Foxa2 and Hif1ab regulate maturation of intestinal goblet cells by modulating agr2 expression in zebrafish embryos

The potential of zebrafish as drug discovery research tool in immune-mediated inflammatory disease

Immune-mediated inflammatory disease (IMID) prevalence is estimated at 3-7% for Westernised populations, with annual incidence reported at almost 1 in 100 people globally. More recently, drug discovery approaches have been evolving towards more targeted therapies with an improved long-term safety profile, while the requirement for individualisation of medicine in complex conditions such as IMIDs, is acknowledged. However, existing preclinical models-such as cellular and in vivo mammalian models-are not ideal for modern drug discovery model requirements, such as real-time in vivo visualisation of drug effects, logistically feasible safety assessment over the course of a lifetime, or dynamic assessment of physiological changes during disease development. Zebrafish share high homology with humans in terms of proteins and disease-causing genes, with high conservation of physiological processes at organ, tissue, cellular and molecular level. These and other unique attributes, such as high fecundity, relative transparency and ease of genetic manipulation, positions zebrafish as the next major role player in IMID drug discovery. This review provides a brief overview of the suitability of this organism as model for human inflammatory disease and summarises the range of approaches used in zebrafish-based drug discovery research. Strengths and limitations of zebrafish as model organism, as well as important considerations in research study design, are discussed. Finally, under-utilised avenues for investigation in the IMID context are highlighted.

Identification of BST2 as a conjunctival epithelial stem/progenitor cell marker

The conjunctival epithelium consists of conjunctival epithelial cells and goblet cells derived from conjunctival epithelial stem/progenitor cells. However, the source of these cells is not well known because no specific markers for conjunctival epithelial stem/progenitor cells have been discovered. Therefore, to identify conjunctival epithelial stem/progenitor cell markers, we performed single-cell RNA sequencing of a conjunctival epithelial cell population derived from human-induced pluripotent stem cells (hiPSCs). The following conjunctival epithelial markers were identified: BST2, SLC2A3, AGR2, TMEM54, OLR1, and TRIM29. Notably, BST2 was strongly positive in the basal conjunctival epithelium, which is thought to be rich in stem/progenitor cells. Moreover, BST2 was able to sort conjunctival epithelial stem/progenitor cells from hiPSC-derived ocular surface epithelial cell populations. BST2-positive cells were highly proliferative and capable of successfully generating conjunctival epithelial sheets containing goblet cells. In conclusion, BST2 has been identified as a specific marker of conjunctival epithelial stem/progenitor cells.

Evaluating Phenotypic and Transcriptomic Responses Induced by Low-Level VOCs in Zebrafish: Benzene as an Example

Urban environments are plagued by complex mixtures of anthropogenic volatile organic compounds (VOCs), such as mixtures of benzene, toluene, ethylene, and xylene (BTEX). Sources of BTEX that drive human exposure include vehicle exhaust, industrial emissions, off-gassing of building material, as well as oil spillage and leakage. Among the BTEX mixture, benzene is the most volatile compound and has been linked to numerous adverse health outcomes. However, few studies have focused on the effects of low-level benzene on exposure during early development, which is a susceptible window when hematological, immune, metabolic, and detoxification systems are immature. In this study, we used zebrafish to conduct a VOC exposure model and evaluated phenotypic and transcriptomic responses following 0.1 and 1 ppm benzene exposure during the first five days of embryogenesis (n = 740 per treatment). The benzene body burden was 2 mg/kg in 1 ppm-exposed larval zebrafish pools and under the detection limit in 0.1 ppm-exposed fish. No observable phenotypic changes were found in both larvae except for significant skeletal deformities in 0.1 ppm-exposed fish (p = 0.01) compared with unexposed fish. Based on transcriptomic responses, 1 ppm benzene dysregulated genes that were implicated with the development of hematological system, and the regulation of oxidative stress response, fatty acid metabolism, immune system, and inflammatory response, including apob, nfkbiaa, serpinf1, foxa1, cyp2k6, and cyp2n13 from the cytochrome P450 gene family. Key genes including pik3c2b, pltp, and chia.2 were differentially expressed in both 1 and 0.1 ppm exposures. However, fewer transcriptomic changes were induced by 0.1 ppm compared with 1 ppm. Future studies are needed to determine if these transcriptomic responses during embryogenesis have long-term consequences at levels equal to or lower than 1 ppm.

