Expression of leptin receptor gene in developing and adult zebrafish

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.

Characterization of adaptive expression regulation of yellowtail kingfish (Seriola lalandi) leptin, receptor, and receptor overlapping transcript genes in response to fasting and re-feeding strategies

Leptins and other related genes have been proven to play vital roles in food intake, weight control, and other life activities. While the function of leptins in yellowtail kingfish (Seriola lalandi) has not yet been explored, in the present study, we investigated the structure and preliminary function of four leptin-related genes in S. lalandi. In detail, the sequence of two leptin genes (lepa and lepb), one leptin receptor gene (lepr), and one leptin receptor overlapping transcript (leprot) gene were obtained by homology cloning and RACE methods, in which lepa and lepb have similar structure. Moreover, homologous sequence alignment and evolutionary analysis of all four genes were clustered with Seriola dumerili. The tissue distribution of these four genes in thirteen tissues of yellowtail kingfish was detected by RT-qPCR. Both lepa and leprot were highly expressed in the brain and ovary, while lepb was highly expressed in the pituitary, gill, muscle, and ovary; lepr was highly expressed in the gill, kidney, and ovary. Additionally, these four genes also played roles in embryo development and early growth and development of larvae and juveniles of yellowtail kingfish. Finally, the function of leptin and leptin-related genes was investigated during fasting and re-feeding adaption of yellowtail kingfish. The results showed that these four genes have different regulation functions in five tissues; for example, the mRNA levels of lepa, lepr, and leprot in the brain decreased during fasting and immediately increased after re-feeding, while the mRNA level of lepb did not show significant fluctuation during starvation but significantly lowered after re-feeding. However, lepa and lepb mRNA levels were significantly elevated during fasting and returned to control levels after re-feeding, and there were no significant changes in the expression of lepr and leprot in the liver during fasting and after re-feeding. Moreover, the body mass of fish in the experimental group was measured, and compensatory growth was found after the resumption of feeding. These results suggested that leptin and receptor genes play different functions in different tissues to regulate the physiological state of fish in food deficiency and gain processes.

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.

Leptin signaling and its central role in energy homeostasis

Leptin plays a critical role in regulating appetite, energy expenditure and body weight, making it a key factor in maintaining a healthy balance. Despite numerous efforts to develop therapeutic interventions targeting leptin signaling, their effectiveness has been limited, underscoring the importance of gaining a better understanding of the mechanisms through which leptin exerts its functions. While the hypothalamus is widely recognized as the primary site responsible for the appetite-suppressing and weight-reducing effects of leptin, other brain regions have also been increasingly investigated for their involvement in mediating leptin's action. In this review, we summarize leptin signaling pathways and the neural networks that mediate the effects of leptin, with a specific emphasis on energy homeostasis.

Investigating Potential Cardiovascular Toxicity of Two Anti-Leukemia Drugs of Asciminib and Ponatinib in Zebrafish Embryos

BCR-ABL, a fusion protein kinase, is a druggable target exclusively expressed in patients with chronic myeloid leukemia (CML). Several anti-leukemia medicines targeting this protein have been developed in recent years. However, therapeutic options are limited for CML patients bearing multiple BCR-ABL1 mutations. Ponatinib (PON), a potent tyrosinase inhibitor, was one of the approved drugs for managing BCR-ABL1 T315I mutant disease. However, treatment of patients with PON reported severe side effects related to cardiovascular events. Asciminib (ASC) was the first allosteric inhibitor approved to target the myristoyl pocket of BCR-ABL protein to inhibit protein activity. The different mechanism of inhibition opens the possibility of co-exposure with both medicines. Reports on cardiovascular side effects due to the combination use of PON + ASC in pre-clinical and clinical studies are minimal. Thus, this study aimed to observe the potential cardiovascular-related side effect after co-exposure to ASC and PON using zebrafish as an animal model. In this study, zebrafish were acutely exposed to both compounds. The cardiovascular physiology parameters and gene expression related to cardiovascular development were evaluated. We demonstrate that combining ASC with PON at no observed effect concentration (NOEC) did not cause any significant change in the cardiac performance parameter in zebrafish. However, a significant increase in nkx2.5 expression level and a substantial decrease in blood flow velocity were recorded, suggesting that combining these compounds at NOEC can cause mild cardiovascular-related side effects.

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.

Impaired leptin signaling causes subfertility in female zebrafish

Reproduction is an energetically costly event across vertebrates and tightly linked to nutritional status and energy reserves. In mammals, the hormone leptin is considered as a link between energy homeostasis and reproduction. However, its role in fish reproduction is still unclear. In this study, we investigated the possible role of leptin in the regulation of reproduction in zebrafish, using a loss of function leptin receptor (lepr) strain. Impaired leptin signaling resulted in severe reproductive deficiencies in female zebrafish. lepr mutant females laid significantly fewer eggs, with low fertilization rates compared to wild-type females. Folliculogenesis was not affected, but oocyte maturation and ovulation were disrupted in lepr mutants. Interestingly, the expression of luteinizing hormone beta (lhb) in the pituitary was significantly lower in mutant females. Analysis of candidate genes in the ovaries and isolated fully grown follicles revealed differential expression of genes involved in steroidogenesis, oocyte maturation and ovulation in the mutants, which are known to be regulated by LH signaling. Moreover, subfertility in lepr mutants could be partially restored by administration of human chorionic gonadotropin. In conclusion, our results show that leptin deficiency does not affect early stages of follicular development, but leptin might be essential in later steps, such as in oocyte maturation and ovulation. To our knowledge, this is the first time that leptin is associated to reproductive deficiencies in zebrafish.

