Specificity and complexity of receptor-G-protein interaction

Sarcolemmal α2-adrenoceptors in feedback control of myocardial response to sympathetic challenge

α2-adrenoceptor (α2-AR) isoforms, abundant in sympathetic synapses and noradrenergic neurons of the central nervous system, are integral in the presynaptic feed-back loop mechanism that moderates norepinephrine surges. We recently identified that postsynaptic α2-ARs, found in the myocellular sarcolemma, also contribute to a muscle-delimited feedback control capable of attenuating mobilization of intracellular Ca²⁺ and myocardial contractility. This previously unrecognized α2-AR-dependent rheostat is able to counteract competing adrenergic receptor actions in cardiac muscle. Specifically, in ventricular myocytes, nitric oxide (NO) and cGMP are the intracellular messengers of α2-AR signal transduction pathways that gauge the kinase-phosphatase balance and manage cellular Ca²⁺ handling preventing catecholamine-induced Ca²⁺ overload. Moreover, α2-AR signaling counterbalances phospholipase C - PKC-dependent mechanisms underscoring a broader cardioprotective potential under sympathoadrenergic and angiotensinergic challenge. Recruitment of such tissue-specific features of α2-AR under sustained sympathoadrenergic drive may, in principle, be harnessed to mitigate or prevent cardiac malfunction. However, cardiovascular disease may compromise peripheral α2-AR signaling limiting pharmacological targeting of these receptors. Prospective cardiac-specific gene or cell-based therapeutic approaches aimed at repairing or improving stress-protective α2-AR signaling may offer an alternative towards enhanced preservation of cardiac muscle structure and function.

Dynamic expression analysis of armc10, the homologous gene of human GPRASP2, in zebrafish embryos

G protein‑coupled receptor‑associated sorting protein 2 (GPRASP2), a member of the GASP family, has been reported to be involved in the modulation of transcription. However, few studies have revealed the role of GPRASP2 in the development and progression of diseases. As a model organism, zebrafish have been widely used to investigate human diseases. In the present study, zebrafish armadillo repeat‑containing 10 (armc10), an orthologous gene of human GPRASP2 was identified, and the spatial and temporal expression patterns of armc10 in zebrafish during early embryonic development were revealed. Bioinformatics analyses showed that ARMC10 protein sequences were highly conserved. Reverse transcription polymerase chain reaction analysis and whole mount in situ hybridization revealed that zebrafish armc10 was maternally expressed and was detected at a weak level up to 12 h post‑fertilization (hpf), however, its expression increased to a high level at 24 hpf. At the 75% epiboly stage and 12 hpf, armc10 was widely expressed in the embryo. At 24 hpf, armc10 mRNA was expressed in the nervous system of the zebrafish head. When the embryo was 2 days old, the wide expression of armc10 was maintained in the nervous system of the zebrafish head. At 72 hpf, the mRNA expression of armc10 was located specifically in the otic vesicles in addition to the nervous system of the head. At 96 hpf, the expression of armc10 was maintained in the otic vesicles and the nervous system of the head. The results of the present study provided novel insight into the spatial and temporal mRNA expression of armc10 in zebrafish, for the further investigation of nervous system diseases.

Expression and Role of Gonadotropin-Releasing Hormone 2 and Its Receptor in Mammals

