Activation of feedforward wiring in adult hippocampal neurons by the bHLH transcription factor Ascl4
Although evidence indicates that the adult brain retains a considerable capacity for circuit formation, adult wiring has not been broadly considered and remains poorly understood. Here, we investigated wiring activation in adult neurons. Based on bioinformatic analyses on a previously published dataset (GSE161619), we identified the basic-helix-loop-helix (bHLH) transcription factor Ascl4 as a potential regulator of adult wiring and show that the AAV-mediated overexpression of Ascl4 can cell autonomously induce wiring in different types of hippocampal neurons of adult mice. The new axons are mainly feedforward and reconfigure synaptic weights in the hippocampal circuit. Mice with rewired circuits do not display signs of pathology and solve spatial problems equally well as controls. Our results demonstrate reprogrammed connectivity by a single transcriptional factor and provide insights into the regulation of brain connectivity in adults. Overall design: This dataset includes single-cell RNAseq data from AAV-EGFP labeled ventral hippocampal granule cells and ventral hippocampal medial CA1 pyramidal neurons of 3-4 months old adult mice. For the granule cell and pyramidal cell experiments Calb1-Cre and Nex-Cre transgenic lines were used, respectively, ensuring genetic access to these cell types. The cells were infected either with intrahippocampal microinjection of Cre-dependent AAV-EGFP and AAV-Ascl4 (experimental group, "AAV-Ascl4") or only Cre-dependent AAV-EGFP in equal amount (control group, "AAV-EGFP"). The cells were collected via electrophysiological patch pipette (patch-RNAseq) from acute brain slices 1 month after AAV injections. The partitioning of the data as follows (total number of cells sequenced / number of cells that passed quality control): (1) AAV-EGFP granule cell (“GFP GC”):60 / 53, (2) AAV-Ascl4 granule cell (“Ascl4 GC”):  56/ 52, (3) AAV-EGFP CA1 pyramidal cell (“GFP CA1”): 54 / 48, (4) AAV-Ascl4 pyramidal cell (“Ascl4 CA1”): 58 / 51.