Goldfish Leptin-AI and Leptin-AII: Function and Central Mechanism in Feeding Control

Characterization of Leptin and Leptin Receptor Gene in the Siberian Sturgeon (Acipenser baerii): Molecular Cloning, Tissue Distribution, and Its Involvement in Feeding Regulation

Leptin is an adipokine known as a regulator of feeding and metabolism in mammals. Previous studies on fish have revealed its role in food intake regulation in limited teleosts. However, its specific function in Siberian sturgeon, an ancient Chondrostei fish, remains poorly understood. This study represents the first successful cloning of sequences for leptin and leptin receptors in Siberian sturgeon, achieved using RT-PCR. The predicted leptin sequence in this species consists of 168 amino acids that exhibit low identity with other fish species, except within the Acipenseriformes order. Tissue distribution analysis revealed a high expression of Siberian sturgeon leptin mRNA in the liver and lepr mRNA in the hypothalamus. Fasting differentially affected the expression of leptin and lepr mRNA, with decreased levels in the hypothalamus and increased levels in the liver (leptin: 3-15 days; lepr: 6-15 days). Recombinant Siberian sturgeon leptin (Ssleptin) was produced via E. coli expression, and intraperitoneal injection (100 ng/g BW) significantly inhibited food intake. The anorectic effect was correlated with changes in hypothalamic gene expression, including downregulation of orexigenic factors (agrp, orexin, npy, and ghrelin) and upregulation of anorexigenic factors (pomc, mch, and insulin). Meanwhile, the peripheral administration of Ssleptin promoted the expression of resistin in the liver and concurrently increased cck and pyy mRNA levels in the valvular intestine. Furthermore, Ssleptin injection stimulated the expression of hypothalamic lepr, jak2, akt, and ampkα2 mRNA. These findings suggest that leptin plays a significant role in the feeding control of Siberian sturgeon and provide new insights into the evolutionary function of leptin in fish.

Physiology, leptin gene expression, and intestinal morphology of pinhead and starved milkfish (Chanos chanos)

"Pinhead" is an abnormal condition of farmed fish which is rarely studied, albeit well known among fish culturists, and is characterized by extreme emaciation and anorexia. In this study, the potential impacts of pinhead condition in milkfish were analyzed and compared to fed, healthy, and a group starved for four weeks. The condition factor and hepatosomatic index of pinhead milkfish were significantly lower compared with fed, healthy individuals. Abnormal plasma osmolality and muscle water content in pinhead milkfish indicated an imbalance in their internal water content. The anorexigenic hormone, leptin A was highly expressed in liver of pinhead milkfish, which could be related to their lack of appetite. Meanwhile, the hepatosomatic index, intestinal somatic index, enterocyte height, number of villi and goblet cells, Na+/K+- ATPase activity, and intestinal protein content of the pinhead milkfish were similar to those of the 4-week starved individuals. Taken together, our results characterized key physiological parameters of pinhead milkfish for the first time. Further investigation is required to understand how environmental or artificial stress can lead to the occurrence of pinhead milkfish, and to develop methods for alleviating this condition.

Leptin gene expression in the brain is associated with the physiological onset of estivation in western sand lance Ammodytes japonicus

Dormancy is an essential ecological characteristic for the survival of organisms that experience harsh environments. Although factors that initiate dormancy vary, suppression or cessation of feeding activities are common among taxa. To distinguish between extrinsic and intrinsic causes of metabolic reduction, we focused on estivation, which occurs in summer when the feeding activity is generally enhanced. Sand lances (genus Ammodytes) are a unique marine fish with a long estivation period from early summer to late autumn. In the present study, we aimed to elucidate the control mechanisms of estivation in western sand lance (A. japonicus), and firstly examined behavioral changes in 8 months including a transition between active and dormant phases. We found that swimming/feeding behavior gradually decreased from June, and completely disappeared by late August, indicating all individuals had entered estivation. Next, we focused on leptin, known as a feeding suppression hormone in various organisms, and examined leptin-A gene (AjLepA) expression in the brain that may regulate the seasonal behavioral pattern. AjLepA expression decreased after 7 days of fasting, suggesting that leptin has a function to regulate feeding in this species. The monthly expression dynamics of AjLepA during the feeding (active) and non-feeding (estivation) periods showed that the levels gradually increased with the onset of estivation and reached its peak when all the experimental fish had estivated. The present study suggests that the suppression of feeding activity by leptin causes shift in the physiological modes of A. japonicus before estivation.

Leptins regulate the migration, proliferation, apoptosis, and synthesis of sexual steroid in tongue sole (Cynoglossus semilaevis) ovarian cells

Leptin is an important hormone in mammals, which plays a key role in regulating reproduction and energy metabolism. However, there are few studies on the function of leptin in reproductive regulation in fish, especially on tongue sole (Cynoglossus semilaevis). Thus, in this study, we firstly exploited the basic function of tongue sole leptins, the migration and growth rate of ovarian cells were reduced after knocking down lepA and lepB in ovarian cells, while increasing the apoptosis rate. Then both rlepA and rlepB were proved to be combined with lepR to further exert functions by dual luciferase assay. Transcriptome sequencing showed that differentially expressed genes (DEGs) were mainly enriched in KEGG pathways related to membrane receptors, fatty acid synthesis, growth, etc. when lepA and lepB were knocked down or additionally added in vitro. Additionally, the estradiol (E2) hormone was increased significantly after knocking down lepB. Finally, based on DEGs and the signaling pathways they participated in, we proposed a hypothesis about the signaling pathways in which leptin may be involved in ovarian cells. Taken together, these results provide new insights into the role of leptin in the regulation of physiological functions such as ovarian growth and development.

