Long-term hyperphagia and caloric restriction caused by low- or high-density husbandry have differential effects on zebrafish postembryonic development, somatic growth, fat accumulation and reproduction

Non-Genetic-Induced Zebrafish Model for Type 2 Diabetes with Emphasis on Tools in Model Validation

The unrelenting increase in the incidence of type 2 diabetes (T2D) necessitates the urgent need for effective animal models to mimic its pathophysiology. Zebrafish possess human-like metabolic traits and share significant genetic similarities, making them valuable candidates for studying metabolic disorders, including T2D. This review emphasizes the critical role of animal models in diabetes research, especially focusing on zebrafish as an alternative model organism. Different approaches to a non-genetic model of T2D in zebrafish, such as the glucose solution, diet-induced, chemical-induced, and combined diet-induced and glucose solution methods, with an emphasis on model validation using indicators of T2D, were highlighted. However, a significant drawback lies in the validation of these models. Some of these models have not extensively demonstrated persistent hyperglycemia or response to insulin resistance and glucose tolerance tests, depicted the morphology of the pancreatic β-cell, or showed their response to antidiabetic drugs. These tools are crucial in T2D pathology. Future research on non-genetic models of T2D in zebrafish must extensively focus on validating the metabolic deficits existing in the model with the same metabolic defects in humans and improve on the existing models for a better understanding of the molecular mechanisms underlying T2D and exploring potential therapeutic interventions.

Loss of Agrp1 in zebrafish: Effects on the growth and reproductive axis

Loss of agouti related neuropeptide (AgRP) does not lead to overt phenotypes in mammals unless AgRP neurons are ablated. In contrast, in zebrafish it has been shown that Agrp1 loss of function (LOF) leads to reduced growth in Agrp1 morphant as well as Agrp1 mutant larvae. Further, it has been shown that multiple endocrine axes are dysregulated upon Agrp1 LOF in Agrp1 morphant larvae. Here we show that adult Agrp1 LOF zebrafish show normal growth and reproductive behavior in spite of a significant reduction in multiple related endocrine axes namely reduced expression in pituitary growth hormone (gh) follicle stimulating hormone (fshb) as well as luteinizing hormone (lhb). We looked for compensatory changes in candidate gene expression but found no changes in growth hormone and gonadotropin hormone receptors that would explain the lack of phenotype. We further looked at expression in the hepatic and muscular insulin-like growth factor (Igf) axis which appears to be normal. Fecundity as well as ovarian histology also appear largely normal while we do see an increase in mating efficiency specifically in fed but not fasted AgRP1 LOF animals. This data shows that zebrafish can grow and reproduce normally in spite of significant central hormone changes and suggests a peripheral compensatory mechanism additional to previously reported central compensatory mechanisms in other zebrafish neuropeptide LOF lines.

High-fat diet feeding triggers a regenerative response in the adult zebrafish brain

Non-alcoholic fatty liver disease (NAFLD) includes a range of liver conditions ranging from excess fat accumulation to liver failure. NAFLD is strongly associated with high-fat diet (HFD) consumption that constitutes a metabolic risk factor. While HFD has been elucidated concerning its several systemic effects, there is little information about its influence on the brain at the molecular level. Here, by using a high-fat diet (HFD)-feeding of adult zebrafish, we first reveal that excess fat uptake results in weight gain and fatty liver. Prolonged exposure to HFD induces a significant increase in the expression of pro-inflammation, apoptosis, and proliferation markers in the liver and brain tissues. Immunofluorescence analyses of the brain tissues disclose stimulation of apoptosis and widespread activation of glial cell response. Moreover, glial activation is accompanied by an initial decrease in the number of neurons and their subsequent replacement in the olfactory bulb and the telencephalon. Long-term consumption of HFD causes activation of Wnt/β-catenin signaling in the brain tissues. Finally, fish fed an HFD induces anxiety, and aggressiveness and increases locomotor activity. Thus, HFD feeding leads to a non-traumatic brain injury and stimulates a regenerative response. The activation mechanisms of a regeneration response in the brain can be exploited to fight obesity and recover from non-traumatic injuries.

