In Vivo Determination of Body Composition in Zebrafish (Danio rerio) by Quantitative Magnetic Resonance

Evaluation of a gel-based versus micro-pellet diet for adult zebrafish (Danio rerio)

Ad libitum feeding of laboratory zebrafish has potential benefits for colony management, but would require a new type of diet, such as a gel that remains in the tank. We hypothesized that adult zebrafish fed a gel diet would have similar body size and reproductive success compared with those fed a standard micro-pellet diet. The gel diet’s impact on water quality was determined to be safe for zebrafish prior to starting a 12-week feeding study. Two hundred adult AB zebrafish of mixed sex were randomly assigned to be fed exclusively either gel or micro-pellet diet. Fish body length and mass were measured every two weeks, and fish were bred within each feed group to assess fecundity. Zebrafish consumed less gel diet than expected. Body length, mass, and breeding success were lower in the gel diet fish than in the micro-pellet diet fish. Low consumption of the gel diet and/or nutritional differences between the two diets may have contributed to reduced growth and fecundity. Though the gel diet could reduce time personnel spend feeding and be safer for fish in static tanks, the tested formulation was not a satisfactory alternative to the control micro-pellet diet in a research zebrafish colony.

Methodological advancements in organ-specific ectopic lipid quantitative characterization: Effects of high fat diet on muscle and liver intracellular lipids

OBJECTIVE

Ectopic lipid accumulation is a hallmark of metabolic diseases, linking obesity to non-alcoholic fatty liver disease, insulin resistance and diabetes. The use of zebrafish as a model of obesity and diabetes is raising due to the conserved properties of fat metabolism between humans and zebrafish, the homologous genes regulating lipid uptake and transport, the implementation of the '3R's principle and their cost-effectiveness. To date, a method allowing the conservation of lipid droplets (LDs) and organs in zebrafish larvae to image ectopic lipids is not available. Our objectives were to develop a novel methodology to quantitatively evaluate organ-specific LDs, in skeletal muscle and liver, in response to a nutritional perturbation.

METHODS

We developed a novel embedding and cryosectioning protocol allowing the conservation of LDs and organs in zebrafish larvae. To establish the quantitative measures, we used a three-arm parallel nutritional intervention design. Zebrafish larvae were fed a control diet containing 14% of nutritional fat or two high fat diets (HFDs) containing 25 and 36% of dietary fats. In muscle and liver, LDs were characterized using immunofluorescence confocal microscopy. In liver, intrahepatocellular lipids were discriminated from intrasinusoid lipids. To complete liver characteristics, fibrosis was identified with Masson's Trichrome staining. Finally, to confirm the conservation and effect of HFD, molecular players of fat metabolism were evaluated by RT-qPCR.

RESULTS

The cryosections obtained after setting up the embedding and cryopreservation method were of high quality, preserving tissue morphology and allowing the visualization of ectopic lipids. Both HFDs were obesogenic, without modifying larvae survival or development. Neutral lipid content increased with time and augmented dietary fat. Intramuscular LD volume density increased and was explained by an increase in LDs size but not in numbers. Intrahepatocellular LD volume density increased and was explained by an increased number of LDs, not by their increased size. Sinusoid area and lipid content were both increased. Hepatic fibrosis appeared with both HFDs. We observed alterations in the expression of genes associated with LD coating proteins, LD dynamics, lipogenesis, lipolysis and fatty acid oxidation.

CONCLUSIONS

In this study, we propose a reproducible and fast method to image zebrafish larvae without losing LD quality and organ morphology. We demonstrate the impact of HFD on LD characteristics in liver and skeletal muscle accompanied by alterations of key players of fat metabolism. Our observations confirm the evolutionarily conserved mechanisms in lipid metabolism and reveal organ specific adaptations. The methodological advancements proposed in this work open the doors to study organelle adaptations in obesity and diabetes related research such as lipotoxicity, organelle contacts and specific lipid depositions.

