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

Oligomeric structure and functional characterization of Caenorhabditis elegans Innexin-6 gap junction protein

Innexin is the molecular component of invertebrate gap junctions. Here we successfully expressed and purified Caenorhabditis elegans innexin-6 (INX-6) gap junction channels and characterized the molecular dimensions and channel permeability using electron microscopy (EM) and microinjection of fluorescent dye tracers, respectively. Negative staining and thin-section EM of isolated INX-6 gap junction membranes revealed a loosely packed hexagonal lattice and a greater cross-sectional width than that of connexin26 and connexin43 (Cx43)-GFP. In gel filtration analysis, the elution profile of purified INX-6 channels in dodecyl maltoside solution exhibited a peak at ∼400 kDa that was shifted to ∼800 kDa in octyl glucose neopentyl glycol. We also obtained the class averages of purified INX-6 channels from these peak fractions by single particle analysis. The class average from the ∼800-kDa fraction showed features of the junction form with a longitudinal height of 220 Å, a channel diameter of 110 Å in the absence of detergent micelles, and an extracellular gap space of 60 Å, whereas the class averages from the ∼400-kDa fraction showed diameters of up to 140 Å in the presence of detergent micelles. These findings indicate that the purified INX-6 channels are predominantly hemichannels in dodecyl maltoside and docked junction channels in octyl glucose neopentyl glycol. Dye transfer experiments revealed that the INX-6-GFP-His channels are permeable to 3- and 10-kDa tracers, whereas no significant amounts of these tracers passed through the Cx43-GFP channels. Based on these findings, INX-6 channels have a larger overall structure and greater permeability than connexin channels.

Autocellular coupling by gap junctions in cultured astrocytes: a new view on cellular autoregulation during process formation

Neocortical astrocytes make two types of gap junctions, intercellular ones create a functional syncytium, while reflexive gap junctions mediate autocellular coupling and serve unknown functions (Rohlmann and Wolff, 1996). Here, the question is addressed whether solitary astrocytes in vitro express connexin43 (Cx43) and establish gap junctions in the absence of intercellular contacts. In all media conditions tested, immunocytochemistry visualized Cx43-expression and gap junctions irrespective of the presence or absence of intercellular contacts. Reflexive gap junctions were associated with mechanical junctions (adherent spots and fascia adherens) connecting surface membranes and cytoskelal components, respectively. Both were characteristically located along incompletely separated borders between developing processes and/or branches. In addition, Cx43-immunoreactivity was found on some non-junctional membranes: i) intracellular vesicle clusters sited to forming processes and at the basis of filopodia; ii) the surface membrane of filopodial subpopulations usually appearing in bunches. Results suggest changes in the resumptive role of Cx43 in cultivated astrocytes: 1) Cx43 is not confined to intercellular gap junctions, it may even selectively compose reflexive ones; 2) from intracellular stores (vesicle aggregates), Cx43 may be incorporated into the surface membrane of filopodia; 3) by contacting other parts of the same cell surface (or neighboring cells), filopodia and membrane patches carrying Cx43-half channels may be essential in initial steps of gap junction formation; 4) the distribution of reflexive gap junctions is compatible with the hypothesis that autocellular coupling serves reorganization of cytoskeleton during the formation of cell processes and branches; 5) in general, gap junctions may be important for coordinating the cytoskeleton across intercellular contacts and within cells with complex shape.

Biophysical properties of heterotypic gap junctions newly formed between two types of insect cells

1. Cell pairs of the insect cell line Sf9 (Spodoptera frugiperda) were chosen to examine the electrical properties of gap junction channels. The dual voltage-clamp method was used to control the membrane potential of each cell (Vm,1 and Vm,2) and hence the junctional voltage gradient (Vj), and to measure intercellular current. 2. Studies with preformed pairs revealed that the gap junction conductance (gj) is controlled by a Vj- and a Vm-sensitive gate. At steady state, gj = f(Vj) was bell shaped and symmetrical (Boltzmann: Vj.0 = -54 and 55 mV, the normalized minimum conductance at large Vj values (gj,min) = 0.24 and 0.23, z = 5.5 and 6.1 for negative and positive Vj, respectively) and gj = f(Vm) was S shaped (Vm.0 = 13 mV, gj,min = 0, z = 1.5). 3. Single channels exhibited two conductances, a main state (gamma j,main) of 224 pS and a residual state (gamma j,residual) of 42 pS. 4. We conclude that gap junctions in Sf9 cells behave similarly to those in the insect cell line C6/36 (Aedes albopictus). 5. An induced cell pair approach was used to examine heterotypic gap junction channels between Sf9 cells and C3/36 cells. 6. Heterotypic channels showed a gamma j,main of 303 pS and a gamma j,residual of 45 and 64 pS, depending on whether the Sf9 cell or C6/36 cell was positive inside. 7. In heterotypic gap junctions, gj = f(Vj) was bell shaped and asymmetrical (gj was more sensitive to Vj when the C6/36 cell was positive inside) and gj = f(Vm) was S shaped (Vm,0 = 2 mV, gj,min = 0, z = 2.9). 8. We conclude that heterotypic channels possess a Vj- and Vm-sensitive gating mechanism. Vj gating involves two gates, one located in each hemi-channel. Vj gates are operated independently and close when the cytoplasmic aspect is made positive. 9. A comparison of homo- and heterotypic channel data suggests that docking of hemi-channels may affect channel gating, but not channel conductance.

