Regulation and physiological functions of G12/13-mediated signaling pathways

Gastrin stimulates pancreatic cancer cell directional migration by activating the Gα12/13-RhoA-ROCK signaling pathway

The mechanism by which gastrin promotes pancreatic cancer cell metastasis is unclear. The process of directing polarized cancer cells toward the extracellular matrix is principally required for invasion and distant metastasis; however, whether gastrin can induce this process and its underlying mechanism remain to be elucidated. In this study, we found that gastrin-induced phosphorylation of paxillin at tyrosine 31/118 and RhoA activation as well as promoted the metastasis of PANC-1 cancer cells. Depletion of Gα12 and Gα13 inhibited the phosphorylation of paxillin and downstream activation of GTP-RhoA, blocked the formation and aggregation of focal adhesions and facilitated polarization of actin filaments induced by gastrin. Suppression of RhoA and ROCK also exhibited identical results. Selective inhibition of the CCKBR–Gα12/13–RhoA–ROCK signaling pathway blocked the reoriented localization of the Golgi apparatus at the leading edge of migrated cancer cells. YM022 and Y-27632 significantly suppressed hepatic metastasis of orthotic pancreatic tumors induced by gastrin in vivo. Collectively, we demonstrate that gastrin promotes Golgi reorientation and directional polarization of pancreatic cancer cells by activation of paxillin via the CCKBR–Gα12/13–RhoA–ROCK signal pathway.

A hormone found in high levels in pancreatic cancer sufferers helps the disease spread by co-ordinating cellular migration. Pancreatic cancer is one of the most deadly forms of cancer, being highly aggressive and likely to metastasize. Honggang Yu at Renmin Hospital of Wuhan University and scientists across China have demonstrated that gastrin, a hormone expressed at higher levels in patients with pancreatic cancer, helps to co-ordinate directional cell migration and ensure the disease spreads effectively. By activating two key molecules via a specific signalling pathway, gastrin ensures the correct orientation of the Golgi apparatus, a cellular organelle tasked with packaging proteins for transportation. This in turn activates directional migration of the cancer cells. The results explain why gastrin is over-expressed in both tumors and blood in cancer patients, and may inform future therapies.

Orexin/Hypocretin Signaling

Orexin/hypocretin peptide (orexin-A and orexin-B) signaling is believed to take place via the two G-protein-coupled receptors (GPCRs), named OX1 and OX2 orexin receptors, as described in the previous chapters. Signaling of orexin peptides has been investigated in diverse endogenously orexin receptor-expressing cells - mainly neurons but also other types of cells - and in recombinant cells expressing the receptors in a heterologous manner. Findings in the different systems are partially convergent but also indicate cellular background-specific signaling. The general picture suggests an inherently high degree of diversity in orexin receptor signaling.In the current chapter, I present orexin signaling on the cellular and molecular levels. Discussion of the connection to (potential) physiological orexin responses is only brief since these are in focus of other chapters in this book. The same goes for the post-synaptic signaling mechanisms, which are dealt with in Burdakov: Postsynaptic actions of orexin. The current chapter is organized according to the tissue type, starting from the central nervous system. Finally, receptor signaling pathways are discussed across tissues, cell types, and even species.

Role of G Protein-Coupled Receptors in the Regulation of Structural Plasticity and Cognitive Function

Cognition and other higher brain functions are known to be intricately associated with the capacity of neural circuits to undergo structural reorganization. Structural remodelling of neural circuits, or structural plasticity, in the hippocampus plays a major role in learning and memory. Dynamic modifications of neuronal connectivity in the form of dendritic spine morphology alteration, as well as synapse formation and elimination, often result in the strengthening or weakening of specific neural circuits that determine synaptic plasticity. Changes in dendritic complexity and synapse number are mediated by cellular processes that are regulated by extracellular signals such as neurotransmitters and neurotrophic factors. As many neurotransmitters act on G protein-coupled receptors (GPCRs), it has become increasingly apparent that GPCRs can regulate structural plasticity through a myriad of G protein-dependent pathways and non-canonical signals. A thorough understanding of how GPCRs exert their regulatory influence on dendritic spine morphogenesis may provide new insights for treating cognitive impairment and decline in various age-related diseases. In this article, we review the evidence of GPCR-mediated regulation of structural plasticity, with a special emphasis on the involvement of common as well as distinct signalling pathways that are regulated by major neurotransmitters.

