Endogenous expression and protein kinase A-dependent phosphorylation of the guanine nucleotide exchange factor Ras-GRF1 in human embryonic kidney 293 cells

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Pleiotropic Roles of Calmodulin in the Regulation of KRas and Rac1 GTPases: Functional Diversity in Health and Disease

Calmodulin is a ubiquitous signalling protein that controls many biological processes due to its capacity to interact and/or regulate a large number of cellular proteins and pathways, mostly in a Ca²⁺-dependent manner. This complex interactome of calmodulin can have pleiotropic molecular consequences, which over the years has made it often difficult to clearly define the contribution of calmodulin in the signal output of specific pathways and overall biological response. Most relevant for this review, the ability of calmodulin to influence the spatiotemporal signalling of several small GTPases, in particular KRas and Rac1, can modulate fundamental biological outcomes such as proliferation and migration. First, direct interaction of calmodulin with these GTPases can alter their subcellular localization and activation state, induce post-translational modifications as well as their ability to interact with effectors. Second, through interaction with a set of calmodulin binding proteins (CaMBPs), calmodulin can control the capacity of several guanine nucleotide exchange factors (GEFs) to promote the switch of inactive KRas and Rac1 to an active conformation. Moreover, Rac1 is also an effector of KRas and both proteins are interconnected as highlighted by the requirement for Rac1 activation in KRas-driven tumourigenesis. In this review, we attempt to summarize the multiple layers how calmodulin can regulate KRas and Rac1 GTPases in a variety of cellular events, with biological consequences and potential for therapeutic opportunities in disease settings, such as cancer.

Pharmacology and Therapeutic Potential of the 5-HT7 Receptor

It is well-documented that serotonin (5-HT) exerts its pharmacological effects through a series of 5-HT receptors. The most recently identified member of this family, 5-HT₇, was first identified in 1993. Over the course of the last 25 years, this receptor has been the subject of intense investigation, and it has been demonstrated that 5-HT₇ plays an important role in a wide range of pharmacological processes. As a result of these findings, modulation of 5-HT₇ activity has been the focus of numerous drug discovery and development programs. This review provides an overview of the roles of 5-HT₇ in normal physiology and the therapeutic potential of this interesting drug target.

RasGRF1 mediates brain-derived neurotrophic factor-induced axonal growth in primary cultured cortical neurons

The appropriate development and regulation of neuronal morphology are important to establish functional neuronal circuits and enable higher brain function of the central nervous system. R-Ras, a member of the Ras family of small GTPases, plays crucial roles in the regulation of axonal morphology, including outgrowth, branching, and guidance. GTP-bound activated R-Ras reorganizes actin filaments and microtubules through interactions with its downstream effectors, leading to the precise control of axonal morphology. However, little is known about the upstream regulatory mechanisms for R-Ras activation in neurons. In this study, we found that brain-derived neurotrophic factor (BDNF) has a positive effect on endogenous R-Ras activation and promotes R-Ras-mediated axonal growth. RNA interference knockdown and overexpression experiments revealed that RasGRF1, a guanine nucleotide exchange factor (GEF) for R-Ras, is involved in BDNF-induced R-Ras activation and the promotion of axonal growth. Phosphorylation of RasGRF1 by protein kinase A at Ser916/898 is needed for the full activation of its GEF activity and to facilitate Ras signaling. We observed that BDNF treatment markedly increased this phosphorylation. Our results suggest that BDNF is one of the critical extrinsic regulators for R-Ras activation, and that RasGRF1 is an intrinsic key mediator for BDNF-induced R-Ras activation and R-Ras-mediated axonal morphological regulation.

Identification of essential residues for binding and activation in the human 5-HT7(a) serotonin receptor by molecular modeling and site-directed mutagenesis

