Multiple signaling pathways of human interleukin-8 receptor A. Independent regulation by phosphorylation

Interleukin-8 (IL-8) receptor A (CXCR1) couples to a pertussis toxin-sensitive G protein to mediate phospholipase Cbeta (PLCbeta) activation and cellular responses. Responses to CXCR1 are attenuated by prior exposure of neutrophils to either IL-8, a cleavage product of the fifth component of complement (C5a) or n-formylated peptides (formylmethionylleucylphenylalanine, fMLP). To characterize the role of receptor phosphorylation in the regulation of the CXCR1, a phosphorylation-deficient mutant, M2CXCR1, was constructed. This receptor, stably expressed in RBL-2H3 cells, coupled more efficiently to G protein and stimulated enhanced phosphoinositide hydrolysis, cAMP production, exocytosis, and phospholipase D activation, and was resistant to IL-8-induced receptor internalization. The rate and total amount of ligand stimulated actin polymerization remained unchanged, but interestingly, chemotaxis was decreased by approximately 30% compared with the wild type receptor. To study the role of receptor phosphorylation in cross-desensitization of chemoattractant receptors, M2CXCR1 was coexpressed with cDNAs encoding receptors for either fMLP (FR), C5a (C5aR), or platelet-activating factor (PAFR). Both C5aR and PAFR were cross-phosphorylated upon M2CXCR1 activation, resulting in attenuated guanosine 5'-3'-O-(thio)triphosphate (GTPgammaS) binding in membranes. In contrast, FR and M2CXCR1 were resistant to cross-phosphorylation and cross-inhibition of GTPgammaS binding by other receptors. Despite the resistance of M2CXCR1 to cross-phosphorylation and receptor/G protein uncoupling, its susceptibility to cross-desensitization of its Ca2+ response by fMLP and C5a, was equivalent to CXCR1. Regardless of the enhancement in certain receptor functions in M2CXCR1 compared with the wild type CXCR1, the mutated receptors mediated equivalent PLCbeta3 phosphorylation and cross-desensitization of Ca2+ mobilization by FR, C5aR, and PAFR. The results herein indicate that phosphorylation of CXCR1 regulates some, but not all of the receptors functions. While receptor phosphorylation inhibits G protein turnover, PLC activation, Ca2+ mobilization and secretion, it is required for normal chemotaxis and receptor internalization. Since phosphorylation of CXCR1 had no effect on its ability to induce phosphorylation of PLCbeta3 or to mediate class-desensitization, these activities may be mediated by independently regulated pathways.

Two distantly positioned PDZ domains mediate multivalent INAD-phospholipase C interactions essential for G protein-coupled signaling

Drosophila INAD, which contains five tandem protein interaction PDZ domains, plays an important role in the G protein-coupled visual signal transduction. Mutations in InaD alleles display mislocalization of signaling molecules of phototransduction which include the essential effector, phospholipase C-beta (PLC-beta), which is also known as NORPA. The molecular and biochemical details of this functional link are unknown. We report that INAD directly binds to NORPA via two terminally positioned PDZ1 and PDZ5 domains. PDZ1 binds to the C-terminus of NORPA, while PDZ5 binds to an internal region overlapping with the G box-homology region (a putative G protein-interacting site). The NORPA proteins lacking binding sites, which display normal basal PLC activity, can no longer associate with INAD in vivo. These truncations cause significant reduction of NORPA protein expression in rhabdomeres and severe defects in phototransduction. Thus, the two terminal PDZ domains of INAD, through intermolecular and/or intramolecular interactions, are brought into proximity in vivo. Such domain organization allows for the multivalent INAD-NORPA interactions which are essential for G protein-coupled phototransduction.

Down-regulation of phospholipase C-beta1 following chronic muscarinic receptor activation

To determine whether prolonged activation of a phospholipase C-coupled receptor can lead to a down-regulation of its effector enzyme, SH-SY5Y neuroblastoma cells were incubated for 24 h with the muscarinic receptor agonist, oxotremorine-M. Under these conditions, significant reductions (46-53%) in muscarinic cholinergic receptor density, G(alphaq/11) and phospholipase C-beta1 (but not the beta3-or gamma1 isoforms) were observed. These results suggest that a selective down-regulation of phospholipase C-beta1 may play a role in adaptation to chronic muscarinic receptor activation.

