Melanocortin receptor-mediated mobilization of intracellular free calcium in HEK293 cells

Measuring GPCR-Induced Intracellular Calcium Signaling Using a Quantitative High-Throughput Assay

Alterations in intracellular calcium are integral to signal transduction pathways for many G-protein-coupled receptors, but this signaling is not well studied. This is mostly due to a lack of reliable, robust, high-throughput, quantitative methods to monitor intracellular calcium concentrations in live cells. Recently, we developed a reliable, robust, quantitative method to measure intracellular calcium levels in which HEK293 cell suspensions loaded with Fura-2/AM are placed in 96-well plates. Minimum and maximum intracellular calcium levels, which are required for converting fluorescence into calcium concentrations, are calibrated using EGTA to chelate calcium and ionomycin to load calcium into cells, respectively. Fluorescence is monitored with a PHERAstar FS plate reader. We provide a detailed method for this high-throughput assay that can be used to quantitate intracellular calcium in endogenous and exogenously (stable or transient) expressed GPCRs in HEK293 cells.

The Role of Melanocortin Plasticity in Pain-Related Outcomes After Alcohol Exposure

The global COVID-19 pandemic has shone a light on the rates and dangers of alcohol misuse in adults and adolescents in the US and globally. Alcohol exposure during adolescence causes persistent molecular, cellular, and behavioral changes that increase the risk of alcohol use disorder (AUD) into adulthood. It is established that alcohol abuse in adulthood increases the likelihood of pain hypersensitivity and the genesis of chronic pain, and humans report drinking alcohol to relieve pain symptoms. However, the longitudinal effects of alcohol exposure on pain and the underlying CNS signaling that mediates it are understudied. Specific brain regions mediate pain effects, alcohol effects, and pain-alcohol interactions, and neural signaling in those brain regions is modulated by neuropeptides. The CNS melanocortin system is sensitive to alcohol and modulates pain sensitivity, but this system is understudied in the context of pain-alcohol interactions. In this review, we focus on the role of melanocortin signaling in brain regions sensitive to alcohol and pain, in particular the amygdala. We also discuss interactions of melanocortins with other peptide systems, including the opioid system, as potential mediators of pain-alcohol interactions. Therapeutic strategies that target the melanocortin system may mitigate the negative consequences of alcohol misuse during adolescence and/or adulthood, including effects on pain-related outcomes.

Quantitative high-throughput assay to measure MC4R-induced intracellular calcium

The melanocortin-4 receptor (MC4R), a critical G-protein-coupled receptor (GPCR) regulating energy homeostasis, activates multiple signalling pathways, including mobilisation of intracellular calcium ([Ca2+]i). However, very little is known about the physiological significance of MC4R-induced [Ca2+]i since few studies measure MC4R-induced [Ca2+]i. High-throughput, read-out assays for [Ca2+]i have proven unreliable for overexpressed GPCRs like MC4R, which exhibit low sensitivity mobilising [Ca2+]i. Therefore, we developed, optimised, and validated a robust quantitative high-throughput assay using Fura-2 ratio-metric calcium dye and HEK293 cells stably transfected with MC4R. The quantitation enables direct comparisons between assays and even between different research laboratories. Assay conditions were optimised step-by-step to eliminate interference from stretch-activated receptor increases in [Ca2+]i and to maximise ligand-activated MC4R-induced [Ca2+]i. Calcium imaging was performed using a PheraStar FS multi-well plate reader. Probenecid, included in the buffers to prevent extrusion of Fura-2 dye from cells, was found to interfere with the EGTA-chelation of calcium, required to determine Rmin for quantitation of [Ca2+]i. Therefore, we developed a method to determine Rmin in specific wells without probenecid, which was run in parallel with each assay. The validation of the assay was shown by reproducible α-melanocyte-stimulating hormone (α-MSH) concentration-dependent activation of the stably expressed human MC4R (hMC4R) and mouse MC4R (mMC4R), inducing increases in [Ca2+]i, for three independent experiments. This robust, reproducible, high-throughput assay that quantitatively measures MC4R-induced mobilisation of [Ca2+]i in vitro has potential to advance the development of therapeutic drugs and understanding of MC4R signalling associated with human obesity.

Lipocalin-2 counteracts metabolic dysregulation in obesity and diabetes

Regulation of food intake is a recently identified endocrine function of bone that is mediated by Lipocalin-2 (LCN2). Osteoblast-secreted LCN2 suppresses appetite and decreases fat mass while improving glucose metabolism. We now show that serum LCN2 levels correlate with insulin levels and β-cell function, indices of healthy glucose metabolism, in obese mice and obese, prediabetic women. However, LCN2 serum levels also correlate with body mass index and insulin resistance in the same individuals and are increased in obese mice. To dissect this apparent discrepancy, we modulated LCN2 levels in mice. Silencing Lcn2 expression worsens metabolic dysfunction in genetic and diet-induced obese mice. Conversely, increasing circulating LCN2 levels improves metabolic parameters and promotes β-cell function in mouse models of β-cell failure acting as a growth factor necessary for β-cell adaptation to higher metabolic load. These results indicate that LCN2 up-regulation is a protective mechanism to counteract obesity-induced glucose intolerance by decreasing food intake and promoting adaptive β-cell proliferation.

Signal pathway analysis of selected obesity-associated melanocortin-4 receptor class V mutants

Mutations in the melanocortin-4 receptor (MC4R) in humans are the single most common cause of rare monogenic 1severe obesity, and polymorphisms in this gene are also associated with obesity in the general population. The MC4R is a G-protein coupled receptor, and in vitro analysis suggests that MC4R can signal through several different G-protein subtypes. In vivo studies show complex outcomes, with different G-proteins in different cells responsible for different physiological responses linked to obesity. There is an emerging consensus that Gαq-linked signals in the paraventricular nucleus of the hypothalamus are essential for normal satiety and the control of feeding behavior. Many MC4R mutations have been analyzed for the molecular defect underlying their association with obesity, which has revealed a group - referred to as class V mutants - with no measurable change in receptor function. However, Gαq-linked signaling leading to Ca²⁺ release has only been examined for a few MC4R mutations. In this study, we have examined seven MC4R class V mutants, as well as two other well-characterized signal-defective mutants as controls, with respect to G-protein signaling coupled to cAMP production, mitogen-activated protein kinase (MAPK) activation, and Ca²⁺ release. These data confirm, with one exception (E308K), the expected pattern of cAMP and MAPK signaling for wild type and mutant MC4R. Our results also demonstrate normal MSH-induced Ca²⁺ signals for wild type as well as all the class V mutants, but not the signal-defective controls. Thus, the means by which class V MC4R mutations lead to obesity remains an open question.

Melanocortin regulation of histaminergic neurons via perifornical lateral hypothalamic melanocortin 4 receptors

OBJECTIVE

Histaminergic neurons of the tuberomammillary nucleus (TMN) are wake-promoting and contribute to the regulation of energy homeostasis. Evidence indicates that melanocortin 4 receptors (MC4R) are expressed within the TMN. However, whether the melanocortin system influences the activity and function of TMN neurons expressing histidine decarboxylase (HDC), the enzyme required for histamine synthesis, remains undefined.

METHODS

We utilized Hdc-Cre mice in combination with whole-cell patch-clamp electrophysiology and in vivo chemogenetic techniques to determine whether HDC neurons receive metabolically relevant information via the melanocortin system.

