Cloning, sequencing, expression, and Gq-independent activation of phospholipase C-beta 2

Gαq signalling from endosomes: A new conundrum

G-protein-coupled receptors (GPCRs) constitute the largest family of membrane receptors, and are involved in the transmission of a variety of extracellular stimuli such as hormones, neurotransmitters, light and odorants into intracellular responses. They regulate every aspect of physiology and, for this reason, about one third of all marketed drugs target these receptors. Classically, upon binding to their agonist, GPCRs are thought to activate G-proteins from the plasma membrane and to stop signalling by subsequent desensitisation and endocytosis. However, accumulating evidence indicates that, upon internalisation, some GPCRs can continue to activate G-proteins in endosomes. Importantly, this signalling from endomembranes mediates alternative cellular responses other than signalling at the plasma membrane. Endosomal G-protein signalling and its physiological relevance have been abundantly documented for Gαs - and Gαi -coupled receptors. Recently, some Gαq -coupled receptors have been reported to activate Gαq on endosomes and mediate important cellular processes. However, several questions relative to the series of cellular events required to translate endosomal Gαq activation into cellular responses remain unanswered and constitute a new conundrum. How are these responses in endosomes mediated in the quasi absence of the substrate for the canonical Gαq -activated effector? Is there another effector? Is there another substrate? If so, how does this alternative endosomal effector or substrate produce a downstream signal? This review aims to unravel and discuss these important questions, and proposes possible routes of investigation.

G protein-coupled receptor interactions and modification of signalling involving the ghrelin receptor, GHSR1a

The growth hormone secretagogue receptor 1a (GHSR1a) is intriguing because of its potential as a therapeutic target and its diverse molecular interactions. Initial studies of the receptor focused on the potential therapeutic ability for growth hormone (GH) release to reduce wasting in aging individuals, as well as food intake regulation for treatment of cachexia. Known roles of GHSR1a now extend to regulation of neurogenesis, learning and memory, gastrointestinal motility, glucose/lipid metabolism, the cardiovascular system, neuronal protection, motivational salience, and hedonic feeding. Ghrelin, the endogenous agonist of GHSR1a, is primarily located in the stomach and is absent from the central nervous system (CNS), including the spinal cord. However, ghrelin in the circulation does have access to a small number of CNS sites, including the arcuate nucleus, which is important in feeding control. At some sites, such as at somatotrophs, GHSR1a has high constitutive activity. Typically, ghrelin-dependent and constitutive GHSR1a activation occurs via Gα_q/11 pathways. In vitro and in vivo data suggest that GHSR1a heterodimerises with multiple G protein-coupled receptors (GPCRs), including dopamine D1 and D2, serotonin 2C, orexin, oxytocin and melanocortin 3 receptors (MCR3), as well as the MCR3 accessory protein, MRAP2, providing possible mechanisms for its many physiological effects. In all cases, the receptor interaction changes downstream signalling and the responses to receptor agonists. This review discusses the signalling mechanisms of GHSR1a alone and in combination with other GPCRs, and explores the physiological consequences of GHSR1a coupling with other GPCRs.

Phospholipase C Beta 2 Protein Overexpression Is a Favorable Prognostic Indicator in Newly Diagnosed Normal Karyotype Acute Myeloid Leukemia

Phospholipase C beta 2 (PLC-β2) regulates various essential functions in cell signaling, differentiation, growth, and mobility. We investigated the clinical implications of PLC-β2 protein expression in newly diagnosed normal karyotype acute myeloid leukemia (NK-AML). The PLC-β2 expression status in bone marrow tissues obtained from 101 patients with NK-AML was determined using semiquantitative immunohistochemistry (IHC). IHC results were compared with those for known prognostic markers. Using a cutoff score for positivity of 7.0, the PLC-β2 overexpression group showed superior overall survival (OS) (72.6% vs. 26.5%; P=0.016) and low hazard ratio (HR) (0.453; P=0.019) compared with the PLC-β2 low-expression group. The PLC-β2 overexpression group showed no significant gain in event-free survival (50.6% vs. 43.0%, P=0.465) and HR (0.735; P=0.464). Among the known prognostic markers, only FLT3-ITD positivity was associated with a significantly low OS and high HR. In conclusion, PLC-β2 overexpression was associated with favorable OS in NK-AML patients. Our results suggest that PLC-β2 expression assessment using IHC allows prognosis prediction in NK-AML.

Phospholipase C

Phospholipase C (PLC) family members constitute a family of diverse enzymes. Thirteen different family members have been cloned. These family members have unique structures that mediate various functions. Although PLC family members all appear to signal through the bi-products of cleaving phospholipids, it is clear that each family member, and at times each isoform, contributes to unique cellular functions. This chapter provides a review of the current literature on PLC. In addition, references have been provided for more in-depth information regarding areas that are not discussed including tyrosine kinase activation of PLC. Understanding the roles of the individual PLC enzymes, and their distinct cellular functions, will lead to a better understanding of the physiological roles of these enzymes in the development of diseases and the maintenance of homeostasis.

Regulation of retinal angiogenesis by phospholipase C-β3 signaling pathway

Angiogenesis has an essential role in many pathophysiologies. Here, we show that phospholipase C-β3 (PLC-β3) isoform regulates endothelial cell function and retinal angiogenesis. Silencing of PLC-β3 in human umbilical vein endothelial cells (HUVECs) significantly delayed proliferation, migration and capillary-like tube formation. In addition, mice lacking PLC-β3 showed impaired retinal angiogenesis with delayed endothelial proliferation, reduced endothelial cell activation, abnormal vessel formation and hemorrhage. Finally, tumor formation was significantly reduced in mice lacking PLC-β3 and showed irregular size and shape of blood vessels. These results suggest that regulation of endothelial function by PLC-β3 may contribute to angiogenesis.

P2Y₆ receptor inhibition perturbs CCL2-evoked signalling in human monocytic and peripheral blood mononuclear cells

The chemokine CCL2 serves to target circulating monocytes and other leukocytes to tissue during innate immune responses, and modulates the progression of chronic inflammatory disease through activation of the receptor CCR2. Here, we show that co-activation of the P2Y₆ purinergic receptor (encoded by P2RY₆) occurs when THP-1 cells and human peripheral blood mononuclear cells sense CCL2 through CCR2. Furthermore, P2Y₆ receptor activation accounts for ∼80% of the intracellular Ca²⁺ signal evoked by CCL2. Scavenging extracellular nucleotides with apyrase caused a fourfold reduction in THP-1 sensitivity to CCL2, whereas inhibition of CD39-like ectonucleotidases potentiated CCL2-evoked Ca²⁺ responses. Pharmacological inhibition of P2Y₆ impaired CCL2-evoked Ca²⁺ signalling and chemotaxis in peripheral blood mononuclear cells and THP-1 cells. Furthermore, stable P2Y₆ receptor knockdown (of twofold) in THP-1 cells impaired CCL2-evoked Ca²⁺ signalling, chemotaxis and adhesion to TNFα-treated HUVECs. We demonstrate that THP-1 cells rapidly secrete ATP during signalling downstream of the CCL2-CCR2 axis and suggest this might act as a mechanism for P2Y₆ receptor co-activation following CCL2 activation of the CCR2 receptor. The discovery that P2Y₆ receptor mediates leukocyte responsiveness to CCL2 represents a new mechanism by which to modulate CCL2 signals.

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.

