Intestinal growth and differentiation in zebrafish

Bone cell differentiation and mineralization in wild-type and osteogenesis imperfecta zebrafish are compromised by per- and poly-fluoroalkyl substances (PFAS)

Perfluorinated compounds (PFAS) are well recognized toxic pollutants for humans, but if their effect is equally harmful for healthy and fragile people is unknown. Addressing this question represents a need for ensuring global health and wellbeing to all individuals in a world facing the progressive increase of aging and aging related diseases. This study aimed to evaluate the impact of perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA) and perfluorohexanoic acid (PFHxA) exposure on development and skeletal phenotype using the osteogenesis imperfecta (OI) zebrafish model Chihuahua (Chi/+), carrying a dominant glycine substitution in the α1 chain of collagen I and their wild-type (WT) littermates. To this purpose Chi/+ and WT zebrafish expressing the green fluorescent protein under the early osteoblast marker osterix were exposed from 1 to 6 days post fertilization to 0.36, 1.5 and 3.0 mg/L PFOS, 0.005 and 0.5 mg/L PFOA and 0.01, 0.48 and 16.0 mg/L PFHxA, and their development and skeletal phenotype investigated. Morphometric measurements, confocal microscopy evaluation of operculum area delimited by the fluorescent preosteoblasts and mineral deposition analysis following alizarin red staining were employed. PFOS and the highest concentration of PFHxA significantly impaired standard length in both genotypes. Osteoblast differentiation was significantly compromised by PFOS and by PFOA only in Chi/+. Limited to WT exposed to PFOA a reduced mineralization was also observed. No effect was detected after PFHxA exposure. Apoptosis was only activated by PFOA, specifically in Chi/+ mutant operculum osteoblasts. Interestingly, an altered lipid distribution in both WT and mutant fish was revealed after exposure to both pollutants. In conclusion, our data demonstrate that PFAS impair operculum development mainly compromising cell differentiation in mutant fish whereas alter lipid hepatic distribution in both genotypes with a more severe effect on Chi/+ preosteoblast survival. These results represent a first warning sign of the negative impact of PFAS exposure in presence of genetically determined skeletal fragility.

Gene expression patterns of the LDL receptor and its inhibitor Pcsk9 in the adult zebrafish brain suggest a possible role in neurogenesis

The low-density lipoprotein receptor (LDLr) is the first member of a closely related transmembrane protein family. It is known for its involvement in various physiological processes, mainly in the regulation of lipid metabolism, especially in the brains of mammals and zebrafish. In zebrafish, two ldlr genes (ldlra and b) have been identified and their distribution in the brain is not well documented. Recently, the roles of ldlr and its inhibitor pcsk9 in regenerative process after telencephalic brain injury have been discussed. In this study, we explored the expression patterns of these genes during zebrafish development. We found that ldlra expression was detected at the end of the pharyngula period (48 hpf) and increased during the larval stage. Conversely, ldlrb expression was observed from zygotic to larval stages. Using techniques like in situ hybridization and taking advantage of transgenic fish, we demonstrated the widespread distribution of ldlra, ldlrb and pcsk9 in the brain of adult zebrafish. Specifically, these genes were expressed in neurons and neural stem cells and also at lower levels in endothelial cells. As expected, intraperitoneal injection of fluorescent-labelled LDLs resulted in their uptake by cerebral endothelial cells in a homeostatic context, whereas they diffused within the brain parenchyma after telencephalic injury. However, after intracerebroventricular injections into animals, LDL particles were not taken up by neural stem cells. In conclusion, our results provide additional evidence for LDLr expression in the brain of adult zebrafish. These results raise the question of the role of LDLr in the cholesterol/lipid imbalance in cerebral complications.

Myosin 1b regulates intestinal epithelial morphogenesis via interaction with UNC45A

Vesicle trafficking and the establishment of apicobasal polarity are essential processes in epithelial morphogenesis. UNC45A deficiency has been reported in a multi-organ syndrome presenting with severe diarrhea associated with enterocyte polarity defects. Myosin 1b, an actin motor able to bind membranes, regulates membrane shaping and vesicle trafficking. Here, we show that MYO1B is part of the UNC45A interactome. In the absence of UNC45A, myosin 1b is degraded and forms aggregates when proteasome activity is inhibited. In 3D Caco-2 cells, lumen formation is impaired in the absence of myosin 1b, associated with spindle orientation defects, Golgi apparatus fragmentation, and trafficking impairment. In zebrafish larvae, loss of myo1b results in intestinal bulb epithelium folding defects associated with terminal web disorganization and vesicle accumulation, reminiscent of villous atrophy. In conclusion, we show that myosin 1b plays an unexpected role in the development of the intestinal epithelium downstream of UNC45A, establishing its contribution in the gut defects reported in UNC45A patients.

Protein absorption in the zebrafish gut is regulated by interactions between lysosome rich enterocytes and the microbiome

Dietary protein absorption in neonatal mammals and fishes relies on the function of a specialized and conserved population of highly absorptive lysosome rich enterocytes (LREs). The gut microbiome has been shown to enhance absorption of nutrients, such as lipids, by intestinal epithelial cells. However, whether protein absorption is also affected by the gut microbiome is poorly understood. Here, we investigate connections between protein absorption and microbes in the zebrafish gut. Using live microscopy-based quantitative assays, we find that microbes slow the pace of protein uptake and degradation in LREs. While microbes do not affect the number of absorbing LRE cells, microbes lower the expression of endocytic and protein digestion machinery in LREs. Using transgene assisted cell isolation and single cell RNA-sequencing, we characterize all intestinal cells that take up dietary protein. We find that microbes affect expression of bacteria-sensing and metabolic pathways in LREs, and that some secretory cell types also take up protein and share components of protein uptake and digestion machinery with LREs. Using custom-formulated diets, we investigated the influence of diet and LRE activity on the gut microbiome. Impaired protein uptake activity in LREs, along with a protein-deficient diet, alters the microbial community and leads to increased abundance of bacterial genera that have the capacity to reduce protein uptake in LREs. Together, these results reveal that diet-dependent reciprocal interactions between LREs and the gut microbiome regulate protein absorption.

Imaging analytical technique to assess gastrointestinal motility in vivo using zebrafish larvae with diabetes mellitus-like traits

In diabetes mellitus (DM), the prevalence of gastrointestinal (GI) complications, including constipation, diarrhoea, gastroparesis, and/or enteropathy, can be up to ~75%. In this study, we compared three zebrafish larvae models of DM and established an analytical protocol for GI motility. Larvae were fed with either a standard diet (SD; control), or one of three diets to induce a DM-like phenotype: excessive feeding of SD food (ED), a high-fat diet (HFD), or exposing SD-fed larvae to 30 mmol/L glucose (SDG). DM was confirmed using a body-mass index, assessment of adipose deposit areas, two glucose assays, and one insulin assay. An analytical technique, whereby GI motility was quantified using pixel differences to track displacement along the centreline of the anterior, middle, and posterior intestine (AI, MI, and PI, respectively), was developed. Our results indicated that clear DM-like traits were observed in the HFD and SGD models, but not the ED model. In the SD controls, the AI showed similar anterograde and retrograde contractions indicating normal GI mixing; the MI exhibited more prominent forward contractions, and the PI showed distinct rectal waves. Compared to the SD, the HFD and SDG models exhibited significantly increased and decreased contraction velocities and could be used as models of diarrhoea and constipation in DM, respectively, while the ED model showed comparatively little change in motility. Together, these data indicate that complex changes in GI motility are associated with diet and therapeutics used to alleviate GI complications in DM should take these into account. Ultimately, the HFD and SDG models can be used to investigate different aspects of GI motility in association to DM. Hence, zebrafish are a useful model for studying GI dysfunctions due to DM and/or DM medication side-effects.