RESULTS

We found that subsets of HDC-expressing neurons were excited by melanotan II (MTII), a non-selective melanocortin receptor agonist. Use of melanocortin receptor selective agonists (THIQ, [D-Trp8]-γ-MSH) and inhibitors of synaptic transmission (TTX, CNQX, AP5) indicated that the effect was mediated specifically by MC4Rs and involved a glutamatergic dependent presynaptic mechanism. MTII enhanced evoked excitatory post-synaptic currents (EPSCs) originating from electrical stimulation of the perifornical lateral hypothalamic area (PeFLH), supportive of melanocortin effects on the glutamatergic PeFLH projection to the TMN. Finally, in vivo chemogenetic inhibition of HDC neurons strikingly enhanced the anorexigenic effects of intracerebroventricular administration of MTII, suggesting that MC4R activation of histaminergic neurons may restrain the anorexigenic effects of melanocortin system activation.

CONCLUSIONS

These experiments identify a functional interaction between the melanocortin and histaminergic systems and suggest that HDC neurons act naturally to restrain the anorexigenic effect of melanocortin system activation. These findings may have implications for the control of arousal and metabolic homeostasis, especially in the context of obesity, in which both processes are subjected to alterations.

Obesity-associated mutant melanocortin-4 receptors with normal Gαs coupling frequently exhibit other discoverable pharmacological and biochemical defects

Mutations in the melanocortin-4 receptor (MC4R) are the most common cause of early syndromic obesity known. Most of these mutations result in a loss of protein expression, α-melanocyte-stimulating hormone binding, receptor trafficking or coupling to the stimulatory G-protein, Gα_s . However, approximately 26% of the obesity-associated mutations characterised to date exhibit none of these pharmacological defects. In the present study, we investigated seven of these apparently normal mutant MC4R in more detail and found that the majority (five of the seven) exhibit marked defects including defective binding of another endogenous melanocortin ligand, defective glycosylation, and defective recruitment of β-arrestin. These data provide support for two hypotheses: (i) that the majority of these rare, obesity-associated mutations are likely defective and causative of obesity and (ii) that β-arrestin recruitment is a valuable marker of normal MC4R function. Recent work has demonstrated a statistical correlation between the efficacy of β-arrestin recruitment to the MC4R and body mass index; however, the data reported here demonstrate both decreased and increased β-arrestin signalling in obesity-associated MC4R mutations.

Involvement of α-Melanocyte-Stimulating Hormone-Thromboxane A2 System on Itching in Atopic Dermatitis

α-Melanocyte-stimulating hormone (α-MSH) is an endogenous peptide hormone involved in cutaneous pigmentation in atopic dermatitis (AD) with severe itching. α-MSH elicits itch-related responses in mice. We, therefore, investigated whether α-MSH was involved in itching in AD. In the skin of AD patients and mice with atopy-like dermatitis, α-MSH and the prohormone convertase 2, which is the key processing enzyme for the production of α-MSH, were distributed mainly in keratinocytes. In the skin of mice with dermatitis, melanocortin receptors (MC1R and MC5R) were expressed at the mRNA level and were distributed in the dermis. In the dorsal root ganglion of mice with dermatitis, mRNAs encoding MC1R, MC3R, and MC5R were also expressed. MC1R antagonist agouti-signaling protein inhibited spontaneous scratching in mice with dermatitis. In healthy mice, intradermal α-MSH elicited itch-associated responses, which were inhibited by thromboxane (TX) A₂ receptor antagonist ONO-3708. In mouse keratinocytes, α-MSH increased the production of TXA₂, which was inhibited by adenylyl cyclase inhibitor SQ-22536 and Ca²⁺ chelator EGTA. In mouse keratinocytes treated with siRNA for MC1R and/or MC5R, α-MSH-induced TXA₂ production was decreased. α-MSH increased intracellular Ca²⁺ ion concentration in dorsal root ganglion neurons and keratinocytes. These results suggest that α-MSH is involved in itching during AD and may elicit itching through the direct action of primary afferents and TXA₂ production by keratinocytes.

Nuclear Entry of CRTC1 as Druggable Target of Acquired Pigmentary Disorder

Rationale: SOX10 (SRY-related HMG-box 10) and MITF-M (microphthalmia-associated transcription factor M) restrict the expression of melanogenic genes, such as TYR (tyrosinase), in melanocytes. DACE (diacetylcaffeic acid cyclohexyl ester) inhibits melanin production in α-MSH (α-melanocyte stimulating hormone)-activated B16-F0 melanoma cells. In this study, we evaluated the antimelanogenic activity of DACE in vivo and elucidated the molecular basis of its action.

Methods: We employed melanocyte cultures and hyperpigmented skin samples for pigmentation assays, and applied chromatin immunoprecipitation, immunoblotting, RT-PCR or siRNA-based knockdown for mechanistic analyses.

Results: Topical treatment with DACE mitigated UV-B-induced hyperpigmentation in the skin with attenuated expression of MITF-M and TYR. DACE also inhibited melanin production in α-MSH- or ET-1 (endothelin 1)-activated melanocyte cultures. As a mechanism, DACE blocked the nuclear import of CRTC1 (CREB-regulated co-activator 1) in melanocytes. DACE resultantly inhibited SOX10 induction, and suppressed the transcriptional abilities of CREB/CRTC1 heterodimer and SOX10 at MITF-M promoter, thereby ameliorating facultative melanogenesis. Furthermore, this study unveiled new issues in melanocyte biology that i) KPNA1 (Impα5) escorted CRTC1 as a cargo across the nuclear envelope, ii) SOX10 was inducible in the melanogenic process, and iii) CRTC1 could direct SOX10 induction at the transcription level.

Conclusion: We propose the targeting of CRTC1 as a unique strategy in the treatment of acquired pigmentary disorders.

Melanocortin-4 Receptor Signalling: Importance for Weight Regulation and Obesity Treatment

The melanocortin-4 receptor (MC4R) - embedded in the leptin-melanocortin pathway - is activated by proopiomelanocortin (POMC)-derived neuropeptides such as α- and β-melanocyte-stimulating hormone (MSH) and plays an important role in hypothalamic body-weight regulation. Accordingly, MC4R is a potential drug target to combat obesity. Previous attempts to develop MC4R agonists failed due to ineffectiveness or severe adverse events. Recently, a new generation of MC4R ligands was developed. Specifically, setmelanotide was found to be effective by inducing biased signalling of the MC4R and thereby reducing feelings of hunger and leading to substantial weight loss in patients with POMC or leptin receptor deficiency. This new potential pharmacological treatment option could be beneficial for further groups of obese patients with defects in the leptin-melanocortin signalling pathway.

