The alpha-subunit of the heterotrimeric G protein G13 activates a phospholipase D isozyme by a pathway requiring Rho family GTPases

Fine Tuning Muscarinic Acetylcholine Receptor Signaling Through Allostery and Bias

The M1 and M4 muscarinic acetylcholine receptors (mAChRs) are highly pursued drug targets for neurological diseases, in particular for Alzheimer's disease and schizophrenia. Due to high sequence homology, selective targeting of any of the M1-M5 mAChRs through the endogenous ligand binding site has been notoriously difficult to achieve. With the discovery of highly subtype selective mAChR positive allosteric modulators in the new millennium, selectivity through targeting an allosteric binding site has opened new avenues for drug discovery programs. However, some hurdles remain to be overcome for these promising new drug candidates to progress into the clinic. One challenge is the potential for on-target side effects, such as for the M1 mAChR where over-activation of the receptor by orthosteric or allosteric ligands can be detrimental. Therefore, in addition to receptor subtype selectivity, a drug candidate may need to exhibit a biased signaling profile to avoid such on-target adverse effects. Indeed, recent studies in mice suggest that allosteric modulators for the M1 mAChR that bias signaling toward specific pathways may be therapeutically important. This review brings together details on the signaling pathways activated by the M1 and M4 mAChRs, evidence of biased agonism at these receptors, and highlights pathways that may be important for developing new subtype selective allosteric ligands to achieve therapeutic benefit.

Phospholipase D signaling pathways and phosphatidic acid as therapeutic targets in cancer

Phospholipase D is a ubiquitous class of enzymes that generates phosphatidic acid as an intracellular signaling species. The phospholipase D superfamily plays a central role in a variety of functions in prokaryotes, viruses, yeast, fungi, plants, and eukaryotic species. In mammalian cells, the pathways modulating catalytic activity involve a variety of cellular signaling components, including G protein-coupled receptors, receptor tyrosine kinases, polyphosphatidylinositol lipids, Ras/Rho/ADP-ribosylation factor GTPases, and conventional isoforms of protein kinase C, among others. Recent findings have shown that phosphatidic acid generated by phospholipase D plays roles in numerous essential cellular functions, such as vesicular trafficking, exocytosis, autophagy, regulation of cellular metabolism, and tumorigenesis. Many of these cellular events are modulated by the actions of phosphatidic acid, and identification of two targets (mammalian target of rapamycin and Akt kinase) has especially highlighted a role for phospholipase D in the regulation of cellular metabolism. Phospholipase D is a regulator of intercellular signaling and metabolic pathways, particularly in cells that are under stress conditions. This review provides a comprehensive overview of the regulation of phospholipase D activity and its modulation of cellular signaling pathways and functions.

Role of phospholipase d in g-protein coupled receptor function

Prolonged agonist exposure of many G-protein coupled receptors induces a rapid receptor phosphorylation and uncoupling from G-proteins. Resensitization of these desensitized receptors requires endocytosis and subsequent dephosphorylation. Numerous studies show the involvement of phospholipid-specific phosphodiesterase phospholipase D (PLD) in the receptor endocytosis and recycling of many G-protein coupled receptors e.g., opioid, formyl or dopamine receptors. The PLD hydrolyzes the headgroup of a phospholipid, generally phosphatidylcholine (PC), to phosphatidic acid (PA) and choline and is assumed to play an important function in cell regulation and receptor trafficking. Protein kinases and GTP binding proteins of the ADP-ribosylation and Rho families regulate the two mammalian PLD isoforms 1 and 2. Mammalian and yeast PLD are also potently stimulated by phosphatidylinositol 4,5-bisphosphate. The PA product is an intracellular lipid messenger. PLD and PA activities are implicated in a wide range of physiological processes and diseases including inflammation, diabetes, oncogenesis or neurodegeneration. This review discusses the characterization, structure, and regulation of PLD in the context of membrane located G-protein coupled receptor function.

Bacterial protein toxins and lipids: role in toxin targeting and activity

All bacterial toxins, which globally are hydrophilic proteins, interact first with their target cells by recognizing a surface receptor, which is either a lipid or a lipid derivative, or another compound but in a lipid environment. Intracellular active toxins follow various trafficking pathways, the sorting of which is greatly dependent on the nature of the receptor, notably lipidic receptor or receptor embedded into a distinct environment such as lipid microdomains. Numerous other toxins act locally on cell membrane. Indeed, phospholipase activity is a common mechanism shared by several membrane-damaging toxins. In addition, many toxins active intracellularly or on cell membrane modulate host cell phospholipid pathways. Unusually, a few bacterial toxins require a lipid post-translational modification to be active. Thereby, lipids are obligate partners of bacterial toxins.

Loss of Lsc/p115 protein leads to neuronal hypoplasia in the esophagus and an achalasia-like phenotype in mice

BACKGROUND & AIMS

Lsc/p115 originally was described as hematopoietic Ras homologous protein guanine exchange factor (Rho-GEF) regulating leukocyte migration, adhesion, and marginal zone B-cell homeostasis. Here we investigate the expression pattern of lsc/p115 in the gastrointestinal tract and the consequences of lsc/p115 deficiency in lsc/p115-knockout mice.

METHODS

The phenotype of lsc/p115-deficient mice was analyzed in vivo with small-animal computed tomography scans and esophageal manometry. The morphology and myenteric plexus were evaluated with immunohistochemistry, morphometry, Western blot analyses, and quantitative reverse-transcription polymerase chain reaction.

RESULTS

lsc/p115 is expressed in the gastrointestinal tract, sparing the segment of the small intestine. Immunohistochemical staining detects lsc/p115 in the muscle layer and the glial fibrillary acidic protein-positive glia in the esophagus. Esophageal manometry uncovers a severe motor dysfunction in lsc/p115-deficient mice. This achalasia-like phenotype is characterized by disturbed peristalsis, hypertension of the lower esophageal sphincter, and impaired relaxation of the lower esophageal sphincter. Lsc/p115-deficient mice develop a progressive dilatation of the esophagus and decrease of the muscle layer. The muscle cell differentiation is not altered in lsc/p115-deficient mice. However, the density of inhibitory and excitatory neurons and glia cells in the myenteric plexus and the muscle layer are reduced in morphometric analyses. This reduced number of glia cells is accompanied by reduced expression of the neurotrophic nerve growth factor.

CONCLUSIONS

lsc/p115 deficiency results in impaired neuronal innervation and in motor dysfunction recapitulating several aspects of esophageal achalasia. Reduced expression of nerve growth factor and a reduced number of glia cells most likely contribute to this phenotype.

Biologic functions of the G12 subfamily of heterotrimeric g proteins: growth, migration, and metastasis

The G12 subfamily of heterotrimeric G proteins has been the subject of intense scientific interest for more than 15 years. During this period, studies have revealed more than 20 potential G12-interacting proteins and numerous signaling axes emanating from the G12 proteins, Galpha12 and Galpha13. In addition, more recent studies have begun to illuminate the various and sundry functions that the G12 subfamily plays in biology. In this review, we summarize the diverse range of proteins that have been identified as Galpha12 and/or Galpha13 interactors and describe ongoing studies designed to dissect the biological roles of specific Galpha-effector protein interactions. Further, we describe and discuss the expanding role of G12 proteins in the biology of cells, focusing on the distinct properties of this subfamily in regulating cell proliferation, cell migration, and metastatic invasion.