Human norovirus disturbs intestinal motility and transit time through its capsid proteins

Human norovirus (HuNoV) accounts for over 700 million cases of gastroenteritis annually. Episodes of HuNoV disease are characterized by vomiting and diarrhea as the two most prominent symptoms. Despite its prevalence, our understanding of the pathophysiological mechanisms triggered upon HuNoV infection is limited, mainly due to a lack of suitable animal models. Our aim was to use the recent HuNoV zebrafish larvae model to study the effect of HuNoV infection on intestinal motility and investigate whether one viral protein could act as an enterotoxin, as seen with rotavirus. We studied whether HuNoV infection affects the contraction frequency of the intestinal bulb and the posterior intestine as well as the transit time. Infection of larvae, following injection of a HuNoV GII.4-containing stool sample in the yolk, resulted in an increased contraction frequency in the intestinal bulb. A comparable effect was observed in serotonin-treated larvae, corresponding to the natural function of serotonin. The higher replication efficacy of HuNoV GII.4 likely explains why they have a more marked effect on gut motility, when compared to other genotypes. Additionally, transit time of fluorescent food was prolonged in HuNoV GII.4 infected larvae, suggesting a loss of coordination in bowel movements upon infection. To identify the proteins responsible for the effect, individual HuNoV non-structural proteins and virus-like particles (VLPs) were injected intraperitoneally (ip). VLPs carrying VP1/VP2, but not those with only VP1, induced increased contraction frequencies in the intestinal bulb in a dose-dependent manner. In conclusion, our findings suggest that the viral capsid and potentially the minor capsid protein VP2 play a crucial role in the aetiology of symptoms associated with HuNoV, potentially acting as a viral enterotoxin. This work contributes to the understanding of the pathophysiological mechanisms in HuNoV-induced disease and further attests zebrafish as a valuable HuNoV disease model.

Enteroendocrine cells regulate intestinal homeostasis and epithelial function

Enteroendocrine cells (EECs) are well-known for their systemic hormonal effects, especially in the regulation of appetite and glycemia. Much less is known about how the products made by EECs regulate their local environment within the intestine. Here, we focus on paracrine interactions between EECs and other intestinal cells as they regulate three essential aspects of intestinal homeostasis and physiology: 1) intestinal stem cell function and proliferation; 2) nutrient absorption; and 3) mucosal barrier function. We also discuss the ability of EECs to express multiple hormones, describe in vitro and in vivo models to study EECs, and consider how EECs are altered in GI disease.

Morphological organisation of the digestive tract in the stream catfish Pseudecheneis sulcatus (McClelland)

We describe the histological organisation and mucin content in the digestive tract of the stream catfish Pseudecheneis sulcatus. The aim is to find the modifications of the digestive tract in relation to food resources of its habitat. The oesophageal mucosa consists of stratified squamous epithelium with many mucous-secreting cells. The thick muscularis contains an inner longitudinal and outer circular, striated muscle cells. The stomach is J-shaped and shows 6-7 thick mucosal folds that are separated from the submucosa by an organised muscularis mucosae. The mucosa consists of superficial cells with mucin granules, and deeper simple tubular gastric glands in cardia and fundus, but absent in pyloric region. The glandular epithelium shows oxynticopeptic cells containing zymogen granules and abundant tubulo-vesicular bodies. We provide evidence that the latter arise by budding from smooth endoplasmic reticulum and reach the apical cytoplasm. The anterior intestine shows longer mucosal folds with goblet cells (GC). GC are more in the posterior intestine and highest in the rectum. Myenteric neurons with myelinated and non-myelinated axons innervate the intrinsic musculature from stomach to rectum. Many stem cells are evident in the basal intestinal epithelium. They show darker nuclei and undifferentiated organelles. Mucin histochemistry reveals the predominance of neutral mucin (PAS+ positive) from oesophagus to rectum, and neutral and acidic mucin (alcian blue+, pH 2.5) in the posterior intestine to the rectum, with few GC colocalizing both. Ultrastructural features suggest that the species is adapted to omnivory and this is reflected in the predominance of neutral mucin in the digestive tract.

Visible Combined Near-Infrared in Situ Imaging Revealed Dynamic Effects of Microplastic Fibers and Beads in Zebrafish

Microplastics (MPs) as emerging contaminants are widely present in the environment and are ubiquitously ingested and accumulated by aquatic organisms. MPs may be quickly eliminated after a brief retention in aquatic animals (such as the digestive tract); thus, understanding the damage caused by MPs during this process and whether the damage can be recovered is important. Here, we proposed the use of visible light imaging to track MPs combined with near-infrared (NIR) imaging to reveal the in situ impacts of MPs. The combination of these two techniques allows for the simultaneous investigation of the localization and functionality of MPs in vivo. We investigated the effects of two types of MPs on zebrafish, microplastic fibers (MFs) and microplastic beads (MBs). The results showed that MPs larger than 10 μm primarily accumulated in the intestines of zebrafish. Both MFs and MBs disrupted the redox balance of the intestine, and the location of the damage was consistent with the heterogeneous accumulation of MPs. MFs caused greater and more difficult-to-recover damage compared to MBs, which was closely related to the slower elimination rate of MFs. Our study highlights the importance of capturing the dynamic toxicological effects of MPs on organisms. Fibrous MPs and spherical MPs clearly had distinct effects on their toxicokinetics and toxicodynamics in fish.

Metabolic disrupting chemicals in the intestine: the need for biologically relevant models: Zebrafish: what can we learn from this small environment-sensitive fish?

Although the concept of endocrine disruptors first appeared almost 30 years ago, the relatively recent involvement of these substances in the etiology of metabolic pathologies (obesity, diabetes, hepatic steatosis, etc.) has given rise to the concept of Metabolic Disrupting Chemicals (MDCs). Organs such as the liver and adipose tissue have been well studied in the context of metabolic disruption by these substances. The intestine, however, has been relatively unexplored despite its close link with these organs. In vivo models are useful for the study of the effects of MDCs in the intestine and, in addition, allow investigations into interactions with the rest of the organism. In the latter respect, the zebrafish is an animal model which is used increasingly for the characterization of endocrine disruptors and its use as a model for assessing effects on the intestine will, no doubt, expand. This review aims to highlight the importance of the intestine in metabolism and present the zebrafish as a relevant alternative model for investigating the effect of pollutants in the intestine by focusing, in particular, on cytochrome P450 3A (CYP3A), one of the major molecular players in endogenous and MDCs metabolism in the gut.

IL-20 is produced by CD3γδ T cells and induced in the mucosal tissues of grass carp during infection with Aeromonas hydrophila

Interleukin (IL) 20 is a multifunctional cytokine and plays a vital role in regulating autoimmune diseases, inflammation, and immune responses. IL-20 homologs have been described in fish. However, due to the lack of antibodies, cellular sources and immunological functions of fish IL-20 in response to infections have not been fully characterized. In this study, a monoclonal antibody (mAb) was generated against the recombinant grass carp (Ctenopharyngodon idella) IL-20 protein and characterized by immunoblotting, immunofluorescent microscopy and flow cytometry. It was shown that the IL-20 mAb specifically recognized recombinant IL-20 proteins expressed in the E. coli cells and HEK293 cells. Using confocal microscopy, the IL-20+ cells were identified in the head kidney, gills and intestine of grass carp, and induced after infection with Aeromonas hydrophila. Moreover, the IL-20 protein was found to be secreted mainly by CD3γδ T cells which were located predominantly in the gill filaments and intestinal mucosa. Taken together, our results suggest that IL-20 producing T cells are required for the mucosal immunity against bacterial infection in fish.

