Abstract. Northern blot analysis of rat heart mRNA probed with a eDNA coding for the principal polypeptide of rat liver gap junctions demonstrated a 3.0-kb band. This band was observed only after hybridization and washing using low stringency conditions; high stringency conditions abolished the hybridization. A rat heart eDNA library was screened with the same eDNA probe under the permissive hybridization conditions, and a single positive clone identified and purified. The clone contained a 220-bp insert, which showed 55 % homology to the original eDNA probe near the 5' end. The 220-bp eDNA was used to rescreen a heart eDNA library under high stringency conditions, and three additional cDNAs that together spanned 2,768 bp were isolated. This composite eDNA contained a single 1,146-bp open reading frame coding for a predicted polypeptide of 382 amino acids with a molecular mass of 43,036 D. Northern analysis of various rat tissues using this heart eDNA as probe showed hybridization to 3.0-kb bands in RNA isolated from heart, ovary, uterus, kidney, and lens epithelium.Comparisons of the predicted amino acid sequences for the two gap junction proteins isolated from heart and liver showed two regions of high homology (58 and 42%), and other regions of little or no homology. A model is presented which indicates that the conserved sequences correspond to transmembrane and extracellular regions of the junctional molecules, while the nonconserved sequences correspond to cytoplasmic regions. Since it has been shown previously that the original eDNA isolated from liver recognizes mRNAs in stomach, kidney, and brain, and it is shown here that the eDNA isolated from heart recognizes mRNAs in ovary, uterus, lens epithelium, and kidney, a nomenclature is proposed which avoids categorization by organ of origin. In this nomenclature, the homologous proteins in gap junctions would be called connexins, each distinguished by its predicted molecular mass in kilodaltons. The gap junction protein isolated from liver would then be called connexin32; from heart, connexin43.AP junctions are composed of collections of membrane channels, called connexons, which join in mirror symmetry with connexons in the membrane of the adjacent cell. These pairs of connexons permit the intercytoplasmic exchange of small metabolites and ions between cells. Each connexon is composed ofa hexamer of an integral membrane protein, whose complete eDNA has been cloned from rat and human liver, with a predicted molecular mass of 32 kD (23, 32). The mRNA coding for this protein is not unique to the liver, but may be detected in other, but not all, organs within the same animal (32). In this paper, we show that a related mRNA is found in abundance in heart and other organs, and that mRNAs coding for both the liver and heart gap junction proteins are in some cases detected in the same organ.Thus, these gap junction mRNAs are not confined to the organs in which they were first observed, necessitating a nomenclature system which avoids mention of source. We propose ...

Cells in tissues share ions, second messengers, and small metabolites through clusters of intercellular channels called gap junctions. This type of intercellular communication permits coordinated cellular activity. Intercellular channels are formed from two oligomeric integral membrane protein assemblies, called connexons, which span two adjacent cells' plasma membranes and join in a narrow, extracellular "gap." Connexons are formed from connexins, a highly related multigene family consisting of at least 13 members. Since the cloning of the first connexin in 1986, considerable progress has been made in our understanding of the complex molecular switches that control the formation and permeability of the intercellular channels. Analysis of the mechanisms of channel assembly has revealed the selectivity of inter-connexin interactions and uncovered novel characteristics of the channel permeability and gating behavior. Structure-function studies provide a molecular understanding of the significance of connexin diversity and demonstrate the unique regulation of connexins by tyrosine kinases and oncogenes.

Adjacent cells share ions, second messengers and small metabolites through intercellular channels which are present in gap junctions. This type of intercellular communication permits coordinated cellular activity, a critical feature for organ homeostasis during development and adult life of multicellular organisms. Intercellular channels are structurally more complex than other ion channels, because a complete cell-to-cell channel spans two plasma membranes and results from the association of two half channels, or connexons, contributed separately by each of the two participating cells, Each connexon, in turn, is a multimeric assembly of protein subunits. The structural proteins comprising these channels, collectively called connexins, are members of a highly related multigene family consisting of at least 13 members.

To examine the functions of electrical synapses in the transmission of signals from rod photoreceptors to ganglion cells, we generated connexin36 knockout mice. Reporter expression indicated that connexin36 was present in multiple retinal neurons including rod photoreceptors, cone bipolar cells, and AII amacrine cells. Disruption of electrical synapses between adjacent AIIs and between AIIs and ON cone bipolars was demonstrated by intracellular injection of Neurobiotin. In addition, extracellular recording in the knockout revealed the complete elimination of rod-mediated, on-center responses at the ganglion cell level. These data represent direct proof that electrical synapses are critical for the propagation of rod signals across the mammalian retina, and they demonstrate the existence of multiple rod pathways, each of which is dependent on electrical synapses.

Gap junctions consist of intercellular channels that connect the cytoplasm of adjacent cells directly and allow the exchange of small molecules. These channels are unique in that they span two plasma membranes--the more orthodox ion or ligand-gated channels span only one. Each cell contributes half of the intercellular channel, and each half is known as a connexon or hemichannel. Recent studies indicate that connexons are also active in single plasma membranes and that they might be essential in intercellular signalling beyond their incorporation into gap junctions.