Polyunsaturated fatty acids and signalling via phospholipase C-beta and A2 in myocardium

Dietary n-6 and n-3 polyunsaturated fatty acids (PUFAs) have potent biological effects on the blood(cells), the vasculature and they myocardium. In the epidemiological studies in which the benefit from the regular ingestion of n-3 PUFAs was reported, the responsible mechanisms remain obscure. A great deal of the PUFA-effect can be explained by the known interference with the eicosanoid metabolism. Many processes, believed to be involved in atherogenesis such as adhesion and infiltration of bloodcells (in)to the vasculature, platelet aggregation, secretion of endothelium-derived factors and mitogenic responses of vascular smooth muscle cells are partially mediated by receptor-activated phospholipases C-beta and A2. As PUFAs take part at many steps of the signalling pathways, the latter could represent important action sites to beneficially interfere with atherogenesis. In this brief review, we have discussed the results of studies on the influence of alteration of PUFA composition of the membrane phospholipids or of exogenously administered non-esterified PURAs on phospholipid signalling. For convenience, we have mainly focused our discussion on those studies available on the myocardium. By changing the PUFA composition of the phospholipids, the endogenous substrates for the membrane-associated phospholipase C-beta and A2 are changed. This is accompanied by changes in their hydrolytic action on these substrates resulting in altered products (the molecular species of 1,2-diacylglycerols and the non-esterified PUFAs) which on their turn evoke changes in events downstream of the signalling cascades: activation of distinct protein kinase C isoenzymes, formation of distinct eicosanoids and non-esterified PUFA effects on Ca2+ channels. It has also become more clear that the membrane physicochemical properties, in terms of fluidity and cholesterol content of the bilayer, might undergo changes due to altered PUFA incorporation into the membrane phospholipids. The latter effects could have consequences for the receptor functioning, receptor-GTP-binding protein coupling, GTP-binding protein-phospholipase C-beta or A2 coupling as well. It should be noted that most of these studies have been carried out with cardiomyocytes isolated from hearts of animals on PUFA diet or incubation of cultured cardiomyocytes with non-esterified PUFAs in the presence of albumin. Studies need to be performed to prove that the PUFA-diet induced modulations of the phospholipid signalling reactions do occur in vivo and that these effects are involved in the mechanism of beneficial effects of dietary PUFAs on the process of atherosclerosis.

Phosphoinositide-generated messengers in cardiac signal transduction

A multitude of agonists like e.g. endothelin-1, angiotensin-II, serotonin, thrombin, histamine and vasopressin as well as alpha 1-adrenergic and muscarinic stimulation lead to stimulation of the phosphoinositide cycle in the heart. Besides the seven membrane spanning-domain receptor-coupled stimulation of the key enzyme of the phosphoinositide cycle, phospholipase C-beta, another class of hormones, growth factors, also couple to the phosphoinositide cycle, now through receptors with intrinsic tyrosine kinase activity that can phosphorylate and stimulate the phospholipase C-gamma isozyme. In this review we summarize the multitude of receptor (sub)types, G-protein-subunit- and phospholipase C-isozymes that are present in the heart. Furthermore, generation of second messengers and cellular responses are described together with the (patho)physiological implications for the heart of phosphoinositide cycle activation and second messenger accumulation.

Molecular, biochemical, and electrophysiological characterization of Drosophila norpA mutants

Inositol phosphate signaling has been implicated in a wide variety of eukaryotic cellular processes. In Drosophila, the phototransduction cascade is mediated by a phosphoinositide-specific phospholipase C (PLC) encoded by the norpA gene. We have characterized eight norpA mutants by electroretinogram (ERG), Western, molecular, and in vitro PLC activity analyses. ERG responses of the mutants show allele-dependent reductions in amplitudes and retardation in kinetics. The mutants also exhibit allele-dependent reductions in in vitro PLC activity levels and greatly reduced or undetectable NorpA protein levels. Three carry a missense mutation and five carry a nonsense mutation within the norpA coding sequence. In missense mutants, the amino acid substitution occurs at residues highly conserved among PLCs. These substitutions reduce the levels of both the NorpA protein and the PLC activity, with the reduction in PLC activity being greater than can be accounted for simply by the reduction in protein. The effects of the mutations on the amount and activity of the protein are much greater than their effects on the ERG, suggesting an amplification of the transduction signal at the effector (NorpA) protein level. Transgenic flies were generated by germline transformation of a null norpA mutant using a P-element construct containing the wild-type norpA cDNA driven by the ninaE promoter. Transformed flies show rescue of the electrophysiological phenotype in R1-R6 photoreceptors, but not in R7 or R8. The degeneration phenotype of R1-R6 photoreceptors is also rescued.