Beneficial Effect of Vitamin D on Non-Alcoholic Fatty Liver Disease (NAFLD) Progression in the Zebrafish Model

A major cause of chronic liver disease, cirrhosis, and hepatocellular carcinoma, non-alcoholic fatty liver disease (NAFLD) results from excessive liver fat accumulation. Vitamin D (VitD) plays multiple important roles in diverse physiologic processes. Here, we describe the role of VitD in the complex pathogenesis of NAFLD and explore the possible therapeutic role of VitD supplementation in NAFLD therapy. To compare the effect of VitD to other interventions such as low-calorie diet, we induced NAFLD in young adult zebrafish (Danio rerio, AB strain) and monitored the effects of VitD supplementation on the disease course. The zebrafish administered with high-dose VitD (1.25 μg) had significantly reduced liver fat compared to those that received low-dose VitD (0.049 μg) or caloric restriction. Gene expression analysis revealed that VitD downregulated several pathways that may play a role in NAFLD etiology, which affected fatty acid metabolism, vitamins and their cofactors, ethanol oxidation, and glycolysis. The pathway analysis revealed that the cholesterol biosynthesis pathway and the isoprenoid biosynthetic process pathway were significantly upregulated whereas the small molecule catabolic process pathway significantly downregulated following the exposure of NAFLD zebrafish model to high VitD dose. Therefore, our findings suggest the association of novel biochemical pathways with NAFLD and highlight the potential of VitD supplementation to reverse the severity of NAFLD, especially in younger people.

The effects of intensive feeding on reproductive performance in laboratory zebrafish (Danio rerio)

The zebrafish (Danio rerio) is among the most widely used model animals in scientific research. Historically, these fish have been reared in the laboratory using simple methods developed by home aquarists. For laboratories with high demand for breeding and generation turn-over, however, there has been a shift away from this approach towards one that leverages techniques, tools, and feeds from commercial aquaculture to help accelerate growth rates and decrease generation times. While these advances have improved efficiency, the effects of feeding zebrafish diets that are designed to grow disparately related cold-water fish species to market size quickly are not well-understood. To explore the impacts that intensive feeding protocols may have on this species, groups of zebrafish larvae from two different wild-type lines were stocked into treatment tanks at a standard density of 10 fish per liter and were administered either a "high" or "low" food diet for a maximum of 63 days. During their growth phase, the "high" food diet group received at least 8x more rotifers and at least 2x more Artemia than the "low" food diet group. Growth, survival, and reproductive performance (fecundity and viability) were measured in these fish and in their offspring. We found that fish that were fed more grew more rapidly and were able to reproduce earlier than fish that were fed less, but they were also more likely to produce higher proportions of non-viable embryos.

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

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

Long-term obesogenic diet leads to metabolic phenotypes which are not exacerbated by catch-up growth in zebrafish

Obesity and metabolic syndrome are of increasing global concern. In order to understand the basic biology and etiology of obesity, research has turned to animals across the vertebrate spectrum including zebrafish. Here, we carefully characterize zebrafish in a long-term obesogenic environment as well as zebrafish that went through early lifetime caloric restriction. We found that long-term obesity in zebrafish leads to metabolic endpoints comparable to mammals including increased adiposity, weight, hepatic steatosis and hepatic lesions but not signs of glucose dysregulation or differences in metabolic rate or mitochondrial function. Malnutrition in early life has been linked to an increased likelihood to develop and an exacerbation of metabolic syndrome, however fish that were calorically restricted from five days after fertilization until three to nine months of age did not show signs of an exacerbated phenotype. In contrast, the groups that were shifted later in life from caloric restriction to the obesogenic environment did not completely catch up to the long-term obesity group by the end of our experiment. This dataset provides insight into a slowly exacerbating time-course of obesity phenotypes.