Ultrastructure of the stomatogastric ganglion neuropil of the crab, Cancer borealis

The stomatogastric ganglion (STG) of the crab, Cancer borealis, contains the neural networks responsible for rhythmic pattern generation of the foregut. Neuron counts indicate that the STG of C. borealis has 25-26 neurons, 4-5 fewer than that found in lobsters. We describe the ultrastructural features of the ganglion by focusing on those that may be involved in storage, release, or range of action of peptide modulators, including a lacunar system and multiple types of intercellular junctions. In the neuropil, we identify five synaptic profile classes that contain the invertebrate presynaptic apparatus (dense bars, small clear vesicles), two of which also contain dense core (modulator-containing) vesicles. These latter two are comprised of multiple immunocytochemical classes that are not easily distinguished by structural criteria. In addition, we find neurohemal-like profiles that contain primarily dense core vesicles. Our finding that multiple profile types in the STG possess modulator-containing vesicles coincides with immunocytochemical results better than do previous ultrastructural studies that report only one such profile type. We show that a single modulatory input, stomatogastric nerve axon 1, makes only classical synapses and not neurohemal-like profiles, although some modulators are found in both these profile types. These data provide the groundwork for understanding the architecture of modulatory input-target interactions and suggest ways that the specificity of modulatory effects within a complex neuropil may be attained.

Structure--function relationships in gap junctions

Gap junctions are metabolic and electrotonic pathways between cells and provide direct cooperation within and between cellular nets. They are among the cellular structures most frequently investigated. This chapter primarily addresses aspects of the assembly of the gap junction channel, considering the insertion of the protein into the membrane, the importance of phosphorylation of the gap junction proteins for coupling modulation, and the formation of whole channels from two hemichannels. Interactions of gap junctions with the subplasmalemmal cytoplasm on the one side and with tight junctions on the other side are closely considered. Furthermore, reviewing the significance and alterations of gap junctions during development and oncogenesis, respectively, including the role of adhesion molecules, takes up a major part of the chapter. Finally, the literature on gap junctions in the central nervous system, especially between astrocytes in the brain cortex and horizontal cells in the retina, is summarized and new aspects on their structure-function relationship included.

Voltage-dependent gap junctional conductance in hepatopancreatic cells of Procambarus clarkii

Properties of gap junction channels present between specific cell types constituting the hepatopancreas of the crayfish (Procambarus clarkii) were investigated using the dual whole cell voltage clamp technique. Four different cell types (E, Fe, R and B) were identified on the basis of their morphology using light and electron microscopy. Although junctional conductance (Gj) could not be measured in B-B cell pairs, junctional currents were resolved in both homologous and heterologous combinations of the other cell types. E-E, Fe-Fe, and E-Fe cell pairs exhibited strong dependence on inside-out voltage (Vi-o), such that Gj increased with hyperpolarization to a maximal plateau reached at approximately -40 mV and was abolished with depolarization > 10 mV. The Gj-Vi-o relationship can be described by a squared Boltzmann relation with A = 0.101 and V0 = 0.135 mV. In this system, sensitivity of the junctions to transjunctional voltage was slight, if present at all. Gating mechanisms were complex, as evidence by the presence of multiple unitary channel conductance states. Single channel recordings showed that large unitary conductances (> 200 pS) were generally found between E-E, Fe-Fe, and E-Fe cell pairs, whereas smaller channel sizes (< 90 pS) were detected between R-R cell pairs.

