Gating properties of gap junction channels assembled from connexin43 and connexin43 fused with green fluorescent protein

Gating and regulation of connexin 43 (Cx43) hemichannels

Connexin 43 (Cx43) nonjunctional or "unapposed" hemichannels can open under physiological or pathological conditions. We characterize hemichannels comprised of Cx43 or Cx43-EGFP (Cx43 with enhanced GFP fused to the C terminus) expressed in HeLa cells. Channel opening was induced at potentials greater than +60 mV. Open probability appeared to be very low. No comparable opening was detected in the parental, nontransfected HeLa cells. Conductance of fully open single hemichannels was approximately 220 pS, which is approximately double that of Cx43 cell-cell channels. Cx43 hemichannels exhibited two types of gating: fast transitions (<1 ms) between the fully open state and a substate of approximately 75 pS and slow transitions (>5 ms) between either open state and the fully closed state. Cx43-EGFP hemichannels exhibited only slow transitions (>5 ms) between closed and fully open states. These properties resemble those of the corresponding Cx43 and Cx43-EGFP cell-cell channels. Cx43 with EGFP on the N terminus (EGFP-Cx43) inserted into the surface and formed plaques but did not form hemichannels or cell-cell channels. Hemichannel blockers, 18beta-glycyrrhetinic acid or La3+, blocked depolarization-induced currents. Uptake of ethidium bromide (i) was faster in Cx43 and Cx43-EGFP than parental and EGFP-Cx43 cells, (ii) was directly correlated with Cx43-EGFP expression, (iii) was reduced by hemichannel blockers, and (iv) occurred at the same low rate in EGFP-Cx43 and parental cells. Although hemichannel opening was not detected electrophysiologically at the resting potential, infrequent or brief opening could account for ethidium bromide uptake. Opening of Cx43 hemichannels may mediate normal signaling or be deleterious.

Gap junction hemichannels in astrocytes of the CNS

Connexins are protein subunits that oligomerize into hexamers called connexons, gap junction hemichannels or just hemichannels. Because some gap junction channels are permeable to negatively and/or positively charged molecules up to approximately 1kDa in size, it was thought that hemichannels should not open to the extracellular space. A growing amount of evidence indicates that opening of hemichannels does occur under both physiological and pathological conditions in astrocytes and other cell types. Electrophysiological studies indicate that hemichannels have a low open probability under physiological conditions but may have a much higher open probability under certain pathological conditions. Some of the physiological behaviours of astrocytes that have been attributed to gap junctions may, in fact, be mediated by hemichannels. Hemichannels constituted of Cx43, the main connexin expressed by astrocytes, are permeable to small physiologically significant molecules, such as ATP, NAD+ and glutamate, and may mediate paracrine as well as autocrine signalling. Hemichannels tend to be closed by negative membrane potentials, high concentrations of extracellular Ca2+ and intracellular H+ ions, gap junction blockers and protein phosphorylation. Hemichannels tend to be opened by positive membrane potentials and low extracellular Ca2+, and possibly by as yet unidentified cytoplasmic signalling molecules. Exacerbated hemichannel opening occurs in metabolically inhibited cells, including cortical astrocytes, which contributes to the loss of chemical gradients across the plasma membrane and speeds cell death.

Evidence for a role of the N-terminal domain in subcellular localization of the neuronal connexin36 (Cx36)

The expression and functional properties of connexin36 (Cx36) have been investigated in two neuroblastoma cell lines (Neuro2A, RT4-AC) and primary hippocampal neurons transfected with a Cx36-enhanced green fluorescent protein (EGFP) expression vector. Transfected cells express Cx36-EGFP mRNA, and Cx36-EGFP protein is localized in the perinuclear area and cell membrane. Upon differentiation of cell lines, Cx36-EGFP protein was detectable in processes with both axonal and dendritic characteristics. Small gap junction plaques were found between adjacent cells, and electrophysiological recordings demonstrated that the electrical properties of these gap junctions were virtually indistinguishable from those reported for native Cx36. Mutagenesis of Cx36 led to the identification of a structural element that interferes with normal protein localization. In contrast, site directed mutagenesis of putative protein phosphorylation motifs did not alter subcellular localization. This excludes phosphorylation/dephosphorylation as a major regulatory step in Cx36 protein transport.

Intercellular calcium signaling mediated by point-source burst release of ATP

Calcium signaling, manifested as intercellular waves of rising cytosolic calcium, is, in many cell types, the result of calcium-induced secretion of ATP and activation of purinergic receptors. The mechanism by which ATP is released has hitherto not been established. Here, we show by real-time bioluminescence imaging that ATP efflux is not uniform across a field of cells but is restricted to brief, abrupt point-source bursts. The ATP bursts emanate from single cells and manifest the transient opening of nonselective membrane channels, which admits fluorescent indicators of < or = 1.5 kDa. These observations challenge the existence of regenerative ATP release, because ATP efflux is finite and restricted to a point source. Transient efflux of cytosolic nucleotides from a subset of cells may represent a conserved pathway for coordinating local activity of electrically nonexcitable cells, because identical patterns of ATP release were identified in human astrocytes, endothelial cells, and bronchial epithelial cells.

