Intestinal brush border revisited

Gastrointestinal Permeation Enhancers Beyond Sodium Caprate and SNAC - What is Coming Next?

Oral peptide delivery is trending again. Among the possible reasons are the recent approvals of two oral peptide formulations, which represent a huge stride in the field. For the first time, gastrointestinal (GI) permeation enhancers (PEs) are leveraged to overcome the main limitation of oral peptide delivery-low permeability through the intestinal epithelium. Despite some success, the application of current PEs, such as salcaprozate sodium (SNAC), sodium caprylate (C8), and sodium caprate (C10), is generally resulting in relatively low oral bioavailabilities (BAs)-even for carefully selected therapeutics. With several hundred peptide-based drugs presently in the pipeline, there is a huge unmet need for more effective PEs. Aiming to provide useful insights for the development of novel PEs, this review summarizes the biological hurdles to oral peptide delivery with special emphasis on the epithelial barrier. It describes the concepts and action modes of PEs and mentions possible new targets. It further states the benchmark that is set by current PEs, while critically assessing and evaluating emerging PEs regarding translatability, safety, and efficacy. Additionally, examples of novel PEs under preclinical and clinical evaluation and future directions are discussed.

The emerging roles of the cytoskeleton in intestinal epithelium homeostasis

The intestinal epithelium must absorb many nutrients and water while forming a barrier that is impermeable to pathogens present in the external environment. Concurrently to fulfill this dual role, the intestinal epithelium is challenged by a rapid renewal of cells and forces resulting from digestion. Hence, intestinal homeostasis requires precise control of tissue integrity, tissue renewal, cell polarity, and force generation/transmission. In this review, we highlight the contribution of the cell cytoskeleton- actin, microtubules, and intermediate filaments- to intestinal epithelium homeostasis. With a focus on enterocytes, we first discuss the role of these networks in the formation and maintenance of cell-cell and cell-matrix junctions. Then, we cover their role in intracellular trafficking related to the apicobasal polarity of enterocytes. Finally, we report on the cytoskeletal changes that occur during tissue renewal. In conclusion, the importance of the cytoskeleton in maintaining intestinal homeostasis is emerging, and we think this field will keep evolving.

Physiological effects of in ovo delivery of bioactive substances in broiler chickens

The poultry industry has improved genetics, nutrition, and management practices, resulting in fast-growing chickens; however, disturbances during embryonic development may affect the entire production cycle and cause irreversible losses to broiler chicken producers. The most crucial time in the chicks' development appears to be the perinatal period, which encompasses the last few days of pre-hatch and the first few days of post-hatch. During this critical period, intestinal development occurs rapidly, and the chicks undergo a metabolic and physiological shift from the utilization of egg nutrients to exogenous feed. However, the nutrient reserve of the egg yolk may not be enough to sustain the late stage of embryonic development and provide energy for the hatching process. In addition, modern hatchery practices cause a delay in access to feed immediately post-hatch, and this can potentially affect the intestinal microbiome, health, development, and growth of the chickens. Development of the in ovo technology allowing for the delivery of bioactive substances into chicken embryos during their development represents a way to accommodate the perinatal period, late embryo development, and post-hatch growth. Many bioactive substances have been delivered through the in ovo technology, including carbohydrates, amino acids, hormones, prebiotics, probiotics and synbiotics, antibodies, immunostimulants, minerals, and microorganisms with a variety of physiological effects. In this review, we focused on the physiological effects of the in ovo delivery of these substances, including their effects on embryo development, gastrointestinal tract function and health, nutrient digestion, immune system development and function, bone development, overall growth performance, muscle development and meat quality, gastrointestinal tract microbiota development, heat stress response, pathogens exclusion, and birds metabolism, as well as transcriptome and proteome. We believe that this method is widely underestimated and underused by the poultry industry.

Bioengineered intestinal tubules as a tool to test intestinal biological efficacy of lettuce species

Lettuce (Lactuca sativa) is one of the most consumed and cultivated vegetables globally. Its breeding is focused on the improvement of yield and disease resistance. However, potential detrimental or beneficial health effects for the consumer are often not targeted in the breeding programs. Here, a bioengineered intestinal tubule was used to assess the intestinal efficacy of extracts from five plant accessions belonging to four Lactuca species. These four species include the domesticated L. sativa, closely related wild species L. serriola, and phylogenetically more distant wild relatives L. saligna and L. virosa. We assessed the epithelial barrier integrity, cell viability, cell attachment, brush border enzyme activity, and immune markers. Extracts from L. sativa cv. Salinas decreased cell attachment and brush border enzyme activity. However, extracts from the non-edible wild species L. saligna and L. virosa reduced the epithelial barrier functions, cell attachment, cell viability, and brush border enzyme activity. Since wild species represent a valuable germplasm pool, the bioengineered intestinal tubules could open ways to evaluate the safety and nutritional properties of the lettuce breeding material originating from crosses with wild Lactuca species.