The human 5-HT₇ receptor is expressed in both the central nervous system and peripheral tissues and is a potential drug target in behavioral and psychiatric disorders. We examined molecular determinants of ligand binding and G protein activation by the human 5-HT_7(a) receptor. The role of several key residues in the 7th transmembrane domain (TMD) and helix 8 were elucidated combining in silico and experimental mutagenesis. Several single and two double point mutations of the 5-HT_7(a) wild type receptor were made (W7.33V, E7.35T, E7.35R, E7.35D, E7.35A, R7.36V, Y7.43A, Y7.43F, Y7.43T, R8.52D, D8.53K; E7.35T-R7.36V, R8.52D-D8.53K), and their effects upon ligand binding were assessed by radioligand binding using a potent agonist (5-CT) and a potent antagonist (SB269970). In addition, the ability of the mutated 5-HT_7(a) receptors to activate G protein after 5-HT-stimulation was determined through activation of adenylyl cyclase. In silico investigation on mutated receptors substantiated the predicted importance of TM7 and showed critical roles of residues E7.35, W7.33, R7.36 and Y7.43 in agonist and antagonist binding and conformational changes of receptor structure affecting adenylyl cyclase activation. Experimental data showed that mutants E7.35T and E7.35R were incapable of ligand binding and adenylyl cyclase activation, consistent with a requirement for a negatively charged residue at this position. The mutant R8.52D was unable to activate adenylyl cyclase, despite unaffected ligand binding, consistent with the R8.52 residue playing an important role in the receptor-G protein interface. The mutants Y7.43A and Y7.43T displayed reduced agonist binding and AC agonist potency, not seen in Y7.43F, consistent with a requirement for an aromatic residue at this position. Knowledge of the molecular interactions important in h5-HT₇ receptor ligand binding and G protein activation will aid the design of selective h5-HT₇ receptor ligands for potential pharmacological use.

The serotonin 5-HT7 receptors: two decades of research

Like most neurotransmitters, serotonin possesses a simple structure. However, the pharmacological consequences are more complex and diverse. Serotonin is involved in numerous functions in the human body including the control of appetite, sleep, memory and learning, temperature regulation, mood, behavior, cardiovascular function, muscle contraction, endocrine regulation, and depression. Low levels of serotonin may be associated with several disorders, namely increase in aggressive and angry behaviors, clinical depression, Parkinson's disease, obsessive-compulsive disorder, eating disorders, migraine, irritable bowel syndrome, tinnitus, and bipolar disease. These effects are mediated via different serotonin (5-HT) receptors. In this review, we will focus on the last discovered member of this serotonin receptor family, the 5-HT7 receptor. This receptor belongs to the G protein-coupled receptor superfamily and was cloned two decades ago. Later, different splice variants were described but no major functional differences have been described so far. All 5-HT7 receptor variants are coupled to Gαs proteins and stimulate cAMP formation. Recently, several interacting proteins have been reported, which can influence receptor signaling and trafficking.

Neuronal Rho GEFs in synaptic physiology and behavior

In the mammalian brain, the majority of excitatory synapses are housed in micron-sized dendritic protrusions called spines, which can undergo rapid changes in shape and number in response to increased or decreased synaptic activity. These dynamic alterations in dendritic spines require precise control of the actin cytoskeleton. Within spines, multidomain Rho guanine nucleotide exchange factors (Rho GEFs) coordinate activation of their target Rho GTPases by a variety of pathways. In this review, we focus on the handful of disease-related Rho GEFs (Kalirin; Trio; Tiam1; P-Rex1,2; RasGRF1,2; Collybistin) localized at synapses and known to affect electrophysiology, spine morphology, and animal behavior. The goal is to integrate structure/function studies with measurements of synaptic function and behavioral phenotypes in animal models.

The RasGrf family of mammalian guanine nucleotide exchange factors

RasGrf1 and RasGrf2 are highly homologous mammalian guanine nucleotide exchange factors which are able to activate specific Ras or Rho GTPases. The RasGrf genes are preferentially expressed in the central nervous system, although specific expression of either locus may also occur elsewhere. RasGrf1 is a paternally-expressed, imprinted gene that is expressed only after birth. In contrast, RasGrf2 is not imprinted and shows a wider expression pattern. A variety of isoforms for both genes are also detectable in different cellular contexts. The RasGrf proteins exhibit modular structures composed by multiple domains including CDC25H and DHPH motifs responsible for promoting GDP/GTP exchange, respectively, on Ras or Rho GTPase targets. The various domains are essential to define their intrinsic exchanger activity and to modulate the specificity of their functional activity so as to connect different upstream signals to various downstream targets and cellular responses. Despite their homology, RasGrf1 and RasGrf2 display differing target specificities and non overlapping functional roles in a variety of signaling contexts related to cell growth and differentiation as well as neuronal excitability and response or synaptic plasticity. Whereas both RasGrfs are activatable by glutamate receptors, G-protein-coupled receptors or changes in intracellular calcium concentration, only RasGrf1 is reported to be activated by LPA, cAMP, or agonist-activated Trk and cannabinoid receptors. Analysis of various knockout mice strains has uncovered a specific functional contribution of RasGrf1 in processes of memory and learning, photoreception, control of post-natal growth and body size and pancreatic β-cell function and glucose homeostasis. For RasGrf2, specific roles in lymphocyte proliferation, T-cell signaling responses and lymphomagenesis have been described.

