Activation of Gz attenuates Rap1-mediated differentiation of PC12 cells

Effects of Gαi2 and Gαz protein knockdown on alpha2A-adrenergic and cannabinoid CB1 receptor regulation of MEK-ERK and FADD pathways in mouse cerebral cortex

BACKGROUND

Alpha_2A-adrenergic (α_2A-AR) and cannabinoid CB₁ (CB₁-R) receptors exert their functions modulating multiple signaling pathways, including MEK-ERK (extracellular signal-regulated kinases) and FADD (Fas-associated protein with death domain) cascades. These molecules are relevant in finding biased agonists with fewer side effects, but the mechanisms involving their modulations by α_2A-AR- and CB₁-R in vivo are unclear. This study investigated the roles of Gαi₂ and Gαz proteins in mediating α_2A-AR- and CB₁-R-induced alterations of MEK-ERK and FADD phosphorylation (p-) in mouse brain cortex.

METHODS

Gαi₂ or Gαz protein knockdown was induced in mice with selective antisense oligodeoxinucleotides (ODNs; 3 nmol/day, 5 days) prior to UK-14,304 (UK or brimonidine; 1 mg/kg) or WIN55212-2 (WIN; 8 mg/kg) acute treatments. Inactivated (p-T²⁸⁶) MEK1, activated (p-S^217/221) MEK1/2, activated (p-T²⁰²/Y²⁰⁴) ERK1/2, p-S¹⁹¹ FADD, and the corresponding total forms of these proteins were quantified by immunoblotting.

RESULTS

Increased (+ 88%) p-T²⁸⁶ MEK1 cortical density, with a concomitant reduction (-43%) of activated ERK was observed in UK-treated mice. Both effects were attenuated by Gαi₂ or Gαz antisense ODNs. Contrastingly, WIN induced Gαi₂- and Gαz-independent upregulations of p-T²⁸⁶ MEK1 (+ 63%), p-S^217/221 MEK1/2 (+ 86%), and activated ERK (+ 111%) in brain. Pro-apoptotic FADD was downregulated (- 34 to 39%) following UK and WIN administration, whereas the neuroprotective p-S¹⁹¹ FADD was increased (+ 74%) in WIN-treated mice only. None of these latter effects required from Gαi₂ or Gαz protein integrity.

CONCLUSION

The results indicate that α_2A-AR (UK), but not CB₁-R (WIN), agonists use Gαi₂ and Gαz proteins to modulate MEK-ERK, but not FADD, pathway in mouse brain cortex.

Biological Pathways of Long-Term Visit-to-Visit Blood Pressure Variability in the American Population: Cardiovascular Health Study and Women’s Health Initiatives

Studies reported a positive relationship between visit-to-visit blood pressure variability (VVBPV) and cardiovascular morbidity and mortality independently of the mean arterial blood pressure across clinical visits. The literature is scarce on the genes and biological mechanisms that regulate long-term VVBPV. We sought to identify biological pathways that regulate visit-to-visit blood pressure variability. We used phenotypic and genotype data from the Women’s Health Initiatives and Cardiovascular Health Studies. We defined VVBPV of systolic and diastolic blood pressure phenotypes as the standard deviation about the participant’s regression line with systolic and diastolic blood pressure regressed separately across visits. We imputed missing genotypes and then conducted a genome-wide association analysis to identify genomic variants related to the VVBPV and detect biological pathways. For systolic VVBPV, we identified a neurological pathway, the GABAergic pathway (P values = 1.1E − 2), and a vascular pathway, the RAP1 signaling pathway (P values = 5.8E − 2). For diastolic VVBPV, the hippo signaling (P values = 4.1E − 2), CDO myogenesis (P values = 7.0E − 2), and O-glycosylation of TSR domain-containing protein pathways (P values = 9.0E − 2) were the significant pathways. Future studies are warranted to validate these results. Further understanding of the roles of the genes regulating the identified pathways will help researchers to improve future pharmacological interventions to treat VVBPV in clinical practice.

C3G dynamically associates with nuclear speckles and regulates mRNA splicing

The first example of a Ras family GTPase and its exchange factor C3G localizing to nuclear speckles and regulating mRNA splicing is presented.

C3G (Crk SH3 domain binding guanine nucleotide releasing factor) (Rap guanine nucleotide exchange factor 1), essential for mammalian embryonic development, is ubiquitously expressed and undergoes regulated nucleocytoplasmic exchange. Here we show that C3G localizes to SC35-positive nuclear speckles and regulates splicing activity. Reversible association of C3G with speckles was seen on inhibition of transcription and splicing. C3G shows partial colocalization with SC35 and is recruited to a chromatin and RNase-sensitive fraction of speckles. Its presence in speckles is dependent on intact cellular actin cytoskeleton and is lost on expression of the kinase Clk1. Rap1, a substrate of C3G, is also present in nuclear speckles, and inactivation of Rap signaling by expression of GFP-Rap1GAP alters speckle morphology and number. Enhanced association of C3G with speckles is seen on glycogen synthase kinase 3 beta inhibition or differentiation of C2C12 cells to myotubes. CRISPR/Cas9-mediated knockdown of C3G resulted in altered splicing activity of an artificial gene as well as endogenous CD44. C3G knockout clones of C2C12 as well as MDA-MB-231 cells showed reduced protein levels of several splicing factors compared with control cells. Our results identify C3G and Rap1 as novel components of nuclear speckles and a role for C3G in regulating cellular RNA splicing activity.