Phospholipase C-beta1 expression correlates with neuronal differentiation and synaptic plasticity in rat somatosensory cortex

Receptor-mediated signal transduction is thought to play an important role in neuronal differentiation and the modification of synaptic connections during brain development. The intracellular signalling molecule phospholipase C-beta1 (PLC-beta1), which is activated via specific neurotransmitter receptors, has recently been implicated in activity-dependent plasticity in the cat visual cortex. PLC-beta1 has been shown to be concentrated in an intermediate compartment-like organelle, the botrysome, which is present in 5-week-old, but not adult, cat cortical neurons. We have characterized the spatial and temporal regulation of PLC-beta1 expression in the developing rat cerebral cortex. PLC-beta1-positive botrysome-like organelles are observed during early postnatal cortical development, but not at postnatal day 14 or later stages. In the postnatal somatosensory cortex, there is also striking spatial variation in diffuse neuropilar immunoreactivity of layer IV and above, in a pattern corresponding to the thalamocortical recipient zones known as barrels. This expression pattern is specific to the developing barrel field and is most distinct at postnatal days 4-7, when cellular components of barrels are capable of activity-dependent modification. During later stages of cortical maturation, stained botrysomes disappear, expression of PLC-beta1 is down-regulated and only diffuse immunoreactivity remains in dendritic processes. Our results are consistent with a role for PLC-beta1 in activity-dependent, receptor-mediated neuronal plasticity during development of the somatosensory cortex.

Identification of a structural element in phospholipase C beta2 that interacts with G protein betagamma subunits

To delineate the specific regions of phospholipase C beta2 (PLC beta2) involved in binding and activation by G protein betagamma subunits, we synthesized peptides corresponding to segments of PLC beta2. Two overlapping peptides corresponding to Asn-564-Lys-583 (N20K) and Glu-574-Lys-593 (E20K) inhibited the activation of PLC beta2 by betagamma subunits (IC50 50 and 150 microM, respectively), whereas two control peptides did not. N20K and E20K, but not the control peptides, inhibited betagamma-dependent ADP-ribosylation of Galphai1 by pertussis toxin and betagamma-dependent activation of phosphoinositide 3-kinase. To demonstrate direct binding of the peptides to betagamma subunits, the peptides were chemically cross-linked to purified beta1gamma2. N20K and E20K cross-linked to both beta1 and gamma2 subunits, whereas the control peptides did not. Cross-linking to beta and gamma was inhibited by incubation with excess PLC beta2 or PLC beta3, whereas cross-linking to gamma but not beta was inhibited by r-myr-alphai1. These data together demonstrate specificity of N20K and E20K for G betagamma binding and inhibition of effector activation by betagamma subunits. The results suggest that an overlapping region of the two active peptides, Glu-574-Lys-583, mimics a region of PLC beta2 that is involved in binding to betagamma subunits. Changing a tyrosine to a glutamine in this overlapping region of the peptides inhibited binding of the peptide to betagamma subunits. Alignment of these peptides with the three-dimensional structure from PLC delta1 identifies a putative alpha helical region on the surface of the catalytic domain of PLC beta2 that could interact with betagamma subunits.

Expression of Gq alpha and PLC-beta in scar and border tissue in heart failure due to myocardial infarction

BACKGROUND

Large transmural myocardial infarction (MI) leads to maladaptive cardiac remodeling and places patients at increased risk of congestive heart failure. Angiotensin II, endothelin, and alpha1-adrenergic receptor agonists are implicated in the development of cardiac hypertrophy, interstitial fibrosis, and heart failure after MI. Because these agonists are coupled to and activate Gq alpha protein in the heart, the aim of the present study was to investigate Gq alpha expression and function in cardiac remodeling and heart failure after MI.

METHODS AND RESULTS

MI was produced in rats by ligation of the left coronary artery, and Gq alpha protein concentration, localization, and mRNA abundance were noted in surviving left ventricle remote from the infarct and in border and scar tissues from 8-week post-MI hearts with moderate heart failure. Immunohistochemical staining localized elevated Gq alpha expression in the scar and border tissues. Western analysis confirmed significant upregulation of Gq alpha proteins in these regions versus controls. Furthermore, Northern analysis revealed that the ratios of Gq alpha/GAPDH mRNA abundance in both scar and viable tissues from experimental hearts were significantly increased versus controls. Increased expression of phospholipase C (PLC)-beta1 and PLC-beta3 proteins was apparent in the scar and viable tissues after MI versus controls and is associated with increased PLC-beta1 activity in experimental hearts. Furthermore, inositol 1,4,5-tris-phosphate is significantly increased in the border and scar tissues compared with control values.

