Loss of the C terminus of melanocortin receptor 2 (MC2R) results in impaired cell surface expression and ACTH insensitivity

Mutations in melanocortin receptor 2 (MC2R) and its related melanocortin receptor accessory protein (MRAP) cause familial glucocorticoid deficiency. We identified a novel MC2R mutation, K289fs. This unique mutation in the C terminus of MC2R is located in the intracellular part of the protein for which the exact function is unknown.

Amino Acid Residues Implicated in the Interaction of Melanocortin Ligands and their Receptors: A Study of MC2R Selectivity

Author: Kristopher D. Veo Title: Amino acid residues implicated in the interaction of Melanocortin ligands and their receptors: A study of MC2R selectivity Advisor: Dr. Robert M. Dores Degree Date: August 2009 ABSTRACT Melanocortin receptor ligand selectivity has been a question not easily answered. The inability to functionally express melanocortin 2 receptor (MC2R) has inhibited the study of why MC2R is only stimulated by ACTH, a melanocortin hormone. With the recent discovery of the MC2R accessory protein (MRAP), creating a heterologous system is now feasible. Using a general cell line like CHO-K1 cells, which do not express endogenous MCRs, we were able to create a heterologous expression system and test the selectivity of MC2R using analog variants of ACTH(1-24). Our results indicate an amino acid requirement in the C-terminal portion of ACTH(1-24) for activation, which supports the 2-step method of activation hypothesized for MC2R. This site, the tetra basic cleavage site, when altered does not stimulate cAMP production and does not compete with ACTH(1-24) for binding. We also demonstrate the potential for a non-mammalian MC2R system in cloning full length Silurana tropicalis MC2R and completed localization studies with this system with MRAP using CHO-K1 cells.

Heterogeneity in the molecular basis of ACTH resistance syndrome

Objective: ACTH resistance syndromes are rare, autosomal, and genetically heterogeneous diseases that include familial glucocorticoid deficiency (FGD) and triple A syndrome. FGD has been shown to segregate with mutations in the gene coding for ACTH receptor (MC2R) or melanocortin 2 receptor accessory protein (MRAP), whereas mutations in the triple A syndrome (AAAS, Allgrove syndrome) gene have been found in segregation with triple A syndrome. We describe the clinical findings and molecular analysis of MC2R, MRAR and AAAS genes in five Brazilian patients with ACTH resistance syndrome. Design and methods: Genomic DNA from patients and their unaffected relatives was extracted from peripheral blood leucocytes and amplified by PCR, followed by automated sequencing. Functional analysis was carried out using Y6 cells expressing wild-type and mutant MC2R. Results: All five patients showed low cortisol and elevated plasma ACTH levels. One patient had achalasia and alacrima, besides the symptoms of adrenal insufficiency. The molecular analysis of FGD patients revealed a novel p.Gly116Val mutation in the MC2R gene in one patient and p.Met1Ile mutation in the MRAP gene in another patient. Expression of p.Glyll.6Val MC2R mutant in Y6 cells revealed that this variant failed to stimulate cAMP production. The analysis of the AAAS gene in the patient with triple A syndrome showed a novel g.782_783deITG deletion. The molecular analysis of DNA from other two patients showed no mutation in MC2R, MRAP or AAAS gene. Conclusions: In conclusion, the molecular basis of ACTH resistance syndrome is heterogeneous, segregating with genes coding for proteins involved with ACTH receptor signaling/expression or adrenal gland development and other unknown genes.

Characterization of a Novel Oral Glucocorticoid System and Its Possible Role in Disease

Synthetic corticosteroids are used widely for the treatment of a variety of diseases of the mouth. However, little is known as to whether the oral mucosa is able to modulate the local concentration of active corticosteroids or to produce steroids de novo. This has important clinical implications, because tissue-specific regulation of glucocorticoids is a key determinant of the clinical efficacy of these drugs. In the present study, we show that oral fibroblasts and keratinocytes expressed ACTH receptor (MC2R), glucocorticoid receptor (GR), and 11 beta-hydroxysteroid dehydrogenases (11 beta-HSDs). Unlike keratinocytes, fibroblasts lacked 11 beta-HSD2 and could not effectively deactivate exogenously administered cortisol. However, both cell types were able not only to activate cortisone into the active form cortisol, but also to synthesize cortisol de novo following stimulation with ACTH. 11 beta-HSD2, the enzyme controlling cortisol deactivation, exhibited different patterns of expression in normal (squamous epithelium and salivary glands) and diseased oral mucosa (squamous cell carcinoma and mucoepidermoid carcinoma). Blocking of endogenous cortisol catabolism in keratinocytes with the 11 beta-HSD2 inhibitor 18 beta-glycyrrhetinic acid mimicked the effect of exogenous administration of hydrocortisone and partially prevented the detrimental effects induced by pemphigus vulgaris sera. Analysis of the data demonstrates that a novel, non-adrenal glucocorticoid system is present in the oral mucosa that may play an important role in disease.

