Neuronal activity-dependent gene expression in learning and excitotoxicity

Abstract

Genes regulated by neuronal activity are the focal point of plasticity-related brain functions, thereby providing the basis for behavioural adaptations such as memory. Cellular functions of activity-regulated genes are diverse; several genes encode transcription factors thus regulating a wide range of downstream processes. Pro-survival genes such as Bdnf rendering neurons more resistant against cellular stress and degeneration are another important class of activity regulated genes. Due to those crucial functions, it is essential to understand the regulation and relevance of activity-induced genes under physiological conditions, but also their response to pathological signals like glutamate mediated excitotoxicity. To examine neuronal activity-dependent gene regulation in physiological as well as pathological environments, this thesis was divided into two parts. The first part investigated whether activity-regulated gene expression is correlated with memory ability by comparing two inbred mouse strains, C57BL/6 and DBA/2. Robust spatial memory impairments were observed for DBA/2. However, the cognitive deficits in DBA/2 were not exclusive for this type of memory. Analysis of basic characteristics such as morphology and electrophysiological properties of CA1 pyramidal neurons, which are crucial for spatial memory formation, revealed no strain difference. Also, activity-dependent gene induction was mostly similar between the strains in cell culture and in vivo after a learning paradigm. Yet, two genes, Inhba and Npas4, showed significantly increased basal expression and increased induction in DBA/2 hippocampal regions, respectively. Since both genes are implicated in excitation/inhibition regulation, inhibitory neuronal input for CA1 neurons via mIPSC electrophysiological recordings was analysed and revealed decreased inhibition in DBA/2. Attenuated inhibition may underlie the DBA/2 learning impairment. Yet, dysregulation of single activity-regulated genes, in particular Inhba and Npas4, correlates with differences in memory ability and might be involved in dysfunction processes. The second part of this thesis analysed activity-dependent gene expression during the pathological condition of excitotoxicity. Intriguingly, differential response pattern of activity-dependent genes towards toxic NMDA stimuli were observed. Single genes like Inhba and Bdnf were intensely downregulated by NMDA-induced active deactivation processes, in contrast, rapid immediate early genes like Arc, Npas4 or Nr4a1 displayed no transcriptional shut-off during excitotoxicity. By analysing the activity of upstream kinases and transcription factors the mechanism underlying the differential regulation was investigated. Fast signalling kinases like ERK1/2 seem to depend predominantly on synaptic activity, transcription factors, however, were broadly dephosphorylated after NMDA application. Furthermore, additional experiments indicate that excitotoxicity-induced mitochondrial depolarization does not act as initiator of active transcriptional downregulation. Together, the presented data illustrate that dysregulation of single activity-dependent genes occurs in a physiological model of memory impairment, but also under the pathological condition of excitotoxicity. Inhba might play an important role in both processes and for memory formation in general since it was dysregulated in both models which are implicated in memory attenuation.