Leptin mRNA and Protein Immunoreactivity in Adipose Tissue and Liver of Rainbow Trout(Oncorhynchus mykiss) and Immunohistochemical Localization in Liver

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

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

Environmental salinity differentiates responses to acute hypothermal stress in milkfish

Global warming has led to an increase in the frequency of cold extremes, causing significant economic losses in aquaculture, particularly in subtropical regions. Milkfish (Chanos chanos) holds significant importance in aquaculture within subtropical Asian regions. Despite milkfish's ability to tolerate varying salinity levels, frequent cold snaps associated with extreme weather events have caused substantial mortality. Understanding the molecular and cellular mechanisms underlying cold stress-induced cell death is crucial for developing effective strategies to mitigate such losses. Given the pivotal role of the liver in fish physiology, we established a primary milkfish hepatocyte culture demonstrating robust proliferation and expressing a unique marker leptin A. The molecular characterization of cold-treated hepatocytes at 18 °C showed that the mRNA levels of superoxide dismutase (sod1) and catalase (cat), responsible for neutralizing reactive oxygen species (ROS), were downregulated in freshwater (FW) conditions, while cat expression was upregulated in seawater (SW) conditions. This differential modulation of ROS signaling implies distinct responses in FW and SW, leading to higher ROS levels and increased cell death in FW condition compared to those in SW. Transcriptomic analysis of liver tissues from milkfish reared in FW or SW, with or without cold stress, revealed distinct gene expression patterns. Although cold stress affected a common set of genes in both FW and SW conditions, SW-specific cold responsive genes are associated with metabolic pathways while FW-specific genes were linked to cell proliferation pathways. Notably, gene set enrichment analysis highlighted the downregulation of cytochrome-related genes, a major source of ROS production, in response to cold stress in SW. In summary, our study unveils an insightful mechanism whereby the salinity of SW counteracts cold stress-induced ROS signaling, emphasizing the feasibility and practicality of preconditioning fish in SW as a preventive measure against cold stress-induced mortality.

Models of body weight and fatness regulation

Body weight and fatness appear to be regulated phenomena. Several different theoretical models are available to capture the essence of this idea. These include the set-point, dynamic equilibrium, adiposity force, control theory-settling point, Hall-Guo, operation point and dual intervention point (DIP) models. The set-point model posits a single reference point around which levels of fat are regulated. The dynamic equilibrium model suggests that the apparent regulation of body fat around a reference point is an illusion owing to the necessary impacts of weight change on energy expenditure. Control theory focuses on the importance of feedback gain and suggests set-point and dynamic equilibrium are ends of a continuum of feedback gain. Control theory models have also been called 'settling point' models. The Hall-Guo, operation point and DIP models also bring together the set-point and dynamic equilibrium ideas into a single framework. The DIP proposes a zone of indifference where dynamic equilibrium 'regulation' predominates, bounded by upper and lower intervention points beyond which physiological mechanisms are activated. The drifty gene hypothesis is an idea explaining where this individual variation in the upper intervention point might come from. We conclude that further experiments to test between the models are sorely required. This article is part of a discussion meeting issue 'Causes of obesity: theories, conjectures and evidence (Part II)'.

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.

Molecular cloning, characterization and evolutionary analysis of leptin gene in Chinese giant salamander, Andrias davidianus

Leptin is an important hormone possessing diverse physiological roles in mammals and teleosts. However, it has been characterized only in a few amphibian species, and its evolutions are still under debate. Here, the full length of the leptin (Adlep) cDNA of Chinese giant salamander (Andrias davidianus), an early diverging amphibian species, is characterized and according to the results of the primary sequence analysis, tertiary structure reconstruction and phylogenetic analysis is confirmed to be an ortholog of mammalian leptin. An intron was identified between the coding exons of A. davidianus leptin, which indicated that the leptin is present in the salamander genome and contains a conserved gene structure in vertebrates. Adlep is widely distributed but expression levels vary among different tissues, with highest expression levels in the muscle. Additionally, the leptin receptor and other genes were mapped to three known leptin signaling pathways, suggesting that the leptin signaling pathways are present in A. davidianus. Phylogenetic topology of leptins are consistent with the generally accepted evolutionary relationships of vertebrates, and multiple leptin members found in teleosts seem to be obtained through a Cluopeocephala-specific gene duplication event. Our results will lay a foundation for further investigations into the physiological roles of leptin in A. davidianus.