Phospholipase D signaling: orchestration by PIP2 and small GTPases

Hydrolysis of phosphatidylcholine by phospholipase D (PLD) leads to the generation of the versatile lipid second messenger, phosphatidic acid (PA), which is involved in fundamental cellular processes, including membrane trafficking, actin cytoskeleton remodeling, cell proliferation and cell survival. PLD activity can be dramatically stimulated by a large number of cell surface receptors and is elaborately regulated by intracellular factors, including protein kinase C isoforms, small GTPases of the ARF, Rho and Ras families and, particularly, by the phosphoinositide, phosphatidylinositol 4,5-bisphosphate (PIP(2)). PIP(2) is well known as substrate for the generation of second messengers by phospholipase C, but is now also understood to recruit and/or activate a variety of actin regulatory proteins, ion channels and other signaling proteins, including PLD, by direct interaction. The synthesis of PIP(2) by phosphoinositide 5-kinase (PIP5K) isoforms is tightly regulated by small GTPases and, interestingly, by PA as well, and the concerted formation of PIP(2) and PA has been shown to mediate receptor-regulated cellular events. This review highlights the regulation of PLD by membrane receptors, and describes how the close encounter of PLD and PIP5K isoforms with small GTPases permits the execution of specific cellular functions.

Dynamic phospholipid signaling by G protein-coupled receptors

G protein-coupled receptors (GPCRs) control a variety of fundamental cellular processes by regulating phospholipid signaling pathways. Essential for signaling by a large number of receptors is the hydrolysis of the membrane phosphoinositide PIP(2) by phospholipase C (PLC) into the second messengers IP(3) and DAG. Many receptors also stimulate phospholipase D (PLD), leading to the generation of the versatile lipid, phosphatidic acid. Particular PLC and PLD isoforms take differential positions in receptor signaling and are additionally regulated by small GTPases of the Ras, Rho and ARF families. It is now recognized that the PLC substrate, PIP(2), has signaling capacity by itself and can, by direct interaction, affect the activity and subcellular localization of PLD and several other proteins. As expected, the synthesis of PIP(2) by phosphoinositide 5-kinases is tightly regulated as well. In this review, we present an overview of how these signaling pathways are governed by GPCRs, explain the molecular basis for the spatially and temporally organized, highly dynamic quality of phospholipid signaling, and point to the functional connection of the pathways.

JNK-interacting leucine zipper protein is a novel scaffolding protein in the Galpha13 signaling pathway

Scaffolding proteins play a critical role in conferring specificity and fidelity to signaling pathways. The JNK-interacting leucine zipper protein (JLP) has been identified as a scaffolding protein involved in linking components of the JNK signaling module. Galpha(12) and Galpha(13), the alpha-subunits of heterotrimeric G proteins G12 and G13, respectively, stimulate the JNK module in diverse cell types. Here, we report that Galpha(13) physically interacts with JLP, and this interaction enhances Galpha(13)-mediated JNK activation. We also demonstrate endogenous interaction between JLP and Galpha(13) in MCF-7 cells. JLP interaction is specific to the G12 family of alpha-subunits via its C-terminal domain (termed GID-JLP), spanning amino acids 1165-1307, and this interaction is more pronounced with the mutationally or functionally activated form of Galpha(13) compared to that of wild-type Galpha(13). The presence of a ternary complex consisting of Galpha(13), JLP, and JNK suggests a role for JLP in tethering Galpha(13) to the signaling components involved in JNK activation. Coexpression of GID-JLP disrupts ternary complex formation in addition to attenuating Galpha(13)-stimulated JNK activity. These findings identify JLP as a novel scaffolding protein in the Galpha(13)-mediated JNK signaling pathway.

Rho GEF Lsc is required for normal polarization, migration, and adhesion of formyl-peptide-stimulated neutrophils

Neutrophil migration requires continuous reorganization of the cytoskeleton and cellular adhesion apparatus. Chemoattractants initiate intracellular signals that direct this reorganization. The signaling pathways that link chemoattractant receptors to the cytoskeleton and cellular adhesion apparatus are now being defined. Formyl-peptide chemoattractants released from bacteria stimulate G-protein-linked receptors on the surface of neutrophils and regulate the neutrophil cytoskeleton and adhesion apparatus through RhoA-dependent pathways. Lsc is a RhoA guanine nucleotide exchange factor that binds the heterotrimeric G-protein alpha-subunits, Galpha12 and Galpha13. We have disrupted the Lsc gene and demonstrated that formyl-peptide-stimulated Lsc knock-out (KO) neutrophils are unable to generate and sustain a single-dominant pseudopod and migrate with increased speed and reduced directionality. Unexpectedly, we also found that Lsc is required for normal beta2- and beta1-integrin-dependent neutrophil adhesion. Lsc-deficient mice have a peripheral leukocytosis and extramedullary hematopoiesis, demonstrating that Lsc is required for leukocyte homeostasis. Lsc-deficient neutrophils are recruited normally to sites of bacterial peritonitis and chemical dermatitis, indicating that other signaling pathways compensate for the Lsc deficiency in some forms of inflammation. These results demonstrate that Lsc links formyl-peptide receptors to RhoA signaling pathways that regulate polarization, migration, and adhesion in neutrophils and that Lsc is required for leukocyte homeostasis.

Mammalian G proteins and their cell type specific functions

Heterotrimeric G proteins are key players in transmembrane signaling by coupling a huge variety of receptors to channel proteins, enzymes, and other effector molecules. Multiple subforms of G proteins together with receptors, effectors, and various regulatory proteins represent the components of a highly versatile signal transduction system. G protein-mediated signaling is employed by virtually all cells in the mammalian organism and is centrally involved in diverse physiological functions such as perception of sensory information, modulation of synaptic transmission, hormone release and actions, regulation of cell contraction and migration, or cell growth and differentiation. In this review, some of the functions of heterotrimeric G proteins in defined cells and tissues are described.

Activation of protein kinase D3 by signaling through Rac and the alpha subunits of the heterotrimeric G proteins G12 and G13

PKD is the founding member of a novel protein kinase family that also includes PKD2 and PKD3. PKD has been the focus of most studies up to date, but little is known about the mechanisms that mediate PKD3 activation. Here, we show that addition of aluminum fluoride to COS-7 cells cotransfected with PKD3 and Galpha13 or Galpha12 induced PKD3 activation, which was associated with a transient plasma membrane translocation of cytosolic PKD3. Treatment with Clostridium difficile toxin B blocked PKD3 activation induced by either bombesin or by aluminum fluoride-stimulated Galpha12/13 but did not affect Galphaq-induced PKD3 activation. Furthermore, PKD3 immunoprecipitated from cells cotransfected with a constitutively active Rac (RacV12) exhibited a marked increase in PKD3 basal catalytic activity. In contrast, cotransfection with active Rho (RhoQ63L), Cdc42 (Cdc42Q61L), or Ras (RasV12) did not promote PKD3 activation. Expression of either COOH-terminal dominant-negative fragment of Galpha13 or dominant negative Rac (Rac N17) attenuated bombesin-induced PKD3 activation. Treatment with protein kinase C (PKC) inhibitors prevented the increase in PKD3 activity induced by RacV12 and aluminum fluoride-stimulated Galpha12/13. The catalytic activation of PKD3 in response to RacV12, alpha12/13 signaling or bombesin correlated with Ser-731/Ser-735 phosphorylation in the activation loop of this enzyme. Our results indicate that Galpha12/13 and Rac are important components in the signal transduction pathways that mediate bombesin receptor-induced PKD3 activation.