Excess feeding increases adipogenesis but lowers leptin transcript abundance in zebrafish larvae

Although fish exposed to municipal wastewater effluents (MWWE) show higher lipid accumulation, whether this is due to adipogenesis is unclear. The objective here was to identify molecular markers of adipogenesis in zebrafish (Danio rerio) larvae for use as high throughput screening tools for environmental contaminants, including obesogens in MWWE. Zebrafish larvae were fed a commercial diet at a maintenance level (5 % body mass) or in excess (25 or 50 % body mass) from day 6 to 30 days post-fertilization (dpf) to stimulate adipogenesis. We monitored fat accumulation and markers of lipid metabolism, including peroxisome proliferator-activated receptor γ (ppar γ), fatty acid synthase (fas), ELOVL fatty acid elongase 2 (elovl2), diacylglycerol O-acyltransferase 2 (dgat2), leptin (lepa and lepb), leptin receptor (lepr), and lipoprotein lipase (lpl). Excess feeding led to a higher growth rate, protein content and an increase in igf1 transcript abundance. Also, these larvae had higher triglyceride levels and accumulated lipids droplets in the abdominal cavity and viscera. The molecular markers of adipogenesis, including fas, elovl2, and dgat2, were upregulated, while the transcript abundance of lpl, a lipolytic gene, was transiently lower due to excess feeding. The increased adiposity seen at 30 dpf due to excess feeding coincided with a lower lep but not lepr transcript abundance in zebrafish. Our results demonstrate that excess feeding alters the developmental programming of key genes involved in lipid homeostasis, leading to excess lipid accumulation in zebrafish larvae. Overall, fas, elovl2, lpl, and dgat2, but not lep or ppar γ, have the potential to be biomarkers of adipogenesis in zebrafish larvae.

Goldfish adiponectin: (I) molecular cloning, tissue distribution, recombinant protein expression, and novel function as a satiety factor in fish model

Adiponectin (AdipoQ) is an adipokine involved in glucose homeostasis and lipid metabolism. In mammals, its role in appetite control is highly controversial. To shed light on the comparative aspects of AdipoQ in lower vertebrates, goldfish was used as a model to study feeding regulation by AdipoQ in fish species. As a first step, goldfish AdipoQ was cloned and found to be ubiquitously expressed at the tissue level. Using sequence alignment, protein modeling, phylogenetic analysis and comparative synteny, goldfish AdipoQ was shown to be evolutionarily related to its fish counterparts and structurally comparable with AdipoQ in higher vertebrates. In our study, recombinant goldfish AdipoQ was expressed in E. coli, purified by IMAC, and confirmed to be bioactive via activation of AdipoQ receptors expressed in HepG2 cells. Feeding in goldfish revealed that plasma levels of AdipoQ and its transcript expression in the liver and brain areas involved in appetite control including the telencephalon, optic tectum, and hypothalamus could be elevated by food intake. In parallel studies, IP and ICV injection of recombinant goldfish AdipoQ in goldfish was effective in reducing foraging behaviors and food consumption. Meanwhile, transcript expression of orexigenic factors (NPY, AgRP, orexin, and apelin) was suppressed with parallel rises in anorexigenic factors (POMC, CART, CCK, and MCH) in the telencephalon, optic tectum and/or hypothalamus. In these brain areas, transcript signals for leptin receptor were upregulated with concurrent drops in the NPY receptor and ghrelin receptors. In the experiment with IP injection of AdipoQ, transcript expression of leptin was also elevated with a parallel drop in ghrelin mRNA in the liver. These findings suggest that AdipoQ can act as a novel satiety factor in goldfish. In this case, AdipoQ signals (both central and peripheral) can be induced by feeding and act within the brain to inhibit feeding behaviors and food intake via differential regulation of orexigenic/anorexigenic factors and their receptors. The feeding inhibition observed may also involve the hepatic action of AdipoQ by modulation of feeding regulators expressed in the liver.

Too stressed to eat: Investigating factors associated with appetite loss in subordinate rainbow trout

Juvenile rainbow trout (Oncorhynchus mykiss) form dominance hierarchies in which subordinates experience chronic social stress and suppression of food intake. Here we tested the hypothesis that inhibition of food intake reflects increased expression of anorexigenic (appetite inhibiting) signals and decreased expression of orexigenic (appetite stimulating) signals. Trout were confined in pairs for 1 or 4 days, or were confined in pairs for 4 days and then allowed to recover from social interactions for 2 or 4 days; sham fish were handled identically but held alone. Subordinates did not feed during social interaction and had lower food intake than dominants or shams during recovery. In parallel, plasma cortisol (∼18-26x) and liver leptin (lep-a1) transcript abundance (∼10-14x) were elevated in subordinates during social interaction but not recovery, suggesting that these factors contributed to the suppression of food intake. Fish deemed likely to become subordinate based on inhibition of food intake in response to a mild stressor also showed elevated liver lep-a1 transcript abundance (∼5x). The moderate response in these fish coupled with a correlation between liver lep-a1 and cortisol suggest that stress-induced elevation of cortisol increased liver lep-a1 transcript abundance in subordinate trout, contributing to stress-induced suppression of food intake.

Appetite regulating genes in zebrafish gut; a gene expression study

The underlying molecular pathophysiology of feeding disorders, particularly in peripheral organs, is still largely unknown. A range of molecular factors encoded by appetite-regulating genes are already described to control feeding behaviour in the brain. However, the important role of the gastrointestinal tract in the regulation of appetite and feeding in connection to the brain has gained more attention in the recent years. An example of such inter-organ connection can be the signals mediated by leptin, a key regulator of body weight, food intake and metabolism, with conserved anorexigenic effects in vertebrates. Leptin signals functions through its receptor (lepr) in multiple organs, including the brain and the gastrointestinal tract. So far, the regulatory connections between leptin signal and other appetite-regulating genes remain unclear, particularly in the gastrointestinal system. In this study, we used a zebrafish mutant with impaired function of leptin receptor to explore gut expression patterns of appetite-regulating genes, under different feeding conditions (normal feeding, 7-day fasting, 2 and 6-hours refeeding). We provide evidence that most appetite-regulating genes are expressed in the zebrafish gut. On one hand, we did not observed significant differences in the expression of orexigenic genes (except for hcrt) after changes in the feeding condition. On the other hand, we found 8 anorexigenic genes in wild-types (cart2, cart3, dbi, oxt, nmu, nucb2a, pacap and pomc), as well as 4 genes in lepr mutants (cart3, kiss1, kiss1r and nucb2a), to be differentially expressed in the zebrafish gut after changes in feeding conditions. Most of these genes also showed significant differences in their expression between wild-type and lepr mutant. Finally, we observed that impaired leptin signalling influences potential regulatory connections between anorexigenic genes in zebrafish gut. Altogether, these transcriptional changes propose a potential role of leptin signal in the regulation of feeding through changes in expression of certain anorexigenic genes in the gastrointestinal tract of zebrafish.