Multiomics Analyses Reveal That Long-Term Intake of Hesperetin-7-O-glucoside Modulates the Gut Microbiota and Bile Acid Metabolism in Mice

Hesperetin-7-O-glucoside (Hes-7-G) is a typical flavonoid monoglucoside, which can be generated from hesperidin with the removal of rhamnose by hydrolysis. Untargeted and targeted metabolomics together with 16S rRNA gene sequencing were employed to explore the exact absorption site of Hes-7-G and its beneficial effect in mice. Intestinal ¹H nuclear magnetic resonance (NMR)-based metabolomics screening showed that Hes-7-G is mainly metabolized in the small intestine of mice, especially the ileum segment. Quantification analysis of bile acids (BAs) in the liver, intestinal tract, feces, and serum of mice suggests that Hes-7-G intake accelerates the processes of biosynthesis and excretion of BAs, thus promoting digestion and lowing hepatic cholesterol and triglyceride. 16S rRNA gene sequencing reveals that Hes-7-G significantly elevates the diversity of the gut microbiota in mice, especially those bacteria associated with BA secondary metabolism. These results demonstrated that long-term dietary Hes-7-G plays beneficial roles in health by modulating the gut bacteria and BA metabolism in mice.

Adaptive differentiation promotes intestinal villus recovery

Loss of differentiated cells to tissue damage is a hallmark of many diseases. In slow-turnover tissues, long-lived differentiated cells can re-enter the cell cycle or transdifferentiate to another cell type to promote repair. Here, we show that in a high-turnover tissue, severe damage to the differentiated compartment induces progenitors to transiently acquire a unique transcriptional and morphological postmitotic state. We highlight this in an acute villus injury model in the mouse intestine, where we identified a population of progenitor-derived cells that covered injured villi. These atrophy-induced villus epithelial cells (aVECs) were enriched for fetal markers but were differentiated and lineage committed. We further established a role for aVECs in maintaining barrier integrity through the activation of yes-associated protein (YAP). Notably, loss of YAP activity led to impaired villus regeneration. Thus, we define a key repair mechanism involving the activation of a fetal-like program during injury-induced differentiation, a process we term "adaptive differentiation."

Human small intestinal organotypic culture model for drug permeation, inflammation, and toxicity assays

The gastrointestinal tract (GIT), in particular, the small intestine, plays a significant role in food digestion, fluid and electrolyte transport, drug absorption and metabolism, and nutrient uptake. As the longest portion of the GIT, the small intestine also plays a vital role in protecting the host against pathogenic or opportunistic microbial invasion. However, establishing polarized intestinal tissue models in vitro that reflect the architecture and physiology of the gut has been a challenge for decades and the lack of translational models that predict human responses has impeded research in the drug absorption, metabolism, and drug-induced gastrointestinal toxicity space. Often, animals fail to recapitulate human physiology and do not predict human outcomes. Also, certain human pathogens are species specific and do not infect other hosts. Concerns such as variability of results, a low throughput format, and ethical considerations further complicate the use of animals for predicting the safety and efficacy xenobiotics in humans. These limitations necessitate the development of in vitro 3D human intestinal tissue models that recapitulate in vivo–like microenvironment and provide more physiologically relevant cellular responses so that they can better predict the safety and efficacy of pharmaceuticals and toxicants. Over the past decade, much progress has been made in the development of in vitro intestinal models (organoids and 3D-organotypic tissues) using either inducible pluripotent or adult stem cells. Among the models, the MatTek’s intestinal tissue model (EpiIntestinal™ Ashland, MA) has been used extensively by the pharmaceutical industry to study drug permeation, metabolism, drug-induced GI toxicity, pathogen infections, inflammation, wound healing, and as a predictive model for a clinical adverse outcome (diarrhea) to pharmaceutical drugs. In this paper, our review will focus on the potential of in vitro small intestinal tissues as preclinical research tool and as alternative to the use of animals.

Differences in human milk peptide release along the gastrointestinal tract between preterm and term infants

BACKGROUND & AIMS

Preterm infants are born with a gastrointestinal tract insufficiently developed to digesting large quantities of human milk proteins. Peptides released from the digestion of human milk proteins have been identified with bioactivities that may be beneficial to the developing infant. However, it is unknown how prematurity affects total and bioactive peptide release along the gastrointestinal tract. The aim of this study was to compare milk peptide release from milk to stomach to stool between preterm and term infants.

METHODS

Milk, gastric, and stool samples were collected from preterm infants as early collection (days 8 and 9 of life) and late collection (days 21 and 22 of life), and from term infants as early collection. Milk peptides were extracted from the samples and identified using Orbitrap mass spectrometry. Peptide abundance and count were compared across digestion and between the three infant groups at each stage of digestion.

RESULTS

Total milk peptide count and abundance increased from milk to stomach then decreased in stool. Total peptide release was similar among the three infant groups for milk and stool samples. In the stomach, preterm early collection had significantly higher peptide abundance and count than the other two groups. Patterns for peptide release from individual milk proteins were distinct from total peptide release both across digestion and among the infant groups. When analyzing single peptides, term early collection gastric samples had significantly higher peptide abundance than preterm early collection for a known antimicrobial peptide, QELLLNPTHQIYPVTQPLAPVHNPISV.

CONCLUSIONS

Preterm and term infants digest milk proteins differently along their gastrointestinal tracts. While preterm infants released more total peptides in the stomach, term infants released specific bioactive peptides at higher abundance. We identified a region at the C-terminus of β-casein that is conserved from milk through stool and from which are released known and potential antimicrobial peptides.

