Corticotropin-releasing hormone-binding protein: biochemistry and function from fishes to mammals

Corticotropin-releasing hormone (CRH) plays multiple roles in vertebrate species. In mammals, it is the major hypothalamic releasing factor for pituitary adrenocorticotropin secretion, and is a neurotransmitter or neuromodulator at other sites in the central nervous system. In non-mammalian vertebrates, CRH not only acts as a neurotransmitter and hypophysiotropin, it also acts as a potent thyrotropin-releasing factor, allowing CRH to regulate both the adrenal and thyroid axes, especially in development. The recent discovery of a family of CRH-like peptides suggests that multiple CRH-like ligands may play important roles in these functions. The biological effects of CRH and the other CRH-like ligands are mediated and modulated not only by CRH receptors, but also via a highly conserved CRH-binding protein (CRH-BP). The CRH-BP has been identified not only in mammals, but also in non-mammalian vertebrates including fishes, amphibians, and birds, suggesting that it is a phylogenetically ancient protein with extensive structural and functional conservation. In this review, we discuss the biochemical properties of the characterized CRH-BPs and the functional roles of the CRH-BP. While much of the in vitro and in vivo data to date support an 'inhibitory' role for the CRH-BP in which it binds CRH and other CRH-like ligands and prevents the activation of CRH receptors, the possibility that the CRH-BP may also exhibit diverse extra- and intracellular roles in a cell-specific fashion and at specific times in development is also discussed.