Purification, molecular cloning, and sequencing of phospholipase C-beta 4

Personal essay of a rookie's journey with Bill Pak and his legacy: tales and perspectives on PI-PLC, NorpA and cyclophilin, NinaA - William L. Pak, PhD., 1932-2023: in memoriam

The neurogenetics and vision community recently mourned William L. Pak, PhD, whose pioneering work spearheaded the genetic, electrophysiological, and molecular bases of biological processes underpinning vision. This essay provides a historical background to the daunting challenges and personal experiences that carved the path to seminal findings. It also reflects on the intellectual framework, mentoring philosophy, and inspirational legacy of Bill Pak's research. An emphasis and perspectives are placed on the discoveries and implications to date of the phosphatidylinositol-specific phospholipase C (PI-PLC), NorpA, and the cyclophilin, NinaA of the fruit fly, Drosophila melanogaster, and their respective mammalian homologues, PI-PLCβ4, and cyclophilin-related protein, Ran-binding protein 2 (Ranbp2) in critical biological processes and diseases of photoreceptors and other neurons.

Molecular regulation of PLCβ signaling

Phospholipase C (PLC) enzymes convert the membrane phospholipid phosphatidylinositol-4,5-bisphosphate (PIP2) into inositol-1,4,5-triphosphate (IP3) and diacylglycerol (DAG). IP3 and DAG regulate numerous downstream pathways, eliciting diverse and profound cellular changes and physiological responses. In the six PLC subfamilies in higher eukaryotes, PLCβ is intensively studied due to its prominent role in regulating crucial cellular events underlying many processes including cardiovascular and neuronal signaling, and associated pathological conditions. In addition to GαqGTP, Gβγ generated upon G protein heterotrimer dissociation also regulates PLCβ activity. Here, we not only review how Gβγ directly activates PLCβ, and also extensively modulates Gαq-mediated PLCβ activity, but also provide a structure-function overview of PLC family members. Given that Gαq and PLCβ are oncogenes, and Gβγ shows unique cell-tissue-organ specific expression profiles, Gγ subtype-dependent signaling efficacies, and distinct subcellular activities, this review proposes that Gβγ is a major regulator of Gαq-dependent and independent PLCβ signaling.

Phospholipase C families: Common themes and versatility in physiology and pathology

Phosphoinositide-specific phospholipase Cs (PLCs) are expressed in all mammalian cells and play critical roles in signal transduction. To obtain a comprehensive understanding of these enzymes in physiology and pathology, a detailed structural, biochemical, cell biological and genetic information is required. In this review, we cover all these aspects to summarize current knowledge of the entire superfamily. The families of PLCs have expanded from 13 enzymes to 16 with the identification of the atypical PLCs in the human genome. Recent structural insights highlight the common themes that cover not only the substrate catalysis but also the mechanisms of activation. This involves the release of autoinhibitory interactions that, in the absence of stimulation, maintain classical PLC enzymes in their inactive forms. Studies of individual PLCs provide a rich repertoire of PLC function in different physiologies. Furthermore, the genetic studies discovered numerous mutated and rare variants of PLC enzymes and their link to human disease development, greatly expanding our understanding of their roles in diverse pathologies. Notably, substantial evidence now supports involvement of different PLC isoforms in the development of specific cancer types, immune disorders and neurodegeneration. These advances will stimulate the generation of new drugs that target PLC enzymes, and will therefore open new possibilities for treatment of a number of diseases where current therapies remain ineffective.

Phospholipase C-β1 Hypofunction in the Pathogenesis of Schizophrenia

Schizophrenia is a mental disorder that is characterized by various abnormal symptoms. Previous studies indicate decreased expression of phospholipase C-β1 (PLC-β1) in the brains of patients with schizophrenia. PLC-β1-null (PLC-β1(-/-)) mice exhibit multiple endophenotypes of schizophrenia. Furthermore, a study of PLC-β1 knockdown in the medial prefrontal cortex of mice has shown a specific behavioral deficit, impaired working memory. These results support the notion that disruption of PLC-β1-linked signaling in the brain is strongly involved in the pathogenesis of schizophrenia. In this review, we broadly investigate recent studies regarding schizophrenia-related behaviors as well as their various clinical and biological correlates in PLC-β1(-/-) and knockdown mouse models. This will provide a better understanding of the pathological relevance of the altered expression of PLC-β1 in the brains of patients with schizophrenia. Evidence accumulated will shed light on future in-depth studies, possibly in human subjects.