Bringing the digestibility of prebiotics into focus: update of carbohydrate digestion models

Oro-gastrointestinal digestion of dietary carbohydrates involves up to six different carbohydrases in a multistage process. Enzymes from the small intestinal brush border membrane play a major role in the digestibility of these substrates. However, to date, the inclusion of these small intestinal enzymes has been dismissed in most in vitro studies carried out, despite their importance in the degradation of carbohydrates. Several in vitro and in vivo studies have demonstrated the capability of brush border enzymes to degrade certain "non-digestible" carbohydrates to a different extent depending on their structural composition (monomeric composition, glycosidic linkage, etc.). In this sense, considering the available evidence, mucosal disaccharidases embedded in the small intestinal brush border membrane vesicles must be considered in addition to α-amylases; therefore, new approaches for the evaluation of the digestibility of carbohydrates have been recently reported. These new methods based on the utilization of the small intestinal enzymes present in the brush border membrane aim to fulfill the final and key step of the digestion of carbohydrates in the small intestine. Here, rat small intestinal extract enzymes as well as brush border membrane vesicles from pig have emerged as very reliable and useful tools to evaluate carbohydrate digestion. Thus, this review aims to go briefly through the most relevant digestion methods for carbohydrates that are currently available and to highlight the new improved methods, which include mammalian intestinal enzymes, and their current use in the evaluation of the digestibility of prebiotics.

Simulating human digestion: developing our knowledge to create healthier and more sustainable foods

The gold standard for nutrition studies is clinical trials but they are expensive and variable, and do not always provide the mechanistic information required, hence the increased use ofin vitroand increasinglyin silicosimulations of digestion.

Quantitative sequence-activity modeling of ACE peptide originated from milk using ACC-QTMS amino acid indices

Up to now, numerous peptides/hydrolysates derived from casein and whey protein have shown angiotensin-I-converting enzyme (ACE) inhibitory. In this research, quantum topological molecular similarity (QTMS) indices of amino acids were utilized in quantitative sequence-activity modeling (QSAM) to predict the activity of a set of milk-driven peptides with ACE inhibition. Since the derived peptides have not the same number of residues, we overcame this issue by auto cross covariance (ACC) methodology. Then, some QSAMs were built to predict the pIC₅₀ value of ACE peptides derived from Bovine Casein and Whey. The model established an acceptable relationship between the selected variables and the pIC₅₀ of the peptides. To estimate the performance of the developed models, casein and whey proteins from human, goat, bovine and sheep were virtually broken by trypsin and chymotrypsin enzymes and the ACE activity of the resultant virtual peptides were predicted and some new ACE peptides were proposed.

"Candida Albicans Interactions With The Host: Crossing The Intestinal Epithelial Barrier"

Formerly a commensal organism of the mucosal surfaces of most healthy individuals, Candida albicans is an opportunistic pathogen that causes infections ranging from superficial to the more life-threatening disseminated infections, especially in the ever-growing population of vulnerable patients in the hospital setting. In these situations, the fungus takes advantage of its host following a disturbance in the host defense system and/or the mucosal microbiota. Overwhelming evidence suggests that the gastrointestinal tract is the main source of disseminated C. albicans infections. Major risk factors for disseminated candidiasis include damage to the mucosal intestinal barrier, immune dysfunction, and dysbiosis of the resident microbiota. A better understanding of C. albicans' interaction with the intestinal epithelial barrier will be useful for designing future therapies to avoid systemic candidiasis. In this review, we provide an overview of the current knowledge regarding the mechanisms of pathogenicity that allow the fungus to reach and translocate the gut barrier.

Trans-β-galactosidase activity of pig enzymes embedded in the small intestinal brush border membrane vesicles

This work highlights the utility of brush border membrane vesicles (BBMV) from the pig small intestine as a reliable model for gathering information about the reaction mechanisms involved in the human digestion of dietary carbohydrates. Concretely, the elucidation of the transgalactosylation mechanism of pig BBMV to synthesize prebiotic galacto-oligosaccharides (GOS) is provided, unravelling the catalytic activity of mammalian small intestinal β-galactosidase towards the hydrolysis of GOS. This study reveals that pig BBMV preferably synthesizes GOS linked by β-(1 → 3) bonds, since major tri- and disaccharide were produced by the transfer of a galactose unit to the C-3 of the non-reducing moiety of lactose and to the C-3 of glucose, respectively. Therefore, these results point out that dietary GOS having β-(1 → 3) as predominant glycosidic linkages could be more prone to hydrolysis by mammalian intestinal digestive enzymes as compared to those linked by β-(1 → 2), β-(1 → 4), β-(1 ↔ 1) or β-(1 → 6). Given that these data are the first evidence on the transglycosylation activity of mammalian small intestinal glycosidases, findings contained in this work could be crucial for future studies investigating the structure-small intestinal digestibility relationship of a great variety of available prebiotics, as well as for designing tailored fully non-digestible GOS.

Oral co-administration of a bacterial protease inhibitor in the vaccine formulation increases antigen delivery at the intestinal epithelial barrier

The study of capture and processing of antigens (Ags) by intestinal epithelial cells is very important for development of new oral administration systems. Efficient oral Ag delivery systems must resist enzymatic degradation by gastric and intestinal proteases and deliver the Ag across biological barriers. The recombinant unlipidated outer membrane protein from Brucella spp. (U-Omp19) is a protease inhibitor with immunostimulatory properties used as adjuvant in oral vaccine formulations. In the present work we further characterized its mechanism of action and studied the interaction and effect of U-Omp19 on the intestinal epithelium. We found that U-Omp19 inhibited protease activity from murine intestinal brush-border membranes and cysteine proteases from human intestinal epithelial cells (IECs) promoting co-administered Ag accumulation within lysosomal compartments of IECs. In addition, we have shown that co-administration of U-Omp19 facilitated the transcellular passage of Ag through epithelial cell monolayers in vitro and in vivo while did not affect epithelial cell barrier permeability. Finally, oral co-delivery of U-Omp19 in mice induced the production of Ag-specific IgA in feces and the increment of CD103⁺ CD11b⁻ CD8α⁺ dendritic cells subset at Peyer's patches. Taken together, these data describe a new mechanism of action of a mucosal adjuvant and support the use of this rationale/strategy in new oral delivery systems for vaccines.

