Discovery of a novel functional leptin protein (LEP) in zebra finches: evidence for the existence of an authentic avian leptin gene predominantly expressed in the brain and pituitary

A bird's-eye overview of molecular mechanisms regulating feed intake in chickens-with mammalian comparisons

In recent decades, a lot of research has been conducted to explore poultry feeding behavior. However, up to now, the processes behind poultry feeding behavior remain poorly understood. The review generalizes modern expertise about the hormonal regulation of feeding behavior in chickens, focusing on signaling pathways mediated by insulin, leptin, and ghrelin and regulatory pathways with a cross-reference to mammals. This overview also summarizes state-of-the-art research devoted to hypothalamic neuropeptides that control feed intake and are prime candidates for predictors of feeding efficiency. Comparative analysis of the signaling pathways that mediate the feed intake regulation allowed us to conclude that there are major differences in the processes by which hormones influence specific neuropeptides and their contrasting roles in feed intake control between two vertebrate clades.

Leptin Resistance Does Not Facilitate Migratory Fattening in Ruby-Throated Hummingbirds (Archilochus Colubris)

In mammals, leptin is an important energy homeostasis hormone produced by adipose tissue. Circulating leptin concentrations correlate positively with fat mass and act in a negative feedback fashion to inhibit food intake and increase energy expenditure, thereby preventing fat gain. For some species, leptin resistance is advantageous during times of year where fat gain is necessary (e.g., prior to hibernation). While the function of leptin in birds remains controversial, seasonal leptin resistance may similarly benefit migratory species. Here, we used the ruby-throated hummingbird (Archilochus colubris) to test the hypothesis that leptin resistance promotes fattening prior to migration. We predicted that during the migratory fattening period, leptin levels should correlate positively with fat mass but should not inhibit food intake or increase energy expenditure, resulting in fattening. We tracked the body (fat) mass, the concentration of leptin-like protein in the urine, and the food intake of 12 captive hummingbirds from August 2021 to January 2022. In a subset of hummingbirds, we also quantified voluntary physical activity as a proxy for energy expenditure. We found remarkable age-related variation in fattening strategies, with juveniles doubling their body fat by mid-September and adults exhibiting only a 50% increase. Changes in fat mass were strongly associated with increased food intake and reduced voluntary activity. However, we found no correlation between leptin-like protein concentration and fat mass, food intake, or voluntary activity. Since increased torpor use has been shown to accelerate migratory fattening in ruby-throated hummingbirds, we also hypothesized that leptin is a mediator of torpor use. In an experimental manipulation of circulating leptin, however, we found no change in torpor use, body fat, or food intake. Overall, our findings suggest that leptin may not act as an adipostat in hummingbirds, nor does leptin resistance regulate how hummingbirds fatten prior to migration.

Leptin administration does not influence migratory behaviour in white-throated sparrows (Zonotrichia albicollis)

Migratory flights by birds are among the most energetically demanding forms of animal movement, and are primarily fueled by fat as an energy source. Leptin is a critical fat-regulation hormone associated with energy balance in non-avian species but its function in birds is highly controversial. Prior research indicated the effects of leptin differed between birds in migratory condition or not, but no research has assessed the effect of leptin on migratory behaviour itself. In this study, our objective was to determine if leptin affects migratory restlessness and fat deposition in migratory songbirds. We used photoperiod manipulation to induce spring migratory condition, and measured migratory restlessness in leptin-injected and saline-injected white-throated sparrows (Zonotrichia albicollis). Leptin treatment had no effect on migratory restlessness nor fat deposition, providing evidence that leptin does not influence avian migratory motivation or behaviour. Our results also further support the idea that birds in a hyperphagic migratory condition may be insensitive to leptin.

Seasonal and sexual variation in mRNA expression of selected adipokine genes affecting fat deposition and metabolism of the emu (Dromaius novaehollandiae)

Emus are farmed for fat production. Oil rendered from their back and abdominal fat pads has good anti-oxidant and anti-inflammatory properties and has ingredients that promote cell growth. Our objective is to examine the mRNA expression of 7 emu adipokine genes (eFABP4, eSCD1, eAdipoQ, eAdipoR1, eAdipoR2, eLEP and eLepR) to identify gene markers that may help improve emu fat production. Back and abdominal fat tissues from 11 adult emus were biopsied at four time points (April, June, August and November). Total RNA was isolated and cDNA was synthesized. Gene specific primers were designed for partial cloning fragments to amplify the open reading frame of the 7 genes. eLEP was not expressed in emu fat tissue. Nucleotides and amino acids sequences of the 6 expressed gene were compared with homologs from other species and phylogenetic relationships established. Seasonal mRNA expression of each gene was assessed by quantitative RT-PCR and differential expression analysed by the 2-ΔΔCT method. The 6 expressed genes showed seasonal variation in expression and showed association of expression level with back fat adiposity. More whole-genome scanning studies are needed to develop novel molecular markers that can be applied to improve fat production in emus.

Adipokines in metabolic and reproductive functions in birds: An overview of current knowns and unknowns

Adipose tissue is now recognized as an active endocrine organ, which synthesizes and secretes numerous peptides factors called adipokines. In mammals, they exert pleiotropic effects affecting energy metabolism but also fertility. In mammals, secretion of adipokines is altered in adipose tissue dysfunctions and may participate to obesity-associated disorders. Thus, adipokines are promising candidates both for novel pharmacological treatment strategies and as diagnostic tools. As compared to mammals, birds exhibit several unique physiological features, which make them an interesting model for comparative studies on endocrine control of metabolism and adiposity and reproductive functions. Some adipokines such as leptin and visfatin may have different roles in avian species as compared to mammals. In addition, some of them found in mammals such as CCL2 (chemokine ligand 2), resistin, omentin and FGF21 (Fibroblast Growth factor 21) have not yet been mapped to the chicken genome model and among its annotated gene models. This brief review aims to summarize data (structure, metabolic and reproductive roles and molecular mechanisms involved) related to main avian adipokines (leptin, adiponectin, visfatin, and chemerin) and we will briefly discuss the adipokines that are still lacking in avian species.

