Expression and characterization of melanin-concentrating hormone receptors on mammalian cell lines

Fast neurotransmitter identity of MCH neurons: Do contents depend on context?

Hypothalamic melanin-concentrating hormone (MCH) neurons participate in many fundamental neuroendocrine processes. While some of their effects can be attributed to MCH itself, others appear to depend on co-released neurotransmitters. Historically, the subject of fast neurotransmitter co-release from MCH neurons has been contentious, with data to support MCH neurons releasing GABA, glutamate, both, and neither. Rather than assuming a position in that debate, this review considers the evidence for all sides and presents an alternative explanation: neurochemical identity, including classical neurotransmitter content, is subject to change. With an emphasis on the variability of experimental details, we posit that MCH neurons may release GABA and/or glutamate at different points according to environmental and contextual factors. Through the lens of the MCH system, we offer evidence that the field of neuroendocrinology would benefit from a more nuanced and dynamic interpretation of neurotransmitter identity.

Determination of the Interaction and Pharmacological Modulation of MCHR1 Signaling by the C-Terminus of MRAP2 Protein

Melanin concentrating hormone (MCH), an orexigenic neuropeptide, is primarily secreted by the hypothalamus and acts on its receptor, the melanin-concentrating hormone receptor 1 (MCHR1), to regulate appetite and energy homeostasis. The Melanocortin Receptor Accessory Protein 2 (MRAP2), a small single transmembrane protein broadly expressed in multiple tissues, has been defined as a vital endocrine modulator of five melanocortin receptors (MC1R-MC5R) and several other GPCRs in the regulation of central neuronal activities and peripheral energy balance. Here, we demonstrated the interaction between MRAP2 and MCHR1 by immunoprecipitation and bimolecular fluorescent assay and found that MRAP2 could inhibit MCHR1 signaling in vitro. A series of functional truncations of different regions further identified that the C-terminal domains of MRAP2 protein were required for the pharmacological modulation of intracellular Ca²⁺ coupled cascades and membrane transport. These findings elucidated the broad regulatory profile of MRAP2 protein in the central nervous system and may provide implications for the modulation of central MCHR1 function in vivo.

The Transition Zone Protein AHI1 Regulates Neuronal Ciliary Trafficking of MCHR1 and Its Downstream Signaling Pathway

The Abelson-helper integration site 1 (AHI1) gene encodes for a ciliary transition zone localizing protein that when mutated causes the human ciliopathy, Joubert syndrome. We prepared and examined neuronal cultures derived from male and female embryonic Ahi1 ^+/+ and Ahi1 ^-/- mice (littermates) and found that the distribution of ciliary melanin-concentrating hormone receptor-1 (MchR1) was significantly reduced in Ahi1 ^-/- neurons; however, the total and surface expression of MchR1 on Ahi1 ^-/- neurons was similar to controls (Ahi1 ^+/+). This indicates that a pathway for MchR1 trafficking to the surface plasma membrane is intact, but the process of targeting MchR1 into cilia is impaired in Ahi1-deficient mouse neurons, indicating a role for Ahi1 in localizing MchR1 to the cilium. Mouse Ahi1 ^-/- neurons that fail to accumulate MchR1 in the ciliary membrane have significant decreases in two downstream MchR1 signaling pathways [cAMP and extracellular signal-regulated kinase (Erk)] on MCH stimulation. These results suggest that the ciliary localization of MchR1 is necessary and critical for MchR1 signaling, with Ahi1 participating in regulating MchR1 localization to cilia, and further supporting cilia as critical signaling centers in neurons.SIGNIFICANCE STATEMENT Our work here demonstrates that neuronal primary cilia are powerful and focused signaling centers for the G-protein-coupled receptor (GPCR), melanin-concentrating hormone receptor-1 (MCHR1), with a role for the ciliary transition zone protein, Abelson-helper integration site 1 (AHI1), in mediating ciliary trafficking of MCHR1. Moreover, our manuscript further expands the repertoire of cilia functions on neurons, a cell type that has not received significant attention in the cilia field. Lastly, our work demonstrates the significant influence of ciliary GPCR signaling in the overall signaling of neurons.

