Intestinal growth and differentiation in zebrafish

Dynamic Lkb1-TORC1 signaling as a possible mechanism for regulating the endoderm-intestine transition

The intestinal epithelium arises from undifferentiated endoderm via a developmental program known as the endoderm-intestine transition (EIT). Previously we found that the target of rapamycin complex 1 (TORC1) regulates intestinal growth and differentiation during the EIT in zebrafish. Here we address a possible role for the tumor-suppressor kinase Lkb1 in regulating TORC1 in this context. We find that TORC1 activity is transiently upregulated during the EIT in both zebrafish and mouse. Concomitantly, Lkb1 becomes transiently localized to the nucleus, suggesting that these two phenomena may be linked. Morpholino-mediated knockdown of lkb1 stimulated intestinal growth via upregulation of TORC1, and also induced precocious intestine-specific gene expression in the zebrafish gut epithelium. Knockdown of tsc2, which acts downstream of lkb1, likewise induced early expression of intestine-specific genes. These data suggest that programmed localization of Lkb1 could represent a novel mechanism for regulating the EIT during intestinal development in vertebrates.

Neurochemical coding of enteric neurons in adult and embryonic zebrafish (Danio rerio)

Although the morphology and development of the zebrafish enteric nervous system have been extensively studied, the precise neurochemical coding of enteric neurons and their proportional enteric distribution are currently not known. By using immunohistochemistry, we determined the proportional expression and coexpression of neurochemical markers in the embryonic and adult zebrafish intestine. Tyrosine hydroxylase (TH), vasoactive intestinal peptide (VIP), and pituitary adenylate cyclase-activating peptide (PACAP) were observed only in nerve fibers, whereas other markers were also detected in neuronal cell bodies. Calretinin and calbindin had similar distributions. In embryos, all markers, except for choline acetyltransferase (ChAT) and TH, were present from 72 hours postfertilization. Nitrergic neurons, evenly distributed and remaining constant in time, constituted the major neuronal subpopulation. The neuronal proportions of the other markers increased during development and were characterized by regional differences. In the adult, all markers examined were expressed in the enteric nervous system. A large percentage of enteric neurons displayed calbindin and calretinin, and serotonin was the only marker showing significant distribution differences in the three intestinal regions. Colocalization studies showed that serotonin was not coexpressed with any of the other markers. At least five neuronal subpopulations were determined: a serotonergic, a nitrergic noncholinergic, two cholinergic nonnitrergic subpopulations along with one subpopulation expressing both ChAT and neuronal nitric oxide synthase. Analysis of nerve fibers revealed that nitrergic neurons coexpress VIP and PACAP, and that nitrergic neurons innervate the tunica muscularis, whereas serotonergic and cholinergic nonnitrergic neurons innervate the lamina propria and the tunica muscularis.

Zebrafish lipid metabolism: from mediating early patterning to the metabolism of dietary fat and cholesterol

Lipids serve essential functions in cells as signaling molecules, membrane components, and sources of energy. Defects in lipid metabolism are implicated in a number of pandemic human diseases, including diabetes, obesity, and hypercholesterolemia. Many aspects of how fatty acids and cholesterol are absorbed and processed by intestinal cells remain unclear and present a hurdle to developing approaches for disease prevention and treatment. Numerous studies have shown that the zebrafish is an excellent model for vertebrate lipid metabolism. In this chapter, we review studies that employ zebrafish to better understand lipid signaling and metabolism.

STORM: a general model to determine the number and adaptive changes of epithelial stem cells in teleost, murine and human intestinal tracts

Intestinal stem cells play a pivotal role in the epithelial tissue renewal, homeostasis and cancer development. The lack of a general marker for intestinal stem cells across species has hampered analysis of stem cell number in different species and their adaptive changes upon intestinal lesions or during development of cancer. Here a two-dimensional model, named STORM, has been developed to address this issue. By optimizing epithelium renewal dynamics, the model examines the epithelial stem cell number by taking experimental input information regarding epithelium proliferation and differentiation. As the results suggest, there are 2.0-4.1 epithelial stem cells on each pocket section of zebrafish intestine, 2.0-4.1 stem cells on each crypt section of murine small intestine and 1.8-3.5 stem cells on each crypt section of human duodenum. The model is able to provide quick results for stem cell number and its adaptive changes, which is not easy to measure through experiments. Its general applicability to different species makes it a valuable tool for analysis of intestinal stem cells under various pathological conditions.

Irf8 regulates macrophage versus neutrophil fate during zebrafish primitive myelopoiesis

In vertebrates, myeloid cells comprise polymorphonuclear and mononuclear lineages that arise from 2 successive waves of development: a transitory primitive wave giving rise to limited myeloid cells during embryonic stage and a definitive wave capable of producing myeloid cells throughout the fetal and adult life. One key unresolved question is what factors dictate polymorphonuclear versus mononuclear lineage fates during myelopoiesis. Here we show that during zebrafish embryogenesis interferon regulatory factor-8 (irf8) is expressed specifically in macrophages but not neutrophils. Suppression of Irf8 function in zebrafish causes a depletion of macrophages and an enhanced output of neutrophils but does not affect the overall number, proliferation, and survival of primitive myeloid cells. These data indicate that the skewed myeloid lineage development in Irf8 knockdown embryos results from a cell-fate switching. Such a conclusion is further supported by the observation showing that overexpression of Irf8 promotes macrophage formation at the expense of neutrophil development. Genetic epistasis analysis reveals that Irf8 acts downstream of Pu.1 but is insufficient to promote macrophage development in the absence of Pu.1. Our findings demonstrate that Irf8 is a critical determinant for neutrophil versus macrophage fate choice during zebrafish primitive myelopoiesis.

Epithelial cell proliferation in the developing zebrafish intestine is regulated by the Wnt pathway and microbial signaling via Myd88

Rates of cell proliferation in the vertebrate intestinal epithelium are modulated by intrinsic signaling pathways and extrinsic cues. Here, we report that epithelial cell proliferation in the developing zebrafish intestine is stimulated both by the presence of the resident microbiota and by activation of Wnt signaling. We find that the response to microbial proliferation-promoting signals requires Myd88 but not TNF receptor, implicating host innate immune pathways but not inflammation in the establishment of homeostasis in the developing intestinal epithelium. We show that loss of axin1, a component of the β-catenin destruction complex, results in greater than WT levels of intestinal epithelial cell proliferation. Compared with conventionally reared axin1 mutants, germ-free axin1 mutants exhibit decreased intestinal epithelial cell proliferation, whereas monoassociation with the resident intestinal bacterium Aeromonas veronii results in elevated epithelial cell proliferation. Disruption of β-catenin signaling by deletion of the β-catenin coactivator tcf4 partially decreases the proliferation-promoting capacity of A. veronii. We show that numbers of intestinal epithelial cells with cytoplasmic β-catenin are reduced in the absence of the microbiota in both WT and axin1 mutants and elevated in animals' monoassociated A. veronii. Collectively, these data demonstrate that resident intestinal bacteria enhance the stability of β-catenin in intestinal epithelial cells and promote cell proliferation in the developing vertebrate intestine.