An updated review of cold shock and cold stress in fish

Temperature is critical in regulating virtually all biological functions in fish. Low temperature stress (cold shock/stress) is an often-overlooked challenge that many fish face as a result of both natural events and anthropogenic activities. In this study, we present an updated review of the cold shock literature based on a comprehensive literature search, following an initial review on the subject by M.R. Donaldson and colleagues, published in a 2008 volume of this journal. We focus on how knowledge on cold shock and fish has evolved over the past decade, describing advances in the understanding of the generalized stress response in fish under cold stress, what metrics may be used to quantify cold stress and what knowledge gaps remain to be addressed in future research. We also describe the relevance of cold shock as it pertains to environmental managers, policymakers and industry professionals, including practical applications of cold shock. Although substantial progress has been made in addressing some of the knowledge gaps identified a decade ago, other topics (e.g., population-level effects and interactions between primary, secondary and tertiary stress responses) have received little or no attention despite their significance to fish biology and thermal stress. Approaches using combinations of primary, secondary and tertiary stress responses are crucial as a research priority to better understand the mechanisms underlying cold shock responses, from short-term physiological changes to individual- and population-level effects, thereby providing researchers with better means of quantifying cold shock in laboratory and field settings.

REV-ERBα Agonist SR9009 Promotes a Negative Energy Balance in Goldfish

REV-ERBα (nr1d1, nuclear receptor subfamily 1 group D member 1) is a transcriptional repressor that in mammals regulates nutrient metabolism, and has effects on energy homeostasis, although its role in teleosts is poorly understood. To determine REV-ERBα's involvement in fish energy balance and metabolism, we studied the effects of acute and 7-day administration of its agonist SR9009 on food intake, weight and length gain, locomotor activity, feeding regulators, plasma and hepatic metabolites, and liver enzymatic activity. SR9009 inhibited feeding, lowering body weight and length gain. In addition, the abundance of ghrelin mRNA decreased in the intestine, and abundance of leptin-aI mRNA increased in the liver. Hypocretin, neuropeptide y (npy), and proopiomelanocortin (pomc) mRNA abundance was not modified after acute or subchronic SR9009 administration, while hypothalamic cocaine- and amphetamine-regulated transcript (cartpt-I) was induced in the subchronic treatment, being a possible mediator of the anorectic effects. Moreover, SR9009 decreased plasma glucose, coinciding with increased glycolysis and a decreased gluconeogenesis in the liver. Decreased triglyceride levels and activity of lipogenic enzymes suggest a lipogenesis reduction by SR9009. Energy expenditure by locomotor activity was not significantly affected by SR9009. Overall, this study shows for the first time in fish the effects of REV-ERBα activation via SR9009, promoting a negative energy balance by reducing energetic inputs and regulating lipid and glucose metabolism.

Leptin receptor-deficient (knockout) zebrafish: Effects on nutrient acquisition

In mammals, knockout of LEPR results in a hyperphagic, morbid obese, and diabetic phenotype, which supports that leptin plays an important role in the control of appetite and energy metabolism, and that its receptor, LEPR, mediates these effects. To date, little is known about the role(s) of lepr in teleost physiology. We investigated a zebrafish (Danio rerio) homozygous lepr knockout (lepr^-/-) line generated by CRISPR/Cas9 in comparison to its wt counterpart with respect to nutrient acquisition, energy allocation, and metabolism. The metabolic characterization included oxygen consumption rate and morphometric parameters (yolk sac area, standard length, wet weight, and condition factor) as proxies for use and allocation of energy in developing (embryos, larvae, and juveniles) zebrafish and showed no particular differences between the two lines, in agreement with previous studies. One exception was found in oxygen consumption at 72 hpf, when zebrafish switch from embryonic to early larval stages and food-seeking behavior could be observed. In this case, the metabolic rate was significantly lower in lepr^-/- than in wt. Both phenotypes showed similar responses, with respect to metabolic rate, to acute alterations (22 and 34 °C) in water temperature (measured in terms of Q₁₀ and activation energy) compared to the standard (28 °C) rearing conditions. To assess lepr involvement in signaling the processing and handling of incoming nutrients when an exogenous meal is digested and absorbed, we conducted an in vivo analysis in lepr^-/- and wt early (8 days post-fertilization) zebrafish larvae. The larvae were administered a bolus of protein hydrolysate (0%, 1%, 5%, and 15% lactalbumin) directly into the digestive tract lumen, and changes in the mRNA expression profile before and after (1 and 3 h) administration were quantified. The analysis showed transcriptional differences in the expressions of genes involved in the control of appetite and energy metabolism (cart, npy, agrp, and mc4r), sensing (casr, t1r1, t1r3, t1r2-1, t1r2-2, pept1a, and pept1b), and digestion (cck, pyy, try, ct, and amy), with more pronounced effects observed in the orexigenic than in the anorexigenic pathways, suggesting a role of lepr in their regulations. Differences in the mRNA levels of these genes in lepr^-/-vs. wt larvae were also observed. Altogether, our analyses suggest an influence of lepr on physiological processes involved in nutrient acquisition, mainly control of food intake and digestion, during early development, whereas metabolism, energy allocation, and growth seem to be only slightly influenced.

Trajectory of leptin and leptin receptor in vertebrates: Structure, function and their regulation

The present review provides a comparative insight into structure, function and control of leptin system in fishes, herptiles, birds and mammals. In general, leptin acts as an anorexigenic hormone since its administration results in decrease of food intake in vertebrates. Nonetheless, functional paradox arises in fishes from contradictory observations on level of leptin during fasting and re-feeding. In addition, leptin is shown to modulate metabolic functions in fishes, reptiles, birds and mammals. Leptin also regulates reproductive and immune functions though more studies are warranted in non-mammalian vertebrates. The expression of leptin and its receptor is influenced by numerous factors including sex steroids, stress and stress-induced catecholamines and glucocorticoids though their effect in non-mammalian vertebrates is hard to be generalized due to limited studies.

