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Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000.

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Molecular Cell Biology. 4th edition.

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Section 15.7Transport across Epithelia

With few exceptions, all the internal and external body surfaces of animals, such as the skin, stomach, and intestines, are covered with a layer of epithelial cells called an epithelium (see Figure 6-4). Many epithelial cells transport ions or small molecules from one side to the other of the epithelium. Those lining the stomach, for instance, secrete hydrochloric acid into the stomach lumen, which after a meal becomes pH 1, while those lining the small intestine transport products of digestion (e.g., glucose and amino acids) into the blood. All epithelial cells in a sheet are interconnected by several types of specialized regions of the plasma membrane called cell junctions. These impart strength and rigidity to the tissue and prevent water-soluble material on one side of the sheet (as in the intestinal lumen) from moving across to the other side. In this section we first describe the polarized nature of epithelia and how different combinations of membrane proteins enable epithelial cells to carry out their transport or secretory functions. Then we discuss the structure and function of the junctions that interconnect epithelial cells.

The Intestinal Epithelium Is Highly Polarized

An epithelial cell is said to be polarized because one side differs in structure and function from the other. In particular, its plasma membrane is organized into at least two discrete regions, each with different sets of transport proteins. In the epithelial cells that line the intestine, for example, that portion of the plasma membrane facing the intestine, the apical surface, is specialized for absorption; the rest of the plasma membrane, the lateral and basal surfaces, often referred to as the basolateral surface, mediates transport of nutrients from the cell to the surrounding fluids which lead to the blood and forms junctions with adjacent cells and the underlying extracellular matrix called the basal lamina (Figure 15-23).

Figure 15-23. Schematic diagram of epithelial cells lining the small intestine and the principal types of cell junctions that connect them.

Figure 15-23

Schematic diagram of epithelial cells lining the small intestine and the principal types of cell junctions that connect them. As in all epithelia, the basal surface of the cells rests on the basal lamina, a fibrous network of collagen and proteoglycans (more...)

Extending from the lumenal (apical) surface of intestinal epithelial cells are numerous fingerlike projections (100 nm in diameter) called microvilli (singular, microvillus). Often referred to collectively as the brush border because of their appearance, microvilli greatly increase the area of the apical surface and thus the number of transport proteins it can contain, enhancing the absorptive capacity of the intestinal epithelium. A bundle of actin filaments that runs down the center of each microvillus gives rigidity to the projection. Overlying the brush border is the glycocalyx, a loose network composed of the oligosaccharide side chains of integral membrane glycoproteins, glycolipids, and enzymes that catalyze the final stages in the digestion of ingested carbohydrates and proteins (Figure 15-24). The action of these enzymes produces monosaccharides and amino acids, which are transported across the intestinal epithelium and eventually into the bloodstream.

Figure 15-24. Micrograph of the microvilli that form the lumenal surface of intestinal epithelial cells, obtained by the deep-etching technique.

Figure 15-24

Micrograph of the microvilli that form the lumenal surface of intestinal epithelial cells, obtained by the deep-etching technique. The surface of each microvillus is covered with a series of bumps believed to be integral membrane proteins. The glycocalyx, (more...)

Transepithelial Movement of Glucose and Amino Acids Requires Multiple Transport Proteins

Movement of monosaccharides and amino acids from the intestinal lumen into the blood is a two-stage transcellular process. The first stage, import of substances from the lumen into intestinal epithelial cells, is carried out by membrane transport proteins in the microvilli on the apical surface of intestinal cells. The second stage, export of substances from the cells into the fluid surrounding the basolateral surface, is carried out by other transport proteins on the basolateral plasma membrane. In order for such transepithelial transport to occur, the epithelial cell must be polarized, with different sets of transport proteins localized in the basolateral and apical surfaces. To illustrate this process, we examine the membrane transport proteins required to move glucose across the epithelial cells lining the intestine and kidney. Similar proteins are used to transport amino acids across these epithelia.

