Functional analysis of melanocortin 2 receptor variants and dimer formation

Abstract

The adrenocorticotropic hormone (ACTH) receptor, also known as the melanocortin 2 receptor (MC2R) is primarily expressed in the adrenal gland and binds to ACTH to activate a signalling cascade that leads to steroidogenesis. The characterisation of the melanocortin 2 receptor has previously been hindered by the failure of cell surface expression in heterologous cells. Unlike other melanocortin receptors, MC2R surface expression requires an accessory protein called melanocortin 2 receptor accessory protein (MRAP). MRAP has been shown to facilitate trafficking of MC2R from the endoplasmic reticulum to the plasma membrane and it also has a role in MC2R signalling. Naturally occurring MC2R mutations occur in Familial Glucocorticoid Deficiency (FGD) type 1. The functional defect remains uncertain for the majority, mainly due to the difficulties in characterisation which has been limited in Y6 cells (mouse derived adrenal cells which are free of MCRs) because of poor transfection efficiency. The first two results chapter of this thesis focus on the molecular biology of FGD mutations and the genotype-phenotype relationship of the disease. The functional defects of these mutations have been analysed in transfectable heterologuous cells stably expressing MRAP. The majority of the missense mutations are trafficking-defective, and yet all appear to interact with MRAP, but some mutations are trafficking-competent and fail to signal. Clinical details including the age of presentation; presenting plasma ACTH and cortisol; height and weight were examined in FGD type 1 and 2 (mutation of MC2R or MRAP respectively) to determine whether there is a genotype-phenotype correlation. Within FGD type 1, there was no clear clinical distinction between the functional defects in vitro and in vivo data. However, there were phenotypic differences between 3 FGD type 1 and 2, with FGD 2 appearing to be more severe with a younger age and a lower height SDS at presentation. The final part of this thesis focuses on MC2R dimerisation and signalling of the receptor. Co-immunoprecipitation studies showed that MC2R homodimerisation is constitutive. This interaction was not enhanced in the presence of MRAP or ACTH. No effect on signalling was observed. When a signalling mutant (R128C) and a ligand binding mutant (D103N) were co-transfected with MRAP and stimulated with ACTH, signalling was not restored, suggesting no cross talk between the two receptors. In the final discussion the implication of these findings are considered with respect to their relevance to disease and novel therapeutic strategies.