Gonadotropin-releasing hormone 1 (GnRH1) and its receptor (GnRHR1) drive mammalian reproduction via regulation of the gonadotropins. Yet, a second form of GnRH (GnRH2) and its receptor (GnRHR2) also exist in mammals. GnRH2 has been completely conserved throughout 500 million years of evolution, signifying high selection pressure and a critical biological role. However, the GnRH2 gene is absent (e.g., rat) or inactivated (e.g., cow and sheep) in some species but retained in others (e.g., human, horse, and pig). Likewise, many species (e.g., human, chimpanzee, cow, and sheep) retain the GnRHR2 gene but lack the appropriate coding sequence to produce a full-length protein due to gene coding errors; although production of GnRHR2 in humans remains controversial. Certain mammals lack the GnRHR2 gene (e.g., mouse) or most exons entirely (e.g., rat). In contrast, old world monkeys, musk shrews, and pigs maintain the coding sequence required to produce a functional GnRHR2. Like GnRHR1, GnRHR2 is a 7-transmembrane, G protein-coupled receptor that interacts with G_αq/11 to mediate cell signaling. However, GnRHR2 retains a cytoplasmic tail and is only 40% homologous to GnRHR1. A role for GnRH2 and its receptor in mammals has been elusive, likely because common laboratory models lack both the ligand and receptor. Uniquely, both GnRH2 and GnRHR2 are ubiquitously expressed; transcript levels are abundant in peripheral tissues and scarcely found in regions of the brain associated with gonadotropin secretion, suggesting a divergent role from GnRH1/GnRHR1. Indeed, GnRH2 and its receptor are not physiological modulators of gonadotropin secretion in mammals. Instead, GnRH2 and GnRHR2 coordinate the interaction between nutritional status and sexual behavior in the female brain. Within peripheral tissues, GnRH2 and its receptor are novel regulators of reproductive organs. GnRH2 and GnRHR2 directly stimulate steroidogenesis within the porcine testis. In the female, GnRH2 and its receptor may help mediate placental function, implantation, and ovarian steroidogenesis. Furthermore, both the GnRH2 and GnRHR2 genes are expressed in human reproductive tumors and represent emerging targets for cancer treatment. Thus, GnRH2 and GnRHR2 have diverse functions in mammals which remain largely unexplored.

The GCR1 and GPA1 participate in promotion of Arabidopsis primary root elongation induced by N-acyl-homoserine lactones, the bacterial quorum-sensing signals

Many gram-negative bacteria use N-acyl-homoserine lactones (AHL) as quorum-sensing signals to coordinate their collective behaviors. Accumulating evidence indicates that plants can respond to AHL. However, little is known about the molecular mechanism of plants reacting to these bacterial signals. In this study, we show that the treatment of Arabidopsis roots with N-3-oxo-hexanoyl-homoserine lactone (3OC6-HSL) and N-3-oxo-octanoyl-homoserine lactone (3OC8-HSL) resulted in significant root elongation. The genetic analysis revealed that the T-DNA insertional mutants of gcr1, encoding a G-protein-coupled receptor GCR1, were insensitive to 3OC6-HSL or 3OC8-HSL in assays of root growth. The loss-of-function mutants of the sole canonical Gα subunit GPA1 showed no response to AHL promotion of root elongation whereas Gα gain-of-function plants overexpressing either the wild type or a constitutively active version of Arabidopsis Gα exhibited the exaggerated effect on root elongation caused by AHL. Furthermore, the expression of GCR1 and GPA1 were significantly upregulated after plants were contacted with both AHL. Taken together, our results suggest that GCR1 and GPA1 are involved in AHL-mediated elongation of Arabidopsis roots. This provides insight into the mechanism of plant responses to bacterial quorum-sensing signals.

Altered expression and function of regulator of G-protein signaling-17 (RGS17) in hepatocellular carcinoma

Guanine nucleotide regulatory proteins (G-proteins) are central to normal hepatocyte function and are implicated in hepatic disease initiation and progression. Regulators of G-protein signaling (RGS) are critical to defining G-protein-dependent signal fidelity, yet the role of RGS proteins in the liver is poorly defined. The aims of this study were to determine RGS17 expression in normal and transformed hepatic tissue and cells, and address the function of RGS17 in hepatic tumorgenicity. RGS17 expression was determined in human and rat HCC tissue and cell lines. Molecular approaches were used to alter RGS17 expression in HCC cells, effects on cell function measured, and RGS17 association with specific Gα-subunits determined. Using these approaches RGS17 mRNA, but not protein, was detectable in human and rat HCC tissue and cells. Conversely, RGS17 mRNA was not detected in normal tissue, isolated hepatocytes, or non-tumorigenic hepatic cells. Subsequent studies using transfected cells demonstrated that RGS17 proteins were not post-translationally modified in HCC cells, and RGS17 expression is governed by protein degradation and not via miRNAs. Notwithstanding inherently low RGS17 protein levels, altering RGS17 expression profoundly affected HCC cell mitogenesis and migration. Analysis of RGS17-G-protein interaction demonstrated RGS17 associates with both Giα- and Gqα-subunits in HCC cells of human and rat origin. In conclusion, these data demonstrate that, despite difficulties in measuring endogenous RGS protein expression, RGS17 is differentially expressed in HCC and plays a central role in regulating transformed hepatocyte tumorgenicity.

