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    • Newt GHS R a proteins expressed

    2022-08-13

    Newt GHS-R1a proteins expressed in a mammalian cell were able to bind homologous newt ghrelin, heterologous rat and bullfrog ghrelin, and a peptidyl GHS-R1a agonist, GHSRP-6, with inducing intracellular Ca signaling. This indicates that the identified cDNA encodes the ghrelin receptor from a view of functionality. It was suggested the possible existence of two lengths of the ghrelin receptors with long- and short-chain as two candidates of the initiation Met codon were found. The Kozak sequence supported that the long-chain receptor may be translated predominantly (Kozak, 1987, De Angioletti et al., 2004). In fact, this possibility needs to examine the existence of the actual protein using a specific antibody recognizing the specific region of each receptor, although we had not examined it in this study. Interestingly, however, ability to increase intracellular Ca in newt GHS-R1a-expressing HEK 293 Iberiotoxin sale was much stronger in short-chain receptor than in long-chain receptor. We do not know the reason, but a primary factor exists in the N-terminal extended 16-amino acids in the long-chain protein. The present result suggests that short-chain receptor may play a main role in the ghrelin receptor in this newt. Newt ghrelin receptors preferred to bind newt and rat ghrelin rather than bullfrog ghrelin, indicating ligand selectivity between Ser3-ghrelin and Thr3-ghrelin. This is a great difference to frog ghrelin receptors: bullfrog and tree frog GHS-R1a did not show such a ligand selectivity (Kaiya et al., 2011a). Our previous study had demonstrated that bullfrog ghrelin shows more potent GH- and PRL-releasing activities than rat ghrelin in bullfrog pituitary cells (Kaiya et al., 2001), suggesting possible presence of the ghrelin receptor that selectively binds Thr3-bullfrog ghrelin in the bullfrog pituitary. So we expected that Thr3-ghrelin (bullfrog ghrelin) could show much higher affinity than Ser3-ghrelin (rat ghrelin) in bullfrog GHS-R1a, and inversely Ser3-ghrelin could activate tree frog GHS-R1a more potently than Thr3-ghrelin because tree frog has Ser3-ghrelin (Kaiya et al., 2011b). However, Thr3-ghrelin and Ser3-ghrelin responded similarly in the identified frog ghrelin receptors. In this regard, the result of the present study was quite reasonable because newt receptors preferred to bind Ser3-ghrelin that newt and rat have. Interestingly, frog GHS-R1a mRNA did not detect in the pituitary, but in the hypothalamus (Kaiya et al., 2011a). Together with the actual ligand selectivity (Kaiya et al., 2001), we have speculated presence of another GHS-R1a. The fact that the newt receptor expressed in the pituitary suggests that it is the receptor showing a ligand selectivity. Newt GHS-R1a mRNA was mainly detected in the brain, pituitary, gastrointestinal tract, pancreas, testis, and fat body, and the distribution of mRNA was similar to that seen in frogs or other animals. Physiological events that the receptor involved have been unknown in the newt at present. The receptor in the brain and pituitary may be involved in regulation of feeding and pituitary hormone secretion (Gnanapavan et al., 2002, Geelissen et al., 2003, Tanaka et al., 2003, Chen and Cheng, 2004, Olszewski et al., 2008, Kaiya et al., 2009a, Kaiya et al., 2009b, Small et al., 2009, Briggs and Andrews, 2011). In the intestinal tract, ghrelin may regulate motility (Peeters, 2005, Kitazawa et al., 2007, Kitazawa et al., 2009). Suzuki and Yamamoto (2011) have reported the presence of ghrelin and GHS-R1a proteins in the pancreas of Xenopus laevis, and ghrelin may regulate islet activity in the pancreas (Dezaki, 2013). Local functions of ghrelin in the testis such as spermatogenesis, gonadotropin secretion and cell proliferation have been suggested in various animals including frogs (García et al., 2007, Tena-Sempere, 2008, Izzo et al., 2011). In newts, weak GHS-R1a mRNA expression was observed in the eye, heart, stomach, liver, gallbladder, kidney and dorsal skin. Sun et al. (2010) have reported that ghrelin contributes to positive inotropic effect on cardiomyocytes. Golestan Jahromi et al. (2010) and Moreno et al. (2010) suggested hepatoprotective effect of ghrelin. Ghrelin may participate in kidney functions that regulate water and mineral balance (Mori et al., 2000, Venables et al., 2011, Seki, 2013). Further study is needed to clarify physiological actions of ghrelin in newts.