Acute and chronic role of nitric oxide, renin-angiotensin system and sympathetic nervous system in the modulation of calcium sensitization in Wistar rats

Principal vasoactive systems - renin-angiotensin system (RAS), sympathetic nervous system (SNS), nitric oxide (NO) and prostanoids - exert their vascular effects through the changes in calcium levels and/or calcium sensitization. To estimate a possible modulation of calcium sensitization by the above vasoactive systems, we studied the influence of acute and chronic blockade of particular vasoactive systems on blood pressure (BP) changes elicited in conscious normotensive rats by acute dose-dependent administration of Rho-kinase inhibitor fasudil. Adult male chronically cannulated Wistar rats were used throughout this study. The acute inhibition of NO synthase (NOS) by L-NAME enhanced BP response to fasudil, the effect being considerably augmented in rats deprived of endogenous SNS. The acute inhibition of prostanoid synthesis by indomethacin modified BP response to fasudil less than the acute NOS inhibition. The chronic NOS inhibition caused moderate BP elevation and a more pronounced augmentation of fasudil-induced BP changes compared to the effect of acute NOS inhibition. This indicates both short-term and long-term NO-dependent attenuation of calcium sensitization. Long-term inhibition of RAS by captopril caused a significant attenuation of BP changes elicited by fasudil. In contrast, a long-term attenuation of SNS by chronic guanethidine treatment (in youth or adulthood) had no effect on BP response to fasudil, suggesting the absence of SNS does not affect calcium sensitization in vascular smooth muscle of normotensive rats. In conclusion, renin-angiotensin system contributes to the long-term increase of calcium sensitization and its effect is counterbalanced by nitric oxide which decreases calcium sensitization in Wistar rats.

G protein-coupled receptors Flop1 and Flop2 inhibit Wnt/β-catenin signaling and are essential for head formation in Xenopus

Patterning of the vertebrate anterior-posterior axis is regulated by the coordinated action of growth factors whose effects can be further modulated by upstream and downstream mediators and the cross-talk of different intracellular pathways. In particular, the inhibition of the Wnt/β-catenin signaling pathway by various factors is critically required for anterior specification. Here, we report that Flop1 and Flop2 (Flop1/2), G protein-coupled receptors related to Gpr4, contribute to the regulation of head formation by inhibiting Wnt/β-catenin signaling in Xenopus embryos. Using whole-mount in situ hybridization, we showed that flop1 and flop2 mRNAs were expressed in the neural ectoderm during early gastrulation. Both the overexpression and knockdown of Flop1/2 resulted in altered embryonic head phenotypes, while the overexpression of either Flop1/2 or the small GTPase RhoA in the absence of bone morphogenetic protein (BMP) signaling resulted in ectopic head induction. Examination of the Flops' function in Xenopus embryo animal cap cells showed that they inhibited Wnt/β-catenin signaling by promoting β-catenin degradation through both RhoA-dependent and -independent pathways in a cell-autonomous manner. These results suggest that Flop1 and Flop2 are essential regulators of Xenopus head formation that act as novel inhibitory components of the Wnt/β-catenin signaling pathway.

PLCε mediated sustained signaling pathways

Phospholipase C-ε (PLCε) integrates signaling from G-protein coupled receptors (GPCRs) to downstream kinases to regulate a broad range of biological and pathophysiological responses. Relative to other PLCs, PLCε is unique in that it not only serves a catalytic function in phosphoinositide hydrolysis but also functions as an exchange factor small the low molecular weight G-protein Rap1. PLCε is selectively stimulated by agonists for GPCRs that couple to RhoA, which bind directly to the enzyme to regulate its activity. Rap1 also regulates PLCε activity by binding to its RA2 domain and this generates a feedback mechanism allowing sustained signaling. As a result of its regulation by inflammatory ligands for GPCRs and its ability to promote chronic signals, PLCε has been implicated in diseases ranging from cancer to ischemia/reperfusion injury. This review will discuss the regulation of PLCε, molecular mechanisms that contribute to sustained signaling, and the role of the enzyme in various disease contexts.

The Fog signaling pathway: insights into signaling in morphogenesis

Epithelia form the building blocks of many tissue and organ types. Epithelial cells often form a contiguous 2-dimensional sheet that is held together by strong adhesions. The mechanical properties conferred by these adhesions allow the cells to undergo dramatic three-dimensional morphogenetic movements while maintaining cell–cell contacts during embryogenesis and post-embryonic development. The Drosophila Folded gastrulation pathway triggers epithelial cell shape changes that drive gastrulation and tissue folding and is one of the most extensively studied examples of epithelial morphogenesis. This pathway has yielded key insights into the signaling mechanisms and cellular machinery involved in epithelial remodeling. In this review, we discuss principles of morphogenesis and signaling that have been discovered through genetic and cell biological examination of this pathway. We also consider various regulatory mechanisms and the system's relevance to mammalian development. We propose future directions that will continue to broaden our knowledge of morphogenesis across taxa.