Morphometric parameters predict body fat proportions in common hamsters

Common hamsters (Cricetus cricetus) are hibernators that rely both on body fat reserves and food stores for the winter period. They face an ongoing population decline in most parts of their distribution and recently were classified as critically endangered. Knowledge on individual body fat proportions in this species is of particular interest for conservation, because it could contribute to better understand the high plasticity in overwintering strategies, overwinter mortality rates, individual variations in reproductive output, and give information on the animals’ health state. To calculate body fat proportions, we validated a method that can be applied in the field without the use of anesthesia. To develop this method, we first analyzed the body fat in carcasses of common hamsters using Soxhlet extractions and measured four morphometric parameters (body mass, head length, tibia length, foot length). The morphometric measurements were then integrated in a linear regression model to predict body fat proportions based on the measured values. The morphometric variables yielded an explained variance (adjusted R²) of 96.42% and body fat proportions were predicted with a mean absolute error of 1.27 ± 0.11% from measured values. We applied the model to predict body fat for available field data, which consistently produced reliable values. By measuring the four morphometric parameters and following the provided instructions, body fat proportions can be reliably and noninvasively estimated in captive or free-ranging common hamsters. Furthermore, the method could be applicable to other rodents after species-specific validation.

The effects of dietary saturated fat source on weight gain and adiposity are influenced by both sex and total dietary lipid intake in zebrafish

The effects of saturated fat intake on obesity and cardiovascular health remain inconclusive, likely due in part to their varied nature and interactions with other nutrients. Investigating the synergistic effects of different saturated fat sources with other dietary lipid components will help establish more accurate nutritional guidelines for dietary fat intake. Over the past two decades, zebrafish (Danio rerio) have been established as an attractive model system to address questions regarding contributions of dietary lipid intake to diet-induced obesity in humans. The goal of the present study was to assess interactions of three different saturated fat sources (milk fat, palm oil, and coconut oil) with sex and total dietary lipid intake on weight gain and body composition in adult zebrafish. Larvae were raised on live feeds until 28 days post fertilization, and then fed a formulated maintenance diet until three months of age. An eight-week feeding trial was then initiated, in which zebrafish were fed nine experimental low- and high-fat diets varying in saturated fatty acid and long-chain polyunsaturated fatty acid content, in addition to a low-fat and high-fat control diet. At termination of the feeding trial, each treatment was evaluated according to body mass, moisture content, and adiposity. Sex and diet significantly interacted in their effects on body mass (P = 0.026), moisture content (P = 0.044), and adiposity (P = 0.035). The influence of saturated fat source on body mass was observed to be dependent on intake of total dietary lipid. In females, all three saturated fat sources had similar effects on adiposity. From these observations, we hypothesize that impacts of saturated fat intake on energy allocation and obesity-related phenotypes are influenced by both sex and intake of other dietary lipid components. Our results suggest that current nutritional guidelines for saturated fat intake may need to be re-evaluated and take sex-specific recommendations into consideration.

Genetic and Genomic Advances in Developmental Models: Applications for Nutrition Research

There is increasing appreciation that dietary components influence and interact with genes important to metabolism. How such influences impact developmental regulation and programming or risks of chronic diseases remains unclear. Nutrition is recognized to affect development and chronic diseases, but our understanding about how genes essential to nutrient metabolism regulate development and impact risks of these diseases remains unclear. Historically, mammalian models, especially rodents such as rats and mice, have been the primary models used for nutrition and developmental nutrition science, although their complexity and relatively slow rate of development often compromise rapid progress in resolving fundamental, genetic-related questions. Accordingly, the objective of this review is to highlight the opportunities for developmental models in the context of uncovering the function of gene products that are relevant to human nutrition and provide the scientific bases for these opportunities. We present recent studies in zebrafish related to obesity as applications of developmental models in nutritional science. Although the control of external factors and dependent variables, such as nutrition, can be a challenge, suggestions for standardizations related to diet are made to improve consistency in findings between laboratories. The review also highlights the need for standardized diets across different developmental models, which could improve consistency in findings across laboratories. Alternative and developmental animal models have advantages and largely untapped potential for the advancement of nutrigenomics and nutritionally relevant research areas.

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.

Influence of Commercial and Laboratory Diets on Growth, Body Composition, and Reproduction in the Zebrafish Danio rerio

The value of the zebrafish (Danio rerio) as a model organism continues to expand. In developing the model, current feeding practice in zebrafish laboratories includes the use of commercially available diets. In this study, we compared outcomes in growth, body composition, and reproduction among zebrafish fed five highly utilized commercial diets and one formulated chemically defined reference diet. Wild-type zebrafish larvae were raised on live feed until 21 days postfertilization and then fed diets for 16 weeks. All fish received a daily ration of >5% of body weight (adjusted biweekly). Growth varied among diets throughout the feeding trial, and at study termination (week 16), significant differences among diets were observed for terminal weight gain, body condition index, body fat deposition, and reproductive outcomes. In addition, the proportion of viable embryos produced from females fed the formulated reference diet was high relative to the commercial diets. These data suggest that metabolic profiles, most likely reflecting nutrient/energy availability, utilization, and allocation, vary relative to diet in zebrafish. Undefined differences in metabolic profiles could result in erroneous predictions of health outcomes and make comparisons among laboratories more challenging. We recommend that dietary standards should be defined for zebrafish to support their common utility in biomedical research.