CONCLUSIONS

Upregulation of the Gq alpha/PLC-beta pathway was observed in the viable, border, and scar tissues in post-MI hearts. Gq alpha and PLC-beta may play important roles in scar remodeling as well as cardiac hypertrophy and fibrosis of the surviving tissue in post-MI rat heart. It is suggested that the Gq alpha/PLC-beta pathway may provide a possible novel target for altering postinfarct remodeling.

Expression of catalytically inactive phospholipase Cbeta disrupts phospholipase Cbeta and mitogen-activated protein kinase signaling and inhibits small cell lung cancer growth

Transformed growth of human small cell lung cancer (SCLC) is mediated by autocrine signaling through multiple G protein-coupled neuropeptide receptors. To define the role of Gq and its effector, phospholipase Cbeta (PLCbeta), in SCLC growth, we expressed a COOH-terminal fragment of PLCbeta1 (PLCbetaCT) that is catalytically inactive and is predicted to behave as a competitive inhibitor of Gq signaling. Using endogenous muscarinic receptors as indicators of Gq-coupled receptor signaling status, we observed that stable expression of PLCbetaCT in NCI-H345 SCLC cells significantly inhibited muscarinic receptor-mediated phospholipase C activation and intracellular Ca2+ mobilization. In addition, PLCbetaCT expression reduced the basal activity of protein kinase C as well as the receptor-stimulated activity of the extracellular signal-regulated kinases, consistent with the sequential requirement for Gq, PLCbeta, and protein kinase C in the regulation of the extracellular signal-regulated kinases by neuropeptide and muscarinic receptors in SCLC. By contrast, muscarinic agonist stimulation of the c-Jun NH2-terminal kinases was not inhibited in SCLC cells expressing PLCbetaCT, indicating that other G proteins such as the G12,13 family members mediate c-Jun NH2-terminal kinase activation by neuropeptides and muscarinic agonists. Finally, soft agar colony formation by the SCLC cells expressing PLCbetaCT, but not growth in suspension culture, was markedly reduced, indicating that signaling through Gq and PLCbeta by autocrine-signaling neuropeptide receptors is a dominant pathway involved in the transformed growth of SCLC.

Homologous and heterologous acute desensitization of vasopressin V1a receptor in Xenopus oocytes

The mechanism of short-term desensitisation of the V1a vasopressin receptor, a phospholipase-C beta linked receptor, was investigated in albino Xenopus oocytes. V1a receptors showed rapid agonist-dependent mobilisation of intracellular calcium, as detected by aequorin photon emission. Agonist-induced homologous short-term desensitisation was evidenced within minutes after stimulation. Injection of the second messengers calcium or inositol triphosphate inside the cell did not desensitise the receptors. In contrast, protein kinase C (PKC) activators 1-oleoyl-2-acetyl-sn-glycerol (OAG) (50 microM) and 1,2-dioctanoyl-glycerol (DIC8) (10 microM), as well as phorbol -12-myristate-13-acetate (1 microM) and phorbol -12,13-dibutyrate (1 microM) blunted the calcium responsiveness of the V1a receptors. The specific PKC inhibitor bisindolylmaleimide (GF109203X) (1 microM) prevented the effect of DIC8 and OAG on V1a receptor desensitisation. Heterologous desensitisation induced by agonist occurred in oocytes that co-expressed the V1a receptor and the PKC-activating M5 muscarinic receptor. It was concluded that PKC activation has a role in short-term desensitisation of the V1a receptor.

Coexpression of ligand-gated P2X and G protein-coupled P2Y receptors in smooth muscle. Preferential activation of P2Y receptors coupled to phospholipase C (PLC)-beta1 via Galphaq/11 and to PLC-beta3 via Gbetagammai3

P2 receptor subtypes and their signaling mechanisms were characterized in dispersed smooth muscle cells. UTP and ATP stimulated inositol 1,4,5-triphosphate formation, Ca2+ release, and contraction that were abolished by U-73122 and guanosine 5'-O-(3-thio)diphosphate, and partly inhibited (50-60%) by pertussis toxin (PTX). ATP analogs (adenosine 5'-(alpha, beta-methylene)triphosphate, adenosine 5'-(beta, gamma-methylene)triphosphate, and 2-methylthio-ATP) stimulated Ca2+ influx and contraction that were abolished by nifedipine and in Ca2+-free medium. Micromolar concentrations of ATP stimulated both Ca2+ influx and Ca2+ release. ATP and UTP activated Gq/11 and Gi3 in gastric and aortic smooth muscle and heart membranes, Gq/11 and Gi1 and/or Gi2 in liver membranes, and Go and Gi1-3 in brain membranes. Phosphoinositide hydrolysis stimulated by ATP and UTP was mediated concurrently by Galphaq/11-dependent activation of phospholipase (PL) C-beta1 and Gbetagammai3-dependent activation of PLC-beta3. Phosphoinositide hydrolysis was partially inhibited by PTX or by antibodies to Galphaq/11, Gbeta, PLC-beta1, or PLC-beta3, and completely inhibited by the following combinations (PLC-beta1 and PLC-beta3 antibodies; Galphaq/11 and Gbeta antibodies; PLC-beta1 and Gbeta antibodies; PTX with either PLC-beta1 or Galphaq/11 antibody). The pattern of responses implied that P2Y2 receptors in visceral, and probably vascular, smooth muscle are coupled to PLC-beta1 via Galphaq/11 and to PLC-beta3 via Gbetagammai3. These receptors co-exist with ligand-gated P2X1 receptors activated by ATP analogs and high levels of ATP.

