Isolation and protein composition of gap junctions from rabbit hearts

Sequence diversity of gap junction proteins

This paper summarizes our understanding of the molecular organization of gap junction proteins. There appear to be overall similarities in the organization of heart and liver junctions in terms of general domains, even though the molecular sizes of the two proteins are quite different. Sequence data on the amino-terminal regions of these two proteins show 43% of the residues to be identical and 25% more to be homologous. The major intrinsic protein of lens (MIP), believed by many to be the lens-fibre junction protein, does not show such sequence homology with the known portions of junction proteins from either heart or liver. Yet the sequence of MIP, which is completely known, suggests a conformation for this molecule quite compatible with a junctional role. It thus appears that molecules potentially involved in junction formation will prove to form a rather diverse family, with special characteristics of organ-specific molecules that may well be related to their function.

Connections with connexins: the molecular basis of direct intercellular signaling

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. 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 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 have begun to provide a molecular understanding of the significance of connexin diversity and demonstrated the unique regulation of connexins by tyrosine kinases and oncogenes. Finally, mutations in two connexin genes have been linked to human diseases. The development of more specific approaches (dominant negative mutants, knockouts, transgenes) to study the functional role of connexins in organ homeostasis is providing a new perception about the significance of connexin diversity and the regulation of intercellular communication.

Isolation and characterization of gap junctions in the osteoblastic MC3T3-E1 cell line

Gap junctions are channels connecting cells that function in cell-to-cell communication. Gap junctions are abundant in osteoblastic cells. Membranes enriched for gap junction plaques were obtained by differential centrifugation, followed by treatment of the membranes with potassium iodide and sarkosyl before sucrose density gradient centrifugation. Electron microscopy showed that the preparation was enriched for electron-dense membranes consistent with gap junctions. Coomassie Blue staining of SDS-PAGE preparations revealed a prominent band at approximately 41 kD. Western analysis with a site-directed antibody, CT-360 (D. Laird, California Institute of Technology, Pasadena, CA), to the C-terminal portion of the rat heart connexin 43 molecule was positive in the MC3T3-E1 cell line, a phenotypic osteoblastic cell line derived from normal neonatal mouse calvariae. Western analysis using a monoclonal antibody, R5.21C, to rat liver connexin 32 was negative. Additionally, a prominent band at 59 kD was detected by CT-360 in both gap junction-enriched preparations and cell lysates. Treatment of diluted samples of gap junction-enriched preparations with sulfhydryl reducing agents in combination with detergents resulted in the enhancement and diminution of the 41 and 59 kD bands, respectively. Immunoprecipitation following [35S]methionine/[35S]cysteine labeling revealed a significant band detected at 122 kD in addition to the 41 kD band. To demonstrate functional gap junctions, transfer of lucifer yellow dye to surrounding cells was monitored after microinjection of a target cell. Between passages 10 and 25 in culture, functional cell coupling was found in approximately 70% of injected cells. Coupling was detected within 1-2 minutes after injection. Simultaneous microinjection of the CT-360 antibody with lucifer yellow resulted in the decoupling of cells. In conclusion, (1) MC3T3-E1 cells possess a 41 kD protein that is recognized by connexin 43 antibody to rat heart gap junction; (2) multimers of the MC3T3-E1 gap junctions occur in the preparation; and (3) functional coupling demonstrated by dye transfer may be regulated by region(s) in the C terminus of the connexin molecule.

Atomic force microscopy and dissection of gap junctions

An atomic force microscope (AFM) was used to study the structure of isolated hepatic gap junctions in phosphate-buffered saline (PBS). The thickness of these gap junctions appears to be 14.4 nanometers, close to the dimensions reported by electron microscopy (EM). When an increasing force is applied to the microscope tip, the top membrane of the gap junction can be "dissected" away, leaving the extracellular domains of the bottom membrane exposed. When such "force dissection" is performed on samples both trypsinized and fixed with glutaraldehyde, the hexagonal array of gap junction hemichannels is revealed, with a center-to-center spacing of 9.1 nanometers.