Cocaine- and amphetamine-regulated transcript peptide- and dopamine-containing systems interact in the ventral tegmental area of the zebra finch, Taeniopygia guttata, during dynamic changes in energy status

The mesolimbic dopamine (DA)-pathway regulates food-reward, feeding-related behaviour and energy balance. Evidence underscores the importance of feeding-related neuropeptides in modulating activity of these DA neurons. The neuropeptide, CART, a crucial regulator of energy balance, modulates DA-release, and influences the activity of ventral tegmental area (VTA) DAergic neurons in the mammalian brain. Whether CART- and DA-containing systems interact at the level of VTA to regulate energy balance, however, is poorly understood. We explored the interaction between CART- and DA-containing systems in midbrain of the zebra finch, Taeniopygia guttata, an interesting model to study dynamic changes in energy balance due to higher BMR/daytime body temperature, and rapid responsiveness of the feeding-related neuropeptides to changes in energy state. Further, its midbrain DA-neurons share similarities with those in mammals. In the midbrain, tyrosine hydroxylase-immunoreactive (TH-i) neurons were seen in the substantia nigra (SN) and VTA [anterior (VTAa), mid (VTAm) and caudal (VTAc)]; those in VTA were smaller. In the VTA, CART-immunoreactive (CART-i)-fibers densely innervated TH-i neurons, and both CART-immunoreactivity (CART-ir) and TH-immunoreactivity (TH-ir) responded to energy status-dependent changes. Compared to fed and fasted birds, refeeding dramatically enhanced TH-ir and the percentage of TH-i neurons co-expressing FOS in the VTA. Increased prepro-CART-mRNA, CART-ir and a transient appearance of CART-i neurons was observed in VTAa of fasted, but not fed birds. To test the functional interaction between CART- and DA-containing systems, ex-vivo superfused midbrain-slices were treated with CART-peptide and changes in TH-ir analysed. Compared to control tissues, CART-treatment increased TH-ir in VTA but not SN. We propose that CART is a potential regulator of VTA DA-neurons and energy balance in T. guttata.

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.

Impairment of testes development in Yangzhou ganders by augmentation of leptin receptor signaling

The aim of this study was to determine the cellular and molecular mechanisms of leptin (LEP) and the leptin receptor (LEPR) in testicular development of prepubertal ganders. In an in vivo animal experiment, active immunization against LEPR severely depressed prepubertal testicular development by significantly reducing testicular weights at 200 and 227 days of age. The number of elongated spermatids in the seminiferous tubules was also significantly decreased by immunization with LEPR at ages of 200 and 227 days. Inhibition of testicular development by LEPR immunization was associated with decreases in LHR, StAR, 3β-HSD, CYP11A1, CYP17A1, and PRLR mRNA expression levels in testicular tissue, which resulted in a significant decrease in testosterone synthesis. In the in vitro experiments, the addition of LEP combined with anti-LEPR antibodies strengthened LEPR signal transduction, and inhibited significantly testosterone production in cultured Leydig cells isolated from prepubertal gander testes. The mRNA expression of LHR, StAR, 3β-HSD, CYP11A1, CYP17A1 also decreased significantly after treatment with LEP combined with anti-LEPR antibodies in cultured Leydig cells. These results suggest that anti-LEPR antibodies strengthen LEPR signaling transduction in the presence of LEP, and immunization against LEPR inhibited testes development and testosterone secretion in prepubertal ganders.

Central signalling cross-talk between insulin and leptin in glucose and energy homeostasis

Energy homeostasis is controlled by an intricate regulatory system centred in the brain. The peripheral adiposity signals insulin and leptin play a crucial role in this system by informing the brain of the energy status of the body and mediating their catabolic effects through signal transduction in hypothalamic areas that control food intake, energy expenditure and glucose metabolism. Disruptions of insulin and leptin signalling can result in diabetes and obesity. The central signalling cross-talk between insulin and leptin is essential for maintenance of normal healthy energy homeostasis. An important role of leptin in glucoregulation has been revealed. Typically regarded as being controlled by insulin, the control of glucose homeostasis critically depends on functional leptin action. Leptin, on the other hand, is able to lower glucose levels in the absence of insulin, although insulin is necessary for long-term stabilisation of euglycaemia. Evidence from rodent models and human patients suggests that leptin improves insulin sensitivity in type 1 diabetes. The signalling cross-talk between insulin and leptin is likely conveyed by the WNT/β-catenin pathway. Leptin activates WNT/β-catenin signalling, leading to inhibition of glycogen synthase kinase-3β, a key inhibitor of insulin action, thereby facilitating improved insulin signal transduction and sensitisation of insulin action. Interestingly, insights into the roles of insulin and leptin in insects and fish indicate that leptin may have initially evolved as a glucoregulatory hormone and that its anorexigenic and body weight regulatory function was acquired throughout evolution. Furthermore, the regulation of both central and peripheral control of energy homeostasis is tightly controlled by the circadian clock, allowing adaptation of homeostatic processes to environmental cues.

β2 adrenergic receptors and leptin interplay to decrease food intake in chicken

1. The present study was designed to examine the effects of intracerebroventricular (ICV) injection of different α and [Formula: see text] adrenergic receptor antagonists on leptin-induced hypophagia in broiler chickens.2. The study consisted of six experiments. In all experiments, chickens were deprived of feed for 3 h prior to the ICV injections and thereafter were returned immediately to the individual cages and cumulative feed intake, based on the percentage of body weight, was measured at 30, 60 and 120 min post-injection.3. In experiment 1, leptin (2.5, 5 or 10 µg) were injected in birds. In experiment 2, groups received either control solution, prazosin (10 nmol), leptin (10 µg) or a co-injection of prazosin (10 nmol) and leptin (10 µg). The other experiments were conducted as experiment 2, but instead of prazosine (10 nmol), yohimbine (13 nmol) was used in experiment 3, metoprolol (24 nmol) in experiment 4, ICI 118,551 (5 nmol) in experiment 5 and SR 59230R (5 nmol) in experiment 6 were injected either in a group or in combination with leptin (10 µg).4. The results of this study revealed a dose-dependent hypophagic effect of leptin and, in experiment 5, ICV co-injection of ICI118, 551 (5 nmol) and leptin (10 µg) significantly attenuated this effect (P˂0.5). These results suggest that the hypophagic effect of leptin is probably mediated by β2 adrenergic receptors in chickens.