G alpha12 interaction with alphaSNAP induces VE-cadherin localization at endothelial junctions and regulates barrier function

The involvement of heterotrimeric G proteins in the regulation of adherens junction function is unclear. We identified alphaSNAP as an interactive partner of G alpha12 using yeast two-hybrid screening. Glutathione S-transferase pull-down assays showed the selective interaction of alphaSNAP with G alpha12 in COS-7 as well as in human umbilical vein endothelial cells. Using domain swapping experiments, we demonstrated that the N-terminal region of G alpha12 (1-37 amino acids) was necessary and sufficient for its interaction with alphaSNAP. G alpha13 with its N-terminal extension replaced by that of G alpha12 acquired the ability to bind to alphaSNAP, whereas G alpha12 with its N terminus replaced by that of G alpha13 lost this ability. Using four point mutants of alphaSNAP, which alter its ability to bind to the SNARE complex, we determined that the convex rather than the concave surface of alphaSNAP was involved in its interaction with G alpha12. Co-transfection of human umbilical vein endothelial cells with G alpha12 and alphaSNAP stabilized VE-cadherin at the plasma membrane, whereas down-regulation of alphaSNAP with siRNA resulted in the loss of VE-cadherin from the cell surface and, when used in conjunction with G alpha12 overexpression, decreased endothelial barrier function. Our results demonstrate a direct link between the alpha subunit of G12 and alphaSNAP, an essential component of the membrane fusion machinery, and implicate a role for this interaction in regulating the membrane localization of VE-cadherin and endothelial barrier function.

G alpha12/G alpha13 subunits of heterotrimeric G proteins mediate parathyroid hormone activation of phospholipase D in UMR-106 osteoblastic cells

PTH, a major regulator of bone remodeling and a therapeutically effective bone anabolic agent, stimulates several signaling pathways in osteoblastic cells. Our recent studies have revealed that PTH activates phospholipase D (PLD) -mediated phospholipid hydrolysis through a RhoA-dependent mechanism in osteoblastic cells, raising the question of the upstream link to the PTH receptor. In the current study, we investigated the role of heterotrimeric G proteins in mediating PTH-stimulated PLD activity in UMR-106 osteoblastic cells. Transfection with antagonist minigenes coding for small peptide antagonists to G alpha 12 and G alpha13 subunits of heterotrimeric G proteins prevented PTH-stimulated activation of PLD, whereas an antagonist minigene to G alphas failed to produce this effect. Effects of pharmacological inhibitors (protein kinase inhibitor, Clostridium botulinum exoenzyme C3) were consistent with a role of Rho small G proteins, but not of cAMP, in the effect of PTH on PLD. Expression of constitutively active G alpha12 and G alpha13 activated PLD, an effect that was inhibited by dominant-negative RhoA. The results identify G alpha12 and G alpha13 as upstream transducers of PTH effects on PLD, mediated through RhoA in osteoblastic cells.

Selective Uncoupling of Gα12 from Rho-mediated Signaling

The heterotrimeric G protein G(12) has been implicated in such cellular regulatory processes as cytoskeletal rearrangement, cell-cell adhesion, and oncogenic transformation. Although the activated alpha-subunit of G(12) has been shown to interact directly with a number of protein effectors, the roles of many of these protein-protein interactions in G(12)-mediated cell physiology are poorly understood. To begin dissecting the specific cellular pathways engaged upon G(12) activation, we produced a series of substitution mutants in the regions of Galpha(12) predicted to play a role in effector binding. Here we report the identification and characterization of an altered form of Galpha(12) that is functionally uncoupled from signaling through the monomeric G protein Rho, a protein known to propagate several Galpha(12)-mediated signals. This mutant of Galpha(12) fails to bind the Rho-specific guanine nucleotide exchange factors p115RhoGEF and LARG (leukemia-associated RhoGEF), fails to stimulate Rho-dependent transcriptional activation, and fails to trigger activation of RhoA and the Rho-mediated cellular responses of cell rounding and c-jun N-terminal kinase activation. Importantly, this mutant of Galpha(12) retains coupling to the effector protein E-cadherin, as evidenced by its ability both to bind E-cadherin in vitro and to disrupt E-cadherin-mediated cell-cell adhesion. Furthermore, this mutant retains the ability to trigger beta-catenin release from the cytoplasmic domain of cadherin. This identification of a variant of Galpha(12) that is selectively uncoupled from one signaling pathway while retaining signaling capacity through a separate pathway will facilitate investigations into the mechanisms through which G(12) proteins mediate diverse biological responses.

Amino acid-stimulated Ca2+ oscillations produced by the Ca2+-sensing receptor are mediated by a phospholipase C/inositol 1,4,5-trisphosphate-independent pathway that requires G12, Rho, filamin-A, and the actin cytoskeleton

The G protein-coupled Ca(2+)-sensing receptor (CaR) is an allosteric protein that responds to two different agonists, Ca(2+) and aromatic amino acids, with the production of sinusoidal or transient oscillations in intracellular Ca(2+) concentration ([Ca(2+)](i)). Here, we examined whether these differing patterns of [Ca(2+)](i) oscillations produced by the CaR are mediated by separate signal transduction pathways. Using real time imaging of changes in phosphatidylinositol 4,5-biphosphate hydrolysis and generation of inositol 1,4,5-trisphosphate in single cells, we found that stimulation of CaR by an increase in the extracellular Ca(2+) concentration ([Ca(2+)](o)) leads to periodic synthesis of inositol 1,4,5-trisphosphate, whereas l-phenylalanine stimulation of the CaR does not induce any detectable change in the level this second messenger. Furthermore, we identified a novel pathway that mediates transient [Ca(2+)](i) oscillations produced by the CaR in response to l-phenylalanine, which requires the organization of the actin cytoskeleton and involves the small GTPase Rho, heterotrimeric proteins of the G(12) subfamily, the C-terminal region of the CaR, and the scaffolding protein filamin-A. Our model envisages that Ca(2+) or amino acids stabilize unique CaR conformations that favor coupling to different G proteins and subsequent activation of distinct downstream signaling pathways.

Gα13 Stimulates Cell Migration through Cortactin-interacting Protein Hax-1

Galpha13, the alpha-subunit of the heterotrimeric G protein G13, has been shown to stimulate cell migration in addition to inducing oncogenic transformation. Cta, a Drosophila ortholog of G13, has been shown to be critical for cell migration leading to the ventral furrow formation in Drosophila embryos. Loss of Galpha13 has been shown to disrupt cell migration associated with angiogenesis in developing mouse embryos. Whereas these observations point to the vital role of G13-orthologs in regulating cell migration, widely across the species barrier, the mechanism by which Galpha13 couples to cytoskeleton and cell migration is largely unknown. Here we show that Galpha13 physically interacts with Hax-1, a cytoskeleton-associated, cortactin-interacting intracellular protein, and this interaction is required for Galpha13-stimulated cell migration. Hax-1 interaction is specific to Galpha13, and this interaction is more pronounced with the mutationally or functionally activated form of Galpha13 as compared with the wild-type Galpha13. Expression of Hax-1 reduces the formation of actin stress fibers and focal adhesion complexes in Galpha13-expressing NIH3T3 cells. Coexpression of Hax-1 also attenuates Galpha(13)-stimulated activity of Rho while potentiating Galpha13-stimulated activity of Rac. The presence of a quadnary complex consisting of Galpha13, Hax-1, Rac, and cortactin indicates the role of Hax-1 in tethering Galpha13 to the cytoskeletal component(s) involved in cell movement. Whereas the expression of Hax-1 potentiates Galpha13-mediated cell movement, silencing of endogenous Hax-1 with Hax-1-specific small interfering RNAs drastically reduces Galpha13-mediated cell migration. These findings, along with the observation that Hax-1 is overexpressed in metastatic tumors and tumor cell lines, suggest a novel role for the association of oncogenic Galpha13 and Hax-1 in tumor metastasis.