Delta/Jagged-mediated Notch signaling induces the differentiation of agr2-positive epidermal mucous cells in zebrafish embryos

Teleosts live in aquatic habitats, where they encounter ionic and acid-base fluctuations as well as infectious pathogens. To protect from these external challenges, the teleost epidermis is composed of living cells, including keratinocytes and ionocytes that maintain body fluid ionic homeostasis, and mucous cells that secret mucus. While ionocyte progenitors are known to be specified by Delta-Notch-mediated lateral inhibition during late gastrulation and early segmentation, it remains unclear how epidermal mucous cells (EMCs) are differentiated and maintained. Here, we show that Delta/Jagged-mediated activation of Notch signaling induces the differentiation of agr2-positive (agr2⁺) EMCs in zebrafish embryos during segmentation. We demonstrated that agr2⁺ EMCs contain cytoplasmic secretory granules and express muc5.1 and muc5.2. Reductions in agr2⁺ EMC number were observed in mib mutants and notch3 MOs-injected notch1a mutants, while increases in agr2⁺ cell number were detected in notch1a- and X-Su(H)/ANK-overexpressing embryos. Treatment with γ-secretase inhibitors further revealed that Notch signaling is required during bud to 15 hpf for the differentiation of agr2⁺ EMCs. Increased agr2⁺ EMC numbers were also observed in jag1a-, jag1b-, jag2a- and dlc-overexpressing, but not jag2b-overexpressing embryos. Meanwhile, reductions in agr2⁺ EMC numbers were detected in jag1a morphants, jag1b mutants, jag2a mutants and dlc morphants, but not jag2b mutants. Reduced numbers of pvalb8-positive epidermal cells were also observed in mib or jag2a mutants and jag1a or jag1b morphants, while increased pvalb8-positive epidermal cell numbers were detected in notch1a-overexpressing, but not dlc-overexpressing embryos. BrdU labeling further revealed that the agr2⁺ EMC population is maintained by proliferation. Cell lineage experiments showed that agr2⁺ EMCs are derived from the same ectodermal precursors as keratinocytes or ionocytes. Together, our results indicate that specification of agr2⁺ EMCs in zebrafish embryos is induced by DeltaC/Jagged-dependent activation of Notch1a/3 signaling, and the cell population is maintained by proliferation.

As aquatic organisms, fish must tolerate environmental challenges that include acid-base fluctuations and water-borne pathogens. The skin provides a first line of defense against these challenges, and specific cell types in the tissue are responsible for different protective functions. For example, keratinocytes provide body coverage, ionocytes are responsible for maintaining body fluid ionic homeostasis, and epidermal mucous cells generate a protective layer of mucus that covers the entire fish surface. In this study, we uncovered the developmental process in zebrafish that underlies the generation of epidermal mucous cells. First, we characterized epidermal mucous cells according to their expression of a particular gene, agr2. Then, we found that these cells differentiate soon after ionocytes and keratinocytes, and the molecular pathways that guide differentiation of all three cell types involve similar signals. While ionocytes and keratinocytes are known to be specified by Delta-Notch-mediated lateral inhibition, we found that epidermal mucous cells are specified by activation of Notch by Delta and Jagged ligands. Thus, our results suggest that the specification of these major cell types in the epidermis occurs via a streamlined Notch-dependent process. This utilization of temporally distinct signaling events can therefore generate diverse cell types in the fish epidermis.

The SARS-CoV-2 receptor and other key components of the Renin-Angiotensin-Aldosterone System related to COVID-19 are expressed in enterocytes in larval zebrafish

People with underlying conditions, including hypertension, obesity, and diabetes, are especially susceptible to negative outcomes after infection with coronavirus SARS-CoV-2, which causes COVID-19. Hypertension and respiratory inflammation are exacerbated by the Renin-Angiotensin-Aldosterone System (RAAS), which normally protects from rapidly dropping blood pressure via Angiotensin II (Ang II) produced by the enzyme Ace. The Ace paralog Ace2 degrades Ang II, counteracting its chronic effects, and serves as the SARS-CoV-2 receptor. Ace, the coronavirus, and COVID-19 comorbidities all regulate Ace2, but we do not yet understand how. To exploit zebrafish (Danio rerio) to help understand the relationship of the RAAS to COVID-19, we must identify zebrafish orthologs and co-orthologs of human RAAS genes and understand their expression patterns. To achieve these goals, we conducted genomic and phylogenetic analyses and investigated single cell transcriptomes. Results showed that most human RAAS genes have one or more zebrafish orthologs or co-orthologs. Results identified a specific type of enterocyte as the specific site of expression of zebrafish orthologs of key RAAS components, including Ace, Ace2, Slc6a19 (SARS-CoV-2 co-receptor), and the Angiotensin-related peptide cleaving enzymes Anpep (receptor for the common cold coronavirus HCoV-229E), and Dpp4 (receptor for the Middle East Respiratory Syndrome virus, MERS-CoV). Results identified specific vascular cell subtypes expressing Ang II receptors, apelin, and apelin receptor genes. These results identify genes and cell types to exploit zebrafish as a disease model for understanding mechanisms of COVID-19.

