Specialized cell junctions occur at points of cell-cell and cell-matrix contact in all tissues, and they are particularly plentiful in epithelia. Cell junctions are best visualized using either conventional or freeze-fracture electron microscopy (discussed in Chapter 9), which reveals that the interacting plasma membranes (and often the underlying cytoplasm and the intervening intercellular space as well) are highly specialized in these regions.
Cell junctions can be classified into three functional groups:
Occluding junctions seal cells together in an epithelium in a way that prevents even small molecules from leaking from one side of the sheet to the other.
Anchoring junctions mechanically attach cells (and their cytoskeletons) to their neighbors or to the extracellular matrix .
Communicating junctions mediate the passage of chemical or electrical signals from one interacting cell to its partner.
The major kinds of intercellular junctions within each group are listed in Table 19-1. We discuss each of them in turn, except for chemical synapses, which are formed exclusively by nerve cells and are considered in Chapters 11 and 15.
A Functional Classification of Cell Junctions.
Occluding Junctions Form a Selective Permeability Barrier Across Epithelial Cell Sheets
All epithelia have at least one important function in common: they serve as selective permeability barriers, separating fluids on either side that have a different chemical composition. This function requires that the adjacent cells be sealed together by occluding junctions. Tight junctions have this barrier role in vertebrates, as we illustrate by considering the epithelium of the mammalian small intestine, or gut.
The epithelial cells lining the small intestine form a barrier that keeps the gut contents in the gut cavity, the lumen . At the same time, however, the cells must transport selected nutrients across the epithelium from the lumen into the extracellular fluid that permeates the connective tissue on the other side (see Figure 19-1 ). From there, these nutrients diffuse into small blood vessels to provide nourishment to the organism. This transcellular transport depends on two sets of membrane -bound membrane transport proteins. One set is confined to the apical surface of the epithelial cell (the surface facing the lumen) and actively transports selected molecules into the cell from the gut. The other set is confined to the basolateral (basal and lateral) surfaces of the cell, and it allows the same molecules to leave the cell by facilitated diffusion into the extracellular fluid on the other side of the epithelium. To maintain this directional transport, the apical set of transport proteins must not be allowed to migrate to the basolateral surface of the cell, and the basolateral set must not be allowed to migrate to the apical surface. Furthermore, the spaces between epithelial cells must be tightly sealed, so that the transported molecules cannot diffuse back into the gut lumen through these spaces (Figure 19-2 ).
The role of tight junctions in transcellular transport. Transport proteins are confined to different regions of the plasma membrane in epithelial cells of the small intestine. This segregation permits a vectorial transfer of nutrients across the epithelium (more. )
The tight junctions between epithelial cells are thought to have both of these roles. First, they function as barriers to the diffusion of some membrane proteins (and lipids) between apical and basolateral domains of the plasma membrane (see Figure 19-2 ). Mixing of such proteins and lipids occurs if tight junctions are disrupted, for example, by removing the extracellular Ca 2+ that is required for tight junction integrity. Second, tight junctions seal neighboring cells together so that, if a low-molecular-weight tracer is added to one side of an epithelium, it will generally not pass beyond the tight junction (Figure 19-3 ). This seal is not absolute, however. Although all tight junctions are impermeable to macromolecules, their permeability to small molecules varies greatly in different epithelia. Tight junctions in the epithelium lining the small intestine, for example, are 10,000 times more permeable to inorganic ions, such as Na +. than the tight junctions in the epithelium lining the urinary bladder. These differences reflect differences in tight junction proteins that form the junctions.
The role of tight junctions in allowing epithelia to serve as barriers to solute diffusion. (A) The drawing shows how a small extracellular tracer molecule added on one side of an epithelium cannot traverse the tight junctions that seal adjacent cells (more. )
Epithelial cells can transiently alter their tight junctions to permit an increased flow of solutes and water through breaches in the junctional barriers. Such paracellular transport is especially important in the absorption of amino acids and monosaccharides from the lumen of the intestine, where their concentration can increase enough after a meal to drive passive transport in the desired direction.
When tight junctions are visualized by freeze-fracture electron microscopy. they seem to be composed of a branching network of sealing strands that completely encircles the apical end of each cell in the epithelial sheet (Figure 19-4A and B ). In conventional electron micrographs, the outer leaflets of the two interacting plasma membranes are seen to be tightly apposed where sealing strands are present (Figure 19-4C ). The ability of tight junctions to restrict the passage of ions through the spaces between cells is found to increase logarithmically with increasing numbers of strands in the network, suggesting that each strand acts as an independent barrier to ion flow.
