Constitutive Receptor Activity series Endogenous inverse agonists and constitutive receptor activity in the melanocortin system

GPR101: Modeling a constitutively active receptor linked to X-linked acrogigantism

GPR101 is a G protein-coupled receptor (GPCR) implicated in a rare form of genetic gigantism known as X-linked acrogigantism, or X-LAG. In particular, X-LAG patients harbor microduplications in the long arm of the X-chromosome that invariably include the GPR101 gene. Duplications of the GPR101 gene lead to the formation of a new chromatin domain that causes over-expression of the receptor in the pituitary tumors of the patients. Notably, GPR101 is a constitutively active receptor, which stimulates cells to produce the second messenger cyclic AMP (cAMP) in the absence of ligands. Moreover, GPR101 was recently reported to constitutively activate not only the cAMP pathway via Gs, but also other G protein subunits (Gq/11 and G12/13). Hence, chemicals that block the constitutive activity of GPR101, known as inverse agonists, have the potential to be useful for the development of pharmacological tools for the treatment of X-LAG. In this study, we provide structural insights into the putative structure of GPR101 based on in-house built homology models, as well as third party models based on the machine learning methods AlphaFold and AlphaFold-Multistate. Moreover, we report a molecular dynamics study, meant to further probe the constitutive activity of GPR101. Finally, we provide a structural comparison with the closest GPCRs, which suggests that GPR101 does not share their natural ligands. While this manuscript was under review, cryo-electron microscopy structures of GPR101 were reported. These structures are expected to enable computer-aided ligand discovery efforts targeting GPR101.

Pharmacological evaluation of MRAP proteins on Xenopus neural melanocortin signaling

Melanocortin-3 receptor (MC3R) and melanocortin-4 receptor (MC4R), two neural G protein-coupled receptors are known to be functionally critical for energy balance in vertebrates. As allosteric regulators of melanocortin receptors, melanocortin accessory proteins (MRAPs) are also involved in energy homeostasis. The interaction of MRAPs and melanocortin signaling was previously shown in mammals and zebrafish, but nothing had been reported in amphibians. As the basal class of tetrapods, amphibians occupy a phylogenetic transition between teleosts and terrestrial animals. Here we examined the evolutionary conservation of MC3R, MC4R, and MRAPs between diploid Xenopus tropicalis (xt-) and other chordates and investigated the pharmacological regulatory properties of MRAPs on the neural MC3R and MC4R signaling. Our results showed that xtMRAP and xtMRAP2 both exerted robust potentiation effect on agonist (α-MSH and adrenocorticotropin [ACTH]) induced activation and modulated the basal activity and cell surface translocation of xtMC3R and xtMC4R. In addition, the presence of two accessory proteins could convert xtMC3R and xtMC4R into ACTH-preferred receptors. These findings suggest that the presence of MRAPs exhibits fine control over the pharmacological activities of the neuronal MC3R and MC4R signaling in the Xenopus tropicalis, which is physiologically relevant with the complicated transition of feeding behaviors during their life history.

Functional Characterization of the Internal Symmetry of MRAP2 Antiparallel Homodimer

The melanocortin receptors are defined as a series of vital pharmaceutical targets to regulate neuronal appetite and maintain controllable body weight for mammals and teleosts. Melanocortin receptor accessory protein 2 (MRAP2) functions as an essential accessory player that modulates the surface translocation and binding to a variety of endogenous or synthetic hormones of central melanocortin-4 receptor (MC4R) signaling. MRAP2 is a single-transmembrane protein and could form a functional symmetric antiparallel homodimer topology. Here, we inverted the N-terminal, transmembrane, and C-terminal domains and generated six distinct conformational variants of the mouse MRAP2 to explore the functional orientations and the internal symmetry of MRAP2 dimers. These remolded MRAP2 mutants showed proper assembly of the antiparallel homodimer and binding to the MC4R, but slightly altered the regulatory profile on the surface expression and the ligand-stimulated cAMP cascades of MC4R. This study elucidated the importance of the orientation of each domain of the single-transmembrane protein and revealed the pharmacological properties of the internal symmetry of the antiparallel homodimer for MRAP2.