Hepatopancreas Transcriptome Profiling Analysis Reveals Physiological Responses to Acute Hypoxia and Reoxygenation in Juvenile Qingtian Paddy Field Carp Cyprinus carpio var qingtianensis

The Qingtian paddy field carp (Cyprinus carpio var qingtianensis) is a local carp cultivated in the rice field of Qingtian county, Zhejiang province, China. Its high tolerance to hypoxia makes it an ideal organism for studying the molecular regulation mechanism during hypoxia process as well as reoxygenation following hypoxia in fish. In this study, we counted the differentially expressed genes (DEGs) altered during hypoxic exposure and reoxygenation process. The results indicated that 2236 genes (1506 up-regulated genes and 730 down-regulated genes) were differentially expressed between the control and hypoxic groups. The results from Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis indicated that 1152 of 2236 genes were enriched, and those genes participated in energy metabolism, reactive oxygen species (ROS) elimination, acceleration of cell apoptosis, inhibition of growth, and other processes. We found activation of the pentose phosphate pathway in hypoxia treatment, suggesting that carbohydrates not only provide energy for metabolism but also provide NADPH for protecting the body from oxidative damage and ribosomes for promoting RNA synthesis. During reoxygenation, 4509 genes (1865 up-regulated genes and 2644 down-regulated genes) were differentially expressed. The results of KEGG enrichment analysis indicated that 2392 of 4509 genes were enriched, and participated in pyruvate and lactic acid metabolism, synthesis of amino acids and lipids, inhibition of cell apoptosis, regulation of cell growth and differentiation, and other processes. These differentially expressed genes effectively alleviate the body acidosis and promote the normal growth and development of the body. Through the analysis of KEGG pathway enrichment, we observed that the physiological regulation of Qingtian paddy field carp during the processes of hypoxia and reoxygenation is not a simple and reversible process. This work first reported the adaptive mechanism of hypoxia and the recovery mechanism of reoxygenation after hypoxia in common carp, and also provided new insights for the physiological regulation of fish under hypoxia treatment.

Effect of TNBS-induced colitis on enteric neuronal subpopulations in adult zebrafish

Inflammatory bowel disease (IBD) includes inflammation of the gastrointestinal (GI) tract and is characterized by periods of acute inflammation and remission. Therapeutic management of IBD is still problematic, because of incomplete understanding its pathogenesis. This study focuses on the effect of 2,4,6-trinitrobenzene sulphonic acid (TNBS)-induced colitis on changes in enteric neuronal subpopulations in adult zebrafish. These changes are suggested to be related to the altered neuro-immune interactions and GI motility, and in IBD pathogenesis. New insights into neuroplasticity will be instrumental in finding appropriate therapeutic treatments. TNBS was intraluminally administered in the distal intestine (DI) of anesthetized adult zebrafish. A histological time course of the intestinal inflammatory response was created to establish optimal TNBS concentration and acute inflammation phase. Using double immunolabelling on whole mounts, the effect of inflammation on neuronal populations was analyzed. Based on intestinal wall thickening, epithelial fold disruption, reduced goblet cell number, and eosinophil infiltration, our analysis indicated that the optimal TNBS concentration (320 mM in 25% ethanol) inducing non-lethal inflammation reached a peak at 6 h post-induction. The inflammatory response returned to baseline values at 3 days post-induction. At the acute inflammation phase, no influence on the distribution or proportion of nitrergic neurons was observed, while only the proportion of cholinergic neurons was significantly reduced in the DI. The proportion of serotonergic neurons was significantly increased in the entire intestine during inflammation. This study describes a method of TNBS-induced colitis in the adult zebrafish. Given that the acute inflammation phase is accompanied by neuroplasticity comparable to changes observed in IBD patients, and the unique and versatile characteristics of the zebrafish, allows this model to be used alongside IBD animal models to unravel IBD pathology and to test new IBD therapies.

Endoplasmic reticulum stress in autoimmune diseases

If the body's immune system is disordered and begins to attack "self" and therefore, its own tissues this is considered to be an autoimmune pathology. The specific mechanisms vary between the different diseases and have not always been elucidated but chronic, non-resolving inflammation is a common theme in the pathogenesis of autoimmune diseases. Interestingly, it has been shown that development and occurrence of various inflammatory responses are closely correlated to endoplasmic reticulum stress. Therefore, this review discusses the current progress of research about the relationship between autoimmune diseases and endoplasmic reticulum stress, specifically the unfolded protein response (UPR).

