Mammalian phosphoinositide-specific phospholipase C

Pharmacological agents that directly modulate insulin secretion

Blood glucose levels are sensed and controlled by the release of hormones from the islets of Langerhans in the pancreas. The beta-cell, the insulin-secreting cell in the islet, can detect subtle increases in circulating glucose levels and a cascade of molecular events spanning the initial depolarization of the beta-cell membrane culminates in exocytosis and optimal insulin secretion. Here we review these processes in the context of pharmacological agents that have been shown to directly interact with any stage of insulin secretion. Drugs that modulate insulin secretion do so by opening the K(ATP) channels, by interacting with cell-surface receptors, by altering second-messenger responses, by disrupting the beta-cell cytoskeletal framework, by influencing the molecular reactions at the stages of transcription and translation of insulin, and/or by perturbing exocytosis of the insulin secretory vesicles. Drugs acting primarily at the K(ATP) channels are the sulfonylureas, the benzoic acid derivatives, the imidazolines, and the quinolines, which are channel openers, and finally diazoxide, which closes these channels. Methylxanthines also work at the cell membrane level by antagonizing the purinergic receptors and thus increase insulin secretion. Other drugs have effects at multiple levels, such as the calcineurin inhibitors and somatostatin. Some drugs used extensively in research, e.g., colchicine, which is used to study vesicular transport, have no effect at the pharmacological doses used in clinical practice. We also briefly discuss those drugs that have been shown to disrupt beta-cell function in a clinical setting but for which there is scant information on their mechanism of action.

Gelsolin suppresses tumorigenicity through inhibiting PKC activation in a human lung cancer cell line, PC10

Gelsolin expression is frequently downregulated in lung cancer and several types of different human cancers. To examine the effects of gelsolin restoration on tumorigenicity, we here stably expressed various levels of gelsolin via gene transfer in lung cancer cells (squamous cell carcinoma line, PC10). We observed the alterations in tumorigenicity in vivo when implanted in nude mice, and the changes in growth properties in vitro. As compared to parental cells and control clones, gelsolin transfectants highly reduced tumorigenicity and repressed cell proliferation. Moreover, we investigated bradykinin-induced responses in gelsolin-overexpressing clones, because agonist-stimulated activation of the phospholipases C (PLC)/protein kinase C (PKC) signal transduction pathway is critical for cell growth and tumorigenicity. Bradykinin promotes phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis by PLC and translocation of various PKC isoforms from the cytosolic fraction to the particulate fraction. Bradykinin treatment did not increase inositoltriphosphate (IP3) production and induce the membrane fractions of PKC alpha and PKC gamma in gelsolin tranfectants, while it induced PIP2 hydrolysis and increased the fractions in parental and control clones. These results suggest that gelsolin suppressed the activation of PKCs involved in phospholipid signalling pathways, inhibiting cell proliferation and tumorigenicity.

Listeria monocytogenes phosphatidylinositol-specific phospholipase C: activation and allostery

The animal and human pathogen Listeria monocytogenes secretes several virulence factors, including a phosphatidylinositol-specific phospholipase C (PI-PLC). Sufficient quantities of L. monocytogenes PI-PLC for biophysical studies were obtained by overexpression of the enzyme in Escherichia coli. The purified PI-PLC was examined in enzyme kinetics experiments using a new fluorogenic substrate, methyl-FLIP. Methyl-FLIP is a water-soluble monomeric substrate cleaved in a manner similar to the natural aggregate substrate, phosphatidylinositol (PI). Michaelis-Menten kinetics were observed with K(M) = 61 +/- 7 microM and V(max) = 120 +/- 5 micromol min(-1) mg(-1), corresponding to k(cat) = 66+/-3 s(-1). The catalysis is activated by the addition of a short-chain phospholipid, dihexanoyl phosphatidylcholine (diC(6)PC). The kinetics were fitted to a two-site model in which the substrate binds to the active site and diC(6)PC binds to a second site, with an interaction between the two sites. The result is a decrease in K(M) and an increase in V(max), producing an overall four to five-fold increase in catalytic efficiency (k(cat)/K(M)). The interaction is not a regulatory mechanism, as is the case for multimeric enzymes; rather, it suggests interfacial cooperativity between the active site and a lipid-binding subsite, presumably adjacent to the active site.

