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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 22.1Cell-Cell Adhesion and Communication

Adhesion of like cells is a primary feature of the architecture of many tissues. A sheet of absorptive epithelial cells, for instance, forms the lining of the small intestine, and sheets of hepatocytes two cells thick make up much of the liver. A number of cell-surface proteins (the CAMs), mediate such homophilic (like-binds-like) adhesion between cells of a single type and heterophilic adhesion between cells of different types. Most CAMs are uniformly distributed along the regions of plasma membranes that contact other cells, and the cytosol-facing domains of these proteins are usually connected to elements of the cytoskeleton.

There are five principal classes of CAMs (Figure 22-2): cadherins, the immunoglobulin (Ig) superfamily, selectins, mucins, and integrins. Cell-cell adhesion involving cadherins and selectins depends on Ca2+ ions, whereas interactions involving integrin and Ig-superfamily CAMs do not. Many cells use several different CAMs to mediate cell-cell adhesion. The integrins mediate cell-matrix interactions whereas the other types of CAMs participate in cell-cell adhesion.

Figure 22-2. Major families of cell-adhesion molecules (CAMs).

Figure 22-2

Major families of cell-adhesion molecules (CAMs). Integral membrane proteins are built of multiple domains. Cadherin and the immunoglobulin (Ig) superfamily of CAMs mediate homophilic cell-cell adhesion. For cadherin, calcium binding to sites (orange) (more...)

Adhesion of cells to one another generally is initiated by one or more of the cell-adhesion molecules described in Figure 22-2. In order for cells in tissues to function in an integrated manner, specialized junctions consisting of clustered cell-adhesion molecules are essential. There are four major classes of junctions: the tight junction, gap junction, cell-cell, and cell-matrix junctions (see Figure 15-23). In Chapter 15, we discussed the structure and function of tight junctions, which connect epithelial cells (e.g., those lining the intestine) and prevent passage of fluids through the cell layer (see Figure 15-28). Here we consider the structure and specialized functions of cell-cell and gap junctions. Cell-cell and cell-matrix junctions perform a simple structural role, to hold cells into a tissue. They carry out this role by connecting the internal cytoskeleton directly to the cell exterior, either another cell or the extracellular matrix, via two cell-adhesion molecules — cadherins and integrins. Rather than inventing unique ways of connecting the actin and intermediate filament (IF) cytoskeletons to cadherin and integrin in the plasma membrane, the cell has instead evolved a common structure that adapts to different partners (see Figure 22-1). Despite their complexity, all cytoskeleton-associated junctions are organized into three parts: cell-adhesion molecules, which connect the cell to another cell or to the extracellular matrix; adapter proteins, which connect the CAMs to actin or keratin filaments; and lastly the bundle of cytoskeletal filaments itself.

In addition to their structural links, cells in tissues are in direct communication through gap junctions. Gap junctions are distributed along the lateral surfaces of adjacent cells and allow them to exchange small molecules. As we discussed in Chapter 21, gap junctions at electric synapses allow ions to pass from one nerve cell to the next, thereby allowing a presynaptic cell to induce an action potential in the postsynaptic cell without a lag period (see Figure 21-35). But gap junctions also are present in many non-neuronal tissues, where they help to integrate the metabolic activities of all the cells in a tissue by permitting exchange of ions and small molecules (e.g., cyclic AMP and precursors of DNA and RNA).

Cadherins Mediate Ca2+-Dependent Homophilic Cell-Cell Adhesion

Cadherins, a family of Ca2+-dependent CAMs, are the major molecules of cell-cell adhesion and play a critical role during tissue differentiation (Chapter 23). The most widely expressed, particularly during early differentiation, are the E-, P-, and N-cadherins. Over 40 different cadherins are known; some of the best understood cadherins are summarized in Table 22-1. The brain expresses the largest number of different cadherins, presumably due to the necessity of forming very specific cell-cell contacts.

Table 22-1. Major Cadherin Molecules on Mammalian Cells.

Table 22-1

Major Cadherin Molecules on Mammalian Cells.