Increases in intracellular calcium via activation of potentially multiple phospholipase C isozymes in mouse olfactory neurons

Phospholipase C (PLC) and internal Ca(2+) stores are involved in a variety of cellular functions. However, our understanding of PLC in mammalian olfactory sensory neurons (OSNs) is generally limited to its controversial role in odor transduction. Here we employed single-cell Ca(2+) imaging and molecular approaches to investigate PLC-mediated Ca(2+) responses and its isozyme gene transcript expression. We found that the pan-PLC activator m-3M3FBS (25 μM) induces intracellular Ca(2+) increases in vast majority of isolated mouse OSNs tested. Both the response amplitude and percent responding cells depend on m-3M3FBS concentrations. In contrast, the inactive analog o-3M3FBS fails to induce Ca(2+) responses. The m-3M3FBS-induced Ca(2+) increase is blocked by the PLC inhibitor U73122, while its inactive analog U73433 has no effect. Removal of extracellular Ca(2+) does not change significantly the m-3M3FBS-induced Ca(2+) response amplitude. Additionally, in the absence of external Ca(2+), we found that a subset of OSNs respond to an odorant mixture with small Ca(2+) increases, which are significantly suppressed by U73122. Furthermore, using reverse transcription polymerase chain reaction and real-time quantitative polymerase chain reaction, we found that multiple PLC isozyme gene transcripts are expressed in olfactory turbinate tissue in various levels. Using RNA in situ hybridization analysis, we further show expression of β4, γ1, γ2 gene transcripts in OSNs. Taken together, our results establish that PLC isozymes are potent enzymes for mobilizing intracellular Ca(2+) in mouse OSNs and provide molecular insight for PLC isozymes-mediated complex cell signaling and regulation in the peripheral olfactory epithelium.

The effect of acteoside on intracellular Ca(2+) mobilization and phospholipase C activity in RBL-2H3 cells stimulated by melittin

This study was performed to investigate the effects of acteoside on various cellular functions such as, intracellular Ca(2+) mobilization, phospholipase C activity, and exocytosis induced by melittin. Melittin (0.1-1 μM) dose-dependently increased intracellular Ca(2+) mobilization in the presence of extracellular Ca(2+), but was not affected by 1 μM U73122, a specific PLC inhibitor. In the absence of extracellular Ca(2+), melittin (1 μM) did not induce a change in intracellular Ca(2+) mobilization, which suggests that melittin-induced intracellular Ca(2+) mobilization may be dependent on the influx of extracellular Ca(2+) rather than on the release of intracellular Ca(2+) storage. Acteoside (10 μM) significantly inhibited 1 μM melittin-induced Ca(2+) mobilization by 33 %. In [(3)H]inositol-labeled cells, 1 μM melittin did not increase inositol phosphate formation, but more than 5 μM melittin significantly increased inositol phosphate formation, which was significantly inhibited by acteoside. Melittin (1 μM) significantly increased histamine release from RBL 2H3 cells in the presence or absence of extracellular Ca(2+). Acteoside significantly inhibited 1-μM-melittin-induced histamine release by 74 % in the presence of extracellular Ca(2+) and by 71 % in the absence of extracellular Ca(2+). These data suggest that the inhibitory effect of acteoside on 1 μM-melittin-induced histamine release may be related to blockage of the calcium-independent pathway. Taken together, these data suggest that melittin has an influence on cellular functions such as intracellular Ca(2+) mobilization, the PLC pathway, and exocytosis via various independent signalling pathways in RBL-2H3 cells, and was significantly inhibited by acteoside.

Structural insights into phospholipase C-β function

Phospholipase C (PLC) enzymes convert phosphatidylinositol-4,5-bisphosphate into the second messengers diacylglycerol and inositol-1,4,5-triphosphate. The production of these molecules promotes the release of intracellular calcium and activation of protein kinase C, which results in profound cellular changes. The PLCβ subfamily is of particular interest given its prominent role in cardiovascular and neuronal signaling and its regulation by G protein-coupled receptors, as PLCβ is the canonical downstream target of the heterotrimeric G protein Gαq. However, this is not the only mechanism regulating PLCβ activity. Extensive structural and biochemical evidence has revealed regulatory roles for autoinhibitory elements within PLCβ, Gβγ, small molecular weight G proteins, and the lipid membrane itself. Such complex regulation highlights the central role that this enzyme plays in cell signaling. A better understanding of the molecular mechanisms underlying the control of its activity will greatly facilitate the search for selective small molecule modulators of PLCβ.

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.

