ACTH signalling and adrenal development: Lessons from mouse models

Abstract

The melanocortin-2-receptor (MC2R), also known as the ACTH receptor, is a critical component of the hypothalamic–pituitary–adrenal axis. The importance of MC2R in adrenal physiology is exemplified by the condition familial gluc ocorticoid deficiency (FGD), a potentially fatal disease characterised by isolated cortisol deficiency. MC2R mutations cause ~25% of cases. The discovery of a MC2R accessory protein MRAP, mutations of which account for ~20% of FGD, has provided insight into MC2R trafficki ng and signalling. MRAP is a single transmembrane domain accessory protein highly expre ssed in the adrenal gland and essential for MC2R expression and function. Mouse models helped elucidate the action of ACTH. The Mc2r-knockout (Mc2r−/−) mice was the first mouse model developed to have adrenal insufficiency with deficiencies in glucocorticoid, mineralocorticoid and catecholamines. We recently reported the generation of the Mrap−/− mice which better mimics the human FGD phenotype with isolated glucocorticoid defi ciency alone. The adrenal glands of adult Mrap−/− mice were grossly dysmorphic with a thickened capsule, deranged zonation and deranged WNT4/beta-catenin and sonic hedg ehog (SHH) pathway signalling. Collectively, these mouse models of FGD highlight t he importance of ACTH and MRAP in adrenal progenitor cell regulation, cortex maintenance and zonation.