Salpingitis Impairs Bovine Tubal Function and Sperm-Oviduct Interaction

Salpingitis is a common cause for subfertility and infertility both in humans and animals. However, the effects of salpingitis on tubal function and reproductive success are largely unknown. Therefore we set out to investigate the effects of inflammation on sperm and oocyte transport and gameto-maternal interaction in the oviduct using the bovine as a model. For this purpose, oviducts revealing mild (n = 45), moderate (n = 55) and severe (n = 45) inflammation were obtained from cows immediately after slaughter and investigated by live cell imaging, histochemistry and scanning electron microscopy. Our studies showed that endometritis was always correlated with salpingitis. Moderate and severe inflammation caused a significant increase in the thickness of tubal folds (p < 0.05). Severe inflammation was characterized by luminal accumulations of mucus and glycoproteins, increased apoptosis, loss of tight junctions and shedding of tubal epithelial cells. The mean ciliary beat frequency (CBF) in the ampulla was significantly reduced as compared to the controls (p < 0.05). The higher the grade of inflammation, the lower was the CBF (p < 0.001). In severe inflammation, spermatozoa were stuck in mucus resulting in decreased sperm motility. Our results imply that tubal inflammation impairs proper tubal function and leads to reduced sperm fertilizing capacity.

The melanin-concentrating hormone receptors: neuronal and non-neuronal functions

Melanin-concentrating hormone (MCH) is a cyclic peptide highly conserved in vertebrates and was originally identified as a skin-paling factor in Teleosts. In fishes, MCH also participates in the regulation of the stress-response and feeding behaviour. Mammalian MCH is a hypothalamic neuropeptide that displays multiple functions, mostly controlling feeding behaviour and energy homeostasis. Transgenic mouse models and pharmacological studies have shown the importance of the MCH system as a potential target in the treatment of appetite disorders and obesity as well as anxiety and psychiatric diseases. Two G-protein-coupled receptors (GPCRs) binding MCH have been characterized so far. The first, named MCH-R1 and also called SLC1, was identified through reverse pharmacology strategies by several groups as a cognate receptor of MCH. This receptor is expressed at high levels in many brain areas of rodents and primates and is also expressed in peripheral organs, albeit at a lower rate. A second receptor, designated MCH-R2, exhibited 38% identity to MCH-R1 and was identified by sequence analysis of the human genome. Interestingly, although MCH-R2 orthologues were also found in fishes, dogs, ferrets and non-human primates, this MCH receptor gene appeared either lacking or non-functional in rodents and lagomorphs. Both receptors are class I GPCRs, whose main roles are to mediate the actions of peptides and neurotransmitters in the central nervous system. However, examples of action of MCH on neuronal and non-neuronal cells are emerging that illustrate novel MCH functions. In particular, the functionality of endogenously expressed MCH-R1 has been explored in human neuroblastoma cells, SK-N-SH and SH-SY5Y cells, and in non-neuronal cell types such as the ependymocytes. Indeed, we have identified mitogen-activated protein kinase (MAPK)-dependent or calcium-dependent signalling cascades that ultimately contributed to neurite outgrowth in neuroblastoma cells or to modulation of ciliary beating in ependymal cells. The putative role of MCH on cellular shaping and plasticity on one side and volume transmission on the other must be now considered.

Inflammation-induced functional connectivity of melanin-concentrating hormone and IL-10

Melanin-concentrating hormone (MCH) was identified in mammals as a hypothalamic neuropeptide regulating appetite and energy balance. However, similarly to most of the brain peptides, MCH is also produced in the gastrointestinal system and can act locally as an immunomodulator. We have previously reported high expression of MCH and its receptor MCHR1 in the affected mucosa of patients with inflammatory bowel disease. Furthermore, MCH deficiency in mice attenuated experimental colitis, pointing to MCH as a mediator of intestinal inflammation. In the present study, in order to gain further insights into the underlying mechanisms of such effects of MCH, we treated mice with established experimental colitis due to IL-10 deficiency with a MCHR1 antagonist (DABA-822). While treatment with the same drug was successful in attenuating TNBS-induced colitis in previous studies, it offered no benefit to the IL-10 knockout mouse model, suggesting that perhaps IL-10 is a downstream target of MCH. Indeed, in experiments focusing on monocytes, we found that treatment with MCH inhibited LPS-mediated IL-10 upregulation. Conversely, in the same cells, exogenous IL-10 prevented LPS-induced MCHR1 expression. Taken together, these findings indicate a functional cross-talk between MCH and IL-10 which prevents resolution of inflammation.

