Insights into mechanisms of corticotropin-releasing hormone receptor signal transduction

Abstract

During evolution, mammals have developed remarkably similar molecular mechanisms to respond to external challenges and maintain survival. Critical regulators of these mechanisms are the family of 'stress'-peptides that consists of the corticotropin-releasing hormone (CRH) and urocortins (Ucns). These neuropeptides 'fine-tune' integration of an intricate series of physiological responses involving the autonomic, endocrine, immune, cardiovascular and reproductive systems, which induce a spectrum of behavioural and homeostatic changes. CRH and Ucns exert their actions by activating two types of CRH receptors (CRH-R), CRH-R1 and CRH-R2, which belong to the class-B1 family of GPCRs. The CRH-Rs exhibit signalling promiscuity facilitated by their ability to couple to multiple G-proteins and regulate diverse intracellular networks that involve intracellular effectors such as cAMP and an array of PKs in an agonist and tissue-specific manner, a property that allows them to exert unique roles in the integration of homeostatic mechanisms. We only now begin to unravel the plethora of CRH-R biological actions and the transcriptional and post-translational mechanisms such as alternative mRNA splicing or phosphorylation-mediated desensitization developed to tightly control CRH-Rs biological activity and regulate their physiological actions. This review summarizes the current understanding of CRH-R signalling complexity and regulatory mechanisms that underpin cellular responses to CRH and Ucns.

Figure 1

Amino acid sequence alignment of CRH and CRH-related peptides. CRH-R1 binds CRH as well as Ucn1, but not Ucn2 or Ucn3, whereas the CRH-R2 binds Ucns with significantly higher binding affinity than CRH. A proline residue at position 11 (coloured red), is found only in CRH-R2 selective ligands Ucn2 and Ucn3. CRH-R2 selective peptides also contain alanine residues at positions 35 and 39 (coloured red), while CRH-R non-selective peptides contain an invariant arginine at position 35 and an acidic amino acid at position 39 (coloured blue).

Figure 2

Amino acid regions, present in the N-terminus EC loops and TMD, important for different agonist (CRH and Ucn1) and antagonist (NBI) binding to CRH-R1. CRH-R1 ligand discrimination is achieved through a mechanism involving a single amino acid Glu-104, which acts as a selectivity filter preventing Ucn2/3 binding because the non-polar Ala-35 is incompatible with the negatively charged Glu-104. Although the CRH-R1 N-terminus is crucial for agonist binding, coupling of the receptor to G proteins requires an additional interaction the juxta-membrane domain of the receptor (the transmembrane helices and intervening loops, the J-domain) and the N-terminal segment of the peptide, which penetrates into the transmembrane segments of the receptor.

Figure 3

The central role of the cAMP pathway in amplifying and diversifying CRH-R signalling. In most tissues, both CRH-R1 and R2 receptor couple to and activate Gs-proteins and stimulate intracellular cAMP levels. Through activation of protein kinase A and EPACs, CRH-R agonists can activate a plethora of downstream intracellular effectors, such as GC, ERK1/2, NF-κB, ion channels, tyrosine hydroxylase phosphorylation and GSK-3β.

Figure 4

CRH-R regulation of ERK1/2 activation via the cAMP/PKA pathway. Examples of tissue/cell specific regulation of ERK1/2 activity by the CRH-Rs mediated via the cAMP/PKA pathway. In some tissues, CRH-activated PKA diminishes ERK1/2 phosphorylation and stimulation, whereas in others (right), ERK1/2 activation requires PKA.

Figure 5

Structural determinants important for intracellular mechanisms regulating CRH-Rs activity. (A) Amino acid residues within the ICL3 and C-terminus of CRH-R1, identified as important phospho-acceptor sites of GRKs and second messenger PKs (PKA and PKC) and are involved in the intracellular mechanism that leads to β-arrestin recruitment and receptor endocytosis. (B) Amino acid casette within the C-terminus of CRH-R2, involved in the intracellular mechanism controlling rate of receptor endocytosis.

Figure 6

An overview of the regulatory mechanisms allowing cells to modulate CRH-R signalling potency and direction. Upon receptor activation by its cognate agonists, cells modify their responsiveness (either through signal termination of generation of alternative signals) through activation of mechanisms that lead to receptor desensitization and decrease of receptor number in the plasma membrane. These mechanisms also potentially involve receptor variants that act as DN regulators of receptor expression or signalling.