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Mechanism of increased BK channel activation from a channel mutation that causes epilepsy.

Wang B, Rothberg BS, Brenner R - J. Gen. Physiol. (2009)

Bottom Line: D369G causes a greater than twofold increase in the closed-to-open equilibrium constant (6.6e(-7)-->1.65e(-6)) and an approximate twofold decrease in Ca(2+)-dissociation constants (closed channel: 11.3-->5.2 microM; open channel: 0.92-->0.54 microM).D369G and beta 4 have opposing effects on BK current recruitment, where D369G reduces and beta 4 increases K(1/2) (K(1/2) microM: alpha(WT) 13.7, alpha(D369G) 6.3, alpha(WT)/beta 4 24.8, and alpha(D369G)/beta 4 15.0).Collectively, our results suggest that the D369G enhancement of intrinsic gating and Ca(2+) binding underlies greater contributions of BK current in the sharpening of action potentials for both alpha and alpha/beta 4 channels.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology, The University of Texas Health Science Center at San Antonio, San Antonio, TX 78229, USA.

ABSTRACT
Concerted depolarization and Ca(2+) rise during neuronal action potentials activate large-conductance Ca(2+)- and voltage-dependent K(+) (BK) channels, whose robust K(+) currents increase the rate of action potential repolarization. Gain-of-function BK channels in mouse knockout of the inhibitory beta 4 subunit and in a human mutation (alpha(D434G)) have been linked to epilepsy. Here, we investigate mechanisms underlying the gain-of-function effects of the equivalent mouse mutation (alpha(D369G)), its modulation by the beta 4 subunit, and potential consequences of the mutation on BK currents during action potentials. Kinetic analysis in the context of the Horrigan-Aldrich allosteric gating model revealed that changes in intrinsic and Ca(2+)-dependent gating largely account for the gain-of-function effects. D369G causes a greater than twofold increase in the closed-to-open equilibrium constant (6.6e(-7)-->1.65e(-6)) and an approximate twofold decrease in Ca(2+)-dissociation constants (closed channel: 11.3-->5.2 microM; open channel: 0.92-->0.54 microM). The beta 4 subunit inhibits mutant channels through a slowing of activation kinetics. In physiological recording solutions, we established the Ca(2+) dependence of current recruitment during action potential-shaped stimuli. D369G and beta 4 have opposing effects on BK current recruitment, where D369G reduces and beta 4 increases K(1/2) (K(1/2) microM: alpha(WT) 13.7, alpha(D369G) 6.3, alpha(WT)/beta 4 24.8, and alpha(D369G)/beta 4 15.0). Collectively, our results suggest that the D369G enhancement of intrinsic gating and Ca(2+) binding underlies greater contributions of BK current in the sharpening of action potentials for both alpha and alpha/beta 4 channels.

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Ca2-dependent effects of β4 on mutant BK channel G-V. (A) A family of currents from αD369G BK channels composed of either the pore forming alone (top) or with the β4 auxiliary subunits (bottom). Recorded in 2.1 µM Ca2+, currents were evoked in response to 200-ms depolarizations. (B, top) Effect of β4 on mslo and hslo mutant G-V relations at 2.1 µM Ca2+ (hslo_αD434G: n = 7; hslo_αD434G β4: n = 9; mslo_αD369G: n = 22; mslo_αD369G β4: n = 32). (B, bottom) Effect of β4 on mslo and hslo mutant G-V relations at 41 µM Ca2+ (hslo_αD434G: n = 10; hslo_αD434G β4: n = 11; mslo_αD369G: n = 18; mslo_αD369G β4: n = 15). Symbols represent mean G/Gmax data, curves represent fits to the Boltzmann function, and error bars represent SEM. (C) Effects of β4 on steady-state gating of mutant channels. Mean V1/2 (top) and mean effective gating charge (Q) values (bottom) plotted as a function of Ca2+. Error bars represent SEM. (D) Effects of β4 on steady-state gating of wild-type channels. Mean V1/2 (top) and mean effective gating charge (Q) values (bottom) plotted as a function of Ca2+. Error bars represent SEM.
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fig5: Ca2-dependent effects of β4 on mutant BK channel G-V. (A) A family of currents from αD369G BK channels composed of either the pore forming alone (top) or with the β4 auxiliary subunits (bottom). Recorded in 2.1 µM Ca2+, currents were evoked in response to 200-ms depolarizations. (B, top) Effect of β4 on mslo and hslo mutant G-V relations at 2.1 µM Ca2+ (hslo_αD434G: n = 7; hslo_αD434G β4: n = 9; mslo_αD369G: n = 22; mslo_αD369G β4: n = 32). (B, bottom) Effect of β4 on mslo and hslo mutant G-V relations at 41 µM Ca2+ (hslo_αD434G: n = 10; hslo_αD434G β4: n = 11; mslo_αD369G: n = 18; mslo_αD369G β4: n = 15). Symbols represent mean G/Gmax data, curves represent fits to the Boltzmann function, and error bars represent SEM. (C) Effects of β4 on steady-state gating of mutant channels. Mean V1/2 (top) and mean effective gating charge (Q) values (bottom) plotted as a function of Ca2+. Error bars represent SEM. (D) Effects of β4 on steady-state gating of wild-type channels. Mean V1/2 (top) and mean effective gating charge (Q) values (bottom) plotted as a function of Ca2+. Error bars represent SEM.