Formyl Peptide Receptor Expression in Birds

N-formyl-methionyl-leucyl-phenylalanine (fMLP) is a major chemotactic factor produced by Escherichia coli and other Gram-negative bacteria. The prototypal human fMLP receptor 1 (FPR1) was cloned in 1990 from a differentiated HL-60 myeloid leukemia cell cDNA library. In transfected cells, FPR1 binds fMLP with high affinity and is activated by picomolar to low nanomolar concentrations of fMLP in chemotaxis and calcium ion mobilization assays. Two additional human genes, designated FPR-like 1 (FPRL1) and FPR-like 2 (FPRL2), were later isolated by low-stringency hybridization using FPR1 cDNA as a probe, and these were shown to cluster with FPR1 on chromosome 19q13.3. In avian models the fMLP effects and the possible expression of FPRs have been poorly investigated. In this study we demonstrated that stimulation with fMLP of cultured cells isolated from the 10-day chick embryo brain causes superoxide anion and nitric oxide release and protein phosphorylation at serine, threonine, and tyrosine residues. These effects were abrogated by pretreatment with pertussis toxin, suggesting the involvement of a G-protein-coupled receptor (GPCR). Although specific N-formyl peptide receptors have so far been demonstrated only in mammals, a specific polyclonal antihuman-FPR1 antibody proved to bind to the membrane of both neurons and glial cells isolated from the chick brain. Immunoblot analysis revealed a single band corresponding to 60 kDa ca. A BLAST search and aa sequence alignments demonstrated that a number of avian 7-transmembrane (7TM) GPCRs share some homologies with the human FPR1. Furthermore, the CXCR4 ligand, SDF-1alpha, seems to compete with the antihuman-FPR1 polyclonal antibody used in our experiments. We thus advance the hypothesis that in birds one (or more) of the expressed 7TM GPCRs, most probably chemokine receptors belonging to the CXCR4 subfamily, also may act as fMLP receptors.

Biological role of the N-formyl peptide receptors

Ligation of N-formyl-methionyl-leucyl-phenylalanine (fMLP) to its specific cell surface receptors triggers different cascades of biochemical events, eventually leading to cellular activation. The formyl peptide receptors (FPRs) are members of the seven-transmembrane, G-protein coupled receptors superfamily, expressed at high levels on polymorphonuclear and mononuclear phagocytes. The main responses elicited upon ligation of formylated peptides, referred to as cellular activation, are those of morphological polarization, locomotion, production of reactive-oxygen species and release of proteolytic enzymes. FPRs have in recent years been shown to be expressed also in several non myelocytic populations, suggesting other unidentified functions for this receptor family, independent of the inflammatory response. Finally, a number of ligands acting as exogenous or host-derived agonists for FPRs, as well as ligands acting as FPRs antagonists, have been described, indicating that these receptors may be differentially modulated by distinct molecules.

Transfected β3- but Not β2-Adrenergic Receptors Regulate Cystic Fibrosis Transmembrane Conductance Regulator Activity via a New Pathway Involving the Mitogen-Activated Protein Kinases Extracellular Signal-Regulated Kinases

