Zebrafish ghrelin is expressed in pancreatic endocrine cells and regulated by metabolic state

Functionally characterizing obesity-susceptibility genes using CRISPR/Cas9, in vivo imaging and deep learning

Hundreds of loci have been robustly associated with obesity-related traits, but functional characterization of candidate genes remains a bottleneck. Aiming to systematically characterize candidate genes for a role in accumulation of lipids in adipocytes and other cardiometabolic traits, we developed a pipeline using CRISPR/Cas9, non-invasive, semi-automated fluorescence imaging and deep learning-based image analysis in live zebrafish larvae. Results from a dietary intervention show that 5 days of overfeeding is sufficient to increase the odds of lipid accumulation in adipocytes by 10 days post-fertilization (dpf, n = 275). However, subsequent experiments show that across 12 to 16 established obesity genes, 10 dpf is too early to detect an effect of CRISPR/Cas9-induced mutations on lipid accumulation in adipocytes (n = 1014), and effects on food intake at 8 dpf (n = 1127) are inconsistent with earlier results from mammals. Despite this, we observe effects of CRISPR/Cas9-induced mutations on ectopic accumulation of lipids in the vasculature (sh2b1 and sim1b) and liver (bdnf); as well as on body size (pcsk1, pomca, irs1); whole-body LDLc and/or total cholesterol content (irs2b and sh2b1); and pancreatic beta cell traits and/or glucose content (pcsk1, pomca, and sim1a). Taken together, our results illustrate that CRISPR/Cas9- and image-based experiments in zebrafish larvae can highlight direct effects of obesity genes on cardiometabolic traits, unconfounded by their - not yet apparent - effect on excess adiposity.

Early nutritional programming in sterlet sturgeon (Acipenser ruthenus) with dietary soybean meal: Assessment of growth performance, body composition, and expression of GH, IGF-I, and Ghrelin genes

This study was performed to assess the impacts of introducing diets containing different levels of soybean meal (SBM) to sterlet sturgeon (Acipenser ruthenus) larvae on growth performance, body composition, and molecular responses in the juvenile stage. The sterlet larvae (57.68 ± 0.66 mg) were weaned onto the formulated diets as follows: a control diet containing 60% fishmeal (FM), and three experimental diets with replacement levels of 15% (SBM15), 30% (SBM30), and 45% (SBM45) of FM with SBM. Then, a total of 260 fish (initial weight: 323.33 ± 11.76 mg) were fed the four different diets for 28 days in triplicates (phase 1, nutritional programming, NP). All treatments were then fed with the FM diet in phase 2 (common phase), and in phase 3 (challenge phase), all experimental groups (6.14 ± 0.08 g) were transitioned to SBM45 for 28 days. At the end of phases 1 and 2, growth performance showed no significant differences among the groups (P > 0.05), while significantly improved in SBM45 than the control at the end of phase 3 (P < 0.05). No significant differences were found among the groups in any phases for whole body composition (P > 0.05). Additionally, the total saturated fatty acids were significantly higher in SBM-based diets than FM at the end of phase 3 (P < 0.05). The mRNA of GH, IGF-I was significantly affected by variation of FM replacement level (P < 0.05). The expression level of Ghrelin was up-regulated in fish fed SBM at the end of phase 3 (P < 0.05). Our findings revealed that NP can positively enhance the adaptation of juvenile sterlet sturgeon to 45% SBM when exposed to the same diets at the larval stage. Further research is being carried out to provide valuable insights into the underlying mechanisms of digestive performance for this species.

The Interplay between Central and Peripheral Systems in Feed Intake Regulation in European Seabass (Dicentrarchus labrax) Juveniles

The present study aimed to evaluate the effects of feeding or feed deprivation on the orexigenic and anorexigenic responses at the central (whole brain) and peripheral (anterior and posterior intestine, stomach, and liver) system levels in European seabass. For this purpose, a group of fish (208 g) was fed a single meal daily for 8 days (fed group) and another group was feed-deprived for 8 days (unfed group). Compared to the fed group, in the whole brain, feed deprivation did not induce changes in npy, agrp1, and cart2 expression, but increased agrp2 and pomc1 expression. In the anterior intestine, feed deprivation increased cck expression, while in the posterior intestine, the npy expression increased and pyyb decreased. In the stomach, the ghr expression decreased regardless of the feeding status. The hepatic lep expression increased in the unfed fish. The present results suggest a feed intake regulation mechanism in European seabass similar to that observed in other teleosts.