Leptin regulates glucose homeostasis via the canonical Wnt pathway in the zebrafish

Leptin is best known for its role in adipostasis, but it also regulates blood glucose levels. The molecular mechanism by which leptin controls glucose homeostasis remains largely unknown. Here, we use a zebrafish model to show that Wnt signaling mediates the glucoregulatory effects of leptin. Under normal feeding conditions, leptin regulates glucose homeostasis but not adipostasis in zebrafish. In times of nutrient excess, however, we found that leptin also regulates body weight and size. Using a Wnt signaling reporter fish, we show that leptin activates the canonical Wnt pathway in vivo. Utilizing two paradigms for hyperglycemia, it is revealed that leptin regulates glucose homeostasis via the Wnt pathway, as pharmacological inhibition of this pathway impairs the glucoregulatory actions of leptin. Our results may shed new light on the evolution of the physiological function of leptin.

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.

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.

A fish with no sex: gonadal and adrenal functions partition between zebrafish NR5A1 co-orthologs

People with NR5A1 mutations experience testicular dysgenesis, ovotestes, or adrenal insufficiency, but we do not completely understand the origin of this phenotypic diversity. NR5A1 is expressed in gonadal soma precursor cells before expression of the sex-determining gene SRY. Many fish have two co-orthologs of NR5A1 that likely partitioned ancestral gene subfunctions between them. To explore ancestral roles of NR5A1, we knocked out nr5a1a and nr5a1b in zebrafish. Single-cell RNA-seq identified nr5a1a-expressing cells that co-expressed genes for steroid biosynthesis and the chemokine receptor Cxcl12a in 1-day postfertilization (dpf) embryos, as does the mammalian adrenal-gonadal (interrenal-gonadal) primordium. In 2dpf embryos, nr5a1a was expressed stronger in the interrenal-gonadal primordium than in the early hypothalamus but nr5a1b showed the reverse. Adult Leydig cells expressed both ohnologs and granulosa cells expressed nr5a1a stronger than nr5a1b. Mutants for nr5a1a lacked the interrenal, formed incompletely differentiated testes, had no Leydig cells, and grew far larger than normal fish. Mutants for nr5a1b formed a disorganized interrenal and their gonads completely disappeared. All homozygous mutant genotypes lacked secondary sex characteristics, including male breeding tubercles and female sex papillae, and had exceedingly low levels of estradiol, 11-ketotestosterone, and cortisol. RNA-seq showed that at 21dpf, some animals were developing as females and others were not, independent of nr5a1 genotype. By 35dpf, all mutant genotypes greatly under-expressed ovary-biased genes. Because adult nr5a1a mutants form gonads but lack an interrenal and conversely, adult nr5a1b mutants lack a gonad but have an interrenal, the adrenal, and gonadal functions of the ancestral nr5a1 gene partitioned between ohnologs after the teleost genome duplication, likely owing to reciprocal loss of ancestral tissue-specific regulatory elements. Identifying such elements could provide hints to otherwise unexplained cases of Differences in Sex Development.

Tissue-Specific Effects of Leptin on Glucose and Lipid Metabolism

The discovery of leptin was intrinsically associated with its ability to regulate body weight. However, the effects of leptin are more far-reaching and include profound glucose-lowering and anti-lipogenic effects, independent of leptin's regulation of body weight. Regulation of glucose metabolism by leptin is mediated both centrally and via peripheral tissues and is influenced by the activation status of insulin signaling pathways. Ectopic fat accumulation is diminished by both central and peripheral leptin, an effect that is beneficial in obesity-associated disorders. The magnitude of leptin action depends upon the tissue, sex, and context being examined. Peripheral tissues that are of particular relevance include the endocrine pancreas, liver, skeletal muscle, adipose tissues, immune cells, and the cardiovascular system. As a result of its potent metabolic activity, leptin is used to control hyperglycemia in patients with lipodystrophy and is being explored as an adjunct to insulin in patients with type 1 diabetes. To fully understand the role of leptin in physiology and to maximize its therapeutic potential, the mechanisms of leptin action in these tissues needs to be further explored.

Combinatorial Normalization of Liver-Derived Cytokine Pathways Alleviates Hepatic Tumor-Associated Cachexia in Zebrafish

The role and significance of liver-derived cytokines in cancer-associated cachexia syndrome remain elusive. Here we report that combinatorial counterbalances of the leptin and Igf1 signaling pathways in hepatocellular carcinoma (HCC) models significantly relieves cachexia. Double transgenic zebrafish models of HCC that stably displayed focal lesions, anorexia, and wasting of adipose and muscle tissues were first generated. Knockout of lepr or mc4r from these zebrafish partially restored appetite and exerted moderate or no effect on tissue wasting. However, genetic replenishment of Igf1 in a lepr-mutant background effectively relieved the cachexia-like phenotype without affecting tumor growth. Similarly, administration of napabucasin, a Stat3/Socs3 inhibitor, on the zebrafish HCC model, mammalian cell lines with exogenous IGF1, and two mouse xenograft models restored insulin sensitivity and rescued the wasting of nontumor tissues. Together, these results describe the synergistic impact of leptin and Igf1 normalization in treating certain HCC-associated cachexia as a practical strategy. SIGNIFICANCE: Disruption of leptin signaling with normalized Igf1 expression significantly rescues anorexia, muscle wasting, and adipose wasting in Ras- and Myc-driven zebrafish models of HCC.

