Relating specific connexin co-expression ratio to connexon composition and gap junction function.

Cardiac connexin 43 (Cx43), Cx40 and Cx45 are co-expressed at distinct ratios in myocytes. This pattern is considered a key factor in regulating the gap junction channels composition, properties and function and remains poorly understood. This work aims to correlate gap junction function with the connexin composition of the channels at accurate ratios Cx43:Cx40 and Cx43:Cx45. Rat liver epithelial cells that endogenously express Cx43 were stably transfected to induce expression of accurate levels of Cx40 or Cx45 that may be present in various areas of the heart (e.g. atria and ventricular conduction system). Induction of Cx40 does not increase the amounts of junctional connexins (Cx43 and Cx40), whereas induction of Cx45 increases the amounts of junctional connexins (Cx43 and Cx45). Interestingly, the non-junctional fraction of Cx43 remains unaffected upon induction of Cx40 and Cx45. Co-immunoprecipitation studies show low level of Cx40/Cx43 heteromerisation and undetectable Cx45/Cx43 heteromerisation. Functional characterisation shows that induction of Cx40 and Cx45 decreases Lucifer Yellow transfer. Electrical coupling is decreased by Cx45 induction, whereas it is decreased at low induction of Cx40 and increased at high induction. These data indicate a fine regulation of the gap junction channel make-up in function of the type and the ratio of co-expressed Cxs that specifically regulates chemical and electrical coupling. This reflects specific gap junction function in regulating impulse propagation in the healthy heart, and a pro-arrhythmic potential of connexin remodelling in the diseased heart.

Gap26, a connexin mimetic peptide, inhibits currents carried by connexin43 hemichannels and gap junction channels.

Connexin mimetic peptides corresponding to short conserved extracellular loop sequences of connexins have been used widely as reversible inhibitors of gap junctional intercellular communication. These peptides also block movement of ATP and Ca(2+) across connexin hemichannels, i.e. hexameric channels yet to dock with partners in aligned cells and to generate the gap junction cell-cell conduit. By means of electrophysiology, we compared the effects of Gap26, a mimetic peptide corresponding to a short linear sequence in the first extracellular loop of connexin43, on connexin channel function in HeLa cells expressing connexin43. We demonstrate that Gap26 inhibited electrical coupling in cell pairs mediated by gap junctions after exposure for 30min. In contrast, Gap26 applied to single cells, inhibited hemichannel currents evoked in low Ca(2+) solution with a response time of less than 5min. The results further support the view that the likely primary and direct inhibitory effect of Gap26 is on connexin hemichannels, with gap junctions becoming inhibited later. The mechanism of action of Gap26 in blocking hemichannels and gap junction channels is discussed in the context of their different functions and locations.

Characterization of zebrafish Cx43.4 connexin and its channels.

Connexins (Cx) form intercellular junctional channels which are responsible for metabolic and electrical coupling. We report here on the biochemical and immunohistochemical characterization of zebrafish connexin zfCx43.4, an orthologue of mammalian and avian Cx45, and the electrophysiological properties of junctional channels formed by this protein. The investigations were performed on transfected COS-7 cells or HeLa cells. Using site-directed antibodies, zfCx43.4 cDNA (GenBank accession no. X96712) was demonstrated to code for a protein with a M(r) of 45 000. In transfected cells, zfCx43.4 was localized in cell-cell contact areas as expected for a gap junction protein. zfCx43.4 channels were shown to transfer Lucifer Yellow. The multichannel currents were sensitive to the transjunctional voltage (V(j)). Their properties were consistent with a two-state model and yielded the following Boltzmann parameters for negative/positive V(j): V(j,0) = -38.4/41.9 mV; g(j,min) = 0.19/0.18; z = 2.6/2.3. These parameters deviate somewhat from those of zfCx43.4 channels expressed in Xenopus oocytes and from those of Cx45, an orthologue of zfCx43.4, expressed in mammalian cells or Xenopus oocytes. Conceivably, the subtle differences may reflect differences in experimental methods and/or in the expression system. The single channel currents yielded two prominent levels attributable to a main conductance state (gamma(j,main) = 33.2 +/- 1.5 pS) and a residual conductance state (gamma(j,residual) = 11.9 +/- 0.6 pS).