The gap junction family: structure, function and chemistry

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.

The mammalian sinoatrial node

The sinoatrial node (SAN) was discovered in 1906 by Keith and Flack. The relation between its ultrastructure and function was first studied by Trautwein and Uchizono in 1963, whereas this relation was definitely established by Taylor and coworkers in 1978. The impulse originates from cells with a relatively low percentage of myofilaments. Earliest discharge is restricted to one site only in rabbit, guinea pig, cat, and pig and presumably also in larger animals. From this primary pacemaker area, the impulse is preferentially conducted towards the crista terminalis. The amount of cells in the primary pacemaker area may vary from a few hundred to a few thousand. In rabbit, guinea pig, cat, and pig, the amount of collagen is considerable. Normal SAN function was observed in the cat although the SAN volume occupied by myocytes was less than 5%. Changes in ionic composition of the perfusion fluid and the addition of autonomic substances may cause pacemaker shifts and altered activation patterns.

The arthropod gap junction and pseudo-gap junction: isolation and preliminary biochemical analysis

The hepatopancreas of the crayfish, Procambarus clarkii, contains an unusual abundance of gap junctions, suggesting that this tissue might provide an ideal source from which to isolate the arthropod-type of gap junction. A membrane fraction obtained by subcellular fractionation of this organ contained smooth septate junctions, zonulae adhaerentes, gap junctions and pentalaminar membrane structures (pseudo-gap junctions) as determined by electron microscopy. A further enrichment of plasma membranes and gap junctions was achieved by the use of linear sucrose gradients and extraction with 5 mM NaOH. The enrichment of gap junctions correlated with the enrichment of a 31 Kd protein band on polyacrylamide gels. Extraction with greater than or equal to 20 mM NaOH or greater than or equal to 0.5% (w/v) Sarkosyl NL97 resulted in the disruption and/or solubilization of gap junctions. Negative staining revealed a uniform population of 9.6 nm diameter subunits within the gap junctions with an apparent sixfold symmetry. Using antisera to the major gap junctional protein of rat liver (32 Kd) and to the lens membrane protein (MP 26), we failed to detect any homologous antigenic components in the arthropod material by immunoblotting-enriched gap junction fractions or by immunofluorescence on tissue sections. The enrichment of another membrane structure (pseudo-gap junctions), closely resembling a gap junction, correlated with the enrichment of two protein bands, 17 and 16 Kd, on polyacrylamide gels. These structures appeared to have originated from intracellular myelin-like figures in phagolysosomal structures. They could be distinguished from gap junctions on the basis of their thickness, detergent-alkali insolubility, and lack of association with other plasma membrane structures, such as the septate junction. Pseudo-gap junctions may be related to a class of pentalaminar contacts among membranes involved in intracellular fusion in many eukaryotic cell types. We conclude that pseudo-gap junctions and gap junctions are different cellular structures, and that gap junctions from this arthropod tissue are uniquely different from mammalian gap junctions of rat liver in their detergent-alkali solubility, equilibrium density on sucrose gradients, and protein content (antigenic properties).

The cardiac gap junction protein (Mr 47,000) has a tissue-specific cytoplasmic domain of Mr 17,000 at its carboxy-terminus

The molecular weight of the heart gap junctional protein subunit was, until recently, believed to be about Mr 28,000-30,000, similar to that of other previously characterized gap junctional proteins. A larger polypeptide of about Mr 44,000-47,000, which undergoes proteolysis during isolation, has recently been proposed as the form of the heart junction protein in vivo. We show here that this entity has the same amino-terminal sequence as the previously characterized Mr 29,000-30,000 component. Thus, the cardiac junctional protein has, at its carboxy-terminus, cytoplasmic domain of Mr 17,000; this domain is absent in the liver protein. These observations provide further evidence that gap junction proteins form a highly diversified family.