Molecular cloning and characterization of the novel adropin from tilapia (Oreochromis niloticus): Involvement in the control of food intake

Adropin has been shown to be involved in the regulation of food intake in mice. However, the mechanism of adropin in feeding regulation is still largely unknown. Using the tilapia, Oreochromis niloticus, we identified and characterized a novel form of adropin (designated adropin-b) encoding a 68-amino acid precursor. Although adropin-b shared low amino acid identities with its tilapia paralog (designated adropin-a), synteny analysis proved that tilapia adropin is orthologous to its human counterpart. The transcripts of adropin-b were ubiquitously expressed in various tissues with the highest levels in the olfactory bulb. A decrease in adropin-b mRNA levels was observed 1 h following a meal in the olfactory bulb, hypothalamus, and optic tectum, whereas fasting for 7 days induced an increase in adropin-b mRNA levels in the olfactory bulb, hypothalamus, and optic tectum of tilapia brain. However, no changes in adropin-a mRNA levels were observed in the postprandial and fasting state. Intraperitoneal injection of tilapia adropin-b was shown to increase food consumption, but adropin-a did not affect feeding. Co-treatment of the fish with adropin-b and neuropeptide Y (NPY) had no additive effects on appetite. The appetite stimulatory effects of adropin-b appeared to be mediated by upregulating the orexigenic Npy, Orexin, and Proapelin gene expression, paralleled by inhibition of the mRNA levels of anorexigenic proopiomelanocortin (Pomc) and cocaine-amphetamine-regulated transcript (Cart) in vivo and in vitro. These observations suggested that adropin-b participated in appetite control and gene regulation of central orexigenic and anorexigenic factors in a fish model.

Leptin signalling in teleost fish with emphasis in food intake regulation

Leptin, the product of the obese (ob or Lep) gene, was first cloned in teleost fish in 2005, more than a decade after its identification in mammals. This was because bony fish and mammalian leptins share a very low amino acid sequence identity, which suggests different functionality of the leptin system in fish compared to that of mammals. Indeed, major differences are evident between the mammalian and fish leptin system. Thus, for instance, mammalian leptin is synthesized and released by the adipose tissue in response to the amount of fat depots, while several tissues (mainly the liver) are the main sources of leptin in fish, whose determining factors of production are still unclear. In mammals, the main physiological role for leptin is its involvement in the maintenance of energy balance by decreasing food intake and increasing energy expenditure, although a wide variety of actions have been attributed to this hormone (e.g., regulation of lipid and carbohydrate metabolism, reproduction and immune functions). In fish, available literature also points towards a multifunctional nature for leptin, although knowledge on its functions is limited. In this review, we offer an overview of teleostean leptin structure and mechanism of action, and discuss the available knowledge on the role of this hormone in food intake regulation in teleost fish, aiming to provide a comparative overview between the functioning of the teleostean and mammalian leptin systems.

The Melanocortin System in Atlantic Salmon (Salmo salar L.) and Its Role in Appetite Control

The melanocortin system is a key neuroendocrine network involved in the control of food intake and energy homeostasis in vertebrates. Within the hypothalamus, the system comprises two main distinct neuronal cell populations that express the neuropeptides proopiomelanocortin (POMC; anorexigenic) or agouti-related protein (AGRP; orexigenic). Both bind to the melanocortin-4 receptor (MC4R) in higher order neurons that control both food intake and energy expenditure. This system is relatively well-conserved among vertebrates. However, in Atlantic salmon (Salmo salar L.), the salmonid-specific fourth round whole-genome duplication led to the presence of several paralog genes which might result in divergent functions of the duplicated genes. In the current study, we report the first comprehensive comparative identification and characterization of Mc4r and extend the knowledge of Pomc and Agrp in appetite control in Atlantic salmon. In silico analysis revealed multiple paralogs for mc4r (a1, a2, b1, and b2) in the Atlantic salmon genome and confirmed the paralogs previously described for pomc (a1, a2, and b) and agrp (1 and 2). All Mc4r paralogs are relatively well-conserved with the human homolog, sharing at least 63% amino acid sequence identity. We analyzed the mRNA expression of mc4r, pomc, and agrp genes in eight brain regions of Atlantic salmon post-smolt under two feeding states: normally fed and fasted for 4 days. The mc4ra2 and b1 mRNAs were predominantly and equally abundant in the hypothalamus and telencephalon, the mc4rb2 in the hypothalamus, and a1 in the telencephalon. All pomc genes were highly expressed in the pituitary, followed by the hypothalamus and saccus vasculosus. The agrp genes showed a completely different expression pattern from each other, with prevalent expression of the agrp1 in the hypothalamus and agrp2 in the telencephalon. Fasting did not induce any significant changes in the mRNA level of mc4r, agrp, or pomc paralogs in the hypothalamus or in other highly expressed regions between fed and fasted states. The identification and wide distribution of multiple paralogs of mc4r, pomc, and agrp in Atlantic salmon brain provide new insights and give rise to new questions of the melanocortin system in the appetite regulation in Atlantic salmon.

Metabolic control of teleost reproduction by leptin and its complements: Understanding current insights from mammals

Reproduction is expensive. Hence, reproductive physiology is sensitive to an array of endogenous signals that provide information on metabolic and nutritional sufficiency. Although metabolic gating of reproductive function in mammals, as evidenced by studies demonstrating delayed puberty and perturbed fertility, has long been understood to be a function of energy sufficiency, an understanding of the endocrine regulators of this relationship have emerged only within recent decades. Peripheral signals including leptin and cortisol have long been implicated in the physiological integration of metabolism and reproduction. Recent studies have begun to explore possible roles for these two hormones in the regulation of reproduction in teleost fishes, as well as a role for leptin as a catabolic stress hormone. In this review, we briefly explore the reproductive actions of leptin and cortisol in mammals and teleost fishes and possible role of both hormones as putative modulators of the reproductive axis during stress events.