Rho kinases in cardiovascular physiology and pathophysiology: the effect of fasudil

Rho kinase (ROCK) is a major downstream effector of the small GTPase RhoA. ROCK family, consisting of ROCK1 and ROCK2, plays central roles in the organization of actin cytoskeleton and is involved in a wide range of fundamental cellular functions, such as contraction, adhesion, migration, proliferation, and apoptosis. Due to the discovery of effective inhibitors, such as fasudil and Y27632, the biological roles of ROCK have been extensively explored with particular attention on the cardiovascular system. In many preclinical models of cardiovascular diseases, including vasospasm, arteriosclerosis, hypertension, pulmonary hypertension, stroke, ischemia-reperfusion injury, and heart failure, ROCK inhibitors have shown a remarkable efficacy in reducing vascular smooth muscle cell hypercontraction, endothelial dysfunction, inflammatory cell recruitment, vascular remodeling, and cardiac remodeling. Moreover, fasudil has been used in the clinical trials of several cardiovascular diseases. The continuing utilization of available pharmacological inhibitors and the development of more potent or isoform-selective inhibitors in ROCK signaling research and in treating human diseases are escalating. In this review, we discuss the recent molecular, cellular, animal, and clinical studies with a focus on the current understanding of ROCK signaling in cardiovascular physiology and diseases. We particularly note that emerging evidence suggests that selective targeting ROCK isoform based on the disease pathophysiology may represent a novel therapeutic approach for the disease treatment including cardiovascular diseases.

Gα12 structural determinants of Hsp90 interaction are necessary for serum response element-mediated transcriptional activation

The G12/13 class of heterotrimeric G proteins, comprising the α-subunits Gα12 and Gα13, regulates multiple aspects of cellular behavior, including proliferation and cytoskeletal rearrangements. Although guanine nucleotide exchange factors for the monomeric G protein Rho (RhoGEFs) are well characterized as effectors of this G protein class, a variety of other downstream targets has been reported. To identify Gα12 determinants that mediate specific protein interactions, we used a structural and evolutionary comparison between the G12/13, Gs, Gi, and Gq classes to identify "class-distinctive" residues in Gα12 and Gα13. Mutation of these residues in Gα12 to their deduced ancestral forms revealed a subset necessary for activation of serum response element (SRE)-mediated transcription, a G12/13-stimulated pathway implicated in cell proliferative signaling. Unexpectedly, this subset of Gα12 mutants showed impaired binding to heat-shock protein 90 (Hsp90) while retaining binding to RhoGEFs. Corresponding mutants of Gα13 exhibited robust SRE activation, suggesting a Gα12-specific mechanism, and inhibition of Hsp90 by geldanamycin or small interfering RNA-mediated lowering of Hsp90 levels resulted in greater downregulation of Gα12 than Gα13 signaling in SRE activation experiments. Furthermore, the Drosophila G12/13 homolog Concertina was unable to signal to SRE in mammalian cells, and Gα12:Concertina chimeras revealed Gα12-specific determinants of SRE activation within the switch regions and a C-terminal region. These findings identify Gα12 determinants of SRE activation, implicate Gα12:Hsp90 interaction in this signaling mechanism, and illuminate structural features that arose during evolution of Gα12 and Gα13 to allow bifurcated mechanisms of signaling to a common cell proliferative pathway.

Recent progress in understanding subtype specific regulation of NMDA receptors by G Protein Coupled Receptors (GPCRs)

G Protein Coupled Receptors (GPCRs) are the largest family of receptors whose ligands constitute nearly a third of prescription drugs in the market. They are widely involved in diverse physiological functions including learning and memory. NMDA receptors (NMDARs), which belong to the ionotropic glutamate receptor family, are likewise ubiquitously expressed in the central nervous system (CNS) and play a pivotal role in learning and memory. Despite its critical contribution to physiological and pathophysiological processes, few pharmacological interventions aimed directly at regulating NMDAR function have been developed to date. However, it is well established that NMDAR function is precisely regulated by cellular signalling cascades recruited downstream of G protein coupled receptor (GPCR) stimulation. Accordingly, the downstream regulation of NMDARs likely represents an important determinant of outcome following treatment with neuropsychiatric agents that target selected GPCRs. Importantly, the functional consequence of such regulation on NMDAR function varies, based not only on the identity of the GPCR, but also on the cell type in which relevant receptors are expressed. Indeed, the mechanisms responsible for regulating NMDARs by GPCRs involve numerous intracellular signalling molecules and regulatory proteins that vary from one cell type to another. In the present article, we highlight recent findings from studies that have uncovered novel mechanisms by which selected GPCRs regulate NMDAR function and consequently NMDAR-dependent plasticity.

Tubulin, actin and heterotrimeric G proteins: coordination of signaling and structure

G proteins mediate signals from membrane G protein coupled receptors to the cell interior, evoking significant regulation of cell physiology. The cytoskeleton contributes to cell morphology, motility, division, and transport functions. This review will discuss the interplay between heterotrimeric G protein signaling and elements of the cytoskeleton. Also described and discussed will be the interplay between tubulin and G proteins that results in atypical modulation of signaling pathways and cytoskeletal dynamics. This will be extended to describe how tubulin and G proteins act in concert to influence various aspects of cellular behavior. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters.This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

PDZ-RhoGEF is essential for CXCR4-driven breast tumor cell motility through spatial regulation of RhoA

