Melanin-concentrating hormone is the cognate ligand for the orphan G-protein-coupled receptor SLC-1

The basic residues in the membrane-proximal C-terminal tail of the rat melanin-concentrating hormone receptor 1 are required for receptor function

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide that plays a key role in food intake. It acts through two G protein-coupled receptors (GPCRs), MCH1R and MCH2R, of which MCH1R is the primary regulator of food intake. We have previously reported that N-linked glycosylation of the extracellular domain of MCH1R is necessary for cell surface expression and signal transduction. We now report a role for the rat MCH1R C-terminal region. We constructed serial C-terminal truncation mutants and determined the resulting changes in protein expression, cell surface expression, ligand binding, and MCH-stimulated calcium influx. By analyzing two mutants, deltaT317 (deletion of 36 C-terminal amino acids) and deltaR321 (deletion of 32 C-terminal amino acids), we found that the region between Phe(318) and Arg(321)) was responsible for signal transduction. A more detailed analysis was performed with single or multiple residue mutations. Single mutations of Arg(319), Lys(320), or Arg(321) exhibited a decrease in the cell surface expression, whereas mutations of either Arg(319) or Lys(320), but not Arg(321), showed a significant reduction in the calcium influx. Furthermore, simultaneous mutations of Arg(319) and Lys(320) produced a pronounced decrease in the efficacy of calcium influx stimulation compared with single mutations. A computational analysis revealed a dibasic amino acid motif that is conserved among many class 1 GPCRs and may be part of the amphiphilic cytoplasmic helix 8 (an eight-cytoplasmic helix). Our results therefore provide new insights into the role of the putative helix 8 in the regulation of GPCR function.

Identification and characterization of single-nucleotide polymorphisms in MCH-R1 and MCH-R2

OBJECTIVE

To identify and functionally characterize single-nucleotide polymorphisms (SNPs) in melanin-concentrating hormone (MCH)-R1 and -R2.

RESEARCH METHODS AND PROCEDURES

The entire coding regions and intron/exon splice junction regions of MCH-R1 and MCH-R2 were sequenced from anonymous white (n=45) and African-American (n=46) individuals. DNA was analyzed, and SNPs were identified using Phred, Phrap, and Consed software. DNA constructs containing MCH-R1 and MCH-R2 SNPs were generated and expressed in CHO cells. The effect of the SNPs in MCH-R1 and MCH-R2 were assessed in receptor binding assays and functional assays measuring changes in intracellular cAMP and Ca2+ levels.

RESULTS

We identified 12 SNPs in the MCH-R1 gene. Two of these SNPs are in coding regions, and one produces an arginine-for-glycine substitution at residue 34 in the MCH-R1 sequence. This SNP is present at a minor allele frequency of 15% in the African-American population tested in this study. We identified eight SNPs in the MCH-R2 gene. Four of these SNPs are in coding regions, and two produce amino acid substitutions. Lysine substitutes for arginine at residue 63 of the African-American population, and glutamine substitutes for arginine at residue 152 in whites (minor allele frequency of 2% for both SNPs). No changes in receptor binding or functional signaling were observed with the SNP mutations in MCH-R1 or MCH-R2.

DISCUSSION

These data indicate that potential therapeutics designed to act at the MCH receptor are unlikely to have altered effects in subpopulations that express variant forms of MCH-R1 or MCH-R2.

Osteoporosis in MCHR1-deficient mice

It is well recognized that the hypothalamus is of central importance in the regulation of food intake and fat mass. Recent studies indicate that it also plays an important role in the regulation of bone mass. Melanin concentrating hormone (MCH) is highly expressed in the hypothalamus and has been implicated in regulation of energy homeostasis. We developed MCHR1 inactivated mice to evaluate the physiological role of this receptor. Interestingly, the MCHR1(-/-) mice have osteoporosis, caused by a reduction in the cortical bone mass, while the amount of trabecular bone is unaffected. The reduction in cortical bone mass is due to decreased cortical thickness. Serum levels of c-telopeptide, a marker of bone resorption, are increased in MCHR1(-/-) mice, indicating that the MCHR1(-/-) mice have a high bone turnover osteoporosis. In conclusion, the MCHR1(-/-) mice have osteoporosis, indicating that MCHR1-signalling is involved in a tonic stimulation of bone mass.

Physiological properties of hypothalamic MCH neurons identified with selective expression of reporter gene after recombinant virus infection

Neurons that synthesize melanin-concentrating hormone (MCH) may modulate arousal and energy homeostasis. The scattered MCH neurons have been difficult to study, as they have no defining morphological characteristics. We have developed a viral approach with AAV for selective long-term reporter gene (GFP) expression in MCH neurons, allowing the study of their cellular physiology in hypothalamic slices. MCH neurons showed distinct membrane properties compared to other neurons infected with the same virus with a cytomegalovirus promoter. Transmitters of extrahypothalamic arousal systems, including norepinephrine, serotonin, and the acetylcholine agonist muscarine, evoked direct inhibitory actions. Orexigenic neuropeptide Y was inhibitory by pre- and postsynaptic mechanisms; an anorexigenic melanocortin agonist had no effect. In contrast, the hypothalamic arousal peptide hypocretin/orexin evoked a direct inward current and increased excitatory synaptic activity and spike frequency in the normally silent MCH neurons. Together, these data support the view that MCH neurons may integrate information within the arousal system in favor of energy conservation.

Mice lacking melanin-concentrating hormone receptor 1 demonstrate increased heart rate associated with altered autonomic activity

Melanin-concentrating hormone (MCH) plays an important role in energy balance. The current studies were carried out on a new line of mice lacking the rodent MCH receptor (MCHR1(-/-) mice). These mice confirmed the previously reported lean phenotype characterized by increased energy expenditure and modestly increased caloric intake. Because MCH is expressed in the lateral hypothalamic area, which also has an important role in the regulation of the autonomic nervous system, heart rate and blood pressure were measured by a telemetric method to investigate whether the increased energy expenditure in these mice might be due to altered autonomic nervous system activity. Male MCHR1(-/-) mice demonstrated a significantly increased heart rate [24-h period: wild type 495 +/- 4 vs. MCHR1(-/-) 561 +/- 8 beats/min (P < 0.001); dark phase: wild type 506 +/- 8 vs. MCHR1(-/-) 582 +/- 9 beats/min (P < 0.001); light phase: wild type 484 +/- 13 vs. MCHR1(-/-) 539 +/- 9 beats/min (P < 0.005)] with no significant difference in mean arterial pressure [wild type 110 +/- 0.3 vs. MCHR1(-/-) 113 +/- 0.4 mmHg (P > 0.05)]. Locomotor activity and core body temperature were higher in the MCHR1(-/-) mice during the dark phase only and thus temporally dissociated from heart rate differences. On fasting, wild-type animals rapidly downregulated body temperature and heart rate. MCHR1(-/-) mice displayed a distinct delay in the onset of this downregulation. To investigate the mechanism underlying these differences, autonomic blockade experiments were carried out. Administration of the adrenergic antagonist metoprolol completely reversed the tachycardia seen in MCHR1(-/-) mice, suggesting an increased sympathetic tone.