A role for Galpha12/Galpha13 in p120ctn regulation

The catenin p120 (p120ctn) is an armadillo repeat domain protein that binds to cadherins and has been shown to facilitate strong cell-cell adhesion. We have investigated a possible link between heterotrimeric G proteins and p120ctn, and found that both Galpha12 and Galpha13 can completely and selectively abrogate the p120ctn-induced branching phenotype in different cell types. Consistent with these observations, the expression of Galpha12 or Galpha13 compensates for the reduction of Rho activity induced by p120ctn. On the other hand, p120ctn can be selectively coimmunoprecipitated with Galpha12, and the coimmunoprecipitation was favored by activation of the G protein. A specific interaction between p120ctn and Galpha12Q231L was also observed in in vitro binding experiments. In addition, p120ctn can be immunoprecipitated along with Galpha12Q231L in L cells in absence of E-cadherin. Interestingly, the expression of Galpha12Q231L increases the amount of p120ctn associated with E-cadherin. These findings demonstrate that Galpha12 and p120ctn are binding partners, and they also suggest a role for Galpha12 in regulating p120ctn activity and its interaction with cadherins. We propose that the Galpha12-p120ctn interaction acts as a molecular switch, which regulates cadherin-mediated cell-cell adhesion.

The Ca2+-sensing receptor couples to Galpha12/13 to activate phospholipase D in Madin-Darby canine kidney cells

The Ca2+-sensing receptor (CaR) couples to multiple G proteins involved in distinct signaling pathways: Galphai to inhibit the activity of adenylyl cyclase and activate ERK, Galphaq to stimulate phospholipase C and phospholipase A2, and Gbetagamma to stimulate phosphatidylinositol 3-kinase. To determine whether the receptor also couples to Galpha12/13, we investigated the signaling pathway by which the CaR regulates phospholipase D (PLD), a known Galpha12/13 target. We established Madin-Darby canine kidney (MDCK) cell lines that stably overexpress the wild-type CaR (CaRWT) or the nonfunctional mutant CaRR796W as a negative control, prelabeled these cells with [3H]palmitic acid, and measured CaR-stimulated PLD activity as the formation of [3H]phosphatidylethanol (PEt). The formation of [3H]PEt increased in a time-dependent manner in the cells that overexpress the CaRWT but not the CaRR796W. Treatment of the cells with C3 exoenzyme inhibited PLD activity, which indicates that the CaR activates the Rho family of small G proteins, targets of Galpha12/13. To determine which G protein(s) the CaR couples to in order to activate Rho and PLD, we pretreated the cells with pertussis toxin to inactivate Galphai or coexpressed regulators of G protein-signaling (RGS) proteins to attenuate G protein signaling (RGS4 for Galphai and Galphaq, and a p115RhoGEF construct containing the RGS domain for Galpha12/13). Overexpression of p115RhoGEF-RGS in the MDCK cells that overexpress CaRWT inhibited extracellular Ca2+-stimulated PLD activity, but pretreatment of cells with pertussis toxin and overexpression of RGS4 were without effect. The involvement of other signaling components such as protein kinase C, ADP-ribosylation factor, and phosphatidylinositol biphosphate was excluded. These findings demonstrate that the CaR couples to Galpha12/13 to regulate PLD via a Rho-dependent mechanism and does so independently of Galphai and Galphaq. This suggests that the CaR may regulate cytoskeleton via Galpha12/13, Rho, and PLD.

Galpha12 regulates epithelial cell junctions through Src tyrosine kinases

Regulation and assembly of the epithelial cell junctional complex involve multiple signaling mechanisms, including heterotrimeric G proteins. Recently, we demonstrated that Galpha12 binds to the tight junction scaffolding protein ZO-1 through the SH3 domain and that activated Galpha12 increases paracellular permeability in Madin-Darby canine kidney (MDCK) cells (Meyer et al. J Biol Chem 277: 24855-24858, 2002). In the present studies, we explore the effects of Galpha12 expression on tight and adherens junction proteins and examine downstream signaling pathways. By confocal microscopy, we detect disrupted tight and adherens junction proteins with increased actin stress fibers in constitutively active Galpha12 (QLalpha12)-expressing MDCK cells. The normal distribution of ZO-1 and Na-K-ATPase was altered in QLalpha12-expressing MDCK cells, consistent with loss of polarity. We found that the tyrosine kinase inhibitor genistein and the Src-specific inhibitor PP-2 reversibly abrogated the QLalpha12 phenotype on the junctional complex. Junctional protein localization was preserved in PP-2- or genistein-treated QLalpha12-expressing cells, and the increase in paracellular permeability as measured by transepithelial resistance and [3H]mannitol flux was prevented by the inhibitors. Src activity was increased in QLalpha12-expressing MDCK cells as assessed by Src autophosphorylation, and beta-catenin tyrosine phosphorylation was also increased, although there was no detectable increase in Rho activity. Taken together, these results indicate that Galpha12 regulates MDCK cell junctions, in part through Src tyrosine kinase pathways.

G-proteins as transducers in transmembrane signalling

The G-protein-mediated signalling system has evolved as one of the most widely used transmembrane signalling mechanisms in mammalian organisms. All mammalian cells express G-protein-coupled receptors as well as several types of heterotrimeric G-proteins and effectors. G-protein-mediated signalling is involved in many physiological and pathological processes. This review summarizes some general aspects of G-protein-mediated signalling and focusses on recent data especially from studies in mutant mice which have elucidated some of the cellular and biological functions of heterotrimeric G-prtoteins.

N-terminal short sequences of alpha subunits of the G12 family determine selective coupling to receptors

The Galpha subunits of the G(12) family of heterotrimeric G proteins, defined by Galpha(12) and Galpha(13), have many cellular functions in common, such as stress fiber formation and neurite retraction. However, a variety of G protein-coupled receptors appear to couple selectively to Galpha(12) and Galpha(13). For example, thrombin and lysophosphatidic acid (LPA) have been shown to induce stress fiber formation via Galpha(12) and Galpha(13), respectively. We recently showed that active forms of Galpha(12) and Galpha(13) interact with Ser/Thr phosphatase type 5 through its tetratricopeptide repeat domain. Here we developed a novel assay to measure the activities of Galpha(12) and Galpha(13) by using glutathione S-transferase-fused tetratricopeptide repeat domain of Ser/Thr phosphatase type 5, taking advantage of the property that tetratricopeptide repeat domain strongly interacts with active forms of Galpha(12) and Galpha(13). By using this assay, we identified that thrombin and LPA selectively activate Galpha(12) and Galpha(13), respectively. Galpha(12) and Galpha(13) show a high amino acid sequence homology except for their N-terminal short sequences. Then we generated chimeric G proteins Galpha(12N/13C) and Galpha(13N/12C), in which the N-terminal short sequences are replaced by each other, and showed that thrombin and LPA selectively activate Galpha(12N/13C) and Galpha(13N/12C), respectively. Moreover, thrombin and LPA stimulate RhoA activity through Galpha(12) and Galpha(13), respectively, in a Galpha(12) family N-terminal sequence-dependent manner. Thus, N-terminal short sequences of the G(12) family determine the selective couplings of thrombin and LPA receptors to the Galpha(12) family.