Each cadherin is a type I integral membrane glycoprotein of 720 – 750 amino acids. The cadherin molecule consists of an N-terminal extracellular region, a single transmembrane spanning segment, and a C-terminal cytoplasmic tail. The extracellular domain contains repeated sequences that are sites necessary for Ca2+ binding and cell-cell adhesion. The cytoplasmic domain associates with the cytoskeleton. On average, 50 – 60 percent of the sequence is identical among different cadherins. Importantly, each cadherin has a characteristic tissue distribution. During differentiation and in some diseases, the amount or nature of the cell-surface cadherins changes, affecting many aspects of cell-cell adhesion and cell migration. For example, the metastasis of tumor cells is correlated with the loss of cadherin on their cell surface.

In adult vertebrates, E-cadherin holds most epithelial sheets together. Sheets of polarized epithelial cells, such as those that line the small intestine or kidney tubules, contain abundant E-cadherin at the sites of cell-cell contact along their lateral surfaces. When a monoclonal antibody to E-cadherin is added to a monolayer of cultured epithelial cells, the cells detach from one another, directly demonstrating the requirement for E-cadherin in cell-cell adhesion. The removal of Ca2+ from the medium also disrupts cell-cell adhesion, showing that E-cadherin-mediated interactions require Ca2+. If E-cadherin-mediated adhesion is blocked during cell aggregation, none of the specialized cell junctions between epithelial cells are generated. Later studies showed that calcium ions cause cadherin to dimerize and that cadherin dimers and not monomers are responsible for cell-cell adhesion.

E-cadherin, like the other cadherins, preferentially mediates homophilic interactions. This phenomenon was demonstrated in experiments with L cells, a line of cultured transformed mouse fibroblasts that express no cadherins and adhere poorly to themselves or to other cultured cells. Lines of transfected L cells that expressed either E-cadherin or P-cadherin were generated; such cells were found to adhere preferentially to cells expressing the same class of cadherin molecules. For instance, E-cadherin-expressing L cells adhere tightly to one another and to epithelial cells from embryonic lung that express E-cadherin; they do not attach to untransfected L cells or to L cells (or other cell types) expressing P-cadherin. L cells expressing P-cadherin adhere to one another, and to other types of cells that express this cadherin. Thus, cadherins directly cause homotypic interactions among cells.

The mechanism for cell-cell adhesion is explained by the atomic structures of the N-terminal domains from E- and N-cadherin. The current model proposes that cadherin molecules associate through their N-terminal domains into a parallel homodimer (see Figure 22-2). The Ca2+-binding sites, located between the cadherin repeats, serve to rigidify the cadherin molecule and expose residues that form the dimer interface. Furthermore, cell-cell adhesion results from head-to-head contact between cadherin dimers in adjacent cell membranes. The two sets of interactions, head-to-head and side-to-side, are postulated to cause the clustering or “zipping” of cadherins in specialized adhesion junctions.

N-CAMs Mediate Ca2+-Independent Homophilic Cell-Cell Adhesion

N-CAMs, a group of Ca2-independent cell-cell adhesion proteins in vertebrates, belong to the Ig superfamily of CAMs (see Figure 22-2). Their full name — nerve-cell adhesion molecule — reflects their particular importance in nervous tissue. Like cadherins, N-CAMs primarily mediate homophilic interactions, binding together cells that express similar N-CAM molecules. Unlike cadherins, N-CAMs are encoded by a single gene; their diversity is generated by alternative mRNA splicing and by differences in glycosylation (Figure 22-3). Like N-cadherin, N-CAMs appear during morphogenesis, playing an important role in differentiation of muscle, glial, and nerve cells. Their role in cell adhesion has been directly demonstrated by use of specific antibodies. For instance, adhesion of cultured retinal neurons is inhibited by addition of antibodies to N-CAMs.

Figure 22-3. Three of the N-CAMs produced by alternative splicing of the primary transcript produced from the single N-CAM gene.