PTEN associates with the vault particles in HeLa cells

PTEN is a tumor suppressor that primarily dephosphorylates phosphatidylinositol 3,4,5-trisphosphate to down-regulate the phosphoinositide 3-kinase/Akt signaling pathway. Although the cellular functions of PTEN as a tumor suppressor have been well characterized, the mechanism by which PTEN activity is modulated by other signal molecules in vivo remains poorly understood. In searching for potential PTEN modulators through protein-protein interaction, we identified the major vault protein (MVP) as a dominant PTEN-binding protein in a yeast two-hybrid screen. MVP is the major structural component of vault, the largest intracellular ribonucleoprotein particle. Co-immunoprecipitation confirmed the interaction between PTEN and MVP in transfected mammalian cells. More importantly, we found that a significant portion of endogenous PTEN associates with vault particles in human HeLa cells. Deletion mutation analysis demonstrated that MVP binds to the C2 domain of PTEN and that PTEN interacts with MVP through its EF hand-like motif. Furthermore, the in vitro binding experiments revealed that the interaction of PTEN with MVP is Ca(2+)-dependent.

Carboxyl-terminal basic amino acids in the X domain are essential for the nuclear import of phospholipase C δ1

BACKGROUND

Although phospholipase C (PLC)delta1 containing a functional nuclear export signal (NES) is normally localized at the plasma membrane and in the cytoplasm, it shuttles between the nucleus and the cytoplasm. Since nucleocytoplasmic shuttling of a molecule is generally regulated by a balance between its NES and the nuclear localization signal (NLS), we examined whether PLCdelta1 contains an NLS sequence.

RESULTS

A region corresponding to the C terminus of the X domain and the XY-linker, which contains clusters of basic amino acid residues, was essential for the nuclear import of PLCdelta1 in Madin-Darby canine kidney cells. A series of point mutations on lysine residues in this region revealed that K432 and K434 in combination were important for the nuclear import. A short synthetic peptide corresponding to residues 429-442, however, was not able to function as an NLS sequence when they were injected into the cytoplasm in a carrier-conjugated form. Neither a longer peptide equivalent to PLCdelta1 412-498 fused to a protein tag consisting of glutathione S-transferase and green fluorescent protein was imported to the nucleus after microinjection into the cytoplasm.

CONCLUSION

The nuclear import of PLCdelta1 requires the C-terminus of the X domain, particularly the amino acid residues K432 and K434, and the XY-linker. The region alone, however, cannot serve as a functional NLS. The machinery for nuclear transport may require additional structural component(s) of the enzyme.

RasGRP4 is a novel Ras activator isolated from acute myeloid leukemia

Although a number of genetic defects are commonly associated with acute myeloid leukemia (AML), a large percentage of AML cases are cytogenetically normal. This suggests a functional screen for transforming genes is required to identify genetic mutations that are missed by cytogenetic analyses. We utilized a retrovirus-based cDNA expression system to identify transforming genes expressed in cytogenetically normal AML patients. We identified a new member of the Ras guanyl nucleotide-releasing protein (RasGRP) family of Ras guanine nucleotide exchange factors, designating it RasGRP4. Subsequently, cDNA sequences encoding rodent and human RasGRP4 proteins were deposited in GenBank. RasGRP4 contains the same protein domain structure as other members of the RasGRP family, including a Ras exchange motif, a CDC25 homology domain, a C1/diacyglycerol-binding domain, and putative calcium-binding EF hands. We show that expression of RasGRP4 induces anchorage-independent growth of Rat1 fibroblasts. RasGRP4 is a Ras-specific activator and, interestingly, is highly expressed in peripheral blood leukocytes and myeloid cell lines. Unlike other RasGRP proteins, RasGRP4 is not expressed in the brain or in lymphoid cells. We demonstrated that 32D myeloid cells expressing RasGRP4 have elevated levels of activated Ras compared with control cells, and phorbol 12-myristate 13-acetate (PMA) treatment greatly enhanced Ras activation. PMA induced membrane localization of RasGRP4 and 32D cells expressing RasGRP4 were capable of cytokine-independent proliferation in the presence of PMA. We conclude that RasGRP4 is a member of the RasGRP family of Ras guanine nucleotide exchange factors that may play a role in myeloid cell signaling growth regulation pathways that are responsive to diacylglycerol levels.