Mentions: The “symmetrical” external recording solution (electrode solution for Figs. 1–6) was composed of the following (in mM): 20 HEPES, 140 KMeSO3, 2 KCl, and 2 MgCl2, pH 7.2. The “physiological” external recording solution (electrode solution for Figs. 7 and 8) was composed of the following (in mM): 10 HEPES, 145 NaCl, 5 KCl, 1 MgCl2, and 2 CaCl2, pH 7.2. Internal solutions were composed of a pH 7.2 solution of the following (in mM): 20 HEPES, 140 KMeSO3, and 2 KCl. Intracellular Ca2+ was buffered with 5 mM EGTA (0.073 and 0.006 µM), HEDTA (2.1 and 0.363 µM) or NTA (41 µM), and free [Ca2+] was measured using a Ca2+-sensitive electrode (Orion Research, Inc.). In low Ca2+ solutions (0.073 and 0.006 µM), Ba2+ was chelated with 40 µM (+)-18-crown-6-tetracarboxylic acid (Cox et al., 1997).


Mechanism of increased BK channel activation from a channel mutation that causes epilepsy.

Wang B, Rothberg BS, Brenner R - J. Gen. Physiol. (2009)

Ca2-dependent effects of β4 on mutant BK channel G-V. (A) A family of currents from αD369G BK channels composed of either the pore forming alone (top) or with the β4 auxiliary subunits (bottom). Recorded in 2.1 µM Ca2+, currents were evoked in response to 200-ms depolarizations. (B, top) Effect of β4 on mslo and hslo mutant G-V relations at 2.1 µM Ca2+ (hslo_αD434G: n = 7; hslo_αD434G β4: n = 9; mslo_αD369G: n = 22; mslo_αD369G β4: n = 32). (B, bottom) Effect of β4 on mslo and hslo mutant G-V relations at 41 µM Ca2+ (hslo_αD434G: n = 10; hslo_αD434G β4: n = 11; mslo_αD369G: n = 18; mslo_αD369G β4: n = 15). Symbols represent mean G/Gmax data, curves represent fits to the Boltzmann function, and error bars represent SEM. (C) Effects of β4 on steady-state gating of mutant channels. Mean V1/2 (top) and mean effective gating charge (Q) values (bottom) plotted as a function of Ca2+. Error bars represent SEM. (D) Effects of β4 on steady-state gating of wild-type channels. Mean V1/2 (top) and mean effective gating charge (Q) values (bottom) plotted as a function of Ca2+. Error bars represent SEM.
© Copyright Policy - openaccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC2654085&req=5

