Functional characterization of growth hormone releasing hormone and its receptor in amphioxus with implication for origin of hypothalamic-pituitary axis

Growth hormone-releasing hormone (GHRH) has been widely shown to stimulate growth hormone (GH) production via binding to GHRH receptor GHRHR in various species of vertebrates, but information regarding the functional roles of GHRH and GHRHR in the protochordate amphioxus remains rather scarce. We showed here that two mature peptides, BjGHRH-1 and BjGHRH-2, encoded by BjGHRH precursor, and a single BjGHRHR protein were identified in the amphioxus Branchiostoma. japonicum. Like the distribution profiles of vertebrate GHRHs and GHRHRs, both the genes Bjghrh and Bjghrhr were widely expressed in the different tissues of amphioxus, including in the cerebral vesicle, Hatschek’s pit, neural tube, gill, hepatic caecum, notochord, testis and ovary. Moreover, both BjGHRH-1 and BjGHRH-2 interacted with BjGHRHR, and triggered the cAMP/PKA signal pathway in a dose-dependent manner. Importantly, BjGHRH-1 and BjGHRH-2 were both able to activate the expression of GH-like gene in the cells of Hatschek’s pit. These indicate that a functional vertebrate-like GHRH-GHRHR axis had already emerged in amphioxus, which is a seminal innovation making physiological divergence including reproduction, growth, metabolism, stress and osmoregulation possible during the early evolution of vertebrates.