Vascular Damage in Obesity and Diabetes: Highlighting Links Between Endothelial Dysfunction and Metabolic Disease in Zebrafish and Man

Endothelial dysfunction is an initial pathophysiological mechanism of vascular damage and is further recognized as an independent predictor of negative prognosis in diabetes-induced micro- and macrovascular complications. Insight into the capability of zebrafish to model metabolic disease like obesity and type II diabetes has increased and new evidence on the induction of vascular pathologies in zebrafish through metabolic disease is available. Here, we raise the question, if zebrafish can be utilized to study the initial impairments of vascular complications in metabolic disorders. In this review, we focus on the advances made to develop models of obesity and type II diabetes in zebrafish, discuss the key points and characteristics of these models, while highlighting the available information linked to the development of endothelial dysfunction in zebrafish and man. We show that larval and adult zebrafish develop metabolic dysregulation in the settings of obesity and diabetes, exhibiting pathophysiological mechanisms, which mimic the human condition. The most important genes related to endothelial dysfunction are present in zebrafish and further display similar functions as in mammals. Several suggested contributors to endothelial dysfunction found in these models, namely hyperinsulinaemia, hyperglycaemia, hyperlipidaemia and hyperleptinaemia are highlighted and the available data from zebrafish are summarised. Many underlying processes of endothelial dysfunction in obesity and diabetes are fundamentally present in zebrafish and provide ground for the assumption, that zebrafish can develop endothelial dysfunction. Conservation of basic biological mechanisms is established for zebrafish, but focused investigation on the subject is now needed as validation and particularly more research is necessary to understand the differences between zebrafish and man. The available data demonstrate the relevance of zebrafish as a model for metabolic disease and their ability to become a proponent for the investigation of vascular damage in the settings of obesity and diabetes.

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.

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.

Photoperiod may regulate growth via leptin receptor A1 in the hypothalamus and saccus vasculosus of Atlantic salmon (Salmo salar)

Photoperiod is believed to regulate growth in fish, although the mechanism involved is still unclear. In this paper, we report a relationship between leptin-receptor A1 (AsLRa1), melatonin-receptor (AsMR) and photoperiod in Atlantic salmon. Atlantic salmon (mean weight 1071.70 ± 155.54 g) were reared under six photoperiod regimes, four constant light regimes 24L:0D, 18L:6D, 12L:12D and 8L:16D, hours of light (L) and dark (D) and two varying light regimes, LL-SL = 24L:0D-8L:16D, and SL-LL = 8L:16D-24L:0D over a period of seven months. The results showed that AsLRa1 transcripts were mainly existed in the hypothalamus and saccus vasculosus (SV), AsMR was mainly expressed in the hypothalamus. Long photoperiod inhibited the expression of AsLRa1 and AsMR transcripts in the Atlantic salmon brain. The expression pattern of AsLRa1 was similar to the expression pattern of AsMR in the hypothalamus. Food intake was higher in fish with lower AsLRa1 transcript levels. This demonstrated that photoperiod influenced somatic growth by changing expression of AsLRa1 in the hypothalamus and SV to affect appetite. In addition, we found that the SV appears to act as a seasonal sensor regulating reproduction in a similar way to the hypothalamus.

Leptin-a mediates transcription of genes that participate in central endocrine and phosphatidylinositol signaling pathways in 72-hour embryonic zebrafish (Danio rerio)

We analyzed microarray expression data to highlight biological pathways that respond to embryonic zebrafish Leptin-a (lepa) signaling. Microarray expression measures for 26,046 genes were evaluated from lepa morpholino oligonucleotide "knockdown", recombinant Leptin-a "rescue", and uninjected control zebrafish at 72-hours post fertilization. In addition to KEGG pathway enrichment for phosphatidylinositol signaling and neuroactive ligand-receptor interactions, Gene Ontology (GO) data from lepa rescue zebrafish include JAK/STAT cascade, sensory perception, nervous system processes, and synaptic signaling. In the zebrafish lepa rescue treatment, we found changes in the expression of homologous genes that align with mammalian leptin signaling cascades including AMPK (prkaa2), ACC (acacb), Ca²⁺/calmodulin-dependent kinase (camkk2), PI3K (pik3r1), Ser/Thr protein kinase B (akt3), neuropeptides (agrp2, cart1), mitogen-activated protein kinase (MAPK), and insulin receptor substrate (LOC794738, LOC100537326). Notch signaling pathway and ribosome biogenesis genes respond to knockdown of Leptin-a. Differentially expressed transcription factors in lepa knockdown zebrafish regulate neurogenesis, neural differentiation, and cell fate commitment. This study presents a role for zebrafish Leptin-a in influencing expression of genes that mediate phosphatidylinositol and central endocrine signaling.

Leptin induces muscle wasting in a zebrafish kras-driven hepatocellular carcinoma (HCC) model

Cancer cachexia affects up to 80% of patients with advanced solid cancer and leads to excessive muscle wasting. Here, using an inducible zebrafish hepatocellular carcinoma (HCC) model driven by oncogenic kras^G12V, we observed a progressive muscle-wasting phenotype in adult zebrafish, characterized by significant loss of body weight and muscle fibers. By differential feeding, we observed that overfeeding caused fatty liver, accelerated carcinogenesis and muscle wasting. Interestingly, leptin, an obesity hormone, was upregulated in oncogenic hepatocytes and overfeeding groups. We also found that leptin expression progressively increased during human liver disease progression. By using leptin receptor (lepr)-knockout fish, we found that tumor fish in the lepr mutant background had a higher survival rate and significantly lower muscle-wasting level after tumor induction than the tumor fish in the wild-type background. Chemical inhibitors targeting leptin signaling also alleviated the muscle-wasting phenotype, indicating that leptin signaling may be a new therapeutic target for cancer patients with muscle wasting.

Summary: Through a zebrafish model, this study demonstrates that leptin plays an important role in cancer-induced muscle wasting and that the leptin pathway may be a therapeutic target in cancer cachexia.