Differential effects of thin and thick filament disruption on zebrafish smooth muscle regulatory proteins

BACKGROUND

 The smooth muscle actin binding proteins Caldesmon and Tropomyosin (Tm) promote thin filament assembly by stabilizing actin polymerization, however, whether filament assembly affects either the stability or activation of these and other smooth muscle regulatory proteins is not known.

METHODS

Measurement of smooth muscle regulatory protein levels in wild type zebrafish larvae following antisense knockdown of smooth muscle actin (Acta2) and myosin heavy chain (Myh11) proteins, and in colourless mutants that lack enteric nerves. Comparison of intestinal peristalsis in wild type and colourless larvae.

KEY RESULTS

Knockdown of Acta2 led to reduced levels of phospho-Caldesmon and Tm. Total Caldesmon and phospho-myosin light chain (p-Mlc) levels were unaffected. Knockdown of Myh11 had no effect on the levels of either of these proteins. Phospho-Caldesmon and p-Mlc levels were markedly reduced in colourless mutants that have intestinal motility comparable with wild type larvae.

CONCLUSIONS & INFERENCES

These in vivo findings provide new information regarding the activation and stability of smooth muscle regulatory proteins in zebrafish larvae and their role in intestinal peristalsis in this model organism.

Antigen-sampling cells in the salmonid intestinal epithelium

Antigen uptake has been shown to occur in the teleost intestine, but so far, limited information is available on the distribution and nature of cells involved in the process, and M cells, known for their antigen-sampling abilities in mammals, have not been identified. Here, different intestinal segments from salmonid fish were exposed to gold-BSA to identify antigen-sampling cells. Sections from exposed intestine were examined by light and electron microscopy. Uptake of gold-BSA was restricted to very few dendritic-like cells and to a limited number of epithelial cells located in the mucosal folds in the second segment of the mid-intestine. Gold-positive epithelial cells displayed diverging and electron-dense microvilli with channels intruding into the cytoplasm. A lectin binding experiment demonstrated the presence of cells with mammalian M-cell characteristics in the identical regions. As the identified epithelial cells shared some morphological similarities with immature mammalian M cells, this phenotype may represent evolutionary early antigen-sampling enterocytes.

Morphological and molecular evidence for functional organization along the rostrocaudal axis of the adult zebrafish intestine

BACKGROUND

The zebrafish intestine is a simple tapered tube that is folded into three sections. However, whether the intestine is functionally similar along its length remains unknown. Thus, a systematic structural and functional characterization of the zebrafish intestine is desirable for future studies of the digestive tract and the intestinal biology and development.

RESULTS

To characterize the structure and function of the adult zebrafish intestine, we divided the intestine into seven roughly equal-length segments, S1-S7, and systematically examined the morphology of the mucosal lining, histology of the epithelium, and molecular signatures from transcriptome analysis. Prominent morphological features are circumferentially-oriented villar ridges in segments S1-S6 and the absence of crypts. Molecular characterization of the transcriptome from each segment shows that segments S1-S5 are very similar while S6 and S7 unique. Gene ontology analyses reveal that S1-S5 express genes whose functions involve metabolism of carbohydrates, transport of lipids and energy generation, while the last two segments display relatively limited function. Based on comparative Gene Set Enrichment Analysis, the first five segments share strong similarity with human and mouse small intestine while S6 shows similarity with human cecum and rectum, and S7 with human rectum. The intestinal tract does not display the anatomical, morphological, and molecular signatures of a stomach and thus we conclude that this organ is absent from the zebrafish digestive system.

CONCLUSIONS

Our genome-wide gene expression data indicate that, despite the lack of crypts, the rostral, mid, and caudal portions of the zebrafish intestine have distinct functions analogous to the mammalian small and large intestine, respectively. Organization of ridge structures represents a unique feature of zebrafish intestine, though they produce similar cross sections to mammalian intestines. Evolutionary lack of stomach, crypts, Paneth cells and submucosal glands has shaped the zebrafish intestine into a simpler but unique organ in vertebrate intestinal biology.

Peroxisomes in zebrafish: distribution pattern and knockdown studies

Peroxisomes are organelles that are essential for normal development in men and mice. In order to explore whether zebrafish could be used as a model system to study the role of peroxisomes, we examined their distribution pattern in developing and adult zebrafish and we tested different approaches to eliminate them during the first days after fertilization. In 4-day-old embryos, catalase-containing peroxisomes were obvious in the liver, the pronephric duct and the wall of the yolk sac, but transcripts for peroxisomal matrix and membrane proteins were also detected in the head region from 24 h post-fertilization. In adult zebrafish, catalase-containing peroxisomes remained prominent in the hepatocytes, the renal proximal tubules and the intestinal epithelium. Several peroxins, essential proteins for the biogenesis of peroxisomes, were targeted using knockdown approaches. Two morpholinos, blocking, respectively, splice sites in pex3 and pex13, only induced a short in frame deletion or insertion in the transcripts and did not result in the elimination of peroxisomes after injection into one-cell embryos. A morpholino blocking translation of pex13 was able to reduce the number of peroxisomes to variable extents. Finally, overexpression of a potential dominant negative fragment of Pex3p did not result in deletion of peroxisomes from developing zebrafish. We conclude that in zebrafish (1) peroxisomes, as visualized by DAB cytochemistry for catalase activity, are most conspicuous in the liver and renal tubular epithelium; this pattern is reminiscent of peroxisome occurrence in mammalian organs, (2) our approaches to eliminate these organelles during development by targeting peroxins were not successful.

Electrochemical quantification of serotonin in the live embryonic zebrafish intestine

We monitored real-time in vivo levels of serotonin release in the digestive system of intact zebrafish embryos during early development (5 days postfertilization, dpf) using differential pulse voltammetry with implanted carbon fiber microelectrodes modified with carbon nanotubes dispersed in nafion. A detection limit of 1 nM, a linear range between 5 and 200 nM, and a sensitivity of 83.65 nA x microM(-1) were recorded. The microelectrodes were implanted at various locations in the intestine of zebrafish embryos. Serotonin levels of up to 29.9 (+/-1.13) nM were measured in vivo in normal physiological conditions. Measurements were performed in intact live embryos without additional perturbation beyond electrode insertion. The sensor was able to quantify pharmacological alterations in serotonin release and provide the longitudinal distribution of this neurotransmitter along the intestine with high spatial resolution. In the presence of fluvoxamine, a selective serotonin reuptake inhibitor (SSRI), concentrations of 54.1 (+/-1.05) nM were recorded while in the presence of p-chloro-phenylalanine (PCPA), a tryptophan hydroxylase inhibitor, the serotonin levels decreased to 7.2 (+/-0.45) nM. The variation of serotonin levels was correlated with immunohistochemical analysis. We have demonstrated the first use of electrochemical microsensors for in vivo monitoring of intestinal serotonin levels in intact zebrafish embryos.

