Evolution of Structural and Functional Diversity of Spexin in Mammalian and Non-mammalian Vertebrate Species

Advances in research on spexin-mediated regulation of reproductive function in vertebrates

Spexin (SPX, NPQ) is a 14-amino acid neuroactive peptide identified using bioinformatics. This amino acid sequence of the mature spexin peptide has been highly conserved during species evolution and is widely distributed in the central nervous system and peripheral tissues and organs. Therefore, spexin may play a role in various biological functions. Spexin, the cognate ligand for GALR2/3, acting as a neuromodulator or endocrine signaling factor, can inhibit reproductive performance. However, controversies and gaps in knowledge persist regarding spexin-mediated regulation of animal reproductive functions. This review focuses on the hypothalamic-pituitary-gonadal axis and provides a comprehensive overview of the impact of spexin on reproduction. Through this review, we aim to enhance understanding and obtain in-depth insights into the regulation of reproduction by spexin peptides, thereby providing a scientific basis for future investigations into the molecular mechanisms underlying the influence of spexin on reproductive function. Such investigations hold potential benefits for optimizing farming practices in livestock, poultry, and fish industries.

The effect of spexin injection and its interaction with nitric oxide, serotonin, and corticotropin receptors on the central regulation of food intake in broilers

Complex homeostatic control mechanisms are tools to adjust the food birds eat and their appetite. Birds and mammals differ in several ways considering food intake regulation. Therefore, this study aimed to investigate the special effects of the intracerebroventricular (ICV) injection of spexin and its interaction with nitric oxide, serotonin and corticotropin receptors on central food intake regulation in broilers. In the test 1, Broilers received ICV injection of saline, PCPA (p-chlorophenylalanine,1.25 µg), spexin (10 nmol) and PCPA+spexin. In test 2-7, 8-OH-DPAT, SB-242084 (5-HT2C, 1.5 µg), L-arginine (Precursor of nitric oxide, 200 nmol), L-NAME (nitric oxide synthetize inhibitor, 100 nmol), Astressin-B (30 µg) and Astressin2-B (30 µg) were injected to Broilers instead of the PCPA. Then, the amount of food received was measured up to 2 h after the injection. The food consumption was significantly decreased by Spexin (10 nmol) (P<0.05). Concomitant injection of SB-242084+spexin attenuated spexin-induced hypophagia (P<0.05). Co-injection of L-arginine+spexin enhanced spexin-induced hypophagia and this effect was reversed by L-NAME (P<0.05). Also, concomitant injection of Astressin-B + spexin or Astressin2-B + spexin enhanced spexin-induced hypophagia (P<0.05). Founded on these observations, spexin-induced hypophagia may be mediated by nitric oxide and 5-HT2C, CRF1, and CRF2 receptors in neonatal broilers.

Serum Spexin Level Is Negatively Associated With Peripheral Neuropathy and Sensory Pain in Type 2 Diabetes

Background: Spexin is a novel peptide hormone and has shown antinociceptive effects in experimental mice. This study is aimed at evaluating the association of serum spexin level with diabetic peripheral neuropathy (DPN) and related pain in a Chinese population. Methods: We enrolled 167 type 2 diabetes mellitus (T2DM) including 56 patients without DPN (non-DPN), 67 painless DPN, and 44 painful DPN. Serum spexin was measured using ELISA. Logistic regression models were performed to analyze the independent effects of spexin on prevalence of DPN and painful DPN. In streptozotocin (STZ)-induced diabetic mice, mechanical pain threshold was measured using electronic von Frey aesthesiometer. Human peripheral blood mononuclear cells (PBMCs) were isolated and further stimulated with lipopolysaccharide without or with spexin. The gene expression was assayed by qPCR. Results: Compared with non-DPN, serum spexin level decreased in painless DPN and further decreased in painful DPN. The odds of DPN was associated with low spexin level in T2DM, which was similar by age, sex, BMI, and diabetes duration, but attenuated in smokers. The odds of having pain was associated with decreased spexin level in DPN, which was similar by age, sex, smoking status, and diabetes duration, but attenuated in normal weight. Furthermore, we observed that mechanical pain threshold increased in spexin-treated diabetic mice. We also found that lipopolysaccharide treatment increased the mRNA level of TNF-α, IL-6, and MCP-1 in human PBMCs, while spexin treatment prevented this increase. Conclusions: These results suggested that spexin might serve as a protective factor for diabetes against neuropathology and pain-related pathogenesis.