PLCζ: a sperm-specific trigger of Ca2+ oscillations in eggs and embryo development

Upon fertilisation by sperm, mammalian eggs are activated by a series of intracellular Ca2+ oscillations that are essential for embryo development. The mechanism by which sperm induces this complex signalling phenomenon is unknown. One proposal is that the sperm introduces an exclusive cytosolic factor into the egg that elicits serial Ca2+ release. The ‘sperm factor’ hypothesis has not been ratified because a sperm-specific protein that generates repetitive Ca2+ transients and egg activation has not been found. We identify a novel, sperm-specific phospholipase C, PLCζ, that triggers Ca2+ oscillations in mouse eggs indistinguishable from those at fertilisation. PLCζ removal from sperm extracts abolishes Ca2+ release in eggs. Moreover, the PLCζ content of a single sperm was sufficient to produce Ca2+ oscillations as well as normal embryo development to blastocyst. Our results are consistent with sperm PLCζ as the molecular trigger for development of a fertilised egg into an embryo.

Networking of phospholipases in plant signal transduction

Phospholipases are activated in response to various cellular and environmental cues. Their activation can affect many cellular processes through their roles in signal transduction. Recent advances in the biochemical and molecular understanding of phospholipase D (PLD) have provided insights into potential networks of PLDs and other phospholipases in plants. PLDs are a family of heterogeneous enzymes, and the activities of the multiple types of PLDs are regulated in distinctly different manners. Phosphoinositides, free fatty acids, lysophospholipids, and calcium are differential modulators of PLDs. Since these modulators are substrates, products, or downstream targets of phospholipase As and phospholipase Cs, there are many potential regulatory and metabolic interrelationships among the various PLDs and other phospholipases.

Role of tryptophan residues in interfacial binding of phosphatidylinositol-specific phospholipase C

The phosphatidylinositol-specific phospholipase C (PI-PLC) from Bacillus thuringiensis exhibits several types of interfacial activation. In the crystal structure of the closely related Bacillus cereus PI-PLC, the rim of the active site is flanked by a short helix B and a loop that show an unusual clustering of hydrophobic amino acids. Two of the seven tryptophans in PI-PLC are among the exposed residues. To test the importance of these residues in substrate and activator binding, we prepared several mutants of Trp-47 (in helix B) and Trp-242 (in the loop). Two other tryptophans, Trp-178 and Trp-280, which are not near the rim, were mutated as controls. Kinetic (both phosphotransferase and cyclic phosphodiesterase activities), fluorescence, and vesicle binding analyses showed that both Trp-47 and Trp-242 residues are important for the enzyme to bind to interfaces, both activating zwitterionic and substrate anionic surfaces. Partitioning of the enzyme to vesicles is decreased more than 10-fold for either W47A or W242A, and removal of both tryptophans (W47A/W242A) yields enzyme with virtually no affinity for phospholipid surfaces. Replacement of either tryptophan with phenylalanine or isoleucine has moderate effects on enzyme affinity for surfaces but yields a fully active enzyme. These results are used to describe how the enzyme is activated by interfaces.

G protein-coupled receptor signalling and cross-talk: achieving rapidity and specificity

Activation of a given type of G protein-coupled receptor (GPCR) triggers a limited set of signalling events in a very rapid and specific manner. The classical paradigm of GPCR signalling was rather linear and sequential. Emerging evidence, however, has revealed that this is only a part of the complex signalling mediated by GPCR. Propagation of GPCR signalling involves cross-regulation of many but specific pathways, including cross-talks between different GPCRs as well as with other signalling pathways. Moreover, it is increasingly apparent that GPCRs can activate both heterotrimeric G protein-dependent and G protein-independent signalling pathways. In this review, we discuss how the signallings initiated by GPCRs achieve rapidity as well as specificity, and how the GPCRs can cross-regulate other specific signalling pathways at the same time. New concepts regarding GPCR signalling have been arising to address this issue, which include multiprotein signalling complex and signalling compartment in microdomain concepts that enable close colocalization or even contact among the proteins engaged in the specific signal transduction. The final outcome of a stimulation of GPCR will thus be the sum of its own specific set of intracellular signalling pathways it regulates.

Nongenomic action of 1 alpha,25(OH)(2)-vitamin D3. Activation of muscle cell PLC gamma through the tyrosine kinase c-Src and PtdIns 3-kinase