Regulators of G-protein signaling (RGS) 4, insertion into model membranes and inhibition of activity by phosphatidic acid

Regulators of G-protein signaling (RGS) proteins are critical for attenuating G protein-coupled signaling pathways. The membrane association of RGS4 has been reported to be crucial for its regulatory activity in reconstituted vesicles and physiological roles in vivo. In this study, we report that RGS4 initially binds onto the surface of anionic phospholipid vesicles and subsequently inserts into, but not through, the membrane bilayer. Phosphatidic acid, one of anionic phospholipids, could dramatically inhibit the ability of RGS4 to accelerate GTPase activity in vitro. Phosphatidic acid is an effective and potent inhibitor of RGS4 in a G alpha(i1)-[gamma-(32)P]GTP single turnover assay with an IC(50) approximately 4 microm and maximum inhibition of over 90%. Furthermore, phosphatidic acid was the only phospholipid tested that inhibited RGS4 activity in a receptor-mediated, steady-state GTP hydrolysis assay. When phosphatidic acid (10 mol %) was incorporated into m1 acetylcholine receptor-G alpha(q) vesicles, RGS4 GAP activity was markedly inhibited by more than 70% and the EC(50) of RGS4 was increased from 1.5 to 7 nm. Phosphatidic acid also induced a conformational change in the RGS domain of RGS4 measured by acrylamide-quenching experiments. Truncation of the N terminus of RGS4 (residues 1-57) resulted in the loss of both phosphatidic acid binding and lipid-mediated functional inhibition. A single point mutation in RGS4 (Lys(20) to Glu) permitted its binding to phosphatidic acid-containing vesicles but prevented lipid-induced conformational changes in the RGS domain and abolished the inhibition of its GAP activity. We speculate that the activation of phospholipase D or diacylglycerol kinase via G protein-mediated signaling cascades will increase the local concentration of phosphatidic acid, which in turn block RGS4 GAP activity in vivo. Thus, RGS4 may represent a novel effector of phosphatidic acid, and this phospholipid may function as a feedback regulator in G protein-mediated signaling pathways.

Cooperation of Gq, Gi, and G12/13 in protein kinase D activation and phosphorylation induced by lysophosphatidic acid

To examine the contribution of different G-protein pathways to lysophosphatidic acid (LPA)-induced protein kinase D (PKD) activation, we tested the effect of LPA on PKD activity in murine embryonic cell lines deficient in Galpha(q/11) (Galpha(q/11) KO cells) or Galpha(12/13) (Galpha(12/13) KO cells) and used cells lacking rhodopsin kinase (RK cells) as a control. In RK and Galpha(12/13) KO cells, LPA induced PKD activation through a phospholipase C/protein kinase C pathway in a concentration-dependent fashion with maximal stimulation (6-fold for RK cells and 4-fold for Galpha(12/13) KO cells in autophosphorylation activity) achieved at 3 microm. In contrast, LPA did not induce any significant increase in PKD activity in Galpha(q/11) KO cells. However, LPA induced a significantly increased PKD activity when Galpha(q/11) KO cells were transfected with Galpha(q). LPA-induced PKD activation was modestly attenuated by prior exposure of RK cells to pertussis toxin (PTx) but abolished by the combination treatments of PTx and Clostridium difficile toxin B. Surprisingly, PTx alone strikingly inhibited LPA-induced PKD activation in a concentration-dependent fashion in Galpha(12/13) KO cells. Similar results were obtained when activation loop phosphorylation at Ser-744 was determined using an antibody that detects the phosphorylated state of this residue. Our results indicate that G(q) is necessary but not sufficient to mediate LPA-induced PKD activation. In addition to G(q), LPA requires additional G-protein pathways to elicit a maximal response with G(i) playing a critical role in Galpha(12/13) KO cells. We conclude that LPA induces PKD activation through G(q), G(i), and G(12) and propose that PKD activation is a point of convergence in the action of multiple G-protein pathways.

Regulation of phospholipase D

Structural studies of plant and bacterial members of the phospholipase D (PLD) superfamily are providing information about the role of the conserved HKD domains in the structure of the catalytic center and the catalytic mechanism of mammalian PLD isozymes (PLD1 and PLD2). Mutagenesis and sequence comparison studies have also defined the presence of pleckstrin homology and phox homology domains in the N-terminus and have demonstrated that a conserved sequence at the C-terminus is required for catalysis. The N- and C-terminal regions of PLD1 also contain interaction sites for protein kinase C, which can directly activate the enzyme through a non-phosphorylating mechanism. Small G proteins of the Rho and ADP-ribosylation factor families also directly regulate the enzyme, with RhoA binding to a sequence in the C-terminus. Certain tyrosine kinases and members of the Ras subfamily of small G proteins can activate the enzyme, but the mechanisms appear to be indirect. The mechanisms by which agonists activate PLD in vivo probably involve multiple pathways.

PGF(2alpha)-induced contraction of cat esophageal and lower esophageal sphincter circular smooth muscle

Lower esophageal sphincter (LES) tone depends on PGF(2alpha) and thromboxane A(2) acting on receptors linked to G(i3) and G(q) to activate phospholipases and produce second messengers resulting in muscle contraction. We therefore examined PGF(2alpha) signal transduction in circular smooth muscle cells isolated by enzymatic digestion from cat esophagus (Eso) and LES. In Eso, PGF(2alpha)-induced contraction was inhibited by antibodies against the alpha-subunit of G(13) and the monomeric G proteins RhoA and ADP-ribosylation factor (ARF)1 and by the C3 exoenzyme of Clostridium botulinum. A [(35)S]GTPgammaS-binding assay confirmed that G(13), RhoA, and ARF1 were activated by PGF(2alpha). Contraction of Eso was reduced by propranolol, a phospholipase D (PLD) pathway inhibitor and by chelerythrine, a PKC inhibitor. In LES, PGF(2alpha)-induced contraction was inhibited by antibodies against the alpha-subunit of G(q) and G(i3), and a [(35)S]GTPgammaS-binding assay confirmed that G(q) and G(i3) were activated by PGF(2alpha). PGF(2alpha)-induced contraction of LES was reduced by U-73122 and D609 and unaffected by propranolol. At low PGF(2alpha) concentration, contraction was blocked by chelerythrine, whereas at high concentration, contraction was blocked by chelerythrine and CGS9343B. Thus, in Eso, PGF(2alpha) activates a PLD- and protein kinase C (PKC)-dependent pathway through G(13), RhoA, and ARF1. In LES, PGF(2alpha) receptors are coupled to G(q) and G(i3), activating phosphatidylinositol- and phosphatidylcholine-specific phospholipase C. At low concentrations, PGF(2alpha) activates PKC. At high concentration, it activates both a PKC- and a calmodulin-dependent pathway.

