By using antibodies directed against gapjunction proteins of liver (connexins 26 and 32) and heart (connexin 43), we have localized immunoreactivity to specific cell types in frozen sections of adult rodent brains. Connexin 32 reactivity was found in oligodendrocytes and also in a few neurons, whereas reactivity to connexins 26 and 43 was localized to leptomeningeal cells, ependymal cells, and pineal gland. Immunoreactivity with antibodies to connexin 43 also occurred in astrocytes. Furthermore, during embryonic and postnatal maturation of brain tissues, gap junction proteins were differentially expressed. Connexins 43 and 26 predominated in the neuroepithelium of embryonic brains, whereas connexin 32 was virtually absent. Between 3 and 6 weeks after birth, connexin 26 largely disappeared from immature brain; this time course corresponded to the increased expression of connexin 32. Expression of connexin 43 remained high throughout embryonic and postnatal development. These findings demonstrate that gap junction expression in the brain is diverse, with specific cell types expressing different connexins; this cell-specific distribution may imply differences in the function of these intercellular channels in different loci and developmental stages.Gap junctions are the structural domains through which electrical transmission and metabolic and ionic cooperation between contiguous cells are thought to be mediated. Recent progress in the understanding of the molecular composition of at least four types of gapjunctions found in liver, heart, and lens fibers has been derived from immunochemical (1-5) and molecular biological studies (6-10). The message emerging from these discoveries is that gap junctions are composed of a family of homologous proteins that are expressed in various amounts in different cell types (11,12). We have shown (11, 12) that at least two gap junction proteins are present in liver.Nervous tissue was among the first organs where gap junction membrane contacts were structurally and physiologically characterized (for review, see ref. 13). Recent studies using antibodies against multiple determinants have demonstrated that gap junctions are abundant in brain (14,15), but the composition of these gap junction proteins was not unambiguously determined.By using affinity-purified polyclonal and monoclonal anticonnexin-32 antibodies, an antibody to the liver protein connexin 26, and a polyclonal antibody directed to the carboxylterminal domain of the heart protein connexin 43, we have evaluated the patterns of expression of these proteins in adult and embryonic brains. Our data provide evidence that specific sets of gap junction proteins are expressed by specific brain cell populations. In addition, we found that during development relative amounts of connexin 26 and connexin 32 shift. Connexin 26, which is abundant in embryonic brain, becomes confined to leptomeningeal and ependymal cells and to pinealocytes. Connexin 32, which is not expressed to a large extent in embryonic brain, is expressed ...

We have cloned a protein from bovine brain, brain-derived voltage-dependent anion channel 1 (BR1-VDAC), that is identical to a recently sequenced plasmalemmal-bound porin from human lymphocytes. mRNA hybridization indicates that BR1-VDAC is widely distributed throughout nervous and nonnervous tissues. In situ localization substantiated that the BR1-VDAC is associated with the plasmalemma of astrocytes. A monoclonal antibody that recognizes the N terminus of the BR1-VDAC protein completely blocks an astrocytic high-conductance anion channel that has electrophysiological similarities with the mitochondrial VDAC. Since the high-conductance anion channel in astrocytes has been shown to respond to hypoosmotic solutions, its molecular identification provides the basis for a better understanding of volume regulation in brain tissue.Astrocytes are involved in the local homeostatic balancing of the interstitial cerebral fluid in brain. The correct condition of the interstitial cerebral fluid is crucial for normal functioning of the neuronal network. Astrocytes have a complement of neurotransmitter receptors and ion channels that form the backbone of this regulative capacity. Although physiological data on the diverse astrocytic receptor and channel types have been collected (1, 2), knowledge of their molecular composition is rare. Determining the molecular mechanisms involved in the homeostatic balancing of the interstitial cerebral fluid is, therefore, indispensible for an understanding of the properties of astrocytes and their concerted action in maintaining normal brain function. Among the various channel types electrophysiologically characterized in astrocytes (2, 3) is a large-conductance anion channel, described in cultured astrocytes (4-6) and cultured rat Schwann cells (7) and reported (8) to be present in astrocytes of intact optic nerves. Jalonen et al. (9) (30-34 kDa) and are considered to provide the pathway for the movement of nucleotides and various other substances into a variety of metabolic pathways (11-13). Compared with the large-conductance anion channel of astrocytes, they exhibit almost identical basic channel properties. (i) Singlechannel conductance is roughly 450 pS. (ii) They show ion selectivity for Cl-in the high-conductance state, which occurs at 0 mV transmembrane voltage. (iii) They respond symmetrically with respect to gating properties; i.e., they are in the high-conductance state at 0 mV and close to lowconductance states in response to application ofboth positive and negative potentials (14). The characteristic voltagedependent gating and the anion preference at low potentials have led to the term VDAC (11,12).VDACs in eukaryotes have been considered to be localized exclusively in mitochondria. Recently, however, an apparently plasmalemmal VDAC (plm-VDAC) protein highly homologous to mitochondrial porins has been sequenced from a human lymphocyte cell line by Edman degradation (15). Subsequently, another VDAC-like protein was found to copurify with the plasmalemmal-bound centra...