A heterotrimeric G protein complex couples the muscarinic m1 receptor to phospholipase C-beta

We addressed the question as to which subtypes of G protein subunits mediate the activation of phospholipase C-beta by the muscarinic m1 receptor. We used the rat basophilic leukemia cell line RBL-2H3-hm1 stably transfected with the human muscarinic m1 receptor cDNA. We microinjected antisense oligonucleotides into the nuclei of the cells to inhibit selectively the expression of G protein subunits; 48 hr later muscarinic receptors were activated by carbachol, and the increase in free cytosolic calcium concentration ([Ca2+]i) was measured. Antisense oligonucleotides directed against the mRNA coding for alpha(q) and alpha11 subunits both suppressed the carbachol-induced increase in [Ca2+]i. In cells injected with antisense oligonucleotides directed against alpha(o1) and alpha14 subunits, the carbachol effect was unchanged. A corresponding reduction of Galpha(q), and Galpha11 proteins by 70-80% compared to uninjected cells was immunochemically detected 2 days after injection of a mixture of alpha(q) and alpha11 antisense oligonucleotides. Expression of Galpha(q) and Galpha11 completely recovered after 4 days. Cells injected with antisense oligonucleotides directed against the mRNAs encoding for beta1, beta4, and gamma4 subunits showed a suppression of the carbachol-induced increase in [Ca2+]i compared to uninjected cells measured at the same time from the same coverslip, whereas in cells injected with antisense oligonucleotides directed against the beta2, beta3, gamma1, gamma2, gamma3, gamma5, and gamma7 subunits, no suppression of carbachol effect was observed. In summary, the results from RBL-2H3-hm1 cells indicate that the m1 receptor utilizes a G protein complex composed of the subunits alpha(q), alpha11, beta1, beta4, and gamma4 to activate phospholipase C.

Isolation and characterization of a novel gene close to the human phosphoinositide-specific phospholipase C beta 3 gene on chromosomal region 11q13

We describe the isolation, characterization, and genomic structure of a gene, Phospholipase C beta 3 Neighboring gene (PNG), located on chromosome 11q13. The cDNA was isolated using a cosmid that also contains the phospholipase C beta 3 gene (PLCB3). PNG does not have any marked similarity to other known genes on the DNA level. However, analysis of hybridization to a panel of somatic cell hybrids indicates the existence of related sequences on chromosomes 2, 4, 7, and 22. PNG showed expression of a 1-kb message in multiple tissues. The predicted protein is 199 amino acids. The gene spans approximately 2.5 kb, divided into four exons and three introns. It is located 4.4 kb upstream of PLCB3, with the 5' ends facing each other. The intergenic region has been completely sequenced, revealing separate CpG islands at both ends of this region. The islands are separated by a stretch of 2 kb, characterized by periodic alteration of the GC content. The 5'-flanking region of PNG does not contain TATA or CCAAT, suggesting a housekeeping promoter structure.

Functional importance of the amino terminus of Gq alpha

Gq alpha is palmitoylated at residues Cys9 and Cys10. Removal of palmitate from purified Gq alpha with palmitoylthioesterase in vitro failed to alter interactions of Gq alpha with phospholipase C-beta 1, the G protein beta gamma subunit complex, or m1 muscarinic cholinergic receptors. Mutants C9A, C10A, C9A/C10A, C9S/C10S, and truncated Gq alpha (removal of residues 1-6) were synthesized in Sf9 cells and purified. Loss of both Cys residues or truncation prevented palmitoylation of Gq alpha. However, truncated Gq alpha and the single Cys mutants activated phospholipase C-beta 1 normally, while the double Cys mutants were poor activators. Loss of both Cys residues impaired but did not abolish interaction of Gq alpha with m1 receptors. These Cys residues are thus important regardless of their state of palmitoylation. When expressed in HEK-293 or Sf9 cells, all of the proteins studied associated entirely or predominantly with membranes, although a minor fraction of nonpalmitoylated Gq alpha proteins accumulated in the cytosol of HEK-293 cells. When subjected to TX-114 phase partitioning, a significant fraction of all of the proteins, including those with no palmitate, was found in the detergent-rich phase. Removal of residues 1-34 of Gq alpha caused a loss of surface hydrophobicity as evidenced by complete partitioning into the aqueous phase. The Cys residues at the amino terminus of Gq alpha are thus important for its interactions with effector and receptor, and the amino terminus conveys a hydrophobic character to the protein distinct from that contributed by palmitate.

