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

The Efficacy and Psychoneuroimmunology Mechanism of Camouflage Combined With Psychotherapy in Vitiligo Treatment

Background and Objectives

The efficacy of camouflage combined with psychotherapy and the underlying mechanisms are poorly understood in vitiligo management. This study aimed to investigate the joint efficacy and further explore psycho-neuro-endocrine-immune-skin interactions.

Patients and Methods

In a prospective, non-randomized and concurrent controlled trial, patients were divided into two groups. Quality of life (QOL) was evaluated using the Chinese version of the Vitiligo Life Quality Index (VLQI-C). Serum levels of neuropeptides and cytokines were detected by enzyme-linked immunosorbent assay.

Results

A total of 149 patients were included for final evaluation. After treatment for 4 weeks, total and subcategory quality of life scores in the intervention group were much lower than in the control group. Serum levels of neuropeptide-Y (NPY) and melanin-concentrating hormone (MCH) significantly decreased, and serum level of adrenocorticotropic hormone (ACTH) increased in both active and stable patients of the intervention group, but not in the control group. In addition, the serum levels of interferon-γ (IFN-γ), CXC chemokine ligand 10 (CXCL10), and interleukin-1β (IL-1β) decreased in both the active and stable patients of the intervention group and only in the active patients of the control group.

Conclusions

The combination of camouflage and psychotherapy provided a clinically meaningful improvement in quality of life and ameliorated the outcome by likely modulating the psycho-neuro-endocrine-immuno-skin system during vitiligo management.

Clinical Trial Registration

www.clinicaltrials.gov/ct2/show/NCT03540966, identifier: NCT03540966.

Genetic deciphering of the antagonistic activities of the melanin-concentrating hormone and melanocortin pathways in skin pigmentation

The genetic origin of human skin pigmentation remains an open question in biology. Several skin disorders and diseases originate from mutations in conserved pigmentation genes, including albinism, vitiligo, and melanoma. Teleosts possess the capacity to modify their pigmentation to adapt to their environmental background to avoid predators. This background adaptation occurs through melanosome aggregation (white background) or dispersion (black background) in melanocytes. These mechanisms are largely regulated by melanin-concentrating hormone (MCH) and α-melanocyte–stimulating hormone (α-MSH), two hypothalamic neuropeptides also involved in mammalian skin pigmentation. Despite evidence that the exogenous application of MCH peptides induces melanosome aggregation, it is not known if the MCH system is physiologically responsible for background adaptation. In zebrafish, we identify that MCH neurons target the pituitary gland-blood vessel portal and that endogenous MCH peptide expression regulates melanin concentration for background adaptation. We demonstrate that this effect is mediated by MCH receptor 2 (Mchr2) but not Mchr1a/b. mchr2 knock-out fish cannot adapt to a white background, providing the first genetic demonstration that MCH signaling is physiologically required to control skin pigmentation. mchr2 phenotype can be rescued in adult fish by knocking-out pomc, the gene coding for the precursor of α-MSH, demonstrating the relevance of the antagonistic activity between MCH and α-MSH in the control of melanosome organization. Interestingly, MCH receptor is also expressed in human melanocytes, thus a similar antagonistic activity regulating skin pigmentation may be conserved during evolution, and the dysregulation of these pathways is significant to our understanding of human skin disorders and cancers.

Melanocytes produce melanin, a natural skin pigment, for body coloration which helps to protect and camouflage an organism and to attract mates. Melanocytes are ubiquitous pigment cells in vertebrates and the genes underlying their development are well conserved, making fishes that possess the ability to modify their pigmentation, biologically relevant and successful models for human skin disorders. Many human skin diseases including albinism, vitiligo, and melanoma are derived from mutations in conserved pigmentation genes. However, much of the conserved molecular mechanisms behind these diseases and human pigmentation remain unknown. For instance, melanin concentrating hormone (MCH) was originally identified as a peptide that when injected, could make fish paler by promoting melanin aggregation but no mutants demonstrating an endogenous function for MCH in pigmentation have been reported. Here, we use zebrafish mutants of MCH and the MCH receptor to determine their specific genetic function in pigmentation. Additionally, we demonstrate that MCH has an antagonistic pigmentation function to the melanocortin system, where MCH expression promotes lighter pigmentation and melanocortin activity promotes darkening. Thus, we find that the balance between the MCH and melanocortin system activities are likely required for skin pigmentation and dysregulation of these pathways could underlie adverse human skin conditions.

