Chimeric-transgenic mice represent a powerful tool for studying how the proliferation and differentiation programs of intestinal epithelial cell lineages are regulated

Exploring the villus

The small intestinal villus and its associated epithelium includes enterocytes as the main cell type and differentiated goblet and argentaffin cells, while the invaginated crypt epithelium is the site of cell division and hence the origin of all epithelial components. Enterocytes form a cohesive monolayer which acts both as a permeability barrier between lumen and the interior, and an important gateway for nutrient digestion, absorption and transport. Differentiation and polarisation of enterocytes depends on cytoskeletal proteins that control cell shape and maintain functionally specialised membrane domains; extracellular matrix (ECM) receptors; channels and transporters regulating ion/solute transfer across the cell. The mesenchymally-derived basement membrane dynamically controls morphogenesis, cell differentiation and polarity, while also providing the structural basis for villi, crypts and the microvasculature of the lamina propria so that tissue morphology, crucially, is preserved in the absence of epithelium. Mucosal re-organisation requires immense cooperation between all elements within the lamina, including marked revisions of the microvasculature and extensive alterations to all basement membranes providing support for endodermal and mesenchymal components. In this context, subepithelial myofibroblasts fulfil important regulatory activities in terms of tissue morphogenesis; remodelling; control of epithelial cell development, polarity and functional attributes; and an intimate involvement in repair, inflammation and fibrosis. This paper reviews the main structural and functional aspects of the villus, including the epithelium and its outer glycocalyx and microvillous border; and subjacent to the epithelium, the basement membrane with its attached web of myo-fibroblasts together with the lamina propria core of the villi, and its microvasculature and lacteals. Finally, some comments on the rapidity with which the overall structure of the villi changes in their response to both external, and internal, influences.

Stem cell dynamics in homeostasis and cancer of the intestine

Intestinal stem cells (ISCs) and colorectal cancer (CRC) biology are tightly linked in many aspects. It is generally thought that ISCs are the cells of origin for a large proportion of CRCs and crucial ISC-associated signalling pathways are often affected in CRCs. Moreover, CRCs are thought to retain a cellular hierarchy that is reminiscent of the intestinal epithelium. Recent studies offer quantitative insights into the dynamics of ISC behaviour that govern homeostasis and thereby provide the necessary baseline parameters to begin to apply these analyses during the various stages of tumour development.

MyD88 adaptor-like (Mal) regulates intestinal homeostasis and colitis-associated colorectal cancer in mice

Toll-like receptors (TLRs) play a central role in the recognition and response to microbial pathogens and in the maintenance and function of the epithelial barrier integrity in the gut. The protein MyD88 adaptor-like (Mal/TIRAP) serves as a bridge between TLR2/TLR4- and MyD88-mediated signaling to orchestrate downstream inflammatory responses. Whereas MyD88 has an essential function in the maintenance of intestinal homeostasis, a role for Mal in this context is less well described. Colitis was induced in wild-type (WT) and Mal-deficient (Mal(-/-)) mice by administration of dextran sodium sulfate (DSS). Colitis-associated cancer was induced by DSS and azoxymethane (AOM) treatment. Chimeric mice were generated by total body gamma irradiation followed by transplantation of bone marrow cells. In the DSS model of colon epithelial injury, Mal(-/-) mice developed increased inflammation and severity of colitis relative to WT mice. Mal(-/-) mice demonstrated the presence of inflammatory cell infiltrates, increased crypt proliferation, and presence of neoformations. Furthermore, in the AOM/DSS model, Mal(-/-) mice had greater incidence of tumors. Mal(-/-) and WT bone marrow chimeras demonstrated that nonhematopoietic cell expression of Mal had an important protective role in the control of intestinal inflammation and inflammation-associated cancer. Mal is essential for the maintenance of intestinal homeostasis and expression of Mal in nonhematopoietic cells prevents chronic intestinal inflammation that may predispose to colon neoplasia.

Adult intestinal stem cells: critical drivers of epithelial homeostasis and regeneration

Small populations of adult stem cells are responsible for the remarkable ability of the epithelial lining of the intestine to be efficiently renewed and repaired throughout life. The recent discovery of specific markers for these stem cells, together with the development of new technologies to track endogenous stem cell activity in vivo and to exploit their ability to generate new epithelia ex vivo, has greatly improved our understanding of stem cell-driven homeostasis, regeneration and cancer in the intestine. These exciting new insights into the biology of intestinal stem cells have the potential to accelerate the development of stem cell-based therapies and ameliorate cancer treatments.

Taking inflammatory bowel disease up a Notch

The epithelium of the gastrointestinal tract, which represents the largest surface area of the body, is constantly exposed to the contents of its surrounding environment. The intestinal epithelium forms barriers that are essential in maintaining equilibrium within the human body. This barrier supports nutrient and water transport while preventing microbial invasion. Intestinal epithelial cells (IECs) sit at the interface between an antigen-rich lumen and a lymphocyte-rich lamina propria (LP). IECs have the capability to discriminate between "peaceful" and "harmful" antigens. The epithelium is constantly sampling luminal contents and making molecular adjustments accordingly. These molecular changes influence the actions of innate and adaptive immune cells. The crosstalk that occurs between the epithelium and the immune compartments serves to maintain intestinal homeostasis. A better understanding of the nature of the interactions between normal LP lymphocytes (LPLs) and IECs will ultimately provide insights into the defects occurring in inflammatory bowel disease patients.