Regulation of Rap GTPases in mammalian neurons

Small GTPases are central regulators of many cellular processes. The highly conserved Rap GTPases perform essential functions in the mammalian nervous system during development and in mature neurons. During neocortical development, Rap1 is required to regulate cadherin- and integrin-mediated adhesion. In the adult nervous system Rap1 and Rap2 regulate the maturation and plasticity of dendritic spine and synapses. Although genetic studies have revealed important roles of Rap GTPases in neurons, their regulation by guanine nucleotide exchange factors (GEFs) that activate them and GTPase activating proteins (GAPs) that inactivate them by stimulating their intrinsic GTPase activity is just beginning to be explored in vivo. Here we review how GEFs and GAPs regulate Rap GTPases in the nervous system with a focus on their in vivo function.

Function and Regulation of Heterotrimeric G Proteins during Chemotaxis

Chemotaxis, or directional movement towards an extracellular gradient of chemicals, is necessary for processes as diverse as finding nutrients, the immune response, metastasis and wound healing. Activation of G-protein coupled receptors (GPCRs) is at the very base of the chemotactic signaling pathway. Chemotaxis starts with binding of the chemoattractant to GPCRs at the cell-surface, which finally leads to major changes in the cytoskeleton and directional cell movement towards the chemoattractant. Many chemotaxis pathways that are directly regulated by Gβγ have been identified and studied extensively; however, whether Gα is just a handle that regulates the release of Gβγ or whether Gα has its own set of distinct chemotactic effectors, is only beginning to be understood. In this review, we will discuss the different levels of regulation in GPCR signaling and the downstream pathways that are essential for proper chemotaxis.

Gαz regulates BDNF-induction of axon growth in cortical neurons

The disruption of neurotransmitter and neurotrophic factor signaling in the central nervous system (CNS) is implicated as the root cause of neuropsychiatric disorders, including schizophrenia, epilepsy, chronic pain, and depression. Therefore, identifying the underlying molecular mechanisms by which neurotransmitter and neurotrophic factor signaling regulates neuronal survival or growth may facilitate identification of more effective therapies for these disorders. Previously, our lab found that the heterotrimeric G protein, Gz, mediates crosstalk between G protein-coupled receptors and neurotrophin signaling in the neural cell line PC12. These data, combined with Gαz expression profiles--predominantly in neuronal cells with higher expression levels corresponding to developmental times of target tissue innervation--suggested that Gαz may play an important role in neurotrophin signaling and neuronal development. Here, we provide evidence in cortical neurons, both manipulated ex vivo and those cultured from Gz knockout mice, that Gαz is localized to axonal growth cones and plays a significant role in the development of axons of cortical neurons in the CNS. Our findings indicate that Gαz inhibits BDNF-stimulated axon growth in cortical neurons, establishing an endogenous role for Gαz in regulating neurotrophin signaling in the CNS.

Deletion of GαZ protein protects against diet-induced glucose intolerance via expansion of β-cell mass

Insufficient plasma insulin levels caused by deficits in both pancreatic β-cell function and mass contribute to the pathogenesis of type 2 diabetes. This loss of insulin-producing capacity is termed β-cell decompensation. Our work is focused on defining the role(s) of guanine nucleotide-binding protein (G protein) signaling pathways in regulating β-cell decompensation. We have previously demonstrated that the α-subunit of the heterotrimeric G(z) protein, Gα(z), impairs insulin secretion by suppressing production of cAMP. Pancreatic islets from Gα(z)-null mice also exhibit constitutively increased cAMP production and augmented glucose-stimulated insulin secretion, suggesting that Gα(z) is a tonic inhibitor of adenylate cyclase, the enzyme responsible for the conversion of ATP to cAMP. In the present study, we show that mice genetically deficient for Gα(z) are protected from developing glucose intolerance when fed a high fat (45 kcal%) diet. In these mice, a robust increase in β-cell proliferation is correlated with significantly increased β-cell mass. Further, an endogenous Gα(z) signaling pathway, through circulating prostaglandin E activating the EP3 isoform of the E prostanoid receptor, appears to be up-regulated in insulin-resistant, glucose-intolerant mice. These results, along with those of our previous work, link signaling through Gα(z) to both major aspects of β-cell decompensation: insufficient β-cell function and mass.

Group II activators of G-protein signalling and proteins containing a G-protein regulatory motif

Beyond the core triad of receptor, Gαβγ and effector, there are multiple accessory proteins that provide alternative modes of signal input and regulatory adaptability to G-protein signalling systems. Such accessory proteins may segregate a signalling complex to microdomains of the cell, regulate the basal activity, efficiency and specificity of signal propagation and/or serve as alternative binding partners for Gα or Gβγ independent of the classical heterotrimeric Gαβγ complex. The latter concept led to the postulate that Gα and Gβγ regulate intracellular events distinct from their role as transducers for cell surface seven-transmembrane span receptors. One general class of such accessory proteins is defined by AGS proteins or activators of G-protein signalling that refer to mammalian cDNAs identified in a specific yeast-based functional screen. The discovery of AGS proteins and related entities revealed a number of unexpected mechanisms for regulation of G-protein signalling systems and expanded functional roles for this important signalling system.

Regulating Rap small G-proteins in time and space

Signaling by the small G-protein Rap is under tight regulation by its GEFs and GAPs. These are multi-domain proteins that are themselves controlled by distinct upstream pathways, and thus couple different extra- and intracellular cues to Rap. The individual RapGEFs and RapGAPs are, in addition, targeted to specific cellular locations by numerous anchoring mechanisms and, consequently, may control different pools of Rap. Here, we review the various activating signals and targeting mechanisms of these proteins and discuss their contribution to the spatiotemporal regulation and biological functions of the Rap proteins.

