Yonsei Med J.  2015 Jan;56(1):1-15. 10.3349/ymj.2015.56.1.1.

Voltage Regulation of Connexin Channel Conductance

• Oh S
• Bargiello

• KMID: 2352783
• DOI: http://doi.org/10.3349/ymj.2015.56.1.1

Abstract

Voltage is an important parameter that regulates the conductance of both intercellular and plasma membrane channels (undocked hemichannels) formed by the 21 members of the mammalian connexin gene family. Connexin channels display two forms of voltage-dependence, rectification of ionic currents and voltage-dependent gating. Ionic rectification results either from asymmetries in the distribution of fixed charges due to heterotypic pairing of different hemichannels, or by channel block, arising from differences in the concentrations of divalent cations on opposite sides of the junctional plaque. This rectification likely underpins the electrical rectification observed in some electrical synapses. Both intercellular and undocked hemichannels also display two distinct forms of voltage-dependent gating, termed Vj (fast)-gating and loop (slow)-gating. This review summarizes our current understanding of the molecular determinants and mechanisms underlying these conformational changes derived from experimental, molecular-genetic, structural, and computational approaches.

Keyword

Connexin; voltage dependence; rectification; gating

MeSH Terms

Animals
Connexins/chemistry/*metabolism
Humans
*Ion Channel Gating
Ion Channels/chemistry/*metabolism
Molecular Dynamics Simulation
Protein Conformation
Connexins
Ion Channels

Figure

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  Fig. 1 Membrane topology of connexin subunits and channel composition. A connexin subunit is composed of four transmembrane domains (TM1, TM2, TM3, and TM4), two extracellular loops (E1 and E2), amino and carboxyl termini (NT and CT, respectively), and a cytoplasmic loop (CL). Six connexin subunits assemble to form a hemichannel or connexon, which is termed homomeric if all subunits are identical and heteromeric if comprised of different connexin subunits. Head to head docking of two connexons forms an intercellular channel, aggregates of which are termed gap junctions and visible as plaques in freeze fracture and transmission electron microscopy. Homotypic intercellular channels are formed by docking two hemichannels of identical subunit composition, be it homomeric or heteromeric. Heterotypic intercellular channels are formed by docking hemichannels that differ in subunit composition.

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  Fig. 2 Representative current traces and conductance-voltage plots of initial and steady state junctional currents obtained from pairs of Xenopus oocytes. (A) Cx32 homotypic channels. (B) Cx26 homotypic channels. (C) Cx32/Cx26 heterotypic channels. Initial conductance (▾) and steady-state conductance (▿) are plotted as the function of the applied Vj relative to the cytoplasm of the right-side hemichannel. Taken with permission from Oh, et al. J Gen Physiol 1999;114:339-64.19

• 

  Fig. 3 Diagram of a gap-junction channel showing presumed isopotential lines resulting from the application of Vi-o (A) and Vj (B). Vi-o was established by simultaneously holding the membrane potential of both cells at -50 mV. Vj does not exist in this case. Vj was established by voltage clamping the left cell to a holding potential of -100 mV, while the right cell was voltage clamped to 0 mV. Note that the generation of Vj also generates Vi-o in regions of the channel, but Vi-o varies with the voltage paradigm. Holding the left cell at -50 mV and the right cell at +50 mV generates the same Vj as the previous example (i.e., the voltage difference across the channel is -100 mV relative to the left cell). Note that Vi-o would differ in this case, because the absolute membrane potentials used to generate Vj differ in the two cases. Taken with permission from Bargiello and Brink.30

• 

  Fig. 4 Single channel records of an undocked Cx32*Cx43E1 hemichannel expressed in a Xenopus oocyte. (A) Outside-out record of a single channel illustrating closures by Vj-gating to three substates at a holding potential of -100 mV. (B) Cell-attached recording of a single wild-type channel illustrating six loop-gating events at a holding potential of -70 mV. Note the slow time course of the transitions and that not all transitions necessarily lead to full channel closure. Taken with permission from Bargiello, et al. Biochim Biophys Acta 2012;1818:1807-22.37

• 

  Fig. 5 (A) Representative macroscopic recording of Cx32*43E1 N2E 219 stop hemichannels in Xenopus oocytes in bath solution containing 100 mM cesium methanesulfonate, 10 mM Hepes, pH 7.6, elicited by voltage steps from 50 to -120 mV in 10 mV increments from a holding potential of 0 mV. Current traces depicted in red correspond to current relaxations corresponding to Vj-gating; black current traces to loop-gating. (B) Semilogarithmic plot of open probability at steady state and membrane potential (n=6 oocytes). The limiting slope is drawn from which gating charge is estimated to be -0.5.

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  Fig. 6 Schematic illustration of open and closed state models of Cx32*43E1 hemichannels cysteine substitutions of residues shown in the open state model are accessible to membrane impermeant thiol modifying reagents. Cysteine substitutions of residues shown in the closed state, with the exception of 56 (red), form Cd2+-thiol metal bridges when the channel resides in the loop-gate closed state. Residues 40 and 43 (green) do not line the open channel pore but enter the pore in the loop-gate closed state. A43C hemichannels form disulfide bridges in western blots. The approximate boundaries of the membrane are shown by dotted lines. This figure was updated to include unpublished data with permission. Previous versions of the figure appeared in Biochim Biophys Acta (BBA)-Biomembranes37 and J Gen Physiol.66

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