Intestinal growth and differentiation in zebrafish

T-cell factor 4 (Tcf7l2) maintains proliferative compartments in zebrafish intestine

Previous studies have shown that Wnt signals, relayed through beta-catenin and T-cell factor 4 (Tcf4), are essential for the induction and maintenance of crypts in mice. We have now generated a tcf4 (tcf7l2) mutant zebrafish by reverse genetics. We first observe a phenotypic defect at 4 weeks post-fertilization (wpf), leading to death at about 6 wpf. The phenotype comprises a loss of proliferation at the base of the intestinal folds of the middle and distal parts of the intestine. The proximal intestine represents an independent compartment, as it expresses sox2 in the epithelium and barx1 in the surrounding mesenchyme, which are early stomach markers in higher vertebrates. Zebrafish are functionally stomach-less, but the proximal intestine might share its ontogeny with the mammalian stomach. Rare adult homozygous tcf4(-/-) 'escapers' show proliferation defects in the gut epithelium, but have no other obvious abnormalities. This study underscores the involvement of Tcf4 in maintaining proliferative self-renewal in the intestine throughout life.

Spatiotemporal expression of smooth muscle markers in developing zebrafish gut

Smooth muscle is important for the contractility and elasticity of visceral organs. The zebrafish is an excellent model for understanding embryonic development, yet due to a lack of appropriate markers, visceral smooth muscle development remains poorly characterized. Here, we develop markers and trace the development of gut and swim bladder smooth muscle in embryonic and juvenile fish. The first smooth muscle marker we detect in the vicinity of the gut is the myoblast marker nonmuscle myosin heavy chain-b at 50 hours postfertilization (hpf), followed by the early smooth muscle markers SM22alpha-b, and alpha-smooth muscle actin at 56 and 60 hpf, respectively. Markers of more differentiated smooth muscle, smoothelin-b and cpi-17, appear by 3 days postfertilization (dpf). Tropomyosin, a relatively late marker, is first expressed at 4 dpf. We find that smooth muscle marker expression in the swim bladder follows the same sequence of marker expression as the gut, but markers have a temporal delay reflecting the later formation of swim bladder smooth muscle.

Kit-like immunoreactivity in the zebrafish gastrointestinal tract reveals putative ICC

Gastrointestinal (GI) motility results from the coordinated actions of enteric neurons, interstitial cells of Cajal (ICC), and smooth muscle cells. The GI tract of the zebrafish has a cellular anatomy that is essentially similar to humans. Although enteric nerves and smooth muscle cells have been described, it is unknown if ICC are present in the zebrafish. Immunohistochemistry and PCR were used determine expression for the zebrafish Kit orthologue in the zebrafish gastrointestinal tract. Cells displaying Kit-like immunoreactivity were identified in the muscular layers of the adult zebrafish gastrointestinal tract. Two layers of Kit-positive cells were identified, one with multipolar cells located between the longitudinal and circular smooth muscle layers and one with simple bipolar cells located deep in the circular muscle layer. Primers specifically designed to amplify mRNA coding for two zebrafish kit genes, kita and kitb, and two kit ligands, kitla and kitlb, amplified the expected transcript from total RNA isolated from zebrafish GI tissues. The Sparse mutant, a kita null mutant, showed reduced contraction frequency and increased size of the GI tract indicating a functional role for kita. These data establish the presence of a cellular network with Kit-like immunoreactivity in the myenteric plexus region of the zebrafish GI tract, adjacent to enteric neurons. Expression of kita and kitb, and the ligands kitla and kitlb, were verified in the adult GI tract. The anatomical arrangement of the Kit-positive cells strongly suggests that they are ICC.