Advances of microplastics ingestion on the morphological and behavioral conditions of model zebrafish: A review

Concerns have been conveyed regarding the availability and hazards of microplastics (MPs) in aquatic biota due to their widespread presence in aquatic habitats. Zebrafish (Danio rerio) are widely used as a model organism to study the adverse impacts of MPs due to their several compelling advantages, such as their small size, ease of breeding, inexpensive maintenance, short life cycle, year-round spawning, high fecundity, fewer legal restrictions, and genetic resemblances to humans. Exposure of organisms to MPs produces physical and chemical toxic effects, including abnormal behavior, oxidative stress, neurotoxicity, genotoxicity, immune toxicity, reproductive imbalance, and histopathological effects. But the severity of the effects is size and concentration-dependent. It has been demonstrated that smaller particles could reach the gut and liver, while larger particles are only confined to the gill, the digestive tract of adult zebrafish. This thorough review encapsulates the current body of literature concerning research on MPs in zebrafish and demonstrates an overview of MPs size and concentration effects on the physiological, morphological, and behavioral characteristics of zebrafish. Finding gaps in the literature paves the way for further investigation.

Ace Deficiency Induces Intestinal Inflammation in Zebrafish

Inflammatory bowel disease (IBD) is a nonspecific chronic inflammatory disease resulting from an immune disorder in the intestine that is prone to relapse and incurable. The understanding of the pathogenesis of IBD remains unclear. In this study, we found that ace (angiotensin-converting enzyme), expressed abundantly in the intestine, plays an important role in IBD. The deletion of ace in zebrafish caused intestinal inflammation with increased expression of the inflammatory marker genes interleukin 1 beta (il1b), matrix metallopeptidase 9 (mmp9), myeloid-specific peroxidase (mpx), leukocyte cell-derived chemotaxin-2-like (lect2l), and chemokine (C-X-C motif) ligand 8b (cxcl8b). Moreover, the secretion of mucus in the ace-/- mutants was significantly higher than that in the wild-type zebrafish, validating the phenotype of intestinal inflammation. This was further confirmed by the IBD model constructed using dextran sodium sulfate (DSS), in which the mutant zebrafish had a higher susceptibility to enteritis. Our study reveals the role of ace in intestinal homeostasis, providing a new target for potential therapeutic interventions.

Zebrafish ApoB-Containing Lipoprotein Metabolism: A Closer Look

Zebrafish have become a powerful model of mammalian lipoprotein metabolism and lipid cell biology. Most key proteins involved in lipid metabolism, including cholesteryl ester transfer protein, are conserved in zebrafish. Consequently, zebrafish exhibit a human-like lipoprotein profile. Zebrafish with mutations in genes linked to human metabolic diseases often mimic the human phenotype. Zebrafish larvae develop rapidly and externally around the maternally deposited yolk. Recent work revealed that any disturbance of lipoprotein formation leads to the accumulation of cytoplasmic lipid droplets and an opaque yolk, providing a visible phenotype to investigate disturbances of the lipoprotein pathway, already leading to discoveries in MTTP (microsomal triglyceride transfer protein) and ApoB (apolipoprotein B). By 5 days of development, the digestive system is functional, making it possible to study fluorescently labeled lipid uptake in the transparent larvae. These and other approaches enabled the first in vivo description of the STAB (stabilin) receptors, showing lipoprotein uptake in endothelial cells. Various zebrafish models have been developed to mimic human diseases by mutating genes known to influence lipoproteins (eg, ldlra, apoC2). This review aims to discuss the most recent research in the zebrafish ApoB-containing lipoprotein and lipid metabolism field. We also summarize new insights into lipid processing within the yolk cell and how changes in lipid flux alter yolk opacity. This curious new finding, coupled with the development of several techniques, can be deployed to identify new players in lipoprotein research directly relevant to human disease.

Comparative histological description of the intestine in platyfish (Xiphophorus maculatus) and swordtail fish (Xiphophorus helleri)

This study aimed to provide a comprehensive analysis of the histological structure of intestinal tissues of platyfish (Xiphophorus maculatus) and swordtail fish (Xiphophorus helleri). Specifically, the objectives were: (1) to compare the structural adaptations of their intestines related to their distinct feeding habits, diet, and digestive strategies; and (2) to explore their potential as animal models for intestinal disease research. Through detailed examination of tissue morphology, cell types, and structural features, this study found that both species lack a stomach, with the intestine directly connected to the esophagus. Additionally, this study proposes a new division of the intestine into anterior and posterior segments based on distinct histological characteristics. The anterior segment may be adapted for temporary food storage and digestion and was characterized by elongated epithelial cells and thin intestinal folds. In contrast, the posterior segment displayed shorter villi and higher concentrations of goblet cells. This study is the first to describe in detail the intestinal morphology of platyfish and swordtail fish. These findings contribute significantly to the understanding of the comparative anatomy and physiology of these fish species, highlighting their potential as valuable models for intestinal biology research.

Zebrafish Feed Intake: A Systematic Review for Standardizing Feeding Management in Laboratory Conditions

Zebrafish are one of the most used animal models in biological research and a cost-effective alternative to rodents. Despite this, nutritional requirements and standardized feeding protocols have not yet been established for this species. This is important to avoid nutritional effects on experimental outcomes, and especially when zebrafish models are used in preclinical studies, as many diseases have nutritional confounding factors. A key aspect of zebrafish nutrition is related to feed intake, the amount of feed ingested by each fish daily. With the goal of standardizing feeding protocols among the zebrafish community, this paper systematically reviews the available data from 73 studies on zebrafish feed intake, feeding regimes (levels), and diet composition. Great variability was observed regarding diet composition, especially regarding crude protein (mean 44.98 ± 9.87%) and lipid content (9.91 ± 5.40%). Interestingly, the gross energy levels of the zebrafish diets were similar across the reviewed studies (20.39 ± 2.10 kilojoules/g of feed). In most of the reviewed papers, fish received a predetermined quantity of feed (feed supplied). The authors fed the fish according to the voluntary intake and then calculated feed intake (FI) in only 17 papers. From a quantitative point of view, FI was higher than when a fixed quantity (pre-defined) of feed was supplied. Also, the literature showed that many biotic and abiotic factors may affect zebrafish FI. Finally, based on the FI data gathered from the literature, a new feeding protocol is proposed. In summary, a daily feeding rate of 9-10% of body weight is proposed for larvae, whereas these values are equal to 6-8% for juveniles and 5% for adults when a dry feed with a proper protein and energy content is used.

Genetic regulation of enteric nervous system development in zebrafish

The enteric nervous system (ENS) is a complex series of interconnected neurons and glia that reside within and along the entire length of the gastrointestinal tract. ENS functions are vital to gut homeostasis and digestion, including local control of peristalsis, water balance, and intestinal cell barrier function. How the ENS develops during embryological development is a topic of great concern, as defects in ENS development can result in various diseases, the most common being Hirschsprung disease, in which variable regions of the infant gut lack ENS, with the distal colon most affected. Deciphering how the ENS forms from its progenitor cells, enteric neural crest cells, is an active area of research across various animal models. The vertebrate animal model, zebrafish, has been increasingly leveraged to understand early ENS formation, and over the past 20 years has contributed to our knowledge of the genetic regulation that underlies enteric development. In this review, I summarize our knowledge regarding the genetic regulation of zebrafish enteric neuronal development, and based on the most current literature, present a gene regulatory network inferred to underlie its construction. I also provide perspectives on areas for future zebrafish ENS research.

Vibrio cholerae arrests intestinal epithelial proliferation through T6SS-dependent activation of the bone morphogenetic protein pathway

To maintain an effective barrier, intestinal progenitor cells must divide at a rate that matches the loss of dead and dying cells. Otherwise, epithelial breaches expose the host to systemic infection by gut-resident microbes. Unlike most pathogens, Vibrio cholerae blocks tissue repair by arresting progenitor proliferation in the Drosophila model. At present, we do not understand how V. cholerae circumvents such a critical antibacterial defense. We find that V. cholerae blocks epithelial repair by activating the growth inhibitor bone morphogenetic protein (BMP) pathway in progenitors. Specifically, we show that interactions between V. cholerae and gut commensals initiate BMP signaling via host innate immune defenses. Notably, we find that V. cholerae also activates BMP and arrests proliferation in zebrafish intestines, indicating an evolutionarily conserved link between infection and failure in tissue repair. Our study highlights how enteric pathogens engage host immune and growth regulatory pathways to disrupt intestinal epithelial repair.

