Cell kinetics in the mouse small intestine during immediate postnatal life

Cataloguing the postnatal small intestinal transcriptome during the first postnatal month

In the first postnatal month, the developing mouse intestine shifts from an immature to a mature intestine that will sustain the organism throughout the lifespan. Here, we surveyed the mouse intestine in C57Bl/6 mice by RNA-Seq to evaluate the changes in gene expression over time from the day of birth through 1 month of age in both the duodenum and ileum. We analyzed gene expression for changes in gene families that correlated with the periods of NEC susceptibility or resistance. We highlight that increased expression of DNA processing genes and vacuolar structure genes, tissue development and morphogenesis genes, and cell migration genes all correlated with NEC susceptibility, while increases in immunity gene sets, intracellular transport genes, ATP production, and intracellular metabolism genes correlated with NEC resistance. Using trends identified in the RNA-Seq analyses, we further evaluated expression of cellular markers and epithelial regulators, immune cell markers, and adenosine metabolism components. We confirmed key changes with qRT-PCR and immunofluorescence. In addition, we compared some findings to humans using human intestinal biopsies and organoids. This dataset can serve as a reference for other groups considering the role of single molecules or molecular families in early intestinal and postnatal development.

Smooth muscle contributes to the development and function of a layered intestinal stem cell niche

Wnt and Rspondin (RSPO) signaling drives proliferation, and bone morphogenetic protein inhibitors (BMPi) impede differentiation, of intestinal stem cells (ISCs). Here, we identify the mouse ISC niche as a complex, multi-layered structure that encompasses distinct mesenchymal and smooth muscle populations. In young and adult mice, diverse sub-cryptal cells provide redundant ISC-supportive factors; few of these are restricted to single cell types. Niche functions refine during postnatal crypt morphogenesis, in part to oppose the dense aggregation of differentiation-promoting BMP+ sub-epithelial myofibroblasts at crypt-villus junctions. Muscularis mucosae, a specialized muscle layer, first appears during this period and supplements neighboring RSPO and BMPi sources. Components of this developing niche are conserved in human fetuses. The in vivo ablation of mouse postnatal smooth muscle increases BMP signaling activity, potently limiting a pre-weaning burst of crypt fission. Thus, distinct and progressively specialized mesenchymal cells together create the milieu that is required to propagate crypts during rapid organ growth and to sustain adult ISCs.

MEX3A regulates Lgr5+ stem cell maintenance in the developing intestinal epithelium

Intestinal stem cells (ISCs) fuel the lifelong self‐renewal of the intestinal tract and are paramount for epithelial repair. In this context, the Wnt pathway component LGR5 is the most consensual ISC marker to date. Still, the effort to better understand ISC identity and regulation remains a challenge. We have generated a Mex3a knockout mouse model and show that this RNA‐binding protein is crucial for the maintenance of the Lgr5⁺ISC pool, as its absence disrupts epithelial turnover during postnatal development and stereotypical organoid maturation ex vivo. Transcriptomic profiling of intestinal crypts reveals that Mex3a deletion induces the peroxisome proliferator‐activated receptor (PPAR) pathway, along with a decrease in Wnt signalling and loss of the Lgr5⁺ stem cell signature. Furthermore, we identify PPARγ activity as a molecular intermediate of MEX3A‐mediated regulation. We also show that high PPARγ signalling impairs Lgr5⁺ISC function, thus uncovering a new layer of post‐transcriptional regulation that critically contributes to intestinal homeostasis.

Development and organization of the zebrafish intestinal epithelial stem cell niche

BACKGROUND

Development of the vertebrate intestinal epithelial stem cell niche begins during embryogenesis but maturation occurs postembryonic. The intestinal mammalian crypt contains stem cells interspersed by secretory cells that play a role in regulation of proliferation. Epithelial cells are specified as either secretory or enterocytes as they migrate up the villi in mammals or fold in zebrafish. Zebrafish forms a functional intestine by the end of embryogenesis but takes another 4 weeks to develop the adult proliferation pattern.

RESULTS

We characterize development of the intestinal epithelial stem cell niche during the postembryonic period. During the first 2-weeks postembryogenesis, different groups of epithelial cells sequentially proceed through one or two cell cycles, appear to become quiescent, and remain at the interfold base. The third week begins asymmetric divisions with proliferative progeny moving up the folds. Apoptotic cells are not observed at the fold tip until the end of the fourth week. Secretory cells intersperse among interfold base proliferative cells, increasing in number during the third and fourth weeks with a coincident change in proliferation pattern.

CONCLUSIONS

Zebrafish postembryonic intestinal epithelial development consists of 2 weeks of slow proliferation followed by 2 weeks of metamorphosis to the adult structure. Developmental Dynamics 2019. © 2019 Wiley Periodicals, Inc.

Cell-Cell Junctions Organize Structural and Signaling Networks

Cell-cell junctions link cells to each other in tissues, and regulate tissue homeostasis in critical cell processes that include tissue barrier function, cell proliferation, and migration. Defects in cell-cell junctions give rise to a wide range of tissue abnormalities that disrupt homeostasis and are common in genetic abnormalities and cancers. Here, we discuss the organization and function of cell-cell junctions primarily involved in adhesion (tight junction, adherens junction, and desmosomes) in two different epithelial tissues: a simple epithelium (intestine) and a stratified epithelium (epidermis). Studies in these tissues reveal similarities and differences in the organization and functions of different cell-cell junctions that meet the requirements for the specialized functions of each tissue. We discuss cell-cell junction responses to genetic and environmental perturbations that provide further insights into their roles in maintaining tissue homeostasis.

