Melanotropins as drugs for the treatment of obesity and other feeding disorders: potential and problems

Melanotan-II reverses memory impairment induced by a short-term HF diet

A high-fat (HF) diet has been shown to increase the risk of neurological impairments and neurodegenerative disorders. The melanotropins used in this study have been associated with diet-related disorders; however, there is an absence of studies on their effect on diet-induced neurobehavioral conditions. Here, we investigated the possible relationship among diet, Melanotan-II (MT-II) targeting melanotropin receptors, and the behavior of zebrafish (Danio rerio). Surprisingly, even a short-term HF diet lasting for ∼ 1 % of the zebrafish's life had a strong developmental effect. Zebrafish fed the HF diet showed an impairment in recognition memory, elevated anxiety levels, and reduced exploratory propensity after just three weeks compared to zebrafish fed the control diet. These HF diet-induced abnormalities were reversed by MT-II. Animals fed a HF diet and treated with MT-II demonstrated recognition memory, anxiety, and exploratory behavior similar to the control group. This study provides evidence that even a short-term HF diet has an impact on memory and emotions and is the first study to show that MT-II reverses these changes.

MC1R Peptide Agonist Self-Assembles into a Hydrogel That Promotes Skin Pigmentation for Treating Vitiligo

Vitiligo, a common skin disease that seriously affects 0.5-2.0% of the worldwide population, lacks approved therapeutics due to a wide range of adverse side effects. As a key regulator of skin pigmentation, MC1R may be an effective therapeutic target for vitiligo. Herein, we report an MC1R peptide agonist that directly self-assembles into nanofibrils that form a hydrogel matrix under normal physiological conditions. This hydrogel exhibits higher stability than free peptides, sustained release, rapid recovery from shear-thinning, and resistance to enzymatic proteolysis. Furthermore, this peptidal MC1R agonist upregulates tyrosinase, tyrosinase-related protein-1 (TYRP-1), and tyrosinase-related protein-2 (TYRP-2) to stimulate melanin synthesis. More importantly, MC1R agonist hydrogel promotes skin pigmentation in mice more potently than free MC1R agonist. This study supports the development of this MC1R agonist hydrogel as a promising pharmacological intervention for vitiligo.

The melanocortin action is biased toward protection from weight loss in mice

The melanocortin action is well perceived for its ability to regulate body weight bidirectionally with its gain of function reducing body weight and loss of function promoting obesity. However, this notion cannot explain the difficulty in identifying effective therapeutics toward treating general obesity via activation of the melanocortin action. Here, we provide evidence that altered melanocortin action is only able to cause one-directional obesity development. We demonstrate that chronic inhibition of arcuate neurons expressing proopiomelanocortin (POMC) or paraventricular hypothalamic neurons expressing melanocortin receptor 4 (MC4R) causes massive obesity. However, chronic activation of these neuronal populations failed to reduce body weight. Furthermore, gain of function of the melanocortin action through overexpression of MC4R, POMC or its derived peptides had little effect on obesity prevention or reversal. These results reveal a bias of the melanocortin action towards protection of weight loss and provide a neural basis behind the well-known, but mechanistically ill-defined, predisposition to obesity development.

Structural Insights into Selective Ligand-Receptor Interactions Leading to Receptor Inactivation Utilizing Selective Melanocortin 3 Receptor Antagonists