Transcriptional profiling of zebrafish intestines identifies macrophages as host cells for human norovirus infection

Human noroviruses (HuNoVs) are a major cause of diarrheal disease, yet critical aspects of their biology, including cellular tropism, remain unclear. Although research has traditionally focused on the intestinal epithelium, the hypothesis that HuNoV infects macrophages has been recurrently discussed and is investigated here using a zebrafish larval model. Through single-cell RNA sequencing of dissected zebrafish intestines, we unbiasedly identified macrophages as host cells for HuNoV replication, with all three open reading frames mapped to individual macrophages. Notably, HuNoV preferentially infects actively phagocytosing inflammatory macrophages. HuNoV capsid proteins and double-stranded RNA colocalized within intestinal macrophages of infected zebrafish larvae, and the negative-strand RNA intermediate was detected within FACS-sorted macrophages. Flow cytometry confirmed viral replication within these macrophages, constituting approximately 23% of HuNoV's host cells. Identifying macrophages as host cells prompts a reevaluation of their role in HuNoV pathogenesis, offering new directions for understanding and controlling this infection.

Cellular evidence of communication strategies for small intestinal high endothelial cells: Ultrastructural insights of nano-scale exosomes and autophagy

Although several immune related cells of small intestine play an essential role in the intestinal homeostasis. However, information related to ultrastructural evidence of Nano-scale exosomes-multivesicular bodies and autophagic pathway in the high endothelial cells (HECs) of the small intestine in laying birds is still ambiguous. In present study, the HECs secreted the early endosome (ee), late endosome (le) and multivesicular bodies (MVBs) in the lamina propria of layer small intestine was confirmed by transmission electron microscopy. Besides that, in the cytoplasm of HECs showed many autophagosomes were directly associated with lysosomes and mitochondria. Further, the immunohistochemistry and immunofluorescence results showed that, the immunoreactivity and immuno-signaling of Nano-scale exosome related proteins, cluster of differentiation (CD63) and tumor susceptibility gene (TSG101), and autophagic related proteins, autophagic related gene (ATG7) and microtubule-associated protein light chain (LC3) were strong positive expression in the lamina propria of small intestine. These results prove that HECs play a well-known immunological role in the maintenance of intestinal homeostasis. In summary, these findings indicate that the small intestine's HECs have developed an innovative way of communication.

Immunohistochemical characterisation of the adult Nothobranchius furzeri intestine

Nothobranchius furzeri is emerging as an exciting vertebrate organism in the field of biomedicine, developmental biology and ecotoxicology research. Its short generation time, compressed lifespan and accelerated ageing make it a versatile model for longitudinal studies with high traceability. Although in recent years the use of this model has increased enormously, there is still little information on the anatomy, morphology and histology of its main organs. In this paper, we present a description of the digestive system of N. furzeri, with emphasis on the intestine. We note that the general architecture of the intestinal tissue is shared with other vertebrates, and includes a folding mucosa, an outer muscle layer and a myenteric plexus. By immunohistochemical analysis, we reveal that the mucosa harbours the same type of epithelial cells observed in mammals, including enterocytes, goblet cells and enteroendocrine cells, and that the myenteric neurons express neurotransmitters common to other species, such as serotonin, substance P and tyrosine hydroxylase. In addition, we detect the presence of a proliferative compartment at the base of the intestinal folds. The description of the normal intestinal morphology provided here constitutes a baseline information to contrast with tissue alterations in future lines of research assessing pathologies, ageing-related diseases or damage caused by toxic agents.

Colonization of larval zebrafish (Danio rerio) with adherent-invasive Escherichia coli prevents recovery of the intestinal mucosa from drug-induced enterocolitis

IMPORTANCE

Although inflammatory bowel diseases are on the rise, what factors influence IBD risk and severity, and the underlying mechanisms remain to be fully understood. Although host genetics, microbiome, and environmental factors have all been shown to correlate with the development of IBD, cause and effect are difficult to disentangle in this context. For example, AIEC is a known pathobiont found in IBD patients, but it remains unclear if gut inflammation during IBD facilitates colonization with AIEC, or if AIEC colonization makes the host more susceptible to pro-inflammatory stimuli. It is critical to understand the mechanisms that contribute to AIEC infections in a susceptible host in order to develop successful therapeutics. Here, we show that the larval zebrafish model recapitulates key features of AIEC infections in other animal models and can be utilized to address these gaps in knowledge.

Single-cell analysis of shared signatures and transcriptional diversity during zebrafish development

During development, animals generate distinct cell populations with specific identities, functions, and morphologies. We mapped transcriptionally distinct populations across 489,686 cells from 62 stages during wild-type zebrafish embryogenesis and early larval development (3-120 h post-fertilization). Using these data, we identified the limited catalog of gene expression programs reused across multiple tissues and their cell-type-specific adaptations. We also determined the duration each transcriptional state is present during development and identify unexpected long-term cycling populations. Focused clustering and transcriptional trajectory analyses of non-skeletal muscle and endoderm identified transcriptional profiles and candidate transcriptional regulators of understudied cell types and subpopulations, including the pneumatic duct, individual intestinal smooth muscle layers, spatially distinct pericyte subpopulations, and recently discovered best4+ cells. To enable additional discoveries, we make this comprehensive transcriptional atlas of early zebrafish development available through our website, Daniocell.

Hhex and Prox1a synergistically dictate the hepatoblast to hepatocyte differentiation in zebrafish

The specification of endoderm cells to prospective hepatoblasts is the starting point for hepatogenesis. However, how a prospective hepatoblast gains the hepatic fate remains elusive. Previous studies have shown that loss-of-function of either hhex or prox1a alone causes a small liver phenotype but without abolishing the hepatocyte differentiation, suggesting that absence of either Hhex or Prox1a alone is not sufficient to block the hepatoblast differentiation. Here, via genetic studies of the zebrafish two single (hhex-/- and prox1a-/-) and one double (hhex-/-prox1a-/-) mutants, we show that simultaneous loss-of-function of the hhex and prox1a two genes does not block the endoderm cells to gain the hepatoblast potency but abolishes the hepatic differentiation from the prospective hepatoblast. Consequently, the hhex-/-prox1a-/- double mutant displays a liverless phenotype that cannot be rescued by the injection of bmp2a mRNA. Taken together, we provide strong evidences showing that Hhex teams with Prox1a to act as a master control of the differentiation of the prospective hepatoblasts towards hepatocytes.

The role of the 5' HoxA genes in the development of the hindgut, vent, and a novel sphincter in a derived teleost (bluebanded goby, Lythrypnus dalli)

Unique expression patterns of the 5' HoxA genes are associated with the evolution and development of novel features including claspers in cartilaginous fishes, modified pectoral fins in batoids, and the yolk sac extension in Cypriniformes. Here, we demonstrate a role for HoxA11a and HoxA13a in demarcating the hindgut in fishes of the family Gobiidae, including a novel sphincter called the intestinal rectal sphincter (IRS). Disruption of 5' HoxA expression, via manipulation of retinoic acid signaling, results in failure of the IRS and/or vent to develop. Furthermore, exposure to HoxA disruptors alters 5' HoxA expression, in association with developmental phenotypes, demonstrating a functional link between 5' HoxA expression and development of a novel feature in the bluebanded goby, Lythrypnus dalli.

Systematic analysis of proximal midgut- and anorectal-originating contractions in larval zebrafish using event feature detection and supervised machine learning algorithms

BACKGROUND

Zebrafish larvae are translucent, allowing in vivo analysis of gut development and physiology, including gut motility. While recent progress has been made in measuring gut motility in larvae, challenges remain which can influence results, such as how data are interpreted, opportunities for technical user error, and inconsistencies in methods.