Establishment of intestinal homeostasis during the neonatal period

The intestinal mucosa faces the challenge of regulating the balance between immune tolerance towards commensal bacteria, environmental stimuli and food antigens on the one hand, and induction of efficient immune responses against invading pathogens on the other hand. This regulatory task is of critical importance to prevent inappropriate immune activation that may otherwise lead to chronic inflammation, tissue disruption and organ dysfunction. The most striking example for the efficacy of the adaptive nature of the intestinal mucosa is birth. Whereas the body surfaces are protected from environmental and microbial exposure during fetal life, bacterial colonization and contact with potent immunostimulatory substances start immediately after birth. In the present review, we summarize the current knowledge on the mechanisms underlying the transition of the intestinal mucosa during the neonatal period leading to the establishment of a stable, life-long host-microbial homeostasis. The environmental exposure and microbial colonization during the neonatal period, and also the influence of maternal milk on the immune protection of the mucosa and the role of antimicrobial peptides, are described. We further highlight the molecular mechanisms of innate immune tolerance in neonatal intestinal epithelium. Finally, we link the described immunoregulatory mechanisms to the increased susceptibility to inflammatory and infectious diseases during the neonatal period.

Current molecular markers for gastric progenitor cells and gastric cancer stem cells

Gastric stem and progenitor cells (GPC) play key roles in the homeostatic renewal of gastric glands and are instrumental in epithelial repair after injury. Until very recently, the existence of GPC could only be inferred by indirect labeling strategies. The last few years have seen significant progress in the identification of biomarkers that allow prospective identification of GPC. The analysis of these unique cell populations is providing new insights into the molecular underpinnings of gastric epithelial homeostasis and repair. Of closely related interest is the potential to identify so-called cancer stem cells, a rare subpopulation of tumor-initiating cells. Here, we review the current useful biomarkers for GPC, including: (a) those that have been demonstrated by lineage tracing to give rise to all gastric cell lineages (e.g., the villin-transgene marker as well as Lgr5); (b) those that give rise to a subset of gastric lineages (e.g., TFF2); (c) markers that recognize cryptic progenitors for metaplasia (e.g., MIST1), and (d) markers that have not yet been analyzed by lineage tracing (e.g., DCKL1/DCAMKL1, CD133/PROM1, and CD44). The study of these markers has been mostly limited to the mouse model, but the hope is that the rapid pace of recent breakthroughs in this animal model will soon lead to a greater understanding of human gastric stem cell biology and to new insights into gastric cancer, the second leading cause of cancer-related death worldwide.

Tissue-resident adult stem cell populations of rapidly self-renewing organs

The epithelial lining of the intestine, stomach, and skin is continuously exposed to environmental assault, imposing a requirement for regular self-renewal. Resident adult stem cell populations drive this renewal, and much effort has been invested in revealing their identity. Reliable adult stem cell biomarkers would accelerate our understanding of stem cell roles in tissue homeostasis and cancer. Membrane-expressed markers would also facilitate isolation of these adult stem cell populations for exploitation of their regenerative potential. Here, we review recent advances in adult stem cell biology, highlighting the promise and pitfalls of the candidate biomarkers of the various stem cell populations.

Notch-1 signaling regulates intestinal epithelial barrier function, through interaction with CD4+ T cells, in mice and humans

BACKGROUND & AIMS

Interactions between lymphocytes and intestinal epithelial cells occur in the subepithelial space of the gastrointestinal tract. Normal human lamina propria lymphocytes (LPLs) induce differentiation of intestinal epithelial cells. The absence of LPLs in mice, such as in RAG1(-/-) mice, results in defects in epithelial cell differentiation. We investigated the role of lymphoepithelial interactions in epithelial differentiation and barrier function.

METHODS

We used adoptive transfer to determine if CD4(+) T cells (CD4(+)CD62L(+)CD45Rb(Hi) and/or CD4(+)CD62L(+)CD45Rb(Lo)) could overcome permeability defect (quantified in Ussing chambers). Immunofluorescence staining was performed to determine expression of cleaved Notch-1, villin, and claudin 5 in colon samples from mice and humans. Caco-2 cells were infected with a lentivirus containing a specific Notch-1 or scrambled short hairpin RNA sequence. Tight junction assembly was analyzed by immunoblot and immunofluorescence analyses, and transepithelial resistance was monitored.

RESULTS

Expression of cleaved Notch-1, villin, or claudin 5 was not detected in RAG1(-/-) colonocytes; their loss correlated with increased intestinal permeability. Transfer of CD45Rb(Hi) and/or CD45Rb(Lo) cells into RAG1(-/-) mice induced expression of cleaved Notch, villin, and claudin 5 in colonocytes and significantly reduced the permeability of the distal colon. Loss of Notch-1 expression in Caco-2 cells correlated with decreased transepithelial resistance and dysregulated expression and localization of tight junction proteins. Levels of cleaved Notch-1 were increased in colonic epithelium of patients with Crohn's disease.

CONCLUSIONS

LPLs promote mucosal barrier function, which is associated with activation of the Notch-1 signaling pathway. LPLs maintain intestinal homeostasis by inducing intestinal epithelial cell differentiation, polarization, and barrier function.

Morphological and functional changes in the colon after massive small bowel resection

PURPOSE

Anecdotal evidence suggests that the colon plays an important role after small bowel resection (SBR). However, colonic changes have not previously been studied. The aim of this study was to characterize morphological and functional changes within the colon after SBR and elucidate the influence of diet complexity on adaptation.

METHOD

In study 1, 4-week-old piglets underwent a 75% SBR or sham operation and were studied at 2, 4, and 6 weeks postoperation to allow analysis of early and late adaptation responses. Piglets received a polymeric infant formula (PIF). In study 2, SBR piglets received an elemental diet and were studied at 6 weeks postoperation and compared with SBR + PIF piglets from study 1. For both studies, immunohistochemistry was used to quantitate intestinal cell types. Changes in functional proteins were measured by Western blot, enteroendocrine/peptide YY (PYY), enterocyte/liver fatty acid binding protein (L-FABP), and goblet cells/trefoil factor 3 (TFF3).

