Differential Roles of Two Leptin Gene Paralogues on Food Intake and Hepatic Metabolism Regulation in Mandarin Fish

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.

Identification of genes related to growth from transcriptome profiles of the muscle and liver of Chinese longsnout catfish (Leiocassis longirostris)

The Chinese longsnout catfish (Leiocassis longirostris) is a commercially important freshwater fish species in China. To understand the molecular mechanisms underlying its growth, we performed a comparative transcriptomic analysis of muscle and liver tissues of fast- and slow-growing L. longirostris. A total of 580 and 511 differentially expressed genes (DEGs) were obtained in the muscle and liver tissues, respectively. We selected 10 DEGs each from muscle and liver tissues by qRT-PCR to verify the reliability of RNA-seq, and it was found that the expression patterns of these genes were consistent with RNA-seq analysis results. According to the differential expression and functional enrichment analysis of genes, we found differences in the expression of several growth-related genes between fast- and slow-growing individuals. These genes may contribute to the differences in the growth of L. longirostris by influencing muscle growth and the metabolism of substances and energy. In particular, the pk and fabp genes were highly expressed in fast-growing individuals, while the cart, leptin, pepck, murf1, trim32, and pparα genes exhibited higher levels in slow-growing individuals. It was speculated that genes related to feeding behavior might be the key genes in regulating the growth of L. longirostris, while glycolytic/gluconeogenic metabolic pathway, lipid metabolism, and ubiquitin-proteasome pathway might be the main pathways involved in regulating body weight of L. longirostris. This study could enrich the available gene resources and provide a valuable basis for further studies on the regulatory mechanisms of growth in L. longirostris.

Phosphofructokinase family genes in grass carp: Molecular identification and tissue-specific expression in response to glucose, insulin and glucagon

It is widely acknowledged that glucose serves as the primary energy source for organisms. However, fish exhibit persistent postprandial hyperglycemia and are thought to have low glucose tolerance. Glycolysis serves as the ubiquitous pathway for glucose catabolism, with phosphofructokinase (PFK) acting as a crucial rate-limiting enzyme in this process and playing an indispensable role. PFK is classified into three isoforms based on their major expression sites, i.e., PFKM (skeletal muscle type), PFKL (liver type) and PFKP (platelet type). In this study, grass carp (Ctenopharyngodon idella) was used as animal model and the open reading frame (ORF) sequences of six PFK genetic isoforms of grass carp were cloned. Real-time PCR was used to detect its tissue distribution, and expression changes in oral glucose tolerance test (OGTT), insulin and glucagon injection experiments. The results showed that the ORF of pfkla, pfklb, pfkma, pfkmb, pfkpa and pfkpb genes was 2343, 2340, 2355, 2331, 2364 and 2349 bp in length, respectively. The results of tissue distribution showed that pfkla and pfklb, homologous to mammalian pfkl, exhibited low expression levels in the liver of grass carp, but were expressed at the highest level in the brain. Muscle-type pfkma and pfkmb mRNA were found to be highly expressed in both red and white muscle, with pfkmb also exhibiting high expression levels in the heart, while platelet type pfkpa and pfkpb showed high mRNA abundances in the brain and heart. Oral glucose administration stimulated pfkma and pfkmb mRNA expression in the red muscle, and up-regulated pfklb mRNA levels in the liver at 3 h post treatment, but it suppressed liver-type and platelet-type PFK genes expression in the brain. The expression of pfkmb in white muscle and pfkmb and pfkpb in heart were promoted by insulin, whereas the expression of pfkla and pfkpb in the brain, pfkma and pfkmb in the red muscle, pfkma in the white muscle, and pfklb in the liver was suppressed by insulin. As for glucagon, it inhibited pfkma and pfkmb mRNA expression in the red muscle, as well as pfklb in the liver, but it up-regulated PFK genes expression in most tissues detected, such as brain (pfklb, pfkpa and pfkpb), white muscle (pfkma and pfkmb), liver (pfkla) and heart (pfkmb and pfkpb). Our results suggest that PFK family genes have different or even opposite expression patterns in response to glucose, insulin and glucagon stimulation in various tissues of grass carp, which may contribute to glucose intolerance in fish.

Regulatory effect of NK homeobox 1 (NKX2.1) on melanocortin 4 receptor (Mc4r) promoter in Mandarin fish

The melanocortin 4 receptor (MC4R) is a G protein-coupled transporter that mediates the regulation of thyroid hormones and leptin on energy balance and food intake. However, the mechanisms of transcriptional regulation of Mc4r by thyroid hormone and leptin in fish have been rarely reported. The messenger RNA expression of Mc4r gene was significantly higher in brain than those in other tissues of mandarin fish. We analyzed the structure and function of a 2029 bp sequence of Mc4r promoter. Meanwhile, overexpression of NKX2.1 and incubation with leptin significantly increased Mc4r promoter activity, but triiodothyronine showed the opposite effect. In addition, mutations in the NKX2.1 binding site abolished not only the activation of Mc4r promoter activity by leptin but also the inhibitory effect of thyroid hormones on Mc4r promoter activity. In summary, these results suggested that thyroid hormones and leptin might regulate the transcriptional expression of Mc4r through NKX2.1.