A novel geranylgeranyl transferase inhibitor in combination with lovastatin inhibits proliferation and induces autophagy in STS-26T MPNST cells

Prenylation inhibitors have gained increasing attention as potential therapeutics for cancer. Initial work focused on inhibitors of farnesylation, but more recently geranylgeranyl transferase inhibitors (GGTIs) have begun to be evaluated for their potential antitumor activity in vitro and in vivo. In this study, we have developed a nonpeptidomimetic GGTI, termed GGTI-2Z [(5-nitrofuran-2-yl)methyl-(2Z,6E,10E)-3,7,11,15-tetramethylhexadeca-2,6,10,14-tetraenyl 4-chlorobutyl(methyl)phosphoramidate], which in combination with lovastatin inhibits geranylgeranyl transferase I (GGTase I) and GGTase II/RabGGTase, without affecting farnesylation. The combination treatment results in a G(0)/G(1) arrest and synergistic inhibition of proliferation of cultured STS-26T malignant peripheral nerve sheath tumor cells. We also show that the antiproliferative activity of drugs in combination occurs in the context of autophagy. The combination treatment also induces autophagy in the MCF10.DCIS model of human breast ductal carcinoma in situ and in 1c1c7 murine hepatoma cells, where it also reduces proliferation. At the same time, there is no detectable toxicity in normal immortalized Schwann cells. These studies establish GGTI-2Z as a novel geranylgeranyl pyrophosphate derivative that may work through a new mechanism involving the induction of autophagy and, in combination with lovastatin, may serve as a valuable paradigm for developing more effective strategies in this class of antitumor therapeutics.

Ras-guanine nucleotide-releasing factor 1 (Ras-GRF1) controls activation of extracellular signal-regulated kinase (ERK) signaling in the striatum and long-term behavioral responses to cocaine

BACKGROUND

Ras-extracellular signal-regulated kinase (Ras-ERK) signaling is central to the molecular machinery underlying cognitive functions. In the striatum, ERK1/2 kinases are co-activated by glutamate and dopamine D1/5 receptors, but the mechanisms providing such signaling integration are still unknown. The Ras-guanine nucleotide-releasing factor 1 (Ras-GRF1), a neuronal specific activator of Ras-ERK signaling, is a likely candidate for coupling these neurotransmitter signals to ERK kinases in the striatonigral medium spiny neurons (MSN) and for modulating behavioral responses to drug abuse such as cocaine.

METHODS

We used genetically modified mouse mutants for Ras-GRF1 as a source of primary MSN cultures and organotypic slices, to perform both immunoblot and immunofluorescence studies in response to glutamate and dopamine receptor agonists. Mice were also subjected to behavioral and immunohistochemical investigations upon treatment with cocaine.

RESULTS

Phosphorylation of ERK1/2 in response to glutamate, dopamine D1 agonist, or both stimuli simultaneously is impaired in Ras-GRF1-deficient striatal cells and organotypic slices of the striatonigral MSN compartment. Consistently, behavioral responses to cocaine are also affected in mice deficient for Ras-GRF1 or overexpressing it. Both locomotor sensitization and conditioned place preference are significantly attenuated in Ras-GRF1-deficient mice, whereas a robust facilitation is observed in overexpressing transgenic animals. Finally, we found corresponding changes in ERK1/2 activation and in accumulation of FosB/DeltaFosB, a well-characterized marker for long-term responses to cocaine, in MSN from these animals.

CONCLUSIONS

These results strongly implicate Ras-GRF1 in the integration of the two main neurotransmitter inputs to the striatum and in the maladaptive modulation of striatal networks in response to cocaine.