RESULTS

In study 1, early and late adaptation-related changes were observed after SBR. Early adaptation included increased numbers of enterocytes (P = .0001), whereas late adaptation included increased proliferative cell numbers (P = .02). Enteroendocrine, goblet, and apoptotic cells numbers were significantly elevated in the resected group at all time-points studied (P < .05). Functional changes included increased levels of L-FABP (P = .04) and PYY (P = .03). There was no change in TFF3 expression. In study 2, feeding with an elemental diet resulted in suboptimal adaptation as evidenced by reduced rate of weight gain and significant reductions in total cell numbers (P = .0001), proliferative (P = .0001) and apoptotic cells (P = .04), enteroendocrine cells (P = .001), and PYY expression (P .004).

CONCLUSION

These findings indicate that significant morphological and functional changes occur in the colon after massive SBR and that these occur as early and late adaptation responses. Elemental diet was associated with suboptimal adaptation suggesting an effect of diet complexity on colonic adaptation.

Cell Lineage metastability in Gfi1-deficient mouse intestinal epithelium

Elucidating the mechanisms determining multipotent progenitor cell fate remains a fundamental project of contemporary biology. Various tissues of mice and men with defects in the zinc-finger transcriptional repressor Gfi1 have dramatic perturbations in the proportions of their differentiated cell types. In Gfi1-deficient intestinal epithelium there is a shift from mucous and Paneth towards enteroendocrine cells, leading to the proposal that Gfi1 functions in the allocation of the progeny derived from a hypothetical common granulocytic progenitor. However, studies of clones have yielded no evidence of such a common progenitor prompting us to investigate alternate mechanisms explaining the Gfi1-deficient phenotype. We report that mucous and Paneth but not enteroendocrine lineage cells normally express Gfi1. Sporadic mucous and Paneth lineage cells in the crypts of Gfi1-deficient mice aberrantly express the pro-enteroendocrine transcription factor Neurog3, indicating that stable repression of Neurog3 in these lineages requires Gfi1. Importantly, we also find mucous and Paneth lineage cells in various stages of cellular reprogramming into the enteroendocrine lineage in Gfi1-deficient mice. We propose that mucous and Paneth cell lineage metastability, rather than reallocation at the level of a hypothetical common granulocytic progenitor, is responsible for the shifts in cell type proportions observed in Gfi1-deficient intestinal epithelium.

Functional morphology of the gastrointestinal tract

The primary function of the gastrointestinal tract is water, electrolyte, and nutrient transport. To perform this function, the epithelium lining the gastrointestinal tract is in close contact with the gastrointestinal lumen. Because the lumen is connected to the external environment and, depending on the site, has a high bacterial and antigen load, the epithelium must also prevent pathogenic agents within the gastrointestinal lumen from gaining access to internal tissues. This creates a unique challenge for the gastrointestinal tract to balance the requirements of forming a barrier to separate the intestinal lumen from underlying tissue while simultaneously setting up a system for moving water, electrolytes, and nutrients across the barrier. In the face of this, the epithelial cells of the gastrointestinal tract form a selectively permeable barrier that is tightly regulated. In addition, the intestinal mucosa actively participates in host defense by engaging the mucosal immune system. Complex tissue organization and diverse cellular composition are necessary to achieve such a broad range of functions. In this chapter, the structure and function of the gastrointestinal tract and their relevance to infectious diseases are discussed.

Good fences make good neighbors: Gastrointestinal mucosal structure

The gastrointestinal lumen is home to over 400 species of microorganisms. The composition of this microbial community varies along the length of the gastrointestinal tract as a function of regional epithelial secretory activity as well as diet and other defined and undefined determinants. Improved understanding of the factors that impact luminal microbial populations and development of means to modulate gut microbes for therapeutic benefit hold great promise. The gastrointestinal epithelium, which regulates interactions between microbes and the mammalian host, is the topic of this review.

Gastrointestinal stem cells in development and cancer

An enormous body of knowledge about the biology of stem cells and their role in development, tissue homeostasis and cancer formation has been gained in the last 20 years. This review gives a comprehensive overview on knowledge about localization and regulation of normal gastrointestinal stem cells and links it to our understanding of gastrointestinal tumourigenesis and malignant progression in the light of the cancer stem cell concept. The focus is on intestinal stem cells and newly identified stem cell factors, such as the beta-catenin target gene Lgr5. The basis of intestinal stem cell regulation is a permanent crosstalk between epithelial and underlying mesenchymal cells in the intestinal stem cell niche. This crosstalk is mediated by crucial pathways, including the Wnt, Hedgehog (HH), Notch, PI3K and BMP pathways. Disturbances in this fine-regulated interaction can both initiate intestinal tumours and, in association with additional genetic alterations or environmental activation of embryonic processes such as epithelial-mesenchymal transition (EMT), lead to tumour invasion and metastasis.

Reconstruction of cell lineage trees in mice

The cell lineage tree of a multicellular organism represents its history of cell divisions from the very first cell, the zygote. A new method for high-resolution reconstruction of parts of such cell lineage trees was recently developed based on phylogenetic analysis of somatic mutations accumulated during normal development of an organism. In this study we apply this method in mice to reconstruct the lineage trees of distinct cell types. We address for the first time basic questions in developmental biology of higher organisms, namely what is the correlation between the lineage relation among cells and their (1) function, (2) physical proximity and (3) anatomical proximity. We analyzed B-cells, kidney-, mesenchymal- and hematopoietic-stem cells, as well as satellite cells, which are adult skeletal muscle stem cells isolated from their niche on the muscle fibers (myofibers) from various skeletal muscles. Our results demonstrate that all analyzed cell types are intermingled in the lineage tree, indicating that none of these cell types are single exclusive clones. We also show a significant correlation between the physical proximity of satellite cells within muscles and their lineage. Furthermore, we show that satellite cells obtained from a single myofiber are significantly clustered in the lineage tree, reflecting their common developmental origin. Lineage analysis based on somatic mutations enables performing high resolution reconstruction of lineage trees in mice and humans, which can provide fundamental insights to many aspects of their development and tissue maintenance.

