Kwon T et al. (2013), Voltage-dependent gating of the Cx32*43E1 hemic...

XB-ART-48212

J Gen Physiol 2013 Feb 01;1412:243-59. doi: 10.1085/jgp.201210839.

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Voltage-dependent gating of the Cx32*43E1 hemichannel: conformational changes at the channel entrances.

Kwon T , Tang Q , Bargiello TA .

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Voltage is an important parameter that regulates the open probability of both intercellular channels (gap junctions) and undocked hemichannels formed by members of the connexin gene family. All connexin channels display two distinct voltage-gating processes, termed loop- or slow-gating and V(j)- or fast-gating, which are intrinsic hemichannel properties. Previous studies have established that the loop-gate permeability barrier is formed by a large conformational change that reduces pore diameter in a region of the channel pore located at the border of the first transmembrane domain and first extracellular loop (TM1/E1), the parahelix (residues 42-51). Here, we use cadmium metal bridge formation to measure conformational changes reported by substituted cysteines at loci demarcating the intracellular (E109 and L108) and extracellular (Q56) entrance of hemichannels formed by the Cx32 chimera (Cx32*43E1). The results indicate that the intracellular pore entrance narrows from ∼15 Å to ∼10 Å with loop-gate but not apparently with V(j)-gate closure. The extracellular entrance does not appear to undergo large conformational changes with either voltage-gating process. The results presented here combined with previous studies suggest that the loop-gate permeability is essentially focal, in that conformational changes in the parahelix but not the intracellular entrance are sufficient to prevent ion flux.

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Species referenced: Xenopus laevis
Genes referenced: gja8 gjb1 gjb2 npnt socs1 tbx2