Zebrafish Mutants Carrying Leptin a (lepa) Gene Deficiency Display Obesity, Anxiety, Less Aggression and Fear, and Circadian Rhythm and Color Preference Dysregulation

Leptin, a hormone secreted by peripheral adipose tissues, regulates the appetite in animals. Recently, evidence has shown that leptin also plays roles in behavioral response in addition to controlling appetite. In this study, we examined the potential function of leptin on non-appetite behaviors in zebrafish model. By using genome editing tool of Transcription activator-like effector nuclease (TALEN), we successfully knocked out leptin a (lepa) gene by deleting 4 bp within coding region to create a premature-translation stop. Morphological and appetite analysis showed the lepa KO fish display a phenotype with obese, good appetite and elevation of Agouti-related peptide (AgRP) and Ghrelin hormones, consistent with the canonical function of leptin in controlling food intake. By multiple behavior endpoint analyses, including novel tank, mirror biting, predator avoidance, social interaction, shoaling, circadian rhythm, and color preference assay, we found the lepa KO fish display an anxiogenic phenotype showing hyperactivity with rapid swimming, less freezing time, less fear to predator, loose shoaling area forming, and circadian rhythm and color preference dysregulations. Using biochemical assays, melatonin, norepinephrine, acetylcholine and serotonin levels in the brain were found to be significantly reduced in lepa KO fish, while the levels of dopamine, glycine and cortisol in the brain were significantly elevated. In addition, the brain ROS level was elevated, and the anti-oxidative enzyme catalase level was reduced. Taken together, by performing loss-of-function multiple behavior endpoint testing and biochemical analysis, we provide strong evidence for a critical role of lepa gene in modulating anxiety, aggression, fear, and circadian rhythm behaviors in zebrafish for the first time.

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.

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.

Distribution and female reproductive state differences in orexigenic and anorexigenic neurons in the brain of the mouth brooding African cichlid fish, Astatotilapia burtoni

Integration of reproduction and metabolism is necessary for species survival. While the neural circuits controlling energy homeostasis are well-characterized, the signals controlling the relay of nutritional information to the reproductive axis are less understood. The cichlid fish Astatotilapia burtoni is ideal for studying the neural regulation of feeding and reproduction because females cycle between a feeding gravid state and a period of forced starvation while they brood developing young inside their mouths. To test the hypothesis that candidate neuropeptide-containing neurons known to be involved in feeding and energy homeostasis in mammals show conserved distribution patterns, we performed immunohistochemistry and in situ hybridization to localize appetite-stimulating (neuropeptide Y, NPY; agouti-related protein, AGRP) and appetite-inhibiting (cocaine and amphetamine-regulated transcript, CART; pro-opiomelanocortin, pomc1a) neurons in the brain. NPY, AGRP, CART, and pomc1a somata showed distribution patterns similar to other teleosts, which included localization to the lateral tuberal nucleus (NLT), the putative homolog of the mammalian arcuate nucleus. Gravid females also had larger NPY and AGRP neurons in the NLT compared to brooding females, but brooding females had larger pomc1a neurons compared to gravid females. Hypothalamic agrp mRNA levels were also higher in gravid compared to brooding females. Thus, larger appetite-stimulating neurons (NPY, AGRP) likely promote feeding while females are gravid, while larger pomc1a neurons may act as a signal to inhibit food intake during mouth brooding. Collectively, our data suggest a potential role for NPY, AGRP, POMC, and CART in regulating energetic status in A. burtoni females during varying metabolic and reproductive demands.

Expression and distribution of leptin and its receptors in the digestive tract of DIO (diet-induced obese) zebrafish

The expression and localization of leptin (A and B) and its receptor family in control and diet-induced obese (DIO) adult male zebrafish gut, after 5-weeks overfeeding, administering Artemia nauplii, as fat-rich food, were investigated. Recently, the obese adult zebrafish was considered an experimental model with pathophysiological pathways similar to mammalian obesity. Currently, there are no reports about leptin in fish obesity, or in a state of altered energy balance. By qRT-PCR, leptin A and leptin B expression levels were significantly higher in DIO zebrafish gut than in the control group (CTRL), and the lowest levels of leptin receptor mRNA appeared in DIO zebrafish gut. The presence of leptin and its receptor proteins in the intestinal tract was detected by western blot analysis in both control and DIO zebrafish. By single immunohistochemical staining, leptin and leptin receptor immunoreactive endocrine cells were identified in the intestinal tract either in DIO or control zebrafish. Moreover, leptin immunopositive enteric nervous system elements were observed in both groups. By double immunohistochemical staining, leptin and its receptor were colocalized especially in DIO zebrafish. Thus, our study represents a starting point in the investigation of a possible involvement of leptin in control of energy homeostasis in control and DIO zebrafish.

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.

Ion transport in the zebrafish kidney from a human disease angle: possibilities, considerations, and future perspectives

Unique experimental advantages, such as its embryonic/larval transparency, high-throughput nature, and ease of genetic modification, underpin the rapid emergence of the zebrafish ( Danio rerio) as a preeminent model in biomedical research. Particularly in the field of nephrology, the zebrafish provides a promising model for studying the physiological implications of human solute transport processes along consecutive nephron segments. However, although the zebrafish might be considered a valuable model for numerous renal ion transport diseases and functional studies of many channels and transporters, not all human renal electrolyte transport mechanisms and human diseases can be modeled in the zebrafish. With this review, we explore the ontogeny of zebrafish renal ion transport, its nephron structure and function, and thereby demonstrate the clinical translational value of this model. By critical assessment of genomic and amino acid conservation of human proteins involved in renal ion handling (channels, transporters, and claudins), kidney and nephron segment conservation, and renal electrolyte transport physiology in the zebrafish, we provide researchers and nephrologists with an indication of the possibilities and considerations of the zebrafish as a model for human renal ion transport. Combined with advanced techniques envisioned for the future, implementation of the zebrafish might expand beyond unraveling pathophysiological mechanisms that underlie distinct genetic or environmentally, i.e., pharmacological and lifestyle, induced renal transport deficits. Specifically, the ease of drug administration and the exploitation of improved genetic approaches might argue for the adoption of the zebrafish as a model for preclinical personalized medicine for distinct renal diseases and renal electrolyte transport proteins.