Re-evaluation of Adrenocorticotropic Hormone and Melanocyte Stimulating Hormone Activation of GPR139 in Vitro

It is now well established that GPR139, a G-protein coupled receptor exclusively expressed in the brain and pituitary, is activated by the essential amino acids L-tryptophan (L-Trp) and L-phenylalanine (L-Phe) via G_αq-coupling. The in vitro affinity and potency values of L-Trp and L-Phe are within the physiological concentration ranges of L-Trp and L-Phe. A recent paper suggests that adrenocorticotropic hormone (ACTH), α and β melanocyte stimulating hormones (α-MSH and β-MSH) and derivatives α-MSH_1-9/α-MSH_1-10 can also activate GPR139 in vitro. We tested this hypothesis using guanosine 5′-O-(3-[³⁵S]thio)-triphosphate binding (GTPγS), calcium mobilization and [³H]JNJ-63533054 radioligand binding assays. In the GTPγS binding assay, α-MSH, α-MSH_1-9/α-MSH_1-10, and β-MSH had no effect on [³⁵S]GTPγS incorporation in cell membranes expressing GPR139 up to 30 μM in contrast to the concentration dependent activation produced by L-Trp, JNJ-63533054, and TC-09311 (two small molecule GPR139 agonists). ACTH slightly decreased the basal level of [³⁵S]GTPγS incorporation at 30 μM. In the GPR139 radioligand binding assay, a moderate displacement of [³H]JNJ-63533054 binding by ACTH and β-MSH was observed at 30 μM (40 and 30%, respectively); α-MSH, α-MSH_1-9/α-MSH_1-10 did not displace any specific binding at 30 μM. In three different host cell lines stably expressing GPR139, α-MSH, and β-MSH did not stimulate calcium mobilization in contrast to L-Trp, JNJ-63533054, and TC-09311. ACTH, α-MSH_1-9/α-MSH_1-10 only weakly stimulated calcium mobilization at 30 μM (<50% of EC₁₀₀). We then co-transfected GPR139 with the three melanocortin (MC) receptors (MC3R, MC4R, and MC5R) to test the hypothesis that ACTH, α-MSH, and β-MSH might stimulate calcium mobilization through a MCR/GPR139 interaction. All three MC peptides stimulated calcium response in cells co-transfected with GPR139 and MC3R, MC4R, or MC5R. The MC peptides did not stimulate calcium response in cells expressing MC3R or MC5R alone consistent with the G_s signaling transduction pathway of these receptors. In agreement with the previously reported multiple signaling pathways of MC4R, including G_q transduction pathway, the MC peptides produced a calcium response in cells expressing MC4R alone. Together, our findings do not support that GPR139 is activated by ACTH, α-MSH, and β-MSH at physiologically relevant concentration but we did unravel an in vitro interaction between GPR139 and the MCRs.

Desacetyl-α-melanocyte stimulating hormone and α-melanocyte stimulating hormone are required to regulate energy balance

Objective

Regulation of energy balance depends on pro-opiomelanocortin (POMC)-derived peptides and melanocortin-4 receptor (MC4R). Alpha-melanocyte stimulating hormone (α-MSH) is the predicted natural POMC-derived peptide that regulates energy balance. Desacetyl-α-MSH, the precursor for α-MSH, is present in brain and blood. Desacetyl-α-MSH is considered to be unimportant for regulating energy balance despite being more potent (compared with α-MSH) at activating the appetite-regulating MC4R in vitro. Thus, the physiological role for desacetyl-α-MSH is still unclear.

Methods

We created a novel mouse model to determine whether desacetyl-α-MSH plays a role in regulating energy balance. We engineered a knock in targeted QKQR mutation in the POMC protein cleavage site that blocks the production of both desacetyl-α-MSH and α-MSH from adrenocorticotropin (ACTH_1-39).

Results

The mutant ACTH_1-39 (ACTH^QKQR) functions similar to native ACTH_1-39 (ACTH^KKRR) at the melanocortin 2 receptor (MC2R) in vivo and MC4R in vitro. Male and female homozygous mutant ACTH_1-39 (Pomc^tm1/tm1) mice develop the characteristic melanocortin obesity phenotype. Replacement of either desacetyl-α-MSH or α-MSH over 14 days into Pomc^tm1/tm1 mouse brain significantly reverses excess body weight and fat mass gained compared to wild type (WT) (Pomc^wt/wt) mice. Here, we identify both desacetyl-α-MSH and α-MSH peptides as regulators of energy balance and highlight a previously unappreciated physiological role for desacetyl-α-MSH.

Conclusions

Based on these data we propose that there is potential to exploit the naturally occurring POMC-derived peptides to treat obesity but this relies on first understanding the specific function(s) for desacetyl-α-MSH and α-MSH.

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KKRR → QKQR mutation in the cleavage site of POMC prevents the production of desacetyl-α-MSH and α-MSH in mice.

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Male and female mutant mice develop characteristic melanocortin obesity.

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Central administration of α-MSH is more potent at reducing body weight in female mutant mice.

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Central administration of desacetyl-α-MSH and α-MSH are similarly potent at reducing body weight in male mutant mice.

Genome-wide Mapping of Protein-DNA Interactions with ChEC-seq in Saccharomyces cerevisiae

Genome-wide mapping of protein-DNA interactions is critical for understanding gene regulation, chromatin remodeling, and other chromatin-resident processes. Formaldehyde crosslinking followed by chromatin immunoprecipitation and high-throughput sequencing (X-ChIP-seq) has been used to gain many valuable insights into genome biology. However, X-ChIP-seq has notable limitations linked to crosslinking and sonication. Native ChIP avoids these drawbacks by omitting crosslinking, but often results in poor recovery of chromatin-bound proteins. In addition, all ChIP-based methods are subject to antibody quality considerations. Enzymatic methods for mapping protein-DNA interactions, which involve fusion of a protein of interest to a DNA-modifying enzyme, have also been used to map protein-DNA interactions. We recently combined one such method, chromatin endogenous cleavage (ChEC), with high-throughput sequencing as ChEC-seq. ChEC-seq relies on fusion of a chromatin-associated protein of interest to micrococcal nuclease (MNase) to generate targeted DNA cleavage in the presence of calcium in living cells. ChEC-seq is not based on immunoprecipitation and so circumvents potential concerns with crosslinking, sonication, chromatin solubilization, and antibody quality while providing high resolution mapping with minimal background signal. We envision that ChEC-seq will be a powerful counterpart to ChIP, providing an independent means by which to both validate ChIP-seq findings and discover new insights into genomic regulation.

Biased signaling at neural melanocortin receptors in regulation of energy homeostasis

The global prevalence of obesity highlights the importance of understanding on regulation of energy homeostasis. The central melanocortin system is an important intersection connecting the neural pathways controlling satiety and energy expenditure to regulate energy homeostasis by sensing and integrating the signals of external stimuli. In this system, neural melanocortin receptors (MCRs), melanocortin-3 and -4 receptors (MC3R and MC4R), play crucial roles in the regulation of energy homeostasis. Recently, multiple intracellular signaling pathways and biased signaling at neural MCRs have been discovered, providing new insights into neural MCR signaling. This review attempts to summarize biased signaling including biased receptor mutants (both naturally occurring and lab-generated) and biased ligands at neural MCRs, and to provide a better understanding of obesity pathogenesis and new therapeutic implications for obesity treatment.

Melanocortin 4 receptor constitutive activity inhibits L-type voltage-gated calcium channels in neurons

The melanocortin 4 receptor (MC4R) is a G protein-coupled receptor (GPCR) that is expressed in several brain nuclei playing a crucial role in the regulation of energy balance controlling the homeostasis of the organism. It displays both agonist-evoked and constitutive activity, and moreover, it can couple to different G proteins. Most of the research on MC4R has been focused on agonist-induced activity, while the molecular and cellular basis of MC4R constitutive activity remains scarcely studied. We have previously shown that neuronal N-type voltage-gated calcium channels (Ca_V2.2) are inhibited by MC4R agonist-dependent activation, while the Ca_V subtypes that carry L- and P/Q-type current are not. Here, we tested the hypothesis that MC4R constitutive activity can affect Ca_V, with focus on the channel subtypes that can control transcriptional activity coupled to depolarization (L-type, Ca_V1.2/1.3) and neurotransmitter release (N- and P/Q-type, Ca_V2.2 and Ca_V2.1). We found that MC4R constitutive activity inhibits specifically Ca_V1.2/1.3 and Ca_V2.1 subtypes of Ca_V. We also explored the signaling pathways mediating this inhibition, and thus propose that agonist-dependent and basal MC4R activation modes signal differentially through G_s and G_i/o pathways to impact on different Ca_V subtypes. In addition, we found that chronic incubation with MC4R endogenous inverse agonist, agouti and agouti-related peptide (AgRP), occludes Ca_V inhibition in a cell line and in amygdaloid complex cultured neurons as well. Thus, we define new mechanisms of control of the main mediators of depolarization-induced calcium entry into neurons by a GPCR that displays constitutive activity.