Postnatal changes in the expression of genes for cryptdins 1-6 and the role of luminal bacteria in cryptdin gene expression in mouse small intestine

Although there have been many fascinating studies on cryptdins, the information for each cryptdin isoform was not completely provided. In this study, the postnatal changes in the gene expression of cryptdin 1-6 were evaluated, and the patterns of change were compared between conventional and germ-free mice. Two patterns of postnatal change were observed: gene expression of cryptdins 1, 3 and 6 increased gradually, and that of cryptdins 2 and 5 increased rapidly. Gene expression of cryptdin 4 increased gradually in the ileum but rapidly in the jejunum. Conventional mice showed significantly higher gene expression for all isoforms than germ-free mice. Interestingly, the difference in the gene expression for cryptdin 2, 4 and 5 between the jejunum and ileum seemed to be increased by the presence of the luminal bacteria. The results indicate that cryptdin isoforms develop differently depending on the isoform type, and that the gene expression of all cryptdin isoforms was affected by the presence of the luminal bacteria.

Current view: intestinal stem cells and signaling

Studies using mice have yielded significant amounts of information regarding signaling pathways, such as Wnt, bone morphogenic protein, PtdIns(3,4,5) kinase, and Notch, involved in intestinal development and homeostasis, including stem cell regulation and lineage specification and maturation. However, attempts to model signals definitively that control intestinal stem cells have been difficult because of a long-standing and recently reenergized debate surrounding their location. Although crypt-based columnar cells have been recently shown to display self-renewal and multipotential capacity, a large body of evidence supports long-term label-retaining cells, located on average at the +4 position just above the Paneth cells, as putative stem cells. Herein, we propose that both these cell types represent true intestinal stem cells maintained in different states (quiescent vs actively cycling), presumably via interactions with different microenvironments. Finally, we review current findings regarding the roles of Wnt, bone morphogenic protein, PtdIns(3,4,5) kinase, and Notch pathways within the intestine.

An amylase/Cre transgene marks the whole endoderm but the primordia of liver and ventral pancreas

Mice bearing a Cre-encoding transgene driven by a compound [SV40 small t antigen/mousealpha-amylase-2] promoter expressed the recombinase at early developmental stages broadly in the embryonic endoderm before the pancreas and lungs begin to outgrow, but not in other germ layers, as determined indirectly by beta-galactosidase and YFP reporter activity, indicating that the transgene is in fact an endodermic marker. Interestingly, the liver and ventral pancreas were excluded from this expression pattern, denoting that the chimerical alpha-amylase-2 promoter was not active in the anterior leading edge of the endoderm (the presumptive region from which liver and ventral pancreas form). These transgenics thus confirm, among other findings, that dorsal and ventral pancreatic primordia have different intrinsic transcriptional capabilities. In conclusion, we have generated a new transgenic mouse that should be useful to target endoderm at early stages, without affecting the liver or ventral pancreas before embryonic day E12.5.

Direct regulation of intestinal fate by Notch

The signals that maintain the proper balance between adult intestinal cell types are poorly understood. Loss-of-function studies have implicated the Notch pathway in the regulation of intestinal fate during development. However, it is unknown whether Notch has a role in maintaining the balance of different cell types in the adult intestine and whether it acts reversibly. To determine whether Notch has a direct effect on intestinal development and adult intestinal cell turnover, we have used a gain-of-function approach to activate Notch. Ectopic Notch signaling in adult intestinal progenitor cells leads to a bias against secretory fates, whereas ectopic Notch activation in the embryonic foregut results in reversible defects in villus morphogenesis and loss of the proliferative progenitor compartment. We conclude that Notch regulates adult intestinal development by controlling the balance between secretory and absorptive cell types. In the embryo, Notch activation perturbs morphogenesis, possibly through effects on stem or progenitor cells.

Improving M cell mediated transport across mucosal barriers: do certain bacteria hold the keys?

Specialized microfold (M) cells of the follicle-associated epithelium (FAE) of the mucosal-associated lymphoid tissue (MALT) in gut and the respiratory system play an important role in the genesis of both mucosal and systemic immune responses by delivering antigenic substrate to the underlying lymphoid tissue where immune responses start. Although it has been shown that dendritic cells (DC) also have the ability to sample antigens directly from the gut lumen, M cells certainly remain the most important antigen-sampling cell to be investigated in order to devise novel methods to improve mucosal delivery of biologically active compounds. Recently, novel information on the interactions between bacteria and FAE have come to light that unveil further the complex cross-talk taking place at mucosal interfaces between bacteria, epithelial cells and the immune system and which are central to the formation and function of M cells. In particular, it has been shown that M cell mediated transport of antigen across the FAE is improved rapidly by exposure to certain bacteria, thus opening the way to identify new means to achieve a more effective mucosal delivery. Here, these novel findings and their potential in mucosal immunity are analysed and discussed, and new approaches to improve antigen delivery to the mucosal immune system are also proposed.

Molecular insights into congenital disorders of the digestive system

Recent work is providing new insights into molecular mechanisms of digestive system development and their alteration in clinically significant disorders. An understanding of these mechanisms has largely been gained through the use of animal models, because many of the basic processes required in embryogenesis are functionally conserved among species. Such conserved factors include cell-cell signaling pathways and the regulation of gene expression. Disruption of these pathways have been implicated in several congenital disorders of the digestive system, including Hirschsprung disease, malrotation, altered sphincter development, Meckel diverticulum, biliary atresia, Alagille syndrome, pancreatic heterotopias, and pancreatic agenesis. In this review, we highlight recent studies in digestive system development, which elucidate mechanisms underlying congenital disorders of the human digestive system.