Leptin as immune mediator: Interaction between neuroendocrine and immune system

Leptin is an adipocyte-derived hormone/cytokine that links nutritional status with neuroendocrine and immune functions. Initially described as an anti-obesity hormone, leptin has subsequently been shown to exert pleiotropic effects, being also able to influence haematopoiesis, thermogenesis, reproduction, angiogenesis, and more importantly immune homeostasis. As a cytokine, leptin can affect both innate and adaptive immunity, by inducing a pro-inflammatory response and thus playing a key role in the regulation of the pathogenesis of several autoimmune/inflammatory diseases. In this review, we discuss the most recent advances on the role of leptin as immune-modulator in mammals and we also provide an overview on its main functions in non-mammalian vertebrates.

Identification of a novel leptin receptor duplicate in Atlantic salmon: Expression analyses in different life stages and in response to feeding status

In recent years rapidly growing research has led to identification of several fish leptin orthologs and numerous duplicated paralogs possibly arisen from the third and fourth round whole genome duplication (3R and 4R WGD) events. In this study we identify in Atlantic salmon a duplicated LepRA gene, named LepRA2, that further extend possible evolutionary scenarios of the leptin and leptin receptor system. The 1121 amino acid sequence of the novel LepRA2 shares 80% sequence identity with the LepRA1 paralog, and contains the protein motifs typical of the functional (long form) leptin receptor in vertebrates. In silico predictions showed similar electrostatic properties of LepRA1 and LepRA2 and high sequence conservation at the leptin interaction surfaces within the CHR/leptin-binding and FNIII domains, suggesting conserved functional specificity between the two duplicates. Analysis of temporal expression profiles during pre-hatching stages indicate that both transcripts are involved in modulating leptin developmental functions, although the LepRA1 paralog may play a major role as the embryo complexity increases. There is ubiquitous distribution of LepRs underlying pleiotropism of leptin in all tissues investigated. LepRA1 and LepRA2 are differentially expressed with LepRA1 more abundant than LepRA2 in most of the tissues investigated, with the only exception of liver. Analysis of constitutive LepRA1 and LepRA2 expression in brain and liver at parr, post-smolt and adult stages reveal striking spatial divergence between the duplicates at all stages investigated. This suggests that, beside increased metabolic requirements, leptin sensitivity in the salmon brain might be linked to important variables such as habitat, ecology and life cycle. Furthermore, leptins and LepRs mRNAs in the brain showed gene-specific variability in response to long term fasting, suggesting that leptin's roles as modulator of nutritional status in Atlantic salmon might be governed by distinct genetic evolutionary processes and distinct functions between the paralogs.

Leptin receptor gene in the European sea bass (Dicentrarchus labrax): Cloning, phylogeny, tissue distribution and neuroanatomical organization

In this study, we report the cloning of three transcripts for leptin receptor in the European sea bass, a marine teleost of economic interest. The two shortest variants, generated by different splice sites, encode all functional extracellular and intracellular domains but missed the transmembrane domain. The resulting proteins are therefore potential soluble binding proteins for leptin. The longest transcript (3605bp), termed sblepr, includes all the essential domains for binding and transduction of the signal. Thus, it is proposed as the ortholog for the human LEPR gene, the main responsible for leptin signaling. Phylogenetic analysis shows the sblepr clustered within the teleost leptin receptor group in 100% of the bootstrap replicates. The neuroanatomical localization of sblepr expressing cells has been assessed by in situ hybridization in brains of sea bass of both sexes during their first sexual maturation. At histological level, the distribution pattern of sblepr expressing cells in the brain shows no clear differences regarding sex or reproductive season. Transcripts of the sblepr have a widespread distribution throughout the forebrain and midbrain until the caudal portion of the hypothalamus. A high hybridization signal is detected in the telencephalon, preoptic area, medial basal and caudal hypothalamus and in the pituitary gland. In a more caudal region, sblepr expressing cells are identified in the longitudinal torus. The expression pattern observed for sblepr suggests that in sea bass, leptin is very likely to be involved in the control of food intake, energy reserves and reproduction.

Leptin signaling regulates glucose homeostasis, but not adipostasis, in the zebrafish

Leptin is the primary adipostatic factor in mammals. Produced largely by adipocytes in proportion to total adipose mass, the hormone informs the brain regarding total energy stored as triglycerides in fat cells. The hormone acts on multiple circuits in the brain to regulate food intake, autonomic outflow, and endocrine function to maintain energy balance. In addition to regulating adipose mass, mammalian leptin also plays a role in the regulation of glucose homeostasis and as a gating factor in reproductive competence. Leptin-deficient mice and people exhibit early onset profound hyperphagia and obesity, diabetes, and infertility. Although leptin and the leptin receptor are found in fish, the hormone is not expressed in adipose tissue, but is found in liver and other tissues. Here, we show that adult zebrafish lacking a functional leptin receptor do not exhibit hyperphagia or increased adiposity, and exhibit normal fertility. However, leptin receptor-deficient larvae have increased numbers of β-cells and increased levels of insulin mRNA. Furthermore, larval zebrafish have been shown to exhibit β-cell hyperplasia in response to high fat feeding or peripheral insulin resistance, and we show here that leptin receptor is required for this response. Adult zebrafish also have increased levels of insulin mRNA and other alterations in glucose homeostasis. Thus, a role for leptin in the regulation of β-cell mass and glucose homeostasis appears to be conserved across vertebrates, whereas its role as an adipostatic factor is likely to be a secondary role acquired during the evolution of mammals.

