caused a striking accumulation of large acidic phase-lucent vacuoles that contain lysosomal membrane proteins LAMP1 and LGP85. Analysis of these structures by electron microscopy suggests that they represent swollen late endosomes that have lost the capacity for inward vesiculation but retain the capacity to fuse with lysosomes. Morphological perturbation of the late endosome compartment was accompanied by a reduced rate of processing of the endosomal intermediate form of cathepsin D to the mature lysosomal form. There was also a reduction in the rate of epidermal growth factor receptor (EGFR) dephosphorylation and degradation following ligand stimulation, consistent with the retention of the EGFR on the limiting membranes of the enlarged late endosomes. By contrast, the suppression of hVps34 expression did not block trafficking of cathepsin D between the TGN and late endosomes, or endocytic uptake of fluid-phase markers, or association of a PtdIns(3)P-binding protein, EEA1, with early endosomes. LAMP1-positive vacuoles were depleted of PtdIns(3)P in the hVps34-knockdown cells, as judged by their inability to bind the PtdIns(3)P probe GFP-2xFYVE. By contrast, LAMP1-negative vesicles continued to bind GFP-2xFYVE in the knockdown cells.

Overall, these findings indicate that hVps34 plays a major role in generating PtdIns(3)P for internal vesicle formation in multivesicular/late endosomes. The findings also unexpectedly suggest that other wortmannin-sensitive kinases and/or polyphosphoinositide phosphatases may be able to compensate for the loss of hVps34 and maintain PtdIns(3)P levels required for vesicular trafficking in the early endocytic pathway or the TGN.

Signaling microdomains regulate inositol 1,4,5-trisphosphate-mediated intracellular calcium transients in cultured neurons

Ca2+ signals in neurons use specific temporal and spatial patterns to encode unambiguous information about crucial cellular functions. To understand the molecular basis for initiation and propagation of inositol 1,4,5-trisphosphate (InsP3)-mediated intracellular Ca2+ signals, we correlated the subcellular distribution of components of the InsP3 pathway with measurements of agonist-induced intracellular Ca2+ transients in cultured rat hippocampal neurons and pheochromocytoma cells. We found specialized domains with high levels of phosphatidylinositol-4-phosphate kinase (PIPKI) and chromogranin B (CGB), proteins acting synergistically to increase InsP3 receptor (InsP3R) activity and sensitivity. In contrast, Ca2+ pumps in the plasma membrane (PMCA) and sarco-endoplasmic reticulum as well as buffers that antagonize the rise in intracellular Ca2+ were distributed uniformly. By pharmacologically blocking phosphatidylinositol-4-kinase and PIPKI or disrupting the CGB-InsP3R interaction by transfecting an interfering polypeptide fragment, we produced major changes in the initiation site and kinetics of the Ca2+ signal. This study shows that a limited number of proteins can reassemble to form unique, spatially restricted signaling domains to generate distinctive signals in different regions of the same neuron. The finding that the subcellular location of initiation sites and protein microdomains was cell type specific will help to establish differences in spatiotemporal Ca2+ signaling in different types of neurons.

Functional control of cold- and menthol-sensitive TRPM8 ion channels by phosphatidylinositol 4,5-bisphosphate

Cold is detected by a small subpopulation of peripheral thermoreceptors. TRPM8, a cloned menthol- and cold-sensitive ion channel, has been suggested to mediate cold transduction in the innocuous range. The channel shows a robust response in whole-cell recordings but exhibits markedly reduced activity in excised membrane patches. Here we report that phosphatidylinositol 4,5-bisphosphate (PIP2) is an essential regulator of the channel function. The rundown of the channel is prevented by lipid phosphatase inhibitors. Application of exogenous PIP2 both activates the channel directly and restores rundown activity. Whole-cell experiments involving intracellular dialysis with polyvalent cations, inhibition of PIP2 synthesis kinases, and receptor-mediated hydrolysis of PIP2 show that PIP2 also modulates the channel activity in the intact cells. The crucial role of PIP2 on the function of TRPM8 suggests that the membrane PIP2 level may be an important regulator of cold transduction in vivo. The opposite effects of PIP2 on the vanilloid receptor TRPV1 and TRPM8 also implies that the membrane lipid may have dual actions as a bimodal switch to selectively control the heat- and cold-induced responses in nociceptors expressing both channels.