Phospholipases of mineralization competent cells and matrix vesicles: roles in physiological and pathological mineralizations

The present review aims to systematically and critically analyze the current knowledge on phospholipases and their role in physiological and pathological mineralization undertaken by mineralization competent cells. Cellular lipid metabolism plays an important role in biological mineralization. The physiological mechanisms of mineralization are likely to take place in tissues other than in bones and teeth under specific pathological conditions. For instance, vascular calcification in arteries of patients with renal failure, diabetes mellitus or atherosclerosis recapitulates the mechanisms of bone formation. Osteoporosis—a bone resorbing disease—and rheumatoid arthritis originating from the inflammation in the synovium are also affected by cellular lipid metabolism. The focus is on the lipid metabolism due to the effects of dietary lipids on bone health. These and other phenomena indicate that phospholipases may participate in bone remodelling as evidenced by their expression in smooth muscle cells, in bone forming osteoblasts, chondrocytes and in bone resorbing osteoclasts. Among various enzymes involved, phospholipases A₁ or A₂, phospholipase C, phospholipase D, autotaxin and sphingomyelinase are engaged in membrane lipid remodelling during early stages of mineralization and cell maturation in mineralization-competent cells. Numerous experimental evidences suggested that phospholipases exert their action at various stages of mineralization by affecting intracellular signaling and cell differentiation. The lipid metabolites—such as arachidonic acid, lysophospholipids, and sphingosine-1-phosphate are involved in cell signaling and inflammation reactions. Phospholipases are also important members of the cellular machinery engaged in matrix vesicle (MV) biogenesis and exocytosis. They may favour mineral formation inside MVs, may catalyse MV membrane breakdown necessary for the release of mineral deposits into extracellular matrix (ECM), or participate in hydrolysis of ECM. The biological functions of phospholipases are discussed from the perspective of animal and cellular knockout models, as well as disease implications, development of potent inhibitors and therapeutic interventions.

Phospholipase C

Phospholipase C (PLC) family members constitute a family of diverse enzymes. Thirteen different family members have been cloned. These family members have unique structures that mediate diverse functions. Although PLC family members all appear to signal through the bi-products of cleaving phospholipids, it is clear that each family member, and at times each isoform, contributes to unique cellular functions. This chapter provides a review of the current literature. In addition, references have been provided for more in depth information regarding areas that are discussed. Ultimately, understanding the roles of the individual PLC enzymes, and their distinct cellular functions, will lead to a better understanding of the development of diseases and the maintenance of homeostasis.

The phospholipase C isozymes and their regulation

The physiological effects of many extracellular neurotransmitters, hormones, growth factors, and other stimuli are mediated by receptor-promoted activation of phospholipase C (PLC) and consequential activation of inositol lipid signaling pathways. These signaling responses include the classically described conversion of phosphatidylinositol(4,5)P(2) to the Ca(2+)-mobilizing second messenger inositol(1,4,5)P(3) and the protein kinase C-activating second messenger diacylglycerol as well as alterations in membrane association or activity of many proteins that harbor phosphoinositide binding domains. The 13 mammalian PLCs elaborate a minimal catalytic core typified by PLC-d to confer multiple modes of regulation of lipase activity. PLC-b isozymes are activated by Gaq- and Gbg-subunits of heterotrimeric G proteins, and activation of PLC-g isozymes occurs through phosphorylation promoted by receptor and non-receptor tyrosine kinases. PLC-e and certain members of the PLC-b and PLC-g subclasses of isozymes are activated by direct binding of small G proteins of the Ras, Rho, and Rac subfamilies of GTPases. Recent high resolution three dimensional structures together with biochemical studies have illustrated that the X/Y linker region of the catalytic core mediates autoinhibition of most if not all PLC isozymes. Activation occurs as a consequence of removal of this autoinhibition.

The correlation between multidomain enzymes and multiple activation mechanisms--the case of phospholipase Cβ and its membrane interactions

Phospholipase Cβ2 (PLCβ2) is a large, multidomain enzyme that catalyzes the hydrolysis of the signaling lipid phosphoinositol 4,5 bisphosphate (PIP2) to promote mitogenic and proliferative changes in the cell. PLCβ2 is activated by Gα and Gβγ subunits of heterotrimeric G proteins, as well as small G proteins and specific peptides. Activation depends on the nature of the membrane surface. Recent crystal structures suggest one model of activation involving the movement of a small autoinhibitory loop upon membrane binding of the enzyme. Additionally, solution studies indicate multiple levels of activation that involve changes in the membrane orientation as well as interdomain movement. Here, we review the wealth of biochemical studies of PLCβ2-G protein activation and propose a comprehensive model that accounts for both the crystallographic and solution results.

Platelet physiology and antiplatelet agents

Apart from the central beneficial role platelets play in hemostasis, they are also involved in atherothrombotic diseases. Here, we review the current knowledge of platelet intracellular signal transduction pathways involved in platelet adhesion, activation, amplification of the activation signal and aggregation, as well as pathways limiting platelet aggregation. A thorough understanding of these pathways allows explanation of the mechanism of action of existing antiplatelet agents, but also helps to identify targets for novel drug development.

Subtype-specific roles of phospholipase C-β via differential interactions with PDZ domain proteins

Since we first identified the PLC-β isozyme, enormous studies have been conducted to investigate the functional roles of this protein (Min et al., 1993; Suh et al.,1988). It is now well-known that the four PLC-β subtypes are major effector molecules in GPCR-mediated signaling, especially for intracellular Ca2+ signaling. Nonetheless, it is still poorly understood why multiple PLC-β subtype exist. Most cells express multiple subtypes of PLC-β in different combinations, and each subtype is involved in somewhat different signaling pathways. Therefore, studying the differential roles of each PLC-β subtype is a very interesting issue. In this regard, we focus here on PDZ domain proteins which are novel PLC-β interacting proteins. As scaffolders, PDZ domain proteins recruit various target proteins ranging from membrane receptors to cytoskeletal proteins to assemble highly organized signaling complexes; this can give rise to efficiency and diversity in cellular signaling. Because PLC-β subtypes have different PDZ-binding motifs, it is possible that they are engaged with different PDZ domain proteins, and in turn participate in distinct physiological responses. To date, several PDZ domain proteins, such as the NHERF family, Shank2, and Par-3, have been reported to selectively interact with certain PLC-β subtypes and GPCRs. Systematic predictions of potential binding partners also suggests differential binding properties between PLC-β subtypes. Furthermore, we elucidated parallel signaling processes for multiple PLC-β subtypes, which still perform distinct functions resulting from differential interactions with PDZ domain proteins within a single cell. Therefore, these results highlight the novel function of PDZ domain proteins as intermediaries in subtype-specific role of PLC-β in GPCR-mediated signaling. Future studies will focus on the physiological meanings of this signaling complex formation by different PDZ domain proteins and PLC-β subtypes. It has been observed for a long time that the expression of certain PLC-β subtype fluctuates during diverse physiological conditions. For example, the expression of PLC-β1 is selectively increased during myoblast and adipocyte differentiation (Faenza et al., 2004; O'Carroll et al., 2009). Likewise, PLC-β2 is highly up-regulated during breast cancer progression and plays a critical role in cell migration and mitosis (Bertagnolo et al., 2007). Although PLC-β3 is selectively down-regulated in neuroendocrine tumors, the expression of PLC-β1 is increased in small cell lung carcinoma (Stalberg et al., 2003; Strassheim et al., 2000). In our hypothetical model, it is most likely that up- and down regulation of certain PLC-β subtypes are due to their selective coupling with specific GPCR-mediated signaling, implicated in these pathophysiologic conditions. Therefore, better understanding of selective coupling between PLC-β subtypes, PDZ domain proteins, and GPCRs will shed light on new prognosis and therapy of diverse diseases, and provide potential targets for drug development.

Pathway-selective suppression of chemokine receptor signaling in B cells by LPS through downregulation of PLC-β2

Lymphocyte activation leads to changes in chemokine receptor expression. There are limited data, however, on how lymphocyte activators can alter chemokine signaling by affecting downstream pathways. We hypothesized that B cell-activating agents might alter chemokine responses by affecting downstream signal transducers, and that such effects might differ depending on the activator. We found that activating mouse B cells using either anti-IgM or lipopolysaccharide (LPS) increased the surface expression of CCR6 and CCR7 with large increases in chemotaxis to their cognate ligands. By contrast, while anti-IgM also led to enhanced calcium responses, LPS-treated cells showed only small changes in calcium signaling as compared with cells that were freshly isolated. Of particular interest, we found that LPS caused a reduction in the level of B-cell phospholipase C (PLC)-β2 mRNA and protein. Data obtained using PLC-β2(-/-) mice showed that the β2 isoform mediates close to one-half the chemokine-induced calcium signal in resting and anti-IgM-activated B cells, and we found that calcium signals in the LPS-treated cells were boosted by increasing the level of PLC-β2 using transfection, consistent with a functional effect of downregulating PLC-β2. Together, our results show activator-specific effects on responses through B-cell chemokine receptors that are mediated by quantitative changes in a downstream signal-transducing protein, revealing an activity for LPS as a downregulator of PLC-β2, and a novel mechanism for controlling chemokine-induced signals in lymphocytes.