Interaction of G alpha(12) with G alpha(13) and G alpha(q) signaling pathways

The G(12) subfamily of heterotrimeric G-proteins consists of two members, G(12) and G(13). Gene-targeting studies have revealed a role for G(13) in blood vessel development. Mice lacking the alpha subunit of G(13) die around embryonic day 10 as the result of an angiogenic defect. On the other hand, the physiological role of G(12) is still unclear. To address this issue, we generated G alpha(12)-deficient mice. In contrast to the G alpha(13)-deficient mice, G alpha(12)-deficient mice are viable, fertile, and do not show apparent abnormalities. However, G alpha(12) does not seem to be entirely redundant, because in the offspring generated from G alpha(12)+/- G alpha(13)+/- intercrosses, at least one intact G alpha(12) allele is required for the survival of animals with only one G alpha(13) allele. In addition, G alpha(12) and G alpha(13) showed a difference in mediating cell migratory response to lysophosphatidic acid in embryonic fibroblast cells. Furthermore, mice lacking both G alpha(12) and G alpha(q) die in utero at about embryonic day 13. These data indicate that the G alpha(12)-mediated signaling pathway functionally interacts not only with the G alpha(13)- but also with the G alpha(q/11)-mediated signaling systems.

Galpha12 and Galpha13 negatively regulate the adhesive functions of cadherin

Cadherins function to promote adhesion between adjacent cells and play critical roles in such cellular processes as development, tissue maintenance, and tumor suppression. We previously demonstrated that heterotrimeric G proteins of the G12 subfamily comprised of Galpha12 and Galpha13 interact with the cytoplasmic domain of cadherins and cause the release of the transcriptional activator beta-catenin (Meigs, T. E., Fields, T. A., McKee, D. D., and Casey, P. J. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 519-524). Because of the importance of beta-catenin in cadherin-mediated cell-cell adhesion, we examined whether G12 subfamily proteins could also regulate cadherin function. The introduction of mutationally activated G12 proteins into K562 cells expressing E-cadherin blocked cadherin-mediated cell adhesion in steady-state assays. Also, in breast cancer cells, the introduction of activated G12 proteins blocked E-cadherin function in a fast aggregation assay. Aggregation mediated by a mutant cadherin that lacks G12 binding ability was not affected by activated G12 proteins, indicating a requirement for direct G12-cadherin interaction. Furthermore, in wound-filling assays in which ectopic expression of E-cadherin inhibits cell migration, the expression of activated G12 proteins reversed the inhibition via a mechanism that was independent of G12-mediated Rho activation. These results validate the G12-cadherin interaction as a potentially important event in cell biology and suggest novel roles for G12 proteins in the regulation of cadherin-mediated developmental events and in the loss of cadherin function that is characteristic of metastatic tumor progression.

Mechanisms of regulation of phospholipase D1 and D2 by the heterotrimeric G proteins G13 and Gq

Our earlier studies of rat brain phospholipase D1 (rPLD1) showed that the enzyme could be activated in cells by alpha subunits of the heterotrimeric G proteins G(13) and G(q). Recently, we showed that rPLD1 is modified by Ser/Thr phosphorylation and palmitoylation. In this study, we first investigated the roles of these post-translational modifications on the activation of rPLD1 by constitutively active Galpha(13)Q226L and Galpha(q)Q209L. Mutations of Cys(240) and Cys(241) of rPLD1, which abolish both post-translational modifications, did not affect the ability of either Galpha(13)Q226L or Galpha(q)Q209L to activate rPLD1. However, the RhoA-insensitive mutants, rPLD1(K946A,K962A) and rPLD1(K962Q), were not activated by Galpha(13)Q226L, although these mutant enzymes responded to phorbol ester and Galpha(q)Q209L. On the contrary, the PKC-insensitive mutant rPLD1(DeltaN168), which lacks the first 168 amino acids of rPLD1, responded to Galpha(13)Q226L but not to Galpha(q)Q209L. In addition, we found that rPLD2 was strongly activated by Galpha(q)Q209L and phorbol ester. However, surprisingly, the enzymatic activity of rPLD2 was suppressed by Galpha(13)Q226L and constitutively active V14RhoA in COS-7 cells. Abolition of the post-translational modifications of rPLD2 did not alter the effects of Galpha(q)Q209L or Galpha(13)Q226L. The suppressive effect of Galpha(13)Q226L on rPLD2 was reversed by dominant negative N19RhoA and the C3 exoenzyme of Clostridium botulinum, further supporting a role for RhoA. In summary, Galpha(13) activation of rPLD1 in COS-7 cells is mediated by Rho, while Galpha(q) activation requires PKC. rPLD2 is activated by Galpha(q), but is inhibited by Galpha(13). Neither Ser/Thr phosphorylation nor palmitoylation is required for these effects.

Distinct regions of the cadherin cytoplasmic domain are essential for functional interaction with Galpha 12 and beta-catenin

Heterotrimeric G proteins of the G(12) subfamily mediate cellular signals leading to events such as cytoskeletal rearrangements, cell proliferation, and oncogenic transformation. Several recent studies have revealed direct effector proteins through which G(12) subfamily members may transmit signals leading to various cellular responses. Our laboratory recently demonstrated that Galpha(12) and Galpha(13) specifically interact with the cytoplasmic domains of several members of the cadherin family of cell adhesion molecules (Meigs, T. E., Fields, T. A., McKee, D. D., and Casey, P. J. (2001) Proc. Natl. Acad. Sci. U. S. A. 98, 519-524). This interaction causes beta-catenin to release from cadherin and relocalize to the cytoplasm and nucleus, where it participates in transcriptional activation. Here we report that two distinct regions of the epithelial cadherin (E-cadherin) tail are required for interaction with beta-catenin and Galpha(12), respectively. Deletion of an acidic, 19-amino acid region of E-cadherin abolishes its ability to bind beta-catenin in vitro, to inhibit beta-catenin-mediated transactivation, or to stabilize beta-catenin; causes subcellular mislocalization of beta-catenin; and disrupts cadherin-mediated cell adhesion. On the other hand, deletion of a distinct 11-amino acid region of E-cadherin dramatically attenuates interaction with Galpha(12); furthermore, Galpha(12) is ineffective in stimulating beta-catenin release from an E-cadherin cytoplasmic domain lacking this putative Galpha(12)-binding region. These findings indicate that Galpha(12) and beta-catenin do not compete for the same binding site on cadherin and provide molecular targets for selectively disrupting the interaction of these proteins with cadherin.

Activation of protein kinase D by signaling through Rho and the alpha subunit of the heterotrimeric G protein G13

Protein kinase D (PKD/PKCmu) immunoprecipitated from COS-7 cells transiently transfected with either a constitutively active mutant of Rho (RhoQ63L) or the Rho-specific guanine nucleotide exchange factor pOnco-Lbc (Lbc) exhibited a marked increase in basal activity. Addition of aluminum fluoride to cells co-transfected with PKD and wild type Galpha(13) also induced PKD activation. Co-transfection of Clostridium botulinum C3 toxin blocked activation of PKD by RhoQ63L, Lbc, or aluminum fluoride-stimulated Galpha(13). Treatment with the protein kinase C inhibitors GF I or Ro 31-8220 prevented the increase in PKD activity induced by RhoQ63L, Lbc, or aluminum fluoride-stimulated Galpha(13). PKD activation in response to Galpha(13) signaling was also completely prevented by mutation of Ser-744 and Ser-748 to Ala in the kinase activation loop of PKD. Co-expression of C. botulinum C3 toxin and a COOH-terminal fragment of Galpha(q) that acts in a dominant-negative fashion blocked PKD activation in response to agonist stimulation of bombesin receptor. Expression of the COOH-terminal region of Galpha(13) also attenuated PKD activation in response to bombesin receptor stimulation. Our results show that Galpha(13) contributes to PKD activation through a Rho- and protein kinase C-dependent signaling pathway and indicate that PKD activation is mediated by both Galpha(q) and Galpha(13) in response to bombesin receptor stimulation.