Intestinal microflora and diversification of the rabbit antibody repertoire

The rabbit establishes its primary Ab repertoire by somatically diversifying an initial repertoire that is limited by restricted VH gene segment usage during VDJ gene rearrangement. Somatic diversification occurs in gut-associated lymphoid tissue (GALT), and by about 1-2 mo of age nearly all Ig VDJ genes are somatically diversified. In other species that are known to establish their primary Ab repertoire by somatic diversification, such as chicken, sheep, and cattle, diversification appears to be developmentally regulated: it begins before birth and occurs independent of exogenous factors. Because somatic diversification in rabbit occurs well after birth in GALT, the diversification process may not be developmentally regulated, but may require interaction with exogenous factors derived from the gut. To test this hypothesis, we examined Ab repertoire diversification in rabbits in which the appendix was ligated shortly after birth to prevent microbial colonization and all other organized GALT was surgically removed. We found that by 12 wk of age nearly 90% of the Ig VDJ genes in PBL were undiversified, indicating that intestinal microflora are required for somatically diversifying the Ab repertoire. We also examined repertoire diversification in sterilely derived remote colony rabbits that were hand raised away from contact with conventional rabbits and thereby acquired a different gut microflora. In these remote colony rabbits, GALT was underdeveloped, and 70% of the Ig VDJ genes in PBL were undiversified. We conclude that specific, currently unidentified intestinal microflora are required for Ab repertoire diversification.

A mouse model of intestinal stem cell function and regeneration

We present here an in vivo mouse model for intestinal stem cell function and differentiation that uses postnatal intestinal epithelial cell aggregates to generate a differentiated murine small intestinal mucosa with full crypt-villus architecture. The process of neomucosal formation is highly similar to that of intestinal regeneration. Both in vivo grafting and primary culture of these cells reveal two different epithelial cell populations, which display properties consistent with intestinal epithelial transit amplifying and stem cell populations. Using this model system with a mixture of wild-type and transgene marked cells, we have shown that neomucosae originally develop from single aggregates, but that over time the mucosae fuse to form chimaeric mucosae. Despite fusion, the chimaeric mucosae maintain crypt clonality and villus polyclonality, demonstrating that clonal segregation persists during intestinal epithelial regeneration.

Analysis of the role of microsomal triglyceride transfer protein in the liver of tissue-specific knockout mice

A deficiency in microsomal triglyceride transfer protein (MTP) causes the human lipoprotein deficiency syndrome abetalipoproteinemia. However, the role of MTP in the assembly and secretion of VLDL in the liver is not precisely understood. It is not clear, for instance, whether MTP is required to move the bulk of triglycerides into the lumen of the endoplasmic reticulum (ER) during the assembly of VLDL particles. To define MTP's role in hepatic lipoprotein assembly, we recently knocked out the mouse MTP gene (Mttp). Unfortunately, achieving our objective was thwarted by a lethal embryonic phenotype. In this study, we produced mice harboring a "floxed" Mttp allele and then used Cre-mediated recombination to generate liver-specific Mttp knockout mice. Inactivating the Mttp gene in the liver caused a striking reduction in VLDL triglycerides and large reductions in both VLDL/LDL and HDL cholesterol levels. The Mttp inactivation lowered apo B-100 levels in the plasma by >95% but reduced plasma apo B-48 levels by only approximately 20%. Histologic studies in liver-specific knockout mice revealed moderate hepatic steatosis. Ultrastructural studies of wild-type mouse livers revealed numerous VLDL-sized lipid-staining particles within membrane-bound compartments of the secretory pathway (ER and Golgi apparatus) and few cytosolic lipid droplets. In contrast, VLDL-sized lipid-staining particles were not observed in MTP-deficient hepatocytes, either in the ER or in the Golgi apparatus, and there were numerous cytosolic fat droplets. We conclude that MTP is essential for transferring the bulk of triglycerides into the lumen of the ER for VLDL assembly and is required for the secretion of apo B-100 from the liver.

Regulatory sequences of the mouse villin gene that efficiently drive transgenic expression in immature and differentiated epithelial cells of small and large intestines

Villin is an early marker of epithelial cells from the digestive and urogenital tracts. Indeed villin is expressed in the stem cells and the proliferative cells of the intestinal crypts. To investigate the underlying molecular mechanisms and particularly those responsible for the restricted tissue specificity, a large genomic region of the mouse villin gene has been analyzed. A 9-kilobase (kb) regulatory region of the mouse villin gene (harboring 3.5 kb upstream the transcription start site and 5.5 kb of the first intron) was able to promote transcription of the LacZ reporter gene in the small and large intestines of transgenic mice, in a transmissible manner, and thus efficiently directed subsequent beta-galactosidase expression in epithelial cells along the entire crypt-villus axis. In the kidney, the transgene was also expressed in the epithelial cells of the proximal tubules but is likely sensitive to the site of integration. A construct lacking the first intron restricted beta-galactosidase expression to the small intestine. Thus, the 9-kb genomic region contains the necessary cis-acting elements to recapitulate the tissue-specific expression pattern of the endogenous villin gene. Hence, these regulatory sequences can be used to target heterologous genes in immature and differentiated epithelial cells of the small and/or large intestinal mucosa.

Development of the human gastrointestinal tract: twenty years of progress

A combination of approaches has begun to elucidate the mechanisms of gastrointestinal development. This review describes progress over the last 20 years in understanding human gastrointestinal development, including data from both human and experimental animal studies that address molecular mechanisms. Rapid progress is being made in the identification of genes regulating gastrointestinal development. Genes directing initial formation of the endoderm as well as organ-specific patterning are beginning to be identified. Signaling pathways regulating the overall right-left asymmetry of the gastrointestinal tract and epithelial-mesenchymal interactions are being clarified. In searching for extrinsic developmental regulators, numerous candidate trophic factors have been proposed, but compelling evidence remains elusive. A critical gene that initiates pancreas development has been identified, as well as a number of genes regulating liver, stomach, and intestinal development. Mutations in genes affecting neural crest cell migration have been shown to give rise to Hirschsprung's disease. Considerable progress has been achieved in understanding specific phenomena, such as the transcription factors regulating expression of sucrase-isomaltase and fatty acid-binding protein. The challenge for the future is to integrate these data into a more complete understanding of the physiology of gastrointestinal development.