Systematic N-methylated derivatives of the melanocortin receptor ligand, SHU9119, lead to multiple binding and functional selectivity toward melanocortin receptors. However, the relationship between N-methylation-induced conformational changes in the peptide backbone and side chains and melanocortin receptor selectivity is still unknown. We conducted comprehensive conformational studies in solution of two selective antagonists of the third isoform of the melanocortin receptor (hMC3R), namely, Ac-Nle-c[Asp-NMe-His⁶-d-Nal(2')⁷-NMe-Arg⁸-Trp⁹-Lys]-NH₂ (15) and Ac-Nle-c[Asp-His⁶-d-Nal(2')⁷-NMe-Arg⁸-NMe-Trp⁹-NMe-Lys]-NH₂ (17). It is known that the pharmacophore (His⁶-DNal⁷-Arg⁸-Trp⁹) of the SHU-9119 peptides occupies a β II-turn-like region with the turn centered about DNal⁷-Arg⁸. The analogues with hMC3R selectivity showed distinct differences in the spatial arrangement of the Trp⁹ side chains. In addition to our NMR studies, we also carried out molecular-level interaction studies of these two peptides at the homology model of hMC3R. Earlier chimeric human melanocortin 3 receptor studies revealed insights regarding the binding and functional sites of hMC3R selectivity. Upon docking of peptides 15 and 17 to the binding pocket of hMC3R, it was revealed that Arg⁸ and Trp⁹ side chains are involved in a majority of the interactions with the receptor. While Arg⁸ forms polar contacts with D154 and D158 of hMC3R, Trp⁹ utilizes π-π stacking interactions with F295 and F298, located on the transmembrane domain of hMC3R. It is hypothesized that as the frequency of Trp⁹-hMC3R interactions decrease, antagonistic activity increases. The absence of any interactions of the N-methyl groups with hMC3R suggests that their primary function is to modulate backbone conformations of the ligands.

Novel approaches to the design of bioavailable melanotropins

INTRODUCTION

The melanocortin system is a primordial and critical system for survival, involved in a wide variety of physiological functions. It includes melanocortin receptors (MCRs) and melanotropin ligands (MCLs). MCRs are important drug targets that can regulate several key physiological processes. Extensive efforts have been made to develop peptide and peptidomimetics targeting melanocortin receptors including MC1R, MC3R, MC4R and MC5R. Most research is focused on developing potent and selective melanotropins. However, developing bioavailable melanotropins remains challenging. Areas covered: Herein, the authors summarize promising strategies for developing bioavailable MCLs by using cyclized N-methylated melanotropins, and using cyclotide and tetrapeptide as templates. They discuss their unique advantages in oral availability and targeting MCRs in the central nervous system or in peripheral tissues. Finally, they discuss the observed differences in thepharmacology of MCRs between in vitro and in vivo tests. Expert opinion: N-methylated cyclized melanotropins have great potential to become bio- available drugs targeting MCRs in the brain, while MCR-grafted cyclotides tend to target MCRs in peripheral tissue. A better understanding of the biased signaling process is a new challenge and opportunity for the future discovery of bioavailable MCLs.

Systematic Backbone Conformational Constraints on a Cyclic Melanotropin Ligand Leads to Highly Selective Ligands for Multiple Melanocortin Receptors

Human melanocortin receptors (hMCRs) have been challenging targets to develop ligands that are explicitly selective for each of their subtypes. To modulate the conformational preferences of the melanocortin ligands and improve the biofunctional agonist/antagonist activities and selectivities, we have applied a backbone N-methylation approach on Ac-Nle-c[Asp-His-D-Nal(2')-Arg-Trp-Lys]-NH2 (Ac-Nle(4)-c[Asp(5),D-Nal(2')(7),Lys(10)]-NH2), a nonselective cyclic peptide antagonist at hMC3R and hMC4R and an agonist at hMC1R and hMC5R. Systematic N-methylated derivatives of Ac-Nle(4)-c[Asp(5),D-Nal(2')(7),Lys(10)]-NH2, with all possible backbone N-methylation combinations, have been synthesized and examined for their binding and functional activities toward melanocortin receptor subtypes 1, 3, 4, and 5 (hMCRs). Several N-methylated analogues are selective and potent agonists or antagonists for hMC1R or hMC5R or have selective antagonist activity for hMC3R. The selective hMC1R ligands show strong binding for human melanoma cells. We have also discovered the first universal antagonist (compound 19) for all subtypes of hMCRs.