Coupling asymmetry of heterotypic connexin 45/ connexin 43-EGFP gap junctions: properties of fast and slow gating mechanisms

Although fast and slow gating mechanisms have been described in gap junctions (GJs), their relative contributions to dependence on transjunctional voltage, V(j), is still unclear. We used cell lines expressing wild-type connexin 45 (Cx45) and connexin 43 fused with enhanced green fluorescent protein (Cx43-EGFP) to examine mechanisms of gating in homo- and heterotypic GJs formed of these connexins. Macroscopically Cx45/Cx45 channels show high sensitivity to V(j). Cx45 channels demonstrate two types of gating: fast transitions between open and residual states and slow transitions between open and completely closed states. Single-channel conductance of the Cx45 channel is approximately 32 pS for the open state and approximately 4 pS for the residual state. Cx45/Cx43-EGFP heterotypic junctions exhibit very asymmetrical V(j) gating with the maximum junctional conductance shifted to V(j) positive on the Cx45 side. Conductance of single Cx45/Cx43-EGFP channels is approximately 55 pS for the open state and approximately 4 pS for the residual state, values consistent with the simple-series connection of Cx45 and Cx43-EGFP hemichannels. At V(j) = 0, the slow gate of many Cx45 hemichannels is closed in both homotypic Cx45/Cx45 and heterotypic Cx45/Cx43-EGFP junctions. Fast and slow V(j) gates of both Cx45 and Cx43 hemichannels close for relative negativity at their cytoplasmic end. Coupling mediated by Cx45/Cx43-EGFP junctions can exhibit asymmetry that can be strongly modulated by small changes in difference of holding potentials. Cx45/Cx43 junctions are likely to be found in brain and heart and may mediate rectifying electrical transmission or modulatable chemical communication.

Segment-specific expression of connexin31 in the embryonic hindbrain is regulated by Krox20

Communication and interaction between cells has been shown to be important during the embryonic development of the vertebrate hindbrain, which becomes transiently subdivided into segments called rhombomeres (r). One gene family allowing intercellular communication and possibly being involved in the control of hindbrain development is the connexin family encoding gap junction channels. Here, we have characterized in detail the previously observed (Dahl et al., 1997) expression of one particular connexin gene, connexin31 (Cx31), in the mouse embryonic hindbrain and compared it with that of Cx43 and Cx36. We found transient Cx31 expression from approximately embryonic day (E) E8-E11 in two small lateral/dorsal subgroups of cells in the hindbrain. We could show that these spots of expression corresponded to r3 and r5 and that Cx31 expression in r3 and r5 was controlled by the transcription factor Krox20. In contrast, expression of Cx43 and Cx36 started later (from E9.5 and E10.5, respectively) and was confined to longitudinal stripes of expression. In addition, from E10.5-E11.5, Cx31 was expressed by a column of cells in ventral r4, most likely representing contralateral vestibulo-acoustic efferent neurons, immediately anterior to a ventral column expressing Cx36 at the same stage. From E11.5 onward, another site of Cx31 expression was detected in the boundary cap cells in the entry/exit points of all mixed sensory/motor and in the entry points of pure sensory nerves. This expression was not present in the boundary cap cells of the exit points of pure motor nerves. So far, our analysis of the hindbrain area of Cx31-deficient embryos in terms of projections of sensory or motor neurons or in the generation or migration of neurons has not yet revealed any obvious defects.

Role of the carboxyl terminal of connexin43 in transjunctional fast voltage gating

Previous studies show that chemical regulation of connexin43 (Cx43) gap junction channels depends on the integrity of the carboxyl terminal (CT) domain. Experiments using Xenopus oocytes show that truncation of the CT domain alters the time course for current inactivation; however, correlation with the behavior of single Cx43 channels has been lacking. Furthermore, whereas chemical gating is associated with a "ball-and-chain" mechanism, there is no evidence whether transjunctional voltage regulation for Cx43 follows a similar model. We provide data on the properties of transjunctional currents from voltage-clamped pairs of mammalian tumor cells expressing either wild-type Cx43 or a mutant of Cx43 lacking the carboxyl terminal domain (Cx43M257). Cx43 transjunctional currents showed bi-exponential decay and a residual steady-state conductance of approximately 35% maximum. Transjunctional currents recorded from Cx43M257 channels displayed a single, slower exponential decay. Long transjunctional voltage pulses caused virtual disappearance of the residual current at steady state. Single channel data revealed disappearance of the residual state, increase in the mean open time, and slowing of the transition times between open and closed states. Coexpression of CxM257 with Cx43CT in a separate fragment restored the lower conductance state. We propose that Cx43CT is an effector of fast voltage gating. Truncation of Cx43CT limits channel transitions to those occurring across the higher energy barrier that separates open and closed states. We further propose that a ball-and-chain interaction provides the fast component of voltage-dependent gating between CT domain and a receptor affiliated with the pore.

