In Vivo Reprogramming of Neuronal Identity and Local Connectivity in the Neocortex

Abstract

Neurons of the mammalian central nervous system are widely considered as canonical examples of terminally differentiated cells. Using excitatory projection neurons (PNs) of the mouse neocortex as a model, this dissertation challenges the assumption that the class-specific identity of postmitotic CNS neurons is immutably determined. By overexpressing the transcription Fezf2 in early postmitotic, embryonic callosal PNs (CPNs) of cortical layer 2/3 in vivo, I demonstrate that they can be induced to acquire molecular, electrophysiological and morphological traits of corticofugal PNs (CFuPNs) of layers 5 and 6. I demonstrate that reprogramming to a CFuPN-like state increases inhibitory input to the converted neurons to levels similar to that of endogenous CFuPNs, and that this differential inhibition is due, at least in part, to increased numbers of inhibitory perisomatic synapses from parvalbumin (PV)-positive interneurons onto successfully reprogrammed neurons, despite their ectopic location in layer 2/3. This work shows that differentiated, postmitotic CPNs can be induced to reprogram a range of molecular and functional traits towards those of another identity in the embryonic cortex. Furthermore, it demonstrates that individual reprogrammed excitatory projection neurons extrinsically modulate afferent input by local PV+ interneurons, suggesting that projection neuron class-specific identity can actively control the wiring of the cortical microcircuit during development. I then sought to test whether postnatal CPNs in the fully developed cortex, which are resistant to Fezf2-induced reprogramming, nonetheless retain the capacity to respond to reprogramming signals. I show that overexpression of Fezf2, Ctip2, Ascl1 and Sox2 in mature CPNs of the juvenile and adult cortex is sufficient to shift several molecular and electrophysiological traits of these CPNs towards a CFuPN identity, demonstrating that PN identity retains at least partial plasticity even at adult stages. Altogether, this dissertation demonstrates that the terminal identity of postmitotic CPNs can be changed in response to defined factors and provides a new set of tools to probe neuronal nuclear plasticity and the rewiring of cortical microcircuits in vivo.