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.

On the Molecular Evolution of Leptin, Leptin Receptor, and Endospanin

Over a decade passed between Friedman's discovery of the mammalian leptin gene (1) and its cloning in fish (2) and amphibians (3). Since 2005, the concept of gene synteny conservation (vs. gene sequence homology) was instrumental in identifying leptin genes in dozens of species, and we now have leptin genes from all major classes of vertebrates. This database of LEP (leptin), LEPR (leptin receptor), and LEPROT (endospanin) genes has allowed protein structure modeling, stoichiometry predictions, and even functional predictions of leptin function for most vertebrate classes. Here, we apply functional genomics to model hundreds of LEP, LEPR, and LEPROT proteins from both vertebrates and invertebrates. We identify conserved structural motifs in each of the three leptin signaling proteins and demonstrate Drosophila Dome protein's conservation with vertebrate leptin receptors. We model endospanin structure for the first time and identify endospanin paralogs in invertebrate genomes. Finally, we argue that leptin is not an adipostat in fishes and discuss emerging knockout models in fishes.

The leptin system and its expression at different nutritional and pregnant stages in lined seahorse (Hippocampus erectus)

Leptin is an essential hormone for the regulation of energy metabolism and food intake in vertebrate animals. To better understand the physiological roles of leptin in nutrient regulation in paternal ovoviviparous fish (family Syngnathidae), the present study cloned the full-length of leptin-a and leptin receptor (lepr) genes in lined seahorse (Hippocampus erectus). Results showed that there was a 576-bp intron between two exons in leptin-a gene but no leptin-b gene in seahorse. Although the primary amino acid sequence conservation of seahorse leptin-a was very low, the 3-D structure modeling of seahorse leptin-a revealed strong conservation of tertiary structure with other vertebrates. Seahorse leptin-a mRNA was highly expressed in brain, whereas lepr mRNA was mainly expressed in ovary and gill. Interestingly, both leptin-a and lepr mRNA were expressed in the brood pouch of male seahorse, suggesting the leptin system plays a role during the male pregnancy. Physiological experiments showed that the expression of hepatic leptin-a and lepr mRNA in unfed seahorses was significantly higher than that in those fed 100%, as well as 60%, of their food during the fasting stage, showing that seahorse might initiate the leptin system to regulate its energy metabolism while starving. Moreover, the expression of leptin-a in the brood pouch of pregnant seahorse was significantly upregulated compared with non-pregnant seahorse, whereas the expression of lepr was downregulated, suggesting that the leptin system might be involved in the male pregnancy. In conclusion, the leptin system plays a role in the energy metabolism and food intake, and might provide new insights into molecular regulation of male pregnancy in seahorse.

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.

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

In mammals, leptin is a peripheral satiety factor that inhibits feeding by regulating a variety of appetite-related hormones in the brain. However, most of the previous studies examining leptin in fish feeding were performed with mammalian leptins, which share very low sequence homologies with fish leptins. To elucidate the function and mechanism of endogenous fish leptins in feeding regulation, recombinant goldfish leptin-AI and leptin-AII were expressed in methylotrophic yeast and purified by immobilized metal ion affinity chromatography (IMAC). By intraperitoneal (IP) injection, both leptin-AI and leptin-AII were shown to inhibit the feeding behavior and to reduce the food consumption of goldfish in 2 h. In addition, co-treatment of leptin-AI or leptin-AII could block the feeding behavior and reduce the food consumption induced by neuropeptide Y (NPY) injection. High levels of leptin receptor (lepR) mRNA were detected in the hypothalamus, telencephalon, optic tectum and cerebellum of the goldfish brain. The appetite inhibitory effects of leptins were mediated by downregulating the mRNA levels of orexigenic NPY, agouti-related peptide (AgRP) and orexin and upregulating the mRNA levels of anorexigenic cocaine-amphetamine-regulated transcript (CART), cholecystokinin (CCK), melanin-concentrating hormone (MCH) and proopiomelanocortin (POMC) in different areas of the goldfish brain. Our study, as a whole, provides new insights into the functions and mechanisms of leptins in appetite control in a fish model.

