Activating Transcription Factor 4 (ATF4) Regulates Neuronal Activity by Controlling GABABR Trafficking

Activating Transcription Factor 4 (ATF4) has been postulated as a key regulator of learning and memory. We previously reported that specific hippocampal ATF4 downregulation causes deficits in synaptic plasticity and memory and reduction of glutamatergic functionality. Here we extend our studies to address ATF4's role in neuronal excitability. We find that long-term ATF4 knockdown in cultured rat hippocampal neurons significantly increases the frequency of spontaneous action potentials. This effect is associated with decreased functionality of metabotropic GABA(B) receptors (GABA(B)Rs). Knocking down ATF4 results in significant reduction of GABA(B)R-induced GIRK currents and increased mIPSC frequency. Furthermore, reducing ATF4 significantly decreases expression of membrane-exposed, but not total, GABA(B)R1a and 1b subunits, indicating that ATF4 regulates GABA(B)R trafficking. In contrast, ATF4 knockdown has no effect on surface expression of GABA(B)R2s, several GABA(B)R-coupled ion channels or beta 2 and gamma 2 GABA(A)Rs. Pharmacologic manipulations confirmed the relationship between GABA(B)R functionality and action potential frequency in our cultures. Specifically, the effects of ATF4 downregulation cited above are fully rescued by transcriptionally active, but not by transcriptionally inactive, shRNA-resistant, ATF4. We previously reported that ATF4 promotes stabilization of the actin-regulatory protein Cdc42 by a transcription-dependent mechanism. To test the hypothesis that this action underlies the mechanism by which ATF4 loss affects neuronal firing rates and GABA(B)R trafficking, we downregulated Cdc42 and found that this phenocopies the effects of ATF4 knockdown on these properties. In conclusion, our data favor a model in which ATF4, by regulating Cdc42 expression, affects trafficking of GABA(B)Rs, which in turn modulates the excitability properties of neurons.