The zebrafish flotte lotte mutant reveals that the local retinal environment promotes the differentiation of proliferating precursors emerging from their stem cell niche

It is currently unclear how intrinsic and extrinsic mechanisms cooperate to control the progression from self-renewing to neurogenic divisions in retinal precursor cells. Here, we use the zebrafish flotte lotte (flo) mutant, which carries a mutation in the elys (ahctf1) gene, to study the relationship between cell cycle progression and neuronal differentiation by investigating how proliferating progenitor cells transition towards differentiation in a retinal stem cell niche termed the ciliary marginal zone (CMZ). In zebrafish embryos without Elys, CMZ cells retain the capacity to proliferate but lose the ability to enter their final neurogenic divisions to differentiate as neurons. However, mosaic retinae composed of wild-type and flo cells show that despite inherent cell cycle defects, flo mutant cells progress from proliferation to differentiation when in the vicinity of wild-type retinal neurons. We propose that the differentiated retinal environment limits the proliferation of precursors emerging from the CMZ in a manner that explains the spatial organisation of cells in the CMZ and ensures that proliferative retinal progenitors are driven towards differentiation.

Growth hormone receptors in zebrafish (Danio rerio): adult and embryonic expression patterns

Growth hormone receptor (GHR) is a critical regulator of growth and metabolism. Although two GHRs have been characterized in many fish species, their functional characteristics, mechanisms of regulation and roles in embryonic development remain unclear. The zebrafish (Danio rerio) is an excellent model organism to study both developmental and physiological processes. In the present work, we characterized the complete cDNA sequences of zebrafish GHRs, ghra and ghrb, and their gene structures. We studied the expression of both receptors in adult tissues, and during embryonic development and larval stages by means of RT-PCR and whole-mount in situ hybridization. We determined that both transcripts are maternal ones, with specific expression patterns during development. Both GHR transcripts are mainly expressed in the notochord, myotomes, anterior structures and in the yolk cell. Interestingly, their expression became undetectable at 96h post-fertilization. Unlike other reports in fish, ghrs expression could not be detected in brain when adult tissues were used, and we detected ghrb but not ghra transcripts in muscle. In addition, we determined alternative transcript sequences for ghra with specific domain deletions, and alternative transcripts for ghrb that generate a premature stop codon and codify for truncated isoforms. These isoforms lack intracellular regions necessary for the activation of signal transducers and activators of transcription (STAT) family transcription factors 5.

Fishing for intestinal cancer models: unraveling gastrointestinal homeostasis and tumorigenesis in zebrafish

Zebrafish has proven to be a highly versatile model for comprehensive studies of gene function in development. Given that the molecular pathways involved in epithelial carcinogenesis appear to be conserved across vertebrates, zebrafish is now considered as a valid model to study tumor biology. Development and homeostasis in multicellular organisms are dependent on a complex interplay between cell proliferation, migration, differentiation, and cell death. The Wnt signaling pathway is a major signaling pathway during embryonic development and is the key regulator of self-renewal homeostasis in several adult tissues. A large body of knowledge on adult stem-cell biology has arisen from the study of the intestinal epithelium over the past 20 years. The Wnt pathway has appeared as its principal regulator of homeostatic self-renewal. Moreover, most cancers of the intestine are caused by activating mutations in the Wnt pathway. Recently, zebrafish models have been developed to study Wnt pathway-induced cancer. An appealing avenue for cancer research in zebrafish is large-scale screens to identify chemotherapeutic and chemopreventive agents in conjunction with the in vivo imaging approaches that zebrafish affords.

Dissection of organs from the adult zebrafish

Over the last 20 years, the zebrafish has become a powerful model organism for understanding vertebrate development and disease. Although experimental analysis of the embryo and larva is extensive and the morphology has been well documented, descriptions of adult zebrafish anatomy and studies of development of the adult structures and organs, together with techniques for working with adults are lacking. The organs of the larva undergo significant changes in their overall structure, morphology, and anatomical location during the larval to adult transition. Externally, the transparent larva develops its characteristic adult striped pigment pattern and paired pelvic fins, while internally, the organs undergo massive growth and remodeling. In addition, the bipotential gonad primordium develops into either testis or ovary. This protocol identifies many of the organs of the adult and demonstrates methods for dissection of the brain, gonads, gastrointestinal system, heart, and kidney of the adult zebrafish. The dissected organs can be used for in situ hybridization, immunohistochemistry, histology, RNA extraction, protein analysis, and other molecular techniques. This protocol will assist in the broadening of studies in the zebrafish to include the remodeling of larval organs, the morphogenesis of organs specific to the adult and other investigations of the adult organ systems.

Host-microbe interactions in the developing zebrafish

The amenability of the zebrafish to in vivo imaging and genetic analysis has fueled expanded use of this vertebrate model to investigate the molecular and cellular foundations of host-microbe relationships. Study of microbial encounters in zebrafish hosts has concentrated on developing embryonic and larval stages, when the advantages of the zebrafish model are maximized. A comprehensive understanding of these host-microbe interactions requires appreciation of the developmental context into which a microbe is introduced, as well as the effects of that microbial challenge on host ontogeny. In this review, we discuss how in vivo imaging and genetic analysis in zebrafish has advanced our knowledge of host-microbe interactions in the context of a developing vertebrate host. We focus on recent insights into immune cell ontogeny and function, commensal microbial relationships in the intestine, and microbial pathogenesis in zebrafish hosts.

Distinct developmental expression of two elongase family members in zebrafish

Despite the known importance of long-chained polyunsaturated fatty acids (LC-PUFA) during development, very little is known about their utilization and biosynthesis during embryogenesis. Combining the advantages of the existence of a complete range of enzymes required for LC-PUFA biosynthesis and the well established developmental biology tools in zebrafish, we examined the expression patterns of three LC-PUFA biosynthesis genes, Elovl2-like elongase (elovl2), Elovl5-like elongase (elovl5) and fatty acyl desaturase (fad) in different zebrafish developmental stages. The presence of all three genes in the brain as early as 24 hours post fertilization (hpf) implies LC-PUFA synthesis activity in the embryonic brain. This expression eventually subsides from 72 hpf onwards, coinciding with the initiation of elovl2 and fad expression in the liver and intestine, 2 organs known to be involved in adult fish LC-PUFA biosynthesis. Collectively, these patterns strongly suggest the necessity for localized production of LC-PUFA in the brain during in early stage embryos prior to the maturation of the liver and intestine. Interestingly, we also showed a specific expression of elovl5 in the proximal convoluted tubule (PCT) of the zebrafish pronephros, suggesting a possible new role for LC-PUFA in kidney development and function.