METHODS

To overcome these challenges, we noninvasively introduced Nile Red fluorescent dye to fill the intraluminal gut space in zebrafish larvae and collected serial confocal microscopic images of gut fluorescence. We automated the detection of fluorescent-contrasted contraction events against the median-subtracted signal and compared it to manually annotated gut contraction events across anatomically defined gut regions. Supervised machine learning (multiple logistic regression) was then used to discriminate between true contraction events and noise. To demonstrate, we analyzed motility in larvae under control and reserpine-treated conditions. We also used automated event detection analysis to compare unfed and fed larvae.

KEY RESULTS

Automated analysis retained event features for proximal midgut-originating retrograde and anterograde contractions and anorectal-originating retrograde contractions. While manual annotation showed reserpine disrupted gut motility, machine learning only achieved equivalent contraction discrimination in controls and failed to accurately identify contractions after reserpine due to insufficient intraluminal fluorescence. Automated analysis also showed feeding had no effect on the frequency of anorectal-originating contractions.

CONCLUSIONS & INFERENCES

Automated event detection analysis rapidly and accurately annotated contraction events, including the previously neglected phenomenon of anorectal contractions. However, challenges remain to discriminate contraction events based on intraluminal fluorescence under treatment conditions that disrupt functional motility.

Apical-basal polarity in the gut

Apical-Basal polarity is a fundamental property of all epithelial cells that underlies both their form and function. The gut is made up of a single layer of intestinal epithelial cells, with distinct apical, lateral and basal domains. Occluding junctions at the apical side of the lateral domains create a barrier between the gut lumen and the body, which is crucial for tissue homeostasis, protection against gastrointestinal pathogens and for the maintenance of the immune response. Apical-basal polarity in most epithelia is established by conserved polarity factors, but recent evidence suggests that the gut epithelium in at least some organisms polarises by novel mechanisms. In this review, we discuss the recent advances in understanding polarity factors by focussing on work in C. elegans, Drosophila, Zebrafish and Mouse.

Integrating 4-D light-sheet fluorescence microscopy and genetic zebrafish system to investigate ambient pollutants-mediated toxicity

Ambient air pollutants, including PM2.5 (aerodynamic diameter d ~2.5 μm), PM10 (d ~10 μm), and ultrafine particles (UFP: d < 0.1 μm) impart both short- and long-term toxicity to various organs, including cardiopulmonary, central nervous, and gastrointestinal systems. While rodents have been the principal animal model to elucidate air pollution-mediated organ dysfunction, zebrafish (Danio rerio) is genetically tractable for its short husbandry and life cycle to study ambient pollutants. Its electrocardiogram (ECG) resembles that of humans, and the fluorescent reporter-labeled tissues in the zebrafish system allow for screening a host of ambient pollutants that impair cardiovascular development, organ regeneration, and gut-vascular barriers. In parallel, the high spatiotemporal resolution of light-sheet fluorescence microscopy (LSFM) enables investigators to take advantage of the transparent zebrafish embryos and genetically labeled fluorescent reporters for imaging the dynamic cardiac structure and function at a single-cell resolution. In this context, our review highlights the integrated strengths of the genetic zebrafish system and LSFM for high-resolution and high-throughput investigation of ambient pollutants-mediated cardiac and intestinal toxicity.

Influence of exposure scenario on the sensitivity to caffeine

The chorion acts as a protective barrier, restricting some chemical absorption into the embryo and the surrounding fluids. In this sense, larvae may only have direct contact with some chemicals after dechorionation. This study aimed to evaluate the effects of caffeine (CAF) (0, 13, 20, 44, 67, and 100 mg.L-1) under different exposure scenarios (embryos with chorion or embryos/larvae already hatched) and rank the stage sensitivity. Thus, three scenarios were investigated: from 2 to 120 hours post fertilization (hpf) (5 days of exposure- 5dE), from 72 to 120 hpf (2dE), and from 96 to 120 hpf (1dE). Heart rate (48 hpf) and energy reserves (120 hpf) were measured in the 5dE scenario, and behavior and acetylcholinesterase (AChE) activity were evaluated at 120 hpf in all scenarios (5dE, 2dE, and 1dE). At 120 hpf, some of the fish was transferred to clean medium for a 10 days depuration period (10dPE). Behavior and AChE activity were assessed after this period. In the 5dE scenario, CAF increased heartbeat (13, 20, and 30 mg.L-1) and reduced carbohydrates (67, and 100 mg.L-1), while inhibiting AChE activity (100 mg.L-1) in the 5dE, 2dE, and 1dE scenarios. CAF reduced the total distance moved in the 5dE (67, and 100 mg.L-1), 2dE (20, 30, 44, 67, and 100 mg.L-1), and 1dE fish (67, and 100 mg.L-1) and increased erratic movements. Based on the lowest observed effect concentration (LOEC) for total distance moved (20 mg.L-1) and higher inhibition of AChE activity (100 mg.L-1) (65%), 2dE fish appear to be more sensitive to CAF. After 10dPE, a recovery in behavior was detected in all scenarios (5dE, 2dE, and 1dE). AChE activity remained inhibited in the 2dE scenario while increasing in the 1dE scenario. This study demonstrated that the presence of the chorion is an important factor for the analysis of CAF toxicity. After the loss of the chorion, organisms show greater sensitivity to CAF and can be used to evaluate the toxicity of various substances, including nanomaterials or chemicals with low capacity to cross the chorion. Therefore, the use of hatched embryos in toxicity tests is suggested, as they allow a shorter and less expensive exposure scenario that provides similar outcome as the conventional scenario.

Single-cell resolution of the adult zebrafish intestine under conventional conditions and in response to an acute Vibrio cholerae infection

Vibrio cholerae is an aquatic bacterium that causes severe and potentially deadly diarrheal disease. Despite the impact on global health, our understanding of host mucosal responses to Vibrio remains limited, highlighting a knowledge gap critical for the development of effective prevention and treatment strategies. Using a natural infection model, we combine physiological and single-cell transcriptomic studies to characterize conventionally reared adult zebrafish guts and guts challenged with Vibrio. We demonstrate that Vibrio causes a mild mucosal immune response characterized by T cell activation and enhanced antigen capture; Vibrio suppresses host interferon signaling; and ectopic activation of interferon alters the course of infection. We show that the adult zebrafish gut shares similarities with mammalian counterparts, including the presence of Best4+ cells, tuft cells, and a population of basal cycling cells. These findings provide important insights into host-pathogen interactions and emphasize the utility of zebrafish as a natural model of Vibrio infection.

A zebrafish model for studying the mechanisms of newborn hyperbilirubinemia and bilirubin-induced neurological damage

Unresolved neonatal hyperbilirubinemia may lead to the accumulation of excess bilirubin in the body, and bilirubin in neural tissues may induce toxicity. Bilirubin-induced neurological damage (BIND) can result in acute or chronic bilirubin encephalopathy, causing temporary or lasting neurological dysfunction or severe damage resulting in infant death. Although serum bilirubin levels are used as an indication of severity, known and unknown individual differences affect the severity of the symptoms. The mechanisms of BIND are not yet fully understood. Here, a zebrafish newborn hyperbilirubinemia model is developed and characterized. Direct exposure to excess bilirubin induced dose- and time-dependent toxicity linked to the accumulation of bilirubin in the body and brain. Introduced bilirubin was processed by the liver, which increased the tolerance of larvae. BIND in larvae was demonstrated by morphometric measurements, histopathological analyses and functional tests. The larvae that survived hyperbilirubinemia displayed mild or severe morphologies associated with defects in eye movements, body posture and swimming problems. Interestingly, a plethora of mild to severe clinical symptoms were reproduced in the zebrafish model.