Europium-labeled melanin-concentrating hormone analogues: ligands for measuring binding to melanin-concentrating hormone receptors 1 and 2

We investigated the use of Eu3+ chelate-labeled analogues of melanin-concentrating hormone (MCH) as ligands for both human MCH receptors (MCHR1 and MCHR2). The analogues employed were Ala17 MCH, S36057 (Y-ADO-RC*MLGRVFRPC*W, where ADO=8-amino-3,6-dioxyoctanoyl and *=disulfide bond), and R2P (RC*MLGRVFRPC*Y-NH2). The peptides were readily labeled on the alpha-amino residue with the Eu3+ chelate of N1-(p-isothiocyanatobenzyl)-diethylenetriamine-N1,N2,N3,N3-tetraacetic acid and then purified by reverse-phase fast-performance liquid chromatography at neutral pH to maintain Eu3+ chelation. Both labeled Ala17 MCH and S36057 had high affinity for MCHR1 ( Kd = 0.37 and 0.059nM, respectively) while Eu3+ -labeled S36057 and R2P had high affinity for MCHR2 ( Kd = 0.16 and 0.10nM, respectively). Labeled Ala17 MCH had little demonstrable binding affinity for MCHR2. Eu3+ -labeled S36057 and R2P were full agonists at MCHR1 when assessed by measurement of agonist-stimulated GTPgamma(35)S binding. Competition binding experiments with both MCHR isoforms, a series of previously characterized alanine scan MCH analogues, and a recently identified nonpeptide MCHR1-selective antagonist T-226296 confirmed the expected receptor selectivity. These studies further extend the utility of Eu3+ chelate time-resolved fluorescence for the development of high-sensitivity, nonradioactive receptor binding assays and demonstrate the need to select the optimal ligand for labeling.

Structural features of the inactive and active states of the melanin-concentrating hormone receptors: Insights from molecular simulations

Comparative molecular dynamics simulations of both subtypes 1 and 2 of the melanin-concentrating hormone receptor (MCHR1 and MCHR2, respectively) in their free and hormone-bound forms have been carried out. The hormone has been used in its full-length and truncated forms, as well as in 16 mutated forms. Moreover, MCHR1 has been simulated in complex with T-226296, a novel orally active and selective antagonist. The comparative analysis of an extended number of receptor configurations suggests that the differences between inactive (i.e., free and antagonist-bound) and active (i.e., agonist-bound) states of MCHRs involve the receptor portions close to the E/DRY and NPxxY motifs, with prominence to the cytosolic extensions of helices 2, 3, 6, and 7. In fact, the active forms of these receptors share the release of selected intramolecular interactions found in the inactive forms, such as that between R3.50 of the E/DRY motif and D2.40, and that between Y7.53 of the NPxxY motif and F7.60. Another feature of the active forms of both MCHRs is the approach of "helix 8" to the cytosolic extension of helix 3. These features of the active forms are concurrent with the opening of a cleft at the cytosolic end of the helix bundle. For both MCHRs, the agonist-induced chemical information transfer from the extracellular to the cytosolic domains is mediated by a cluster of aromatic amino acids in helix 6, following the ligand interaction with selected amino acids in the extracellular half of the receptor.

Promoter characterization of the mouse melanin-concentrating hormone receptor 1

The gene encoding the mouse melanin-concentrating hormone receptor 1 was isolated and its structural organization and flanking regions were characterized. The 3' flanking region is marked by the presence of two polyadenylation signals but used with different frequencies. RNase protection and 5' rapid amplification of cDNA ends (RACE) identified multiple transcription initiation sites between -150 and -203 bp upstream of the ATG initiation codon. Functional analysis of deletion mutants reveals a cell independent transcriptional activity localized between nucleotide -305 and -589. The proximal 1.5 kb region does not possess consensus TATA or CAAT boxes but has several consensus sequences for regulatory elements including USF, GATA, AP1, AP4, MyoD, GKLF and Ikaros that could explain the broad expression of the receptor.

Mechanisms of action of melanin-concentrating hormone in the teleost fish erythrophoroma cell line (GEM-81)

Melanin-concentrating hormone (MCH) evokes an increase of GEM-81 cell proliferation. This action of 10(-6)M MCH was inhibited in the presence of the following blockers: U-73122 (phospholipase C), Ro-31-8220 (PKC) or KN-93 (Ca(2+)/calmodulin-dependent kinase). The more selective PKC inhibitors, HBDDE and Go-6983, which block, respectively, PKC alpha/gamma isoform and beta1 isoform, were used. HBDDE was ineffective whereas Go-6983 reversed the proliferative response promoted by MCH. Flow cytometry assays demonstrated that MCH induces a slow and long-lasting rise in intracellular calcium, which can be blocked by U-73122. Our results also show a cAMP increase evoked by MCH. Our data support the assumption that MCH exerts its effect on GEM-81 erythrophoroma cells through activation of phosholipase C, beta1 PKC, and Ca(2+)/calmodulin-dependent PKC, and eliciting a slow, long-lasting rise in calcium, which may trigger the proliferative signal.

Hypothalamic control of energy balance: different peptides, different functions

Energy balance is maintained via a homeostatic system involving both the brain and the periphery. A key component of this system is the hypothalamus. Over the past two decades, major advances have been made in identifying an increasing number of peptides within the hypothalamus that contribute to the process of energy homeostasis. Under stable conditions, equilibrium exists between anabolic peptides that stimulate feeding behavior, as well as decrease energy expenditure and lipid utilization in favor of fat storage, and catabolic peptides that attenuate food intake, while stimulating sympathetic nervous system (SNS) activity and restricting fat deposition by increasing lipid metabolism. The equilibrium between these neuropeptides is dynamic in nature. It shifts across the day-night cycle and from day to day and also in response to dietary challenges as well as peripheral energy stores. These shifts occur in close relation to circulating levels of the hormones, leptin, insulin, ghrelin and corticosterone, and also the nutrients, glucose and lipids. These circulating factors together with neural processes are primary signals relaying information regarding the availability of fuels needed for current cellular demand, in addition to the level of stored fuels needed for long-term use. Together, these signals have profound impact on the expression and production of neuropeptides that, in turn, initiate the appropriate anabolic or catabolic responses for restoring equilibrium. In this review, we summarize the evidence obtained on nine peptides in the hypothalamus that have emerged as key players in this process. Data from behavioral, physiological, pharmacological and genetic studies are described and consolidated in an attempt to formulate a clear statement on the underlying function of each of these peptides and also on how they work together to create and maintain energy homeostasis.

THEIDENTIFICATION OFLIGANDS ATORPHANG-PROTEINCOUPLEDRECEPTORS

The completion of the human genome sequencing project has identified approximately 720 genes that belong to the G-protein coupled receptor (GPCR) superfamily. Approximately half of these genes are thought to encode sensory receptors. Of the remaining 360 receptors, the natural ligand has been identified for approximately 210 receptors, leaving 150 so-called orphan GPCRs with no known ligand or function. The identification of ligands active at orphan GPCRs has been achieved through the development of a number of experimental approaches, including the screening of putative small molecule and peptide ligands, reverse pharmacology, and the use of bioinformatics to predict candidate ligands. In this review, we discuss the methodologies developed for the identification of ligands at orphan GPCRs and include examples of their successful application.

Translational medicine in fish-derived peptides: from fish endocrinology to human physiology and diseases

Recent studies have revealed the importance of fish-derived peptide hormones to human endocrinology. These peptides include melanin-concentrating hormone (MCH), urocortins (human urotensin-I), and urotensin-II. MCH, a hypothalamic peptide, is a potent stimulator on appetite. Urocortins, e.g. urocortin 1 and urocortin 3 (stresscopin), are endogenous ligands for the corticotropin-releasing factor (CRF) receptors, particularly CRF type 2 receptor, that mediates a vasodilator action, a positive inotropic action and a central appetite-inhibiting action. These actions mediated by CRF type 2 receptor may ameliorate the stress response. Human urotensin-II is a potent vasoconstrictor peptide, while it acts as a vasodilator on some arteries. Human urotensin-II is expressed in various types of cells and tissues, including cardiovascular tissues, as well as many types of tumor cells. Thus, these fish-derived peptides appear to play important roles in human physiology, such as appetite regulation, stress response and cardiovascular regulation, and also in diseases, for example, obesity, cardiovascular diseases and tumors. Development of antagonists/agonists against the receptors for these peptides may open new strategies for the treatment of various diseases, including obesity-related diseases, hypertension, heart failure and malignant tumors.

