In vivo functions of heterotrimeric G-proteins: studies in Galpha-deficient mice

Structure-based identification of a G protein-biased allosteric modulator of cannabinoid receptor CB1

Cannabis sativa is known for its therapeutic benefit in various diseases including pain relief by targeting cannabinoid receptors. The primary component of cannabis, Δ9-tetrahydrocannabinol (THC), and other agonists engage the orthosteric site of CB1, activating both Gi and β-arrestin signaling pathways. The activation of diverse pathways could result in on-target side effects and cannabis addiction, which may hinder therapeutic potential. A significant challenge in pharmacology is the design of a ligand that can modulate specific signaling of CB1. By leveraging insights from the structure-function selectivity relationship (SFSR), we have identified Gi signaling-biased agonist-allosteric modulators (ago-BAMs). Further, two cryoelectron microscopy (cryo-EM) structures reveal the binding mode of ago-BAM at the extrahelical allosteric site of CB1. Combining mutagenesis and pharmacological studies, we elucidated the detailed mechanism of ago-BAM-mediated biased signaling. Notably, ago-BAM CB-05 demonstrated analgesic efficacy with fewer side effects, minimal drug toxicity and no cannabis addiction in mouse pain models. In summary, our finding not only suggests that ago-BAMs of CB1 provide a potential nonopioid strategy for pain management but also sheds light on BAM identification for GPCRs.

Serotonin signals through postsynaptic Gαq, Trio RhoGEF, and diacylglycerol to promote Caenorhabditis elegans egg-laying circuit activity and behavior

Activated Gαq signals through phospholipase-Cβ and Trio, a Rho GTPase exchange factor (RhoGEF), but how these distinct effector pathways promote cellular responses to neurotransmitters like serotonin remains poorly understood. We used the egg-laying behavior circuit of Caenorhabditis elegans to determine whether phospholipase-Cβ and Trio mediate serotonin and Gαq signaling through independent or related biochemical pathways. Our genetic rescue experiments suggest that phospholipase-Cβ functions in neurons while Trio Rho GTPase exchange factor functions in both neurons and the postsynaptic vulval muscles. While Gαq, phospholipase-Cβ, and Trio Rho GTPase exchange factor mutants fail to lay eggs in response to serotonin, optogenetic stimulation of the serotonin-releasing HSN neurons restores egg laying only in phospholipase-Cβ mutants. Phospholipase-Cβ mutants showed vulval muscle Ca2+ transients while strong Gαq and Trio Rho GTPase exchange factor mutants had little or no vulval muscle Ca2+ activity. Treatment with phorbol 12-myristate 13-acetate that mimics 1,2-diacylglycerol, a product of PIP2 hydrolysis, rescued egg-laying circuit activity and behavior defects of Gαq signaling mutants, suggesting both phospholipase-C and Rho signaling promote synaptic transmission and egg laying via modulation of 1,2-diacylglycerol levels. 1,2-Diacylglycerol activates effectors including UNC-13; however, we find that phorbol esters, but not serotonin, stimulate egg laying in unc-13 and phospholipase-Cβ mutants. These results support a model where serotonin signaling through Gαq, phospholipase-Cβ, and UNC-13 promotes neurotransmitter release, and that serotonin also signals through Gαq, Trio Rho GTPase exchange factor, and an unidentified, phorbol 12-myristate 13-acetate-responsive effector to promote postsynaptic muscle excitability. Thus, the same neuromodulator serotonin can signal in distinct cells and effector pathways to coordinate activation of a motor behavior circuit.

The emerging roles of Gα12/13 proteins on the hallmarks of cancer in solid tumors

G12 proteins comprise a subfamily of G-alpha subunits of heterotrimeric GTP-binding proteins (G proteins) that link specific cell surface G protein-coupled receptors (GPCRs) to downstream signaling molecules and play important roles in human physiology. The G12 subfamily contains two family members: Gα12 and Gα13 (encoded by the GNA12 and GNA13 genes, respectively) and, as with all G proteins, their activity is regulated by their ability to bind to guanine nucleotides. Increased expression of both Gα12 and Gα13, and their enhanced signaling, has been associated with tumorigenesis and tumor progression of multiple cancer types over the past decade. Despite these strong associations, Gα12/13 proteins are underappreciated in the field of cancer. As our understanding of G protein involvement in oncogenic signaling has evolved, it has become clear that Gα12/13 signaling is pleotropic and activates specific downstream effectors in different tumor types. Further, the expression of Gα12/13 proteins is regulated through a series of transcriptional and post-transcriptional mechanisms, several of which are frequently deregulated in cancer. With the ever-increasing understanding of tumorigenic processes driven by Gα12/13 proteins, it is becoming clear that targeting Gα12/13 signaling in a context-specific manner could provide a new strategy to improve therapeutic outcomes in a number of solid tumors. In this review, we detail how Gα12/13 proteins, which were first discovered as proto-oncogenes, are now known to drive several "classical" hallmarks, and also play important roles in the "emerging" hallmarks, of cancer.

The G-protein βγ subunits regulate platelet function

AIMS

G-protein coupled receptors (GPCRs) tightly regulate platelet function by interacting with various physiological agonists. An essential mediator of GPCR signaling is the G protein αβγ heterotrimers, in which the βγ subunits are central players in downstream signaling. Herein, we investigated the role of Gβγ subunits in platelet function, hemostasis and thrombogenesis.

METHODS

To achieve this goal, platelets from both mice and humans were employed in the context of a small molecule inhibitor of Gβγ, namely gallein. We used an aggregometer to examine aggregation and dense granules secretion. We also used flow cytometry for P-selectin and PAC1 to determine the impact of inhibiting Gβγ on α -granule secretion and αIIbβ3 activation. Clot retraction and the platelet spreading assay were used to examine Gβγ role in outside-in platelet signaling, whereas Western blot was employed to examine its role in Akt activation. Finally, we used the bleeding time assay and the FeCl₃-induced carotid-artery injury thrombosis model to determine Gβγ contribution to in vivo platelet function.

RESULTS

We observed that gallein inhibits platelet aggregation and secretion in response to agonist stimulation, in both mouse and human platelets. Furthermore, gallein also exerted inhibitory effects on integrin αIIbβ3 activation, clot retraction, platelet spreading and Akt activation/phosphorylation. Finally, gallein's inhibitory effects manifested in vivo, as documented by its ability to modulate physiological hemostasis and delay thrombus formation.

CONCLUSION

Our findings demonstrate, for the first time, that Gβγ subunits directly regulate GPCR-dependent platelet function, in vitro and in vivo. Moreover, these data highlight Gβγ as a novel therapeutic target for managing thrombotic disorders.

Heterotrimeric Gα12/13 proteins in kidney injury and disease

Gα₁₂ and Gα₁₃ are ubiquitous members of the heterotrimeric guanine nucleotide-binding protein (G protein) family that play central and integrative roles in the regulation of signal transduction cascades within various cell types in the kidney. Gα₁₂/Gα₁₃ proteins enable the kidney to adapt to an ever-changing environment by transducing stimuli from cell surface receptors and accessory proteins to effector systems. Therefore, perturbations in Gα₁₂/Gα₁₃ levels or their activity can contribute to the pathogenesis of various renal diseases, including renal cancer. This review will highlight and discuss the complex and expanding roles of Gα₁₂/Gα₁₃ proteins on distinct renal pathologies, with emphasis on more recently reported findings. Deciphering how the different Gα₁₂/Gα₁₃ interaction networks participate in the onset and development of renal diseases may lead to the discovery of new therapeutic strategies.