Molecular cloning of cDNA for rat liver gap junction protein

An affinity-purified antibody directed against the 27-kD protein associated with isolated rat liver gap junctions was produced. Light and electron microscopic immunocytochemistry showed that this antigen was localized specifically to the cytoplasmic surfaces of gap junctions. The antibody was used to select cDNA from a rat liver library in the expression vector lambda gt11. The largest cDNA selected contained 1,494 bp and coded for a protein with a calculated molecular mass of 32,007 daltons. Northern blot analysis indicated that brain, kidney, and stomach express an mRNA with similar size and homology to that expressed in liver, but that heart and lens express differently sized, less homologous mRNA.

Rat heart gap junctions as disulfide-bonded connexon multimers: their depolymerization and solubilization in deoxycholate

Unproteolyzed gap junctions isolated from rat heart and liver were analyzed for the presence of inter-subunit disulfide bonds by sodium dodecylsulfate polyacrylamide gel electrophoresis. Rat cardiac junctions contained multiple disulfide bonds connecting the Mr 47,000 subunits of the same connexon and of different connexons. Inter-subunit disulfide bonds were absent in liver junctions. Unproteolyzed rat heart gap junctions were resistant to deoxycholate in their oxidized state, but dissolved readily in the detergent when the disulfide bonds were cleaved with beta-mercaptoethanol. Disulfide bonding in proteolyzed cardiac junctions was limited to pairs of Mr 29,500 subunits. These junctions were not soluble in deoxycholate even in the presence of beta-mercaptoethanol. These results show that heart and liver junctions differ in their quarternary organization.

Preparation of a gap junction fraction from uteri of pregnant rats: the 28-kD polypeptides of uterus, liver, and heart gap junctions are homologous

A procedure for the preparation of a gap junction fraction from the uteri of pregnant rats is described. The uterine gap junctions, when examined by electron microscopy of thin sections and in negatively stained preparations, were similar to gap junctions isolated from heart and liver. Major proteins of similar apparent molecular weight (Mr 28,000) were found in gap junction fractions isolated from the uterus, heart, and liver, and were shown to have highly homologous structures by two-dimensional mapping of their tryptic peptides. An Mr 10,000 polypeptide, previously deduced to be a proteolytic product of the Mr 28,000 polypeptide of rat liver (Nicholson, B. J., L. J. Takemoto, M. W. Hunkapiller, L. E. Hood, and J.-P. Revel, 1983, Cell, 32:967-978), was also studied and shown by chymotryptic mapping to be homologous in the uterine, heart, and liver gap junction fractions. An antibody raised in rabbits to a synthetic peptide corresponding to an amino-terminal sequence of the liver gap junction protein recognized Mr 28,000 proteins in the three tissues studied, showing that the proteins shared common antigenic determinants. These results indicate that gap junctions are biochemically conserved plasma membrane specializations. The view that gap junctions are tissue-specific plasma membrane organelles based on previous comparisons of Mr 26,000-30,000 polypeptides is not sustained by the present results.

Proteolysis of cardiac gap junctions during their isolation from rat hearts

Gap junctions (GJ) isolated from rat hearts in presence of the protease inhibitor phenylmethylsulfonylfluoride (PMSF) contain a Mr 44,000 to 47,000 major polypeptide and have a urea-resistant layer of fuzz on their cytoplasmic surfaces, whereas junctions isolated without PMSF are proteolyzed to a Mr 29,500 polypeptide by a serine protease and have smooth cytoplasmic surfaces (C.K. Manjunath, G.E. Goings & E. Page Am. J. Physiol. 246:H865-H875, 1984). Rat liver GJ isolated with or without PMSF contain a Mr 28,000 polypeptide and have smooth cytoplasmic surfaces. Here we examine the origin, type and inhibitor sensitivity of the heart protease; why similar proteolysis is absent during isolation of rat liver gap junctions; and whether the Mr 44,000 to 47,000 cardiac GJ polypeptide is the precursor of the Mr 29,500 subunit. We show that the Mr 44,000 to 47,000 polypeptide corresponds to the unproteolyzed connexon subunit; that proteolysis of this polypeptide occurs predominantly during exposure to high ionic strength solution (0.6 M KI) which releases serine protease from mast cell granules; that this protease is inhibitable with PMSF and (less completely) soybean trypsin inhibitor and chymostatin; and that in vivo degranulation of mast cells by injecting rats with compound 48/80 fails to prevent breakdown of cardiac GJ during isolation. The results support the concept that GJ from rat heart and liver differ in protein composition.