Intracellular domains of mouse connexin26 and -30 affect diffusional and electrical properties of gap junction channels.

To evaluate the influence of intracellular domains of connexin (Cx) on channel transfer properties, we analyzed mouse connexin (Cx) Cx26 and Cx30, which show the most similar amino acid sequence identities within the family of gap junction proteins. These connexin genes are tightly linked on mouse chromosome 14. Functional studies were performed on transfected HeLa cells stably expressing both mouse connexins. When we examined homotypic intercellular transfer of microinjected neurobiotin and Lucifer yellow, we found that gap junctions in Cx30-transfected cells, in contrast to Cx26 cells, were impermeable to Lucifer yellow. Furthermore, we observed heterotypic transfer of neurobiotin between Cx30-transfectants and HeLa cells expressing mouse Cx30.3, Cx40, Cx43 or Cx45, but not between Cx26 transfectants and HeLa cells of the latter group. The main differences in amino acid sequence between Cx26 and Cx30 are located in the presumptive cytoplasmic loop and C-terminal region of these integral membrane proteins. By exchanging one or both of these domains, using PCR-based mutagenesis, we constructed Cx26/30 chimeric cDNAs, which were also expressed in HeLa cells after transfection. Homotypic intercellular transfer of injected Lucifer yellow was observed exclusively with those chimeric constructs that coded for both cytoplasmic domains of Cx26 in the Cx30 backbone polypeptide chain. In contrast, cells transfected with a construct that coded for the Cx26 backbone with the Cx30 cytoplasmic loop and C-terminal region did not show transfer of Lucifer yellow. Thus, Lucifer yellow transfer can be conferred onto chimeric Cx30 channels by exchanging the cytoplasmic loop and the C-terminal region of these connexins. In turn, the cytoplasmic loop and C-terminal domain of Cx30 prevent Lucifer yellow transfer when swapped with the corresponding domains of Cx26. In chimeric Cx30/Cx26 channels where the cytoplasmic loop and C-terminal domains had been exchanged, the unitary channel conductance was intermediate between those of the parental channels. Moreover, the voltage sensitivity was slightly reduced. This suggests that these cytoplasmic domains interfere directly or indirectly with the diffusivity, the conductance and voltage gating of the channels.

Dendro-somatic distribution of calcium-mediated electrogenesis in purkinje cells from rat cerebellar slice cultures.

The role of Ca2+ entry in determining the electrical properties of cerebellar Purkinje cell (PC) dendrites and somata was investigated in cerebellar slice cultures. Immunohistofluorescence demonstrated the presence of at least three distinct types of Ca2+ channel proteins in PCs: the alpha1A subunit (P/Q type Ca2+ channel), the alpha1G subunit (T type) and the alpha1E subunit (R type). In PC dendrites, the response started in 66 % of cases with a slow depolarization (50 +/- 15 ms) triggering one or two fast (approximately 1 ms) action potentials (APs). The slow depolarization was identified as a low-threshold non-P/Q Ca2+ AP initiated, most probably, in the dendrites. In 16 % of cases, this response propagated to the soma to elicit an initial burst of fast APs. Somatic recordings revealed three modes of discharge. In mode 1, PCs display a single or a short burst of fast APs. In contrast, PCs fire repetitively in mode 2 and 3, with a sustained discharge of APs in mode 2, and bursts of APs in mode 3. Removal of external Ca2+ or bath applications of a membrane-permeable Ca2+ chelator abolished repetitive firing. Tetraethylammonium (TEA) prolonged dendritic and somatic fast APs by a depolarizing plateau sensitive to Cd2+ and to omega-conotoxin MVII C or omega-agatoxin TK. Therefore, the role of Ca2+ channels in determining somatic PC firing has been investigated. Cd2+ or P/Q type Ca2+ channel-specific toxins reduced the duration of the discharge and occasionallyinduced the appearance of oscillations in the membrane potential associated with bursts of APs. In summary, we demonstrate that Ca2+ entry through low-voltage gated Ca2+ channels, not yet identified, underlies a dendritic AP rarelyeliciting a somatic burst of APs whereas Ca2+ entry through P/Q type Ca2+ channels allowed a repetitive firing mainly by inducing a Ca2+-dependent hyperpolarization.