Regulation of the muscarinic K+ channel by extracellular ATP through membrane phosphatidylinositol 4,5-bisphosphate in guinea-pig atrial myocytes

1 The present study was designed to examine the functional role of membrane phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2)) in the regulation of the muscarinic K(+) channel (I(K,ACh)) by extracellular ATP and adenosine in guinea-pig atrial myocytes, using the whole-cell patch-clamp method. 2 Bath application of ATP in micromolar concentrations typically evoked a transient activation of I(K,ACh); a rapid activation phase was consistently followed by a progressive decline even to the baseline level despite the continued presence of ATP. This progressive decline of I(K,ACh) was significantly attenuated either by blockade of phospholipase C (PLC) with compound 48/80 (100 microM) or by addition of PtdIns(4,5)P(2) (50 microM) to the cell inside, suggesting that depletion of membrane PtdIns(4,5)P(2) via PLC activation is mainly, if not totally, responsible for the progressive decline of I(K,ACh) during the presence of ATP. 3 When atrial myocytes were exposed to wortmannin (50 microM) following ATP (50 microM) application to impair the resynthesis of PtdIns(4,5)P(2), the activation of I(K,ACh) evoked by subsequently applied ATP (50 microM) was greatly reduced. Activation of I(K,ACh) by adenosine (100 microM) was partially reduced by pretreatment of atrial myocytes with ATP (100 microM) and was largely abolished by a further addition of wortmannin (50 microM) in the presence of ATP (100 microM). These results support the view that the activation of I(K,ACh) by ATP and adenosine depends on membrane PtdIns(4,5)P(2) that is subject to reduction by extracellular ATP. 4 The present study thus provides functional evidence to suggest that extracellular ATP activates PLC and thereby depletes membrane PtdIns(4,5)P(2) that is critically involved in the activation process of I(K,ACh) by its agonists ATP and adenosine in guinea-pig atrial myocytes.

Neuronal calcium sensor-1 potentiates glucose-dependent exocytosis in pancreatic beta cells through activation of phosphatidylinositol 4-kinase beta

Cytosolic free Ca2+ plays an important role in the molecular mechanisms leading to regulated insulin secretion by the pancreatic beta cell. A number of Ca2+-binding proteins have been implicated in this process. Here, we define the role of the Ca2+-binding protein neuronal Ca2+ sensor-1 (NCS-1) in insulin secretion. In pancreatic beta cells, NCS-1 increases exocytosis by promoting the priming of secretory granules for release and increasing the number of granules residing in the readily releasable pool. The effect of NCS-1 on exocytosis is mediated through an increase in phosphatidylinositol (PI) 4-kinase beta activity and the generation of phosphoinositides, specifically PI 4-phosphate and PI 4,5-bisphosphate. In turn, PI 4,5-bisphosphate controls exocytosis through the Ca2+-dependent activator protein for secretion present in beta cells. Our results provide evidence for an essential role of phosphoinositide synthesis in the regulation of glucose-induced insulin secretion by the pancreatic beta cell. We also demonstrate that NCS-1 and its downstream target, PI 4-kinase beta, are critical players in this process by virtue of their capacity to regulate the release competence of the secretory granules.

Sustained diacylglycerol accumulation resulting from prolonged G protein-coupled receptor agonist-induced phosphoinositide breakdown in hepatocytes

Studies in various cells have led to the idea that agonist-stimulated diacylglycerol (DAG) generation results from an early, transient phospholipase C (PLC)-catalyzed phosphoinositide breakdown, while a more sustained elevation of DAG originates from phosphatidylcholine (PC). We have examined this issue further, using cultured rat hepatocytes, and report here that various G protein-coupled receptor (GPCR) agonists, including vasopressin (VP), angiotensin II (Ang.II), prostaglandin F2alpha, and norepinephrine (NE), may give rise to a prolonged phosphoinositide hydrolysis. Preincubation of hepatocytes with 1-butanol to prevent conversion of phosphatidic acid (PA) did not affect the agonist-induced DAG accumulation, suggesting that phospholipase D-mediated breakdown of PC was not involved. In contrast, the GPCR agonists induced phosphoinositide turnover, assessed by accumulation of inositol phosphates, that was sustained for up to 18 h, even under conditions where PLC was partially desensitized. Pretreatment of hepatocytes with wortmannin, to inhibit synthesis of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (PIP2), prevented agonist-induced inositol phosphate and DAG accumulation. Upon VP stimulation the level of PIP) declined, but only transiently, while increases in inositol 1,4,5-trisphosphate (InsP3) and DAG mass were sustained, suggesting that efficient resynthesis of PIP2 allowed sustained PLC activity. This was confirmed when cells were pretreated with wortmannin to prevent resynthesis of PIP2. Furthermore, metabolism of InsP3 was rapid, compared to that of DAG, with a more than 20-fold difference in half-life. Thus, rapid metabolism of InsP3 and efficient resynthesis of PIP2 may account for the larger amount of DAG generated and the more sustained time course, compared to InsP3. The results suggest that DAG accumulation that is sustained for many hours in response to VP, Ang.II, NE, and prostaglandin F2alpha in hepatocytes is mainly due to phosphoinositide breakdown.