Histamine receptors of cones and horizontal cells in Old World monkey retinas

In primates the retina receives input from histaminergic neurons in the posterior hypothalamus that are active during the day. In order to understand how this input contributes to information processing in Old World monkey retinas, we have been localizing histamine receptors (HR) and studying the effects of histamine on the neurons that express them. Previously, we localized HR3 to the tips of ON bipolar cell dendrites and showed that histamine hyperpolarizes the cells via this receptor. We raised antisera against synthetic peptides corresponding to an extracellular domain of HR1 between the 4th and 5th transmembrane domains and to an intracellular domain near the carboxyl terminus of HR2. Using these, we localized HR1 to horizontal cells and a small number of amacrine cells and localized HR2 to puncta closely associated with synaptic ribbons inside cone pedicles. Consistent with this, HR1 mRNA was detected in horizontal cell perikarya and primary dendrites and HR2 mRNA was found in cone inner segments. We studied the effect of 5 μM exogenous histamine on primate cones in macaque retinal slices. Histamine reduced I(h) at moderately hyperpolarized potentials, but not the maximal current. This would be expected to increase the operating range of cones and conserve ATP in bright, ambient light. Thus, all three major targets of histamine are in the outer plexiform layer, but the retinopetal axons containing histamine terminate in the inner plexiform layer. Taken together, the findings in these three studies suggest that histamine acts primarily via volume transmission in primate retina.

Cellular neurochemical characterization and subcellular localization of phospholipase C β1 in rat brain

The present study describes a complete and detailed neuroanatomical distribution map of the phospholipase C beta1 (PLCβ1) isoform along the adult rat neuraxis, and defines the phenotype of cells expressing PLCβ1, along with its subcellular localization in cortical neurons as assessed by double-immunofluorescence staining and confocal laser scanning. Immunohistochemical labeling revealed a considerable morphological heterogeneity among PLCβ1-positive cells in the cortex, even though there was a marked predominance of pyramidal morphologies. As an exception to the general non-matching distribution of GFAP and PLCβ1, a high degree of co-expression was observed in radial glia-like processes of the spinal cord white matter. In the somatosensory cortex, the proportion of GABAergic neurons co-stained with PLCβ1 was similar (around 2/3) in layers I, II-III, IV and VI, and considerably lower in layer V (around 2/5). Double immunofluorescence against PLCβ1 and nuclear speckle markers SC-35 and NeuN/Fox3 in isolated nuclei from the rat cortex showed a high overlap of both markers with PLCβ1 within the nuclear matrix. In contrast, there was no apparent co-localization with markers of the nuclear envelope and lamina. Finally, to assess whether the subcellular expression pattern of PLCβ1 involved specifically one of the two splice variants of PLCβ1, we carried out Western blot experiments in cortical subcellular fractions. Notably, PLCβ1a/1b ratios were statistically higher in the cytoplasm than in the nuclear and plasma membrane fractions. These results provide a deeper knowledge of the cellular distribution of the PLCβ1 isoform in different cell subtypes of the rat brain, and of its presence in the neuronal nuclear compartment.

Effect of Extremely Low Frequency Electromagnetic Fields (EMF) on Phospholipase Activity in the Cultured Cells

This study was conducted to investigate the effects of extremely low frequency electromagnetic fields (EMF) on signal pathway in plasma membrane of cultured cells (RAW 264.7 cells and RBL 2H3 cells), by measuring the activity of phospholipase A(2) (PLA(2)), phospholipase C (PLC) and phospholipase D (PLD). The cells were exposed to the EMF (60 Hz, 0.1 or 1 mT) for 4 or 16 h. The basal and 0.5 µM melittin-induced arachidonic acid release was not affected by EMF in both cells. In cell-free PLA(2) assay, we failed to observe the change of cPLA(2) and sPLA(2) activity. Also both PLC and PLD activities did not show any change in the two cell lines exposed to EMF. This study suggests that the exposure condition of EMF (60 Hz, 0.1 or 1 mT) which is 2.4 fold higher than the limit of occupational exposure does not induce phospholipases-associated signal pathway in RAW 264.7 cells and RBL 2H3 cells.

Phospholipase Cbeta4 isozyme is expressed in human, rat, and murine heart left ventricles and in HL-1 cardiomyocytes

Phospholipase C-beta (PLCbeta) isozymes (PLCbeta(1) and PLCbeta(3)) have been extensively characterized in cardiac tissue, but no data are available for the PLCbeta(4) isozyme. In this study, PLCbeta((1-4)) isozymes mRNA relative expression was studied by real-time PCR (RT-PCR) in human, rat, and murine left ventricle and the presence of PLCbeta(4) isozyme at the protein level was confirmed by Western blotting in all species studied. Confocal microscopy experiments carried out in HL-1 cardiomyocytes revealed a sarcoplasmic subcellular distribution of PLCbeta(4). Although there were unexpected significant interspecies differences in the PLCbeta((1-4)) mRNA expression, PLCbeta(4) mRNA was the main transcript expressed in all left ventricles studied. Thus, whereas in human and rat left ventricles PLCbeta(4) > PLCbeta(3) > PLCbeta(2) > PLCbeta(1) mRNA pattern of expression was found, in murine left ventricle the pattern of expression was different, i.e., PLCbeta(4) > PLCbeta(1) > PLCbeta(3) > PLCbeta(2). However, results obtained in mouse HL-1 cardiomyocytes showed PLCbeta(3) approximately PLCbeta(4) > PLCbeta(1) > PLCbeta(2) pattern of mRNA expression indicating a probable cell type specific expression of the different PLCbeta isozymes in cardiomyocytes. Finally, RT-PCR experiments showed a trend, even though not significant (P = 0.067), to increase PLCbeta(4) mRNA levels in HL-1 cardiomyocytes after angiotensin II treatment. These results demonstrate the presence of PLCbeta(4) in the heart and in HL-1 cardiomyocytes showing a different species-dependent pattern of expression of the PLCbeta((1-4)) transcripts. We discuss the relevance of these findings in relation to the development of cardiac hypertrophy.

