The small intestine is the longest section of the digestive tube and consists of three segments forming a passage from the pylorus to the large intestine. The duodenum is a short section starting immediately after the stomach and which receives secretions from the pancreas and liver via the pancreatic and common bile ducts. It is followed by the jejunum, considered to be roughly 40% of the small intestine in man, but closer to 90% in animals, and by the ileum, which connects to the large intestine. The ileum is considered to be about 60% of the intestine in man. However, veterinary anatomists usually refer to it as being only the short terminal section of the small intestine.
The structure of the small intestine looks on the first sight quite similar to other regions of the digestive tube. However, three features account for its huge absorptive surface area. The inner surface of the small intestine is not flat but wrinkled into circular folds (mucosal folds), which increase its surface area several-folds. The mucosa itself is composed of multitudes of projections (villi) which protrude into the lumen and are covered with epithelial cells. Finally, the lumenal plasma membrane of those absorptive epithelial cells is also folded and densely-packed in microdomains named “microvilli”, whose border is commonly referred to as the
"brush border" due to its appearance in the microscope (see Fig. 1.1).
Figure 1.1: The small intestine inner surface. The panels above depict the bulk of the small intestine surface area expansion, showing villi, the epithelial cells that cover the villi, and the microvilli of the epithelial cells (downloaded from web site
http://www.vivo.colostate.edu:80/hbooks/pathphys/digestion/smallgut/anatomy.html ).
The epithelial cells of the small intestine mature into absorptive epithelial cells that cover the villi. These are the cells that take up and deliver to the blood stream virtually all nutrients from the diet. Two other major cell types populate the small intestinal epithelium: the enteroendocrine cells which, as part of the enteric endocrine system, sense the lumenal environment and secrete hormones such as cholecystokinin and gastrin into blood; and the Goblet cells, which secrete lubricating mucus into the intestinal lumen.
1.1.2 Brush Border Membrane: Location and Function
Intestinal epithelial cells are polar in their cellular organization. The intestinal brush border (synonyms: microvillus, luminal, apical) membranes of the enterocytes differ in protein and lipid composition from the inner side of the plasma membrane, the basolateral membrane (BLM) (see Fig. 1.2). The apical surface of polarized intestinal epithelial cells (the surface facing the intestinal lumen) is characterized by structurally distinct cell protrusions referred as microvilli or brush border membranes (BBMs), responsible for digestion and absorption of nutrients.
Figure 1.2: Schematic representation of a typical intestinal epithelial cell. The apical membrane (BBM) has a different protein and lipid composition from the basolateral
membrane. The BBM can be isolated from the BLM using protocols that take advantage of the difference in polarity between the two membranes (figure downloaded from web site:
http://www.vivo.colostate.edu:80/hbooks/pathphys/digestion/smallgut/anatomy.html ).
The processing capacity of enterocytes is directly proportional to the surface of absorptive epithelia BBM. BBM are supported by cytoskeletal actin filaments which are organized into
both more or less permanent and rapidly rearranging bundles. Cytoskeleton bundles are in turn interconnected with transmembrane protein complexes forming a highly organized import–export membrane interface specialized for a variety of digestive and absorptive functions, such as protein and peptide degradation, absorption of minerals, amino acids, sugars, lipids and cholesterol (1). Shortcomings in these mechanisms may cause a variety of pathological conditions such as disorders in the metabolism of saccharides (glucose galactose malabsorption, lactose intolerance) amino acids (Hartnup disease, aminoacidurias), ions (sodium and potassium in the case of familiar diarrhea), metals (zinc in acrodermatitis enteropathica) and cholesterol lipids (cardiovascular diseases).
Recently, several proteomics studies have reported the identification of proteins localized in the BBM membrane (2, 3). Until now, however, these approaches have failed to identify transporters and receptors that are known to be located in the BBM membrane based on kinetic studies, immunological assays and in gene data, probably because of the complexity of the analyzed samples.
1.1.3 Brush Border Membrane and Lipid Rafts microdomains
Recent studies have suggested that plasma membranes might be organized into heterogeneous functional microdomains. One type of these microdomains, called lipid rafts, is stated to be enriched in glycosphingolipids/cholesterol and in typical sets of proteins, among them also cholesterol transporters (4). Lipid rafts can be isolated by taking advantage of their resistance to nonionic detergent extraction at cold and by their differential buoyancy on a density gradient ultracentrifugation. The lipid rafts hypothesis was originally proposed to explain how proteins and lipids were sorted to the apical surface of polarized cells. However, in recent years, several functions including signaling, cholesterol homeostasis, cell trafficking or even docking sites on mammalian cells for certain pathogens and toxins have also been attributed to lipid rafts (5, 6). Despite accumulated experimental data from biophysical, biochemical, and fluorescent microscopy studies supporting the fact that lipid rafts may exist in vivo, the lipid rafts hypothesis remains controversial at least for their size, stability and the mechanism of their formation (7).
Lipid rafts isolated from the BBM have also been the subject of several recent proteomics studies (3, 8, 9). While these studies have reported the identification of proteins that were localized in the lipid rafts, almost none of these proteins were described to be involved in cholesterol absorption, a major area of interest for the analysis of the lipid rafts.