Spexin acts as a novel glucose-lowering factor in grass carp (Ctenopharyngodon idella)

At present, the physiological roles of various hormones in fish glucose metabolism have been elucidated. Spexin, a 14-amino acids polypeptide, is highly conserved in many species and has functions such as reducing body weight and improving insulin resistance. In this paper, the open reading frame (ORF) of spx21 in grass carp (Ctenopharyngodon idella) was cloned, and the tissue distribution of spx1 and spx2, their direct and indirect regulatory effects on glucose metabolism of grass carp were investigated. The ORF of spx2 gene in grass carp was 279 bp in length. Moreover, spx1 was highly expressed in the adipose tissue, while spx2 was highly expressed in the brain. In vitro, SPX1 and SPX2 showed opposite effects on the glycolytic pathway in the primary hepatocytes. In vivo, intraperitoneal injection of SPX1 and SPX2 significantly reduced serum glucose levels and increased hepatopancreas glycogen contents. Meanwhile, SPX1 and SPX2 promoted the expression of key genes of glycolysis (pk) and glycogen synthesis (gys) in the hepatopancreas at 3 h post injection. As for indirect effects, 1000 nM SPX1 and SPX2 significantly increased insulin-mediated liver type phosphofructokinase (pfkla) mRNA expression and enhanced the inhibitory effects of insulin on glucose-6-phosphatase (g6pase), phosphoenolpyruvate carboxykinase (pepck), glycogen phosphorylase L (pygl) mRNA expression. Our results show that SPX1 and SPX2 have similar indirect effects on the regulation of glucose metabolism that enhance insulin activity, but they exhibit opposite roles in terms of direct effects.

Evolution of GCGR family ligand-receptor extensive cross-interaction systems suggests a therapeutic direction for hyperglycemia in mammals

Glucose is essential to the physiological processes of vertebrates. Mammalian physiological stability requires a relatively stable blood glucose level (~5 mM), whereas other vertebrates have greater flexibility in regulating blood glucose (0.5-25 mM). GCGR family receptors play an important role in vertebrate glucose regulation. Here, we examine the evolution of the GCGR family ligand-receptor systems in different species. Comparatively, we discover that the conserved sequences among GCG family ligands lead to the non-specific activation of ligands across species. In particular, we observe that glucagon-like peptide 1 receptor (GLP1R), glucagon-like peptide 2 receptor (GLP2R), and glucagon-like receptor (GCGLR, also called GCRPR) are arbitrarily activated by other members of the ligand family in birds. Moreover, we reveal that Gallus gallus GLP2 (gGLP2) effectively activates mammalian GLP1R and improves glucose tolerance in diabetic mice. Our study has important implications for understanding blood glucose stabilization in vertebrates and demonstrates that gGLP2 may be a potential drug for treating type 2 diabetes.

Immunoexpression of Spexin in Selected Segments of the Bovine (Bos taurus taurus) Gastrointestinal Tract

In the expansive domain of neuropeptide investigation, spexin (SPX) has emerged as a captivating subject, exerting a significant impact on diverse physiological processes. Initially identified in mice, SPX's distribution transcends various organs, suggesting its potential regulatory roles. Despite extensive research in smaller species, a notable gap exists in our comprehension of SPX in larger mammals, particularly ruminants. Our study meticulously explores the immunolocalization of SPX within the gastrointestinal organs of bovines, with a specific focus on the abomasum, jejunum, and colon. Tissue samples from Holstein-Friesian cattle underwent careful processing, and gene mRNA expression levels, particularly GALR2 and SPX, were assessed. Intriguingly, our findings revealed that GALR2 expression was highest in the jejunum, signifying a potentially critical role in this digestive segment. Immunohistochemistry further unveiled distinct patterns of SPX immunoreactivity in each examined region-abomasum, jejunum, and colon-highlighting nuanced, region-specific responses. Notably, the abomasum and jejunum predominantly exhibited positive immunoreactivity in the submucosal plexus, while the colon, in contrast, demonstrated a higher degree of immunoreactivity in myenteric plexus neurons. Our investigation, grounded in the hypothesis of ubiquitous SPX distribution in ruminants, delves deeper into the intricate role of SPX within the enteric nervous system. This study meticulously explores the spatial distribution of SPX within the myenteric and submucosal plexuses, integral components of the enteric nervous system. These findings significantly enhance our understanding of SPX's potential roles in gastrointestinal regulation in bovines, providing a unique perspective on larger mammals and enriching our comprehension of this intriguing neuropeptide's significance in various physiological processes.

Exploring the Potential Link between Acute Central Serous Chorioretinopathy and Trimethylamine N-Oxide, Phoenixin, Spexin, and Alarin Molecules

PURPOSE

Acute central serous chorioretinopathy (ACSCR) is a condition characterized by decreased visual acuity, macular thickening, and edema under the retinal layer. Although the underlying mechanisms of the disease are not fully understood, oxidative stress is considered to be a critical risk factor. The aim of this study was to shed light on the pathophysiology of ACSCR by investigating the levels of circulating trimethylamine N-oxide (TMAO), phoenixin (PNX), alarin (ALA), and spexin (SPX) molecules in ACSCR patients.

METHODS

The study included 30 ACSCR patients and 30 healthy individuals as controls. ACSCR was diagnosed using optical coherence tomography (OCT) imaging. Five mL blood samples were collected from all participants following overnight fasting. The levels of TMAO, PNX, ALA, and SPX in the blood samples were measured using the ELISA method.

