Knockdown of leptin A expression dramatically alters zebrafish development

Leptin signaling promotes blood vessel formation in the Xenopus tail during the embryo-larval transition

The signals that regulate peripheral blood vessel formation during development are still under investigation. The hormone leptin promotes blood vessel formation, adipose tissue establishment and expansion, tumor growth, and wound healing, but the underlying mechanisms for these actions are currently unknown. We investigated whether leptin promotes angiogenesis in the developing tail fin using embryonic transgenic xflk-1:GFP Xenopus laevis, which express a green fluorescent protein on vascular endothelial cells to mark blood vessels. We found that leptin protein is expressed in endothelial cells of developing blood vessels and that leptin treatment via injection increased phosphorylated STAT3 signaling, which is indicative of leptin activation of its receptor, in blood vessels of the larval tail fin. Leptin administration via media increased vessel length, branching, and reconnection with the cardinal vein, while decreased leptin signaling via immunoneutralization had an opposing effect on vessel development. We also observed disorganization of major vessels and microvessels of the tail fin and muscle when leptin signaling was decreased. Reduced leptin signaling lowered mRNA expression of cenpk, gpx1, and mmp9, markers for cell proliferation, antioxidation, and extracellular matrix remodeling/cell migration, respectively, in the developing tail, providing insight into three possible mechanisms underlying leptin's promotion of angiogenesis. Together these results illustrate that leptin levels are correlated with embryonic angiogenesis and that leptin coordinates multiple aspects of blood vessel growth and development, showing that leptin is an important morphogen during embryonic development.

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

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

Transcriptomic sequencing reveals the potential molecular mechanism by which Tetrabromobisphenol A bis (2-hydroxyethyl ether) exposure exerts developmental neurotoxicity in developing zebrafish (Danio rerio)

Tetrabromobisphenol A bis (2-hydroxyethyl ether) (TBBPA-DHEE) is a derivative of Tetrabromobisphenol A (TBBPA) used as an intermediate flame retardant in engineering polymers. The mechanism of neurodevelopmental toxicity of TBBPA-DHEE remains unclear due to limited toxicological data. We performed behavioral and transcriptomic analyses to assess the neurodevelopmental effects of TBBPA-DHEE on developing zebrafish and potential toxicity mechanisms. Our result shows that exposure to TBBPA-DHEE significantly increased mortality, deformity rate, and reduction in hatch rate, hatchability, and body length relative to the DMSO control. The behavior analysis indicates that TBBPA-DHEE significantly reduced the spontaneous movement of larva compared to the control. The TSH and GH levels were significantly reduced in all the exposure groups in a concentration-dependent manner relative to the DMSO control. TBBPA-DHEE exhibited a significant reduction in locomotor activity across all the exposure groups in the light/dark locomotion test. The transcriptomic analysis result shows that 579 genes were differentially expressed. KEGG analysis shows the enrichment of complement cascade, JAK-STAT signaling pathway, cytokine-cytokine interaction, and phototransduction pathway resulting in a change in mRNA expression of their genes. These observed changes in developmental endpoints, hormonal level, and alteration in mRNA expression of component genes involved in neurodevelopmental pathways could be part of the possible mechanism of the observed toxic effects of TBBPA-DHEE exposure on zebrafish. This study could reveal the possible neurodevelopmental toxicity of TBBPA-DHEE to aquatic species, which could help uncover the health implications of emerging environmental contaminants.

The Mineralocorticoid Receptor Functions as a Key Glucose Regulator in the Skeletal Muscle of Zebrafish