Novel somatic mutations in heterotrimeric G proteins in melanoma

Heterotrimeric guanine nucleotide-binding proteins (G proteins) mediate signals between G-protein coupled receptors and their downstream pathways, and have been shown to be mutated in cancer. In particular, GNAQ was found to be frequently mutated in blue nevi of the skin and uveal melanoma, acting as an oncogene in its mutated form. To further examine the role of heterotrimeric G proteins in malignant melanoma, we performed a comprehensive mutational analysis of the 35 genes in the heterotrimeric G protein gene family in a panel of 80 melanoma samples. Somatic alterations in a G protein subunit were detected in 17% of samples spanning 7 genes. The highest rates of somatic, non-synonymous mutations were found in GNG10 and GNAZ, neither of which has been previously reported to be mutated in melanoma. Our study is the first systematic analysis of the heterotrimeric G proteins in melanoma and indicates that multiple mutated heterotrimeric G proteins may be involved in melanoma progression.

RapGAPs in brain: multipurpose players in neuronal Rap signalling

Small Rap guanosine-tri-phosphate (GTP)ases are crucially involved in many cellular processes, including cell proliferation, differentiation, survival, adhesion and movement. In line, it has been shown that Rap signalling is involved in various aspects of neuronal differentiation, like the establishment of neuronal polarity or axonal growth cone movement. Rap GTPases can be activated by a wide variety of external stimuli, and this is mediated by specific guanine nucleotide exchange factors (RapGEFs). Inactivation of RapGTP can be achieved with the aid of specific GTPase-activating proteins (RapGAPs). In the brain, the most prominent RapGAPs are Rap1GAP and those of the spine-associated RapGAP (SPAR) family. This latter family consists of three members (SPAR1-3), from which two of them, namely SPAR1 and 2, have been investigated in more detail. As such, the localization of RapGAPs is crucially important in regulating Rap signalling at various sites in the cell and, for both SPAR1 and 2, enrichment at synaptic sites has been demonstrated. In recent years particularly the role of SPAR1 in shaping dendritic spine morphology has attracted considerable interest. In this review we will summarize the described actions of different RapGAPs expressed in the brain, and we will focus in particular on the SPAR family members.

ERK5 activity is required for nerve growth factor-induced neurite outgrowth and stabilization of tyrosine hydroxylase in PC12 cells

Extracellular signal-regulated kinases (ERKs) play important physiological roles in proliferation, differentiation, and gene expression. ERK5 is approximately twice the size of ERK1/2, and its amino-terminal half contains the kinase domain that shares homology with ERK1/2 and TEY activation motif, whereas the carboxyl-terminal half is unique. In this study, we examined a physiological role of ERK5 in rat pheochromocytoma cells (PC12), comparing it with ERK1/2. Nerve growth factor (NGF) induced phosphorylation of both ERK5 and ERK1/2, whereas the cAMP analog dibutyryl cAMP (Bt(2)cAMP) caused only ERK1/2 phosphorylation. U0126, at 30 mum, that blocks ERK1/2 signaling selectively attenuated neurite outgrowth induced by NGF and Bt(2)cAMP, but BIX02188 and BIX02189, at 30 mum, that block ERK5 signaling and an ERK5 dominant-negative mutant suppressed only NGF-induced neurite outgrowth. Next, we examined the expression of tyrosine hydroxylase, a rate-limiting enzyme of catecholamine biosynthesis. Both NGF and Bt(2)cAMP increased tyrosine hydroxylase gene promoter activity in an ERK1/2-dependent manner but was ERK5-independent. However, when both ERK5 and ERK1/2 signalings were inhibited, tyrosine hydroxylase protein up-regulation by NGF and Bt(2)cAMP was abolished, because of the loss of stabilization of tyrosine hydroxylase protein by ERK5. Taking these results together, ERK5 is involved in neurite outgrowth and stabilization of tyrosine hydroxylase in PC12 cells, and ERK5, along with ERK1/2, plays essential roles in the neural differentiation process.

Phosphorylation of Rap1GAP, a striatally enriched protein, by protein kinase A controls Rap1 activity and dendritic spine morphology

Protein kinase A (PKA)-dependent signaling cascades play an important role in mediating the effects of dopamine and other neurotransmitters in striatal medium spiny neurons. We have identified a prominent striatal PKA substrate as Rap1-GTPase activating protein (Rap1GAP), a negative regulator of Rap1 signaling. Although present throughout the brain, Rap1GAP is enriched in striatal medium spiny neurons and is phosphorylated by PKA at Ser-441 and Ser-499 in response to activation of D1 dopamine receptors. Phosphorylation of Rap1GAP is associated with inhibition of GAP activity, as demonstrated by increased Rap1 activity in striatal neurons. Phosphorylation of Rap1GAP is also associated with decreased [corrected] dendritic spine head size in cultured neurons. These findings suggest that phosphorylation of Rap1GAP by PKA plays an important role in striatal neurons by modulating Rap1 actions.