Spatiotemporal dynamics of Ca2+ signaling and its physiological roles

Changes in the intracellular Ca(2+) concentration regulate numerous cell functions and display diverse spatiotemporal dynamics, which underlie the versatility of Ca(2+) in cell signaling. In many cell types, an increase in the intracellular Ca(2+) concentration starts locally, propagates within the cell (Ca(2+) wave) and makes oscillatory changes (Ca(2+) oscillation). Studies of the intracellular Ca(2+) release mechanism from the endoplasmic reticulum (ER) showed that the Ca(2+) release mechanism has inherent regenerative properties, which is essential for the generation of Ca(2+) waves and oscillations. Ca(2+) may shuttle between the ER and mitochondria, and this appears to be important for pacemaking of Ca(2+) oscillations. Importantly, Ca(2+) oscillations are an efficient mechanism in regulating cell functions, having effects supra-proportional to the sum of duration of Ca(2+) increase. Furthermore, Ca(2+) signaling mechanism studies have led to the development of a method for specific inhibition of Ca(2+) signaling, which has been used to identify hitherto unrecognized functions of Ca(2+) signals.

Targeting lipid metabolism by the lipoprotein lipase inhibitor orlistat results in apoptosis of B-cell chronic lymphocytic leukemia cells

Constitutively activated pathways contribute to apoptosis resistance in chronic lymphocytic leukemia (CLL). Little is known about the metabolism of lipids and function of lipases in CLL cells. Performing gene expression profiling including B-cell receptor (BCR) stimulation of CLL cells in comparison to healthy donor CD5+ B cells, we found significant overexpression of lipases and phospholipases in CLL cells. In addition, we observed that the recently defined prognostic factor lipoprotein lipase (LPL) is induced by stimulation of BCR in CLL cells but not in CD5+ normal B cells. CLL cellular lysates exhibited significantly higher lipase activity compared to healthy donor controls. Incubation of primary CLL cells (n=26) with the lipase inhibitor orlistat resulted in induction of apoptosis, with a half-maximal dose (IC(50)) of 2.35 microM. In healthy B cells a significantly higher mean IC(50) of 148.5 microM of orlistat was observed, while no apoptosis was induced in healthy peripheral blood mononuclear cells (PBMCs; P<0.001). Orlistat-mediated cytotoxicity was decreased by BCR stimulation. Finally, the cytotoxic effects of orlistat on primary CLL cells were enhanced by the simultaneous incubation with fludarabine (P=0.003). In summary, alterations of lipid metabolism are involved in CLL pathogenesis and might represent a novel therapeutic target in CLL.

Alternative splice variants of phospholipase C-beta2 are expressed in platelets: effect on Galphaq-dependent activation and localization

Phospholipase C (PLC) beta2 plays a pivotal role in G-protein dependent signal transduction in platelets. We have previously demonstrated in platelets, leukocytes and human erythroleukemia cells the presence of transcripts of two forms of PLC-beta2 generated by alternative splicing. They differ by 45 nucleotides in the carboxyl-terminal region and are designated as PLC-beta2a and PLC-beta2b, with and without by 15 amino acid residues (corresponding to 864-878). The presence of the two variants has not been shown at the protein level in cells. Moreover, the carboxy-terminal region of PLC-beta has been implicated in Galphaq activation, particulate association, and nuclear localization, suggesting that the PLC-beta2 splice variants may be regulated differentially. We demonstrate for the first time that both PLC-beta2 isoforms are expressed in platelets at the protein level. Studies in CV-1 cells transfected with PLC-beta2a or beta2b cDNAs, along with constitutively activated Galphaq (Q209L), showed that inositolphosphate formation was comparable between the two variants. However, the nuclear localization of the two isoforms was different with a higher cytoplasmic to nuclear ratio for PLC-beta2b compared to PLC-beta2a, suggesting that a great proportion of the total PLC-beta2a was in the nucleus relative to PLC-beta2b. There was no difference in the relative distribution of the two variants between the cytosol and particulate fractions. Both PLC-beta2 alternative splice variants are expressed at the protein level in platelets. In transfected CV-1 cells, PLC-beta2a is relatively more enriched in the nuclei than PLC-beta2b suggesting that the two variants may have different effects in cell proliferation and differentiation.

G proteins in signal transduction: the regulation of phospholipase C

The hydrolysis of phosphatidylinositol 4,5-bisphosphate by specific phospholipase C (PLC) enzymes produces two second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. Heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins) of the Gq subfamily activate the PLC beta 1 isoform of PLC. We have purified three isozymes of PLC beta: PLC beta 1 and PLC beta 3 from rat brain and PLC beta 2 from HL-60 cells. Whereas the beta 1 and beta 2 isozymes appear restricted to a few cell types, beta 3 is broadly distributed. Gq alpha (the alpha subunit of the Gq subfamily) can activate all three isoforms but PLC beta 2 is much less sensitive. Thus all three enzymes are potential effectors for pertussis toxin-insensitive regulation by hormones. The three beta isozymes can also be activated by purified beta gamma subunits. The PLC beta 3 isoform gives the greatest activation with beta gamma; PLC beta 1 is least responsive. The results indicate that all the known isoforms of mammalian PLC beta can be regulated at unique sites by both Gq alpha and beta gamma subunits. The effect of beta gamma subunits may provide a pathway for the regulation of PLC beta isozymes by pertussis toxin-sensitive G proteins or may indicate that the alpha subunit of Gq and its associated beta gamma both participate in regulation of the same phospholipase molecule.

Receptor- and calcium-dependent induced inositol 1,4,5-trisphosphate increases in PC12h cells as shown by fluorescence resonance energy transfer imaging

The production and further metabolism of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] require several calcium-dependent enzymes, but little is known about subsequent calcium-dependent changes in cellular Ins(1,4,5)P3. To study the calcium dependence of muscarinic acetylcholine receptor-induced Ins(1,4,5)P3 increases in PC12h cells, we utilized an Ins(1,4,5)P3 imaging system based on fluorescence resonance energy transfer and using green fluorescent protein variants fused with the pleckstrin homology domain of phospholipase C-delta1. The intracellular calcium concentration, monitored by calcium imaging, was adjusted by thapsigargin pretreatment or alterations in extracellular calcium concentration, enabling rapid receptor-independent changes in calcium concentration via store-operated calcium influx. We found that Ins(1,4,5)P3 production was increased by a combination of receptor- and calcium-dependent components, rather than by calcium alone. The level of Ins(1,4,5)P3 induced by the receptor was found to be half that induced by the combined receptor and calcium components. Increases in calcium levels prior to receptor activation did not affect the subsequent receptor-induced Ins(1,4,5)P3 increase, indicating that calcium does not influence Ins(1,4,5)P3 production without receptor activation. Removal of both the receptor agonists and calcium rapidly restored calcium and Ins(1,4,5)P3 levels, whereas removal of calcium alone restored calcium to its basal concentration. Similar calcium-dependent increases in Ins(1,4,5)P3 were also observed in Chinese hamster ovary cells expressing m1 muscarinic acetylcholine receptor, indicating that the observed calcium dependence is common to Ins(1,4,5)P3 production. To our knowledge, our results are the first showing receptor- and calcium-dependent components within cellular Ins(1,4,5)P3.