Orphan G protein-coupled receptors: targets for new therapeutic interventions

With the completion of the human genome, many genes will be uncovered with unknown functions. The 'orphan' G protein coupled receptors (GPCRs) are examples of genes without known functions. These are genes that exhibit the seven helical conformation hallmark of the GPCRs but that are called 'orphans' because they are activated by none of the primary messengers known to activate GPCRs in vivo. They are the targets of undiscovered transmitters and this lack of knowledge precludes understanding their function. Yet, because they belong to the supergene family that has the widest regulatory role in the organism, the orphan GPCRs have generated much excitement in academia and industry. They hold much hope for revealing new intercellular interactions that will open new areas of basic research which ultimately will lead to new therapeutic applications. However, the first step in understanding the function of orphan GPCRs is to 'deorphanize' them, to identify their natural transmitters. Here we review the search for the natural primary messengers of orphan GPCRs and focus on two recently deorphanized GPCR systems, the melanin-concentrating hormone (MCH) and prolactin-releasing peptide (PrRP) systems, to illustrate the strategies applied to solve their function and to exemplify the therapeutic potentials that such systems hold.

Neuropeptides in hypothalamic neuronal disorders

A few examples of hypothalamic, peptidergic disorders leading to clinical signs and symptoms are presented in this review. Increased activity of corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus (PVN) and decreased activity of the vasopressin neurons in the biological clock and of the thyroxine-releasing hormone (TRH) neurons in the PVN contribute to the signs and symptoms of depression. In men, the central nucleus of the bed nucleus of the stria terminalis (BSTc) is about twice as large and contains twice as many somatostatin neurons as in women. In transsexuals this sex difference is reversed, pointing to a role of this structure in gender. Luteinizing hormone-releasing hormone (LHRH) neurons are formed in the fetal olfactory placade and migrate along the terminal nerve fibers into the hypothalamus. In Kallmann's syndrome the migration process of the LHRH (gonadotropin-releasing hormone) neurons is aborted, which explains the joint occurrence of hypogonadotropic hypogonadism and anosmia in this syndrome. In postmenopausal women, the neurons of the infundibular nucleus hypertrophy and become hyperactive because of the disappearance of the estrogen feedback and contain hyperactive peptidergic neurons. Climacteric flushes may be caused by hyperactivity of the neurokinin-B or LHRH neurons in this nucleus. The hypocretin (orexin) neurons in the perifornical area are involved in sleep. In narcolepsy with cataplexy, a loss of these neurons, probably due to an autoimmune process, is found. Obese subjects with a mutation in the gene that encodes for leptin, the preproghrelin gene, or the alpha-melanocyte-stimulating hormone (alpha-MSH) gene have been described. Decreased numbers and activity of the oxytocin neurons in the PVN may be responsible for the absence of satiety in Prader-Willi syndrome. Moreover, a glucocorticoid receptor polymorphism is associated with obesitas and dysregulation of the hypothalamus-pituitary-adrenal axis. In contrast, two single nucleotide polymorphisms (SNPs) of the AGRP gene have been associated with anorexia nervosa.

Identification of relaxin-3/INSL7 as an endogenous ligand for the orphan G-protein-coupled receptor GPCR135

GPCR135, publicly known as somatostatin- and angiotensin-like peptide receptor, is expressed in the central nervous system and its cognate ligand(s) has not been identified. We have found that both rat and porcine brain extracts stimulated 35S-labeled guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) incorporation in cells over-expressing GPCR135. Multiple rounds of extraction, purification, followed by N-terminal sequence analysis of the ligand from porcine brain revealed that the ligand is a product of the recently identified gene, relaxin-3 (aka insulin-7 or INSL7). Recombinant human relaxin-3 potently stimulates GTPgammaS binding and inhibits cAMP accumulation in GPCR135 overexpressing cells with EC50 values of 0.25 and 0.35 nM, respectively. 125I-Relaxin-3 binds GPCR135 at high affinity with a Kd value of 0.31 nM. Relaxin-3 is the only member of the insulin/relaxin superfamily that can activate GPCR135. In situ hybridization showed that relaxin-3 mRNA is predominantly expressed in the dorsomedial ventral tegmental nucleus of the brainstem (aka nucleus incertus), as well as in discrete cells in the lateral periaqueductal gray and in the central gray nucleus. GPCR135 is expressed abundantly in the hypothalamus with discrete expression in the paraventricular nucleus of the hypothalamus and supraoptic nucleus, as well as in the cortex, septal nucleus, and preoptical area. Relaxin-3 has previously been shown to bind and activate the LGR7 relaxin receptor. However, we believe that neuroanatomical colocalization of GPCR135 and relaxin-3, coupled with a clear high affinity interaction, suggest that GPCR135 is the receptor for relaxin-3. The identification of relaxin-3 as the ligand for GPCR135 provides the framework for the discovery of a new brainstem/hypothalamus circuitry.

Increased susceptibility to LTP generation and changes in NMDA-NR1 and -NR2B subunits mRNA expression in rat hippocampus after MCH administration

The present study attempts to determine which mechanisms underlie the retrograde facilitation of memory induced by microinjection hippocampal melanin-concentrating hormone (MCH) on the inhibitory avoidance paradigm. Previous reports using this test on the hippocampus suggest that NMDA receptor-mediated mechanisms are involved in memory processing and are also necessary for the induction of long-term potentiation (LTP) of the hippocampal dentate gyrus. In addition, alterations in expression of synaptic NMDA subunits in the hippocampus have been associated with memory formation of an inhibitory avoidance task. We have studied the effects of the neuropeptide upon the electrophysiological parameters using hippocampal slices from rats injected with the peptide and tested in step-down tests as well as possible changes in the mRNA expression of NMDA receptor subunits. We postulate that the increased facility to induce LTP, and the overexpression of this N-methyl-D-aspartate mRNA receptor subunits induced by MCH, could be behind the retrograde facilitation observed after MCH hippocampal microinjection.

Chronic MCH-1 receptor modulation alters appetite, body weight and adiposity in rats

Central administration of the neuropeptide melanin-concentrating hormone (MCH) stimulates feeding in rodents. We studied the effects of intracerebroventricular (i.c.v.) administration of an MCH-1 receptor agonist (Compound A) and an MCH-1 receptor antagonist (Compound B) on feeding in satiated rats. Compound B (10 microg, i.c.v.) blocked the acute orexigenic effect of Compound A (5 microg, i.c.v.). In an experiment designed to either stimulate or inhibit MCH-1 receptor signaling over an extended period, rats received continuous i.c.v. infusions of vehicle (saline), Compound A (30 microg/day), Compound B (30 or 48 microg/day) or neuropeptide Y (24 microg/day, as positive control) via implantable infusion pumps. Continuous MCH-1 receptor activation recapitulated the obese phenotype of MCH-over-expressor mice, manifest as enhanced feeding (+23%, P<0.001), caloric efficiency and body weight gain (+38%, P<0.005) over the 14-day period relative to controls. Chronic MCH-1 receptor activation also elevated plasma insulin and leptin levels significantly. Conversely, continuous MCH-1 receptor antagonism led to sustained reductions in food intake (-16%, P<0.001), body weight gain (-35%, P<0.01), and body fat gain relative to controls, without an effect on lean mass. Antagonism of the MCH-1 receptor may be an effective approach for the treatment of obesity.