Molecular Characterization and Expression Pattern of leptin in Yellow Cheek Carp (Elopichthys bambusa) and Its Transcriptional Changes in Response to Fasting and Refeeding

Leptin, a secretory protein encoded by obese genes, plays an important role in regulating feeding and energy metabolism in fish. To study the structure and function of the Leptin gene in yellow cheek carp (Elopichthys bambusa), the full-length cDNA sequence of leptin was cloned, named EbLep. The full-length cDNA of Eblep was 1140 bp, and the length of the open reading frame (ORF), which can encode a protein of 174 amino acids, was 525 bp. The signal peptide was predicted to contain 33 amino acids. Sequence alignment showed that the amino acid sequence of Leptin was conserved in cyprinid fish. Despite large differences between primary structures, the tertiary structure of the EbLep protein was similar to that of the human protein and had four α-helices. The EbLep mRNA transcript was detected in all tested tissues, with the highest expression in the liver and lowest expression in the spleen. In this study, short-term fasting significantly increased the mRNA expression of EbLep in the liver, which returned to a normal level after 6 days of refeeding and was significantly lower than the normal level after 28 days of refeeding. In the brain, the mRNA expression of EbLep significantly decreased during short-term fasting and significantly increased to a higher value than the control group after 1 h of refeeding. It then rapidly decreased to a lower value than the control group after 6 h of refeeding, returning to the normal level after 1 day of refeeding, and significantly decreasing to a lower value than the control group after 28 days of refeeding. To sum up, the change in the mRNA expression of EbLep in the brain and liver may be an adaptive strategy for different energy levels.

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.

Leptin Receptor Deficiency Results in Hyperphagia and Increased Fatty Acid Mobilization during Fasting in Rainbow Trout (Oncorhynchus mykiss)

Leptin is a pleiotropic hormone known for regulating appetite and metabolism. To characterize the role of leptin signaling in rainbow trout, we used CRISPR/Cas9 genome editing to disrupt the leptin receptor (LepR) genes, lepra1 and lepra2. We compared wildtype (WT) and mutant fish that were either fed to satiation or feed deprived for six weeks. The LepR mutants exhibited a hyperphagic phenotype, which led to heavier body weight, faster specific growth rate, increased viscero- and hepatosomatic indices, and greater condition factor. Muscle glycogen, plasma leptin, and leptin transcripts (lepa1) were also elevated in fed LepR mutant fish. Expression levels of several hypothalamic genes involved in feed regulation were analyzed (agrp, npy, orexin, cart-1, cart-2, pomc-a1, pomc-b). No differences were detected between fed WT and mutants except for pomc-b (proopiomelanocortin-b), where levels were 7.5-fold higher in LepR fed mutants, suggesting that pomc-b expression is regulated by leptin signaling. Fatty acid (FA) content did not statistically differ in muscle of fed mutant fish compared to WT. However, fasted mutants exhibited significantly lower muscle FA concentrations, suggesting that LepR mutants exhibit increased FA mobilization during fasting. These data demonstrate a key role for leptin signaling in lipid and energy mobilization in a teleost fish.

Roles of leptin in initiation of acquired growth hormone resistance and control of metabolism in rainbow trout

Catabolic conditions often induce concomitant changes in plasma leptin (Lep), growth hormone (GH) and insulin growth factor I (IGF-I) levels in teleost fish, but it is unclear whether these parts of the endocrine system are responding independently or functionally linked. In this study, fasted rainbow trout was used to study the effects of Lep on the GH-IGF-I system and metabolism. Fish were implanted intraperitoneally with recombinant rainbow trout Lep pellets and remained unfed. After 4 days, plasma GH levels were elevated in the Lep-treated fish in a dose-dependent manner; the expression of hepatic igf1 and plasma IGF-I levels were suppressed accordingly. In vitro Lep treatment reversed ovine GH (oGH)-stimulated expression of igf1 and igf2 in hepatocytes isolated from fasted fish, similar to the inhibitory effects of the MEK1/2 inhibitor U0126 treatment. However, Lep treatment alone had no effect on the expression of igfs or oGH-stimulated ghr2a expression in the hepatocytes. These results demonstrate an additive effect of Lep on suppression of IGF-I under catabolic conditions, indicating that Lep is likely involved in initiation of acquired GH resistance. Although the Lep-implant treatment had no effect on standard metabolic rate, it significantly suppressed gene expression of hepatic hydroxyacyl-CoA dehydrogenase, phosphoenolpyruvate carboxykinase and glucose 6-phosphatase, which are key enzymes in lipid utilization and gluconeogenesis, in different patterns. Overall, this study indicates that the Lep increase in fasting salmonids is an important regulatory component for physiological adaptation during periods of food deprivation, involved in suppressing growth and hepatic metabolism to spare energy expenditure.

The feedback regulation of carbohydrates intake on food intake and appetite in grass carp (Ctenopharyngodon idella)

Improving carbohydrate utilization can contribute to sustainability of aquaculture. In order to explore the feedback mechanism of glucose homeostasis in fish, one control diet (25% carbohydrate and 40% protein), one relatively high carbohydrate diet named HG (42% carbohydrate and 40% protein), and one high dietary carbohydrate coupled with relatively low protein diet named HGP (42% carbohydrate and 25% protein) were fed to grass carp for 40 days. After the feeding trial, HG group impeded the food intake and growth performance of fish compared with the other two groups. Meanwhile, the serum glucose and insulin level were both significantly elevated under the condition of high carbohydrates intake when compared HG with control group. However, although no significant difference was observed in peripheral glucose or insulin between HG and HGP groups, fish fed with HGP diet increased growth performance and food intake compared with the HG group. Gene expression data indicated that fish selectively regulated the expressions of the cerebral anorexigenic genes (cart and pomc) to adapt to the HG and HGP intake. Therefore, the HGP diet with high carbohydrate and low protein was more suitable for grass carp feeding and growth when compared with the other two diets, possibly because the diet composition was closer to the natural food of this fish. In addition, the serum leptin level was highly consistent with changes in food intake and anorexigenic gene expressions when comparing the three experimental diets, indicating that leptin might be the key to mediate the feedback regulation of carbohydrates intake on food intake and appetite in fish.

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.

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.