Design of peptide and peptidomimetic ligands with novel pharmacological activity profiles

Peptide hormones and neurotransmitters are of central importance in most aspects of intercellular communication and are involved in virtually all degenerative diseases. In this review, we discuss physicochemical approaches to the design of novel peptide and peptidomimetic agonists, antagonists, inverse agonists, and related compounds that have unique biological activity profiles, reduced toxic side effects, and, if desired, the ability to cross the blood-brain barrier. Designing ligands for specific biological and medical needs is emphasized, as is the close collaboration of chemists and biologists to maximize the chances for success. Special emphasis is placed on the use of conformational (ϕ-ψ space) and topographical (χ space) considerations in design.

Retinal conformation governs pKa of protonated Schiff base in rhodopsin activation

We have explored the relationship between conformational energetics and the protonation state of the Schiff base in retinal, the covalently bound ligand responsible for activating the G protein-coupled receptor rhodopsin, using quantum chemical calculations. Guided by experimental structural determinations and large-scale molecular simulations on this system, we examined rotation about each bond in the retinal polyene chain, for both the protonated and deprotonated states that represent the dark and photoactivated states, respectively. Particular attention was paid to the torsional degrees of freedom that determine the shape of the molecule, and hence its interactions with the protein binding pocket. While most torsional degrees of freedom in retinal are characterized by large energetic barriers that minimize structural fluctuations under physiological temperatures, the C6-C7 dihedral defining the relative orientation of the β-ionone ring to the polyene chain has both modest barrier heights and a torsional energy surface that changes dramatically with protonation of the Schiff base. This surprising coupling between conformational degrees of freedom and protonation state is further quantified by calculations of the pKa as a function of the C6-C7 dihedral angle. Notably, pKa shifts of greater than two units arise from torsional fluctuations observed in molecular dynamics simulations of the full ligand-protein-membrane system. It follows that fluctuations in the protonation state of the Schiff base occur prior to forming the activated MII state. These new results shed light on important mechanistic aspects of retinal conformational changes that are involved in the activation of rhodopsin in the visual process.

An unusual conformation of γ-melanocyte-stimulating hormone analogues leads to a selective human melanocortin 1 receptor antagonist for targeting melanoma cells

γ-MSH (γ-melanocyte-stimulating hormone, H-Tyr-Val-Met-Gly-His-Phe-Arg-Trp-Asp-Arg-Phe-Gly-OH), with its exquisite specificity and potency, has recently created much excitement as a drug lead. However, this peptide is like most peptides susceptible to proteolysis in vivo, which potentially decreases its beneficial activities. In our continued effort to design a proteolytically stable ligand with specific receptor binding, we have engineered peptides by cyclizing γ-MSH using a thioether bridge. A number of novel cyclic truncated γ-MSH analogues were designed and synthesized, in which a thioether bridge was incorporated between a cysteine side chain and an N-terminal bromoacyl group. One of these peptides, cyclo-[(CH(2))(3)CO-Gly(1)-His(2)-D-Phe(3)-Arg(4)-D-Trp(5)-Cys(S-)(6)]-Asp(7)-Arg(8)-Phe(9)-Gly(10)-NH(2), demonstrated potent antagonist activity and receptor selectivity for the human melanocortin 1 receptor (hMC1R) (IC(50) = 17 nM). This novel peptide is the most selective antagonist for the hMC1R to date. Further pharmacological studies have shown that this peptide can specifically target melanoma cells. The nuclear magnetic resonance analysis of this peptide in a membrane-like environment revealed a new turn structure, specific to the hMC1R antagonist, at the C-terminus, where the side chain and backbone conformation of D-Trp(5) and Phe(9) of the peptide contribute to hMC1R selectivity. Cyclization strategies represent an approach for stabilizing bioactive peptides while keeping their full potencies and should boost applications of peptide-based drugs in human medicine.

Discovery of a spiroindane based compound as a potent, selective, orally bioavailable melanocortin subtype-4 receptor agonist

We report the design, synthesis and properties of spiroindane based compound 1, a potent, selective, orally bioavailable, non-peptide melanocortin subtype-4 receptor agonist. Compound 1 shows excellent erectogenic activity in the rodent models.