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	Figure 1. Side view of Cx26 (A) and Cx3243E1 N2E ΔCT (B) hemichannels after equilibration by all atom molecular dynamics in a fully hydrated POPC membrane. The side chains of residues 108 (green balls), 109 (blue balls), and 56 (orange balls) are shown. The backbone of the parahelix (residues 42–51) is depicted by the red ribbon. The C terminus of Cx32 was not included in the homology model. The Cx3243E1 N2E hemichannel was incorporated into a fully hydrated POPC membrane as described previously (Kwon et al., 2011). A tetragonal periodic boundary box, 103 × 103 × 109 Å including the protein, lipid membrane, TIP3 waters, and 150 mM KCl, was constructed with a program contained in CHARMM-GUI. All protein atoms in this system used the CHARMM 22 force field with CMAP corrections. The CHARMM 36 force field was used for lipid molecules (Feller and MacKerell, 2000). The system was equilibrated for 1 µs at 310 K using NPnT dynamics in Desmond using an Anton computer (Shaw et al., 2007). NPnT is the ensemble name for constant number of particles (N), pressure in normal direction (Pn), and temperature (T).
	Figure 2. Cadmium stabilizes the loop-gate closed state of Cx32*43E1 N2E E109C hemichannels. (A) A segment of a continuous current trace evoked by a train of alternating voltage polarizations from −10 mV (10 s duration) to −70 mV (10 s duration) with different [Cd2+] indicated by the colored solid bars. (B) Expansion of steady-state current traces shown in A. Current reductions at −70 mV (upward, positive going current relaxations) reflect closure of loop-gates. Increase in currents at −10 mV (downward, negative going current relaxations) reflect opening of loop-gates. (C) Steady-state peak currents measured at −70 mV are plotted against [Cd2+]. Half-maximal current reduction is obtained at [Cd2+] ∼10 µM. (D) Plots of normalized fitted currents of steady-state current relaxations at −70 mV. The time constants of current relaxations are shortened as [Cd2+] is increased. Current relaxations were well fitted by standard exponential function with two terms in Clampfit 9.0. (E) Plots of normalized fitted currents of steady-state current relaxations at −10 mV. Current relaxations in 0 and 10 µM Cd2+ were well fitted by single exponential functions. Current relaxations obtained with 10-s voltage applications became linear in higher [Cd2+] and could not be fitted to an exponential function.
	Figure 3. MTSEA modification of E109C attenuates Cd2+ stabilization of the loop-gate closed state. (A) A segment of a continuous current trace evoked by a train of alternating voltage polarizations between −10 mV (15 s duration) and −70 mV (5 s duration). The colored solid bars indicate the onset and duration of specified treatments. Capacitive transients were reduced but not fully eliminated by 100× data reduction in Clampfit. (B) Expansion of steady-state current traces shown in A as indicated by arrows. Peak currents obtained at steady-state at −70 mV are marked by arrows.
	Figure 4. Endogenous cysteines in the CT are not required for Cd2+ coordination by E109C. (A) A segment of a continuous current trace evoked by a train of alternating voltage polarizations from −10 mV (10 s duration) to −70 mV (10 s duration). 20 µM Cd2+ was applied for the time indicated by the red bar. (B) Plots of normalized fitted currents of steady-state current relaxations at −70 mV corresponding to loop-gate closure. (C) Plots of normalized fitted currents of steady-state current relaxations at −10 mV corresponding to loop-gate opening.
	Figure 5. Cd2+ does not substantially alter Vj-gating of Cx3243E1 N2E 109C hemichannels. (A) Segments of a continuous current trace evoked by a train of alternating voltage polarizations from 10 mV (15 s duration) to 50 mV (5 s duration). The current relaxations elicited at 50 mV correspond to the closure of V-gates. The time course of currents elicited at 10 mV is nearly linear and suggests that the time course of Vj-gate opening is fast at this potential. The colored solid bars indicate the onset and duration of the specified treatments. (B) Expansion of steady-state current traces shown in A. Peak currents at 50 mV are marked by arrows. (C) Plots of normalized fitted currents of steady-state current relaxations at 50 mV. Current elicited by a 40-mV polarization. The oocyte was perfused with bath solution containing 10 µM CdCl2 at the time indicated by the arrow. (D) A continuous trace of Cx3243E1 E109C hemichannel evoked by a 40-mV polarization from a holding potential of 10 mV. The observed current relaxation reflects closure of Vj-gates. 10 µM Cd2+ was perfused at the time indicated by the arrow.
	Figure 6. Cadmium stabilizes a closed state but has no effect on the open state of Cx43*43E1 L109C hemichannels. (A) A segment of a continuous current trace evoked by a train of alternating voltage polarizations from 10 mV (10 s duration) to −70 mV (10 s duration) in bath solution containing 1.8 mM MgCl2. The time of perfusion with 60 µM Cd2+ is depicted by the red bar. (B) Same oocyte as in A, but voltage was stepped between −10 and −70 mV. (C) Plot of normalized fitted currents at steady-state of loop-gate closure at −70 mV for the current trace shown in B. Steady currents were fitted to an exponential function with two terms. Current relaxations reach steady-state faster in the presence of Cd2+ (red), indicating a shortening of the time constants of channel closure. (D) Plot of normalized fitted currents at steady-state of loop-gate opening at −10 mV. Steady currents were fitted to an exponential function with one term. Current relaxations reach steady-state slower in the presence of Cd2+, which indicates a lengthening of time constant of channel opening. (E) Time series of current traces evoked by a train of alternating voltage polarizations from 10 mV (10 s duration) to 50 mV (10 s duration) in bath solution containing 1.8 mM MgCl2. The time of perfusion with 60 µM Cd2+ is depicted by the red bar.
	Figure 7. Cadmium stabilizes the loop-gate but not Vj-gate closed state of Cx32*43E1 N2E L108C hemichannels. (A) A segment of a continuous current trace evoked by a train of alternating voltage polarizations from −10 mV (15 s duration) to −70 mV (5 s duration). The time of perfusion with 10 µM Cd2+ is depicted by the red bar. (B) Same as A only with perfusion of 20 µM Cd2+. (C) Expanded segments of the current trace shown in B at steady-state. Initial currents before perfusion of Cd2+ (black trace), steady-state current after perfusion of Cd2+ (red trace), and steady-state current after wash (blue trace). Arrows mark the level of peak current. Capacitive transients were not removed. (D) Plot of normalized fitted currents at steady-state of loop-gate closure at −70 mV. Steady currents were fitted to an exponential function with two terms. Current relaxations reach steady-state faster in the presence of Cd2+, which indicates a shortening of the time constants of channel closure. (E) Plot of normalized fitted currents at steady-state of loop-gate opening at −10 mV. Steady currents were fitted to an exponential function with one term. Current relaxations reach steady-state slower in the presence of Cd2+, which indicates a lengthening of time constant of channel opening. (F) A segment of a continuous current trace elicited by a polarization from 10 to 30 mV. Current relaxation reflects closure of Vj-gates. 20 µM Cd2+ was perfused at the time indicated by the arrow followed by washing with Cd2+-free bath solution. Cd2+ had no effect on the time course to reach steady-state.
	Figure 8. Cadmium does not stabilize closed conformations of Cx32*43E1 Q56C hemichannels. (A) A segment of a continuous current trace evoked by a train of alternating voltage polarizations from −10 mV (15 s duration) to −70 mV (5 s duration) in bath solution containing 1.8 mM MgCl2. The time of perfusion with 20 µM Cd2+ is depicted by the red bar, 60 µM Cd2+ by the green bar. (B) Same as in A only the bath solution contained 1.8 mM CaCl2. (C) Plot of normalized fitted currents at steady-state of loop-gate closure at −70 mV in 1.8 mM MgCl2 bath solutions with no added Cd2+ (black trace) and 60 µM Cd2+ (green trace). Steady currents were fitted to an exponential function with two terms. Current relaxations reach steady-state more slowly in 60 µM Cd2+, which indicates that the reductions in peak current shown in A do not result from the stabilization of closed states by Cd2+ coordination of thiol groups. (D) Plot of normalized fitted currents at steady-state of loop-gate closure at −70 mV in 1.8 mM CaCl2 bath solutions with no added Cd2+ (black trace) and 60 µM Cd2+ (green trace). Steady currents were fitted to an exponential function with one term. The similarity of the time course of current relaxations in the presence and absence of Cd2+ indicate that the reductions in peak current shown in B do not result from the stabilization of closed states by Cd2+ coordination of thiol groups.
	Figure 9. Schematic representation of open and loop-gate closed state of the Cx3243E1 hemichannel. Residues reported in this study are indicated in red. Residues in black are those reported in Tang et al. (2009). The position of the 45th and 50th residues in the closed state are based on results from Cx50 hemichannels reported in Verselis et al. (2009) and unpublished studies of Cx3243E1 hemichannels (see also Bargiello et al., 2012).

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Abrams, Gap junctions in inherited human disorders of the central nervous system. 2012, Pubmed