Roles of leptin and ghrelin in adipogenesis and lipid metabolism of rainbow trout adipocytes in vitro

Leptin and ghrelin are important regulators of energy homeostasis in mammals, whereas their physiological roles in fish have not been fully elucidated. In the present study, the effects of leptin and ghrelin on adipogenesis, lipolysis and on expression of lipid metabolism-related genes were examined in rainbow trout adipocytes in vitro. Leptin expression and release increased from preadipocytes to mature adipocytes in culture, but did not affect the process of adipogenesis. While ghrelin and its receptor were identified in cultured differentiated adipocytes, ghrelin did not influence either preadipocyte proliferation or differentiation, indicating that it may have other adipose-related roles. Leptin and ghrelin increased lipolysis in mature freshly isolated adipocytes, but mRNA expression of lipolysis markers was not significantly modified. Leptin significantly suppressed the fatty acid transporter-1 expression, suggesting a decrease in fatty acid uptake and storage, but did not affect expression of any of the lipogenesis or β-oxidation genes studied. Ghrelin significantly increased the mRNA levels of lipoprotein lipase, fatty acid synthase and peroxisome proliferator-activated receptor-β, and thus appears to stimulate synthesis of triglycerides as well as their mobilization. Overall, the study indicates that ghrelin, but not leptin seems to be an enhancer of lipid turn-over in adipose tissue of rainbow trout, and this regulation may at least partly be mediated through autocrine/paracrine mechanisms. The mode of action of both hormones needs to be further explored to better understand their roles in regulating adiposity in fish.

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.

Effects of nutritional status on plasma leptin levels and in vitro regulation of adipocyte leptin expression and secretion in rainbow trout

As leptin has a key role on appetite, knowledge about leptin regulation is important in order to understand the control of energy balance. We aimed to explore the modulatory effects of adiposity on plasma leptin levels in vivo and the role of potential regulators on leptin expression and secretion in rainbow trout adipocytes in vitro. Fish were fed a regular diet twice daily ad libitum or a high-energy diet once daily at two ration levels; satiation (SA group) or restricted (RE group) to 25% of satiation, for 8weeks. RE fish had significantly reduced growth (p<0.001) and adipose tissue weight (p<0.001), and higher plasma leptin levels (p=0.022) compared with SA fish. Moreover, plasma leptin levels negatively correlated with mesenteric fat index (p=0.009). Adipocytes isolated from the different fish were treated with insulin, ghrelin, leucine, eicosapentaenoic acid or left untreated (control). In adipocytes from fish fed regular diet, insulin and ghrelin increased leptin secretion dose-dependently (p=0.002; p=0.033, respectively). Leptin secretion in control adipocytes was significantly higher in RE than in SA fish (p=0.022) in agreement with the in vivo findings, indicating that adipose tissue may contribute to the circulating leptin levels. No treatment effects were observed in adipocytes from the high-energy diet groups, neither in leptin expression nor secretion, except that leptin secretion was significantly reduced by leucine in RE fish adipocytes (p=0.025). Overall, these data show that the regulation of leptin in rainbow trout adipocytes by hormones and nutrients seems to be on secretion, rather than at the transcriptional level.

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.

A comparative perspective on lipid storage in animals

Lipid storage is an evolutionary conserved process that exists in all organisms from simple prokaryotes to humans. In Metazoa, long-term lipid accumulation is restricted to specialized cell types, while a dedicated tissue for lipid storage (adipose tissue) exists only in vertebrates. Excessive lipid accumulation is associated with serious health complications including insulin resistance, type 2 diabetes, cardiovascular diseases and cancer. Thus, significant advances have been made over the last decades to dissect out the molecular and cellular mechanisms involved in adipose tissue formation and maintenance. Our current understanding of adipose tissue development comes from in vitro cell culture and mouse models, as well as recent approaches to study lipid storage in genetically tractable lower organisms. This Commentary gives a comparative insight into lipid storage in uni- and multi-cellular organisms with a particular emphasis on vertebrate adipose tissue. We also highlight the molecular mechanisms and nutritional signals that regulate the formation of mammalian adipose tissue.

