Inhibition of melanocortin-4 receptor dimerization by substitutions in intracellular loop 2

Obesity is one of the most challenging global health problems. One key player in energy homeostasis is the melanocortin-4 receptor (MC4R), which is a family A G-protein-coupled receptor (GPCR). It has recently been shown that MC4R has the capacity to form homo- or heterodimers. Dimerization of GPCRs is of great importance for signaling regulation, with major pharmacological implications. Unfortunately, not enough is yet known about the detailed structural properties of MC4R dimers or the functional consequences of receptor dimerization. Our goal, therefore, was to explore specific properties related to MC4R dimerization. First, we aimed to induce the dissociation of dimers to monomers and to compare the functional parameters of wild-type and MC4R variants. To inhibit homodimerization, we designed MC4R chimeras with the cannabinoid-1 receptor, a receptor that does not interact with MC4R. Indeed, we identified several substitutions in the intracellular loop 2 (ICL2) and adjacent regions of transmembrane helix 3 (TMH3) and TMH4 that lead to partial dimer dissociation. Interestingly, the capacity for signaling activity was generally increased in these MC4R variants, although receptor expression remained unchanged. This increase in activity for dissociated receptors might indicate a link between receptor dimerization and signaling capacity. Moreover, dimer dissociation was also observed in a naturally occurring activating MC4R mutation in ICL2. Taken together, this study provides new information on the structural prerequisites for MC4R dimerization and identifies an approach to induce the dissociation of MC4R dimers. This might be useful for further investigation of pharmacological properties.