fig5: Ca2-dependent effects of β4 on mutant BK channel G-V. (A) A family of currents from αD369G BK channels composed of either the pore forming alone (top) or with the β4 auxiliary subunits (bottom). Recorded in 2.1 µM Ca2+, currents were evoked in response to 200-ms depolarizations. (B, top) Effect of β4 on mslo and hslo mutant G-V relations at 2.1 µM Ca2+ (hslo_αD434G: n = 7; hslo_αD434G β4: n = 9; mslo_αD369G: n = 22; mslo_αD369G β4: n = 32). (B, bottom) Effect of β4 on mslo and hslo mutant G-V relations at 41 µM Ca2+ (hslo_αD434G: n = 10; hslo_αD434G β4: n = 11; mslo_αD369G: n = 18; mslo_αD369G β4: n = 15). Symbols represent mean G/Gmax data, curves represent fits to the Boltzmann function, and error bars represent SEM. (C) Effects of β4 on steady-state gating of mutant channels. Mean V1/2 (top) and mean effective gating charge (Q) values (bottom) plotted as a function of Ca2+. Error bars represent SEM. (D) Effects of β4 on steady-state gating of wild-type channels. Mean V1/2 (top) and mean effective gating charge (Q) values (bottom) plotted as a function of Ca2+. Error bars represent SEM.
Mentions: The “symmetrical” external recording solution (electrode solution for Figs. 1–6) was composed of the following (in mM): 20 HEPES, 140 KMeSO3, 2 KCl, and 2 MgCl2, pH 7.2. The “physiological” external recording solution (electrode solution for Figs. 7 and 8) was composed of the following (in mM): 10 HEPES, 145 NaCl, 5 KCl, 1 MgCl2, and 2 CaCl2, pH 7.2. Internal solutions were composed of a pH 7.2 solution of the following (in mM): 20 HEPES, 140 KMeSO3, and 2 KCl. Intracellular Ca2+ was buffered with 5 mM EGTA (0.073 and 0.006 µM), HEDTA (2.1 and 0.363 µM) or NTA (41 µM), and free [Ca2+] was measured using a Ca2+-sensitive electrode (Orion Research, Inc.). In low Ca2+ solutions (0.073 and 0.006 µM), Ba2+ was chelated with 40 µM (+)-18-crown-6-tetracarboxylic acid (Cox et al., 1997).

Bottom Line: D369G causes a greater than twofold increase in the closed-to-open equilibrium constant (6.6e(-7)-->1.65e(-6)) and an approximate twofold decrease in Ca(2+)-dissociation constants (closed channel: 11.3-->5.2 microM; open channel: 0.92-->0.54 microM).D369G and beta 4 have opposing effects on BK current recruitment, where D369G reduces and beta 4 increases K(1/2) (K(1/2) microM: alpha(WT) 13.7, alpha(D369G) 6.3, alpha(WT)/beta 4 24.8, and alpha(D369G)/beta 4 15.0).Collectively, our results suggest that the D369G enhancement of intrinsic gating and Ca(2+) binding underlies greater contributions of BK current in the sharpening of action potentials for both alpha and alpha/beta 4 channels.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology, The University of Texas Health Science Center at San Antonio, San Antonio, TX 78229, USA.

ABSTRACT
Concerted depolarization and Ca(2+) rise during neuronal action potentials activate large-conductance Ca(2+)- and voltage-dependent K(+) (BK) channels, whose robust K(+) currents increase the rate of action potential repolarization. Gain-of-function BK channels in mouse knockout of the inhibitory beta 4 subunit and in a human mutation (alpha(D434G)) have been linked to epilepsy. Here, we investigate mechanisms underlying the gain-of-function effects of the equivalent mouse mutation (alpha(D369G)), its modulation by the beta 4 subunit, and potential consequences of the mutation on BK currents during action potentials. Kinetic analysis in the context of the Horrigan-Aldrich allosteric gating model revealed that changes in intrinsic and Ca(2+)-dependent gating largely account for the gain-of-function effects. D369G causes a greater than twofold increase in the closed-to-open equilibrium constant (6.6e(-7)-->1.65e(-6)) and an approximate twofold decrease in Ca(2+)-dissociation constants (closed channel: 11.3-->5.2 microM; open channel: 0.92-->0.54 microM). The beta 4 subunit inhibits mutant channels through a slowing of activation kinetics. In physiological recording solutions, we established the Ca(2+) dependence of current recruitment during action potential-shaped stimuli. D369G and beta 4 have opposing effects on BK current recruitment, where D369G reduces and beta 4 increases K(1/2) (K(1/2) microM: alpha(WT) 13.7, alpha(D369G) 6.3, alpha(WT)/beta 4 24.8, and alpha(D369G)/beta 4 15.0). Collectively, our results suggest that the D369G enhancement of intrinsic gating and Ca(2+) binding underlies greater contributions of BK current in the sharpening of action potentials for both alpha and alpha/beta 4 channels.

Show MeSH
Related in: MedlinePlus