Knockdown of leptin A expression dramatically alters zebrafish development

Using morpholino antisense oligonucleotide (MO) technology, we blocked leptin A or leptin receptor expression in embryonic zebrafish, and analyzed consequences of leptin A knock-down on fish development. Embryos injected with leptin A or leptin receptor MOs (leptin A or leptin receptor morphants) had smaller bodies and eyes, undeveloped inner ear, enlarged pericardial cavity, curved body and/or tail and larger yolk compared to control embryos of the same stages. The defects persisted in 6-9 days old larvae. We found that blocking leptin A function had little effect on the development of early brain (1 day old), but differentiation of both the morphant dorsal brain and retinal cells was severely disrupted in older (2 days old) embryos. Despite the enlarged pericardial cavity, differentiation of cardiac cells appeared to be similar to control embryos. Formation of the morphants' inner ear is also severely disrupted, which corroborates existing reports of leptin receptor expression in inner ear of both zebrafish and mammals. Co-injection of leptin A MO and recombinant leptin results in partial rescue of the wild-type phenotype. Our results suggest that leptin A plays distinct roles in zebrafish development.

Seasonal appetite regulation in the anadromous Arctic charr: evidence for a role of adiposity in the regulation of appetite but not for leptin in signalling adiposity

The aim of this study was to investigate whether the seasonal feeding cycle of the anadromous Arctic charr (Salvelinus alpinus) is regulated by a lipostatic mechanism and if leptin (Lep) might act as an endocrine signal of adiposity. Offspring of anadromous Arctic charr with a body mass of 121 g were divided into two treatment groups; one was given feed in excess from March to November, and the other was fasted between April and early June and fed in excess thereafter. In the continuously fed group there was an 8-fold increase in body mass, and a doubling of percentage body fat, from March to August, after which there was no further increase. Fish in the other group lost weight and body fat during fasting, but grew rapidly on being fed, and had partially compensated for their deficit in body mass by August. Differences in percentage body fat between treatment groups were eliminated by August, providing evidence for a lipostatic regulation of feeding and energy homeostasis in Arctic charr. Neither liver total LepA gene expression nor plasma Lep concentrations correlated positively with fish adiposity, so there was no evidence that Lep acts as a signal of adiposity in this species. On the other hand, there was a strong increase in liver LepA1 gene expression at the end of the fasting period, concomitant with fat mobilization and increased plasma glucose, indicating that LepA1 may play a role in regulating metabolic processes associated with fasting.

Effects of long-term restricted feeding on plasma leptin, hepatic leptin expression and leptin receptor expression in juvenile Atlantic salmon (Salmo salar L.)

Leptin is a pleiotropic hormone and plays a key role in body weight regulation, energy homeostasis and lipid store utilization in mammals. In this study, we investigated the effect of feed-restriction on leptin genes (lepa1 and lepa2), leptin receptor (lepr) gene expression and plasma leptin levels in juvenile Atlantic salmon parr. Feed restriction was performed from late April to mid-June, in order to gain insight into the role of the leptin system in energy balance regulation and adiposity in juvenile salmon. A significant increase in lepa1 expression as well as higher levels of plasma leptin was found in feed-restricted fish in June compared to fully fed controls, while lepa2 gene expression decreased in both groups during the treatment period. Lepa2 was, however significantly higher in the feed-restricted group in June. Leptin receptor expression was up regulated during the period of enhanced growth and lipid deposition in the fully fed control, indicating a seasonal effect on the receptor expression in the brain. Both lepa1 and lepa2 genes very mainly expressed in the liver in juvenile salmon, while lepr was expressed in the brain but showed also considerable expression in various peripheral tissues. The study provides evidence that the leptin system is sensitive to the metabolic status of the fish as both season and restricted feeding affect lepa1 and lepa2 gene expression in the liver and brain leptin receptor expression, however, for lepa1 expression and leptin plasma level in an opposite way as that observed in the mammalian system.

The role of growth hormone in growth, lipid homeostasis, energy utilization and partitioning in rainbow trout: interactions with leptin, ghrelin and insulin-like growth factor I