The CXCL12-CXCR4 chemokine signaling pathway is a well-established driver of cancer progression. One key process promoted by CXCR4 stimulation is tumor cell motility; however, the specific signaling pathways leading to migration remain poorly understood. Previously, we have shown that CXCL12 stimulation of migration depends on temporal regulation of RhoA. However, the specific RhoGEF that translates CXCR4 signaling into RhoA activity and cell motility is unknown. We screened the three regulator of G-protein signaling RhoGEFs (LSC, LARG and PRG) and found that PRG selectively regulated the migration and invasion of CXCR4-overexpressing breast tumor cells. Interestingly, we found that PDZ-RhoGEF (PRG) was required for spatial organization of F-actin structures in the center, but not periphery of the cells. The effects on the cytoskeleton were mirrored by the spatial effects on RhoA activity that were dependent upon PRG. Loss of PRG also enhanced adherens junctions in the epithelial-like MCF7-CXCR4 cell line, and inhibited directional persistence and polarity in the more mesenchymal MDA-MB-231 cell line. Thus, PRG is essential for CXCR4-driven tumor cell migration through spatial regulation of RhoA and the subsequent organization of the cytoskeletal structures that support motility. Furthermore, immunohistochemical analysis of human breast tumor tissues shows a significant increase of PRG expression in the invasive areas of the tumors, suggesting that this RhoGEF is associated with breast tumor invasion in vivo.

Determinants at the N- and C-termini of Gα12 required for activation of Rho-mediated signaling

BACKGROUND

Heterotrimeric guanine nucleotide binding proteins of the G12/13 subfamily, which includes the α-subunits Gα12 and Gα13, stimulate the monomeric G protein RhoA through interaction with a distinct subset of Rho-specific guanine nucleotide exchange factors (RhoGEFs). The structural features that mediate interaction between Gα13 and RhoGEFs have been examined in crystallographic studies of the purified complex, whereas a Gα12:RhoGEF complex has not been reported. Several signaling responses and effector interactions appear unique to Gα12 or Gα13, despite their similarity in amino acid sequence.

METHODS

To comprehensively examine Gα12 for regions involved in RhoGEF interaction, we screened a panel of Gα12 cassette substitution mutants for binding to leukemia-associated RhoGEF (LARG) and for activation of serum response element mediated transcription.

RESULTS

We identified several cassette substitutions that disrupt Gα12 binding to LARG and the related p115RhoGEF. These Gα12 mutants also were impaired in activating serum response element mediated signaling, a Rho-dependent response. Most of these mutants matched corresponding regions of Gα13 reported to contact p115RhoGEF, but unexpectedly, several RhoGEF-uncoupling mutations were found within the N- and C-terminal regions of Gα12. Trypsin protection assays revealed several mutants in these regions as retaining conformational activation. In addition, charge substitutions near the Gα12 N-terminus selectively disrupted binding to LARG but not p115RhoGEF.

CONCLUSIONS

Several structural aspects of the Gα12:RhoGEF interface differ from the reported Gα13:RhoGEF complex, particularly determinants within the C-terminal α5 helix and structurally uncharacterized N-terminus of Gα12. Furthermore, key residues at the Gα12 N-terminus may confer selectivity for LARG as a downstream effector.

G protein-coupled receptors engage the mammalian Hippo pathway through F-actin: F-Actin, assembled in response to Galpha12/13 induced RhoA-GTP, promotes dephosphorylation and activation of the YAP oncogene

The Hippo pathway, a cascade of protein kinases that inhibits the oncogenic transcriptional coactivators YAP and TAZ, was discovered in Drosophila as a major determinant of organ size in development. Known modes of regulation involve surface proteins that mediate cell-cell contact or determine epithelial cell polarity which, in a tissue-specific manner, use intracellular complexes containing FERM domain and actin-binding proteins to modulate the kinase activities or directly sequester YAP. Unexpectedly, recent work demonstrates that GPCRs, especially those signaling through Galpha12/13 such as the protease activated receptor PAR1, cause potent YAP dephosphorylation and activation. This response requires active RhoA GTPase and increased assembly of filamentous (F-)actin. Morever, cell architectures that promote F-actin assembly per se also activate YAP by kinase-dependent and independent mechanisms. These findings unveil the ability of GPCRs to activate the YAP oncogene through a newly recognized signaling function of the actin cytoskeleton, likely to be especially important for normal and cancerous stem cells.

Expression of Gα12 predicts progression and prognosis of oral squamous cell carcinomas in Taiwan

BACKGROUND

Expression of Gα12 is found to be associated with cancer cell proliferation, migration, invasion, and metastasis.

METHODS

This study used immunohistochemistry to examine the expression of Gα12 protein in 100 specimens of oral squamous cell carcinoma (OSCC), 45 specimens of oral epithelial dysplasia (OED), and 36 specimens of normal oral mucosa (NOM).

