Identification and characterization of a novel receptor for chicken GHRH and the potential receptor(s) for chicken GHRHlike peptide (GHRH-L) and peptide histidine isoleucine (PHI)

Abstract

GHRH and its structurally related peptides, GHRH-like peptide (GHRH-L, also named PRP), PHI, VIP, and PACAP, are demonstrated to play important roles in the pituitary and extra-pituitary tissues of vertebrates; however, the molecular basis of their actions in various tissues of birds remains largely unknown. In this study, 6 highly homologous receptors for GHRH-L(GHRH-LR), PACAP(PAC1), GHRH(GHRHR1 and GHRHR2), VIP and PHI (VPAC1 and VPAC2), respectively, were cloned from chicken brain (or pituitary) and their functionalities were examined in CHO cells using the pGL3-CRE luciferase reporter system. The results showed that: 1) all receptors are coupled to the cAMP-PKA pathway; 2) 2 GHRH receptors, named GHRHR1 and GHRHR2, respectively, are co-existed in the chicken genome and both of them could be activated by chicken GHRH potently and specifically; 3) Interestingly, 2 VIP receptors, namely VPAC1 and VPAC2, could be potently activated not only by chicken VIP and ovine PACAP38, but also by turkey PHI, suggesting that 2 VIP receptors may also function as the potential receptors specific for PHI in birds; and 4) chicken GHRH-L can activate its receptor, GHRH-LR; however, it also activates the 2 chicken GHRH receptors. Using RT-PCR assay, we further examined the expressions of these 6 receptors in adult chicken tissues. Interestingly, the expressions of GHRH-LR, GHRHR1 and GHRHR2 are mainly restricted to the pituitary and/or different brain regions, while PAC1, VPAC1, and VPAC2 are expressed virtually in all tissues examined. Together, our study not only identified a novel receptor for GHRH and the potential receptor(s) for GHRH-L and PHI in chickens, but also establishes a molecular basis to elucidate the physiological roles of these structurally and functionally related peptides in different tissues, including their actions in the pituitary of chicken.