Glucocorticoids (GCs) are essential for maintaining energy homeostasis as part of the adaptive stress response. Most work to date has characterized the metabolic role of GCs via the activation of the glucocorticoid receptor (nr3c1; GR), which is activated under high GC conditions. However, GCs also bind to the mineralocorticoid receptor (nr3c2; MR), a high-affinity corticosteroid receptor active under basal GC conditions. Despite the expression of MR in skeletal muscles, almost nothing is known about its physiological role. Here we tested the hypothesis that the MR promotes anabolic processes during resting cortisol levels and curtails the catabolic actions of the GR during high (stressed) levels of cortisol. To determine the effect of MR, a zebrafish line with a ubiquitous MR knockout (MRca402/ca402) was utilized. The GR was activated in the same group by chronically treating fish with exogenous cortisol. In the muscle, MR primarily promoted nutrient storage, and restricted energy substrate mobilization under resting conditions, whereas GR activation resulted in increased nutrient utilization. Interestingly, MR loss improved GR-driven metabolic flexibility, suggesting that the activation state of these receptors is a key determinant of skeletal muscle ability to switch fuel sources. To determine if the anabolism-promoting nature of MR was due to an interaction with insulin, fish were co-injected with insulin and the fluorescent glucose analogue 2-NBDG. A loss of MR abolished insulin-stimulated glucose uptake in the skeletal muscle. Taken together, we postulate that MR acts as a key modulator of glucose metabolism in the musculature during basal and stress conditions.

Endospanin Is a Candidate for Regulating Leptin Sensitivity

The hypothesis advanced is that endospanin, a highly conserved vesicle traffic protein in vertebrates, regulates leptin sensitivity in bone signaling. The effects of leptin on bones are well-studied but without consensus on whether the increases in leptin signaling stimulate bone gain or loss. The bone response may depend on leptin sensitivity, and endospanin is an established modulator of leptin sensitivity. An argument is advanced to develop zebrafish models for specific leptin signaling pathways. Zebrafish have well-developed molecular tools (e.g., CRISPR) and the advantage of non-destructive sampling of bones in the form of scales. Using these tools, experiments are described to substantiate the role of endospanin in zebrafish bone dynamics.

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

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

Tissue-Specific Effects of Leptin on Glucose and Lipid Metabolism

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

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

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

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.

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

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

Presence and distribution of leptin and its receptor in the gut of adult zebrafish in response to feeding and fasting

Leptin is an anorectic hormone secreted mainly by peripheral adipocytes but also by other central and peripheral tissues. It acts by means of a receptor called OB-R, influencing not only appetite and body mass but being also involved in many fields like endocrinology, metabolism and reproduction. Immunohistochemistry and qRT-PCR techniques were, respectively, used to demonstrate the presence of leptin and its receptor in the gut of adult zebrafish and to evaluate the leptin gene expression response to feeding and fasting. Immunoreactivity for the antibodies utilized was demonstrated in feeding but not in fasting fish, and the gene expression analysis corroborates the data obtained by immunohistochemistry. Therefore, all the obtained results support the hypothesis of the role of this hormone in food regulation in zebrafish.

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.

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.

Genome-Wide Meta-Analysis Unravels Interactions between Magnesium Homeostasis and Metabolic Phenotypes

Magnesium (Mg²⁺) homeostasis is critical for metabolism. However, the genetic determinants of the renal handling of Mg²⁺, which is crucial for Mg²⁺ homeostasis, and the potential influence on metabolic traits in the general population are unknown. We obtained plasma and urine parameters from 9099 individuals from seven cohorts, and conducted a genome-wide meta-analysis of Mg²⁺ homeostasis. We identified two loci associated with urinary magnesium (uMg), rs3824347 (P=4.4×10^-13) near TRPM6, which encodes an epithelial Mg²⁺ channel, and rs35929 (P=2.1×10^-11), a variant of ARL15, which encodes a GTP-binding protein. Together, these loci account for 2.3% of the variation in 24-hour uMg excretion. In human kidney cells, ARL15 regulated TRPM6-mediated currents. In zebrafish, dietary Mg²⁺ regulated the expression of the highly conserved ARL15 ortholog arl15b, and arl15b knockdown resulted in renal Mg²⁺ wasting and metabolic disturbances. Finally, ARL15 rs35929 modified the association of uMg with fasting insulin and fat mass in a general population. In conclusion, this combined observational and experimental approach uncovered a gene-environment interaction linking Mg²⁺ deficiency to insulin resistance 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.