Influence of the aquatic environment and 1α,25(OH)2 vitamin D3 on calcium influx in the intestine of adult zebrafish

We investigated the effects of environment calcium challenge and 1α,25(OH)2 vitamin D3 (1,25-D3) on 45Ca2+ influx in the intestine of zebrafish (ZF). In vitro45Ca2+ influx was analyzed using intestines from fed and fasted fish. ZF were held in water containing Ca2+ (0.02, 0.7, 2.0 mM) to analyze the ex vivo45Ca2+ influx in the intestine and for histology. Intestines from fish held in water with Ca2+ were incubated ex vivo to characterize ion channels, receptors, ATPases and ion exchangers that orchestrate 45Ca2+ influx. For in vitro studies, intestines were incubated with antagonists/agonist or inhibitors to study the mechanism of 1,25-D3 on 45Ca2+ influx. Fasted ZF reached a plateau for 45Ca2+ influx at 30 min. In vivo fish at high Ca2+ stimulated ex vivo45Ca2+ influx and increased the height of intestinal villi in low calcium. In the normal calcium, 45Ca2+ influx was maintained by the reverse-mode Na+/Ca2+ (NCX) activation, Na+/K+-ATPase pump and sarco/endoplasmic reticulum calcium ATPase (SERCA) pump. However, Ca2+ hyperosmolarity is supported by L-type voltage-dependent calcium channels (L-VDCC), transient receptor potential vanilloid subfamily 1 (TRPV1) and Na+/K+-ATPase activity. The calcium challenge causes morphological alteration and changes the ion type-channels involved in the intestine to maintain hyperosmolarity. 1,25-D3 stimulates Ca2+ influx in normal osmolarity coordinated by L-VDCC activation and SERCA inhibition to keeps high intracellular calcium in intestine. Our data showed that the adult ZF regulates the calcium challenge (per se osmolarity), independently of the hormonal regulation to maintain the calcium balance through the intestine to support ionic adaptation.

The Distribution of IgT mRNA+ Cells in the Gut of the Atlantic Salmon (Salmo salar L.)

The newly discovered IgT+ B cell is thought to play a dominant role in mucosal immunity, but limited studies have examined its distribution in fish species, hindering our understanding of its function. This study investigated IgT and poly Ig receptor (pIgR) mRNA+ cell distribution in Atlantic salmon (Salmo salar) gut using RNAscope in situ hybridization (ISH) and assessed the effects of vaccination. The pyloric caeca, mid-intestine (first and second parts), and posterior segment in two weight stages (Group 1: avg. 153 g, Group 2: avg. 1717 g) were examined in both vaccinated and unvaccinated fish. ISH revealed more IgT mRNA+ cells in the second part of the midgut compared to other intestinal segments, as well as a higher number of positive cells in Group 2 (older fish). In line with previous findings, intraperitoneal vaccination had no significant impact on the number of IgT+ transcripts. IgT mRNA+ cells were found mostly in the lamina propria and near capillaries, while pIgR was registered in both the lamina propria and mucosa. Interestingly, vaccinated fish presented adhesions and granulomatous tissue in the peritoneum, with both IgT and pIgR mRNA+ cells. Taken together, these results suggest that the distribution of IgT mRNA+ cells in the intestine of Atlantic salmon is region-specific and is not affected by intraperitoneal vaccination but varies with fish age.

Intestinal expression patterns of transcription factors and markers for interstitial cells in the larval zebrafish

For the digestion of food, it is important for the gut to be differentiated regionally and to have proper motor control. However, the number of transcription factors that regulate its development is still limited. Meanwhile, the interstitial cells of the gastrointestinal (GI) tract are necessary for intestinal motility in addition to the enteric nervous system. There are anoctamine1 (Ano1)-positive and platelet-derived growth factor receptor α (Pdgfra)-positive interstitial cells in mammal, but Pdgfra-positive cells have not been reported in the zebrafish. To identify new transcription factors involved in GI tract development, we used RNA sequencing comparing between larval and adult gut. We isolated 40 transcription factors that were more highly expressed in the larval gut. We demonstrated expression patterns of the 13 genes, 7 of which were newly found to be expressed in the zebrafish larval gut. Six of the 13 genes encode nuclear receptors. The osr2 is expressed in the anterior part, while foxP4 in its distal part. Also, we reported the expression pattern of pdgfra for the first time in the larval zebrafish gut. Our data provide fundamental knowledge for studying vertebrate gut regionalization and motility by live imaging using zebrafish.

A mucin-regulated adhesin determines the spatial organization and inflammatory character of a bacterial symbiont in the vertebrate gut

In a healthy gut, microbes are often aggregated with host mucus, yet the molecular basis for this organization and its impact on intestinal health are unclear. Mucus is a viscous physical barrier separating resident microbes from epithelia, but it also provides glycan cues that regulate microbial behaviors. Here, we describe a mucin-sensing pathway in an Aeromonas symbiont of zebrafish, Aer01. In response to the mucin-associated glycan N-acetylglucosamine, a sensor kinase regulates the expression of an aggregation-promoting adhesin we named MbpA. Upon MbpA disruption, Aer01 colonizes to normal levels but is largely planktonic and more pro-inflammatory. Increasing cell surface MbpA rescues these traits. MbpA-like adhesins are common in human-associated bacteria, and the expression of an Akkermansia muciniphila MbpA-like adhesin in MbpA-deficient Aer01 restores lumenal aggregation and reverses its pro-inflammatory character. Our work demonstrates how resident bacteria use mucin glycans to modulate behaviors congruent with host health.

Wdr5-mediated H3K4me3 coordinately regulates cell differentiation, proliferation termination, and survival in digestive organogenesis

Food digestion requires the cooperation of different digestive organs. The differentiation of digestive organs is crucial for larvae to start feeding. Therefore, during digestive organogenesis, cell identity and the tissue morphogenesis must be tightly coordinated but how this is accomplished is poorly understood. Here, we demonstrate that WD repeat domain 5 (Wdr5)-mediated H3K4 tri-methylation (H3K4me3) coordinately regulates cell differentiation, proliferation and apoptosis in zebrafish organogenesis of three major digestive organs including intestine, liver, and exocrine pancreas. During zebrafish digestive organogenesis, some of cells in these organ primordia usually undergo differentiation without apoptotic activity and gradually reduce their proliferation capacity. In contrast, cells in the three digestive organs of wdr5^-/- mutant embryos retain progenitor-like status with high proliferation rates, and undergo apoptosis. Wdr5 is a core member of COMPASS complex to implement H3K4me3 and its expression is enriched in digestive organs from 2 days post-fertilization (dpf). Further analysis reveals that lack of differentiation gene expression is due to significant decreases of H3K4me3 around the transcriptional start sites of these genes; this histone modification also reduces the proliferation capacity in differentiated cells by increasing the expression of apc to promote the degradation of β-Catenin; in addition, H3K4me3 promotes the expression of anti-apoptotic genes such as xiap-like, which modulates p53 activity to guarantee differentiated cell survival. Thus, our findings have discovered a common molecular mechanism for cell fate determination in different digestive organs during organogenesis, and also provided insights to understand mechanistic basis of human diseases in these digestive organs.

The development of the digestive system and the fate of the yolk syncytial layer in postembryogenesis of Stenodus leucichthys nelma (Teleostei)

Stenodus leucichthys nelma is an economically important species for cold-water aquaculture. Unlike other Coregoninae, S. leucichthys nelma is a piscivore. Here, we describe in detail the development of the digestive system and the yolk syncytial layer from hatching to early juvenile stage using histological and histochemical methods to determine their common and specific characteristics and to test the hypothesis that the digestive system of S. leucichthys nelma rapidly acquires adult features. The digestive tract differentiates at hatching and starts to function before the transition to mixed feeding. The mouth and anus are open, mucous cells and taste buds are present in the buccopharyngeal cavity and esophagus, pharyngeal teeth have erupted, the stomach primordium is seen, the intestinal epithelium with mucous cells is folded and the intestinal valve is observed; the epithelial cells of the postvalvular intestine contain supranuclear vacuoles. The liver blood vessels are filled with blood. The cells of exocrine pancreas are loaded with zymogen granules, and at least two islets of Langerhans are present. However, the larvae remain dependent on maternal yolk and lipids for a long time. The adult features of the digestive system develop gradually, the most significant changes take place approximately from 31 to 42 days posthatching. Then, the gastric glands and pyloric caeca buds appear, the U-shaped stomach with glandular and aglandular regions develops, the swim bladder inflates, the number of islets of Langerhans increases, the pancreas becomes scattered, and the yolk syncytial layer undergoes programmed death during the larval-to-juvenile transition. During postembryonic development, the mucous cells of the digestive system contain neutral mucosubstances.