Effects of Holding Density on the Welfare of Zebrafish: A Systematic Review

The zebrafish is becoming an increasingly popular research animal around the world. Its welfare is affected by an array of environmental factors, such as food access and water quality. Holding density is an important welfare aspect, not least due to its interaction with other housing conditions. Despite the extensive use of zebrafish in research, little is known of how densities affect its welfare. In this systematic review, we have performed a large literature search, compiled, and evaluated all publications regarding zebrafish holding density. We have analyzed how density effects growth, reproduction, and stress response, including behavior, water quality, and pathogenic outbreaks in young and adult fish. Our review shows that the holding densities tested vary largely depending on the research focus, for example, body growth or behavior. In fact, research indicates that future recommendations on holding density could depend on which welfare aspects are considered. Overall, there is a need for more studies investigating the interactive effects of density on welfare indicators, such as reproduction coupled with stress response. We stress the necessity of including holding density in universal housing guidelines and reporting information on holding conditions of larvae and adults when publishing zebrafish work.

Environmental enrichment applied with sensory components prevents age-related decline in synaptic dynamics: Evidence from the zebrafish model organism

Progression of cognitive decline with or without neurodegeneration varies among elderly subjects. The main aim of the current study was to illuminate the molecular mechanisms that promote and retain successful aging in the context of factors such as environment and gender, both of which alter the resilience of the aging brain. Environmental enrichment (EE) is one intervention that may lead to the maintenance of cognitive processing at older ages in both humans and animal subjects. EE is easily applied to different model organisms, including zebrafish, which show similar age-related molecular and behavioral changes as humans. Global changes in cellular and synaptic markers with respect to age, gender and 4-weeks of EE applied with sensory stimulation were investigated using the zebrafish model organism. Results indicated that EE increases brain weight in an age-dependent manner without affecting general body parameters like body mass index (BMI). Age-related declines in the presynaptic protein synaptophysin, AMPA-type glutamate receptor subunits and a post-mitotic neuronal marker were observed and short-term EE prevents these changes in aged animals, as well as elevates levels of the inhibitory scaffolding protein, gephyrin. Gender-driven alterations were observed in the levels of the glutamate receptor subunits. Oxidative stress markers were significantly increased in the old animals, while exposure to EE did not alter this pattern. These data suggest that EE with sensory stimulation exerts its effects mainly on age-related changes in synaptic dynamics, which likely increase brain resilience through specific cellular mechanisms.

Feed Restriction Modulates Growth, Gut Morphology and Gene Expression in Zebrafish

A reduction in daily caloric or nutrient intake has been observed to promote health benefits in mammals and other vertebrates. Feed Restriction (FR), whereby the overall food intake of the organism is reduced, has been explored as a method to improve metabolic and immune health, as well as to optimize productivity in farming. However, less is known regarding the molecular and physiological consequences of FR. Using the model organism, Danio rerio, we investigated the impact of a short-term (month-long) FR on growth, gut morphology and gene expression. Our data suggest that FR has minimal effects on the average growth rates, but it may affect weight and size heterogeneity in a sex-dependent manner. In the gut, we observed a significant reduction in gut circumference and generally lower mucosal heights, whereas other parameters remained unchanged. Gene Ontology (GO), EuKaryotic Orthologous Groups (KOG), and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis identified numerous metabolic, reproductive, and immune response pathways that were affected by FR. These results broaden our understanding of FR and contribute towards growing knowledge of its effects on vertebrate health.

Is sodium carboxymethyl cellulose (CMC) really completely innocent? It may be triggering obesity

The toxicity of sodium carboxymethyl cellulose (CMC), which has GRAS status and has been determined as "ADI non specified", was re-evaluated with a new modelling and molecular-based data. For this purpose, CMC, a food additive, was injected to the yolk sac (food) of the zebrafish embryo by the microinjection method at the 4th hour of fertilization at different concentrations. As a result, it was found that CMC showed no toxic effects within the framework of the parameters studied. But, we determined increasing lipid accumulation in zebrafish embryos exposed to CMC in a dose-dependent manner. To elucidate the mechanism underlying this lipid accumulation, the expression levels of genes related to obesity-linked lipid metabolism were examined. Our findings show that while CMC does not cause a toxic effect in zebrafish embryos, it can lead important effects on lipid metabolism by causing changes in the expression of some genes associated with obesity.