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The bacterial protease inhibitor U-Omp19 limits antigens proteolysis by enterocytes.

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Oral co-administration of U-Omp19 increases antigen half-life inside enterocytes.

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U-Omp19 oral administration does not affect epithelial cell barrier permeability.

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Oral co-delivery of U-Omp19 increases frequency of dendritic cells bearing antigen.

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U-Omp19 increases the half-life and immunogenicity of cholera toxin subunit B antigen.

Fructose metabolism, cardiometabolic risk, and the epidemic of coronary artery disease

Despite strong indications that increased consumption of added sugars correlates with greater risks of developing cardiometabolic syndrome (CMS) and cardiovascular disease (CVD), independent of the caloric intake, the worldwide sugar consumption remains high. In considering the negative health impact of overconsumption of dietary sugars, increased attention is recently being given to the role of the fructose component of high-sugar foods in driving CMS. The primary organs capable of metabolizing fructose include liver, small intestine, and kidneys. In these organs, fructose metabolism is initiated by ketohexokinase (KHK) isoform C of the central fructose-metabolizing enzyme KHK. Emerging data suggest that this tissue restriction of fructose metabolism can be rescinded in oxygen-deprived environments. In this review, we highlight recent progress in understanding how fructose metabolism contributes to the development of major systemic pathologies that cooperatively promote CMS and CVD, reference recent insights into microenvironmental control of fructose metabolism under stress conditions and discuss how this understanding is shaping preventive actions and therapeutic approaches.

Human Primary Cell-Based Organotypic Microtissues for Modeling Small Intestinal Drug Absorption

PURPOSE

The study evaluates the use of new in vitro primary human cell-based organotypic small intestinal (SMI) microtissues for predicting intestinal drug absorption and drug-drug interaction.

METHODS

The SMI microtissues were reconstructed using human intestinal fibroblasts and enterocytes cultured on a permeable support. To evaluate the suitability of the intestinal microtissues to model drug absorption, the permeability coefficients across the microtissues were determined for a panel of 11 benchmark drugs with known human absorption and Caco-2 permeability data. Drug-drug interactions were examined using efflux transporter substrates and inhibitors.

RESULTS

The 3D-intestinal microtissues recapitulate the structural features and physiological barrier properties of the human small intestine. The microtissues also expressed drug transporters and metabolizing enzymes found on the intestinal wall. Functionally, the SMI microtissues were able to discriminate between low and high permeability drugs and correlated better with human absorption data (r2 = 0.91) compared to Caco-2 cells (r2 = 0.71). Finally, the functionality of efflux transporters was confirmed using efflux substrates and inhibitors which resulted in efflux ratios of >2.0 fold and by a decrease in efflux ratios following the addition of inhibitors.

CONCLUSION

The SMI microtissues appear to be a useful pre-clinical tool for predicting drug bioavailability of orally administered drugs.

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.

Diagnostic and Research Aspects of Small Intestinal Disaccharidases in Coeliac Disease

Disaccharidases (DS) are brush border enzymes embedded in the microvillous membrane of small intestinal enterocytes. In untreated coeliac disease (CD), a general decrease of DS activities is seen. This manuscript reviews different aspects of DS activities in CD: their utility in the diagnosis and their application to in vitro toxicity testing. The latter has never been established in CD research. However, with the recent advances in small intestinal organoid techniques, DS might be employed as a biomarker for in vitro studies. This includes establishment of self-renewing epithelial cells raised from tissue, which express differentiation markers, including the brush border enzymes. Determining duodenal DS activities may provide additional information during the diagnostic workup of CD: (i) quantify the severity of the observed histological lesions, (ii) provide predictive values for the grade of mucosal villous atrophy, and (iii) aid diagnosing CD where minor histological changes are seen. DS can also provide additional information to assess the response to a gluten-free diet as marked increase of their activities occurs four weeks after commencing it. Various endogenous and exogenous factors affecting DS might also be relevant when considering investigating the role of DS in other conditions including noncoeliac gluten sensitivity and DS deficiencies.

Salmonella Co-opts Host Cell Chaperone-mediated Autophagy for Intracellular Growth

Salmonella enterica are invasive intracellular pathogens that replicate within a membrane-bound compartment inside infected host cells known as the Salmonella-containing vacuole. How Salmonella obtains nutrients for growth within this intracellular niche despite the apparent isolation is currently not known. Recent studies have indicated the importance of glucose and related carbon sources for tissue colonization and intracellular proliferation within host cells during Salmonella infections, although none have been found to be essential. We found that wild-type Salmonella are capable of replicating within infected host cells in the absence of both exogenous sugars and/or amino acids. Furthermore, mutants defective in glucose uptake or dependent upon peptides for growth also showed no significant loss in intracellular replication, suggesting host-derived peptides can supply both carbon units and amino acids. Here, we show that intracellular Salmonella recruit the host proteins LAMP-2A and Hsc73, key components of the host protein turnover pathway known as chaperone-mediated autophagy involved in transport of cytosolic proteins to the lysosome for degradation. Host-derived peptides are shown to provide a significant contribution toward the intracellular growth of Salmonella The results reveal a means whereby intracellular Salmonella gain access to the host cell cytosol from within its membrane-bound compartment to acquire nutrients. Furthermore, this study provides an explanation as to how Salmonella evades activation of autophagy mechanisms as part of the innate immune response.

