Comparative Functional Alanine Positional Scanning of the α-Melanocyte Stimulating Hormone and NDP-Melanocyte Stimulating Hormone Demonstrates Differential Structure–Activity Relationships at the Mouse Melanocortin Receptors

Abstract

The melanocortin system has been implicated in the regulation of various physiological functions including melanogenesis, steroidogenesis, energy homeostasis, and feeding behavior. Five melanocortin receptors have been identified to date and belong to the family of G protein-coupled receptors (GPCR). Post-translational modification of the proopiomelanocortin (POMC) prohormone leads to the biosynthesis of the endogenous melanocortin agonists, including α-melanocyte stimulating hormone (α-MSH), β-MSH, γ-MSH, and adrenocorticotropic hormone (ACTH). All the melanocortin agonists derived from the POMC prohormone contain a His-Phe-Arg-Trp tetrapeptide sequence that has been implicated in eliciting the pharmacological responses at the melanocortin receptors. Herein, an alanine (Ala) positional scan is reported for the endogenous α-MSH ligand and the synthetic, more potent, NDP-MSH peptide (Ac-Ser1-Tyr2-Ser3-Nle4-Glu5-His6-DPhe7-Arg8-Trp9-Gly10-Lys11-Pro12-Val13-NH2) at the cloned mouse melanocortin receptors to test the assumption that the structure-activity relationships of one ligand would apply to the other. Several residues outside of the postulated pharmacophore altered potency at the melanocortin receptors, most notably the 1560-, 37-, and 15-fold potency loss when the Glu5 position of α-MSH was substituted with Ala at the mMC1R, mMC3R, and mMC4R, respectively. Importantly, the altered potencies due to Ala substitutions in α-MSH did not necessarily correlate with equivalent Ala substitutions in NDP-MSH, indicating that structural modifications and corresponding biological activities in one of these melanocortin ligands may not be predictive for the other agonist.

Bench-top to clinical therapies: A review of melanocortin ligands from 1954 to 2016

“…Acetylation of the N-terminal has been demonstrated to increase the stability of α-MSH compared to des-acetylated α-MSH [44, 45]. The full length peptide possesses nonselective sub-nanomolar to nanomolar potencies at the MC1R, MC3R, MC4R, and MC5R [46, 47]. Alanine scans of α-MSH have also indicated the importance of the Met 4 , Phe 7 , Arg 8 , and Trp 9 positions for binding and functional activity at the mouse MC1R and rat MC3R [48, 49].…”

“…Alanine scans of α-MSH have also indicated the importance of the Met 4 , Phe 7 , Arg 8 , and Trp 9 positions for binding and functional activity at the mouse MC1R and rat MC3R [48, 49]. A 2016 report examining the cloned mouse receptors indicated that in addition to positions Met 4 , Phe 7 , Arg 8 , and Trp 9 , the Glu 5 and His 6 positions also affect functional activity [47]. Expression of α-MSH in the central nervous system is predominantly in the hypothalamus [50].…”

“…Since the sequence of α-MSH was reported in 1957 [27], this peptide has been subjected to numerous structure-activity relationship studies, including the classic truncation and alanine-positional scanning experiments [32, 33, 47-49]. Although α-MSH possesses nanomolar to sub-nanomolar potencies at the MCRs, key discoveries led to synthetic compounds with increased potency, length of activity, and altered receptor pharmacological profiles.…”

“…Truncation studies of NDP-MSH indicated an Ac-DPhe-Arg-Trp-NH 2 tripeptide sequence to be the minimally active fragment in both the frog skin bioassay and at the cloned MCRs [46, 114]. An alanine-positional scan of NDP-MSH reported decreased potencies when either the DPhe 7 or Trp 9 positions were substituted, indicating the importance of these two aromatic residues for the high potency of NDP-MSH [47]. Thirty-four years after its discovery, NDP-MSH was approved in the European Union as a treatment for adult erythropoietic protoporphria in 2014 [115].…”

“…Two selective agonists have been reported for the hMC3R (Table 1): a partial agonist at the hMC3R was selective over the hMC4R [119] and one compound has been reported selective for the hMC3R over the hMC1R and hMC4R [127]. At the mMC3R (Table 2), one compound has also shown to be selective for the mMC3R over the mMC1R [47]. Of the MC3R selective compounds, two were derivatives of α-MSH [47, 119] and one based upon the MTII/SHU9119 template [127].…”