Melanin-concentrating hormone receptor 1 activates extracellular signal-regulated kinase and synergizes with G(s)-coupled pathways

Melanin-concentrating hormone (MCH) is a hypothalamic neuropeptide that plays a key role in energy homeostasis. Like many neuropeptides, it signals through two G protein-coupled receptors. MCH receptor 1 (MCHR1) is the sole receptor expressed in rodents and couples to G(i) and G(q) proteins. Little is known about the intracellular pathways engaged by MCH and its receptor. Using HEK293 cells stably expressing MCHR1, we demonstrate that MCH, acting through MCHR1, antagonizes the action of forskolin, an adenylate cyclase activator that increases intracellular levels of cAMP. MCH also inhibits cAMP induction by the G(s)-coupled beta-adrenergic receptor. Activation of either the G(i)- or G(s)-dependent pathway typically results in ERK phosphorylation in HEK293 cells. In contrast to opposing actions on cAMP synthesis, simultaneous MCH and forskolin treatment results in synergistic activation of ERK. This synergy proceeds through pertussis toxin-independent pathways and requires several enzymatic activities such as protein kinase A, protein kinase C, phospholipase C, and Src kinase. Finally, we provide evidence that such positive interactions are not limited to cell lines but can also be observed in the brain.

Identification and characterization of a novel RF-amide peptide ligand for orphan G-protein-coupled receptor SP9155

Orphan G-protein-coupled receptors are a large class of receptors whose cognate ligands are unknown. SP9155 (also referred to as AQ27 and GPR103) is an orphan G-protein-coupled receptor originally cloned from a human brain cDNA library. SP9155 was found to be predominantly expressed in brain, heart, kidney, retina, and testis. Phylogenetic analysis shows that SP9155 shares high homology with Orexin, NPFF, and cholecystokinin (CCK) receptors, but identification of the endogenous ligand for SP9155 has not been reported. In this study, we have used a novel method to predict peptides from genome data bases. From these predicted peptides, a novel RF-amide peptide, P52 was shown to selectively activate SP9155-transfected cells. We subsequently cloned the precursor gene of the P52 ligand and characterized the activity of other possible peptides encoded by the precursor. This revealed an extended peptide, P518, which exhibited high affinity for SP9155 (EC50 = 7 nm). mRNA expression analysis revealed that the peptide P518 precursor gene is predominantly expressed in various brain regions, coronary arteries, thyroid and parathyroid glands, large intestine, colon, bladder, testes, and prostate. These results indicate the existence of a novel RF-amide neuroendocrine peptide system, and suggest that SP9155 is likely the relevant G-protein-coupled receptor for this peptide.

Behavioural analysis of melanin-concentrating hormone in rats: evidence for orexigenic and anxiolytic properties

Melanin-concentrating hormone (MCH) is a cyclic neuropeptide, predominantly expressed in hypothalamus, and recognized as a key regulator in feeding behaviour and energy balance. In this study, we examined the behavioural effects of intracerebroventricularly administered MCH on food intake, anxiety, exploratory behaviour and body core temperature in rats. MCH (0.15-10.0 microg, i.c.v.) acutely increased food intake in a dose-dependent manner. In addition, MCH (0.6-10.0 microg, i.c.v.) produced effects similar to anxiolytics in an animal model of anxiety, Vogel's punished drinking test. Thus, punished drinking episodes were significantly increased. We found no effects of MCH (5.0-20.0 microg, i.c.v.) on locomotor activity either in habituated or non-habituated animals. Furthermore, MCH did not produce any changes in body core temperature. Together, these observations further support a role for MCH as an orexigenic neuropeptide and also suggest anti-anxiety properties for MCH.

Lateral hypothalamic neuropeptides in reward and drug addiction

The hypothalamus has been long considered important in feeding and other motivated behaviors. The identification of neuropeptides expressed in the hypothalamus has initiated efforts to better elucidate the underlying molecular mechanisms involved. The neuropeptides orexin and melanin-concentrating hormone (MCH) are expressed in the lateral hypothalamus (LH) and have been implicated in regulation of feeding behavior. Neurons expressing these neuropeptides have extensive projections to regions of the brain important for behavioral responses to drugs of abuse, raising the possibility that the pathways may also be important in addiction. Regulation of LH intracellular signaling pathways in response to drugs of abuse supports a role for the LH neuropeptides in addiction.

Novel human G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) are important mediators of signal transduction and targets for pharmacological therapeutics. Novel receptor-ligand systems have been discovered through the identification and analysis of orphan GPCRs (oGPCRs). Here we describe the discovery of seven novel human genes encoding oGPCRs. Each novel oGPCR gene was discovered using customized searches of the GenBank genomic databases with previously known GPCR-encoding sequences. The expressed genes can now be used in assays to determine endogenous and pharmacological ligands. GPR133, GPR134, GPR135, GPR136, and GPR137 share identities with a prostate-specific odorant-like GPCR-encoding gene (PSGR). GPR138 and GPR139 share identities with an odorant-like gene derived from human erythroid cells. Transcripts encoding GPR133, GPR134, GPR135, GPR136, and GPR137 were detected in various CNS tissues. The expression of odorant-like genes in non-olfactory tissues requires further clarification, which may be achieved through the search for endogenous cognate ligands for these and other oGPCRs.

Exploiting the co-evolution of interacting proteins to discover interaction specificity

Protein interactions are fundamental to the functioning of cells, and high throughput experimental and computational strategies are sought to map interactions. Predicting interaction specificity, such as matching members of a ligand family to specific members of a receptor family, is largely an unsolved problem. Here we show that by using evolutionary relationships within such families, it is possible to predict their physical interaction specificities. We introduce the computational method of matrix alignment for finding the optimal alignment between protein family similarity matrices. A second method, 3D embedding, allows visualization of interacting partners via spatial representation of the protein families. These methods essentially align phylogenetic trees of interacting protein families to define specific interaction partners. Prediction accuracy depends strongly on phylogenetic tree complexity, as measured with information theoretic methods. These results, along with simulations of protein evolution, suggest a model for the evolution of interacting protein families in which interaction partners are duplicated in coupled processes. Using these methods, it is possible to successfully find protein interaction specificities, as demonstrated for >18 protein families.

Effect of direct injection of melanin-concentrating hormone into the paraventricular nucleus: further evidence for a stimulatory role in the adrenal axis via SLC-1

Melanin-concentrating hormone (MCH) is implicated in the control of a number of hormonal axes including the hypothalamic-pituitary adrenal (HPA) axis. Previous studies have shown that there is evidence for both a stimulatory and an inhibitory action on the HPA axis; therefore, we attempted to further characterize the effects of MCH on this axis. Intracerebroventricular injection of MCH increased circulating adrenocorticotropic hormone (ACTH) at 10 min post injection. Injection of MCH directly into the paraventricular nucleus (PVN) was found to increase both circulating ACTH and corticosterone 10 min after injection. Additionally, MCH was found to increase corticotropin-releasing factor (CRF) release from hypothalamic explants, and this effect was abolished by the specific SLC-1 antagonist SB-568849. Neuropeptide EI, a peptide from the same precursor as MCH was also found to increase CRF release from explants. These results suggest that MCH has a stimulatory role in the HPA axis via SLC-1, and that MCH exerts its effects predominantly through the PVN CRF neuronal populations

Chronic intracerebroventricular infusion of MCH causes obesity in mice. Melanin-concentrating hormone

Melanin-concentrating hormone (MCH) is a cyclic amino acid neuropeptide localized in the lateral hypothalamus. Although MCH is thought to be an important regulator of feeding behavior, the involvement of this peptide in body weight control has been unclear. To examine the role of MCH in the development of obesity, we assessed the effect of chronic intracerebroventricular infusion of MCH in C57BL/6J mice that were fed with regular or moderately high-fat (MHF) diets. Intracerebroventricular infusion of MCH (10 microg/day for 14 days) caused a slight but significant increase in body weight in mice maintained on the regular diet. In the MHF diet-fed mice, MCH more clearly increased the body weight accompanied by a sustained hyperphagia and significant increase in fat and liver weights. Plasma glucose, insulin, and leptin levels were also increased in the MCH-treated mice fed the MHF diet. These results suggest that chronic stimulation of the brain MCH system causes obesity in mice and imply that MCH may have a major role in energy homeostasis.