Adipokines and melanocortins in the hepatic manifestation of metabolic syndrome: nonalcoholic fatty liver disease

Metabolic syndrome is associated with nonalcoholic fatty liver disease and its more aggressive form, nonalcoholic steatohepatitis. Adipokines produced by white adipose tissue possess broad physiological activity and play an important autocrine role in obesity-associated complications, including metabolic syndrome, nonalcoholic fatty liver disease and cardiovascular disease. Various adipokines may have beneficial or harmful effects. Other tissues, particularly stomach and intestine, produce active molecules that can influence the function of adipocytes and, possibly, the levels of adipokine secretion. In some cases, the production sites of these molecules remain unknown. The review focuses on our current understanding of the disease-related effects of the adipokines and the melanocortins on various peripheral tissues, and discusses some of their potential interactions with each other. Potential therapeutic applications are also considered.

Signalling pathway of goldfish melanin-concentrating hormone receptors 1 and 2

Melanin-concentrating hormone (MCH) is the natural ligand for the MCH-1 receptor (MCHR1) and MCH-2 receptor (MCHR2). The MCH-MCHR1 system plays a central role in energy metabolism in rodents. Recently, we identified MCHR1 and MCHR2 orthologues in goldfish, designated gfMCHR1 and gfMCHR2. In a mammalian cell-based assay, calcium mobilization was evoked by gfMCHR2 via both Gαi/o and Gαq, while the gfMCHR1-mediated response was exclusively dependent on Gαq. This coupling capacity to G proteins is in contrast to human MCHR1 and MCHR2. Here, we extended our previous characterization of the two gfMCHRs by examining their different signalling pathway. We found that MCH caused activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) via both gfMCHR1 and gfMCHR2 in dose-dependent manners. Unlike the case for gfMCHR2, gfMCHR1 signalling was not sensitive to pertussis toxin, suggesting Gαq coupling of gfMCHR1 in the ERK1/2 pathway as well as a calcium mobilization system. Cyclic AMP assays revealed that gfMCHR2 was efficiently coupled to Gαi/o, while gfMCHR1 was weakly coupled to Gαs. Finally, we investigated the transduction features stimulated by two mammalian MCH analogues. As expected, Compound 15, which is a full agonist of human MCHR1, was a potent gfMCHR1 agonist in multiple signalling pathways. On the other hand, Compound 30, which is a human MCHR1-selective antagonist with negligible agonist potency, unexpectedly acted as a selective agonist of gfMCHR1. These results are the first to demonstrate that gfMCHR1 and gfMCHR2 have quite different signalling properties from human MCHRs.

Cellular models for the study of the pharmacology and signaling of melanin-concentrating hormone receptors

Cellular models for the study of the neuropeptide melanin-concentrating hormone (MCH) have become indispensable tools for pharmacological profiling and signaling analysis of MCH and its synthetic analogues. Although expression of MCH receptors is most abundant in the brain, MCH-R(1) is also found in different peripheral tissues. Therefore, not only cell lines derived from nervous tissue but also from peripheral tissues that naturally express MCH receptors have been used to study receptor signaling and regulation. For screening of novel compounds, however, heterologous expression of MCH-R(1) or MCH-R(2) genes in HEK293, Chinese hamster ovary, COS-7, or 3T3-L1 cells, or amplified MCH-R(1) expression/signaling in IRM23 cells transfected with the G(q) protein gene are the preferred tools because of more distinct pharmacological effects induced by MCH, which include inhibition of cAMP formation, stimulation of inositol triphosphate production, increase in intracellular free Ca(2+) and/or activation of mitogen-activated protein kinases. Most of the published data originate from this type of model system, whereas data based on studies with cell lines endogenously expressing MCH receptors are more limited. This review presents an update on the different cellular models currently used for the analysis of MCH receptor interaction and signaling.