Corticotropin releasing factor-induced ERK phosphorylation in AtT20 cells occurs via a cAMP-dependent mechanism requiring EPAC2

CRF-induced ERK phosphorylation has been shown to be an important mechanism underlying expression of pro-opiomelanocortin, a key precursor molecule in the hypothalamic pituitary adrenal axis. In AtT20 cells, CRF signalling has been investigated but the mechanism behind CRF-induced ERK activity is not fully understood. This paper elucidates the signalling cascade involved in this phenomenon. Involvement of CRF(1) receptor on ERK phosphorylation was shown by using CRF and urocortin 1. The lack of inhibitory effect of pertussis toxin and BAPTA-AM excluded involvement of G(i)-coupling and calcium mobilization respectively. In contrast, the process is suggested to be driven by cAMP since treatment of AtT20 cells with forskolin triggered strong ERK phosphorylation. Treatment with PKA inhibitors had a minor effect on CRF-induced ERK signalling while phosphorylation of CREB was completely abolished. This ruled out involvement of PKA and suggested a role for exchange protein directly activated by cAMP (EPAC). Moreover, an activator of EPACs 8-(4-methoxyphenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate mimicked CRF-induced ERK phosphorylation. Gene expression analysis showed high levels of EPAC2 mRNA and protein but low levels of EPAC1. Knockdown of EPAC2 expression by the use of specific siRNAs abolished CRF- and forskolin-induced ERK phosphorylation. The current study demonstrates a clear cAMP-dependent but PKA-independent mechanism underlying CRF-induced ERK activity that proceeds via EPAC2 signalling. Further research will provide more insight in the role of EPAC2 in CRF signalling.

Many faces of Ras activation

Ras proteins were originally identified as the products of oncogenes capable of inducing cell transformation. Over the last twenty-five years they have been studied in great detail because mutant Ras proteins are associated with many types of human cancer. Wild type Ras proteins play a central role in the regulation of proliferation and differentiation of various cell types. They alternate between an active GTP-bound state and an inactive GDP-bound state. Their activation is catalysed by a specialized group of enzymes known as guanine nucleotide exchange factors (GEFs). To date, four subfamilies of GEF molecules have been identified. Although all of them are able to activate Ras, their structure, tissue expression and regulation are significantly diverse. In this review we will summarize the various mechanisms by which these exchange factors activate Ras.

Relations between the mitogen-activated protein kinase and the cAMP-dependent protein kinase pathways: comradeship and hostility

Inter- and intracellular communications and responses to environmental changes are pivotal for the orchestrated and harmonious operation of multi-cellular organisms. These well-tuned functions in living organisms are mediated by the action of signal transduction pathways, which are responsible for receiving a signal, transmitting and amplifying it, and eliciting the appropriate cellular responses. Mammalian cells posses numerous signal transduction pathways that, rather than acting in solitude, interconnect with each other, a phenomenon referred to as cross-talk. This allows cells to regulate the distribution, duration, intensity and specificity of the response. The cAMP/cAMP-dependent protein kinase (PKA) pathway and the mitogen-activated protein kinase (MAPK) cascades modulate common processes in the cell and multiple levels of cross-talk between these signalling pathways have been described. The first- and best-characterized interconnections are the PKA-dependent inhibition of the MAPKs ERK1/2 mediated by RAF-1, and PKA-induced activation of ERK1/2 interceded through B-RAF. Recently, novel interactions between components of these pathways and new mechanisms for cross-talk have been elucidated. This review discusses both known and novel interactions between compounds of the cAMP/PKA and MAPKs signalling pathways in mammalian cells.

G protein regulation of MAPK networks

G proteins provide signal-coupling mechanisms to heptahelical cell surface receptors and are critically involved in the regulation of different mitogen-activated protein kinase (MAPK) networks. The four classes of G proteins, defined by the G(s), G(i), G(q) and G(12) families, regulate ERK1/2, JNK, p38MAPK, ERK5 and ERK6 modules by different mechanisms. The alpha- as well as betagamma-subunits are involved in the regulation of these MAPK modules in a context-specific manner. While the alpha- and betagamma-subunits primarily regulate the MAPK pathways via their respective effector-mediated signaling pathways, recent studies have unraveled several novel signaling intermediates including receptor tyrosine kinases and small GTPases through which these G-protein subunits positively as well as negatively regulate specific MAPK modules. Multiple mechanisms together with specific scaffold proteins that can link G-protein-coupled receptors or G proteins to distinct MAPK modules contribute to the context-specific and spatio-temporal regulation of mitogen-activated protein signaling networks by G proteins.