Under- to over-eating: how do serotonin receptors contribute?

In numerous pathological states, the brain can restrict food intake to a lethal level despite mounting requirements for energy as seen in adolescents with anorexia nervosa. How the brain reduces food intake to the point of death while eating is a cornerstone of survival that remains just as ‘cryptic’ as the association between anorexia and overeating. This review provides a recent snapshot of the neural underpinnings of the rewarding effects of anorexia that may compete with the adaptive decision-making process to eat, and with survival instinct. Among a plethora of factors, impaired activity of the serotonin receptors in the reward system underlies the ability of animals to self-impose food restriction, and the transition from under- to over-eating. However, the triumvirate association between serotonin, overeating and addiction appears unlikely. Considering the implication of the serotonin receptors in the hypothalamus, anorexia and bulimia nervosa could result from an impairment of a ‘synchronic activity’ between the autonomic and voluntary nervous systems.

Comparison of the activation kinetics of the M3 acetylcholine receptor and a constitutively active mutant receptor in living cells

Activation of G-protein-coupled receptors is the first step of the signaling cascade triggered by binding of an agonist. Here we compare the activation kinetics of the G(q)-coupled M(3) acetylcholine receptor (M(3)-AChR) with that of a constitutively active mutant receptor (M(3)-AChR-N514Y) using M(3)-AChR constructs that report receptor activation by changes in the fluorescence resonance energy transfer (FRET) signal. We observed a leftward shift in the concentration-dependent FRET response for acetylcholine and carbachol with M(3)-AChR-N514Y. Consistent with this result, at submaximal agonist concentrations, the activation kinetics of M(3)-AChR-N514Y were significantly faster, whereas at maximal agonist concentrations the kinetics of receptor activation were identical. Receptor deactivation was significantly faster with carbachol than with acetylcholine and was significantly delayed by the N514Y mutation. Receptor-G-protein interaction was measured by FRET between M(3)-AChR-yellow fluorescent protein (YFP) and cyan fluorescent protein (CFP)-Gγ(2). Agonist-induced receptor-G-protein coupling was of a time scale similar to that of receptor activation. As observed for receptor deactivation, receptor-G-protein dissociation was slower for acetylcholine than that for carbachol. Acetylcholine-stimulated increases in receptor-G-protein coupling of M(3)-AChR-N514Y reached only 12% of that of M(3)-AChR and thus cannot be kinetically analyzed. G-protein activation was measured using YFP-tagged Gα(q) and CFP-tagged Gγ(2). Activation of G(q) was significantly slower than receptor activation and indistinguishable for the two agonists. However, G(q) deactivation was significantly prolonged for acetylcholine compared with that for carbachol. Consistent with decreased agonist-stimulated coupling to G(q), agonist-stimulated G(q) activation by M(3)-AChR-N514Y was not detected. Taken together, these results indicate that the N514Y mutation produces constitutive activation of M(3)-AChR by decreasing the rate of receptor deactivation, while having minimal effect on receptor activation.

The neuroendocrine circuitry controlled by POMC, MSH, and AGRP

Obesity is one of the most challenging health problems worldwide. Over the past few decades, our knowledge concerning mechanisms of weight regulation has increased tremendously leading to the identification of the leptin-melanocortin pathway. The filling level of energy stores is signaled to the brain, and the information is integrated by hypothalamic nuclei, resulting in a well-orchestrated response to food intake and energy expenditure to ensure constant body weight. One of the key players in this system is proopiomelanocortin (POMC), a precursor of a variety of neuropeptides. POMC-derived alpha- and beta-MSH play an important role in energy homeostasis by activating melanocortin receptors expressed in the arcuate nucleus (MC3R) and in the nucleus paraventricularis (MC4R). Activation of these two G protein-coupled receptors is antagonized by agouti-related peptide (AgRP). Naturally occurring mutations in this system were identified in patients suffering from common obesity as well as in patients demonstrating a phenotype of severe early-onset obesity, adrenal insufficiency, red hair, and pale skin. Detailed understanding of the complex system of POMC-AgRP-MC3R-MC4R and their interaction with other hypothalamic as well as peripheral signals is a prerequisite to combat the obesity epidemic.