Regulation of gene expression in the intestinal epithelium

Regulation of gene expression within the intestinal epithelium is complex and controlled by various signaling pathways that regulate the balance between proliferation and differentiation. Proliferation is required both to grow and to replace cells lost through apoptosis and attrition, yet in all but a few cells, differentiation must take place to prevent uncontrolled growth (cancer) and to provide essential functions. In this chapter, we review the major signaling pathways underlying regulation of gene expression within the intestinal epithelium, based primarily on data from mouse models, as well as specific morphogens and transcription factor families that have a major role in regulating intestinal gene expression, including the Hedgehog family, Forkhead Box (FOX) factors, Homeobox (HOX) genes, ParaHox genes, GATA transcription factors, canonical Wnt/β-catenin signaling, EPH/Ephrins, Sox9, BMP signaling, PTEN/PI3K, LKB1, K-RAS, Notch pathway, HNF, and MATH1. We also briefly highlight important emerging areas of gene regulation, including microRNA (miRNA) and epigenetic regulation.

Oxazolone-induced enterocolitis in zebrafish depends on the composition of the intestinal microbiota

BACKGROUND & AIMS

The pathogenesis of inflammatory bowel disease involves dysfunctional mucosal immune responses to commensal bacteria in genetically predisposed hosts. Interactions between host cells and bacteria are complicated, making it a challenge to assess their relative contribution to intestinal pathology. We developed a zebrafish model of enterocolitis to study these interactions.

METHODS

Enterocolitis was induced by intrarectal administration of the hapten oxazolone in adult wild-type and myeloperoxidase-reporter transgenic zebrafish in the presence or absence of antibiotics. Intestinal inflammation was evaluated by histological and flow cytometry analyses and cytokine profiling with quantitative real-time polymerase chain reaction. Changes in the composition of the intestinal microbiota following antibiotic administration were assessed by 16SrRNA sequencing and bacterial load was quantified by culture on nonselective media (colony-forming units).

RESULTS

In zebrafish, the infiltrate and severity of oxazolone-induced enterocolitis are influenced by the composition of the microbiota. Inflammation is characterized by granulocyte influx; epithelial damage; goblet cell depletion; and increased expression of interleukin-1beta, tumor necrosis factor-alpha, and interleukin-10. Zebrafish given vancomycin had bacterial populations dominated by Fusobacteria and reduced enterocolitis scores, intestinal damage, and percentages of infiltrating neutrophils and eosinophils. In contrast, zebrafish given colistin sulphate had a predominance of proteobacteria and reduced eosinophil and lymphocyte infiltration, but enterocolitis scores were not reduced.

CONCLUSIONS

In zebrafish with oxazolone-induced enterocolitis, components of the intestinal microbiota affect the severity and composition of the intestinal infiltrate.

miR-145 directs intestinal maturation in zebrafish

The rapid specification and differentiation of the embryonic zebrafish gut is essential to provide contractility for the digestion of food. The role of microRNAs in modulating gut epithelial or smooth muscle differentiation is currently not known. Here we show that the microRNA miR-145 is strongly expressed in zebrafish gut smooth muscle and regulates its development. Modulation of miR-145 levels results in gut smooth muscle and epithelium maturation defects. Loss of miR-145 results in defects of smooth muscle function as measured by decreased nitric oxide production but also leads to increased expression of the embryonic smooth muscle markers sm22alpha-b, nm-mhc-b, and smoothelin. Defects in gut epithelial maturation are also present as observed by immature morphology and a complete loss of alkaline phosphatase expression. Loss or gain of miR-145 function phenocopies defects observed with altered gata6 expression and accordingly, we show that miR-145 directly represses gata6, and that gata6 is a major miR-145 target in vitro and in vivo. miR-145 therefore plays a critical role in promoting the maturation of both layers of the gut during development through regulation of gata6.

FACS-assisted microarray profiling implicates novel genes and pathways in zebrafish gastrointestinal tract development

BACKGROUND & AIMS

The zebrafish Danio rerio is an excellent model system for mammalian gastrointestinal development. To identify differentially regulated genes important in gastrointestinal organogenesis, we profiled the transcriptome of the zebrafish developing gastrointestinal tract.

METHODS

Embryos from a transgenic zebrafish line expressing green fluorescent protein (GFP) in the developing intestine, liver, and pancreas were dissociated at 4 developmental time points, their cells sorted based on GFP expression with fluorescence-activated cell sorting (FACS), and analyzed with microarrays. To improve our analysis, we annotated the Affymetrix Zebrafish GeneChip with human orthologs.

RESULTS

Transcriptional profiling showed significant differences between GFP(+) and GFP(-) cells. Up-regulated genes and pathways were consistent with mammalian gastrointestinal development, such as hepatic nuclear factor gene networks and cancer. We implicate the phosphatidylinositol 3 kinase (PI3K) pathway and show that inhibition with LY294002 causes gastrointestinal defects in zebrafish. We identified novel genes, such as the microRNAs miR-217 and miR-122, the tight junction protein claudin c, the gene fam136a, and a zebrafish tetraspanin. Novel pathways include genes containing a putative transcription factor binding sequence, GGAANCGGAANY, and a nucleolar gene network. The zebrafish microarrays also identify a set of 32 genes that may mediate the effects of gain of chromosome arm 8q in human colon, liver, and pancreatic cancers.

CONCLUSIONS

We successfully combine FACS and microarray profiling to follow organogenesis throughout development. These experiments identify novel genes and pathways that probably play a role in mammalian gastrointestinal development and are potential targets for therapeutic intervention in the management of gastrointestinal disease and cancer.

Evolutionary structural and functional conservation of an ortholog of the GLUT2 glucose transporter gene (SLC2A2) in zebrafish

In mammals, GLUT2 plays an essential role in glucose homeostasis. From an evolutionary perspective, relatively little is known about the biology of GLUT2, or other GLUTs, in nonmammalian vertebrates. Here, we have conducted studies to functionally characterize GLUT2 in zebrafish. First, we cloned the zebrafish ortholog of GLUT2 (zfGLUT2) encoding a protein of 504 amino acids with high-sequence identity to other known vertebrate GLUT2 proteins. The zfGLUT2 gene consists of 11 exons and 10 introns, spanning 20 kb and mapping to a region of chromosome 2 that exhibits conserved synteny with human chromosome 3. When expressed in Xenopus oocytes, zfGLUT2 transported 2-deoxyglucose (2-DG) with similar affinity than mammalian GLUT2 (K(m) of 11 mM). Transport of 2-DG was competed mostly by D-fructose and D-mannose and was inhibited by cytochalasin B. During early development, zfGLUT2 expression was detected already at 10 h postfertilization and remained elevated in 5-day larvae, when it was clearly localized to the liver and intestinal bulb. In the adult, zfGLUT2 expression was highest in testis, brain, skin, kidney, and intestine, followed by liver and muscle. In the intestine, zfGLUT2 transcripts were detected in absorptive enterocytes, and its mRNA levels were altered by fasting and refeeding, suggesting that its expression in the intestine may be regulated by the nutritional status. These results indicate that the structure and function of GLUT2 has been remarkably well conserved during vertebrate evolution and open the way for the use of zebrafish as a model species in which to study the biology and pathophysiology of GLUT2.