RESULTS

The mean Gα12 labeling indices (LIs, defined as the percentage of positive cells in total cells) increased significantly from NOM (7 ± 11%) through OED (21 ± 20%) to OSCC samples (53 ± 33%, P < 0.001). The higher mean Gα12 LI was significantly associated with OSCCs with larger tumor size (P = 0.003), positive lymph node metastasis (P = 0.002), or more advanced clinical stages (P = 0.003). Positive lymph node metastasis (P = 0.039) and Gα12 LI > 50% (P = 0.009) were identified as independent unfavorable prognosis factors by multivariate analyses with Cox regression model. Moreover, Kaplan-Meier curve showed that OSCC patients with a Gα12 LI > 50% had a significantly poorer cumulative survival than those with a Gα12 LI ≤ 50% (log-rank test, P = 0.009).

CONCLUSIONS

Our results showed a stepwise and significant elevation in Gα12 protein expression from NOM through OED to OSCCs, suggesting that overexpression of Gα12 protein may be an early event in oral carcinogenesis and may play a pivotal role in oral cancer development. Moreover, the Gα12 protein can be a biomarker for prediction of the progression of OSCCs and the prognosis of patients with OSCC in Taiwan.

S1pr2/Gα13 signaling controls myocardial migration by regulating endoderm convergence

A key process during vertebrate heart development is the migration of bilateral populations of myocardial precursors towards the midline to form the primitive heart tube. In zebrafish, signaling mediated by sphingosine-1-phosphate (S1P) and its cognate G protein-coupled receptor (S1pr2/Mil) is essential for myocardial migration, but the underlying mechanisms remain undefined. Here, we show that suppression of Gα(13) signaling disrupts myocardial migration, leading to the formation of two bilaterally located hearts (cardia bifida). Genetic studies indicate that Gα(13) acts downstream of S1pr2 to regulate myocardial migration through a RhoGEF-dependent pathway. Furthermore, disrupting any component of the S1pr2/Gα(13)/RhoGEF pathway impairs endoderm convergence during segmentation, and the endodermal defects correlate with the extent of cardia bifida. Moreover, endoderm transplantation reveals that the presence of wild-type anterior endodermal cells in Gα(13)-deficient embryos is sufficient to rescue the endoderm convergence defect and cardia bifida, and, conversely, that the presence of anterior endodermal cells defective for S1pr2 or Gα(13) in wild-type embryos causes such defects. Thus, S1pr2/Gα(13) signaling probably acts in the endoderm to regulate myocardial migration. In support of this notion, cardiac-specific expression of Gα(13) fails to rescue cardia bifida in the context of global Gα(13) inhibition. Our data demonstrate for the first time that the Gα(13)/RhoGEF-dependent pathway functions downstream of S1pr2 to regulate convergent movement of the endoderm, an event that is crucial for coordinating myocardial migration.

Fine-tuning of GPCR signals by intracellular G protein modulators

Heterotrimeric G proteins convey receptor signals to intracellular effectors. Superimposed over the basic GPCR-G protein-effector scheme are three types of auxiliary proteins that also modulate Gα. Regulator of G protein signaling proteins and G protein signaling modifier proteins respectively promote GTPase activity and hinder GTP/GDP exchange to limit Gα activation. There are also diverse proteins that, like GPCRs, can promote nucleotide exchange and thus activation. Here we review the impact of these auxiliary proteins on GPCR signaling. Although their precise physiological functions are not yet clear, all of them can produce significant effects in experimental systems. These signaling changes are generally consistent with established effects on isolated Gα; however, the activation state of Gα is seldom verified and many such changes appear also to reflect the physical disruption of or indirect effects on interactions between Gα and its associated GPCR, Gβγ, and/or effector.

Modification of heterotrimeric G-proteins in Swiss 3T3 cells stimulated with Pasteurella multocida toxin

Many bacterial toxins covalently modify components of eukaryotic signalling pathways in a highly specific manner, and can be used as powerful tools to decipher the function of their molecular target(s). The Pasteurella multocida toxin (PMT) mediates its cellular effects through the activation of members of three of the four heterotrimeric G-protein families, G(q), G(12) and G(i). PMT has been shown by others to lead to the deamidation of recombinant Gα(i) at Gln-205 to inhibit its intrinsic GTPase activity. We have investigated modification of native Gα subunits mediated by PMT in Swiss 3T3 cells using 2-D gel electrophoresis and antibody detection. An acidic change in the isoelectric point was observed for the Gα subunit of the G(q) and G(i) families following PMT treatment of Swiss 3T3 cells, which is consistent with the deamidation of these Gα subunits. Surprisingly, PMT also induced a similar modification of Gα(11), a member of the G(q) family of G-proteins that is not activated by PMT. Furthermore, an alkaline change in the isoelectric point of Gα(13) was observed following PMT treatment of cells, suggesting differential modification of this Gα subunit by PMT. G(s) was not affected by PMT treatment. Prolonged treatment with PMT led to a reduction in membrane-associated Gα(i), but not Gα(q). We also show that PMT inhibits the GTPase activity of G(q).