Melanin-concentrating hormone receptor 2 affects 3T3-L1 preadipocyte differentiation

To investigate the influence of melanin-concentrating hormone receptor 2 (MCHR2) on preadipocyte proliferation and differentiation, a 3T3-L1-MCHR2 cell line stably expressing human MCHR2 was established. The expression level and biological activity of MCHR2 were evaluated by immunofluorescence and radioligand binding assays. A 3-(4,5-dimethylthiazol-2yl)-2,5-diphenyltetrazolium bromide (MTT) assay and cell cycle analysis showed that MCH did not affect 3T3-L1-MCHR2 and 3T3-L1-mock cell proliferation. MCH accelerated preadipocyte differentiation of the two cell lines when they were administered various doses of standard adipogenic inducers. The rate and degree of differentiation of the 3T3-L1-MCHR2 cells were higher than those of the 3T3-L1-mock cells. The expression levels of adipocyte-specific marker genes, including peroxisome proliferator-activated receptorgamma2 (PPARgamma2), CCAAT enhancer-binding proteinalpha (C/EBPalpha), adipocyte-specific fatty acid-binding protein (aP(2)) and leptin were significantly up-regulated, especially in 3T3-L1-MCHR2 cells. These findings suggest that MCHR2 positively mediates the regulation of MCH during preadipocyte differentiation and is involved in energy balance regulation without affecting preadipocyte proliferation.

Electrophysiological effects of MCH on neurons in the hypothalamus

Melanin concentrating hormone (MCH) has been implicated in many brain functions and behaviors essential to the survival of animals. The hypothalamus is one of the primary targets where MCH-containing nerve fibers and MCH receptors are extensively expressed and its actions in the brain are exerted. Since the identification of MCH receptors as orphan G protein coupled receptors, the cellular effects of MCH have been revealed in many non-neuronal expression systems (including Xenopus oocytes and cell lines), however, the mechanism by which MCH modulates the activity in the neuronal circuitry of the brain is still under investigation. This review summarizes our current knowledge of electrophysiological effects of MCH on neurons in the hypothalamus, particularly in the lateral hypothalamus. Generally, MCH exerts inhibitory effects on neurons in this structure and may serve as a homeostatic regulator in the lateral hypothalamic area. Given the contrast between the limited data on cellular functions of MCH in the hypothalamus versus a fast growing body of evidence on the vital role of MCH in animal behavior, further investigations of the former are warranted.

Melanin-concentrating hormone induces neurite outgrowth in human neuroblastoma SH-SY5Y cells through p53 and MAPKinase signaling pathways

Melanin-concentrating hormone (MCH) peptide plays a major role in energy homeostasis regulation. Little is known about cellular functions engaged by endogenous MCH receptor (MCH-R1). Here, MCH-R1 mRNA and cognate protein were found expressed in human neuroblastoma SH-SY5Y cells. Electrophysiological experiments demonstrated that MCH modulated K(+) currents, an effect depending upon the time of cellular growth. MCH treatments induced a transient phosphorylation of MAPKinases, abolished by PD98059, and partially blocked by PTX, suggesting a Galphai/Galphao protein contribution. MCH stimulated expression and likely nuclear localization of phosphorylated p53 proteins, an effect fully dependent upon MAPKinase activities. MCH treatment also increased phosphorylation of Elk-1 and up-regulated Egr-1, two transcriptional factors targeted by the MAPKinase pathway. Finally, MCH provoked neurite outgrowth after 24h-treatment of neuroblastoma cells. This effect and transcriptional factors activation were partly prevented by PD98059. Collectively, our results provide the first evidence for a role of MCH in neuronal differentiation of endogenously MCH-R1-expressing cells via non-exclusive MAPKinase and p53 signaling pathways.