The structure and evolution of the melanocortin and MCH receptors in fish and mammals

Zebrafish are an excellent genetic model system for studying developmental and physiological processes. Pigment patterns in zebrafish are affected by mutations in three types of chromatophores. The behavior of these cells is influenced by alpha-melanocyte-stimulating hormone (alphaMSH) and melanin-concentrating hormone (MCH). Mammals have five alphaMSH receptors (melanocortin receptors) and one or two MCH receptors. We have identified the full complement of melanocortin and MCH receptors in both zebrafish and the pufferfish, Fugu. Zebrafish have six melanocortin receptors, including two MC5R orthologues, while Fugu, lacking MC3R, has only four. We also demonstrate that Fugu and zebrafish have two and three MCHR genes, respectively. MC2R and MC5R are physically linked in all species examined. Unlike other species, we find the Fugu genes contain introns, one of which is in a conserved location and is probably ancestral. We also detail the differential expression of the zebrafish genes throughout development.

Different structural requirements for melanin-concentrating hormone (MCH) interacting with rat MCH-R1 (SLC-1) and mouse B16 cell MCH-R

Melanin-concentrating hormone (MCH) is a neuropeptide occurring in all vertebrates and some invertebrates and is now known to stimulate pigment aggregation in teleost melanophores and food-intake in mammals. Whereas the two MCH receptor subtypes hitherto cloned, MCH-R1 and MCH-R2, are thought to mediate mainly the central effects of MCH, the MCH-R on pigment cells has not yet been identified, although in some studies MCH-R1 was reported to be expressed by human melanocytes and melanoma cells. Here we present data of a structure-activity study in which 12 MCH peptides were tested on rat MCH-R1 and mouse B16 melanoma cell MCH-R, by comparing receptor binding affinities and biological activities. For receptor binding analysis with HEK-293 cells expressing rat MCH-R1 (SLC-1), the radioligand was [125I]-[Tyr13]-MCH with the natural sequence. For B16 cells (F1 and G4F sublines) expressing B16 MCH-R, the analog [125I]-[D-Phe13, Tyr19]-MCH served as radioligand. The bioassay used for MCH-R1 was intracellular Ca2+ mobilization quantified with the FLIPR instrument, whereas for B16 MCH-R the signal determined was MAP kinase activation. Our data show that some of the peptides displayed a similar relative increase or decrease of potency in both cell types tested. For example, linear MCH with Ser residues at positions 7 and 16 was almost inactive whereas a slight increase in side-chain hydrophilicity at residues 4 and 8, or truncation of MCH at the N-terminus by two residues hardly changed binding affinity or bioactivity. On the other hand, salmonic MCH which also lacks the first two residues of the mammalian sequence but in addition has different residues at positions 4, 5, 9, and 18 exhibited a 5- to 10-fold lower binding activity than MCH in both cell systems. A striking difference in ligand recognition between MCH-R1 and B16 MCH-R was however observed with modifications at position 13 of MCH: whereas L-Phe13 in [Phe13, Tyr19]-MCH was well tolerated by both MCH-R1 and B16 MCH-R, change of configuration to D-Phe13 in [D-Phe13, Tyr19]-MCH or [D-Phe13]-MCH led to a complete loss of biological activity and to a 5- to 10-fold lower binding activity with MCH-R1. By contrast, the D-Phe13 residue increased the affinity of [D-Phe13, Tyr19]-MCH to B16 MCH-R about 10-fold and elicited MAP kinase activation as observed with [Phe13, Tyr19]-MCH or MCH. These data demonstrate that ligand recognition by B16 MCH-R differs from that of MCH-R1 in several respects, indicating that the B16 MCH-R represents an MCH-R subtype different from MCH-R1.

Methods to optimize the generation of cDNA from postmortem human brain tissue

The analysis of gene transcript levels in postmortem human brain is a valuable tool for the study of neurological and psychiatric diseases. Optimization of the methods of RNA extraction and cDNA generation is particularly important in this application because postmortem human brain tissue is in limited supply and generally yields less RNA than many other human tissues. We compared column extraction and solvent extraction for total RNA, reverse transcription (RT) with random hexamers versus oligo-dT priming, and incubation of the RNA with or without DNase for effect on the cDNA product derived from the same homogenized pool of postmortem human frontal cortex. The total RNA obtained from the solvent method was found to be less stable at room temperature and to contain a higher proportion of non-messenger RNA than that obtained from the column method. Evaluating the RT-PCR results per wet weight of tissue extracted, we found that the signal strength was increased >20-fold by a protocol of Qiagen RNeasy column extraction, random hexamer RT priming and omitting DNase treatment of the RNA.

Identification of natural ligands for the orphan G protein-coupled receptors GPR7 and GPR8

GPR7 and GPR8 are two structurally related orphan G protein-coupled receptors, presenting high similarities with opioid and somatostatin receptors. Two peptides, L8 and L8C, derived from a larger precursor, were recently described as natural ligands for GPR8 (Mori, M., Shimomura, Y., Harada, M., Kurihara, M., Kitada, C., Asami, T., Matsumoto, Y., Adachi, Y., Watanabe, T., Sugo, T., and Abe, M. (December, 27, 2001) World Patent Cooperation Treaty, Patent Application WO 01/98494A1). L8 is a 23-amino acid peptide, whereas L8C is the same peptide with a C terminus extension of 7 amino acids, running through a dibasic motif of proteolytic processing. Using as a query the amino acid sequence of the L8 peptide, we have identified in DNA databases a human gene predicted to encode related peptides and its mouse ortholog. By analogy with L8 and L8C, two peptides, named L7 and L7C could result from the processing of a 125-amino acid human precursor through the alternative usage of a dibasic amino acid motif. The activity of these four peptides was investigated on GPR7 and GPR8. In binding assays, L7, L7C, L8, and L8C were found to bind with low nanomolar affinities to the GPR7 and GPR8 receptors expressed in Chinese hamster ovary (CHO)-K1 cells. They inhibited forskolin-stimulated cAMP accumulation through a pertussis toxin-sensitive mechanism. The tissue distribution of prepro-L7 (ppL7) and prepro-L8 (ppL8) was investigated by reverse transcription-PCR. Abundant ppL7 transcripts were found throughout the brain as well as in spinal cord, spleen, testis, and placenta; ppL8 transcripts displayed a more restricted distribution in brain, with high levels in substantia nigra, but were more abundant in peripheral tissues. The ppL7 and ppL8 genes therefore encode the precursors of a class of peptide ligands, active on two receptor subtypes, GPR7 and GPR8. The distinct tissue distribution of the receptor and peptide precursors suggest that each ligand and receptor has partially overlapping but also specific roles in this signaling system.