Both Dietary Ratio of n-6 to n-3 Fatty Acids and Total Dietary Lipid Are Positively Associated with Adiposity and Reproductive Health in Zebrafish

BACKGROUND

Controversial findings have been reported in human and animal studies regarding the influence of n-6 (ω-6) to n-3 (ω-3) fatty acid ratios on obesity and health. Two confounding factors may be related to interactions with other dietary lipid components or sex-specific differences in fatty acid metabolism.

OBJECTIVE

This study investigated main and interactive effects of total dietary lipid, ratio of n-6 to n-3 fatty acids, and sex on growth, adiposity, and reproductive health in wild-type zebrafish.

METHODS

Male and female zebrafish (3 wk old) were fed 9 diets consisting of 3 ratios of n-6 to n-3 fatty acids (1.4:1, 5:1, and 9.5:1) varied within 3 total lipid amounts (80, 110, and 140 g/kg) for 16 wk. Data were then collected on growth, body composition (determined by chemical carcass analysis), and female reproductive success (n = 32 breeding events/diet over 4 wk). Main and interactive effects of dietary lipid and sex were evaluated with regression methods. Significant differences within each dietary lipid component were relative to the intercept/reference group (80 g/kg and 1.4:1 ratio).

RESULTS

Dietary lipid and sex interacted in their effects on body weight (P = 0.015), total body length (P = 0.003), and total lipid mass (P = 0.029); thus, these analyses were stratified by sex. Female spawning success decreased as dietary total lipid and fatty acid ratio increased (P = 0.030 and P = 0.026, respectively). While total egg production was not associated with either dietary lipid component, females fed the 5:1 ratio produced higher proportions of viable embryos compared with the 1.4:1 ratio [median (95% CI): 0.915 (0.863, 0.956) vs 0.819 (0.716, 0.876); P < 0.001].

CONCLUSIONS

Further characterization of dietary lipid requirements will help define healthy balances of dietary lipid, while the sex-specific responses to dietary lipid identified in this study may partially explain sex disparities in the development of obesity and its comorbidities.

Developmental exposure to cannabidiol (CBD) alters longevity and health span of zebrafish (Danio rerio)

Consumption of cannabinoid-containing products is on the rise, even during pregnancy. Unfortunately, the long-term, age-related consequences of developmental cannabidiol (CBD) exposure remain largely unknown. This is a critical gap given the established Developmental Origins of Health and Disease (DOHaD) paradigm which emphasizes that stressors, like drug exposure, early in life can instigate molecular and cellular changes that ultimately lead to adverse outcomes later in life. Thus, we exposed zebrafish (Danio rerio) to varying concentrations of CBD (0.02, 0.1, 0.5 μM) during larval development and assessed aging in both the F0 (exposed generation) and their F1 offspring 30 months later. F0 exposure to CBD significantly increased survival (~ 20%) and reduced size (wet weight and length) of female fish. While survival was increased, the age-related loss of locomotor function was unaffected and the effects on fecundity varied by sex and dose. Treatment with 0.5 μM CBD significantly reduced sperm concentration in males, but 0.1 μM increased egg production in females. Similar to other model systems, control aged zebrafish exhibited increased kyphosis as well as increased expression markers of senescence, and inflammation (p16^ink4ab, tnfα, il1b, il6, and pparγ) in the liver. Exposure to CBD significantly reduced the expression of several of these genes in a dose-dependent manner relative to the age-matched controls. The effects of CBD on size, gene expression, and reproduction were not reproduced in the F1 generation, suggesting the influence on aging was not cross-generational. Together, our results demonstrate that developmental exposure to CBD causes significant effects on the health and longevity of zebrafish.

Nutrient Sensing and Response Drive Developmental Progression in Caenorhabditis elegans

In response to nutrient limitation, many animals, including Caenorhabditis elegans, slow or arrest their development. This process requires mechanisms that sense essential nutrients and induce appropriate responses. When faced with nutrient limitation, C. elegans can induce both short and long-term survival strategies, including larval arrest, decreased developmental rate, and dauer formation. To select the most advantageous strategy, information from many different sensors must be integrated into signaling pathways, including target of rapamycin (TOR) and insulin, that regulate developmental progression. Here, how nutrient information is sensed and integrated into developmental decisions that determine developmental rate and progression in C. elegans is reviewed.