Cellular Stress Responses and Gut Microbiota in Inflammatory Bowel Disease

Progresses in the past two decades have greatly expanded our understanding of inflammatory bowel disease (IBD), an incurable disease with multifaceted and challenging clinical manifestations. The pathogenesis of IBD involves multiple processes on the cellular level, which include the stress response signaling such as endoplasmic reticulum (ER) stress, oxidative stress, and hypoxia. Under physiological conditions, the stress responses play key roles in cell survival, mucosal barrier integrity, and immunomodulation. However, they can also cause energy depletion, trigger cell death and tissue injury, promote inflammatory response, and drive the progression of clinical disease. In recent years, gut microflora has emerged as an essential pathogenic factor and therapeutic target for IBD. Altered compositional and metabolic profiles of gut microbiota, termed dysbiosis, are associated with IBD. Recent studies, although limited, have shed light on how ER stress, oxidative stress, and hypoxic stress interact with gut microorganisms, a potential source of stress in the microenvironment of gastrointestinal tract. Our knowledge of cellular stress responses in intestinal homeostasis as well as their cross-talks with gut microbiome will further our understanding of the pathogenesis of inflammatory bowel disease and probably open avenues for new therapies.

Evaluating the Capacity of Human Gut Microorganisms to Colonize the Zebrafish Larvae (Danio rerio)

In this study we evaluated if zebrafish larvae can be colonized by human gut microorganisms. We tested two strategies: (1) through transplantation of a human fecal microbiota and (2) by successively transplanting aerotolerant anaerobic microorganisms, similar to the colonization in the human intestine during early life. We used conventionally raised zebrafish larvae harboring their own aerobic microbiota to improve the colonization of anaerobic microorganisms. The results showed with the fecal transplant, that some members of the human gut microbiota were transferred to larvae. Bacillus, Roseburia, Prevotella, Oscillospira, one unclassified genus of the family Ruminococcaceae and Enterobacteriaceae were detected in 3 days post fertilization (dpf) larvae; however only Bacillus persisted to 7 dpf. Successive inoculation of Lactobacillus, Bifidobacterium and Clostridioides did not improve their colonization, compared to individual inoculation of each bacterial species. Interestingly, the sporulating bacteria Bacillus clausii and Clostridioides difficile were the most persistent microorganisms. Their endospores persisted at least 5 days after inoculating 3 dpf larvae. However, when 5 dpf larvae were inoculated, the proportion of vegetative cells in larvae increased, revealing proliferation of the inoculated bacteria and better colonization of the host. In conclusion, these results suggest that it is feasible to colonize zebrafish larvae with some human bacteria, such as C. difficile and Bacillus and open an interesting area to study interactions between these microorganisms and the host.

AGR2 ameliorates tumor necrosis factor-α-induced epithelial barrier dysfunction via suppression of NF-κB p65-mediated MLCK/p-MLC pathway activation

Intestinal epithelial barrier dysfunction plays a critical role in the pathogenesis of inflammatory bowel disease (IBD). Anterior gradient protein 2 homologue (AGR2) assists in maintaining intestinal homeostasis in dextran sulphate sodium-induced mouse ileocolitis; however, it is unclear whether it modulates intestinal barrier function. Our study aimed to investigate the protective role of AGR2 in tumor necrosis factor (TNF)-α-induced intestinal epithelial barrier injury. Caco-2 cell monolayers were pre-transfected with an AGR2 plasmid and then exposed to TNF-α. Epithelial permeability was assessed by detecting transepithelial electrical resistance and fluorescein isothiocyanate-dextran (40 kDa) flux. The protein expression levels of zonula occludens-1 (ZO-1), occludin, claudin-1, myosin light chain kinase (MLCK)/p-MLC, and nuclear factor (NF)-κB p65 were determined by western blotting. In addition, the cellular distributions of ZO-1, occludin, F-actin, and NF-κB p65 were evaluated by immunofluorescence staining. The results showed that the AGR2 mRNA and protein expression levels were both decreased in the Caco-2 cell monolayers, while AGR2 overexpression significantly ameliorated TNF-α-induced epithelial barrier hyperpermeability, increased the expression of tight junction (TJ) proteins and stabilized the cytoskeletal structure. Furthermore, AGR2 inhibited the changes in MLCK, MLC and p-MLC expression in response to TNF-α stimulation. Collectively, our study suggests that AGR2 inhibits TNF-α-induced Caco-2 cell hyperpermeability by regulating TJ and that this protective mechanism may be promoted by inhibition of NF-κB p65-mediated activation of the MLCK/p-MLC signaling pathway.