Comparative Physiology of Energy Metabolism: Fishing for Endocrine Signals in the Early Vertebrate Pool

Energy is the common currency of life. To guarantee a homeostatic supply of energy, multiple neuro-endocrine systems have evolved in vertebrates; systems that regulate food intake, metabolism, and distribution of energy. Even subtle (lasting) dysregulation of the delicate balance of energy intake and expenditure may result in severe pathologies. Feeding-related pathologies have fueled research on mammals, including of course the human species. The mechanisms regulating food intake and body mass are well-characterized in these vertebrates. The majority of animal life is ectothermic, only birds and mammals are endotherms. What can we learn from a (comparative) study on energy homeostasis in teleostean fishes, ectotherms, with a very different energy budget and expenditure? We present several adaptation strategies in fish. In recent years, the components that regulate food intake in fishes have been identified. Although there is homology of the major genetic machinery with mammals (i.e., there is a vertebrate blueprint), in many cases this does not imply analogy. Although both mammals and fish must gain their energy from food, the expenditure of the energy obtained is different. Mammals need to spend vast amounts of energy to maintain body temperature; fishes seem to utilize a broader metabolic range to their advantage. In this review, we briefly discuss ecto- and endothermy and their consequences for energy balance. Next, we argue that the evolution of endothermy and its (dis-)advantages may explain very different strategies in endocrine regulation of energy homeostasis among vertebrates. We follow a comparative and evolutionary line of thought: we discuss similarities and differences between fish and mammals. Moreover, given the extraordinary radiation of teleostean fishes (with an estimated number of 33,400 contemporary species, or over 50% of vertebrate life forms), we also compare strategies in energy homeostasis between teleostean species. We present recent developments in the field of (neuro)endocrine regulation of energy balance in teleosts, with a focus on leptin.

Leptin as immune mediator: Interaction between neuroendocrine and immune system

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

Identification of 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.

Mechanisms of otoconia and otolith development

BACKGROUND

Otoconia are bio-crystals that couple mechanic forces to the sensory hair cells in the utricle and saccule, a process essential for us to sense linear acceleration and gravity for the purpose of maintaining bodily balance. In fish, structurally similar bio-crystals called otoliths mediate both balance and hearing. Otoconia abnormalities are common and can cause vertigo and imbalance in humans. However, the molecular etiology of these illnesses is unknown, as investigators have only begun to identify genes important for otoconia formation in recent years.

RESULTS

To date, in-depth studies of selected mouse otoconial proteins have been performed, and about 75 zebrafish genes have been identified to be important for otolith development.

CONCLUSIONS

This review will summarize recent findings as well as compare otoconia and otolith development. It will provide an updated brief review of otoconial proteins along with an overview of the cells and cellular processes involved. While continued efforts are needed to thoroughly understand the molecular mechanisms underlying otoconia and otolith development, it is clear that the process involves a series of temporally and spatially specific events that are tightly coordinated by numerous proteins. Such knowledge will serve as the foundation to uncover the molecular causes of human otoconia-related disorders.

Mechanisms of otoconia and otolith development

BACKGROUND

Otoconia are bio-crystals that couple mechanic forces to the sensory hair cells in the utricle and saccule, a process essential for us to sense linear acceleration and gravity for the purpose of maintaining bodily balance. In fish, structurally similar bio-crystals called otoliths mediate both balance and hearing. Otoconia abnormalities are common and can cause vertigo and imbalance in humans. However, the molecular etiology of these illnesses is unknown, as investigators have only begun to identify genes important for otoconia formation in recent years.

RESULTS

To date, in-depth studies of selected mouse otoconial proteins have been performed, and about 75 zebrafish genes have been identified to be important for otolith development.

CONCLUSIONS

This review will summarize recent findings as well as compare otoconia and otolith development. It will provide an updated brief review of otoconial proteins along with an overview of the cells and cellular processes involved. While continued efforts are needed to thoroughly understand the molecular mechanisms underlying otoconia and otolith development, it is clear that the process involves a series of temporally and spatially specific events that are tightly coordinated by numerous proteins. Such knowledge will serve as the foundation to uncover the molecular causes of human otoconia-related disorders.