Dietary methionine restriction: Effects on glucose tolerance, lipid content and micro-RNA composition in the muscle of rainbow trout

Lean muscle mass plays an important role in overall health, as altered skeletal muscle metabolism can impact both the incidence and prevention of conditions related to metabolic health. Intriguingly, dietary methionine restriction (MR) has been shown to ameliorate this phenotype over time potentially through mechanisms related to changes in myogenic precursor cell (MPC) differentiation status. Recently the role of micro-RNAs (miRs) in regulating the expression of muscle specific transcription factors myoD and myogenin as well as signaling molecules involved in skeletal muscle differentiation has been reported in vitro. We performed an 8week feeding trial to determine if MR in vivo could alter miR abundance as well as change metabolic markers. Results show changes in muscle miR abundance for miR-133a at 4weeks with no significant difference seen in miR-210 or miR-206. After 8weeks of MR feeding fish demonstrated increased clearance of glucose, increased fat accumulation in the liver, and decreased fat accumulation in the muscle. These data demonstrate conservation of MR effects on fish metabolism, and suggest, for the first time, that miR-133a might play a role in tissue response to MR.

Tectal corticotropin-releasing factor (CRF) neurons respond to fasting and a reactive stressor in the African Clawed Frog, Xenopus laevis

It is well established that hypothalamic neurons producing the peptide corticotropin-releasing factor (CRF) play a key role in stress adaptation, including reduction of food intake when a threat or stressor is present. We have previously reported on the presence of an intrinsic CRF signaling system within the optic tectum (OT), a brain area that plays a key role in visually guided prey capture/predator avoidance decisions. To better understand the potential role of tectal CRF neurons in regulating adaptive behavior and energy balance during stress we examined evidence for modulation of tectal CRF neuronal activity after stressor exposure and food deprivation in the African clawed frog Xenopus laevis. We tested two predictions, 1) that exposure to categorically distinct stressors (ether vapors and shaking) will reduce food intake and modulate the activity of tectal CRF cells, and 2) that food deprivation will modulate the activity of tectal CRF cells. Exposure to ether increased tectal content of CRF and CRF transcript, but lowed CRFR1 transcript abundance. Two weeks of food deprivation reduced total fat stores in frogs and decreased tectal content of CRF content while having no effect on CRF and CRFR1 transcript abundance. Our data are consistent with a role for tectal CRF neurons in modulating food intake in response to certain stressors.

Validation of Body Condition Indices and Quantitative Magnetic Resonance in Estimating Body Composition in a Small Lizard

Measurements of body condition are typically used to assess an individual's quality, health, or energetic state. Most indices of body condition are based on linear relationships between body length and mass. Although these indices are simple to obtain, nonlethal, and useful indications of energetic state, their accuracy at predicting constituents of body condition (e.g., fat and lean mass) are often unknown. The objectives of this research were to (1) validate the accuracy of another simple and noninvasive method, quantitative magnetic resonance (QMR), at estimating body composition in a small-bodied lizard, Anolis sagrei, and (2) evaluate the accuracy of two indices of body condition (based on length-mass relationships) at predicting body fat, lean, and water mass. Comparisons of results from QMR scans to those from chemical carcass analysis reveal that QMR measures body fat, lean, and water mass with excellent accuracy in male and female lizards. With minor calibration from regression equations, QMR will be a reliable method of estimating body composition of A. sagrei. Body condition indices were positively related to absolute estimates of each constituent of body composition, but these relationships showed considerable variation around regression lines. In addition, condition indices did not predict fat, lean, or water mass when adjusted for body mass. Thus, our results emphasize the need for caution when interpreting body condition based upon linear measurements of animals. Overall, QMR provides an alternative noninvasive method for accurately measuring fat, lean, and water mass in these small-bodied animals.