Temporal expression profiles of leptin and its receptor genes during early development and ovarian maturation of Cynoglossus semilaevis

Leptin (Lep) plays a key role in the regulation of food intake and energy homeostasis in vertebrates. Our previous studies have provided evidence for the existence of two leptin genes (lepa and lepb) and one leptin receptor (lepr) gene in a flatfish, the half-smooth tongue sole (Cynoglossus semilaevis). However, the spatial-temporal expression patterns and possible roles of the leptin system during early development and ovarian maturation are still poorly understood in teleosts. In the current study, we evaluated dynamic expression profiles of lepa, lepb, and lepr mRNAs during various developmental stages in this species. Quantitative RT-PCR analysis indicated that both ligand (lepa and lepb) and receptor (lepr) genes were detected in unfertilized eggs and during embryogenesis but with different expression profiles. In addition, lepa, lepb, and lepr transcripts levels increased significantly during larval development, reaching the peak at 10, 25, and 30 days post-hatching (dph), respectively. On the other hand, changes in mRNA expression of these three genes at the brain-pituitary-gonad (BPG) axis were also investigated during ovarian maturation, and lepa, lepb, and lepr mRNAs varied greatly. Taken together, our results encompass the first study reporting the dynamic expression patterns of leptin and its receptor mRNAs in the order Pleuronectiformes, providing evidence that the leptin system could be functional and play important roles during early development and ovarian maturation in tongue sole.

Two Distinct C-Type Lysozymes in Goldfish: Molecular Characterization, Antimicrobial Potential, and Transcriptional Regulation in Response to Opposing Effects of Bacteria/Lipopolysaccharide and Dexamethasone/Leptin

Lysozymes are key antimicrobial peptides in the host innate immune system that protect against pathogen infection. In this study, the full-length cDNAs of two c-type lysozymes (gfLyz-C1 and gfLyz-C2) were cloned from goldfish (Carassius auratus). The structural domains, three-dimensional structures, and amino acid sequences of gfLyz-C1 and gfLyz-C2 were highly comparable, as the two proteins shared 89.7% sequence identity. The gfLyz-C1 and gfLyz-C2 recombinant proteins were generated in the insoluble fractions of an Escherichia coli system. Based on the results of lysoplate and turbidimetric assays, gfLyz-C1 and gfLyz-C2 showed broad-spectrum antimicrobial properties with high levels of activity against Micrococcus lysodeikticus, Vibrio parahemolyticus, and Edwardsiella tarda, and relatively low activity against E. coli. Both gfLyz-C1 and gfLyz-C2 mRNAs were mainly expressed in the trunk kidney and head kidney, and gfLyz-C1 was expressed at much higher levels than gfLyz-C2 in the corresponding tissues. The expression of the gfLyz-C1 and gfLyz-C2 transcripts in the trunk kidney and head kidney was induced in these tissues by challenge with heat-inactivated E. coli and lipopolysaccharides (LPS), and the transcriptional responses of gfLyz-C1 were more intense. In goldfish primary trunk kidney cells, the levels of the gfLyz-C1 and gfLyz-C2 transcripts were upregulated by heat-inactivated E. coli, V. parahemolyticus, and E. tarda, as well as LPS, and downregulated by treatment with dexamethasone and leptins. Overall, this study may provide new insights that will improve our understanding of the roles of c-type lysozymes in the innate immunity of cyprinid fish, including the structural and phylogenetic characteristics, antimicrobial effects, and regulatory mechanism.

Structure and function analysis of various brain subregions and pituitary in grass carp (Ctenopharyngodon idellus)

It has been generally acknowledged that environment could alter the morphology and functional differentiation of vertebrate brain. However, as the largest group of all vertebrates, studies about the structures and functions of various brain subregions in teleost are still scarce. In this study, using grass carp as a model, histology method and RNA-sequencing were recruited to examine the microstructure and transcript levels among different brain subregions and pituitary. Histological results showed that the grass carp brain was composed of six parts, including olfactory bulb, telencephalon, hypothalamus, optic tectum, cerebellum, and medulla oblongata. In addition, compared to elasmobranchs and non-teleost bony ray-finned fishes, grass carp lost the hypothalamo-hypophyseal portal system, instead the hypophysiotropic neurons were directly terminated in the pituitary cells. At the transcriptomic level, our results suggested that the olfactory bulb might be related to reproduction and immune function. The telencephalon was deemed to be involved in the regulation of appetite and reproduction. The optic tectum might play important roles in the vision system and feeding. The hypothalamus could regulate feeding, and reproduction process. The medulla oblongata was related with the auditory system. The pituitary seemed to play pivotal roles in energy metabolism, organ development and reproduction. Finally, the correlation analysis suggested that the hypothalamus and the telencephalon were highly related, and close anatomical connection and overlapping functions suggested that the telencephalon and hypothalamus might be the regulation center of feeding and reproduction among teleost brain. This study provided a global view of the microstructures and specific functions of various brain subregions and pituitary in teleost. These results will be very helpful for further study in the neuroendocrinology regulation of growth and reproduction in teleost brain-pituitary axis.

Effects of short-term exercise on food intake and the expression of appetite-regulating factors in goldfish

In mammals, growing evidence indicates that exercise affects food intake, metabolism and the expression and blood levels of appetite regulators. In this study, we examined the effects of short-term (30 min, at low and high water flow) exercise on food intake, glucose levels and the expressions of appetite regulators in goldfish hypothalamus (irisin, orexin, CART, leptin), intestine (CCK, PYY, proglucagon/GLP-1), muscle (irisin) and liver (leptin), of brain-derived neurotrophic factor (BDNF) in brain, interleukin-6 (IL6) in muscle and hypothalamus, and major metabolic enzymes, the glycolytic enzyme glucokinase (GCK) and its regulatory protein (GCKR) in liver, the lipolytic enzyme lipoprotein lipase in intestine and muscle, and trypsin in intestine. Fish submitted to high flow exercise had a lower post-exercise food intake compared to control fish but no differences were seen in glucose levels between groups. Exercise induced an increase in hypothalamic expression levels of CART, IL6 and BDNF, but not orexin, irisin, CRF, leptin and NPY. High flow exercise induced an increase in intestine CCK, PYY and GLP-1, and muscle irisin and IL-6 expression levels. Exercise had no effects on expression levels of hepatic leptin or any of the metabolic enzymes examined. Our results suggest that, in goldfish, short-term exercise might decrease feeding in part by affecting the expressions of myokines and peripheral, but not central appetite regulators or metabolic enzyme/hormones.