Rap1a null mice have altered myeloid cell functions suggesting distinct roles for the closely related Rap1a and 1b proteins

The Ras-related GTPases Rap1a and 1b have been implicated in multiple biological events including cell adhesion, free radical production, and cancer. To gain a better understanding of Rap1 function in mammalian physiology, we deleted the Rap1a gene. Although loss of Rap1a expression did not initially affect mouse size or viability, upon backcross into C57BL/6J mice some Rap1a-/- embryos died in utero. T cell, B cell, or myeloid cell development was not disrupted in Rap1a-/- mice. However, macrophages from Rap1a null mice exhibited increased haptotaxis on fibronectin and vitronectin matrices that correlated with decreased adhesion. Chemotaxis of lymphoid and myeloid cells in response to CXCL12 or CCL21 was significantly reduced. In contrast, an increase in FcR-mediated phagocytosis was observed. Because Rap1a was previously copurified with the human neutrophil NADPH oxidase, we addressed whether GTPase loss affected superoxide production. Neutrophils from Rap1a-/- mice had reduced fMLP-stimulated superoxide production as well as a weaker initial response to phorbol ester. These results suggest that, despite 95% amino acid sequence identity, similar intracellular distribution, and broad tissue distribution, Rap1a and 1b are not functionally redundant but rather differentially regulate certain cellular events.

NRG induces membrane targeting of Galphaz in muscle: implication in myogenesis

The neuregulins (NRGs) constitute a family of trophic factors that are known to play critical roles during neural development. We recently reported that Gbeta subunit regulates NRG-mediated signaling and gene transcription in cultured C2C12 myotubes. In this study, we demonstrated that NRG treatment of C2C12 myotubes stimulates a rapid translocation of Galphaz protein to the plasma membranes. In addition, Galphaz protein is localized to the postsynaptic regions at adult neuromuscular junctions and is prominently expressed in rat skeletal muscle during early postnatal stages. Interestingly, we found that expression of the constitutively activated Galphaz in C2C12 myoblasts attenuates myogenic differentiation. Taken together, our observations reveal an unanticipated role of Galphaz in mediating the actions of NRG during neural development.

Independent regulation of Rap1 and mitogen-activated protein kinase by the alpha chain of Go

Receptors coupled to G(i/o) proteins stimulate the mitogen-activated protein kinase (MAPK) cascade. The intracellular pathways linking the alpha chains of these G proteins to MAPK activation are not completely understood. One of the signaling molecules which has been suggested to act downstream of Galpha(i/o) is the small G protein Rap1. We investigated the role of Rap1 in MAPK stimulation by Galpha(o) in Chinese hamster ovary (CHO) cells. Our previous results have shown that in this cell system activated Galpha(o) strongly potentiates the MAPK response to the epidermal growth factor (EGF) receptor. Rap1 regulation was examined in cells transfected with Rap1 and wild-type Galpha(o) or the activated mutant Galpha(o)-Q205L. Immunocytochemical analysis detected both Rap1 and the Galpha(o) subunit at the plasma membrane as well as on perinuclear cytoplasmic vesicles. Expression of wild-type Galpha(o) had no significant effect on the levels of activated Rap1. In contrast, Galpha(o)-Q205L virtually abolished the activation of Rap1 induced by EGF. Further experiments showed that MAPK stimulation by EGF was greatly inhibited by expression of activated Rap1, suggesting that Rap1 inhibition could mediate the effect of Galpha(o) on the MAPK cascade. However, Galpha(o)-Q205L efficiently inhibited the activation of Rap1 induced by fibroblast growth factor (FGF). We have previously found that the ability of FGF to activate MAPK is not modified by Galpha(o). In addition, expression of the GAP protein RAP1GAPII blocked Rap1 activation without affecting EGF- or FGF-dependent MAPK stimulation. These findings provide evidence for independent regulation of Rap1 and MAPK by the G(o )alpha chain.

Differential G-alpha interaction capacities of the GoLoco motifs in Rap GTPase activating proteins

GoLoco motif proteins act as guanine nucleotide dissociation inhibitors (GDIs) for G-protein alpha subunits of the adenylyl cyclase-inhibitory (Galpha(i/o)) class. Rap1GAP2 is a newly identified GoLoco motif- and RapGAP domain-containing protein, and thus is considered a potential integrator of heterotrimeric and monomeric GTPase signaling. Primary sequence analysis indicated that the Rap1GAP2 GoLoco motif contains a lysine (Lys-75), rather than an arginine, at the crucial residue responsible for binding the alpha and beta phosphates of GDP and exerting GDI activity. To determine the functional outcome of this sequence variation we conducted a biophysical analysis of the human Rap1GAP2b/c GoLoco motif. We found that human Rap1GAP2b/c was deficient in GDI activity and Galpha interaction capability. Mutation of lysine-75 to arginine could not regain functional activity of the Rap1GAP2b/c GoLoco motif. Thus, the Rap1GAP2b/c GoLoco motif can be classed as inactive towards Galpha subunits. We also found that the Rap1GAP1a GoLoco motif, which lacks seven N-terminal amino acid residues present in canonical GoLoco motifs, does not interact with Galpha(i1). In contrast, the GoLoco motif of Rap1GAP1b, which is canonical in primary sequence, was found to interact with Galpha(i1).GDP.

Activation of heterotrimeric G-proteins independent of a G-protein coupled receptor and the implications for signal processing

Heterotrimeric G-proteins are key transducers for signal transfer from outside the cell, mediating signals emanating from cell-surface G-protein coupled receptors (GPCR). Many, if not all, subtypes of heterotrimeric G-proteins are also regulated by accessory proteins that influence guanine nucleotide binding, guanosine triphosphate (GTP) hydrolysis, or subunit interactions. One subgroup of such accessory proteins (activators of G-protein signaling; AGS proteins) refer to a functionally defined group of proteins that activate selected G-protein signaring systems in the absence of classical G-protein coupled receptors. AGS and related proteins provide unexpected insights into the regulation of the G-protein activation-deactivation cycle. Different AGS proteins function as guanine nucleotide exchange factors or guanine nucleotide dissociation inhibitors and may also influence subunit interactions by interaction with GBgamma. These proteins play important roles in the generation or positioning of signaling complexes and of the regulation of GPCR signaling, and as alternative binding partners for G-protein subunits. Perhaps of even broader impact is the discovery that AGS proteins provide a foundation for the concept that heterotrimeric G-protein subunits are processing signals within the cell involving intrinsic cues that do not involve the classical signal input from a cell surface GPCR.

