Current Molecular Pharmacology - Volume 10, Issue 4, 2017

The Corticotropin-releasing Hormone (CRH) mammalian family members include CRH, urocortin I, Stresscopin (SCP) and Stresscopin-related peptide (SRP), along with the CRH receptors type 1 (CRHR1) and type 2 (CRHR2), and CRH-binding protein (CRH-BP). These family members differ in their tissue distribution and pharmacology. Several studies have provided evidence supporting an important role of this family in the regulation of the neuroendocrine and behavioral responses to stress. Regulation of the relative contribution of CRH and its homologs and the two CRH receptors in brain CRH pathways may be essential in coordinating physiologic responses to stress. The development of disorders related to heightened stress sensitivity and dysregulation of stress-coping mechanisms appears to involve regulatory mechanisms of the CRH family members. Therapeutic agents that target CRH family members may offer a new approach to the treatment of these disorders. The purpose of this review is to summarize the most significant discoveries related to CRH over time.

It is noteworthy that thirty three years of efforts in corticotropin releasing factor (CRF) research by academia and the pharmaceutical industry resulted in several thousand papers and patents, yet little progress has been made to identify and market diagnostic or therapeutic CRF peptides and small molecule ligands. We document the potential relevance of CRF peptide antagonists to reinvigorate stress/anxiety affected “anatomy systems” in order to boost their efficacy.

Corticotropin-releasing factor (CRF) can be considered a very important hormone or a chemical mediator. It works closely with other systems to regulate the manner through which the body may respond to stress. Thus it affects many biological processes associated with stress. Dysfunction of this system has also been correlated with various diseases such as major depression, anxiety, drug addiction and eating disorders. Rationally, this means that interfering with binding of CRF to its intended receptors can be an attractive target for drug design aiming at developing new medications for many ailments that are associated with stress such as depression, anxiety and stress-induced relapse in drug addiction. Hundreds of accounts of small molecule antagonists have appeared in the literature and the preclinical and clinical pharmacology have been reported for many of these agents. Several classes of small molecule CRF receptor antagonists which belong to the non-peptide class have been developed with many being widely used for research purposes. Currently several major pharmaceutical companies are pursuing development of small non-peptide CRF1 receptor antagonists. In this review article we explain the importance of development of non-peptide CRF antagonists and their clinical relevance with emphasis on those members that showed great promise or those that were advanced to clinical trials.

G-Protein coupled receptors (GPCRs) have been, and remain a key target of drug discovery programs for human disease. While many drugs have been developed that interact with these proteins in the simple classic manner - that is - physically blocking the cognate ligand from simply binding to its target receptor, drug discovery approaches have elucidated alternative more complex methods by which small molecules can interact with these receptors and block their function. This is most evident in the Class B GPCRs where the cognate ligands are relatively large peptides with multiple points of contact on the GPCR spanning both hydrophilic and hydrophobic domains on the same protein to elicit function. It has therefore been difficult to precisely determine not only the mechanism by which a small molecule can inhibit the function of a large peptide but also the nature of that mechanism that drives the differences in efficacy. This review will examine in detail the nature of small molecule inhibition of corticotropin-releasing factor receptors and illustrate the role that allosteric binding and kinetics play in the functional inhibition of this Class B GPCR.

To maintain homeostatic equilibrium, living organisms have evolved complex adaptation systems that control an array of behavioural, autonomic, neuroendocrine and immune responses. One of the important switches of this system is the hypothalamic hormone corticotropin-releasing hormone (CRH), which together with a family of related peptides (urocortins, UCNs) orchestrate stress-coping responses that reinstate homeostasis. Persistent disturbances in the homeostatic equilibrium either due to inadequate or persistently uncontrolled responses have been associated with pathogenic mechanisms of disease. CRH and UCNs exert their actions by activating two receptors of the Class B1 GPCRs, CRH-R1 and CRH-R2. Their signalling versatility allows activation of multiple and diverse signalling pathways characterized by ‘cell-specific agonist-dependent signalling’ responses. Alternative mRNA splicing, interactions with intracellular protein partners and mechanisms that allow selective regulation of signalling potency and termination, provide additional levels of regulation to fine-tune cellular responses. Although understanding of CRH-R signalling is still incomplete, recent important advances in decoding CRH-R structure and signalling properties uncovered key important functions and roles in physiology and pathobiology.