Serotonin and neuronal growth factors ? a convergence of signaling pathways

Monoamines, including serotonin (5-HT), have traditionally been associated with short-term signaling pathways in neurons, such as the modulation of cAMP and Ca(2+) levels. In contrast, neuronal growth factors, such as neurotrophins, have been traditionally associated with signaling pathways, such as those for activation of extracellular-regulated kinase (ERK) and Akt (protein kinase B), which are known to induce long-term protective changes. It has therefore been unclear how antidepressants that increase serotonin (5-HT), induce such changes as hippocampal neuroprotection and neurogenesis. It has been hypothesized, that the actions of 5-HT may be mediated indirectly through increased synthesis of peptide growth factors. However, there is increasing evidence that some subtypes of 5-HT receptors can directly couple to activation of the ERK and Akt pathways. Such coupling suggests a more direct potential role for 5-HT in mediating the long-term actions induced by antidepressants.

Novel aspects of G-protein-coupled receptor signalling--different ways to achieve specificity

Our understanding of signal transduction via G-protein-coupled receptors (GPCR) has developed dramatically during the last decades. The initial idea of linear signalling pathways transferring information from the cell membrane to the nucleus has evolved into a complicated network of signalling pathways offering the possibility of crosstalk, fine tuning and specific regulation at multiple levels. During the recent meeting on GPCRs at the Karolinska Institutet, Stockholm novel aspects of GPCR signalling were presented and discussed. Here, we will discuss several possibilities for GPCRs to achieve specificity in signal transduction, such as the phenomenon of biased agonism, receptor multimerization, the role of co-receptors, the regulation of heterotrimeric G proteins as well as multiple G(s)-dependent pathways to extracellular single-regulated protein kinases.

Laser microdissection and microarray analysis of the hippocampus of Ras-GRF1 knockout mice reveals gene expression changes affecting signal transduction pathways related to memory and learning

We used manual macrodissection or laser capture microdissection (LCM) to isolate tissue sections of the hippocampus area of Ras-GRF1 wild type and knockout mice brains, and analyzed their transcriptional patterns using commercial oligonucleotide microarrays. Comparison between the transcriptomes of macrodissected and microdissected samples showed that the LCM samples allowed detection of significantly higher numbers of differentially expressed genes, with higher statistical rates of significance. These results validate LCM as a reliable technique for in vivo genomic studies in the brain hippocampus, where contamination by surrounding areas (not expressing Ras-GRF1) increases background noise and impairs identification of differentially expressed genes. Comparison between wild type and knockout LCM hippocampus samples revealed that Ras-GRF1 elimination caused significant gene expression changes, mostly affecting signal transduction and related neural processes. The list of 36 most differentially expressed genes included loci concerned mainly with Ras/G protein signaling and cytoskeletal organization (i.e. 14-3-3gamma/zeta, Kcnj6, Clasp2) or related, cross-talking pathways (i.e. jag2, decorin, strap). Consistent with the phenotypes shown by Ras-GRF1 knockout mice, many of these differentially expressed genes play functional roles in processes such as sensory development and function (i.e. Sptlc1, antiquitin, jag2) and/or neurological development/neurodegeneration processes affecting memory and learning. Indeed, potential links to neurodegenerative diseases such as Alzheimer disease (AD) or Creutzfeldt-Jacobs disease (CJD), have been reported for a number of differentially expressed genes identified in this study (Ptma, Aebp2, Clasp2, Hebp1, 14-3-3gamma/zeta, Csnk1delta, etc.). These data, together with the previously described role of IRS and insulin (known Ras-GRF1 activators) in AD, warrant further investigation of a potential functional link of Ras-GRF1 to neurodegenerative processes.

The requirement of Ras and Rap1 for the activation of ERKs by cAMP, PACAP, and KCl in cerebellar granule cells

In cerebellar granule cells, the mitogen-activated protein kinase (MAPK) or extracellular signal-regulated kinase (ERK) cascade mediates multiple functions, including proliferation, differentiation, and survival. In these cells, ERKs are activated by diverse stimuli, including cyclic adenosine monophosphate (cAMP), pituitary adenylate cyclase activating protein (PACAP), depolarization induced by elevated extracellular potassium (KCl), and the neurotrophin brain-derived neurotrophic factor. Extensive studies in neuronal cell lines have implicated the small G proteins Ras and Rap1 in the activation of ERKs by cAMP, PACAP, and KCl. However, the requirement of Ras and Rap1 in these pathways in cerebellar granule cells has not been addressed. In this study, we utilize multiple biochemical assays to determine the mechanisms of action and requirement of Ras and Rap1 in cultured cerebellar granule cells. We show that both Ras and Rap1 can be activated by cAMP or PACAP via protein kinase (PKA)-dependent mechanisms. KCl activation of Ras also required PKA. Using both adenoviral and transgenic approaches, we show that Ras plays a major role in ERK activation by cAMP, PACAP, and KCl, while Rap1 also mediates activation of a selective membrane-associated pool of ERKs. Furthermore, Rap1, but not Ras, activation by PKA appears to require the action of Src family kinases.