Melanocortins and body weight regulation: glucocorticoids, Agouti-related protein and beyond

In the intervening three decades since Panksepp observed for the first time that centrally administered α-melanocyte stimulating hormone decreased food intake (Panksepp and Meeker, 1976), a wealth of data have accrued to firmly establish melanocortin signaling as a central regulator of food intake and fat mass. Advances in molecular biology have not only allowed detailed studies of spontaneously occurring obese mice with altered melanocortin signaling to be undertaken but also permitted the generation of a plethora of mouse models with precise perturbations at critical steps in the melanocortin system to finesse further the cellular and molecular architecture of relevant pathways. In this article we focus in upon a number of these mouse models which continue to help us tease apart the complexities of this critical system. Further, we review data on the important interaction between pro-opiomelanocortin derived peptides and the adrenal system and the relationship between agonist and antagonist peptides acting at central melanocortin receptors.

The melanocortin-4 receptor: physiology, pharmacology, and pathophysiology

The melanocortin-4 receptor (MC4R) was cloned in 1993 by degenerate PCR; however, its function was unknown. Subsequent studies suggest that the MC4R might be involved in regulating energy homeostasis. This hypothesis was confirmed in 1997 by a series of seminal studies in mice. In 1998, human genetic studies demonstrated that mutations in the MC4R gene can cause monogenic obesity. We now know that mutations in the MC4R are the most common monogenic form of obesity, with more than 150 distinct mutations reported thus far. This review will summarize the studies on the MC4R, from its cloning and tissue distribution to its physiological roles in regulating energy homeostasis, cachexia, cardiovascular function, glucose and lipid homeostasis, reproduction and sexual function, drug abuse, pain perception, brain inflammation, and anxiety. I will then review the studies on the pharmacology of the receptor, including ligand binding and receptor activation, signaling pathways, as well as its regulation. Finally, the pathophysiology of the MC4R in obesity pathogenesis will be reviewed. Functional studies of the mutant MC4Rs and the therapeutic implications, including small molecules in correcting binding and signaling defect, and their potential as pharmacological chaperones in rescuing intracellularly retained mutants, will be highlighted.

Neutralizing endogenous chemokines with small molecules. Principles and potential therapeutic applications

Regulation of cellular responses to external stimuli such as hormones, neurotransmitters, or cytokines is achieved through the control of all steps of the complex cascade starting with synthesis, going through maturation steps, release, distribution, degradation and/or uptake of the signalling molecule interacting with the target protein. One possible way of regulation, referred to as scavenging or neutralization of the ligand, has been increasingly studied, especially for small protein ligands. It shows innovative potential in chemical biology approaches as well as in disease treatment. Neutralization of protein ligands, as for example cytokines or chemokines can lead to the validation of signalling pathways under physiological or pathophysiological conditions, and in certain cases, to the development of therapeutic molecules now used in autoimmune diseases, chronic inflammation and cancer treatment. This review explores the field of ligand neutralization and tries to determine to what extent small chemical molecules could substitute for neutralizing antibodies in therapeutic approaches.

Involvement of the V2 vasopressin receptor in adaptation to limited water supply

Mammals adapted to a great variety of habitats with different accessibility to water. In addition to changes in kidney morphology, e.g. the length of the loops of Henle, several hormone systems are involved in adaptation to limited water supply, among them the renal-neurohypophysial vasopressin/vasopressin receptor system. Comparison of over 80 mammalian V2 vasopressin receptor (V2R) orthologs revealed high structural and functional conservation of this key component involved in renal water reabsorption. Although many mammalian species have unlimited access to water there is no evidence for complete loss of V2R function indicating an essential role of V2R activity for survival even of those species. In contrast, several marsupial V2R orthologs show a significant increase in basal receptor activity. An increased vasopressin-independent V2R activity can be interpreted as a shift in the set point of the renal-neurohypophysial hormone circuit to realize sufficient water reabsorption already at low hormone levels. As found in other desert mammals arid-adapted marsupials show high urine osmolalities. The gain of basal V2R function in several marsupials may contribute to the increased urine concentration abilities and, therefore, provide an advantage to maintain water and electrolyte homeostasis under limited water supply conditions.