Gα-13 induces CXC motif chemokine ligand 5 expression in prostate cancer cells by transactivating NF-κB

GNA13, the α subunit of a heterotrimeric G protein, mediates signaling through G-protein-coupled receptors (GPCRs). GNA13 is up-regulated in many solid tumors, including prostate cancer, where it contributes to tumor initiation, drug resistance, and metastasis. To better understand how GNA13 contributes to tumorigenesis and tumor progression, we compared the entire transcriptome of PC3 prostate cancer cells with those cells in which GNA13 expression had been silenced. This analysis revealed that GNA13 levels affected multiple CXC-family chemokines. Further investigation in three different prostate cancer cell lines singled out pro-tumorigenic CXC motif chemokine ligand 5 (CXCL5) as a target of GNA13 signaling. Elevation of GNA13 levels consistently induced CXCL5 RNA and protein expression in all three cell lines. Analysis of the CXCL5 promoter revealed that the -505/+62 region was both highly active and influenced by GNA13, and a single NF-κB site within this region of the promoter was critical for GNA13-dependent promoter activity. ChIP experiments revealed that, upon induction of GNA13 expression, occupancy at the CXCL5 promoter was significantly enriched for the p65 component of NF-κB. GNA13 knockdown suppressed both p65 phosphorylation and the activity of a specific NF-κB reporter, and p65 silencing impaired the GNA13-enhanced expression of CXCL5. Finally, blockade of Rho GTPase activity eliminated the impact of GNA13 on NF-κB transcriptional activity and CXCL5 expression. Together, these findings suggest that GNA13 drives CXCL5 expression by transactivating NF-κB in a Rho-dependent manner in prostate cancer cells.

The Ric-8A/Gα13/FAK signalling cascade controls focal adhesion formation during neural crest cell migration in Xenopus

Ric-8A is a pleiotropic guanine nucleotide exchange factor involved in the activation of various heterotrimeric G-protein pathways during adulthood and early development. Here, we sought to determine the downstream effectors of Ric-8A during the migration of the vertebrate cranial neural crest (NC) cells. We show that the Gα13 knockdown phenocopies the Ric-8A morphant condition, causing actin cytoskeleton alteration, protrusion instability, and a strong reduction in the number and dynamics of focal adhesions. In addition, the overexpression of Gα13 is sufficient to rescue Ric-8A-depleted cells. Ric-8A and Gα13 physically interact and colocalize in protrusions of the cells leading edge. The focal adhesion kinase FAK colocalizes and interacts with the endogenous Gα13, and a constitutively active form of Src efficiently rescues the Gα13 morphant phenotype in NC cells. We propose that Ric-8A-mediated Gα13 signalling is required for proper cranial NC cell migration by regulating focal adhesion dynamics and protrusion formation.

Platelet Signaling and Disease: Targeted Therapy for Thrombosis and Other Related Diseases

Platelets are essential for clotting in the blood and maintenance of normal hemostasis. Under pathologic conditions such as atherosclerosis, vascular injury often results in hyperactive platelet activation, resulting in occlusive thrombus formation, myocardial infarction, and stroke. Recent work in the field has elucidated a number of platelet functions unique from that of maintaining hemostasis, including regulation of tumor growth and metastasis, inflammation, infection, and immune response. Traditional therapeutic targets for inhibiting platelet activation have primarily been limited to cyclooxygenase-1, integrin αIIbβ3, and the P2Y12 receptor. Recently identified signaling pathways regulating platelet function have made it possible to develop novel approaches for pharmacological intervention in the blood to limit platelet reactivity. In this review, we cover the newly discovered roles for platelets as well as their role in hemostasis and thrombosis. These new roles for platelets lend importance to the development of new therapies targeted to the platelet. Additionally, we highlight the promising receptor and enzymatic targets that may further decrease platelet activation and help to address the myriad of pathologic conditions now known to involve platelets without significant effects on hemostasis.

The good and bad faces of the CXCR4 chemokine receptor

Chemokines are chemotactic cytokines that promote cell migration and activation under homeostatic and inflammatory conditions. Chemokines bind to seven transmembrane-spanning receptors that are coupled to heterotrimeric guanine nucleotide-binding (G) proteins, which are the responsible for intracellularly transmitting the activating signals for cell migration. Hematopoiesis, vascular development, lymphoid organ morphogenesis, cardiogenesis and neural differentiation are amongst the processes involving chemokine function. In addition, immune cell trafficking from bone marrow to blood circulation, and from blood and lymph to lymphoid and inflamed tissues, is tightly regulated by chemokines both under physiological conditions and also in autoimmune diseases. Furthermore, chemokine binding to their receptors stimulate trafficking to and positioning of cancer cells into target tissues and organs during tumour dissemination. The CXCL12 chemokine (also known as stromal-cell derived factor-1α, SDF-1α) plays key roles in hematopoiesis and lymphoid tissue architecture, in cardiogenesis, vascular formation and neurogenesis, as well as in the trafficking of solid and hematological cancer cell types. CXCL12 binds to the CXCR4 receptor, a multi-facetted molecule which tightly mirrors CXCL12 functions in homeostasis and disease. This review addresses the important roles of the CXCR4-CXCL12 axis in homeostasis, specially focusing in hematopoiesis, as well as it provides a picture of CXCR4 as mediator of cancer cell spreading, and a view of the available CXCR4 antagonists in different cancer types.

Gα12 regulates osteoclastogenesis by modulating NFATc1 expression

The G12 family of G protein alpha subunits has been shown to participate in the regulation of various physiological processes. However, the role of Gα12 in bone physiology has not been well described. Here, by micro‐CT analysis, we discovered that Gα12‐knockout mice have an osteopetrotic phenotype. Histological examination showed lower osteoclast number in femoral tissue of Gα12‐knockout mice compared to wild‐type mice. Additionally, in vitro osteoclastic differentiation of precursor cells with receptor activator of nuclear factor‐κB ligand (RANKL) showed that Gα12 deficiency decreased the number of osteoclast generated and the bone resorption activity. The induction of nuclear factor of activated T‐cell c1 (NFATc1), the key transcription factor of osteoclastogenesis, and the activation of RhoA by RANKL was also significantly suppressed by Gα12 deficiency. We further found that the RANKL induction of NFATc1 was not dependent on RhoA signalling, while osteoclast precursor migration and bone resorption required RhoA in the Gα12‐mediated regulation of osteoclasts. Therefore, Gα12 plays a role in differentiation through NFATc1 and in cell migration and resorption activity through RhoA during osteoclastogenesis.