Feeding Rate and Protein Quality Differentially Affect Growth and Feeding Efficiency Response Variables of Zebrafish (Danio rerio)

Manipulating feeding rate and protein quality may improve growth and feeding efficiency of cultured species. However, whether feeding rate, protein quality, or their interaction has a greater effect on growth and feeding efficiency response variables is unknown. To determine whether feeding rate and protein quality individually or interactively affect growth and feeding efficiency, juvenile Zebrafish (Danio rerio) were either offered nutritionally similar diet consisting of either menhaden fishmeal protein or a 100% replacement of fishmeal with soybean meal-based protein restrictively or to satiation. Total length, weight, feed intake, and feed conversion ratio (FCR) were measured throughout the duration of the study. Protein quality and feeding rate individually and interactively affected feed intake and FCR: Zebrafish offered feed to satiation had higher growth and FCR than those fed restrictively, and Zebrafish fed soybean meal-based diet showed lower growth and higher FCR and feed intake compared to those fed fishmeal-based diet, although magnitude of response depended on feeding rate. These findings likely indicate lower digestibility of soybean meal or the presence of antinutritional factors in soybean meal that led to impaired nutrient absorption of fish offered soybean meal-based diet. Differences in measured response variables between protein qualities and feeding rates highlight the importance of determining interactive effects in nutritional studies.

Nutritional programming improves dietary plant protein utilization in zebrafish Danio rerio

Nutritional Programming (NP) has been shown to counteract the negative effects of dietary plant protein (PP) by introducing PP at an early age towards enhancement of PP utilization during later life stages. This study explored the effect of NP and its induction time on growth, expression of appetite-stimulating hormones, and any morphological changes in the gut possibly responsible for improved dietary PP utilization. At 3 days post-hatch (dph) zebrafish were distributed into 12 (3 L) tanks, 100 larvae per tank. This study included four groups: 1) The control (NP-FM) group received fishmeal (FM)-based diet from 13–36 dph and was challenged with PP-based diet during 36–66 dph; 2) The NP-PP group received NP with dietary PP in larval stage via live food enrichment during 3–13 dph followed by FM diet during 13–36 dph and PP diet during 36–66 dph; 3) The T-NP group received NP between 13–23 dph through PP diet followed by FM diet during 23–36 dph and PP diet during 36–66 dph; and 4) The PP group received PP diet from 13–66 dph. During the PP challenge the T-NP group achieved the highest weight gain compared to control and PP. Ghrelin expression in the brain was higher in T-NP compared to NP-FM and NP-PP, while in the gut it was reduced in both NP-PP and T-NP groups. Cholecystokinin expression showed an opposite trend to ghrelin. The brain neuropeptide Y expression was lower in NP-PP compared to PP but not different with NP-FM and T-NP groups. The highest villus length to width ratio in the middle intestine was found in T-NP compared to all other groups. The study suggests that NP induced during juvenile stages improves zebrafish growth and affects digestive hormone regulation and morphology of the intestinal lining–possible mechanisms behind the improved PP utilization in pre-adult zebrafish stages.

Fasting Upregulates npy, agrp, and ghsr Without Increasing Ghrelin Levels in Zebrafish (Danio rerio) Larvae

Food intake in fish and mammals is orchestrated by hypothalamic crosstalk between orexigenic (food intake stimulation) and anorexigenic (food intake inhibition) signals. Some of these signals are released by peripheral tissues that are associated with energy homeostasis or nutrient availability. During the fish larva stage, orexigenic stimulation plays a critical role in individual viability. The goal of this study was to assess the mRNA levels of the main neuropeptides involved in food intake regulation (npy, agrp, carppt, and pomc), in concert with the mRNA levels and peptide levels of ghrelin, under a fasting intervention at the larval stage in zebrafish (Danio rerio). Prior to the fasting intervention, the zebrafish larva cohort was reared for 20 days post fertilization (dpf) and then randomly divided into two groups of 20 individuals. One group was subjected to a fasting intervention for 5 days (fasted group), and the other group was fed normally (fed group); this experimental protocol was performed twice independently. At the end of the fasting period, individuals from each experimental group were divided into different analysis groups, for evaluations such as relative gene expression, immunohistochemistry, and liquid chromatography coupled to nano high-resolution mass spectrometry (nLC-HRMS) analyses. The relative expression levels of the following genes were assessed: neuropeptide Y (npy), agouti-related peptide (agrp), proopiomelanocortin (pomc), cocaine and amphetamine-regulated transcript (cartpt), ghrelin (ghrl), ghrelin O-acyltransferase (mboat4), growth hormone secretagogue receptor (ghsr), and glucokinase (gck). In the fasted group, significant upregulation of orexigenic peptides (npy - agrp) and ghsr was observed, which was associated with significant downregulation of gck. The anorexigenic peptides (pomc and cartpt) did not show any significant modulation between the groups, similar to mboat4. Contrary to what was expected, the relative mRNA upregulation of the orexigenic peptides observed in the fasted experimental group could not be associated with significant ghrelin modulation as assessed by three different approaches: qPCR (relative gene expression of ghrelin), nLC-HRMS (des-acyl-ghrelin levels), and immunohistochemistry (integrated optical density of prepropeptides in intestinal and hepatopancreas tissues). Our results demonstrate that zebrafish larvae at 25 dpf exhibit suitable modulation of the relative mRNA levels of orexigenic peptides (npy and agrp) in response to fasting intervention; nevertheless, ghrelin was not coregulated by fasting. Therefore, it can be suggested that ghrelin is not an essential peptide for an increase in appetite in the zebrafish larva stage. These results give rise to new questions about food intake regulation factors in the early stages of fish.