Gap junctions are aggregates of transmembranous channels which bypass the extracellular space by transporting messenger molecules and ions from one cytoplasmic source to an adjacent cytoplasmic interior. The channels join the plasma membranes of adjacent cells by bridging the extracellular space between them. Thereby, cellular "compartments" which were once considered to be individual units are, in actuality, interconnected by a system of pathways which form a functional cellular syncytium. The evolutionary importance of a generalized intercellular communication system can be appreciated when one considers the widespread prevalence of gap junctions within animals of all multicellular phyla, and within almost all tissues of vertebrates. Only a few population of cells such as skeletal muscle cells (which are fused to form functional syncytia) and circulating blood cells are not equipped with gap junctions. This paper provides a brief review of the diverse structural, molecular and functional aspects of gap junctions as revealed by current research.

To gain insight into the function of gap junctions' connexin43, connexin32 and connexin26 in a neural structure that retains neuronal turnover capacities throughout adulthood, the expression of these molecules has been investigated in the developing and adult olfactory system by immunocytochemical and biochemical methods. Connexin43 was detectable from the olfactory placode stage. During early embryonic development, the levels of connexin43 expression remained low. An increase in the expression of this connexin occurred perinatally. Expression of connexin43 became very high during the postnatal stages and adulthood. Electron microscopy (EM) immunocytochemistry of the olfactory system showed connexin43 expression in non-neuronal cells. Strong regional differences in the expression of connexin43 in the olfactory epithelium were observed. No apparent relationship between connexin43 expression and turnover activity of olfactory neurons was detected. Western blots of olfactory tissues revealed the presence of three different isoforms of connexin43. Connexin32 was detected in the olfactory bulb at late postnatal stages including adulthood. Connexin32 was observed on some cells tentatively identified as oligodendrocytes. Connexin26 was localized onto leptomeninges. Some immunofluorescence was also obtained in the periglomerular region and in the subependymal layer of the bulb. Northern blot analysis revealed the presence of mRNA of connexin32 and connexin26 in the adult olfactory system. Our results substantiate the cell specific expression of these three types of connexins and they document the primary of connexin43 in olfactory tissues. Moreover, our findings indicate that although expression of connexin43 in the olfactory system is developmentally regulated, it is not directly associated with the neuronal cell turnover of the olfactory epithelium.

A 16 kDa protein from an enriched gap junction preparation was isolated from bovine brain tissues. N-terminal amino acid microsequencing of the first 20 amino acids showed a complete homology with a recently published sequence of a proteolipid from a vacuolar H ÷-ATPase from chromaffin granules. Incubation of the brain gap junction preparation with 14C-N,N'-dicyclohexylcarbodiimide showed a significant binding of this compound to the 16 kDa protein, indicating that a proton binding site also occurs within that particular protein. The data suggest that this 16 kDa protein, which has also been described in gap junction preparations from various other tissues, belongs to the proton transporting ATPase.