What allows for communication and the passage of small molecules between adjacent cells?

Gap Junction Channels and Intercalated Discs

Another family of ion channel proteins is that containing the gap junctional channels. These dodecameric channels are found in the intercalated discs between adjacent cells (Fig. 34.7A, B). Three types of specialized junctions make up each intercalated disc. The macula adherens or desmosome and the fascia adherens form areas of strong adhesion between cells and may provide a linkage for the transfer of mechanical energy from one cell to the next. Thenexus, also called the tight or gap junction (Fig. 34.7C-E), is a region in the intercalated disc where cells are in functional contact with each other. Membranes at these junctions are separated by only about 10 to 20 Å and are connected by a series of hexagonally packed subunit bridges or gap junction channels that provide biochemical and low-resistance electrical coupling between adjacent cells, by establishing aqueous pores that directly link the cytoplasm of these adjacent cells. Gap junctions allow the movement of ions (e.g., Na+, Cl−, K+, Ca2+) and small molecules (e.g., cAMP, cGMP, inositol 1,4,5-triphosphate [IP3]) between cells, thereby linking the interiors of adjacent cells.

Gap junctions permit a multicellular structure such as the heart to function electrically as an orderly, synchronized, interconnected unit and are responsible in part for conduction in the myocardium being anisotropic; that is, its anatomic and biophysical properties vary according to the direction in which they are measured. Usually, conduction velocity is two to three times faster longitudinally, in the direction of the long axis of the fiber, than it is transversely, in the direction perpendicular to this long axis. Resistivity is lower longitudinally than transversely. Interestingly, thesafety factor for propagation is greater transversely than horizontally. Thesafety factor for conduction determines the success of action potential propagation and has been defined as the ratio of electrical charge that is generated to charge that is consumed during the excitation cycle of a single myocyte in tissue. Conduction delay or block occurs more frequently in the longitudinal direction than it does transversely. Cardiac conduction is discontinuous because of resistive discontinuities created by the gap junctions, which have an anisotropic distribution on the cell surface. Because of anisotropy, propagation is discontinuous and can be a cause of reentry.1

Gap junctions also provide “biochemical coupling,” which permits cell-to-cell movement of ATP (or other high-energy phosphates), cyclic nucleotides, and IP3, the activator of the IP3-sensitive SR Ca2+-release channel. This demonstrates that diffusion of second-messenger substances through gap junctional channels constitutes a mechanism enabling coordinated responses of the myocardial syncytium to physiologic stimuli.1

Gap junctions can also change their electrical resistance. When the intracellular calcium level rises, as in myocardial infarction (MI), the gap junction may close to help seal off injured from noninjured cells. Acidosis increases and alkalosis decreases gap junctional resistance. Increased gap junctional resistance tends to slow the rate of action potential propagation, a condition that could lead to conduction delay or block. Cardiac-restricted inactivation of gap junctions decreases transverse conduction velocity to a greater degree than longitudinal conduction, thereby resulting in an increased anisotropic ratio, which may play a role in premature sudden death from ventricular arrhythmias.

Connexins are the proteins that form the intercellular channels of gap junctions. An individual channel is created by two hemichannels (connexons), each located in the plasma membrane of adjacent cells and composed of six integral membrane protein subunits (connexins). The hemichannels surround an aqueous pore and thereby create a transmembrane channel (Fig. 34.7A).Connexin 43, a 43-kDa polypeptide, is the most abundant cardiac connexin, with connexins 40 and 45 being found in smaller amounts. Ventricular muscle expresses connexins 43 and 45, whereas atrial muscle and components of the specialized conduction system express connexins 43, 45, and 40. Individual cardiac connexins form gap junctional channels with characteristic unitary conductances, voltage sensitivities, and permeabilities. Tissue-specific connexin expression and the spatial distribution of gap junctions determine the disparate conduction properties of cardiac tissue. The functional diversity of cardiac gap junctions is further enhanced by the ability of different connexin isoforms to form hybrid gap junctional channels with unique electrophysiologic properties (Fig. 34.7B). These channel chimeras appear to have a major function in controlling impulse transmission at the SA node–atrium border, the atrium–AV node transitional zone, and the Purkinje-myocyte border.1

