Twists and turns in the development and maintenance of the mammalian small intestine epithelium

Loss of HCN2 leads to delayed gastrointestinal motility and reduced energy intake in mice

Hyperpolarization-activated Cyclic Nucleotide-gated (HCN) channels are important regulators of excitability in neural, cardiac, and other pacemaking cells, which are often altered in disease. In mice, loss of HCN2 leads to cardiac dysrhythmias, persistent spike-wave discharges similar to those seen in absence epilepsy, ataxia, tremor, reduced neuropathic and inflammatory pain, antidepressant-like behavior, infertility, and severely restricted growth. While many of these phenotypes have tissue-specific mechanisms, the cause of restricted growth in HCN2 knockout animals remains unknown. Here, we characterize a novel, 3kb insertion mutation of Hcn2 in the Tremor and Reduced Lifespan 2 (TRLS/2J) mouse that leads to complete loss of HCN2 protein, and we show that this mutation causes many phenotypes similar to other mice lacking HCN2 expression. We then demonstrate that while TRLS/2J mice have low blood glucose levels and impaired growth, dysfunction in hormonal secretion from the pancreas, pituitary, and thyroid are unlikely to lead to this phenotype. Instead, we find that homozygous TRLS/2J mice have abnormal gastrointestinal function that is characterized by less food consumption and delayed gastrointestinal transit as compared to wildtype mice. In summary, a novel mutation in HCN2 likely leads to impaired GI motility, causing the severe growth restriction seen in mice with mutations that eliminate HCN2 expression.

CRS-peptides: unique defense peptides of mouse Paneth cells

The intestine is the most densely colonized site in both mice and man. Recent data suggest that the intestinal flora is, in part, controlled by antimicrobial substances secreted by the intestinal epithelium. The defense system of the small intestine includes a protective mucus layer, a high turnover of epithelial cells, and a regulated secretion of effector molecules, notably antimicrobial peptides. Human and mouse small intestines share many similarities in their intestinal defense micro-organization, including the secretion of the well-known α-defensins. Mice, however, produce an additional unique antimicrobial peptide family, the CRS (cryptdin-related sequences)-peptides, not found in man. This review comprises a detailed presentation of the peptide-based defense of the gut, with specific emphasis on the CRS-peptide family. The first part presents the current knowledge of the CRS-peptide family's biochemical characteristics and nomenclature, and the second part is devoted to the possible role of this family in the homeostasis of the gut.

Altered intestinal epithelial homeostasis in mice with intestine-specific deletion of the Krüppel-like factor 4 gene

The zinc finger transcription factor, Krüppel-like factor 4 (KLF4), is expressed in the post-mitotic, differentiated epithelial cells lining the intestinal tract and exhibits a tumor suppressive effect on intestinal tumorigenesis. Here we report a role for KLF4 in maintaining homeostasis of intestinal epithelial cells. Mice with conditional ablation of the Klf4 gene from the intestinal epithelium were viable. However, both the rates of proliferation and migration of epithelial cells were increased in the small intestine of mutant mice. In addition, the brush-border alkaline phosphatase was reduced as was expression of ephrine-B1 in the small intestine, resulting in mispositioning of Paneth cells to the upper crypt region. In the colon of mutant mice, there was a reduction of the differentiation marker, carbonic anhydrase-1, and failure of differentiation of goblet cells. Mechanistically, deletion of Klf4 from the intestine resulted in activation of genes in the Wnt pathway and reduction in expression of genes encoding regulators of differentiation. Taken together, these data provide new insights into the function of KLF4 in regulating postnatal proliferation, migration, differentiation, and positioning of intestinal epithelial cells and demonstrate an essential role for KLF4 in maintaining normal intestinal epithelial homeostasis in vivo.

Conserved genetic pathways controlling the development of the diffuse endocrine system in vertebrates and Drosophila

The midgut epithelium is formed by absorptive enterocytes, secretory cells and endocrine cells. Each of these lineages is derived from the pluripotent progenitors that constitute the embryonic endoderm; the mature midgut retains pools of self-renewing stem cells that continue to produce all lineages. Recent findings in vertebrates and Drosophila shed light on the genetic mechanism that specifies the fate of the different lineages. A pivotal role is played by the Notch signaling pathway that, in a manner that appears to be very similar to the way in which Notch signaling selects neural progenitors within the neurectoderm, distinguishes the fate of secretory/endocrine cells and enterocytes. Proneural genes encoding bHLH transcription factors are expressed and required in prospective endocrine cells; activation of the Notch pathways restricts the number of these cells and promotes enterocyte development. In this review we compare the development of the intestinal endocrine cells in vertebrates and insects and summarize recent findings dealing with genetic pathways controlling this cell type.