Inositol lipid cycle and autonomous nuclear signalling

The involvement of phospholipids and in particular polyphosphoinositides in cellular signalling has been documented in detail in the last 20 years. In addition to the plasma membrane localization also the nucleus is shown to be a site for both synthesis and hydrolysis of the phosphorylated forms of phosphatidylinositol. Previous observation have established that the nucleus possesses a specific PLC for inositol lipids, i.e., the PLC beta 1 isoform, which undergoes rapid and transient activation after IGF-I stimulation of quiescent Swiss 3T3 cells and is down-regulated after treatment of Friend erythroleukemia cells with DMSO. Here we have reviewed: (i) the potential of nuclear PLC beta 1 to be a target for anti-cancer drug, (ii) the capability of this PLC isoform, when activated by IGF-I, to be a key signalling molecule in the onset of DNA synthesis, via DAG generation and PKC alpha translocation to the nucleus, (iii) the chromosome mapping of PLC beta 1 gene. The differentiation program of Friend cells can be activated by other agents besides DMSO including tiazofurin, an anti-tumor drug, also capable of affecting the nuclear inositol lipid cycle. Tiazofurin induces a lowering of the activity of PLC beta 1 due to down regulation of this isoform as revealed by both Western blotting and Northern blotting analyses. Using Swiss 3T3 cells stably transformed with an antisense PLC beta 1 construct, the knock-out of the PLC beta 1 gene induces both a loss of PLC beta 1 expression, as determined by Western blots, and a loss of the mitogenic responsiveness to IGF-I. These events show a direct relationship between nuclear PLC beta 1 evoked signals and IGF-I induced cell proliferation. Finally, the assignment of the PLC beta 1 gene to the band q35-36 of rat chromosome 3 paves the way for further genetic studies given the fact that the region where PLC beta 1 gene maps is a hot spot for genetic alterations in a number of experimentally induced rat tumors. Taken as a whole, these results assign a key role to the regulation of nuclear PLC activity and expression both in growth-factor activated mitogenesis and in in vitro erythroid differentiation.

Protein kinase C inhibits the Ca(2+)-dependent stimulation of phospholipase C-beta 1 in vitro

Protein kinase C (PKC) inhibited the Ca(2+)-dependent stimulation of a 600-fold purified phospholipase C beta 1 (PLC-beta 1). Inhibition by PKC was time-dependent, and required ATP and diacylglycerol. Inhibition was more pronounced when the PLC assay was conducted with a PIP2 substrate mixture containing phosphatidylserine, then with a substrate mixture containing phosphatidyle-thanolamine. Cyclic AMP-dependent protein kinase A did not inhibit PLC-beta 1 activity. PKC did not affect the rate of PLC-beta 1 activation by Ca2+ or the rate of PLC-beta 1 deactivation by EGTA. PLC-beta 1 purified 1700-fold was less sensitive to inhibition by PKC despite stoichiometric phosphorylation. These results demonstrate that PKC inhibits the Ca(2+)-dependent stimulation of a 600-fold purified PLC-beta 1 in vitro. Furthermore, purification of PLC-beta 1 to homogeneity results in a diminished sensitivity to inhibition by PKC, indicating that other components may participate in mediating the effect of PKC on the Ca(2+)-dependent stimulation of PLC-beta 1 in vitro.

Adrenal lesion in multiple endocrine neoplasia type 1

BACKGROUND

Multiple endocrine neoplasia (MEN) type 1 is accompanied by adrenal involvement, but characteristics and clinical handling of this lesion have been insufficiently explored.

METHODS

Patients with MEN 1 (n = 43) were monitored (mean, 6.3 years) with annual biochemical and radiologic adrenal evaluation. Adrenal specimens were examined by in situ RNA-RNA hybridization for expression of the MEN1 candidate gene phospholipase C beta 3 (PLC beta 3) and immunostaining for insulin-like growth factor-1 receptor.

RESULTS

Altogether 17 patients (40%) displayed adrenal enlargement, which was limited to the adrenal cortex and showed signs of progression, marked atypia, and cancer development in three of them. Only the carcinoma exhibited adrenocortical hormone excess. PLC beta 3 was expressed in the hyperplastic and adenomatous proliferation but not the carcinoma. Pancreatic endocrine tumors with insulin-proinsulin excess were overrepresented in the patients with adrenocortical involvement, but significant insulin-like growth factor-1 receptor immunoreactivity was restricted to the carcinoma.

CONCLUSIONS

The prevalent adrenocortical lesion associated with MEN 1 requires regular attention because of malignant potential. It was unrelated to loss of constitution heterozygosity for the MEN1 locus (11q13) and PLC beta 3 expression, except for the cortical carcinoma exhibiting allelic losses involving also the Wiedemann-Beckwith gene at 11p15. Mechanisms for mitogenic relationships between the pancreatic and adrenal lesions of MEN 1 demand further clarification.