Conformational features of human melanin-concentrating hormone: an NMR and computational analysis

The conformational features of human melanin-concentrating hormone (hMCH) [Asp1-Phe2-Asp3-Met4-Leu5-Arg6-cyclo(S[bond]S)(Cys7-Met8-Leu9-Gly10-Arg11-Val12-Tyr13-Arg14-Pro15-Cys16)-Trp17-Gln18-Val19], in water and in a CD(3)CN/H(2)O (1:1 v/v) mixture at 298 K, have been determined by NMR spectroscopy followed by simulated annealing and molecular dynamics analyses to identify conformer populations. Backbone clustering analysis of NMR-spectroscopy-derived structures in the 7-16 peptide region led to the identification of a single representative structure in each solvent. Both root mean square deviation clustering and secondary structure analysis of the final conformers in both solvents show substantial convergence of most conformers into a single fold in the 4-17 region, with a limited variability around Gly10 and Tyr13 on going from CD(3)CN/H(2)O to pure water. The main feature deduced from the analysis of secondary structures is the occurrence of an N-terminal alpha helix of variable length, which spans an overall residue range of 2-9. A comparative analysis in the two solvents highlights that these structures are substantially different from that reported in the literature for the cyclic MCH(5-14) subunit of salmon MCH, which was used to perform a molecular characterization of the MCH/receptor complex. Our conformational data call for a critical revision of the proposed MCH/receptor complex model.

Functional role of N-linked glycosylation on the rat melanin-concentrating hormone receptor 1

Melanin-concentrating hormone (MCH) is known to act through two G-protein-coupled receptors MCHR1 and MCHR2. MCHR1 has three potential sites (Asn13, Asn16 and Asn23) for N-linked glycosylation in its extracellular amino-terminus which may modulate its reactivity. Site-directed mutagenesis of the rat MCHR1 cDNA at single or multiple combinations of the three potential glycosylation sites was used to examine the role of the putative carbohydrate chains on receptor activity. It was found that all three potential N-linked glycosylation sites in MCHR1 were glycosylated, and that N-linked glycosylation of Asn23 was necessary for full activity. Furthermore, disruption of all three glycosylation sites impaired proper expression at the cell surface and receptor activity. These data outline the importance of the N-linked glycosylation of the MCHR1.

Emerging roles for orphan G-protein-coupled receptors in the cardiovascular system

Despite current drug therapies, including those that target enzymes, channels and known G-protein-coupled receptors (GPCRs), cardiovascular disease remains the major cause of ill health, which suggests that other transmitter systems might be involved in this disease. In humans, approximately 175 genes have been predicted to encode 'orphan' GPCRs, where the endogenous ligand is not yet known. As a result of intensive screening using 'reverse pharmacology', an increasing number of orphan receptors are being paired with their cognate ligands, many of which are peptides. The existence of some of these peptides such as urotensin-II and relaxin had been known for some time but others, including ghrelin and apelin, represent novel sequences. The pharmacological characterization of these emerging peptide-receptor systems is a tantalising area of cardiovascular research, with the prospect of identifying new therapeutic targets.

Melanin-concentrating hormone, hippocampal nitric oxide levels and memory retention

The present study attempts to determine, if the effect of melanin-concentrating hormone (MCH) upon memory retention is correlated with changes in nitric oxide synthase (NOS) activity and tissue levels of nitric oxide (NO) and cGMP. We used a behavioral experiment using a step-down inhibitory avoidance test, the biochemical determinations of NO and cGMP, and electrophysiological model. Results of behavioral studies (step-down test) showed that MCH administration reverts the amnesic effects induced by N(G)-nitro-L-arginine (L-NOArg). Moreover, electrophysiological studies demonstrated that L-NOArg did not block the potentiation induced by the peptide. Hippocampal NO and cGMP levels increased after MCH injection.

Neuroendocrine regulation of eating behavior

The dual center hypothesis in the central control of energy balance originates from the first observations performed more than 5 decades ago with brain lesioning and stimulation experiments. On the basis of these studies the "satiety center" was located in the ventromedial hypothalamic nucleus, since lesions of this region caused overfeeding and excessive weight gain, while its electrical stimulation suppressed eating. On the contrary, lesioning or stimulation of the lateral hypothalamus elicited the opposite set of responses, thus leading to the conclusion that this area represented the "feeding center". The subsequent expansion of our knowledge of specific neuronal subpopulations involved in energy homeostasis has replaced the notion of specific "centers" controlling energy balance with that of discrete neuronal pathways fully integrated in a more complex neuronal network. The advancement of our knowledge on the anatomical structure and the function of the hypothalamic regions reveals the great complexity of this system. Given the aim of this review, we will focus on the major structures involved in the control of energy balance.

Endogenous receptor for melanin-concentrating hormone in human neuroblastoma Kelly cells

Melanin-concentrating hormone (MCH), a cyclic nonadecapeptide, is predominantly expressed in mammalian neurons located in the zona incerta and lateral hypothalamus. Current interest in MCH relates to its role in the control of feeding behaviour. Two receptors for MCH were recently found: MCH-R(1) and MCH-R(2). We show here by RT-PCR analysis and immunofluorescence studies that the human neuroblastoma cell line Kelly expresses MCH and MCH-R(1) but not MCH-R(2). In competition assays using 125I-labelled MCH an inhibitory concentration 50% (IC(50)) of 76nM was determined for MCH, indicating a high affinity of Kelly cells for MCH. MCH induces mitogen-activated protein kinase (MAPK) phosphorylation in Kelly cells but no increase in the intracellular free Ca(2+) concentration. This suggests that MCH signals via Galpha(i)/Galpha(0) in these cells. The presence and functionality of MCH-R(1) renders this neuronal cell a very useful model for future structure-activity studies in a physiological environment mimicking the human brain for the evaluation of potential appetite-regulating drugs.

Hypothalamic ventromedial and arcuate neurons of normal and postnatally overnourished rats differ in their responses to melanin-concentrating hormone

Melanin-concentrating hormone (MCH) is a neuropeptide involved in regulation of food intake and body weight. The study aimed to detect possible differences in responses of hypothalamic ventromedial and arcuate neurons to MCH, depending on the short-term nutritional state (fed versus food-deprived) and on the long-term state in overweight rats due to early postnatal overnutrition. The effect of MCH on a single-unit activity was studied in brain slices of normal and overweight rats. The latter (n=16) were raised till weaning in small litters (SL) of 3 pups compared to 10 pups in control litters (CL) and gained significantly greater body mass. Whereas MCH in effective concentrations in the pico- to nanomolar range could increase or suppress the activity of ventromedial or arcuate neurons studied in male normal fed or food-deprived (24 h) rats, its action became shaped in an unidirectional way in overweight, hyperphagic rats. Medial arcuate neurons (n=25) from hyperphagic rats were predominantly activated by MCH (p<0.05, paired t-test). This effect differed significantly from that induced on neurons (n=27) of control rats. Ventromedial neurons (n=34) of overweight rats were predominantly inhibited. Activation of arcuate neurons may induce feeding in particular through release of neuropeptide Y (NPY). Inhibition of ventromedial neurons may contribute to reduced energy expenditure. The increased expression of one response type to MCH by a neuronal population in overweight, hyperphagic rats might reflect a general mechanism of neurochemical plasticity and also suggest a participation of the peptide in long-term regulation of food intake and body weight in this model of obesity.