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

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

Avian Leptin: Bird's-Eye View of the Evolution of Vertebrate Energy-Balance Control

Discovery of the satiety hormone leptin in 1994 and its characterization in mammals provided a key tool to deciphering the complex mechanism governing adipose tissue regulation of appetite and energy expenditure. Surprisingly, despite the perfectly logical notion of an energy-storing tissue announcing the amount of fat stores using leptin signaling, alternate mechanisms were chosen in bird evolution. This conclusion emerged based on the recent discovery and characterization of genuine avian leptin - after it had been assumed missing by some, and erroneously identified by others. Critical evaluation of the past and present indications of the role of leptin in Aves provides a new perspective on the evolution of energy-balance control in vertebrates; proposing a regulation strategy alternative to the adipostat mechanism.

Avian Expression Patterns and Genomic Mapping Implicate Leptin in Digestion and TNF in Immunity, Suggesting That Their Interacting Adipokine Role Has Been Acquired Only in Mammals

In mammals, leptin and tumor-necrosis factor (TNF) are prominent interacting adipokines mediating appetite control and insulin sensitivity. While TNF pleiotropically functions in immune defense and cell survival, leptin is largely confined to signaling energy stores in adipocytes. Knowledge about the function of avian leptin and TNF is limited and they are absent or lowly expressed in adipose, respectively. Employing radiation-hybrid mapping and FISH-TSA, we mapped TNF and its syntenic genes to chicken chromosome 16 within the major histocompatibility complex (MHC) region. This mapping position suggests that avian TNF has a role in regulating immune response. To test its possible interaction with leptin within the immune system and beyond, we compared the transcription patterns of TNF, leptin and their cognate receptors obtained by meta-analysis of GenBank RNA-seq data. While expression of leptin and its receptor (LEPR) were detected in the brain and digestive tract, TNF and its receptor mRNAs were primarily found in viral-infected and LPS-treated leukocytes. We confirmed leptin expression in the duodenum by immunohistochemistry staining. Altogether, we suggest that whereas leptin and TNF interact as adipokines in mammals, in birds, they have distinct roles. Thus, the interaction between leptin and TNF may be unique to mammals.

Peripherally injected ghrelin and leptin reduce food hoarding and mass gain in the coal tit (Periparus ater)

In birds little is known about the hormonal signals that communicate nutritional state to the brain and regulate appetitive behaviours. In mammals, the peptide hormones ghrelin and leptin elevate and inhibit consumption and food hoarding, respectively. But in birds, administration of both ghrelin and leptin inhibit food consumption. The role of these hormones in the regulation of food hoarding in avian species has not been examined. To investigate this, we injected wild caught coal tits (Periparus ater) with leptin, high-dose ghrelin, low-dose ghrelin and a saline control in the laboratory. We then measured food hoarding and mass gain, as a proxy of food consumption, every 20 min for 2 h post-injection. Both high-dose ghrelin and leptin injections significantly reduced hoarding and mass gain compared with controls. Our results provide the first evidence that hoarding behaviour can be reduced by both leptin and ghrelin in a wild bird. These findings add to evidence that the hormonal control of food consumption and hoarding in avian species differs from that in mammals. Food hoarding and consumptive behaviours consistently show the same response to peripheral signals of nutritional state, suggesting that the hormonal regulation of food hoarding has evolved from the consumption regulatory system.

Comparative omics and feeding manipulations in chicken indicate a shift of the endocrine role of visceral fat towards reproduction

Background

The mammalian adipose tissue plays a central role in energy-balance control, whereas the avian visceral fat hardly expresses leptin, the key adipokine in mammals. Therefore, to assess the endocrine role of adipose tissue in birds, we compared the transcriptome and proteome between two metabolically different types of chickens, broilers and layers, bred towards efficient meat and egg production, respectively.

Results

Broilers and layer hens, grown up to sexual maturation under free-feeding conditions, differed 4.0-fold in weight and 1.6-fold in ovarian-follicle counts, yet the relative accumulation of visceral fat was comparable. RNA-seq and mass-spectrometry (MS) analyses of visceral fat revealed differentially expressed genes between broilers and layers, 1106 at the mRNA level (FDR ≤ 0.05), and 203 at the protein level (P ≤ 0.05). In broilers, Ingenuity Pathway Analysis revealed activation of the PTEN-pathway, and in layers increased response to external signals. The expression pattern of genes encoding fat-secreted proteins in broilers and layers was characterized in the RNA-seq and MS data, as well as by qPCR on visceral fat under free feeding and 24 h-feed deprivation. This characterization was expanded using available RNA-seq data of tissues from red junglefowl, and of visceral fat from broilers of different types. These comparisons revealed expression of new adipokines and secreted proteins (LCAT, LECT2, SERPINE2, SFTP1, ZP1, ZP3, APOV1, VTG1 and VTG2) at the mRNA and/or protein levels, with dynamic gene expression patterns in the selected chicken lines (except for ZP1; FDR/P ≤ 0.05) and feed deprivation (NAMPT, SFTPA1 and ZP3) (P ≤ 0.05). In contrast, some of the most prominent adipokines in mammals, leptin, TNF, IFNG, and IL6 were expressed at a low level (FPKM/RPKM< 1) and did not show differential mRNA expression neither between broiler and layer lines nor between fed vs. feed-deprived chickens.

Conclusions

Our study revealed that RNA and protein expression in visceral fat changes with selective breeding, suggesting endocrine roles of visceral fat in the selected phenotypes. In comparison to gene expression in visceral fat of mammals, our findings points to a more direct cross talk of the chicken visceral fat with the reproductive system and lower involvement in the regulation of appetite, inflammation and insulin resistance.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4675-0) contains supplementary material, which is available to authorized users.

