G-Protein-coupled receptor-protein interactions: basis for new concepts on receptor structure and function

New Strains of Akkermansia muciniphila and Faecalibacterium prausnitzii are Effective for Improving the Muscle Strength of Mice with Immobilization-Induced Muscular Atrophy

Muscular atrophy is a muscle disease in which muscle mass and strength decrease due to aging, injury, metabolic disorders, or chronic conditions. Proteins in muscle tissue are degraded by the ubiquitin-proteasome pathway, and atrophy accelerates this pathway. Akkermansia muciniphila and Faecalibacterium prausnitzii strains are effective agents against metabolic and inflammatory diseases in next-generation probiotic research. In this study, we evaluated the efficacy of A. muciniphila strain EB-AMDK19 and F. prausnitzii strain EB-FPDK11 in a mouse model of muscular atrophy, since atrophy inhibits energy metabolism and immune activation. After oral administration of each strain for 4 weeks, the hind legs of the mice were fixed with a plaster cast to immobilize them for a week. As a result, the administration of EB-AMDK19 and EB-FPDK11 strains improved grip strength but did not increase muscle mass. At the molecular level, A. muciniphila and F. prausnitzii treatments decreased the expression levels of ubiquitin-proteasome genes, atrogin-1, MuRF, and cathepsin L. They increased the expression level of the mitochondrial biogenesis regulatory gene, PGC-1α. The effect of the strains was confirmed by a decrease in myostatin. Furthermore, A. muciniphila and F. prausnitzii modulated the immune function by enhancing ZO-1 and inhibiting IL-6. In particular, EB-AMDK19 promoted the expression of IL-10, an anti-inflammatory cytokine. These results suggest that A. muciniphila and F. prausnitzii may have beneficial effects on muscular atrophy, verified by newly isolated EB-AMDK19 and EB-FPDK11 as potential next-generation probiotics.

Exogenous SERP1 attenuates restenosis by restoring GLP-1 receptor activity in diabetic rats following vascular injury

The activity of glucagon-like peptide 1 (GLP-1R) is essential for preventing restenosis following vascular injury; however, the mechanism of dysfunctional GLP-1R glycosylation and ways to enhance the activity of GLP-1R on vascular surfaces in diabetic patients are poorly understood. In the present study, we investigated the N-glycosylation level and role of stress-associated endoplasmic reticulum protein 1 (SERP1) in preventing restenosis following carotid injury in diabetic rats. Our results showed that N-glycosylation levels in both rat aortic endothelial cells (RAOECs) and rat vascular smooth muscle cells (VSMCs) decreased gradually following glucose treatment in a concentration dependant manner. Furthermore, co-immunoprecipitation (Co-IP) analyses indicated that SERP1 could interact with GLP-1R in RAOECs and VSMCs. Moreover, SERP1 enhanced GLP-1R N-glycosylation and increased the production of phosphorylated endothelial nitric oxide synthase (eNOS) as well as proliferation of RAOECs. SERP1 also increased phosphorylated adenosine monophosphate activated protein kinase (AMPK) and decreased the migration of VSMCs. Importantly, intima media thickness (IMT) and neointimal hyperplasia were alleviated in the carotid artery of diabetic rats injected with SERP1 following balloon injury. We also found an increase in re-endothelialization and a decrease in VSMC proliferation in the carotid artery of diabetic rats injected with SERP1. In summary, the remarkable effects of SERP1 on reducing restenosis following vascular injury may contribute to future advancements in the treatment of diabetic vascular complications.

The GPRC6A receptor displays constitutive internalization and sorting to the slow recycling pathway