PDGFRα+ pericryptal stromal cells are the critical source of Wnts and RSPO3 for murine intestinal stem cells in vivo

Tissue stem cells in vivo reside in highly structured niches that provide signals for proliferation and differentiation. Understanding the role of the niche requires identifying the key cell types that provide these regulators. In the intestine, R-spondins and Wnts are essential regulators of the stem-cell niche. Here we identify subepithelial myofibroblasts of the PDGF receptor α lineage as the specific stromal cell type that secretes these ligands. These data demonstrate the close interaction between epithelial stem cells and the underlying regulatory stroma niche and provide insights into both normal homeostasis and tissue recovery after injury.

Wnts and R-spondins (RSPOs) support intestinal homeostasis by regulating crypt cell proliferation and differentiation. Ex vivo, Wnts secreted by Paneth cells in organoids can regulate the proliferation and differentiation of Lgr5-expressing intestinal stem cells. However, in vivo, Paneth cell and indeed all epithelial Wnt production is completely dispensable, and the cellular source of Wnts and RSPOs that maintain the intestinal stem-cell niche is not known. Here we investigated both the source and the functional role of stromal Wnts and RSPO3 in regulation of intestinal homeostasis. RSPO3 is highly expressed in pericryptal myofibroblasts in the lamina propria and is several orders of magnitude more potent than RSPO1 in stimulating both Wnt/β-catenin signaling and organoid growth. Stromal Rspo3 ablation ex vivo resulted in markedly decreased organoid growth that was rescued by exogenous RSPO3 protein. Pdgf receptor alpha (PdgfRα) is known to be expressed in pericryptal myofibroblasts. We therefore evaluated if PdgfRα identified the key stromal niche cells. In vivo, Porcn excision in PdgfRα⁺ cells blocked intestinal crypt formation, demonstrating that Wnt production in the stroma is both necessary and sufficient to support the intestinal stem-cell niche. Mice with Rspo3 excision in the PdgfRα⁺ cells had decreased intestinal crypt Wnt/β-catenin signaling and Paneth cell differentiation and were hypersensitive when stressed with dextran sodium sulfate. The data support a model of the intestinal stem-cell niche regulated by both Wnts and RSPO3 supplied predominantly by stromal pericryptal myofibroblasts marked by PdgfRα.

MT1-MMP and its potential role in the vertebrate intestinal morphogenesis

Membrane type 1-matrix metalloproteinase (MT1-MMP) is involved in numerous biological processes, including morphogenesis. However, the role of MT1-MMP in the development of the vertebrate intestine is poorly understood. This study aimed to evaluate the expression of MT1-MMP in the intestine of rats and chickens along the embryonic and postnatal periods using immunohistochemistry. Results revealed a remarkable spatiotemporal correlation between MT1-MMP expression and intestinal villi morphogenesis in both vertebrates. However, the villi morphogenesis process was found to be different in chickens to that of rats. Moreover, extensive MT1-MMP labeling was observed in the entire villus epithelium from birth until the complete maturation of the small intestinal mucosa in both vertebrates. From these results, we suggest that MT1-MMP contributes to intestinal development, particularly to villi morphogenesis, in both vertebrates. However, further studies are necessary to confirm the role of MT1-MMP in this cellular process. In addition, we performed validation of the primary antibody against human MT1-MMP for adult chickens.

Molecular and cytological analyses reveal distinct transformations of intestinal epithelial cells during Xenopus metamorphosis

Background

The thyroid hormone (T3)-induced formation of adult intestine during amphibian metamorphosis resembles the maturation of the mammalian intestine during postembryonic development, the period around birth when plasma T3 level peaks. This process involves de novo formation of adult intestinal stem cells as well as the removal of the larval epithelial cells through apoptosis. Earlier studies have revealed a number of cytological and molecular markers for the epithelial cells undergoing different changes during metamorphosis. However, the lack of established double labeling has made it difficult to ascertain the identities of the metamorphosing epithelial cells.

Results

Here, we carried out different double-staining with a number of cytological and molecular markers during T3-induced and natural metamorphosis in Xenopus laevis. Our studies demonstrated conclusively that the clusters of proliferating cells in the epithelium at the climax of metamorphosis are undifferentiated epithelial cells and express the well-known adult intestinal stem cell marker gene Lgr5. We further show that the adult stem cells and apoptotic larval epithelial cells are distinct epithelial cells during metamorphosis.

Conclusions

Our findings suggest that morphologically identical larval epithelial cells choose two alternative paths: programmed cell death or dedifferentiation to form adult stem cells, in response to T3 during metamorphosis with apoptosis occurring prior to the formation of the proliferating adult stem cell clusters (islets).

Dnmt1 is essential to maintain progenitors in the perinatal intestinal epithelium

The DNA methyltransferase Dnmt1 maintains DNA methylation patterns and genomic stability in several in vitro cell systems. Ablation of Dnmt1 in mouse embryos causes death at the post-gastrulation stage; however, the functions of Dnmt1 and DNA methylation in organogenesis remain unclear. Here, we report that Dnmt1 is crucial during perinatal intestinal development. Loss of Dnmt1 in intervillus progenitor cells causes global hypomethylation, DNA damage, premature differentiation, apoptosis and, consequently, loss of nascent villi. We further confirm the crucial role of Dnmt1 during crypt development using the in vitro organoid culture system, and illustrate a clear differential requirement for Dnmt1 in immature versus mature organoids. These results demonstrate an essential role for Dnmt1 in maintaining genomic stability during intestinal development and the establishment of intestinal crypts.

Broader expression of the mouse platelet factor 4-cre transgene beyond the megakaryocyte lineage

BACKGROUND

Transgenic mice expressing cre recombinase under the control of the platelet factor 4 (Pf4) promoter, in the context of a 100-kb bacterial artificial chromosome, have become a valuable tool with which to study genetic modifications in the platelet lineage. However, the specificity of cre expression has recently been questioned, and the time of its onset during megakaryopoiesis remains unknown.