Genome-wide expression analysis detects eight genes with robust alterations specific to bipolar I disorder: relevance to neuronal network perturbation

The limited number of genome-wide transcriptome analyses using the postmortem brains of bipolar disorder sufferers has not produced a clear consensus on the molecular pathways affected by the disorder. To expand the knowledge in this area, we examined the expression levels of more than 12 000 genes in Brodmann's Area (BA), 46 (dorsolateral prefrontal cortex) from bipolar I disorder and control samples using Affymetrix GeneChips. This analysis detected 108 differentially expressed genes in bipolar brains. Validation studies using quantitative RT-PCR on the two original diagnostic cohorts plus tissue from schizophrenic subjects, confirmed the differential expressions of eight genes (RAP1GA1, SST, HLA-DRA, KATNB1, PURA, NDUFV2, STAR and PAFAH1B3) in a bipolar-specific manner and one gene (CCL3) which was downregulated in both bipolar and schizophrenic brains. Of these, protein levels of RAP1GA1 (RAP1 GTPase activating protein 1) showed a trend of increase in BA46 from bipolar brains, in keeping with mRNA transcript levels. Transmission disequilibrium analysis of the nine genes using 43 single nucleotide polymorphisms (SNPs) in 229 National Institute of Mental Health bipolar trios exposed nominal SNP association and modest empirical haplotypic association (P=0.033) between SST (somatostatin) and disease. Finally, gene network analysis using the currently obtained expression data highlighted cellular growth and nervous system development pathways as potential targets in the molecular pathophysiology of bipolar disorder.

Accessory proteins for G proteins: partners in signaling

Accessory proteins involved in signal processing through heterotrimeric G proteins are generally defined as proteins distinct from G protein-coupled receptor (GPCR), G protein, or classical effectors that regulate the strength/efficiency/specificity of signal transfer upon receptor activation or position these entities in the right microenvironment, contributing to the formation of a functional signal transduction complex. A flurry of recent studies have implicated an additional class of accessory proteins for this system that provide signal input to heterotrimeric G proteins in the absence of a cell surface receptor, serve as alternative binding partners for G protein subunits, provide unexpected modes of G protein regulation, and have introduced additional functional roles for G proteins. This group of accessory proteins includes the recently discovered Activators of G protein Signaling (AGS) proteins identified in a functional screen for receptor-independent activators of G protein signaling as well as several proteins identified in protein interaction screens and genetic screens in model organisms. These accessory proteins may influence GDP dissociation and nucleotide exchange at the G(alpha) subunit, alter subunit interactions within heterotrimeric G(alphabetagamma) independent of nucleotide exchange, or form complexes with G(alpha) or G(betagamma) independent of the typical G(alphabetagamma) heterotrimer. AGS and related accessory proteins reveal unexpected diversity in G protein subunits as signal transducers within the cell.

Rap1GAP inhibits tumor growth in oropharyngeal squamous cell carcinoma

Rap1, a growth regulatory protein that is strongly expressed in human squamous cell carcinoma (SCC), is inactivated by rap1GAP. Recent evidence in normal rat cells suggests that rap1GAP regulates proliferation. The objective of the current study was to investigate whether rap1GAP functions as a tumor suppressor in SCC. Using a pull-down assay, active GTP-bound rap1 was up-regulated in SCC compared to normal or immortalized keratinocytes. Because both rap1A and rap1B isoforms of rap1 are expressed in SCC, the rap1GAP inactivation of both rap1 isoforms was verified using cells transfected with EGFP-rap1A or EGFP-rap1B or co-transfected with FLAG-tagged rap1GAP. The results demonstrate that expression of rap1GAP in oropharyngeal SCC down-regulated active rap1, ERK activation, and proliferation. Incubation of stably transfected SCC cells with nocodazole, an inhibitor of mitosis, caused a slower accumulation of rap1GAP-transfected cells in the G2 phase, in comparison to the vector control, indicating that rap1GAP-transfected cells have slower progression through the cell cycle. This was supported by down-regulation of cyclin D1, cdk4, and cdk6 in rap1GAP-transfected SCC cells. Furthermore, SCC cells transfected with rap1GAP produced significantly smaller tumors in nude mice as compared to controls (P < 0.01). These novel findings suggest that rap1GAP acts as a tumor suppressor protein in SCC.