The class C G protein-coupled receptor GPRC6A is a putative nutrient-sensing receptor and represents a possible new drug target in metabolic disorders. However, the specific physiological role of this receptor has yet to be identified, and the mechanisms regulating its activity and cell surface availability also remain enigmatic. In the present study, we investigated the trafficking properties of GPRC6A by use of both a classical antibody feeding internalization assay in which cells were visualized using confocal microscopy and a novel internalization assay that is based on real-time measurements of fluorescence resonance energy transfer. Both assays revealed that GPRC6A predominantly undergoes constitutive internalization, whereas the agonist-induced effects were imperceptible. Moreover, postendocytic sorting was investigated by assessing the co-localization of internalized GPRC6A with selected Rab protein markers. Internalized GPRC6A was mainly co-localized with the early endosome marker Rab5 and the long loop recycling endosome marker Rab11 and to a much lesser extent with the late endosome marker Rab7. This suggests that upon agonist-independent internalization, GPRC6A is recycled via the Rab11-positive slow recycling pathway, which may be responsible for ensuring a persistent pool of GPRC6A receptors at the cell surface despite chronic agonist exposure. Distinct trafficking pathways have been reported for several of the class C receptors, and our results thus substantiate that non-canonical trafficking mechanisms are a common feature for the nutrient-sensing class C family that ensure functional receptors in the cell membrane despite prolonged agonist exposure.

The identification of novel proteins that interact with the GLP-1 receptor and restrain its activity

Glucagon-like peptide 1 receptor (GLP-1R) controls diverse physiological functions in tissues including the pancreatic islets, brain, and heart. To understand the mechanisms that control glucagon-like peptide 1 (GLP-1) signaling better, we sought to identify proteins that interact with the GLP-1R using a membrane-based split ubiquitin yeast two-hybrid (MYTH) assay. A screen of a human fetal brain cDNA prey library with an unliganded human GLP-1R as bait in yeast revealed 38 novel interactor protein candidates. These interactions were confirmed in mammalian Chinese hamster ovarian cells by coimmunoprecipitation. Immunofluorescence was used to show subcellular colocalization of the interactors with GLP-1R. Cluster analysis revealed that the interactors were primarily associated with signal transduction, metabolism, and cell development. When coexpressed with the GLP-1R in Chinese hamster ovarian cells, 15 interactors significantly altered GLP-1-induced cAMP accumulation. Surprisingly, all 15 proteins inhibited GLP-1-activated cAMP. Given GLP-1's prominent role as an incretin, we then focused on 3 novel interactors, SLC15A4, APLP1, and AP2M1, because they are highly expressed and localized to the membrane in mouse insulinoma β-cells. Small interfering RNA-mediated knockdown of each candidate gene significantly enhanced GLP-1-induced insulin secretion. In conclusion, we have generated a novel GLP-1R-protein interactome, identifying several interactors that suppress GLP-1R signaling. We suggest that the inhibition of these interactors may serve as a novel strategy to enhance GLP-1R activity.

A simple method for quantifying functional selectivity and agonist bias

Activation of seven-transmembrane (7TM) receptors by agonists does not always lead to uniform activation of all signaling pathways mediated by a given receptor. Relative to other ligands, many agonists are "biased" toward producing subsets of receptor behaviors. A hallmark of such "functional selectivity" is cell type dependence; this poses a particular problem for the profiling of agonists in whole cell test systems removed from the therapeutic one(s). Such response-specific cell-based variability makes it difficult to guide medicinal chemistry efforts aimed at identifying and optimizing therapeutically meaningful agonist bias. For this reason, we present a scale, based on the Black and Leff operational model, that contains the key elements required to describe 7TM agonism, namely, affinity (K(A) (-1)) for the receptor and efficacy (τ) in activating a particular signaling pathway. Utilizing a "transduction coefficient" term, log(τ/K(A)), this scale can statistically evaluate selective agonist effects in a manner that can theoretically inform structure-activity studies and/or drug candidate selection matrices. The bias of four chemokines for CCR5-mediated inositol phosphate production versus internalization is quantified to illustrate the practical application of this method. The independence of this method with respect to receptor density and the calculation of statistical estimates of confidence of differences are specifically discussed.

GPCR and voltage-gated calcium channels (VGCC) signaling complexes

Voltage-gated ion channels are transmembrane proteins that control nerve impulses and cell homeostasis. Signaling molecules that regulate ion channel activity and density at the plasma membrane must be specifically and efficiently coupled to these channels in order to control critical physiological functions such as action potential propagation. Although their regulation by G-protein receptor activation has been extensively explored, the assembly of ion channels into signaling complexes of GPCRs plays a fundamental role, engaging specific downstream -signaling pathways that trigger precise downstream effectors. Recent work has confirmed that GPCRs can intimately interact with ion channels and serve as -chaperone proteins that finely control their gating and trafficking in subcellular microdomains. This chapter aims to describe examples of GPCR-ion channel co-assembly, focusing mainly on signaling complexes between GPCRs and voltage-gated calcium channels.