Anti-leptin receptor antibodies strengthen leptin biofunction in growing chickens

Antibodies against the extracellular domains of the chicken leptin receptor were used to study the biological function of leptin in growing chickens. Both polyclonal and monoclonal anti-LEPR antibodies were administered intramuscularly to 30-d-old Chinese indigenous Gushi pullets. Both antibody preparations increased feed intake for 6 h after injection and reduced plasma concentrations of glucose, triglycerides, and both high- and low-density lipoproteins. The antibody treatments also upregulated agouti-related peptide and neuropeptide Y in the hypothalamus and downregulated proopiomelanocortin, melanocortin 4 receptor, and leptin receptor. The treatments also upregulated leptin receptor, acetyl CoA carboxylase beta, and acyl-CoA oxidase in the liver, abdominal fat, and breast muscle and downregulated sterol regulatory element-binding protein-1 and fatty acid synthase. Furthermore, even though the anti-leptin receptor antibodies failed to affect leptin receptor signaling transduction when administered alone, they did augment the induction of leptin receptor signaling transduction by leptin. These results demonstrate that antibodies against the extracellular domains of leptin-specific receptor enhance, but do not mimic, the ability of leptin to activate receptors. Furthermore, the enhanced leptin bioactivity observed after the intramuscular injection of anti-LEPR antibodies confirmed the occurrence of de novo leptin in the peripheral tissues and blood of treated chickens.

Obese super athletes: fat-fueled migration in birds and bats

Migratory birds are physiologically specialized to accumulate massive fat stores (up to 50-60% of body mass), and to transport and oxidize fatty acids at very high rates to sustain flight for many hours or days. Target gene, protein and enzyme analyses and recent -omic studies of bird flight muscles confirm that high capacities for fatty acid uptake, cytosolic transport, and oxidation are consistent features that make fat-fueled migration possible. Augmented circulatory transport by lipoproteins is suggested by field data but has not been experimentally verified. Migratory bats have high aerobic capacity and fatty acid oxidation potential; however, endurance flight fueled by adipose-stored fat has not been demonstrated. Patterns of fattening and expression of muscle fatty acid transporters are inconsistent, and bats may partially fuel migratory flight with ingested nutrients. Changes in energy intake, digestive capacity, liver lipid metabolism and body temperature regulation may contribute to migratory fattening. Although control of appetite is similar in birds and mammals, neuroendocrine mechanisms regulating seasonal changes in fuel store set-points in migrants remain poorly understood. Triacylglycerol of birds and bats contains mostly 16 and 18 carbon fatty acids with variable amounts of 18:2n-6 and 18:3n-3 depending on diet. Unsaturation of fat converges near 70% during migration, and unsaturated fatty acids are preferentially mobilized and oxidized, making them good fuel. Twenty and 22 carbon n-3 and n-6 polyunsaturated fatty acids (PUFA) may affect membrane function and peroxisome proliferator-activated receptor signaling. However, evidence for dietary PUFA as doping agents in migratory birds is equivocal and requires further study.

Avian genomics lends insights into endocrine function in birds

The genomics era has brought along the completed sequencing of a large number of bird genomes that cover a broad range of the avian phylogenetic tree (>30 orders), leading to major novel insights into avian biology and evolution. Among recent findings, the discovery that birds lack a large number of protein coding genes that are organized in highly conserved syntenic clusters in other vertebrates is very intriguing, given the physiological importance of many of these genes. A considerable number of them play prominent endocrine roles, suggesting that birds evolved compensatory genetic or physiological mechanisms that allowed them to survive and thrive in spite of these losses. While further studies are needed to establish the exact extent of avian gene losses, these findings point to birds as potentially highly relevant model organisms for exploring the genetic basis and possible therapeutic approaches for a wide range of endocrine functions and disorders.

Chicken Is a Useful Model to Investigate the Role of Adipokines in Metabolic and Reproductive Diseases

Reproduction is a complex and essential physiological process required by all species to produce a new generation. This process involves strict hormonal regulation, depending on a connection between the hypothalamus-pituitary-gonadal axis and peripheral organs. Metabolic homeostasis influences the reproductive functions, and its alteration leads to disturbances in the reproductive functions of humans as well as animals. For a long time, adipose tissue has been recognised as an endocrine organ but its ability to secrete and release hormones called adipokines is now emerging. Adipokines have been found to play a major role in the regulation of metabolic and reproductive processes at both central and peripheral levels. Leptin was initially the first adipokine that has been described to be the most involved in the metabolism/reproduction interrelation in mammals. In avian species, the role of leptin is still under debate. Recently, three novel adipokines have been discovered: adiponectin (ADIPOQ, ACRP30), visfatin (NAMPT, PBEF), and chemerin (RARRES2, TIG2). However, their mode of action between mammalian and nonmammalian species is different due to the different reproductive and metabolic systems. Herein, we will provide an overview of the structure and function related to metabolic and reproductive mechanisms of the latter three adipokines with emphasis on avian species.

Gene expression profiling in embryonic chicken ovary during asymmetric development

The reproductive system in female birds arises as bilateral asymmetrical anlagen, excluding the birds of prey. Earlier, histological and messenger RNA (mRNA) expression profile studies of several genes related to gonadal sex differentiation in chicken embryos tried to elucidate the query of this asymmetry in a scattered manner. To understand the matter precisely, we have focused on mRNA expression of a cohort of genes (FSHR, CYP19A1, caspase 3, caspase 8) in second half of the embryonic days (E10-E18). The established role of leptin in development of the embryo and its expression in the embryonic ovary also drove us to check leptin receptor (LEPR) expression in the ovary. Increased expression of FSHR and CYP19A1 in the left ovary compared with that in the right ovary was identified (P < 0.05), promoting preferential left ovarian development and functionality. Significant high expression (P < 0.05) of the apoptotic genes in the right ovary were also involved here. Leptin probably has no direct influence on ovarian asymmetry as no significant variation in gonadal mRNA expression of LEPR was observed within the same experimental days. We propose that asymmetric expression of this cohort of genes (FSHR, CYP19A1, caspase 3, caspase 8) leads to the development of dimorphic gonads during embryogenesis.