G-protein alpha subunit interaction and guanine nucleotide dissociation inhibitor activity of the dual GoLoco motif protein PCP-2 (Purkinje cell protein-2)

Purkinje cell protein-2 (PCP-2; L7/GPSM4) is a GoLoco motif-containing protein that is specifically expressed in Purkinje and retinal ON bipolar cells. An alternative splice variant of PCP-2 has recently been isolated which contains two GoLoco motifs. Although the second GoLoco motif (GL2) of PCP-2 has been reported to interact with Galpha-subunits, a complete biochemical analysis of each individual motif of PCP-2 has not been performed. We demonstrate that the first GoLoco motif (GL1) of PCP-2 is equipotent as a guanine nucleotide dissociation inhibitor (GDI) towards Galphai1 and Galphai2, while it has sevenfold lower GDI activity for Galphai3 and greater than 20-fold lower GDI activity against Galphao. In contrast we found PCP-2 GL2 to be essentially equipotent as a GDI for all Galphai subunits, but it had negligible activity toward Galphao. Using co-immunoprecipitation from COS-7 cells, we found that PCP-2 was only able to interact with Galphai1 but not Galphao nor Galpha-subunits from other families (Galphas, Galphaq, or Galpha12). Mutational analysis of a non-canonical residue (glycine 24) in human PCP-2 GL1 provided evidence for heterogeneity in mechanisms of Galphai interactions with GoLoco motifs. Collectively, the data demonstrate that PCP-2 is a comparatively weak GoLoco motif protein that exhibits highest affinity interactions and GDI activity toward Galphai1, Galphai2, and Galphai3 subunits.

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.

A Role for Gz in Pancreatic Islet β-Cell Biology

Glucose-stimulated insulin secretion and beta-cell growth are important facets of pancreatic islet beta-cell biology. As a result, factors that modulate these processes are of great interest for the potential treatment of Type 2 diabetes. Here, we present evidence that the heterotrimeric G protein G(z) and its effectors, including some previously thought to be confined in expression to neuronal cells, are present in pancreatic beta-cells, the largest cellular constituent of the islets of Langerhans. Furthermore, signaling pathways upon which G alpha(z) impacts are intact in beta-cells, and G alpha(z) activation inhibits both cAMP production and glucose-stimulated insulin secretion in the Ins-1(832/13) beta-cell-derived line. Inhibition of glucose-stimulated insulin secretion by prostaglandin E (PGE1) is pertussis-toxin insensitive, indicating that other G alpha(i) family members are not involved in this process in this beta-cell line. Indeed, overexpression of a selective deactivator of G alpha(z), the RGS domain of RGSZ1, blocks the inhibitory effect of PGE1 on glucose-stimulated insulin secretion. Finally, the inhibition of glucose-stimulated insulin secretion by PGE1 is substantially blunted by small interfering RNA-mediated knockdown of G alpha(z) expression. Taken together, these data strongly imply that the endogenous E prostanoid receptor in the Ins-1(832/13) beta-cell line couples to G(z) predominantly and perhaps even exclusively. These data provide the first evidence for G(z) signaling in pancreatic beta-cells, and identify an endogenous receptor-mediated signaling process in beta-cells that is dependent on G alpha(z) function.

Differentiation of neuroblastoma cell line N1E-115 involves several signaling cascades

No systematic searches for differential expression of signaling proteins (SP) in undifferentiated vs. differentiated cell lineages were published and herein we used protein profiling for this purpose. The NIE-115 cell line was cultivated and an aliquot was differentiated with dimethylsulfoxide (DMSO), that is known to lead to a neuronal phenotype. Cell lysates were prepared, run on two-dimensional gel electrophoresis followed by MALDI-TOF-TOF identification of proteins and maps of identified SPs were generated. Seven SPs were comparable, 27 SPs: GTP-binding/Ras-related proteins, kinases, growth factors, calcium binding proteins, phosphatase-related proteins were observed in differentiated NIE-115 cells and eight SPs of the groups mentioned above were observed in undifferentiated cells only. Switching-on/off of several individual SPs from different signaling cascades during the differentiation process is a key to understand mechanisms involved. The findings reported herein are challenging in vitro and in vivo studies to confirm a functional role for deranged SPs.

The GAPs, GEFs, and GDIs of heterotrimeric G-protein alpha subunits

The heterotrimeric G-protein alpha subunit has long been considered a bimodal, GTP-hydrolyzing switch controlling the duration of signal transduction by seven-transmembrane domain (7TM) cell-surface receptors. In 1996, we and others identified a superfamily of “regulator of G-protein signaling” (RGS) proteins that accelerate the rate of GTP hydrolysis by Gα subunits (dubbed GTPase-accelerating protein or “GAP” activity). This discovery resolved the paradox between the rapid physiological timing seen for 7TM receptor signal transduction in vivo and the slow rates of GTP hydrolysis exhibited by purified Gα subunits in vitro. Here, we review more recent discoveries that have highlighted newly-appreciated roles for RGS proteins beyond mere negative regulators of 7TM signaling. These new roles include the RGS-box-containing, RhoA-specific guanine nucleotide exchange factors (RGS-RhoGEFs) that serve as Gα effectors to couple 7TM and semaphorin receptor signaling to RhoA activation, the potential for RGS12 to serve as a nexus for signaling from tyrosine kinases and G-proteins of both the Gα and Ras-superfamilies, the potential for R7-subfamily RGS proteins to couple Gα subunits to 7TM receptors in the absence of conventional Gβγ dimers, and the potential for the conjoint 7TM/RGS-box Arabidopsis protein AtRGS1 to serve as a ligand-operated GAP for the plant Gα AtGPA1. Moreover, we review the discovery of novel biochemical activities that also impinge on the guanine nucleotide binding and hydrolysis cycle of Gα subunits: namely, the guanine nucleotide dissociation inhibitor (GDI) activity of the GoLoco motif-containing proteins and the 7TM receptor-independent guanine nucleotide exchange factor (GEF) activity of Ric‑8/synembryn. Discovery of these novel GAP, GDI, and GEF activities have helped to illuminate a new role for Gα subunit GDP/GTP cycling required for microtubule force generation and mitotic spindle function in chromosomal segregation.