Synergistic Signaling by Light and Acetylcholine in Mouse Iris Sphincter Muscle

The mammalian pupillary light reflex (PLR) involves a bilateral brain circuit whereby afferent light signals in the optic nerve ultimately drive iris-sphincter-muscle contraction via excitatory cholinergic parasympathetic innervation [1, 2]. Additionally, the PLR in nocturnal and crepuscular sub-primate mammals has a "local" component in the isolated sphincter muscle [3-5], as in amphibians, fish, and bird [6-10]. In mouse, this local PLR requires the pigment melanopsin [5], originally found in intrinsically photosensitive retinal ganglion cells (ipRGCs) [11-19]. However, melanopsin's presence and effector pathway locally in the iris remain uncertain. The sphincter muscle itself may express melanopsin [5], or its cholinergic parasympathetic innervation may be modulated by suggested intraocular axonal collaterals of ipRGCs traveling to the eye's ciliary body or even to the iris [20-22]. Here, we show that the muscarinic receptor antagonist, atropine, eliminated the effect of acetylcholine (ACh), but not of light, on isolated mouse sphincter muscle. Conversely, selective genetic deletion of melanopsin in smooth muscle mostly removed the light-induced, but not the ACh-triggered, increase in isolated sphincter muscle's tension and largely suppressed the local PLR in vivo. Thus, sphincter muscle cells are bona fide, albeit unconventional, photoreceptors. We found melanopsin expression in a small subset of mouse iris sphincter muscle cells, with the light-induced contractile signal apparently spreading through gap junctions into neighboring muscle cells. Light and ACh share a common signaling pathway in sphincter muscle. In summary, our experiments have provided details of a photosignaling process in the eye occurring entirely outside the retina.

Expression profiles of the Gα subunits during Xenopus tropicalis embryonic development

Heterotrimeric G protein signaling plays major roles during different cellular events. However, there is a limited understanding of the molecular mechanisms underlying G protein control during embryogenesis. G proteins are highly conserved and can be grouped into four subfamilies according to sequence homology and function. To further studies on G protein function during embryogenesis, the present analysis identified four Gα subunits representative of the different subfamilies and determined their spatiotemporal expression patterns during Xenopus tropicalis embryogenesis. Each of the Gα subunit transcripts was maternally and zygotically expressed, and, as development progressed, dynamic expression patterns were observed. In the early developmental stages, the Gα subunits were expressed in the animal hemisphere and dorsal marginal zone. While expression was observed at the somite boundaries, in vascular structures, in the eye, and in the otic vesicle during the later stages, expression was mainly found in neural tissues, such as the neural tube and, especially, in the cephalic vesicles, neural crest region, and neural crest-derived structures. Together, these results support the pleiotropism and complexity of G protein subfamily functions in different cellular events. The present study constitutes the most comprehensive description to date of the spatiotemporal expression patterns of Gα subunits during vertebrate development.

G-protein genomic association with normal variation in gray matter density

While detecting genetic variations underlying brain structures helps reveal mechanisms of neural disorders, high data dimensionality poses a major challenge for imaging genomic association studies. In this work, we present the application of a recently proposed approach, parallel independent component analysis with reference (pICA-R), to investigate genomic factors potentially regulating gray matter variation in a healthy population. This approach simultaneously assesses many variables for an aggregate effect and helps to elicit particular features in the data. We applied pICA-R to analyze gray matter density (GMD) images (274,131 voxels) in conjunction with single nucleotide polymorphism (SNP) data (666,019 markers) collected from 1,256 healthy individuals of the Brain Imaging Genetics (BIG) study. Guided by a genetic reference derived from the gene GNA14, pICA-R identified a significant SNP-GMD association (r=-0.16, P=2.34×10(-8)), implying that subjects with specific genotypes have lower localized GMD. The identified components were then projected to an independent dataset from the Mind Clinical Imaging Consortium (MCIC) including 89 healthy individuals, and the obtained loadings again yielded a significant SNP-GMD association (r=-0.25, P=0.02). The imaging component reflected GMD variations in frontal, precuneus, and cingulate regions. The SNP component was enriched in genes with neuronal functions, including synaptic plasticity, axon guidance, molecular signal transduction via PKA and CREB, highlighting the GRM1, PRKCH, GNA12, and CAMK2B genes. Collectively, our findings suggest that GNA12 and GNA14 play a key role in the genetic architecture underlying normal GMD variation in frontal and parietal regions.

Molecular approaches for manipulating astrocytic signaling in vivo

Astrocytes are the predominant glial type in the central nervous system and play important roles in assisting neuronal function and network activity. Astrocytes exhibit complex signaling systems that are essential for their normal function and the homeostasis of the neural network. Altered signaling in astrocytes is closely associated with neurological and psychiatric diseases, suggesting tremendous therapeutic potential of these cells. To further understand astrocyte function in health and disease, it is important to study astrocytic signaling in vivo. In this review, we discuss molecular tools that enable the selective manipulation of astrocytic signaling, including the tools to selectively activate and inactivate astrocyte signaling in vivo. Lastly, we highlight a few tools in development that present strong potential for advancing our understanding of the role of astrocytes in physiology, behavior, and pathology.

Harnessing the platelet signaling network to produce an optimal hemostatic response

Once released into the circulation by megakaryocytes, circulating platelets can undergo rapid activation at sites of vascular injury and resist unwarranted activation, which can lead to heart attacks and strokes. Historically, the signaling mechanisms underlying the regulation of platelet activation have been approached as a collection of individual pathways unique to agonist. This review takes a different approach, casting platelet activation as the product of a signaling network, in which activating and restraining mechanisms interact in a flexible network that regulates platelet adhesiveness, cohesion between platelets, granule secretion, and the formation of a stable hemostatic thrombus.

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.

New Fundamentals in Hemostasis

Hemostasis encompasses the tightly regulated processes of blood clotting, platelet activation, and vascular repair. After wounding, the hemostatic system engages a plethora of vascular and extravascular receptors that act in concert with blood components to seal off the damage inflicted to the vasculature and the surrounding tissue. The first important component that contributes to hemostasis is the coagulation system, while the second important component starts with platelet activation, which not only contributes to the hemostatic plug, but also accelerates the coagulation system. Eventually, coagulation and platelet activation are switched off by blood-borne inhibitors and proteolytic feedback loops. This review summarizes new concepts of activation of proteases that regulate coagulation and anticoagulation, to give rise to transient thrombin generation and fibrin clot formation. It further speculates on the (patho)physiological roles of intra- and extravascular receptors that operate in response to these proteases. Furthermore, this review provides a new framework for understanding how signaling and adhesive interactions between endothelial cells, leukocytes, and platelets can regulate thrombus formation and modulate the coagulation process. Now that the key molecular players of coagulation and platelet activation have become clear, and their complex interactions with the vessel wall have been mapped out, we can also better speculate on the causes of thrombosis-related angiopathies.

Inactivation of Pde8b enhances memory, motor performance, and protects against age-induced motor coordination decay

Phosphodiesterases (PDEs) are critical regulatory enzymes in cyclic nucleotide signaling. PDEs have diverse expression patterns within the central nervous system (CNS), show differing affinities for cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), and regulate a vast array of behaviors. Here, we investigated the expression profile of the PDE8 gene family members Pde8a and Pde8b in the mouse brain. We find that Pde8a expression is largely absent in the CNS; by contrast, Pde8b is expressed in select regions of the hippocampus, ventral striatum, and cerebellum. Behavioral analysis of mice with Pde8b gene inactivation (PDE8B KO) demonstrate an enhancement in contextual fear, spatial memory, performance in an appetitive instrumental conditioning task, motor-coordination, and have an attenuation of age-induced motor coordination decline. In addition to improvements observed in select behaviors, we find basal anxiety levels to be increased in PDE8B KO mice. These findings indicate that selective antagonism of PDE8B may be an attractive target for enhancement of cognitive and motor functions; however, possible alterations in affective state will need to be weighed against potential therapeutic value.