Intercalated discs of mammalian heart: a review of structure and function

Intercalated discs are exceptionally complex entities, and possess considerable functional significance in terms of the workings of the myocardium. Examination of different species and heart regions indicates that the original histological term has become out-moded; it is likely, however, that all such complexes will continue to fall under the generic heading of 'intercalated discs'. The membranes of the intercalated discs establish specific associations with a variety of intracellular and extracellular structures, as well as with numerous types of proteins and glycoproteins. Characterization of discs and their components has already brought together a large number of research disciplines, including microscopy, cytochemistry, morphometry, cell isolation and culture, cell fractionation, cryogenics, immunology, biochemistry, and electrophysiology. The continued dissection of substance and function of intercalated discs will depend on such interdisciplinary approaches. The intercalated disc component which continues to attract the greatest amount of interest is the so-called gap junction. All indications thus far point to a great deal of inherent lability in the architecture of the gap junction. There is thus considerable potential for the creation of artefact while preserving and observing gap junctions, and this problem will doubtless continue to hamper the understanding of their functions. A question of special interest concerns whether the gap junctions of intercalated discs are required for transfer of electrical excitation between cells, or maintain cell-to-cell adhesion, or in fact subserve both electrical and structural phenomena. Two schools of thought exist with respect to cell-to-cell coupling in the heart. One proposes that low-resistance junctions in the discs mediate electrical coupling, whereas the other supports the possibility of coupling across ordinary high-resistance membranes. Thus the intercalated discs continue to be a source of controversy, just as they have been since they were originally discovered in heart muscle over a century ago.

Intercellular junctions and the cardiac intercalated disk

Cardiac muscle cells are equipped with three distinct types of intercellular junction--gap junctions, "spot" desmosomes, and "sheet" desmosomes (or fasciae adherentes)--located in a specialized portion of the plasma membrane, the intercalated disk. Gap junctions are responsible for electrical coupling and the transfer of small molecules between cells, whereas the desmosomelike junctions (also known as adherens junctions) provide strong intercellular adhesion. The adhesion sites formed by the "spot" desmosome anchor the intermediate-filament cytoskeleton of the cell; those formed by the fascia adherens anchor the contractile apparatus. An understanding of the ultrastructure of these junctions helps explain how they carry out their functions, and new observations in this field have been made through the application of ultrarapid freezing techniques in conjunction with freeze-fracture electron microscopy. With recent findings from biochemical and immunocytochemical studies, this understanding is now being extended to the molecular level.

pH dependence of transmission at electrotonic synapses of the crayfish septate axon

Gap junctions between segments of the crayfish septate axon mediate electrotonic transmission of impulses propagating along the length of the nerve cord. We simultaneously measured intracellular pH (pHi) and gap junctional conductance (gj) while axons were exposed to saline equilibrated with CO2, weak acids, and the weak base ammonium chloride. Normal pHi is about 7.1. When pHi is elevated, gj is unaffected. When pHi is reduced, gj declines with an apparent pK of about 6.7 and a Hill coefficient of about 2.7. We also measured effects of pHi on non-junctional conductance (gnj) and on the coupling coefficient, k. Over the pHi range 6.2-8, gnj increases approximately linearly with pHi. Since k is a function of gj and gnj, it reached a maximum at about pHi 7.1, decreasing at higher and lower pHi. The pHi dependence of gj in crayfish septate axon is less steep and has a lower apparent pK than the gj-pHi relation in two vertebrate embryos previously examined. This finding illustrates a difference in gating among analogous and possibly homologous membrane channels.