Effects of Melanocortin 3 and 4 Receptor Deficiency on Energy Homeostasis in Rats

Melanocortin-3 and 4 receptors (MC3R and MC4R) can regulate energy homeostasis, but their respective roles especially the functions of MC3R need more exploration. Here Mc3r and Mc4r single and double knockout (DKO) rats were generated using CRISPR-Cas9 system. Metabolic phenotypes were examined and data were compared systematically. Mc3r KO rats displayed hypophagia and decreased body weight, while Mc4r KO and DKO exhibited hyperphagia and increased body weight. All three mutants showed increased white adipose tissue mass and adipocyte size. Interestingly, although Mc3r KO did not show a significant elevation in lipids as seen in Mc4r KO, DKO displayed even higher lipid levels than Mc4r KO. DKO also showed more severe glucose intolerance and hyperglycaemia than Mc4r KO. These data demonstrated MC3R deficiency caused a reduction of food intake and body weight, whereas at the same time exhibited additive effects on top of MC4R deficiency on lipid and glucose metabolism. This is the first phenotypic analysis and systematic comparison of Mc3r KO, Mc4r KO and DKO rats on a homogenous genetic background. These mutant rats will be important in defining the complicated signalling pathways of MC3R and MC4R. Both Mc4r KO and DKO are good models for obesity and diabetes research.

60 YEARS OF POMC: Regulation of feeding and energy homeostasis by α-MSH

The melanocortin peptides derived from pro-opiomelanocortin (POMC) were originally understood in terms of the biological actions of α-melanocyte-stimulating hormone (α-MSH) on pigmentation and adrenocorticotrophic hormone on adrenocortical glucocorticoid production. However, the discovery of POMC mRNA and melanocortin peptides in the CNS generated activities directed at understanding the direct biological actions of melanocortins in the brain. Ultimately, discovery of unique melanocortin receptors expressed in the CNS, the melanocortin-3 (MC3R) and melanocortin-4 (MC4R) receptors, led to the development of pharmacological tools and genetic models leading to the demonstration that the central melanocortin system plays a critical role in the regulation of energy homeostasis. Indeed, mutations in MC4R are now known to be the most common cause of early onset syndromic obesity, accounting for 2-5% of all cases. This review discusses the history of these discoveries, as well as the latest work attempting to understand the molecular and cellular basis of regulation of feeding and energy homeostasis by the predominant melanocortin peptide in the CNS, α-MSH.

Molecular cloning and pharmacological characterization of giant panda (Ailuropoda melanoleuca) melanocortin-4 receptor

The melanocortin-4 receptor (MC4R) is critical in regulating mammalian food intake and energy expenditure. Giant panda (Ailuropoda melanoleuca), famous as the living fossil, is an endangered species endemic to China. We are interested in exploring the functions of the giant panda MC4R (amMC4R) in regulating energy homeostasis and report herein the molecular cloning and pharmacology of the amMC4R. Sequence analysis revealed that amMC4R was highly homologous (>88%) at nucleotide and amino acid sequences to several mammalian MC4Rs. Western blot revealed that the expression construct myc-amMC4R in pcDNA3.1 was successfully constructed and expressed in HEK293T cells. With human MC4R (hMC4R) as a control, pharmacological characteristics of amMC4R were analyzed with binding and signaling assays. Four agonists, including [Nle(4), D-Phe(7)]-α-melanocyte stimulating hormone (NDP-MSH), α- and β-MSH, and a small molecule agonist, THIQ, were used in binding and signaling assays. We showed that amMC4R bound NDP-MSH with the highest affinity followed by THIQ, α-MSH, and β-MSH, with the same ranking order as hMC4R. Treatment of HEK293T cells expressing amMC4R with different concentrations of agonists resulted in dose-dependent increase of intracellular cAMP levels, with similar EC50s for the four agonists. The results suggested that the cloned amMC4R encoded a functional MC4R. The availability of amMC4R and its binding and signaling properties will facilitate the investigation of amMC4R in regulating food intake and energy homeostasis.

ChEC-seq kinetics discriminates transcription factor binding sites by DNA sequence and shape in vivo

Chromatin endogenous cleavage (ChEC) uses fusion of a protein of interest to micrococcal nuclease (MNase) to target calcium-dependent cleavage to specific genomic loci in vivo. Here we report the combination of ChEC with high-throughput sequencing (ChEC-seq) to map budding yeast transcription factor (TF) binding. Temporal analysis of ChEC-seq data reveals two classes of sites for TFs, one displaying rapid cleavage at sites with robust consensus motifs and the second showing slow cleavage at largely unique sites with low-scoring motifs. Sites with high-scoring motifs also display asymmetric cleavage, indicating that ChEC-seq provides information on the directionality of TF-DNA interactions. Strikingly, similar DNA shape patterns are observed regardless of motif strength, indicating that the kinetics of ChEC-seq discriminates DNA recognition through sequence and/or shape. We propose that time-resolved ChEC-seq detects both high-affinity interactions of TFs with consensus motifs and sites preferentially sampled by TFs during diffusion and sliding.

In chromatin endogenous cleavage (ChEC), micrococcal nuclease (MNase) is fused to a protein of interest and its cleavage is thus targeted to specific genomic loci in vivo. Here, the authors show that time-resolved ChEC-seq (high-throughput sequencing after ChEC) can detect DNA shape patterns regardless of motif strength.

Temporal cAMP Signaling Selectivity by Natural and Synthetic MC4R Agonists

The melanocortin-4 receptor (MC4R) is a G protein-coupled receptor expressed in the brain, where it controls energy balance through pathways including α-melanocyte-stimulating hormone (α-MSH)-dependent signaling. We have reported that the MC4R can exist in an active conformation that signals constitutively by increasing cAMP levels in the absence of receptor desensitization. We asked whether synthetic MC4R agonists differ in their ability to increase intracellular cAMP over time in Neuro2A cells expressing endogenous MC4R and exogenous, epitope-tagged hemagglutinin-MC4R-green fluorescent protein. By analyzing intracellular cAMP in a temporally resolved Förster resonance energy transfer assay, we show that withdrawal of α-MSH leads to a quick reversal of cAMP induction. By contrast, the synthetic agonist melanotan II (MTII) induces a cAMP signal that persists for at least 1 hour after removal of MTII from the medium and cannot be antagonized by agouti related protein. Similarly, in mHypoE-42 immortalized hypothalamic neurons, MTII, but not α-MSH, induced persistent AMP kinase signal, which occurs downstream of increased cAMP. By using a fluorescence recovery after photobleaching assay, it appears that the receptor exposed to MTII continues to signal after being internalized. Similar to MTII, the synthetic MC4R agonists, THIQ and BIM-22511, but not LY2112688, induced prolonged cAMP signaling after agonist withdrawal. However, agonist-exposed MC4R desensitized to the same extent, regardless of the ligand used and regardless of differences in receptor intracellular retention kinetics. In conclusion, α-MSH and LY2112688, when compared with MTII, THIQ, and BIM-22511, vary in the duration of the acute cAMP response, showing distinct temporal signaling selectivity, possibly linked to specific cell compartments from which cAMP signals may originate.