Melanin-concentrating hormone neuron system: the Wide Web that controls the feeding

Recent evidence indicates that the hypothalamic control system of food intake involves many feeding-related neuropeptides. Melanin-concentrating hormone (MCH), one of a group of potent orexigenic peptides, is exclusively produced in neurons of the lateral hypothalamic area that give off fibers to the widespread brain regions. The receptor of MCH was recently identified to be localized in cell bodies and dendritic processes of particular neurons throughout the brain, in close spatial relation to MCH fibers. Leptin, an anorectic hormone secreted from the adipose tissue, acts on the specific receptor present on its target neurons in the brain, and suppresses the expression of both MCH and its receptor. Leptin receptor and STAT3, a transcription factor mediating the leptin signaling, are distributed in the widespread brain regions including the cerebral neocortex, hippocampal formation and lower brainstem as well as the hypothalamus where MCH fibers and the MCH receptor are abundantly present. These findings suggest that MCH exerts the effect through its specific receptor distributed throughout the brain and that the function of MCH is influenced by the condition of peripheral energy balance via leptin, the leptin receptor and STAT3, not only in the hypothalamus, but also in other brain regions. In the brain, MCH might be involved in various feeding-related functions, such as appetite, food-searching behavior, eating muscle movement, and control of energy balance, depending upon the physiological role of each region.

Melanin-concentrating hormone activates signaling pathways in 3T3-L1 adipocytes

Energy homeostasis is regulated by peripheral signals, such as leptin, and by several orexigenic and anorectic neuropeptides. Recently, we reported that the orexigenic neuropeptide melanin-concentrating hormone (MCH) stimulates leptin production by rat adipocytes and that the MCH receptor (MCH-R1) is present on these cells. Here, we show that MCH-R1 is present on murine 3T3-L1 adipocytes. Treatment of 3T3-L1 adipocytes with 1 micromolar MCH for up to 2 h acutely downregulated MCH-R1, indicating a mechanism of ligand-induced receptor downregulation. Potential signaling pathways mediating MCH-R1 action in adipocytes were investigated. Treatment of 3T3-L1 adipocytes with 1 micromolar MCH rapidly induced a threefold and a fivefold increase in p44/42 MAPK and pp70 S6 kinase activities, respectively. In addition, 3T3-L1 adipocytes transiently transfected with a murine leptin-luciferase promoter construct showed a fourfold and a sixfold increase in leptin promoter-reporter gene expression at 1 h and 4 h, respectively, in response to MCH. Activity decreased to basal levels at 8 h. Furthermore, MCH-stimulated leptin promoter-driven luciferase activity was diminished in the presence of the MAP/ERK kinase inhibitor PD-98059 and in the presence of rapamycin, an inhibitor of pp70 S6 kinase activation. These results provide further evidence for a functional MCH signaling pathway in adipocytes.

Melanin-concentrating hormone and its receptor are expressed and functional in human skin

In this study, we have demonstrated the presence of melanin-concentrating hormone (MCH) and melanin-concentrating hormone receptor (MCHR1) transcripts in human skin. Sequence analysis confirmed that the transcripts of both genes were identical to those previously found in human brain. In culture, endothelial cells showed pro-MCH expression whereas no signal was found in keratinocytes, melanocytes, and fibroblasts. MCHR1 expression was restricted to melanocytes and melanoma cells. Stimulation of cultured human melanocytes with MCH reduced the alpha-MSH-induced increase in cAMP production. Furthermore, the melanogenic actions of alpha-MSH were inhibited by MCH. We propose that the MCH/MCHR1 signalling system is present in human skin and may have a role with the melanocortins in regulating the melanocyte.

Cloning and characterization of rhesus monkey MCH-R1 and MCH-R2

Rhesus monkey MCH-R1 and MCH-R2 receptors were cloned. Amino acid homology is 98.8% between monkey and human MCH-R1, while monkey and human MCH-R2 are 98% homologous. Binding and intracellular signaling characteristics of the monkey receptors were compared with the human homologues. The results demonstrate that MCH binds to the monkey MCH-R1 receptor with a K(d) of 6.5 nM and monkey MCH-R2 with a K(d) of 2.2 nM similar to K(d) values for human MCH-R1 and MCH-R2. Additionally, monkey MCH-R1 couples through G(i)/G(o) and G(q)-type G proteins similar to human MCH-R1 whereas monkey and human MCH-R2 utilize the G(q) signaling pathway.

Antidepressant, anxiolytic and anorectic effects of a melanin-concentrating hormone-1 receptor antagonist

Melanin concentrating hormone (MCH) is an orexigenic hypothalamic neuropeptide, which plays an important role in the complex regulation of energy balance and body weight. Here we show that SNAP-7941, a selective, high-affinity MCH1 receptor (MCH1-R) antagonist, inhibited food intake stimulated by central administration of MCH, reduced consumption of palatable food, and, after chronic administration to rats with diet-induced obesity, resulted in a marked, sustained decrease in body weight. In addition, after mapping the binding sites for [(3)H]SNAP-7941 in rat brain, we evaluated its effects in a series of behavioral models. SNAP-7941 produced effects similar to clinically used antidepressants and anxiolytics in three animal models of depression/anxiety: the rat forced-swim test, rat social interaction and guinea pig maternal-separation vocalization tests. Given these observations, an MCH1-R antagonist may be useful not only in the management of obesity but also as a treatment for depression and/or anxiety.

Appetite-boosting property of pro-melanin-concentrating hormone(131-165) (neuropeptide-glutamic acid-isoleucine) is associated with proteolytic resistance

Melanin-concentrating hormone (MCH) is a cyclic neuropeptide, with a major role in stimulation of feeding behavior in mammals. MCH signals in the brain occur via two seven-transmembrane G protein-coupled receptors, namely MCH1 (SLC-1, MCH(1), MCH-R1, or MCH-1R) and MCH2 (SLT, MCH(2), MCH-R2, or MCH-2R). In this study, we demonstrate that the pro-MCH(131-165) peptide neuropeptide-glutamic acid-isoleucine (NEI)-MCH is more potent than MCH in stimulating feeding in the rat. Using rat MCH1-expressed human embryonic kidney 293 cells, we show that NEI-MCH exhibits 5-fold less affinity in a binding assay and 2-fold less potency in a cAMP assay than MCH. A similar 7- to 8-fold shift in potency was observed in a Ca(2+)(i) assay using rat MCH1 or human MCH2-transfected Chinese hamster ovary cell models. This demonstrates that NEI-MCH is not a better agonist than MCH at either of the MCH receptors. Then, we compared the proteolysis resistance of MCH and NEI-MCH to rat brain membrane homogenates and purified proteases. Kinetics of peptide degradation using brain extracts indicated a t(1/2) of 34.8 min for MCH and 78.5 min for NEI-MCH with a specific pattern of cleavage of MCH but not NEI-MCH by exo- and endo-proteases. Furthermore, MCH was found highly susceptible to degradation by aminopeptidase M and endopeptidase 24.11, whereas NEI-MCH was fully resistant to proteolysis by these enzymes. Therefore, our results strongly suggest that reduced susceptibility to proteases of NEI-MCH compared with MCH account for its enhanced activity in feeding behavior. NEI-MCH represents therefore the first MCH natural functional "superagonist" so far described.

The use of constitutively active GPCRs in drug discovery and functional genomics

The complete sequencing of the human genome has afforded researchers the opportunity to identify novel G-protein-coupled receptors (GPCRs) that are expressed in human tissues. The successful identification of hundreds of GPCRs represents the single greatest opportunity for novel drug development today. However, the lack of identified ligands for these GPCRs has limited their utility for traditional drug discovery approaches that focus on ligand-based assay methods to discover and pharmacologically characterize drug candidates. Here, we review the use of constitutively activated GPCRs in the discovery pathway, both as a means to overcome the limitations of traditional drug discovery at novel GPCRs and as a tool to investigate the functionality of these receptors.