Reorganization of the actin cytoskeleton upon G-protein coupled receptor signaling

The actin cytoskeleton is involved in a multitude of cellular responses besides providing structural support. While the role of the actin cytoskeleton in cellular processes such as trafficking and motility has been extensively studied, reorganization of the actin cytoskeleton upon signaling by G-protein coupled receptors (GPCRs) represents a relatively unexplored area. The G-protein coupled receptor superfamily is an important protein family in mammals, involved in signal transduction across membranes. G-protein coupled receptors act as major signaling hubs and drug targets. The serotonin(1A) receptor is a representative member of the G-protein coupled receptor superfamily and plays a crucial role in the generation and modulation of various cognitive, developmental and behavioral functions. In order to monitor the changes in the actin cytoskeleton upon serotonin(1A) receptor signaling in a quantitative manner, we developed an approach based on high magnification imaging of F-actin in cells, followed by image reconstruction. Our results suggest that the actin cytoskeleton is reorganized in response to serotonin(1A) receptor signaling. In addition, we show that reorganization of the actin cytoskeleton is strongly dependent on adenosine 3',5'-cyclic monophosphate level, and is mediated by the activation of protein kinase A. Our results are consistent with the possibility of a feedback mechanism involving the actin cytoskeleton, adenosine 3',5'-cyclic monophosphate level and the serotonin(1A) receptor.

Enhanced hypotensive, bradycardic, and hypnotic responses to alpha2-adrenergic agonists in spinophilin-null mice are accompanied by increased G protein coupling to the alpha2A-adrenergic receptor

We previously identified spinophilin as a regulator of alpha(2) adrenergic receptor (alpha(2)AR) trafficking and signaling in vitro and in vivo (Science 304:1940-1944, 2004). To assess the generalized role of spinophilin in regulating alpha(2)AR functions in vivo, the present study examined the impact of eliminating spinophilin on alpha(2)AR-evoked cardiovascular and hypnotic responses, previously demonstrated to be mediated by the alpha(2A)AR subtype, after systemic administration of the alpha(2)-agonists 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK14,304) and clonidine in spinophilin-null mice. Mice lacking spinophilin expression display dramatically enhanced and prolonged hypotensive, bradycardic, and sedative-hypnotic responses to alpha(2)AR stimulation. Whereas these changes in sensitivity to alpha(2)AR agonists occur independent of any changes in alpha(2A)AR density or intrinsic affinity for agonist in the brains of spinophilin-null mice compared with wild-type control mice, the coupling of the alpha(2A)AR to cognate G proteins is enhanced in spinophilin-null mice. Thus, brain preparations from spinophilin-null mice demonstrate enhanced guanine nucleotide regulation of UK14,304 binding and evidence of a larger fraction of alpha(2A)AR in the guanine-nucleotide-sensitive higher affinity state compared with those from wild-type mice. These findings suggest that eliminating spinophilin expression in native tissues leads to an enhanced receptor/G protein coupling efficiency that contributes to sensitization of receptor mediated responses in vivo.

In search of analgesia: emerging roles of GPCRs in pain

Of all clinically marketed drugs, greater than thirty percent are modulators of G protein-coupled receptors (GPCRs). Nearly 400 GPCRs (i.e., excluding odorant and light receptors) are encoded within the human genome, but only a small fraction of these seven-transmembrane proteins have been identified as drug targets. Chronic pain affects more than one-third of the population, representing a substantial societal burden in use of health care resources and lost productivity. Furthermore, currently available treatments are often inadequate, underscoring the significant need for better therapeutic strategies. The expansion of the identified human GPCR repertoire, coupled with recent insights into the function and structure of GPCRs, offers new opportunities for the development of novel analgesic therapeutics.

