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. 2014 Jul 16;3:e03255.
doi: 10.7554/eLife.03255.

High Temperature Sensitivity Is Intrinsic to Voltage-Gated Potassium Channels

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Free PMC article

High Temperature Sensitivity Is Intrinsic to Voltage-Gated Potassium Channels

Fan Yang et al. Elife. .
Free PMC article

Abstract

Temperature-sensitive transient receptor potential (TRP) ion channels are members of the large tetrameric cation channels superfamily but are considered to be uniquely sensitive to heat, which has been presumed to be due to the existence of an unidentified temperature-sensing domain. Here we report that the homologous voltage-gated potassium (Kv) channels also exhibit high temperature sensitivity comparable to that of TRPV1, which is detectable under specific conditions when the voltage sensor is functionally decoupled from the activation gate through either intrinsic mechanisms or mutations. Interestingly, mutations could tune Shaker channel to be either heat-activated or heat-deactivated. Therefore, high temperature sensitivity is intrinsic to both TRP and Kv channels. Our findings suggest important physiological roles of heat-induced variation in Kv channel activities. Mechanistically our findings indicate that temperature-sensing TRP channels may not contain a specialized heat-sensor domain; instead, non-obligatory allosteric gating permits the intrinsic heat sensitivity to drive channel activation, allowing temperature-sensitive TRP channels to function as polymodal nociceptors.

Keywords: TRP channel; allosteric coupling; conformational changes; gating; potassium channel; temperature sensing.

Conflict of interest statement

The authors declare that no competing interests exist.

