Intrinsic membrane hyperexcitability of amyotrophic lateral sclerosis patient-derived motor neurons

Abstract

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease of the motor nervous system. We show using multielectrode array and patch-clamp recordings that hyperexcitability detected by clinical neurophysiological studies of ALS patients is recapitulated in induced pluripotent stem cell-derived motor neurons from ALS patients harboring superoxide dismutase 1 (SOD1), C9orf72, and fused-in-sarcoma mutations. Motor neurons produced from a genetically corrected but otherwise isogenic SOD1(+/+) stem cell line do not display the hyperexcitability phenotype. SOD1(A4V/+) ALS patient-derived motor neurons have reduced delayed-rectifier potassium current amplitudes relative to control-derived motor neurons, a deficit that may underlie their hyperexcitability. The Kv7 channel activator retigabine both blocks the hyperexcitability and improves motor neuron survival in vitro when tested in SOD1 mutant ALS cases. Therefore, electrophysiological characterization of human stem cell-derived neurons can reveal disease-related mechanisms and identify therapeutic candidates.

Figure 1. Multi-Electrode Array (MEA) Recording Reveals Increased Spontaneous Firing in ALS-Derived Neurons Compared to Control-Derived Neurons

(A) Schematic of differentiation and recording. (B) Representative recordings from 4 out of 64 MEA electrodes in control (11a, 18a) and ALS (39b, RB9d)-derived neurons cultured for 28 days on the arrays. (C) Total action potential firing rate during one minute of recording from MEAs (11a, n=3; 18a, n=3; control mean 6,510 ± 3,131 spikes/minute; 39b, n=3; RB9d, n=3; ALS mean 20,528 ± 5,069 spikes/minute; p<0.05, t-test). (D) Mean firing rate histograms of individual neurons from MEAs in B. See also Figure S1. (E) Average of mean firing rate for patient-derived neurons (11a, n=381; 18a, n=191; control mean 1.17 ± 0.04 Hz; 39b, n= 520; RB9d, n=662; ALS mean 1.76 ± 0.05 Hz; p<10⁻¹⁵, t-test). (F) Total action potential firing rate during one minute recordings from MEAs of FACS-sorted 18a Hb9::GFP and 39b Hb9::GFP motor neurons recorded every four days (repeated measures ANOVA F-test p=1×10⁻⁴ for difference between lines; post-hoc t-tests with Bonferroni correction for multiple testing indicated as * for p<0.05 and ** for p<0.01). See also Figures S2–S3. (G) Total action potential firing rate during one minute of recording from MEAs cultured for 14 days on the arrays (39b-Cor, n=4; mean 775 ± 712 spikes/minute; 39b, n=4; 39b mean 6,278 ± 1,758 spikes/minute; p=0.01, t-test).

Figure 2. ALS Patient-Derived Motor Neurons are Hyperexcitable and Have Reduced Delayed-Rectifier Potassium Currents Compared to Control-Derived Motor Neurons

(A) An iPSC-derived motor neuron identified by Hb9::RFP lentiviral transduction (right) and during patch clamp recording (left) after culture for 28 days. Scale bar 20 μm. (B) Representative current clamp recordings during ramp depolarization from control and ALS patient-derived motor neurons (upper four panels); sample recordings from separate experiments comparing the isogenic correction of the 39b SOD1^A4V mutation (39b-Cor) and 39b (lower two panels). (C) Upper panel: Average number of action potentials elicited by ramp depolarization from control (11a, n=12; 18a, n=11; control mean 2.5 ± 0.4) and ALS (39b, n=13; RB9d, n=12; ALS mean 4.2 ± 0.5) motor neurons obtained from four separate differentiations (p<0.05, Mann-Whitney U test). Lower panel: Separate experiments showing average number of action potentials during ramp depolarization from 39b-Cor (n=17; mean 4.1 ± 0.5) and 39b (n= 19; mean 6.4 ± 0.9) motor neurons from three additional differentiations (p<0.05, Mann-Whitney U test). (D) Sample voltage clamp recordings from control and ALS-derived Hb9::RFP-positive motor neurons cultured for 28 days. (E) Average delayed-rectifier (DR) steady-state potassium current amplitude relative to peak sodium current amplitude in control (11a, n=12; 18a, n=11; control mean 0.88 ± 0.087) and ALS (39b, n=13; RB9d, n=12; ALS mean 0.44 ± 0.054) patient-derived motor neurons from four differentiations (p<0.001, t-test). (F) Experiments from three separate differentiations showing average delayed-rectifier steady-state potassium current amplitude relative to peak sodium current amplitude in 39b-Cor (n=18; mean 0.54 ± 0.061) and 39b (n=19; mean 0.32 ± 0.036; p<0.005, t-test). (G) Direct measurement of delayed-rectifier voltage-gated potassium current isolated by holding at −30 mV, stepping to a test-potential of +40 mV for 2 s and normalizing steady state current amplitude to cell capacitance in 39b-Cor (n=19; mean 42.6 ± 4.3 pA/pF) and 39b (n=18; mean 30.3 ± 3.1 pA/pF; p<0.05, t-test) derived motor neurons using cells from two additional separate differentiations. (H) Peak sodium current amplitude normalized to cell capacitance in 39b-Cor (n=16; mean 400.4 ± 44.7 pA/pF) and 39b (n=15; mean 387.1 ± 50.5 pA/pF; p=0.8, t-test) derived motor neurons.

