The Role of Bacteria in the Development of Intestinal Protective Function

Isolauri E, Walker WA (eds): Allergic Diseases and the Environment. Nestlé Nutrition Workshop Series Pediatric Program, Vol. 53, pp. 153–177, Nestec Ltd.; Vevey/S. Karger AG, Basel, © 2004. The Role of Bacteria in the Development of Intestinal Protective Function N. Nanda Nanthakumar and W. Allan Walker Harvard Medical School and Developmental Gastroenterology Laboratory of the Combined Program in Pediatric Gastrointestinal and Nutrition, Massachusetts General Hospital, Charlestown, Mass., USA Introduction The primary function of the gastrointestinal tract is to complete the digestion and absorption of nutrients so as to provide a source of energy and substrate for growth and maintenance of the complete organism. Therefore, diseases that affect intestinal function have a major impact on body systems [1, 2]. This challenge is further compounded by the fact that the gut is directly in contact with a microbial and nutritional rich external environment. Under normal circumstances, a large number of bacterial species reside in the intestinal lumen in a symbiotic relationship with the host [3]. In addition, the gut is continuously exposed to foreign antigens, derived from luminal microbes, diet and ingested toxic substances [4]. In contrast to other organ systems, with the exception of skin, the gut is continually exposed to this external environment with an epithelial surface juxtaposed between the lumen and the interstitium and circulation. Unlike the skin, the intestinal epithelium is made up of a single polarized monolayer [2]. The apical surface of the epithelium is exposed to luminal contents including commensal flora [2, 3] but these substances are restricted from the basolateral surface by tight and adherent junction proteins [5]. These two junctional complexes are specialized structures unique to polarized cells and provide not a rigid structure, but an active flexible surface that allows migration of activated polymorphonuclear cells [6] during infection and access by dendritic cells to sample foreign antigen in the lumen [7]. 153 Microbes in Gut Development Structure and Function of the Intestine The intestinal epithelium which separates luminal contents from the underlying mucosa consists of absorptive enterocytes (93–95% of cells), mucus-secreting goblet cells (3–5% of cells) and gastrointestinal hormone- producing enteroendocrine cells (1–2% of cells) [2]. Unlike the colon, the surface area of the small intestine is increased by invagination into ‘tongue- like’ structures called villi. Mucus produced by the goblet cells is secreted as a layer of highly glycosylated proteins onto the intestinal surface and functions as a lubricant and protective layer on the epithelial surface. Undifferentiated proliferative cells including stem cells exit in a pit-like structure called the crypts of Lieberkühn both in the small intestine and colon [8, 9]. However, only in the small intestine unique cells called paneth cells are located at the bottom of each crypt [10]. Paneth cells produce a number of unique antibacterial proteins which act to protect nearby stem cells from microbial damage. The crypt epithelium is also polarized but only acquires microvilli, also known as the brush border, on its apical surface as cells migrate from the crypt to villus [2, 8, 9]. As these cells emerge from the crypts and undergo epithelial differentiation they begin to express specialized apical proteins such as digestive enzymes and transporters [1, 2]. These glycoproteins, anchored on the apical surface of the epithelium, are responsible for the digestive and absorptive functions of this tissue. These highly glycosylated proteins and glycolipids that enrich the apical surface of the epithelium can also function as receptors for commensal microflora that begin to colonize the gut lumen shortly after birth [11, 12]. In addition to the multi-lineage epithelial cells described, the epithelial monolayer infrequently displays a dome-like surface called the follicle- associated epithelium (FAE) [2, 13]. Unlike the villus epithelium, the epithelial surface of those domes display, with varying frequency, a unique type of epithelial cell called the microfold cell (M cell). In humans and rodents 10% of FAE are made up of M cells [2, 13]. These cells have no lysosomes and are capable of invaginating upon attachment of microorganisms and large proteins. The M cells are a specialized lineage of epithelium dedicated to antigen sampling [13]. Unlike the adjacent enterocytes, M cells have fewer and shorter microvilli on their apical surface and the basolateral surface display numerous invaginations in which mucosal lymphocytes reside [14]. M cells are never seen on differentiated villus epithelium. The FAE and M cells