Characterization of phospholipases C beta and gamma and their possible roles in Chaetopterus egg activation

Intracellular calcium release from the endoplasmic reticulum is a hallmark at egg activation of both vertebrates and invertebrates. This fertilization-associated calcium release results from generation of the second messenger inositol 1,4,5-trisphosphate (IP(3)) by one or more phospholipases C (PLC). We characterized Chaetopterus PLCbeta and gamma by reverse transcription/degenerate oligonucleotide primed PCR and rapid amplification of cDNA end PCR. Phylogenetic analyses suggested that the deduced PLCbeta protein shared the greatest homology with mammalian PLCbeta4; the deduced PLCgamma protein shared the greatest homology with starfish PLCgamma and diverged from mammalian PLCgamma before mammalian the PLCgamma1 and gamma2 isoforms diverged. Western blot analyses with specific anti-PLCbeta and gamma antibodies, respectively, revealed that 135 and 150 kDa proteins were expressed in eggs. The general PLC antagonist U-73122 blocked fertilization-induced egg activation; however, the inactive analog, U-73343, had no effect on egg activation. We further tested whether egg activation was G protein-PLCbeta and/or protein tyrosine kinase-PLCgamma dependent. Cholera and pertussis toxins, well-known effectors of G proteins, had no effect on egg activation; while two antagonists of PTK, genistein and tyrphostin B42, inhibited both fertilization-induced and artificial egg activation. Taken together, our studies suggested that PLC activity from eggs contributes to Chaetopterus egg activation and PLCgamma might play an important role during this biological process.

Melanopsin Ganglion Cells Use a Membrane-Associated Rhabdomeric Phototransduction Cascade

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are photoreceptors of the mammalian eye that drive pupillary responses, synchronization of circadian rhythms, and other reflexive responses to daylight. Melanopsin is the ipRGC photopigment, but the signaling cascade through which this invertebrate-like opsin triggers the photocurrent in these cells is unknown. Here, using patch-clamp recordings from dissociated ipRGCs in culture, we show that a membrane-associated phosphoinositide cascade lies at the heart of the ipRGC phototransduction mechanism, similar to the cascade in rhabdomeric photoreceptors of invertebrate eyes. When ipRGCs were illuminated, melanopsin activated a G protein of the Gq/11class, stimulating the effector enzyme phospholipase C. The presence of these signaling components in ipRGCs was confirmed by single-cell RT-PCR and immunofluorescence. The photoresponse was fully functional in excised inside-out patches of ipRGC membrane, indicating that all core signaling components are within or tightly coupled to the plasma membrane. The striking similarity of phototransduction in ipRGCs and invertebrate rhabdomeric photoreceptors reinforces the emerging view that these cells have a common evolutionary origin.

Evidence for two populations of bitter responsive taste cells in mice

Taste receptor cells use multiple signaling mechanisms to detect different taste stimuli in the oral cavity. Ionic stimuli (sour, salty) interact directly with ion channels to elicit responses, whereas bitter, sweet, and umami tastants activate G protein-coupled receptors to initiate phospholipase C (PLC)-dependent release of calcium from intracellular stores. However, the precise role for PLC in taste responses remains unclear. One study reported that bitter, sweet, and umami detection is abolished in PLCbeta2 knock-out animals, indicating that the perception of these stimuli depends solely on PLCbeta2. In contrast, another study found that PLCbeta2 knock-out mice have a reduced, but not abolished, capacity to detect these taste qualities, suggesting a PLCbeta2-independent signaling pathway may be involved in the detection of taste stimuli. Since PLCbeta2-expressing taste cells do not have conventional synapses or express voltage-gated calcium channels (VGCCs), we sought to determine if any taste cells responding to bitter express VGCCs. We characterized calcium responses generated by bitter stimuli to activate the PLC pathway and 50 mM KCl to activate VGCCs. Comparisons of evoked calcium responses found that these two stimuli generated significantly different responses. Surprisingly, although most responsive taste cells responded to bitter or 50 mM KCl, some taste cells responded to both. Analysis of dual responsive cells found that bitter responses were inhibited by the PLC inhibitor U73122. Immunocytochemical analysis detected PLCbeta3 and IP(3)R1, indicating the presence of multiple PLC signaling pathways in taste cells.