The interaction of MC3R and MC4R with MRAP2, ACTH, α-MSH and AgRP in chickens

The interaction of melanocortin-4 (MC4R) and melanocortin-3 (MC3R) receptors with proopiomelanocortin (POMC)-derived peptides (e.g. α-MSH), agouti-related protein (AgRP) and melanocortin-2 receptor accessory protein 2 (MRAP2) is suggested to play critical roles in energy balance of vertebrates. However, evidence on their interaction in birds remains scarce. Our study aims to reveal their interaction in chickens and the results showed that (1) chicken (c-)MC3R and cMC4R expressed in Chinese hamster ovary (CHO) cells can be activated by α-MSH and ACTH_1-39 equipotently, monitored by a pGL3-CRE-luciferase reporter system; (2) cMC3R and cMC4R, when co-expressed with cMRAP2 (or cMRAP, a cMRAP2 homolog), show increased sensitivity to ACTH treatment and thus likely act as ACTH-preferring receptors, and the interaction between cMC3R/cMC4R and cMRAP2 was demonstrated by co-immunoprecipitation assay; (3) both cMC3R and cMC4R display constitutive activity when expressed in CHO cells, as monitored by dual-luciferase reporter assay, and cMRAP2 (and cMRAP) can modulate their constitutive activity; (4) AgRP inhibits the constitutive activity of cMC3R/cMC4R, and it also antagonizes ACTH/α-MSH action on cMC4R/cMC3R, indicating that AgRP functions as the inverse agonist and antagonist for both receptors. These findings, together with the co-expression of cMC4R, cMC3R, cMRAP2, cAgRP and cPOMC in chicken hypothalamus detected by quantitative real-time PCR, suggest that within the hypothalamus, α-MSH/ACTH, AgRP and MRAP2 may interact at the MC4R(/MC3R) interface to control energy balance. Furthermore, our data provide novel proof for the involvement of MRAP2 (and MRAP) in fine-tuning the constitutive activity and ligand sensitivity and selectivity of both MC3R and MC4R in vertebrates.

Central injection of a synthetic chicken partial leptin peptide does not affect food intake in chicks

Leptin is an adipose tissue-derived hormone in mammals that plays an important role in whole body energy balance via its inhibitory effects on food intake mediated through the hypothalamus. Chicken leptin has a low sequence homology to mammalian leptin and its role in appetite regulation is not reported; hence the objective of this study was to determine effects of central injection of chicken leptin on food and water intake and associated behaviors in chicks. Chicks were intracerebroventricularly injected with 0 (vehicle), 0.3, 1.0, or 3.0 nmol of a synthetic chicken leptin partial peptide and food and water intake were monitored. There were no effects observed and a second experiment was conducted to evaluate food and water intake at higher doses; after injection of 0, 2.5, 5.0, or 10.0 nmol leptin. Again, there were no effects on food or water intake. In the third experiment, behaviors were analyzed during the first 30 min post-injection of vehicle or 10 nmol leptin. At 5 min post-injection, vehicle-injected chicks spent more time sitting than leptin-injected chicks. A wide dose range was evaluated however, the absence of an effect on food intake or behavior suggests that the chicken leptin peptide that was tested does not mediate effects on appetite in the brain and that chicken leptin likely has a different physiological role in birds than in mammals.

Gut Hormones and Regulation of Food Intake in Birds

Gut hormones act as appetite regulatory hormones in mammals. For example, the hunger hormone ghrelin, which is released from the stomach before food intake, stimulates appetite. In contrast, satiety hormones such as cholecystokinin, glucagon-like peptide-1, and peptide YY, which are released from the intestines after food intake, suppress appetite. The effects of these peptides on food intake have been shown to be similar in both mammals and fishes. However, evidence suggests that the physiological roles of these gut hormones may be different between birds and other vertebrates. This review summarizes the current information on the roles of gut hormones in the regulation of food intake in birds, especially in chickens.

Influence of leptin and GABAB-receptor agonist and antagonist on neurons of the hypothalamic infundibular nucleus in the chicken

In birds and mammals, the neuroendocrine regulation of energy balance is conserved in many aspects. Despite significant similarities between the two groups, differences in the regulatory mechanisms were detected. The present study was performed to carry out investigations of the influence of human leptin and GABA_B-receptor agonist and antagonist on the firing rate of neurons of the Nucleus infundibuli hypothalami in brain slices from juvenile chickens. For the first time, we demonstrated a clear, dose-related change in the firing rate of hypothalamic neurons in juvenile chickens after the acute application of recombinant human leptin (1, 10, and 100 nM). All investigated neurons increased their subsequent firing rate. Application of GABA_B-receptor agonist baclofen (1 µM) blocked, while antagonist CGP 35348 (10 µM) increased the spontaneous neuronal activity. Simultaneous application of baclofen and leptin reduced the effect observed from single leptin application. This was not found after simultaneously application of leptin and CGP. Altogether, our results indicate that in bird brain slices, and exemplarily in those of the chicken, hypothalamic neurons show mammalian-like responsiveness after acute leptin and GABA application. GABA_B-mechanisms involved in GABA release play a likely important role in the leptin-mediated effects on NI neurons via functional leptin receptors.

Ghrelin affects stopover decisions and food intake in a long-distance migrant

Billions of birds migrate long distances to either reach breeding areas or to spend the winter at more benign places. On migration, most passerines frequently stop over to rest and replenish their fuel reserves. To date, we know little regarding how they decide that they are ready to continue their journey. What physiological signals tell a bird's brain that its fuel reserves are sufficient to resume migration? A network of hormones regulates food intake and body mass in vertebrates, including the recently discovered peptide hormone, ghrelin. Here, we show that ghrelin reflects body condition and influences migratory behavior of wild birds. We measured ghrelin levels of wild garden warblers (Sylvia borin) captured at a stopover site. Further, we manipulated blood concentrations of ghrelin to test its effects on food intake and migratory restlessness. We found that acylated ghrelin concentrations of garden warblers with larger fat scores were higher than those of birds without fat stores. Further, injections of unacylated ghrelin decreased food intake and increased migratory restlessness. These results represent experimental evidence that appetite-regulating hormones control migratory behavior. Our study lays a milestone in migration physiology because it provides the missing link between ecologically dependent factors such as condition and timing of migration. In addition, it offers insights in the regulation of the hormonal system controlling food intake and energy stores in vertebrates, whose disruption causes eating disorders and obesity.