Modulation of opioid receptor function by protein-protein interactions

Opioid receptors, MORP, DORP and KORP, belong to the family A of G protein coupled receptors (GPCR), and have been found to modulate a large number of physiological functions, including mood, stress, appetite, nociception and immune responses. Exogenously applied opioid alkaloids produce analgesia, hedonia and addiction. Addiction is linked to alterations in function and responsiveness of all three opioid receptors in the brain. Over the last few years, a large number of studies identified protein-protein interactions that play an essential role in opioid receptor function and responsiveness. Here, we summarize interactions shown to affect receptor biogenesis and trafficking, as well as those affecting signal transduction events following receptor activation. This article also examines protein interactions modulating the rate of receptor endocytosis and degradation, events that play a major role in opiate analgesia. Like several other GPCRs, opioid receptors may form homo or heterodimers. The last part of this review summarizes recent knowledge on proteins known to affect opioid receptor dimerization.

Receptor activity-modifying proteins differentially modulate the G protein-coupling efficiency of amylin receptors

Receptor activity-modifying proteins (RAMPs) 1, 2, and 3 are prototypic G protein-coupled receptor accessory proteins that can alter not only receptor trafficking but also receptor phenotype. Specific RAMP interaction with the calcitonin receptor (CTR) generates novel and distinct receptors for the peptide amylin; however, the role of RAMPs in receptor signaling is not understood. The current study demonstrates that RAMP interaction with the CTRa in COS-7 or HEK-293 cells leads to selective modulation of signaling pathways activated by the receptor complex. There was a 20- to 30-fold induction in amylin potency at CTR/RAMP1 (AMY1) and CTR/RAMP3 (AMY3) receptors, compared with CTR alone, for formation of the second-messenger cAMP that parallels an increase in amylin binding affinity. In contrast, only 2- to 5-fold induction of amylin potency was seen for mobilization of intracellular Ca++ or activation of ERK1/2. In addition, in COS-7 cells, the increase in amylin potency for Ca++ mobilization was 2-fold greater for AMY3 receptors, compared with AMY1 receptors and this paralleled the relative capacity of overexpression of Galphaq proteins to augment induction of high affinity 125I-amylin binding. These data demonstrate that RAMP-complexed receptors have a different signaling profile to CTRs expressed in the absence of RAMPs, and this is likely due to direct effects of the RAMP on G protein-coupling efficiency.

Reviews of anti-infective agents: maraviroc: the first of a new class of antiretroviral agents

Maraviroc is the first US Food and Drug Administration-approved drug from a new class of antiretroviral agents that targets a host protein, the chemokine receptor CCR5, rather than a viral target. Binding of maraviroc to this cell-surface protein results in blocking human immunodeficiency virus type 1 (HIV-1) attachment to the coreceptor and prevents the virus from entering CD4+ cells. In this review, we include the details of the discoveries that led to the development of this drug. The drug's pharmacology, including pharmacokinetics and drug interactions, is discussed, as are the clinical efficacy studies that led to licensure. HIV-1 mechanisms of resistance to maraviroc, assays to determine viral coreceptor use (tropism), drug safety, and clinical use of maraviroc are discussed at length.

GPCR-Kir channel signaling complexes: defining rules of engagement

Ion channels and G protein-coupled receptors (GPCRs) are integral transmembrane proteins vital to a multitude of cell signaling and physiological functions. Members of these large protein families are known to interact directly with various intracellular protein partners in a dynamic and isoform-dependent manner, ultimately shaping their life cycle and signal output. The family of G protein-gated inwardly rectifying potassium channels (Kir3 or GIRK) expressed in brain, heart, and endocrine tissues were recently shown to stably associate with several different GPCRs, forming the basis of a macromolecular ion channel-GPCR signaling complex. The molecular determinants that mediate and maintain GPCR-Kir3 channel complexes are currently not well understood. Recent findings and emerging hypotheses on the assembly and stability of multiprotein GPCR-Kir channel signaling complexes are discussed, highlighting distinct mechanisms used by different Kir channel families. These protein-protein interaction processes are crucial in determining both the synaptic response times and the extent of GPCR "cross-talk" in Kir3-mediated inhibitory synaptic transmission.