Extensive in vitro gastrointestinal digestion markedly reduces the immune-toxicity of Triticum monococcum wheat: implication for celiac disease

SCOPE

The ancient diploid Triticum monococcum is of special interest as a candidate low-toxic wheat species for celiac disease patients. Here, we investigated how an in vitro gastro-intestinal digestion, affected the immune toxic properties of gliadin from diploid compared to hexaploid wheat.

METHODS AND RESULTS

Gliadins from Triticum monococcum, and Triticum aestivum cultivars were digested using either a partial proteolysis with pepsin-chymotrypsin, or an extensive degradation that used gastrointestinal enzymes including the brush border membrane enzymes. The immune stimulatory properties of the digested samples were investigated on T-cell lines and jejunal biopsies from celiac disease patients. The T-cell response profile to the Triticum monococcum gliadin was comparable to that obtained with Triticum aestivum gliadin after the partial pepsin-chymotrypsin digestion. In contrast, the extensive gastrointestinal hydrolysis drastically reduced the immune stimulatory properties of Triticum monococcum gliadin. MS-based analysis showed that several Triticum monococcum peptides, including known T-cell epitopes, were degraded during the gastrointestinal treatment, whereas many of Triticum aestivum gliadin survived the gastrointestinal digestion.

CONCLUSION

The pattern of Triticum monococcum gliadin proteins is sufficiently different from those of common hexaploid wheat to determine a lower toxicity in celiac disease patients following in vitro simulation of human digestion.

The Secretion and Action of Brush Border Enzymes in the Mammalian Small Intestine

Microvilli are conventionally regarded as an extension of the small intestinal absorptive surface, but they are also, as latterly discovered, a launching pad for brush border digestive enzymes. Recent work has demonstrated that motor elements of the microvillus cytoskeleton operate to displace the apical membrane toward the apex of the microvillus, where it vesiculates and is shed into the periapical space. Catalytically active brush border digestive enzymes remain incorporated within the membranes of these vesicles, which shifts the site of BB digestion from the surface of the enterocyte to the periapical space. This process enables nutrient hydrolysis to occur adjacent to the membrane in a pre-absorptive step. The characterization of BB digestive enzymes is influenced by the way in which these enzymes are anchored to the apical membranes of microvilli, their subsequent shedding in membrane vesicles, and their differing susceptibilities to cleavage from the component membranes. In addition, the presence of active intracellular components of these enzymes complicates their quantitative assay and the elucidation of their dynamics. This review summarizes the ontogeny and regulation of BB digestive enzymes and what is known of their kinetics and their action in the peripheral and axial regions of the small intestinal lumen.

Digestive system dysfunction in cystic fibrosis: challenges for nutrition therapy

Cystic fibrosis can affect food digestion and nutrient absorption. The underlying mutation of the cystic fibrosis trans-membrane regulator gene depletes functional cystic fibrosis trans-membrane regulator on the surface of epithelial cells lining the digestive tract and associated organs, where Cl(-) secretion and subsequently secretion of water and other ions are impaired. This alters pH and dehydrates secretions that precipitate and obstruct the lumen, causing inflammation and the eventual degradation of the pancreas, liver, gallbladder and intestine. Associated conditions include exocrine pancreatic insufficiency, impaired bicarbonate and bile acid secretion and aberrant mucus formation, commonly leading to maldigestion and malabsorption, particularly of fat and fat-soluble vitamins. Pancreatic enzyme replacement therapy is used to address this insufficiency. The susceptibility of pancreatic lipase to acidic and enzymatic inactivation and decreased bile availability often impedes its efficacy. Brush border digestive enzyme activity and intestinal uptake of certain disaccharides and amino acids await clarification. Other complications that may contribute to maldigestion/malabsorption include small intestine bacterial overgrowth, enteric circular muscle dysfunction, abnormal intestinal mucus, and intestinal inflammation. However, there is some evidence that gastric digestive enzymes, colonic microflora, correction of fatty acid abnormalities using dietary n-3 polyunsaturated fatty acid supplementation and emerging intestinal biomarkers can complement nutrition management in cystic fibrosis.

Stem cell differentiation and lumen formation in colorectal cancer cell lines and primary tumors

Single cancer stem-like cells (CSC) from colorectal cancers can be functionally identified by their ability to form large lumen-containing colonies in three-dimensional Matrigel cultures. These colonies contain the three types of differentiated colorectal epithelial cells, and single cells obtained from them can reproduce themselves and form tumors efficiently in immunodeficient mice. In this study, we show how hypoxia affects these CSC-derived lumens to control differentiation of stem-like cells and enterocytes via the homeobox gene CDX1. Lumens were identified by F-actin staining and they expressed many characteristics associated with normal differentiated intestinal epithelium, including brush border enzymes, polarization, and tight junctions. RNA interference-mediated silencing of CDX1 reduced lumen formation. Inhibitory effects of hypoxia on lumen formation and stem cell differentiation, including suppression of CDX1 expression, could be mimicked by inhibiting prolyl-hydroxylases that activate HIF1, suggesting that HIF1 is a critical mediator of the effects of hypoxia in this setting. Cell line-derived lumens were phenotypically indistinguishable from colorectal tumor glandular structures used by pathologists to grade tumor differentiation. Parallel results to those obtained with established cell lines were seen with primary cultures from fresh tumors. This in vitro approach to functional characterization of CSCs and their differentiation offers a valid model to study colorectal tumor differentiation and differentiation of colorectal CSCs, with additional uses to enable high-throughput screening for novel anticancer compounds.