Characterisation of the melanocortin-1 receptor gene in alpaca and identification of possible markers associated with phenotypic variations in colour

The aim of this study was to determine if any correlation exists between melanocortin-1 receptor (MC1R) polymorphisms and skin and fibre colour in alpacas. Primers capable of amplifying the entire alpaca MC1R gene were designed from a comparative alignment of Bos taurus and Mus musculus MC1R gene sequences. The complete MC1R gene of 41 alpacas exhibiting a range of fibre colours, and which were sourced from farms across Australia, was sequenced from PCR products. Twenty-one single nucleotide polymorphisms were identified within MC1R. Two of these polymorphisms (A82G and C901T) have the potential to reduce eumelanin production by disrupting the activity of MC1R. No agreement was observed between fibre colour alone and MC1R genotype in the 41 animals in this study. However, when the animals were assigned to groups based on the presence or absence of eumelanin in their fibre and skin, only animals that had at least one allele with the A82/C901 combination expressed eumelanin. We propose that A82/C901 is the wild-type dominant ‘E’ MC1R allele, while alpacas with either G82/T901 or G82/Y901 are homozygous for the recessive ‘e’ MC1R allele and are therefore unable to produce eumelanin.

Constitutive activation of G protein-coupled receptors and diseases: insights into mechanisms of activation and therapeutics

The existence of constitutive activity for G protein-coupled receptors (GPCRs) was first described in 1980s. In 1991, the first naturally occurring constitutively active mutations in GPCRs that cause diseases were reported in rhodopsin. Since then, numerous constitutively active mutations that cause human diseases were reported in several additional receptors. More recently, loss of constitutive activity was postulated to also cause diseases. Animal models expressing some of these mutants confirmed the roles of these mutations in the pathogenesis of the diseases. Detailed functional studies of these naturally occurring mutations, combined with homology modeling using rhodopsin crystal structure as the template, lead to important insights into the mechanism of activation in the absence of crystal structure of GPCRs in active state. Search for inverse agonists on these receptors will be critical for correcting the diseases cause by activating mutations in GPCRs. Theoretically, these inverse agonists are better therapeutics than neutral antagonists in treating genetic diseases caused by constitutively activating mutations in GPCRs.

Endosomal colocalization of melanocortin-3 receptor and beta-arrestins in CAD cells with altered modification of AKT/PKB

The melanocortin 3-receptor is involved in regulating energy metabolism, body fluid composition and inflammatory responses. Melanocortin receptors function by activating membrane bound adenylate cyclase. However, the literature reports indicate that some G protein coupled receptors (GPCRs) can also activate mitogen activated protein kinase (MAPK) or phosphoinositide 3 kinase (PI3K) signaling pathways consequent to their endocytosis. These studies were undertaken to evaluate the role of these pathways in MC3R signaling in brain-stem neuronal cells. Recruitment of arrestins is implicated in the activation of secondary pathways by GPCRs and our data shows the colocalization of either arrestin B1 or B2 with MC3R in endosomes. An alteration in PKB phosphorylation pattern was observed in MC3R expressing cells independent of agonist stimulation. MC3R transfectants exhibited increased proliferation rates and inhibition of PKB pathway with triciribine abrogated cell proliferation in both vector control and MC3R transfectants. PKB is constitutively active in proliferating CAD cells but could be further activated by culturing the cells in differentiation medium. These studies suggest that the AKT/PKB pathway plays an important role in the proliferation of CAD cells and suggest a link between MC3R and cell growth pathways that may involve the alteration of AKT/PKB signaling pathway.