Activation of a phospholipase Cbeta2 deletion mutant by limited proteolysis

All phosphoinositide-specific phospholipases C (PLC) identified until today exhibit a high degree of similarity within two regions of 170 and 260 residues, respectively, which are designated regions X and Y. The PLCbeta family, including four members designated PLCbeta1, PLCbeta2, PLCbeta3 and PLCbeta4, is regulated by heterotrimeric G proteins. In order to investigate structure-function relationships of PLCbeta2, we expressed PLCbeta2Delta, a deletion mutant of PLCbeta2 which lacks most of the sequence downstream of region Y, in the baculovirus/insect cell system. The mutant was present in both soluble and particulate fractions of Sf9 cells and was demonstrated to be catalytically active and sensitive to betagamma-subunits. Sf9 cytosol containing this mutant was subjected to limited proteolysis by trypsin and S. aureus protease V8, respectively. Immunochemical analysis revealed that both proteases cleaved the enzyme between the regions X and Y. Most interestingly, proteolytic cleavage at this site by both proteases stimulated the catalytic activity of PLC2beta2Delta. The proteolytically activated enzyme was still sensitive to betagamma-subunits and showed an unchanged concentration dependence on Ca2+. Gel filtration chromatography indicated that the fragments generated by cleavage between the regions X and Y were still connected and formed a functional heterodimeric complex. In order to visualize all fragments generated by protease V8, PLCbeta2Delta was purified to homogeneity from Sf9 cytosol. Limited proteolysis of the purified enzyme by S. aureus protease V8 and subsequent SDS/PAGE and silver staining revealed that several cuts take place between the regions X and Y and that the resulting fragments remain intact. We hypothesize that the activating proteolytic cut induces a conformational change of the enzyme which might facilitate hydrolysis of the phospholipid substrate.

A cytosolic, galphaq- and betagamma-insensitive splice variant of phospholipase C-beta4

Phospholipase C (PLC)-beta4 has been considered to be a mammalian homolog of the NorpA PLC, which is responsible for visual signal transduction in Drosophila. We reported previously the cloning of a cDNA encoding rat phospholipase C-beta4 (PLC-beta4) (Kim, M. J., Bahk, Y. Y., Min, D. S., Lee, S. J., Ryu, S. H., and Suh, P.-G. (1993) Biochem. Biophys. Res. Commun. 194, 706-712). We report now the isolation and characterization of a splice variant (PLC-beta4b). PLC-beta4b is identical to the 130-kDa PLC-beta4 (PLC-beta4a) except that the carboxyl-terminal 162 amino acids of PLC-beta4a are replaced by 10 distinct amino acids. The existence of PLC-beta4b transcripts in the rat brain was demonstrated by reverse transcription-polymerase chain reaction analysis. Immunological analysis using polyclonal antibody specific for PLC-beta4b revealed that this splice variant exists in rat brain cytosol. To investigate functional differences between the two forms of PLC-beta4, transient expression studies in COS-7 cells were conducted. We found that PLC-beta4a was localized mainly in the particulate fraction of the cell, and it could be activated by Galphaq, whereas PLC-beta4b was localized exclusively in the soluble fraction, and it could not be activated by Galphaq. In addition, both PLC-beta4a and PLC-beta4b were not activated by G-protein betagamma-subunits purified from rat brain. These results suggest that PLC-beta4b may be regulated by a mechanism different from that of PLC-beta4a, and therefore it may play a distinct role in PLC-mediated signal transduction.