Protective mechanisms of a microbial oil against hypercholesterolemia: evidence from a zebrafish model

A Western diet elevates the circulating lipoprotein and triglyceride levels which are the major risk factors in cardiovascular disease (CVD) development. Consumption of long-chain omega-3 fatty acids can stall the disease progression. Although these fatty acids can significantly impact the intestine under a hypercholesterolemic condition, the associated changes have not been studied in detail. Therefore, we investigated the alterations in the intestinal transcriptome along with the deviations in the plasma lipids and liver histomorphology of zebrafish offered DHA- and EPA-rich oil. Fish were allocated to 4 dietary treatments: a control group, a high cholesterol group and microbial oil groups with low (3.3%) and high (6.6%) inclusion levels. We quantified the total cholesterol, lipoprotein and triglyceride levels in the plasma. In addition, we assessed the liver histology, intestinal transcriptome and plasma lipidomic profiles of the study groups. The results suggested that higher levels of dietary microbial oil could control the CVD risk factor indices in zebrafish plasma. Furthermore, microbial oil-fed fish had fewer liver vacuoles and higher mRNA levels of genes involved in β-oxidation and HDL maturation. Analyses of the intestine transcriptome revealed that microbial oil supplementation could influence the expression of genes altered by a hypercholesterolemic diet. The plasma lipidomic profiles revealed that the higher level of microbial oil tested could elevate the long-chain poly-unsaturated fatty acid content of triglyceride species and lower the concentration of several lysophosphatidylcholine and diacylglycerol molecules. Our study provides insights into the effectiveness of microbial oil against dyslipidemia in zebrafish.

How Do Alternative Protein Resources Affect the Intestine Morphology and Microbiota of Atlantic Salmon?

The availability and cost of fishmeal constitute a bottleneck in Atlantic salmon production expansion. Fishmeal is produced from wild fish species and constitutes the major feed ingredient in carnivorous species such as the Atlantic salmon. These natural stocks are at risk of depletion and it is therefore of major importance to find alternative protein sources that meet the nutritional requirements of the Atlantic salmon, without compromising the animals' health. Terrestrial animal by-products have been used in aquaculture feed, but their use is limited by the lack of several essential amino acids and consumer acceptance. In the case of plant ingredients, it is necessary to take into account both their concentration and the extraction methodologies, since, if not dosed correctly, they can cause macro- and microscopic alterations of the structure of the gastrointestinal tract and can also negatively modulate the microbiota composition. These alterations may compromise the digestive functions, growth of the animal, and, ultimately, its well-being. An updated revision of alternative protein sources is provided, with the respective impact on the intestine health in terms of both morphology and microbiota composition. Such information may constitute the premise for the choice and development of Atlantic salmon feeds that guarantee fish health and growth performance without having a significant impact on the surrounding environment, both in terms of depletion of the fish's natural stocks and in terms of pressure on the terrestrial agriculture. The sustainability of aquaculture should be a priority when choosing next-generation ingredients.

Small fish, big discoveries: zebrafish shed light on microbial biomarkers for neuro-immune-cardiovascular health

Zebrafish (Danio rerio) have emerged as a powerful model to study the gut microbiome in the context of human conditions, including hypertension, cardiovascular disease, neurological disorders, and immune dysfunction. Here, we highlight zebrafish as a tool to bridge the gap in knowledge in linking the gut microbiome and physiological homeostasis of cardiovascular, neural, and immune systems, both independently and as an integrated axis. Drawing on zebrafish studies to date, we discuss challenges in microbiota transplant techniques and gnotobiotic husbandry practices. We present advantages and current limitations in zebrafish microbiome research and discuss the use of zebrafish in identification of microbial enterotypes in health and disease. We also highlight the versatility of zebrafish studies to further explore the function of human conditions relevant to gut dysbiosis and reveal novel therapeutic targets.

Erk5 functions in modulation of zebrafish intestinal permeability

The intestine of zebrafish consists of mucosa, muscularis and serosa. Intestinal epithelial cells (IECs) act as a physical and biochemical barrier to protect against invasion by external commensal bacteria. Cell junction is one of the crucial basis of the barrier function. When cell junctions were disrupted, intestinal permeability would be naturally impeded. Extracellular signal-regulated kinase 5 (ERK5), belonging to the Mitogen-activated protein kinase (MAPK) family, is involved in the normal physiological development of the cardiovascular system and nervous system. But the role of erk5 in intestinal morphogenesis and intestinal function is yet to know. Here, we showed that knockout of the erk5 in zebrafish larvae resulted in intestinal wall hypoplasia, including the thinned intestinal wall, reduced intestinal folds, and disrupted cell junctions. In addition, the intestinal permeability assay demonstrated that knockout of erk5 resulted in increased intestinal permeability. All of these showed that erk5 plays an essential role in the maintenance of intestinal barrier function. Thus, our data indicate that erk5 is a critical effector in intestinal morphogenesis and intestinal function, and dysfunction of erk5 would lead to intestinal diseases.

The manipulation of cell suspensions from zebrafish intestinal mucosa contributes to understanding enteritis

Background

Although zebrafish are commonly used to study intestinal mucosal immunity, no dedicated procedure for isolating immune cells from zebrafish intestines is currently available. A speedy and simple operating approach for preparing cell suspension from mucosa has been devised to better understanding of intestinal cellular immunity in zebrafish.

Methods and results

The mucosal villi were separated away from the muscle layer by repeated blows. The complete deprivation of mucosa was done and evidenced by HE and qPCR results. Higher expression of both innate (mpeg1, mpx, and lck) and adaptive immune genes (zap70, blnk, foxp3a, and foxp3b) was revealed compared to cells obtained by typical mesh rubbing. The cytometric results also revealed that the tested operation group had a higher concentration and viability. Further, fluorescent-labelled immune cells from 3mo Tg(lyz:DsRED2), Tg(mpeg1:EGFP), Tg(Rag2:DsRED), and Tg(lck:EGFP), were isolated and evaluated for the proportion, and immune cells' type could be inferred from the expression of marker genes. The transcriptomic data demonstrated that the intestinal immune cell suspension made using the new technique was enriched in immune-related genes and pathways, including il17a/f, il22, cd59, and zap70, as well as pattern recognition receptor signaling and cytokine-cytokine receptor interaction. In addition, the low expression of DEG for the adherent and close junctions indicated less muscular contamination. Also, lower expression of gel-forming mucus-associated genes in the mucosal cell suspension was consistent with the current less viscous cell suspension. To apply and validate the developed manipulation, enteritis was induced by soybean meal diet, and immune cell suspensions were analyzed by flow cytometry and qPCR. The finding that in enteritis samples, there was inflammatory increase of neutrophils and macrophages, was in line with upregulated cytokines (il8 and il10) and cell markers (mpeg1 and mpx).

Conclusion

As a result, the current work created a realistic technique for studying intestinal immune cells in zebrafish. The immune cells acquired may aid in further research and knowledge of intestinal illness at the cellular level.