Figure 3. Retigabine Reduces Motor Neuron Excitability and Increases Survival

(A) Rheobase measurements in a 39b Hb9::RFP-positive ALS-derived motor neuron in whole-cell patch clamp before (left) and after (right) the application of 10 μM retigabine (baseline rheobase 4.8 ± 1.5 pA vs post-retigabine rheobase 8.4 ± 2.2 pA; n=11; p<0.05, Wilcoxon signed rank test). (B) Representative current clamp recording showing effect of 10 μM retigabine on membrane voltage and spontaneous firing (baseline Vm −60.4 ± 2.9 mV vs post-retigabine Vm −66.3 ± 3.6 mV, n=11; p=0.001, t-test). In (A–B), CNQX (15 μM), D-AP5 (20 μM), bicuculline (25 μM), and strychnine (2.5 μM) were added to the external solution. (C) Dose response curve for retigabine on suppression of spontaneous action potentials in MEA recording and Hill plot fit of mean data from 39b (n=4) and RB9d (n=4) with EC50 1.5 ± 0.8 μM. (D) Effect of vehicle (open circles) and 1μM retigabine (filled circles) treatment from days 14–28 of culture on the survival of Islet-positive, Tuj1-positive motor neurons measured at day 30 (total control n=11; total ALS n=9; F-test for effect of retigabine on all cells p=3.8×10⁻⁴; effect of retigabine in ALS motor neurons, red, 25.3% (SD 5.6; t-test p=6.4×10⁻⁵); effect of retigabine in control motor neurons, black, 6.1% (SD 5.1, p=0.23). Cell counts are from individual wells for four separate differentiations. See also Figure S4.

Figure 4. Hyperexcitability of C9orf72 Repeat Expansion-Derived Motor Neurons

(A)Representative recordings from four/64 MEA electrodes recorded from control (11a, 18a) and C9orf72 expansion ALS-derived neurons (19f, RB8b) cultured for 14 days. (B) Total action potential firing rate during one minute of recording from MEAs (11a, n=1; 18a, n=3; control mean 4,752 ± 2,786 spikes/minute; 19f, n=3; RB8b, n=3; ALS mean 20,022 ± 3,775 spikes/minute; p<0.05, t-test). (C) Average of mean firing rate for control and C9orf72-derived neurons (11a, n=82; 18a, n=203; control mean 1.11 ± 0.06 Hz; 19f, n=407; RB9d, n=929; ALS mean 1.50 ± 0.04; p<10⁻⁵, t-test).

Figure 5. Motor Neuron Hyperexcitability and Block by Retigabine are Broad Properties of ALS Variants

(A) Multi-electrode array recordings of motor neurons derived from control (11a, n=8; 15b, n=2; 17a, n=5; 18a, n=7; 18b, n=10; 20b, n=6), SOD1 (25b, D90A, n=3; 27d, G85S, n=7; 39b, A4V, n=3; RB9d, A4V, n=7), C9orf72 expansion (19f, n=2; RB8B, n=13) and FUS (MGH5b, frameshift mutation at residue 511, n=10; RB21, H517Q, n=4) subjects cultured for 14 days. ANOVA, p <10⁻⁷; Tukey’s post-hoc tests for control vs SOD1 p<0.01, control vs C9orf72 p<0.01, control vs FUS p<0.05. For subject 18, motor neurons from two different iPSC lines were recorded. Error bars are 95% CI. See also Figure S5. (B) Dose response curve for retigabine on suppression of spontaneous action potentials in MEA recording and Hill plot fit of mean data from SOD1 (n=10; EC50 1.9 ± 0.5 μM), C9orf72 (n=9; EC50 2.6 ± 0.8 μM) and FUS (n=4; EC50 1.9 ± 1.1 μM). (C) Dose response curve for flupirtine on suppression of spontaneous action potentials in MEA recording and Hill plot fit of mean data from SOD1 (n=5; EC50 9.8 μM), C9orf72 (n=4; EC50 19.4 μM) and FUS (n=2; EC50 15 μM).