OBJECTIVES/METHODS

To characterize the expression of this transgene, we used double-fluorescent cre reporter mice.

RESULTS

In the bone marrow, Pf4-cre-mediated recombination had occurred in all CD42-positive megakaryocytes as early as stage I of maturation, and in rare CD42-negative cells. In circulating blood, all platelets had recombined, along with only a minor fraction of CD45-positive cells. However, we found that all tissues contained recombined cells of monocyte/macrophage origin. When recombined, these cells might potentially modify the function of the tissues under particular conditions, especially inflammatory conditions, which further increase recombination in immune cells. Unexpectedly, a subset of epithelial cells from the distal colon showed signs of recombination resulting from endogenous Pf4-cre expression. This is probably the basis of the unexplained colon tumors developed by Apc(flox/flox) ;Pf4-cre mice, generated in a separate study on the role of Apc in platelet formation.

CONCLUSION

Altogether, our results indicate early recombination with full penetrance in megakaryopoiesis, and confirm the value of Pf4-cre mice for the genetic engineering of megakaryocytes and platelets. However, care must be taken when investigating the role of platelets in processes outside hemostasis, especially when immune cells might be involved.

Meta-analysis of the turnover of intestinal epithelia in preclinical animal species and humans

Due to the rapid turnover of the small intestinal epithelia, the rate at which enterocyte renewal occurs plays an important role in determining the level of drug-metabolizing enzymes in the gut wall. Current physiologically based pharmacokinetic (PBPK) models consider enzyme and enterocyte recovery as a lumped first-order rate. An assessment of enterocyte turnover would enable enzyme and enterocyte renewal to be modeled more mechanistically. A literature review together with statistical analysis was employed to establish enterocyte turnover in human and preclinical species. A total of 85 studies was identified reporting enterocyte turnover in 1602 subjects in six species. In mice, the geometric weighted combined mean (WX) enterocyte turnover was 2.81 ± 1.14 days (n = 169). In rats, the weighted arithmetic mean enterocyte turnover was determined to be 2.37 days (n = 501). Humans exhibited a geometric WX enterocyte turnover of 3.48 ± 1.55 days for the gastrointestinal epithelia (n = 265), displaying comparable turnover to that of cytochrome P450 enzymes in vitro (0.96-4.33 days). Statistical analysis indicated humans to display longer enterocyte turnover as compared with preclinical species. Extracted data were too sparse to support regional differences in small intestinal enterocyte turnover in humans despite being indicated in mice. The utilization of enterocyte turnover data, together with in vitro enzyme turnover in PBPK modeling, may improve the predictions of metabolic drug-drug interactions dependent on enzyme turnover (e.g., mechanism-based inhibition and enzyme induction) as well as absorption of nanoparticle delivery systems and intestinal metabolism in special populations exhibiting altered enterocyte turnover.

Transport of particles in intestinal mucus under simulated infant and adult physiological conditions: impact of mucus structure and extracellular DNA

The final boundary between digested food and the cells that take up nutrients in the small intestine is a protective layer of mucus. In this work, the microstructural organization and permeability of the intestinal mucus have been determined under conditions simulating those of infant and adult human small intestines. As a model, we used the mucus from the proximal (jejunal) small intestines of piglets and adult pigs. Confocal microscopy of both unfixed and fixed mucosal tissue showed mucus lining the entire jejunal epithelium. The mucus contained DNA from shed epithelial cells at different stages of degradation, with higher amounts of DNA found in the adult pig. The pig mucus comprised a coherent network of mucin and DNA with higher viscosity than the more heterogeneous piglet mucus, which resulted in increased permeability of the latter to 500-nm and 1-µm latex beads. Multiple-particle tracking experiments revealed that diffusion of the probe particles was considerably enhanced after treating mucus with DNase. The fraction of diffusive 500-nm probe particles increased in the pig mucus from 0.6% to 64% and in the piglet mucus from ca. 30% to 77% after the treatment. This suggests that extracellular DNA can significantly contribute to the microrheology and barrier properties of the intestinal mucus layer. To our knowledge, this is the first time that the structure and permeability of the small intestinal mucus have been compared between different age groups and the contribution of extracellular DNA highlighted. The results help to define rules governing colloidal transport in the developing small intestine. These are required for engineering orally administered pharmaceutical preparations with improved delivery, as well as for fabricating novel foods with enhanced nutritional quality or for controlled calorie uptake.

Unraveling intestinal stem cell behavior with models of crypt dynamics

The definition, regulation and function of intestinal stem cells (ISCs) has been hotly debated. Recent discoveries have started to clarify the nature of ISCs, but many questions remain. This review discusses the current advances and controversies of ISC biology as well as theoretical compartmental models that have been coupled with in vivo experimentation to investigate the mechanisms of ISC dynamics during homeostasis, tumorigenesis, repair and development. We conclude our review by discussing the key lingering questions in the field and proposing how many of these questions can be addressed using both compartmental models and experimental techniques.

Cytological and morphological analyses reveal distinct features of intestinal development during Xenopus tropicalis metamorphosis

Background

The formation and/or maturation of adult organs in vertebrates often takes place during postembryonic development, a period around birth in mammals when thyroid hormone (T3) levels are high. The T3-dependent anuran metamorphosis serves as a model to study postembryonic development. Studies on the remodeling of the intestine during Xenopus (X.) laevis metamorphosis have shown that the development of the adult intestine involves de novo formation of adult stem cells in a process controlled by T3. On the other hand, X. tropicalis, highly related to X. laevis, offers a number of advantages for studying developmental mechanisms, especially at genome-wide level, over X. laevis, largely due to its shorter life cycle and sequenced genome. To establish X. tropicalis intestinal metamorphosis as a model for adult organogenesis, we analyzed the morphological and cytological changes in X. tropicalis intestine during metamorphosis.