Alterations in the distribution and function of cardiac gap junctions are associated with increased susceptibility to arrhythmias. Conduction slowing and arrhythmogenesis have been associated with redistribution of connexin 43 (Cx43) gap junctions from the end of cardiomyocytes to the lateral borders and with decreased phosphorylation of Cx43 in a dog model of nonischemic dilated cardiomyopathy (Fig. 34.7C-E). Adult mice genetically engineered to express progressively decreasing levels of cardiac Cx43 exhibited increased susceptibility to the induction of fatal tachyarrhythmias. Side-to-side electrical coupling between cardiomyocytes from the epicardial border zone of healing infarcts has been shown to be reduced, thereby exaggerating anisotropy and facilitating reentrant activity.1 Lastly, mutations in the atrial-specific connexin 40 gene that exhibit altered function have been associated with AF.27 Studies have suggested that normal electrical coupling of cardiomyocytes through gap junctions depends on normal mechanical coupling through cell-cell adhesion junctions. A defect in cell-cell adhesion or a discontinuity in the linkage between intercellular junctions and the cytoskeleton prevents normal localization of connexins in gap junctions, which in turn could contribute tachyarrhythmias causing sudden death. Mutations indesmoplakin, a protein that links desmosomal adhesion molecules todesmin, a filament protein of the cardiomyocyte cytoskeleton, andplakoglobin, a protein that connects N-cadherins to actin and desmosomal cadherins to desmin, produce recessive variants of arrhythmogenic right ventricular cardiomyopathy (ARVC), Cavajal disease, and Naxos disease, respectively28 (seeChapter 77). Notably, restoring plakoglobin (JUP gene) levels in a mouse model of Naxos disease caused by a truncation of plakoglobin prevented cardiac dysfunction, consistent with a loss of function defect of the truncated protein.29 Approximately 40% of the pathogenic variants linked to familial ARVC are in the gene encoding the desmosomal protein plakophilin-2.30 Demonstration of the important role of other adhesion proteins in stabilizing gap junctions comes from a study in which conditional loss of N-cadherin expression in mouse hearts resulted in a decrease in Cx43 gap junctions and changes in conduction velocity, with a concomitant increase in arrhythmogenicity (eFig. 34.6).

Gap Junctions among Glia

Gap junctions connect astrocytes into a functional syncytium that extends throughout the brain; moreover, astrocytes and oligodendrocytes are coupled by gap junctions. The purpose of this strong glial coupling has long been hypothesized to maximize the range of K+ buffering so that at the site of local neuronal activity the astrocytes would take up K+, distribute it freely among the interconnected population, and perhaps deposit K+ through astrocytic end feet in contact with blood vessels.

Although intercellular communication among glial cells is generally regarded as passive, at least one presumed function, intercellular Ca2+ wave spread, involves relay of a signal that can trigger a suprathreshold, perhaps regenerative, chemical event (Fig. 2). Both Ca2+ and the Ca2+-releasing intermediate IP3 can diffuse through astrocytic gap junction channels, where Ca2+ signals generated in one cell by mechanical, electrical, or pharmacological stimuli may spread to neighboring cells with a velocity of approximately 10–20 μm/sec measured in confluent cell monolayers. Amplitudes of Ca2+ responses recorded in responding cells are generally relatively constant and the extent of the Ca2+ waves is generally limited, indicating that the concentration of the messenger initiating the wave (most likely IP3) is progressively diluted until it no longer reaches threshold for eliciting the all-or-none Ca2+ increase. In addition to this intercellular gap junction-mediated pathway, an extracellular pathway can contribute to the communication of Ca2+ waves, working in parallel with the intercellular pathway. This paracrine route involves release of an extracellular messenger, such as ATP, other adenosine nucleotides, glutamate, or other neurotransmitters or hormones, that relays the Ca2+ signal through the activation of cell surface membrane receptors. In certain cell types or under certain conditions, the extracellular route may be the primary or sole pathway for the slow transmission of Ca2+ signals; moreover, interplay between inter- and extracellular routes of Ca2+ signaling may provide compensation in cases in which one pathway is interrupted.

Gap junctions between astrocytes also mediate the diffusion of metabolites within the population. Various energetic metabolites are gap junction permeant (including gutamine, glutamate, lactate, glucose, and glucose-6-phosphate), and gap junctions may facilitate the delivery of such metabolites from the pericapillary glia to energy-consuming neurons. Toxic metabolites are also gap junction permeant; intercellular diffusion of such molecules can mediate the phenomenon of “bystander” cell killing, in which small molecules generated in one cell may spread death to neighboring, otherwise healthy cells. A therapeutic strategy currently undergoing clinical trials involves the exploitation of the bystander effect. The anticancer drug acyclovir applied to cells transduced with herpes thymide kinase produces a phosphorylated apoptotic by-product that is gap junction permeant, thereby optimizing the efficacy of glioma irradication.

Connexin43 Gap Junctions

The smooth muscle cells in the corpora cavernosa are connected by connexin43 gap junctions that allow the ions and some signaling molecules such as IP3 to diffuse freely across smooth muscle cells103 (Fig. 20.7). The ionic changes induced by a stimulus in one smooth muscle cell are communicated rapidly across other smooth muscle cells, resulting in coordinate regulation of the entire corpora cavernosa.103 Thus, the corpora cavernosa can be viewed functionally as a syncytium of interconnected smooth muscle cells103 (seeFig. 20.7).

