Corticotropin-Releasing Factor Family: A Stress Hormone-Receptor System's Emerging Role in Mediating Sex-Specific Signaling

Abstract

No organ in the body is impervious to the effects of stress, and a coordinated response from all organs is essential to deal with stressors. A dysregulated stress response that fails to bring systems back to homeostasis leads to compromised function and ultimately a diseased state. The components of the corticotropin-releasing factor (CRF) family, an ancient and evolutionarily conserved stress hormone-receptor system, helps both initiate stress responses and bring systems back to homeostasis once the stressors are removed. The mammalian CRF family comprises of four known agonists, CRF and urocortins (UCN1-3), and two known G protein-coupled receptors (GPCRs), CRF₁ and CRF₂. Evolutionarily, precursors of CRF- and urocortin-like peptides and their receptors were involved in osmoregulation/diuretic functions, in addition to nutrient sensing. Both CRF and UCN1 peptide hormones as well as their receptors appeared after a duplication event nearly 400 million years ago. All four agonists and both CRF receptors show sex-specific changes in expression and/or function, and single nucleotide polymorphisms are associated with a plethora of human diseases. CRF receptors harbor N-terminal cleavable peptide sequences, conferring biased ligand properties. CRF receptors have the ability to heteromerize with each other as well as with other GPCRs. Taken together, CRF receptors and their agonists due to their versatile functional adaptability mediate nuanced responses and are uniquely positioned to orchestrate sex-specific signaling and function in several tissues.

Keywords: BNST; CRF; GPCR; cardiovascular; diabetes; gut; inflammatory bowel disease; pancreas; reproduction; sexually dimorphic; urocortins.

Figure 1

Origin of the corticotropin-releasing factor (CRF) family of peptide hormones: (A) Phylogenetic tree of showing evolution of prototype urotensin-I (UI), urocortins, and CRF with respect to geological times (in million years). The phylogenetic tree was generated using the TimeTree, which is a public knowledge base as described by Kumar et al. [4]. Prototype diuretic peptide hormones are present in the fruit fly, Drosophila melanogaster. Putative rounds of whole genome duplication events are depicted as 1R–4R. Earth impact events, changes in oxygen and carbon dioxide levels, as well as solar luminosity that might have influenced evolution are also shown. (B) A cluster dendogram of the CRF family of peptides was generated in Cobalt using the National Center for Biotechnology Information (NCBI) database. These data suggest that UCN3 and UCN2 evolved earlier than UCN1 and CRF, with UCN3 and UCN2 being more homologous to each other than to CRF and UCN1. UCN3 appears to be most conserved evolutionarily. (C) Cluster dendograms of CRF receptors and their orthologs. Boxed Inset: CRF receptors are more closely related to diuretic hormone 44 (DH44) receptors found in flies and arose from a common GPCR ancestral protein, seb-2, which is present in worms. CRF receptor orthologs: GIP: gastric inhibitory polypeptide receptor; GLP-1/2 R: glucagon-like peptide 1/2 receptors; PTH2: parathyroid hormone 2 receptor; PAC₁: pituitary adenylate cyclase-activating polypeptide type I receptors; VPAC_1/2; vasoactive intestinal polypeptide receptors 1 and 2.

Figure 1

Origin of the corticotropin-releasing factor (CRF) family of peptide hormones: (A) Phylogenetic tree of showing evolution of prototype urotensin-I (UI), urocortins, and CRF with respect to geological times (in million years). The phylogenetic tree was generated using the TimeTree, which is a public knowledge base as described by Kumar et al. [4]. Prototype diuretic peptide hormones are present in the fruit fly, Drosophila melanogaster. Putative rounds of whole genome duplication events are depicted as 1R–4R. Earth impact events, changes in oxygen and carbon dioxide levels, as well as solar luminosity that might have influenced evolution are also shown. (B) A cluster dendogram of the CRF family of peptides was generated in Cobalt using the National Center for Biotechnology Information (NCBI) database. These data suggest that UCN3 and UCN2 evolved earlier than UCN1 and CRF, with UCN3 and UCN2 being more homologous to each other than to CRF and UCN1. UCN3 appears to be most conserved evolutionarily. (C) Cluster dendograms of CRF receptors and their orthologs. Boxed Inset: CRF receptors are more closely related to diuretic hormone 44 (DH44) receptors found in flies and arose from a common GPCR ancestral protein, seb-2, which is present in worms. CRF receptor orthologs: GIP: gastric inhibitory polypeptide receptor; GLP-1/2 R: glucagon-like peptide 1/2 receptors; PTH2: parathyroid hormone 2 receptor; PAC₁: pituitary adenylate cyclase-activating polypeptide type I receptors; VPAC_1/2; vasoactive intestinal polypeptide receptors 1 and 2.

