Structural and Functional Insights into CRF Peptides and Their Receptors

Abstract

Corticotropin-releasing factor or hormone (CRF or CRH) and the urocortins regulate a plethora of physiological functions and are involved in many pathophysiological processes. CRF and urocortins belong to the family of CRF peptides (CRF family), which includes sauvagine, urotensin, and many synthetic peptide and non-peptide CRF analogs. Several of the CRF analogs have shown considerable therapeutic potential in the treatment of various diseases. The CRF peptide family act by interacting with two types of plasma membrane proteins, type 1 (CRF₁R) and type 2 (CRF₂R), which belong to subfamily B1 of the family B G-protein-coupled receptors (GPCRs). This work describes the structure of CRF peptides and their receptors and the activation mechanism of the latter, which is compared with that of other GPCRs. It also discusses recent structural information that rationalizes the selective binding of various ligands to the two CRF receptor types and the activation of receptors by different agonists.

Keywords: CRF-peptides; CRF-receptors; activation; agonists; antagonists; binding; structure.

Figure 1

Model of CRF₁R receptor (blue) in complex with the CRF (orange) and the three subunits of G-proteins, namely the G_αs protein (green), G_β protein (brown), and G_γ protein (red). This generalized model of receptor has been created based on the crystal structure of the J-Domain of CRF₁R in complex with CRF peptide and G-proteins (PDB: 6P9X) and the crystal structure of the ECD domain of CRF₁R in complex with the CRF (PDB: 3EHU). The place between the two dotted lines is the lipid bilayer of the plasma membrane.

Figure 2

Structures of CRF analog peptides Astressin-2B (PDB: 2RM9), Astressin-B (PDB: 2RMD), Stressin1-A (PDB: 2RME), UcnI (PDB: 2RMF), UcnII (PDB: 2RMG) and UcnIII (PDB: 2RMH). The C-segment of the peptides is depicted in blue color, whereas the I-segment is depicted in green color and the N-segment is depicted in orange color [68].

Figure 3

Model of CRF₁R receptor (green) in complex with UcnI-EK (cyan). Salt bridges between peptide amino acids are shown in pink sticks.

Figure 4

Structures of reported small molecule antagonists of CRF₁R [84,85,86,87].

Figure 5

Crystal structure of the ECD (cyan) of human CRF₁R (PDB: 3EHU) in complex with CRF (orange). Among the interactions between ECD and peptide, functionally important ones are the H-bonds (yellow sticks) between the main chain atoms of Val97, and the oxygen and nitrogen atoms of the C-terminal amide group (of residue 41) of CRF.

Figure 6

Allosteric binding of CP-376395 (C₂₁H₃₀N₂O) to CRF₁R (PDB: 4K5Y). Receptor residues are denoted by superscripts indicating their positions in receptor transmembrane domains (TMs). In subfamily B1 GPCRs, the most conserved residue in each TM is labeled as position index .50, preceded by the TM number (TM1-TM7). For example, Phe284^5.51b denotes Phe284 located in TM5, one residue after the most conserved residue, Asn283^5.50b.

Figure 7

Molecular interactions of crucial CRF amino acids (orange) with CRF₁R (light blue) (PDB: 6P9X). CRF forms an inserted loop consisting of residues Ser^7P, Leu^8P, and Asp^9P, to accommodate accordingly and form selective polar and hydrophobic interactions with amino acids of mainly TMs 3, 5, and 6. Polar interactions are depicted in yellow dashes.

Figure 8

(A) Cryo-EM structure of UcnI in complex with TMD of CRF₁R (PDB: 6PB0). (B) Cryo-EM structure of UcnI in complex with TMD of CRF₂R (PDB: 6PB1). (C) Molecular interactions of crucial UcnI amino acids (magenta) with CRF₁R intracellular residues (wheat). (D) Molecular interactions of UcnI amino acids (pink) with CRF₂R (green). Polar interactions are shown in yellow dashes.

Figure 9

Molecular interactions of G_as protein (green) with CRF₁R (cyan) (PDB: 6P9X). CRF (orange) and the three subunits of G-proteins, G_β protein (brown), and G_γ protein (red) are also shown as part of the general complex in the left side.