Role of third intracellular loop of the melanocortin 4 receptor in the regulation of constitutive activity

The melanocortin 4 receptor (MC4R) has been reported to display constitutive activity, which is probably relevant to the maintenance of a normal energy balance. Among the clinically reported mutants of MC4R in human obesity patients, we investigated the functional characteristics of seven mutants characterized by mutations in the third intracellular (i3) loop of MC4R. Via a CRE (cAMP responsive element)-mediated luciferase reporter gene assay, we show that most of these mutants displayed significantly reduced basal activity with reduced reporter gene activity, whereas the P230L mutant manifested significantly increased basal activity. When the dominant negative G(s) mutant was co-expressed, the majority of the mutants, including the P230L mutant, showed reduced basal activity. These results suggest that the i3 loop of MC4R is essential not only for the functional activity but also for the regulation and maintenance of an optimal constitutive activity of MC4R in association with G protein coupling, in the control of energy homeostasis.

Inhibition of milk ingestion and growth after administration of a neutral cannabinoid CB1 receptor antagonist on the first postnatal day in the mouse

We have shown previously that neonatal exposure to the cannabinoid CB1 receptor antagonist/inverse agonist rimonabant (SR141716) interfered with suckling and development. However, it was not clear whether the developmental deficiencies were induced by neutral CB1 receptor blockade, thereby inhibiting endogenous cannabinoid "tone," or by inverse agonist reduction of constitutive CB1 receptors. CB1 receptor blockade supports our hypothesis that low CB1 receptor concentrations and/or reduced endocannabinoid levels underlie infant nonorganic failure to thrive (NOFTT). Inverse agonism implies that lower constitutive CB1 receptor activity may be responsible for impaired food intake in newborns. In the present study, we injected the neutral CB1 receptor antagonist 5-(4-chlorophenyl)-3-[(E)-2-cyclohexylethenyl]-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole (VCHSR1) to 1-d-old mouse pups and recorded weight gain, gastric milk contents (milkbands), axillary temperature, and survival between age 1 and 10 d. The results showed a dose-related interference with all measures. These data show that (1) growth failure induced by rimonabant is generalized to another CB1 antagonist and (2) cannabinoid CB1 receptor activation by endocannabinoids is essential for normal milk ingestion and development in mice. This supports our hypothesis that endocannabinoid deficiency and perhaps CB1 receptor dysfunction represents the uncharacterized biologic vulnerability, which underlies NOFTT.

Versatility of GPCR recognition by drugs: from biological implications to therapeutic relevance

Most drugs acting on G-protein-coupled receptors (GPCRs) are classically defined as agonists, partial agonists or antagonists. This simplified classification seems sufficient to explain most of their therapeutic properties. The more recent description of inverse agonism has helped to revise theoretical models of GPCR function, but the therapeutic implications of the new concepts remain clearly restricted. Further complexity has arisen with demonstrations that a given receptor can adopt various conformations that support coupling with distinct G proteins. Because the related signaling pathways seem to be differentially affected by some ligands, the concept of 'functional selectivity' has been proposed, calling for a revision of the definitions of agonism and intrinsic efficacy. Evidence of complexity in G-protein coupling and examples of functional selectivity are accumulating, opening perspectives for drug development. Although such complexity should be regarded as an opportunity to gain pharmacological specificity, unraveling the physiological implications of these concepts is essential before their therapeutic relevance can be defined.

Pharmacogenomic and Structural Analysis of Constitutive G Protein–Coupled Receptor Activity

G protein-coupled receptors (GPCRs) respond to a chemically diverse plethora of signal transduction molecules. The notion that GPCRs also signal without an external chemical trigger, i.e., in a constitutive or spontaneous manner, resulted in a paradigm shift in the field of GPCR pharmacology. The discovery of constitutive GPCR activity and the fact that GPCR binding and signaling can be strongly affected by a single point mutation drew attention to the evolving area of GPCR pharmacogenomics. For a variety of GPCRs, point mutations have been convincingly linked to human disease. Mutations within conserved motifs, known to be involved in GPCR activation, might explain the properties of some naturally occurring, constitutively active GPCR variants linked to disease. In this review, we provide a brief historical introduction to the concept of constitutive receptor activity and the pharmacogenomic and structural aspects of constitutive receptor activity.