We have previously demonstrated that the steroid hormone 1 alpha,25(OH)(2)-vitamin D(3)[1 alpha,25(OH)(2)D(3)] stimulates the production of inositol trisphosphate (InsP(3)), the breakdown product of phosphatidylinositol 4,5-biphosphate (PtdInsP(2)) by phospholipase C (PtdIns-PLC), and activates the cytosolic tyrosine kinase c-Src in skeletal muscle cells. In the present study we examined whether 1 alpha,25(OH)(2)D(3) induces the phosphorylation and membrane translocation of PLC gamma and the mechanism involved in this isozyme activation. We found that the steroid hormone triggers a significant phosphorylation on tyrosine residues of PLC gamma and induces a rapid increase in membrane-associated PLC gamma immunoreactivity with a time course that correlates with that of phosphorylation in muscle cells. Genistein, a tyrosine kinase inhibitor, blocked the phosphorylation of PLC gamma. Inhibition of 1 alpha,25(OH)(2)D(3)-induced c-Src activity by its specific inhibitor PP1 or muscle cell transfection with an antisense oligodeoxynucleotide directed against c-Src mRNA, prevented hormone stimulation of PLC gamma tyrosine phosphorylation. The isozyme phosphorylation is also blocked by both wortmannin and LY294002, two structurally different inhibitors of phosphatidyl inositol 3-kinase (PtdIns3K), the enzyme that produces PtdInsP(3) known to activate PLC gamma isozymes specifically by interacting with their SH2 and pleckstrin homology domains. The hormone also increases the physical association of c-Src and PtdIns3K with PLC gamma and induces a c-Src-dependent tyrosine phosphorylation of the p85 regulatory subunit of PtdIns3K. The time course of hormone-dependent PLC gamma phosphorylation closely correlates with the time course of its redistribution to the membrane, suggesting that phosphorylation and redistribution to the membrane of PLC gamma are two interdependent events. 1 alpha,25(OH)(2)D(3)-induced membrane translocation of PLC gamma was prevented to a great extent by c-Src and PtdIns3K inhibitors, PP1 and LY294002. Taken together, the present data indicates that the cytosolic tyrosine kinase c-Src and PtdIns 3-kinase play indispensable roles in 1 alpha,25(OH)(2)D(3) signal transduction cascades leading to PLC gamma activation.

Membrane targeting of C2 domains of phospholipase C-delta isoforms

The C2 domain is a Ca(2+)-dependent membrane-targeting module found in many cellular proteins involved in signal transduction or membrane trafficking. To understand the mechanisms by which the C2 domain mediates the membrane targeting of PLC-delta isoforms, we measured the in vitro membrane binding of the C2 domains of PLC-delta1, -delta3, and -delta4 by surface plasmon resonance and monolayer techniques and their subcellular localization by time-lapse confocal microscopy. The membrane binding of the PLC-delta1-C2 is driven by nonspecific electrostatic interactions between the Ca(2+)-induced cationic surface of protein and the anionic membrane and specific interactions involving Ca(2+), Asn(647), and phosphatidylserine (PS). The PS selectivity of PLC-delta1-C2 governs its specific Ca(2+)-dependent subcellular targeting to the plasma membrane. The membrane binding of the PLC-delta3-C2 also involves Ca(2+)-induced nonspecific electrostatic interactions and PS coordination, and the latter leads to specific subcellular targeting to the plasma membrane. In contrast to PLC-delta1-C2 and PLC-delta3-C2, PLC-delta4-C2 has significant Ca(2+)-independent membrane affinity and no PS selectivity due to the presence of cationic residues in the Ca(2+)-binding loops and the substitution of Ser for the Ca(2+)-coordinating Asp in position 717. Consequently, PLC-delta4-C2 exhibits unique pre-localization to the plasma membrane prior to Ca(2+) import and non-selective Ca(2+)-mediated targeting to various cellular membranes, suggesting that PLC-delta4 might have a novel regulatory mechanism. Together, these results establish the C2 domains of PLC-delta isoforms as Ca(2+)-dependent membrane targeting domains that have distinct membrane binding properties that control their subcellular localization behaviors.

Alteration of second messengers during acute cerebral ischemia - adenylate cyclase, cyclic AMP-dependent protein kinase, and cyclic AMP response element binding protein