Cell and molecular cell biology of melanin-concentrating hormone

Recent advances in the study of melanin-concentrating hormone (MCH) have depended largely on molecular biological techniques. In mammals, which have attracted the most attention, novel findings concern (i) the MCH gene, which can yield several peptides by either posttranslational cleavage or alternative splicing, as well as bidirectional transcription; (ii) the identification of two G protein-coupled MCH receptors in the brain and peripheral tissues; and (iii) the evidence for subpopulations of MCH neurons in the central nervous system, characterized by their chemical phenotypes, connections, and individual physiological responses to different physiological paradigms. The involvement of central MCH in various functions, including feeding, reproduction, stress, and behavior patterns, is reviewed. The stage during evolution at which MCH may have acquired hypophysiotrophic and hormonal functions in lower vertebrates is considered in light of morphological data. Evidence that MCH also has peripheral paracrine/autocrine effects in mammals is provided.

SVK14 cells express an MCH binding site different from the MCH1 or MCH2 receptor

Melanin-concentrating hormone (MCH) is a cyclic peptide, mainly involved in the regulation of skin pigmentation in teleosts and feeding behavior in mammals. The human keratinocyte SVK14 cell line has been previously shown to express binding sites for the MCH analog [125I]-[Phe13,3-iodo-Tyr19]MCH. We report here that: (1) this binding site similarly recognized [125I]-[3-iodo-Tyr13]MCH; (2) its pharmacological profile clearly differed from those observed at the two human MCH receptor subtypes, MCH1-R and MCH2-R; (3) MCH did not induce any effect on second messenger systems (including cAMP, calcium, and MAP kinase signaling pathways), and (4) no mRNAs corresponding to the MCH receptors were found. In conclusion, the binding site characterized in the SVK14 cell line is distinct from the MCH1 and MCH2 receptors and deserves therefore further investigation.

Melanin-concentrating hormone depresses L-, N-, and P/Q-type voltage-dependent calcium channels in rat lateral hypothalamic neurons

Melanin-concentrating hormone (MCH), a cyclic 19-amino-acid peptide, is synthesized exclusively by neurons in the lateral hypothalamic (LH) area. It is involved in a number of brain functions and recently has raised interest because of its role in energy homeostasis. MCH axons and receptors are found throughout the brain. Previous reports set the foundation for understanding the cellular actions of MCH by using non-neuronal cells transfected with the MCH receptor gene; these cells exhibited an increase in cytoplasmic calcium in response to MCH, suggesting an excitatory action for the peptide. In the study presented here, we have used whole-cell recording in 117 neurons from LH cultures and brain slices to examine the actions of MCH. MCH decreased the amplitude of voltage-dependent calcium currents in almost all tested neurons. The inhibition desensitized rapidly (18 s to half maximum at 100 nM concentration) and was dose-dependent (IC(50) = 7.8 nM) when activated with a pulse from -80 mV to 0 mV. A priori activation of G-proteins with GTPgammaS completely eliminated the MCH-induced effect at low MCH concentrations and reduced the MCH-induced effect at high MCH concentrations. Inhibition of G-proteins with pertussis toxin (PTX) blocked the MCH-induced inhibitory effect at high MCH concentrations. Pre-pulse depolarization resulted in an attenuation of the MCH-induced inhibition of calcium currents in most neurons. These data suggest that MCH exerts an inhibitory effect on calcium currents via PTX-sensitive G-protein pathways, probably the G(i)/G(o) pathway, in LH neurons. L-, N- and P/Q-type calcium channels were identified in LH neurons, with L- and N-type channels accounting for most of the voltage-activated current (about 40 % each); MCH attenuated each of the three types (mean 50 % depression), with the greatest inhibition found for N-type currents. In contrast to previous data on non-neuronal cells showing an MHC-evoked increase in calcium, our data suggest that the reverse occurs in LH neurons. The attenuation of calcium currents is consistent with an inhibitory action for the peptide in neurons.

G protein-coupled receptors: dominant players in cell-cell communication

The G protein-coupled receptors (GPCRs) are the most numerous and the most diverse type of receptors (1-5% of the complete invertebrate and vertebrate genomes). They transduce messages as different as odorants, nucleotides, nucleosides, peptides, lipids, and proteins. There are at least eight families of GPCRs that show no sequence similarities and that use different domains to bind ligands and activate a similar set of G proteins. Homo- and heterodimerization of GPCRs seem to be the rule, and in some cases an absolute requirement, for activation. There are about 100 orphan GPCRs in the human genome which will be used to find new message molecules. Mutations of GPCRs are responsible for a wide range of genetic diseases. The importance of GPCRs in physiological processes is illustrated by the fact that they are the target of the majority of therapeutical drugs and drugs of abuse.

Melanin-concentrating hormone receptor subtypes 1 and 2: species-specific gene expression

To assess the contribution of potential central nervous system pathways implicated in the control of appetite regulation and energy metabolism, it is essential to first identify appropriate animal models. Melanin-concentrating hormone (MCH), a conserved cyclic neuropeptide implicated in the modulation of food intake, has been shown to bind and activate two G-protein-coupled receptors, called GPR24 and MCHR2, expressed in human brain and other tissues. Here we show that several non-human species (rat, mouse, hamster, guinea pig, and rabbit) do not have functional MCHR2 receptors, or encode a nonfunctional MCHR2 pseudogene while retaining GPR24 expression. We identified three species for further evaluation that express both MCH receptor subtypes. We cloned and functionally characterized dog, ferret, and rhesus GPR24 and MCHR2 in mammalian cells and studied their brain distribution patterns by in situ hybridization. The homology, expression profile, and functional similarity of the receptors in the dog, ferret, and rhesus to that of human support the potential use of these species as preclinical animal models in the development of therapeutic agents for obesity or other MCH-mediated disorders.

Synthesis and biological evaluation in vitro of selective, high affinity peptide antagonists of human melanin-concentrating hormone action at human melanin-concentrating hormone receptor 1

Human melanin-concentrating hormone (hMCH) and many of its analogues are potent but nonspecific ligands for human melanin-concentrating hormone receptors 1 and 2 (hMCH-1R and hMCH-2R). To differentiate between the physiological functions of these receptors, selective antagonists are needed. In this study, analogues of Ac-Arg(6)-cyclo(S-S)(Cys(7)-Met(8)-Leu(9)-Gly(10)-Arg(11)-Val(12)-Tyr(13)-Arg(14)-Pro(15)-Cys(16))-NH(2), a high affinity but nonselective agonist at hMCH-1R and hMCH-2R, were prepared and tested in binding and functional assays on cells expressing these receptors. In the new analogues, 5-aminovaleric acid (Ava) was incorporated in place of the Leu(9)-Gly(10) and/or Arg(14)-Pro(15) segments of the disulfide ring. Several of these compounds turned out to be high affinity antagonists selective for hMCH-1R. Moreover, even at micromolar concentrations, they were devoid of agonist potency at both hMCH receptors and not effective as hMCH-2R antagonists. For example, peptide 14, Gva(6)- cyclo(S-S)(Cys(7)-Met(8)-Leu(9)-Gly(10)-Arg(11)-Val(12)-Tyr(13)-Ava(14,15)-Cys(16))-NH(2), (Gva = 5-guanidinovaleric acid), was a full competitive hMCH-1R antagonist (IC(50) = 14 nM, K(B) = 0.9 nM) with more than 1000-fold selectivity over hMCH-2R. Examination of various compounds with Ava in positions 9,10 and/or 14,15 revealed that the Leu(9)-Gly(10) and Arg(14)-Pro(15) segments of the disulfide ring are the principal structural elements determining hMCH-1R selectivity and ability to act as a hMCH-1R antagonist.