Expression of the Melanocortin 2 Receptor Accessory Proteins in the Mouse Brain

Abstract

The melanocortin 2 receptor accessory protein α1 (MRAPα1) is critical for the expression and signalling of the MC2R in the mouse adrenal gland cortex. MRAPα1 and MRAPα2 are splice variants of the mouse MRAP gene and MRAP2 is a paralogue of MRAP. MRAP2 can also allow for cell surface expression of MC2R but only allows signalling of MC2R in vitro at pharmacological concentrations of the MC2R ligand, adrenocorticotropin hormone. In vitro, MRAPs can interact with the other four-melanocortin receptors (MC1R, MC3R-MC5R) that have wide tissue distributions. This suggestes MRAPs have functions beyond the MC2R and the adrenal gland. Human and rodent studies detected MRAPs in the brain using RT-PCR, where all five MCRs have also been found. Functions for MRAPs in the brain are unknown and we hypothesised MRAPs are expressed in regions where MCRs are, and they may interact with MCRs in vivo to modulate the neural melanocortin system. This study aimed to map mRNA expression of the three mouse MRAPs and the five MCRs in brains dissected from male C57BL/6J mice aged 15-150 days. Antisense riboprobes to specifically detect each MRAP was synthesised with 33P-UTP and used for high stringency in situ hybridisation (ISH) performed on five series of coronal brain sections. Each adjacent series was probed with a different probe. Control ISH was performed on adrenal gland sections where MRAPα1 and MC2R are expressed. The results of the study showed MRAPα1 was absent in the mouse brain and MRAPα2 was expressed in the hippocampus and piriform cortex. MRAP2 was seen in the olfactory bulbs, thalamus, hippocampus, hypothalamus, midbrain and brainstem regions. The expression of MRAP2 overlapped with some MC3R and MC4R in the thalamus and hypothalamus. MRAP2 was also present in regions that do not express MCRs. Many of the regions that expressed MRAP2 are important for regulation of feeding and appetite, learning and memory, autonomic regulation, stress responses, circadian rhythms and neurogenesis. Thus, MRAP2 appears to have a myriad of roles in the brain that are likely to involve the neural MCRs, but it is also likely that it has functions in the brain independent of MCRs.