Diet-Induced Growth Is Regulated via Acquired Leptin Resistance and Engages a Pomc-Somatostatin-Growth Hormone Circuit

Anorexigenic pro-opiomelanocortin (Pomc)/alpha-melanocyte stimulating hormone (αMSH) neurons of the hypothalamic melanocortin system function as key regulators of energy homeostasis, also controlling somatic growth across different species. However, the mechanisms of melanocortin-dependent growth control still remain ill-defined. Here, we reveal a thus-far-unrecognized structural and functional connection between Pomc neurons and the somatotropic hypothalamo-pituitary axis. Excessive feeding of larval zebrafish causes leptin resistance and reduced levels of the hypothalamic satiety mediator pomca. In turn, this leads to reduced activation of hypophysiotropic somatostatin (Sst)-neurons that express the melanocortin receptor Mc4r, elevated growth hormone (GH) expression in the pituitary, and enhanced somatic growth. Mc4r expression and αMSH responsiveness are conserved in Sst-expressing hypothalamic neurons of mice. Thus, acquired leptin resistance and attenuation of pomca transcription in response to excessive caloric intake may represent an ancient mechanism to promote somatic growth when food resources are plentiful.

High-fat diet impairs cognitive function of zebrafish

An unhealthy diet with excessive fat intake has often been claimed to induce not only obesity but also cognitive dysfunction in mammals; however, it is not known whether this is the case in zebrafish. Here, we investigated the effect of excessive fat in the diet on cognitive function and on gene expression in the telencephalon of zebrafish. Cognitive function, as measured by active avoidance test, was impaired by feeding of a high-fat diet compared with a control diet. In RNA sequencing analysis of the telencephalon, 97 genes were identified with a fold change in expression greater than 2 and a p-value less than 0.05 between the two diets. In quantitative real-time PCR analysis of the telencephalon, genes related to neuronal activity, anti-oxidative stress, blood–brain barrier function and amyloid-β degradation were found to be downregulated, whereas genes related to apoptosis and amyloid-β production were found to be upregulated, in the high-fat diet group, which are changes known to occur in mammals fed a high-fat diet. Collectively, these results are similar to those found in mammals, suggesting that zebrafish can serve as a suitable animal model in research into cognitive impairment induced by excessive fat in the diet.

Human Semaphorin 3 Variants Link Melanocortin Circuit Development and Energy Balance

Hypothalamic melanocortin neurons play a pivotal role in weight regulation. Here, we examined the contribution of Semaphorin 3 (SEMA3) signaling to the development of these circuits. In genetic studies, we found 40 rare variants in SEMA3A-G and their receptors (PLXNA1-4; NRP1-2) in 573 severely obese individuals; variants disrupted secretion and/or signaling through multiple molecular mechanisms. Rare variants in this set of genes were significantly enriched in 982 severely obese cases compared to 4,449 controls. In a zebrafish mutagenesis screen, deletion of 7 genes in this pathway led to increased somatic growth and/or adiposity demonstrating that disruption of Semaphorin 3 signaling perturbs energy homeostasis. In mice, deletion of the Neuropilin-2 receptor in Pro-opiomelanocortin neurons disrupted their projections from the arcuate to the paraventricular nucleus, reduced energy expenditure, and caused weight gain. Cumulatively, these studies demonstrate that SEMA3-mediated signaling drives the development of hypothalamic melanocortin circuits involved in energy homeostasis.

