In vitro modeling of pallium patterning and specification of distinct cortical identities using neuronal populations derived from mouse ESCs and human iPSCs

The cerebral cortex is one of the most complex and fascinating tissues in mammals due to its intricate diversity and enigmatic evolutionary origin. Although cortical development has been extensively studied in several vertebrate animal models, the mechanisms that determine distinct pallial identities during embryonic corticogenesis are not fully understood. One of the major mechanisms regulating cortical patterning is the establishment of specific transcription factor expression domains in the ventricular zone of the neuroepithelium by a relatively small number of signaling molecules. In vitro modeling of neurogenesis can facilitate the dissection of the effects of different signaling pathways on the specification of neural progenitor cells. For example, the central role of Wnt signaling in directing dorsal telencephalic progenitors to the isocortex or hippocampus has been elucidated. The present study shows that the timely inhibition of MAPK/ERK and BMP signaling in neuralized mouse embryonic stem cells (ESCs) specifies a cell identity characteristic of the caudal allocortex. Comparison of the global gene expression profiles of neural cells obtained by MAPK/ERK and BMP inhibition (MiBi cells) with those of cells from early postnatal encephalic regions emphasizes a pallial identity of MiBi cells, distinct from isocortical and hippocampal cells. MiBi cells exhibit a unique pattern of gene expression and connectivity, as well as molecular and electrophysiological features consistent with the entorhinal cortex. Surprisingly, MiBi neuronal networks in adherent cultures develop distinct patterns of electrical activity compared to isocortical and hippocampal neuronal networks. Finally, assembly of in vitro-derived entorhinal-hippocampal cell cultures generated functional neuronal networks with emergent properties that differed from their single-culture counterparts. Taken together, our results indicate that early changes in cell signaling can specify distinct pallial fates that are maintained by specific neuronal lineages independent of subsequent embryonic morphogenetic interactions and can determine their functional connectivity.