The corticotropin releasing factor (CRF) receptors belong to the large family of G proteincoupled receptors (GPCRs) and must be transported to the plasma membrane to function properly. The first step of the intracellular transport of GPCRs is their insertion into the membrane of the endoplasmic reticulum (ER). This process is mediated by the translocon complex of the ER membrane and the signal sequences of the receptors. Most GPCRs possess signal sequences which form part of the mature proteins, the so called signal anchor sequences (usually transmembrane domain 1). The CRF receptors possess instead signal sequences at their extreme N tails which were thought to be cleaved off following integration of the receptors into the ER membrane (signal peptides, SPs, also called cleaved signal sequences). Recent work, however, showed that not all subtypes of CRF receptors stick to this rule. Whereas the corticotropin-releasing factor receptor type 1 (CRF1R) and the corticotropin-releasing factor receptor type 2b (CRF2(b)R) possess conventional SPs which are indeed cleaved off following ER insertion, the SP of the cortictropin-releasing factor receptor type 2a (CRF2(a)R) remains uncleaved. It forms a unique N-terminal domain (pseudo signal peptide, PSP) which has surprising functions beyond the ER level. Its presence not only influences expression levels at the plasma membrane but also receptor homodimerisation and, as a consequence, G protein selectivity. In this mini-review, we summarize the progress in understanding the functions of SPs of CRF receptors. Recent data also allow deriving hypotheses for a physiological significance of these sequences.

Corticotropin releasing factor (CRF) receptors belong to the secretin family of G proteincoupled receptors (GPCRs) and are responsible for initiating endocrine stress responses and mediating anxiety related behaviors upon activation via stressors. The main binding site for the CRF ligands is the first extracellular domain (ECD) of the receptors. Several structures of ligand-free and ligand-bound ECDs were recently determined either by nuclear magnetic resonance (NMR) spectroscopy or X-ray crystallography. They reveal how the ligands bind through both hydrophobic and hydrophilic interactions to the ECDs, which is highly dynamic in the absence of ligands. It is the purpose of this review to discuss these structures with a particular focus on ligand-receptor interaction.

The corticotropin-releasing factor type 1 and 2 receptors (CRF1R and CRF2R) belong to the secretin-like family, also known as class B1, of G protein-coupled receptors (GPCRs). Several endogenous hormones mediate their responses through the CRF receptors, such as CRF and the urocortins. The structures for the N-terminus extracellular domain of both CRF1R and CRF2R in complex with peptidic ligands were released a few years ago and permitted the study of hormone binding to the orthosteric binding site. Until the crystal structure of the transmembrane domain of human CRF1R in its inactive state bound to an allosteric antagonist became available. Together with the crystal structures of the transmembrane domain of the glucagon receptor (GCGR), they have enabled the structural alignment between the rhodopsin and secretin-like families, which permits the direct comparison of the functional domains in both classes. In this report, we review the current structural landscape, in addition to the knowledge regarding activation of both CRF receptors and the generalization to secretin-like GPCRs in general. Thus, significant effort was put in trying to identify possible analogous microswitches in the class B1, with the hypothesis that both families could maintain a similar arrangement of their functional domain.

The structural analysis of class B G protein-coupled receptors (GPCR), cell surface proteins responding to peptide hormones, has until recently been restricted to the extracellular domain (ECD). Corticotropin-releasing factor receptor type 1 (CRF1R) is a class B receptor mediating stress response and also considered a drug target for depression and anxiety. Here we report the crystal structure of the transmembrane domain of human CRF1R in complex with the small-molecule antagonist CP-376395 in a hexagonal setting with translational non-crystallographic symmetry. Molecular dynamics and metadynamics simulations on this novel structure and the existing TMD structure for CRF1R provides insight as to how the small molecule ligand gains access to the induced-fit allosteric binding site with implications for the observed selectivity against CRF2R. Furthermore, molecular dynamics simulations performed using a full-length receptor model point to key interactions between the ECD and extracellular loop 3 of the TMD providing insight into the full inactive state of multidomain class B GPCRs.