Functional and Molecular Impact of Foxg1 Over-expression in Neocortical Projection Neurons

Foxg1 is an evolutionary conserved transcription factor gene playing a crucial role in a variety of neurodevelopmental processes taking place in the rostral brain. Mice overexpressing Foxg1 in their neocortical projection neurons were previously generated and characterized in our laboratory. These mice showed an increased activity in baseline conditions and a higher incidence of limbic motor seizures upon kainic acid administration (Tigani, Pinzan et al., submitted). In this study we modulated Foxg1 gene expression in primary cultures of neocortical neurons to model its impact on activity of neocortical pyramids. We evaluated neuronal activity through Ca2+ imaging assay and found that the overexpression of our gene of interest led to hyperactivity and hyper-synchronization of the culture. Interestingly, neuronal hyperactivity was also achieved when the up-regulation of our gene was limited to glutamatergic cells. Given this, we wondered whether GABAergic neurons were affected by Foxg1 overexpression. We observed a pronounced depletion of GABAergic interneurons, indeed, when the genetic manipulation occurred in the whole neuronal compartment of the culture. Aiming to dissect molecular mechanisms leading to Foxg1-dependent neuronal hyperactivity, we profiled the transcriptome of Foxg1-GOF neurons and detected a misregulation of a number of key genes, encoding for crucial players such as ion channels, GABA and glutamate receptors. We also detected a transient Foxg1 up-regulation presumably ignited by neuronal activity, upon the administration of KCl. We monitored the response to the same treatment of some immediate early genes and found that they are necessary to Foxg1 mRNA increase and contribute to its early articulation. Taken all together, these data suggest that even small changes of Foxg1 levels result in a deep impact on pyramids' activity, an issue relevant to neuronal physiology and neurological aberrancies associated to FOXG1 copy number variations.