The rat phospholipase C beta 4 gene is expressed at high abundance in cerebellar Purkinje cells

Inositol phospholipid-specific phospholipase C (PLC) generates two important second messengers, inositol triphosphate and diacylglycerol. The recently cloned rat PLC beta 4 cDNA is highly homologous to the norpA cDNA of Drosophila melanogaster. We have mapped the PLC beta 4 gene expression in rat brain tissue sections by in situ hybridization. The PLC beta 4 gene is expressed at high abundance in cerebellar Purkinje cells and neurones of the substantia nigra, the median geniculate bodies and the thalamic nuclei. PLC beta 4 transcripts are also detected in the mammillary nuclei, the neocortex, the habenula and the olfactory bulbs. The specific pattern of gene expression we have observed should help to clarify the relationships between the PLC beta 4 and various constituents of second-messenger systems involved in transduction mechanisms triggered by the stimulation of seven transmembrane domain receptors. The strong gene expression in Purkinje cells and retinal neurones suggests that PLC beta 4 may be involved in the pathogenesis of mouse and human neurological diseases characterized by ataxia and retinal degeneration.

Decreased m3-muscarinic and alpha 1-adrenergic receptor stimulation of PIP2 hydrolysis in parotid gland membranes from aged rats: defect in activation of G alpha q/11

m3-Muscarinic cholinergic receptor (m3-AChR) and alpha 1-adrenergic receptor (alpha 1-AR) stimulation of phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis (by a PIP2-specific phospholipase C, PLC) in rat parotid gland membranes is mediated via activation of alpha subunits of the Gq/11 family of G-proteins. This study examines m3-AChR and alpha 1-AR stimulation of PIP2 hydrolysis in membranes isolated from parotid glands of old (24 months) and young (3 months) rats (old and young rat membranes). Old rat membranes exhibited reduced stimulation of PIP2 hydrolysis in response to the addition of guanosine-5'-O-(3-thiotrisphosphate) (GTP gamma S) alone or GTP gamma S plus either carbachol (m3-AChR agonist) or epinephrine (alpha 1-AR agonist). This reduction in receptor-stimulated PIP2 hydrolysis was not due to a decrease in PLC activity per se since cholate-solubilized PLC activity was similar in old and young rat membranes. Additionally, these membranes exhibited comparable, immunologically detectable, levels of PLC beta 3, G alpha q/11, and G beta. In the presence of 10 microM AlCl3 and 10 mM NaF, stimulation of PIP2 hydrolysis in both old and young rat membranes was similar. Preincubation of membranes from old rats with GTP gamma S induced a time-dependent increase in the rate of PIP2 hydrolysis and, with 20 min preincubation, the rates of hydrolysis in old and young rat membranes were not statistically different. In aggregate, these data indicate that there is a defect in the activation of G alpha q/11 in parotid gland membranes from old rats.

The intranuclear amount of phospholipase C beta 1 decreases following cell differentiation in Friend cells, whereas gamma 1 isoform is not affected

The existence of a signal transduction system in the nucleus, based on polyphosphoinositide breakdown mediated by specific phosphoinositidases (PLC), has been widely documented. In different cell systems, nuclear PLCs can be modulated, in response to agonists, either by enhancing or by down-regulating their activity, thus leading to DNA replication or to cell differentiation. Friend cells, induced to erythroid differentiation by dimethyl sulfoxide (DMSO), show a down-regulation of PLC beta 1 isoform, as indicated by the reduction of the transcription of its mRNA and of the in vitro synthesis of its translation product. The intracellular localization and the amount of different PLC isoforms have been evaluated by electron microscope immunocytochemistry. In untreated Friend cells, PLC beta 1 and gamma 1 isoforms are both present within the nucleus, whereas mainly the gamma 1 isoform is detected in the cytoplasm. The small amount of cytoplasmic PLC beta 1 is probably representative only of the newly synthesized enzyme. Quantitative immunolabeling analyses demonstrate that erythroid differentiation is associated with a significant decrease of the PLC beta 1 amount in the nucleus and with an almost complete disappearance of that isoform in the cytoplasm, whereas the PLC gamma 1 isoform is unaffected. The two PLC isoforms, moreover, appear to be differently associated with the nuclear components, PLC beta 1 being steadily bound to the inner nuclear matrix, whereas PLC gamma 1 is almost completely soluble.

