MRAP deficiency impairs adrenal progenitor cell differentiation and gland zonation

Abstract

Melanocortin 2 receptor accessory protein (MRAP) is a single transmembrane domain accessory protein and a critical component of the hypothamo-pituitary-adrenal axis. MRAP is highly expressed in the adrenal gland and is essential for adrenocorticotropin hormone (ACTH) receptor expression and function. Human loss-of-function mutations in MRAP cause familial glucocorticoid (GC) deficiency (FGD) type 2 (FGD2), whereby the adrenal gland fails to respond to ACTH and to produce cortisol. In this study, we generated Mrap-null mice to study the function of MRAP in vivo. We found that the vast majority of Mrap−/− mice died at birth but could be rescued by administration of corticosterone to pregnant dams. Surviving Mrap−/− mice developed isolated GC deficiency with normal mineralocorticoid and catecholamine production, recapitulating FGD2. The adrenal glands of adult Mrap−/− mice were small, with grossly impaired adrenal capsular morphology and cortex zonation. Progenitor cell differentiation was significantly impaired, with dysregulation of WNT4/β-catenin and sonic hedgehog pathways. These data demonstrate the roles of MRAP in both steroidogenesis and the regulation of adrenal cortex zonation. This is the first mouse model of isolated GC deficiency and reveals the role of MRAP in adrenal progenitor cell regulation and cortex zonation.—Novoselova, T. V., Hussain, M., King, P. J., Guasti, L., Metherell, L. A., Charalambous, M., Clark, A. J. L., Chan, L. F. MRAP deficiency impairs adrenal progenitor cell differentiation and gland zonation.

ACTH signalling and adrenal development: lessons from mouse models

“…Adult Mrap − / − mice of both genders exhibited a grossly dysmorphic adrenal cortex with the glucocorticoid synthesis pathway severely downregulated and unable to produce glucocorticoids in the presence of high plasma ACTH levels. Surprisingly, unlike the Mc2r − / − mice, circulating aldosterone and catecholamine levels were unaffected (41, 43). The enzyme phenylethanolamine N-methyltransferase (PNMT), which is responsible for conversion of norepinephrine to epinephrine, appeared to be unaffected in Mrap − / − mice even though PNMT expression is known to be dependent on glucocorticoid action.…”

“…We recently reported a Mrap KO ( Mrap − / − ) mouse model created by targeting the first coding exon of Mrap, which led to complete absence of transcript and protein in homozygote mice (43). Intercrossing heterozygote ( Mrap +/ − ), also on a C57BL/6J background, demonstrated high neonatal mortality.…”

“…Intercrossing heterozygote ( Mrap +/ − ), also on a C57BL/6J background, demonstrated high neonatal mortality. Out of 325 mice generated from breeding Mrap +/ − mice, only three Mrap − / − mice (<1%) survived until weaning (43). The new-born pups born from heterozygous parents with normal adrenal function died before postnatal day 1 and morphologically showed immature lungs and lack of hepatic glycogen stores (43).…”

“…Out of 325 mice generated from breeding Mrap +/ − mice, only three Mrap − / − mice (<1%) survived until weaning (43). The new-born pups born from heterozygous parents with normal adrenal function died before postnatal day 1 and morphologically showed immature lungs and lack of hepatic glycogen stores (43). Glucocorticoid treatment of pregnant Mrap +/ − dams rescued the phenotype, resulting in Mrap − / − mice born at the expected ratio of 25%.…”

“…Assessment of the adrenal glands of Mrap − / − mice at embryonic day 17.5 shows that the adrenal sizes are comparable to Mrap +/+ mice, derived from intercrosses of heterozygous mice with normal HPA activity. However, when assessed at 8 weeks of age the gland is significantly reduced in size with a thickened adrenal capsule (43). One major issue is the separation of the effects of absent ACTH action and glucocorticoid deficiency that are both present in adult Mrap − / − mice.…”