A variety of neurotransmitters and other chemical substances are released into the extracellular space in the brain in response to acute ischemic stress, and the biological actions of these substances are exclusively mediated by receptor-linked second messenger systems. One of the well-known second messenger systems is adenylate cyclase, which catalyzes the generation of cyclic AMP, triggering the activation of cyclic AMP-dependent protein kinase (PKA). PKA controls a number of cellular functions by phosphorylating many substrates, including an important DNA-binding transcription factor, cyclic AMP response element binding protein (CREB). CREB has recently been shown to play an important role in many physiological and pathological conditions, including synaptic plasticity and neuroprotection against various insults, and to constitute a convergence point for many signaling cascades. The autoradiographic method developed in our laboratory enables us to simultaneously quantify alterations of the second messenger system and local cerebral blood flow (lCBF). Adenylate cyclase is diffusely activated in the initial phase of acute ischemia (< or = 30 min), and its activity gradually decreases in the late phase of ischemia (2-6 h). The areas of reduced adenylate cyclase activity strictly coincide with infarct areas, which later become visible. The binding activity of PKA to cyclic AMP, which reflects the functional integrity of the enzyme, is rapidly suppressed during the initial phase of ischemia in the ischemic core, especially in vulnerable regions, such as the CA1 of the hippocampus, and it continues to decline. By contrast, PKA binding activity remains enhanced in the peri-ischemia area. These changes occur in a clearly lCBF-dependent manner. CREB phosphorylation at a serine residue, Ser(133), which suggests the activation of CREB-mediated transcription of genes containing a CRE motif in the nuclei, remains enhanced in the peri-ischemia area, which is spared of infarct damage. On the other hand, CREB phosphorylation at Ser133 rapidly diminishes in the ischemic core before the histological damage becomes manifest. The Ca2+ influx during membrane depolarization contributes to CREB phosphorylation in the initial phase of post-ischemic recirculation, while PKA activation and other signaling elements seem to be responsible in the later phase. These findings suggest that derangement of cyclic AMP-related intracellular signal transduction closely parallels ischemic neuronal damage and that persistent enhancement of this signaling pathway is important for neuronal survival in acute cerebral ischemia.

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Regulation of phospholipase C gamma isoforms in haematopoietic cells: why one, not the other?

Phospholipase C gamma (PLCgamma) isoforms are critical for the generation of calcium signals in haematopoietic systems in response to the stimulation of immune receptors. PLCgamma is unique amongst phospholipases in that it is tightly regulated by the action of a number of tyrosine kinases. It is itself directly phosphorylated on a number of tyrosines and contains several domains through which it can interact with other signalling proteins and lipid products such as phosphatidylinositol 3,4,5-trisphosphate. Through this network of interactions, PLCgamma is activated and recruited to its substrate, phosphatidylinositol 4,5-bisphosphate, at the membrane. Both isoforms of PLCgamma, PLCgamma1 and PLCgamma2, are present in haematopoietic cells. The signalling cascade involved in the regulation of these two isoforms varies between cells, though the systems are similar for both PLCgamma1 and PLCgamma2. We will compare these cascades for both PLCgamma1 and PLCgamma2 and discuss possible reasons as to why one form of PLCgamma and not the other is required for signalling in specific haematopoietic cells, including T lymphocytes, B lymphocytes, platelets, and mast cells.

PLANT PHOSPHOLIPASES

Phospholipases are a diverse series of enzymes that hydrolyze phospholipids. Multiple forms of phospholipases D, C, and A have been characterized in plants. These enzymes are involved in a broad range of functions in cellular regulation, lipid metabolism, and membrane remodeling. In recent years, increasing attention has been paid to the many roles of phospholipases in signal transduction. This review highlights recent developments in the understanding of biochemical, molecular biological, and functional aspects of various phospholipases in plants.

N-terminal EF-hand-like domain is required for phosphoinositide-specific phospholipase C activity in Arabidopsis thaliana

Phosphoinositide-specific phospholipase C's (PI-PLCs) are ubiquitous in eukaryotes, from plants to animals, and catalyze the hydrolysis of phosphatidylinositol 4,5-bisphosphate into the two second messengers inositol 1,4,5-trisphosphate and diacylglycerol. In animals, four distinct subfamilies of PI-PLCs have been identified, and the three-dimensional structure of one rat isozyme, PLC-delta1, determined. Plants appear to contain only one gene family encoding PI-PLCs. The catalytic properties of plant PI-PLCs are very similar to those of animal enzymes. However, very little is known about the regulation of plant PI-PLCs. All plant PI-PLCs comprise three domains, X, Y and C2, which are also conserved in isoforms from animals and yeast. We here show that one PI-PLC isozyme from Arabidopsis thaliana, AtPLC2, is predominantly localized in the plasma membrane, and that the conserved N-terminal domain may represent an EF-hand domain that is required for catalytic activity but not for lipid binding.