Figure 22-3

Three of the N-CAMs produced by alternative splicing of the primary transcript produced from the single N-CAM gene. N-CAM 180 (180,000 MW) and N-CAM 140 are anchored in the membrane by a single hydrophobic α helix and differ in the length of their (more...)

The adhesive properties of N-CAMs are modulated by long chains of sialic acid, a negatively charged sugar (see Figure 17-31). N-CAMs that are heavily sialylated form weaker homophilic interactions than do less sialylated forms, possibly because of repulsion between the negatively charged sialic acid residues. In embryonic tissues such as brain, polysialic acid constitutes as much as 25 percent of the mass of N-CAMs; in contrast, N-CAMs from adult tissues contain only one-third as much sialic acid. The lower adhesive properties of embryonic N-CAMs enable cell-cell contacts to be made and then broken, a property necessary for specific cell contacts to form in the developing nervous system. The higher adhesive properties of the adult forms of N-CAM stabilize these contacts. Thus, the strength of cell-cell adhesions is modified during differentiation by differential glycosylation of the N-CAMs.

Selectins and Other CAMs Participate in Leukocyte Extravasation

Thus far we have considered cell interactions in solid tissues, such as epithelia and neuronal tissue. Once these interactions form during differentiation, they generally are stable for the life of the cells. In adult organisms, many types of cells that participate in defense against foreign invaders (e.g., bacteria and viruses) must move rapidly from the blood, where they circulate as unattached cells, into the underlying tissue at sites of infection or inflammation. Movement into tissue, termed extravasation, of four types of leukocytes (white blood cells) is particularly important: monocytes, the precursors of macrophages, which can ingest foreign particles; neutrophils, which release several antibacterial proteins; and T and B lymphocytes, the antigen-specific cells of the immune system.

Extravasation requires the successive formation and breakage of cell-cell contacts between leukocytes in the blood and endothelial cells lining the vessels. These contacts are mediated by selectins, a class of cell-adhesion molecules that are specific for leukocyte – vascular cell interactions. A key protein in this process, P-selectin, is localized to the blood-facing surface of endothelial cells. Like other members of the selectin family of CAMs, P-selectin is a lectin, a protein that binds to carbohydrates. Each type of selectin binds to specific oligosaccharide sequences in glycoproteins or glycolipids. As with cadherins, binding of selectins to their ligands is Ca2+-dependent. The sugar-binding lectin domain in selectins generally is at the end of the extracellular region of the molecule (see Figure 22-2). The ligand for P-selectin is a specific oligosaccharide sequence, called the sialyl Lewis-x antigen, that is part of longer oligosaccharides present in abun-dance on leukocyte glycoproteins and glycolipids.

As illustrated in Figure 22-4, in normal endothelial cells P-selectin is localized to intracellular vesicles and is not present on the plasma membrane. These cells are activated by various inflammatory signals released by surrounding cells in areas of infection or inflammation. Once endothelial cells are activated, the vesicles containing P-selectin undergo exocytosis within seconds, and P-selectin appears on the plasma membrane. As a consequence, passing leukocytes adhere weakly to the endothelium; because of the force of the blood flow, these “trapped” leukocytes are slowed but not stopped and seem to roll along the surface of the endothelium.

Figure 22-4. Interactions between cell-adhesion molecules during the initial binding and tight binding of T cells, a kind of leukocyte, to activation endothelial cells.

Figure 22-4

Interactions between cell-adhesion molecules during the initial binding and tight binding of T cells, a kind of leukocyte, to activation endothelial cells. Once a T cell has firmly adhered to the endothelium, it can move (extravasate) into the underlying (more...)

In order for tight adhesion to occur between activated endothelial cells and leukocytes, β2-containing integrins on the surface of leukocytes also must be activated. For example, activation of the αLβ2 integrin, which is expressed by T lymphocytes, is induced by platelet-activating factor (PAF), a phospholipid released by activated endothelial cells at the same time that P-selectin is exocytosed. Binding of PAF to its receptor on T lymphocytes activates integrin αLβ2 through the Rho signaling pathway. The activated integrin then binds to ICAM-1 and ICAM-2, which are Ig-superfamily CAMs expressed constitutively on the surface of endothelial cells. The tight adhesion mediated by the Ca2+-independent interaction of αLβ2 and the ICAMs leads to spreading of T lymphocytes on the surface of the endothelium; soon the adhered T lymphocytes move between adjacent endothelial cells and into the underlying tissue.