Differential expression of rat brain phospholipase C isozymes in development and aging

Phosphoinositide-specific phospholipase C (PLC) is a key enzyme in signal transduction. In the present study we examined developmental and aging changes in three PLC isozymes (beta 1, gamma 1, and delta 1) in the rat brain. Enzyme assays and immunoblot analyses after gel filtration chromatography of brain extracts from embryonic day 19 and postnatal 4- and 48-week rats indicated that gamma-specific activity was highest in fetal brain and decreased with aging, that beta 1-specific activity was high at 4 weeks but essentially undetected in fetal brain, and that delta 1-specific activity was high at both 4 and 48 weeks with faint detection in fetal brain. Our results suggest that the gamma 1 isozyme may be particularly involved in cell division and growth during the histo-genesis of the central nervous system, while beta 1 and delta 1 isozymes may take part in processes of its maturation and maintenance.

RGS4 inhibits signaling by group I metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) couple to heterotrimeric G-proteins and regulate cell excitability and synaptic transmission in the CNS. Considerable effort has been focused on understanding the cellular and biochemical mechanisms that underlie regulation of signaling by G-proteins and their linked receptors, including the mGluRs. Recent findings demonstrate that regulators of G-protein signaling (RGS) proteins act as effector antagonists and GTPase-activating proteins for Galpha subunits to inhibit cellular responses by G-protein-coupled receptors. RGS4 blocks Gq activation of phospholipase Cbeta and is expressed broadly in rat brain. The group I mGluRs (mGluRs 1 and 5) couple to Gq pathways to regulate several effectors in the CNS. We examined the capacity of RGS4 to regulate group I mGluR responses. In Xenopus oocytes, purified RGS4 virtually abolishes the mGluR1a- and mGluR5a-mediated but not the inositol trisphospate-mediated activation of a calcium-dependent chloride current. Additionally, RGS4 markedly attenuates the mGluR5-mediated inhibition of potassium currents in hippocampal CA1 neurons. This inhibition is dose-dependent and occurs at concentrations that are virtually identical to those required for inhibition of phospholipase C activity in NG108-15 membranes and reconstituted systems using purified proteins. These findings demonstrate that RGS4 can modulate mGluR responses in neurons, and they highlight a previously unknown mechanism for regulation of G-protein-coupled receptor signaling in the CNS.

Localization of two forms of phospholipase C-beta1, a and b, in C6Bu-1 cells

Phospholipase C-beta1 (PLC-beta1), one of the PLC-beta isozymes, exists as two immunologically distinguishable polypeptides of 150 (PLC-beta1a) and 140 kDa (PLC-beta1b) which are encoded in two distinct transcripts and generated by alternative splicing of a single gene. In this study, the subcellular localization of the two phospholipases C-beta1 proteins was examined in rat C6Bu-1 glioma cells using immunological techniques. Immunoblot analysis revealed that the two forms of PLC-beta1 were detectable in both cytosolic and nuclear fractions. PLC-beta1a appeared to be located preferentially in the cytosol, whereas PLC-beta1b was found predominantly in the nuclei of C6Bu-1 cells. Immunocytochemical experiments confirmed the differential localization of the two PLC-beta1 species in C6Bu-1 cells. These results suggest that the two PLC-beta1 proteins may have different physiological roles in the cell.

Gastrin induces IP3 formation through phospholipase C gamma 1 and pp60c-src kinase

We have previously reported that gastrin induces a rapid and transient tyrosine phosphorylation of phospholipase C gamma 1 (PLC gamma 1) in association with inositol 1,4,5-trisphosphate (IP3) formation in rat colonic epithelial cells (34). In this study, we demonstrate that gastrin regulates IP3 formation mainly through PLC gamma 1 isozyme. Immunoblotting analysis revealed the expression of PLC beta 3 and -gamma 1, but not PLC beta 1, -beta 2, or -beta 4 in the rat colonic epitheliums. To explore what PLC isozyme(s) modulates gastrin effect on IP3, immunoneutralizing antibody to PLC beta 1, -beta 3, or -gamma 1 was introduced into the colonic cells using a lipid carrier. The gastrin-stimulated increase in IP3 concentration was specifically prevented by anti-PLC gamma 1 but not by anti-PLC beta 1 or -beta 3 antibody. Immunoprecipitation assays have also revealed that gastrin promoted an increase in tyrosine phosphorylation and co-precipitation of a 60 kDa src kinase with PLC gamma 1. Administration of antibody specific to pp60c-src into the colonic cells prevented the gastrin-stimulated increases in IP3. Tyrosine phosphorylation of PLC gamma 1 may be a major mechanism through which gastrin regulates IP3 level in the colonic cells. Pretreatment of cells with the tyrosine kinase inhibitor genistein abrogated gastrin's effect on IP3, while extended pretreatment with pertussis toxin, a G-protein inhibitor, did not affect the ability of gastrin to stimulate IP3 formation. Colonic cells expressed the G alpha i subunits1-3; however, immunoblotting analysis did not reveal any difference in G alpha i proteins' expression between control and gastrin treated cells. The results provide direct evidence that gastrin regulates IP3 level by a signaling mechanism that involves PLC gamma 1 and pp60c-src kinase.