Lysophospholipid receptor activation of RhoA and lipid signaling pathways

The lysophospholipids sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) signal through G-protein coupled receptors (GPCRs) which couple to multiple G-proteins and their effectors. These GPCRs are quite efficacious in coupling to the Gα(12/13) family of G-proteins, which stimulate guanine nucleotide exchange factors (GEFs) for RhoA. Activated RhoA subsequently regulates downstream enzymes that transduce signals which affect the actin cytoskeleton, gene expression, cell proliferation and cell survival. Remarkably many of the enzymes regulated downstream of RhoA either use phospholipids as substrates (e.g. phospholipase D, phospholipase C-epsilon, PTEN, PI3 kinase) or are regulated by phospholipid products (e.g. protein kinase D, Akt). Thus lysophospholipids signal from outside of the cell and control phospholipid signaling processes within the cell that they target. Here we review evidence suggesting an integrative role for RhoA in responding to lysophospholipids upregulated in the pathophysiological environment, and in transducing this signal to cellular responses through effects on phospholipid regulatory or phospholipid regulated enzymes. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

Rho kinase as a therapeutic target in cardiovascular disease

Rho kinase (ROCK) belongs to the AGC (PKA/PKG/PKC) family of serine/threonine kinases and is a major downstream effector of the small GTPase RhoA. ROCK plays central roles in the organization of the actin cytoskeleton and is involved in a wide range of fundamental cellular functions such as contraction, adhesion, migration, proliferation and gene expression. Two ROCK isoforms, ROCK1 and ROCK2, are assumed to be functionally redundant, based largely on the major common activators, the high degree of homology within the kinase domain and studies from overexpression with kinase constructs and chemical inhibitors (e.g., Y27632 and fasudil), which inhibit both ROCK1 and ROCK2. Extensive experimental and clinical studies support a critical role for the RhoA/ROCK pathway in the vascular bed in the pathogenesis of cardiovascular diseases, in which increased ROCK activity mediates vascular smooth muscle cell hypercontraction, endothelial dysfunction, inflammatory cell recruitment and vascular remodeling. Recent experimental studies, using ROCK inhibitors or genetic mouse models, indicate that the RhoA/ROCK pathway in myocardium contributes to cardiac remodeling induced by ischemic injury or persistent hypertrophic stress, thereby leading to cardiac decompensation and heart failure. This article, based on recent molecular, cellular and animal studies, focuses on the current understanding of ROCK signaling in cardiovascular diseases and in the pathogenesis of heart failure.

Serotonin and molecular neuroimaging in humans using PET

The serotonergic system is one of the most important modulatory neurotransmitter systems in the human brain. It plays a central role in major physiological processes and is implicated in a number of psychiatric disorders. Along with the dopaminergic system, it is also one of the phylogenetically oldest human neurotransmitter systems and one of the most diverse, with 14 different receptors identified up to this day, many of whose function remains to be understood. The system's functioning is even more diverse than the number of its receptors, since each is implicated in a number of different processes. This review aims at illustrating the distribution and summarizing the main functions of the serotonin (5-hydroxytryptamin, 5-HT) receptors as well as the serotonin transporter (SERT, 5-HTT), the vesicular monoamine transporter 2, monoamine oxidase type A and 5-HT synthesis in the human brain. Recent advances in in vivo quantification of these different receptors and enzymes that are part of the serotonergic system using positron emission tomography are described.

Regulators of G-protein signaling in the heart and their potential as therapeutic targets

Signal transduction through G-protein-coupled receptors (GPCRs) is central for the regulation of virtually all cellular functions and has been widely implicated in human disease. Regulators of G-protein signaling (RGS proteins) belong to a diverse protein family that was originally discovered for their ability to accelerate signal termination in response to GPCR stimulation, thereby reducing the amplitude and duration of GPCR effects. All RGS proteins share a common RGS domain that interacts with G protein α subunits and mediates their biological regulation of GPCR signaling. However, RGS proteins differ widely in size and the organization of their sequences flanking the RGS domain, which contain several additional functional domains that facilitate protein-protein (or protein-lipid) interactions. RGS proteins are subject to posttranslational modifications, and, in addition, their expression, activity, and subcellular localization can be dynamically regulated. Thus, there exists a wide array of mechanisms that facilitate their proper function as modulators and integrators of G-protein signaling. Several RGS proteins have been implicated in the cardiac remodeling response and heart rate regulation, and changes in RGS protein expression and/or function are believed to participate in the pathophysiology of cardiac hypertrophy, failure and arrhythmias as well as hypertension. This review is based on recent advances in our understanding of the expression pattern, regulation, and functional role of canonical RGS proteins, with a special focus on the healthy heart and the diseased heart. In addition, we discuss their potential and promise as therapeutic targets as well as strategies to modulate their expression and function.