Thus, the selective adhesion of T lymphocytes to the endothelium near sites of infection or inflammation depends on the sequential appearance and activation of several different CAMs on the surface of the interacting cells. Other leukocytes that express specific integrins containing the β2 subunit move into tissues by a similar mechanism; αMβ2, for instance, is found primarily on macrophages. As might be expected, humans with a genetic defect in synthesis of the integrin β2 subunit, termed leukocyte-adhesion deficiency, are susceptible to repeated bacterial infections.

Cadherin-Containing Junctions Connect Cells to One Another

Although hundreds of individual cell-cell adhesions are sufficient to cause cells to adhere, specialized junctions consisting of dense clusters of cell-adhesion molecules are required for the function of tissues. In electron micrographs, the plasma membranes of cell-cell junctions are parallel and only 15 – 20 nm apart. Concentrated in this region is E-cadherin, which links the plasma membranes of adjacent cells and via catenin adapter proteins attaches to actin filaments in adherens junctions or keratin filaments in desmosomes (Figure 22-5).

Figure 22-5. Adhesion molecules in junctions involved in cell-cell adhesion.

Figure 22-5

Adhesion molecules in junctions involved in cell-cell adhesion. Adherens junctions and desmosomes are specialized cell-cell junctions that consist of clustered-cadherin dimers. Cadherin is connected to either the circumferential belt of actin filaments (more...)

Adherens Junctions

Epithelial cells contain a continuous band of cadherin molecules, usually located near the apical surface just below the tight junction, that connects the lateral membranes of epithelial cells (see Figure 15-23). Known as the adherens junction, this region contains α- and β-catenins that link E-cadherin in the plasma membrane to the circumferential belt of actin and myosin filaments (see Figure 18-35). Adherens junctions contain many of the same proteins found at focal adhesions, including vinculin, tropomyosin, and α-actinin. As a complex with the adherens junction, the circumferential belt functions as a tension cable that can internally brace the cell and thereby control its shape.


A desmosome consists of proteinaceous adhesion plaques (15 – 20 nm thick) attached to the cytosolic face of the plasma membranes of adjacent cells and connected by transmembrane linker proteins (Figure 22-6). Plakoglobin is a major constituent of the plaques; it is very similar to β-catenin. The transmembrane linker proteins, called desmoglein and desmocollin, belong to the cadherin family of cell-adhesion molecules. They bind to plakoglobin and other proteins in the plaques and extend into the intercellular space, where they interact, forming an interlocking network that binds two cells together.

Figure 22-6. Desmosomes.

Figure 22-6

Desmosomes. (a) Schematic model showing components of a desmosome between epithelial cells and attachments to the sides of keratin intermediate filaments, which crisscross the interior of cells. The transmembrane linker proteins, desmoglein and desmocollin, belong (more...)

Image med.jpgDesmoglein was first identified by an unusual but revealing skin disease called pemphigus vulgaris, an autoimmune disease. Patients with autoimmune disorders synthesize antibodies that bind to a normal body protein. In this case, the autoantibodies disrupt adhesion between epithelial cells, causing blisters of the skin and mucous membranes. The predominant autoantibody was shown to be specific for desmoglein, a major protein in the skin desmosomes; indeed, addition of such antibodies to normal skin induces formation of blisters and disruption of cell adhesion.

In epithelial cells, keratin intermediate filaments course near the cytoplasmic plaques of desmosomes and apparently are linked to them by the desmoplakin proteins. Some of these filaments run parallel to the cell surface, and others penetrate and traverse the cytoplasm. They are thought to be part of the internal structural framework of the cell, giving it shape and rigidity. If so, desmosomes also could transmit shearing forces from one region of a cell layer to the epithelium as a whole; they thus provide strength and rigidity to the entire epithelial cell layer.