Intracellular signaling mechanisms of the melanocortin receptors: current state of the art

The melanocortin system is composed by the agonists adrenocorticotropic hormone and α, β and γ-melanocyte-stimulating hormone, and two naturally occurring antagonists, agouti and agouti-related protein. These ligands act by interaction with a family of five melanocortin receptors (MCRs), assisted by MCRs accessory proteins (MRAPs). MCRs stimulation activates different signaling pathways that mediate a diverse array of physiological processes, including pigmentation, energy metabolism, inflammation and exocrine secretion. This review focuses on the regulatory mechanisms of MCRs signaling, highlighting the differences among the five receptors. MCRs signal through G-dependent and independent mechanisms and their functional coupling to agonists at the cell surface is regulated by interacting proteins, namely MRAPs and β-arrestins. The knowledge of the distinct modulation pattern of MCRs signaling and function may be helpful for the future design of novel drugs able to combine specificity, safety and effectiveness in the course of their therapeutic use.

Structural insight into the MC4R conformational changes via different agonist-mediated receptor signaling

The melanocortin-4 receptor (MC4R) plays a key role in the regulation of food intake and body weight. Previous studies indicate that α-melanocyte stimulating hormone (α-MSH) binds to MC4R and activates three signal pathways (cAMP, calcium, and mitogen-activated protein kinase pathways), whereas MC4R synthetic agonist THIQ can activate only the cAMP pathway. The molecular basis of the MC4R responsible for different ligand-mediated signaling is unknown. We hypothesize that different MC4R agonists can stabilize different MC4R conformations and result in ligand-mediated signal transduction. In this study, we examined the effect of the MC4R conformational change in cAMP signaling pathways mediated by different agonists by cross-linking two transmembrane helices (TM3 and TM6). We generated and tested 11 single and 8 double mutations that are located at the end of TM3 and beginning of TM6 in MC4R. Our results indicate that (1) single or double mutations of the MC4R did not significantly alter cAMP production induced by NDP-MSH compared to that of wild-type MC4R except single mutation 243H (the mutation 243H significantly decreased cAMP production mediated by NDP-MSH or THIQ due to a low level of receptor expression at the cell surface), (2) the mutation 247H significantly decreased THIQ-mediated cAMP production but not NDP-MSH, and (3) the receptor cAMP signaling pathway activation by THIQ is blocked in the presence of Zn(2+) with the double mutation I150/242H but activation by NDP-MSH is not, suggesting that the activated conformation of MC4R mediated by NDP-MSH and THIQ is different. This study provides insight into the molecular basis of MC4R responsible for receptor signaling mediated by different agonists.

An essential role for the K+-dependent Na+/Ca2+-exchanger, NCKX4, in melanocortin-4-receptor-dependent satiety

K(+)-dependent Na(+)/Ca(2+)-exchangers are broadly expressed in various tissues, and particularly enriched in neurons of the brain. The distinct physiological roles for the different members of this Ca(2+) transporter family are, however, not well described. Here we show that gene-targeted mice lacking the K(+)-dependent Na(+)/Ca(2+)-exchanger, NCKX4 (gene slc24a4 or Nckx4), display a remarkable anorexia with severe hypophagia and weight loss. Feeding and satiety are coordinated centrally by melanocortin-4 receptors (MC4R) in neurons of the hypothalamic paraventricular nucleus (PVN). The hypophagic response of Nckx4 knock-out mice is accompanied by hyperactivation of neurons in the PVN, evidenced by high levels of c-Fos expression. The activation of PVN neurons in both fasted Nckx4 knock-out and glucose-injected wild-type animals is blocked by Ca(2+) removal and MC4R antagonists. In cultured hypothalamic neurons, melanocyte stimulating hormone induces an MC4R-dependent and sustained Ca(2+) signal, which requires phospholipase C activity and plasma membrane Ca(2+) entry. The Ca(2+) signal is enhanced in hypothalamic neurons from Nckx4 knock-out animals, and is depressed in cells in which NCKX4 is overexpressed. Finally, MC4R-dependent oxytocin expression in the PVN, a key essential step in satiety, is prevented by blocking phospholipase C activation or Ca(2+) entry. These findings highlight an essential, and to our knowledge previously unknown, role for Ca(2+) signaling in the MC4R pathway that leads to satiety, and a novel non-redundant role for NCKX4-mediated Ca(2+) extrusion in controlling MC4R signaling and feeding behavior. Together, these findings highlight a novel pathway that potentially could be exploited to develop much needed new therapeutics to tackle eating disorders and obesity.

Astrocytes: new targets of melanocortin 4 receptor actions

Astrocytes exert a wide variety of functions with paramount importance in brain physiology. After injury or infection, astrocytes become reactive and they respond by producing a variety of inflammatory mediators that help maintain brain homeostasis. Loss of astrocyte functions as well as their excessive activation can contribute to disease processes; thus, it is important to modulate reactive astrocyte response. Melanocortins are peptides with well-recognized anti-inflammatory and neuroprotective activity. Although melanocortin efficacy was shown in systemic models of inflammatory disease, mechanisms involved in their effects have not yet been fully elucidated. Central anti-inflammatory effects of melanocortins and their mechanisms are even less well known, and, in particular, the effects of melanocortins in glial cells are poorly understood. Of the five known melanocortin receptors (MCRs), only subtype 4 is present in astrocytes. MC4R has been shown to mediate melanocortin effects on energy homeostasis, reproduction, inflammation, and neuroprotection and, recently, to modulate astrocyte functions. In this review, we will describe MC4R involvement in anti-inflammatory, anorexigenic, and anti-apoptotic effects of melanocortins in the brain. We will highlight MC4R action in astrocytes and discuss their possible mechanisms of action. Melanocortin effects on astrocytes provide a new means of treating inflammation, obesity, and neurodegeneration, making them attractive targets for therapeutic interventions in the CNS.

Curbing Inflammation through Endogenous Pathways: Focus on Melanocortin Peptides

The resolution of inflammation is now known to be an active process, armed with a multitude of mediators both lipid and protein in nature. Melanocortins are peptides endowed with considerable promise with their proresolution and anti-inflammatory effects in preclinical models of inflammatory disease, with tissue protective effects. These peptides and their targets are appealing because they can be seen as a natural way of inducing these effects as they harness endogenous pathways of control. Whereas most of the information generated about these mediators derives from several acute models of inflammation (such as zymosan induced peritonitis), there is some indication that these mediators may inhibit chronic inflammation by modulating cytokines, chemokines, and leukocyte apoptosis. In addition, proresolving mediators and their mimics have often been tested alongside therapeutic protocols, hence have been tested in settings more relevant to real life clinical scenarios. We provide here an overview on some of these mediators with a focus on melanocortin peptides and receptors, proposing that they may unveil new opportunities for innovative treatments of inflammatory arthritis.

Melanocortin 5 receptor signaling and internalization: role of MAPK/ERK pathway and β-arrestins 1/2

The Melanocortin 5 receptor (MC5R) is a G-protein coupled receptor (GPCR) that exhibits high affinity for α-MSH. Here we present evidence for MC5R-GFP internalization and subsequent recycling to cell surface, in α-MSH-stimulated HeLa cells. This melanocortin induces a biphasic activation of ERK1/2 with an early peak at 15min, a G(i)-protein driven, β-arrestins 1/2 independent process, and a late sustained activation that is regulated by β-arrestins 1/2. ERK1/2 lead to downstream phosphorylation of 90-kDa ribosomal S6 kinases (p90RSK) and mitogen- and stress-activated protein kinase 1 (MSK1). Only a small fraction (10%) of phosphorylated p90RSK and ERK1/2 translocates to the nucleus inducing c-Fos expression. α-MSH also activates CREB through cAMP/PKA pathway. In 3T3-L1 adipocytes, where MC5R is endogenously expressed, α-MSH also induces phosphorylation and cytosolic retention of the same signaling molecules. These findings provide new evidence on the signaling mechanisms underlying MC5R biological response to α-MSH.