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.

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.

Localization of phospholipase Cβ1 on the detergent-resistant membrane microdomain prepared from the synaptic plasma membrane fraction of rat brain

The membrane microdomain (MD), such as detergent-resistant low-density membrane microdomain fraction (DRM), has been paid much attention because many signal-transducing molecules are recovered in this fraction, although precise localization and interactions of these molecules are largely unclear. To identify neuronal MD-localized proteins, monoclonal antibodies (mAbs) against the DRM-components of synaptic plasma membrane fraction (SPM) were produced and the antigens were characterized. One of the antigens reacted with two closely positioned bands of about 140 kDa in SDS-PAGE and the antigen showed age-dependent localization on DRM. The antigen was immunoprecipitated with the mAb after partial solubilization with 0.6 M NaCl from SPM-derived DRM and identified as phospholipase C beta 1 through mass analysis. The identity was further confirmed with Western blotting using a specific polyclonal antibody. The enzyme purified from the DRM was activated by the alpha subunit of trimeric G protein, Gq, expressed in HEK293 cells. The lipid composition of the liposomes affected the enzymatic activity and the addition of NAP-22, a neuronal DRM-localized protein, inhibited the activity. These results suggest that there exists a signal-transducing MD that performs important roles in neuronal functions through PIP(2) signaling and Ca(2+) mobilization.

Complementary stripes of phospholipase Cβ3 and Cβ4 expression by Purkinje cell subsets in the mouse cerebellum

Transverse boundaries divide the cerebellar cortex into four transverse zones, and within each zone the cortex is further subdivided into a symmetrical array of parasagittal stripes. Several molecules believed to mediate long-term depression at the parallel fiber-Purkinje cell synapse are known to be expressed in stripes. We have therefore explored the distributions of phospholipase Cbeta3 and phospholipase Cbeta4, key components in the transduction of type 1 metabotropic glutamate receptor-mediated responses. The data reveal that both phospholipase Cbeta isotypes are expressed strongly in the mouse cerebellum in subsets of Purkinje cells. The two distributions are distinct and largely nonoverlapping. The pattern of phospholipase Cbeta3 expression is unique, revealing stripes in three of the four transverse zones and a uniform distribution in the fourth. In contrast, phospholipase Cbeta4 appears to be confined largely to the Purkinje cells that are phospholipase Cbeta3-negative. PLCbeta3 is restricted to the zebrin II-immunopositive Purkinje cell subset. Not all zebrin II-immunoreactive Purkinje cells express PLCbeta3: in lobules IX and X it is restricted to that zebrin II-immunopositive subset that also expresses the small heat shock protein HSP25. PLCbeta4 expression is restricted to, and coextensive with, the zebrin II-immunonegative Purkinje cell subset. These nonoverlapping expression patterns suggest that long-term depression may be manifested differently between cerebellar modules.

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.

Cloning and characterization of a phospholipase C-beta isoform from the sea urchin Lytechinus pictus

Calcium is a ubiquitous intracellular signaling molecule controlling a wide array of cellular processes including fertilization and egg activation. The mechanism for triggering intracellular Ca(2+) release in sea urchin eggs during fertilization is the generation of inositol-1,4,5-trisphosphate by phospholipase C (PLC) hydrolysis of phosphatidylinositol-4,5-bisphosphate. Of the five PLC isoforms identified in mammals (beta, gamma, delta, epsilon and zeta), only PLCgamma and PLCdelta have been detected in echinoderms. Here, we provide direct evidence of the presence of a PLCbeta isoform, named suPLCbeta, within sea urchin eggs. The coding sequence was cloned from eggs of Lytechinus pictus and determined to have the greatest degree of homology and identity with the mammalian PLCbeta4. The presence of suPLCbeta within the egg was verified using a specifically generated antibody. The majority of the enzyme is localized in the non-soluble fraction, presumably the plasma membrane of the unfertilized egg. This distribution remains unchanged 1 min postfertilization. Unlike PLCbeta4, suPLCbeta is activated by G protein betagamma subunits, and this activity is Ca(2+)-dependent. In contrast to all known PLCbeta enzymes, suPLCbeta is not activated by Galphaq-GTPgammaS subunit suggesting other protein regulators may be present in sea urchin eggs.

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.