Gap Junctions Allow Small Molecules to Pass between Adjacent Cells

Early electron micrographs showed that almost all animal cells that come in contact with each other have regions of junctional specialization characterized by an intercellular gap, which is filled by a well-defined set of cylindrical particles (Figure 22-7). Morphologists named these regions gap junctions, but in retrospect the gap is not their most important feature. The cylindrical particles, which are water-filled channels, are the key to the function of gap junctions. These channels directly link the cytosol of one cell with that of an adjacent cell, providing a passageway for movement of very small molecules and ions between the cells.

Figure 22-7. Electron micrograph of a thin section through a gap junction connecting two mouse liver cells.

Figure 22-7

Electron micrograph of a thin section through a gap junction connecting two mouse liver cells. The two plasma membranes are closely associated for a distance of several hundred nanometers, separated by a “gap” of 2 – 3 (more...)

The size of these intercellular channels can be measured by injecting a cell with a fluorescent dye covalently linked to molecules of various sizes and using a fluorescence microscope to observe whether they pass into neighboring cells. Gap junctions between mammalian cells permit the passage of molecules as large as 1.2 nm in diameter. In insects, these junctions are permeable to molecules as large as 2 nm in diameter. Generally speaking, molecules with a molecular weight less than 1200 pass freely, and those with a molecular greater than 2000 do not pass; the passage of intermediate-sized molecules is variable and limited. Thus ions and many low-molecular-weight building blocks of cellular macromolecules, such as amino acids and nucleoside phosphates, can pass from cell to cell.

A vivid example of this cell-cell transfer is the phenomenon of metabolic coupling, or metabolic cooperation, in which a cell transfers molecules to a neighboring cell that is unable to synthesize them. This phenomenon can be demonstrated with mutant cells unable to synthesize dATP, an immediate precursor of DNA, from hypoxanthine via a nucleotide-salvage pathway (see Figure 6-9). When cultured alone, these cells cannot incorporate radioactivity from hypoxanthine into their DNA. However, when the mutant cells are co-cultured with wild-type cells, radioactivity is frequently found in the nuclear DNA of the mutant cells. (The mutant and wild-type cells can be differentiated by their distinct morphologies or by feeding one of the cell lines carbon particles before mixing it with the other line.) The dATP derived from hypoxanthine is incorporated only into the DNA of mutant cells that are in direct or indirect contact (through an intermediate cell) with wild-type cells. It is thought that labeled adenosine mono-, di-, or triphosphate is synthesized from the labeled hypoxanthine by wild-type cells and then passed through gap junctions to the mutant cells.

Another important compound transferred from cell to cell through gap junctions is cyclic AMP (cAMP), which acts as an intracellular second messenger. As discussed in Chapter 20, the amount of cellular cAMP increases in response to stimulation of cells by binding of many different hormones. The fact that cAMP can pass through gap junctions means that the hormonal stimulation of just one or a few cells in an epithelium initiates a metabolic reaction in many of them. For instance, binding of secretory hormones, such as secretin, to receptors on the basal plasma membranes of pancreatic acinar cells leads to increase in the intracellular concentration of either cAMP or Ca2+ ions, both of which trigger secretion of the contents of secretory vesicles. Because Ca2+ and cAMP can pass through the gap junctions, hormonal stimulation of one cell triggers secretion by many. As we saw in Chapter 18, an elevation in cytosolic Ca2+ in smooth muscle cells induces contraction. Gap junction – mediated transfer of Ca2+ ions between adjacent smooth muscle cells thus allows the coordinated contractile waves in the intestine during peristalsis and in the uterus during birth.