The major intrinsic protein (MIP) of the bovine lens fiber membrane: characterization and structure based on cDNA cloning

Synthetic oligonucleotide probes have been used to identify two overlapping cDNA clones that represent the entire coding region of the mRNA for the major intrinsic protein (MIP) of bovine lens cell membrane. Hybridization studies indicate that bovine MIP is encoded by a single-copy gene. The cDNA hybridizes to the rat genome, but MIP mRNA is not detected in rat liver. Analysis of the deduced amino acid sequence provides support for the potential role of MIP as a junctional protein. The structure predicted for MIP suggests that it traverses the lipid bilayer six times with both carboxy and amino termini on the cytoplasmic side, and that it has at least one amphiphilic transmembrane segment, as expected if the protein were to participate in the formation of an aqueous channel.

A protein homologous to the 27,000 dalton liver gap junction protein is present in a wide variety of species and tissues

The species and tissue specificities of gap junction polypeptides were investigated with antibodies raised against the 27,000 dalton rat liver gap junction protein. Cross-reacting 27,000 dalton polypeptides were detected in liver from mammalian, fish, and avian species by immunoreplica analyses and were localized to punctate regions of the plasma membrane by indirect immunofluorescence. They were also found in homogenates of other rat tissues, including pancreas, heart, brain, kidney, stomach, and adrenal gland, but not in lens fiber material. Localization of antibody binding in pancreas was similar to that of liver, while in heart ventricle the immunofluorescence pattern was consistent with binding to the intercalated disc. These findings indicate that homologous gap junction polypeptides may be widely distributed among vertebrate tissues.

Detergent sensitivity and splitting of isolated liver gap junctions

Isolated rat liver gap junctions were split by two methods. In the first method, isolated gap junctions were stabilized by cross-linking their cytoplasmic surfaces with glutaraldehyde under conditions that prevented the entry of glutaraldehyde into the "gap" region. The "stabilized" junctions were then split in the junctional gap with SDS. In the second procedure, unfixed gap junctions were split by incubation in urea-containing solutions. Junctional splitting was monitored by electron microscopy of thin sectioned and freeze fractured membrane pellets. Sidedness of the split junctional membranes was defined by labeling their cytoplasmic surfaces with glutaraldehyde-activated ferritin before splitting with urea. Gap junctional splitting did not result in any loss of protein components as determined by SDS-gel electrophoresis. The glutaraldehyde cross-linking procedure was also used to determine the effects of various detergents on the protein-protein interactions in the "gap" region. Of the detergents tested, only SDS caused junctional splitting.

Comparative analysis of the gap junction protein from rat heart and liver: is there a tissue specificity of gap junctions?

Gap junctions have been isolated from both rat heart and liver, tissues where junctions are typical in appearance and physiology. The purity of the fractions obtained was monitored by electron microscopy (thin-sectioning and negative staining) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The myocardial gap junctions are comprised of a single polypeptide of Mr 28,000, apparently derived from a protein of Mr 30,000. Hepatic gap junctions are also comprised of a single native protein of Mr 28,000 as previously reported. Exhaustive trypsin digestion of the isolated junctions cleaves both of these proteins similarly, while leaving their characteristic junctional lattice structures intact. However, comparison of heart and liver junctional proteins by two-dimensional peptide mapping of tryptic and alpha-chymotryptic fragments, followed by high pressure liquid chromatography, reveals no homology between these proteins.

Differences between liver gap junction protein and lens MIP 26 from rat: implications for tissue specificity of gap junctions

Liver gap junctions and gap-junction-like structures from eye lenses are each comprised of a single major protein (Mr 28,000 and 26,000, respectively). These proteins display different two-dimensional peptide fingerprints, distinct amino acid compositions, nonhomologous N-terminal amino acid sequences and different sensitivities to proteases when part of the intact junction. However, the junctional protein of each tissue is well conserved between species, as demonstrated previously for lens and now for liver in several mammalian species. The possiblity of tissue-specific gap junction proteins is discussed in the light of data suggesting that rat heart gap junctions are comprised of yet a third protein.