Presence of different phospholipase C isoforms in the nucleus and their activation during compensatory liver growth

Phospholipase C (PLC) was purified from the membrane-depleted rat liver nuclei. About 60% of the total PLC-activity corresponded to beta1b isoform, 30% to PLC-gamma1 and less than 10% to PLC-delta1. PLC-beta1b and -gamma1 were found in the nuclear matrix, while PLC-delta1 was detected in the chromatin. Two peaks of an increase in the total PLC-activity were detected occurring at 6 and 20 h after partial hepatectomy. An early increase in PLC-beta1b activity in the nuclear matrix was associated with serine phosphorylation of the enzyme, while the later increase paralleled the increase in the amount of protein. The increase in the PLC-gamma1 activity measured at 6 and 20 h after partial hepatectomy was associated with tyrosine phosphorylation of the enzyme. The activity of PLC-delta1 and the amount of the protein found in the chromatin was increased only at 20 h after partial hepatectomy.

Activation of platelet-activating factor receptor-coupled G alpha q leads to stimulation of Src and focal adhesion kinase via two separate pathways in human umbilical vein endothelial cells

Platelet-activating factor (PAF), a phospholipid second messenger, has diverse physiological functions, including responses in differentiated endothelial cells to external stimuli. We used human umbilical vein endothelial cells (HUVECs) as a model system. We show that PAF activated pertussis toxin-insensitive G alpha(q) protein upon binding to its seven transmembrane receptor. Elevated cAMP levels were observed via activation of adenylate cyclase, which activated protein kinase A (PKA) and was attenuated by a PAF receptor antagonist, blocking downstream activity. Phosphorylation of Src by PAF required G alpha(q) protein and adenylate cyclase activation; there was an absolute requirement of PKA for PAF-induced Src phosphorylation. Immediate (1 min) PAF-induced STAT-3 phosphorylation required the activation of G alpha(q) protein, adenylate cyclase, and PKA, and was independent of these intermediates at delayed (30 min) and prolonged (60 min) PAF exposure. PAF activated PLC beta 3 through its G alpha(q) protein-coupled receptor, whereas activation of phospholipase C gamma 1 (PLC gamma 1) by PAF was independent of G proteins but required the involvement of Src at prolonged PAF exposure (60 min). We demonstrate for the first time in vascular endothelial cells: (i) the involvement of signaling intermediates in the PAF-PAF receptor system in the induction of TIMP2 and MT1-MMP expression, resulting in the coordinated proteolytic activation of MMP2, and (ii) a receptor-mediated signal transduction cascade for the tyrosine phosphorylation of FAK by PAF. PAF exposure induced binding of p130(Cas), Src, SHC, and paxillin to FAK. Clearly, PAF-mediated signaling in differentiated endothelial cells is critical to endothelial cell functions, including cell migration and proteolytic activation of MMP2.

Calmodulin is a phospholipase C-beta interacting protein

Phospholipase C-beta 3 (PLC beta 3) is an important effector enzyme in G protein-coupled signaling pathways. Activation of PLC beta 3 by G alpha and G beta gamma subunits has been fairly well characterized, but little is known about other protein interactions that may also regulate PLC beta 3 function. A yeast two-hybrid screen of a mouse brain cDNA library with the amino terminus of PLC beta 3 has yielded potential PLC beta 3 interacting proteins including calmodulin (CaM). Physical interaction between CaM and PLC beta 3 is supported by a positive secondary screen in yeast and the identification of a CaM binding site in the amino terminus of PLC beta 3. Co-precipitation of in vitro translated and transcribed amino- and carboxyl-terminal PLC beta 3 revealed CaM binding at a putative amino-terminal binding site. Direct physical interaction of PLC beta 3 and PLC beta 1 isoforms with CaM is supported by pull-down of both isoenzymes with CaM-Sepharose beads from 1321N1 cell lysates. CaM inhibitors reduced M1-muscarinic receptor stimulation of inositol phospholipid hydrolysis in 1321N1 astrocytoma cells consistent with a physiologic role for CaM in modulation of PLC beta activity. There was no effect of CaM kinase II inhibitors, KN-93 and KN-62, on M1-muscarinic receptor stimulation of inositol phosphate hydrolysis, consistent with a direct interaction between PLC beta isoforms and CaM.

G protein subunit G gamma 13 is coexpressed with G alpha o, G beta 3, and G beta 4 in retinal ON bipolar cells

We investigated the expression of Ggamma13, a recently discovered G protein subunit, and a selection of Gbeta subunits in retinal bipolar cells, by using a transgenic mouse strain in which green fluorescent protein is strongly expressed in a single type of cone bipolar cell. The cells have ON morphology, and patch-clamp recordings in slices confirmed that they are of the physiological ON type. Immunohistochemistry showed that Ggamma13 is expressed in rod bipolar cells and ON cone bipolar cells, where it is colocalized in the dendrites with Galphaomicron. ON and OFF cone bipolar cells and rod bipolar cells were identified among dissociated cells by their green fluorescence and/or distinct morphology. Hybridization of single-cell polymerase chain reaction products with cDNA probes for G protein subunits Gbeta1 to 5 showed that Gbeta3, Gbeta4, and Ggamma13 are coexpressed in ON bipolar cells but not present in OFF bipolar cells. Gbeta1, 2, and 5 are expressed in partially overlapping subpopulations of cone bipolar cells. Ggamma13 and Gbeta3 and/or Gbeta4, thus, seem selectively to participate in signal transduction by ON bipolar cells.