Toxicity and developmental defects of different sizes and shape nickel nanoparticles in zebrafish

Metallic nanoparticles such as nickel are used in catalytic sensing, and electronic applications, but health and environmental affects have not been fully investigated. While some metal nanoparticles result in toxicity, it is also important to determine whether nanoparticles of the same metal but of different size and shape changes toxicity. Three different size nickel nanoparticle (Ni NPs) of 30, 60, and 100 nm and larger particle clusters of aggregated 60 nm entities with a dendritic structure were synthesized and exposed to zebrafish embryos assessing mortality and developmental defects. Ni NPs exposure was compared to soluble nickel salts. All three 30, 60, and 100 nm Ni NPs are equal to or less toxic than soluble nickel while dendritic clusters were more toxic. With each Ni NP exposure, thinning of the intestinal epithelium first occurs around the LD10 continuing into the LD50. LD50 exposure also results in skeletal muscle fiber separation. Exposure to soluble nickel does not cause intestinal defects while skeletal muscle separation occurs at concentrations well over LD50. These results suggest that configuration of nanoparticles may affect toxicity more than size and defects from Ni NPs exposure occur by different biological mechanisms than soluble nickel.

Zebrafish cdx1b regulates differentiation of various intestinal cell lineages

Both antisense morpholino oligonucleotide (MO)-mediated knockdown and overexpression experiments were performed to analyze zebrafish cdx1b's function in intestinal cell differentiation. Substantial reductions in goblet cell numbers were detected in intestines of 102- and 120-hours post-fertilization (hpf) cdx1b MO-injected embryos (morphants) compared to cdx1b-4-base mismatched (4mm)-MO-injected and wild type embryos. A significant decrease in enteroendocrine cell numbers was also observed in intestines of 96-hpf cdx1b morphants. Furthermore, ectopic cdx1b expression caused notable increases in respective cell numbers of enteroendocrine and goblet cells in intestines of 96- and 98-hpf injected embryos. Decreased PepT1 expression was detected in enterocytes of intestines in cdx1b morphants from 80 to 102 hr of development. In addition, increased cell proliferation was detected in intestines of cdx1b morphants. Overall, our results suggest that zebrafish cdx1b plays important roles in regulating intestinal cell proliferation and the differentiation of various intestinal cell lineages.

T-cell factor 4 (tcf7l2) is the main effector of Wnt signaling during zebrafish intestine organogenesis

The Wnt pathway orchestrates cell fate decisions during embryonic development, organogenesis, and adult tissues homeostasis. T-cell factor (Tcf )/lymphoid enhancer-binding factor (Lef) transcription factors are the downstream effectors of canonical Wnt signaling. Upon Wnt signal activation, beta-catenin stabilizes and translocates to the nucleus, where it interacts with Tcfs activating the transcription of Wnt target genes. In the absence of Wnt, levels of stable beta-catenin are reduced by the action of adenomatous polyposis coli (Apc) and other cytoplasmic proteins. Mutations in Apc cause constitutive accumulation of beta-catenin and inappropriate activation of the Wnt pathway. apc(mcr/mcr) fish embryos show absence of expression of tissue-specific differentiation markers in the intestine, suggesting that inappropriate activation of Wnt signaling abrogates gut organogenesis. Which Tcf transcription factor mediates Wnt signaling during zebrafish gut organogenesis remains unclear. We studied the combined effect of loss of Tcf family members and Apc in the developing embryo. Tcf4 (tcf7l2) loss rescues the apc(mcr/mcr) phenotype in the intestine. Single depletion of Tcf1 (tcf7) and Tcf3 (tcf7l1a) function in an Apc mutant background had no effect on endoderm development. This study reveals that Tcf4 (tcf7l2) is the major effector of Wnt signaling in the intestine during zebrafish organogenesis.

The role of the visceral mesoderm in the development of the gastrointestinal tract

The gastrointestinal (GI) tract forms from the endoderm (which gives rise to the epithelium) and the mesoderm (which develops into the smooth muscle layer, the mesenchyme, and numerous other cell types). Much of what is known of GI development has been learned from studies of the endoderm and its derivatives, because of the importance of epithelial biology in understanding and treating human diseases. Although the necessity of epithelial-mesenchymal cross talk for GI development is uncontested, the role of the mesoderm remains comparatively less well understood. The transformation of the visceral mesoderm during development is remarkable; it differentiates from a very thin layer of cells into a complex tissue comprising smooth muscle cells, myofibroblasts, neurons, immune cells, endothelial cells, lymphatics, and extracellular matrix molecules, all contributing to the form and function of the digestive system. Understanding the molecular processes that govern the development of these cell types and elucidating their respective contribution to GI patterning could offer insight into the mechanisms that regulate cell fate decisions in the intestine, which has the unique property of rapid cell renewal for the maintenance of epithelial integrity. In reviewing evidence from both mammalian and nonmammalian models, we reveal the important role of the visceral mesoderm in the ontogeny of the GI tract.

Autonomic innervation of the fish gut

The enteric nervous system follows a similar overall arrangement in all vertebrate groups. In fish, the majority of nerve cell bodies are found in the myenteric plexus, innervating muscles, blood vessels and glands. In this review, I describe similarities and differences in size, shape and transmitter content in enteric neurons in different fish species and also in comparison with other vertebrates, foremost mammals. The use of different histological and immunochemical methods is reviewed in a historical perspective including advantages and disadvantages of different methods. Lately, zebrafish have become an important model species for developmental studies of the nervous system, including the enteric nervous system, and this is briefly discussed. Finally, examples of how the enteric nervous system controls gut activity in fish is presented, focussing on the effect on gastrointestinal motility.

On the diabetic menu: zebrafish as a model for pancreas development and function

Development of the vertebrate pancreas is a complex stepwise process comprising regionalization, cell differentiation, and morphogenesis. Studies in zebrafish are contributing to an emerging picture of pancreas development in which extrinsic signaling molecules influence intrinsic transcriptional programs to allow ultimate differentiation of specific pancreatic cell types. Zebrafish experiments have revealed roles for several signaling molecules in aspects of this process; for example our own work has shown that retinoic acid signals specify the pre-pancreatic endoderm. Time-lapse imaging of live zebrafish embryos has started to provide detailed information about early pancreas morphogenesis. In addition to modeling embryonic development, the zebrafish has recently been used as a model for pancreas regeneration studies. Here, we review the significant progress in these areas and consider the future potential of zebrafish as a diabetes research model.

