Balanced Activity between Kv3 and Nav Channels Determines Fast-Spiking in Mammalian Central Neurons

Abstract

Fast-spiking (FS) neurons can fire action potentials (APs) up to 1,000 Hz and play key roles in vital functions such as sound location, motor coordination, and cognition. Here we report that the concerted actions of Kv3 voltage-gated K⁺ (Kv) and Na⁺ (Nav) channels are sufficient and necessary for inducing and maintaining FS. Voltage-clamp analysis revealed a robust correlation between the Kv3/Nav current ratio and FS. Expressing Kv3 channels alone could convert ∼30%-60% slow-spiking (SS) neurons to FS in culture. In contrast, co-expression of either Nav1.2 or Nav1.6 together with Kv3.1 or Kv3.3, but not alone or with Kv1.2, converted SS to FS with 100% efficiency. Furthermore, RNA-sequencing-based genome-wide analysis revealed that the Kv3/Nav ratio and Kv3 expression levels strongly correlated with the maximal AP frequencies. Therefore, FS is established by the properly balanced activities of Kv3 and Nav channels and could be further fine-tuned by channel biophysical features and localization patterns.

Keywords: Biophysics; Molecular Neuroscience; Neuroscience.

Graphical abstract

Figure 1

Kv3 Channels Display Different Efficacies in Converting Young SS Neurons to FS (A) Example AP traces were recorded from pyramidal neuron (left; n = 16), SST+ neuron (middle; n = 15), and PV+ neuron (right; n = 6) in brain slices under current-clamp mode. (B) The voltage-clamp traces recorded from the same neurons in (A). (C) Example traces of APs induced by current injection (105 pA; 1 s) in cultured hippocampal neurons (7 DIV) transfected with different Kv3 constructs. (D) An example trace of APs recorded from older neurons (17 DIV) transfected with Kv3.3. (E) Waveforms of single APs from transfected young neurons. (F–H) Summary of the quantification of AP firing recorded from young neurons (7 DIV) transfected with different Kv3 constructs; after-hyperpolarization (AHP) potential (F), % SS neurons converted to FS (G), and the input-output relationship (H). One-way ANOVA followed by Dunnett's test was used in (F). **p < 0.01.

Figure 2

Biophysical Properties of Four Kv3 Channels Two days after transfection, HEK293 cells that were transfected with Kv3.1b, Kv3.2b, Kv3.3a, or Kv3.4a were recorded with the voltage-clamp mode. (A) Voltage-clamp recording traces. (B) Tail currents of different Kv3 constructs. (C) Summary of current amplitude in HEK293 cells. (D) The conductance-voltage relationship of the four Kv3 channels. (E) Activation time constant (τ_on) at +30 mV. (F) Deactivation time constant (τ_off). (G) The range of Kv3.2b τ_on. (H) The range of Kv3.2b τ_off; n = 25. One-way ANOVA followed by Dunnett's test was used in (E) and (F). **p < 0.01.

Figure 3

Inward and Outward Currents in SS and FS Neurons That Were Transfected with Kv3 Channels (A) Example traces of voltage-clamp recording (within the initial 6 ms) from young neurons (7 DIV) transfected with different Kv3 constructs. (B) Summary of peak I_out and I_in from SS and FS neurons measured from the traces in each voltage command (from −40 to 60 mV with 10-mV increment). (C) The average maximal firing frequencies of SS (n = 41) and FS (n = 38) neurons at 7 DIV. (D) Example traces of voltage-clamp recording (within the initial 6 ms) from old neurons (17 DIV) transfected with different Kv3 constructs. (E) Summary of peak I_out and I_in from SS (n = 40) and FS (n = 37) neurons measured from the traces in each voltage command (from −40 to 60 mV with 10-mV increment). (F) The average maximal firing frequencies of SS and FS neurons at 17 DIV. Unpaired t test was used. *p < 0.05, **p < 0.01.

Figure 4

Concerted Action of Nav and Kv3 Channels Is Completely Sufficient for FS Induction This experiment was carried out in young neurons (7 DIV). (A–D) Example traces of current-clamp (CC, left) and voltage-clamp (VC, right) recordings that were carried out in neurons transfected with Nav1.2 alone (A), or plus Kv3.1b (B), Kv3.3 (C), or YFP-Kv1.2 (D). (E–H) Example traces of current-clamp (CC, left) and voltage-clamp (VC, right) recordings that were carried out in neurons transfected with Nav1.6 alone (E), or plus Kv3.1b (F), Kv3.3 (G), or YFP-Kv1.2 (H). (I) The input-output relationship of neurons transfected with Nav1.2 and others. (J) The input-output relationship of neurons transfected with Nav1.6 and others. (K) Summary of percentage of neurons converted into FS under different conditions.