Melanin-concentrating hormone and melanin-concentrating hormone receptors in mammalian skin physiopathology

To date, there is a dearth of evidence to support functions for melanin-concentrating hormone (MCH) and melanin-concentrating hormone receptors (MCH-R) in mammalian skin physiology including pigmentation, inflammation and immune responses and skin cell proliferation. Much research is therefore still needed to define the roles of the hormone and its receptors in mammalian skin. This will be a crucial step to identifying pathogenic mechanisms that may involve the MCH/MCH-R system in the context of inflammatory and autoimmune skin diseases as well as skin cancers. The following review summarizes the studies which have been carried out to examine the expression and function of MCH and MCH-R in mammalian skin. Recent findings with regard to humoral immune responses to the MCH-R1 in patients with the skin depigmenting disease vitiligo are also discussed.

The melanin-concentrating hormone system and its physiological functions

Melanin-concentrating hormone (MCH) is a neuropeptide that was originally isolated from salmon pituitary where it causes pigment aggregation. MCH is also abundantly present in mammalian neurons and expressed in the lateral hypothalamus and zona incerta, brain regions that are known to be at the center of feeding behavior. MCH binds to and activates two G protein-coupled receptors, MCH1R and MCH2R. Although MCH2R is non-functional in rodents, genetic and pharmacological studies have demonstrated that rodent MCH1R is involved in the regulation of feeding behavior and energy balance. Unexpectedly, some antagonists have provided evidence that MCH signaling participates in the regulation of other processes, such as emotion and stress. The discovery of MCH receptors has extensively promoted the progress of MCH studies and may represent an ideal example of how deorphanized receptors can open new directions toward more detailed physiological studies.

The melanin-concentrating hormone receptor 2 (MCH-R2) mediates the effect of MCH to control body color for background adaptation in the barfin flounder

Melanin-concentrating hormone (MCH) is a neuropeptide generated in neurons originating in the hypothalamus, from which axons project to the entire brain and neurohypophysis in fish. MCH has both central and peripheral roles such as food intake and body color change. Here we cloned two MCH receptors (MCH-R) from the barfin flounder, Verasper moseri, Pleuronectiformes. The phylogenetic analysis shows that these are orthologues to the mammalian MCH-R1 and MCH-R2 showing 49 and 30% amino acid sequence identity to the corresponding human receptors while they have 31% amino acid sequence identify between them. Essential amino acid residues for ligand binding, signal transduction and receptor conformation, which have been shown in mammalian MCH-R, are well conserved in the flounder MCH-Rs. MCH-R1 has one intron in the extracellular N-terminal region and MCH-R2 has one intron in the DRY motif, which is a homologous position to one of the five introns of human MCH-R2. Orthologues of MCH-R1 and MCH-R2 may have appeared by gene duplication of the ancestry of MCH-Rs having at least two introns, and then MCH-R1 and MCH-R2 inherited different introns in flounder strains. We also determined their tissue distribution and functional role in rearing condition. Reverse transcription PCR revealed that the expression of MCH-R1 is confined to the brain of the barfin flounder, while transcripts of MCH-R2 were detected in the brain, pituitary, eyeball, gill, atrium, ventricle, head kidney, body kidney, spleen, intestine, inclinator, skeletal muscle testis, ovary, eyed-side skin, and non-eyed-side skin. The expression of MCH-R2 in eyed-side skin was higher in fish reared in a black tank (121 days) than in a white tank while the expression levels of MCH in the brain were significantly greater in the group reared with the white background suggesting down-regulation of this receptor gene with increased levels of MCH. The results suggest that the MCH-R2 mediates the effect of MCH to control body color for background adaptation in the eyed-side skin of the barfin flounder.