Adrenal gland development and defects

The network regulating human adrenal development is complex. Studies of patients with adrenal insufficiency due to gene mutations established a central role for transcription factors GLI3, SF1 and DAX1 in the initial steps of adrenal formation. Adrenal differentiation seems to depend on adrenocorticotropic hormone (ACTH) stimulation and signalling, including biosynthesis and action of POMC, PC1, TPIT, MC2R, MRAP and ALADIN, all of which cause adrenocortical hypoplasia when mutated in humans. Studies of knockout mice revealed many more factors involved in adrenal development; however, in contrast to rodents, in humans several of those factors had no adrenal phenotype when mutated (e.g. WT1, WNT4) or, alternatively, human mutations have not (yet) been identified. Tissue profiling of fetal and adult adrenals suggested 69 genes involved in adrenal development. Among them were genes coding for steroidogenic enzymes, transcription and growth factors, signalling molecules, regulators of cell cycle and angiogenesis, and extracellular matrix proteins; however, the exact role of most of them remains to be elucidated.

Clinical, genetic, and functional characterization of adrenocorticotropin receptor mutations using a novel receptor assay.

The ACTH receptor (MC2R) is expressed predominantly in the adrenal cortex, but is one of five G protein-coupled, seven-transmembrane melanocortin receptors (MCRs), all of which bind ACTH to some degree. Testing of MC2R activity is difficult because most cells express endogenous MCRs; hence, ACTH will elicit background activation of assayable reporter systems. Inactivating mutations of MC2R lead to hereditary unresponsiveness to ACTH, also known as familial glucocorticoid deficiency (FGD). These patients are usually seen in early childhood with very low cortisol concentrations, normal mineralocorticoids, hyperpigmentation, and increased bodily growth. Several MC2R mutations have been reported in FGD, but assays of the activities of these mutants are cumbersome. We saw two patients with typical clinical findings of FGD. Genetic analysis showed that patient 1 was homozygous for the mutation R137W, and patient 2 was a compound heterozygote for S74I and Y254C. We tested the activity of these mutations in OS-3 cells, which are unresponsive to ACTH but have intact downstream cAMP signal transduction. OS-3 cells transfected with a cAMP-responsive luciferase reporter plasmid (pCREluc) were unresponsive to ACTH, but cotransfection with a vector expressing human MC2R increased luciferase activity more than 40-fold. Addition of ACTH to cells cotransfected with the pCREluc reporter and wild-type MC2R activated luciferase expression with a 50% effective concentration of 5.5 x 10(-9) M ACTH, which is similar to previously reported values. By contrast, the MC2R mutant R137W had low activity, and the S74I or Y254C mutants elicited no measurable response. This assay provides excellent sensitivity in an easily assayed transient transfection system, providing a more rapid and efficient measurement of ACTH receptor activity.

Evolution of Melanocortin-2 Receptor Activations: Studies on a Mammal and a Fish

The structure and function relationship between melanocortin-2 receptor (MC2R) and ACTH are the most complicated in melanocortin receptor gene family. A comparative study on the activation of human and rainbow trout MC2R will provide a useful model system for understanding how ACTH emerged as the sole ligand for the MC2R of bony vertebrates. This dissertation will discuss how studies utilizing analogs of hACTH(1-24) have revealed two critical amino acid motifs in this ligand (HFRW and KKRRP) which are required for the activation of MC2R. In addition, the KKRRP motif functioned as the unique binding site for MC2R that directly contributes to the ligand selectivity feature, as revealed from studies on an ACTH antagonist which exclusively targets MC2R. Finally, based on our model for the interaction of ACTH and MC2R, the amino acid residues within TM4, EC2, and TM5 domains responsible for ACTH ligand selectivity will be evaluated by site-directed mutagenesis studies.