Figure 15-25 depicts the transport of glucose from the intestinal lumen to the blood. Glucose is imported against its concentration gradient from the intestinal lumen across the apical surface of the epithelial cells by a two-Na+/one-glucose symporter located in the microvillar membranes. As noted above, this symporter couples the energetically unfavorable inward movement of one glucose molecule to the energetically favorable inward transport of two Na+ ions (see Figure 15-19). In the steady state, all the Na+ ions transported from the intestinal lumen into the cell during Na+/glucose symport, or the similar process of Na+/amino acid symport, are pumped out across the basolateral membrane, often called the serosal (blood-facing) membrane. Thus the low intracellular Na+ concentration is maintained. The Na+/K+ ATPase that accomplishes this is found in these cells exclusively on the basolateral surface of the plasma membrane. The coordinated operation of these transporters allows uphill movement of glucose and amino acids from the intestine into the cell, and ultimately is powered by ATP hydrolysis by the Na+/K+ ATPase.

Figure 15-25. Transport of glucose from the intestinal lumen into the blood.

Figure 15-25

Transport of glucose from the intestinal lumen into the blood. Activity of the Na+/K+ ATPase (green) in the basolateral surface membrane generates Na+ and K+ concentration gradients, and the K+ gradient generates an inside-negative membrane potential. (more...)

Glucose and amino acids concentrated inside intestinal cells by symporters are exported down their concentration gradients into the blood via uniport proteins in the basolateral membrane. In the case of glucose, this movement is mediated by GLUT2, a glucose transporter that is localized in the basal and lateral membranes of intestinal cells (see Figure 5-1c). (GLUT2 is a homolog of GLUT1; as discussed earlier; however, GLUT1 generally functions to import glucose into many body cells.) The net result of the operation of these various transport proteins is movement of Na+ ions, amino acids, and glucose from the intestinal lumen across the intestinal epithelium into the interstitial spaces surrounding the cells, and eventually into the blood. Tight junctions between the epithelial cells prevent these molecules from diffusing back into the intestinal lumen.

The epithelial cells lining kidney tubules, which have an architecture similar to that of intestinal epithelial cells, reabsorb glucose from the blood filtrate that is the forming urine and return it to the blood. In the first part of a kidney tubule, the epithelial cells transport glucose against a relatively small glucose concentration gradient. These cells utilize a second type of Na+/glucose symport protein — a one-Na+/one-glucose symporter, which has a high transport rate but cannot transport glucose against a steep concentration gradient. At the intracellular Na+ concentration and membrane potential depicted in Figure 15-9, this symporter can generate an intracellular glucose concentration ≈100 times that of the extracellular medium (here the forming urine). In the latter part of a kidney tubule, however, the epithelial cells take up the remaining glucose against a more than 100-fold glucose concentration gradient. To accomplish this, these cells contain in their apical membrane the same two-Na+/one-glucose symporter found in intestinal epithelial cells. The two types of Na+/glucose symport proteins are similar in amino acid sequence, predicted structure, and mechanism but have evolved to transport glucose under different conditions.

Parietal Cells Acidify the Stomach Contents While Maintaining a Neutral Cytosolic pH

The mammalian stomach contains a 0.1 M solution of hydrochloric acid (H+Cl). This strongly acidic medium denatures many ingested proteins before they are degraded by proteolytic enzymes in the stomach (e.g., pepsin) that function at acidic pH. Hydrochloric acid is secreted into the stomach by parietal cells (also known as oxyntic cells) in the gastric lining. These cells contain a H+/K+ ATPase in their apical membrane, which faces the stomach lumen and generates a concentration of H+ ions 106 times greater in the stomach lumen than in the cell cytosol (pH = 1.0 versus pH = 7.0). This enzyme is a P-class ATPase, similar in structure and function to the Na+/K+ ATPase discussed earlier. Operation of the Na+/K+ ATPase results in a net outward movement of one charged ion per ATP (see Figure 15-13). In contrast, the action of the H+/K+ ATPase, which exports one H+ ion and imports one K+ ion for each ATP hydrolyzed, produces no net movement of electric charge. The numerous mitochondria in parietal cells produce abundant ATP for use by the H+/K+ ATPase.