Vasopressin-stimulated Ca2+ spiking in vascular smooth muscle cells involves phospholipase D

Physiological concentrations of [Arg(8)]vasopressin (AVP; 10-500 pM) stimulate oscillations of cytosolic free Ca2+ concentration (Ca2+ spikes) in A7r5 vascular smooth muscle cells. We previously reported that this effect of AVP was blocked by a putative phospholipase A2 (PLA2) inhibitor, ONO-RS-082 (5 microM). In the present study, the products of PLA2, arachidonic acid (AA), and lysophospholipids were found to be ineffective in stimulating Ca2+ spiking, and inhibitors of AA metabolism did not prevent AVP-stimulated Ca2+ spiking. Thin layer chromatography was used to monitor the release of AA and phosphatidic acid (PA), which are the products of PLA2 and phospholipase D (PLD), respectively. AVP (100 pM) stimulated both AA and PA formation, but only PA formation was inhibited by ONO-RS-082 (5 microM). Exogenous PLD (type VII; 2.5 U/ml) stimulated Ca2+ spiking equivalent to the effect of 100 pM AVP. AVP stimulated transphosphatidylation of 1-butanol (a PLD-catalyzed reaction) but not 2-butanol, and 1-butanol (but not 2-butanol) completely prevented AVP-stimulated Ca2+ spiking. Protein kinase C (PKC) inhibition, which completely prevents AVP-stimulated Ca2+ spiking, did not inhibit AVP-stimulated phosphatidylbutanol formation. These results suggest that AVP-stimulated Ca2+ spiking depends on activation of PLD rather than PLA2 and that PKC activation may be downstream of PLD in the signaling cascade.

Transforming G proteins

Heterotrimeric guanine nucleotide binding proteins, commonly known as G proteins form a super-family of signal transduction proteins. They are peripherally associated with the plasma membrane and provide signal coupling to seven transmembrane surface receptors. G proteins are composed of monomers of alpha, beta, and gamma subunits. The beta- and gamma-subunits are tightly associated. The receptors activated by the appropriate "signal", interact catalytically with specific G-proteins to mediate guanine nucleotide exchange at the GDP/GTP binding site of the G-protein alpha-subunits, thus displacing the bound GDP for GTP. The GTP bound form of the g-protein alpha-subunit and in some cases the free betagamma-subunits initiate cellular response by altering the activity of specific effector molecules. Recent studies have indicated that the asyncronous activation of these proteins can lead to the oncogenic transformation of different cell types. The mechanism by which G-proteins regulate the various cell functions appear to involve a complex net-working between different signaling pathways. This review summarizes the signaling mechanisms involved in the regulation of cell proliferation by these transforming G proteins.

Enhancement of lysophosphatidic acid-induced ERK phosphorylation by phospholipase D1 via the formation of phosphatidic acid

We made stable cell lines overexpressing PLD1 (GP-PLD1) from GP+envAm12 cell, a derivative of NIH 3T3 cell. PLD1 activity and extracellular signal-regulated kinase (ERK) phosphorylation were enhanced in GP-PLD1 cells by the treatment of lysophosphatidic acid (LPA). In contrast, these LPA-induced effects were attenuated with the pretreatment of pertussis toxin (PTX) or protein kinase C (PKC) inhibitor. Moreover, accumulation of phosphatidic acid (PA), a product of PLD action, potentiated the LPA-induced ERK activation in GP-PLD1 cells while blocking of PA production with the treatment of 1-butanol attenuated LPA-induced ERK phosphorylation. From these results, we suggest that LPA activate PLD1 through pertussis toxin-sensitive G protein and PKC-dependent pathways, then PA produced from PLD1 activation facilitate ERK phosphorylation.

Cloning and direct G-protein regulation of phospholipase D from tobacco

Phospholipase D (PLD) and heterotrimeric G-proteins are involved in plant signal transduction pathways at the plasma membrane. There is evidence suggesting that PLD acts downstream from G-proteins, but a direct interaction of specific members has not been shown. In the present paper, a PLD cDNA clone was isolated from tobacco, expressed as a GST fusion in bacteria, and the recombinant protein was purified by glutathione affinity. Its enzymatic properties identified it as an alpha-type PLD. The alpha-subunit of a G-protein from tobacco was isolated in a similar way. Both proteins were functional in biochemical assays. When the G-protein was included in the PLD assay, a strong dosage-dependent inhibition of the PLD activity was observed. Different control proteins did not exhibit this inhibitory effect. When GST-NtGPalpha1 was activated by incubation with GTPgammaS the inhibitory activity was greatly reduced. These results provide a first indication for a direct regulation of PLDalpha by a heterotrimeric G-protein alpha-subunit in plants.

The M3 muscarinic acetylcholine receptor expressed in HEK-293 cells signals to phospholipase D via G12 but not Gq-type G proteins: regulators of G proteins as tools to dissect pertussis toxin-resistant G proteins in receptor-effector coupling

The M(3) muscarinic acetylcholine receptor (mAChR) expressed in HEK-293 cells couples to G(q) and G(12) proteins and stimulates phospholipase C (PLC) and phospholipase D (PLD) in a pertussis toxin-insensitive manner. To determine the type of G protein mediating M(3) mAChR-PLD coupling in comparison to M(3) mAChR-PLC coupling, we expressed various Galpha proteins and regulators of the G protein signaling (RGS), which act as GTPase-activating proteins for G(q)- or G(12)-type G proteins. PLD stimulation by the M(3) mAChR was enhanced by the overexpression of Galpha(12) and Galpha(13), whereas the overexpression of Galpha(q) strongly increased PLC activity without affecting PLD activity. Expression of the RGS homology domain of Lsc, which acts specifically on Galpha(12) and Galpha(13), blunted the M(3) mAChR-induced PLD stimulation without affecting PLC stimulation. On the other hand, overexpression of RGS4, which acts on Galpha(q)- but not Galpha(12)-type G proteins, suppressed the M(3) mAChR-induced PLC stimulation without altering PLD stimulation. We conclude that the M(3) mAChR in HEK-293 cells apparently signals to PLD via G(12)- but not G(q)-type G proteins and that G protein subtype-selective RGS proteins can be used as powerful tools to dissect the pertussis toxin-resistant G proteins and their role in receptor-effector coupling.

Interaction of the type Ialpha PIPkinase with phospholipase D: a role for the local generation of phosphatidylinositol 4, 5-bisphosphate in the regulation of PLD2 activity

Phosphoinositides are localized in various intracellular compartments and can regulate a number of intracellular functions, such as cytoskeletal dynamics and membrane trafficking. Phospholipase Ds (PLDs) are regulated enzymes that hydrolyse phosphatidylcholine (PtdCho) to generate the putative second messenger phosphatidic acid (PtdOH). In vitro, PLDs have an absolute requirement for higher phosphorylated inositides, such as phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)]. Whether this lipid is able to regulate the activity of PLD in vivo is contentious. To examine this hypothesis we studied the relationship between PLD and an enzyme critical for the intracellular synthesis of PtdIns(4,5)P(2): phosphatidylinositol 4-phosphate 5-kinase alpha (Type Ialpha PIPkinase). We find that both PLD1 and PLD2 interact with the Type Ialpha PIPkinase and that PLD2 activity in vivo can be regulated solely by the expression of this lipid kinase. Moreover, PLD2 is able to recruit the Type Ialpha PIPkinase to its intracellular location. We show that the physiological requirement of PLD enzymes for PtdIns(4,5)P(2) is critical and that PLD2 activity can be regulated solely by the levels of this key intracellular lipid.