Digestion of kiwifruit fiber

Dietary fiber affects the digestion and absorption of nutrients in the gastrointestinal tract. Moreover, it is generally believed that fiber largely escapes digestion in the human small intestine and is therefore mainly a substrate for microbial fermentation in the hindgut. Kiwifruit is a food naturally high in dietary fiber, yet the impact of dietary kiwifruit on nutrient availability has not been reported. The digestion of kiwifruit has been investigated but only in in vitro digestion studies. With its naturally high nonstarch polysaccharide content, it would be expected that kiwifruit would possess the characteristics of a good source of fiber for nutrition and health. Kiwifruit contains soluble and nonsoluble fiber components, both of which would be expected to affect the physical attributes of digesta as it transits the gastrointestinal tract. This chapter summarizes fiber digestion in general and current knowledge of kiwifruit fiber digestion in the gastrointestinal tract.

SAP-1 is a microvillus-specific protein tyrosine phosphatase that modulates intestinal tumorigenesis

SAP-1 (PTPRH) is a receptor-type protein tyrosine phosphatase (RPTP) with a single catalytic domain in its cytoplasmic region and fibronectin type III-like domains in its extracellular region. The cellular localization and biological functions of this RPTP have remained unknown, however. We now show that mouse SAP-1 mRNA is largely restricted to the gastrointestinal tract and that SAP-1 protein localizes to the microvilli of the brush border in gastrointestinal epithelial cells. The expression of SAP-1 in mouse intestine is minimal during embryonic development but increases markedly after birth. SAP-1-deficient mice manifested no marked changes in morphology of the intestinal epithelium. In contrast, SAP-1 ablation inhibited tumorigenesis in mice with a heterozygous mutation of the adenomatous polyposis coli gene. These results thus suggest that SAP-1 is a microvillus-specific RPTP that regulates intestinal tumorigenesis.

Immunopathology of giardiasis: the role of lymphocytes in intestinal epithelial injury and malfunction

T lymphocyte-mediated pathogenesis is common to a variety of enteropathies, including giardiasis, cryptosporidiosis, bacterial enteritis, celiac's disease, food anaphylaxis, and Crohn's disease. In giardiasis as well as in these other disorders, a diffuse loss of microvillous brush border, combined or not with villus atrophy, is responsible for disaccharidase insufficiencies and malabsorption of electrolytes, nutrients, and water, which ultimately cause diarrheal symptoms. Other mucosal changes may include crypt hyperplasia and increased infiltration of intra-epithelial lymphocytes. Recent studies using models of giardiasis have shed new light on the immune regulation of these abnormalities. Indeed, experiments using an athymic mouse model of infection have found that these epithelial injuries were T cell-dependent. Findings from further research indicate that that the loss of brush border surface area, reduced disaccharidase activities, and increase crypt-villus ratios are mediated by CD8+ T cells, whereas both CD8+ and CD4+ small mesenteric lymph node T cells regulate the influx of intra-epithelial lymphocytes. Future investigations need to characterize the CD8+ T cell signaling cascades that ultimately lead to epithelial injury and malfunction in giardiasis and other malabsorptive disorders of the intestine.

Regulation and role of brush border-associated ERK1/2 in intestinal epithelial cells

We have recently shown that elevated extracellular signal-regulated kinase (ERK) activities stimulate proliferation of intestinal cells whereas low sustained levels of ERK activities correlate with Gl arrest and are required for expression of several enterocyte differentiation proteins. In an attempt to clarify how ERK1/2 regulates intestinal differentiation, the present study assessed the subcellular distribution and regulation of ERK proteins and activities in differentiated enterocytes. We report that (1) ERK1/2 and their upstream modulators Ras, p85 (PI-3K), Rac1, and MEK1 are found in the brush border; (2) brush border-associated ERK1/2 are stimulated by EGF and feeding; (3) immunoblotting of proteins phosphorylated on SP/K motif suggests the presence of ERK substrates in the brush border, one of which could be actin; and (4) pharmacological inhibition of ERK alters microvilli architecture. Our results suggest that ERK may play important roles in the control of microvilli structure and possibly, in brush border-associated responses in differentiated intestinal epithelial cells.

