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Modeling interactions between voltage-gated Ca (2+) channels and KCa1.1 channels.

Engbers JD, Zamponi GW, Turner RW - Channels (Austin) (2013)

Bottom Line: We recently found that low voltage-activated Cav3 calcium channels also create KCa1.1-Cav3 complexes.Combined with the effect of EGTA, these results suggest that the Ca (2+) domains of several KCa1.1-Cav3 complexes need to cooperate to generate sufficient [Ca (2+)]i, despite the physical association between KCa1.1 and Cav3 channels.By comparison, Cav2.2 channels were twice as effective at activating KCa1.1 channels and a single KCa1.1-Cav2.2 complex would be self-sufficient.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology & Anatomy; Hotchkiss Brain Institute; University of Calgary; Calgary, Canada.

ABSTRACT
High voltage-activated (HVA) Cav channels form complexes with KCa1.1 channels, allowing reliable activation of KCa1.1 current through a nanodomain interaction. We recently found that low voltage-activated Cav3 calcium channels also create KCa1.1-Cav3 complexes. While coimmunoprecipitation studies again supported a nanodomain interaction, the sensitivity to calcium chelating agents was instead consistent with a microdomain interaction. A computational model of the KCa1.1-Cav3 complex suggested that multiple Cav3 channels were necessary to activate KCa1.1 channels, potentially causing the KCa1.1-Cav3 complex to be more susceptible to calcium chelators. Here, we expanded the model and compared it to a KCa1.1-Cav2.2 model to examine the role of Cav channel conductance and kinetics on KCa1.1 activation. As found for direct recordings, the voltage-dependent and kinetic properties of Cav3 channels were reflected in the activation of KCa1.1 current, including transient activation from lower voltages than other KCa1.1-Cav complexes. Substantial activation of KCa1.1 channels required the concerted activity of several Cav3.2 channels. Combined with the effect of EGTA, these results suggest that the Ca (2+) domains of several KCa1.1-Cav3 complexes need to cooperate to generate sufficient [Ca (2+)]i, despite the physical association between KCa1.1 and Cav3 channels. By comparison, Cav2.2 channels were twice as effective at activating KCa1.1 channels and a single KCa1.1-Cav2.2 complex would be self-sufficient. However, even though Cav3 channels generate small, transient currents, the regulation of KCa1.1 activity by Cav3 channels is possible if multiple complexes cooperate through microdomain interactions.

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Figure 3. Temporal properties of Cav-KCa1.1 currents. (A) A KCa1.1-Cav3.2(4×; 20 nm) complex generates a transient current which inactivates within 100 ms. KCa1.1 activation can be observed for voltages over -60 mV. (B and C) KCa1.1-Cav2.2 complexes (1 or 4 channels; 20 nm) generate long-lasting KCa1.1 activation with slow inactivation kinetics. When 4 channels are included in the model, the rate of inactivation of KCa1.1 is slowed for depolarized voltages. Significant KCa1.1 activation can only be seen beyond -40 mV, as expected for a K+ current that follows the voltage-dependence of the HVA Ca2+ source.
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Figure 3: Figure 3. Temporal properties of Cav-KCa1.1 currents. (A) A KCa1.1-Cav3.2(4×; 20 nm) complex generates a transient current which inactivates within 100 ms. KCa1.1 activation can be observed for voltages over -60 mV. (B and C) KCa1.1-Cav2.2 complexes (1 or 4 channels; 20 nm) generate long-lasting KCa1.1 activation with slow inactivation kinetics. When 4 channels are included in the model, the rate of inactivation of KCa1.1 is slowed for depolarized voltages. Significant KCa1.1 activation can only be seen beyond -40 mV, as expected for a K+ current that follows the voltage-dependence of the HVA Ca2+ source.

Mentions: Cav3.2 and Cav2.2 channels differ significantly in their kinetic properties. Cav3.2 channels show fast activation and a short time constant of inactivation while Cav2.2 channels show inactivation, but to a lesser extent and over a greater timeframe. A series of voltage steps provided to the KCa1.1-Cav3.2(4×; 20 nm) model revealed a KCa1.1 channel current that also showed fast activation followed by rapid inactivation within 100 ms. This is important in confirming that the properties of the Cav3 channel were conferred to the KCa1.1 channel (Fig. 3A), as found in MVN neuron and tsA-210 cell recordings.4 On the other hand, the KCa1.1-Cav2.2(1×; 20 nm) model showed slower inactivation of K+ current with significant activation still observed after 300 ms (Fig. 3B). As previously reported, no significant activation was detected below -30 mV for the KCa1.1-Cav2.2(1×; 20 nm) complex, again confirming direct recordings.15 Increasing the number of Cav2.2 channels resulted in a slowing of inactivation for the KCa1.1-Cav2.2 complex, which was not seen for an increased number of Cav3.2 channels (Fig. 3C). This is due to the larger conductance of Cav2.2 channels and build-up of [Ca2+]i with a large number of Cav2.2 channels.