RESULTS

Visual acuity was found to be significantly reduced in ACSCR patients compared to the control group (<0.05), while macular thickness was increased (<0.05). Furthermore, TMAO, PNX, and ALA levels were significantly higher in ACSCR patients (<0.05), while SPX levels were significantly lower compared to the control group (<0.05). In ACSCR patients, there was a positive correlation between macular thickness and TMAO, PNX, and ALA; there was, however, a negative correlation with SPX. Additionally, visual acuity was negatively correlated with TMAO, PNX, and ALA, while SPX levels decreased as visual acuity decreased.

CONCLUSIONS

These results demonstrate a correlation between the TMAO, PNX, ALA, and SPX levels of ACSCR patients and their visual acuity and macular thickness. Given the role of these molecules in ACSCR's pathophysiology, they hold promise as potential diagnostic, therapeutic, and follow-up markers in the future.

Spexin in the European sea bass, Dicentrarchus labrax: Characterization, brain distribution, and interaction with Gnrh and Gnih neurons

Spexin (Spx) is a recently characterized neuropeptide implicated in multiple physiological processes in vertebrates, including reproduction, food intake, and regulation of anxiety and stress. Two orthologs (Spx1 and Spx2) are present in some nonmammalian vertebrates, including teleosts. However, information on the distribution of Spx in the brain and its interactions with other neuroendocrine systems in fish is still scarce. In this work, we cloned and sequenced the sea bass (Dicentrarchus labrax) Spx1, which included a 27 aa signal peptide and a mature peptide of 14 aa that is C-terminal amidated. spx1 transcripts were higher in the diencephalon/caudal preoptic area/hypothalamus and medulla but were also detected in the olfactory bulbs, telencephalon/rostral preoptic area, optic tectum/tegmentum, cerebellum/pons, and pituitary. The immunohistochemical study revealed Spx1-immunoreactive (ir) cells in different nuclei of the preoptic area, habenula, prethalamus, mesencephalic tegmentum and in the proximal pars distalis (PPD) and pars intermedia of the pituitary. Spx1-ir fibers were widely distributed throughout the brain being particularly abundant in the midbrain and hindbrain, in close contact with tegmental gonadotropin-releasing hormone 2 (Gnrh2) cells and isthmic gonadotropin-inhibitory hormone (Gnih) cells of the secondary gustatory nucleus. Moreover, Gnih fibers were observed innervating Spx1-ir cells lying in several subdivisions of the magnocellular preoptic nucleus and in the lateral nucleus of the valvula, whereas ventrolateral prethalamic Spx1-ir cells received immunopositive Gnrh2 fibers. In the pituitary, Gnrh1-ir fibers were observed closely associated with Spx1-ir cells of the PPD. These results suggest that Spx1 could be involved in both reproductive and nonreproductive (i.e., food intake, behavior) functions in sea bass.

Social stress-induced serotonin dysfunction activates spexin in male Nile tilapia (Oreochromis Niloticus)

Social disturbance in interpersonal relationships is the primary source of stress in humans. Spexin (SPX, SPX1a in cichlid), an evolutionarily conserved neuropeptide with diverse physiological functions, is up-regulated in the brain during chronic social defeat stress in teleost. On the other hand, repeated exposure to social stress can lead to dysregulation of the monoaminergic system and increase the vulnerability of developing depression. Since dysfunction of the serotonin (5-hydroxytryptamine, 5-HT) system is associated with social stress and the pathophysiology of depression, the present study investigated the regulatory relationship between the central 5-HT system and SPX1a in the male teleost, Nile tilapia (Oreochromis niloticus). To identify stress factors that regulate SPX1a gene expression, cortisol, dexamethasone (DEX), and 5-HT were used to treat tilapia brain primary cultures. Our study shows cortisol and DEX treatment had no effect on SPX1a gene expression, but SPX1a gene expression was down-regulated following 5-HT treatment. Anatomical localization showed a close association between 5-HT immunoreactive projections and SPX1a neurons in the semicircular torus. In addition, 5-HT receptors (5-HT2B) were expressed in SPX1a neurons. SPX1a immunoreactive neurons and SPX1a gene expression were significantly increased in socially defeated tilapia. On the other hand, citalopram (antidepressant, 5-HT antagonist) treatment to socially defeated tilapia normalized SPX1a gene expression to control levels. Taken together, the present study shows that 5-HT is an upstream regulator of SPX1a and that the inhibited 5-HT activates SPX1a during social defeat.

Impact of intraspecific variation in teleost fishes: aggression, dominance status and stress physiology

Dominance-based social hierarchies are common among teleost fishes. The rank of an animal greatly affects its behaviour, physiology and development. The outcome of fights for social dominance is affected by heritable factors and previous social experience. Divergent stress-coping styles have been demonstrated in a large number of teleosts, and fish displaying a proactive coping style have an advantage in fights for social dominance. Coping style has heritable components, but it appears to be largely determined by environmental factors, especially social experience. Agonistic behaviour is controlled by the brain's social decision-making network, and its monoaminergic systems play important roles in modifying the activity of this neuronal network. In this Review, we discuss the development of dominance hierarchies, how social rank is signalled through visual and chemical cues, and the neurobiological mechanisms controlling or correlating with agonistic behaviour. We also consider the effects of social interactions on the welfare of fish reared in captivity.