Transcriptional study of appetite regulating genes in the brain of zebrafish (Danio rerio) with impaired leptin signalling

The hormone leptin is a key regulator of body weight, food intake and metabolism. In mammals, leptin acts as an anorexigen and inhibits food intake centrally by affecting the appetite centres in the hypothalamus. In teleost fish, the regulatory connections between leptin and other appetite-regulating genes are largely unknown. In the present study, we used a zebrafish mutant with a loss of function leptin receptor to investigate brain expression patterns of 12 orexigenic and 24 anorexigenic genes under different feeding conditions (normal feeding, 7-day fasting, 2 and 6-hours refeeding). Expression patterns were compared to wild-type zebrafish, in order to identify leptin-dependent differentially expressed genes under different feeding conditions. We provide evidence that the transcription of certain orexigenic and anorexigenic genes is influenced by leptin signalling in the zebrafish brain. We found that the expression of orexigenic genes was not affected by impaired leptin signalling under normal feeding conditions; however, several orexigenic genes showed increased transcription during fasting and refeeding, including agrp, apln, galr1a and cnr1. This suggests an inhibitory effect of leptin signal on the transcription of these orexigenic genes during short-term fasting and refeeding in functional zebrafish. Most pronounced effects were observed in the group of anorexigenic genes, where the impairment of leptin signalling resulted in reduced gene expression in several genes, including cart family, crhb, gnrh2, mc4r, pomc and spx, in the control group. This suggests a stimulatory effect of leptin signal on the transcription of these anorexigenic genes under normal feeding condition. In addition, we found multiple gain and loss in expression correlations between the appetite-regulating genes, in zebrafish with impaired leptin signal, suggesting the presence of gene regulatory networks downstream of leptin signal in zebrafish brain. The results provide the first evidence for the effects of leptin signal on the transcription of various appetite-regulating genes in zebrafish brain, under different feeding conditions. Altogether, these transcriptional changes suggest an anorexigenic role for leptin signal, which is likely to be mediated through distinct set of appetite-regulating genes under different feeding conditions.

Suppression of leptin-AI/AII transcripts by insulin in goldfish liver: A fish specific response of leptin under food deprivation

Leptin is primarily considered a peripheral satiety hormone and is also found to perform important roles in energy homeostasis in vertebrates ranging from fish to mammals. The liver is a major source of leptin production in teleost fish. Using goldfish as a model, a previous report by our group illustrated the positive regulation of leptin mRNA levels by treatment with the hyperglycemic hormone glucagon, and our present study provided evidence for the negative regulation of hepatic leptin-AI and leptin-AII transcripts through the administration of the hypoglycemic hormone insulin. This study is the first to demonstrate changes in the hepatopancreatic insulin, glucagon, leptin-AI and leptin-AII mRNA levels in goldfish during fasting and refeeding. Insulin was found to be effective in suppressing leptin-AI and leptin-AII transcript levels in goldfish liver via both in vivo intraperitoneal injection and in vitro cell incubation approaches. Only the insulin receptor, not the IGF-I receptor, was involved in insulin-inhibited leptin mRNA level. The suppression of leptin levels by insulin was caused by the activation of MKK_3/6/p³⁸MAPK and MEK_1/2/Erk_1/2 cascades. Insulin treatment could eliminate the stimulation of glucagon on leptin mRNA level. Our study describes the regulation and signal transduction mechanism of insulin on leptin mRNA levels in the goldfish liver, suggesting that the leptin function in fish is speculated to be not only an anorexigenic factor but also a metabolic mediator. This also supports the hypothesis that the poikilothermal fish use a passive survival strategy during the periods of food deprivation, which is mediated by the fish-specifically high leptin levels induced by the cooperation of insulin and glucagon.

Fish as models for understanding the vertebrate endocrine regulation of feeding and weight

The frequencies of eating disorders and obesity have increased worldwide in recent years. Their pathophysiologies are still unclear, but recent evidence suggests that they might be related to changes in endocrine and neural factors that regulate feeding and energy homeostasis. In order to develop efficient therapeutic drugs, a more thorough knowledge of the neuronal circuits and mechanisms involved is needed. Although to date, rodents have mostly been used models in the area of neuroscience and neuroendocrinology, an increasing number of studies use non-mammalian vertebrates, in particular fish, as model systems. Fish present several advantages over mammalian models and they share genetic and physiological homology to mammals with close similarities in the mechanisms involved in the neural and endocrine regulation of appetite. This review briefly describes the regulation of feeding in two model species, goldfish and zebrafish, how this regulation compares to that in mammals, and how these fish could be used for studies on endocrine regulation of eating and weight and its dysregulations.

Mechanisms for Temperature Modulation of Feeding in Goldfish and Implications on Seasonal Changes in Feeding Behavior and Food Intake

