Figure 4.

Characteristic Firing Pattern of iPSC-MNs as a Function of Stimulus Strength

(A and B) Heatmaps showing universal shape of the F-I curves for control and SOD1 A4V mutant iPSC-MNs. F-I curves from each cell were rescaled along the x and y axes as follows. Firing rate was expressed as a percentage of the cell's maximum firing rate. Stimulus intensities were aligned to the lowest intensity stimulus at which this maximum firing rate was achieved. This rescaling revealed typical F-I trajectory shapes in a manner that was independent of changes in CheRiff expression level. Control (A) and SOD1 A4V mutant iPSC-MNs (B) showed a linear increase in firing rate versus stimulus strength for weak stimuli, a plateau in firing rate for moderate stimuli, and a collapse in firing under strong stimuli. Data based on n = 834 control neurons, n = 331 SOD1 A4V, and six rounds of differentiation.

(C) Top: fluorescence traces from a single representative cell passed through three distinct stages of firing in response to monotonically increasing optogenetic stimulus strength. Bottom: fit of piecewise-continuous F-I curves to the data in (A) and (B) for (black) genome-corrected and (red) SOD1 A4V mutant cell lines. Curves were constructed from measurements of spontaneous rate, average slope of the F-I curve (controlling for expression level), maximum firing rate, and the number of stimulus steps spent at the maximum.