Phospholipase Cβ3 is distributed in both somatodendritic and axonal compartments and localized around perisynapse and smooth endoplasmic reticulum in mouse Purkinje cell subsets

Phospholipase Cbeta3 (PLCbeta3) and PLCbeta4 are the two major isoforms in cerebellar Purkinje cells (PCs), displaying reciprocal expression across the cerebellum. Here, we examined subcellular distribution of PLCbeta3 in the mouse cerebellum by producing specific antibody. PLCbeta3 was detected as a particulate pattern of immunostaining in various PC elements. Like PLCbeta4, PLCbeta3 was richly distributed in somatodendritic compartments, where it was colocalized with molecules constituting the metabotropic glutamate receptor (mGluR1) signalling pathway, i.e. mGluR1alpha, G alpha q/G alpha 11 subunits of G q protein, inositol 1,4,5-trisphosphate receptor IP3R1, Homer1, protein kinase C PKCgamma, and diacylglycerol lipase DAGLalpha. Unlike PLCbeta4, PLCbeta3 was also distributed at low to moderate levels in PC axons, which were intense for IP3R1 and PKCgamma, low for G alpha q/G alpha 11, and negative for mGluR1alpha, Homer1, and DAGLalpha. By immunoelectron microscopy, PLCbeta3 was preferentially localized around the smooth endoplasmic reticulum in spines, dendrites, and axons of PCs, and also accumulated at the perisynapse of parallel fibre-PC synapses. Consistent with the ultrastructural localization, PLCbeta3 was biochemically enriched in the microsomal and postsynaptic density fractions. These results suggest that PLCbeta3 plays a major role in mediating mGluR1-dependent synaptic transmission, plasticity, and integration in PLCbeta3-dominant PCs, through eliciting Ca2+ release, protein phosphorylation, and endocannabinoid production at local somatodendritic compartments. Because PLCbeta3 can be activated by G betagamma subunits liberated from Gi/o and Gs proteins as well, axonal PLCbeta3 seems to modulate the conduction of action potentials through mediating local Ca2+ release and protein phosphorylation upon activation of a variety of G protein-coupled receptors other than mGluR1.

Signalling through phospholipase C beta 4 is not essential for midbrain dopaminergic neuron survival

The most prominent progressive neurodegenerative movement disorder, Parkinson's disease, is attributed to selective loss of dopamine neurons in the substantia nigra pars compacta, resulting in severe deficiency of dopamine. The homeo-domain gene, Pit x 3, is essential for proper development of midbrain dopaminergic neurons in the substantia nigra pars compacta and might be involved in midbrain dopaminergic survival pathways. The mGluR1-signaling downstream-effector phospholipase C beta 4 was identified in a suppression subtractive hybridization screen comparing wild-type and Pit x 3-deficient Aphakia midbrain dopaminergic neurons. Expression pattern analysis revealed that phospholipase C beta 4 was expressed in midbrain dopaminergic neurons of the substantia nigra pars compacta and part of the ventral tegmental area, whereas expression of mGluR1alpha was predominantly observed in the more vulnerable midbrain dopaminergic neurons in the lateral substantia nigra pars compacta. However, clear expression of phospholipase C beta 4 in spared midbrain dopaminergic neurons of Aphakia mice located in the ventral tegmental area, indicated that induction and maintenance of phospholipase C beta 4 expression is Pit x 3-independent in these neurons. Furthermore, we report here a normal distribution of midbrain dopaminergic cell bodies and axonal projection to the striatum in phospholipase C beta 4-/- mice, indicating that signaling of phospholipase C beta 4 is not essential for the survival of midbrain dopaminergic neurons.

Mechanosensitivity of human osteosarcoma cells and phospholipase C β2 expression

Bone adapts to mechanical load by osteosynthesis, suggesting that osteoblasts might respond to mechanical stimuli. We therefore investigated cell proliferation and phospholipase C (PLC) expression in osteoblasts. One Hertz uniaxial stretching at 4000 microstrains significantly increased the proliferation rates of human osteoblast-like osteosarcoma cell line MG-63 and primary human osteoblasts. However, U-2/OS, SaOS-2, OST, and MNNG/HOS cells showed no significant changes in proliferation rate. We investigated the expression pattern of different isoforms of PLC in these cell lines. We were able to detect PLC beta1, beta3, gamma1, gamma2, and delta1 in all cells, but PLC beta2 was only detectable in the mechanosensitive cells. We therefore investigated the possible role of PLC beta2 in mechanotransduction. Inducible antisense expression for 24h inhibited the translation of PLC beta1 in U-2/OS cells by 35% and PLC beta2 in MG-63 by 29%. Fluid shear flow experiments with MG-63 lacking PLC beta2 revealed a significantly higher level of cells losing attachment to coverslips and a significantly lower number of cells increasing intracellular free calcium.

Molecular cloning and characterization of a novel phospholipase C, PLC-eta

PLC (phospholipase C) plays an important role in intracellular signal transduction by hydrolysing phosphatidylinositol 4,5-bisphosphate, a membrane phospholipid. To date, 12 members of the mammalian PLC isoforms have been identified and classified into five isotypes beta, gamma, delta, epsilon and zeta, which are regulated by distinct mechanisms. In the present study, we describe the identification of a novel PLC isoform in the brains of human and mouse, named PLC-eta, which contains the conserved pleckstrin homology domain, X and Y domains for catalytic activity and the C2 domain. The first identified gene encoded 1002 (human) or 1003 (mouse) amino acids with an estimated molecular mass of 115 kDa. The purified recombinant PLC-eta exhibited Ca2+-dependent catalytic activity on phosphatidylinositol 4,5-bisphosphate. Furthermore, molecular biological analysis revealed that the PLC-eta gene was transcribed to several splicing variants. Although some transcripts were detected in most of the tissues we examined, the transcript encoding 115 kDa was restricted to the brain and lung. In addition, the expression of the 115 kDa protein was defined in only nerve tissues such as the brain and spinal cord. In situ hybridization analysis with brain revealed that PLC-eta was abundantly expressed in various regions including cerebral cortex, hippocampus, zona incerta and cerebellar Purkinje cell layer, which are neuronal cell-enriched regions. These results suggest that PLC-eta may perform fundamental roles in the brain.

Intramolecular interaction between phosphorylated tyrosine-783 and the C-terminal Src homology 2 domain activates phospholipase C-gamma1

Phospholipase C-gamma1 (PLC-gamma1) contains two tandem Src homology 2 (SH2) domains. The NH(2)-terminal SH2 domain has been known to mediate the binding of PLC-gamma1 to receptor protein tyrosine kinases, which then activate PLC-gamma1 via phosphorylation at Y783. We now show that the phosphorylated Y783 residue (pY783) associates with the COOH-terminal SH2 domain [SH2(C)] within the same molecule of PLC-gamma1. The specificity of this intramolecular interaction is demonstrated in several ways. The mutation of SH2(C), but not of the NH(2)-terminal SH2 domain, exposes pY783 and makes it available for binding by anti-pY783 antibodies, for intermolecular association with a GST fusion protein containing the tandem SH2 domains of PLC-gamma1 and for dephosphorylation by phosphatases. The intramolecular interaction between pY783 and SH2(C) induces a rearrangement of surface charge such that PLC-gamma1 molecules phosphorylated at Y783 are retained more strongly by heparin resins than are unphosphorylated molecules. Finally, the intramolecular interaction of pY783 with SH2(C) results in activation of phospholipase activity. Our results thus clarify the molecular mechanism of PLC-gamma1 activation, revealing the specific function of pY783 and the distinct roles of the two SH2 domains in this process.