Evidence suggesting a role for phospholipase C isozyme, PLC-delta 1 in corticomedullary osmotic gradients in rat kidneys

We determined the distributional patterns of phospholipase C isozymes within the rat kidneys. PLC-beta1 was localized in the inner medulla at the highest degree followed by the inner stripe of the outer medulla, the cortex and the outer stripe of the outer medulla. PLC-gamma1 was distributed homogeneously along the corticomedullary axis. PLC-delta1 showed gradual increase from the cortex to the inner medulla. Tissue osmotic gradients were measured using 4 slices, resulting in gradual increase from the cortex to the inner medulla. The pattern of PLC-delta1 appeared very similar to the osmotic gradient in the kidney. The results suggest that the distinct patterns of the PLC isozymes may be associated with different signal transduction pathways along the corticomedullary axis and PLC-delta1 may play a role in the osmoregulation of the medullary region.

cDNA sequence and gene locus of the human retinal phosphoinositide-specific phospholipase-C beta 4 (PLCB4)

Defects in the Drosophila norpA (no receptor potential A) gene encoding a phosphoinositide-specific phospholipase C (PLC) block invertebrate phototransduction and lead to retinal degeneration. The mammalian homolog, PLCB4, is expressed in rat brain, bovine cerebellum, and the bovine retina in several splice variants. To determine a possible role of PLCB4 gene defects in human disease, we isolated several overlapping cDNA clones from a human retina library. The composite cDNA sequence predicts a human PLC beta 4 polypeptide of 1022 amino acid residues (MW 117,000). This PLC beta 4 variant lacks a 165-amino-acid N-terminal domain characteristic for the rat brain isoforms, but has a distinct putative exon 1 unique for human and bovine retina isoforms. A PLC beta 4 monospecific antibody detected a major (130 kDa) and a minor (160 kDa) isoform in retina homogenates. Somatic cell hybrids and deletion panels were used to localize the PCLB4 gene to the short arm of chromosome 20. The gene was further sublocalized to 20p12 by fluorescence in situ hybridization.

Inositol 1,4,5-trisphosphate receptors selectively localized to the acrosomes of mammalian sperm

Calcium flux is required for the mammalian sperm acrosome reaction, an exocytotic event triggered by egg binding, which results in a dramatic rise in sperm intracellular calcium. Calcium-dependent membrane fusion results in the release of enzymes that facilitate sperm penetration through the zona pellucida during fertilization. We have characterized inositol 1,4,5-trisphosphate (IP3)-gated calcium channels and upstream components of the phosphoinositide signaling system in mammalian sperm. Peptide antibodies colocalized G alpha q/11 and the beta 1 isoform of phospholipase C (PLC beta 1) to the anterior acrosomal region of mouse sperm. Western blotting using a polyclonal antibody directed against purified brain IP3 receptor (IP3R) identified a specific 260 kD band in 1% Triton X-100 extracts of rat, hamster, mouse and dog sperm. In each species, IP3R immunostaining localized to the acrosome cap. Scatchard analysis of [3H]IP3 binding to rat sperm sonicates revealed a curvilinear plot with high affinity (Kd = 26 nM, Bmax = 30 pmol/mg) and low affinity (Kd = 1.6 microM, Bmax = 550 pmol/mg) binding sites, reflecting among the highest receptor densities in mammalian tissue. Immunoelectron microscopy confirmed the acrosomal localization in rat sperm. The IP3R fractionated with acrosomes by discontinuous sucrose gradient centrifugation and was enriched in the medium of acrosome-reacted sperm. ATP-dependent 45Ca2+ loading of digitonin permeabilized rat sperm was decreased by 45% in the presence of 10 microM IP3. The IP3-mediated release of calcium was blocked by heparin. Thapsigargin, a sequiterpene lactone inhibitor of the microsomal Ca(2+)-ATPase, stimulated the acrosome reaction of mouse sperm to the same extent as the Ca2+ ionophore, A23187. The failure of caffeine and ryanodine to affect calcium accumulation suggested that thapsigargin acted through an IP3-sensitive store. The presence of G alpha q/11, PLC beta 1 and a functional IP3R in the anterior acrosomal region of mammalian sperm, as well as thapsigargin's induction of the acrosome reaction, implicate IP3-gated calcium release in the mammalian acrosome reaction.

Phosphoinositide signaling in nuclei of Friend cells: tiazofurin down-regulates phospholipase C beta 1

Previous investigations have demonstrated the presence of conventional lipid kinases and phospholipase C (PLC) activities in nuclei of Friend erythroleukemia cells. Moreover, when Friend erythroleukemia cells are treated for 96 h with the antitumor drug tiazofurin, the induction of erythroid differentiation is accompanied by changes in amounts of both phosphatidylinositol and phosphatidylinositol 4,5-bisphosphate due to the inhibition of an uncharacterized nuclear PLC activity. Here, we show that the nuclear PLC beta 1 isoform is down-regulated by tiazofurin (5 microM) treatment of Friend erythroleukemia cells as shown by both Western blot and Northern blot analyses for PLC beta 1 message. This indicates that PLC beta 1 down-regulation is tightly linked with erythroid differentiation of Friend erythroleukemia cells and that the autonomous nuclear signaling via inositol lipid cycle can be controlled by the antitumor drug tiazofurin.