Nutrient Regulation of Endocrine Factors Influencing Feeding and Growth in Fish

Endocrine factors regulate food intake and growth, two interlinked physiological processes critical for the proper development of organisms. Somatic growth is mainly regulated by growth hormone (GH) and insulin-like growth factors I and II (IGF-I and IGF-II) that act on target tissues, including muscle, and bones. Peptidyl hormones produced from the brain and peripheral tissues regulate feeding to meet metabolic demands. The GH-IGF system and hormones regulating appetite are regulated by both internal (indicating the metabolic status of the organism) and external (environmental) signals. Among the external signals, the most notable are diet availability and diet composition. Macronutrients and micronutrients act on several hormone-producing tissues to regulate the synthesis and secretion of appetite-regulating hormones and hormones of the GH-IGF system, eventually modulating growth and food intake. A comprehensive understanding of how nutrients regulate hormones is essential to design diet formulations that better modulate endogenous factors for the benefit of aquaculture to increase yield. This review will discuss the current knowledge on nutritional regulation of hormones modulating growth and food intake in fish.

Critical Role for a Subset of Intestinal Macrophages in Shaping Gut Microbiota in Adult Zebrafish

The gut microbiota is strongly influenced by environmental factors, although host contribution is far less understood. We leveraged macrophage-deficient interferon regulatory factor irf8 zebrafish mutants to investigate the role of macrophages in this process. In conventionally raised adult irf8-deficient mutants, we found a significant loss of intestinal macrophages associated with a strikingly altered gut microbiota when compared to co-housed siblings. The destabilization of the gut commensal microbiota was associated with a severe reduction in complement C1q genes and outgrowth of a rare bacterial species. Consistent with a critical function of irf8 in adult intestinal macrophages, irf8 is abundantly expressed in these cells normally, and restoring macrophage irf8 expression in irf8 mutants was sufficient to recover commensal microbes and C1q genes expression. This study reports an important subpopulation of intestinal macrophages that requires irf8 to establish in the gut, ensure normal colonization of gut microbes, and prevent immune dysregulation.

Loss of stomach, loss of appetite? Sequencing of the ballan wrasse (Labrus bergylta) genome and intestinal transcriptomic profiling illuminate the evolution of loss of stomach function in fish

BACKGROUND

The ballan wrasse (Labrus bergylta) belongs to a large teleost family containing more than 600 species showing several unique evolutionary traits such as lack of stomach and hermaphroditism. Agastric fish are found throughout the teleost phylogeny, in quite diverse and unrelated lineages, indicating stomach loss has occurred independently multiple times in the course of evolution. By assembling the ballan wrasse genome and transcriptome we aimed to determine the genetic basis for its digestive system function and appetite regulation. Among other, this knowledge will aid the formulation of aquaculture diets that meet the nutritional needs of agastric species.

RESULTS

Long and short read sequencing technologies were combined to generate a ballan wrasse genome of 805 Mbp. Analysis of the genome and transcriptome assemblies confirmed the absence of genes that code for proteins involved in gastric function. The gene coding for the appetite stimulating protein ghrelin was also absent in wrasse. Gene synteny mapping identified several appetite-controlling genes and their paralogs previously undescribed in fish. Transcriptome profiling along the length of the intestine found a declining expression gradient from the anterior to the posterior, and a distinct expression profile in the hind gut.