The melanocortins and melanin-concentrating hormone in the central regulation of feeding behavior and energy homeostasis

A number of different neuropeptides exert powerful concerted controls on feeding behavior and energy balance, most of them being produced in hypothalamic neuronal networks under stimulation by anabolic and catabolic peripheral hormones such as ghrelin and leptin, respectively. These peptide-expressing neurons interconnect extensively to integrate the multiple opposing signals that mediate changes in energy expenditure. In the present review I have summarized our current knowledge about two key peptidic systems involved in regulating appetite and energy homeostasis, the melanocortin system (alpha-MSH, agouti and Agouti-related peptides, MC receptors and mahogany protein) and the melanin-concentrating hormone system (proMCH-derived peptides and MCH receptors) that contribute to satiety and feeding-initiation, respectively, with concurrent effects on energy expenditure. I have focused particularly on recent data concerning transgenic mice and the ongoing development of MC/MCH receptor antagonists/agonists that may represent promising drugs to treat human eating disorders on both sides of the energy balance (anorexia, obesity).

Characterization of a neuronal cell line expressing native human melanin-concentrating hormone receptor 1 (MCHR1)

Melanin-concentrating hormone (MCH), an orexigenic neuropeptide in mammals, activates a G-protein coupled receptor, MCHR1. It is expected that antagonists of MCHR1 function will prove therapeutically useful as anti-obesity agents. Intracellular signaling by MCHR1 has been investigated primarily using non-neural cell lines expressing the recombinant receptor, in which MCHR1 has been shown to couple to G alpha(i/o) and G alpha(q) G-proteins. While these cell lines have been widely utilized to discover and optimize small molecule antagonists, it is unknown whether the intracellular signaling pathways in these cells accurately reflect those in neurons. Thus, we sought to develop a neurally derived cell line endogenously expressing MCHR1. IMR32, a human neuroblastoma cell line, has been shown to express MCHR1 mRNA; however, we were unable to detect either MCH-binding or MCH-stimulated Ca++-mobilization in these cells. Following transfection of IMR32 cells with a plasmid encoding human G alpha(16) G-protein, we isolated a cell line, I3.4.2, which responded to MCH in Ca++-mobilization assays. We found that the expression level of MCHR1 mRNA in I3.4.2 cells was 2000-fold higher than in the parent cell line. Using [125I]MCH saturation-binding to I3.4.2 cell membranes, we estimated the Bmax as 0.72 pmol/mg protein and the Kd as 0.35 nM. We report that Ca++-mobilization in I3.4.2 cells was insensitive to pertussis toxin (Ptx) treatment, indicating that signaling was via G alpha(q) G-proteins. Furthermore, negative results in cAMP accumulation assays confirmed the lack of signaling via the G alpha(i/o) G-proteins. Our results suggest that the I3.4.2 cell line may be useful for characterization of MCHR1 activity in a neural-derived cell line.

Identification of melanin-concentrating hormone receptor and its impact on drug discovery

The neuropeptide melanin-concentrating hormone (MCH) was originally isolated from the pituitary of salmon, in which it causes skin paling. MCH is also found abundantly in mammalian neurons, and has been detected in the lateral hypothalamus and zona incerta, brain regions that are at the center of feeding behavior. Acute central administration of MCH leads to a rapid and significant increase in food intake, while MCH expression changes in states of altered energy balance, such as fasting and obesity. Furthermore, MCH knockout mice tend toward hypophagia and leanness. In 1999, we and four other groups identified an orphan G-protein-coupled receptor (GPCR) as a specific receptor for MCH (MCH-1 receptor). Although a second MCH receptor (MCH-2 receptor) was isolated in humans, it was found to be non-functional or encode a non-functional pseudogene in non-human species, including rodents. The discovery of these MCH receptors permitted the launch of a broad array of drug screening efforts and three MCH-1 receptor antagonists were identified to reduce food intake and body weight. Interestingly, some antagonists unexpectedly produced evidence that blockade of these receptors has antidepressant and anxiolytic activities. The expressions of the MCH receptors, which have been implicated in regulating emotion, stress and motivation, make MCH an excellent candidate for integrating the various homeostatic stimuli necessary for maintaining the proper conditions of energy metabolism and other physiological functions. Finally, the speed at which MCH receptor studies have been undertaken exemplifies the impact that this deorphanized GPCR will have on setting the stage for more detailed physiological studies.