Rap1 and SPA-1 in hematologic malignancy

Rap1 is a member of the Ras family of GTPases and, depending on the cellular context, has an important role in the regulation of proliferation or cell adhesion. In lymphohematopoietic tissues, SPA-1 is a principal Rap1 GTPase-activating protein. Mice that are deficient for the SPA-1 gene develop age-dependent progression of T-cell immunodeficiency followed by a spectrum of late onset myeloproliferative disorders, mimicking human chronic myeloid leukemia. Recent studies reveal that deregulated Rap1 activation in SPA-1-deficient mice causes enhanced expansion of the bone marrow hematopoietic progenitors, but induces progressive unresponsiveness or anergy in T cells. Rap1 and its regulator, SPA-1, could, therefore, provide unique molecular targets for the control of human hematologic malignancy.

Cannabinoid receptor-induced neurite outgrowth is mediated by Rap1 activation through G(alpha)o/i-triggered proteasomal degradation of Rap1GAPII

The G(alpha)o/i-coupled CB1 cannabionoid receptor induces neurite outgrowth in Neuro-2A cells. The mechanisms of signaling through G(alpha)o/i to induce neurite outgrowth were studied. The expression of G(alpha)o/i reduces the stability of its direct interactor protein, Rap1GAPII, by targeting it for ubiquitination and proteasomal degradation. This results in the activation of Rap1. G(alpha)o/i-induced activation of endogenous Rap1 in Neuro-2A cells is blocked by the proteasomal inhibitor lactacystin. G(alpha)o/i stimulates neurite outgrowth that is blocked by the expression of dominant negative Rap1. Expression of Rap1GAPII also blocks the G(alpha)o/i-induced neurite outgrowth and treatment with proteasomal inhibitors potentiates this inhibition. The endogenous G(alpha)o/i-coupled cannabinoid (CB1) receptor in Neuro-2A cells stimulates the degradation of Rap1GAPII; activation of Rap1 and treatment with pertussis toxin or lactacystin blocks these effects. The CB1 receptor-stimulated neurite outgrowth is blocked by treatment with pertussis toxin, small interfering RNA for Rap, lactacystin, and expression of Rap1GAPII. Thus, the G(alpha)o/i-coupled cannabinoid receptor, by regulating the proteasomal degradation of Rap1GAPII, activates Rap1 to induce neurite outgrowth.

Rap1GAP2 is a new GTPase-activating protein of Rap1 expressed in human platelets

The Ras-like guanine-nucleotide-binding protein Rap1 controls integrin alpha(IIb)beta3 activity and platelet aggregation. Recently, we have found that Rap1 activation can be blocked by the nitric oxide/cyclic guanosine monophosphate (NO/cGMP) signaling pathway by type 1 cGMP-dependent protein kinase (cGKI). In search of possible targets of NO/cGMP/cGKI, we studied the expression of Rap1-specific GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs) in platelets. We could detect mRNAs for a new protein most closely related to Rap1GAP and for postsynaptic density-95 discs-large and zona occludens protein 1 (PDZ)-GEF1 and CalDAG-GEFs I and III. Using 5'-rapid amplification of cDNA ends (RACE), we isolated the complete cDNA of the new GAP encoding a 715-amino acid protein, which we have termed Rap1GAP2. Rap1GAP2 is expressed in at least 3 splice variants, 2 of which are detectable in platelets. Endogenous Rap1GAP2 protein partially colocalizes with Rap1 in human platelets. In transfected cells, we show that Rap1GAP2 exhibits strong GTPase-stimulating activity toward Rap1. Rap1GAP2 is highly phosphorylated, and we have identified cGKI as a Rap1GAP2 kinase. cGKI phosphorylates Rap1GAP2 exclusively on serine 7, a residue present only in the platelet splice variants of Rap1GAP2. Phosphorylation of Rap1GAP2 by cGKI might mediate inhibitory effects of NO/cGMP on Rap1. Rap1GAP2 is the first GTPase-activating protein of Rap1 found in platelets and is likely to have an important regulatory role in platelet aggregation.

PKA phosphorylation of Src mediates Rap1 activation in NGF and cAMP signaling in PC12 cells

Recent studies suggest that the tyrosine kinase Src plays an important role in the hormonal regulation of extracellular signal-regulated kinases (ERKs) via cyclic AMP (cAMP). Src has also been proposed to mediate signals downstream of nerve growth factor (NGF). Here, we report that the cAMP-dependent protein kinase A (PKA) induced the phosphorylation of Src at residue serine17 (S17) in multiple cell types including PC12, Hek293, AtT-20 and CHO cells. In PC12 cells, Src phosphorylation on S17 participates in the activation of the small G protein Rap1 by both cAMP and NGF. In these cells, Rap1 is required for cAMP/PKA signaling to ERKs and also for the sustained activation of ERKs by NGF. The activation of Rap1 by both cAMP and NGF was blocked by PP2, an inhibitor of Src family kinases, and by a Src mutant incapable of being phosphorylated by PKA (SrcS17A), consistent with the requirement of PKA phosphorylation of Src at S17 in these actions. PP2 and SrcS17A also inhibited the Rap1-dependent activation of ERKs by both agents. These results strongly indicate that PKA phosphorylation of Src at S17 is essential for cAMP and NGF signaling in PC12 cells and identify PKA as an important downstream target of NGF. PKA phosphorylation of Src may therefore be required for Rap1 activation in PC12 cells.