Conductance and permeability of the residual state of connexin43 gap junction channels

We used cell lines expressing wild-type connexin43 and connexin43 fused with the enhanced green fluorescent protein (Cx43-EGFP) to examine conductance and perm-selectivity of the residual state of Cx43 homotypic and Cx43/Cx43-EGFP heterotypic gap junction channels. Each hemichannel in Cx43 cell-cell channel possesses two gates: a fast gate that closes channels to the residual state and a slow gate that fully closes channels; the transjunctional voltage (V(j)) closes the fast gate in the hemichannel that is on the relatively negative side. Here, we demonstrate macroscopically and at the single-channel level that the I-V relationship of the residual state rectifies, exhibiting higher conductance at higher V(j)s that are negative on the side of gated hemichannel. The degree of rectification increases when Cl(-) is replaced by Asp(-) and decreases when K(+) is replaced by TEA(+). These data are consistent with an increased anionic selectivity of the residual state. The V(j)-gated channel is not permeable to monovalent positively and negatively charged dyes, which are readily permeable through the fully open channel. These data indicate that a narrowing of the channel pore accompanies gating to the residual state. We suggest that the fast gate operates through a conformational change that introduces positive charge at the cytoplasmic vestibule of the gated hemichannel, thereby producing current rectification, increased anionic selectivity, and a narrowing of channel pore that is largely responsible for reducing channel conductance and restricting dye transfer. Consequently, the fast V(j)-sensitive gating mechanism can serve as a selectivity filter, which allows electrical coupling but limits metabolic communication.

Function of the voltage gate of gap junction channels: selective exclusion of molecules

Gap junction channels span the membranes of two adjacent cells and allow the gated transit of molecules as large as second messengers from cell to cell. In vertebrates, gap junctions are composed of proteins from the connexin (cx) gene family. Gap junction channels formed by most connexins are affected by transjunctional voltage. The function of the voltage gate is unclear, because substantial electrical coupling typically remains with activated gates because of the channels dwelling in subconductance rather than closed states. Here, we find in Xenopus oocytes expressing cx43 or cx46 that the activated voltage gate preferentially restricts the passage of larger ions, such as fluorescent tracer molecules and cAMP, while having little effect on the electrical coupling arising from the passage of small electrolytes. Thus, a conceivable physiological role of the voltage gate is to selectively restrict the passage of large molecules between cells while allowing electrical coupling.

Metabolic inhibition induces opening of unapposed connexin 43 gap junction hemichannels and reduces gap junctional communication in cortical astrocytes in culture

Rat cortical astrocytes in pure culture are functionally coupled to neighboring cells via connexin (Cx) 43 gap junctions under ordinary conditions. Small fluorescent molecules such as Lucifer yellow (LY) pass between cell interiors via gap junctions, but do not enter the cells when externally applied. Subjecting rat and mouse cortical astrocytes to "chemical ischemia" by inhibition of glycolytic and oxidative metabolism induced permeabilization of cells to Lucifer yellow and ethidium bromide before loss of membrane integrity determined by dextran uptake and lactate dehydrogenase release. The gap junction blockers octanol and 18alpha-glycyrrhetinic acid markedly reduced dye uptake, suggesting that uptake was mediated by opening of unapposed hemichannels. Extracellular La(3+) also reduced dye uptake and delayed cell death. The purinergic blocker, oxidized ATP, was ineffective. Astrocytes isolated from mice with targeted deletion of the Cx43 coding DNA exhibited greatly reduced dye coupling and ischemia-induced dye uptake, evidence that dye uptake is mediated by Cx43 hemichannels. Dye coupling was reduced but not blocked by metabolic inhibition. Blockade of lipoxygenases or treatment with free radical scavengers reduced dye uptake by rat astrocytes, suggesting a role for arachidonic acid byproducts in hemichannel opening. Furthermore, permeabilization was accompanied by reduction in ATP levels and dephosphorylation of Cx43. Although hemichannel opening would tend to collapse electrochemical and metabolic gradients across the plasma membrane of dying cells, healthy cells might rescue dying cells by transfer of ions and essential metabolites via Cx43 gap junctions. Alternatively, dying astrocytes might compromise the health of neighboring cells via Cx43 gap junctions, thereby promoting the propagation of cell death.