We have shown previously that in a heterologous mammalian expression system A549 cells, beta3-adrenoceptor (beta3-AR) stimulation regulates the activity of cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel. The present investigation was carried out to determine the signaling pathway involved in this regulation. A549 cells were intranuclearly injected with plasmids encoding human CFTR and beta3-AR. CFTR activity was functionally assessed by microcytofluorimetry. The application of 1 microM 4-[3-t-butylamino-2-hydroxypropoxy]benzimidazol-2-1 hydrochloride (CGP-12177), a beta3-AR agonist, produced a CFTR activation that was not abolished by protein kinase A inhibitors. In pertussis toxin-pretreated cells, the CFTR activation induced by CGP-12177 was abolished. The overexpression of beta-adrenoceptor receptor kinase, an inhibitor of betagamma subunits, abolished the CGP-12177-induced CFTR activation, suggesting the involvement of betagamma subunits of Gi/o proteins. The pretreatment of A549 cells with selective inhibitors of either phosphoinositide 3-kinase (PI3K), wortmannin, and 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride (LY294002), or extracellular signal-regulated kinases 1 and 2 (ERK1/2) mitogen-activated protein kinase (MAPK), 2'-amino-3'-methoxyflavone (PD98059), and 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophynyltio)butadiene (U0126), abolished the effects of CGP-12177 on the CFTR activity. Immunohistochemical assays showed that only the cells expressing beta3-AR exhibited MAPK activation in response to CGP-12177. Furthermore, CFTR activity increased in cells pretreated with 10% fetal bovine serum both in A549 cells injected only with CFTR and in T84 cells, which endogenously express CFTR, indicating that CFTR activity can be regulated by the MAPK independently of the beta3-AR stimulation. In conclusion, we have demonstrated that CFTR is regulated through a Gi/o/PI3K/ERK1/2 MAPK signaling cascade dependently or not on an activation of beta3-ARs. This pathway represents a new regulation for CFTR.

Identification of a novel family of G protein-coupled receptor associated sorting proteins

During the past few years several new interacting partners for G protein-coupled receptors (GPCRs) have been discovered, suggesting that the activity of these receptors is more complex than previously anticipated. Recently, candidate G protein-coupled receptor associated sorting protein (GASP-1) has been identified as a novel interacting partner for the delta opioid receptor and has been proposed to determine the degradative fate of this receptor. We show here that GASP-1 associates in vitro with other opioid receptors and that the interaction domain in these receptors is restricted to a small portion of the carboxyl-terminal tail, corresponding to helix 8 in the three-dimensional structure of rhodopsin. In addition, we show that GASP-1 interacts with COOH-terminus of several other GPCRs from subfamilies A and B and that two conserved residues within the putative helix 8 of these receptors are critical for the interaction with GASP-1. In situ hybridization and northern blot analysis indicate that GASP-1 mRNA is mainly distributed throughout the central nervous system, consistent with a potential interaction with numerous GPCRs in vivo. Finally, we show that GASP-1 is a member of a novel family comprising at least 10 members, whose genes are clustered on chromosome X. Another member of the family, GASP-2, also interacts with the carboxyl-terminal tail of several GPCRs. Therefore, GASP proteins may represent an important protein family regulating GPCR physiology.

The human formyl peptide receptor as model system for constitutively active G-protein-coupled receptors

According to the two-state model of G-protein-coupled receptor (GPCR) activation, GPCRs isomerize from an inactive (R) state to an active (R*) state. In the R* state, GPCRs activate G-proteins. Agonist-independent R/R* isomerization is referred to as constitutive activity and results in an increase in basal G-protein activity, i.e. GDP/GTP exchange. Agonists stabilize the R* state and further increase, whereas inverse agonists stabilize the R state and decrease, basal G-protein activity. Constitutive activity is observed in numerous wild-type GPCRs and disease-causing GPCR mutants with increased constitutive activity. The human formyl peptide receptor (FPR) exists in several isoforms (FPR-26, FPR-98 and FPR-G6) and activates chemotaxis and cytotoxic cell functions of phagocytes through G(i)-proteins. Studies in HL-60 leukemia cell membranes demonstrated inhibitory effects of Na(+) and pertussis toxin on basal G(i)-protein activity, suggesting that the FPR is constitutively active. However, since HL-60 cells express several constitutively active chemoattractant receptors, analysis of constitutive FPR activity was difficult. Sf9 insect cells do not express chemoattractant receptors and G(i)-proteins and provide a sensitive reconstitution system for FPR/G(i)-protein coupling. Such expression studies showed that FPR-26 is much more constitutively active than FPR-98 and FPR-G6 as assessed by the relative inhibitory effects of Na(+) and of the inverse agonist cyclosporin H on basal G(i)-protein activity. Site-directed mutagenesis studies suggest that the E346A exchange in the C-terminus critically determines dimerization and constitutive activity of FPR. Moreover, N-glycosylation of the N-terminus seems to be important for constitutive FPR activity. Finally, we discuss some future directions of research.