Leptin and IL-6 family cytokines synergize to stimulate Müller glia reprogramming and retina regeneration

Unlike mammals, zebrafish can regenerate a damaged retina. This remarkable regenerative response is mediated by Müller glia (MG) that undergo a reprogramming event that drives their proliferation and the generation of multipotent progenitors for retinal repair. The mechanisms that drive MG reprogramming are poorly understood. Here, we report that Leptin and Gp130-coupled receptors, acting via a Jak/Stat signaling pathway, stimulate MG reprogramming and progenitor formation in the injured retina. Importantly, we find that ascl1a gene expression, which drives MG reprogramming in fish and mammals, is regulated in a Jak/Stat-dependent manner and requires consensus Stat-binding sites for injury-dependent activation. Finally, we identify cytokines that are induced by retinal injury and exhibit a remarkable synergy in their ability to activate Jak/Stat signaling and MG reprogramming in the uninjured retina. Our study not only furthers our understanding of retina regeneration in zebrafish but also suggests new strategies for awakening retina regeneration in mammals.

Comparative endocrinology of leptin: assessing function in a phylogenetic context

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

Leptin in teleostean fish, towards the origins of leptin physiology

Teleostean leptin was first cloned in 2005, more than a decade after the discovery of mammalian leptin. The reason for this delay lies in the very poor primary sequence conservation (∼13-25%) between mammalian and fish leptins. These low sequence conservations indicate a high degree of molecular evolvability and warrant a search for different and original functions of leptin in teleosts. Indeed, new and original insights are obtained because of the unique phylogenetic position of teleostean fish as the earliest vertebrates and because of their ectothermy, which means that teleosts are more flexible in changing their metabolism than mammals and leptin could play a role in this flexibility. Research during the last decade reveals that leptin is a truly pleiotropic hormone in fish and mammals alike, with functions among others in the regulation of food intake and body weight, development, but also in the regulation of the stress axis and acclimation processes to for instance low oxygen levels in the water. In this review, we provide an overview of the teleostean leptin work done in the last ten years, and demonstrate that the power of a comparative approach leads to new insights on the origins of leptin physiology.

The impact of leptin on perinatal development and psychopathology

Leptin has long been associated with metabolism as it is a critical regulator of both food intake and energy expenditure, but recently, leptin dysregulation has been proposed as a mechanism of psychopathology. This review discusses the evidence supporting a role for leptin in mental health disorders and describes potential mechanisms that may underlie this association. Leptin plays a critical role in pregnancy and in fetal growth and development. Leptin's role and profile during development is examined in available human studies, and the validity of applying studies conducted in animal models to the human population are discussed. Rodents experience a postnatal leptin surge, which does not occur in humans or larger animal models. This suggests that further research using large mammal models, which have a leptin profile across pregnancy and development similar to humans, are of high importance. Maternal obesity and hyperleptinemia correlate with increased leptin levels in the umbilical cord, placenta, and fetus. Leptin levels are thought to impact fetal brain development; likely by activating proinflammatory cytokines that are known to impact many of the neurotransmitter systems that regulate behavior. Leptin is likely involved in behavioral regulation as leptin receptors are widely distributed in the brain, and leptin influences cortisol release, the mesoaccumbens dopamine pathway, serotonin synthesis, and hippocampal synaptic plasticity. In humans, both high and low levels of leptin are reported to be associated with psychopathology. This inconsistency is likely due to differences in the metabolic state of the study populations. Leptin resistance, which occurs in the obese state, may explain how both high and low levels of leptin are associated with psychopathology, as well as the comorbidity of obesity with numerous mental illnesses. Leptin resistance is likely to influence disorders such as depression and anxiety where high leptin levels have been correlated with symptomatology. Schizophrenia is also associated with both low and high leptin levels. However, as anti-psychotics pharmacotherapy induces weight gain, which elevates leptin levels, drug-naïve populations are needed for further studies. Elevated circulating leptin is consistently found in childhood neurodevelopmental disorders including autism spectrum disorders and Rhett disorder. Further, studies on the impact of leptin and leptin resistance on psychopathology and neurodevelopmental disorders are important directions for future research. Studies examining the mechanisms by which exposure to maternal obesity and hyperleptinemia during fetal development impact brain development and behavior are critical for the health of future generations.

