Broad transcriptomic effects of Egr1 knockdown in the mouse nucleus accumbens core and its role in cocaine locomotor sensitization

Immediate early genes are widely used markers of neuronal activation, but their function in neurons is not well understood. We focused on the role of Egr1 in the nucleus accumbens core (NAc-c) in the long-lasting behavioral effects of cocaine, using an AAV expressing short hairpin RNA (Egr1-shRNA). Egr1 knockdown did not alter acute cocaine locomotor effects or conditioned place preference. In contrast, shEgr1 markedly decreased the locomotor sensitization induced by repeated cocaine administration. Because EGR1 is a transcription factor, we explored the transcriptomic alterations using RNAseq completed by RT-qPCR and protein studies. Egr1 knockdown modified the expression of numerous genes. Analysis of the upregulated genes revealed indirect activation of astrocytes and microglia evidenced by immunohistofluorescence, but shEgr1-induced dampening of cocaine sensitization was unaffected by minocycline, a microglia inhibitor. Proteasome genes were upregulated by shEgr1, possibly contributing to its functional consequences. Downregulated genes included potential EGR1 targets and comprised many genes characteristic of striatal neurons, including those coding signaling proteins (DARPP-32, CDK5 activator p35), glutamate ionotropic (NMDA NR1/2B, AMPA GluA1-3) and metabotropic (mGluR1/5) receptors, and postsynaptic proteins (PSD-95). We confirmed these alterations at the protein level and found decreased cocaine-induced phospho-Ser845-GluA1. Thus our study shows the broad transcriptional consequences of silencing Egr1 in neurons. It provides a mechanism by which Egr1 knockdown in the NAc-c can alter cocaine-induced locomotor sensitization, through downregulation of many genes including key components of glutamate neurotransmission. This broad role of EGR1 in regulating transcription provides clues about its function and role in learning, memory, and synaptic plasticity.