A functional assay for G-protein-coupled receptors using stably transformed insect tissue culture cell lines

Insect cells are an underexplored resource for functional G-protein-coupled receptor (GPCR) assays, despite a strong record in biochemical (binding) assays. Here we describe the use of vectors capable of creating stably transformed insect cell lines to generate a cell-based functional GPCR assay. This assay employs the luminescent photoprotein aequorin and the promiscuous G-protein subunit Galpha16 and is broadly applicable to human GPCRs. We demonstrate that the assay can quantitate ligand concentration-activity relationships for seven different human GPCRs, can differentiate between partial and full agonists, and can determine rank order potencies for both agonists and antagonists that match those seen with other assay systems. Human Galpha16 improves signal strength but is not required for activity with some receptors. The coexpression of human and bovine betagamma subunits and/or phospholipase Cbeta makes no difference to agonist efficacy or potency. Two different receptors expressed in the same cell line respond to their specific agonists, and two different cell lines (Sf9 and High 5) are able to functionally detect the same expressed GPCR. Sf9 cells have the capability to produce fully functional human receptors, allied to a low background of endogenous receptors, and so are a valuable system for investigating orphan GPCRs and receptor dimerization.

Cross signaling, cell specificity, and physiology

The literature on intracellular signal transduction presents a confusing picture: every regulatory factor appears to be regulated by all signal transduction cascades and to regulate all cell processes. This contrasts with the known exquisite specificity of action of extracellular signals in different cell types in vivo. The confusion of the in vitro literature is shown to arise from several causes: the inevitable artifacts inherent in reductionism, the arguments used to establish causal effect relationships, the use of less than adequate models (cell lines, transfections, acellular systems, etc.), and the implicit assumption that networks of regulations are universal whereas they are in fact cell and stage specific. Cell specificity results from the existence in any cell type of a unique set of proteins and their isoforms at each level of signal transduction cascades, from the space structure of their components, from their combinatorial logic at each level, from the presence of modulators of signal transduction proteins and of modulators of modulators, from the time structure of extracellular signals and of their transduction, and from quantitative differences of expression of similar sets of factors.

Molecular, structural, and cellular biology of follitropin and follitropin receptor

Follitropin and the follitropin receptor are essential for normal gamete development in males and females. This review discusses the molecular genetics and structural and cellular biology of the follitropin/follitropin receptor system. Emphasis is placed on the human molecules when possible. The structure and regulation of the genes for the follitropin beta subunit and the follitropin receptor is discussed. Control of systemic and cellular protein levels is explained. The structural biology of each protein is described, including protein structure, motifs, and activity relationships. Finally, the follitropin/follitropin receptor signal transduction system is discussed.

Gbeta gamma isoforms selectively rescue plasma membrane localization and palmitoylation of mutant Galphas and Galphaq

Mutation of Galpha(q) or Galpha(s) N-terminal contact sites for Gbetagamma resulted in alpha subunits that failed to localize at the plasma membrane or undergo palmitoylation when expressed in HEK293 cells. We now show that overexpression of specific betagamma subunits can recover plasma membrane localization and palmitoylation of the betagamma-binding-deficient mutants of alpha(s) or alpha(q). Thus, the betagamma-binding-defective alpha is completely dependent on co-expression of exogenous betagamma for proper membrane localization. In this report, we examined the ability of beta(1-5) in combination with gamma(2) or gamma(3) to promote proper localization and palmitoylation of mutant alpha(s) or alpha(q). Immunofluorescence localization, cellular fractionation, and palmitate labeling revealed distinct subtype-specific differences in betagamma interactions with alpha subunits. These studies demonstrate that 1) alpha and betagamma reciprocally promote the plasma membrane targeting of the other subunit; 2) beta(5), when co-expressed with gamma(2) or gamma(3), fails to localize to the plasma membrane or promote plasma membrane localization of mutant alpha(s) or alpha(q); 3) beta(3) is deficient in promoting plasma membrane localization of mutant alpha(s) and alpha(q), whereas beta(4) is deficient in promoting plasma membrane localization of mutant alpha(q); 4) both palmitoylation and interactions with betagamma are required for plasma membrane localization of alpha.