Giardia lamblia rearranges F-actin and alpha-actinin in human colonic and duodenal monolayers and reduces transepithelial electrical resistance

The mechanisms of epithelial injury in giardiasis remain unknown. The effects of live Giardia lamblia on cellular G-actin, F-actin, alpha-actinin, and electrical resistance of human intestinal epithelial monolayers were investigated using SCBN and Caco2 cell lines grown on chamber slides or Transwell filter membranes. In separate experiments, some monolayers were also exposed to sonicated trophozoites, some to supernatant from live G. lamblia cultures, and some with or without the Ca2+ channel blocker verapamil. After 2, 24, or 48 hr of coincubation with G. lamblia, monolayers were assessed for cytoskeletal arrangement under fluorescence and confocal laser microscopy, and transepithelial electrical resistance was measured. Exposure to live G. lamblia trophozoites induced localized condensation of F-actin and loss of perijunctional alpha-actinin while G-actin remained unchanged. Confocal laser microscopy indicated that F-actin rearrangement was not affected by verapamil and was localized within the terminal web area. Coincubation of monolayers with G. lamblia lysates or with spent medium alone similarly rearranged F-actin. Verapamil alone did not alter F-actin. Electrical resistance of SCBN and Caco2 monolayers exposed to G. lamblia was significantly decreased versus controls regardless of whether live or lysed trophozoite samples were used. The results indicate that G. lamblia-induced epithelial injury is associated with F-actin and alpha-actinin rearrangements in the terminal web area via mechanisms independent of extracellular Ca2+. These alterations are associated with reduced transepithelial electrical resistance and are due at least in part to trophozoite products.

Jejunal brush border microvillous alterations in Giardia muris-infected mice: role of T lymphocytes and interleukin-6

Intestinal colonization with the protozoan Giardia causes diffuse brush border microvillous alterations and disaccharidase deficiencies, which in turn are responsible for intestinal malabsorption and maldigestion. The role of T cells and/or cytokines in the pathogenesis of Giardia-induced microvillous injury remains unclear. The aim of this study was to assess the role of T cells and interleukin-6 (IL-6) in the brush border pathophysiology of acute murine giardiasis in vivo. Athymic nude (nu(-)/nu(-)) CD-1 mice and isogenic immunocompetent (nu(+)/nu(+)) CD-1 mice (4 weeks old) received an axenic Giardia muris trophozoite inoculum or vehicle (control) via orogastric gavage. Weight gain and food intake were assessed daily. On day 6, segments of jejunum were assessed for parasite load, brush border ultrastructure, IL-6 content, maltase and sucrase activities, villus-crypt architecture, and intraepithelial lymphocyte (IEL) infiltration. Despite similar parasitic loads on day 6, infected immunocompetent animals, but not infected nude mice, showed a diffuse loss of brush border microvillous surface area, which was correlated with a significant reduction in maltase and sucrase activities and a decrease in jejunal IL-6 concentration. In both athymic control and infected mice, jejunal brush border surface area and disaccharidases were high, but levels of tissue IL-6 were low and comparable to the concentration measured in immunocompetent infected animals. In both immunocompetent and nude mice, infection caused a small but significant increase in the numbers of IELs. These findings suggest that the enterocyte brush border injury and malfunction seen in giardiasis is, at least in part, mediated by thymus-derived T lymphocytes and that suppressed jejunal IL-6 does not necessarily accompany microvillous shortening.

An aminopeptidase N deficiency in dog small intestine

This study has identified a naturally occurring, specific deficiency of a brush border aminopeptidase N (ApN) in the small intestines of five clinically healthy dogs. ApN activity in mucosal homogenates of dog small intestine was reduced significantly in deficient animals (13.4 (1.1) nmol min-1 mg-1 protein, n = 5, P < 0.002) compared to healthy control dogs (95.1 (6.7), n = 22). Alkaline phosphatase, gamma-glutamyl transferase, zinc-resistant alpha-glucosidase, maltase, sucrase and lactase in the ApN deficient dogs exhibited comparable activities to those in the control dogs. Microvillar membranes were analysed by one- and two-dimensional electrophoresis. ApN was represented by a single 145kDa band in all control dogs, identified by immunoblotting and immunoprecipitation. Protein maps from deficient dogs were normal apart from the virtual absence of an ApN spot and there were no apparent abnormalities in the glycosylation of microvillar proteins. The findings suggest that intestinal ApN deficiency in these dogs is a primary lesion involving diminished expression of an otherwise normal enzyme protein.

Lectin histochemistry and carcinoembryonic antigen in spontaneous colonic cancers of cotton-top tamarins (Saguinus oedipus)

The glycoconjugate-binding properties and the expression of carcinoembryonic antigen in colonic cancers of cotton-top tamarins at the German Primate Center are described. Deviating from findings in non-tumorous colonic epithelia of cotton-top tamarins was the presence of UEA-, SBA-, or DBA-binding sites in the infranuclear cytoplasm of colonic epithelial cells, staining of entire cells, and the occurrence of subepithelial extracellular glycoconjugates in and adjacent to colonic cancers. Carcinoembryonic antigen, which normally was limited to the fuzzy coat of the colonic epithelium, was demonstrated in the cytoplasm of the cancer cells.

Comparison of the gastrointestinal anatomy, physiology, and biochemistry of humans and commonly used laboratory animals

In addition to metabolic differences, the anatomical, physiological, and biochemical differences in the gastrointestinal (G.I.) tract of the human and common laboratory animals can cause significant variation in drug absorption from the oral route. Among the physiological factors, pH, bile, pancreatic juice, and mucus and fluid volume and content can modify dissolution rates, solubility, transit times, and membrane transport of drug molecules. The microbial content of the G.I. tract can significantly affect the reductive metabolism and enterohepatic circulation of drugs and colonic delivery of formulations. The transit time of dosage forms can be significantly different between species due to different dimensions and propulsive activities of the G.I. tract. The lipid/protein composition of the enterocyte membrane along the G.I. tract can alter binding and passive, active, and carrier-mediated transport of drugs. The location and number of Peyer's patches can also be important in the absorption of large molecules and particulate matter. While small animals, rats, mice, guinea pigs, and rabbits, are most suitable for determining the mechanism of drug absorption and bioavailability values from powder or solution formulations, larger animals, dogs, pigs, and monkeys, are used to assess absorption from formulations. The understanding of physiological, anatomical, and biochemical differences between the G.I. tracts of different animal species can lead to the selection of the correct animal model to mimic the bioavailability of compounds in the human. This article reviews the anatomical, physiological, and biochemical differences between the G.I. tracts of humans and commonly used laboratory animals.