A role for GATA-6 in vertebrate chondrogenesis

The GATA family of transcription factors are known to play multiple critical roles in vertebrate developmental processes, including erythropoiesis, endoderm formation and cardiogenesis. There have been no previous demonstrations of a functional role for any GATA family member being associated with musculoskeletal development but we now identify a possible role for GATA-6 in chondrogenesis. We detect abundant levels of GATA-6 mRNA in precartilaginous condensations (PCCs) in both the axial and appendicular skeleton of mouse embryos and in committed primary chondrocyte precursors. We also show that the G-protein coupled receptor, Gpr49, is a target of GATA-6 regulation in differentiating embryonal carcinoma cells and that, in vivo, the expression domains of the two genes overlap within PCCs. Finally, we have identified conserved, canonical GATA binding sites within the Gpr49 gene locus, and show by EMSAs that GATA-6 can bind to these sites in vitro. These data therefore suggest that GATA-6 also plays a role in chondrogenesis and that Gpr49 is a potential direct target of GATA regulation in this process.

G protein-coupled receptor oligomerization provides the framework for signal discrimination

The idea that G protein-coupled receptors (GPCRs) may undergo homo- or hetero-oligomerization, although highly controversial up to a few years ago, has recently gained wide acceptance. The recognition that GPCRs may exhibit either dimeric or oligomeric structures is based upon a large body of biochemical and biophysical evidence. While much effort has been spent to demonstrate the mechanism(s) by which GPCRs interact with each other, the physiological relevance of this phenomenon remains rather elusive. GPCR oligomerization has been proposed to play a role in receptor ontogeny by either chaperoning protein folding or controlling trafficking to the cell surface. However, the acquisition of these roles does not rule out the possibility that oligomeric receptors may have additional functions, once they are brought to the cell surface. Herein, we propose that protein-protein as well as protein-lipid interactions may provide the structural basis for organizing distinct cell compartments along the plasma membrane where different extracellular signals may be perceived and discriminated.

Involvement of the 90-kDa Heat Shock Protein (Hsp-90) in CB2 Cannabinoid Receptor-Mediated Cell Migration: A New Role of Hsp-90 in Migration Signaling of a G Protein-Coupled Receptor

The endocannabinoid 2-arachidonoylglycerol (2-AG) enhances cell migration through the CB2 cannabinoid receptor. In this study, using an immunoprecipitation and mass spectrometry-based proteomic approach, we first identified the 90-kDa heat shock protein (Hsp90), a chaperone protein with novel signaling functions, as a CB2-interacting protein. The CB2/Hsp90 interaction was confirmed in human embryonic kidney 293 cells expressing transfected CB2 and in differentiated HL-60 cells expressing endogenous CB2, by coimmunoprecipitation and Western blot experiments, as well as by treatment with geldanamycin (GA), a specific Hsp90 inhibitor. Disruption of the CB2/Hsp90 interaction by treatment with GA or reducing Hsp90 levels with specific short interfering RNAs markedly inhibited 2-AG-induced cell migration, demonstrating that Hsp90 is crucial for 2-AG-induced cell migration. 2-AG treatment resulted in a CB2-mediated stimulation of Rac1 activity, and treatment with GA blocked 2AG-induced activation of Rac1. It is noteworthy that expression of the dominant-negative form of Rac1 reduced 2-AG-induced cell migration. These data demonstrate that 2-AG-induced activation of Rac1 is essential for 2-AG-induced cell migration, and the CB2/Hsp90 interaction is needed for 2-AG-induced activation of Rac1. Furthermore, 2-AG-induced Rac1 activation was sensitive to pertussis toxin treatment, hence involving G(i) proteins. In addition, treatment with GA significantly inhibited the CB2/Galpha(i2) interaction. As a whole, our data indicate that Hsp90 may serve as scaffold to keep the CB2 receptor and its signaling components, including Galpha(i2), in proximity, thus facilitating CB2-mediated signaling to cell migration through the G(i)-Rac1 pathway. By demonstrating that Hsp90 is essential for CB2-mediated signaling to cell migration, this study reveals a novel role of Hsp90 in the signaling events mediated by a G protein-coupled receptor.