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.

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.

Localization of two distinct type III phosphatidylinositol 4-kinase enzyme mRNAs in the rat

Inositol lipid kinases generate polyphosphoinositides, important regulators of several cellular functions. We have recently cloned two distinct phosphatidylinositol (PI) 4-kinase enzymes, the 210-kDa PI4KIIIalpha and the 110-kDa PI4KIIIbeta, from bovine tissues. In the present study, the distribution of mRNAs encoding these two enzymes was analyzed by in situ hybridization histochemistry in the rat. PI4KIIIalpha was found predominantly expressed in the brain, with low expression in peripheral tissues. PI4KIIIbeta was more uniformly expressed being also present in various peripheral tissues. Within the brain, PI4KIIIbeta showed highest expression in the gray matter, especially in neurons of the olfactory bulb and the hippocampus, but also gave a signal in the white matter indicating its presence in glia. PI4KIIIalpha was highly expressed in neurons, but lacked a signal in the white matter and the choroid plexus. Both enzymes showed expression in the pigment layer and nuclear layers as well as in the ganglion cells of the retina. In a 17-day-old rat fetus, PI4KIIIbeta was found to be more widely distributed and PI4KIIIalpha was primarily expressed in neurons. These results indicate that PI4KIIIbeta is more widely expressed than PI4KIIIalpha, and that the two enzymes are probably coexpressed in many neurons. Such expression pattern and the conservation of these two proteins during evolution suggest their nonredundant functions in mammalian cells.

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.

Inositides in the nucleus: presence and characterisation of the isozymes of phospholipase beta family in NIH 3T3 cells

Previous reports from our laboratories and others have hinted that the nucleus is a site for an autonomous signalling system acting through the activation of the inositol lipid cycle. Among phospholipases (PLC) it has been shown previously that PLCbeta1 is specifically localised in the nucleus as well as at the plasma membrane. Using NIH 3T3 cells, it has been possible to obtain, with two purification strategies, in the presence or in the absence of Nonidet P-40, both intact nuclei still maintaining the outer membrane and nuclei completely stripped of their envelope. In these nuclei, we show that not only PLCbeta1 is present, but also PLCbeta2, PLCbeta3 and PLCbeta4. The more abounding isoform is PLCbeta1 followed by PLCbeta3, PLCbeta2 and PLCbeta4, respectively. All the isoforms are enriched in nuclear preparations free from nuclear envelope and cytoplasmatic debris, indicating that the actual localisation of the PLCbeta isozymes is in the inner nuclear compartment.

Regulation of squid visual phospholipase C by activated G-protein alpha

Phospholipase C (PLC) is the key enzyme in the phototransduction cascade of invertebrate rhabdomeric photoreceptors. In addition to 130 kDa PLC, a 95 kDa protein recognized by antibody against the catalytic site of PLC was found in the squid retina. The PLC-like 95 kDa protein (95 kDa PLC) was produced from 130 kDa PLC by an intrinsic protease in the presence of calcium. The 130 kDa PLC was stimulated by the active form of Gq-class G-protein alpha (Gq alpha), but the 95 kDa PLC was not, although their PLC activity was similar. A 35 kDa fragment, the counterpart of 95 kDa PLC, was not recognized by antibodies against catalytic site or N-terminal site of the 130 kDa PLC, indicating that the cleavage site is on the C-terminal side beyond the catalytic site. In the presence of a large excess of the 35 kDa fragment, 95 kDa PLC was stimulated by Gq alpha to a similar extent as intact 130 kDa PLC. These results indicate that the C-terminal polypeptide of PLC is necessary for regulation of its enzyme activity by Gq alpha. The uncoupling of PLC from Gq alpha, caused by limited proteolysis, is therefore a candidate regulatory mechanism of the phototransduction cascade in rhabdomeric photoreceptors.

A unique isoform of phospholipase Cbeta4 highly expressed in the cerebellum and eye

We report a unique isoform of PLCbeta4 in rat, PLCbeta4c, that has an additional 37-nucleotide exon inserted between nucleotides 3459-3460 of the previously published PLCbeta4a coding sequence. This insertion results in replacement of 22 amino acid residues at the carboxyl terminal tail of PLCbeta4a with 41 unique residues. A human EST for PLCbeta4 also contains this exon and this exon was mapped to within a 5.5 kb intron of the human PLCbeta4 gene. PLCbeta4c is the third PLCbeta4 isoform to be identified which has a unique carboxyl-terminal tail. PLCbeta4b differs from PLCbeta4a by truncation 162 amino acid residues from the carboxyl terminus which are replaced with 10 distinct amino acid residues. Reverse transcription-polymerase chain reaction experiments show that both PLCbeta4a and PLCbeta4c mRNA are expressed throughout the rat brain and that PLCbeta4c mRNA is highly expressed in the eye and cerebellum. RNase protection assays demonstrate that both PLCbeta4a and PLCbeta4c transcripts are abundant in the cerebellum. The different carboxyl terminal tails of PLCbeta4 isoforms may allow for differential targeting and subcellular localization, contributing to regulation of PLC beta4-mediated signal transduction.