Regulation of interleukin-12 production by G-protein-coupled receptors

G-protein-coupled receptors have long been known to play a critical role in the recruitment and migration of leukocytes to areas of inflammation. This review will focus, however, on emerging data that G-protein-coupled receptors can modulate cytokine production by antigen-presenting cells including interleukin-12 and tumor necrosis factor-alpha and may therefore play a significant role in the regulation of innate and acquired immunity.

Electroconvulsive seizures modulate levels of thyrotropin releasing hormone and related peptides in rat hypothalamus, cingulate and lateral cerebellum

We have studied the neuroanatomic extent of electroconvulsive (ECS)-responsive prepro-TRH and TRH-related gene expression and its possible interaction with forced swimming. Young adult male Wistar rats were treated in a 2x2 Latin square protocol of swimming, no swimming, three daily ECS or sham ECS. Sixteen different brain regions were dissected and immunoreactivity measured for TRH (pGlu-His-Pro-NH(2)); TRH-Gly, a TRH precursor; Ps4, a prepro-TRH-derived TRH-enhancing decapeptide, and EEP (pGlu-Glu-Pro-NH(2)). ECS, in addition to elevating TRH-immunoreactivity (TRH-IR), TRH-Gly-IR, Ps4-IR and EEP-IR levels in the limbic regions, as we have previously reported, also significantly increased Ps4-IR levels in hypothalamus, posterior cingulate and lateral cerebellum, and increased TRH-Gly-IR levels in hypothalamus. Interestingly, the combination of ECS and swimming significantly reduced the levels of TRH-Gly-IR in the anterior cingulate compared to the sham ECS-no swim group. The combined use of high-pressure liquid chromatography and the EEP radioimmunoassay (RIA) revealed that pGlu-Tyr-Pro-NH(2) and/or pGlu-Phe-Pro-NH(2) occur in amygdala, anterior cingulate, frontal cortex, entorhinal cortex, lateral cerebellum and striatum and make a substantial contribution to the EEP-IR and TRH-IR. We conclude that ECS can alter the expression and secretion of TRH-related peptides in the hypothalamus, cingulate and lateral cerebellum. Such effects have not previously been reported in these limbic and extra-limbic regions which are increasingly implicated in the autonomic, behavioral and volitional changes which accompany severe depression and its treatment.

G(i) activator region of alpha(2A)-adrenergic receptors: distinct basic residues mediate G(i) versus G(s) activation

The structural determinants of G protein coupling versus activation by G protein-coupled receptors are not well understood. We examine the role of two distinct basic regions in the carboxyl terminal portion of the third intracellular loop of the alpha(2A)-adrenergic receptor to dissect these aspects of function. Changing three arginines to alanines by mutagenesis and stable expression in Chinese hamster ovary-K1 cells impaired the alpha(2)-adrenergic receptor G(s)-mediated stimulation of cyclic AMP (cAMP) accumulation, whereas G(i)-mediated inhibition was normal. When two (B2) or three (B3) basic residues closer to transmembrane span 6 were mutated to alanine, normal ligand binding was observed, but G(i)-mediated inhibition of cAMP accumulation showed 20-fold and 50-fold decreases in agonist potency for the B2 and B3 mutants, respectively. Surprisingly, a normal G(s) response was seen for the B2 mutant, and the B3 mutant showed only a 6-fold decrease in agonist potency. Mutation of both the three alanines and B3 residues to alanines showed a 200-fold decrease in agonist potency for G(i)-mediated inhibition of cAMP accumulation, whereas the G(s) response was nearly completely eliminated. The three basic residues (which include the BB of the BBXXF motif) play a role as G(i) activators rather than in receptor-G protein coupling, because high-affinity agonist binding is intact. Thus, we have identified three basic residues required for activation of G(i) but not required for receptor-G protein coupling. Also, distinct basic residues are required for optimal G(i) and G(s) responses, defining a microspecificity determinant within the carboxyl terminal portion of the third intracellular loop of the alpha(2a) adrenergic receptor.