Genetic screen for mutations affecting development and function of the enteric nervous system

An intact enteric nervous system is required for normal gastrointestinal tract function. Several human conditions result from decreased innervation by enteric neurons; however, the genetic basis of enteric nervous system development and function is incompletely understood. In an effort to increase our understanding of the mechanisms underlying enteric nervous system development, we screened mutagenized zebrafish for changes in the number or distribution of enteric neurons. We also established a motility assay and rescreened mutants to learn whether enteric neuron number is correlated with gastrointestinal motility in zebrafish. We describe mutations isolated in our screen that affect enteric neurons specifically, as well as mutations that affect other neural crest derivatives or have pleiotropic effects. We show a correlation between the severity of enteric neuron loss and gastrointestinal motility defects. This screen provides biological tools that serve as the basis for future mechanistic studies.

In vivo imaging and genetic analysis link bacterial motility and symbiosis in the zebrafish gut

Complex microbial communities reside within the intestines of humans and other vertebrates. Remarkably little is known about how these microbial consortia are established in various locations within the gut, how members of these consortia behave within their dynamic ecosystems, or what microbial factors mediate mutually beneficial host-microbial interactions. Using a gnotobiotic zebrafish-Pseudomonas aeruginosa model, we show that the transparency of this vertebrate species, coupled with methods for raising these animals under germ-free conditions can be used to monitor microbial movement and localization within the intestine in vivo and in real time. Germ-free zebrafish colonized with isogenic P. aeruginosa strains containing deletions of genes related to motility and pathogenesis revealed that loss of flagellar function results in attenuation of evolutionarily conserved host innate immune responses but not conserved nutrient responses. These results demonstrate the utility of gnotobiotic zebrafish in defining the behavior and localization of bacteria within the living vertebrate gut, identifying bacterial genes that affect these processes, and assessing the impact of these genes on host-microbial interactions.

TTX-sensitive and TTX-insensitive control of spontaneous gut motility in the developing zebrafish (Danio rerio) larvae

Spontaneous regular gut motility in zebrafish begins around 4 days post fertilisation (d.p.f.) and is modulated by release of acetylcholine and nitric oxide. The role of intrinsic or extrinsic innervation for initiating and propagating the spontaneous contractions, however, is not well understood. By creating spatiotemporal maps, we could examine spontaneous motility patterns in zebrafish larvae in vivo at 4 and 7 d.p.f. in more detail. Tetrodotoxin (TTX) was added to elucidate the importance of nervous control. Anterograde and retrograde contraction waves originated in the same region,just posterior to the intestinal bulb. This area correlates well with the distribution of Hu (human neuronal protein C/D)-immunoreactive nerve cell bodies. Whereas numerous immunoreactive nerve cells were present in the mid and distal intestine at both 4 and 7 d.p.f., fewer cells were seen anterior to the origin of contractions. The overall frequency of contractions(1.16±0.15 cycles min–1, N=14 at 4 d.p.f.;1.05±0.09 cycles min–1, N=13 at 7 d.p.f.) and the interval between individual anterograde contraction waves (54.8±7.9 s at 4 d.p.f., N=14; 56.9±4.4 s, N=13 at 7 d.p.f.)did not differ between the two stages but the properties of the contractions were altered. The distance travelled by each wave increased from 591.0±43.8 μm at 4 d.p.f. (N=14) to 719.9±33.2 μm at 7 d.p.f. (N=13). By contrast, the velocity decreased from 4 d.p.f.(49.5±5.5 μm s–1, N=12) to 7 d.p.f.(27.8±3.6 μm s–1, N=13). At 4 d.p.f., TTX did not affect any of the parameters whereas at 7 d.p.f. anterograde frequency(control 1.07±0.12 cycles min–1, N=8; TTX 0.55±0.13 cycles min–1, N=8) and distance travelled (control 685.1±45.9 μm, N=8; TTX 318.7±88.7 μm, N=6) were decreased. In conclusion, enteric or extrinsic innervation does not seem to be necessary to initiate spontaneous contractions of the gut in zebrafish larvae. However, later in development,nerves have an increasingly important role as modulators of intestinal activity.)

We know you are in there: conversing with the indigenous gut microbiota

The vertebrate gut harbors a coevolved consortium of microbes that plays critical roles in the development and health of this organ. Here we discuss recent insights into the microbial-host molecular dialogs that shape the digestive tracts of the model vertebrates, mice and zebrafish, and consider the parallels between vertebrate-microbial mutualisms and the well-studied squid-Vibrio symbiosis.