Listeria monocytogenes virulence factors that stimulate endothelial cells

Listeria monocytogenes infection of endothelial cells upregulates surface expression of adhesion molecules and stimulates neutrophil adhesion to infected cell monolayers. The experiments presented here tested the roles of specific bacterial virulence factors as triggers for this inflammatory phenotype and function. Human umbilical vein endothelial cell (HUVEC) monolayers were infected with wild-type L. monocytogenes or L. monocytogenes mutants; then surface expression of E-selectin and neutrophil adhesion were measured. The results showed that delta hly and prfA mutants were the most crippled, requiring 100-fold more mutant bacteria than wild-type bacteria for analogous stimulation. By comparison, L. monocytogenes mutants with deletions of actA, inlA, inlB, inlAB, plcA, and plcB resembled their parent strains, and a delta plcA delta plcB mutant displayed decreased intracellular growth rate but only a minor decrease in stimulation of E-selectin or neutrophil adhesion. Other experiments showed that cytochalasin D-treated HUVEC monolayers bound bacteria, but internalization and increased surface E-selectin and intercellular adhesion molecule-1 expression were profoundly inhibited. However, cytochalasin D had no effect on the HUVEC response to stimulation with lipopolysaccharide or tumor necrosis factor alpha. These data suggest that listeriolysin O production by infecting L. monocytogenes contributes to increased expression of surface E-selectin and intercellular adhesion molecule-1, but neither it nor intracellular replication are directly responsible for this event. Nonetheless it is possible that listeriolysin O potentiates the effect(s) of an other molecule(s) that directly triggers this response. Additionally, cellular invasion by L. monocytogenes appears to be critical for initiating the HUVEC response, potentially by providing a signal which results in upregulation of the necessary bacterial genes.

Promoter region of the Drosophila melanogaster norpA gene

We have sequenced the 5'-flanking region of the Drosophila melanogaster norpA gene and characterized its promoter. The potential promoter region, which was deduced from the determination of the transcription start point (tsp), lacks a distinct TATA box sequence. Deletion analysis of the promoter region suggests that the minimal promoter necessary for efficient expression of the gene is located between -138 (PstI) and +278 relative to the tsp. Within this minimal promoter region, at least two downstream regulatory elements responsible for the stimulation of gene expression seem to exist in the DNA fragments between +44 and +121 and between +214 and +278. Among these, the DNA fragment between +44 and +121 affects promoter activity more dramatically (about 6-7 fold). This DNA fragment contains the consensus promoter element previously reported to be important for photoreceptor cell-specific expression, and this promoter element seems to be working in the norpA gene expression.

Control of expression of PLCbeta1 by LAC repressor system: relationship between nuclear PLCbeta1 overexpression and growth factor stimulation

Swiss 3T3 cells have a nuclear phosphoinositide signalling system which is under the control of insulin-like growth factor I (IGF-I) and acts separately from that at the plasma membrane. By using the Lac repressor system we were able both to obtain the inducible overexpression of phospholipase C beta1 (PLC beta1) and to determine its subcellular localisation and partitioning. Moreover, by comparing the level of expression at the nucleus and the percentage of cells actively incorporating bromodeoxyuridine (BrdU) in S phase it has strengthened the issue of the importance of this PLC in the onset of DNA synthesis mediated by IGF-I. In addition, this system appears to be a very powerful tool for further analysis of the downstream events following the activation of nuclear PLC beta1.

