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Contributions of intracellular ions to kv channel voltage sensor dynamics.

Goodchild SJ, Fedida D - Front Pharmacol (2012)

Bottom Line: However, several other factors not directly linked to the voltage-dependent movement of charged residues within the voltage sensor impact the dynamics of the voltage sensor, such as inactivation, ionic conductance, intracellular ion identity, and block of the channel by intracellular ligands.There is a significant amount of variability in the reported kinetics of voltage sensor deactivation kinetics of Kv channels attributed to different mechanisms such as open state stabilization, immobilization, and relaxation processes of the voltage sensor.These considerations are of critical importance in understanding the molecular determinants of the complete channel gating cycle from activation to deactivation.

View Article: PubMed Central - PubMed

Affiliation: Department of Anesthesiology, Pharmacology and Therapeutics, University of British Columbia Vancouver, BC, Canada.

ABSTRACT
Voltage-sensing domains (VSDs) of Kv channels control ionic conductance through coupling of the movement of charged residues in the S4 segment to conformational changes at the cytoplasmic region of the pore domain, that allow K(+) ions to flow. Conformational transitions within the VSD are induced by changes in the applied voltage across the membrane field. However, several other factors not directly linked to the voltage-dependent movement of charged residues within the voltage sensor impact the dynamics of the voltage sensor, such as inactivation, ionic conductance, intracellular ion identity, and block of the channel by intracellular ligands. The effect of intracellular ions on voltage sensor dynamics is of importance in the interpretation of gating current measurements and the physiology of pore/voltage sensor coupling. There is a significant amount of variability in the reported kinetics of voltage sensor deactivation kinetics of Kv channels attributed to different mechanisms such as open state stabilization, immobilization, and relaxation processes of the voltage sensor. Here we separate these factors and focus on the causal role that intracellular ions can play in allosterically modulating the dynamics of Kv voltage sensor deactivation kinetics. These considerations are of critical importance in understanding the molecular determinants of the complete channel gating cycle from activation to deactivation.

No MeSH data available.


Related in: MedlinePlus

Gating currents demonstrating immobilization. (A) A model of Shaker gating used to simulate currents (Zagotta et al., 1994a). The transitions from R to A carry the majority of gating charge and the channel opens once all four subunits have transitioned to A. The extra state O:B represents the an intracellular blocking particle as discussed in the text. Ionic (B) and gating (C) currents generated from the model in response to 12 ms depolarization from a holding potential of −100 mV with the program Ionchannellab (De Santiago-Castillo et al., 2010). (D) Gating currents display severe immobilization when the model is run in the presence of an intracellular blocker [B] according to the scheme shown in (A). (E) Cartoon representation of the mechanism by which the N-terminus inactivation peptide or QA ions can cause an immobilization of gating charges. The S1–S3 segments of the VSD is colored blue, S4 shown in purple (resting) or red (activated), and the pore domain S5–S6 grey. At resting negative membrane potentials the pore is closed at the bundle crossing and the S4 charge carrying segments of the VSD are in a resting state. Upon depolarization the S4 segments move outwards into the activated position before the pore opens at the intracellular gate. Once the pore is opened the N-terminus inactivation peptide or QA ions can enter the cavity. The pore is unable to close when these particles in the cavity and has the effect of slowing the voltage sensors return from the activated state.
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Figure 2: Gating currents demonstrating immobilization. (A) A model of Shaker gating used to simulate currents (Zagotta et al., 1994a). The transitions from R to A carry the majority of gating charge and the channel opens once all four subunits have transitioned to A. The extra state O:B represents the an intracellular blocking particle as discussed in the text. Ionic (B) and gating (C) currents generated from the model in response to 12 ms depolarization from a holding potential of −100 mV with the program Ionchannellab (De Santiago-Castillo et al., 2010). (D) Gating currents display severe immobilization when the model is run in the presence of an intracellular blocker [B] according to the scheme shown in (A). (E) Cartoon representation of the mechanism by which the N-terminus inactivation peptide or QA ions can cause an immobilization of gating charges. The S1–S3 segments of the VSD is colored blue, S4 shown in purple (resting) or red (activated), and the pore domain S5–S6 grey. At resting negative membrane potentials the pore is closed at the bundle crossing and the S4 charge carrying segments of the VSD are in a resting state. Upon depolarization the S4 segments move outwards into the activated position before the pore opens at the intracellular gate. Once the pore is opened the N-terminus inactivation peptide or QA ions can enter the cavity. The pore is unable to close when these particles in the cavity and has the effect of slowing the voltage sensors return from the activated state.

Mentions: For the purposes of the review we have used an established model of ShakerIR gating to simulate typical ionic and gating currents from a channel to illustrate typical immobilized gating charge. Figure 2 illustrates currents simulated using an established model of shaker gating (Zagotta et al., 1994a). Figure 2A shows a simplified shaker gating scheme in which voltage sensors in the each four subunits undergo transitions between resting and activated states upon depolarization, followed by a sequential opening step that represents the pore opening. The transitions between resting and active carry the majority of gating charge. In Figure 2B the simulated ionic currents show a rapid increase in the open probability of the channel at depolarizing potentials more positive than −40 mV. The gating currents correlating to the same gating model are displayed in Figure 2C and show a mild immobilization of gating charge return, which is a typical feature of Shaker gating currents. The immobilization is seen in the return of gating charge (OFF gating currents) that display a rapid return after depolarizations to potentials where the channel has not opened (<−40 mV) followed by a slowing of the gating charge return after openings to potentials with significant channel probability of opening (>−20 mV). This slowing of OFF gating currents observed experimentally can be clearly linked to the channel opening process as it occurs only after the channel has opened.