Target of rapamycin (TOR) signaling controls epithelial morphogenesis in the vertebrate intestine

The target of rapamycin (TOR) signaling pathway regulates cell growth and proliferation, however the extent to which TOR signaling mediates particular organogenesis programs remains to be determined. Here we report an examination of TOR signaling during zebrafish development, using a combination of small molecule treatment and morpholino-mediated gene knockdown. First, we amplified and sequenced the full-length cDNA for the zebrafish TOR ortholog (ztor). By in situ hybridization, we found that ztor is expressed ubiquitously in the early embryo, but displays a dynamic pattern in the gut between 48 and 72 h post-fertilization (hpf). Treatment of zebrafish embryos with rapamycin induced only a mild general developmental delay up to 72 hpf, but digestive tract development became arrested at the primitive gut tube stage. Rapamycin inhibited intestinal epithelial growth, morphogenesis and differentiation. Using morpholino-mediated gene knockdown of TOR pathway components, we show that this effect is mediated specifically by the rapamycin-sensitive TOR complex 1 (TORC1). Thus, in addition to regulating cell growth and proliferation, TOR signaling controls the developmental program guiding epithelial morphogenesis in the vertebrate intestine.

The effects of endogenous and exogenous nitric oxide on gut motility in zebrafish Danio rerio embryos and larvae

Using motion analysis, the ontogeny of the nitrergic control system in the gut was studied in vivo in zebrafish Danio rerio embryos and larvae. For the first time we show the presence of a nitrergic tonus,modulating both anterograde and retrograde contraction waves in the intestine of developing zebrafish. At 4 d.p.f. (days post fertilisation), the nitric oxide synthase (NOS) inhibitor l-NAME (three boluses of 50–100 nl, 10–3 mol l–1) increased the anterograde contraction wave frequency by 0.50±0.10 cycles min–1. Subsequent application of the NO donor sodium nitroprusside (SNP; three boluses of 50–100 nl, 10–4mol l–1) reduced the frequency of propagating anterograde waves (–0.71±0.20 cycles min–1). This coincided with the first appearance of an excitatory cholinergic tonus, observed in an earlier study. One day later, at 5 d.p.f., in addition to the effect on anterograde contraction waves, application of l-NAME increased(0.39±0.15 cycles min–1) and following SNP application reduced (–1.61±0.36 cycles min–1) the retrograde contraction wave frequency. In contrast, at 3 d.p.f., when no spontaneous motility is observed, application of l-NAME did not induce contraction waves in either part of the gut, indicating the lack of a functional inhibitory tonus at this early stage. Gut neurons expressing NOS-like immunoreactivity were present in the distal and middle intestine as early as 2 d.p.f., and at 1 day later in the proximal intestine. In conclusion, the present study suggests that a nitrergic inhibitory tonus develops shortly before or at the time for onset of exogenous feeding.

Distinct signals from the microbiota promote different aspects of zebrafish gut differentiation

All animals exist in intimate associations with microorganisms that play important roles in the hosts' normal development and tissue physiology. In vertebrates, the most populous and complex community of microbes resides in the digestive tract. Here, we describe the establishment of the gut microbiota and its role in digestive tract differentiation in the zebrafish model vertebrate, Danio rerio. We find that in the absence of the microbiota, the gut epithelium is arrested in aspects of its differentiation, as revealed by the lack of brush border intestinal alkaline phosphatase activity, the maintenance of immature patterns of glycan expression and a paucity of goblet and enteroendocrine cells. In addition, germ-free intestines fail to take up protein macromolecules in the distal intestine and exhibit faster motility. Reintroduction of a complex microbiota at later stages of development or mono-association of germ-free larvae with individual constituents of the microbiota reverses all of these germ-free phenotypes. Exposure of germ-free zebrafish to heat-killed preparations of the microbiota or bacterial lipopolysaccharide is sufficient to restore alkaline phosphatase activity but not mature patterns of Gal alpha1,3Gal containing glycans, indicating that the host perceives and responds to its associated microbiota by at least two distinct pathways.