Abnormal nuclear pore formation triggers apoptosis in the intestinal epithelium of elys-deficient zebrafish

BACKGROUND & AIMS

Zebrafish mutants generated by ethylnitrosourea-mutagenesis provide a powerful tool for dissecting the genetic regulation of developmental processes, including organogenesis. One zebrafish mutant, "flotte lotte" (flo), displays striking defects in intestinal, liver, pancreas, and eye formation at 78 hours postfertilization (hpf). In this study, we sought to identify the underlying mutated gene in flo and link the genetic lesion to its phenotype.

METHODS

Positional cloning was employed to map the flo mutation. Subcellular characterization of flo embryos was achieved using histology, immunocytochemistry, bromodeoxyuridine incorporation analysis, and confocal and electron microscopy.

RESULTS

The molecular lesion in flo is a nonsense mutation in the elys (embryonic large molecule derived from yolk sac) gene, which encodes a severely truncated protein lacking the Elys C-terminal AT-hook DNA binding domain. Recently, the human ELYS protein has been shown to play a critical, and hitherto unsuspected, role in nuclear pore assembly. Although elys messenger RNA (mRNA) is expressed broadly during early zebrafish development, widespread early defects in flo are circumvented by the persistence of maternally expressed elys mRNA until 24 hpf. From 72 hpf, elys mRNA expression is restricted to proliferating tissues, including the intestinal epithelium, pancreas, liver, and eye. Cells in these tissues display disrupted nuclear pore formation; ultimately, intestinal epithelial cells undergo apoptosis.

CONCLUSIONS

Our results demonstrate that Elys regulates digestive organ formation.

A new high-content model system for studies of gastrointestinal transit: the zebrafish

The zebrafish gastrointestinal (GI) tract displays an anatomy and cellular architecture that is similar to the human GI tract, with concentric layers of inner epithelia, connective tissue, circular muscle and outer longitudinal muscle layers. Propulsion of luminal content results from the integrated activity of smooth muscle cells, enteric neurons and the interstitial cells of Cajal (ICC). Zebrafish larvae are transparent and propagating contractions in the entire GI tract are easily visualized. A new moderate-throughput zebrafish-based GI transit assay is described in this issue of Neurogastroenterology and Motility. This assay utilizes intact zebrafish larvae which contain essential regulatory elements (ICC and enteric neurons). Forward genetic analysis, which identifies genes underlying specific phenotypes, is possible using the zebrafish system. The zebrafish model system compliments existing models for studies of GI motility and will contribute to the understanding of the regulation of GI motility, and to identification of novel drug targets.

hnRNP I inhibits Notch signaling and regulates intestinal epithelial homeostasis in the zebrafish

Regulated intestinal stem cell proliferation and differentiation are required for normal intestinal homeostasis and repair after injury. The Notch signaling pathway plays fundamental roles in the intestinal epithelium. Despite the fact that Notch signaling maintains intestinal stem cells in a proliferative state and promotes absorptive cell differentiation in most species, it remains largely unclear how Notch signaling itself is precisely controlled during intestinal homeostasis. We characterized the intestinal phenotypes of brom bones, a zebrafish mutant carrying a nonsense mutation in hnRNP I. We found that the brom bones mutant displays a number of intestinal defects, including compromised secretory goblet cell differentiation, hyperproliferation, and enhanced apoptosis. These phenotypes are accompanied by a markedly elevated Notch signaling activity in the intestinal epithelium. When overexpressed, hnRNP I destabilizes the Notch intracellular domain (NICD) and inhibits Notch signaling. This activity of hnRNP I is conserved from zebrafish to human. In addition, our biochemistry experiments demonstrate that the effect of hnRNP I on NICD turnover requires the C-terminal portion of the RAM domain of NICD. Our results demonstrate that hnRNP I is an evolutionarily conserved Notch inhibitor and plays an essential role in intestinal homeostasis.

Genetic model system studies of the development of the enteric nervous system, gut motility and Hirschsprung's disease

The enteric nervous system (ENS) is the largest and most complicated subdivision of the peripheral nervous system. Its action is necessary to regulate many of the functions of the gastrointestinal tract including its motility. Whilst the ENS has been studied extensively by developmental biologists, neuroscientists and physiologists for several decades it has only been since the early 1990s that the molecular and genetic basis of ENS development has begun to emerge. Central to this understanding has been the use of genetic model organisms. In this article, we will discuss recent advances that have been achieved using both mouse and zebrafish model genetic systems that have led to new insights into ENS development and the genetic basis of Hirschsprung's disease.

In vivo imaging of zebrafish digestive organ function using multiple quenched fluorescent reporters

Optical clarity of larvae makes the zebrafish ideal for real-time analyses of vertebrate organ function through the use of fluorescent reporters of enzymatic activities. A key function of digestive organs is to couple the generation of enzymes with mechanical processes that enable nutrient availability and absorption. However, it has been extremely difficult, and in many cases not possible, to directly observe digestive processes in a live vertebrate. Here we describe a new method to visualize intestinal protein and lipid processing simultaneously in live zebrafish larvae using a quenched fluorescent protein (EnzChek) and phospholipid (PED6). By employing these reagents, we found that wild-type larvae exhibit significant variation in intestinal phospholipase and protease activities within a group but display a strong correlation between the activities within individuals. Furthermore, we found that pancreas function is essential for larval digestive protease activity but not for larval intestinal phospholipase activity. Although fat-free (ffr) mutant larvae were previously described to exhibit impaired lipid processes, we found they also had significantly reduced protease activity. Finally, we selected and evaluated compounds that were previously suggested to have altered phospholipase activity and are known or suspected to have inflammatory effects in the intestinal tract including nonsteroidal anti-inflammatory drugs, and identified a compound that significantly increases intestinal phospholipid processing. Thus the multiple fluorescent reporter-based methodology facilitates the rapid analysis of digestive organ function in live zebrafish larvae.

Morphologic analysis of the zebrafish digestive system

The zebrafish provides an ideal model for the study of vertebrate organogenesis, including the formation of the digestive tract and its associated organs. Despite optical transparency of embryos, the internal position of the developing digestive system and its close juxtaposition with the yolk initially made morphological analysis relatively challenging, particularly during the first 3 d of development. However, methodologies have been successfully developed to address these problems and comprehensive morphologic analysis of the developing digestive system has now been achieved using a combination of light and fluorescence microscope approaches-including confocal analysis-to visualize wholemount and histological preparations of zebrafish embryos. Furthermore, the expanding number of antibodies that cross-react with zebrafish proteins and the generation of tissue-specific transgenic green fluorescent protein reporter lines that mark specific cell and tissue compartments have greatly enhanced our ability to successfully image the developing zebrafish digestive system.