Emerging central and peripheral actions of spexin in feeding behavior, leptin resistance and obesity

Consumption of a high calorie diet with irregular eating and sedentary behavior habits is typical of the current suboptimal lifestyle, contributing to the development of metabolic diseases such as obesity and type 2 diabetes mellitus. Most notably, the disorder of adipokine secretion in visceral adiposity is a major contributor to metabolic diseases with advancing age. In this regard, spexin and leptin are established as anorexigenic adipokines that can modulate adipogenesis and glucose metabolism by suppressing food intake or increasing energy expenditure, respectively. Emerging evidence points out that spexin levels are lower in the serum and adipose tissue of patients with obesity and/or insulin resistance, whereas circulating levels of leptin are higher in obesity and comorbidities. In turn, spexin and leptin pharmacologically induce beneficial effects on the brain's modulation of food intake and energy expenditure. On the other hand, endocrine crosstalk via spexin and leptin has also been reported in patients suffering from obesity and diabetes. Spexin plays a crucial role in the regulation of leptin secretion and leptin resistance. It should therefore be taken into account that studying the role of spexin in leptin regulation will help us combat the pathologies of obesity caused by leptin resistance.

Effects of Intracerebroventricular Injection of Spexin and its Interaction with NPY, GalR2 and GalR3 Receptors on the Central Food Intake Regulation and Nutritional Behavior in Broiler Chickens

Food intake and appetite in birds can be adjusted by the complex homeostatic control mechanisms. There seem to be many similarities between mammalian and avian species in terms of the regulatory feeding systems. Therefore, the aim of this study was to investigate the effects of ICV injection of spexin and its interaction with GalR and NPY receptors on central food intake regulation and nutritional behavior in broiler chickens. In experiment 1, chicken received ICV injection of saline, spexin (2.5nmol), spexin (5nmol) and spexin (10nmol). In experiment 2, birds received ICV injection of saline, B5063 (NPY₁ receptor antagonist 1.25µg), spexin (10nmol) and B5063+spexin. In experiments 3-6, SF22 (NPY₂ receptor antagonist ,1.25µg), ML0891 (NPY₅ receptor antagonist ,1.25µg), M871 (GalR₂ receptor antagonist ,10nmol) and SNAP37889 (GalR₃ receptor antagonist,10nmol) were injected in chickens instead of B5063. Then food intake was measured until 120 minutes after the injection and nutritional behavior was monitored at 30 minutes after the injection. Based on the data, a dose-dependent hypophagia was observed by the injection of spexin (P<0.05). Concomitant injection of B5063+spexin enhanced spexin-induced hypophagia (P<0.05). Co-injection of SNAP37889+spexin (10nmol) attenuated -induced hypophagia (P<0.05). Spexin (5 and 10 nmol) decreased number of steps, jumps, the exploratory food and pecks at 15 minutes after the injection (P<0.05). Spexin (5 and 10nmol) decreased standing time while siting time and rest time increased at 10 minutes after injection (P<0.05). Based on observations, spexin-induced hypophagia could be mediated by NPY₁ and GalR₃ receptors in neonatal broiler chickens.

Mutation of spexin2 promotes feeding, somatic growth, adiposity and insulin resistance in zebrafish

Spexin2 (spx2) is a newly identified gene in vertebrates, but its biological functions remain unclear. In this study, we cloned the full-length cDNA of spx2 in zebrafish. The 288 bp open reading frame encodes a protein of 95 amino acids (aa) that contains a 14 aa mature peptide. Spx2 is highly expressed in brain and testis. Its expression was significantly down-regulated in the hypothalamus after a 1 h feeding treatment and 7 days of food deprivation. Using a zebrafish spx2^-/- mutant line, we observed a greater amount of food intake and changes in mRNA levels of feeding factors. We found that, SPX2 acts as a satiety factor that inhibits food intake by downregulating the expression of agouti related neuropeptide (agrp). Moreover, spx2 mutant fish exhibited a larger body size, excessive lipid accumulation, and insulin resistance. Taken together, our results revealed that SPX2 functions as a satiety factor involved in energy metabolic regulation in zebrafish.