In fish models, seasonal change in feeding is under the influence of water temperature. However, the effects of temperature on appetite control can vary among fish species and the mechanisms involved have not been fully characterized. Using goldfish (Carassius auratus) as a model, seasonal changes in feeding behavior and food intake were examined in cyprinid species. In our study, foraging activity and food consumption in goldfish were found to be reduced with positive correlation to the gradual drop in water temperature occurring during the transition from summer (28.4 ± 2.2°C) to winter (15.1 ± 2.6°C). In goldfish with a 4-week acclimation at 28°C, their foraging activity and food consumption were notably higher than their counterparts with similar acclimation at 15°C. When compared to the group at 28°C during summer, the attenuation in feeding responses at 15°C during the winter also occurred with parallel rises of leptin I and II mRNA levels in the liver. Meanwhile, a drop in orexin mRNA along with concurrent elevations of CCK, MCH, POMC, CART, and leptin receptor (LepR) transcript expression could be noted in brain areas involved in feeding control. In short-term study, goldfish acclimated at 28°C were exposed to 15°C for 24 h and the treatment was effective in reducing foraging activity and food intake. The opposite was true in reciprocal experiment with a rise in water temperature to 28°C for goldfish acclimated at 15°C. In parallel time-course study with lowering of water temperature from 28 to 15°C, short-term exposure (6-12 h) of goldfish to 15°C could also increase leptin I and II mRNA levels in the liver. Similar to our seasonality study, transcript level of orexin was reduced along with up-regulation of CCK, MCH, POMC, CART, and LepR gene expression in different brain areas. Our results, as a whole, suggest that temperature-driven regulation of leptin output from the liver in conjunction with parallel modulations of orexigenic/anorexigenic signals and leptin responsiveness in the brain may contribute to the seasonal changes of feeding behavior and food intake observed in goldfish.

Evidence for leptin receptor immunoreactivity in the gastrointestinal tract and gastric leptin regulation in the rainbow trout (Oncorhynchus mykiss)

In this study, evidence for leptin receptor (LR) and gastric leptin immunoreactivity along the digestive tract of the rainbow trout (Oncorhynchus mykiss), is reported. Besides this, the regulation of gastric leptin and its transcript by fatty acids was analyzed in vitro. LR was detected mainly in the cells of the stomach gastric glands and in the brush border of the epithelium of the anterior, middle and distal intestine. In the stomach LR was co-distributed with leptin. The regulation of gastric leptin and its transcript by fatty acids was analyzed by in vitro incubations. Rabbit polyclonal antibodies anti rainbow trout leptin were developed and employed to detect leptin concentration in the stomach and in the incubation medium. Stomach slices were incubated with butyric (4:0), oleic (18:1n-9), α-linolenic (18:3n-3) and arachidonic fatty acids (20:4n-6). All fatty acids caused an increase in the protein in both the stomach and culture medium, while leptin transcript was not modified. Overall, the results confirm the gastric leptin release upon nutritional modulation.

Leptin and leptin receptor genes in tongue sole (Cynoglossus semilaevis): Molecular cloning, tissue distribution and differential regulation of these genes by sex steroids

Leptin (Lep) is a key factor for the regulation of food intake and energy homeostasis in mammals. To date, a number of studies have provided evidence for the existence of multiple leptin genes in teleosts, but not much information is available in fish regarding the regulation of leptin genes by sex steriods. As a first step, two leptin genes (lepa and lepb) and a leptin receptor (lepr) gene were cloned from the half-smooth tongue sole (Cynoglossus semilaevis), a representative species of the order Pleuronectiformes. The full-length cDNAs of tongue sole lepa and lepb were 1265 bp and 1157 bp in length, encoding for proteins of 160 aa and 158 aa, respectively. The three-dimensional structures modeling of tongue sole LepA and LepB showed strong conservation of tertiary structure with other vertebrates. The full-length cDNA of tongue sole lepr was 4576 bp, encoding a protein of 1133 aa which contained all functionally important domains conserved among vertebrate LepRs. Tissue distribution analysis showed that tongue sole lepa mRNA was highly detectable in the ovary and brain, while lepb mRNA was ubiquitously expressed in various tissues. Notably, the tongue sole lepr mRNA was most abundant in the ovary. Using a primary hepatocyte culture system, we evaluated the effects of sex steroids on lep/lepr gene expression. Both 17β-estradiol (E2) and testosterone (T) inhibited hepatic lepa and lepr mRNAs without affecting lepb mRNA levels. In addition, T also suppressed growth hormone receptor 1 (ghr1), ghr2, and insulin-like growth factor 2 (igf-2) mRNA levels, and stimulated expression of igf-1 gene. On the other hand, none of these four genes were altered by E2. To the best of our knowledge, this is the first description of a direct and differential regulation of lep/lepr gene expression by sex steroids at the hepatocyte level of a flatfish, supporting that individual leptin peptide may possess different biological roles in teleosts.

Molecular identification of the Dyn/Kor system and its potential role in the reproductive axis of goldfish

To ascertain the significance of the dynorphin/kappa-opioid receptor (Dyn/Kor) system in fish reproduction, prodynorphin (pdyn) cDNA was cloned from goldfish. Two Dyn peptides (DynA and DynB) are present in the goldfish prodynorphin precursor. Both DynA and DynB are biologically active as they are able to functionally interact with the goldfish Kor expressed in cultured eukaryotic cells to suppress forskolin-induced CRE promoter activity. RT-PCR analysis showed that pdyn is widely expressed in brain regions, with the highest expression in hypothalamus. During ovarian development, hypothalamic pdyn and kor mRNA levels are lower in the early vitellogenic stage. Then the biological effects of Dyn peptides on salmon gonadotropin releasing hormone (sgnrh), luteinizing hormone beta (lhb) and follicle stimulating hormone beta (fshb) mRNA synthesis were further investigated in goldfish. Intraperitoneal injections of DynA and DynB significantly reduced hypothalamic sgnrh and pituitary lhb and fshb mRNA levels in male goldfish, but these two peptides only down-regulated sgnrh and lhb mRNA expression in female goldfish. In vitro studies revealed that DynA also decreased lhb mRNA levels in primary cultures of pituitary cells, indicating that this peptide can exert its actions at the pituitary level. Our findings suggest that the Dyn/Kor system plays a negative role in regulating the reproductive axis in goldfish.

Leptin Selectively Regulates Nutrients Metabolism in Nile Tilapia Fed on High Carbohydrate or High Fat Diet

Leptin is known to inhibit appetite and promote energy metabolism in vertebrates. Leptin resistance (LR) commonly occurs in diet-induced obesity (DIO) in mammals. However, the roles of leptin in the energy homeostasis in DIO animals with LR remain unclear. Here we first verified the high expression of leptin in subcutaneous adipose tissue (SCAT) as in liver in Nile tilapia. Furthermore, we produced two types of DIO Nile tilapia by using a high-carbohydrate diet (HCD) or a high-fat diet (HFD), and confirmed the existence of LR in both models. Notably, we found that HCD-DIO fish retained leptin action in the activation of lipid metabolism and showed LR in glucose metabolism regulation, while this selective leptin action between lipid and glucose metabolism was reversed in HFD-DIO fish. Fasting the fish for 1 week completely recovered leptin actions in the regulation of lipid and glucose metabolism. Therefore, leptin may retain more of its activities in animals with LR than previously believed. Evolutionally, this selective regulation of leptin in nutrients metabolism could be an adaptive mechanism in animals to store surplus calories when different types of food are abundant.

