Spexin (SPX) is a 14 amino acid peptide hormone that has pleiotropic functions across vertebrates, one of which is involvement in the brain-pituitary-gonad axis of fish. SPX(1) has been identified in each class of vertebrates, and a second SPX (named SPX2) has been found in some non-mammalian species. We have cloned two spexin paralogs, designated as Spx1a and Spx1b, from Nile tilapia (Oreochromis niloticus) that have varying tissue distribution patterns. Spx1b is a novel peptide only identified in cichlid fish, and is more closely related to Spx1 than Spx2 homologs as supported by phylogenetic, synteny, and functional analyses. Kisspeptin, Spx, and galanin (Gal) peptides and their corresponding kiss receptors and Gal receptors (Galrs), respectively, are evolutionarily related. Cloning of six tilapia Galrs (Galr1a, Galr1b, Galr2a, Galr2b, Galr type 1, and Galr type 2) and subsequent in vitro second-messenger reporter assays for Gα s , Gα q , and Gα i suggests that Gal and Spx activate Galr1a/Galr2a and Galr2b, respectively. A decrease in plasma follicle stimulating hormone and luteinizing hormone concentrations was observed with injections of Spx1a or Spx1b in vivo. Additionally, application of Spx1a and Spx1b to pituitary slices decreased the firing rate of LH cells, suggesting that the peptides can act directly at the level of the pituitary. These data collectively suggest an inhibitory mechanism of action against the secretion of gonadotropins for a traditional and a novel spexin paralog in cichlid species.
21Spexin (SPX) is a 14 amino acid peptide hormone that has pleiotropic functions 22 across vertebrates, one of which is involvement in the brain-pituitary-gonad axis of fish. 23 SPX(1) has been identified in each class of vertebrates, and a second SPX (named SPX2) 24 has been found in some non-mammalian species. We have cloned two spexin paralogs, 25 designated as Spx1a and Spx1b, from Nile tilapia (Oreochromis niloticus) that have 26 varying tissue distribution patterns. Spx1b is a novel peptide only identified in cichlid 27 fish, and is more closely related to Spx1 than Spx2 homologs as supported by 28 phylogenetic, synteny, and functional analyses. Kisspeptin, Spx, and galanin (Gal) 29 peptides and their corresponding kiss receptors and Gal receptors (Galrs), respectively, 30 are evolutionarily related. Cloning of six tilapia Galrs (Galr1a, Galr1b, Galr2a, Galr2b, 31 Galr type 1, and Galr type 2) and subsequent in vitro second-messenger reporter assays 32 for Gαs, Gαq, and Gαi suggests that Gal and Spx activate Galr1a/Galr2a and Galr2b, 33 respectively. A decrease in plasma follicle stimulating hormone and luteinizing hormone 34 concentrations was observed with injections of Spx1a or Spx1b in vivo. Additionally, 35 application of Spx1a to pituitary slices decreased the firing rate of LH cells, suggesting 36 direct inhibition at the pituitary level. These data collectively suggest an inhibitory 37 mechanism of action against the secretion of gonadotropins for a traditional and a novel 38 spexin paralog in cichlid species. 39 40 42 methods [1], and then also by chemical methods [2] in humans. These computational 43 methods have been attempted mostly on the basis of the characteristics of the 44 prohormones from which active neuropeptides are processed. The mature peptide 45 sequence contains 14 amino acids that are flanked by monobasic and dibasic proteolytic 46 cleavage sites. The mature spexin peptide was found to be identical in all tetrapods and 47 elephant shark, and differs in only one amino acid (A 13 T) in piscine species. A paralog of 48 spexin, termed Spx2, was later identified in non-mammalian species [3]. In mammals, 49 Spx was found to participate in inducing stomach contraction [1], inhibiting 50 adrenocortical cell proliferation[4], postnatal hypoxia response [5], cardiovascular and 51 renal modulation[6], nociceptive response [7], fatty acid absorption and weight regulation 52 [8], and diabetes [9]. In teleosts, functional studies of Spx1 mainly focused on its 53 inhibitory role in the regulation of reproduction [10] and food intake [2] [11]. However, a 54 recent study reported that spx1 knock-out zebrafish exhibited normal reproductive 55 capability but higher food intake than wild type fish, an effect mediated via increased 56 expression of the appetite stimulant, agouti-related peptide AgRP1 [12].57The galanergic neurotransmission system is one of the newest described signaling 58 systems. Today, the galanin family consists of galanin (Gal), galanin-like peptide (GalP), 59...
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