Inflammatory diseases modelling in zebrafish

The ingest of diets with high content of fats and carbohydrates, low or no physical exercise and a stressful routine are part of the everyday lifestyle of most people in the western world. These conditions are triggers for different diseases with complex interactions between the host genetics, the metabolism, the immune system and the microbiota, including inflammatory bowel diseases (IBD), obesity and diabetes. The incidence of these disorders is growing worldwide; therefore, new strategies for its study are needed. Nowadays, the majority of researches are in use of murine models for understand the genetics, physiopathology and interaction between cells and signaling pathways to find therapeutic solutions to these diseases. The zebrafish, a little tropical water fish, shares 70% of our genes and conserves anatomic and physiological characteristics, as well as metabolical pathways, with mammals, and is rising as a new complementary model for the study of metabolic and inflammatory diseases. Its high fecundity, fast development, transparency, versatility and low cost of maintenance makes the zebrafish an interesting option for new researches. In this review, we offer a discussion of the existing genetic and induced zebrafish models of two important Western diseases that have a strong inflammatory component, the IBD and the obesity.

Probiotic treatment reduces appetite and glucose level in the zebrafish model

The gut microbiota regulates metabolic pathways that modulate the physiological state of hunger or satiety. Nutrients in the gut stimulate the release of several appetite modulators acting at central and peripheral levels to mediate appetite and glucose metabolism. After an eight-day exposure of zebrafish larvae to probiotic Lactobacillus rhamnosus, high-throughput sequence analysis evidenced the ability of the probiotic to modulate the microbial composition of the gastrointestinal tract. These changes were associated with a down-regulation and up-regulation of larval orexigenic and anorexigenic genes, respectively, an up-regulation of genes related to glucose level reduction and concomitantly reduced appetite and body glucose level. BODIPY-FL-pentanoic-acid staining revealed higher short chain fatty acids levels in the intestine of treated larvae. These results underline the capability of the probiotic to modulate the gut microbiota community and provides insight into how the probiotic interacts to regulate a novel gene network involved in glucose metabolism and appetite control, suggesting a possible role for L. rhamnosus in the treatment of impaired glucose tolerance and food intake disorders by gut microbiota manipulation.

Discovery and functional characterization of leptin and its receptors in Japanese quail (Coturnix japonica)

Leptin is an important endocrine regulation factor of food intake and energy homeostasis in mammals; however, the existence of a poultry leptin gene (LEP) is still debated. Here, for the first time, we report the cloning of a partial exon 3 sequence of LEP (qLEP) and four different leptin receptor splicing variants, including a long receptor (qLEPRl) and three soluble receptors (qLEPR-a, qLEPR-b and qLEPR-c) in Japanese quail (Coturnix japonica). The qLEP gene had high GC content (64%), which is similar to other reported avian leptin genes. The encoded qLEP protein possessed the conserved pair of cysteine residues that are required to form a lasso knot for full biological activity, but shared relatively low identities with LEPs of other vertebrates. The translated qLEPRl protein contained 1143 amino acids and shared high amino acid sequence identity with a chicken homolog (89% identity). qLEPRl also contained all the motifs, domains, and basic tyrosine residues that are conserved in the LEPRl proteins of other vertebrates. qRT-PCR analysis showed that LEP and the four LEPR variants were expressed extensively in all tissues examined; the expression levels of LEP were relatively high in hypothalamus, skeletal muscle, and pancreas, while the expression levels of the LEPRs were highest in the pituitary. Compared with the expression levels of juvenile qLEP and total qLEPR (including all LEPR variants), the expression levels of mature qLEP and total qLEPR were up-regulated in the hypothalamus and pituitary, and down-regulated in the ovary. The expressions of LEP/LEPR increased when fasting and decreased when refeeding in the brain and peripheral tissues of juvenile quail, which suggested that the LEP/LEPR system modulated food intake and energy expenditure, although, unlike in mammals, LEP may actually act to inhibit food intake during fasting, at least in juvenile quail. The results indicate that qLEP and qLEPR have unique expression patterns and that the encoded proteins play important roles in the regulation of reproduction and energy status in Japanese quail.

The Mammalian "Obesogen" Tributyltin Targets Hepatic Triglyceride Accumulation and the Transcriptional Regulation of Lipid Metabolism in the Liver and Brain of Zebrafish

Recent findings indicate that different Endocrine Disrupting Chemicals (EDCs) interfere with lipid metabolic pathways in mammals and promote fat accumulation, a previously unknown site of action for these compounds. The antifoulant and environmental pollutant tributyltin (TBT), which causes imposex in gastropod snails, induces an “obesogenic” phenotype in mammals, through the activation of the nuclear receptors retinoid X receptor (RXR) and peroxisome proliferator-activated receptor gamma (PPARγ). In teleosts, the effects of TBT on the lipid metabolism are poorly understood, particularly following exposure to low, environmental concentrations. In this context, the present work shows that exposure of zebrafish to 10 and 50 ng/L of TBT (as Sn) from pre-hatch to 9 months of age alters the body weight, condition factor, hepatosomatic index and hepatic triglycerides in a gender and dose related manner. Furthermore, TBT modulated the transcription of key lipid regulating factors and enzymes involved in adipogenesis, lipogenesis, glucocorticoid metabolism, growth and development in the brain and liver of exposed fish, revealing sexual dimorphic effects in the latter. Overall, the present study shows that the model mammalian obesogen TBT interferes with triglyceride accumulation and the transcriptional regulation of lipid metabolism in zebrafish and indentifies the brain lipogenic transcription profile of fish as a new target of this compound.