Epac- and Rap- independent ERK1/2 phosphorylation induced by Gs-coupled receptor stimulation in HEK293 cells

Serotonin activates Ras and Ras-dependent ERK1/2 phosphorylation in HEK293 cells expressing G(s)-coupled 5-HT(4) or 5-HT(7) serotonin receptors through unknown mechanisms. Both Epac/Rap-dependent and -independent pathways for Ras-dependent ERK1/2 activation have been suggested. Epac overexpression or Epac-specific 8-CPT-2'-O-Me-cAMP did not cause ERK1/2 phosphorylation, despite Rap activation. The data did not support a role for PLCepsilon or DAG-dependent Ras GEFs of the Ras-GRP family in Ras-dependent ERK1/2 phosphorylation. However, serotonin stimulated phosphorylation of endogenous and recombinant Ras-GRF1, increased [Ca(2+)](i) and caused Ca(2+)- and calmodulin-dependent ERK1/2 phosphorylation. Different signalling pathways seem to be utilised by G(s)-coupled receptors in various isolates of HEK293 cells.

Cholesterol depletion reduces serotonin binding and signaling via human 5-HT7(a) receptors

Lipids, including cholesterol, are critical components of the cell membrane where they are enriched in microdomains, lipid rafts, which organize and concentrate receptors and intracellular proteins involved in signal transduction. The present study examined the effects of cholesterol depletion on serotonin (5-HT) binding and signaling via 5-hydroxytryptamine(7) (5-HT(7)) receptors in stably transfected HeLa cells. Immunohistochemical, ligand-binding and biotinylation experiments demonstrated that the studied cells expressed high levels of 5-HT(7) receptors at their surface with a pharmacological profile resembling 5-HT(7) receptors in native tissue. Depletion of cholesterol, by combined treatment with mevastatin, fumonisin B(1) and mevalonate or methyl-beta-cyclodextrin (MbetaCD), caused highly significant reductions in the B(max) values of [(3)H]5-HT- and [(3)H]-(R)-3-(2-(2-(4-methylpiperidin-1-yl)-ethyl)pyrrolidine-1-sulfonyl)phenol ([(3)H]SB269970)-binding to 5-HT(7) receptors. Cholesterol depletion also reduced the total level of 5-HT(7) receptor protein determined by Western blot analysis. None of the examined treatments affected the affinity of [(3)H]5-HT- or [(3)H]SB269970-binding to 5-HT(7) receptors. Treatment with serotonin caused strong inductions in the phosphorylation states of Ser(63)-ATF-1 and Ser(133)-CREB. These effects of serotonin on signal transduction were significantly counteracted by pre-treatment with cholesterol synthesis inhibitors. Altogether, the present study demonstrates that cholesterol depletion decreases binding of both agonist and antagonist radioligands to 5-HT(7) receptors and counteract 5-HT(7) receptor-mediated intracellular signaling.

Activation of STAT3 by Gαs Distinctively Requires Protein Kinase A, JNK, and Phosphatidylinositol 3-Kinase

Signal transducer and activator of transcription 3 (STAT3) can be stimulated by several G(s)-coupled receptors, but the precise mechanism of action has not yet been elucidated. We therefore examined the ability of Galpha(s)Q226L (Galpha(s)QL), a constitutively active mutant of Galpha(s), to stimulate STAT3 Tyr705 and Ser727 phosphorylations in human embryonic kidney 293 cells. Apart from Galpha(s)QL, the stimulation of Galpha(s) by cholera toxin or beta2-adrenergic receptor and the activation of adenylyl cyclase by forskolin, (Sp)-cAMP, or dibutyryl-cAMP all promoted both STAT3 Tyr705 and Ser727 phosphorylations. Moreover, the removal of Galpha(s) by RNA interference significantly reduced the beta2-adrenergic receptor-mediated STAT3 phosphorylations, denoting its capacity to regulate STAT3 activation by a G protein-coupled receptor. The possible downstream signaling molecules involved were assessed by using specific inhibitors and dominant negative mutants. Induction of STAT3 Tyr705 and Ser727 phosphorylations by Galpha(s)QL was suppressed by inhibition of protein kinase A, Janus kinase 2/3, Rac1, c-Jun N-terminal kinase (JNK), or phosphatidylinositol 3-kinase, and a similar profile was observed in response to beta2-adrenergic receptor stimulation. In contrast to the Galpha16-mediated regulation of STAT3 in HEK 293 cells (Lo, R. K., Cheung, H., and Wong, Y. H. (2003) J. Biol. Chem. 278, 52154-52165), the Galpha(s)-mediated responses, including STAT3-driven luciferase activation, were resistant to inhibition of phospholipase Cbeta. Surprisingly, Galpha(s)-mediated phosphorylation at Tyr705, but not at Ser727, was resistant to inhibition of c-Src, Raf-1, and MEK1/2 as well as to the expression of dominant negative Ras. Therefore, as with other Galpha-mediated activations of STAT3, the stimulatory signal arising from Galpha(s) is transduced via multiple signaling pathways. However, unlike the mechanisms employed by Galpha(i) and Galpha(14/16), Galpha(s) distinctively requires protein kinase A, JNK, and phosphatidylinositol 3-kinase for STAT3 activation.

