for the glutamateric synaptic marker vGlut1 (Figure 2C). Non-neuronal cells showed typical fibroblast morphologies and Vimentin expression; cell division was not involved in the iN process (Figure S1). Following gentle relocation onto a layer of astrocytes during week 4 of conversion, iNs displayed pronounced human synapsin-I promoter-driven red fluorescent protein (RFP) fluorescence (LV-hSyn∷RFP), mature neuronal morphologies, and punctate staining of synapsin-I at the intersections of neurites (Figure 2D). They also showed both evoked and spontaneous action potential firing (Figure 2E). We next applied this iN paradigm to our fibroblast cohort and found that cells from all ages efficiently converted into iNs that showed solid neuronal marker expression within 3 weeks (Figure 2F). Quantification revealed that, independent of donor age, typically over 50% of all DAPI-positive cells were positive for βIII-tubulin and hTau, around 40% expressed NeuN, and over 30% were consistently positive for MAP2ab (Figures 2G and S1). Equally, iNs generated from all tested donors (n = 13) displayed Na+/K+ channel-mediated inward/outward currents as well as multiple evoked action potentials with no apparent differences between iNs derived from young and old donors’ fibroblasts (Figure 2H). Quantification of neuronal subtype markers revealed that a consistent majority of the neurons adopted a