Platelet signaling

This chapter summarizes current ideas about the intracellular signaling that drives platelet responses to vascular injury. After a brief overview of platelet activation intended to place the signaling pathways into context, the first section considers the early events of platelet activation leading up to integrin activation and platelet aggregation. The focus is on the G protein-mediated events utilized by agonists such as thrombin and ADP, and the tyrosine kinase-based signaling triggered by collagen. The second section considers the events that occur after integrin engagement, some of which are dependent on close physical contact between platelets. A third section addresses the regulatory events that help to avoid unprovoked or excessive platelet activation, after which the final section briefly considers individual variations in platelet reactivity and the role of platelet signaling in the innate immune response and embryonic development.

G12 signaling through c-Jun NH2-terminal kinase promotes breast cancer cell invasion

Signaling through the heterotrimeric G protein, G12, via Rho induces a striking increase in breast cancer cell invasion. In this study, evidence is provided that the c-Jun NH₂-terminal kinase (JNK) is a key downstream effector of G12 on this pathway. Expression of constitutively-active Gα12 or activation of G12 signaling by thrombin leads to increased JNK and c-Jun phosphorylation. Pharmacologic inhibition of JNK or knockdown of JNK expression by siRNA significantly decreases G12-induced JNK activation as well as the ability of breast cancer cells to invade a reconstituted basement membrane. Furthermore, expression of dominant-negative Rho or treatment of cells with an inhibitor of the Rho kinase, ROCK, reduces G12-induced JNK and c-Jun activation, and ROCK inhibitor treatment also inhibits G12-induced cellular invasion. JNK knockdown or ROCK inhibitor treatment has no effect on activation of Rho by G12. Taken together, our data indicate that JNK activation is required for G12-induced invasion of breast cancer cells and that JNK is downstream of Rho and ROCK on this pathway. This study implicates a G12-stimulated mitogen-activated protein kinase cascade in cancer cell invasion, and supports a role for JNK in cancer progression.

Signalling mechanisms of RhoGTPase regulation by the heterotrimeric G proteins G12 and G13

G protein-mediated signal transduction can transduce signals from a large variety of extracellular stimuli into cells and is the most widely used mechanism for cell communication at the membrane. The RhoGTPase family has been well established as key regulators of cell growth, differentiation and cell shape changes. Among G protein-mediated signal transduction, G12/13-mediated signalling is one mechanism to regulate RhoGTPase activity in response to extracellular stimuli. The alpha subunits of G12 or G13 have been shown to interact with members of the RH domain containing guanine nucleotide exchange factors for Rho (RH-RhoGEF) family of proteins to directly connect G protein-mediated signalling and RhoGTPase signalling. The G12/13-RH-RhoGEF signalling mechanism is well conserved over species and is involved in critical steps for cell physiology and disease conditions, including embryonic development, oncogenesis and cancer metastasis. In this review, we will summarize current progress on this important signalling mechanism.

RGS10 restricts upregulation by chemokines of T cell adhesion mediated by α4β1 and αLβ2 integrins

Chemokines rapidly and transiently upregulate α4β1 and αLβ2 integrin-mediated adhesion during T lymphocyte extravasation by activating Gα-dependent inside-out signaling. To limit and terminate Gα-mediated signaling, cells can use several mechanisms, including the action of regulator of G protein signaling (RGS) proteins, which accelerate the GTPase activity of Gα subunits. Using human T cells silenced for or overexpressing RGS10, we show in this article that RGS10 functions as an inhibitor of Gα(i)-dependent, chemokine-upregulated T cell adhesion mediated by α4β1 and αLβ2. Shear stress-dependent detachment and cell spreading analyses revealed that RGS10 action mainly targets the adhesion strengthening and spreading phases of α4β1-mediated cell attachment. Associated with these observations, chemokine-stimulated Vav1-Rac1 activation was longer sustained and of higher intensity in RGS10-silenced T cells, or inhibited in cells overexpressing RGS10. Of importance, expression of constitutively activated Rac1 forms in cells overexpressing RGS10 led to the rescue of CXCL12-stimulated adhesion to VCAM-1 to levels similar to those in control transfectants. Instead, adhesion under flow conditions, soluble binding experiment, flow cytometry, and biochemical analyses revealed that the earlier chemokine-triggered integrin activation step was mostly independent of RGS10 actions. The data strongly suggest that RGS10 opposes activation by chemokines of the Vav1-Rac1 pathway in T cells, leading to repression of adhesion strengthening mediated by α4β1. In addition to control chemokine-upregulated T cell attachment, RGS10 also limited adhesion-independent cell chemotaxis and activation of cdc42. These results identify RGS10 as a key molecule that contributes to the termination of Gα-dependent signaling during chemokine-activated α4β1- and αLβ2-dependent T cell adhesion.

SNPs in genes encoding G proteins in pharmacogenetics

Heterotrimeric guanine-binding proteins (G proteins) transmit signals from the cell surface to intracellular signal cascades and are involved in various physiological and pathophysiological processes. Polymorphisms in the genes GNB3 (encoding the Gβ3 subunit), GNAS (encoding the Gαs subunit) and GNAQ (encoding the Gαq subunit) have been the primary focus of investigation. Polymorphisms in these genes could be associated with different complex phenotypes underlining that alterations in G-protein signaling can cause multiple disorders. G proteins present a point of convergence or ‘bottleneck’ between various receptors and effectors, thus making them a sensible tool for pharmacogenetic studies. The pharmacogenetic studies performed to date mostly demonstrate an association between G-protein polymorphisms and response to therapy or occurrence of adverse drug effects. Therefore, polymorphisms in genes encoding G-protein subunits may help to individualize drug treatment in various diseases with regard to both efficacy and safety.

A functional GNAQ promoter haplotype is associated with altered Gq expression and with insulin resistance and obesity in women with polycystic ovary syndrome

OBJECTIVES

The G-protein Gq, encoded by GNAQ, is involved in glucose metabolism. The GNAQ promoter harbours three polymorphisms. The TT(-695/-694)GC polymorphism was already shown to affect Gq transcription. Accordingly, we (i) characterized the GNAQ promoter polymorphisms G(-173)A and G(-168)A, (ii) investigated potential influences upon the TT(-695/-694)GC polymorphism and (iii) studied the associations with metabolic abnormalities in polycystic ovary syndrome (PCOS).

METHODS

Characterization of the polymorphisms was performed with electrophoretic mobility shift assays and reporter assays. Inhibition of lipolysis and Gq expression were measured in adipocytes isolated from female mammary tissue. We genotyped 266 healthy Caucasians, 265 women with PCOS, and 293 healthy, age-matched female controls to associate GNAQ promoter polymorphisms and haplotypes with anthropometric and metabolic variables.

RESULTS

The A(-168) allele was associated with significantly decreased transcriptional activity and altered transcription factor binding, whereas the G(-173)A polymorphism appeared functionally silent. Linkage and haplotype frequencies analysis resulted in four common haplotypes. In adipose tissue, a 44% higher Gq mRNA concentration was observed in homozygous GC(-695/-694)-G(-168) haplotypes compared with homozygous TT(-695/-694)-G(-168) haplotypes (P=0.046). This was associated with increased insulin inhibition of lipolysis in isolated adipocytes. In PCOS patients, the homozygous GC-G haplotype was associated with decreased insulin resistance and body mass index (BMI) compared with the homozygous TT-G haplotype (homeostatic model assessment of insulin resistance: 3.4+/-0.4 vs. 5.6+/-0.7 mmol/l x mmol/l2, P=0.001; fasting insulin: 86.6+/-11.9 vs. 128.8+/-16.5 pmol/l, P=0.003; BMI: 29.3+/-1.2 vs. 33.9+/-1.3 kg/m2, P=0.002). No association with BMI was found in healthy women.