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.

Ets transcription factors control epithelial maturation and transit and crypt-villus morphogenesis in the mammalian intestine

Members of the Ets transcription factor family are widely expressed in both the developing and mature mammalian intestine, but their biological functions remain primarily uncharacterized. We used a dominant repressor transgene approach to probe the function of epithelial Ets factors in the homeostasis of the crypt-villus unit, the functional unit of the small intestine. We show that targeted expression in small intestinal epithelium of a fusion protein composed of the Engrailed repressor domain and the Erm DNA-binding domain (En/Erm) results in marked disruption of normal crypt-villus homeostasis, including a cell-autonomous disturbance of epithelial maturation, increased epithelial transit, severe villus dysmorphogenesis, and crypt dysmorphogenesis. The epithelial maturation disturbance is independent of the regulation of TGFbetaRII levels, in contrast to Ets-mediated epithelial differentiation during development; rather, regulation of Cdx2 expression may play a role. The villus dysmorphogenesis is independent of alterations in the crypt-villus boundary and inappropriate beta-catenin activation, and thus appears to represent a new mechanism controlling villus architectural organization. An Analysis of animals mosaic for En/Erm expression suggests that crypt nonautonomous mechanisms underlie the crypt dysmorphogenesis phenotype. Our studies thus uncover novel Ets-regulated pathways of intestinal homeostasis in vivo. Interestingly, the overall En/Erm phenotype of disturbed crypt-villus homeostasis is consistent with recently identified Ets function(s) in the restriction of intestinal epithelial tumorigenesis.

Loss of Kindlin-1 causes skin atrophy and lethal neonatal intestinal epithelial dysfunction

Kindler Syndrome (KS), characterized by transient skin blistering followed by abnormal pigmentation, skin atrophy, and skin cancer, is caused by mutations in the FERMT1 gene. Although a few KS patients have been reported to also develop ulcerative colitis (UC), a causal link to the FERMT1 gene mutation is unknown. The FERMT1 gene product belongs to a family of focal adhesion proteins (Kindlin-1, -2, -3) that bind several β integrin cytoplasmic domains. Here, we show that deleting Kindlin-1 in mice gives rise to skin atrophy and an intestinal epithelial dysfunction with similarities to human UC. This intestinal dysfunction results in perinatal lethality and is triggered by defective intestinal epithelial cell integrin activation, leading to detachment of this barrier followed by a destructive inflammatory response.

Mutations in FERMT1, coding for the Kindlin-1 protein, cause Kindler Syndrome in humans, characterized by skin blistering, atrophy, and cancer. Recent reports showed that some Kindler Syndrome patients additionally suffer from ulcerative colitis. However, it is unknown whether this is caused by loss of Kindlin-1 or by unrelated abnormalities such as infections or additional mutations. We ablated the Fermt1 gene in mice to directly analyze the pathological consequences and the molecular mode of action of Kindlin-1. Kindlin-1–deficient mice develop a severe epidermal atrophy, but lack blisters. All mutant mice die shortly after birth from a dramatic, shear force-induced detachment of intestinal epithelial cells followed by a profound inflammation and organ destruction. The intestinal phenotype is very similar to, although more severe than, the one observed in Kindler Syndrome patients. In vitro studies revealed that impaired integrin activation, and thus impaired adhesion, to the extracellular matrix of the intestinal wall causes intestinal epithelial cell detachment. Therefore, we demonstrate that intestinal epithelial cells require adhesive function of integrins to resist the shear force applied by the stool. Furthermore, we provide evidence that the colitis associated with Kindler Syndrome is caused by a dysfunction of Kindlin-1 rather than by a Kindlin-1–independent event.

Regional variations in Paneth cell antimicrobial peptide expression along the mouse intestinal tract

Background

Enteric antimicrobial peptides secreted from Paneth cells, including α-defensins (in mice named cryptdins), are key effector molecules of innate immunity in the small intestine. The importance of Paneth cells α-defensins emerged from studies of enteric bacterial infection in genetically modified mice, as well as from recent studies linking reduced levels of these α-defensins to Crohn's disease localized to the ileum. However, analysis of expression of Paneth cell α-defensins is incomplete. We therefore performed a comprehensive evaluation of the distribution of antimicrobial molecules along the mouse small intestinal tract to identify potential variations in regional expression.

