Molecular signatures of Id2- and Ascl4-induced wiring of adult hippocampal neurons.

While circuit formation is generally thought to be absent in the adult brain, there is evidence that the adult brain retains a considerable capacity for neuronal wiring. Among other findings, molecular programming by basic helix-loop-helix (bHLH) proteins can reactivate axon growth in adult neurons, allowing the investigation of the mechanisms and potential of adult brain rewiring. Here, we analyzed proteomic and transcriptomic changes during Id2- (a bHLH transcriptional regulator) and Ascl4- (a bHLH transcription factor) induced feedback and feedforward wiring in the hippocampus of adult male and female mice. We find that Id2 and Ascl4 share a molecular cascade through the transcription factor Stat1 and co-regulate a set of proteins that may constitute a core program for adult wiring. Unexpectedly, we also identify shared molecular changes that do not originate directly from the programmed neurons, but from endothelial cells and microglia. Taken together, our results suggest that adult brain wiring is regulated by specific molecular programs and dynamic interactions between the wiring neurons and the circuit environment.Significance statement Contrary to conventional views, axonal wiring and circuit formation is not limited to the developing brain but can also occur in the adult brain. Although this area of research remains largely unexplored in the naive brain, technologies to rewire the adult brain have the potential to help overcome severe limitations imposed by brain disease, injury, or aging. In this study, we investigate the signatures of adult brain wiring induced by molecular cell programming using different factors and reveal common molecular changes to better understand the underlying biological mechanisms.