Association of INAD with NORPA is essential for controlled activation and deactivation of Drosophila phototransduction in vivo

Visual transduction in Drosophila is a G protein-coupled phospholipase C-mediated process that leads to depolarization via activation of the transient receptor potential (TRP) calcium channel. Inactivation-no-afterpotential D (INAD) is an adaptor protein containing PDZ domains known to interact with TRP. Immunoprecipitation studies indicate that INAD also binds to eye-specific protein kinase C and the phospholipase C, no-receptor-potential A (NORPA). By overlay assay and site-directed mutagenesis we have defined the essential elements of the NORPA-INAD association and identified three critical residues in the C-terminal tail of NORPA that are required for the interaction. These residues, Phe-Cys-Ala, constitute a novel binding motif distinct from the sequences recognized by the PDZ domain in INAD. To evaluate the functional significance of the INAD-NORPA association in vivo, we generated transgenic flies expressing a modified NORPA, NORPAC1094S, that lacks the INAD interaction. The transgenic animals display a unique electroretinogram phenotype characterized by slow activation and prolonged deactivation. Double mutant analysis suggests a possible inaccessibility of eye-specific protein kinase C to NORPAC1094S, undermining the observed defective deactivation, and that delayed activation may similarly result from NORPAC1094S being unable to localize in close proximity to the TRP channel. We conclude that INAD acts as a scaffold protein that facilitates NORPA-TRP interactions required for gating of the TRP channel in photoreceptor cells.

Regulation of human chemokine receptors CXCR4. Role of phosphorylation in desensitization and internalization

Members of the chemokine receptor family CCR5 and CXCR4 have recently been shown to be involved in the entry of human immunodeficiency virus (HIV) into target cells. Here, we investigated the regulation of CXCR4 in rat basophilic leukemia cells (RBL-2H3) stably transfected with wild type (Wt CXCR4) or a cytoplasmic tail deletion mutant (DeltaCyto CXCR4) of CXCR4. The ligand, stromal cell derived factor-1 (SDF-1) stimulated higher G-protein activation, inositol phosphate generation, and a more sustained calcium elevation in cells expressing DeltaCyto CXCR4 relative to Wt CXCR4. SDF-1 and phorbol 12-myristate 13-acetate (PMA), but not a membrane permeable cAMP analog induced rapid phosphorylation as well as desensitization of Wt CXCR4. Phosphorylation of DeltaCyto CXCR4 was not detected under any of these conditions. Despite lack of receptor phosphorylation, calcium mobilization by SDF-1 in DeltaCyto CXCR4 cells was partially desensitized by prior treatment with SDF-1. Of interest, the rapid release of calcium was inhibited without affecting the sustained calcium elevation, indicating independent regulatory pathways for these processes. PMA completely inhibited phosphoinositide hydrolysis and calcium mobilization in Wt CXCR4 but only partially inhibited these responses in DeltaCyto CXCR4. cAMP also partially inhibited these responses in both Wt CXCR4 and DeltaCyto CXCR4. SDF-1, PMA, and cAMP caused phosphorylation of phospholipase Cbeta3 in Wt and DeltaCyto CXCR4 cells. Both SDF-1 as well as PMA induced rapid internalization of Wt CXCR4. SDF-1 but not PMA induced internalization of DeltaCyto CXCR4 albeit at reduced levels relative to Wt CXCR4. These results indicate that signaling and internalization of CXCR4 are regulated by receptor phosphorylation dependent and independent mechanisms. Desensitization of CXCR4 signaling, independent of receptor phosphorylation, appears to be a consequence of the phosphorylation of phospholipase Cbeta3.

Attenuation of renomedullary phospholipase C isozyme, PLC-delta 1, in spontaneously hypertensive rats

The distributional patterns of PLC isozymes within the kidney were investigated using spontaneously hypertensive rats (SHRs) and normotensive Wistar-Kyoto (WKY) rats at 4 and 12 weeks of age. PLC-beta 1, PLC-beta 3 and PLC-delta 1 quantified by Western blot analysis, were present in the highest concentrations in the inner medulla of rats at both 4 and 12 weeks of age. On the other hand, PLC-beta 4, PLC-gamma 1 and PLC-gamma 2 were distributed almost equally among the regions for the rats of both ages. When compared with WKY rats at 12 weeks of age, the amounts of PLC-beta 1, PLC-beta 3, PLC-gamma 1, PLC-gamma 2, and PLC-delta 1 in the inner medulla of SHRs were significantly lower, and the amount of PLC-delta 1 in the inner stripe of the outer medulla was also significantly lower. Even at the prehypertensive stage at 4 weeks of age, the inner medullary concentration of PLC-delta 1 was significantly lower in SHRs than WKY rats. These results suggest that PLC-delta 1 would play an important role in the development of hypertension.