Signaling complex formation of phospholipase Cβ4 with metabotropic glutamate receptor type 1α and 1,4,5-trisphosphate receptor at the perisynapse and endoplasmic reticulum in the mouse brain

Upon activation of cell surface receptors coupled to the Gq subclass of G proteins, phospholipase C (PLC) beta hydrolyses membrane phospholipid to yield a pair of second messengers, inositol 1,4,5-trisphosphate (IP3) and 1,2-diacylglycerol. PLCbeta4 has been characterized as the isoform enriched in cerebellar Purkinje cells (PCs) and the retina and involved in motor and visual functions. Here we examined cellular and subcellular distributions of PLCbeta4 in adult mouse brains. Immunohistochemistry showed that high levels of PLCbeta4 were detected in the somatodendritic domain of neuronal populations expressing the metabotropic glutamate receptor (mGluR) type 1alpha, including olfactory periglomerular cells, neurons in the bed nucleus anterior commissure, thalamus, substantia nigra, inferior olive, and unipolar brush cells and PCs in the cerebellum. Low to moderate levels were detected in many other mGluR1alpha-positive neurons and in a few mGluR1alpha-negative neurons. In PCs, immunogold electron microscopy localized PLCbeta4 to the perisynapse, at which mGluR1alpha is concentrated, and to the smooth endoplasmic reticulum in dendrites and spines, an intracellular Ca2+ store gated by IP3 receptors. In the cerebellum, immunoblot demonstrated its concentrated distribution in the post-synaptic density and microsomal fractions, where mGluR1alpha and type 1 IP3 receptor were also greatly enriched. Furthermore, PLCbeta4 formed coimmunoprecipitable complexes with mGluR1alpha, type 1 IP3 receptor and Homer 1. These results suggest that PLCbeta4 is preferentially localized in the perisynapse and smooth endoplasmic reticulum as a component of the physically linked phosphoinositide signaling complex. This close molecular relationship might provide PLCbeta4 with a high-fidelity effector function to mediate various neuronal responses under physiological and pathophysiological conditions.

Identification of Phospholipase C-γ1 as a Mitogen-activated Protein Kinase Substrate

The discovery of sequence motifs that mediate protein-protein interactions, coupled with the availability of protein amino acid sequence data, allows for the identification of putative protein binding pairs. The present studies were based on our identification of an amino acid sequence in phosphatidylinositol-specific phospholipase C-gamma1 (PLC-gamma1) that fits the consensus sequence for a mitogen-activated protein kinase (MAPK) binding site, termed the D-domain. Extracellular signal-regulated kinase 2 (ERK2), an MAPK, and phospho-ERK2 were bound by an immobilized peptide sequence containing the identified PLC-gamma1 D-domain. Furthermore, a peptide containing the PLC-gamma1 D-domain was able to competitively inhibit the in vitro phosphorylation of recombinant PLC-gamma1 by recombinant phospho-ERK2, whereas a control peptide derived from a distant region of PLC-gamma1 was ineffective. Similarly, the peptide containing the PLC-gamma1 D-domain, but not the control peptide, competitively inhibited the in vitro phosphorylation of Elk-1 and c-Jun catalyzed by recombinant phospho-ERK2 and phospho-c-Jun N-terminal kinase 3 (phospho-JNK3), another type of MAPK, respectively. Incubation of anti-PLC-gamma1 immunocomplexes isolated from rat brain with recombinant phospho-ERK2 opposed the increase in PLC-gamma1-catalyzed hydrolysis of phosphatidylinositol 4,5-P(2) (PtdIns(4,5)P(2)), which was produced by a tyrosine kinase associated with the immunocomplexes, whereas in vitro phosphorylation of recombinant PLC-gamma1 by recombinant phospho-ERK2 did not alter PLC-gamma1-catalyzed PtdIns(4,5)P(2) hydrolysis. These studies have uncovered a previously unidentified mechanism for the integration of PLC-gamma1- and ERK2-dependent signaling.

Association analysis of two candidate phospholipase genes that map to the chromosome 15q15.1-15.3 region associated with reading disability

Molecular genetic studies have suggested a reading disability (RD, dyslexia) susceptibility locus on chromosome 15q. We have previously mapped this locus by association to the region surrounding D15S994. Very little is known about the neurobiological processes involved in RD, and therefore selecting positional candidate genes for analysis based upon function is difficult. Nevertheless we were able to identify two functional candidates based upon existing hypotheses. Both were phospholipase genes, phospholipase C beta 2 (PLCB2) and phospholipase A2, group IVB (cytosolic; PLA2G4B). D15S944 is located within PLCB2 and is 1.6 Mb from PLA2G4B. We examined each gene for association using a mixed direct and indirect association approach, a case (n = 164)/control (n = 174) sample, and a partially overlapping sample of 178 RD parent-proband trios from South Wales and England. Mutation analysis revealed 14 sequence variants in PLCB2 and 33 variants in PLA2G4B. All non-synonymous SNPs were genotyped as were SNPs across each gene with maximum distance between SNPs of 6 kb. Case-control analyses revealed modest evidence (0.01 < P < 0.05) for association between a single variant in PLCB2 and two variants in PLA2G4B. However, association was not confirmed in the family based sample. As the latter sample has previously generated replicated significant evidence for association between RD and markers/haplotypes surrounding D15S944, it should have sufficient power to detect association to variants in susceptibility gene itself. We conclude that neither gene accounts for the association signal we previously observed. As these are the only clear cut functional candidate genes in the region, identification of the putative susceptibility locus for RD on 15q will require more methodical non-hypothesis driven positional cloning approaches.

Expression of G protein alpha subunits in the lateral wall of the rat cochlea

Expression of five G protein alpha subunits was investigated in the rat cochlea by reverse transcription-polymerase chain reaction (RT-PCR) in order to understand their role in the cochlear signal transduction mechanisms. Immunohistochemical techniques were employed to study their distribution in the lateral wall of the cochlea. Total RNA was extracted with guanidine thiocyanate from cochleas and brains of 14-21-day-old rats. The extract was treated with DNase to degrade genomic DNA. After RT, the resulting cDNA was amplified by PCR using primers specific for the nucleotide sequences representing alpha subunits of heterotrimeric G proteins. The results indicated that mRNA for all five alpha subunits was expressed in the brain and cochlear samples. For immunohistochemical localization, temporal bones of 6-week-old rats were fixed in 4% paraformaldehyde and 0.1% glutaraldehyde and processed for embedding in paraffin wax. The dewaxed, midmodiolar sections of the cochlea were incubated with subunit-specific polyclonal antibodies. The pattern of immunoreactivity varied for the five G protein alpha subunits studied in the stria vascularis and spiral ligament. The significance of these findings and the role of G protein alpha subunits in cochlear fluid homeostasis are discussed.

Specificity and structural requirements of phospholipase C-beta stimulation by Rho GTPases versus G protein beta gamma dimers

Phospholipase C-beta(2) (PLC beta(2)) is activated both by heterotrimeric G protein alpha- and beta gamma- subunits and by Rho GTPases. In this study, activated Rho GTPases are shown to stimulate PLC beta isozymes with the rank order of PLC beta(2) > PLC beta(3) > or = PLC beta(1). The sensitivity of PLC beta isozymes to Rho GTPases was clearly different from that observed for G protein beta gamma dimers, which decreased in the following order: PLC beta(3) > PLC beta(2) > PLC beta(1) for beta(1)gamma(1/2) and PLC beta(2) > PLC beta(1) >>> PLC beta(3) for beta(5)gamma(2). Rac1 and Rac2 were found to be more potent and efficacious activators of PLC beta(2) than was Cdc42Hs. The stimulation of PLC beta(2) by Rho GTPases and G protein beta gamma dimers was additive, suggesting that PLC beta(2) activation can be augmented by independent regulation of the enzyme by the two stimuli. Using chimeric PLC beta(1)-PLC beta(2) enzymes, beta gamma dimers, and Rho GTPases are shown to require different regions of PLC beta(2) to mediate efficient stimulation of the enzyme. Although the catalytic subdomains X and Y of PLC beta(2) were sufficient for efficient stimulation by beta gamma, the presence of the putative pleckstrin homology domain of PLC beta(2) was absolutely required for the stimulation of the enzyme by Rho GTPases. Taken together, these results identify Rho GTPases as novel PLC beta regulators, which mediate PLC beta isozyme-specific stimulation and are potentially involved in coordinating the activation of PLC beta(2) by extracellular mediators in intact cells.