The -700/-310 fragment of the apolipoprotein A-IV gene combined with the -890/-500 apolipoprotein C-III enhancer is sufficient to direct a pattern of gene expression similar to that for the endogenous apolipoprotein A-IV gene

Spatial gene expression in the intestine is mediated by specific regulatory sequences. The three genes of the apoA-I/C-III/A-IV cluster are expressed in the intestine following cephalocaudal and crypt-to-villus axes. Previous studies have shown that the -780/-520 enhancer region of the apoC-III gene directs the expression of the apoA-I gene in both small intestinal villi and crypts, implying that other unidentified elements are necessary for a normal intestinal pattern of apoA-I gene expression. In this study, we have characterized transgenic mice expressing the chloramphenicol acetyltransferase gene under the control of different regions of the apoC-III and apoA-IV promoters. We found that the -890/+24 apoC-III promoter directed the expression of the reporter gene in crypts and villi and did not follow a cephalocaudal gradient of expression. In contrast, the -700/+10 apoA-IV promoter linked to the -500/-890 apoC-III enhancer directed the expression of the reporter gene in enterocytes with a pattern of expression similar to that of the endogenous apoA-IV gene. Furthermore, linkage of the -700/-310 apoA-IV distal promoter region to the -890/+24 apoC-III promoter was sufficient to restore the appropriate pattern of intestinal expression of the reporter gene. These findings demonstrate that the -700/-310 distal region of the apoA-IV promoter contains regulatory elements that, in combination with proximal promoter elements and the -500/-890 enhancer, are necessary and sufficient to restrict apoC-III and apoA-IV gene expression to villus enterocytes of the small intestine along the cephalocaudal axis.

Polymerase chain reaction-based detection of clonality as a non-morphologic diagnostic tool for fine-needle aspiration of the breast

BACKGROUND

Fine-needle aspiration (FNA) of breast specimens can be difficult and between 10-25% of the lesions ultimately are classified as "atypical," even by the most experienced cytopathologist. The goal of this study was to identify a molecular mechanism that reliably distinguishes benign and malignant (or pre-malignant) lesions and that could be used as an adjunct in these morphologically ambiguous cases.

METHODS

Because all malignancies represent clonal proliferations, assessment of clonality represents a potential molecular mechanism for making this distinction. Excess material preserved from breast FNAs was examined using the human androgen receptor locus clonality assay. This assay allows determination of clonality on the basis of X chromosome inactivation as detected by polymerase chain reaction analysis of genomic DNA after methylase-sensitive restriction digestion.

RESULTS

In this pilot study, 25 cases showed reproducible results. All malignant cases (9 of 9) were monoclonal, whereas 10 of 12 benign cases were polyclonal. Of four atypical cases, two were monoclonal and both were found to be malignant after surgical resection. Monoclonality was observed in two benign cases that were hyperplastic lesions.

CONCLUSIONS

These preliminary results suggest that this test may provide a non-morphologic molecular mechanism for the objective categorization of breast FNAs.

Phenotype of mice lacking functional Deleted in colorectal cancer (Dcc) gene

The DCC (Deleted in colorectal cancer) gene was first identified as a candidate for a tumour-suppressor gene on human chromosome 18q. More recently, in vitro studies in rodents have provided evidence that DCC might function as a receptor for the axonal chemoattractant netrin-1. Inactivation of the murine Dcc gene caused defects in axonal projections that are similar to those observed in netrin-1-deficient mice but did not affect growth, differentiation, morphogenesis or tumorigenesis in mouse intestine. These observations fail to support a tumour-suppressor function for Dcc, but are consistent with the hypothesis that DCC is a component of a receptor for netrin-1.

Stem cells of the respiratory epithelium and their in vitro cultivation

Parenchymal (epithelial or mesenchyma) stem cells are rapidly drawing both scientific and clinical attention in solid organs like the liver, skin, intestine and abdominal mesothelium, just as has been the case in the hematopoietic system. For the stem cells of these organs various definitions, markers for identification, methods of isolation and in vitro cultivation, and lineage mechanisms have been proposed and some of them are now proven to be valid and useful. In this article attempts will be made to explore whether there are stem cells in the lower respiratory system (from the trachea to the lung periphery) and what they look like. Because of its anatomical and functional complexity the stem cell concept for the respiratory system has been developing rather slowly. Nevertheless, the data available seem to indicate that in analogy to the above mentioned organs there is only one type of epithelial stem cells throughout all sections of the lower respiratory system during fetal through adult stages. They are multipotent for cell differentiation and able to yield lineage progenitors for ciliated, goblet, basal. Clara neuroendocrine, alveolar type 1 and alveolar type 2 cells.

Forced expression of the tumor suppressor adenomatosis polyposis coli protein induces disordered cell migration in the intestinal epithelium

Mutations of the human adenomatosis polyposis coli (APC) gene are associated with the development of familial as well as sporadic intestinal neoplasia. To examine the in vivo function of APC, 129/Sv embryonic stem (ES) cells were transfected with DNA encoding the wild-type human protein under the control of a promoter that is active in all four of the small intestine's principal epithelial lineages during their migration-associated differentiation. ES-APC cells were then introduced into C57BL/6-ROSA26 blastocysts. Analyses of adult B6-ROSA26<-->129/Sv-APC chimeric mice revealed that forced expression of APC results in markedly disordered cell migration. When compared with the effects of forced expression of E-cadherin, the data suggest that APC-catenin and E-cadherin-catenin complexes have opposing effects on intestinal epithelial cell movement/adhesiveness; augmentation of E-cadherin-beta-catenin complexes produces a highly ordered, "adhesive" migration, whereas augmentation of APC-beta-catenin complexes produces a disordered, nonadhesive migratory phenotype. We propose that APC mutations may promote tumorigenesis by increasing the relative activity of cadherin-catenin complexes, resulting in enhanced adhesiveness and functional anchorage of initiated cells within the intestinal crypt. Our studies also indicate that chimeric mice generated from B6-ROSA26 blastocysts and genetically manipulated ES cells should be useful for auditing gene function in the gastrointestinal tract and in other tissues.