The Ras-GRF1 exchange factor coordinates activation of H-Ras and Rac1 to control neuronal morphology

The Ras-GRF1 exchange factor has regulated guanine nucleotide exchange factor (GEF) activity for H-Ras and Rac1 through separate domains. Both H-Ras and Rac1 activation have been linked to synaptic plasticity and thus could contribute to the function of Ras-GRF1 in neuronal signal transduction pathways that underlie learning and memory. We defined the effects of Ras-GRF1 and truncation mutants that include only one of its GEF activities on the morphology of PC12 phaeochromocytoma cells. Ras-GRF1 required coexpression of H-Ras to induce morphological effects. Ras-GRF1 plus H-Ras induced a novel, expanded morphology in PC12 cells, which was characterized by a 10-fold increase in soma size and by neurite extension. A truncation mutant of Ras-GRF1 that included the Ras GEF domain, GRFdeltaN, plus H-Ras produced neurite extensions, but did not expand the soma. This neurite extension was blocked by inhibition of MAP kinase activation, but was independent of dominant-negative Rac1 or RhoA. A truncation mutant of Ras-GRF1 that included the Rac GEF domains, GRFdeltaC, produced the expanded phenotype in cotransfections with H-Ras. Cell expansion was inhibited by wortmannin or dominant-negative forms of Rac1 or Akt. GRFdeltaC binds H-Ras.GTP in both pulldown assays from bacterial lysates and by coimmunoprecipitation from HEK293 cells. These results suggest that coordinated activation of H-Ras and Rac1 by Ras-GRF1 may be a significant controller of neuronal cell size.

Rap1-mediated activation of extracellular signal-regulated kinases by cyclic AMP is dependent on the mode of Rap1 activation

Like other small G proteins of the Ras superfamily, Rap1 is activated by distinct guanine nucleotide exchange factors (GEFs) in response to different signals to elicit cellular responses. Activation of Rap1 by cyclic AMP (cAMP) can occur via cAMP-dependent protein kinase A (PKA)-independent and PKA-dependent mechanisms. PKA-independent activation of Rap1 by cAMP is mediated by direct binding of cAMP to Rap1-guanine nucleotide exchange factors (Rap1-GEFs) Epac1 (exchange protein directly activated by cAMP 1) and Epac2 (Epac1 and Epac2 are also called cAMP-GEFI and -GEFII). The availability of cAMP analogues that selectively activate Epacs, but not PKA, provides a specific tool to activate Rap1. It has been argued that the inability of these analogues to regulate extracellular signal-regulated kinases (ERKs) signaling despite activating Rap1 provides evidence that Rap1 is incapable of regulating ERKs. We confirm that the PKA-independent activation of Rap1 by Epac1 activates a perinuclear pool of Rap1 and that this does not result in ERK activation. However, we demonstrate that this inability to regulate ERKs is not a property of Rap1 but is rather a property of Epacs themselves. The addition of a membrane-targeting motif to Epac1 (Epac-CAAX) relocalizes Epac1 from its normal perinuclear locale to the plasma membrane. In this new locale it is capable of activating ERKs in a Rap1- and cAMP-dependent manner. Rap1 activation by Epac-CAAX, but not wild-type Epac, triggers its association with B-Raf. Therefore, we propose that its intracellular localization prevents Epac1 from activating ERKs. C3G (Crk SH3 domain Guanine nucleotide exchanger) is a Rap1 exchanger that is targeted to the plasma membrane upon activation. We show that C3G can be localized to the plasma membrane by cAMP/PKA, as can Rap1 when activated by cAMP/PKA. Using a small interfering RNA approach, we demonstrate that C3G is required for the activation of ERKs and Rap1 by cAMP/PKA. This activation requires the GTP-dependent association of Rap1 with B-Raf. These data demonstrate that B-Raf is a physiological target of Rap1, but its utilization as a Rap1 effector is GEF specific. We propose a model that specific GEFs activate distinct pools of Rap1 that are differentially coupled to downstream effectors.

