In another experiment, we tested whether synapses in Ngn2 produced iN cells can be modulated. Recent studies revealed that retinoic acid rapidly upregulates postsynaptic AMPA-type glutamate receptors via a direct synaptic action, and that this signaling pathway is activated by activity blockade (Aoto et al., 2008; Wang et al., 2011). We thus explored the ability of exogenously applied retinoic acid to upregulate postsynaptic glutamate receptors (Fig. 5F). Indeed, acute treatment (~45 min) of iN cells with retinoic acid significantly enhanced the amplitude of postsynaptic mEPSCs that are mediated by AMPA-type glutamate receptors without changing the frequency of mEPSCs, demonstrating that the retinoic acid-dependent synaptic signaling pathway is operational in iN cells and thus also applies to humans. The effect was equally observed with iN cells derived from H1 ES cells and with iN cells derived from two different iPS cell lines (Figs. 5F and S5).