Methodology/Principal Findings

We observed that in X. tropicalis, the premetamorphic intestine was made of mainly a monolayer of larval epithelial cells surrounded by little connective tissue except in the single epithelial fold, the typhlosole. During metamorphosis, the larval epithelium degenerates and adult epithelium develops to form a multi-folded structure with elaborate connective tissue and muscles. Interestingly, typhlosole, which is likely critical for adult epithelial development, is present along the entire length of the small intestine in premetamorphic tadpoles, in contrast to X. laevis, where it is present only in the anterior 1/3. T3-treatment induces intestinal remodeling, including the shortening of the intestine and the typhlosole, just like in X. laevis.

Conclusions/Significance

Our observations indicate that the intestine undergoes similar metamorphic changes in X. laevis and X. tropicalis, making it possible to use the large amount of information available on X. laevis intestinal metamorphosis and the genome sequence information and genetic advantages of X. tropicalis to dissect the pathways governing adult intestinal development.

Optimality in the development of intestinal crypts

Intestinal crypts in mammals are comprised of long-lived stem cells and shorter-lived progenies. These two populations are maintained in specific proportions during adult life. Here, we investigate the design principles governing the dynamics of these proportions during crypt morphogenesis. Using optimal control theory, we show that a proliferation strategy known as a "bang-bang" control minimizes the time to obtain a mature crypt. This strategy consists of a surge of symmetric stem cell divisions, establishing the entire stem cell pool first, followed by a sharp transition to strictly asymmetric stem cell divisions, producing nonstem cells with a delay. We validate these predictions using lineage tracing and single-molecule fluorescence in situ hybridization of intestinal crypts in infant mice, uncovering small crypts that are entirely composed of Lgr5-labeled stem cells, which become a minority as crypts continue to grow. Our approach can be used to uncover similar design principles in other developmental systems.

Reduced expression of ferroportin-1 mediates hyporesponsiveness of suckling rats to stimuli that reduce iron absorption

BACKGROUND & AIMS

Suckling mammals absorb high levels of iron to support their rapid growth. In adults, iron absorption is controlled by systemic signals that alter expression of the iron-regulatory hormone hepcidin. We investigated whether hepcidin and absorption respond appropriately to systemic stimuli during suckling.

METHODS

In Sprague-Dawley rats, iron levels increased following administration of iron dextran, and inflammation was induced with lipopolysaccharide. Gene expression was measured by quantitative reverse-transcription polymerase chain reaction; protein levels were measured by immunoblot analyses. Iron absorption was determined based on retention of an oral dose of 59Fe.

RESULTS

Iron absorption was high during suckling and reduced to adult levels upon weaning. In response to iron dextran or lipopolysaccharide, iron absorption in adults decreased substantially, but, in suckling animals, the changes were minimal. Despite this, expression of hepcidin messenger RNA was strongly induced by each agent, before and after weaning. The hyporesponsiveness of iron absorption to increased levels of hepcidin during suckling correlated with reduced or absent duodenal expression of ferroportin 1 (Fpn1), normally a hepcidin target. Fpn1 expression was robust in adults. Predominance of the Fpn1A splice variant, which is under iron-dependent translational control, accounts for the low level of Fpn1 in the iron-deficient intestine of suckling rats.

CONCLUSIONS

Iron absorption during suckling is largely refractory to changes in expression of the systemic iron regulator hepcidin, and this in turn reflects limited expression of Fpn1 protein in the small intestine. Iron absorption is therefore not always controlled by hepcidin.

Effects of fasting at different stages of lighting regimen on the proliferation of jejunal epithelial cells during rat pup weaning

The lifespan of intestinal epithelial cells is predetermined by the process of cell proliferation that occurs constantly in the crypt. The control of this process involves some endogenous factors, such as hormones, as well as exogenous factors, like food and natural light variations. These last two exogenous factors seem to be the major modulators of the cell proliferation process. Fasting treatment was conducted to assess the role of food and its effect on the metaphase index (MI) of the intestinal epithelium at different times and periods (light and dark) of the day. The effects of short- (5 hr) and long-term (25 hr) fasting on the MI in the jejunal epithelium of young rats were investigated at 09:00 h, 15:00 hr, 21:00 hr, and 02:00 hr using the arrested metaphases method. The present study demonstrates that 5 hr and 25 hr of fasting treatment decrease the MI at 09:00 hr. It was observed from MI analysis that there is an interaction between the fed/fasted status of the animal and the different times of the day. This result suggests that during the transition from youth to adulthood, the control of MI by the light/dark cycle seems to be more pronounced as compared with control by food intake at some periods of the day, although at other times food had a greater impact on the MI.

Role of apolipoprotein E4 in protecting children against early childhood diarrhea outcomes and implications for later development

Our group and others have reported a series of studies showing that heavy burdens of diarrheal diseases in the formative first two years of life in children in urban shantytowns have profound consequences of impaired physical and cognitive development lasting into later childhood and schooling. Based on these previous studies showing that apolipoprotein E4 (APOE4) is relatively common in favela children, we review recent data suggesting a protective role for the APOE4 allele in the cognitive and physical development of children with heavy burdens of diarrhea in early childhood. Despite being a marker for cognitive decline with Alzheimer's and cardiovascular diseases later in life, APOE4 appears to be important for cognitive development under the stress of heavy diarrhea. The reviewed findings provide a potential explanation for the survival advantage in evolution of the thrifty APOE4 allele and raise questions about its implications for human development under life-style changes and environmental challenges.

