Spexin2 Is a Novel Food Regulator in Gallus gallus

Abstract

Spexin2 (SPX2), a paralog of SPX1, is a newly identified gene in non-mammalian vertebrates. Limited studies in fish have evidenced its important role in food intake and energy balance modulation. However, little is known about its biological functions in birds. Using the chicken (c-) as a model, we cloned the full-length cDNA of SPX2 by using RACE-PCR. It is 1189 base pair (bp) in length and predicted to generate a protein of 75 amino acids that contains a 14 amino acids mature peptide. Tissue distribution analysis showed that cSPX2 transcripts were detected in a wide array of tissues, with abundant expression in the pituitary, testis, and adrenal gland. cSPX2 was also observed to be ubiquitously expressed in chicken brain regions, with the highest expression in the hypothalamus. Its expression was significantly upregulated in the hypothalamus after 24 or 36 h of food deprivation, and the feeding behavior of chicks was obviously suppressed after peripheral injection with cSPX2. Mechanistically, further studies evidenced that cSPX2 acts as a satiety factor via upregulating cocaine and amphetamine regulated transcript (CART) and downregulating agouti-related neuropeptide (AGRP) in hypothalamus. Using a pGL4-SRE-luciferase reporter system, cSPX2 was demonstrated to effectively activate a chicken galanin II type receptor (cGALR2), a cGALR2-like receptor (cGALR2L), and a galanin III type receptor (cGALR3), with the highest binding affinity for cGALR2L. Collectively, we firstly identified that cSPX2 serves as a novel appetite monitor in chicken. Our findings will help clarify the physiological functions of SPX2 in birds as well as its functional evolution in vertebrates.

Recent Research on Mechanisms of Feeding Regulation in Chicks

“…The mature peptide of SPX2 also contains 14 amino acids; however, its sequence differs from that of SPX1 at four positions[ 33 ]. Luciferase reporter assays have demonstrated that SPXs activate GAL receptors, GALR 2 and GALR3, but not GALR1 in humans, zebrafish[ 33 ], and chickens[ 35 , 36 ]. Interestingly, the potency of SPXs against GALR2 and GALR3 is similar to or greater than those of GAL in humans and zebrafish[ 33 ].…”

“…SPX mRNA is widely distributed in the central nervous system and peripheral tissues of chickens[ 34 , 35 , 36 ]. Fasting significantly increases the plasma SPX concentration[ 34 ] and hypothalamic mRNA levels of SPXs in chickens[ 35 , 36 ].…”

“…SPX mRNA is widely distributed in the central nervous system and peripheral tissues of chickens[ 34 , 35 , 36 ]. Fasting significantly increases the plasma SPX concentration[ 34 ] and hypothalamic mRNA levels of SPXs in chickens[ 35 , 36 ]. Generally, fasting upregulates orexigenic factors and downregulates anorexigenic factors; for example, the expressions of orexigenic neuropeptides such as neuropeptide Y (NPY) and agouti-related peptide (AgRP) are significantly increased by fasting in chicken hypothalamus[ 37 , 38 ], whereas fasting significantly decreases those of proopiomelanocortin (POMC, the precursor of α-melanocyte stimulating hormone (α-MSH), an anorexigenic peptide) in the hypothalamus and gut hormone such as cholecystokinin (CCK) and peptide YY in chicken gastrointestinal tract[ 37 , 38 , 39 , 40 ].…”

“…Generally, fasting upregulates orexigenic factors and downregulates anorexigenic factors; for example, the expressions of orexigenic neuropeptides such as neuropeptide Y (NPY) and agouti-related peptide (AgRP) are significantly increased by fasting in chicken hypothalamus[ 37 , 38 ], whereas fasting significantly decreases those of proopiomelanocortin (POMC, the precursor of α-melanocyte stimulating hormone (α-MSH), an anorexigenic peptide) in the hypothalamus and gut hormone such as cholecystokinin (CCK) and peptide YY in chicken gastrointestinal tract[ 37 , 38 , 39 , 40 ]. However, intracerebroventricular (ICV) and intravenous injections of SPXs significantly decrease food intake in a dose-dependent manner in chicks[ 35 , 36 , 41 ], in contrast to GAL, an orexigenic peptide in vertebrates[ 24 ]. The decrease following ICV injection of SPX is attenuated by the co-injection of GALR3 antagonists, but not GALR2 antagonists, suggesting that SPX inhibits food intake via GALR3 in chicks[ 41 ].…”

“…The decrease following ICV injection of SPX is attenuated by the co-injection of GALR3 antagonists, but not GALR2 antagonists, suggesting that SPX inhibits food intake via GALR3 in chicks[ 41 ]. Furthermore, SPXs control food intake via the gene expression of appetite-regulating neuropeptides in the hypothalamus of chicks[ 35 , 36 ]. Intravenous injection of SPXs increases the mRNA levels of cocaine- and amphetamine-regulated transcript (CART, an anorexigenic neuropeptide) in the hypothalamus of chicks, whereas those of AgRP decrease after injection, although no significant change is observed in the levels of NPY and POMC[ 35 , 36 ].…”