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.

Regulation of Agouti-Related Protein and Pro-Opiomelanocortin Gene Expression in the Avian Arcuate Nucleus

The arcuate nucleus is generally conserved across vertebrate taxa in its neuroanatomy and neuropeptide expression. Gene expression of agouti-related protein (AGRP), neuropeptide Y (NPY), pro-opiomelanocortin (POMC), and cocaine- and amphetamine-regulated transcript (CART) has been established in the arcuate nucleus of several bird species and co-localization demonstrated for AGRP and NPY. The proteins encoded by these genes exert comparable effects on food intake in birds after central administration to those seen in other vertebrates, with AGRP and NPY being orexigenic and CART and α-melanocyte-stimulating hormone anorexigenic. We have focused on the measurement of arcuate nucleus AGRP and POMC expression in several avian models in relation to the regulation of energy balance, incubation, stress, and growth. AGRP mRNA and POMC mRNA are, respectively, up- and downregulated after energy deprivation and restriction. This suggests that coordinated changes in the activity of AGRP and POMC neurons help to drive the homeostatic response to replace depleted energy stores in birds as in other vertebrates. While AGRP and POMC expression are generally positively and negatively correlated with food intake, respectively, we review here situations in some avian models in which AGRP gene expression is dissociated from the level of food intake and may have an influence on growth independent of changes in appetite. This suggests the possibility that the central melanocortin system exerts more pleiotropic functions in birds. While the neuroanatomical arrangement of AGRP and POMC neurons and the sensitivity of their activity to nutritional state appear generally conserved with other vertebrates, detailed knowledge is lacking of the key nutritional feedback signals acting on the avian arcuate nucleus and there appear to be significant differences between birds and mammals. In particular, recently identified avian leptin genes show differences between bird species in their tissue expression patterns and appear less closely linked in their expression to nutritional state. It is presently uncertain how the regulation of the central melanocortin system in birds is brought about in the situation of the apparently reduced importance of leptin and ghrelin compared to mammals.

Effect of ghrelin and leptin receptors genes polymorphisms on production results and physicochemical characteristics of M. pectoralis superficialis in broiler chickens

Ghrelin and leptin and their receptors GHSR and LEPR regulate food intake, the processes in adipose tissue, and the body’s energy homeostasis in mammals. The aim of the present study was to determine the effect of GHSR/Csp6I and LEPR/Bsh1236I polymorphisms on the meat production parameters of broiler chickens reared to 42 days of age. In 318 fast-growing Hubbard Flex and Ross 308 chickens, g.3051C > T substitution at the GHSR locus and a GGTCAA deletion at positions g.3407_3409del and g.3411_3413del were identified. The use of restriction enzyme Bsh1236I showed the presence of two transitions g.352C > T and g.427G > A in LEPR locus. The chickens were classified into four GHSR/Csp6I and into five LEPR/Bsh1236I diplotypes. GHSR and LEPR polymorphisms were found to influence final bodyweight, daily gain, dressing percentage without giblets, proportion of giblets and the quality characteristics of M. pectoralis superficialis. GHSR/Csp6I and LEPR/Bsh1236I had an effect on pH24 h (P < 0.05) and lightness (L*) of M. pectoralis superficialis (P < 0.05), whereas GHSR/Csp6I influenced shear force (P < 0.05) and thawing loss (P < 0.05). GHSR/Csp6I and LEPR/Bsh1236I were found to have no effect on the abdominal fat content in chicken carcasses. Single nucleotide polymorphisms reported in the present study could be used in breeding programs as selection markers for growth traits and poultry meat quality.

Molecular characterization of neuropeptide Y (NPY) receptors (Y1, Y4 and Y6) and investigation of the tissue expression of their ligands (NPY, PYY and PP) in chickens

Neuropeptide Y (NPY) receptors and its ligands, NPY, peptide YY (PYY) and pancreatic polypeptide (PP), are suggested to regulate many physiological processes including food intake in birds. However, our knowledge regarding this avian NPY system remains rather limited. Here, we examined the tissue expression of NPY, PYY and PP and the gene structure, expression and signaling of three NPY receptors (cY1, cY4 and cY6) in chickens. The results showed that 1) NPY is widely expressed in chicken tissues with abundance noted in the hypothalamus via quantitative real-time PCR, whereas PYY is highly expressed in the pancreas, gastrointestinal tract and various brain regions, and PP is expressed almost exclusively in the pancreas; 2) cY1, cY4 and cY6 contain novel non-coding exon(s) at their 5'-UTR; 3) The wide tissue distribution of cY1 and cY4 and cY6 were detected in chickens by quantitative real-time PCR and their expression is controlled by the promoter near exon 1, which displays strong promoter activity in DF-1 cells as demonstrated by Dual-luciferase reporter assay; 4) Monitored by luciferase reporter assays, activation of cY1 and cY4 expressed in HEK293 cells by chicken NPY_1-36, PYY_1-37, and PP_1-36 treatment inhibits cAMP/PKA and activates MAPK/ERK signaling pathways, while cY6-expressing cells show little response to peptide treatment, indicating that cY1 and cY4, and not cY6, can transmit signals in vitro. Taken together, our study offers novel information about the expression and functionality of cY1, cY4, cY6 and their ligands in birds, and helps to decipher their conserved roles in vertebrates.

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.