Leptin Stimulates Prolactin mRNA Expression in the Goldfish Pituitary through a Combination of the PI3K/Akt/mTOR, MKK3/6/p38MAPK and MEK1/2/ERK1/2 Signalling Pathways

Leptin actions at the pituitary level have been extensively investigated in mammalian species, but remain insufficiently characterized in lower vertebrates, especially in teleost fish. Prolactin (PRL) is a pituitary hormone of central importance to osmoregulation in fish. Using goldfish as a model, we examined the global and brain-pituitary distribution of a leptin receptor (lepR) and examined the relationship between expression of lepR and major pituitary hormones in different pituitary regions. The effects of recombinant goldfish leptin-AI and leptin-AII on PRL mRNA expression in the pituitary were further analysed, and the mechanisms underlying signal transduction for leptin-induced PRL expression were determined by pharmacological approaches. Our results showed that goldfish lepR is abundantly expressed in the brain-pituitary regions, with highly overlapping PRL transcripts within the pituitary. Recombinant goldfish leptin-AI and leptin-AII proteins could stimulate PRL mRNA expression in dose- and time-dependent manners in the goldfish pituitary, by both intraperitoneal injection and primary cell incubation approaches. Moreover, the PI3K/Akt/mTOR, MKK_3/6/p³⁸MAPK, and MEK_1/2/ERK_1/2—but not JAK2/STAT 1, 3 and 5 cascades—were involved in leptin-induced PRL mRNA expression in the goldfish pituitary.

Hypothalamic Integration of Metabolic, Endocrine, and Circadian Signals in Fish: Involvement in the Control of Food Intake

The regulation of food intake in fish is a complex process carried out through several different mechanisms in the central nervous system (CNS) with hypothalamus being the main regulatory center. As in mammals, a complex hypothalamic circuit including two populations of neurons: one co-expressing neuropeptide Y (NPY) and Agouti-related peptide (AgRP) and the second one population co-expressing pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) is involved in the integration of information relating to food intake control. The production and release of these peptides control food intake, and the production results from the integration of information of different nature such as levels of nutrients and hormones as well as circadian signals. The present review summarizes the knowledge and recent findings about the presence and functioning of these mechanisms in fish and their differences vs. the known mammalian model.

Appetite-Controlling Endocrine Systems in Teleosts

Mammalian studies have shaped our understanding of the endocrine control of appetite and body weight in vertebrates and provided the basic vertebrate model that involves central (brain) and peripheral signaling pathways as well as environmental cues. The hypothalamus has a crucial function in the control of food intake, but other parts of the brain are also involved. The description of a range of key neuropeptides and hormones as well as more details of their specific roles in appetite control continues to be in progress. Endocrine signals are based on hormones that can be divided into two groups: those that induce (orexigenic), and those that inhibit (anorexigenic) appetite and food consumption. Peripheral signals originate in the gastrointestinal tract, liver, adipose tissue, and other tissues and reach the hypothalamus through both endocrine and neuroendocrine actions. While many mammalian-like endocrine appetite-controlling networks and mechanisms have been described for some key model teleosts, mainly zebrafish and goldfish, very little knowledge exists on these systems in fishes as a group. Fishes represent over 30,000 species, and there is a large variability in their ecological niches and habitats as well as life history adaptations, transitions between life stages and feeding behaviors. In the context of food intake and appetite control, common adaptations to extended periods of starvation or periods of abundant food availability are of particular interest. This review summarizes the recent findings on endocrine appetite-controlling systems in fish, highlights their impact on growth and survival, and discusses the perspectives in this research field to shed light on the intriguing adaptations that exist in fish and their underlying mechanisms.

Signal transduction mechanism for glucagon-induced leptin gene expression in goldfish liver

Leptin is a peripheral satiety hormone that also plays important roles in energy homeostasis in vertebrates ranging from fish to mammals. In teleost fish, however, the regulatory mechanism for leptin gene expression still remains unclear. In this study, we found that glucagon, a key hormone in glucose homeostasis, was effective at elevating the leptin-AI and leptin-AII transcript levels in goldfish liver via both in vivo intraperitoneal injection and in vitro cells incubation approaches. The responses of leptin-AI and leptin-AII mRNA to glucagon treatment were highly comparable. In contrast, blockade of local glucagon action could reduce the basal and induced leptin-AI and leptin-AII mRNA expression. The stimulation of leptin levels by glucagon was caused by the activation of adenylate cyclase (AC)/cyclic-AMP (cAMP)/ protein kinase A (PKA), and probably cAMP response element-binding protein (CREB) cascades. Our study described the effect and signal transduction mechanism of glucagon on leptin gene expression in goldfish liver, and may also provide new insight into leptin as a mediator in the regulatory network of energy metabolism in the fish model.

The Neuroendocrine Regulation of Food Intake in Fish: A Review of Current Knowledge

Fish are the most diversified group of vertebrates and, although progress has been made in the past years, only relatively few fish species have been examined to date, with regards to the endocrine regulation of feeding in fish. In fish, as in mammals, feeding behavior is ultimately regulated by central effectors within feeding centers of the brain, which receive and process information from endocrine signals from both brain and peripheral tissues. Although basic endocrine mechanisms regulating feeding appear to be conserved among vertebrates, major physiological differences between fish and mammals and the diversity of fish, in particular in regard to feeding habits, digestive tract anatomy and physiology, suggest the existence of fish- and species-specific regulating mechanisms. This review provides an overview of hormones known to regulate food intake in fish, emphasizing on major hormones and the main fish groups studied to date.