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

Abstract

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, Gαs. 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.

The Asp298Asn polymorphism of melanocortin‐4 receptor (MC4R) in pigs: evidence for its potential effects on MC4R constitutive activity and cell surface expression

“…In sharp contrast, the basal constitutive activity of pMC4R Asn was not altered by pMRAP2. As our finding in pigs are different from those of previous studies in humans (Gillyard et al 2019) (Sebag et al 2013), in which the basal constitutive activity of MC4R could be inhibited by MRAP2, we further tested the effects of chicken MRAP2 on the pMC4R Asp /pMC4R Asn . As shown in Fig.…”

Differential Signaling Profiles of MC4R Mutations with Three Different Ligands

“…For S127L, some publications identified a constitutive activity in G S [58,59], which could not be determined in the present study. One reason might be that in our HEK293 cell system MRAP2 is expressed in low amounts, which might reduce basal signaling [22,53,60]. We did, however, reproduce the increase in basal G S coupling that was reported for H158R [46].…”

“…The second endogenous agonist for MC4R, β-MSH has been neglected in most MC4R studies so far, but is proven to be important for weight regulation [22,48,49] and was therefore included in this study ( Figure 5). A β-MSH challenge created different responses for MC4R mutations V103I and H158R.…”

“…The endogenous ligands αand β-MSH share a core amino acid sequence (His-Phe-Arg-Trp) but differ in length and protein stability. They are both involved in the anorexigenic effect of MC4R signaling [1,22,48,49], though the reason for two different peptides that convey a rather similar function within the pathway is not entirely clear. For the MC4R WT there are no strong differences in the signaling profile of the two ligands; however, G S signaling of β-MSH in the presence of 100 nM AgRP is not as strongly affected compared to the one of α-MSH.…”

“…Interestingly, in some of the tested mutations no functional defect in G S signaling had been described before [21]. Recently, bias towards β-arrestin has been studied, revealing mutations with unremarkable changes in G S signaling but with a markedly affected ability to recruit β-arrestins [22,23].…”