Forced expression of E-cadherin in the mouse intestinal epithelium slows cell migration and provides evidence for nonautonomous regulation of cell fate in a self-renewing system

The adult mouse small intestinal epithelium is self-renewing. Its crypt-villus unit provides a model for studying many of the processes that occur during tissue morphogenesis such as control of proliferative status, specification of cell fate, regulation of differentiation, and induction of death. To assess the contributions of cell-cell and cell-substratum interactions to the coordinated control of these processes, 129/Sv embryonic stem (ES) cells, transfected with a recombinant DNA consisting of a fatty acid-binding protein gene (Fabp1) promoter that functions along the entire length of the crypt-villus axis linked to mouse E-cadherin, were introduced into normal C57Bl/6 (B6) blastocysts. Analyses of adult B6 <--> 129/Sv mice indicated that forced expression of E-cadherin suppresses proliferation and induces apoptosis in the crypt, and slows cell movement up the villus. The slowed migration is not accompanied by a change in distribution of terminal differentiation markers along the crypt-villus axis suggesting that differentiation is largely cell nonautonomous. To determine whether the slowed migration was a direct effect of forced expression of E-cadherin or a secondary effect of reduced crypt cell production, another Fabp promoter was used to restrict overproduction of E-cadherin to the villus epithelium of transgenic mice. Enterocytic migration was slowed, although proliferation and apoptosis were not perturbed in crypts. Augmentation of cellular E-cadherin pools was accompanied by an increase in beta-catenin levels. These findings establish that cadherins and their associated proteins modulate cellular migration, proliferation, and death programs in an adult vertebrate organ.

Genetic engineering of carbohydrate biosynthetic pathways in transgenic mice demonstrates cell cycle-associated regulation of glycoconjugate production in small intestinal epithelial cells

Proliferation, migration-associated differentiation, and cell death occur continuously and in a spatially well-organized fashion along the crypt-villus axis of the mouse small intestine, making it an attractive system for studying how these processes are regulated and interrelated. A pathway for producing glycoconjugates was engineered in adult FVB/N transgenic mice by expressing a human alpha 1,3/4-fucosyltransferase (alpha 1,3/4-FT; EC 2.4.1.65) along the length of this crypt-villus axis. The alpha 1,3/4-FT can use lacto-N-tetraose or lacto-neo-N-tetraose core chains to generate Lewis (Le) blood group antigens Le(a) or Le(x), respectively, and H type 1 or H type 2 core chains to produce Leb and Le(y). Single- and multilabel immunohistochemical studies revealed that expression of the alpha 1,3/4-FT results in production of Le(a) and Leb antigens in both undifferentiated proliferated crypt cells and in differentiated postmitotic villus-associated epithelial cells. In contrast, Le(x) antigens were restricted to crypt cells. Villus enterocytes can be induced to reenter the cell cycle by expression of simian virus 40 tumor antigen under the control of a promoter that only functions in differentiated members of this lineage. Bitransgenic animals, generated from a cross of FVB/N alpha 1,3/4-FT with FVB/N simian virus 40 tumor antigen mice, expand the range of Le(x) expression to include villus-associated enterocytes that have reentered the cell cycle. Thus, the fucosylations unveil a proliferation-dependent switch in oligosaccharide production, as defined by a monoclonal antibody specific for the Le(x) epitope. These findings show that genetic engineering of oligosaccharide biosynthetic pathways can be used to define markers for entry into, or progression through, the cell cycle and to identify changes in endogenous carbohydrate metabolism that occur when proliferative status is altered in a manner that is not deleterious to the system under study.

Inflammatory bowel disease and adenomas in mice expressing a dominant negative N-cadherin

Cadherins mediate cell adhesion and are essential for normal development. Embryonic stem cells were transfected with a dominant negative N-cadherin mutant (NCAD delta) under the control of promoters active in small intestinal epithelial cells and then introduced into C57BL/6 mouse blastocysts. Analysis of adult chimeric mice revealed that expression of NCAD delta along the entire crypt-villus axis, but not in the villus epithelium alone, produced an inflammatory bowel disease resembling Crohn's disease. NCAD delta perturbed proliferation, migration, and death programs in crypts, which lead to adenomas. This model provides insights about cadherin function in an adult organ and the factors underlying inflammatory bowel disease and intestinal neoplasia.

Intestinal epithelial cell differentiation: new insights from mice, flies and nematodes

Decisions commonly made during development that affect proliferation, cell fate specification, differentiation, migration, and death are made repeatedly in the mouse small intestinal epithelium throughout adulthood. The results of these decisions are a stratification of proliferation, differentiation, and death along the mouse small intestine's crypt/villus axis. Recent genetic studies in Caenorhabditis elegans and Drosophila melanogaster have identified factors involved in determining cell fate and differentiation in gut endoderm. The stem cell hierarchy of the adult mouse intestinal epithelium makes it ideally suited for using chimeric animals to examine the functions of homologs of these lower eukaryotic (and other) proteins.

In vivo analysis of cadherin function in the mouse intestinal epithelium: essential roles in adhesion, maintenance of differentiation, and regulation of programmed cell death

A model system is described for defining the physiologic functions of mammalian cadherins in vivo. 129/Sv embryonic stem (ES) cells, stably transfected with a dominant negative N-cadherin mutant (NCAD delta) under the control of a promoter that only functions in postmitotic enterocytes during their rapid, orderly, and continuous migration up small intestinal villi, were introduced into normal C57B1/6 (B6) blastocysts. In adult B6<->129/Sv chimeric mice, each villus receives the cellular output of several surrounding monoclonal crypts. A polyclonal villus located at the boundary of 129/Sv- and B6-derived intestinal epithelium contains vertical coherent bands of NCAD delta-producing enterocytes plus adjacent bands of normal B6-derived enterocytes. A comparison of the biological properties of these cell populations established that NCAD delta disrupts cell-cell and cell-matrix contacts, increases the rate of migration of enterocytes along the crypt-villus axis, results in a loss of their differentiated polarized phenotype, and produces precocious entry into a death program. These data indicate that enterocytic cadherins are critical cell survival factors that actively maintain intestinal epithelial function in vivo.