[D-Arg(1),D-Trp(5,7,9),Leu(11)]Substance P inhibits bombesin-induced mitogenic signal transduction mediated by both G(q) and G(12) in Swiss 3T3cells

Substance P (SP) analogues including [d-Arg(1),d-Trp(5,7,9), Leu(11)]SP are broad spectrum neuropeptide antagonists and potential anticancer agents, but their mechanism of action is not fully understood. Here, we examined the mechanism of action of [d-Arg(1), d-Trp(5,7,9),Leu(11)]SP as an inhibitor of G protein-coupled receptor (GPCR)-mediated signal transduction and cellular DNA synthesis in Swiss 3T3 cells. Addition of [d-Arg(1),d-Trp(5,7,9), Leu(11)]SP, at 10 micrometer, caused a striking rightward shift in the dose-response curves of DNA synthesis induced by bombesin, bradykinin, or vasopressin and markedly inhibited the activation of p42(mapk) (ERK-2) and p44(mapk) (ERK-1) induced by these GPCR agonists. In addition, this SP analogue also prevented the protein kinase C-dependent activation of protein kinase D induced by these agonists. [d-Arg(1),d-Trp(5,7,9),Leu(11)]SP, at a concentration (10 micrometer) that inhibited these G(q)-mediated events, also prevented GPCR agonist-induced responses mediated through the G proteins of the G(12) subfamily. These include bombesin-induced assembly of focal adhesions, formation of parallel arrays of actin stress fibers, increase in the tyrosine phosphorylation of focal adhesion kinase (FAK), p130(Cas), and paxillin, and formation of a complex between FAK and Src. We conclude that [d-Arg(1),d-Trp(5,7,9),Leu(11)]SP acts as a mitogenic antagonist of neuropeptide GPCRs blocking signal transduction via both G(q) and G(12).

The role of Rho in G protein-coupled receptor signal transduction

Low molecular weight G proteins of the Rho subfamily are regulators of actin cytoskeletal organization. In contrast to the heterotrimeric G proteins, the small GTPases are not directly activated through ligand binding to G protein-coupled receptors (GPCRs). However, a subset of GPCRs, including those for lysophosphatidic acid and thrombin, induce stress fibers, focal adhesions, and cell rounding through Rho-dependent pathways. C3 exoenzyme has been a useful tool for demonstrating Rho involvement in these and other responses, including Ca2+ sensitization of smooth muscle contraction, cell migration, transformation, and serum response element-mediated gene expression. Most of the GPCRs that induce Rho-dependent responses can activate Gq, but this is not a sufficient signal. Recent data demonstrate that G alpha 12/13 can induce Rho-dependent responses. Furthermore, G alpha 12/13 can bind and activate Rho-specific guanine nucleotide exchange factors, providing a mechanism by which GPCRs that couple to G alpha 12/13 could activate Rho and its downstream responses.

Association of the N- and C-terminal domains of phospholipase D. Contribution of the conserved HKD motifs to the interaction and the requirement of the association for Ser/Thr phosphorylation of the enzyme

Rat brain phospholipase D1 (rPLD1) belongs to a superfamily defined by the highly conserved catalytic motif (H(X)K(X)(4)D, denoted HKD. rPLD1 contains two HKD domains, located in the N- and C-terminal regions. The integrity of the two HKD domains is essential for enzymatic activity. Our previous studies showed that the N-terminal half of rPLD1 containing one HKD motif can associate with the C-terminal half containing the other HKD domain to reconstruct wild type PLD activity (Xie, Z., Ho, W.-T. and Exton, J. H. (1998) J. Biol. Chem. 273, 34679-34682). In the present study, we have shown by mutagenesis that conserved amino acids in the HKD domains are important for both the catalytic activity and the association between the two halves of rPLD1. Furthermore, we found that rPLD1 could be modified by Ser/Thr phosphorylation. The modification occurred at the N-terminal half of the enzyme, however, the association of the N-terminal domain with the C-terminal domain was required for the modification. The phosphorylation of the enzyme was not required for its catalytic activity or response to PKCalpha and small G proteins in vitro, although the phosphorylated form of rPLD1 was localized exclusively in the crude membrane fraction. In addition, we found that the individually expressed N- and C-terminal fragments did not interact when mixed in vitro and were unable to reconstruct PLD activity under these conditions. It is concluded that the association of the N- and C-terminal halves of rPLD1 requires their co-expression in vivo and depends on conserved residues in the HKD domains. The association is also required for Ser/Thr phosphorylation of the enzyme.

Galpha(12) and galpha(13) inhibit Ca(2+)-dependent exocytosis through Rho/Rho-associated kinase-dependent pathway

The release of neurotransmitters is known to be regulated by activation of heterotrimeric G protein-coupled receptors, although precise mechanisms have not yet been elucidated. To assess the role of the G(12) family of heterotrimeric G proteins in the regulation of neurotransmitter release, we established PC12 cell lines that expressed constitutively active Galpha(12) or Galpha(13) using an isopropyl-beta-D-thiogalactoside-inducible expression system. In the cells, expression of constitutively active Galpha(12) or Galpha(13) inhibited the high K(+)-evoked [(3)H]dopamine release without any effect on the high K(+)-induced increase in intracellular Ca(2+) concentration. A Ca(2+) ionophore ionomycin-induced [(3)H]dopamine release was also inhibited by the expression of active Galpha(12) or Galpha(13). These inhibitory effects of Galpha(12) and Galpha(13) on [(3)H]dopamine release were mimicked by the expression of constitutively active RhoA. In addition, Y-27632, and inhibitor of Rho-associated kinase, a downstream Rho effector, completely abolished the inhibition of [(3)H]dopamine release by Galpha(12), Galpha(13), and RhoA. These results indicate that Ca(2+)-dependent exocytosis is regulated by Galpha(12) and Galpha(13) through a Rho/Rho-associated kinase-dependent pathway.

Activation of phospholipase D1 by Cdc42 requires the Rho insert region

Members of the Rho subfamily of GTP-binding proteins are implicated in the regulation of phospholipase D (PLD). In the present study, we demonstrate a physical association between a Rho family member, Cdc42, and PLD1. Binding of Cdc42 to PLD1 and subsequent activation are GTP-dependent. Although binding of Cdc42 to PLD1 does not require geranylgeranylation, activation of PLD1 is dependent on this lipid modification of Cdc42. Specific point mutations in the switch I region of Cdc42 abolish binding to and, therefore, activation of PLD1 by Cdc42. Deletion of the Rho insert region, which consists of residues 120-139, from Cdc42 does not interfere with binding to PLD1 but inhibits Cdc42 stimulated PLD1 activity. Interestingly, deletion of the insert region from Cdc42 also inhibits activation of PLD1 by Arf and protein kinase C. With the lack of specific inhibitors of PLD activity, the insert deletion mutant of Cdc42 (designated (DeltaL8)Cdc42) is a novel reagent for in vitro studies of PLD1 regulation, as well as for in vivo studies of Cdc42-mediated signaling pathways leading to PLD1 activation. Because the insert region is required for the transforming activity of Cdc42, regulation of PLD1 by this region on Cdc42 is of major interest.