The growth-promoting effects of in vivo growth hormone (GH) treatment were studied in relation to size and lipid content of energy stores including liver, mesentery, white muscle and belly flap in rainbow trout. In order to elucidate endocrine interactions and links to regulation of growth, adiposity and energy metabolism, plasma levels of GH, insulin-like growth factor I (IGF-I), leptin (Lep) and ghrelin, were assessed and correlated to growth and energy status. In addition tissue-specific expression of lepa1 mRNA was examined. Juvenile rainbow trout were implanted with sustained-release bovine GH implants and terminally sub-sampled at 1, 3 and 6 weeks. GH increased specific growth rate, reduced condition factor (CF) and increased feed conversion efficiency resulting in a redistribution of energy stores. Thus, GH decreased mesenteric (MSI) and liver somatic index (LSI). Lipid content of the belly flap increased following GH-treatment while liver and muscle lipid content decreased. Independent of GH substantial growth was accompanied by an increase in muscle lipids and a decrease in belly flap lipids. The data suggest that the belly flap may function as an energy buffering tissue during episodes of feeding and lean growth. Liver and muscle lipids were positively correlated to body weight, indicating a size-dependent change in adiposity. Hepatic lepa1 mRNA positively correlated to MSI and CF and its expression decreased following GH treatment, coinciding with decreased hepatic lipid content. Plasma Lep was positively correlated to MSI and belly flap lipid content, suggesting that Lep may communicate energy status. In summary, the observed GH tissue-specific effects on lipid metabolism in rainbow trout highlight the complex physiology of the energy reserves and their endocrine control.

Leptin in teleost fishes: an argument for comparative study

All organisms face tradeoffs with regard to how limited energy resources should be invested. When is it most favorable to grow, to reproduce, how much lipid should be allocated to storage in preparation for a period of limited resources (e.g., winter), instead of being used for growth or maturation? These are a few of the high consequence fitness "decisions" that represent the balance between energy acquisition and allocation. Indeed, for animals to make favorable decisions about when to grow, eat, or reproduce, they must integrate signals among the systems responsible for energy acquisition, storage, and demand. We make the argument that leptin signaling is a likely candidate for an integrating system. Great progress has been made understanding the leptin system in mammals, however our understanding in fishes has been hampered by difficulty in cloning fish orthologs of mammalian proteins and (we assert), underutilization of the comparative approach.

An immunohistochemical study of the gut neuroendocrine system in juvenile pejerrey Odontesthes bonariensis (Valenciennes)

In this study, several neuropeptides were identified by immunohistochemistry in neuroendocrine cells (NEC) located in the gut epithelium and nerve cell bodies of the enteric nervous system of pejerrey Odontesthes bonariensis, a species that is a promising candidate for intensive aquaculture. The neuropeptides involved in orexigenic or anorexigenic action, i.e. gastrin, cholecystokinin-8, neuropeptide Y and calcitonin gene-related peptide (CGRP), displayed a significantly higher number of immunoreactive NECs in the anterior intestine, suggesting that this region of the gut plays an important role in the peripheral control of food intake. On the other hand, leu-enkephalin and vasoactive intestinal peptide (VIP), both associated with the modulation of the enteric immune system, showed no significant variations in the mean value of immunopositive NECs between the anterior and posterior intestine. This may indicate that their activity is required at a similar level along the entire gut. In addition, CGRP and VIP-immunoreactive neurons and nerve fibres were observed in the myenteric plexus, which might exert synergistic effects with the neuropeptides immunolocalized in NECs.

Evolution of leptin structure and function

Leptin, the protein product of the obese(ob or Lep) gene, is a hormone synthesized by adipocytes that signals available energy reserves to the brain, and thereby influences development, growth, metabolism and reproduction. In mammals, leptin functions as an adiposity signal: circulating leptin fluctuates in proportion to fat mass, and it acts on the hypothalamus to suppress food intake. Orthologs of mammalian Lep genes were recently isolated from several fish and two amphibian species, and here we report the identification of two Lep genes in a reptile, the lizard Anolis carolinensis. While vertebrate leptins show large divergence in their primary amino acid sequence, they form similar tertiary structures, and may have similar potencies when tested in vitro on heterologous leptin receptors (LepRs). Leptin binds to LepRs on the plasma membrane, activating several intracellular signaling pathways. Vertebrate LepRs signal via the Janus kinase (Jak) and signal transducer and activator of transcription (STAT) pathway. Three tyrosine residues located within the LepR cytoplasmic domain are phosphorylated by Jak2 and are required for activation of SH2-containing tyrosine phosphatase-2, STAT5 and STAT3 signaling. These tyrosines are conserved from fishes to mammals, demonstrating their critical role in signaling by the LepR. Leptin is anorexigenic in representatives of all vertebrate classes, suggesting that its role in energy balance is ancient and has been evolutionarily conserved. In addition to its integral role as a regulator of appetite and energy balance, leptin exerts pleiotropic actions in development, physiology and behavior.