Apolipoprotein E knockout mice have accentuated malnutrition with mucosal disruption and blunted insulin-like growth factor I responses to refeeding

Apolipoprotein E (apoE) is synthesized mainly in the liver and in the brain and is critical for cholesterol metabolism and recovery from brain injury. However, although apoE mRNA increases at birth, during suckling, and after fasting in rat liver, little is known about its role in early postnatal development. Using an established postnatal malnutrition model and apoE knock-out (ko) mice, we examined the role of apoE in intestinal adaptation responses to early postnatal malnutrition. Wild-type and apoE-ko mice were separated from their lactating dams for defined periods each day (4 hours on day 1, 8 hours on day 2, and 12 hours thereafter). We found significant growth deficits, as measured by weight gain or tail length, in the apoE-ko mice submitted to a malnutrition challenge, as compared with malnourished wild type, especially during the second week of postnatal development (P < .05). In addition, apoE-ko animals failed to show growth catch-up after refeeding, compared with wild-type malnourished controls. Furthermore, we found shorter crypts and reduced villus height and area in the apoE-ko malnourished mice, compared with controls, after refeeding. Insulinlike growth factor 1 expression was also blunted in the ileum in apoE-ko mice after refeeding, compared with wild-type controls, which exhibited full insulinlike growth factor 1 expression along the intestinal crypts, villi, and in the muscular layer. Taken together, these findings suggest the importance of apoE in coping with a malnutrition challenge and during the intestinal adaptation after refeeding.

Intestinal tumorigenesis in Min mice is enhanced by X-irradiation in an age-dependent manner

We examined the effect of X-irradiation on intestinal tumorigenesis in Min (multiple intestinal neoplasia) mice. Single whole-body irradiation was given to mice of various ages from newborn to young adults. On the C57BL/6J (B6) background, X-irradiation increased tumor multiplicity of the small intestine exposed at ages from 2-3 days to 24-25 days, with a peak of 2.7-fold increase at 10-12 days of age; exposure at later ages resulted in only a slight increase. X-irradiation also increased colonic tumors; however, the susceptible age period appeared earlier than that of the small intestine; the peak value of 4.6-fold increase was observed in the exposure at around 2-3 days of age. Irradiation at 24 days or later ages showed almost no effect on the colonic tumor induction. On the (B6 x MSM)F1 background, X-irradiation resulted in 2.7-fold increase in the small intestinal tumors, but no increase in the colonic tumors, and besides, the age dependency observed in the small intestinal tumors was much attenuated. Collectively, we conclude that tumorigenic efficacy of X-irradiation in Min mice was determined by the combination of the target organ, the age at exposure, and the genetic background.

Cell cycle time and rate of entry of cells into mitosis in the small intestine of young rats

Cell cycle time (T(C)) and the rate of entry of cells into mitosis (r(M)) in the jejunum and duodenum of young rats were investigated using the stathmokinetic method. The cell cycle times in the jejunum were 24.3 and 28.3 h in light and dark periods, respectively. Cell cycle times in the duodenum were 17.1 and 21.5 h in light and dark periods, respectively. Rates of entry of cells into mitosis in the jejunum were 1.2 and 1.1 cells/cell/h in light and dark periods and rates of entry of cells into mitosis in the duodenum were 1.4 and 1.8 cells/cell/h in light and dark periods, respectively. Although these changes to cell cycle time values are not statistically significant, the variation between the two periods should be considered in relation to its possible biological effects.

Differential appearance of T cell subsets in the large and small intestine of neonatal mice

We examined the appearance of intestinal intraepithelial lymphocytes (IEL) during the first 12 wk of life to gain insight into postnatal factors that contribute to the differences found between IEL in the large and small intestines of adult mice. Intestinal T cells were very infrequent at birth, but increased in number in the large and small intestine during the first 4 wk of life and then stabilized. The small intestinal epithelium at 2 wk of age contained mostly T cell receptor (TCR) alphabeta+, CD2+ T cells, unlike IEL in adult mice, which were composed of nearly equal proportions of CD2-, TCR alphabeta+ and TCR gammadelta+ cells. Between 2 and 3 wk of age, TCR gammadelta+, CD2- IEL increased greatly in the small intestine, whereas TCR alphabeta+ cells expressing CD2 decreased. By contrast, IEL in the large intestine at 2 and 3 wk of age were mostly TCR alphabeta+, CD2+ T cells similar to large intestinal IEL in adult mice. And finally, the expression of CD69 increased earlier and to higher levels on TCR alphabeta+ and TCR gammadelta+ IEL in the small intestine than in the large intestine. Our results demonstrate that IEL in the large and small intestine are phenotypically similar during suckling and that differences between these populations are established after weaning. Furthermore, the earlier accumulation of IEL with an activated adult IEL phenotype in the small intestine suggests that these T cells mature or expand in the gut and contribute to the maturation of immune function during postnatal life in mice.

Creating and maintaining the gastrointestinal ecosystem: what we know and need to know from gnotobiology

Studying the cross talk between nonpathogenic organisms and their mammalian hosts represents an experimental challenge because these interactions are typically subtle and the microbial societies that associate with mammalian hosts are very complex and dynamic. A large, functionally stable, climax community of microbes is maintained in the murine and human gastrointestinal tracts. This open ecosystem exhibits not only regional differences in the composition of its microbiota but also regional differences in the differentiation programs of its epithelial cells and in the spatial distribution of its component immune cells. A key experimental strategy for determining whether "nonpathogenic" microorganisms actively create their own regional habitats in this ecosystem is to define cellular function in germ-free animals and then evaluate the effects of adding single or several microbial species. This review focuses on how gnotobiotics-the study of germ-free animals-has been and needs to be used to examine how the gastrointestinal ecosystem is created and maintained. Areas discussed include the generation of simplified ecosystems by using genetically manipulatable microbes and hosts to determine whether components of the microbiota actively regulate epithelial differentiation to create niches for themselves and for other organisms; the ways in which gnotobiology can help reveal collaborative interactions among the microbiota, epithelium, and mucosal immune system; and the ways in which gnotobiology is and will be useful for identifying host and microbial factors that define the continuum between nonpathogenic and pathogenic. A series of tests of microbial contributions to several pathologic states, using germ-free and ex-germ-free mice, are proposed.