Lineage-specific defect in gene expression in human platelet phospholipase C-beta2 deficiency

Phospholipase C (PLC)-beta2 plays a major role in platelet activation. Previous studies have described a unique patient with impaired receptor-mediated platelet aggregation, secretion, calcium mobilization, and phospholipase C (PLC) activation associated with a selective decrease in platelet PLC-beta2 isozyme. To identify the mechanisms leading to the defect, platelet RNA from the patient and healthy subjects was subjected to reverse transcription-polymerase chain reaction (RT-PCR) and the products sequenced. The PLC-beta2 cDNA sequence in the patient showed no abnormalities. Platelet PLC-beta2 and beta-actin (internal control) mRNA levels were assessed by RT-PCR; the ratio of PLC-beta2 to beta-actin mRNA levels was 0.80 to 0.95 in 4 healthy subjects and 0.28 in the patient. PLC-beta2 mRNA levels were similarly reduced compared with GPIIb and Galphaq mRNA levels. PLC-gamma2 and platelet factor 4 mRNA levels were normal. Calcium mobilization was studied in neutrophils upon activation with formyl-Met-Leu-Phe (fMLP), adenosine diphosphate (ADP), platelet-activating factor (PAF), interleukin-8 (IL-8), C5a, and leukotriene B(4) (LTB(4)), and it was normal. Neutrophil elastase secretion upon activation with fMLP, ADP, PAF, IL-8, C5a, and LTB(4) was normal, as were neutrophil PLC-beta2 mRNA and PLC-beta2 on immunoblotting. Thus, responses to activation, PLC-beta2 protein, and PLC-beta2 mRNA are decreased in patient platelets but not in neutrophils, providing evidence for a hitherto undescribed lineage (platelet)-specific defect in PLC-beta2 gene expression. These studies provide a physiologically relevant model to delineate regulation of PLC-beta2 gene and its tissue-specific expression. (Blood. 2002;99:905-911)

Inhibition of TASK-1 potassium channel by phospholipase C

The two-pore-domain K(+) channel, TASK-1, was recently shown to be a target of receptor-mediated regulation in neurons and in adrenal glomerulosa cells. Here, we demonstrate that TASK-1 expressed in Xenopus laevis oocytes is inhibited by different Ca(2+)-mobilizing agonists. Lysophosphatidic acid, via its endogenous receptor, and ANG II and carbachol, via their heterologously expressed ANG II type 1a and M(1) muscarinic receptors, respectively, inhibit TASK-1. This effect can be mimicked by guanosine 5'-O-(3-thiotriphosphate), indicating the involvement of GTP-binding protein(s). The phospholipase C inhibitor U-73122 reduced the receptor-mediated inhibition of TASK-1. Downstream signals of phospholipase C action (inositol 1,4,5-trisphosphate, cytoplasmic Ca(2+) concentration, and diacylglycerol) do not mediate the inhibition. Unlike the G(q)-coupled receptors, stimulation of the G(i)-activating M(2) muscarinic receptor coexpressed with TASK-1 results in an only minimal decrease of the TASK-1 current. However, additional coexpression of phospholipase C-beta(2) (which is responsive also to G(i) beta gamma-subunits) renders M(2) receptor activation effective. This indicates the significance of phospholipase C activity in the receptor-mediated inhibition of TASK-1.

KN-93 inhibition of G protein signaling is independent of the ability of Ca2+/calmodulin-dependent protein kinase II to phosphorylate phospholipase Cbeta3 on 537-Ser

Stimulation of the phospholipase Cbeta (PLC) signaling pathway results in intracellular Ca2+ release and subsequent activation of calmodulin (CaM) and CaM kinase II (CaMK II). KN-93, an inhibitor of CaMK II, reduced the stimulation of phosphatidylinositide (PI) turnover by Galphai-coupled (formyl-Met-Leu-Phe, fMLP) or Galphaq-coupled [M1 muscarinic and oxytocin (OT)] receptors. The inhibitory effect of KN-93 was also observed when PLCbeta3 was stimulated directly by Galphaq or Gbetagamma in overexpression assays. CaMK II phosphorylated PLCbeta3 but not PLCbeta1 in vitro. Phosphorylation occurred exclusively on 537Ser in the X-Y linker region of PLCbeta3. 537Ser was also phosphorylated in the basal state in cells and phosphorylation was enhanced by ionomycin treatment. However, mutation of 537Ser to Glu had no effect on inhibition of Galphaq or Gbetagamma-stimulated PLCbeta3 activity by KN-93. KN-93 also inhibited Galphaq -stimulated PLCbeta1 activity, even though this enzyme is not a substrate for CaMK II. These data indicate that phosphorylation of PLCbeta3 by CaMK II is not directly involved in the inhibitory effect of KN-93 on phosphatidylinositide turnover.

Distributional patterns of phospholipase C isozymes in rat pancreas

Phospholipase C (PLC) isozymes are believed to play a role in regulating pancreatic exocrine and endocrine secretion. In an attempt to investigate the role of PLC, we examined the distribution patterns of PLC isozymes in the normal rat pancreas by Western blot analysis and immunohistochemistry. Western blot analysis was performed on pancreatic acinar tissues and the islet of Langerhans, which were separated from each other. PLC-beta isozymes (beta1, beta2, beta3, and beta4), delta1, and delta2 were detected in both acinar and islet cells, whereas PLC-gamma1 and gamma2 were observed only in acinar tissues. On immunohistochemistry, the immunoreactivities of PLC isozymes except for PLC-gamma1 were observed as follows: PLC-beta1, in both the exocrine and endocrine tissues; PLC-beta2, mainly in the periphery of the islet and acinar cells; PLC-beta3, in the periphery of the islet and in some ductal epithelium; PLC-beta4, through the islet of Langerhans and ductal epithelium; PLC-gamma1, not detected in pancreatic tissue; PLC-gamma2, mainly in acinar cells; PLC-delta1 and delta2, in the islet and in ductal epithelium. These results suggest that the intrapancreatic site-specific existence of PLC isozymes may modulate pancreatic exocrine and endocrine functions through a PLC-mediated signal transduction.

The orexin OX1 receptor activates a novel Ca2+ influx pathway necessary for coupling to phospholipase C

Ca(2+) elevations in Chinese hamster ovary cells stably expressing OX(1) receptors were measured using fluorescent Ca(2+) indicators fura-2 and fluo-3. Stimulation with orexin-A led to pronounced Ca(2+) elevations with an EC(50) around 1 nm. When the extracellular [Ca(2+)] was reduced to a submicromolar concentration, the EC(50) was increased 100-fold. Similarly, the inositol 1,4,5-trisphosphate production in the presence of 1 mm external Ca(2+) was about 2 orders of magnitude more sensitive to orexin-A stimulation than in low extracellular Ca(2+). The shift in the potency was not caused by depletion of intracellular Ca(2+) but by a requirement of extracellular Ca(2+) for production of inositol 1,4,5-trisphosphate. Fura-2 experiments with the "Mn(2+)-quench technique" indicated a direct activation of a cation influx pathway by OX(1) receptor independent of Ca(2+) release or pool depletion. Furthermore, depolarization of the cells to +60 mV, which almost nullifies the driving force for Ca(2+) entry, abolished the Ca(2+) response to low concentrations of orexin-A. The results thus suggest that OX(1) receptor activation leads to two responses, (i) a Ca(2+) influx and (ii) a direct stimulation of phospholipase C, and that these two responses converge at the level of phospholipase C where the former markedly enhances the potency of the latter.

Structure, function, and control of phosphoinositide-specific phospholipase C

Phosphoinositide-specific phospholipase C (PLC) subtypes beta, gamma, and delta comprise a related group of multidomain phosphodiesterases that cleave the polar head groups from inositol lipids. Activated by all classes of cell surface receptor, these enzymes generate the ubiquitous second messengers inositol 1,4, 5-trisphosphate and diacylglycerol. The last 5 years have seen remarkable advances in our understanding of the molecular and biological facets of PLCs. New insights into their multidomain arrangement and catalytic mechanism have been gained from crystallographic studies of PLC-delta(1), while new modes of controlling PLC activity have been uncovered in cellular studies. Most notable is the realization that PLC-beta, -gamma, and -delta isoforms act in concert, each contributing to a specific aspect of the cellular response. Clues to their true biological roles were also obtained. Long assumed to function broadly in calcium-regulated processes, genetic studies in yeast, slime molds, plants, flies, and mammals point to specific and conditional roles for each PLC isoform in cell signaling and development. In this review we consider each subtype of PLC in organisms ranging from yeast to mammals and discuss their molecular regulation and biological function.