Heterologous desensitization is evoked by both agonist and antagonist stimulation of the human 5-HT(7) serotonin receptor

Previously, we demonstrated that human serotonin (5-HT) 5-HT(7) receptors display marked constitutive activity. Here, we tested if the constitutive activation of adenylyl cyclase by 5-HT(7) receptors influenced both the desensitization properties of transfected 5-HT(7) receptors and the ability of endogenous G(s)-coupled receptors to activate adenylyl cyclase. Using membranes from stably transfected HEK293 cells expressing the recombinant human 5-HT(7) receptor splice variants (5-HT(7(a)), 5-HT(7(b)) and 5-HT(7(d))), we compared the effects of 1-h or 24-h preincubation of the agonist 5-HT, partial inverse agonists mesulergine and SB269970, and full inverse agonists clozapine and methiothepin on subsequent activation of adenylyl cyclase by both 5-HT through transfected 5-HT(7) receptors and the endogenous G(s)-coupled beta-adrenoceptors and prostaglandin receptors of HEK293 cells. The data show that stable expression of 5-HT(7) receptors is sufficient to attenuate adenylyl cyclase activation by endogenous G(s)-coupled receptors. Interestingly, preincubation with inverse agonists not only failed to result in the predicted resensitization of all receptor mediated adenylyl cyclase activation, but some inverse agonists further attenuated (desensitized) beta-adrenoceptor and prostaglandin-stimulated adenylyl cyclase activation similar to long-term agonist exposure by 5-HT. These effects were not correlated with inverse agonist efficacy, were not accompanied by receptor down-regulation and appear to be mediated by a protein kinase A (PKA) independent mechanism. It is concluded that the human 5-HT(7) receptor mediates heterologous desensitization of endogenous G(s)-coupled receptors through an unknown and potentially novel mechanism.

Activation of adenylyl cyclase by endogenous G(s)-coupled receptors in human embryonic kidney 293 cells is attenuated by 5-HT(7) receptor expression

Human 5-hydroxytryptamine(7) (5-HT(7)) receptors display characteristics shared with receptors believed to form a tight physical coupling with G protein in the absence of ligand. Some receptors apparently preassociated with G(i/o) and G(q/11) are reported to inhibit the signaling of other similarly coupled G protein-coupled receptors by limiting their access to activate a common G protein pool. Therefore, we determined whether 5-HT(7) receptor expression was sufficient to limit signaling of endogenously expressed G(s)-coupled receptors in human embryonic kidney (HEK) 293 cells. Using the ecdysone-inducible expression system, which allows for the titration of increasing receptor density in the same clonal cell line, we compared the effects of 5-HT(4(b)) and 5-HT(7(a,b,d)) receptor expression on adenylyl cyclase (AC) stimulation by the endogenous G(s)-coupled beta-adrenergic (betaAR) and prostanoid EP (EPR) receptors. betaAR- and EPR-stimulated AC activity was attenuated by 5-HT(7) receptor expression in both membrane preparations and intact HEK293 cells. betaAR- and EPR-stimulated AC activity was unaffected by expression of the G(s)-coupled 5-HT(4) receptor. The mechanism of this heterologous desensitization seems independent of protein kinase A activation, nor does it occur at the level of G protein activation because 1) betaAR- and EPR-stimulated AC activity was not restored to control values when Galpha(s) was overexpressed; and 2) beta(1)AR and beta(2)AR activation of Galpha(s) was unaffected by the expression of 5-HT(7) receptors. In addition, overexpression of AC isoforms was unable to rescue betaAR- and EPR-stimulated AC activity. Therefore, 5-HT(7) receptors probably limit access and/or impede activation of AC by betaAR and EP receptors. Although the 5-HT(7) receptor may preassociate with G protein and/or AC, the mechanism of this heterologous desensitization remains elusive.