CONCLUSIONS

We showed gene loss has occurred for all known genes related to stomach function in the ballan wrasse, while the remaining functions of the digestive tract appear intact. The results also show appetite control in ballan wrasse has undergone substantial changes. The loss of ghrelin suggests that other genes, such as motilin, may play a ghrelin like role. The wrasse genome offers novel insight in to the evolutionary traits of this large family. As the stomach plays a major role in protein digestion, the lack of genes related to stomach digestion in wrasse suggests it requires formulated diets with higher levels of readily digestible protein than those for gastric species.

The effects of dietary Myrtle (Myrtus communis) on skin mucus immune parameters and mRNA levels of growth, antioxidant and immune related genes in zebrafish (Danio rerio)

Myrtle (Myrtus communis L., Myrtaceae) is a significant plant which naturally distributed around the globe. Although numerous studies have demonstrated the benefits of myrtle in different species, studies using the oral route are rare in the literature. In the present study, we evaluated the effect of myrtle intake on the antioxidant, immune, appetite and growth related genes as well as mucosal immune responses in zebrafish (Danio rerio) model. Zebrafish were fed control or myrtle (5, 10 and 20 g kg^-1 myrtle) supplemented diets for sixty days. The results showed that, oral administration of Myrtle significantly improved mucosal immune responses (the activity of lysozyme, total Ig and protease). Furthermore, fish fed 20 g kg^-1 showed remarkably higher antioxidant (sod and cat) enzymes gene expression compared other treatment. There were significant difference between myrtle fed fish and control group regarding tnf-alpha and lyz expression. Also, evaluation of growth (gh and igf1) related genes revealed remarkable upregulation in 20 g kg^-1 myrtle treatment compared other myrtle treatments and control group. Similar results was observed regarding the mRNA levels of appetite related genes (ghrl) in zebrafish fed 20 g kg^-1 myrtle. The present results indicated that dietary administration of myrtle improved mucosal immune parameters and altered mRNA levels of selected genes. These results on zebrafish model also highlights the potential use of Myrtle supplements as additive in human diets.

Dietary sodium propionate affects mucosal immune parameters, growth and appetite related genes expression: Insights from zebrafish model

Propionate is a short-chain fatty acid (SCFA) that improves physiological and pathophysiological properties. However, there is limited information available about the effects of SCFAs on mucosal immune parameters as well as growth and appetite related genes expression. The aim of the present study was to evaluate the effect of sodium propionate (SP) intake on the mucosal immune parameters, growth and appetite related genes expression using zebrafish (Danio rerio) as model organism. Zebrafish fed control or diet supplemented with different levels (0.5, 1 and 2%) of SP for 8weeks. At the end of feeding trial, the expression of the key genes related to growth and appetite (GH, IGF1, MYSTN and Ghrl) was evaluated. Also, mucosal immune parameters (Total Ig, lysozyme and protease activity) were studied in skin mucus of zebrafish. The results showed that dietary administration of SP significantly (P<0.05) up-regulated the expression of GH, IGF1 and down-regulated MYSTN gene. Also, feeding zebrafish with SP supplemented diet significantly increased appetite related gene expression (P<0.05) with a more pronounced effect in higher inclusion levels. Compared with control group, the expression of appetite related gene (Ghrl) was remarkably (P<0.05) higher in SP fed zebrafish. Also, elevated mucosal immune parameters was observed in zebrafish fed SP supplemented diet. The present results revealed beneficial effects of dietary SP on mucosal immune response and growth and appetite related genes expression. These results also highlighted the potential use of SP as additive in human diets.

Interplay between the endocrine and circadian systems in fishes

The circadian system is responsible for the temporal organisation of physiological functions which, in part, involves daily cycles of hormonal activity. In this review, we analyse the interplay between the circadian and endocrine systems in fishes. We first describe the current model of fish circadian system organisation and the basis of the molecular clockwork that enables different tissues to act as internal pacemakers. This system consists of a net of central and peripherally located oscillators and can be synchronised by the light-darkness and feeding-fasting cycles. We then focus on two central neuroendocrine transducers (melatonin and orexin) and three peripheral hormones (leptin, ghrelin and cortisol), which are involved in the synchronisation of the circadian system in mammals and/or energy status signalling. We review the role of each of these as overt rhythms (i.e. outputs of the circadian system) and, for the first time, as key internal temporal messengers that act as inputs for other endogenous oscillators. Based on acute changes in clock gene expression, we describe the currently accepted model of endogenous oscillator entrainment by the light-darkness cycle and propose a new model for non-photic (endocrine) entrainment, highlighting the importance of the bidirectional cross-talking between the endocrine and circadian systems in fishes. The flexibility of the fish circadian system combined with the absence of a master clock makes these vertebrates a very attractive model for studying communication among oscillators to drive functionally coordinated outputs.