Effects of prenatal ethanol exposure on phospholipase C-beta 1 and phospholipase A2 in hippocampus and medial frontal cortex of adult rat offspring

Previous studies in our laboratory using a rat model of fetal alcohol exposure (FAE) suggest that FAE-induced behavioral deficits are, in part, linked to neurochemical and electrophysiological deficits in long-term potentiation (LTP) in the entorhinal cortical perforant path projection to the hippocampal formation. Several findings suggest that signal-activated phospholipase C (PLC) and phospholipase A2 (PLA2) are critical to the induction and maintenance of LTP. Thus, alterations in phospholipid metabolism may play a significant role in the LTP deficits observed in FAE offspring. To test this hypothesis, we measured PLC-beta 1 and PLA2 activities in the hippocampus and medial frontal cortex of adult rats prenatally exposed to ethanol. PLC-beta 1 activities were significantly decreased by 20 to 30% in both the hippocampus and medial frontal cortex of FAE rats, compared with ad libitum and pair-fed controls. Total Ca(2+)-dependent PLA2 activity was 25% lower in the medial frontal cortex of FAE rats, but did not significantly differ from controls in the hippocampal formation. Approximately 30% of the measured activity in both the medial frontal cortex and hippocampal formation of ad libitum and pair-fed animals was associated with an 85 kDa cytosolic PLA2 form. Cytosolic PLA2 activities were significantly reduced in both the medial frontal cortex and hippocampal formation of FAE rats, compared with controls. These changes in Ca(2+)-dependent PLA 2 and PLC-beta 1 activities, coupled with reports of FAE-induced deficits in protein kinase C activity, indicate that prenatal exposure to moderate quantities of ethanol causes profound and long-lasting deficits in the cellular signaling mechanisms associated with activity-dependent synaptic plasticity and memory formation.

Membrane-binding properties of phospholipase C-beta1 and phospholipaseC-beta2: role of the C-terminus and effects of polyphosphoinositides, G-proteins and Ca2+

We have studied the binding of two G-protein-regulated phospholipase C (PLC) enzymes, PLCs-beta1 and -beta2, to membrane surfaces using sucrose-loaded bilayer phospholipid vesicles of varying compositions. Neither enzyme binds appreciably to pure phosphatidylcholine vesicles at lipid concentrations up to 10(-3) M. PLC-beta1 and PLC-beta2 bind vesicles composed of phosphatidylcholine, phosphatidylserine and phosphatidylethanolamine (molar ratio 1:1:1) with an approximate Kd of 10(-5) M. Inclusion of 2% PtdIns(4,5)P2 in these vesicles had no effect on the affinity of this interaction. As reported by others, removal of the C-terminus of PLC-beta1 and PLC-beta2 produces catalytically active fragments. The affinity of these truncated proteins for phospholipid vesicles is dramatically reduced suggesting that this region of the proteins contains residues important for membrane binding. Inclusion of G-protein alpha- and betagamma-subunit activators in the phospholipid vesicles does not increase the binding of PLC-beta1 or PLC-beta2, and the magnitude of G-protein-mediated PLC activation observed at low phospholipid concentrations (10(-6) M) is comparable to that observed at concentrations at which the enzymes are predominantly membrane-bound (10(-3) M). PLC-beta1 and -beta2 contain C2 domains but Ca2+ does not enhance binding to the vesicles. Our results indicate that binding of these enzymes to membranes involves the C-temini of the proteins and suggest that activation of these enzymes by G-proteins results from a regulated interaction between the membrane-bound proteins rather than G-protein-dependent recruitment of soluble enzymes to a substrate-containing phospholipid surface.

The coiled-coil region of the G protein beta subunit. Mutational analysis of Ggamma and effector interactions

The beta and gamma subunits of the heterotrimeric G proteins remain tightly associated throughout the signaling cycle as the betagamma dimer interacts with Galpha, receptors, and effectors. A coiled-coil structure involving alpha-helical segments at the N termini of the beta and gamma subunits contributes to the dimerization interface and has been implicated in effector signaling in yeast. Scanning mutagenesis of the coiled-coil region of the mammalian beta1 subunit was performed to examine the effect of point mutations on betagamma assembly and effector signaling in COS cell cotransfection assays. In addition to the E10K mutation described previously, mutations A11E, L14E, and I18E in beta1 were found to block betagamma association, as evidenced by the failure of the Gbeta mutants to undergo cytosolic translocation with cotransfected nonisoprenylated Ggamma. Although none of 14 beta1 point mutations prevented the betagamma-dependent activation of the c-Jun N-terminal kinase (JNK) effector pathway, the D20K point mutation enhanced JNK but not phospholipase C-beta2 activation. These findings implicate the coiled-coil region of Gbeta in JNK signaling, provide further evidence that the structural features of the betagamma complex mediating effector regulation may differ among effectors, and identify single codons in the mammalian beta subunit where mutation might yield a phenotype of defective signal transduction.