The structure of a tight junction between epithelial cells of the small intestine. The junctions are shown (A) schematically, (B) in a freeze-fracture electron micrograph, and (C) in a conventional electron micrograph. Note that the cells are oriented (more. )
Each tight junction sealing strand is composed of a long row of transmembrane adhesion proteins embedded in each of the two interacting plasma membranes. The extracellular domains of these proteins join directly to one another to occlude the intercellular space (Figure 19-5 ). The major transmembrane proteins in a tight junction are the claudins, which are essential for tight junction formation and function and differ in different tight junctions. A specific claudin found in kidney epithelial cells, for example, is required for Mg 2+ to be resorbed from the urine into the blood. A mutation in the gene encoding this claudin results in excessive loss of Mg 2+ in the urine. A second major transmembrane protein in tight junctions is occludin, the function of which is uncertain. Claudins and occludins associate with intracellular peripheral membrane proteins called ZO proteins (a tight junction is also known as a zonula occludens ), which anchor the strands to the actin cytoskeleton.
A current model of a tight junction. (A) This drawing shows how the sealing strands hold adjacent plasma membranes together. The strands are composed of transmembrane proteins that make contact across the intercellular space and create a seal. (B) This (more. )
In addition to claudins, occludins, and ZO proteins, several other proteins can be found associated with tight junctions. These include some that regulate epithelial cell polarity and others that help guide the delivery of components to the appropriate domain of the plasma membrane. Thus, the tight junction may serve as a regulatory center to help in coordinating multiple cell processes.
In invertebrates, septate junctions are the main occluding junction. More regular in structure than a tight junction. they likewise form a continuous band around each epithelial cell. But their morphology is distinct because the interacting plasma membranes are joined by proteins that are arranged in parallel rows with a regular periodicity (Figure 19-6 ). A protein called Discs-large, which is required for the formation of septate junctions in Drosophila, is structurally related to the ZO proteins found in vertebrate tight junctions. Mutant flies that are deficient in this protein not only lack septate junctions but also develop epithelial tumors. This observation suggests that the normal regulation of cell proliferation in epithelial tissues may depend, in part, on intracellular signals that emanate from occluding junctions.
A septate junction. A conventional electron micrograph of a septate junction between two epithelial cells in a mollusk. The interacting plasma membranes, seen in cross section, are connected by parallel rows of junctional proteins. The rows, which have (more. )
Anchoring Junctions Connect the Cytoskeleton of a Cell Either to the Cytoskeleton of Its Neighbors or to the Extracellular Matrix
The lipid bilayer is flimsy
and cannot by itself transmit large forces from cell to cell or from cell to extracellular matrix. Anchoring junctions solve the problem by forming a strong membrane -spanning structure that is tethered inside the cell to the tension-bearing filaments of the cytoskeleton (Figure 19-7 ).
Anchoring junctions in an epithelium. This drawing illustrates, in a very general way, how anchoring junctions join cytoskeletal filaments from cell to cell and from cells to the extracellular matrix.
Anchoring junctions are widely distributed in animal tissues and are most abundant in tissues that are subjected to severe mechanical stress, such as heart, muscle, and epidermis. They are composed of two main classes of proteins (Figure 19-8 ). Intracellular anchor proteins form a distinct plaque on the cytoplasmic face of the plasma membrane and connect the junctional complex to either actin filaments or intermediate filaments. Transmembrane adhesion proteins have a cytoplasmic tail that binds to one or more intracellular anchor proteins and an extracellular domain that interacts with either the extracellular matrix or the extracellular domains of specific transmembrane adhesion proteins on another cell. In addition to anchor proteins and adhesion proteins, many anchoring junctions contain intracellular signaling proteins that enable the junctions to signal to the cell interior.
The construction of an anchoring junction from two classes of proteins. This drawing shows how intracellular anchor proteins and transmembrane adhesion proteins form anchoring junctions.
Anchoring junctions occur in two functionally different forms:
Adherens junctions and desmosomes hold cells together and are formed by transmembrane adhesion proteins that belong to the cadherin family .
Focal adhesions and hemidesmosomes bind cells to the extracellular matrix and are formed by transmembrane adhesion proteins of the integrin family .
On the intracellular side of the membrane. adherens junctions and focal adhesions serve as connection sites for actin filaments, while desmosomes and hemidesmosomes serve as connection sites for intermediate filaments (see Table 19-1. p. 1067).
Adherens Junctions Connect Bundles of Actin Filaments from Cell to Cell
Adherens junctions occur in various forms. In many nonepithelial tissues, they take the form of small punctate or streaklike attachments that indirectly connect the cortical actin filaments beneath the plasma membranes of two interacting cells. But the prototypical examples of adherens junctions occur in epithelia, where they often form a continuous adhesion belt (or zonula adherens) just below the tight junctions, encircling each of the interacting cells in the sheet. The adhesion belts are directly apposed in adjacent epithelial cells, with the interacting plasma membranes held together by the cadherins that serve here as transmembrane adhesion proteins.
Within each cell, a contractile bundle of actin filaments lies adjacent to the adhesion belt. oriented parallel to the plasma membrane. The actin is attached to this membrane through a set of intracellular anchor proteins, including catenins. vinculin. and α-actinin. which we consider later. The actin bundles are thus linked, via the cadherins and anchor proteins, into an extensive transcellular network (Figure 19-9 ). This network can contract with the help of myosin motor proteins (discussed in Chapter 16), and it is thought to help in mediating a fundamental process in animal morphogenesis—the folding of epithelial cell sheets into tubes and other related structures (Figure 19-10 ).
