Human fibroblasts were obtained from commercial sources (Supplementary Table S3) and
maintained in fibroblast medium (DMEM supplemented with 10% or 15% fetal bovine
serum and penicillin/streptomycin) according to the vendor’s instructions for cell culture.
Cells were seeded onto gelatin-, fibronectin-, or Matrigel-coated culture vessels (4.8×105
per
24- or 48-well plate, or 8×105
per 10-cm dish) with or without glass cover slips. They were
transduced the next day with retroviral supernatants in the presence of 6 μg/ml polybrene.
After overnight incubation, culture media were refreshed. One day later, the cells were then
switched to C2 medium, which consists of DMEM:F12:Neurobasal (2:2:1), 0.8% N2
(Invitrogen), 0.4% B27 (Invitrogen), and penicillin/streptomycin. Screens for small
molecules were conducted by incubating the indicated chemicals (Supplementary Table S1)
in C2 medium for 5 days starting at 2 days post infection (dpi). The resulting neural
induction medium for human fetal fibroblasts consist of DM (1 μM, from 2 dpi to 6 dpi) and
FSK (10 μM) in C2 medium. For postnatal and adult human fibroblasts, FGF2 (10 ng/ml)
and DM was included in the above induction medium from 2 dpi to 12 dpi. Induction
medium was half-changed every other day. For inducible expression, cells were transduced
overnight with lentiviruses. The day after medium change, Dox (0.1 μg/ml) was added to
induce gene expression for the indicated period of time during the course of reprogramming.
For gene expression analysis, electrophysiology, or coculture with myotubes, induced
neurons were purified by differential plating. Briefly, cells at 14-21 dpi were rinsed with
DPBS and dissociated with 0.025% trypsin for 15 minutes at 37°C. After blocking trypsin
activity with FBS-containing fibroblast medium, the cell suspension was plated onto a
gelatin-coated culture dish, to which fibroblasts tightly attached. About 30 min later, floating
cells, which mainly consisted of induced neurons, were replated onto a new coated dish to
remove residual fibroblasts or were directly collected by centrifugation at 500 g for 2 min.
Cells were resuspended into C2 medium and centrifuged again to remove cell debris.
Finally, cells were plated onto a coated dish in C2 medium supplemented with BDNF,
GDNF and NT3 (10 ng/ml each, Peprotech). Unless indicated otherwise, C2 media with
neurotrophic factors were half-changed twice a week. The human neural stem cell line,
K048, was a kind gift of Dr. P. Wu (UT Medical Branch) and Dr. C.N. Svendsen
(University of Wisconsin).