If parietal cells simply exported H+ ions in exchange for K+ ions, a rise in the concentration of OH ions and thus a marked rise in cytosolic pH would occur, since in the cytosol, as in all aqueous solutions, the product of the H+ and OH concentrations is a constant (10−14 M2). However, during acidification of the stomach lumen, the pH of the parietal-cell cytosol remains neutral. Parietal cells accomplish this feat by means of a Cl/HCO3 antiporter in the basolateral membrane (Figure 15-26). The “excess” cytosolic OH, generated by exporting protons, combines with CO2 that diffuses into the cell from the blood, forming HCO3 in a reaction catalyzed by cytosolic carbonic anhydrase. The HCO3 ion is transported across the basolateral membrane of the cell into the blood in exchange for an incoming Cl ion by means of an anion antiporter that is similar in structure and function to the erythrocyte AE1. The Cl ions thus imported into the cell exit through Cl channels in the apical membrane, entering the stomach lumen. To preserve electroneutrality, each Cl ion that moves into the stomach lumen across the apical membrane is accompanied by a K+ ion that moves outward through a separate K+ channel. In this way, the excess K+ ions pumped inward by the H+/K+ ATPase are returned to the stomach lumen, thus maintaining the intracellular K+ concentration. The net result is accumulation of both H+ and Cl ions (i.e., HCl) in the stomach lumen, while the pH of the cytosol remains neutral and the excess OH ions, as HCO3, are transported into the blood.

Figure 15-26. Acidification of the stomach lumen by parietal cells in the gastric lining.

Figure 15-26

Acidification of the stomach lumen by parietal cells in the gastric lining. The apical membrane of parietal cells contains a H+/K+ ATPase (a P-class pump) as well as Cl and K+ channel proteins. Note the cyclic K+ transport across the apical membrane: (more...)

Tight Junctions Seal Off Body Cavities and Restrict Diffusion of Membrane Components

For polarized epithelial cells to carry out their transport functions, extracellular fluids surrounding their apical and basolateral membranes must be kept separate. This is accomplished by tight junctions, which connect adjacent epithelial cells and usually are located just below the apical surface (see Figure 15-23). These specialized regions of the plasma membrane form a barrier that seals off body cavities such as the intestine, the stomach lumen, ductules in pancreatic acini, and the bile duct in the liver. For example, tight junctions prevent diffusion of small molecules directly from the intestinal lumen into the interstitial spaces that surround the basolateral plasma membrane and that lead to the blood. Thus intestinal epithelial cells must transport nutrients through the cells as previously described. In the pancreas, tight junctions between acinar cells likewise prevent leakage of secreted proteins, including digestive enzymes, from the central ductules into the blood (Figure 15-27). Tight junctions also prevent diffusion of membrane proteins and glycolipids between the apical and basolateral regions of the plasma membrane, ensuring that these regions contain different membrane components.

Figure 15-27. Diagram of pancreatic acinar cells.

Figure 15-27

Diagram of pancreatic acinar cells. An acinus is a spherical aggregate of about a dozen cells; the lumen of an acinus is connected to a ductule that merges with other ductules and eventually leads into a main pancreatic duct, which empties into the lumen (more...)

Structure of Tight Junctions

Tight junctions are composed of thin bands of plasma-membrane proteins that completely encircle a polarized cell and are in contact with similar thin bands on adjacent cells. When thin sections of cells are viewed in an electron microscope, the plasma membranes of adjacent cells appear to touch each other at intervals and even to fuse (Figure 15-28a). Freeze-fracture electron microscopy affords a striking view of the tight junction. The microvillar tight junction shown in Figure 15-28b appears to comprise an interlocking network of ridges in the plasma membrane. More specifically, there appear to be ridges on the cytosolic face of the plasma membrane of each of the two contacting cells. (Corresponding grooves not shown here are found on the exoplasmic face.) High magnification reveals that these ridges are made up of protein particles 3 – 4 nm in diameter. In the model shown in Figure 15-28c, the tight junction is formed by a double row of these particles, one row donated by each cell.