An important aspect of gap-junction physiology is that the channels close in the presence of very high concentrations of Ca2+ in the cytosol. Recall that the Ca2+ concentration in extracellular fluids in quite high (from 1×10−3 M to 2×10−3 M), whereas normally the concentration of Ca2+ free in the cytosol is lower than 10−6 M (see Table 15-1). If the membrane of one cell in an epithelium is ruptured, Ca2+ enters the cell, closing the channels that connect the cell with its neighbors and thus preventing leakage of the low-molecular-weight substances that are present in the cytoplasm of all epithelium cells. Even slight increases in the level of cytosolic Ca2+ ions or decreases in cytosolic pH can decrease the permeability of gap junctions. Thus cells may modulate the degree of coupling with their neighbors, but precisely why and how they accomplish this is a matter of debate.

Connexin, a Transmembrane Protein, Forms Cylindrical Channels in Gap Junctions

In the liver and many other tissues, large numbers of individual gap junction channels cluster together in an area about 0.3 mm in diameter. This property has enabled researchers to separate gap junctions from other components of the plasma membrane by shearing the purified plasma membrane into small fragments. Owing to their relatively high protein content, fragments containing gap junctions have a higher density than the bulk of the plasma membrane and can be purified on an equilibrium density gradient (see Figure 5-24). Electron micrographs of stained, isolated gap junctions reveal a lattice of hexagonal particles with hollow cores as intercellular channels (see Figure 21-35).

A current model of the structure of the gap junction is shown in Figure 22-8a. The transmembrane particles from purified liver gap junctions are composed of connexin subunits, proteins with molecular weights between 25,000 and 50,000. Each hexagonal particle consists of 12 connexin molecules: 6 of the molecules are arranged in a connexon hemichannel, a hexagonal cylinder in one plasma membrane, and joined to a connexon hemichannel in the adjacent cell membrane.

Figure 22-8. Structure of gap junctions.

Figure 22-8

Structure of gap junctions. (a) In this model, a gap junction is a cluster of channels between two plasma membranes that are separated by a gap of about 2 – 3 nm. (b) Both membranes contain connexon hemichannels, cylinders of six (more...)

The sequences of several connexin proteins, expressed in different tissues, have been determined from their cDNAs. All connexins have related amino acid sequences. Experiments suggest that each connexin polypeptide spans the plasma membrane four times (see Figure 22-8b) and that one conserved transmembrane α helix lines the aqueous channel. The connexins differ mainly in the length and sequence of their most C-terminal segment, which faces the cytosol. At least 12 different genes in the connexin family have been cloned; many are expressed in specific types of cells.

Some cells express a single connexin, consequently their gap junction channels are homotypic, consisting of identical connexons. However, most cells express at least two connexin genes. Consequently, different connexin polypeptides can assemble into hetero-oligomeric connexons, which in turn form heterotypic gap-junction channels. This diversity in channel composition leads to differences in permeability of the channels to different molecules.


  •  Cell-cell interactions involve multiple ligands and cell-adhesion molecules (CAMs), which are a diverse group of integral membrane proteins. CAMs fall into five main classes: the cadherins, Ig-superfamily CAMs, selectins, mucins and integrins (see Figure 22-2).
  •  Cadherins are responsible for Ca2+-dependent homophilic interactions between cells.
  •  Ca2+-independent homophilic interactions between cells are mediated by N-CAMs, which belong to the Ig superfamily.
  •  Selectins, which bind to carbohydrate groups on mucin-like CAMs via their distal lectin domain, mediate Ca2+-dependent heterophilic cell-cell interactions. P-selectin on the surface of activated vascular endothelial cells plays an important role in the extravasation of leukocytes into tissues (see Figure 22-4).
  •  In all cell-adhesion junctions, clusters of transmembrane cell-adhesion molecules are linked via various adapter proteins in cytoplasmic plaques to the cytoskeleton.
  •  Adherens junctions and desmosomes are cadherin-containing junctions that bind the membranes of adjacent cells in a way that gives strength and rigidity to the entire tissue (see Figure 22-5).
  •  Gap junctions are constructed of 12 copies of a single protein, connexin, formed into a transmembrane channel that interconnects the cytoplasm of two adjacent cells (see Figure 22-8). Small molecules can pass through gap junctions, permitting metabolic coupling of adjacent cells.
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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: NBK21599