Phosphatidylinositol transfer protein dictates the rate of inositol trisphosphate production by promoting the synthesis of PIP2

BACKGROUND

Phosphatidylinositol transfer protein (PI-TP), which has the ability to transfer phosphatidylinositol (PI) from one membrane compartment to another, is required in the inositol lipid signalling pathway through phospholipase C-beta (PLC-beta) that is regulated by GTP-binding protein(s) in response to extracellular signals. Here, we test the hypothesis that the principal role of PI-TP is to couple sites of lipid hydrolysis to sites of synthesis, and so to replenish depleted substrate for PLC-beta.

RESULTS

We have designed an experimental protocol that takes advantage of the different rates of release of endogenous PI-TP and PLC-beta from HL60 cells permeabilized with streptolysin O. We have examined the kinetics of stimulated inositol lipid hydrolysis in cells depleted of PI-TP, but not of endogenous PLC-beta, in the presence and absence of exogenous PI-TP. Linear time-courses were observed in the absence of any added protein, and the rate was accelerated by PI-TP using either guanosine 5'[gamma-thio]-triphosphate (GTP gamma S) or the receptor-directed agonist fMetLeuPhe as activators. In addition, depletion from the cells of both PI-TP and PLC-beta isoforms by extended permeabilization (40 minutes) allowed us to control the levels of PLC-beta present in the cells. Once again, PI-TP increased the rates of reactions. To identify whether the role of PI-TP was to make available the substrate phosphatidylinositol bisphosphate (PIP2) for the PLC, we examined the synthesis of PIP2 in cells depleted of PI-TP. We found that PI-TP was essential for the synthesis of PIP2.

CONCLUSIONS

The predicted function of PI-TP in inositol lipid signalling is the provision of substrate for PLC-beta from intracellular sites where PI is synthesized. We propose that PI-TP is in fact a co-factor in inositol lipid signalling and acts by interacting with the inositol lipid kinases. We hypothesize that the preferred substrate for PLC-beta is not the lipid that is resident in the membrane but that provided through PI-TP.

Multiple subtypes of phospholipase C are encoded by the norpA gene of Drosophila melanogaster

The norpA gene of Drosophila melanogaster encodes a phosphatidylinositol-specific phospholipase C that is essential for phototransduction. Besides being found abundantly in retina, norpA gene products are expressed in a variety of tissues that do not contain phototransduction machinery, implying that norpA is involved in signaling pathways in addition to phototransduction. We have identified a second subtype of norpA protein that is generated by alternative splicing of norpA RNA. The alternative splicing occurs at a single exon that is excluded from mature norpA transcripts when a substitute exon of equal size is retained. The net difference between the two subtypes of norpA protein is 14 amino acid substitutions occurring between amino acid positions 130 and 155 of the enzyme. Results from Northern analyses suggest that norpA subtype I transcripts are most abundantly expressed in adult retina, while subtype II transcripts are most abundant in adult body. Moreover, norpA subtype I RNA can be detected by the reverse transcription-polymerase chain reaction in extracts of adult head tissue but not adult body nor at earlier stages of Drosophila development. Conversely, norpA subtype II RNA can be detected by reverse transcription-polymerase chain reaction throughout development as well as in heads and bodies of adults. Furthermore, norpA subtype I RNA is easily detected in retina using tissue in situ hybridization analysis, while subtype II RNA is not detectable in retina but is found in brain. Since only norpA subtype I RNA is found in retina, we conclude that subtype I protein is utilized in phototransduction. Since norpA subtype II RNA is not found in retina but is expressed in a variety of tissues not known to contain phototransduction machinery, subtype II protein is likely to be utilized in signaling pathways other than phototransduction. The amino acid differences between the two subtypes of norpA protein may reflect the need for each subtype to interact with signaling components of different signal-generating pathways.

Phospholipase C rescues visual defect in norpA mutant of Drosophila melanogaster

Mutations in the norpA gene of Drosophila melanogaster severely affect the light-evoked photoreceptor potential with strong mutations rendering the fly blind. The norpA gene has been proposed to encode phosphatidylinositol-specific phospholipase C (PLC), which enzymes play a pivotal role in one of the largest classes of signaling pathways known. A chimeric norpA minigene was constructed by placing the norpA cDNA behind an R1-6 photoreceptor cell-specific rhodopsin promoter. This minigene was transferred into norpAP24 mutant by P-element-mediated germline transformation to determine whether it could rescue the phototransduction defect concomitant with restoring PLC activity. Western blots of head homogenates stained with norpA antiserum show that norpA protein is restored in heads of transformed mutants. Moreover, transformants exhibit a large amount of measurable PLC activity in heads, whereas heads of norpAP24 mutant exhibit very little to none. Immunohistochemical staining of tissue sections using norpA antiserum confirm that expression of norpA protein in transformants localizes in the retina, more specifically in rhabdomeres of R1-6 photoreceptor cells, but not R7 or R8 photoreceptor cells. Furthermore, electrophysiological analyses reveal that transformants exhibit a restoration of light-evoked photoreceptor responses in R1-6 photoreceptor cells, but not in R7 or R8 photoreceptor cells. This is the strongest evidence thus far supporting the hypothesis that the norpA gene encodes phospholipase C that is utilized in phototransduction.