Cell proliferation and migration in the jejunum of suckling rats submitted to progressive fasting

Cell proliferation and migration in the intestinal crypts, and cell migration in the villus are controlled by different mechanisms in adult rats. In the present study, weanling rats and fasting rats were used to quantitatively study the correlation of cell cycle parameters and epithelial cell migration in crypts and intestinal villi. Eighteen-day-old rats received a single injection of tritiated thymidine [3H]TdR (23:00 h); half of the pups were submitted to fasting 5 h earlier. Cell proliferation was determined in radioautographs of jejunal crypts, on the basis of the labeling indices (LI) taken 1, 8, 13 and 19 h after [3H]TdR. The results showed that the labeling index did not differ 1 h or 19 h after [3H]TdR between the fed (38.7% or 48%) and fasting groups (34.6% or 50.4%). The modified method of grain count halving indicated that cell cycle time did not differ between fed (16.5 h) and fasting rats (17.8 h); the growth fraction, however, had lower values in fasting (59%) than in fed rats (77%). Cell migration in the crypt, estimated by the LI obtained for each cell position, did not change with treatment. As for the villi, the cell migration rate was significantly retarded by 3 cell positions (8%). These results suggest that the cell migration in the villi of weanling pups does not depend directly on the cell proliferation and migration in the intestinal crypt, but is directly affected by the absence of food in the lumen.

An investigation into the reversibility of the morphological and cytokinetic changes seen in the small intestine of riboflavin deficient rats

BACKGROUND

Impaired iron handling in riboflavin deficiency is thought to be partially a result of significant morphological and cytokinetic changes within the small intestine.

AIMS

The aim of the study was to find out if the responses of the rat small intestine to riboflavin deficiency induced at weaning could be reversed upon repletion.

SUBJECTS

48 female weanling Wistar rats were used for the purpose of the study.

METHODS

Rats were fed a riboflavin deficient diet or a complete control diet for a period of five weeks followed by a repletion period of up to three weeks. Rats were killed on day 0, 2, 7, or 21 of repletion. The duodenum was removed and fixed for subsequent analysis.

RESULTS

Five weeks of riboflavin deficiency significantly changed the morphology and cytokinetics of the duodenum; the changes were not reversed within the 21 day repletion period despite biochemical evidence for a correction of the deficiency.

CONCLUSIONS

The results show that the small intestine cannot readily recover from a period of riboflavin deficiency induced at weaning, supporting the notion that the weaning period is a critical time for gastrointestinal development and highlighting the importance of adequate nutrition during infancy.

Paneth cell differentiation in the developing intestine of normal and transgenic mice

Paneth cells represent one of the four major epithelial lineages in the mouse small intestine. It is the only lineage that migrates downward from the stem-cell zone located in the lower portion of the crypt of Lieberkühn to the crypt base. Mature Paneth cells release growth factors, digestive enzymes, and antimicrobial peptides from their apical secretory granules. Some of these factors may affect the crypt stem cell, its transit-cell descendants, differentiating villus-associated epithelial lineages, and/or the gut microflora. We used single and multilabel immunocytochemical methods to study Paneth cell differentiation during and after completion of gut morphogenesis in normal, gnotobiotic, and transgenic mice as well as in intestinal isografts. This lineage emerges coincident with cytodifferentiation of the fetal small intestinal endoderm, formation of crypts from an intervillus epithelium, and establishment of a stem-cell hierarchy. The initial differentiation program involves sequential expression of cryptdins, a phospholipase A2 (enhancing factor), and lysozyme. A dramatic increase in Paneth cell number per crypt occurs during postnatal days 14-28, when crypts proliferate by fission. Accumulation of fucosylated and sialylated glycoconjugates during this period represents the final evolution of the lineage's differentiation program. Establishment of this lineage is not dependent upon instructive interactions from the microflora. Transgenic mice containing nucleotides -6500 to +34 of the Paneth cell-specific mouse cryptdin 2 gene linked to the human growth hormone gene beginning at its nucleotide +3 inappropriately express human growth hormone in a large population of proliferating and nonproliferating cells in the intervillus epithelium up to postnatal day 5. Transgene expression subsequently becomes restricted to the Paneth cell lineage in the developing crypt. Cryptdin 2 nucleotides -6500 to +34 should be a useful marker of crypt morphogenesis and a valuable tool for conducting gain-of-function or loss-of-function experiments in Paneth cells.

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.

Colonic cell proliferation and growth fraction in young, adult and old rats

Colonic epithelial proliferation was investigated in three groups of rats, aged 3, 60 and 121 weeks. As reported in previous work, the crypts were markedly longer in the young rats, and the number of labelled cells per crypt was significantly greater. There was an upward movement of the marker positions derived from the distribution of labelled cells within the crypt of the young rats. This was a consequence of the increased crypt length, so that the growth fraction, as expressed as a percentage of crypt length, was the same. The proliferative changes between the young rats and the other aged rats were therefore effected by altering the size of the crypts, while maintaining the kinetic organisation. There was no evidence of any proliferative changes or changes in the growth fraction when the colons of the old rats were compared with those of the 60 week old rats.