Organizing cell renewal in the intestine: stem cells, signals and combinatorial control

The lining of the intestine is renewed at an extraordinary rate, outpacing all other tissues in the vertebrate body. The renewal process is neatly organized in space, so that the whole production line, from the ever-youthful stem cells to their dying, terminally differentiated progeny, is laid out to view in histological sections. A flurry of recent papers has clarified the key regulatory signals and brought us to the point where we can begin to give a coherent account, for at least one tissue, of how these signals collaborate to organize the architecture and behaviour of a stem-cell system.

Adenomatous polyposis coli-deficient zebrafish are susceptible to digestive tract neoplasia

Truncation of the tumour suppressor adenomatous polyposis coli (APC) constitutively activates the Wnt/beta-catenin signalling pathway. This event constitutes the primary transforming event in sporadic colorectal cancer in humans. Moreover, humans or mice carrying germline truncating mutations in APC develop large numbers of intestinal adenomas. Here, we report that zebrafish that are heterozygous for a truncating APC mutation spontaneously develop intestinal, hepatic and pancreatic neoplasias that are highly proliferative, accumulate beta-catenin and express Wnt target genes. Treatment with the chemical carcinogen 7,12-dimethylbenz[a]anthracene accelerates the induction of these lesions. These observations establish apc-mutant zebrafish as a bona fide model for the study of digestive tract cancer.

Loss of function of def selectively up-regulates Delta113p53 expression to arrest expansion growth of digestive organs in zebrafish

Transcription factor p53 forms a network with associated factors to regulate the cell cycle and apoptosis in response to environmental stresses. However, there is currently no direct genetic evidence to show if or how the p53 pathway functions during organogenesis. Here we present evidence to show that the zebrafish def (digestive-organ expansion factor) gene encodes a novel pan-endoderm-specific factor. A loss-of-function mutation in def confers hypoplastic digestive organs and selectively up-regulates the expression of Delta113p53, counterpart to a newly identified isoform of p53 produced by an alternative internal promoter in intron 4 of the p53 gene in human. The increased Delta113p53 expression is limited to within the mutant digestive organs, and this increase selectively induces the expression of p53-responsive genes to trigger the arrest of the cell cycle but not apoptosis, resulting in compromised organ growth in the mutant. Our data demonstrate that, while induction of expression of p53 and/or its isoforms is crucial to suppress abnormal cell growth, Delta113p53 is tightly regulated by an organ/tissue-specific factor Def, especially during organogenesis, to prevent adverse inhibition of organ/tissue growth.

Exocrine pancreas development in zebrafish

Although many of the genes that regulate development of the endocrine pancreas have been identified, comparatively little is known about how the exocrine pancreas forms. Previous studies have shown that exocrine pancreas development may be modeled in zebrafish. However, the timing and mechanism of acinar and ductal differentiation and morphogenesis have not been described. Here, we characterize zebrafish exocrine pancreas development in wild type and mutant larvae using histological, immunohistochemical and ultrastructural analyses. These data allow us to identify two stages of zebrafish exocrine development. During the first stage, the exocrine anlage forms from rostral endodermal cells. During the second stage, proto-differentiated progenitor cells undergo terminal differentiation followed by acinar gland and duct morphogenesis. Immunohistochemical analyses support a model in which the intrapancreatic ductal system develops from progenitors that join to form a contiguous network rather than by branching morphogenesis of the pancreatic epithelium, as described for mammals. Contemporaneous appearance of acinar glands and ducts in developing larvae and their disruption in pancreatic mutants suggest that common molecular pathways may regulate gland and duct morphogenesis and differentiation of their constituent cells. By contrast, analyses of mind bomb mutants and jagged morpholino-injected larvae suggest that Notch signaling principally regulates ductal differentiation of bipotential exocrine progenitors.

