SUMMARYHuman pluripotent stem cells (hPSCs) have opened new opportunities for understanding human development, modeling disease processes and developing new therapeutics. However, these applications are hindered by low-efficiency and heterogeneity of target cell types differentiated from hPSCs, such as motor neurons (MNs), as well as our inability to maintain the potency of lineage committed progenitors. Here, by using a combination of small molecules that regulate multiple signaling pathways, we develop a method to guide human embryonic stem cells to a near-pure population (>95%) of motor neuron progenitors (MNPs) in 12 days, and an enriched population (>90%) of functionally mature MNs in an additional 16 days. More importantly, the MNPs can be expanded for at least 5 passages so that a single MNP can be amplified to 1×104. This method is reproducible in human induced pluripotent stem cells and is applied to model MNdegenerative diseases and in proof-of-principle drug screening assays.

SUMMARY Amyotrophic lateral sclerosis (ALS) presents motoneuron (MN)-selective protein inclusions and axonal degeneration but the underlying mechanisms of such are unknown. Using induced pluripotent cells (iPSCs) from patients with mutation in the Cu/Zn superoxide dismutase (SOD1)gene, we show that spinal MNs, but rarely non-MNs, exhibited neurofilament (NF) aggregation followed by neurite degeneration when glia were not present. These changes were associated with decreased stability of NF-L mRNA and binding of its 3′ UTR by mutant SOD1 and thus altered protein proportion of NF subunits. Such MN-selective changes were mimicked by expression of a single copy of the mutant SOD1 in human embryonic stem cells and were prevented by genetic correction of the SOD1 mutation in patient’s iPSCs. Importantly, conditional expression of NF-L in the SOD1 iPSC-derived MNs corrected the NF subunit proportion, mitigating NF aggregation and neurite degeneration. Thus, NF misregulation underlies mutant SOD1-mediated NF aggregation and axonal degeneration in ALS MNs.