Spexin and Galanin in Metabolic Functions and Social Behaviors With a Focus on Non-Mammalian Vertebrates

Spexin (SPX) and galanin (GAL) are two neuropeptides that are phylogenetically related and have descended from a common ancestral gene. Considerable attention has been given to these two multifunctional neuropeptides because they share GAL receptors 1,2, and 3. Since GAL and SPX-synthesizing neurons have been detected in several brain areas, therefore, it can be speculated that SPX and GAL are involved in various neurophysiological functions. Several studies have shown the functions of these two neuropeptides in energy regulation, reproduction, and response to stress. SPX acts as a satiety factor to suppress food intake, while GAL has the opposite effect as an orexigenic factor. There is evidence that SPX acts as an inhibitor of reproductive functions by suppressing gonadotropin release, while GAL modulates the activity of gonadotropin-releasing hormone (GnRH) neurons in the brain and gonadotropic cells in the pituitary. SPX and GAL are responsive to stress. Furthermore, SPX can act as an anxiolytic factor, while GAL exerts anti-depressant and pro-depressive effects depending on the receptor it binds. This review describes evidence supporting the central roles of SPX and GAL neuropeptides in energy balance, reproduction, stress, and social behaviors, with a particular focus on non-mammalian vertebrate systems.

Functions of habenula in reproduction and socio-reproductive behaviours

Habenula is an evolutionarily conserved structure in the brain of vertebrates. Recent reports have drawn attention to the habenula as a processing centre for emotional decision-making and its role in psychiatric disorders. Emotional decision-making process is also known to be closely associated with reproductive conditions. The habenula receives innervations from reproductive centres within the brain and signals from key reproductive neuroendocrine regulators such as gonadal sex steroids, gonadotropin-releasing hormone (GnRH), and kisspeptin. In this review, based on morphological, biochemical, physiological, and pharmacological evidence we discuss an emerging role of the habenula in reproduction. Further, we discuss the modulatory role of reproductive endocrine factors in the habenula and their association with socio-reproductive behaviours such as mating, anxiety and aggression.

New evidence for SPX2 in regulating the brain-pituitary reproductive axis of half-smooth tongue sole (Cynoglossus semilaevis)

Spexin (SPX) is an evolutionarily conserved neuropeptide, which was first identified in human proteome by data mining. Two orthologs (SPX1 and SPX2) are present in some non-mammalian species, including teleosts. It has been demonstrated that SPX1 is involved in reproduction and food intake, whereas the functional role of SPX2 is still absent in any vertebrate. The aim of the current study was to evaluate the actions of intraperitoneal injection of endogenous SPX2 peptide on the expression levels of some key reproductive genes of the brain-pituitary axis in half-smooth tongue sole. Our data showed an inhibitory action of SPX2 on brain gnih, spx1, tac3 and pituitary gthα, lhβ mRNA levels. However, SPX2 had no significant effect on brain gnihr, gnrh2, gnrh3, kiss2, kiss2r, spx2 expression or pituitary gh expression. On the other hand, SPX2 induced an increase in pituitary fshβ expression. Taken together, our results provide initial evidence for the involvement of SPX2 in the regulation of reproduction in vertebrates, which is in accordance with previous studies on SPX1.

Spexin: Its role, regulation, and therapeutic potential in the hypothalamus

Spexin is the most recently discovered member of the galanin/kisspeptin/spexin family of peptides. This 14-amino acid peptide is highly conserved and is implicated in homeostatic functions including, but not limited to, metabolism, energy homeostasis, and reproduction. Spexin is expressed by neurons in the hypothalamus, which coordinate energy homeostasis and reproduction. Critically, levels of spexin appear to be altered in disorders related to energy homeostasis and reproduction, such as obesity, diabetes, and polycystic ovarian syndrome. In this review, we discuss the evidence for the involvement of spexin in the hypothalamic control of energy homeostasis and reproduction. The anorexigenic properties of spexin have been attributed to its effects on the energy-regulating neuropeptide Y/agouti-related peptide neurons and proopiomelanocortin neurons. While the role of spexin in reproduction remains unclear, there is evidence that gonadotropin-releasing hormone expressing neurons may produce and respond to spexin. Furthermore, we discuss the disorders and concomitant treatments, which have been reported to alter spexin expression, as well as the underlying signaling mechanisms that may be involved. Finally, we discuss the biochemical basis of spexin, its interaction with its cognate receptors, and how this information can be adapted to develop therapeutics for disorders related to the alteration of energy homeostasis and reproduction.

Novel insights on the role of spexin as a biomarker of obesity and related cardiometabolic disease

Spexin (SPX) is a 14-amino acid neuropeptide, discovered recently using bioinformatic techniques. It is encoded by the Ch12:orf39 gene that is widely expressed in different body tissues/organs across species, and secreted into systemic circulation. Recent reports have highlighted a potentially important regulatory role of SPX in obesity and related comorbidities. SPX is also ubiquitously expressed in human tissues, including white adipose tissue. The circulating concentration of SPX is significantly lower in individuals with obesity compared to normal weight counterparts. SPX's role in obesity appears to be related to various factors, such as the regulation of energy expenditure, appetite, and eating behaviors, increasing locomotion, and inhibiting long-chain fatty acid uptake into adipocytes. Recent reports have also suggested SPX's relationship with novel biomarkers of cardiovascular disease (CVD) and glucose metabolism and evoked the potential role of SPX as a key biomarker/player in the early loss of cardiometabolic health and development of CVD and diabetes later in life. Data on age-related changes in SPX and SPX's response to various interventions are also emerging. The current review focuses on the role of SPX in obesity and related comorbidities across the life span, and its response to interventions in these conditions. It is expected that this article will provide new ideas for future research on SPX and its metabolic regulation, particularly related to cardiometabolic diseases.