Subcellular trafficking and transfection efficacy of polyethylenimine-polyethylene glycol polyplex nanoparticles with a ligand to melanocortin receptor-1

We have synthesized and investigated properties of new PEI-PEG-based polyplexes containing MC1SP-peptide, a ligand specific for melanocortin receptor-1 (targeted polyplexes), and control polyplexes without this ligand peptide (non-targeted polyplexes). The targeted polyplexes demonstrated receptor-mediated transfection of Cloudman S91 (clone M-3) murine melanoma cells that was more efficient than with the non-targeted ones. Transfection with the targeted polyplexes was inhibited by chlorpromazine, an inhibitor of the clathrin-mediated endocytosis pathway, and, to a lesser extent, by filipin III or nystatin, inhibitors of the lipid-raft endocytosis pathway, whereas transfection with the non-targeted polyplexes was inhibited mainly by nystatin or filipin III. The targeted polyplexes caused significantly higher in vivo transfection of melanoma tumor cells after intratumoral administration compared to the non-targeted control. The targeted polyplexes carrying the HSVtk gene, after ganciclovir administration, more efficiently inhibited melanoma tumor growth and prolonged the lifespan of DBA/2 tumor-bearing mice compared to the non-targeted ones. Packed targeted polyplexes appeared and accumulated in the melanoma cells 6h earlier than the non-targeted ones. The targeted polyplexes enter into the nuclei of the melanoma cells more rapidly than the non-targeted control, and this difference may also be attributed to processes of receptor-mediated endocytosis. We believe that these data may be useful for the optimization of polyplex systems.

Role of proopiomelanocortin-derived peptides and their receptors in the osteoarticular system: from basic to translational research

Proopiomelanocortin (POMC)-derived peptides such as melanocortins and β-endorphin (β-ED) exert their pleiotropic effects via binding to melanocortin receptors (MCR) and opioid receptors (OR). There is now compelling evidence for the existence of a functional POMC system within the osteoarticular system. Accordingly, distinct cell types of the synovial tissue and bone have been identified to generate POMC-derived peptides like β-ED, ACTH, or α-MSH. MCR subtypes, especially MC1R, MC2R (the ACTH receptor), MC3R, and MC4R, but also the μ-OR and δ-OR, have been detected in various cells of the synovium, cartilage, and bone. The respective ligands of these POMC-derived peptide receptors mediate an increasing number of newly recognized biological effects in the osteoarticular system. These include bone mineralization and longitudinal growth, cell proliferation and differentiation, extracellular matrix synthesis, osteoprotection, and immunomodulation. Importantly, bone formation is also regulated by the central melanocortin system via a complex hormonal interplay with other organs and tissues involved in energy metabolism. Among the POMC-derived peptides examined in cell culture systems from osteoarticular tissue and in animal models of experimentally induced arthritis, α-MSH, ACTH, and MC3R-specific agonists appear to have the most promising antiinflammatory actions. The effects of these melanocortin peptides may be exploited in future for the treatment of patients with inflammatory and degenerative joint diseases.

Melanocortin-induced PKA activation inhibits AMPK activity via ERK-1/2 and LKB-1 in hypothalamic GT1-7 cells

α-Melanocyte-stimulating hormone (α-MSH)-induced activation of the melanocortin-4 receptor in hypothalamic neurons increases energy expenditure and inhibits food intake. Active hypothalamic AMP-activated protein kinase (AMPK) has recently been reported to enhance food intake, and in vivo experiments suggested that intrahypothalamic injection of melanocortins decreased food intake due to the inhibition of AMPK activity. However, it is not clear whether α-MSH affects AMPK via direct intracellular signaling cascades or if the release of paracrine factors is involved. Here, we used a murine, hypothalamic cell line (GT1-7 cells) and monitored AMPK phosphorylation at Thr(172), which has been suggested to increase AMPK activity. We found that α-MSH dephosphorylated AMPK at Thr(172) and consequently decreased phosphorylation of the established AMPK substrate acetyl-coenzyme A-carboxylase at Ser(79). Inhibitory effects of α-MSH on AMPK were blocked by specific inhibitors of protein kinase A (PKA) or ERK-1/2, pointing to an important role of both kinases in this process. Because α-MSH-induced activation of ERK-1/2 was blunted by PKA inhibitors, we propose that ERK-1/2 serves as a link between PKA and AMPK in GT1-7 cells. Furthermore, down-regulation of liver kinase B-1, but not inhibition of calcium-calmodulin-dependent kinase kinase-β or TGFβ-activated kinase-1 decreased basal phosphorylation of AMPK and its dephosphorylation induced by α-MSH. Thus, we propose that α-MSH inhibits AMPK activity via a linear pathway, including PKA, ERK-1/2, and liver kinase B-1 in GT1-7 cells. Given the importance of the melanocortin system in the formation of adipositas, detailed knowledge about this pathway might help to develop drugs targeting obesity.

The neuroendocrine circuitry controlled by POMC, MSH, and AGRP

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

Modulation of basophil activity: a novel function of the neuropeptide α-melanocyte-stimulating hormone

BACKGROUND

Little is known about the effect of neuropeptides on basophils, which are important effector cells in immune and allergic responses.

OBJECTIVE

This study aimed at revealing the role of α-melanocyte-stimulating hormone (α-MSH) on basophil function.

METHODS

Expression of melanocortin receptors and proopiomelanocortin (POMC) was analyzed by means of RT-PCR, Western immunoblotting, fluorescence-activated cell sorting, and double-immunofluorescence analysis. Signal transduction studies included cyclic AMP and Ca(2+) mobilization assays. Basophil activity was assessed based on CD63 surface expression and cytokine release.

RESULTS

MC-1R expression was detectable in basophils isolated from human peripheral blood, as well as in basophils within nasal tissue. In isolated basophils from human blood, truncated POMC transcripts were present, but there was no POMC protein. Treatment of basophils with α-MSH increased intracellular Ca(2+) but not cyclic AMP levels. α-MSH at physiologic doses potently suppressed basophil activation induced by N-formyl-methionyl-leucyl-phenylalanine, phorbol 12-myristate 13-acetate, or grass pollen allergen in whole blood of healthy or allergic subjects, respectively. The effect of α-MSH on basophil activation was MC-1R mediated (as shown by blockade with a peptide analogue of agouti-signaling protein) and imitated by adrenocorticotropic hormone but not elicited by the tripeptides KPV and KdPT, both of which lack the central pharmacophore of α-MSH. Moreover, α-MSH at physiologic doses significantly suppressed secretion of 3 proallergic cytokines, IL-4, IL-6, and IL-13, in basophils stimulated with anti-IgE, N-formyl-methionyl-leucyl-phenylalanine, or phorbol 12-myristate 13-acetate.

CONCLUSION

Our findings highlight a novel functional activity of α-MSH, which acts as a natural antiallergic basophil-response modifier. These findings might point to novel therapeutic strategies in treating allergic diseases.