CONCLUSION

G(-168)A is functionally relevant and in linkage with TT(-695/-694)GC. GNAQ promoter diplotypes are associated with insulin resistance and obesity in PCOS.

Adenosine A2A receptor signaling and golf assembly show a specific requirement for the gamma7 subtype in the striatum

The adenosine A(2A) receptor (A(2A)R) is increasingly recognized as a novel therapeutic target in Parkinson disease. In striatopallidal neurons, the G-protein α(olf) subtype is required to couple this receptor to adenylyl cyclase activation. It is now well established that the βγ dimer also performs an active role in this signal transduction process. In principal, sixty distinct βγ dimers could arise from combinatorial association of the five known β and 12 γ subunit genes. However, key questions regarding which βγ subunit combinations exist and whether they perform specific signaling roles in the context of the organism remain to be answered. To explore these questions, we used a gene targeting approach to specifically ablate the G-protein γ(7) subtype. Revealing a potentially new signaling paradigm, we show that the level of the γ(7) protein controls the hierarchial assembly of a specific G-protein α(olf)β(2)γ(7) heterotrimer in the striatum. Providing a probable basis for the selectivity of receptor signaling, we further demonstrate that loss of this specific G-protein heterotrimer leads to reduced A(2A)R activation of adenylyl cyclase. Finally, substantiating an important role for this signaling pathway in pyschostimulant responsiveness, we show that mice lacking the G-protein γ(7) subtype exhibit an attenuated behavioral response to caffeine. Collectively, these results further support the A(2A)R G-protein α(olf)β(2)γ(7) interface as a possible therapeutic target for Parkinson disease.

Role of G12 proteins in oncogenesis and metastasis

The G12 subfamily of heterotrimeric guanine nucleotide-binding proteins consists of two alpha subunits, G alpha12 and G alpha13. These proteins mediate signalling via G protein-coupled receptors and have been implicated in various physiological and pathophysiological processes. A number of direct and indirect effectors of G alpha12 and G alpha13 have been identified that mediate, or have been proposed to mediate, the diverse cellular responses accompanying activation of G12 proteins. This review describes the signalling pathways and cellular events stimulated by G12 proteins, with a particular emphasis on processes that are important in regulating cell migration and invasion, and could potentially be involved in the pathophysiology of cancer metastasis. Experimental findings directly implicating G12 proteins in the spread of metastatic disease are also summarized, indicating the importance of targeted inhibition of G12 signalling as a potential therapeutic option for locally advanced and metastatic disease.

Chemokine signaling in cancer: one hump or two?

Chemokines and their receptors play essential roles in the development and function of multiple tissues. Chemokine expression, particularly CXCL12 and its receptor CXCR4, has prognostic significance in several cancers apparently due to chemokine mediated growth and metastatic spread. These observations provide the rationale for pursuing CXCR4 inhibition for cancer chemotherapy. However, the multiple homeostatic functions of CXCR4 may preclude global inhibition as a therapeutic strategy. Here I review CXCR4 signaling and how it might differ in normal and transformed cells with special emphasis on the role that altered CXCR4 counter-regulation might play in tumor biology. I propose that CXCR4 mediates unique signals in cancer cells as a consequence of abnormal counter-regulation and that this results in novel biological responses. The importance of testing this hypothesis lies in the possibility that targeting abnormal CXCR4 signaling might provide an anti-tumor effect without disturbing normal CXCR4 functions.

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

Accumulating data indicate that G12 subfamily (Galpha12/13)-mediated signaling pathways play pivotal roles in a variety of physiological processes, while aberrant regulation of this pathway has been identified in various human diseases. It has been demonstrated that Galpha12/13-mediated signals form networks with other signaling proteins at various levels, from cell surface receptors to transcription factors, to regulate cellular responses. Galpha12/13 have slow rates of nucleotide exchange and GTP hydrolysis, and specifically target RhoGEFs containing an amino-terminal RGS homology domain (RH-RhoGEFs), which uniquely function both as a GAP and an effector for Galpha12/13. In this review, we will focus on the mechanisms regulating the Galpha12/13 signaling system, particularly the Galpha12/13-RH-RhoGEF-Rho pathway, which can regulate a wide variety of cellular functions from migration to transformation.

Characterization of the GNAQ promoter and association of increased Gq expression with cardiac hypertrophy in humans

AIMS

Transgenic mice with cardiac overexpression of Gq develop cardiac hypertrophy, apoptosis, and heart failure. Similar mechanisms may contribute to human left ventricular hypertrophy (LVH). However, mechanisms regulating transcription of the human GNAQ gene encoding the Gq protein are unknown and single-nucleotide polymorphisms have not been reported.

METHODS AND RESULTS

We delineated essential elements for transcription in the human GNAQ promoter using reporter assays and showed promoter induction by serum and angiotensin II. Sequencing of the whole promoter revealed a common (minor allele frequency 0.48) dinucleotide polymorphism at position -694/-695, resulting in an exchange of two adjacent nucleotides (TT > GC). The GC allele had increased transcription factor binding and was associated with enhanced transcriptional activation by serum or angiotensin II, resulting in enhanced Gq expression and intracellular signalling. Genotyping a population-based survey (n = 1204) revealed a higher prevalence of LVH in individuals with the GC/GC genotype [odds ratio (OR) 4.07; 95% CI 1.63-10.16; P = 0.003], this effect being more pronounced in women (OR 5.52; P = 0.005).

CONCLUSION

A novel polymorphism in the Gq promoter region is associated with enhanced promoter activity, Gq expression, intracellular signal transduction, and increased prevalence of LVH, particularly in women.

Gα13 activation rescues moesin-depletion induced apoptosis in F9 teratocarcinoma cells

Mouse F9 cells differentiate into primitive endoderm when treated with retinoic acid (RA) and into parietal endoderm in response to RA and dibutyryl (db-) cAMP. G protein signaling either blocks or mimics RA-induced differentiation, the latter signaling through the Wnt-beta-catenin pathway. In our study, we found that a constitutively active Galpha13 mutant induces F9 cells to differentiate into parietal endoderm in the absence of exogenous agents. Galpha13 expression and subsequent differentiation are accompanied by beta-catenin translocation to the nucleus. Differentiation and changes in cell morphology are supported by rearrangements to the F-actin cytoskeleton. ERM (ezrin-radixin-moesin) proteins, known to link F-actin to transmembrane receptors, are also redistributed during differentiation. Furthermore, morpholino antisense and shRNA approaches show that moesin expression is essential since its knockdown leads to altered F-actin distribution and subsequent apoptosis. Moesin-depleted cells, however, remain attached to the substrate when Galpha13 is constitutively expressed, but they do not differentiate into extraembryonic endoderm. Our study demonstrates a link between Galpha13 signaling that regulates differentiation of F9 cells through primitive to parietal endoderm and a moesin requirement for cell survival.