Performance of Debaryomyces hansenii as a Diet for Rotifers for Feeding Zebrafish Larvae

The zebrafish larval stage is a critical moment due to high mortality rates associated with inadequate supplies of nutritional requirements. Larval feeding has important challenges associated with such factors as small mouth gape (≈100 μm), the low activity of digestive enzymes, and the intake of live food. A common zebrafish live food at the onset of exogenous feeding is rotifers, mainly Brachionus plicatilis. These rotifers should be fed with other microorganisms such as microalgae or yeast, mostly from the Saccharomyces genus. In the laboratory, the culture of microalgae is more expensive than the culture of yeast. The aim of this study was to evaluate the performance of Debaryomyces hansenii as a diet for rotifers in comparison to a microalgae-based diet (Rotigrow^®). To achieve this aim, we assessed the rotifer total protein content, the rotifers fatty acid profile, zebrafish larval growth performance, the expression of key growth, and endocrine appetite regulation genes. The total protein and fatty acids content were similar in both rotifer cultures, averaging 35% of dry matter (DM) and 18% of DM, respectively. Interestingly, the fatty acids profile showed differences between the two rotifer cultures: omega-3 fatty acids were only observed in the Microalgae/rotifer, whereas, omega-6 fatty acids presented similar levels in both rotifer cultures. No differences were observed in the larval body length distribution or mortalities between the rotifer cultures. However, gh, igf-1, and cck gene expression showed significantly higher upregulation in zebrafish fed the Microalgae/rotifer diet compared with those fed the Debaryomyces/rotifer diet. In conclusion, D. hansenii could be an alternative diet for rotifer used as a live food in zebrafish larvae at the onset of exogenous feeding. The gene responses observed in this work open up the opportunity to study the effect of omega-3 supply on growth regulation in zebrafish.

Effect of dietary macronutrients on the expression of cholecystokinin, leptin, ghrelin and neuropeptide Y in gilthead sea bream (Sparus aurata)

Endocrine factors released from the central nervous system, gastrointestinal tract, adipose tissue and other peripheral organs mediate the regulation of food intake. Although many studies have evaluated the effect of fed-to-starved transition on the expression of appetite-related genes, little is known about how the expression of appetite-regulating peptides is regulated by the macronutrient composition of the diet. The aim of the present study was to examine the effect of diet composition and nutritional status on the expression of four peptides involved in food intake control in gilthead sea bream (Sparus aurata): neuropeptide Y (NPY), ghrelin, cholecystokinin (CCK) and leptin. Quantitative real-time RT-PCR showed that high protein/low carbohydrate diets stimulated the expression of CCK and ghrelin in the intestine and leptin in the adipose tissue, while downregulation of ghrelin and NPY mRNA levels was observed in the brain. Opposite effects were found for the expression of the four genes in fish fed low protein/high carbohydrate diets or after long-term starvation. Our findings indicate that the expression pattern of appetite-regulating peptides, particularly CCK and ghrelin, is modulated by the nutritional status and diet composition in S. aurata.

Ghrelin O-Acyl Transferase in Zebrafish Is an Evolutionarily Conserved Peptide Upregulated During Calorie Restriction

Ghrelin is a multifunctional orexigenic hormone with a unique acyl modification enabled by ghrelin O-acyl transferase (GOAT). Ghrelin is well-characterized in nonmammals, and GOAT sequences of several fishes are available in the GenBank. However, endogenous GOAT in non-mammals remains poorly understood. In this research, GOAT sequence comparison, tissue-specific GOAT expression, and its regulation by nutrient status and exogenous ghrelin were studied. It was found that the bioactive core of zebrafish GOAT amino acid sequence share high identity with that of mammals. GOAT mRNA was most abundant in the gut. GOAT-like immunoreactivity (i.r.) was found colocalized with ghrelin in the gastric mucosa. Food deprivation increased, and feeding decreased GOAT and preproghrelin mRNA expression in the brain and gut. GOAT and ghrelin peptides in the gut and brain showed corresponding decrease in food-deprived state. Intraperitoneal injection of acylated fish ghrelin caused a significant decrease in GOAT mRNA expression, suggesting a feedback mechanism regulating its abundance. Together, these results provide the first in-depth characterization of GOAT in a non-mammal. Our results demonstrate that endogenous GOAT expression is responsive to metabolic status and availability of acylated ghrelin, providing further evidences for GOAT in the regulation of feeding in teleosts.