Characterization of microvillar membrane proteins of dog small intestine by two-dimensional electrophoresis

A method for analysing microgram amounts of microvillar membranes by two-dimensional electrophoresis (protein mapping) is described, and has been used to characterize the microvillar proteins of the small intestine of German shepherd, corgi, and beagle dogs. Detergent-solubilized microvillar membranes were radiolabelled with 14C and separated by isoelectric focussing followed by SDS-PAGE. Proteins were detected fluorographically and glycoproteins by lectin-affinity staining. The microvillar hydrolases alkaline phosphatase and dipeptidyl aminopeptidase IV were identified by active-site labelling and aminopeptidase N by immunoprecipitation. Changes following pancreatic duct diversion were consistent with accumulation of pro-sucrase-isomaltase and diminished expression of the sucrase and isomaltase subunits. Cytoskeletal proteins were concentrated in the core fraction remaining after extraction of microvillar membranes with Triton X-100. There were no consistent differences between dogs of different breed, and the canine protein maps were similar to the human.

Amino acid derivatives of 5-ASA as novel prodrugs for intestinal drug delivery

In an attempt to obtain site-specific delivery of 5-ASA in the intestinal tract, we have determined the extent of absorption and metabolism of a number of novel 5-ASA derivatives, namely, (N-L-glutamyl)-amino-2-salicylic acid (1), (N-L-aspartyl)-amino-2-salicylic-acid (2), 5-aminosalicyl-L-proline-L-leucine (3), and 5-(N-L-glutamyl)-aminosalicyl-L-proline-L-leucine (4), which are selectively cleaved by intestinal brush border aminopeptidase A and carboxypeptidases. These novel prodrugs, 5-ASA, and sulfasalazine were administered to adult Fisher rats (N = 30) and to animals that had undergone prior colostomy (N = 30). Urine and feces were collected at timed intervals for 48 hr and the metabolites, 5-ASA, and N-acetyl-5-ASA were measured by high-performance liquid chromatography. The absorption and metabolism of all compounds were essentially identical in colostomized and normal animals. 5-ASA exhibited a rapid proximal intestinal absorption as evidenced by the high cumulative urinary excretion (> 65%) and low fecal excretion. Sulfasalazine, as expected, exhibited a lower urinary recovery (< 35%) and higher fecal excretion of 5-ASA and its metabolite. The novel glutamate and aspartate derivatives (1 and 2) behaved similarly to sulfasalazine, while administration of the proline-leucine derivative (3) resulted in urinary and fecal recovery values intermediate with respect to those observed with 5-ASA and sulfasalazine. 5-(N-L-Glutamyl)-aminosalicyl-L-proline-L-leucine yielded the highest fecal recovery of 5-ASA and its N-acetyl derivative, indicating a more efficient delivery to the distal bowel. Amino acid derivatives of 5-ASA appear to be potentially useful prodrugs for the site-specific delivery of 5-ASA to different regions of the intestinal tract.

Structural changes of duodenum brush border in lung cancer patients treated with cisplatin plus etoposide

The effects of cisplatin plus etoposide chemotherapy (PE) on the structure of proximal intestine villi and brush border were investigated in 10 patients with lung cancer. The day before starting chemotherapy (time 1); 8 days after its initiation (time 2), and one month after the 3rd course of PE (time 3) they underwent esophagogastroduodenoscopy and three biopsies were taken from the descending duodenum. Intestinal villi were examined by light microscopy; brush border by transmission electron microscopy. No significant histological changes of villous pattern were observed at times 2 and 3. The height of microvilli was reduced in seven patients at time 2 (P < 0.05). Microvilli abnormalities (i.e. rarefaction and/or heterogeneity in their height) were present in nine patients at time 2 (P < 0.05). Brush border appearance at time 3 did not differ from that at time 1. PE chemotherapy seems to have short-term toxic effects on small intestine brush border, but does not cause chronic enteropathy.

Intestinal epithelial function: the case for immunophysiological regulation. Implications for disease (2)

Substantial amounts of data have been reported showing a role for immunomodulation of epithelial function (particularly ion secretion and permeability) using animal models of anaphylactic reactions. In part one of this review we outlined the main immune cell types and mediators/cytokines that are currently known to influence epithelial physiology either directly, or indirectly via an intermediate cell type. Here we will expand on the significance of these studies and show how antigenic activation of the mucosal immune system can evoke changes in epithelial function that may be beneficial to the host by mediating loss/inactivation of the antigen. However, a continued and inappropriate immune stimulation can lead to pathophysiological reactions and disease. Thus, we will present data on immune regulation of epithelial function with direct applicability to understanding the mechanism underlying human intestinal inflammatory and secretory disease. Finally, we highlight key strategic points in the cascade of immune events that can control epithelial function and thus may be of relevance in the formulation of new therapeutic approaches to intestinal inflammation.