Results

In conventionally reared mice, the repertoire of Paneth cell antimicrobials differs between duodenum and ileum. In contrast to the uniform expression of most Paneth cell antimicrobials, both cryptdin 4 and cryptdin-related sequences (CRS) 4C peptides were expressed at progressively increasing amounts (10¹- and 10⁴-fold, respectively) comparing duodenum and ileum. In tissues other than the small intestine, expression of CRS peptides was noted in thymus and caecum. Most Paneth cell products were also produced in the small intestine of germ-free mice at levels similar to those in controls, however CRS4C and RegIIIγ had reduced levels in the former (3- and 8-fold, respectively). No significant changes in expression levels of Paneth cell antimicrobial peptides was observed after oral challenge with either Salmonella enterica serovar typhimurium or Listeria monocytogenes, supporting current notions on the constitutive nature of this defensive system.

Conclusion

The repertoire of antimicrobial peptides changes along the small intestinal tract, and a subset of these molecules are up-regulated upon colonization, but not in response to enteric bacterial pathogens. The changes detected upon colonization suggest that Paneth cell antimicrobial peptides may play an important role in commensal microbial homeostasis, in addition to their proposed role in protection against infection. In addition, the differential expression of CRS4C along the small intestine suggests mechanisms of regulation that are distinct from other Paneth cell derived antimicrobial peptides.

Early transcriptional response in the jejunum of germ-free piglets after oral infection with virulent rotavirus

Germ-free piglets were orally infected with virulent rotavirus to collect jejunal mucosal scrapings at 12 and 18 hours post infection (two piglets per time point). IFN-gamma mRNA expression was stimulated in the mucosa of all four infected piglets, indicating that they all responded to the rotavirus infection. RNA pools prepared from two infected piglets were used to compare whole mucosal gene expression at 12 and 18 hpi to expression in uninfected germ-free piglets (n = 3) using a porcine intestinal cDNA microarray. Microarray analysis identified 13 down-regulated and 17 up-regulated genes. Northern blot analysis of a selected group of genes confirmed the data of the microarray. Genes were functionally clustered in interferon-regulated genes, proliferation/differentiation genes, apoptosis genes, cytoskeleton genes, signal transduction genes, and enterocyte digestive, absorptive, and transport genes. Down-regulation of the transport gene cluster reflected in part the loss of rotavirus-infected enterocytes from the villous tips. Data mining suggested that several genes were regulated in lower- or mid-villus immature enterocytes and goblet cells, probably to support repair of the damaged epithelial cell layer at the villous tips. Furthermore, up-regulation was observed for IFN-γ induced guanylate binding protein 2, a protein that effectively inhibited VSV and EMCV replication in vitro (Arch Virol 150:1213–1220, 2005). This protein may play a role in the small intestine’s innate defense against enteric viruses like rotavirus.

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.

Establishment and characterization of a primary canine duodenal epithelial cell culture

Many mechanisms involved in the pathogenesis of chronic enteropathies or host-pathogen interactions in canine intestine have not been elucidated so far. Next to the clinical and in vivo research tools, an in vitro model of canine intestinal cell culture would be very helpful for studies at the cellular level. Therefore, the purpose of this study was to establish and characterize a primary canine duodenal epithelial cell culture. Neonatal duodenum was disrupted with trypsin-ethylenediaminetetraacetic acid (EDTA) and the mucosa scraped off and digested with collagenase and dispase. After centrifugation on a 2% sorbitol gradient, the cells were incubated at 37 degrees C in OptiMEM supplemented with Primocin, epidermal growth factor, insulin, hydrocortisone, and 10% fetal calf serum (FCS). After 24 h, the FCS concentration was reduced to 2.5%, and the temperature decreased to 33 degrees C. With this method, the cultures were growing to confluent monolayers within 5-6 d and remained viable for an average of 2 wk. Their epithelial nature was confirmed by electron microscopy and immunofluorescence staining using antibodies directed against specific cytokeratins, desmosomes, and tight junctions. The intestinal cells proliferated, as evidenced by immunolabeling with a Ki-67 antibody, and cryptal cell subpopulations could be identified. Furthermore, alkaline phosphatase and sucrase activity were detected.