Plasma membrane association of p63 Rho guanine nucleotide exchange factor (p63RhoGEF) is mediated by palmitoylation and is required for basal activity in cells

Activation of G protein-coupled receptors at the cell surface leads to the activation or inhibition of intracellular effector enzymes, which include various Rho guanine nucleotide exchange factors (RhoGEFs). RhoGEFs activate small molecular weight GTPases at the plasma membrane (PM). Many of the known G protein-coupled receptor-regulated RhoGEFs are found in the cytoplasm of unstimulated cells, and PM recruitment is a critical aspect of their regulation. In contrast, p63RhoGEF, a Gα(q)-regulated RhoGEF, appears to be constitutively localized to the PM. The objective of this study was to determine the molecular basis for the localization of p63RhoGEF and the impact of its subcellular localization on its regulation by Gα(q). Herein, we show that the pleckstrin homology domain of p63RhoGEF is not involved in its PM targeting. Instead, a conserved string of cysteines (Cys-23/25/26) at the N terminus of the enzyme is palmitoylated and required for membrane localization and full basal activity in cells. Conversion of these residues to serine relocates p63RhoGEF from the PM to the cytoplasm, diminishes its basal activity, and eliminates palmitoylation. The activity of palmitoylation-deficient p63RhoGEF can be rescued by targeting to the PM by fusion with tandem phospholipase C-δ1 pleckstrin homology domains or by co-expression with wild-type Gα(q) but not with palmitoylation-deficient Gα(q). Our data suggest that p63RhoGEF is regulated chiefly through allosteric control by Gα(q), as opposed to other known Gα-regulated RhoGEFs, which are instead sequestered in the cytoplasm, perhaps because of their high basal activity.

Determining the activation of rho as an index of receptor coupling to G₁₂/₁₃ proteins

Heterotrimeric G proteins are composed of α, β, and γ subunits. G proteins can be activated by a large number of cell-surface hepathelical receptors and can transduce signals from these receptors to various intracellular signaling molecules. When G protein-coupled receptors are bound by their cognate ligand, interaction with specific subtypes of G protein leads to dissociation of the α subunit of the heterotrimeric G protein from the βγ dimer, and both Gα-GTP and Gβγ are capable of initiating their own signal transduction pathways. G proteins are functionally divided into four groups based on the nature of α subunit into G(s), G(i), G(q), and G₁₂ families. The members of the G₁₂ subfamily are G₁₂ and G₁₃. Increasing evidence indicates that G₁₂/₁₃ proteins play critical roles in various physiological functions. G₁₂ and G₁₃ regulate the small GTPase Rho through modulation of guanine nucleotide exchange factor (RhoGEF) activity to regulate various cellular responses, such as cytoskeletal changes and cell growth. Therefore, Rho activity can often represent a sensitive marker of G₁₂/₁₃ activity. Here, we describe the Rho activation assay to monitor the activity of G₁₂/₁₃ proteins.

Heterotrimeric G proteins, focal adhesion kinase, and endothelial barrier function

Ligands by binding to G protein coupled receptors (GPCRs) stimulate dissociation of heterotrimeric G proteins into Gα and Gβγ subunits. Released Gα and Gβγ subunits induce discrete signaling cues that differentially regulate focal adhesion kinase (FAK) activity and endothelial barrier function. Activation of G proteins downstream of receptors such as protease activated receptor 1 (PAR1) and histamine receptors rapidly increases endothelial permeability which reverses naturally within the following 1-2 h. However, activation of G proteins coupled to the sphingosine-1-phosphate receptor 1 (S1P1) signal cues that enhance basal barrier endothelial function and restore endothelial barrier function following the increase in endothelial permeability by edemagenic agents. Intriguingly, both PAR1 and S1P1 activation stimulates FAK activity, which associates with alteration in endothelial barrier function by these agonists. In this review, we focus on the role of the G protein subunits downstream of PAR1 and S1P1 in regulating FAK activity and endothelial barrier function.

Ethanol impairs the assembly and disassembly of actin cytoskeleton and cell adhesion via the RhoA signaling pathway, catenin p120 and E-cadherin in CCK-stimulated pancreatic acini

The purpose of the present study was to evaluate the effects of EtOH on RhoA, actin cytoskeleton, catenin p120 and E-cadherin and their interactions in CCK-stimulated rat pancreatic acini. In isolated rat pancreatic acinar cells, CCK stimulation enhanced protein expression and association of RhoA, G(α13), Vav-2, catenin p120 and E-cadherin. CCK induced translocation and activation of RhoA and actin-filamentous assembly and disassembly. RhoA was diffusely localized throughout the acinar cell in the resting state and redistributed to the apical site in response to submaximal CCK stimulation and to a lesser extent in response to supramaximal CCK stimulation. Ethanol and subsequent submaximal CCK stimulation mimicked the effect of supramaximal CCK stimulation in terms of amylase secretion and morphologic effects. However, inhibition of RhoA translocation and activation were observed only with ethanol pretreatment. Ethanol followed by supramaximal CCK stimulation disrupted the well-defined localization of catenin p120 and E-cadherin around the lateral plasma membrane. These data suggest that ethanol impaired the assembly and disassembly of actin cytoskeleton and impaired cell-cell adhesion via the RhoA signaling pathways, catenin p120 and E-cadherin in CCK-stimulated pancreatic acini.