Growth inhibition, enhancement of intercellular adhesion, and increased expression of carcinoembryonic antigen by overexpression of phosphoinositides-specific phospholipase C beta 1 in LS174T human colon adenocarcinoma cell line

By using a retrovirus-derived system we generated derivatives of the human colon adenocarcinoma cell line LS174T (ATCC CL 188) that stably overexpress a full-length cDNA encoding the beta 1 isoform of bovine phosphoinositides-specific phospholipase C (PI-PLC). This was confirmed by the elevated levels of catalytic activity to release phosphoinositides from phosphatidylinositol (PI-PLC) or phosphatidylinositol-bis-phosphate (PIP2-PLC), and the enhanced expressions of messenger RNA and protein. PI-PLC beta 1 overexpresser clones grew to form cell clumps floating in liquid medium, whereas the pMV7-introduced control clones displayed morphologic characteristics that were very similar to those of the parent LS174T cell line. Three individual PI-PLC beta 1 overexpresser cell lines displayed increased doubling time (18.0 h, 21.5 h, and 23.8 h) when compared with 4 individual pMV7-introduced control cell lines (13.1 h, 10.7 h, 12.9 h, and 9.3 h). Anchorage-independent growth ability in soft agar medium was dramatically suppressed by overexpression of PLC beta 1, and the ability of PLC-overproducer clones to form aggregates when cultured in liquid medium was dramatically enhanced when compared with that of pMV7-introduced control clones. Tumorigenicity of PLC beta 1-overproducers was much weaker than that of vector-transduced control clones. The spontaneous release of carcinoembryonic antigen from PLC beta 1-overproducer clones was much higher than that from pMV7 control clones. The ability of PLC beta 1-overproducer clones to form aggregates during suspension culture was much stronger than that of the control clones. These results provide the first evidence that elevated levels of endogenous PI-PLC beta 1 suppress tumor cell growth, but enhance the ability to form cell aggregates and to release carcinoembryonic antigen, an intercellular adhesion molecule.

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.

P2Y and P2U receptors differentially release intracellular Ca2+ via the phospholipase c/inositol 1,4,5-triphosphate pathway in astrocytes from the dorsal spinal cord

In astrocytes, raising intracellular Ca2+ concentration is a principal mechanism for transducing extracellular signals following activation of cell-surface receptors. Receptors that may be activated by purine nucleotides, P2 receptors, are known to be expressed by astrocytes from dorsal spinal cord; these astrocytes express two distinct subtypes of P2 receptor, P2Y and P2U. A main goal of the present study was to determine the intracellular signalling pathways mediating the Ca2+ responses produced by stimulating these receptors. Experiments were done using cultured astrocytes from rat dorsal spinal cord. Ca2+ responses were evoked by 2-methylthio-ATP or UTP, nucleotides previously shown to selectively activate P2Y and P2U receptors, respectively, in these cells. P2Y- and P2U-evoked Ca2+ responses were found not to depend upon extracellular Ca2+ and were blocked by thapsigargin, a Ca2+-ATPase inhibitor known to deplete inositol 1,4,5-triphosphate-sensitive Ca2+ stores. Intracellular application of the inositol 1,4,5-triphosphate-sensitive receptor antagonist, heparin, or of the G-protein inhibitor guanosine 5'-O-(2-thiodiphosphate), blocked the P2Y- and P2U-evoked Ca2+ responses. Moreover, the responses were prevented by the phospholipase C inhibitor, U-73122, but were unaffected by the inactive analogue, U-73343. These results indicate that P2Y and P2U receptors on dorsal spinal astrocytes are linked via G-protein coupling to release of intracellular Ca2+ via the phospholipase C/inositol 1,4,5-triphosphate pathway. When we assessed the releasable pools of intracellular Ca2+, by repeated agonist applications in zero extracellular Ca2+, we found that the pool accessed by activating P2U receptors was only a subpool of that accessed by activating P2Y receptors. This implies that there are separable inositol 1,4,5-triphosphate-releasable pools of Ca2+ in dorsal spinal astrocytes and that these may be differentially released by activating distinct metabotropic P2 receptors. This differential release of Ca2+ may be important for physiological as well as pathophysiological events occurring within the spinal cord.

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.