Figure 15-28. Tight junctions.

Figure 15-28

Tight junctions. (a) Thin-section electron micrograph of the apical region of two liver epithelial cells, illustrating the tight junction just below the microvilli and the adherens junction. From the apical region of these liver cells, which faces the (more...)

The two principal integral membrane proteins found in tight junctions are occludin and claudin. Each of these proteins has four membrane-spanning α helices. Although the molecular structure of the junction is not known, the extracellular domains of rows of occludin and claudin proteins in the plasma membrane of one cell probably form extremely tight links with similar rows of claudin and occludin in the adjacent cell, essentially fusing two adjacent cells and creating an impenetrable seal. Treatment of an epithelium with the protease trypsin destroys the tight junctions, supporting the proposal that proteins are essential structural components of these junctions.

The long C-terminal cytosolic-facing domain of occludin is bound to one of a group of large cytosolic proteins (ZO-1, ZO-2, and ZO-3) that, in turn, are bound to other cytoskeletal proteins and to actin fibers. These interactions appear to stabilize the linkage between occludin molecules that is essential for integrity of the tight junction (Chapter 22).

Impermeability of Tight Junctions to Aqueous Solutions

That tight junctions are impermeable to most water-soluble substances can be demonstrated in an experiment in which lanthanum hydroxide (an electron-dense colloid of high molecular weight) is injected into the pancreatic blood vessel of an experimental animal; a few minutes later the pancreatic acinar cells are fixed and prepared for microscopy. As shown in Figure 15-29, the lanthanum hydroxide diffuses from the blood into the space that separates the lateral surfaces of adjacent acinar cells, but cannot penetrate past the outermost tight junction.

Figure 15-29. Experimental demonstration that tight junctions prevent passage of water-soluble substances.

Figure 15-29

Experimental demonstration that tight junctions prevent passage of water-soluble substances. Pancreatic acinar tissue is fixed and prepared for microscopy a few minutes after electron-opaque lanthanum hydroxide is injected into the blood of an experimental (more...)

Other studies have shown that tight junctions also are impermeable to salts. For instance, when MDCK cells are grown in a medium containing very low concentrations of Ca2+, they form a monolayer in which the cells are not connected by tight junctions; as a result, fluids and salts flow freely across the cell layer. When Ca2+ is added to such a monolayer, tight junctions form within an hour, and the cell layer becomes impermeable to fluids and salts (see Figure 6-7).

Ability of Tight Junctions to Block Diffusion of Proteins and Lipids in the Plane of the Plasma Membrane

When liposomes containing a fluorescent-tagged glycoprotein are added to the medium in contact with the apical surface of a monolayer of MDCK cells, some spontaneously fuse with the plasma membrane. Fluorescent glycoprotein is detectable in the apical but not in the basolateral surface of the cells so long as the tight junctions between adjacent cells are intact. However, if the tight junctions are destroyed by removing Ca2+ from the medium, the fluorescent protein is soon detectable in the basolateral surface, indicating that it can diffuse from the apical to the basolateral regions of the plasma membrane. These results indicate that plasma membrane proteins cannot diffuse through tight junctions.

Lipids in the cytosolic leaflets of the apical and basolateral membranes of epithelial cells have the same composition and apparently can diffuse from one region of the membrane to the other. In contrast, the lipid compositions of the exoplasmic leaflets of the apical and basolateral membrane regions are very different, and membrane lipids in the exoplasmic leaflets cannot diffuse through tight junctions. All the glycolipid in MDCK cells, for instance, is present in the exoplasmic face of the apical membrane, as are all proteins anchored to the membrane by fatty acids linked to a glycosylphosphatidylinositol group (see Figure 3-36a). In fact, the only lipids in the exoplasmic leaflet of the apical plasma membrane are glycolipids, fatty acid components of glycosylphosphatidylinositol anchors, and cholesterol. Phosphatidylcholine, conversely, is present almost exclusively in the exoplasmic face of the basolateral plasma membrane.