GPR35 as a Novel Therapeutic Target

G protein-coupled receptors (GPCRs) remain the best studied class of cell surface receptors and the most tractable family of proteins for novel small molecule drug discovery. Despite this, a considerable number of GPCRs remain poorly characterized and in a significant number of cases, endogenous ligand(s) that activate them remain undefined or are of questionable physiological relevance. GPR35 was initially discovered over a decade ago but has remained an "orphan" receptor. Recent publications have highlighted novel ligands, both endogenously produced and synthetic, which demonstrate significant potency at this receptor. Furthermore, evidence is accumulating which highlights potential roles for GPR35 in disease and therefore, efforts to characterize GPR35 more fully and develop it as a novel therapeutic target in conditions that range from diabetes and hypertension to asthma are increasing. Recently identified ligands have shown marked species selective properties, indicating major challenges for future drug development. As we begin to understand these issues, the continuing efforts to identify novel agonist and antagonist ligands for GPR35 will help to decipher its true physiological relevance; translating multiple assay systems in vitro, to animal disease systems in vivo and finally to man.

Structure and function of heterotrimeric G protein-regulated Rho guanine nucleotide exchange factors

Activation of certain classes of G protein-coupled receptors (GPCRs) can lead to alterations in the actin cytoskeleton, gene transcription, cell transformation, and other processes that are known to be regulated by Rho family small-molecular-weight GTPases. Although these responses can occur indirectly via cross-talk from canonical heterotrimeric G protein cascades, it has recently been demonstrated that Dbl family Rho guanine nucleotide exchange factors (RhoGEFs) can serve as the direct downstream effectors of heterotrimeric G proteins. Heterotrimeric Galpha(12/13), Galpha(q), and Gbetagamma subunits are each now known to directly bind and regulate RhoGEFs. Atomic structures have recently been determined for several of these RhoGEFs and their G protein complexes, providing fresh insight into the molecular mechanisms of signal transduction between GPCRs and small molecular weight G proteins. This review covers what is currently known about the structure, function, and regulation of these recently recognized effectors of heterotrimeric G proteins.

Identification of a novel amino acid response pathway triggering ATF2 phosphorylation in mammals

It has been well established that amino acid availability can control gene expression. Previous studies have shown that amino acid depletion induces transcription of the ATF3 (activation transcription factor 3) gene through an amino acid response element (AARE) located in its promoter. This event requires phosphorylation of activating transcription factor 2 (ATF2), a constitutive AARE-bound factor. To identify the signaling cascade leading to phosphorylation of ATF2 in response to amino acid starvation, we used an individual gene knockdown approach by small interfering RNA transfection. We identified the mitogen-activated protein kinase (MAPK) module MEKK1/MKK7/JNK2 as the pathway responsible for ATF2 phosphorylation on the threonine 69 (Thr69) and Thr71 residues. Then, we progressed backwards up the signal transduction pathway and showed that the GTPase Rac1/Cdc42 and the protein Galpha12 control the MAPK module, ATF2 phosphorylation, and AARE-dependent transcription. Taken together, our data reveal a new signaling pathway activated by amino acid starvation leading to ATF2 phosphorylation and subsequently positively affecting the transcription of amino acid-regulated genes.

RHO protein regulation of contraction in the human uterus

The state of contraction in smooth muscle cells of the human uterus is dependent on the interaction of activated forms of actin and myosin. Ras homology (RHO) proteins are small monomeric GTP-binding proteins that regulate actin polymerisation and myosin phosphorylation in smooth muscle cells. Their action is determined by their level of expression, GTP-bound state, intracellular localisation and phosphorylated status. Agonist activated RHO proteins bind to effector kinases such as RHO kinase (ROCK) and diaphanous proteins (DIAPH) to regulate smooth muscle contraction by two mechanisms: ROCK activates smooth muscle myosin either by direct phosphorylation at Ser19/Thr18 or through inhibition of myosin phosphatase which is a trimeric protein regulated by ROCK and by other protein kinases. Actin-polymerising proteins such as DIAPH homolog 1 increase filamentous actin assembly to enhance acto-myosin cross bridge formation and contraction. This review explores recent advances in RHO protein signalling in human myometrium and proposes areas of further research to investigate the involvement of these proteins in the regulation of uterine contractility in pregnancy and labour.

G protein signaling controls the differentiation of multiple cell lineages

G protein-coupled receptors (GPCRs) detect a great diversity of extracellular stimuli ranging from hormonal peptides, chemokines, neurotransmitters, lipids, nucleotides, amino acids, biogenic amines to ions. G protein-coupled pathways regulate a rich collection of biological processes involved in normal physiological function of the body as well as in pathological progression of diseases. In addition to their function in postmitotic steady-state tissues, GPCRs have been implicated in the differentiation of stem cells and tissue specific progenitor cells during development. Examples of these include the functions of nucleotides and neuropeptides in neuronal differentiation and axon growth, chemokines in lymphocyte differentiation and activation, and other GPCR-mediated processes in the differentiation of adipocytes, osteoblasts and smooth muscle cells. This review summarizes the recent advances in our understanding of the importance of GPCR-linked signaling cascades in the differentiation of different cell lineages.