Impaired central leptin signaling and sensitivity in rainbow trout with high muscle adiposity

The hormone leptin has been identified in all vertebrate classes, but its physiological roles in non-mammalian vertebrates are not well defined. To elucidate leptin regulation in energy homeostasis in a teleost fish species, this study compares hypothalamic and pituitary leptin signaling systems in energetically divergent rainbow trout lines selected for low (lean line, LL) and high (fat line, FL) muscle adiposity under feeding and starvation conditions. In fed fish, hypothalamic gene expression and protein density of the full-functional leptin receptor (LepRL), as well as a leptin binding protein (LepBP) expression, are lower in FL than LL fish. The FL fish have also lower activation of leptin-relevant signaling pathways involving protein kinase B (Akt) and extracellular signal-related kinase. These observations suggests impaired central leptin action in FL fish. During fasting, hypothalamic LepRL and LepBP expression, as well as active Akt levels are downregulated after one week, while pituitary LepRL expression is upregulated, in the LL fish only. After four weeks, hypothalamic LepRL protein levels return to normal levels in both fish lines and Akt is reactivated, although not to the same extent in FL as in LL fish, indicating that FL fish have low leptin sensitivity to nutritional changes. Neuropeptide Y and orexin expression is downregulated to similar levels in both fish lines after one-week fasting. The divergent leptin system profiles between the two fish lines demonstrate that phenotypic selection for high muscle adiposity affects leptin endocrinology, indicating regulatory roles for leptin in rainbow trout energy homeostasis.

Identification of the Long-Sought Leptin in Chicken and Duck: Expression Pattern of the Highly GC-Rich Avian leptin Fits an Autocrine/Paracrine Rather Than Endocrine Function

More than 20 years after characterization of the key regulator of mammalian energy balance, leptin, we identified the leptin (LEP) genes of chicken (Gallus gallus) and duck (Anas platyrhynchos). The extreme guanine-cytosine content (∼70%), the location in a genomic region with low-complexity repetitive and palindromic sequence elements, the relatively low sequence conservation, and low level of expression have hampered the identification of these genes until now. In vitro-expressed chicken and duck leptins specifically activated signaling through the chicken leptin receptor in cell culture. In situ hybridization demonstrated expression of LEP mRNA in granular and Purkinje cells of the cerebellum, anterior pituitary, and in embryonic limb buds, somites, and branchial arches, suggesting roles in adult brain control of energy balance and during embryonic development. The expression patterns of LEP and the leptin receptor (LEPR) were explored in chicken, duck, and quail (Coturnix japonica) using RNA-sequencing experiments available in the Short Read Archive and by quantitative RT-PCR. In adipose tissue, LEP and LEPR were scarcely transcribed, and the expression level was not correlated to adiposity. Our identification of the leptin genes in chicken and duck genomes resolves a long lasting controversy regarding the existence of leptin genes in these species. This identification was confirmed by sequence and structural similarity, conserved exon-intron boundaries, detection in numerous genomic, and transcriptomic datasets and characterization by PCR, quantitative RT-PCR, in situ hybridization, and bioassays. Our results point to an autocrine/paracrine mode of action for bird leptin instead of being a circulating hormone as in mammals.

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.

Leptin activates chicken growth hormone promoter without chicken STAT3 in vitro

Leptin is an adipocyte-derived hormone that not only regulates food intake and energy homeostasis but also induces growth hormone (GH) mRNA expression and release, thereby controlling growth and metabolism in mammals. The molecular mechanism of leptin-induced regulation of GH gene transcription is unclear. The current study investigated the effects of leptin on the chicken GH (cGH) promoter and the molecular mechanism underlying leptin-induced cGH gene expression in vitro. Leptin activated the cGH promoter in the presence of chPit-1α in CHO cells stably expressing the chicken leptin receptor. Promoter activation did not require STAT-binding elements in the cGH promoter or STAT3 activity. However, JAK2 activation was required for leptin-dependent activity. JAK2-dependent pathways include p42/44 MAPK and PI3K, and inhibition of these pathways partially blocked leptin-induced cGH gene transcription. Although CK2 directly activates JAK2, a CK2 inhibitor blocked leptin-dependent activation of the cGH gene without affecting JAK2 phosphorylation. The CK2 inhibitor suppressed Erk1/2 and Akt phosphorylation. Additional data implicate Src family kinases in leptin-dependent cGH gene activation. These results suggest that leptin activates the cGH gene in the presence of chPit-1α via several leptin-activated kinases. Although further study is required, we suggest that the leptin-induced JAK2/p42/44 MAPK and JAK2/PI3K cascades are activated by Src-meditated CK2, leading to CBP phosphorylation and interaction with chPit-1α, resulting in transactivation of the cGH promoter.

Regulation of the avian central melanocortin system and the role of leptin

The avian central melanocortin system is well conserved between birds and mammals in terms of the component genes, the localisation of their expression in the hypothalamic arcuate nucleus, the effects on feeding behaviour of their encoded peptides and the sensitivity of agouti-related protein (AGRP) and pro-opiomelanocortin (POMC) gene expression to changes in energy status. Our recent research has demonstrated that AGRP gene expression precisely differentiates between broiler breeder hens with different histories of chronic food restriction and refeeding. We have also shown that the sensitivity of AGRP gene expression to loss of energy stores is maintained even when food intake has been voluntarily reduced in chickens during incubation and in response to a stressor. However, the similarity between birds and mammals does not appear to extend to the way AGRP and POMC gene expression are regulated. In particular, the preliminary evidence from the discovery of the first avian leptin (LEP) genes suggests that LEP is more pleiotropic in birds and may not even be involved in regulating energy balance. Similarly, ghrelin exerts inhibitory, rather than stimulatory, effects on food intake. The fact that the importance of these prominent long-term regulators of AGRP and POMC expression in mammals appears diminished in birds suggests that the balance of regulatory inputs in birds may have shifted to more short-term influences such as the tone of cholecystokinin (CCK) signalling. This is likely to be related to the different metabolic fuelling required to support flight.

