MC2R — the ACTH receptor

Tissue

Adrenal cortex

Ligands

ACTH only (uniquely selective)

Function

The ACTH receptor — drives adrenal steroidogenesis and cortisol output at the bottom of the HPA stress axis.

When it fails

Mutations cause familial glucocorticoid deficiency; the receptor needs the accessory protein MRAP to reach the cell surface.

MC2R is the odd one out of the melanocortin family. Where the other four receptors respond to several melanocortin peptides, MC2R answers to just one — ACTH — and it sits in just one main place: the cortex of the adrenal gland, where it forms the final step of the body's stress-hormone axis.^[1]

The end of the stress axis

Stress and circadian cues drive the hypothalamus to release CRH, which makes the pituitary secrete ACTH, which travels in the blood to the adrenal cortex and binds MC2R. Like its relatives, MC2R couples to Gs and raises cyclic AMP; through protein kinase A this switches on the steroidogenic machinery that produces cortisol (and contributes to adrenal androgens).^[2] This hypothalamic–pituitary–adrenal (HPA) loop is the circuit that theoverview pillar describes ACTH serving — and MC2R is its receiving end.

Why only ACTH?

The melanocortins share a common His-Phe-Arg-Trp core, which is enough to activate MC1R and MC3R–MC5R. MC2R is different: it additionally requires a stretch of ACTH present only in the full ACTH molecule and not in the shorter MSH peptides, so α-, β-, and γ-MSH cannot meaningfully switch it on.^[1]That selectivity is what keeps the pigment-and-appetite arm of the system from accidentally triggering cortisol release.

It needs a chaperone: MRAP

On its own, MC2R cannot reach the cell surface. It depends on a small single-pass accessory protein, the melanocortin-2-receptor accessory protein (MRAP), which forms an unusual antiparallel dimer that escorts the receptor out of the endoplasmic reticulum and helps it respond to ACTH.^[2] The discovery of MRAP reframed MC2R as a two-component system — receptor plus chaperone — and explained a class of disease that maps not to the receptor itself but to its escort.

When it fails: familial glucocorticoid deficiency

Loss-of-function mutations in MC2R cause familial glucocorticoid deficiency (FGD) type 1, in which the adrenal cortex cannot mount a normal cortisol response to ACTH — presenting with low cortisol, very high ACTH, and risks such as hypoglycaemia and recurrent infection. Most disease-causing MC2R mutations work by stranding the receptor inside the cell rather than abolishing its function outright.^[3] Mutations in MRAP cause a closely related form, FGD type 2 — a reminder that, here, the chaperone is as load-bearing as the receptor.^[2]

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References

1. 1.Cone RD. Studies on the physiological functions of the melanocortin system. Endocr Rev. 27(7):736–49. 2006. link ↗
2. 2.Cooray SN, Clark AJL. The melanocortin receptors and their accessory proteins. Front Endocrinol (Lausanne). 4:9. 2013. link ↗
3. 3.Chan LF, Metherell LA, Clark AJL, et al. The majority of ACTH receptor (MC2R) mutations found in familial glucocorticoid deficiency type 1 lead to defective trafficking of the receptor to the cell surface. J Clin Endocrinol Metab. 94(9):3865–71. 2009. link ↗