Alpha2 macroglobulin-like is essential for liver development in zebrafish

Background

Alpha 2 Macroglobulin family members have been studied extensively with respect to their roles in physiology and human disease including innate immunity and Alzheimer's disease, but little is known about a possible role in liver development loss-of-function in model systems.

Principal Findings

We report the isolation of the zebrafish α2 macroglobulin-like (A2ML) gene and its specific expression in the liver during differentiation. Morpholino-based knock-down of A2ML did not block the initial formation of the liver primordium, but inhibited liver growth and differentiation.

Significance

This report on A2ML function in zebrafish development provides the first evidence for a specific role of an A2M family gene in liver formation during early embryogenesis in a vertebrate.

Intestinal differentiation in zebrafish requires Cdx1b, a functional equivalent of mammalian Cdx2

BACKGROUND & AIMS

The ParaHox transcription factor Cdx2 is an essential determinant of intestinal phenotype in mammals throughout development, influencing gut function, homeostasis, and epithelial barrier integrity. Cdx2 expression demarcates the zones of intestinal stem cell proliferation in the adult gut, with deregulated expression implicated in intestinal metaplasia and cancer. However, in vivo analysis of these prospective roles has been limited because inactivation of Cdx2 in mice leads to preimplantation embryonic lethality. We used the zebrafish, a valuable model for studying gut development, to generate a system to further understanding of the role of Cdx2 in normal intestinal function and in disease states.

METHODS

We isolated and characterized the zebrafish cdx1b ortholog and analyzed its function by antisense morpholino gene knockdown.

RESULTS

We showed that zebrafish Cdx1b replaces the role of Cdx2 in gut development. Evolutionary studies have indicated that the zebrafish cdx2 loci were lost following the genome-wide duplication event that occurred in teleosts. Zebrafish Cdx1b is expressed exclusively in the developing intestine during late embryogenesis and regulates intestinal cell proliferation and terminal differentiation.

CONCLUSIONS

This work established an in vivo system to explore further the activity of Cdx2 in the gut and its impact on processes such as inflammation and cancer.

Mutation of the zebrafish nucleoporin elys sensitizes tissue progenitors to replication stress

The recessive lethal mutation flotte lotte (flo) disrupts development of the zebrafish digestive system and other tissues. We show that flo encodes the ortholog of Mel-28/Elys, a highly conserved gene that has been shown to be required for nuclear integrity in worms and nuclear pore complex (NPC) assembly in amphibian and mammalian cells. Maternal elys expression sustains zebrafish flo mutants to larval stages when cells in proliferative tissues that lack nuclear pores undergo cell cycle arrest and apoptosis. p53 mutation rescues apoptosis in the flo retina and optic tectum, but not in the intestine, where the checkpoint kinase Chk2 is activated. Chk2 inhibition and replication stress induced by DNA synthesis inhibitors were lethal to flo larvae. By contrast, flo mutants were not sensitized to agents that cause DNA double strand breaks, thus showing that loss of Elys disrupts responses to selected replication inhibitors. Elys binds Mcm2-7 complexes derived from Xenopus egg extracts. Mutation of elys reduced chromatin binding of Mcm2, but not binding of Mcm3 or Mcm4 in the flo intestine. These in vivo data indicate a role for Elys in Mcm2-chromatin interactions. Furthermore, they support a recently proposed model in which replication origins licensed by excess Mcm2-7 are required for the survival of human cells exposed to replication stress.

DNA replication is a complex process that requires activation of cell cycle checkpoints and DNA repair pathways. Genetic analyses in fungi have suggested that nucleoporins, the proteins that make up the nuclear pore complex (NPC), play a role in the cellular response to agents that disrupt cell proliferation or damage DNA. Here we show that mutation of the Elys nucleoporin causes widespread apoptosis in the intestine and other tissues of zebrafish flotte lotte (flo) mutants. Intestinal apoptosis occurs in the absence of the DNA damage marker γH2X, and levels of chromatin bound Mcm2, a component of the DNA replication helicase, were also reduced in flo mutants. These findings suggested that flo intestinal cells cannot repair endogenous replication errors. Consistent with this idea, flo mutants were highly sensitized to treatment with DNA replication inhibitors such as hydroxyurea, UV irradiation, or cisplatin, but not agents that cause DNA double strand breaks, such as γ-irradiation or camptothecin. These data point to a conserved role for nucleoporins in the cellular response to replication stress in eukaryote cells.

Zebrafish: an emerging technology for in vivo pharmacological assessment to identify potential safety liabilities in early drug discovery

The zebrafish is a well-established model organism used in developmental biology. In the last decade, this technology has been extended to the generation of high-value knowledge on safety risks of novel drugs. Indeed, the larval zebrafish appear to combine advantages of whole organism phenotypic assays and those (rapid production of results with minimal resource engagement) of in vitro high-throughput screening techniques. Thus, if appropriately evaluated, it can offer undeniable advantages in drug discovery for identification of target and off-target effects. Here, we review some applications of zebrafish to identify potential safety liabilities, particularly before lead/candidate selection. For instance, zebrafish cardiovascular system can be used to reveal decreases in heart rate and atrial-ventricular dissociation, which may signal human ether-a-go-go-related gene (hERG) channel blockade. Another main area of interest is the CNS, where zebrafish behavioural assays have been and are further being developed into screening platforms for assessment of locomotor activity, convulsant and proconvulsant liability, cognitive impairment, drug dependence potential and impaired visual and auditory functions. Zebrafish also offer interesting possibilities for evaluating effects on bone density and gastrointestinal function. Furthermore, available knowledge of the renal system in larval zebrafish can allow identification of potential safety issues of drug candidates on this often neglected area in early development platforms. Although additional validation is certainly needed, the zebrafish is emerging as a versatile in vivo animal model to identify off-target effects that need investigation and further clarification early in the drug discovery process to reduce the current, high degree of attrition in development.

Development of enteric and vagal innervation of the zebrafish (Danio rerio) gut

The autonomic nervous system develops following migration and differentiation of precursor cells originating in the neural crest. Using immunohistochemistry on intact zebrafish embryos and larvae we followed the development of the intrinsic enteric and extrinsic vagal innervation of the gut. At 3 days postfertilization (dpf), enteric nerve cell bodies and fibers were seen mainly in the middle and distal intestine, while the innervation of the proximal intestine was scarcer. The number of fibers and cell bodies gradually increased, although a large intraindividual variation was seen in the timing (but not the order) of development. At 11-13 dpf most of the proximal intestine received a similar degree of innervation as the rest of the gut. The main intestinal branches of the vagus were similarly often already well developed at 3 dpf, entering the gut at the transition between the proximal and middle intestine and projecting posteriorly along the length of the gut. Subsequently, fibers branching off the vagus innervated all regions of the gut. The presence of several putative enteric neurotransmitters was suggested by using markers for neurokinin A (NKA), pituitary adenylate cyclase-activating polypeptide (PACAP), vasoactive intestinal polypeptide (VIP), nitric oxide, serotonin (5-hydroxytryptamine, 5-HT), and calcitonin gene-related peptide (CGRP). The present results corroborate the belief that the enteric innervation is well developed before the onset of feeding (normally occurring around 5-6 dpf). Further, the more detailed picture of how development proceeds at stages previously not examined suggests a correlation between increasing innervation and more regular and elaborated motility patterns.