Hypothalamic Agouti-Related Peptide mRNA is Elevated During Natural and Stress-Induced Anorexia

As part of their natural lives, animals can undergo periods of voluntarily reduced food intake and body weight (i.e. animal anorexias) that are beneficial for survival or breeding, such as during territorial behaviour, hibernation, migration and incubation of eggs. For incubation, a change in the defended level of body weight or 'sliding set point' appears to be involved, although the neural mechanisms reponsible for this are unknown. We investigated how neuropeptide gene expression in the arcuate nucleus of the domestic chicken responded to a 60-70% voluntary reduction in food intake measured both after incubation and after an environmental stressor involving transfer to unfamiliar housing. We hypothesised that gene expression would not change in these circumstances because the reduced food intake and body weight represented a defended level in birds with free access to food. Unexpectedly, we observed increased gene expression of the orexigenic peptide agouti-related peptide (AgRP) in both incubating and transferred animals compared to controls. Also pro-opiomelanocortin (POMC) mRNA was higher in incubating hens and significantly increased 6 days after exposure to the stressor. Conversely expression of neuropeptide Y and cocaine- and amphetamine-regulated transcript gene was unchanged in both experimental situations. We conclude that AgRP expression remains sensitive to the level of energy stores during natural anorexias, which is of adaptive advantage, although its normal orexigenic effects are over-ridden by inhibitory signals. In the case of stress-induced anorexia, increased POMC may contribute to this inhibitory role, whereas, for incubation, reduced feeding may also be associated with increased expression in the hypothalamus of the anorexigenic peptide vasoactive intestinal peptide.

Characterization of the Two CART Genes (CART1 and CART2) in Chickens (Gallus gallus)

Cocaine- and amphetamine-regulated transcript (CART) peptide is implicated in the control of avian energy balance, however, the structure and expression of CART gene(s) remains largely unknown in birds. Here, we cloned and characterized two CART genes (named cCART1 and cCART2) in chickens. The cloned cCART1 is predicted to generate two bioactive peptides, cCART1(42-89) and cCART1(49-89), which share high amino acid sequence identity (94-98%) with their mammalian counterparts, while the novel cCART2 may produce a bioactive peptide cCART2(51-91) with 59% identity to cCART1. Interestingly, quantitative RT-PCR revealed that cCART1 is predominantly expressed in the anterior pituitary and less abundantly in the hypothalamus. In accordance with this finding, cCART1 peptide was easily detected in the anterior pituitary by Western blot, and its secretion from chick pituitaries incubated in vitro was enhanced by ionomycin and forskolin treatment, indicating that cCART1 is a novel peptide hormone produced by the anterior pituitary. Moreover, cCART1 mRNA expression in both the pituitary and hypothalamus is down-regulated by 48-h fasting, suggesting its expression is affected by energy status. Unlike cCART1, cCART2 is only weakly expressed in most tissues examined by RT-PCR, implying a less significant role of cCART2 in chickens. As in chickens, 2 or more CART genes, likely generated by gene and genome duplication event(s), were also identified in other non-mammalian vertebrate species including coelacanth. Collectively, the identification and characterization of CART genes in birds helps to uncover the roles of CART peptide(s) in vertebrates and provides clues to the evolutionary history of vertebrate CART genes.

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.

Creating leptin-like biofunctions by active immunization against chicken leptin receptor in growing chickens

In this study, immunization against chicken leptin receptor (cLEPR) extracellular domain (ECD) was applied to investigate leptin regulation and LEPR biofunction in growing chicken pullets. A recombinant protein (cLEPR ECD) based on the cLEPR complemenary DNA sequence corresponding to the 582nd to 796th amino acid residues of cLEPR mature peptide was prepared and used as antigen. Immunization against cLEPR ECD in growing chickens increased anti-cLEPR ECD antibody titers in blood, enhanced proportions of phosphorylated janus kinase 2 (JAK2) and served as signal transducer and activator of transcription 3 (STAT3) protein in liver tissue. Chicken live weight gain and abdominal fat mass were significantly decreased (P < 0.05), but feed intake was stimulated by cLEPR ECD immunization (P < 0.05). The treatment also upregulated the gene expression levels of lepR, AMP-activated protein kinase (AMPK), acetyl CoA carboxylase-2 (ACC2), and uncoupling protein 3 (UCP3) in liver, abdominal fat, and breast muscle (P < 0.05) but decreased fasn expression levels (P < 0.01). Apart from that of lepR, the expression of appetite-regulating genes, such as orexigenic genes, agouti-related peptide (AgRP) and neuropeptide Y (NPY), were upregulated (P < 0.01), whereas the anorexigenic gene proopiomelanocortin (POMC) was downregulated in the hypothalamic tissue of cLEPR-immunized pullets (P < 0.01). Blood concentrations of metabolic molecules, such as glucose, triglycerides, and very-low-density lipoprotein, were significantly decreased in cLEPR-immunized pullets but those of cholesterol, high-density lipoprotein, and low-density lipoprotein increased. These results demonstrate that antibodies to membrane proximal cLEPR ECD enhance cLEPR signal transduction, which stimulates metabolism and reduces fat deposition in chickens.

Quack leptin

BACKGROUND

A LEP transcript up-regulated in lungs of ducks (Anas platyrhynchos) infected by avian influenza A virus was recently described in the Nature Genetics manuscript that reported the duck genome. In vertebrates, LEP gene symbol is reserved for leptin, the key regulator of energy balance in mammals.

RESULTS

Launching an extensive search for this gene in the genome data that was submitted to the public databases along with duck genome manuscript and extending this search to all avian genomes in the whole-genome shotgun-sequencing database, we were able to report the first identification of coding sequences capable of encoding the full leptin protein precursor in wild birds. Gene structure, synteny and sequence-similarity (up to 54% identity and 68% similarity) to reptilian leptin evident in falcons (Falco peregrinus and cherrug), tits (Pseudopodoces humilis), finches (Taeniopygia guttata) and doves (Columba livia) confirmed that the bird leptin was a true ortholog of its mammalian form. Nevertheless, in duck, like other domestic fowls the LEP gene was not identifiable.

CONCLUSION

Lack of the LEP gene in poultry suggests that birds that have lost it are particularly suited to domestication. Identification of an intact avian gene for leptin in wild birds might explain in part the evolutionary conservation of its receptor in leptin-less fowls.