Transcriptomic Analyses Reveal Novel Genes with Sexually Dimorphic Expression in Yellow Catfish (Pelteobagrus fulvidraco) Brain

Yellow catfish (Pelteobagrus fulvidraco) is a pivotal freshwater aquaculture species in China. It shows sexual size dimorphism favoring male in growth. Whole transcriptome approach is required to get the overview of genetic toolkit for understanding the sex determination mechanism aiming at devising its monosex production. Beside gonads, the brain is also considered as a major organ for vertebrate reproduction. Transcriptomic analyses on the brain and of different developmental stages will provide the dynamic view necessary for better understanding its sex determination. In this regard, we have performed a de novo assembly of yellow catfish brain transcriptome by high throughput Illumina sequencing. A total number of 154,507 contigs were obtained with the lengths ranging from 201 to 27,822 bp and N50 of 2,101 bp, as well as 20,699 unigenes were identified. Of these unigenes, 13 and 54 unigenes were detected to be XY-specifically expressed genes (SEGs) for one and 2-year-old yellow catfish, while the corresponding numbers of XX-SEGs for those two stages were 19 and 13, respectively. Our work identifies a set of annotated genes that are candidate factors affecting sexual dimorphism as well as simple sequence repeat (SSR) and single nucleotide variation (SNV) in yellow catfish. To validate the expression patterns of the sex-related genes, we performed quantitative real-time PCR (qRT-PCR) indicating the reliability and accuracy of our analysis. The results in our study may enhance our understanding of yellow catfish sex determination and potentially help to improve the production of all-male yellow catfish for aquaculture.

Duplicated leptin receptors in two species of eel bring new insights into the evolution of the leptin system in vertebrates

Since its discovery in mammals as a key-hormone in reproduction and metabolism, leptin has been identified in an increasing number of tetrapods and teleosts. Tetrapods possess only one leptin gene, while most teleosts possess two leptin genes, as a result of the teleost third whole genome duplication event (3R). Leptin acts through a specific receptor (LEPR). In the European and Japanese eels, we identified two leptin genes, and for the first time in vertebrates, two LEPR genes. Synteny analyses indicated that eel LEPRa and LEPRb result from teleost 3R. LEPRb seems to have been lost in the teleost lineage shortly after the elopomorph divergence. Quantitative PCRs revealed a wide distribution of leptins and LEPRs in the European eel, including tissues involved in metabolism and reproduction. Noticeably, leptin1 was expressed in fat tissue, while leptin2 in the liver, reflecting subfunctionalization. Four-month fasting had no impact on the expression of leptins and LEPRs in control European eels. This might be related to the remarkable adaptation of silver eel metabolism to long-term fasting throughout the reproductive oceanic migration. In contrast, sexual maturation induced differential increases in the expression of leptins and LEPRs in the BPG-liver axis. Leptin2 was strikingly upregulated in the liver, the central organ of the reproductive metabolic challenge in teleosts. LEPRs were differentially regulated during sexual maturation, which may have contributed to the conservation of the duplicated LEPRs in this species. This suggests an ancient and positive role of the leptin system in the vertebrate reproductive function. This study brings new insights on the evolutionary history of the leptin system in vertebrates. Among extant vertebrates, the eel represents a unique case of duplicated leptins and leptin receptors as a result of 3R.

Gonadal transcriptomic analysis of yellow catfish (Pelteobagrus fulvidraco): identification of sex-related genes and genetic markers

Yellow catfish (Pelteobagrus fulvidraco) has been recognized as a vital freshwater aquaculture species in East and Southeast Asia. In addition to its commercial interest, it is also attracted much attention because of its value in studying sex-determination mechanisms. A comprehensive gonadal transcriptome analysis is believed to provide a resource for genome annotation, candidate gene identification, and molecular marker development. Herein, we performed a de novo assembly of yellow catfish gonad transcriptome by high-throughput Illumina sequencing. A total of 82,123 contigs were obtained, ranging from 351 to 21,268 bp, and N50 of 2,329 bp. Unigenes of 21,869 in total were identified. Of these, 229 and 1,188 genes were found to be specifically expressed in XY gonad tissue for 1 yr and 2 yr old yellow catfish, respectively; correspondingly, 51 and 40 genes were identified in XX gonad tissue at those two stages. Gene ontology and KEGG analysis were conducted and classified all contigs into different categories. A large number of unigenes involved in sex determination were identified, as well as microsatellites and SNP variants. The expression patterns of sex-related genes were then validated by quantitative real-time PCR (qRT-PCR) suggesting the high reliability of RNA-Seq results. In this study, the transcriptome of yellow catfish gonad was first sequenced, assembled, and characterized; it provides a valuable genomic resource for better understanding of yellow catfish sex determination as well as development of molecular markers, thereby assisting in the production of monosex yellow catfish for aquaculture.

Regulation of the seasonal leptin and leptin receptor expression profile during early sexual maturation and feed restriction in male Atlantic salmon, Salmo salar L., parr

In mammals, leptin acts as an adiposity signal and is a crucial link between nutritional status and the reproductive axis. So far the link between leptin and energy balance during sexual maturation in teleosts has been poorly investigated. In this study, seasonal gene expression changes in two leptin genes (lepa1 and lepa2) and the leptin receptor were investigated during early sexual maturation in male Atlantic salmon parr under fully fed (control) and feed restricted conditions from April through September. Both Atlantic salmon lepa1 and lepa2 in the liver and lepr in the brain were significantly down-regulated in non-maturing control males in early spring, coinciding with the start of the growth and fat accumulation. In maturing control males, hepatic leptin expression increased during mid-spermatogenesis and lepa1 and lepa2 mRNA levels were up-regulated by 7.7 and 49 times respectively during final maturation. For the first time in a fish species, a significant up-regulation of lepr expression was observed in the testis throughout mid to late spermatogenesis. Feed restriction decreased the incidence of sexual maturation by 53% and highly up-regulated both leptin genes in the liver and the leptin receptor in the pituitary. This study shows that hepatic lepa1 and lepa2 expression and lepr expression in the testis is affected by early sexual maturation in male Atlantic salmon parr. Fast growth and high fat stores are associated with low leptin levels while feed restriction has a stimulatory effect on hepatic leptin and leptin receptor gene expression in the pituitary, suggesting a role for leptin other than that as an adiposity signal.