Contributions of intracellular ions to kv channel voltage sensor dynamics.

Goodchild SJ, Fedida D - Front Pharmacol (2012)

Gating currents demonstrating immobilization. (A) A model of Shaker gating used to simulate currents (Zagotta et al., 1994a). The transitions from R to A carry the majority of gating charge and the channel opens once all four subunits have transitioned to A. The extra state O:B represents the an intracellular blocking particle as discussed in the text. Ionic (B) and gating (C) currents generated from the model in response to 12 ms depolarization from a holding potential of −100 mV with the program Ionchannellab (De Santiago-Castillo et al., 2010). (D) Gating currents display severe immobilization when the model is run in the presence of an intracellular blocker [B] according to the scheme shown in (A). (E) Cartoon representation of the mechanism by which the N-terminus inactivation peptide or QA ions can cause an immobilization of gating charges. The S1–S3 segments of the VSD is colored blue, S4 shown in purple (resting) or red (activated), and the pore domain S5–S6 grey. At resting negative membrane potentials the pore is closed at the bundle crossing and the S4 charge carrying segments of the VSD are in a resting state. Upon depolarization the S4 segments move outwards into the activated position before the pore opens at the intracellular gate. Once the pore is opened the N-terminus inactivation peptide or QA ions can enter the cavity. The pore is unable to close when these particles in the cavity and has the effect of slowing the voltage sensors return from the activated state.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3376422&req=5

Figure 2: Gating currents demonstrating immobilization. (A) A model of Shaker gating used to simulate currents (Zagotta et al., 1994a). The transitions from R to A carry the majority of gating charge and the channel opens once all four subunits have transitioned to A. The extra state O:B represents the an intracellular blocking particle as discussed in the text. Ionic (B) and gating (C) currents generated from the model in response to 12 ms depolarization from a holding potential of −100 mV with the program Ionchannellab (De Santiago-Castillo et al., 2010). (D) Gating currents display severe immobilization when the model is run in the presence of an intracellular blocker [B] according to the scheme shown in (A). (E) Cartoon representation of the mechanism by which the N-terminus inactivation peptide or QA ions can cause an immobilization of gating charges. The S1–S3 segments of the VSD is colored blue, S4 shown in purple (resting) or red (activated), and the pore domain S5–S6 grey. At resting negative membrane potentials the pore is closed at the bundle crossing and the S4 charge carrying segments of the VSD are in a resting state. Upon depolarization the S4 segments move outwards into the activated position before the pore opens at the intracellular gate. Once the pore is opened the N-terminus inactivation peptide or QA ions can enter the cavity. The pore is unable to close when these particles in the cavity and has the effect of slowing the voltage sensors return from the activated state.
Mentions: For the purposes of the review we have used an established model of ShakerIR gating to simulate typical ionic and gating currents from a channel to illustrate typical immobilized gating charge. Figure 2 illustrates currents simulated using an established model of shaker gating (Zagotta et al., 1994a). Figure 2A shows a simplified shaker gating scheme in which voltage sensors in the each four subunits undergo transitions between resting and activated states upon depolarization, followed by a sequential opening step that represents the pore opening. The transitions between resting and active carry the majority of gating charge. In Figure 2B the simulated ionic currents show a rapid increase in the open probability of the channel at depolarizing potentials more positive than −40 mV. The gating currents correlating to the same gating model are displayed in Figure 2C and show a mild immobilization of gating charge return, which is a typical feature of Shaker gating currents. The immobilization is seen in the return of gating charge (OFF gating currents) that display a rapid return after depolarizations to potentials where the channel has not opened (<−40 mV) followed by a slowing of the gating charge return after openings to potentials with significant channel probability of opening (>−20 mV). This slowing of OFF gating currents observed experimentally can be clearly linked to the channel opening process as it occurs only after the channel has opened.

Bottom Line: However, several other factors not directly linked to the voltage-dependent movement of charged residues within the voltage sensor impact the dynamics of the voltage sensor, such as inactivation, ionic conductance, intracellular ion identity, and block of the channel by intracellular ligands.There is a significant amount of variability in the reported kinetics of voltage sensor deactivation kinetics of Kv channels attributed to different mechanisms such as open state stabilization, immobilization, and relaxation processes of the voltage sensor.These considerations are of critical importance in understanding the molecular determinants of the complete channel gating cycle from activation to deactivation.

View Article: PubMed Central - PubMed

Affiliation: Department of Anesthesiology, Pharmacology and Therapeutics, University of British Columbia Vancouver, BC, Canada.

ABSTRACT
Voltage-sensing domains (VSDs) of Kv channels control ionic conductance through coupling of the movement of charged residues in the S4 segment to conformational changes at the cytoplasmic region of the pore domain, that allow K(+) ions to flow. Conformational transitions within the VSD are induced by changes in the applied voltage across the membrane field. However, several other factors not directly linked to the voltage-dependent movement of charged residues within the voltage sensor impact the dynamics of the voltage sensor, such as inactivation, ionic conductance, intracellular ion identity, and block of the channel by intracellular ligands. The effect of intracellular ions on voltage sensor dynamics is of importance in the interpretation of gating current measurements and the physiology of pore/voltage sensor coupling. There is a significant amount of variability in the reported kinetics of voltage sensor deactivation kinetics of Kv channels attributed to different mechanisms such as open state stabilization, immobilization, and relaxation processes of the voltage sensor. Here we separate these factors and focus on the causal role that intracellular ions can play in allosterically modulating the dynamics of Kv voltage sensor deactivation kinetics. These considerations are of critical importance in understanding the molecular determinants of the complete channel gating cycle from activation to deactivation.

No MeSH data available.


Related in: MedlinePlus