Protein kinase C alpha -mediated negative feedback regulation is responsible for the termination of insulin-like growth factor I-induced activation of nuclear phospholipase C beta1 in Swiss 3T3 cells

Previous studies from several independent laboratories have demonstrated the existence of an autonomous phosphoinositide (PI) cycle within the nucleus, where it is involved in both cell proliferation and differentiation. Stimulation of Swiss 3T3 cells with insulin-like growth factor-I (IGF-I) has been shown to induce a transient and rapid increase in the activity of nuclear-localized phospholipase C (PLC) beta1, which in turn leads to the production of inositol trisphosphate and diacylglycerol in the nucleus. Nuclear diacylglycerol provides the driving force for the nuclear translocation of protein kinase C (PKC) alpha. Here, we report that treatment of Swiss 3T3 cells with Go6976, a selective inhibitor of PKC alpha, caused a sustained elevation of IGF-I-stimulated nuclear PLC activity. A time course study revealed an inverse relationship between nuclear PKC activity and the activity of nuclear PLC in IGF-I-treated cells. A time-dependent association between PKC alpha and PLC beta1 in the nucleus was also observed following IGF-I treatment. Two-dimensional phosphopeptide mapping and site-directed mutagenesis demonstrated that PKC promoted phosphorylation of PLC beta1 at serine 887 in the nucleus of IGF-I-treated cells. Overexpression of either a PLC beta1 mutant in which the PKC phosphorylation site Ser(887) was replaced by alanine, or a dominant-negative PKC alpha, resulted in a sustained activation of nuclear PLC following IGF-I stimulation. These results indicate that a negative feedback regulation of PLC beta1 by PKC alpha plays a critical role in the termination of the IGF-I-dependent signal that activates the nuclear PI cycle.

Sperm factor induces intracellular free calcium oscillations by stimulating the phosphoinositide pathway

Injection of a porcine cytosolic sperm factor (SF) or of a porcine testicular extract into mammalian eggs triggers oscillations of intracellular free calcium ([Ca(2+)](i)) similar to those initiated by fertilization. To elucidate whether SF activates the phosphoinositide (PI) pathway, mouse eggs or SF were incubated with U73122, an inhibitor of events leading to phospholipase C (PLC) activation and/or of PLC itself. In both cases, U73122 blocked the ability of SF to induce [Ca(2+)](i) oscillations, although it did not inhibit Ca(2+) release caused by injection of inositol 1,4,5-triphosphate (IP(3)). The inactive analogue, U73343, had no effect on SF-induced Ca(2+) responses. To determine at the single cell level whether SF triggers IP(3) production concomitantly with a [Ca(2+)](i) rise, SF was injected into Xenopus oocytes and IP(3) concentration was determined using a biological detector cell combined with capillary electrophoresis. Injection of SF induced a significant increase in [Ca(2+)](i) and IP(3) production in these oocytes. Using ammonium sulfate precipitation, chromatographic fractionation, and Western blotting, we determined whether PLCgamma1, PLCgamma2, or PLCdelta4 and/or its splice variants, which are present in sperm and testis, are responsible for the Ca(2+) activity in the extracts. Our results revealed that active fractions do not contain PLCgamma1, PLCgamma2, or PLCdelta4 and/or its splice variants, which were present in inactive fractions. We also tested whether IP(3) could be the sensitizing stimulus of the Ca(2+)-induced Ca(2+) release mechanism, which is an important feature of fertilized and SF-injected eggs. Eggs injected with adenophostin A, an IP(3) receptor agonist, showed enhanced Ca(2+) responses to CaCl(2) injections. Thus, SF, and probably sperm, induces [Ca(2+)](i) rises by persistently stimulating IP(3) production, which in turn results in long-lasting sensitization of Ca(2+)-induced Ca(2+) release. Whether SF is itself a PLC or whether it acts upstream of the egg's PLCs remains to be elucidated.

Ras effectors: Buying shares in Ras plc

The debate over whether activated Ras can regulate phosphoinositide-specific phospholipase C (PLC) has been contentious and at times heated. The argument may be resolved by the recent identification of a novel Ras-regulated PLC, but some unexpected properties of this protein are sure to stimulate further controversy.

Bitter taste transduced by PLC-beta(2)-dependent rise in IP(3) and alpha-gustducin-dependent fall in cyclic nucleotides

Current evidence points to the existence of multiple processes for bitter taste transduction. Previous work demonstrated involvement of the polyphosphoinositide system and an alpha-gustducin (Galpha(gust))-mediated stimulation of phosphodiesterase in bitter taste transduction. Additionally, a taste-enriched G protein gamma-subunit, Ggamma(13), colocalizes with Galpha(gust) and mediates the denatonium-stimulated production of inositol 1,4,5-trisphosphate (IP(3)). Using quench-flow techniques, we show here that the bitter stimuli, denatonium and strychnine, induce rapid (50-100 ms) and transient reductions in cAMP and cGMP and increases in IP(3) in murine taste tissue. This decrease of cyclic nucleotides is inhibited by Galpha(gust) antibodies, whereas the increase in IP(3) is not affected by antibodies to Galpha(gust). IP(3) production is inhibited by antibodies specific to phospholipase C-beta(2) (PLC-beta(2)), a PLC isoform known to be activated by Gbetagamma-subunits. Antibodies to PLC-beta(3) or to PLC-beta(4) were without effect. These data suggest a transduction mechanism for bitter taste involving the rapid and transient metabolism of dual second messenger systems, both mediated through a taste cell G protein, likely composed of Galpha(gust)/beta/gamma(13), with both systems being simultaneously activated in the same bitter-sensitive taste receptor cell.

