Familial glucocorticoid deficiency (FGD), or hereditary unresponsiveness to adrenocorticotropin (ACTH; OMIM 202200), is an autosomal recessive disorder resulting from resistance to the action of ACTH on the adrenal cortex, which stimulates glucocorticoid production. Affected individuals are deficient in cortisol and, if untreated, are likely to succumb to hypoglycemia or overwhelming infection in infancy or childhood. Mutations of the ACTH receptor (melanocortin 2 receptor, MC2R) account for approximately 25% of cases of FGD. FGD without mutations of MC2R is called FGD type 2. Using SNP array genotyping, we mapped a locus involved in FGD type 2 to chromosome 21q22.1. We identified mutations in a gene encoding a 19-kDa single-transmembrane domain protein, now known as melanocortin 2 receptor accessory protein (MRAP). We show that MRAP interacts with MC2R and may have a role in the trafficking of MC2R from the endoplasmic reticulum to the cell surface.

GPCR accessory proteins ͉ receptor signalling ͉ receptor trafficking T he melanocortin receptor (MCR) family is involved in a diverse range of physiologic and disease processes (1). MC1R is important in pigmentation, MC2R in steroidogenesis, and MC5R has an exocrine function especially in sebaceous gland secretion. MC3R and MC4R are both highly expressed in the brain and play key roles in energy homeostasis. Mutations in MC4R are the most common cause of monogenic obesity. More recently, fat mass, weight, risk of obesity, and insulin resistance were associated with common variants near the MC4R locus (2, 3). Both MC4R knockout mice and humans with MC4R mutations display early-onset obesity associated with hyperphagia (4, 5). MC3R knockout mice, however, develop a milder phenotype with later-onset obesity (6, 7).We previously identified MRAP (melanocortin-2-receptor accessory protein), a small transmembrane protein, as an MC2R accessory protein, enabling the functional expression of MC2R in transfected cells. The identification of MRAP provides a molecular explanation for the difficulties encountered in the expression of the MC2R in nonadrenal cell lines (8). Furthermore, mutations in MRAP result in the autosomal recessive disorder familial glucocorticoid deficiency type 2 (9).Here we report the identification and characterization of a unique MRAP homologue encoded by C6orf117 on human chromosome 6q14.3, which we have named MRAP2. We show that both MRAP and MRAP2 can modulate the signaling of all 5 MCRs. MRAP2 is primarily expressed in human brain and adrenal gland. In the brain, MRAP2 expression is seen in the hypothalamus (10), a site that also expresses a high level of MC3R and MC4R (6,11). MRAP expression in the hypothalamus has also been demonstrated by in situ hybridization (12). These findings suggest that MRAP and MRAP2 may regulate MC3R and MC4R function in the central nervous system. Results MRAP2:A Unique Homologue of MRAP. The human MRAP2 gene consists of 4 exons, and its protein product comprises 205 aa residues, with a predicted molecular mass of 23.5 kDa (Fig. 1A). MRAP2 is homologous to MRAP, with 39% amino acid identity to MRAP in the N-terminal and transmembrane domains (Fig. 1B). The protein is highly conserved through vertebrates (supporting information Fig. S1) and like MRAP has no predicted signal sequence.

Formyl-peptide receptor type 2 (FPR2), also called ALX (the lipoxin A4 receptor), conveys the proresolving properties of lipoxin A 4 and annexin A1 (AnxA1) and the proinflammatory signals elicited by serum amyloid protein A and cathelicidins, among others. We tested here the hypothesis that ALX might exist as homo-or heterodimer with FPR1 or FPR3 (the two other family members) and operate in a ligand-biased fashion. Coimmunoprecipitation and bioluminescence resonance energy transfer assays with transfected HEK293 cells revealed constitutive dimerization of the receptors; significantly, AnxA1, but not serum amyloid protein A, could activate ALX homodimers. A p38/MAPK-activated protein kinase/heat shock protein 27 signaling signature was unveiled after AnxA1 application, leading to generation of IL-10, as measured in vitro (in primary monocytes) and in vivo (after i.p. injection in the mouse). The latter response was absent in mice lacking the ALX ortholog. Using a similar approach, ALX/FPR1 heterodimerization evoked using the panagonist peptide Ac2-26, identified a JNK-mediated proapoptotic path that was confirmed in primary neutrophils. These findings provide a molecular mechanism that accounts for the dual nature of ALX and indicate that agonist binding and dimerization state contribute to the conformational landscape of FPRs.inflammation | leukocyte | resolution signaling G -protein-coupled receptors (GPCRs) constitute a large family of cell surface receptors that share structural characteristics and perform pivotal biological functions, transducing signals from hormones, autacoids, and chemokines. The human GPCR termed "ALX/FPR2" (formyl peptide receptor type 2 or lipoxin A 4 receptor, hereafter referred to as "ALX") is a unique GPCR, shown to convey signals induced by proteins, peptides, and lipid ligands (1). ALX belongs to a small family of receptors that is also activated by formylated peptides, short amino acid sequences with an N-terminal formyl group released by pathogenic and commensal bacteria, as well as by mitochondria upon cell damage. There are three human FPRs and they are termed FPR1, ALX, and FPR3 (2). In view of their different nature and potential engagement with a large number endogenous and exogenous ligands, elucidation of FPR functions may reveal important biological pathways.ALX is an unconventional receptor for the diversity of its agonists and because it can convey contrasting biological signals. The proresolving and anti-inflammatory properties of the protein annexin A1 (AnxA1) and the lipid lipoxin A 4 (LXA 4 ), which include neutrophil apoptosis and macrophage efferocytosis, are mediated by this receptor, as shown using pharmacological approaches (1, 3) and more recently with knockout mouse models (4). At the same time, the proinflammatory responses elicited by the cathelicidin-associated antimicrobial peptide LL-37 and serum amyloid protein A (SAA) are also mediated by ALX, which modulates leukocyte activation, recruitment to the site of inflammation, and lifespan (5-7). Moreover,...

The ACTH receptor [melanocortin 2 receptor (MC2R)] gene produces a functional receptor only when transfected into cells of adrenocortical origin, implying that it may require an adrenal-specific accessory factor. Recently we showed that the MC2R accessory protein (MRAP) is essential for the cell surface expression of the MC2R in such models. Using RNA interference (RNAi) technology, we have further explored the action of MRAP in the functioning of the MC2R in Y1 mouse adrenocortical cells that endogenously express MRAP and MC2R. We created stable cell lines expressing mouse MRAP short hairpin RNA (shRNAs) by transfecting cells with an expression vector containing the MRAP small interfering RNA sequence. The knockdown of MRAP resulted in a reduction in MC2R signaling. The overexpression of a mouse MRAP-Flag construct did not restore the expression of MRAP due to its degradation by the mouse shRNAs. The introduction of human MRAP that is resistant to silencing by mouse MRAP shRNAs resulted in the rescue of the MC2R signaling. MRAP migrates on SDS-PAGE with markedly lower mobility than predicted for a 14.1-kDa protein. Coimmunoprecipitation and mass spectroscopy suggests that MRAP exists as a homodimer that is resistant to dissociation by sodium dodecyl sulfate and reducing agents.