Neuronal induction via the overexpression of the pro-neuronal transcription factors (first demonstrated using the combination of ASCL1, BRN2, MYT1L and NEUROD1) can induce human fibroblasts into neurons and is now a viable alternative to directed differentiation [17]. Moreover, further maturity can be achieved by co-expressing microRNA-9/9* and microRNA-124 [18]. Recently, overexpression of mouse Neurogenin 2 (mNgn2) or human NEURO-GENIN 2 (hNGN2) in hiPSCs, combined with puromycin selection to increase the purity of the cultures, yielded populations of induced neurons (iNs) comprised of more than 90% Microtubule-associated protein 2AB (MAP2AB)-positive neurons within 14 days [19–21]. These mNgn2-iNs express glutamatergic synaptic proteins such as vesicular glutamate transporter 1 (vGLUT1), postsynaptic density-95 (PSD95) and synapsin1 (SYN1) [19]. In addition, iNs exhibited excitatory synaptic function, when co-cultured with mouse cortical neurons, and integrated into the mouse brain following transplantation [19], indicating that this approach is capable of rapidly and efficiently generating highly pure populations of functional excitatory neurons.