Depressed responsiveness of phospholipase C isoenzymes to phosphatidic acid in congestive heart failure

The cardiac sarcolemmal membrane cis -unsaturated fatty acid-sensitive phospholipase D hydrolyzes phosphatidylcholine to form phosphatidic acid. The functional significance of phosphatidic acid is indicated by its ability to increase [Ca(2+)](i)and augment cardiac contractile performance via the activation of phospholipase C. Accordingly, we tested the hypothesis that a defect occurs in the membrane level of phosphatidic acid and/or the responsiveness of cardiomyocytes to phosphatidic acid in congestive heart failure due to myocardial infarction. Myocardial infarction was produced in rats by ligation of the left coronary artery while sham-operated animals served as control. At 8 weeks after surgery, the experimental animals were at a stage of moderate congestive heart failure. Compared to sham controls, phosphatidic acid-mediated increase in [Ca(2+)](i), as determined by the fura 2-AM technique, was significantly reduced in failing cardiomyocytes. Immunoprecipitation of sarcolemmal phospholipase C isoenzymes using specific monoclonal antibodies revealed that the stimulation of phospholipase C gamma(1)and delta(1)phosphatidylinositol 4,5-bisphosphate hydrolyzing activities by phosphatidic acid was decreased in the failing heart. Although the activity of phospholipase C beta(1)in the failing heart was higher than the control, phosphatidic acid did not stimulate this isoform in control sarcolemma, and produced an inhibitory action in the failing heart preparation. Furthermore, the specific binding of phosphatidic acid to phospholipase C gamma(1)and delta(1)isoenzymes was decreased, whereas binding to phospholipase beta(1)was absent in the failing heart. A reduction in the intramembranal level of phosphatidic acid derived via cis -unsaturated fatty acid-sensitive phospholipase D was also seen in the failing heart. These findings suggest that a defect in phosphatidic acid-mediated signal pathway in sarcolemma may represent a novel mechanism of heart dysfunction in congestive heart failure.

PTH stimulates PLCbeta and PLCgamma isoenzymes in rat enterocytes: influence of ageing

We previously reported that in rat duodenal cells (enterocytes), parathyroid hormone (PTH [1-34]: PTH) stimulates the hydrolysis of polyphosphoinositides by phospholipase C (PLC), generating the second messengers inositol trisphosphate (IP(3)) and diacylglycerol (DAG) and that this mechanism is severely altered in old animals. In the present study, we show that PTH [1-34]-dependent IP(3) release in young rats was blocked to a great extent by an antibody against guanine nucleotide binding protein Galphaq/11, indicating that the hormone activates a beta isoform of PLC coupled to the alpha subunit of Gq/11. In addition, PTH rapidly (within 30 s, with maximal effects at 1 min) stimulated tyrosine phosphorylation of PLCgamma in a dose-dependent fashion (10(-10)-10(-7) M). The hormone response was specific as PTH [7-34] was without effects. The tyrosine kinase inhibitors, genistein (100 microM) and herbimycin (2 microM), suppressed PTH-dependent PLCgamma tyrosine phosphorylation. Stimulation of PLCgamma tyrosine phosphorylation by PTH [1-34] greatly decreased with ageing. PP1 (10 microM), a specific inhibitor of the Src family of tyrosine kinases, completely abolished PLCgamma phosphorylation. The hormone-induced Src tyrosine dephosphorylation, a major mechanism of Src activation, an effect that was blunted in old animals. These results indicate that in rat enterocytes PTH generates IP(3) mainly through G-protein-coupled PLCbeta and stimulates PLCgamma phosphorylation via the nonreceptor tyrosine kinase Src. Impairment of PTH activation of both PLC isoforms upon ageing may result in abnormal hormone regulation of cell Ca(2+) and proliferation in the duodenum.