Use of transgenic mice to map cis-acting elements in the intestinal fatty acid binding protein gene (Fabpi) that control its cell lineage-specific and regional patterns of expression along the duodenal-colonic and crypt-villus axes of the gut epithelium

The mouse intestinal epithelium is able to establish and maintain complex lineage-specific, spatial, and temporal patterns of gene expression despite its rapid and continuous renewal. A multipotent stem cell located near the base of each intestinal crypt gives rise to progeny which undergo amplification and allocation to either enterocytic, Paneth cell, goblet cell, or enteroendocrine cell lineages. Differentiation of these four lineages occurs during their geographically ordered migration along the crypt-villus axis. Gut stem cells appear to have a "positional address" which is manifested by differences in the differentiation programs of their lineal descendants along the duodenal-colonic (cephalocaudal) axis. We have used the intestinal fatty acid binding protein gene (Fabpi) as a model to identify cis-acting elements which regulate cell- and region-specific patterns of gene expression in the gut. Nucleotides -1178 to +28 of rat Fabpi direct a pattern of expression of a reporter (human growth hormone [hGH]) which mimics that of mouse Fabpi (a) steady-state levels of hGH mRNA are highest in the distal jejunum of adult transgenic mice and fall progressively toward both the duodenum and the mid-colon; and (b) hGH is confined to the enterocytic lineage and first appears as postmitotic, differentiating cells exit the crypt and migrate to the base of small intestinal villi or their colonic homologs, the surface epithelial cuffs. Nucleotides -103 to +28, which are highly conserved in rat, mouse and human Fabpi, are able to correctly initiate transgene expression in late fetal life, restrict hGH to the enterocytic lineage, and establish an appropriate cephalocaudal gradient of reporter expression. This cephalocaudal gradient is also influenced by cis-acting elements located between nucleotides -1178 and -278, and -277 and -185 that enhance and suppress (respectively) expression in the ileum and colon and by element(s) located upstream of nucleotide -277 that are needed to sustain high levels of hGH production after weaning. Nucleotides -277 to -185 contain part of a domain conserved between the three orthologous Fabpi genes (nucleotides -240 to -159), a 24-bp element (nucleotides -212 to -188) that binds nuclear factors present in colonic but not small intestinal epithelial cells, and a portion of a CCAAT/enhancer binding protein footprint (C/EBP alpha, nucleotides -188 to -167). Removal of nucleotides -277 to -185 (yielding I-FABP-184 to +28/hGH+3) results in inappropriate expression of hGH in proliferating and nonproliferating epithelial cells located in the mid and upper portions of duodenal, jejunal, ileal, and colonic crypts without affecting the "shape" of the cephalocaudal gradient of transgene expression.(ABSTRACT TRUNCATED AT 400 WORDS)

Intestinal transferrin receptors and iron absorption in the neonatal rat

The transferrin receptor is a major protein found on the basolateral membranes of intestinal epithelial cells, yet its possible role in intestinal iron metabolism and also in iron absorption is unclear. We have studied intestinal transferrin receptor expression during the peri- and postnatal development of the small intestine of the rat using immunohistochemistry with a monoclonal antibody to the rat receptor. Two major changes in transferrin receptor expression in the developing small intestine were found, a decrease in receptor expression associated with birth, and an increase at the time of weaning. Around the time of weaning there was a large decrease in iron absorption, but there was no direct correlation between absorption and transferrin receptor expression. However, at both birth and weaning there were major changes in intestinal cell kinetics, and the distribution of receptor correlated well with the distribution of proliferating cell populations. In addition, as the intestinal epithelial cells differentiated and stopped dividing, there was a redistribution of transferrin receptors from the cell surface to intracellular sites. These data suggest that the most likely role of the transferrin receptor in the neonatal intestine is in the supply of iron to the developing epithelial cells in the crypts, and that the receptor does not play a direct role in iron transit across the intestinal epithelium.

Migration of fetal intestinal intervillous cells in neonatal mice

The migration of intestinal intervillous epithelial cells labeled in the fetus was followed in neonatal mice. At 17 days of gestation, a first group of pregnant mice received three intraperitoneal injections of 3H-thymidine (150 microCi/injection) administered at 30 min intervals. Two mothers were sacrificed 3 hours after the first injection. Mice from different litters were also sacrificed on days 0, 2, 4, 8, 12, 14, and 16 after birth. A second group of pregnant mice was injected at 18 1/2 days of gestation and offspring were sacrificed on days 6, 8, 10, 12, 14, and 16 after birth. Segments of duodenum and ileum were fixed in glutaraldehyde, postfixed in osmium tetroxide, dehydrated, and embedded in Epon. Sections were stained with aldehyde fuchsin and processed for radioautography. By following the leading front and trailing edge of labeled cells in the longest villi of the duodenum and ileum, we observed that 1) extrusion zones become active immediately after birth and 2) the longest villi do not elongate until 10 days after birth in the duodenum and 14 days in the ileum, that is, when all labeled epithelial cells originally present in the fetus have been extruded. Moreover, by measuring the distance between the internal limit of the inner circular layer of smooth muscle and the intervillous epithelium at 17 days of gestation (12.95 +/- 1.18 microns) or the bottom of the crypts at day 3 (14.81 +/- 0.91 microns), we propose that crypts do not develop as downgrowths: rather the intervillous epithelium is reshaped and the crypt-villus junction moves upward, away from the muscularis externa.(ABSTRACT TRUNCATED AT 250 WORDS)

Specific expression of lactase in the jejunum and colon during postnatal development and hormone treatments in the rat