Adherens junctions. (A) Adherens junctions, in the form of adhesion belts, between epithelial cells in the small intestine. The beltlike junction encircles each of the interacting cells. Its most obvious feature is a contractile bundle of actin filaments (more. )
The folding of an epithelial sheet to form an epithelial tube. The oriented contraction of the bundles of actin filaments running along adhesion belts causes the epithelial cells to narrow at their apex and helps the epithelial sheet to roll up into a (more. )
The assembly of tight junctions between epithelial cells seems to require the prior formation of adherens junctions. Anti-cadherin antibodies that block the formation of adherens junctions, for example, also block the formation of tight junctions.
Desmosomes Connect Intermediate Filaments from Cell to Cell
Desmosomes are buttonlike points of intercellular contact that rivet cells together (Figure 19-11A ). Inside the cell, they serve as anchoring sites for ropelike intermediate filaments, which form a structural framework of great tensile strength (Figure 19-11B ). Through desmosomes, the intermediate filaments of adjacent cells are linked into a net that extends throughout the many cells of a tissue. The particular type of intermediate filaments attached to the desmosomes depends on the cell type: they are keratin filaments in most epithelial cells, for example, and desmin filaments in heart muscle cells.
Desmosomes. (A) An electron micrograph of three desmosomes between two epithelial cells in the intestine of a rat. (B) An electron micrograph of a single desmosome between two epidermal cells in a developing newt, showing clearly the attachment of intermediate (more. )
The general structure of a desmosome is illustrated in Figure 19-11C. and some of the proteins that form it are shown in Figure 19-11D. The junction has a dense cytoplasmic plaque composed of a complex of intracellular anchor proteins (plakoglobin and desmoplakin ) that are responsible for connecting the cytoskeleton to the transmembrane adhesion proteins. These adhesion proteins (desmoglein and desmocollin ), like those at an adherens junction. belong to the cadherin family. They interact through their extracellular domains to hold the adjacent plasma membranes together.
The importance of desmosome junctions is demonstrated by some forms of the potentially fatal skin disease pemphigus. Affected individuals make antibodies against one of their own desmosomal cadherin proteins. These antibodies bind to and disrupt the desmosomes that hold their skin epithelial cells (keratinocytes) together. This results in a severe blistering of the skin, with leakage of body fluids into the loosened epithelium.
Anchoring Junctions Formed by Integrins Bind Cells to the Extracellular Matrix: Focal Adhesions and Hemidesmosomes
Some anchoring junctions bind cells to the extracellular matrix rather than to other cells. The transmembrane adhesion proteins in these cell-matrix junctions are integrins —a large family of proteins distinct from the cadherins. Focal adhesions enable cells to get a hold on the extracellular matrix through integrins that link intracellularly to actin filaments. In this way, muscle cells, for example, attach to their tendons at the myotendinous junction. Likewise, when cultured fibroblasts migrate on an artificial substratum coated with extracellular matrix molecules, they also grip the substratum at focal adhesions, where bundles of actin filaments terminate. At all such adhesions, the extracellular domains of transmembrane integrin proteins bind to a protein component of the extracellular matrix, while their intracellular domains bind indirectly to bundles of actin filaments via the intracellular anchor proteins talin, α-actinin, filamin, and vinculin (Figure 19-12B ).
Focal adhesions. (A) In these immunofluorescence micrographs, cells in culture have been labeled with antibodies against both actin (green) and the intracellular anchor protein vinculin (red). Note that vinculin is located at focal adhesions, which is (more. )
Hemidesmosomes. or half-desmosomes, resemble desmosomes morphologically and in connecting to intermediate filaments, and, like desmosomes, they act as rivets to distribute tensile or shearing forces through an epithelium. Instead of joining adjacent epithelial cells, however, hemidesmosomes connect the basal surface of an epithelial cell to the underlying basal lamina (Figure 19-13 ). The extracellular domains of the integrins that mediate the adhesion bind to a laminin protein (discussed later) in the basal lamina, while an intracellular domain binds via an anchor protein (plectin) to keratin intermediate filaments. Whereas the keratin filaments associated with desmosomes make lateral attachments to the desmosomal plaques (see Figure 19-11C and D ), many keratin filaments associated with hemidesmosomes have their ends buried in the plaque (see Figure 19-13 ).
Desmosomes and hemidesmosomes. The distribution of desmosomes and hemidesmosomes in epithelial cells of the small intestine. The keratin intermediate filament networks of adjacent cells are indirectly connected to one another through desmosomes and to (more. )
Although the terminology for the various anchoring junctions can be confusing, the molecular principles (for vertebrates, at least) are relatively simple (Table 19-2 ). Integrins in the plasma membrane anchor a cell to extracellular matrix molecules; cadherin family members in the plasma membrane anchor it to the plasma membrane of an adjacent cell. In both cases, there is an intracellular coupling to cytoskeletal filaments, either actin filaments or intermediate filaments, depending on the types of intracellular anchor proteins involved.