Discovery of the elusive leptin in birds: identification of several 'missing links' in the evolution of leptin and its receptor

Leptin is a pleiotropic protein best known for regulation of appetite and fat storage in mammals. While many leptin orthologs have been identified among vertebrates, an authentic leptin in birds has remained elusive and controversial. Here we identify leptin sequence from the Peregrine falcon, Falco peregrinus (pfleptin), and identify sequences from two other birds (mallard and zebra finch), and 'missing' vertebrates (elephant shark, alligator, Indian python, Chinese soft-shelled turtle, and coelacanth). The pattern of genes surrounding leptin (snd1, rbm28) is syntenic between the falcon and mammalian genomes. Phylogenetic analysis of all known leptin protein sequences improves our understanding of leptin's evolution. Structural modeling of leptin orthologs highlights a highly conserved hydrophobic core in the four-helix cytokine packing domain. A docked model of leptin with the leptin receptor for Peregrine falcon reveals several conserved amino acids important for the interaction and possible coevolution of leptin with its receptor. We also show for the first time, an authentic avian leptin sequence that activates the JAK-STAT signaling pathway. These newly identified sequences, structures, and tools for avian leptin and its receptor will allow elucidation of the function of these proteins in feral and domestic birds.

Role for leptin in promoting glucose mobilization during acute hyperosmotic stress in teleost fishes

Osmoregulation is critical for survival in all vertebrates, yet the endocrine regulation of this metabolically expensive process is not fully understood. Specifically, the function of leptin in the regulation of energy expenditure in fishes, and among ectotherms, in general, remains unresolved. In this study, we examined the effects of acute salinity transfer (72  h) and the effects of leptin and cortisol on plasma metabolites and hepatic energy reserves in the euryhaline fish, the tilapia (Oreochromis mossambicus). Transfer to 2/3 seawater (23  ppt) significantly increased plasma glucose, amino acid, and lactate levels relative to those in the control fish. Plasma glucose levels were positively correlated with amino acid levels (R2=0.614), but not with lactate levels. The mRNA expression of liver leptin A (lepa), leptin receptor (lepr), and hormone-sensitive and lipoprotein lipases (hsl and lpl) as well as triglyceride content increased during salinity transfer, but plasma free fatty acid and triglyceride levels remained unchanged. Both leptin and cortisol significantly increased plasma glucose levels in vivo, but only leptin decreased liver glycogen levels. Leptin decreased the expression of liver hsl and lpl mRNAs, whereas cortisol significantly increased the expression of these lipases. These findings suggest that hepatic glucose mobilization into the blood following an acute salinity challenge involves both glycogenolysis, induced by leptin, and subsequent gluconeogenesis of free amino acids. This is the first study to report that teleost leptin A has actions that are functionally distinct from those described in mammals acting as a potent hyperglycemic factor during osmotic stress, possibly in synergism with cortisol. These results suggest that the function of leptin may have diverged during the evolution of vertebrates, possibly reflecting differences in metabolic regulation between poikilotherms and homeotherms.

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

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

Molecular cloning and genomic characterization of novel leptin-like genes in salmonids provide new insight into the evolution of the Leptin gene family

In the current study we describe the identification of novel leptin B homologous gene/s in the four salmonid species Atlantic salmon (Salmo salar), rainbow trout (Oncorhynchus mykiss), brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus). Homology modeling of Salmo salar (Ss) LepB1/B2 suggests that the protein satisfies parameters as long-chain four helical cytokine family and that the basic structural pattern of the protein follows that of human leptin (Zhang et al., 1997). Importantly, the docking studies suggested the SsLepB has binding affinity to the AA residues that identify the leptin binding and FNIII domains of the SsLep receptor (Rønnestad et al., 2010). Phylogenetic analyses support that LepB paralogs have most probably originated by 4R whole genome duplication (WGD) before speciation of the salmonid lineages. LepB1 and LepB2 genes are both present in the two closest relatives, the Atlantic salmon and the brown trout, while rainbow trout and charr have only preserved the long LepB1 variant in their genome. We have defined the sites of SsLepB mRNA expression at key life stages in Atlantic salmon and found that SsLepB1 and SsLepB2, although to different extent, were expressed in redundant and mostly complementary fashion in brain and gills throughout the lifecycle, suggesting that this pair of paralogs is likely undergoing early stages of subfunctionalization. Furthermore, we have quantified the expression profiles of SsLepB genes and of other two recently duplicated salmon leptins (SsLepA1, SsLepA2) during early development and show evidence that in fish, as in mammals and amphibians, leptin could play important roles in growth and development. This study provides an essential groundwork to further elucidate structural and functional evolution of this important hormone in salmonids as well as in other teleosts.