Formation of the digestive system in zebrafish: III. Intestinal epithelium morphogenesis

Recent analysis of a novel strain of transgenic zebrafish (gutGFP) has provided a detailed description of the early morphological events that occur during the development of the liver and pancreas. In this paper, we aim to complement these studies by providing an analysis of the morphological events that shape the zebrafish intestinal epithelium. One of our goals is to provide a framework for the future characterization of zebrafish mutant phenotypes in which intestinal epithelial morphogenesis has been disrupted. Our analysis encompasses the period between 26 and 126 h post-fertilization (hpf) and follows the growth, lumen formation and differentiation of a continuous layer of endoderm into a functional intestinal epithelium with three morphologically distinct segments: the intestinal bulb, mid-intestine and posterior intestine. Between 26 hpf and 76 hpf, the entire intestinal endoderm is a highly proliferative organ. To make a lumen, the zebrafish endoderm cells undergo apical membrane biogenesis, adopt a bilayer configuration and form small cavities that coalesce without cell death. Thereafter, the endoderm cells polarize and differentiate into distinct cell lineages. Enteroendocrine cells are distinguished first at 52 hpf in the caudal region of the intestine in a new stable transgenic line, Tg[nkx2.2a:mEGFP]. The differentiation of mucin-containing goblet cells is first evident at 100 hpf and is tightly restricted to a middle segment of the intestine, designated the mid-intestine, that is also demarcated by the presence of enterocytes with large supranuclear vacuoles. Meanwhile, striking expansion of the lumen in the rostral intestine forms the intestinal bulb. Here the epithelium elaborates folds and proliferating cells become progressively restricted to a basal compartment analogous to the crypts of Lieberkühn in mammals. At 126 hpf, the posterior intestine remains an unfolded monolayer of simple columnar epithelium.

Mutation of Smooth Muscle Myosin Causes Epithelial Invasion and Cystic Expansion of the Zebrafish Intestine

Zebrafish meltdown (mlt) mutants develop cystic expansion of the posterior intestine as a result of stromal invasion of nontransformed epithelial cells. Positional cloning identified zebrafish smooth muscle myosin heavy chain (myh11) as the responsible gene. The mlt mutation constitutively activates the Myh11 ATPase, which disrupts smooth muscle cells surrounding the posterior intestine. Adjacent epithelial cells ectopically express metalloproteinases, integrins, and other genes implicated in human cancer cell invasion. Knockdown and pharmacological inhibition of these genes restores intestinal structure in mlt mutants despite persistent smooth muscle defects. These data identify an essential role for smooth muscle signaling in the maintenance of epithelial architecture and support gene expression analyses and other studies that identify a role for stromal genes in cancer cell invasion. Furthermore, they suggest that high-throughput screens to identify regulators of cancer cell invasion may be feasible in zebrafish.

Delta-Notch signalling controls commitment to a secretory fate in the zebrafish intestine

The transparency of the juvenile zebrafish and its genetic advantages make it an attractive model for study of cell turnover in the gut. BrdU labelling shows that the gut epithelium is renewed in essentially the same way as in mammals: the villi are lined with non-dividing differentiated cells, while cell division is confined to the intervillus pockets. New cells produced in the pockets take about 4 days to migrate out to the tips of the villi, where they die. We have generated monoclonal antibodies to identify the absorptive and secretory cells in the epithelium, and we have used these antibodies to examine the part that Delta-Notch signalling plays in producing the diversity of intestinal cell types. Several Notch receptors and ligands are expressed in the gut. In particular, the Notch ligand DeltaD (Delta1 in the mouse) is expressed in cells of the secretory lineage. In an aei mutant, where DeltaD is defective, secretory cells are overproduced. In mind bomb (mib), where all Delta-Notch signalling is believed to be blocked, almost all the cells in the 3-day gut epithelium adopt a secretory character. Thus, secretory differentiation appears to be the default in the absence of Notch activation, and lateral inhibition mediated by Delta-Notch signalling is required to generate a balanced mixture of absorptive and secretory cells. These findings demonstrate the central role of Notch signalling in the gut stem-cell system and establish the zebrafish as a model for study of the mechanisms controlling renewal of gut epithelium.