Regulation of expression of 1,25D3-MARRS/ERp57/PDIA3 in rat IEC-6 cells by TGF beta and 1,25(OH)2D3

We examined the transcriptional regulation of expression of the redox-sensitive Membrane-Associated-Rapid Response, Steroid-binding (1,25D(3)-MARRS) protein specific for 1,25(OH)(2)D(3) in a rat small intestinal cell line, IEC-6, that demonstrates rapid responses to 1,25(OH)(2)D(3). 1,25D(3)-MARRS binds and is activated by 1,25(OH)(2)D(3), but is not itself up-regulated by treatment with 1,25(OH)(2)D(3), nor is there a Vitamin D response element (VDRE) in its proximal promoter. We previously reported that transforming growth factor beta (TGFbeta) increased steady state levels of 1,25D(3)-MARRS transcript and protein approximately two-fold [Rohe B, Safford SE, Nemere I, Farach-Carson, MC. Identification and characterization of 1,25D(3)-membrane-associated rapid response, steroid (1,25D(3)-MARRS)-binding protein in rat IEC-6 cells. Steroids 2005;70:458-63]. To determine if this up-regulation could be attributed to the function of a highly conserved consensus smad 3 binding element present in the proximal promoter of the 1,25D(3)-MARRS gene, we created a promoter-reporter [SEAP] construct that was responsive to TGFbeta (200 pM). Deletion or mutation of the smad 3 element greatly reduced the response of the 1,25D(3)-MARRS promoter to TGFbeta. Subsequent studies found that the smad 3 response element is bound by a protein found in the IEC-6 nuclear extract, most likely smad 3. Interestingly, although 1,25(OH)(2)D(3) alone did not increase expression of the 1,25D(3)-MARRS promoter-reporter, co-treatment of transfected IEC-6 cells with 1,25(OH)(2)D(3) and TGFbeta shifted the dose-response curve to a lower effective concentration (100 pM peptide). We conclude that TGFbeta is a transcriptional regulator of 1,25D(3)-MARRS expression via a functional smad 3 element and that cross-talk with non-classical 1,25(OH)(2)D(3)-stimulated pathways occurs. The findings have broad implications for redox-sensitive signaling phenomena including those that regulate phosphate transport in the intestine.

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Kynurenic acid as a ligand for orphan G protein-coupled receptor GPR35

Local catabolism of the essential amino acid tryptophan is considered an important mechanism in regulating immunological and neurological responses. The kynurenine pathway is the main route for the non-protein metabolism of tryptophan. The intermediates of the kynurenine pathway are present at micromolar concentrations in blood and are regulated by inflammatory stimuli. Here we show that GPR35, a previously orphan G protein-coupled receptor, functions as a receptor for the kynurenine pathway intermediate kynurenic acid. Kynurenic acid elicits calcium mobilization and inositol phosphate production in a GPR35-dependent manner in the presence of G(qi/o) chimeric G proteins. Kynurenic acid stimulates [35S]guanosine 5'-O-(3-thiotriphosphate) binding in GPR35-expressing cells, an effect abolished by pertussis toxin treatment. Kynurenic acid also induces the internalization of GPR35. Expression analysis indicates that GPR35 is predominantly detected in immune cells and the gastrointestinal tract. Furthermore, we show that kynurenic acid inhibits lipopolysaccharide-induced tumor necrosis factor-alpha secretion in peripheral blood mononuclear cells. Our results suggest unexpected signaling functions for kynurenic acid through GPR35 activation.

Evaluation of intestinal trophism: review of current methods and techniques

PURPOSE OF REVIEW

This review aims to cover the methods and techniques currently used by researchers to evaluate intestinal trophism.

RECENT FINDINGS

Although subjected to criticisms, conventional gut trophism evaluation by both morphological and histological parameters continues to be largely reported. The metaphase arrest technique and the detection of H-tymidine have been used less frequently in recent years due to increasing interest in the immunohistochemical methods such as 5-bromo-2'deoxy-uridine, Ki-67, and proliferating cell nuclear antigen. Along with common techniques for the evaluation of intestinal trophism, the combination of methods that includes a measurement of cell proliferation and an apoptotic index in the same study are most valuable and are increasing in the current literature.

SUMMARY

Evaluation of intestinal trophism is an important issue for both clinical and experimental nutritional studies. The immunohistochemical techniques are the gold standard to access cell proliferation. Evaluation of proliferation and apoptosis at the same time may offer a more reliable evaluation of the actual status of gut trophism.