Figure 2

CRF family of peptide hormones and their two G protein-coupled receptors (GPCR) receptors, CRF₁ and CRF₁. Based on in vitro binding assays, CRF binds mainly to CRF₁, whereas urocortin 1 (UCN1) binds to both receptors, but with 10-fold higher affinity than other ligands. UCN2 and UCN3 bind exclusively to CRF₂. The activation of CRF₁ receptors by CRF in the hypothalamus initiates the hypothalamic–pituitary–adrenal (HPA) axis, ultimately resulting in the release of glucocorticoids (cortisol in primates and corticosterone in rodents) from the adrenal cortex. Activation of CRF₂ by urocortins is key for recovery from stress responses and returning to homeostasis.

Figure 3

A comparison of examples of primary amino acid sequence from each group of CRF and urocortin precursor peptides found in the Protein data bank and generated using the multiple alignment tool from NCBI Blast. Red and blue are alignment columns with no gaps, whereas grey residues are with gaps. Highly conserved amino acids are shown in red and less conserved amino acids are shown in blue based on the residues’ relative entropy thresholds.

Figure 4

A comparison of examples of primary amino acid sequence from CRF receptors and their closely related GPCRs, seb-2, and diuretic hormone 44 receptors found in the Protein data bank and generated using the multiple alignment tool from NCBI Blast. Red and blue are alignment columns with no gaps, whereas grey residue are aligned with gaps. Highly conserved amino acids are shown in red, and less conserved are shown in blue based on the residues’ relative entropy thresholds.

Figure 5

Electron micrographs showing representative pancreatic acinar cell sections from control and pancreatitis male and female mice. Endoplasmic reticulum (RER) shows whorling and the distortion of cisternae; mitochondrial (M) swelling is also evident. N: nucleus; SG: secretory granules; WT: wild-type; ZG: zymogen granules. Scale bar: 1 µM. Modified from Kubat et al. [93].

Figure 6

A scatterplot of the correlation between perceived stress score (PSS) and placental UCN1 levels. Analysis revealed that placental UCN1 levels were negatively correlated with PSS scores (r = 0.531, p = 0.07). Higher PSS scores indicate higher levels of stress experienced during the third trimester of pregnancy. Nulliparous women (n = 12) were recruited at the University of California San Francisco (UCSF) Mission Bay Hospital under UCSF’s institutional review board protocol # 16-10957. All women carried to term and completed PSS questionnaires between 37 and 40 weeks of gestation. Placentae were collected after delivery, and UCN1 levels were measured using ELISA.

Figure 7

A summary schema showing the evolution of CRF and urocortins hormones and their functions. Diuretic hormones (DH44) and DH44 receptors in the fruit flies probably served as prototypes for urocortin 3 and 2, and CRF receptors, respectively, which first appeared in Actinoptergyii (fish) and Sarcoptergyii after a genome-wide duplication event nearly 400 million years ago. Distinct CRF, urotensin I (UI), urocortins, and sauvagine peptides first seemed to have appeared nearly 500 million years ago. The innervation of hypothalamic cells by CRF family members occurred in class Actinoptergyii along with appearance of urocortin-like peptides. The release of CRF into the portal circulation appeared in Sarcoptergyii, whereas UI-like and sauvagine peptides transitioned to being expressed in the skin in the amphibians. The innervation and descending of CRF fibers in the spinal cord seemed to have occurred nearly 290 million years ago [2]. As the peptides evolved, the functions appeared to have diversified to include osmo and vasoregulation, as well as regulation of HPA axis-mediated stress responses. By virtue of having sex hormone response elements in the promoter regions of all four hormones and two receptors, sex-specific differences in the expression and function mediated by the CRF system are noted.