Mouse Paneth cell defensins: primary structures and antibacterial activities of numerous cryptdin isoforms

Cryptdins are antimicrobial peptides of the defensin family that are produced by intestinal Paneth cells. mRNAs encoding 17 cryptdin isoforms have been characterized from a cDNA library generated from a single jejunal crypt. Six cryptdin cDNAs correspond to known peptides, and the remainder predict 11 novel Paneth cell defensins. Most cryptdin cDNAs have > or = 93% nucleotide sequence identity overall, except for cryptdin 4 and 5 cDNAs, whose respective mature peptide-encoding regions are only 74 and 78% identical to that of cryptdin 1. Cryptdin cDNAs differ at a small number of nucleotide positions: frequent substitutions were found in codons 38 and 52 of the propiece and in codons 68, 73, 76, 87, and 89 of the deduced peptides; cDNA clones with changes in codons 74, 83, and 88 were found, but there were fewer of these. The antimicrobial activities of cryptdins 1 to 6 were tested against Escherichia coli ML35 in two assays. In an agar diffusion assay, the potencies of cryptdins 1 to 3, 5, and 6 were approximately equivalent to that of rabbit neutrophil defensin NP-1 but cryptdin 4 was 30 times more active than NP-1. In a bactericidal assay system, cryptdins 1 and 3 to 6 were equally active at 10 micrograms/ml but cryptdin 2 and rabbit NP-1 were not active at this concentration. Since cryptdins 2 and 3 differ only at residue 10 (Thr and Lys, respectively), this amino acid appears to function in bactericidal interaction with E. coli. The demonstration that Paneth cells express a diverse population of microbicidal defensins further implicates cryptdins in restricting colonization or invasion of small intestinal epithelium by bacteria.

Keratinocyte growth factor induces proliferation of hepatocytes and epithelial cells throughout the rat gastrointestinal tract

Keratinocyte growth factor (KGF), a member of the fibroblast growth factor (FGF) family, was identified as a specific keratinocyte mitogen after isolation from a lung fibroblast line. Recently, recombinant (r)KGF was found to influence proliferation and differentiation patterns of multiple epithelial cell lineages within skin, lung, and the reproductive tract. In the present study, we designed experiments to identify additional target tissues, and focused on the rat gastrointestinal (GI) system, since a putative receptor, K-sam, was originally identified in a gastric carcinoma. Expression of KGF receptor and KGF mRNA was detected within the entire GI tract, suggesting the gut both synthesized and responded to KGF. Therefore, rKGF was administered to adult rats and was found to induce markedly increased proliferation of epithelial cells from the foregut to the colon, and of hepatocytes, one day after systemic treatment. Daily treatment resulted in the marked selective induction of mucin-producing cell lineages throughout the GI tract in a dose-dependent fashion. Other cell lineages were either unaffected (e.g., Paneth cells), or relatively decreased (e.g., parietal cells, enterocytes) in rKGF-treated rats. The direct effect of rKGF was confirmed by demonstrating markedly increased carcinoembryonic antigen production in a human colon carcinoma cell line, LIM1899. Serum levels of albumin were specifically and significantly elevated after daily treatment. These results demonstrate rKGF can induce epithelial cell activation throughout the GI tract and liver. Further, endogenous KGF may be a normal paracrine mediator of growth within the gut.

The mouse ileal lipid-binding protein gene: a model for studying axial patterning during gut morphogenesis

Normal, chimeric-transgenic, and transgenic mice have been used to study the axial patterns of ileal lipid-binding protein gene (Ilbp) expression during and after completion of gut morphogenesis. Ilbp is initially activated in enterocytes in bidirectional wave that expands proximally in the ileum and distally to the colon during late gestation and the first postnatal week. This activation occurs at the same time that a wave of cytodifferentiation of the gut endoderm is completing its unidirectional journey from duodenum to colon. The subsequent contraction of Ilbp's expression domain, followed by its reexpansion from the distal to proximal ileum, coincides with a critical period in gut morphogenesis (postnatal days 7-28) when its proliferative units (crypts) form, establish their final stem cell hierarchy, and then multiply through fission. The wave of reactivation is characterized by changing patterns of Ilbp expression: (a) at the proximal most boundary of the wave, villi contain a mixed population of scattered ileal lipid-binding protein (ILBP)-positive and ILBP-negative enterocytes derived from the same monoclonal crypt; (b) somewhat more distally, villi contain vertical coherent stripes of wholly ILBP-positive enterocytes derived from monoclonal crypts and adjacent, wholly ILBP-negative stripes of enterocytes emanating from other monoclonal crypts; and (c) more distally, all the enterocytes on a villus support Ilbp expression. Functional mapping studies of Ilbp's promoter in transgenic mice indicate that nucleotides -145 to +48 contain cis-acting elements sufficient to produce an appropriately directed distal-to-proximal wave of Ilbp activation in the ileum, to maintain an appropriate axial distribution of monophenotypic wholly reporter-positive villi in the distal portion of the ileum, as well as striped and speckled villi in the proximal portion of its expression domain, and to correctly support reporter production in villus-associated ileal enterocytes. Nucleotides -417 to -146 of Ilbp contain a "temporal" suppressor that delays initial ileal activation of the gene until the second postnatal week. Nucleotides -913 to -418 contain a temporal suppressor that further delays initial activation of the gene until the third to fourth postnatal week, a spatial suppressor that prohibits gene expression in the proximal quarter of the ileum and in the proximal colon, and a cell lineage suppressor that prohibits expression in goblet cells during the first two postnatal weeks.