Molecular mechanism of the inhibition of phospholipase C beta 3 by protein kinase C

Activation of protein kinase C (PKC) can result from stimulation of the receptor-G protein-phospholipase C (PLCbeta) pathway. In turn, phosphorylation of PLCbeta by PKC may play a role in the regulation of receptor-mediated phosphatidylinositide (PI) turnover and intracellular Ca(2+) release. Activation of endogenous PKC by phorbol 12-myristate 13-acetate inhibited both Galpha(q)-coupled (oxytocin and M1 muscarinic) and Galpha(i)-coupled (formyl-Met-Leu-Phe) receptor-stimulated PI turnover by 50-100% in PHM1, HeLa, COSM6, and RBL-2H3 cells expressing PLCbeta(3). Activation of conventional PKCs with thymeleatoxin similarly inhibited oxytocin or formyl-Met-Leu-Phe receptor-stimulated PI turnover. The PKC inhibitory effect was also observed when PLCbeta(3) was stimulated directly by Galpha(q) or Gbetagamma in overexpression assays. PKC phosphorylated PLCbeta(3) at the same predominant site in vivo and in vitro. Peptide sequencing of in vitro phosphorylated recombinant PLCbeta(3) and site-directed mutagenesis identified Ser(1105) as the predominant phosphorylation site. Ser(1105) is also phosphorylated by protein kinase A (PKA; Yue, C., Dodge, K. L., Weber, G., and Sanborn, B. M. (1998) J. Biol. Chem. 273, 18023-18027). Similar to PKA, the inhibition by PKC of Galpha(q)-stimulated PLCbeta(3) activity was completely abolished by mutation of Ser(1105) to Ala. In contrast, mutation of Ser(1105) or Ser(26), another putative phosphorylation target, to Ala had no effect on inhibition of Gbetagamma-stimulated PLCbeta(3) activity by PKC or PKA. These data indicate that PKC and PKA act similarly in that they inhibit Galpha(q)-stimulated PLCbeta(3) as a result of phosphorylation of Ser(1105). Moreover, PKC and PKA both inhibit Gbetagamma-stimulated activity by mechanisms that do not involve Ser(1105).

Evidence for two alternatively spliced forms of phospholipase C-beta2 in haematopoietic cells

Alternatively spliced forms have been reported for several phospholipase C (PLC) isozymes, but not for PLC-beta2, the most abundant PLC-beta in platelets. PLC-beta2 cDNA cloned from the HL-60-cell cDNA library is 3543 bases long, coding for 1181 amino acids. Compared with the published sequence, a deletion of 45 nucleotides (2755-2799 nt, amino acids 864-878) was detected in platelet and leucocyte mRNA amplified by reverse transcription (RT) polymerase chain reaction (PCR) using primers corresponding to 1814-1838 nt (forward) and 3328-3352 nt (reverse). Amplification of genomic DNA using primers corresponding to 2575-2596 nt and 2864-2885 nt yielded a approximately 750 bp product; restriction analysis and sequencing revealed the 45-bp exon flanked by introns of 198 bp and 118 bp. Amplification of leucocyte and platelet cDNA using the same primers yielded products of approximately 310 nt and approximately 265 nt, with (PLC-beta2a) and without (PLC-beta2b) the 45-nt sequence. Thus, two alternatively spliced forms (1181 and 1166 amino acids) of PLC-beta2 are generated in haematopoietic cells. They differ in the carboxyl terminal sequence implicated in interaction of PLC-beta enzymes with Galphaq, particulate association and nuclear localization. We propose that the PLC-beta2 splice variants may be regulated differentially with distinct roles in signal transduction.

Preterm labour

Spontaneous preterm labour remains a major obstetric problem because of the high incidence of neonatal mortality or long-term handicap associated with it. The drugs available for the management of preterm labour are poorly effective and have potentially serious side-effects for the mother or fetus. In recent years, there has been a remarkable increase in the knowledge of the biochemical mechanism underlying uterine quiescence and contractility. Many of the G protein-coupled receptors that participate in the regulation of myometrial activity have been cloned and characterized, and their intracellular signalling pathways have been elucidated. The role of G protein receptor kinases in uterine tachyphylaxis is better understood. New developments in our understanding of the cellular mechanisms involved in uterine contractions in idiopathic and infection-associated preterm labour are expected, which will lead to better, more selective therapy for this problem. However, much research remains to be done before the mechanism of human parturition is fully understood.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

Effects of a water-soluble antitumor ether phosphonoinositide, D-myo-inositol 4-(hexadecyloxy)-3(S)-methoxybutanephosphonate (C4-PI), on inositol lipid metabolism in breast epithelial cancer cell lines

We have demonstrated previously that D-myo-inositol 4-(hexadecyloxy)-3(S)-methoxybutanephosphonate (C4-PI), an isosteric phosphonate analog of phosphatidylinositol developed to inhibit inositol lipid metabolism, was unable to inhibit phosphatidylinositol (PI) 3-kinase activity. We now report the effects of the compound on other aspects of inositol metabolism. We demonstrated that C4-PI inhibits the activity of purified recombinant PI-phospholipase C-beta (PLC-beta) at all concentrations tested; it enhanced the activity of PI-PLC-gamma and PI-PLC-delta at low concentrations (10 microM), while severely inhibiting their activities at higher concentrations. In the breast cancer cell lines MCF-7 (estrogen receptor positive) and MDA-MB-468 (estrogen receptor negative), C4-PI had no effect on the uptake of D-myo-inositol but severely inhibited its incorporation into PI. In spite of the drastic decrease in PI synthesis, C4-PI did not affect the levels of inositol incorporated into phosphatidylinositol 4,5-bisphosphate (PIP2) in the cells. In vitro assays showed that C4-PI inhibited PI synthase activity (inhibition of 35% at 50 microM) but had little effect on PI 4-kinase activity (inhibition of 13% at 150 microM). C4-PI inhibited the proliferation of MCF-7 and MDA-MB-468 cell lines with IC(50) values of 12 and 18 microM. Taken together, the results suggest that the accumulation of [3H]inositol in PIP2 in cells incubated with C4-PI may be due to the inhibition of PIP2 hydrolysis in the cells with no effect on its synthesis. The role of these C4-PI-induced effects in the mechanism of growth inhibition by C4-PI remains to be established.

AHNAK, a protein that binds and activates phospholipase C-gamma1 in the presence of arachidonic acid

We have recently shown that phospholipase C-gamma (PLC-gamma) is activated by tau, a neuronal cell-specific microtubule-associated protein, in the presence of arachidonic acid. We now report that non-neuronal tissues also contain a protein that can activate PLC-gamma in the presence of arachidonic acid. Purification of this activator from bovine lung cytosol yielded several proteins with apparent molecular sizes of 70-130 kDa. They were identified as fragments derived from an unusually large protein (approximately 700 kDa) named AHNAK, which comprises about 30 repeated motifs each 128 amino acids in length. Two AHNAK fragments containing one and four of the repeated motifs, respectively, were expressed as glutathione S-transferase fusion proteins. Both recombinant proteins activated PLC-gamma1 at nanomolar concentrations in the presence of arachidonic acid, suggesting that an intact AHNAK molecule contains multiple sites for PLC-gamma activation. The role of arachidonic acid was to promote a physical interaction between AHNAK and PLC-gamma1, and the activation by AHNAK and arachidonic acid was mainly attributable to reduction in the enzyme's apparent Km toward the substrate phosphatidylinositol 4,5-bisphosphate. Our results suggest that arachidonic acid liberated by phospholipase A2 can act as an additional trigger for PLC-gamma activation, constituting an alternative mechanism that is independent of tyrosine phosphorylation.