Bisphenol C induces developmental defects in liver and intestine through mTOR signaling in zebrafish (Danio rerio)

Bisphenol A (BPA) was widely used in the plastic products and banned in infant food containers in many countries due to the environmental and biological toxicity. As a common substitute of BPA to manufacture products, Bisphenol C (BPC) is frequently detected in human samples like infants and toddlers' urine, indicating infants and young children are at risk of BPC exposure. However, the understanding of effects of BPC exposure on early development is limited. Herein, we evaluated the early developmental toxicity of BPC and studied the underlying mechanism in a zebrafish model. We found BPC exposure leading to liver and intestinal developmental defects in zebrafish, which occurred via disruption of GPER-AKT-mTOR-RPS6 pathway. Specifically, BPC downregulated phosphorylated and total levels of mTOR, which synergistically reduced the phosphorylation of RPS6, suppressing the translation of genes essential for cell proliferation in liver and intestine such as yap1 and tcf4. Collectively, our results not only observed clear toxicity of BPC during liver and intestinal development but also demonstrated the underlying mechanism of BPC-mediated defects via disrupting the GPER-AKT-mTOR-RPS6 pathway.

The comparative analysis of gastrointestinal toxicity of azithromycin and 3'-decladinosyl azithromycin on zebrafish larvae

The most commonly reported side effect of azithromycin is gastrointestinal (GI) disorders, and the main acid degradation product is 3'-Decladinosyl azithromycin (impurity J). We aimed to compare the GI toxicity of azithromycin and impurity J on zebrafish larvae and investigate the mechanism causing the differential GI toxicity. Results of our study showed that the GI toxicity induced by impurity J was higher than that of azithromycin in zebrafish larvae, and the effects of impurity J on transcription in the digestive system of zebrafish larvae were significantly stronger than those of azithromycin. Additionally, impurity J exerts stronger cytotoxic effects on GES-1 cells than azithromycin. Simultaneously, impurity J significantly increased ghsrb levels in the zebrafish intestinal tract and ghsr levels in human GES-1 cells compared to azithromycin, and ghsr overexpression significantly reduced cell viability, indicating that GI toxicity induced by azithromycin and impurity J may be correlated with ghsr overexpression induced by the two compounds. Meanwhile, molecular docking analysis showed that the highest -CDOCKER interaction energy scores with the zebrafish GHSRb or human GHSR protein might reflect the effect of azithromycin and impurity J on the expression of zebrafish ghsrb or human ghsr. Thus, our results suggest that impurity J has higher GI toxicity than azithromycin due to its greater ability to elevate ghsrb expression in zebrafish intestinal tract.

Single-cell analysis of shared signatures and transcriptional diversity during zebrafish development

During development, animals generate distinct cell populations with specific identities, functions, and morphologies. We mapped transcriptionally distinct populations across 489,686 cells from 62 stages during wild-type zebrafish embryogenesis and early larval development (3-120 hours post-fertilization). Using these data, we identified the limited catalog of gene expression programs reused across multiple tissues and their cell-type-specific adaptations. We also determined the duration each transcriptional state is present during development and suggest new long-term cycling populations. Focused analyses of non-skeletal muscle and the endoderm identified transcriptional profiles of understudied cell types and subpopulations, including the pneumatic duct, individual intestinal smooth muscle layers, spatially distinct pericyte subpopulations, and homologs of recently discovered human best4+ enterocytes. The transcriptional regulators of these populations remain unknown, so we reconstructed gene expression trajectories to suggest candidates. To enable additional discoveries, we make this comprehensive transcriptional atlas of early zebrafish development available through our website, Daniocell.

Differential roles of the fish chitinous membrane in gut barrier immunity and digestive compartments

The chitin-based peritrophic matrix (PM) is a structure critical for both gut immunity and digestion in invertebrates. PM was traditionally considered lost in all vertebrates, but a PM-like chitinous membrane (CM) has recently been discovered in fishes, which may increase the knowledge on vertebrate gut physiology and structural evolution. Here, we show that in zebrafish, the CM affects ingestion behavior, microbial homeostasis, epithelial renewal, digestion, growth, and longevity. Young mutant fish without CM appear healthy and are able to complete their life cycle normally, but with increasing age they develop gut inflammation, resulting in gut atrophy. Unlike mammals, zebrafish have no visible gel-forming mucin layers to protect their gut epithelia, but at least in young fish, the CM is not a prerequisite for the antibacterial gut immunity. These findings provide new insights into the role of the CM in fish prosperity and its eventual loss in tetrapods. These findings may also help to improve fish health and conservation, as well as to advance the understanding of vertebrate gut physiology and human intestinal diseases.

Exposure to the herbicide fluridone induces cardiovascular toxicity in early developmental stages of zebrafish

Fluridone is a systemic herbicide used to control a range of invasive aquatic plants in irrigation systems, lake, and reservoirs. Since aquatic herbicides are more likely to have a hazardous impact on ecosystems than terrestrially applied herbicides, a risk assessment is needed to determine whether to expand or limit their use. The aim of this study was to investigate the developmental toxicity of fluridone using zebrafish. Diverse toxicological results were observed for the sub-lethal endpoints, including lack of hatching, reduced heartbeat and disturbed blood circulation through dysmorphic heart, and edema formation. Abnormal apoptosis was observed in the brain and yolk sac of fluridone-exposed larvae. A computational analysis was used to predict chemical properties in non-target organisms and revealed that fluridone was highly relevant in the cardiovascular system. Double transgenic zebrafish (fli1a:EGFP;cmlc2:dsRed) were used to evaluate the effects of fluridone on the cardiovascular system during embryonic development. Ectopic growth of sub-intestinal vessels and sprouting angiogenesis in the hindbrain region were highly inhibited. Additionally, essential genes involved in the VEGF signaling and heart development were differentially expressed in dose-dependent manner. Collectively, our toxicological findings in fluridone exposure highlight defects in the cardiovascular development causing embryonic lethality that could damage aquatic communities and natural ecosystems.

The Meteorin-like cytokine is upregulated in grass carp after infection with Aeromonas hydrophila

Meteorin-like (Metrnl) is a novel immune regulatory factor or adipokine which is mainly produced by activated macrophages. In teleost fish, two homologs are present. In this study, monoclonal antibodies were prepared against recombinant grass carp (Ctenopharyngodon idella, Ci) Metrnl-a in mice and characterized by Western blotting, flow cytometry and immunofluorescent microscopy. In grass carp infected with Aeromonus hydrophila (A. hydrophila), the cells expressing CiMetrnl-a markedly increased in the gills, head kidney and intestine. In the inflamed intestine caused by A. hydrophila infection, the CiMetrnl-a producing cells were detected mainly in the mucosal layer of anterior, middle and posterior segments. Consistently, qRT-PCR analysis showed that the mRNA expression of CiMetrnl-a was markedly induced. Our results suggest that CiMetrnl-a is involved in regulating intestine inflammation caused by bacterial infection.

On the Novel Process of Pristine Microplastic Bio-fragmentation by Zebrafish (Danio rerio)

Microplastics are highly persistent particles that deliberately contaminate our ecosystem. These small-sized particles can pass through filtering systems into the water bodies, affecting various forms of aquatic and terrestrial life. However, little is known about their fragmentation process within the organism's body. In previous studies, commercially available microplastics were used that are rarely found in the environment naturally, hence they cannot mimic the effects on our surroundings. Therefore, using the zebrafish, Danio rerio we have evaluated the process of bio-fragmentation of ingested pristine polyethene microplastics which are widely used in our daily life. We have also examined their faecal pellets through Field Emission Scanning Electron Microscopy (FE-SEM) and Fourier transform infrared spectroscopy (FTIR). Our results show that zebrafish can potentially bio-fragment the pristine microplastic particles into nano-plastic within a short period of 24 h. Additionally, zebrafish cannot recognize the pristine microplastic particles and can ingest them as food. No mortality occurred during the experiment. Thus, we have identified a natural pathway of microplastic bio-fragmentation, introducing an emerging role of zebrafish in biogeochemical cycling and the fate of plastics.