appear above aggregates of lymphocytes in Peyer’s patches [14]. Since a single stem cell resides in each crypt, how these two lineages are derived during epithelial differentiation is not known, because the lack of a suitable in vitro model system has precluded the elucidation of the mechanism of this form of epithelial differentiation. However, recently exciting in vitro studies have shown that a differentiated enterocyte cell line can trans-differentiate into M cells under the influence of luminal pathogens and basolateral exposure of B cells suggesting 154 Microbes in Gut Development that luminal microbial attachments and paracrine action by B cell may be responsible for M-cell differentiation [15]. Development of the Small Intestine Morphological development, cytodifferentiation and enterocyte-specific differentiation are established by the end of the first trimester in humans [16] and at birth in rodents (rats and mice) [2, 8]. Gestation is 21 days in rodents, whereas it is 40 weeks in humans. The functional maturation of the gut is divided into 2 periods. Details of the development of the gastrointestinal tract is beyond the scope of this review and provided in several recent reviews [1, 2, 9, 16]. By the end of the first trimester the epithelium begins to form a monolayer and a crypt–villus architecture appears. The epithelium starts to differentiate and tissue-specific markers appear [2]. Proliferating epithelium is confined to the crypts where multiple stem cells reside [9, 16]. This phase of development occurs during the second and third trimester in humans and during the first 2 weeks of postnatal development in rodents [2, 9]. The early phase of functional maturation of the small intestine can be defined using differentiation- specific enterocyte markers [1, 2]. For example, disaccharidases are first detected with initial cytodifferentiation of the enterocytes, but the levels of disaccharidases vary depending on species. In humans, lactase remains low in utero but sucrase is high during this period, e.g. equivalent to levels found in infants [16]. In contrast, in rodents sucrase is undetectable with high- lactase activity until weaning [2]. The second and final phase of development begins at birth for humans and at the time of weaning (3rd postnatal week) in rodents [2]. During this period, lactase activity rapidly declines to the levels seen in adult rodents but in humans this enzyme increases and reaches a maximal level in the newborn [16]. In rodents the expression of sucrase increases to adult levels by the end of weaning [2]. At the same time, most of the enzymes and transporters responsible for digesting solid food are rapidly established at mature levels. Terminal maturation of the small intestine temporally coincides with weaning in rats and mice. These changes, coinciding with ‘hard-wired’ development of enzymatic expression, reflect the adaptive process necessary for survival on solid food [1, 2]. Regulation of Intestinal Development The functional development of the gut is regulated by a number of factors. To unravel the complex mechanism(s) of development, extensive studies have been done in the rodent model [1, 2, 16]. However, little objective data are available for human gut development because of the inaccessibility of human tissues and inadequate intestinal models. The regulators of intestinal 155 Microbes in Gut Development development can be either extrinsic (luminal) factors such amniotic fluid, colostrum/milk and microbial flora or intrinsic factors such as circulating growth factors, e.g. glucocorticoids, intrinsic timing mechanisms (a biological clock), and/or epithelial–mesenchyme interactions. The role of these divergent regulators are briefly discussed below. Colostrum and Mature Milk Colostrum and mother’s milk are complex biological fluids that contain many substances which provide nutrition but also protect and stimulate cell turnover including proteins such as casein, micelles, membranes, membrane- bound globules, and viable cells [4]. A complete description of the macro- and micronutrients in milk has been published recently [17]. However, in this review we will focus only on trophic factors present in colostrum/breast milk that play a critical role in intestinal development. These factors are present in physiologic quantities and their role(s) in intestinal development is not fully understood, again in part because of lack of availability of a model that recapitulates the newborn human gut. Commensal Flora At birth, commensal bacteria begins to colonize the gastrointestinal tract [18]. The composition of the flora changes at the time of weaning [19]. This is in part due to the changing luminal environment contributed to by diet and the epithelium itself. However, a symbiotic relationship likely

Load more