Coverslips were placed in a 24-well plate, coated with Poly-L-Lysine for 5 minutes and dried overnight after washing four times with water. SH-SY5Y cells were plated at a density of 2.5 × 105 in 500 ul of DMEM with 10% FBS and grown overnight. The cells were fixed in 4% paraformaldehyde/4% sucrose in phosphate-buffered saline (PBS) at room temperature for 15 minutes and washed three times for 5 minutes in PBS. iPSC-derived neurons: Approximately 15,000 neurons were seeded into each well of an 8-well chamber slide (Thermo Fisher), previously coated in Matrigel (BD). Neurons received either mock treatment or application of 50 mM KCl in N2B27 neurobasal media. Depolarized cells were fixed at one, three and six, hours post application of KCl. Cells were washed once gently, so as not to disturb fragile cell extensions, in PBS and fixed in 4% paraformaldehyde, 4% sucrose for 15 minutes at room temperature. Immunohistochemistry: Cells were then permeabilized in 0.1% Triton X-100/0.1% Na-Citrate/PBS for 3 minutes at room temperature and washed three times for 5 minutes with PBS. Cells were next blocked in