Effects of α-melanocyte-stimulating hormone on calcium concentration in SZ95 sebocytes

Melanocortins have been implicated in human sebum secretion for a long time. However, the signalling pathways of α-melanocyte-stimulating hormone (α-MSH) in human sebocytes expressing melanocortin receptors (MC-Rs) are still poorly understood. Because calcium ions play a central role in MC-R signalling, we investigated whether α-MSH affects calcium signalling in the immortalized human sebocyte cell line SZ95. In addition, we investigated the impact of α-MSH on MC-1R expression and lipid synthesis in these cells. α-MSH increased intracellular calcium levels. α-MSH-mediated calcium mobilization originated from intracellular calcium stores and was mediated by inositol triphosphate. Moreover, α-MSH increased MC-1R immunoreactivity and lipid synthesis in SZ95 sebocytes in the presence of testosterone. Our data demonstrate that α-MSH in human sebocytes controls a key cellular signalling pathway, the calcium ion response, which may coordinate MC-1R-mediated sebum secretion.

Alternative G protein coupling and biased agonism: new insights into melanocortin-4 receptor signalling

The melanocortin-4 receptor (MC4R) is a prototypical G protein-coupled receptor (GPCR) that plays a considerable role in controlling appetite and energy homeostasis. Signalling initiated by MC4R is orchestrated by multiple agonists, inverse agonism and by interactions with accessory proteins. The exact molecular events translating MC4R signalling into its physiological role, however, are not fully understood. This review is an attempt to summarize new aspects of MC4R signalling in the context of its recently discovered alternative G protein coupling, and to give a perspective on how future research could improve our knowledge about the intertwining molecular mechanisms that are responsible for the regulation of energy homeostasis by the melanocortin system.

Physiological roles of the melanocortin MC₃ receptor

The melanocortin MC(3) receptor remains the most enigmatic of the melanocortin receptors with regard to its physiological functions. The receptor is expressed both in the CNS and in multiple tissues in the periphery. It appears to be an inhibitory autoreceptor on proopiomelanocortin neurons, yet global deletion of the receptor causes an obesity syndrome. Knockout of the receptor increases adipose mass without a readily measurable increase in food intake or decrease in energy expenditure. And finally, no melanocortin MC(3) receptor null humans have been identified and associations between variant alleles of the melanocortin MC(3) receptor and diseases remain controversial, so the physiological role of the receptor in humans remains to be determined.

Adipocyte-brain: crosstalk

The initial discovery of leptin, an appetite-suppressing hormone originating from fat tissue, substantially supported the idea that fat-borne factors act on the brain to regulate food intake and energy expenditure. Since then, a growing number of cytokines have been found to be released from adipose tissue, thus acting in an endocrine manner. These adipocytokines include not only, e.g., adiponectin, apelin, resistin, and visfatin, but also inflammatory cytokines and steroid hormones such as estrogens and glucocorticoids. They are secreted from their adipose depots and differ in terms of release stimuli, downstream signaling, and their action on the brain. Clearly, adipocytokines play a prominent role in the central control of body weight, and the deregulation of this circuit may lead to the development of obesity and related disorders. In this chapter, we will focus on crosstalk mechanisms and the deregulation of adipocytokines at the expression level and/or sites of central action that eventually will lead to the development and perpetuation of obesity and diabetes.

Melanocortin-4 receptor activation promotes insulin-stimulated mTOR signaling

The melanocortin signaling system is integral in regulating energy homeostasis. The melanocortin-4 receptor (MC4R) activates several signaling pathways in performance of this function. The effect of MC4R on insulin-stimulated mammalian target of rapamycin (mTOR), a cellular energy sensor, signaling was investigated. The GT1-1 cell line which expresses MC4R expression was utilized. mTOR signaling was measured by Western blotting analysis using Phospho-mTOR (Ser2448) antibody. NDP-MSH dose-dependently enhanced insulin-stimulated mTOR phosphorylation. The MC4R antagonist SHU9119 blocked this effect, demonstrating specificity. The protein kinase A - cyclic AMP pathway and the MAP kinase pathway were not involved in NDP-MSH actions on insulin-stimulated mTOR phosphorylation. In contrast, the AMP-activated protein kinase agonist, AICAR, attenuated this effect. MC4R activation potentiates insulin-stimulated mTOR signaling via the AMPK pathway.

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

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

The melanocortin-1 receptor gene polymorphism and association with human skin cancer

The melanocortin-1 receptor (MC1R) is a key gene involved in the regulation of melanin synthesis and encodes a G-protein coupled receptor expressed on the surface of the melanocyte in the skin and hair follicles. MC1R activation after ultraviolet radiation exposure results in the production of the dark eumelanin pigment and the tanning process in humans, providing physical protection against DNA damage. The MC1R gene is highly polymorphic in Caucasian populations with a number of MC1R variant alleles associated with red hair, fair skin, freckling, poor tanning, and increased risk of melanoma and nonmelanoma skin cancer. Variant receptors have shown alterations in biochemical function, largely due to intracellular retention or impaired G-protein coupling, but retain some signaling ability. The association of MC1R variant alleles with skin cancer risk remains after correction for pigmentation phenotype, indicating regulation of nonpigmentary pathways. Notably, MC1R activation has been linked to DNA repair and may also contribute to the regulation of immune responses.

Melanocortin-4 receptor activation inhibits c-Jun N-terminal kinase activity and promotes insulin signaling

The melanocortin system is crucial to regulation of energy homeostasis. The melanocortin receptor type 4 (MC4R) modulates insulin signaling via effects on c-Jun N-terminal kinase (JNK). The melanocortin agonist NDP-MSH dose-dependently inhibited JNK activity in HEK293 cells stably expressing the human MC4R; effects were reversed by melanocortin receptor antagonist. NDP-MSH time- and dose-dependently inhibited IRS-1(ser307) phosphorylation, effects also reversed by a specific melanocortin receptor antagonist. NDP-MSH augmented insulin-stimulated AKT phosphorylation in vitro. The melanocortin agonist melanotan II increased insulin-stimulated AKT phosphorylation in the rat hypothalamus in vivo. NDP-MSH increased insulin-stimulated glucose uptake in hypothalamic GT1-1 cells. The current study shows that the melanocortinergic system interacts with insulin signaling via novel effects on JNK activity.

Agouti protein, mahogunin, and attractin in pheomelanogenesis and melanoblast-like alteration of melanocytes: a cAMP-independent pathway

Melanocortin-1 receptor (MC1R) and its ligands, α-melanocyte stimulating hormone (αMSH) and agouti signaling protein (ASIP), regulate switching between eumelanin and pheomelanin synthesis in melanocytes. Here we investigated biological effects and signaling pathways of ASIP. Melan-a non agouti (a/a) mouse melanocytes produce mainly eumelanin, but ASIP combined with phenylthiourea and extra cysteine could induce over 200-fold increases in the pheomelanin to eumelanin ratio, and a tan-yellow color in pelletted cells. Moreover, ASIP-treated cells showed reduced proliferation and a melanoblast-like appearance, seen also in melanocyte lines from yellow (A^y/a and Mc1r^e/ Mc1r^e) mice. However ASIP-YY, a C-terminal fragment of ASIP, induced neither biological nor pigmentary changes. As, like ASIP, ASIP-YY inhibited the cAMP rise induced by αMSH analog NDP-MSH, and reduced cAMP level without added MSH, the morphological changes and depigmentation seemed independent of cAMP signaling. Melanocytes genetically null for ASIP mediators attractin or mahogunin (Atrn^mg-3J/mg-3J or Mgrn1^md-nc/md-nc) also responded to both ASIP and ASIP-YY in cAMP level, while only ASIP altered their proliferation and (in part) shape. Thus, ASIP–MC1R signaling includes a cAMP-independent pathway through attractin and mahogunin, while the known cAMP-dependent component requires neither attractin nor mahogunin.