Figure 5

Subcellular Localization Patterns of Nav and Different Kv3 Channels (A and B) In young neurons (7 DIV), localization of surface Kv3.1aHA (A) and Kv3.1bHA (B) versus endogenous pan-Nav channels was examined. Anti-HA staining was performed under non-permeabilized condition. The AIS (red arrows) is shown in the insets at the top right corners. (C) Expression of Kv3.2b (green), Kv3.3 (green), and Kv3.4aHA (green) in young neurons differentially changed the endogenous level of Nav1.2 (red). Red arrowheads, transfected neurons; white arrowheads, untransfected neurons. (D) Summary of the effects of Kv3 channel expression on endogenous Nav1.2 level at soma. 1, Kv3.1bHA (10); 2, Kv3.2b (9); 3, Kv3.3 (11); 4, Kv3.4aHA (10). One-way ANOVA followed by Dunnett's test: *p < 0.05. (E–H) In old neurons (17 DIV), localization of expressed Kv3.1bHA (E), Kv3.2b (F), Kv3.3 (G), and Kv3.4aHA (H) versus endogenous pan-Nav channels was examined.

Figure 6

Knocking Down Ankyrin-G Significantly Altered the Balance of Nav and Kv3.1 Channels, Leading to Disrupted FS (A) Example traces of current- (top) and voltage- (bottom) clamp recording of neurons transfected with YFP alone, Nav1.6 + Kv3.1b, or Nav1.6 + Kv3.1b + AnkGsiR. (B) The input-output relationship. (C) Summary of AP amplitude. (D) Summary of AP duration. (E) Extended current traces of voltage-clamp recording (the initial 6 ms) at the command potentials of 0 mV (left) and +40 mV (right). (F) Summary of inward and outward currents under three different conditions. (G) Localization of Nav1.6 and Kv3.1bHA in neurons with (top) and without (bottom) AnkGSiR. (H) Summary of Nav1.6 and Kv3.1bHA staining intensities at the AIS. Unpaired t test was used. *p < 0.05, **p < 0.01, ***p < 0.001.

Figure 7

The mRNA Ratio between Kv3 and Nav1 Channels Best Correlates with the Firing Rate among all K⁺ Channel Subfamilies RNA-seq datasets were obtained from the Allen Institute for Brain Science. (A) Correlation between Kv1 and Nav1 channels in 14 different types of neurons. The FPKM values of all the isoforms of Kv1 channel subfamily were added up with equal weight to represent the overall Kv1 channel expression. The Fragments Per Kilobase of transcript per Million mapped reads (FPKM) values of all the isoforms of Nav1 channels were added up to represent the overall Nav1 channel expression. (B) Correlation between Kv3 and Nav1 channels. The FPKM values of all the isoforms (Kv3.1–Kv3.4) of Kv3 channel subfamily were added up to represent the overall Kv3 channel expression. (C) Correlation between Kv4 and Nav1 channels. (D) Correlation between Kv7 and Nav1 channels. High-firing neurons are shown in red, medium-firing neurons in yellow, and low-firing neurons in black. (E) Correlations between the K⁺/Nav1 ratio and the firing rate in different neurons. (F) Correlations between the K⁺ channel subfamilies and Nav1 channels in different neurons, regardless of the firing rate. In (E) and (F), each color indicates a channel subfamily.

Figure 8

Correlation of the Ratio between Individual K⁺ and Nav1 Channels with the Firing Frequency RNA-seq datasets were obtained from the Allen Institute for Brain Science. (A) The heatmap shows correlation of the ratios between individual K⁺ and Nav1 channels with the firing frequency. Positive correlations are shown in red, whereas negative correlations are shown in green. Two best correlations, Kv3.1 and Kv3.2, are indicated with “*.” The second-tier genes of good correlations are indicated with black arrowheads. (B) mRNA levels of individual Nav1 and Kv(1–4) channels in neurons with high (top) and low (bottom) firing rate. (C) mRNA levels of Navβ subunits, including Navβ4, in neurons with high, medium and low firing rates.