Allosteric GPCR modulators: taking advantage of permissive receptor pharmacology

The past decade has witnessed a dramatic increase in the identification of allosteric modulators of G-protein-coupled receptor (GPCR) activity. Concomitantly, several new perspectives and hypotheses regarding the way ligands regulate GPCR signalling have also emerged. Here, we briefly discuss how the concepts of collateral efficacy and permissive agonism-antagonism intersect the field of allosteric GPCR modulation. Despite the challenges associated with detecting and quantifying the myriad of possible allosteric effects on GPCR activity, it is proposed that allosteric ligands offer the exciting prospect of engendering stimulus-bias in orthosteric ligand signalling, thus paving the way for not only receptor-selective but also signalling-pathway-selective therapies.

Opposite effects of PSD-95 and MPP3 PDZ proteins on serotonin 5-hydroxytryptamine2C receptor desensitization and membrane stability

PSD-95/Disc large/Zonula occludens 1 (PDZ) domain-containing proteins (PDZ proteins) play an important role in the targeting and the trafficking of transmembrane proteins. Our previous studies identified a set of PDZ proteins that interact with the C terminus of the serotonin 5-hydroxytryptamine (5-HT)(2C) receptor. Here, we show that the prototypic scaffolding protein postsynaptic density-95 (PSD-95) and another membrane-associated guanylate kinase, MAGUK p55 subfamily member 3 (MPP3), oppositely regulate desensitization of the receptor response in both heterologous cells and mice cortical neurons in primary culture. PSD-95 increased desensitization of the 5-HT(2C) receptor-mediated Ca(2+) response, whereas MPP3 prevented desensitization of the Ca(2+) response. The effects of the PDZ proteins on the desensitization of the Ca(2+) response were correlated with a differential regulation of cell surface expression of the receptor. Additional experiments were performed to assess how PDZ proteins globally modulate desensitization of the 5-HT(2C) receptor response in neurons, by using a peptidyl mimetic of the 5-HT(2C) receptor C terminus fused to the human immunodeficiency virus type-1 Tat protein transduction domain, which disrupts interaction between the 5-HT(2C) receptor and PDZ proteins. Transduction of this peptide inhibitor into cultured cortical neurons increased the desensitization of the 5-HT(2C) receptor-mediated Ca(2+) response. This indicates that, overall, interaction of 5-HT(2C) receptors with PDZ proteins inhibits receptor desensitization in cortical neurons.

Modeling activated states of GPCRs: the rhodopsin template

Activation of G Protein-Coupled Receptors (GPCRs) is an allosteric mechanism triggered by ligand binding and resulting in conformational changes transduced by the transmembrane domain. Models of the activated forms of GPCRs have become increasingly necessary for the development of a clear understanding of signal propagation into the cell. Experimental evidence points to a multiplicity of conformations related to the activation of the receptor, rendered important physiologically by the suggestion that different conformations may be responsible for coupling to different signaling pathways. In contrast to the inactive state of rhodopsin (RHO) for which several high quality X-ray structures are available, the structure-related information for the active states of rhodopsin and all other GPCRs is indirect. We have collected and stored such information in a repository we maintain for activation-specific structural data available for rhodopsin-like GPCRs, http://www.physiology.med.cornell.edu/GPCRactivation/gpcrindex.html . Using these data as structural constraints, we have applied Simulated Annealing Molecular Dynamics to construct a number of different active state models of RHO starting from the known inactive structure. The common features of the models indicate that TM3 and TM5 play an important role in activation, in addition to the well-established rearrangement of TM6. Some of the structural changes observed in these models occur in regions that were not involved in the constraints, and have not been previously tested experimentally; they emerge as interesting candidates for further experimental exploration of the conformational space of activated GPCRs. We show that none of the normal modes calculated from the inactive structure has a dominant contribution along the path of conformational rearrangement from inactive to the active forms of RHO in the models. This result may differentiate rhodopsin from other GPCRs, and the reasons for this difference are discussed in the context of the structural properties and the physiological function of the protein.