A Role for the G12 Family of Heterotrimeric G Proteins in Prostate Cancer Invasion

Many studies have suggested a role for the members of the G12 family of heterotrimeric G proteins (Galpha12 and Galpha13) in oncogenesis and tumor cell growth. However, few studies have examined G12 signaling in actual human cancers. In this study, we examined the role of G12 signaling in prostate cancer. We found that expression of the G12 proteins is significantly elevated in prostate cancer. Interestingly, expression of the activated forms of Galpha12 or Galpha13 in the PC3 and DU145 prostate cancer cell lines did not promote cancer cell growth. Instead, expression of the activated forms of Galpha12 or Galpha13 in these cell lines induced cell invasion through the activation of the RhoA family of G proteins. Furthermore, inhibition of G12 signaling by expression of the RGS domain of the p115-Rho-specific guanine nucleotide exchange factor (p115-RGS) in the PC3 and DU145 cell lines did not reduce cancer cell growth. However, inhibition of G12 signaling with p115-RGS in these cell lines blocked thrombin- and thromboxane A2-stimulated cell invasion. These observations identify the G12 family proteins as important regulators of prostate cancer invasion and suggest that these proteins may be targeted to limit invasion- and metastasis-induced prostate cancer patient mortality.

The G12 family of heterotrimeric G proteins promotes breast cancer invasion and metastasis

Although the prognosis for patients with early-stage breast cancer has improved, the therapeutic options for patients with locally advanced and metastatic disease are limited. To improve the treatment of these patients, the molecular mechanisms underlying breast cancer invasion and metastasis must be understood. In this study, we report that signaling through the G12 family of heterotrimeric G proteins (Galpha12 and Galpha13) promotes breast cancer cell invasion. Moreover, we demonstrate that inhibition of G12 signaling reduces the metastatic dissemination of breast cancer cells in vivo. Finally, we demonstrate that the expression of Galpha12 is significantly up-regulated in the earliest stages of breast cancer, implying that amplification of G12 signaling may be an early event in breast cancer progression. Taken together, these observations identify the G12 family proteins as important regulators of breast cancer invasion and suggest that these proteins may be targeted to limit invasion- and metastasis-induced patient morbidity and mortality.

G proteins in development

The focus of developmental biologists has expanded from the analysis of gene expression to include the analysis of cell signalling. Heterotrimeric G proteins (G proteins) mediate signalling from a superfamily of heptahelical receptors (G-protein-coupled receptors) to a smaller number of effector units that include adenylyl cyclases, phospholipase C and various ion channels. The convergence of developmental biology with cell signalling has now revealed overlaps in which G proteins mediate complex pathways in embryonic development.

Two G12 family G proteins, G alpha12 and G alpha13, show different subcellular localization

The G alpha subunits of the G12 family of heterotrimeric G proteins, G alpha12 and G alpha13, are closely related in sequences and some effectors, but they often act through different pathways or bind to different proteins. We have examined subcellular distribution of these two G proteins and found that endogenous G alpha12 and G alpha13 localize in membrane and cytoplasmic fractions, respectively. Exogenously expressed G alpha12 and G alpha13 also localize in membrane and cytoplasmic fractions, respectively, in COS-7 cells. Stimulation of lysophosphatidic acid receptor coupled to G alpha13 markedly promotes the translocation of G alpha13 from cytoplasm to membrane. This different localization of G alpha12 and G alpha13 may explain some of the nonoverlapping actions of G alpha12 and G alpha13.

Silencing the expression of multiple Gbeta-subunits eliminates signaling mediated by all four families of G proteins

The Gbetagamma-subunit complex derived from heterotrimeric G proteins can act to regulate the function of a variety of protein targets. We established lentiviral-based RNA interference in J774A.1 mouse macrophages to characterize the role of Gbeta in G protein-coupled receptor signaling. The expression of Gbeta1 and Gbeta2, the major subtypes present in J774A.1 cells, was eliminated by sequential treatment with small hairpin RNA expressing lentivirus. These betagamma complex-deficient cells lost the ability to respond to G protein-mediated signals. Chemotaxis and the phosphorylation of Akt in response to C5a were both blocked. Similarly, C5a-mediated actin polymerization, C5a- and UTP-stimulated intracellular calcium mobilization, and the stimulation of cAMP formation by isoproterenol were all eliminated in the absence of the Gbeta-subunits. In addition, stabilization and membrane localization of several Galpha- and Ggamma-subunit proteins was strongly effected. Furthermore, in DNA microarray analysis, regulation of gene expression stimulated by prostaglandin E2 and UTP was not observed in cells lacking Gbeta-subunits. In contrast, phagocytotic activity, serum-dependent cell growth and the patterns of gene expression induced by stimulating the Toll receptors with LPS were similar in wild-type cells and small hairpin RNA-containing cells. Thus, ablation of the Gbeta-subunits destabilized Galpha- and Ggamma-subunits and effectively eliminated G protein-mediated signaling responses. Unrelated ligand regulated pathways remained intact. These cells provide a system that can be used to study signaling in the absence of most G protein-mediated functions.

Essential roles of G{alpha}12/13 signaling in distinct cell behaviors driving zebrafish convergence and extension gastrulation movements

Gα_12/13 have been implicated in numerous cellular processes, however, their roles in vertebrate gastrulation are largely unknown. Here, we show that during zebrafish gastrulation, suppression of both Gα₁₂ and Gα₁₃ signaling by overexpressing dominant negative proteins and application of antisense morpholino-modified oligonucleotide translation interference disrupted convergence and extension without changing embryonic patterning. Analyses of mesodermal cell behaviors revealed that Gα_12/13 are required for cell elongation and efficient dorsalward migration during convergence independent of noncanonical Wnt signaling. Furthermore, Gα_12/13 function cell-autonomously to mediate mediolateral cell elongation underlying intercalation during notochord extension, likely acting in parallel to noncanonical Wnt signaling. These findings provide the first evidence that Gα₁₂ and Gα₁₃ have overlapping and essential roles in distinct cell behaviors that drive vertebrate gastrulation.

Protease-activated receptors: regulation of neuronal function

Certain serine proteases from the circulation (e.g., coagulation factors), inflammatory cells (e.g., mast-cell tryptase, neutrophil proteinase 3), and from many other cell types (e.g., trypsins) can specifically signal to cells by cleaving protease-activated receptors (PARs), a family of four G protein-coupled receptors. Proteases cleave PARs at specific sites within the extracellular amino-terminus to expose amino-terminal tethered ligand domains that bind to and activate the cleaved receptors. The proteases that activate PARs are often generated and released during injury and inflammation, and activated PARs orchestrate tissue responses to injury, including hemostasis, inflammation, pain, and repair. This review concerns protease and PAR signaling in the nervous system. Neurons of the central and peripheral nervous systems express all four PARs. Proteases that may derive from the circulation, inflammatory cells, or neural tissues can cleave PARs on neurons and thereby activate diverse signaling pathways that control survival, morphology, release of neurotransmitters, and activity of ion channels. In this manner proteases and PARs regulate neurodegeneration, neurogenic inflammation, and pain transmission. Thus, PARs may participate in disease states and PAR antagonists or agonists may be useful therapies for certain disorders.