Mouse Spexin: (III) Differential Regulation by Glucose and Insulin in Glandular Stomach and Functional Implication in Feeding Control

Spexin (SPX), a neuropeptide with diverse functions, is a novel satiety factor in fish models and its role in feeding control has been recently confirmed in mammals. In mouse, food intake was shown to trigger SPX expression in glandular stomach with parallel rise in serum SPX and these SPX signals could inhibit feeding via central actions within the hypothalamus. However, the mechanisms for SPX regulation by food intake are still unclear. To examine the role of insulin signal caused by glucose uptake in SPX regulation, the mice were IP injected with glucose and insulin, respectively. In this case, serum SPX was elevated by glucose but not altered by insulin. Meanwhile, SPX transcript expression in the glandular stomach was up-regulated by glucose but the opposite was true for insulin treatment. Using in situ hybridization, the differential effects on SPX gene expression were located in the gastric mucosa of glandular stomach. Co-injection experiments also revealed that glucose stimulation on serum SPX and SPX mRNA expressed in glandular stomach could be blocked by insulin. In gastric mucosal cells prepared from glandular stomach, the opposite effects on SPX transcript expression by glucose and insulin could still be noted with similar blockade of the stimulatory effects of glucose by insulin. In this cell model, SPX gene expression induced by glucose was mediated by glucose uptake via GLUT, ATP synthesis by glycolysis/respiratory chain, and subsequent modulation of KATP channel activity, but the voltage-sensitive Ca2+ channels were not involved. The corresponding inhibition by insulin, however, was mediated by PI3K/Akt, MEK1/2/ERK1/2, and P38MAPK cascades coupled to insulin receptor but not IGF-1 receptor. Apparently, glucose uptake in mice can induce SPX expression in the glandular stomach through ATP synthesis via glucose metabolism and subsequent modification of KATP channel activity, which may contribute to SPX release into circulation to act as the satiety signal after food intake. The insulin rise caused by glucose uptake, presumably originated from the pancreas, may serve as a negative feedback to inhibit the SPX response by activating MAPK and PI3K/Akt pathways in the stomach.

Effect of Fasting on the Spexin System in Broiler Chickens

Simple Summary

The regulation of physiological processes by biologically active substances such as peptides, proteins, or hormones is very important in the context of both the development of the basic sciences and their subsequent use in improving animal husbandry. One such substance is spexin (SPX), a recently discovered, very conservative peptide that has been shown in mammalian studies to be able to regulate food intake, as well as carbohydrate–lipid metabolism. Because there is no information on the role of SPX in the metabolism of birds in the literature, we first decided to determine whether the expression of the gene encoding this peptide is present in the various tissues of birds. A further object of the study was to determine whether the concentration of SPX in serum blood changes during the disturbance of the carbohydrate metabolism caused by starvation and whether these changes correlate with other metabolic parameters. These studies will help us fully understand the role of SPX in bird physiology, and this research should be further extended.

Abstract

Spexin (SPX) is a highly conservative peptide hormone containing 14 amino acids and was discovered in 2007 by bioinformatics methods. However, nothing is yet known about its role in the metabolism of birds, including broilers. The aim of this study was to investigate the effect of short-term fasting (2, 4, and 8 h) on the concentration of SPX in blood serum and the expression levels of the genes encoding this peptide (SPX1) and its receptors, GALR2 and GALR3, in the tissues involved in carbohydrate and lipid metabolism (muscles, adipose tissue, and liver). We also analyzed the mRNA expression of these genes in various chicken tissues. Moreover, we studied the correlation between the serum level of SPX and other metabolic parameters (insulin, glucagon, glucose, triglycerides, and cholesterol). Using RT-qPCR, we found that SPX1, GALR2, and GALR3 are expressed in all investigated tissues in broiler chicken. Moreover, using a commercially available radio-immunoassay, we noted an increase of the SPX level in blood serum after 4 and 8 h of fasting compared to nonfasted animals (p < 0.05). This increase was positively correlated with glucagon concentration (r = 0.341; p < 0.05) and negatively with glucose concentration (r = −0.484; p < 0.01). Additionally, we discovered that in the short term, food deprivation leads to the expression regulation of SPX1, GALR2, and GLAR3 in tissues associated with metabolism of carbohydrates and lipids. The obtained results indicate that SPX is involved in the regulation of metabolism in broiler chickens.

