Compact transcription factor cassettes generate functional, engraftable motor neurons by direct conversion

Direct conversion generates patient-specific, disease-relevant cell types such as neurons that are rare, limited, or difficult to isolate from common and easily accessible cells such as skin cells. However, low rates of direct conversion and complex protocols limit scalability and thus the potential of cell-fate conversion for biomedical applications. Systems-level optimization of conversion protocols across molecular and cellular scales can support scalable cell manufacturing for diverse application. We use insights from our work on how proliferation and transcription factor levels act together to drive direct conversion to optimize the conversion protocol by examining process parameters including transcript design, delivery via AAV, retrovirus and lentivirus, cell seeding density, and the impact of media conditions. Thus, we report a compact, portable conversion process that boosts proliferation and increases direct conversion of mouse fibroblasts to induced motor neurons to achieve high conversion rates of above 1,000%, corresponding to more than 10 motor neurons yielded per cell seeded which we achieve through expansion. Our optimized, direct conversion process generates functional motor neurons at scales relevant for cell therapies (>107 cells) that graft with the mouse central nervous system. High-efficiency, compact, direct conversion systems will support scaling to patient-specific, neural cell therapies. Overall design: Transcriptional profiling of cells during the process of direct conversion of mouse embryonic fibroblasts to induced motor neurons (iMNs) and at the terminal stage at 14 dpi. Terminal samples at 14 dpi include Hb9::GFP+ sorted iMNs by fluorescence-activated cell sorting (FACS) generated with LNI + DDRR (LNI is mLhx3-P2A-mNgn2x3HA-T2A-Isl1; DDRR is HRAS(G12V)-IRES-p53DD) in the presence and absence of supplements (neurotrophic growth factors: BDNF, CNTF, GDNF, FGF; and RepSox). Samples at 4 dpi in conversion for LNI + DDRR with supplements for CellTrace-FarRed-low population sorted by FACS represent cells in the intermediate stages of direct conversion.