Figures

Figure 1.
Figure 1.. Heat sensitivity of TRPV1 and Shaker channel exhibits distinct voltage dependence.
(A) TRPV1 current was elicited by a voltage ramp at varying temperatures (upper panel). Q10, quantified between 36.6°C and 46.0°C, remains high across the entire voltage range from −100 mV to +150 mV (lower panel). (B) Shaker channels exhibit a low heat sensitivity at most voltages as quantified by Q10 between 22.4°C and 27.7°C. However, Q10 transiently peaks (red arrow) around the voltage (−70 mV) where the channel just starts to open. Simplified gating schemes involving two transitions are shown on the bottom. The C→C′ transition is weakly voltage-dependent for TRPV1 but highly voltage-dependent for Shaker. (C) TRPV1 and Shaker channels show distinct voltage-dependent activation behaviors. TRPV1 conductance (open circle) has a shallow voltage dependence (with an apparent gating charge of 0.76 ± 0.06 e0, n = 4) that occurs in a highly depolarized range (V1/2 = 114.9 ± 10.9 mV, n = 4). Shaker activation has a steep voltage dependence (with an apparent gating charge 5.1 ± 1.3 e0, n = 5) that occurs at hyperpolarized voltages (V1/2 = −54.2 ± 4.7 mV, n = 5). The Q-V curve for Shaker (dotted curve) is reproduced from a published study (Schoppa and Sigworth, 1998). DOI: http://dx.doi.org/10.7554/eLife.03255.003
Figure 2.
Figure 2.. Voltage dependence of BK channel temperature sensitivity shifts with intracellular Ca2+.
Q10 (blue trace, right axis) calculated from temperature-dependent changes in the G-V curves at labeled temperatures (left axis) measured in the absence of Ca2+ (lower panel) are larger in value and shifted more to the right in voltage dependence compared to those measured in presence of saturating Ca2+ (200 μM, upper panel). DOI: http://dx.doi.org/10.7554/eLife.03255.004
Figure 3.
Figure 3.. Both Kv2.1 and Kv4.3 channels are highly heat-sensitive within a narrow voltage range.
(A) and (E) Large increases in current were observed from both Kv2.1 and Kv4.3 when temperature was raised. (B) Kv2.1 channels were substantially activated by heat at −50 mV (filled circles) but not at −80 mV (open circles). (C) Representative current traces at time points shown in (B). (F) and (G) Similar behaviors as Kv2.1 were observed with Kv4.3. (D) and (H) Voltage dependence of G-V curves (left axis) and Q10 (right axis) for Kv2.1 (D) and Kv4.3 (H). G-V curves are color-coded as in (A) and (E), respectively. Q10 peaks in the range where the channel just starts to be voltage-activated. DOI: http://dx.doi.org/10.7554/eLife.03255.005
Figure 3—figure supplement 1.
Figure 3—figure supplement 1.. High heat sensitivity of Kv2.1 channels was robustly observed when the length of voltage ramp and extracellular K+ concentration were changed.
(A) The length of voltage ramp from −100 mV to +50 mV was increased from 200 ms to 1 s. With this slowed voltage ramp large increases in current were still observed from Kv2.1 when temperature was raised. (B) Voltage dependence of G-V curves (left axis) and Q10 (right axis) for Kv2.1 with the slow voltage ramp. (C) 130 mM extracellular K+ was replaced with 130 mM Na+. Large increases in current were still observed from Kv2.1 when temperature was raised. (D) Voltage dependence of G-V curves (left axis) and Q10 (right axis) for Kv2.1 with 130 mM extracellular Na+. DOI: http://dx.doi.org/10.7554/eLife.03255.006
Figure 4.
Figure 4.. Heat-induced shift in V1/2 is not responsible for high temperature sensitivity.
(A) Changes in V1/2 were induced by raising temperature from the room temperature to a level that maximally activated the channels (n = 3-to-4). (B) Corresponding changes in free energy were calculated as zFΔV1/2, where z is the total gating charge, F is Faraday's constant. n = 3-to-4. Values of z for Kv channels are based on published work (Schoppa and Sigworth, 1998; Islas and Sigworth, 1999; Dougherty and Covarrubias, 2006). **p<0.01; ***p<0.001; n.s., no significance. (C) Q10 value of each channel type was plotted against the temperature-induced shift in V1/2. At similar levels of shift in V1/2, both large and small Q10 values were observed, suggesting that shift in V1/2 is not responsible for high temperature sensitivity. n = 3-to-4. DOI: http://dx.doi.org/10.7554/eLife.03255.007
Figure 5.
Figure 5.. Closed state inactivation (CSI) and channel activation overlap within specific voltage ranges.
(A) Representative current recordings of Kv2.1 CSI. Length of the P2 segment was 10 s. Current measured at P3 was normalized to that measured at P1. The normalized current was then plotted against P2 voltage for voltage dependence of CSI. (B) For Kv2.1, the voltage dependence of both CSI (open squares) and activation (filled circles) follows a single-Boltzmann function (left axis). After normalization the two Boltzmann functions were multiplied to yield the red curve (right axis). This combined probability curve surges in the voltage range where the channels are only slightly activated while significant CSI occurs, which overlaps with the voltage range where high heat sensitivity is observed. (C) Similar voltage dependences of CSI and activation were observed for Kv4.3. n = 3-to-5. DOI: http://dx.doi.org/10.7554/eLife.03255.008
Figure 6.
Figure 6.. Shaker ILT and V2 mutants exhibit large but opposite heat responses.
(A) and (F) Conductance-voltage (G-V; n = 3-to-6) and gating charge-voltage (Q-V) curves for ILT (A) and V2 (F). Q-V curves for ILT and V2 are reproduced from published studies (Schoppa and Sigworth, 1998; Ledwell and Aldrich, 1999). (B) Large increases in current are observed from ILT when temperature was raised. To increase current amplitude when driving voltage was small, extracellular solution contained 130 mM KCl, while intracellular solution contained 130 mM NaCl. (C) Voltage dependence of Q10 for ILT based on G-V curves derived from (B). (D) and (E) ILT channels were substantially activated by heat at +40 mV. (G) In contrast to the ILT current, V2 current decreases upon temperature rise. (H) Voltage dependence of Q10 for V2 based on G-V curves derived from (G). Q10 drops below 1 when voltage reaches a level that the channel starts to open, as increasing temperature deactivates the channel. (I) and (J) V2 channels were substantially deactivated by heat at +20 mV. DOI: http://dx.doi.org/10.7554/eLife.03255.009
Figure 7.
Figure 7.. Comparison of Q10 (A) and thermodynamic measurements (B) between TRPV1, Kv and CLC-0 channels.
Data for CLC-0 are reproduced from published results (Yang et al., 2010). n = 3-to-6; *p<0.05; **p<0.01; ***p<0.001; n.s., no significance. DOI: http://dx.doi.org/10.7554/eLife.03255.010
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