Dravet Variant SCN1AA1783V Impairs Interneuron Firing Predominantly by Altered Channel Activation

Figure 1. Biophysical properties of SCN1A wild type and mutant NaV1.1A1783V channels recorded in tsA201 cells display a clear loss of function. (A) Representative current traces were recorded from tsA201 cells transfected with NaV1.1WT (black traces) or NaV1.1A1783V (blue traces). (B) Peak Na+ currents normalized by cell capacitance were plotted vs. command voltage and revealed no changes between wild-type (WT) and mutant NaV1.1A1783V channels. (C, right) Voltage-dependence of steady-state activation curves represented as normalized conductance fit by a Boltzmann function to the data points (lines). (C, left curve) Voltage-dependent state of fast inactivation fit by a Boltzmann function to the data points (lines). (D) Time course of recovery from fast inactivation at −100 mV. (E) Voltage-dependence of the time constant of fast inactivation τh analyzed by a one exponential fit to inactivating phase of the Na+ currents. (G) Entry into slow inactivation is represented as a first-order exponential fit to the data points. (H) Voltage-dependent steady-state of slow inactivation fit by a Boltzmann function as in (C). (F) Na-current use dependence evoked by 50 pulses at a frequency of 40 Hz as…

Figure 2. In silico modeling of neuron excitability. (A) Action potentials of simulated cortical fast-spiking interneurons (upper traces) and cortical pyramidal cells (PCs) (lower traces) in response to square pulse current injections of 320 pA with various modifications of NaV1.1 current kinetics based on findings in tsA recordings. Colors indicate different model conditions: WT (black), the heterozygous shift of the voltage-dependence of steady-state activation (orange), the heterozygous shift of the voltage-dependence of the steady-state slow inactivation (green), the combination of both changes (blue), a heterozygous protein knockout (red), and a combination of respective changes in sodium current and a decreased input resistance of the cell (brown). The exact parameters for all simulated conditions are summarized in Table 1. (B) f−I curves of interneurons (left) and PCs (right) constructed from simulation studies displayed in (A). (C–G) Time courses of model variables during the first action potential after a 300 pA current injection, t = 0 indicates the peak of the action potential. (C) Membrane voltage; (D) activation gate m—ratio of activated sodium channels; (E) absolute INa; (F)…

Figure 3. In vitro studies of neuronal excitability. (A) Representative AP trains in response to 800 ms step current injections from cortical PCs and (C) fast-spiking interneurons recorded in brain slice cultures of WT (black traces) and heterozygous NaV1.1+/A1783V animals (blue traces). (B) f−I curves of PCs from floxed heterozygous mice showed the same frequency of action potential firing while (D) fast-spiking inhibitory neurons showed higher and maintained lower AP frequencies compared to cells of WT animals. A number of recorded cells, exact values of statistically analyzed parameters, and p-values are listed in Supplementary Tables 2, 3.