The expression of lactase was compared in the jejunum and colon of the rat at the levels of enzyme activity and protein and RNA content. We found that the enzyme proteins and the corresponding mRNAs share common features and are encoded by a single gene in both intestinal segments. In the jejunum, large amounts of lactase mRNA and proteins were detected during postnatal development as well as in adult rats, despite the 10-fold decline in lactase specific activity which occurs at weaning. In contrast, in the colon the expression of lactase was restricted to early postnatal development. In the colon, the enzymic activity of lactase and the amounts of protein and mRNA followed parallel development profiles with a peak at day 4 after birth. Injections of thryoxine or epidermal growth factor into neonates led to small modifications in the expression of lactase in the jejunum. On the other hand, these treatments caused a large decline in lactase activity in the colon that paralleled a decrease in the amount of lactase protein and mRNA. These data indicate that the expression of lactase is mainly regulated at the post-transcriptional level in the jejunum, whereas it is controlled at the pretranslational level in the colon.

Maturation of the rat small intestine at weaning: changes in epithelial cell kinetics, bacterial flora, and mucosal immune activity

The relationship between maturation of the small intestine and change in mucosal immune activity was examined in the DA rat during the weaning period from 12 to 30 days. Two stages of jejunal maturation were observed: an initial stage of morphological development and crypt proliferation (days 12 to 22), followed by a period of stabilisation (days 24 to 30). By day 22 of the initial phase, villi increased principally in width but not in length, crypt length increased, and crypt cell production rate increased from 0.5 (day 12) to 11.1 (day 22) cells/crypt/hour. Various measures of mucosal immune activity showed a biphasic response. Up to days 20 to 22, the weight of the mesenteric lymph node increased seven-fold (p less than 0.0001), counts of jejunal eosinophils and goblet cells increased 3- (p less than 0.0001) and 19-fold (p less than 0.0001) respectively, and mean serum rat mucosal mast cell protease II, released from mucosal mast cells, increased from 24 (day 12) to 313 (day 22) ng/ml (p less than 0.0001). After day 22, mesenteric lymph node weight stabilised, eosinophil count stabilised and goblet cells decreased, serum rat mucosal mast cell protease II decreased three-fold (p less than 0.0001), and mean jejunal count of intraepithelial lymphocytes increased from 26 (day 22) to 54 (day 24) cells per mm of muscularis mucosae (p less than 0.0001), before stabilising. These results demonstrated a close association between maturation of the small intestine and change in activity of the mucosal immune system.

The growth and life of a monoclonal crypt

A computer simulation of a highly dynamic model for the birth, growth and adult life of a monoclonal crypt in the intestine was developed starting with a single precursor stem cell. The intestinal epithelial system was studied and observed in analogy to 'in vivo' experiments. The model output, e.g. the geometric shape of a crypt, mitotic index, labelling index and the crypt length distribution in adult state, was compared with experimental data. There was evidence from the simulation that a certain steady state in the adult life could be reached regardless of some harmless influences in post-natal life, e.g. the influence of being weaned or non-weaned. The model is based on our hypothesis of the generation-controlled proliferation mechanism and at the same time is a confirmation of it.

Effect of graft versus host reaction on cell cycle time in neonatal mouse jejunum

The duration of cell cycle parameters in control mouse jejunum has been compared with that found following induction of a graft-versus-host reaction (GvHR) during the first 3 weeks of postnatal life. Values for tc and tG1 were found to decrease progressively during normal development: estimates for the whole crypt column in 21-day-old mice were approximately half to one quarter those found 6 days after birth 12.1 +/- 0.5 hr and 24.2 +/- 0.3 hr for tc; 2.8 +/- 0.3 hr and 12.1 +/- 0.3 hr for tG1 respectively; (means +/- SE). tS and tG2 were found to remain approximately constant during this period of neonatal development. Injecting foreign spleen cells into 3-day-old mice produced no effect on crypt cell proliferation or cell cycle parameters measured 3 days later. GvHR mice studied 8 days after spleen cell injection, however, showed both an increase in crypt cell proliferation and decreases in the values for tc and tG1 to levels similar to those normally found in 21-day-old control animals (tc 12.4 +/- 0.4 hr and tG1 5.4 +/- 0.4 hr for 11-day-old GvHR mice). The possible mechanism leading to these changes is discussed. The ability of GvHR to stimulate cell proliferation is used in the present work to test the hypothesis that the total number of cell divisions taking place after birth determines the temporal sequence of changes in disaccharidase content produced during neonatal development.

Crypt cell development in newborn rat small intestine

Three monoclonal antibodies were prepared against luminal membranes from small intestinal cells of 3-d-old rats (YBB 1/27, YBB 3/10) and crypt cell membranes from adult rats (CC 4/80). The antibodies were shown to define specific stages of development of the intestinal crypt cells. The YBB 1/27 antigen was first detected at the luminal membrane of the epithelial cells in fetal intestine at day 20 of gestation; it was confined to the crypt cells and lower villus cells between 1 and 20-22 d after birth, and could not be detected in any region of the intestine in older animals. The YBB 3/10 antigen, identified as a set of high Mr proteins, was localized over the entire surface membrane of fetal intestinal cells and of crypt and villus cells after birth; after weaning (20-22 d after birth) it gradually disappeared from the villus cells and became confined to the region of the crypts. The CC 4/80 antigen, identified as a protein (or a set of related proteins) of molecular mass 28-34 kD, was shown to appear in the crypt cells 10-14 d after birth. Its distribution changed after weaning, when it disappeared from the crypts, and was localized in the absorptive lower villus cells. This change in pattern could, in part, be prematurely elicited by cortisone injection in younger animals. These results have demonstrated the presence of specific surface membrane components on the intestinal crypt cells, and suggested that fetal antigens may be retained in these cells after birth.