Identification and characterization of regulator of G protein signaling 4 (RGS4) as a novel inhibitor of tubulogenesis: RGS4 inhibits mitogen-activated protein kinases and vascular endothelial growth factor signaling

Tubulogenesis by epithelial cells regulates kidney, lung, and mammary development, whereas that by endothelial cells regulates vascular development. Although functionally dissimilar, the processes necessary for tubulation by epithelial and endothelial cells are very similar. We performed microarray analysis to further our understanding of tubulogenesis and observed a robust induction of regulator of G protein signaling 4 (RGS4) mRNA expression solely in tubulating cells, thereby implicating RGS4 as a potential regulator of tubulogenesis. Accordingly, RGS4 overexpression delayed and altered lung epithelial cell tubulation by selectively inhibiting G protein-mediated p38 MAPK activation, and, consequently, by reducing epithelial cell proliferation, migration, and expression of vascular endothelial growth factor (VEGF). The tubulogenic defects imparted by RGS4 in epithelial cells, including its reduction in VEGF expression, were rescued by overexpression of constitutively active MKK6, an activator of p38 MAPK. Similarly, RGS4 overexpression abrogated endothelial cell angiogenic sprouting by inhibiting their synthesis of DNA and invasion through synthetic basement membranes. We further show that RGS4 expression antagonized VEGF stimulation of DNA synthesis and extracellular signal-regulated kinase (ERK)1/ERK2 and p38 MAPK activation as well as ERK1/ERK2 activation stimulated by endothelin-1 and angiotensin II. RGS4 had no effect on the phosphorylation of Smad1 and Smad2 by bone morphogenic protein-7 and transforming growth factor-beta, respectively, indicating that RGS4 selectively inhibits G protein and VEGF signaling in endothelial cells. Finally, we found that RGS4 reduced endothelial cell response to VEGF by decreasing VEGF receptor-2 (KDR) expression. We therefore propose RGS4 as a novel antagonist of epithelial and endothelial cell tubulogenesis that selectively antagonizes intracellular signaling by G proteins and VEGF, thereby inhibiting cell proliferation, migration, and invasion, and VEGF and KDR expression.

Reciprocal Signaling between the Transcriptional Co-Factor Eya2 and Specific Members of the Gαi Family

As part of a program to elucidate signaling processes controlled by the heterotrimeric G protein Galphaz, a human fetal brain cDNA library was screened for proteins that specifically interact with the activated form of Galphaz. One of the most-encountered molecules in this screen was Eya2, a member of the Eyes absent family of proteins. Mammalian Eya proteins are predominantly cytosolic proteins that are known to interact with members of the Sine oculis (Six) family of homeodomain transcription factors. This interaction facilitates the translocation of Eya into the nucleus, where the Eya/Six complex regulates transcription during critical stages of embryonic development. In vitro binding studies confirmed that Galphaz interacts with Eya2 in an activation-dependent fashion; furthermore, most other members of the Galphai family including Galphai1, Galphai2, and Galphai3 were found to interact with Eya2. It is interesting that one of the most abundant Galphai proteins, Galphao, did not interact with Eya2. Coexpression of the activated forms of Galphai1, Galphai2, and Galphai3, but not Galphao, with Eya2 recruited Eya2 to the plasma membrane, prevented Eya2 translocation into the nucleus, and abrogated Eya2/Six4-mediated transcription. In addition, Eya2 impinged on G protein-mediated signaling, as evidenced by its ability to relieve Galphai2-mediated inhibition of adenylyl cyclase. These results demonstrate that the interaction between the Galphai proteins and Eya2 may impact on seemingly disparate regulatory events involving both classes of proteins.

Genetic factors in cardiac hypertrophy

Cardiac hypertrophy is an adaptive response to any cardiac insult or stress that increases hemodynamic load. Cardiac hypertrophy can exist in a state of compensation or progress to a decompensated state (i.e., heart failure) over time. It has been established through transgenic overexpression and gene ablation studies that multiple signaling pathways are involved in the induction of hypertrophy as well as its decompensation. This article reviews the role of G alpha q in the development of pressure overload hypertrophy and discusses the relationships between G alpha q and beta-adrenergic receptors, RGS proteins, and the proapoptotic factor, Nix/Bnip3L.

Functional genomic dissection of multimeric protein families in zebrafish

The study of multimeric protein function in the postgenomicera has become complicated by the discovery of multiple isoforms for each subunit of those proteins. A correspondingly large number of potential isoform combinations offer the multicellular organism a constellation of protein assemblies from which to generate a variety of functions across different cells, tissues, and organs. At the same time, the multiplicity of potential subunit isoform combinations presents a significant challenge when attempting to dissect the functions of particular isoform combinations. Biochemical and cell culture methods have brought us to a significant state of understanding of multimeric proteins but are unable to answer questions of function within the context of the many tissues and developmental stages of the multicellular organism. Answering those questions can be greatly facilitated in model systems in which expression can be determined over time, in the context of the whole organism, and in which hypomorphic function of each subunit can be studied individually and in combination. Fortunately, the potential for high-throughput in situ hybridization studies and antisense-based reverse genetic knockdowns in zebrafish offers exciting opportunities to meet this challenge. Some of these opportunities, along with cautions of interpretation and gaps in the existing technologies, are discussed in the context of ongoing investigations of the dimeric Na,K-ATPases and heterotrimeric G proteins.

Integrin alpha4beta1 involvement in stromal cell-derived factor-1alpha-promoted myeloma cell transendothelial migration and adhesion: role of cAMP and the actin cytoskeleton in adhesion

The chemokine stromal cell-derived factor-1alpha (SDF-1alpha) is expressed by bone marrow (BM) stromal cells and plays key roles in cell homing to and retention into the bone marrow. In multiple myeloma, blood-borne malignant plasma cells home to the BM and accumulate in contact with stromal cells, implicating myeloma cell migration across endothelium. Myeloma cells express the SDF-1alpha receptor CXCR4, as well as the integrin alpha4beta1, which mediates their attachment to BM stroma. We show here that SDF-1alpha promotes transendothelial migration of purified BM myeloma cells and myeloma-derived NCI-H929 cells, involving a transient upregulation of alpha4beta1-dependent cell adhesion to the endothelium. Characterization of intracellular signaling pathways involved in the modulation by SDF-1alpha of alpha4beta1-mediated myeloma cell adhesion revealed that intracellular cAMP amounts associated with the activation of protein kinase A play key roles in this modulation. Furthermore, a functional link between cAMP actions on the dynamics of actin cytoskeleton, RhoA activation, and alpha4beta1-dependent cell adhesion in response to SDF-1alpha has been found. The regulation of alpha4beta1-mediated myeloma cell adhesion by SDF-1alpha could play key roles during myeloma cell homing into and trafficking inside the BM, and characterization of the molecular events involved in SDF-1alpha-activated modulation of this adhesion will contribute to a better understanding of mechanisms participating in cell migration.

Protease-activated receptors: contribution to physiology and disease

Proteases acting at the surface of cells generate and destroy receptor agonists and activate and inactivate receptors, thereby making a vitally important contribution to signal transduction. Certain serine proteases that derive from the circulation (e.g., coagulation factors), inflammatory cells (e.g., mast cell and neutrophil proteases), and from multiple other sources (e.g., epithelial cells, neurons, bacteria, fungi) can cleave protease-activated receptors (PARs), a family of four G protein-coupled receptors. Cleavage within the extracellular amino terminus exposes a tethered ligand domain, which binds to and activates the receptors to initiate multiple signaling cascades. Despite this irreversible mechanism of activation, signaling by PARs is efficiently terminated by receptor desensitization (receptor phosphorylation and uncoupling from G proteins) and downregulation (receptor degradation by cell-surface and lysosomal proteases). Protease signaling in tissues depends on the generation and release of proteases, availability of cofactors, presence of protease inhibitors, and activation and inactivation of PARs. Many proteases that activate PARs are produced during tissue damage, and PARs make important contributions to tissue responses to injury, including hemostasis, repair, cell survival, inflammation, and pain. Drugs that mimic or interfere with these processes are attractive therapies: selective agonists of PARs may facilitate healing, repair, and protection, whereas protease inhibitors and PAR antagonists can impede exacerbated inflammation and pain. Major future challenges will be to understand the role of proteases and PARs in physiological control mechanisms and human diseases and to develop selective agonists and antagonists that can be used to probe function and treat disease.