Gap junction protein tissue distribution and abundance in the adult brain in Drosophila

The distribution of gap junction (GJ) protein in Drosophila tissues and developmental stages was determined by probing immuno-blots with an anti-Drosophila GJ protein antibody (R2AP18). All tissues and developmental stages examined contained 18, 24 or 72 kD GJ protein. GJ protein was notably abundant in immuno-blots of homogenates of adult brain tissue. This was confirmed by the direct visualization of GJs in thin sections of adult brain by electron microscopy. GJs were particularly large and numerous between glial cells in the optic lobes and peripheral glial sheath. R2AP18 reactivity was used to identify GJ protein in immunoblots of cell fractions from isolated adult heads. The final GJ-enriched pellets, derived by extracting crude membrane fractions with urea and N-lauroyl sarcosine, contained GJs with reduced profile widths (13-15 nm vs 16-18 nm for native GJs) and which, unlike native GJs in the crude membrane fractions, were immuno-labelled by R2AP18. Immuno-blots of the urea-sarcosine extracted GJ pellets and supernatant contained higher molecular weight R2AP18 immuno-reactive proteins in addition to the 18 kD form which was present in the tissue homogenate and crude membrane fractions. The results confirm previous observations that urea-sarcosine causes alterations in GJ structure and suggest that urea-sarcosine treatment exposes antigenic determinant(s) which are unavailable for R2AP18 binding in non-extracted native GJs. The abundance of GJs in the adult brain and the relatively simple R2AP18 staining patterns in immuno-blots of GJ-enriched fractions from isolated adult heads suggest that this tissue will be useful for further biochemical and molecular studies of GJs in Drosophila.

The gap junction-like form of a vacuolar proton channel component appears not to be an artifact of isolation: an immunocytochemical localization study

Gap junctional structures containing a 16-kDa intrinsic membrane protein have been isolated from the hepatopancreas of the crustacean Nephrops norvegicus. These structures are double membranes 14-15 nm thick and composed of hexagonal arrays of particles which have a central pore that is penetrated by a cationic negative stain. Membrane preparations have also been isolated from the hepatopancreas and these contain similar gap junctional regions of uniform width. Affinity purified antibodies to the 16-kDa protein bind principally to these gap junctional regions. Antiserum raised against the isolated gap junctional structures binds strongly to the lateral surfaces of the columnar epithelial cells and in particular to gap junction-like regions.

Sequence and tissue distribution of a second protein of hepatic gap junctions, Cx26, as deduced from its cDNA

While a number of different gap junction proteins have now been identified, hepatic gap junctions are unique in being the first demonstrated case where two homologous, but distinct, proteins (28,000 and 21,000 Mr) are found within a single gap junctional plaque (Nicholson, B. J., R. Dermietzel, D. Teplow, O. Traub, K. Willecke, and J.-P. Revel. 1987. Nature [Lond.]. 329:732-734). The cDNA for the major 28,000-Mr component has been cloned (Paul, D. L. 1986. J. Cell Biol. 103:123-134) (Kumar, N. M., and N. B. Gilula. 1986. J. Cell Biol. 103:767-776) and, based on its deduced formula weight of 32,007, has been designated connexin 32 (or Cx32 as used here). We now report the selection and characterization of clones for the second 21,000-Mr protein using an oligonucleotide derived from the amino-terminal protein sequence. Together the cDNAs represent 2.4 kb of the single 2.5-kb message detected in Northern blots. An open reading frame of 678 bp coding for a protein with a calculated molecular mass of 26,453 D was identified. Overall sequence homology with Cx32 and Cx43 (64 and 51% amino acid identities, respectively) and a similar predicted tertiary structure confirm that this protein forms part of the connexin family and is consequently referred to as Cx26. Consistent with observations on Cx43 (Beyer, E. C., D. L. Paul, and D. A. Goodenough. 1987. J. Cell Biol. 105:2621-2629) the most marked divergence between Cx26 and other members of the family lies in the sequence of the cytoplasmic domains. The Cx26 gene is present as a single copy per haploid genome in rat and, based on Southern blots, appears to contain at least one intron outside the open reading frame. Northern blots indicate that Cx32 and Cx26 are typically coexpressed, messages for both having been identified in liver, kidney, intestine, lung, spleen, stomach, testes, and brain, but not heart and adult skeletal muscle. This raises the interesting prospect of having differential modes of regulating intercellular channels within a given tissue and, at least in the case of liver, a given cell.

A 16 kDa protein co-isolating with gap junctions from brain tissue belonging to the class of proteolipids of the vacuolar H+-ATPases

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.