Mouse Spexin: (II) Functional Role as a Satiety Factor inhibiting Food Intake by Regulatory Actions Within the Hypothalamus

Spexin (SPX) is a pleiotropic peptide with highly conserved protein sequence from fish to mammals and its biological actions are mediated by GalR2/GalR3 receptors expressed in target tissues. Recently, SPX has been confirmed to be a novel satiety factor in fish species but whether the peptide has a similar function in mammals is still unclear. Using the mouse as a model, the functional role of SPX in feeding control and the mechanisms involved were investigated. After food intake, serum SPX in mice could be up-regulated with elevations of transcript expression and tissue content of SPX in the glandular stomach but not in other tissues examined. As revealed by immunohistochemical staining, food intake also intensified SPX signals in the major cell types forming the gastric glands (including the foveolar cells, parietal cells, and chief cells) within the gastric mucosa of glandular stomach. Furthermore, IP injection of SPX was effective in reducing food intake with parallel attenuation in transcript expression of NPY, AgRP, NPY type 5 receptor (NPY5R), and ghrelin receptor (GHSR) in the hypothalamus, and these inhibitory effects could be blocked by GalR3 but not GalR2 antagonism. In agreement with the central actions of SPX, similar inhibition on feeding and hypothalamic expression of NPY, AgRP, NPY5R, and GHSR could also be noted with ICV injection of SPX. In the same study, in contrast to the drop in NPY5R and GHSR, SPX treatment could induce parallel rises of transcript expression of leptin receptor (LepR) and melanocortin 4 receptor (MC4R) in the hypothalamus. These findings, as a whole, suggest that the role of SPX as a satiety factor is well conserved in the mouse. Apparently, food intake can induce SPX production in glandular stomach and contribute to the postprandial rise of SPX in circulation. Through GalR3 activation, this SPX signal can act within the hypothalamus to trigger feedback inhibition on feeding by differential modulation of feeding regulators (NPY and AgRP) and their receptors (NPY5R, GHSR, LepR, and MC4R) involved in the feeding circuitry within the CNS.

Mouse Spexin: (I) NMR Solution Structure, Docking Models for Receptor Binding, and Histological Expression at Tissue Level

Spexin (SPX), a highly conserved neuropeptide, is known to have diverse functions and has been implicated/associated with pathological conditions, including obesity, diabetes, anorexia nervosa, and anxiety/mood disorders. Although most of the studies on SPX involved the mouse model, the solution structure of mouse SPX, structural aspects for SPX binding with its receptors GalR2/3, and its cellular expression/distribution in mouse tissues are largely unknown. Using CD and NMR spectroscopies, the solution structure of mouse SPX was shown to be in the form of a helical peptide with a random coil from Asn¹ to Pro⁴ in the N-terminal followed by an α-helix from Gln⁵ to Gln¹⁴ in the C-terminus. The molecular surface of mouse SPX is largely hydrophobic with Lys¹¹ as the only charged residue in the α-helix. Based on the NMR structure obtained, docking models of SPX binding with mouse GalR2 and GalR3 were constructed by homology modeling and MD simulation. The models deduced reveal that the amino acids in SPX, especially Asn¹, Leu⁸, and Leu¹⁰, could interact with specific residues in ECL_1&2 and TMD_2&7 of GalR2 and GalR3 by H-bonding/hydrophobic interactions, which provides the structural evidence to support the idea that the two receptors can act as the cognate receptors for SPX. For tissue distribution of SPX, RT-PCR based on 28 tissues/organs harvested from the mouse demonstrated that SPX was ubiquitously expressed at the tissue level with notable signals detected in the brain, GI tract, liver, gonad, and adrenal gland. Using immunohistochemical staining, protein signals of SPX could be located in the liver, pancreas, white adipose tissue, muscle, stomach, kidney, spleen, gonad, adrenal, and hypothalamo-pituitary axis in a cell type-specific manner. Our results, as a whole, not only can provide the structural information for ligand/receptor interaction for SPX but also establish the anatomical basis for our on-going studies to examine the physiological functions of SPX in the mouse model.

Identification, tissue distribution, periprandial expression, and anorexigenic effect of spexin in Siberian sturgeon, Acipenser baeri

Spexin (Spx), an endogenous peptide, is considered to be a neuropeptide. In a few fish and mammals, it has been proved to play a role in the regulation of animal feeding. However, the possible mechanisms of spexin regulating food intake are mostly blurry in vertebrates including Siberian sturgeon. In this study, firstly, the coding sequence of spexin cDNA was cloned and sequenced in Siberian sturgeon. Then, we detected that spexin mRNA was widely expressed in the hypothalamus, gastrointestinal tract, and liver, with the highest expression in the hypothalamus. The expression of spexin mRNA in the hypothalamus was significantly increased after food intake. At 1 h, 3 h, and 6 h after injection, the food intake in the spexin group (0.10, 0.30, and 0.90 μg/g BW) was significantly lower than that in the saline group. Moreover, compared with the saline group, the mRNA expression of anorectic nucb2, cart, ucn3, and pyy in the hypothalamus was significantly upregulated and orectic npy was significantly downregulated at 1 h after spexin injection; in the stomach, the mRNA expression of nucb2 and pyy was significantly upregulated. All in all, these results provide evidence for the anorexic effect of spexin on Siberian sturgeon.

