Novel aspects of spexin functions in fish models

Abstract

Spexin (SPX), a novel peptide first identified using bioinformatics, is widely expressed at tissue level but its biological functions are still in the early phase of investigation. Based on limited studies in mammals, SPX has emerged as a neuropeptide with pleiotropic functions, and to date, little is known regarding the comparative aspects of SPX in lower vertebrates. In this study, using goldfish and grass carp as representatives for the Cyprinid family, the possible role of SPX in feeding control, reproduction and somatotropic actions in fish models was examined. In my recent studies, SPX was confirmed to be a satiety factor in goldfish and its expression could inhibit food intake via differential regulation of orexigenic and anorexigenic signals within the brain. In current study with goldfish, insulin induced by glucose uptake after feeding was found to up-regulate SPX expression in brain areas for appetite control as well as in the liver. These stimulatory effects were mediated by insulin receptor and to a lesser extent by IGF-I receptor via activation of p38 MAPK and PI3K/Akt cascades. At the pituitary level, insulin could also induce growth hormone (GH) secretion and lead to subsequent rises of IGF-I and IGF-II expression in the liver. At the hepatic level, insulin was shown to increase IGF-I with parallel inhibition on IGF-II expression via PI3K/Akt pathway, and local expression of IGF-I/-II was also effective in triggering SPX expression in goldfish liver. These findings indicate that insulin can serve as the functional link between food intake and SPX expression in the fish model via direct actions in the brain and liver as well as indirect actions mediated by GH and IGF-I/-II. In the goldfish pituitary, SPX immunoreactivity was detected in nerve fibers as well as in pituitary cells including gonadotrophs. In goldfish pituitary cells, SPX treatment was found to increase luteinizing hormone (LH) secretion without affecting its gene expression. This stimulatory response was additive to the LH-releasing effect of PACAP but not GnRH and could be suppressed by dopamine and sex steroids. Using a pharmacological approach, the LH-releasing effect of SPX was confirmed to be mediated by galanin-2 receptor (GalR2) coupled to PLC/PKC, Ca2+/CaMK-II, PI3K/Akt and MAPK cascades. These results, taken together, provide evidence that SPX can regulate reproductive functions in the fish model by stimulating LH release at the pituitary level. In parallel studies using grass carp as a model, carp SPX (single-copy gene) and GalR2 were cloned. At the tissue level, SPX was ubiquitously expressed whereas GalR2 exhibited a restricted pattern of tissue expression with highest level in the liver. In carp hepatocytes, SPX was found to up-regulate GH receptor (GHR) expression and this stimulatory effect was mediated by GalR2 coupled to cAMP/PKA, PLC/PKC, Ca2+/CaMK-II, PI3K/Akt and MAPK cascades. Similar post-receptor signaling pathways were also involved in SPX-induced GHR promoter activity expressed in HEK293 cells, implying that SPX-induced GHR expression may involve GHR gene transcription, and can probably lead to enhancement of hepatic sensitivity to GH induction and subsequent stimulation on body growth.