Modeling interactions between voltage-gated Ca (2+) channels and KCa1.1 channels.

Engbers JD, Zamponi GW, Turner RW - Channels (Austin) (2013)

Figure 3. Temporal properties of Cav-KCa1.1 currents. (A) A KCa1.1-Cav3.2(4×; 20 nm) complex generates a transient current which inactivates within 100 ms. KCa1.1 activation can be observed for voltages over -60 mV. (B and C) KCa1.1-Cav2.2 complexes (1 or 4 channels; 20 nm) generate long-lasting KCa1.1 activation with slow inactivation kinetics. When 4 channels are included in the model, the rate of inactivation of KCa1.1 is slowed for depolarized voltages. Significant KCa1.1 activation can only be seen beyond -40 mV, as expected for a K+ current that follows the voltage-dependence of the HVA Ca2+ source.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
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Figure 3: Figure 3. Temporal properties of Cav-KCa1.1 currents. (A) A KCa1.1-Cav3.2(4×; 20 nm) complex generates a transient current which inactivates within 100 ms. KCa1.1 activation can be observed for voltages over -60 mV. (B and C) KCa1.1-Cav2.2 complexes (1 or 4 channels; 20 nm) generate long-lasting KCa1.1 activation with slow inactivation kinetics. When 4 channels are included in the model, the rate of inactivation of KCa1.1 is slowed for depolarized voltages. Significant KCa1.1 activation can only be seen beyond -40 mV, as expected for a K+ current that follows the voltage-dependence of the HVA Ca2+ source.
Mentions: Cav3.2 and Cav2.2 channels differ significantly in their kinetic properties. Cav3.2 channels show fast activation and a short time constant of inactivation while Cav2.2 channels show inactivation, but to a lesser extent and over a greater timeframe. A series of voltage steps provided to the KCa1.1-Cav3.2(4×; 20 nm) model revealed a KCa1.1 channel current that also showed fast activation followed by rapid inactivation within 100 ms. This is important in confirming that the properties of the Cav3 channel were conferred to the KCa1.1 channel (Fig. 3A), as found in MVN neuron and tsA-210 cell recordings.4 On the other hand, the KCa1.1-Cav2.2(1×; 20 nm) model showed slower inactivation of K+ current with significant activation still observed after 300 ms (Fig. 3B). As previously reported, no significant activation was detected below -30 mV for the KCa1.1-Cav2.2(1×; 20 nm) complex, again confirming direct recordings.15 Increasing the number of Cav2.2 channels resulted in a slowing of inactivation for the KCa1.1-Cav2.2 complex, which was not seen for an increased number of Cav3.2 channels (Fig. 3C). This is due to the larger conductance of Cav2.2 channels and build-up of [Ca2+]i with a large number of Cav2.2 channels.

Bottom Line: We recently found that low voltage-activated Cav3 calcium channels also create KCa1.1-Cav3 complexes.Combined with the effect of EGTA, these results suggest that the Ca (2+) domains of several KCa1.1-Cav3 complexes need to cooperate to generate sufficient [Ca (2+)]i, despite the physical association between KCa1.1 and Cav3 channels.By comparison, Cav2.2 channels were twice as effective at activating KCa1.1 channels and a single KCa1.1-Cav2.2 complex would be self-sufficient.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology & Anatomy; Hotchkiss Brain Institute; University of Calgary; Calgary, Canada.

ABSTRACT
High voltage-activated (HVA) Cav channels form complexes with KCa1.1 channels, allowing reliable activation of KCa1.1 current through a nanodomain interaction. We recently found that low voltage-activated Cav3 calcium channels also create KCa1.1-Cav3 complexes. While coimmunoprecipitation studies again supported a nanodomain interaction, the sensitivity to calcium chelating agents was instead consistent with a microdomain interaction. A computational model of the KCa1.1-Cav3 complex suggested that multiple Cav3 channels were necessary to activate KCa1.1 channels, potentially causing the KCa1.1-Cav3 complex to be more susceptible to calcium chelators. Here, we expanded the model and compared it to a KCa1.1-Cav2.2 model to examine the role of Cav channel conductance and kinetics on KCa1.1 activation. As found for direct recordings, the voltage-dependent and kinetic properties of Cav3 channels were reflected in the activation of KCa1.1 current, including transient activation from lower voltages than other KCa1.1-Cav complexes. Substantial activation of KCa1.1 channels required the concerted activity of several Cav3.2 channels. Combined with the effect of EGTA, these results suggest that the Ca (2+) domains of several KCa1.1-Cav3 complexes need to cooperate to generate sufficient [Ca (2+)]i, despite the physical association between KCa1.1 and Cav3 channels. By comparison, Cav2.2 channels were twice as effective at activating KCa1.1 channels and a single KCa1.1-Cav2.2 complex would be self-sufficient. However, even though Cav3 channels generate small, transient currents, the regulation of KCa1.1 activity by Cav3 channels is possible if multiple complexes cooperate through microdomain interactions.

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