Up- or Downregulation of Melanin Synthesis Using Amino Acids, Peptides, and Their Analogs

Harmonious synthesis and distribution of melanin in the skin contribute to the expression of beauty and the maintenance of health. When skin pigmentary disorders occur because of internal or external factors or, when there is a need to artificially increase or reduce the pigmentation level of the skin for aesthetic or therapeutic purposes, various pharmacological therapies are applied but the results are not always satisfactory. Studies have been conducted to improve the efficacy and safety of these treatment strategies. In this review, we present the latest studies regarding peptides and related compounds that may be useful in artificially increasing or reducing skin melanin levels. Certain analogs of α-melanocyte stimulating hormone (MSH) and oligopeptides with the sequences derived from the hormone were shown to promote melanin synthesis in cells and in vivo models. Various amino acids, peptides, their analogs, and their hybrid compounds with other chemical moieties were shown to inhibit tyrosinase (TYR) catalytic activity or downregulate TYR gene expression. Certain peptides were shown to inhibit melanosome biogenesis or induce autophagy, leading to decreased pigmentation. In vivo and clinical evidence are available for some compounds, including [Nle⁴-_D-Phe⁷]-α-MSH, glutathione disulfide, and glycinamide hydrochloride. For many other compounds, additional studies are required to verify their efficacy and safety in vivo and in clinical trials. The accumulating information regarding pro- and antimelanogenic activity of peptides and related compounds will lead to the development of novel drugs for the treatment of skin pigmentary disorders.

Discovery of a Highly Selective MC1R Agonists Pentapeptide to Be Used as a Skin Pigmentation Enhancer and with Potential Anti-Aging Properties

One of the first lines of cutaneous defense against photoaging is (a) the synthesis of melanin and (b) the initiation of an oxidative stress response to protect skin against the harmful effects of solar radiation. Safe and selective means to stimulate epidermal pigmentation associated with oxidative stress defense are; however, scarce. Activation of the melanocortin-1 receptor (MC1R) on epidermal melanocytes represents a key step in cutaneous pigmentation initiation and, additionally, it regulates cellular defense mechanisms like oxidative stress and DNA-repair. Thus, making the activation of MC1R an attractive strategy for modulating skin pigmentation and oxidative stress. In this context, we designed and synthesized pentapeptides that act as MC1R agonists. These peptides bound, with high potency, to MC1R and activated cAMP synthesis in CHO cells expressing human MC1R. Using one lead pentapeptide, we could show that this activation of MC1R was specific as testing the activation of other G-protein coupled receptors, including the MC-receptor family, was negative. In vitro efficacy on mouse melanoma cells showed similar potency as for the synthetic MC1R agonist alpha-melanocyte stimulating hormone (NDP-alpha-MSH). Moreover, we could reproduce this activity in human skin tissue culture. The lead pentapeptide was able to induce ex-vivo protein expression of key melanogenesis markers melanocyte inducing transcription factor (MITF), tyrosinase (TYR), and tyrosinase-related protein 1 (TYRP-1). Concerning oxidative stress response, we found that the pentapeptide enhanced the activation of Nrf2 after UVA-irradiation. Our results make this pentapeptide an ideal candidate as a skin pigmentation enhancer that mimics alpha-MSH and may also have anti-photoaging effects on the skin.

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.

Vertebrate melanophores as potential model for drug discovery and development: a review

Drug discovery in skin pharmacotherapy is an enormous, continually expanding field. Researchers are developing novel and sensitive pharmaceutical products and drugs that target specific receptors to elicit concerted and appropriate responses. The pigment-bearing cells called melanophores have a significant contribution to make in this field. Melanophores, which contain the dark brown or black pigment melanin, constitute an important class of chromatophores. They are highly specialized in the bidirectional and coordinated translocation of pigment granules when given an appropriate stimulus. The pigment granules can be stimulated to undergo rapid dispersion throughout the melanophores, making the cell appear dark, or to aggregate at the center, making the cell appear light. The major signals involved in pigment transport within the melanophores are dependent on a special class of cell surface receptors called G-protein-coupled receptors (GPCRs). Many of these receptors of adrenaline, acetylcholine, histamine, serotonin, endothelin and melatonin have been found on melanophores. They are believed to have clinical relevance to skin-related ailments and therefore have become targets for high throughput screening projects. The selective screening of these receptors requires the recognition of particular ligands, agonists and antagonists and the characterization of their effects on pigment motility within the cells. The mechanism of skin pigmentation is incredibly intricate, but it would be a considerable step forward to unravel its underlying physiological mechanism. This would provide an experimental basis for new pharmacotherapies for dermatological anomalies. The discernible stimuli that can trigger a variety of intracellular signals affecting pigment granule movement primarily include neurotransmitters and hormones. This review focuses on the role of the hormone and neurotransmitter signals involved in pigment movement in terms of the pharmacology of the specific receptors.

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.