Adenosine A1 receptor-mediated activation of phospholipase C-beta 3 in intestinal muscle: dual requirement for alpha and beta gamma subunits of Gi3

Four native and cloned adenosine receptors (ARs), designated A1AR, A2aAR, A2bAR, and A3AR, have been characterized functionally and by radioligand binding. In the present study, we have used selective antibodies to identify the G protein subunits and phospholipase C (PLC)-beta isoform coupled to A1ARs in smooth muscle membranes and permeabilized muscle cells from rabbit intestine. Immunoblot analysis disclosed the presence of a full complement of G proteins. Adenosine caused contraction of dispersed muscle cells and increases in D-myo-inositol-1,4,5-trisphosphate, intracellular calcium, and cAMP levels. Contraction and the increases in D-myo-inositol-1,4,5-trisphosphate and intracellular calcium levels were abolished by the A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine and augmented by the A2 antagonist CGS-15943; the reverse occurred with cAMP. A selective A1AR agonist, cyclopentyladenosine, inhibited forskolin-stimulated cAMP accumulation; the inhibition was reversed by treatment of the cells with pertussis toxin or a G alpha i3-specific antibody. The pattern of inhibition implied coexistence of A1ARs and A2ARs coupled to interactive signaling pathways, with A2ARs mediating activation of adenylyl cyclase and A1ARs mediating activation of PLC and inhibition of adenylyl cyclase. Adenosine-stimulated PLC activity in muscle membranes was selectively blocked by G alpha i3- and G beta-specific antibodies, as well as by a PLC-beta 3-specific antibody, but not by antibodies to other PLC-beta isoforms or G proteins. A combination of maximally effective concentrations of G alpha i3- and G beta-specific antibodies did not elicit greater inhibition than did either alone. In contrast, cholecystokinin-stimulated PLC activity was selectively blocked by PLC-beta 1- and G alpha q/11-specific antibodies. Adenosine-stimulated contraction and 45Ca2+ efflux in permeabilized muscle cells were also selectively blocked by G alpha i3-, G beta-, and PLC-beta 3-specific antibodies, whereas cholecystokinin-stimulated contraction was selectively blocked by PLC-beta 1- and G alpha q/11-specific antibodies. The results indicate that A1ARs are coupled to PLC-beta 3 via both alpha and beta gamma subunits of Gi3.

G protein control of Drosophila photoreceptor phospholipase C

Light stimulates phosphatidylinositol bisphosphate phospholipase C (PLC) activity in Drosophila photoreceptors. We have investigated the mechanism of this reaction by assaying PLC activity in Drosophila head membranes using exogenous phospholipid substrates. PLC activation depends on the photoconversion of rhodopsin to metarhodopsin and is reduced in norpAEE5 PLC and ninaEP332 rhodopsin mutants. NorpA PLC is stimulated by light at free Ca2+ concentrations between 10 nM and 1 microM. This finding is consistent with a Ca(2+)-mediated positive feedback mechanism that contributes to the rapid temporal response of invertebrate photoreceptor cells. The guanyl nucleotide dependence of light-stimulated PLC activity indicates that a G protein regulates NorpA. This was confirmed by the observation that light stimulation of PLC activity is deficient in mutants that lack the eye-specific G protein beta subunit G beta e. These results indicate that G beta e functions as the beta subunit of the G protein coupling rhodopsin to NorpA PLC.

Kinetic analysis of phospholipase C beta isoforms using phospholipid-detergent mixed micelles. Evidence for interfacial catalysis involving distinct micelle binding and catalytic steps