SH2 domain-mediated activation of phospholipase Cgamma is not required to initiate Ca2+ release at fertilization of mouse eggs

The initiation of Ca2+ release at fertilization of mammalian eggs requires inositol trisphosphate (Miyazaki et al., 1992, Science 257, 251-255), indicating that an enzyme of the phospholipase C family is probably activated. Because Ca2+ release at fertilization in echinoderm eggs is initiated by SH2 domain-mediated activation of phospholipase Cgamma (Carroll et al., 1997, J. Cell Biol. 138, 1303-1311), we examined the possible role of PLCgamma in initiating Ca2+ release at fertilization in mouse eggs. Both PLCgamma isoforms, PLCgamma1 and PLCgamma2, are present in mouse eggs and sperm, and stimulation of these enzymes in the egg by way of an exogenously expressed PDGF receptor causes Ca2+ release. Recombinant SH2 domains of PLCgamma1 and PLCgamma2 inhibit PLCgamma1 and PLCgamma2 activation by the PDGF receptor, completely preventing Ca2+ release in response to PDGF when injected at an approximately 20- to 40-fold excess over the concentrations of endogenous proteins. However, even at an approximately 100- to 400-fold excess over endogenous protein levels, PLCgamma1 and PLCgamma2 SH2 domains do not inhibit Ca2+ release at fertilization. These findings indicate that Ca2+ release at fertilization of mouse eggs does not require SH2-domain-mediated activation of PLCgamma. However, activation of PLCgamma in the egg by an alternative pathway, or introduction of activated PLCgamma from the sperm, may be important.

Identification of a phospholipase C beta subtype in rat taste cells

From rat circumvallate papillae a novel phospholipase C (PLC) subtype has been cloned and identified as most closely related to human PLC beta2. The corresponding mRNA was only detected in sensory lingual tissue but not in non-taste lingual tissue or any other tissues examined by Northern blot analysis. In situ hybridization revealed that a subset of taste receptor cells of circumvallate papillae was specifically labeled. A functional involvement of this PLC beta subtype in taste signal transduction emerged from biochemical analysis monitoring the second messenger response in circumvallate preparations induced by denatonium benzoate. This bitter agent elicited a rapid and transient increase of the inositol 1,4,5-trisphosphate level; this response was blocked by U73122, a potent inhibitor of PLC, and by PLC beta2-specific antibodies. These data indicate that a phospholipase C beta2 isoform mediates a denatonium benzoate-induced second messenger response of taste sensory cells in the circumvallate papillae.

Patterns of expression for the mRNA corresponding to the four isoforms of phospholipase Cbeta in mouse brain

Ligand binding to neurotransmitter and hormone receptors which couple to the Gq subclass of GTP-binding protein leads to the activation of phospholipase Cbeta (PLCbeta) which hydrolyses phosphatidyl-inositol 4,5-bisphosphate, yielding a pair of second messengers, diacylglycerol and inositol 1,4,5-trisphosphate (IP3). The expression of PLCbeta1-4 mRNAs was comparatively examined by in situ hybridization in the mouse brain. In adults, PLCbeta1 mRNA was expressed predominantly in the telencephalon, including the cerebral cortex, hippocampus, amygdala, lateral septum and olfactory bulb, with little expression in most thalamic nuclei. PLCbeta2 mRNA was distributed in the white matter, suggesting its expression in non-neuronal cells, most likely oligodendrocytes. PLCbeta3 mRNA was specifically expressed in cerebellar Purkinje cells. The highest levels of PLCbeta4 mRNA were detected in Purkinje cells. High levels of PLCbeta4 mRNA were also found in the thalamus and medial septum, whereas weak signals were detected in most telencephalic regions, thus showing an expression pattern almost reciprocal to that of PLCbeta1 mRNA. During development, such characteristic regional expression of PLCbeta1 and PLCbeta4 were observed starting in late foetal stages, while specific expression of PLCbeta2 and PLCbeta3 appeared in early postnatal stages. We conclude that despite the existence of four PLCbeta isoforms, only one or two of them is expressed in individual neurons and glial cells. The distinct expression of PLCbetas provides a molecular basis for analysing the nature of the specific signal transduction pathway leading to the production of diacylglycerol and IP3 in distinct cell types and in different regions of the brain.

Differential regulation of formyl peptide and platelet-activating factor receptors. Role of phospholipase Cbeta3 phosphorylation by protein kinase A

Formylated peptides (e.g. n-formyl-Met-Leu-Phe (fMLP)) and platelet-activating factor (PAF) mediate chemotactic and cytotoxic responses in leukocytes through receptors coupled to G proteins that activate phospholipase C (PLC). In RBL-2H3 cells, fMLP utilizes a pertussis toxin (ptx)-sensitive G protein to activate PLC, whereas PAF utilizes a ptx-insensitive G protein. Here we demonstrate that fMLP, but not PAF, enhanced intracellular cAMP levels via a ptx-sensitive mechanism. Protein kinase A (PKA) inhibition by H-89 enhanced inositol phosphate formation stimulated by fMLP but not PAF. Furthermore, a membrane-permeable cAMP analog 8-(4-chlorophenylthio)-cAMP (cpt-cAMP) inhibited phosphoinositide hydrolysis and secretion stimulated by fMLP but not PAF. Both cpt-cAMP and fMLP stimulated PLCbeta3 phosphorylation in intact RBL cells. The purified catalytic subunit of PKA phosphorylated PLCbeta3 immunoprecipitated from RBL cell lysate. Pretreatment of intact cells with cpt-cAMP and fMLP, but not PAF, resulted in an inhibition of subsequent PLCbeta3 phosphorylation by PKA in vitro. These data demonstrate that fMLP receptor, which couples to a ptx-sensitive G protein, activates both PLC and cAMP production. The resulting PKA activation phosphorylates PLCbeta3 and appears to block the ability of Gbetagamma to activate PLC. Thus, both fMLP and PAF generate stimulatory signals for PLCbeta3, but only fMLP produces a PKA-dependent inhibitory signal. This suggests a novel mechanism for the bidirectional regulation of receptors which activate PLC by ptx-sensitive G proteins.

Activation of phospholipase C-gamma by phosphatidylinositol 3,4,5-trisphosphate

Signal transduction across cell membranes often involves the activation of both phosphatidylinositol (PI)-specific phospholipase C (PLC) and phosphoinositide 3-kinase (PI 3-kinase). Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2), a substrate for both enzymes, is converted to phosphatidylinositol 3,4, 5-trisphosphate (PI(3,4,5)P3) by the action of PI 3-kinase. Here, we show that PI(3,4,5)P3 activates purified PLC-gamma isozymes by interacting with their Src homology 2 domains. Furthermore, the expression of an activated catalytic subunit of PI 3-kinase in COS-7 cells resulted in an increase in inositol phosphate formation, whereas platelet-derived growth factor-induced PLC activation in NIH 3T3 cells was markedly inhibited by the specific PI 3-kinase inhibitor LY294002. These results suggest that receptors coupled to PI 3-kinase may activate PLC-gamma isozymes indirectly, in the absence of PLC-gamma tyrosine phosphorylation, through the generation of PI(3,4,5)P3.

Age-related defect of phospholipase C activity, differential expression of the beta 2 isoform in active T lymphocytes from aged humans

The activation of phosphoinositide-specific PLC is one of the early cellular responses to various growth factors and mitogens. It is known that these functions are altered with ageing. Here we show that the beta 2 isozyme is decreased with ageing in a peculiar T lymphocyte subpopulation involved in the immune response, namely the active T lymphocytes. The presence and cellular distribution of PLC isozymes were investigated with immunochemical and immunocytochemical methods. The in situ immunocytochemistry displayed the presence of the beta and gamma isoforms in the cytoplasm, while no reactivity for the delta isoform was observed regardless of the age. The immunoblot analysis showed an increased expression in the beta 2 isoform in the young and an equal expression of the gamma 1 isoform in either age group. Our findings suggest that the age-related defect of PLC activity is possibly due to an impaired expression of isozymes in aged active T lymphocytes.