Comparative endocrinology of leptin: assessing function in a phylogenetic context

As we approach the end of two decades of leptin research, the comparative biology of leptin is just beginning. We now have several leptin orthologs described from nearly every major clade among vertebrates, and are moving beyond gene descriptions to functional studies. Even at this early stage, it is clear that non-mammals display clear functional similarities and differences with their better-studied mammalian counterparts. This review assesses what we know about leptin function in mammals and non-mammals, and gives examples of how these data can inform leptin biology in humans.

Neurotransmitter map of the asymmetric dorsal habenular nuclei of zebrafish

The role of the habenular nuclei in modulating fear and reward pathways has sparked a renewed interest in this conserved forebrain region. The bilaterally paired habenular nuclei, each consisting of a medial/dorsal and lateral/ventral nucleus, can be further divided into discrete subdomains whose neuronal populations, precise connectivity, and specific functions are not well understood. An added complexity is that the left and right habenulae show pronounced morphological differences in many non-mammalian species. Notably, the dorsal habenulae of larval zebrafish provide a vertebrate genetic model to probe the development and functional significance of brain asymmetry. Previous reports have described a number of genes that are expressed in the zebrafish habenulae, either in bilaterally symmetric patterns or more extensively on one side of the brain than the other. The goal of our study was to generate a comprehensive map of the zebrafish dorsal habenular nuclei, by delineating the relationship between gene expression domains, comparing the extent of left-right asymmetry at larval and adult stages, and identifying potentially functional subnuclear regions as defined by neurotransmitter phenotype. Although many aspects of habenular organization appear conserved with rodents, the zebrafish habenulae also possess unique properties that may underlie lateralization of their functions.

The emerging use of zebrafish to model metabolic disease

The zebrafish research community is celebrating! The zebrafish genome has recently been sequenced, the Zebrafish Mutation Project (launched by the Wellcome Trust Sanger Institute) has published the results of its first large-scale ethylnitrosourea (ENU) mutagenesis screen, and a host of new techniques, such as the genome editing technologies TALEN and CRISPR-Cas, are enabling specific mutations to be created in model organisms and investigated in vivo. The zebrafish truly seems to be coming of age. These powerful resources invoke the question of whether zebrafish can be increasingly used to model human disease, particularly common, chronic diseases of metabolism such as obesity and type 2 diabetes. In recent years, there has been considerable success, mainly from genomic approaches, in identifying genetic variants that are associated with these conditions in humans; however, mechanistic insights into the role of implicated disease loci are lacking. In this Review, we highlight some of the advantages and disadvantages of zebrafish to address the organism’s utility as a model system for human metabolic diseases.

CART in the brain of vertebrates: circuits, functions and evolution

Cocaine- and amphetamine-regulated transcript peptide (CART) with its wide distribution in the brain of mammals has been the focus of considerable research in recent years. Last two decades have witnessed a steady rise in the information on the genes that encode this neuropeptide and regulation of its transcription and translation. CART is highly enriched in the hypothalamic nuclei and its relevance to energy homeostasis and neuroendocrine control has been understood in great details. However, the occurrence of this peptide in a range of diverse circuitries for sensory, motor, vegetative, limbic and higher cortical areas has been confounding. Evidence that CART peptide may have role in addiction, pain, reward, learning and memory, cognition, sleep, reproduction and development, modulation of behavior and regulation of autonomic nervous system are accumulating, but an integration has been missing. A steady stream of papers has been pointing at the therapeutic potentials of CART. The current review is an attempt at piecing together the fragments of available information, and seeks meaning out of the CART elements in their anatomical niche. We try to put together the CART containing neuronal circuitries that have been conclusively demonstrated as well as those which have been proposed, but need confirmation. With a view to finding out the evolutionary antecedents, we visit the CART systems in sub-mammalian vertebrates and seek the answer why the system is shaped the way it is. We enquire into the conservation of the CART system and appreciate its functional diversity across the phyla.

Enhancer turnover and conserved regulatory function in vertebrate evolution

Mutations in regulatory regions including enhancers are an important source of variation and innovation during evolution. Enhancers can evolve by changes in the sequence, arrangement and repertoire of transcription factor binding sites, but whole enhancers can also be lost or gained in certain lineages in a process of turnover. The proopiomelanocortin gene (Pomc), which encodes a prohormone, is expressed in the pituitary and hypothalamus of all jawed vertebrates. We have previously described that hypothalamic Pomc expression in mammals is controlled by two enhancers-nPE1 and nPE2-that are derived from transposable elements and that presumably replaced the ancestral neuronal Pomc regulatory regions. Here, we show that nPE1 and nPE2, even though they are mammalian novelties with no homologous counterpart in other vertebrates, nevertheless can drive gene expression specifically to POMC neurons in the hypothalamus of larval and adult transgenic zebrafish. This indicates that when neuronal Pomc enhancers originated de novo during early mammalian evolution, the newly created cis- and trans-codes were similar to the ancestral ones. We also identify the neuronal regulatory region of zebrafish pomca and confirm that it is not homologous to the mammalian enhancers. Our work sheds light on the process of gene regulatory evolution by showing how a locus can undergo enhancer turnover and nevertheless maintain the ancestral transcriptional output.

Leptin expression affects metabolic rate in zebrafish embryos (D. rerio)

We used antisense morpholino oligonucleotide technology to knockdown leptin-(A) gene expression in developing zebrafish embryos and measured its effects on metabolic rate and cardiovascular function. Using two indicators of metabolic rate, oxygen consumption was significantly lower in leptin morphants early in development [<48 hours post-fertilization (hpf)], while acid production was significantly lower in morphants later in development (>48 hpf). Oxygen utilization rates in <48 hpf embryos and acid production in 72 hpf embryos could be rescued to that of wildtype embryos by recombinant leptin coinjected with antisense morpholino. Leptin is established to influence metabolic rate in mammals, and these data suggest leptin signaling also influences metabolic rate in fishes.