A G-protein-coupled 130 kDa phospholipase C isozyme, PLC-beta 4, from the particulate fraction of bovine cerebellum

Gαq signalling from endosomes: A new conundrum

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

Desensitization of angiotensin-stimulated inositol phosphate accumulation in human vascular smooth muscle cells

The effect of angiotensin II treatment on desensitization of phospholipase C (PLC)-mediated inositol phosphate accumulation has not been quantitated in human aortic vascular smooth muscle (HVSM) cells. We determined the angiotensin II pretreatment dose dependency and time course for desensitization of PLC activation in HVSM cells and the effect of protein kinase C (PKC) activators on angiotensin II-mediated inositol phosphate accumulation. Our results with PKC activators and direct G protein stimulators suggest that PKC activation may play a negative feedback role in desensitization of angiotensin II-activated signaling in HVSM cells by modifying the Gq transducer, PLC-beta effector, or related proteins in the signaling pathway. However, neither angiotensin II nor PKC activator affected basal phosphorylation levels of PLC-beta1 or PLC-beta3 in HVSM cells; PLC-beta isoenzymes were shown to be phosphorylated in unstimulated cells independent of PKC inhibition. We suggest that desensitization of G protein-stimulated inositol phosphate accumulation in HVSM differs from other cell types in which phosphorylation of PLC-beta isoenzymes accompanies desensitization.

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.

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

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

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.

Identification of the peptides that stimulate the phosphoinositide hydrolysis in lymphocyte cell lines from peptide libraries

Peptides which stimulate the formation of inositol phosphates (InoPs) in lymphocyte cell lines were identified by screening synthetic peptide libraries composed of random sequences of hexapeptides. The peptides containing the consensus sequence XKYX(P/V)M were found to be most active in the phospholipase C (PLC)-mediated formation of InoPs in a human B myeloma cell line, U266. The peptides also stimulated the phosphoinositide hydrolysis and the release of [Ca2+]i in HL60 and U937 cell lines. On the other hand, these peptides showed no effect in the following cell lines: NIH3T3, PC12, Daudi, Sp2, Jurkat, H9, Molt-4, SupT-1, K562, and RBL-2H3. The result suggests the possibility that the peptides may have cell type specificity. Experiments with one of the active peptides, WKYMVM-NH2 showed that its action mimics the effect of AlF4- which is a G-protein activator in the InoPs generation, and pertussis toxin partially blocked the InoPs accumulation and [Ca2+]i release induced by the peptide in the U266 cells. Binding assays with the peptide labeled with 125I showed that U266 cells have a saturable number of binding sites for the peptide. Taken together, these results suggest that the peptides could activate PLC-mediated signal transduction via a pertussis toxin-sensitive G-protein coupled receptor in certain cell types.

Expression and immunohistochemical localization of eight phospholipase C isoforms in adult male mouse cerebellar cortex

By means of specific polyclonal or monoclonal antibodies we have investigated the expression and the localization of phospholipase C isoforms in the adult mice cerebellar cortex. Western-blot analysis revealed that mouse cerebellum expressed eight phospholipase C isozymes: -beta 1, -beta 2, -beta 3, -beta 4, -gamma 1, -gamma 2, -delta 1, -delta 2. Immunohistochemical analysis carried out on cryosections showed a distinct pattern of expression for each of the isoforms. Purkinje cells had high levels of -beta 1, -beta 3, -gamma 2 and -delta 2 isotypes. The -gamma 2 isozyme was the only one that was identified also in the dendrites of Purkinje cells. In the molecular layer we detected mostly -beta 1 and -gamma 1 isozymes whereas in the granular layer -gamma 1 and -gamma 2 isoforms prodominated. These results indicate a heterogeneity of the phospholipase C isoforms expressed in the layers of mouse cerebellar cortex conceivably due to the fact that these enzymes are coupled to different receptors and perform selective tasks in regulating cell signalling events taking place in the cerebellar cortex of mice.

The active role of beta gamma in signal transduction

Many receptors that sense the environment effect intracellular regulation through stimulation of heterotrimeric G proteins and the consequences thereof. While prominence was originally given to the alpha-subunits of G proteins as the pathway for downstream regulation, very active roles for the beta gamma-subunits have emerged in the past year. Recent experiments highlight the versatility of beta gamma-subunits in these regulatory pathways, but also emphasize some fundamental questions that remain.