Gap Junctions

Gap junctions are sites of intercellular communication where cells are interconnected by small aqueous channels (Figure 1b). The term is morphological, for in transmission electron micrographs of thin sections, the unit membranes of the joined cells are seen to remain separated by a gap of about 2 nm (Figure 1a, inset). The gap of gap junctions distinguishes them from tight junctions, or zonulae occludentes, where extracellular space is completely occluded. The gap plays no part in the function, because it is crossed by bridging structures that contain the channels running between the cytoplasmic compartments of the two cells. These channels allow the diffusion of small molecules up to a molecular weight of about 1 kDa and diameter of about 1.5 nm and flow of electric current, carried mostly by potassium ions, when there is transjunctional voltage. Gap junctions in the protostome line (nematodes, mollusks, arthropods) are formed of a family of proteins termed pannexins or innexins; gap junctions in the deuterostome line (echinoderms, ascidians, vertebrates) are formed of connexins, which are encoded by a different gene family. Mammals and presumably less-differentiated deuterostomes also express pannexins (hence the name), but no gap junctions formed of these proteins have yet been demonstrated in vertebrates. The many convergences and few differences between connexin gap junctions in deuterostomes and pannexins in protostomes will be noted at the end of this article. Elsewhere in this article, gap junction refers to the connexin-based organelle. Gap junctions are found widely in inexcitable tissues, such as liver, epidermis, exocrine pancreas, salivary glands, neuroglia, and lining of the gut from mouth to anus. Although gap junctions are very common in these tissues, there are many contacts between specific kinds of cell where they are absent. With respect to excitable tissues in vertebrates, gap junctions interconnect cells in cardiac muscle, in many kinds of smooth muscle, and the islets of Langerhans. As electrical synapses between neurons, they occur at many sites in mammals as well as in lower vertebrates. They are not found between adult vertebrate skeletal muscle fibers, and are rare or absent between neurons and glia and between many kinds of neurons that come into close contact. The basic structure of gap junctions was derived from several techniques of electron microscopy. The junctions were first defined in thin section as close membrane appositions (Figure 1a), and at higher magnification a gap between the apposed membranes could be seen. The gap could be penetrated by small extracellular markers, such as colloidal La(OH)3. Freeze-fracture replicas in the plane of the junction show junctional particles on the protoplasmic face and corresponding pits on the external face (Figures 2 and 3). These structures are often hexagonally packed. With good resolution the particles show a central depression that presumably is the site of the aqueous channel (Figure 2, inset). Gap junctions can be isolated by several techniques that dissolve away surrounding membrane. Isolated junctions, when negatively stained, usually show domains of hexagonal symmetry (Figure 4). There is a central dot at the presumed site of the channel lumen and a hexagonal lattice surrounding the dots that results from stain in the intercellular gap, which is interrupted by the bridging structures. Isolated junctions have been analyzed by low-dose electron microscopy of isolated junctions and separated junctional membranes. The data indicate that each channel is a dodecamer comprised of a hexamer in each of the apposed membranes, as diagrammed in Figure 1b. When hemichannels come together to form a cell–cell channel, one hemichannel is rotated by 30 relative to the other (Figure 5). The hexamers are termed hemichannels or connexons. Antibodies against the junctional protein or connexin can be used to label gap junctions at the light and electronmicroscopic levels; the intercalated discs of cardiac ventricle are illustrated in Figure 1c and d). Freezefracture (followed by) immunolabeling (FRIL) of residual protein on the replica allows much more precise localization of particular connexins in identifiable gap junctions (Figure 3).