Metabolic and bone effects of high-fat diet in adult zebrafish

An increase of visceral fat affects human bone health causing fragility, mechanical strength reduction, and increased propensity to fractures because of impaired bone matrix microstructure and aberrant bone cell function. Adult Danio rerio (zebrafish) represents a powerful model to study both metabolic diseases and bone metabolism. The aim of this study was to generate an obese adult zebrafish by high-fat diet and evaluate metabolic and bone tissue effects. Fish blood glucose and insulin levels were found to be altered in high-fat diet fish revealing a failure in β-cells insulin production. Blood analysis of adipokines revealed significant alterations in adiponectin and leptin levels that are common in human and other obesity animal models. Advanced glycation end products (AGEs), derived from hyperglycemia condition, were found to be altered too. All these alterations were associated with an impaired bone metabolism. The scales of high-fat diet fish shown bone resorption lacunae associated with an intense osteoclastic tartrate-resistant acid phosphatase (TRAP) activity, whereas alkaline phosphatase (ALP) decreased. These data suggest that an imbalance of fat metabolism alters energy metabolism generating an osteoporosis-like phenotype in adult zebrafish scales. The zebrafish obesity model can contribute to elucidate in vivo the molecular mechanisms of metabolic changes in human obese patients.

Effects of overfeeding and high-fat diet on cardiosomatic parameters and cardiac structures in young and adult zebrafish

Obesity is a complex global health problem because it is a risk factor for multiple chronic pathologies such as cardiovascular, endocrine, metabolic, and neoplastic diseases. It is considered a multicausal disease, and one of the determining factors is nutritional imbalances, which include high-fat diets. In this paper, we use the zebrafish model to assess the impact of overfeeding and a high-fat diet in somatic and cardiac parameters in young and adult zebrafish. The results show that fish receiving a high-fat diet showed greater weight gain compared to fish receiving a standard fat diet. Additionally, changes in the heart, including increases in size, a change in the triangular shape of the ventricle to a globular shape, and an increase in the thickness of the trabeculae of the spongy myocardium were observed. These changes could be indicators of cardiovascular overload. The results show that there is a direct relationship between the intake of a high-fat diet and obesity, which in turn can induce cardiac changes, supporting the hypothesis of the relationship between high-fat diets and cardiovascular risk factors. Given the genetic similarity between zebrafish and humans, these results could be extrapolated to human beings, and the findings similarly highlight the importance of incorporating a balanced diet from the early life stages to reduce the risk of cardiovascular disease.

LITTLE FISH, BIG DATA: ZEBRAFISH AS A MODEL FOR CARDIOVASCULAR AND METABOLIC DISEASE

The burden of cardiovascular and metabolic diseases worldwide is staggering. The emergence of systems approaches in biology promises new therapies, faster and cheaper diagnostics, and personalized medicine. However, a profound understanding of pathogenic mechanisms at the cellular and molecular levels remains a fundamental requirement for discovery and therapeutics. Animal models of human disease are cornerstones of drug discovery as they allow identification of novel pharmacological targets by linking gene function with pathogenesis. The zebrafish model has been used for decades to study development and pathophysiology. More than ever, the specific strengths of the zebrafish model make it a prime partner in an age of discovery transformed by big-data approaches to genomics and disease. Zebrafish share a largely conserved physiology and anatomy with mammals. They allow a wide range of genetic manipulations, including the latest genome engineering approaches. They can be bred and studied with remarkable speed, enabling a range of large-scale phenotypic screens. Finally, zebrafish demonstrate an impressive regenerative capacity scientists hope to unlock in humans. Here, we provide a comprehensive guide on applications of zebrafish to investigate cardiovascular and metabolic diseases. We delineate advantages and limitations of zebrafish models of human disease and summarize their most significant contributions to understanding disease progression to date.

A classification system for zebrafish adipose tissues

The zebrafish model system offers significant utility for in vivo imaging of adipose tissue (AT) dynamics and for screening to identify chemical and genetic modifiers of adiposity. In particular, AT can be quantified accurately in live zebrafish using fluorescent lipophilic dyes. Although this methodology offers considerable promise, the comprehensive identification and classification of zebrafish ATs has not been performed. Here, we use fluorescent lipophilic dyes and in vivo imaging systematically to identify, classify and quantify the zebrafish AT pool. We identify 34 regionally distinct zebrafish ATs, including five visceral ATs and 22 subcutaneous ATs. For each of these ATs, we describe detailed morphological characteristics to aid their identification in future studies. Furthermore, we quantify the areas for each AT and construct regression models to allow prediction of expected AT size and variation across a range of developmental stages. Finally, we demonstrate the utility of this resource for identifying effects of strain variation and high-fat diet on AT growth. Altogether, this resource provides foundational information on the identity, dynamics and expected quantities of zebrafish ATs for use as a reference for future studies.