Endoderm-derived Sonic hedgehog and mesoderm Hand2 expression are required for enteric nervous system development in zebrafish

The zebrafish enteric nervous system (ENS), like those of all other vertebrate species, is principally derived from the vagal neural crest cells (NCC). The developmental controls that govern the migration, proliferation and patterning of the ENS precursors are not well understood. We have investigated the roles of endoderm and Sonic hedgehog (SHH) in the development of the ENS. We show that endoderm is required for the migration of ENS NCC from the vagal region to the anterior end of the intestine. We show that the expression of shh and its receptor ptc-1 correlate with the development of the ENS and demonstrate that hedgehog (HH) signaling is required in two phases, a pre-enteric and an enteric phase, for normal ENS development. We show that HH signaling regulates the proliferation of vagal NCC and ENS precursors in vivo. We also show the zebrafish hand2 is required for the normal development of the intestinal smooth muscle and the ENS. Furthermore we show that endoderm and HH signaling, but not hand2, regulate gdnf expression in the intestine, highlighting a central role of endoderm and SHH in patterning the intestine and the ENS.

Ret isoform function and marker gene expression in the enteric nervous system is conserved across diverse vertebrate species

The enteric nervous system (ENS) derives from migratory neural crest cells that colonize the developing gut tube, giving rise to an integrated network of neurons and glial cells, which together regulate important aspects of gut function, including coordinating the smooth muscle contractions of the gut wall. The absence of enteric neurons in portions of the gut (aganglionosis) is the defining feature of Hirschsprung's disease (HSCR) and has been replicated in a number of mouse models. Mutations in the RET tyrosine kinase account for over half of familial cases of HSCR and mice mutant for Ret exhibit aganglionosis. RET exists in two main isoforms, RET9 and RET51 and studies in mouse have shown that RET9 is sufficient to allow normal development of the ENS. In the last several years, zebrafish has emerged as a model of vertebrate ENS development, having been supported by a number of demonstrations of conservation of gene function between zebrafish, mouse and human. In this study we further analyse the potential similarities and differences between ENS development in zebrafish, mouse and human. We demonstrate that zebrafish Ret is required in a dose-dependent manner to regulate colonization of the gut by neural crest derivatives, as in human. Additionally, we show that as in mouse and human, zebrafish ret is produced as two isoforms, ret9 and ret51. Moreover, we show that, as in mouse, the Ret9 isoform is sufficient to support colonization of the gut by enteric neurons. Finally, we identify zebrafish orthologues of genes previously identified to be expressed in the mouse ENS and demonstrate that these genes are expressed in the developing zebrafish ENS, thereby identifying useful ENS markers in this model organism. These studies reveal that the similarities between gene expression and gene function across vertebrate species is more extensive than previously appreciated, thus supporting the use of zebrafish as a general model for vertebrate ENS development and the use of zebrafish genetic screens as a way to identify candidate genes mutated in HSCR cases.

Intestinal alkaline phosphatase detoxifies lipopolysaccharide and prevents inflammation in zebrafish in response to the gut microbiota

Vertebrates harbor abundant lipopolysaccharide (LPS) in their gut microbiota. Alkaline phosphatases can dephosphorylate and detoxify the endotoxin component of LPS. Here, we show that expression of the zebrafish intestinal alkaline phosphatase (Iap), localized to the intestinal lumen brush border, is induced during establishment of the gut microbiota. Iap-deficient zebrafish are hypersensitive to LPS toxicity and exhibit the excessive intestinal neutrophil influx characteristic of wild-type zebrafish exposed to LPS. Both of these Iap mutant phenotypes are dependent on Myd88 and Tumor Necrosis Factor Receptor (Tnfr), proteins also involved in LPS sensitivity in mammals. When reared germ-free, the intestines of Iap-deficient zebrafish are devoid of neutrophils. Together, these findings demonstrate that the endogenous microbiota establish the normal homeostatic level of neutrophils in the zebrafish intestine through a process involving Iap, Myd88, and Tnfr. Thus, by preventing inflammatory responses, Iap plays a crucial role in promoting mucosal tolerance to resident gut bacteria.

Mutation of RNA Pol III subunit rpc2/polr3b Leads to Deficiency of Subunit Rpc11 and disrupts zebrafish digestive development

The role of RNA polymerase III (Pol III) in developing vertebrates has not been examined. Here, we identify a causative mutation of the second largest Pol III subunit, polr3b, that disrupts digestive organ development in zebrafish slim jim (slj) mutants. The slj mutation is a splice-site substitution that causes deletion of a conserved tract of 41 amino acids in the Polr3b protein. Structural considerations predict that the slj Pol3rb deletion might impair its interaction with Polr3k, the ortholog of an essential yeast Pol III subunit, Rpc11, which promotes RNA cleavage and Pol III recycling. We engineered Schizosaccharomyces pombe to carry an Rpc2 deletion comparable to the slj mutation and found that the Pol III recovered from this rpc2-delta yeast had markedly reduced levels of Rpc11p. Remarkably, overexpression of cDNA encoding the zebrafish rpc11 ortholog, polr3k, rescued the exocrine defects in slj mutants, indicating that the slj phenotype is due to deficiency of Rpc11. These data show that functional interactions between Pol III subunits have been conserved during eukaryotic evolution and support the utility of zebrafish as a model vertebrate for analysis of Pol III function.

Nkx2.2 regulates cell fate choice in the enteroendocrine cell lineages of the intestine

Nkx2.2 is a homeodomain-containing transcription factor essential for pancreatic islet cell specification. In this study we investigate the role of Nkx2.2 within the small intestine. We have determined that Nkx2.2 is expressed at the onset of intestinal epithelial cell differentiation in specific intestinal cell populations, including a subset of enteroendocrine cells. Similar to its role in the pancreatic islet, Nkx2.2 regulates cell fate choices within the intestinal enteroendocrine population; in the Nkx2.2 null mice, several hormone-producing enteroendocrine cell populations are absent or reduced and the ghrelin-producing cell population is upregulated. The remaining intestinal cell populations, including the paneth cells, goblet cells, and enterocytes appear to be unaffected by the loss of Nkx2.2. Furthermore, similar to the pancreatic islet, Nkx2.2 appears to function upstream of Pax6 in regulating intestinal cell fates; Pax6 mRNA and protein expression is decreased in the Nkx2.2 null mice. These studies identify a novel role for Nkx2.2 in intestinal endocrine cell development and reveal the regulatory similarities between cell type specification in the pancreatic islet and in the enteroendocrine population of the intestine.