No evidence that Spexin impacts LH release and seasonal breeding in the ewe

Spexin (SPX) is a recently identified peptide hormone of 14 amino acids. Interestingly, Spx and Kiss1 genes share a common ancestor gene. Considering that KISS1 peptides are key controllers of breeding in mammals and circumstantial evidence that SPX regulates gonadotropins in some fish species, we hypothesized that SPX may play a KISS1-related role in sheep. Here, we cloned the ovine Spx cDNA, performed in vivo injection and infusion of SPX (i.c.v. route, with or without concomittant KISS1 presence) and assessed a potential regulation of Spx expression by season, thyroid hormone and estradiol in the medio-basal hypothalamus of the ewe. Our data do not provide support for a role of SPX in the control of the gonadotropic axis in the ewe.

Chronic Social Defeat Stress Up-Regulates Spexin in the Brain of Nile Tilapia (Oreochromis niloticus)

Spexin (SPX), a neuropeptide evolutionarily conserved from fish to mammals, is widely distributed in the brain and peripheral tissues and associated with various physiological functions. Recently SPX has been suggested to be involved in neurological mechanism of stress. The current study investigates the involvement of SPX in chronic social defeat stress, using male teleost, the Nile tilapia (Oreochromis niloticus) as an animal model due to its distinct social hierarchy of dominant and subordinate relationship. The tilapia genome has SPX1a and SPX1b but has no SPX2. In the Nile tilapia, we localized SPX1a and SPX1b in the brain using in-situ hybridization. Next, using qPCR we examined gene expression of SPX1a and SPX1b in chronically stress (socially defeated) fish. SPX1a expressing cells were localized in the semicircular torus of the midbrain region and SPX1b expressing cells in the telencephalon. Chronically stress fish showed elevated plasma cortisol levels; with an upregulation of SPX1a and SPX1b gene expression in the brain compared to non-stress (control) fish. Since social defeat is a source of stress, the upregulated SPX mRNA levels during social defeat suggests SPX as a potentially inhibitory neuropeptide capable of causing detrimental changes in behaviour and physiology.

Maternal and umbilical cord blood subfatin and spexin levels in patients with gestational diabetes mellitus

Subfatin and spexin are two novel adipokines implicated in glucose homeostasis. This study was designed to investigate changes in blood subfatin and spexin levels during gestational diabetes mellitus (GDM) and childbirth, and define the mechanisms of these hormones in the physiopathology of GDM. A total of 60 pregnant women, comprising 30 diagnosed with GDM and 30 with normal gestation, were included in the study. The diagnosis of GDM was made through a 75-g oral glucose tolerance test (OGTT) administered between 24 and 28 weeks of pregnancy. The amounts of subfatin, spexin, and insulin were measured in blood samples by enzyme-linked immunosorbent assays; lipid profiles, glucose, and other biochemical parameters were measured by using an autoanalyzer. Levels of subfatin and spexin were significantly higher in blood samples drawn at baseline (before OGTT) in mothers with GDM compared to those with normal gestation. Similar observations were made in maternal and cord blood sampled at the end of pregnancy. However, at delivery, the increase in subfatin and spexin concentrations observed at baseline was abrogated in both groups of pregnant women, although levels in mothers with GDM were comparatively higher. These results show that levels of subfatin and spexin increased because of GDM and suggest that these hormones could be potential biomarkers for the diagnosis and management of GDM.

Peripheral Spexin Inhibited Food Intake in Mice

Spexin (SPX, NPQ), a novel endogenous neuropeptide, was firstly identified by bioinformatics. Spexin gene and protein widely distributed in the central nervous system and peripheral tissues, such as the hypothalamus and digestive tract. The role of spexin in appetite regulation in mammalian is still unclear. The present study was designed to investigate the mechanism and effect of peripheral spexin on food intake in mice. During the light period, an intraperitoneal (i.p.) injection of spexin (10 nmol/mouse) significantly inhibited cumulative food intake at 2, 4, and 6 h after treatment in fasted mice. During the dark period, spexin (1 and 10 nmol/mouse, i.p.) significantly suppressed cumulative food intake at 4 and 6 h after treatment in freely feeding mice. The GALR3 antagonist SNAP37889, not GALR2 antagonist, significantly antagonized the inhibitory effect on cumulative food intake (0-6 h) induced by spexin. Spexin significantly reduced the mRNA level of Npy mRNA, not Agrp, Pomc, Cart, Crh, Orexin, or Mch, in the hypothalamus. Spexin (10 nmol/mouse, i.p.) increased the number of c-Fos positive neurons in hypothalamic AHA and SCN, but not in ARC, DMN, LHA, PVN, SON, or VMH. The hypothalamic p-CaMK2 protein expression was upregulated by spexin. This study indicated that acute peripheral injection of spexin inhibited mouse food intake. The anorectic effect may be mediated by GALR3, and inhibiting neuropeptide Y (NPY) via p-CaMK2 and c-Fos in the hypothalamus.

Spexin and a Novel Cichlid-Specific Spexin Paralog Both Inhibit FSH and LH Through a Specific Galanin Receptor (Galr2b) in Tilapia

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.