Return of the GDI: the GoLoco motif in cell division

The GoLoco motif is a 19-amino-acid sequence with guanine nucleotide dissociation inhibitor activity against G-alpha subunits of the adenylyl-cyclase-inhibitory subclass. The GoLoco motif is present as an independent element within multidomain signaling regulators, such as Loco, RGS12, RGS14, and Rap1GAP, as well as in tandem arrays in proteins, such as AGS3, G18, LGN, Pcp-2/L7, and Partner of Inscuteable (Pins/Rapsynoid). Here we discuss the biochemical mechanisms of GoLoco motif action on G-alpha subunits in light of the recent crystal structure of G-alpha-i1 bound to the RGS14 GoLoco motif. Currently, there is sparse evidence for GoLoco motif regulation of canonical G-protein-coupled receptor signaling. Rather, studies of asymmetric cell division in Drosophila and Caenorhabditis elegans, as well as mammalian mitosis, implicate GoLoco proteins, such as Pins, GPR-1/GPR-2, LGN, and RGS14, in mitotic spindle organization and force generation. We discuss potential mechanisms by which GoLoco/Galpha complexes might modulate spindle dynamics.

Rap1 up-regulation and activation on plasma membrane regulates T cell adhesion

Rap1 and Ras are closely related GTPases that share some effectors but have distinct functions. We studied the subcellular localization of Rap1 and its sites of activation in living cells. Both GFP-tagged Rap1 and endogenous Rap1 were localized to the plasma membrane (PM) and endosomes. The PM association of GFP-Rap1 was dependent on GTP binding, and GFP-Rap1 was rapidly up-regulated on this compartment in response to mitogens, a process blocked by inhibitors of endosome recycling. A novel fluorescent probe for GTP-bound Rap1 revealed that this GTPase was transiently activated only on the PM of both fibroblasts and T cells. Activation on the PM was blocked by inhibitors of endosome recycling. Moreover, inhibition of endosome recycling blocked the ability of Rap1 to promote integrin-mediated adhesion of T cells. Thus, unlike Ras, the membrane localizations of Rap1 are dynamically regulated, and the PM is the principle platform from which Rap1 signaling emanates. These observations may explain some of the biological differences between these GTPases.

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.

Thyroid-stimulating hormone/cAMP and glycogen synthase kinase 3beta elicit opposing effects on Rap1GAP stability

Beyond regulating Rap activity, little is known regarding the regulation and function of the Rap GTPase-activating protein Rap1GAP. Tuberin and E6TP1 protein levels are tightly regulated through ubiquitin-mediated proteolysis. A role for these RapGAPs, along with SPA-1, as tumor suppressors has been demonstrated. Whether Rap1GAP performs a similar role was investigated. We now report that Rap1GAP protein levels are dynamically regulated in thyroid-stimulating hormone (TSH)-dependent thyroid cells. Upon TSH withdrawal, Rap1GAP undergoes a net increase in phosphorylation followed by proteasome-mediated degradation. Sequence analysis identified two putative destruction boxes in the Rap1GAP C-terminal domain. Glycogen synthase kinase 3beta (GSK3beta) phosphorylated Rap1GAP immunoprecipitated from thyroid cells, and GSK3beta inhibitors prevented phosphorylation and degradation of endogenous Rap1GAP. Co-expression of GSK3beta and Rap1GAP in human embryonic kidney 293 cells stimulated proteasome-dependent Rap1GAP turnover. Mutational analysis established a role for serine 525 in the regulation of Rap1GAP stability. Overexpression of Rap1GAP in thyroid cells impaired TSH/cAMP-stimulated p70S6 kinase activity and cell proliferation. These data are the first to show that Rap1GAP protein levels are tightly regulated and are the first to support a role for Rap1GAP as a tumor suppressor.

Phosphorylation-regulated inhibition of the Gz GTPase-activating protein activity of RGS proteins by synapsin I

Synapsins are neuronal proteins that bind and cluster synaptic vesicles in the presynaptic space, presumably by anchoring to actin filaments, but specific regulatory functions of the synapsins are unknown. We found that a sub-population of brain synapsin Ia, a splice variant of one of three synapsin isoforms, inhibits the GTPase-activating protein (GAP) activity of several RGS proteins. Inhibition is highly selective for Galphaz, a member of the Gi family that is found in neurons, platelets, adrenal chromaffin cells, and a few other neurosecretory cells. Gz has been indirectly implicated in the regulation of secretion. Synapsin Ia constitutes a major fraction of the total GAP-inhibitory activity in brain, and its inhibitory activity is absent from the brains of synapsin I(-/-)/II(-/-) mice. Inhibition depends on the cationic D/E domain of synapsin. Phosphorylation of synapsin Ia at serine 9 by either cyclic AMP-dependent protein kinase or p21-activated protein kinase (PAK1) attenuates its potency as a GAP inhibitor more than 7-fold. Synapsin can thus act as a phosphorylation-modulated mediator of feedback regulation of Gz signaling by the synaptic machinery.

Does Rap1 deserve a bad Rap?

The Ras superfamily of small G proteins is remarkable for both its diversity and physiological functions. One member, Rap1, has been implicated in a particularly wide range of biological processes, from cell proliferation and differentiation to cell adhesion. But the diversity of Rap1 has lead to contradictory reports of its effects. Originally identified as an antagonist of Ras-induced transformation, Rap1 can oppose other actions of Ras including regulation of cell growth and differentiation, integrin-dependent responses and synaptic plasticity. Furthermore, recent evidence confirms that Rap1, like Ras, can activate the MAP kinase cascade (ERK) in several cell types. These diverse functions of Rap1 underscore that the activation and action of Rap1 are regulated by complex factors that are cell-type specific.