Other Junctions Interconnect Epithelial Cells and Control Passage of Molecules between Them

In order to function in an integrated manner, the individual cells composing epithelia and other organized tissues must adhere to one another and to the surrounding extracellular matrix and also control the movement of ions and small molecules between them. Several specialized cell junctions are critical to adhesion and passage of molecules between cells in tissues (see Figure 15-23).

Three types of cell junctions, called desmosomes, function in cell-cell and cell-matrix adhesion. Epithelial and some other types of cells, such as smooth muscle, are bound tightly together by spot desmosomes. These are buttonlike points of contact between cells, often thought of as a “spot-weld” between adjacent plasma membranes, that confer mechanical strength on these tissues. Hemidesmosomes, similar in structure to spot desmosomes, anchor the plasma membrane to regions of the extracellular matrix. Bundles of intermediate filaments course through the cell, interconnecting spot desmosomes and hemidesmosomes. Finally, adherens junctions (also known as belt desmosomes), which are found primarily in epithelial cells, form a belt of cell-cell adhesion just under the tight junctions.

The lateral surfaces of adjacent cells contain numerous gap junctions. These junctions help to integrate the metabolic activities of all cells in a tissue by allowing the direct passage of ions and small molecules from the cytosol of one cell to that of another (see the chapter opening figure). Among these are intracellular signaling molecules (e.g., cyclic AMP and Ca2+) and precursors of DNA and RNA.

Electron micrographs of animal tissue sections have shown that a space of about 20 nm ordinarily is present between the nonjunctional regions of plasma membranes of adjacent cells. This space contains integral membrane and extracellular surface glycoproteins that assist junctions in intercellular adhesion.

An understanding of the structure and function of desmosomes requires knowledge about actin microfilaments and intermediate filaments. Likewise, an understanding of gap junctions and their equivalent in plant cells (plasmodesmata) depends on knowledge of cellular metabolism and signaling. Therefore, we defer detailed discussion of these junctions until later chapters when these related topics are examined.

SUMMARY

  •  The apical and basolateral plasma membrane domains of epithelial cells contain different transport proteins and carry out quite different transport processes.
  •  In the intestinal epithelial cell, Na+/glucose and Na+/amino acid symporters are in the apical membrane region facing the intestinal lumen, while Na+/K+ ATPases and glucose and amino acid uniporters are in the basolateral membrane region facing the blood capillaries. The coordinated operation of these membrane transport proteins allows the uphill transepithelial movement of amino acids and glucose from the lumen to the blood, powered by ATP hydrolysis by the Na+/K+ ATPase (see Figure 15-25).
  •  Parietal cells in the stomach lining, which secrete HCl into the lumen, have ATP-powered H+/K+ pumps, K+ channels, and Cl channels on the apical membrane and pH-sensitive Cl/HCO3 antiporters on the basolateral membrane. The combined action of these proteins allows the cytosolic pH to be maintained near neutrality, despite the active export of protons from the cells into the stomach lumen, causing its acidification (see Figure 15-26).
  •  The plasma membrane contains specialized regions that form various types of cell junctions between adjacent cells (see Figure 15-23).
  •  Tight junctions interconnecting epithelial and other polarized cells seal off body cavities and restrict diffusion of plasma-membrane proteins from the apical to the basolateral surfaces. Tight junctions also prevent diffusion of lipids in the exoplasmic (but not the cytosolic leaflet) from the apical to the basolateral domains of the plasma membrane.
  •  Adherens junctions and spot desmosomes bind the plasma membranes of adjacent cells in a way that gives strength and rigidity to the entire tissue. Hemidesmosomes help connect cells to the extracellular matrix.
  •  Gap junctions in animal cells and plasmodesmata in plant cells interconnect the cytosol of two adjacent cells, allowing small molecules and ions to pass between them.
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By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2000, W. H. Freeman and Company.
Bookshelf ID: NBK21502