P2Y2 purinergic and M3 muscarinic acetylcholine receptors activate different phospholipase C-beta isoforms that are uniquely susceptible to protein kinase C-dependent phosphorylation and inactivation

Activation of phospholipase C-beta (PLC-beta) by G protein-coupled receptors typically results in rapid but transient second messenger generation. Although PLC-beta deactivation may contribute to the transient nature of this response, the mechanisms governing PLC-beta deactivation are poorly characterized. We investigated the involvement of protein kinase C (PKC) in the termination of PLC-beta activation induced by endogenous P2Y(2) purinergic receptors and transfected M(3) muscarinic acetylcholine receptors (mAChR) in Chinese hamster ovary cells. Activation of P2Y(2) receptors causes Galpha(q/11) to associate with PLC-beta3, whereas M(3) mAChR activation causes Galpha(q/11) to associate with both PLC-beta1 and PLC-beta3 in these cells. Phosphorylation of PLC-beta3, but not PLC-beta1, is induced by activating either P2Y(2) receptors or M(3) mAChR. We demonstrate that PKC rather than protein kinase A mediates the G protein-coupled receptor-induced phosphorylation of PLC-beta3. The PKC-mediated phosphorylation of PLC-beta3 diminishes the interaction of Galpha(q/11) with PLC-beta3, thereby contributing to the termination PLC-beta3 activity. These findings indicate that the distinct temporal profiles of PLC activation by P2Y(2) receptors and mAChR may arise from the differential activation of PLC-beta1 and PLC-beta3 by the receptors, coupled with a selective PKC-mediated negative feedback mechanism that targets PLC-beta3 but not PLC-beta1.

Cytoplasmic and nuclear phospholipase C-beta 1 relocation: role in resumption of meiosis in the mouse oocyte

The location of the phospholipase C beta 1-isoform (PLC-beta 1) in the mouse oocyte and its role in the resumption of meiosis were examined. We used specific monoclonal antibodies to monitor the in vitro dynamics of the subcellular distribution of the enzyme from the release of the oocyte from the follicle until breakdown of the germinal vesicle (GVBD) by Western blotting, electron microscope immunohistochemistry, and confocal microscope immunofluorescence. PLC-beta 1 became relocated to the oocyte cortex and the nucleoplasm during the G2/M transition, mainly in the hour preceding GVBD. The enzyme was a 150-kDa protein, corresponding to PLC-beta 1a. Its synthesis in the cytoplasm increased during this period, and it accumulated in the nucleoplasm. GVBD was dramatically inhibited by the microinjection of anti-PLC-beta1 monoclonal antibody into the germinal vesicle (GV) only when this accumulation was at its maximum. In contrast, PLC-gamma 1 was absent from the GV from the time of release from the follicle until 1 h later, and microinjection of anti-PLC-gamma 1 into the GV did not affect GVBD. Our results demonstrate a relationship between the relocation of PLC-beta 1 and its role in the first step of meiosis.

Increasing molecular diversity of secreted phospholipases A(2) and their receptors and binding proteins

Secreted phospholipases A(2) (sPLA(2)s) form a large family of structurally related enzymes which are widespread in nature. Snake venoms are known for decades to contain a tremendous molecular diversity of sPLA(2)s which can exert a myriad of toxic and pharmacological effects. Recent studies indicate that mammalian cells also express a variety of sPLA(2)s with ten distinct members identified so far, in addition to the various other intracellular PLA(2)s. Furthermore, scanning of nucleic acid databases fueled by the different genome projects indicates that several sPLA(2)s are also present in invertebrate animals like Drosophila melanogaster as well as in plants. All of these sPLA(2)s catalyze the hydrolysis of glycerophospholipids at the sn-2 position to release free fatty acids and lysophospholipids, and thus could be important for the biosynthesis of biologically active lipid mediators. However, the recent identification of a variety of membrane and soluble proteins that bind to sPLA(2)s suggests that the sPLA(2) enzymes could also function as high affinity ligands. So far, most of the binding data have been accumulated with venom sPLA(2)s and group IB and IIA mammalian sPLA(2)s. Collectively, venom sPLA(2)s have been shown to bind to membrane and soluble mammalian proteins of the C-type lectin superfamily (M-type sPLA(2) receptor and lung surfactant proteins), to pentraxin and reticulocalbin proteins, to factor Xa and to N-type receptors. Venom sPLA(2)s also associate with three distinct types of sPLA(2) inhibitors purified from snake serum that belong to the C-type lectin superfamily, to the three-finger protein superfamily and to proteins containing leucine-rich repeats. On the other hand, mammalian group IB and IIA sPLA(2)s can bind to the M-type receptor, and group IIA sPLA(2)s can associate with lung surfactant proteins, factor Xa and proteoglycans including glypican and decorin, a mammalian protein containing a leucine-rich repeat.