Phosphatidylinositol 4,5-bisphosphate (PtdIns (4,5)-P2) hydrolysis by three different beta-isoforms of phospholipase C (PLC) was examined to investigate the catalytic action of these extracellular signal-regulated enzymes. Depletion of phospholipase C from solution by incubation with sucrose-loaded vesicles of differing compositions followed by ultracentrifugation demonstrated stable attachment of PLC to the vesicles from which an equilibrium association constant of PLC with PtdIns (4,5)P2 could be determined. A mixed micellar system was established to assay PLC activity using dodecyl maltoside, which behaved as an essentially inert diluent of PtdIns (4,5)P2 with respect to PLC beta activity. Kinetic analyses were performed to test whether PLC beta activity was dependent on both bulk PtdIns (4,5)P2 concentration and surface concentration in the micelles as has been shown for other lipid metabolising enzymes. Each of the PLC beta isoforms behaved similarly in these analyses, which indicated the involvement of at least two binding events. Interfacial Michaelis constants were calculated to be between 0.1-0.2 mol fraction for all three enzymes, and Ks (the equilibrium dissociation constant of PLC for lipid) ranged between 100-200 microM. The apparent multiple interfacial binding events did not appear to result from lipid-induced PLC beta oligomerization implying that PLC beta monomers possess more than one lipid-binding site. Surface dilution of PLC-catalyzed PtdIns (4,5)P2 hydrolysis was assessed in the presence of increasing concentrations of various nonsubstrate phospholipids, which profoundly reduced PLC activity, suggesting that these lipids may inhibit enzyme action. The data indicate that G protein-regulated isoforms of PLC operate with separate lipid binding and catalytic steps and imply that under physiological conditions, PLC beta isoforms operate under first-order conditions. These findings may have implications for the mechanisms of regulation of PLC beta s by G protein subunits.

Mapping regions of G alpha q interacting with PLC beta 1 using multiple overlapping synthetic peptides

The heterotrimeric G-protein alpha-chain G alpha q plays a critical role mediating receptor-linked activation of the beta isoforms of PLC which hydrolyse membrane inositol-containing phospholipids to generate the second messengers inositol 1,4,5-trisphosphate and diacylglycerol. Despite knowledge of the three-dimensional structure of two G-protein alpha-chains (G alpha t and G alpha i1) as well as high regional amino acid conservation between members of the G-protein alpha-chain family, the precise molecular domains of G alpha q mediating activation of PLC beta 1 are unknown. To map sites responsible for effector interaction we employed 188 peptides each of 15 residues and corresponding to overlapping regions of the complete G alpha q sequence. These were tested for their ability to inhibit G alpha q-dependent activation of recombinant PLC beta 1 using an in vitro reconstitution assay. Peptides from two regions of G alpha q mediated up to 100% inhibition of GTP gamma S-stimulated PLC beta 1 activity, and representative peptides from each of these regions were half-maximally effective at 69.3 +/- 27.4 microM (n = 4) (G alpha q: 251-265) and 110.0 +/- 41.9 microM (n = 4) (G alpha q: 306-319). G alpha q regions described by inhibitory peptides are conserved selectively in other G-protein alpha-chains linked to PLC beta 1 activation (G alpha 11, G alpha 14) and correspond spatially to sites of effector interaction identified in G alpha s by scanning mutagenesis and in transducin using site-specific antibodies and peptides. Computer transducin using site-specific antibodies and peptides. Computer homology modelling of G alpha q based on the crystal structure of transducin indicates that regions interacting with PLC beta 1 form two parallel alpha-helices lying at the surface of the G alpha q structure. These observations provide the first description of two regions within G alpha q critically important for activating PLC beta 1, and moreover, indicate that effector binding domains identified in transducin and G alpha s are also conserved spatially in G alpha q.

Characterization of putative polyphosphoinositide binding motifs from phospholipase C beta 2

Several phosphatidylinositol 4,5-bisphosphate (PtdInsP2)-regulated actin-binding proteins and most phosphoinositide-specific phospholipases C (PI-PLCs) comprise a basic amino acid motif (KxxxKxKK, where x denotes any amino acid), which was previously suggested to represent a PtdInsP2-binding site commonly present in these proteins. We have shown earlier that a peptide corresponding to amino acids 448-464 of human PLC beta 2 (LPSPEDLRGKILIKNKK, peptide P1) markedly and specifically stimulated the activity of this enzyme [Simões et al. (1993) FEBS Lett. 331, 248]. Here, we present a detailed analysis of the effects of various peptides related to peptide P1 aimed at understanding the mechanisms of peptide-mediated PLC beta 2 stimulation. Peptide KILIKNKK (P2), which comprises only the basic amino acid consensus motif, also stimulated PLC beta 2, although higher concentrations were required to observe this stimulatory effect. The effects of P1 and P2 were not additive, indicating that the two peptides affect PLC beta 2 activity via the same mechanism. Peptide LPSPEDLRG (P3), composed of the amino-terminal half of P1, did not affect the activity of PLC beta 2. Peptide KILIKNKKQFSGPTSS (P4), which includes the nine amino acids flanking the carboxy-terminus of the KILIKNKK motif within the sequence of PLC beta 2, stimulated the enzyme but was indistinguishable in potency from P2. Circular dichroism analysis revealed that peptide P1 changes its conformation in the presence of PtdInsP2 but not in the presence of other phospholipids including phosphatidylinositol 4-phosphate. The results suggest that the basic amino acid sequence physically interacts with PtdInsP2.(ABSTRACT TRUNCATED AT 250 WORDS)