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. 2000 Dec 15;529 Pt 3(Pt 3):533-9.
doi: 10.1111/j.1469-7793.2000.00533.x.

A sodium channel mutation causing epilepsy in man exhibits subtle defects in fast inactivation and activation in vitro

A Alekov  1 , M M Rahman, N Mitrovic, F Lehmann-Horn, H Lerche
Affiliations

A sodium channel mutation causing epilepsy in man exhibits subtle defects in fast inactivation and activation in vitro

A Alekov et al. J Physiol. .

Abstract

Generalized epilepsy with febrile seizures plus (GEFS+) is a benign epileptic syndrome of humans. It is characterized by febrile and afebrile generalized seizures that occur predominantly in childhood and respond well to standard antiepileptic therapy. A mutation in the b1-subunit of the voltage-gated sodium channel, linked to chromosome 19q13 (GEFS+ type 1) has been found in one family. For four other families, linkage was found to chromosome 2q21-33 (GEFS+ type 2) where three genes encoding neuronal sodium channel a-subunits are located (SCN1-3A). Recently, the first two mutations were identified in SCN1A. We introduced one of these mutations, which is highly conserved to SCN1A, into the cDNA of the gene SCN4A encoding the a-subunit of the human skeletal muscle sodium channel (hSkm1). The mutation is located in the S4 voltage sensor of domain IV, predicting substitution of histidine for the fifth of eight arginines (R1460H in hSkm1). Functional studies were performed by expressing the a-subunit alone in the mammalian tsA201 cell line using the whole-cell patch clamp technique. Compared to wild-type (WT), mutant R1460H channels showed small defects in fast inactivation. The time course of inactivation was slightly (1.5-fold) slowed and its voltage dependence reduced, and recovery from inactivation was accelerated 3-fold. However, there was no increase in persistent sodium current as observed for SCN4A mutations causing myotonia or periodic paralysis. The activation time course of R1460H channels was slightly accelerated. Slow inactivation was slightly but significantly stabilized, confirming the importance of this region for slow inactivation. The combination of activation and fast inactivation defects can explain the occurrence of epileptic seizures, but the effects were much more subtle than the inactivation defects described previously for mutations in SCN4A causing disease in skeletal muscle. Hence, with regard to pathological excitability, our results suggest a greater vulnerability of the central nervous system compared to muscle tissue.

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Figures

Figure 1
Figure 1. Mutation R1460H in segment IV/S4
A, amino acid sequence comparison of the IV/S4 segment of various sodium channel α-subunits derived from different genes. Sequence changes are underlined. Residue R1648 (SCN1A numbering) corresponding to R1460 (SCN4A numbering) is marked by an arrow. B, representative whole-cell sodium current families recorded from cells transfected with either WT or mutant channel cDNA.
Figure 2
Figure 2. Parameters of fast inactivation
A, voltage dependence of the inactivation time constant, τh. Values at 0 mV were 0.28 ± 0.01 for WT vs. 0.42 ± 0.03 ms for R1460H; n = 8, 14; P < 0.001. B, recovery from inactivation at -100 mV. Lines are fits to a first order exponential function with recovery time constants (τrec) of 13.7 ± 1.9 vs. 4.4 ± 0.3 ms; n = 7, 11; P < 0.001; and an initial delay of 1.0 ± 0.1 vs. 0.59 ± 0.04 ms, P < 0.001, for WT and R1460H, respectively. The inset shows the voltage dependence of τrec, n = 4-11. C, steady-state inactivation was determined using 300 ms prepulses to the potentials indicated, followed by a short test pulse to -20 mV. Lines are fits to a standard Boltzmann function: I/Imax= 1/(1 + exp[(V–V0.5)/kV]), where V0.5 is the voltage of half-maximal inactivation and kV is a slope factor. V0.5 was -89.0 ± 1.4 vs. -96.0 ± 1.6 mV; n = 7, 16; P < 0.02; kV was 5.5 ± 0.3 vs. 7.2 ± 0.2 mV, P < 0.001, for WT and R1460H, respectively.
Figure 3
Figure 3. Activation and deactivation parameters
A, 10-90% rise time of the sodium current as a function of test potential. The differences between mutant and WT channels over the range -52.5 to -30 mV are statistically significant at P < 0.05; n = 7, 8; T= 15°C. B, in order to measure deactivation, a short depolarizing pulse (0.5 to -10 mV) was followed by the test pulse to the indicated potentials. The deactivation time constant, τdeact, was obtained from a first order exponential fit to the tail currents; n = 4, P > 0.05, T= 15°C. C, voltage dependence of activation for WT and mutant sodium channels, obtained by 25 ms depolarizing pulses to the indicated potentials from a holding potential of -140 mV. Lines are fits to a standard Boltzmann function:V0.5 was -45.3 ± 1.9 vs. -44.4 ± 2.1 mV; n = 7, 6; P > 0.05; kV was 6.2 ± 0.3 vs. 7.8 ± 0.2 mV, P < 0.01, for WT and R1460H, respectively; T= 22°C.
Figure 4
Figure 4. Parameters of slow inactivation
A, entry into slow inactivation at 0 mV. Cells were held at -100 mV, depolarized to 0 mV for increasing durations as indicated on the abscissa, repolarized for 100 ms to -100 mV to let the channels recover from fast inactivation and then depolarized again to -10 mV to determine the fraction of slow inactivated channels. The lines represent fits to a first order exponential function with the following time constants: 1.9 ± 0.3 for WT vs. 2.2 ± 0.1 s for R1460H; n = 3, 6; P > 0.05. B, recovery from slow inactivation measured at -100 mV after a 30 s conditioning pulse to 0 mV. Curves were best fitted to a second order exponential function with the following slow recovery time constants: τsrec1= 0.31 ± 0.11 vs. 0.46 ± 0.06 s, τsrec2= 4.2 ± 1.8 vs. 7.5 ± 1.3 s; relative amplitude of τsrec1= 54 ± 1 vs. 56 ± 4%; n = 3, 5; P > 0.05, for WT and R1460H, respectively. C, steady-state slow inactivation was determined using 30 s prepulses to potentials indicated on the abscissa, followed by a 20 ms repolarizing pulse to the holding potential of -140 mV to let the channels recover from fast inactivation, and a short test pulse to -20 mV. The data were fitted to a standard Boltzmann function: V0.5= -73.0 ± 1.1 for WT vs. -80.9 ± 2.1 mV for R1460H, n = 4, P < 0.02.
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References

    1. Baulac S, Gourfinkel-An I, Picard F, Rosenberg-Bourgin M, Prud’homme J-F, Baulac M, Brice A, Leguern E. A second locus for familial generalized epilepsy with febrile seizures plus maps to chromosome 2q21-q33. American Journal of Human Genetics. 1999;65:1078–1085. - PMC - PubMed
    1. Biervert C, Schroeder BC, Kubisch C, Berkovic SF, Propping P, Jentsch TJ, Steinlein OK. A potassium channel mutation in neonatal human epilepsy. Science. 1998;279:403–406. - PubMed
    1. Cannon SC. From mutation to myotonia in sodium channel disorders. Neuromuscular Disorders. 1997;7:241–249. - PubMed
    1. Catterall WA. From ionic currents to molecular mechanisms: the structure and function of voltage-gated sodium channels. Neuron. 2000;26:13–25. - PubMed
    1. Cha A, Ruben PC, George AL, Jr, Fujimoto E, Bezanilla F. Voltage sensors in domains III and IV, but not I and II, are immobilized by Na+ channel fast inactivation. Neuron. 1999;22:73–87. - PubMed

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