Xanthine nucleotide-specific G-protein alpha-subunits: a novel approach for the analysis of G-protein-mediated signal transduction

Pro- and eukaryotic cells express multiple GTP-binding proteins that play crucial roles in signal transduction. GTP-binding proteins possess a highly conserved NKX D motif critically involved in guanine binding. In order to selectively activate a defined GTP-binding protein, base-specificity can be switched from guanine to xanthine by mutating the conserved aspartate into asparagine (D/N-mutation). This approach was very successful at elucidating the function of structurally diverse GTP-binding proteins in complex systems. However, attempts to generate functional xanthine nucleotide-specific alpha-subunits of heterotrimeric GTP-binding proteins (G-proteins) met more difficulties. Recent studies have shown that a sufficiently high GDP-affinity is critical for functional expression of xanthine nucleotide-selective G-protein mutants. Moreover, xanthosine 5'-[gamma-thio]triphosphate and xanthosine 5'-[gamma, beta-imido]triphosphate are not functionally equivalent activators of D/N-G-protein mutants. We are now in the position to exploit xanthine nucleotide-specific G-proteins to dissect signaling pathways activated by a given G-protein in complex systems.

Lipid Lowering by Pravastatin Increases Parasympathetic Modulation of Heart Rate

Background— We have previously demonstrated in an in vitro model for lipid lowering that lipoprotein depletion resulted in a marked increase in the negative chronotropic response to the acetylcholine analogue carbamylcholine. In this study we used heart rate variability analysis to determine the effect of lipid lowering by statins on the response of the heart to parasympathetic stimulation. In parallel, we examined whether changes in parasympathetic responsiveness correlated with changes in the expression of Gα i2 , a molecular component of the parasympathetic signaling pathway in the heart.

Methods and Results— Patients were randomized in a crossover study of pravastatin and simvastatin. R-R interval analysis of Holter monitor studies demonstrated that in patients treated initially with pravastatin, the peak high-frequency power fraction during sleep, which reflects parasympathetic modulation of heart rate, increased by 24.0±5.02% (SEM, n=13, P <0.001) compared with the untreated control value. Simvastatin had no significant effect. Western blot analysis of lymphocytes from patients treated with pravastatin demonstrated a 90.1±27.3% (n=10, P =0.009) increase in Gα i2 expression, whereas simvastatin had no effect. Relative changes in Gα i2 correlated significantly with the changes in the fraction of high-frequency power (ρ=0.574, P =0.016).

Conclusions— Taken together with our in vitro data, these data are the first to suggest that cholesterol lowering by pravastatin might increase the response of the heart to parasympathetic stimulation and that changes in Gα i2 expression might serve as a molecular marker for this effect.

Thrombin protease-activated receptor-1 signals through Gq- and G13-initiated MAPK cascades regulating c-Jun expression to induce cell transformation

Although the ability of G protein-coupled receptors to stimulate normal and aberrant cell growth has been intensely investigated, the precise nature of the molecular mechanisms underlying their transforming potential are still not fully understood. In this study, we have taken advantage of the potent mitogenic effect of thrombin and the focus-forming activity of one of its receptors, protease-activated receptor-1, to dissect how this receptor coupled to Galphai, Galphaq/11, and Galpha12/13 transduces signals from the membrane to the nucleus to initiate transcriptional events involved in cell transformation. Using endogenous and transfected thrombin receptors in NIH 3T3 cells, ectopic expression of muscarinic receptors coupled to Galphaq and Galphai, and chimeric G protein alpha subunits and murine fibroblasts deficient in Galphaq/11, and Galpha12/13, we show here that, although coupling to Galphai is sufficient to induce ERK activation, the ability to couple to Galphaq and/or Galpha13 is necessary to induce c-jun expression and cell transformation. Furthermore, we show that Galphaq and Galpha13 can initiate the activation of MAPK cascades, including JNK, p38, and ERK5, which in turn regulate the activity of transcription factors controlling expression from the c-jun promoter. We also present evidence that c-Jun and the kinases regulating its expression are integral components of the transforming pathway initiated by protease-activated receptor-1.

Dimers of class A G protein-coupled receptors function via agonist-mediated trans-activation of associated G proteins

The histamine H1 receptor and the alpha1b-adrenoreceptor are G protein-coupled receptors that elevate intracellular [Ca2+] via activation of Gq/G11. Assessed by co-immunoprecipitation and time-resolved fluorescence resonance energy transfer they both exist as homo-dimers. The addition of the G protein G11alpha to the C terminus of these receptors did not prevent dimerization. Agonists produced a large stimulation of guanosine 5'-3-O-([35S]thio)triphosphate ([35S]GTPgammaS) binding to receptor-G protein fusions containing wild type forms of both polypeptides. For both receptors this was abolished by incorporation of G208AG11alpha into the fusions. Mutation of a highly conserved leucine in intracellular loop 2 of each receptor also eliminated agonist function but not binding. Co-expression of the two non-functional but complementary fusion constructs reconstituted agonist-mediated binding of [35S]GTPgammaS in membranes of HEK293 cells and elevation of [Ca2+]i in mouse embryo fibroblasts lacking both Gq and G11. Co-expression of the histamine H1 receptor- and the alpha1b-adrenoreceptor-G11alpha fusions allowed detection of functional hetero-dimeric complexes, whereas co-expression of histamine H1 receptor-G11alpha with increasing amounts of L151Dalpha1b-adrenoreceptor resulted in decreasing levels of histamine-stimulated [35S]GTPgammaS binding. Co-expression of the alpha1b-adrenoreceptor with a fusion protein incorporating the N-terminal domain and transmembrane helix 1 of the alpha1b-adrenoreceptor and G11alpha did not result in agonist activation of the G protein but did indicate a role for transmembrane helix 1 in dimerization. These data demonstrate that dimers of these class A receptors function via trans-activation of associated G proteins.

Light-dependent translocation of arrestin in the absence of rhodopsin phosphorylation and transducin signaling

Visual arrestin plays a crucial role in the termination of the light response in vertebrate photoreceptors by binding selectively to light-activated, phosphorylated rhodopsin. Arrestin localizes predominantly to the inner segments and perinuclear region of dark-adapted rod photoreceptors, whereas light induces redistribution of arrestin to the rod outer segments. The mechanism by which arrestin redistributes in response to light is not known, but it is thought to be associated with the ability of arrestin to bind photolyzed, phosphorylated rhodopsin in the outer segment. In this study, we show that light-driven translocation of arrestin is unaffected in two different mouse models in which rhodopsin phosphorylation is lacking. We further show that arrestin movement is initiated by rhodopsin but does not require transducin signaling. These results exclude passive diffusion and point toward active transport as the mechanism for light-dependent arrestin movement in rod photoreceptor cells.

Loss of G protein gamma 7 alters behavior and reduces striatal alpha(olf) level and cAMP production

The G protein beta gamma-dimer is required for receptor interaction and effector regulation. However, previous approaches have not identified the physiologic roles of individual subtypes in these processes. We used a gene knockout approach to demonstrate a unique role for the G protein gamma(7)-subunit in mice. Notably, deletion of Gng7 caused behavioral changes that were associated with reductions in the alpha(olf)-subunit content and adenylyl cyclase activity of the striatum. These data demonstrate that an individual gamma-subunit contributes to the specificity of a given signaling pathway and controls the formation or stability of a particular G protein heterotrimer.