Abstract

Electrotonic coupling between cells is accomplished through the formation of gap junctions (GJs) between cells. Composed of connexin proteins, GJs are found on multiple cell types and connexin distribution is cell type-specific. All GJs play a role in cellular communication, but astrocytic GJs are thought to play a role in K+ and glutamate redistribution, synaptic strength regulation, and memory formation. Both human tissue studies and animal models of epilepsy have shown considerable changes in connexin expression after seizure activity. Electrophysiological studies have implicated GJs in the generation of very fast oscillations that precede seizures. Knockout and GJ inhibitors studies have demonstrated potential anticonvulsant effects, although these results are mixed and suffer from lack of specificity of many of the currently available GJ inhibitors. Findings implicating GJs in epilepsy as well as differences in the roles of neuronal versus glial GJs in tissue excitability are considered in this chapter.

Fluid Transfer Through Gap Junctions

The gap junctions linking PE to NPE cells are more numerous than those linking cells within the PE and NPE layers.10 These PE–NPE gap junctions are formed of connexin proteins (Cx43 and Cx40), with Cx43 likely playing a dominant role.13,14 The connexins comprising the links between adjacent NPE cells are different (Cx26 and Cx31), and evidence for these connexins within the PE-cell layer is still lacking.15 The PE–NPE gap junctions are not only more numerous but possibly more robust to experimental perturbation16 than the PE–PE and NPE–NPE gap junctions. This suggests that aqueous humor is fundamentally formed by parallel couplets of PE–NPE cells.16 Interruption of the PE–NPE gap junctions with heptanol or octanol markedly reduces ionic current and net Cl− movement across the ciliary epithelium.17 However, these inhibitors are too nonselective to be clinically relevant.

Introduction

Gap junctions (GJ) are unique large channels that connect the cytoplasm of two adjacent cells. Half GJs, called connexons or hemichannels (HC), expressed by individual cells may also be functionally active, and when open would connect a cell’s cytoplasm to extracellular space. Gap junctions mediate a primitive and important form of intercellular communication that is seen across phyla. Discovered in 1952 in heart muscle cells, GJs are expressed in most mammalian cells; mature skeletal muscle cells, spermatozoa and erythrocytes are the only exceptions. Gap junction mediated coupling between glial cells was first noted in the leech central nervous system (CNS) by Stephen Kuffler and colleagues in 1964. In the mammalian CNS, connexins, the proteins that form GJs and HCs, are most abundantly expressed by glial cells. All subtypes of glia express connexins and GJs, with the possible exception of the newly defined NG2 cell. The characteristics and possible functions of GJs and HCs in glial cells are the focus of this review. First, however, some important general principles about these channels and the proteins that make them will be discussed.

Gap junctions

Gap junctions constitute another junctional complex that bridges the intercellular space between adjacent endothelial cells. However, unlike the other junctional structures described above, which mediate cell–cell adhesion, gap junctions promote intercellular communication. Metabolites, ions, and second messengers, including Ca2+, cAMP, and inositol triphosphate, are able to pass through gap junction channels from one cell to another, enabling the coordination of multicellular responses. Gap junction channels are dodecameric structures made up of the connexin family of proteins. Six individual connexin proteins oligomerize in the plasma membrane of one cell to form a hemichannel or connexon, and the docking of two connexons, one from each opposing cell, results in a complete gap junction channel.

Summary

Gap junctions in nervous tissue synchronize neuronal activity, provide pathways for second messenger and metabolite exchange, and may modulate cell growth, differentiation, and organization. Abnormal gap junction expression or function is associated with both genetic and somatic disease states, presumably contributing to the pathology through loss of the important intercellular pathway provided by these channels. As listed in Table 15.2, mice in which connexins are deleted by molecular genetic manipulation provide model systems in which the roles of specific gap junction types can be explored in detail and in which the contribution of functionally ablated gap junctions to different pathological situations can be directly assessed.

Gap junctions

Gap junctions are specialized cell–cell junctions which form from a mirror image of protein units (connexons) between plasma membranes of cells. The cytoplasms of the cells are connected by narrow water-filled channels. These channels allow passage of small signalling molecules such as calcium and cyclic AMP, but not of large molecules such as proteins. In the myometrium, gap junctions provide low-resistance pathways between the smooth muscle cells, thereby increasing their electrical coupling to allow increased coordination of myometrial contractility. During pregnancy, gap junctions are present at very low numbers in the myometrium; however labour is associated with increased numbers and size of gap junctions. This has led to the idea that gap junctions are essential, but not sufficient, for effective labour and delivery.

Abstract

Gap junctions in the nervous system fulfill vital functions of signal transmission and metabolite delivery and buffering. Between neurons, gap junctions form electrotonic synapses providing rapid bidirectional relay that is essential for rapid and synchronous activities. Between glial cells, gap junctions provide a route for long-range intercellular calcium signaling, as well as delivery of glucose and buffering of potassium ions. A family of connexin proteins comprises gap junctions with cell-specific expression, and mutations in the genes that encode connexins are responsible for a number of neurological diseases including the X-linked form of Charcot–Marie–Tooth disease, hereditary nonsyndromic deafness, and oculodentodigital dysplasia.