On the Molecular Pharmacology of the Melanocortin-4 Receptor

Abstract

Melanocortin hormones, like alpha-MSH and ACTH (melanocyte stimulating hormone and adrenocorticotropic hormone), regulate several biological processes, including bodyweight homeostasis, neuropathic pain and nerve regeneration after nerve damage. Five melanocortin receptor subtypes have been cloned, which transduce the hormone signals via G-proteins to the appropriate tissues. Based on its biological functions, the melanocortin system provides a target for treating obesity, nerve damage or neuropathic pain. Before such treatments can be developed, detailed information on the working mechanisms of the melanocortins is needed. This thesis describes the molecular pharmacological characterization of the melanocortin-4 receptor (MC4R). This receptor is mainly expressed in the central nervous system, where it is involved in bodyweight regulation, but it has also been implicated to mediate the trophic effects of melanocortins on regenerating neurons and to regulate neuropathic pain. Ligands that selectively bind to or activate a receptor subtype are very useful to characterize the biological function of that receptor. For long, no selective ligands were available for the melanocortin receptor subtypes, which has hampered our knowledge on which receptor subtype mediates which melanocortin effect. This thesis describes the development and in vivo use of new MC4R selective peptides. Systematic mutation of a melanocortin-based peptide yielded MC4R selective agonists and subsequently selective antagonists were developed. The peptides were active in vivo, as determined in a rat grooming assay. Using these new selective peptides, it was shown that the MC4R is probably the melanocortin receptor subtype that mediates the effects of melanocortin ligands on neuropathic pain. The MC4R however, seems not to be involved in the stimulating effects of melanocortins on peripheral nerve regeneration, since the pharmacology of the peptides for the MC4R did not correlate with their effects in a rat model for peripheral nerve regeneration. A new concept in receptor pharmacology is the existence of ligand-independent activity of receptors. Some ligands, now referred to as "inverse agonists", can inhibit this constitutive receptor activity. It was shown that the MC4R is constitutively active, and that an endogenous "antagonist" of the MC4R, AgRP, is in fact an inverse agonist. Since AgRP is the first endogenous inverse agonist working at a endogenous receptor, this provides important evidence that inverse agonism may be relevant for the biological function of receptor ligands. Furthermore, for the development of melanocortin-based drugs, inverse agonism may be a relevant property to screen for. Mutations in the MC4R are associated with obesity in humans. Functional characterization of the mutated receptors is needed to strengthen involvement of the MC4R in development of obesity and to gain insight into the molecular mechanism underlying this form of obesity. Eleven mutated MC4Rs were found to generally have normal pharmacology for the endogenous ligands, but cell surface expression was decreased for all mutants. Since the mutations affect receptor function, it is plausible that these mutations indeed predispose for obesity. Restoring cell surface expression of the MC4R may thus be effective in treating obesity of humans bearing a MC4R mutation, but also for treatment of obesity in general, this is an interesting option.