Summary: A standardized nomenclature and classification system for zebrafish adipose tissues and regression models to predict expected adipose size during the course of zebrafish development.

Short-term overfeeding of zebrafish with normal or high-fat diet as a model for the development of metabolically healthy versus unhealthy obesity

BACKGROUND

Obese individuals differ in their risk of developing metabolic and cardiovascular complications depending on fat distribution (subcutaneous versus visceral) and adipose tissue (AT) phenotype (hyperplasic versus hypertrophic). However, the exact mechanisms which determine whether an obese individual is metabolically healthy or unhealthy are not clear, and analyses of the underlying pathomechanisms are limited by the lack of suitable in vivo models in which metabolically healthy versus metabolically unhealthy AT accumulation can be specifically induced. In the current study, we aimed to establish a protocol for the use of zebrafish as a model for obesity-related metabolically healthy versus metabolically unhealthy AT accumulation.

METHODS

We overfed adult male zebrafish of the AB strain with normal fat diet (NFD) or high fat diet (HFD) for 8 weeks and compared parameters related to obesity, i.e. body weight, body mass index, condition index and body fat percentage, to control zebrafish fed under physiological conditions. In addition, we investigated the presence of early obesity-related metabolic alterations by quantifying blood glucose levels, plasma triglyceride and cholesterol levels, and by assessing ectopic lipid accumulation in the liver of zebrafish. Finally, we determined gene expression levels of marker genes related to lipid metabolism, inflammation and fibrosis in visceral AT and liver.

RESULTS

We show that 8-weeks overfeeding with either NFD or HFD leads to a significant increase in body weight and AT mass compared to controls. In contrast to NFD-overfed zebrafish, HFD-overfed zebrafish additionally present metabolic alterations, e.g. hyperglycemia and ectopic lipid accumulation in the liver, and a metabolically unhealthy AT phenotype with adipocyte hypertrophy especially in the visceral AT depot, which is accompanied by changes in the expression of marker genes for lipid metabolism, inflammation and fibrosis.

CONCLUSIONS

In summary, we have established a method for the specific induction of metabolically distinct obesity phenotypes in zebrafish. Our results indicate that zebrafish represents an attractive model to study regulatory mechanisms involved in the determination of AT phenotype during development of metabolically healthy versus metabolically unhealthy obesity.

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

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

Pharmacological evaluation of the mechanisms involved in increased adiposity in zebrafish triggered by the environmental contaminant tributyltin

One proposed contributing factor to the rise in overweight and obesity is exposure to endocrine disrupting chemicals. Tributyltin chloride (TBT), an organotin, induces adipogenesis in cell culture models and may increases adipose mass in vivo in vertebrate model organisms. It has been hypothesized that TBT acts via the peroxisome proliferator activated receptor (PPAR)γ-dependent pathway. However, the mechanisms involved in the effects of TBT exposure on in vivo adipose tissue metabolism remain unexplored. Semitransparent zebrafish larvae, with their well-developed white adipose tissue, offer a unique opportunity for studying the effects of toxicant chemicals and pharmaceuticals on adipocyte biology and whole-organism adiposity in a vertebrate model. Within hours, zebrafish larvae, treated at environmentally-relevant nanomolar concentrations of TBT, exhibited a remarkable increase in adiposity linked to adipocyte hypertrophy. Under the experimental conditions used, we also demonstrated that zebrafish larvae adipose tissue proved to be highly responsive to selected human nuclear receptor agonists and antagonists. Retinoid X receptor (RXR) homodimers and RXR/liver X receptor heterodimers were suggested to be in vivo effectors of the obesogenic effect of TBT on zebrafish white adipose tissue. RXR/PPARγ heterodimers may be recruited to modulate adiposity in zebrafish but were not a necessary requirement for the short term in vivo TBT obesogenic effect. Together, the present results suggest that TBT may induce the promotion of triacylglycerol storage in adipocytes via RXR-dependent pathways without necessary using PPAR isoforms.