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

Identification of amino acids that are selectively involved in Gi/o activation by rat melanin-concentrating hormone receptor 1

Many G-protein-coupled receptors (GPCRs) are known to functionally couple to multiple G-protein subfamily members. Although promiscuous G-protein coupling enables GPCRs to mediate diverse signals, only a few GPCRs have been identified with differential determinants for coupling to distinct Gα proteins. Mammalian melanin-concentrating hormone receptor 1 (MCHR1) couples to dual G-protein subfamilies. However, the selectivity mechanisms between MCHR1 and different subtypes of Gα proteins are unclear. Our previous studies demonstrated that mammalian MCHR1 couples to both Gi/o and Gq, whereas goldfish MCHR1 exclusively couples to Gq. In this study, we analyzed multiple sequence alignments between rat and goldfish MCHR1s, and designed three multisubstituted mutants of rat MCHR1 by replacing corresponding residues with those in goldfish MCHR1, focusing on regions around the cytosolic intracellular loops. By measurement of intracellular Ca(2+) mobilization, we found that two MCHR1 mutants, i2_6sub and i3_6sub, which contained six simultaneously substituted residues in the second intracellular loop or a combination of substituted residues in the third intracellular loop and fifth transmembrane domain, respectively, significantly reduced Gi/o-sensitive pertussis toxin responsiveness without altering Gq-mediated activity. Analyses of 10 other substitutions revealed that the multiple substitutions in i2_6sub and i3_6sub were necessary for Gi/o-selective responses. As judged by Gi/o-dependent GTPγS binding and cyclic AMP assays, i2_6sub and i3_6sub elicited phenotypes for impaired Gi/o-mediated signaling. We also monitored the dynamic mass redistribution (DMR) in living cells, which reveals receptor activity as an optical trace containing activation of all GPCR coupling classes. Cells transfected with i2_6sub or i3_6sub exhibited reduced Gi/o-mediated DMR responses compared with those transfected with MCHR1. These data suggest that two different regions independently affect the Gi/o-protein preference, and that multiple residues comprise a conformation favoring Gi/o-protein coupling and subsequently result in Gi/o-selective signaling.

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 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 immune function

To date, melanin-concentrating hormone (MCH) has been generally considered as peptide acting almost exclusively in the central nervous system. In the present paper, we revise the experimental evidence, demonstrating that MCH and its receptors are expressed by cells of the immune system and directly influence the response of these cells in some circumstances. This therefore supports the idea that, as with other peptides, MCH could be considered as a modulator of the immune system. Moreover, we suggest that this could have important implications in several immune-mediated disorders and affirm that there is a clear need for further investigation.

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.

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

The neuropeptide melanin-concentrating hormone (MCH) is expressed in central and peripheral tissues where it participates in the complex network regulating energy homeostasis as well as in other physiologically important functions. Two MCH receptor subtypes, MCH-R1 and MCH-R2, have been cloned which signal through activation of Gi/o/q proteins and hence regulate different intracellular signals, such as inhibition of cAMP formation, stimulation of IP3 production, increase in intracellular free Ca2+ and/or activation of MAP kinases. Most of the data were obtained with cell systems heterologously expressing either of the MCH receptors. Fewer reports exist on studies with cell lines which endogenously express MCH receptors. Here, we describe human and other mammalian cell lines with which MCH receptor activation can be studied under "natural" conditions and we summarize the characteristics and signaling pathways of the MCH receptors in the different cell systems.

Comparative analysis of melanin-concentrating hormone structure and activity in fishes and mammals

A comparative analysis of the structure of the melanin-concentrating hormone (MCH) precursor reveals that this sequence has been subjected to a higher selection pressure in mammals than in teleosts, suggesting that the structural constraints have not been the same throughout the vertebrate lineage. In contrast, the MCH peptide sequence has been very well conserved in all species. A sensitive and reproducible eel skin assay was developed and allowed us to define the structural features needed for a full MCH bioactivity. It was shown that the minimal structure carrying the critical residues was the same in fishes and in mammals. A pharmacological approach confirmed that MCH receptor activation decreased the cAMP levels in the fish skin, but this effect appeared to be independent from a Galphai protein. We propose that one of the intracellular signaling pathways of the MCH receptor in fish skin is the activation of one or several cellular phosphodiesterases.