Melanocortin 5 receptor activates ERK1/2 through a PI3K-regulated signaling mechanism

Melanocortin 5 receptor (MC5R) is a G protein coupled receptor (GPCR) with high affinity for alpha-melanocyte-stimulating hormone (alpha-MSH). To unravel some of the downstream cell-signaling pathways activated by this receptor, HEK293 cells were transiently and stably transfected with a vector encoding green fluorescent protein (GFP)-tagged MC5R. In these cells the receptor was correctly addressed to the cell surface and was functional, as shown by the MC5R-induced formation of intracellular cAMP. In fact, the MC5R agonist alpha-MSH induced a 10- or 16-fold increase (transient or stable cells, respectively) above the cAMP levels found in unstimulated cells. Moreover, in cells stably expressing MC5R-GFP, alpha-MSH promoted ERK1/2 phosphorylation in a dose-dependent manner (EC50=7.3 nM) with the maximal effect occurring after 5 min of agonist incubation. The signaling pathway conveyed through ERK1/2 is not linked to cAMP, since the phosphorylation of these kinases is unchanged by the inhibition of adenylyl cyclase. Also, ERK1/2 activation is not significantly affected by protein kinase A (PKA), protein kinase C (PKC) and protein kinase B or Akt (Akt/PKB) specific inhibitors. However, alpha-MSH-induced ERK1/2 activation is abolished by the phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin and LY294002. Altogether, these findings demonstrate that MC5R signals through a PI3K-regulated Akt-independent pathway leading to downstream activation of ERK1/2. The involvement of these MAPK suggests that MC5R could be implicated in cellular proliferation or differentiation mechanisms.

Binding of guanylyl cyclase activating protein 1 (GCAP1) to retinal guanylyl cyclase (RetGC1). The role of individual EF-hands

Guanylyl cyclase activating protein 1 (GCAP1), after substitution of Ca(2+) by Mg(2+) in its EF-hands, stimulates photoreceptor guanylyl cyclase, RetGC1, in response to light. We inactivated metal binding in individual EF-hands of GCAP1 tagged with green fluorescent protein to assess their role in GCAP1 binding to RetGC1 in co-transfected HEK293 cells. When expressed alone, GCAP1 was uniformly distributed throughout the cytoplasm and the nuclei of the cells, but when co-expressed with either fluorescently tagged or non-tagged RetGC1, it co-localized with the cyclase in the membranes. The co-localization did not occur when the C-terminal portion of RetGC1, containing its regulatory and catalytic domains, was removed. Mutations that preserved Mg(2+) binding in all three metal-binding EF-hands did not affect GCAP1 association with the cyclase in live cells. Locking EF-hand 4 in its apo-conformation, incapable of binding either Ca(2+) or Mg(2+), had no effect on GCAP1 association with the cyclase. In contrast to EF-hand 4, inactivation of EF-hand 3 reduced the efficiency of the co-localization, and inactivation of EF-hand 2 drastically suppressed GCAP1 binding to the cyclase. These results directly demonstrate that metal binding in EF-hand 2 is crucial for GCAP1 attachment to RetGC1, and that in EF-hand 3 it is less critical, although it enhances the efficiency of the GCAP1 docking on the target enzyme. Metal binding in EF-hand 4 has no role in the primary attachment of GCAP1 to the cyclase, and it only triggers the activator-to-inhibitor functional switch in GCAP1.

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

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

Activation and endocytic internalization of melanocortin 3 receptor in neuronal cells

Melanocortins play a central role in autonomic modulation of metabolism by acting through a family of highly homologous G protein-coupled receptors. Studies with gene knockout mice have implicated neural melanocortin receptors, MC3R and MC4R, in the etiology of obesity, insulin resistance, and salt-sensitive hypertension. In an attempt to better understand the mechanisms of function of these receptors, we expressed MC3R and MC4R in neuronal cells and demonstrated their co-localization to several membrane regions. We now show that in cultured neuronal cells, MC3R localizes to lipid rafts and undergoes endocytic internalization upon activation by gamma-MSH through a protein kinase-sensitive pathway. The appearance of the internalized receptor in lysosomes suggests that it is subsequently degraded. The expression of protein kinase A regulatory subunits and of c-Jun and c-Fos was analyzed by either immunoblotting or real-time PCR. No discernable changes were observed in the expression levels of these protein kinase A and protein kinase C responsive genes. Immunohistochemical studies showed a robust expression of MC3R protein in brain nuclei with relevance to cardiovascular function and fluid homeostasis further supporting the notion that the physiological effects of melanocortins on the cardiovascular system arise from effects on the central nervous system.

Melanocortin-3 receptor activates MAP kinase via PI3 kinase

HEK 293 cells stably expressing human melanocortin-3 receptor (MC3R) were exposed to melanocortin receptor agonist, NDP-MSH (10(-)(10)-10(-)(6) M). ERK1/2 was phosphorylated in a dose-dependent manner with an EC(50) of 3.3+/-1.5 x 10(-)(9) M, similar to the IC(50) of NDP-MSH binding to the MC3R. ERK1/2 phosphorylation was blocked by the melanocortin receptor antagonists SHU9119. NDP-MSH-induced ERK1/2 phosphorylation was sensitive to pertussis toxin and the PI3K inhibitor, wortmannin. Rp-cAMPS, BAPTA-AM and Myr-PKC did not inhibit the NDP-MSH-induced ERK1/2 phosphorylation. NDP-MSH stimulated cellular proliferation in a dose-dependent manner with a similar EC(50) to ERK1/2 phosphorylation, 2.1+/-0.6 x 10(-)(9) M. Cellular proliferation was blocked by AGRP (86-132) and by the MEK inhibitor, PD98059. The NDP-MSH did not inhibit serum deprivation-induced apoptosis. MC3R activation induces ERK1/2 phosphorylation via PI3K and this pathway is involved in cellular proliferation in HEK cells expressing MC3R.

Peripherally administered desacetyl alpha-MSH and alpha-MSH both influence postnatal rat growth and associated rat hypothalamic protein expression

Desacetyl alpha-MSH predominates over alpha-MSH during development, but whether it is biologically active and has a physiological role is unclear. We compared the effects of 0.3 microg.g(-1).day(-1) desacetyl alpha-MSH with that of 0.3 microg.g(-1).day(-1) alpha-MSH on postnatal body growth by administering the peptides subcutaneously daily for postnatal days 0-14 and also used a two-dimensional gel electrophoresis gel-based proteomic approach to analyze protein changes in hypothalami, the relay center for body weight and growth regulation, after 14 days of treatment. We found that the growth rate between days 1 and 10 was significantly decreased by desacetyl alpha-MSH but not by alpha-MSH, but by day 14, a time reported for development of a mature pattern of hypothalamic innervation, both peptides had significantly increased neonatal growth compared with PBS-treated control rats. Desacetyl alpha-MSH significantly increased spleen weight, but alpha-MSH had no effect. alpha-MSH significantly decreased kidney weight, but desacetyl alpha-MSH had no effect. Both desacetyl alpha-MSH and alpha-MSH significantly decreased brain weight. By 14 days, both peptides significantly changed expression of a number of hypothalamic proteins, specifically metabolic enzymes, cytoskeleton, signaling, and stress response proteins. We show that peripherally administered desacetyl alpha-MSH is biologically active and induces responses that can differ from those for alpha-MSH. In conclusion, desacetyl alpha-MSH appears to be an important regulator of neonatal rat growth.