Amyloid Precursor Protein (APP) controls the expression of the transcriptional activator Neuronal PAS Domain Protein 4 (NPAS4) and synaptic GABA release.

The Amyloid Precursor Protein (APP) has been extensively studied as the precursor of the beta-amyloid peptide (Abeta) peptide, the major component of the senile plaques found in the brain of Alzheimer's disease (AD) patients. However, the function of APP per se in neuronal physiology remains to be fully elucidated. APP is expressed at high levels in the brain. It resembles a cell adhesion molecule or a membrane receptor, suggesting that its function relies on cell-cell interaction and/or activation of intracellular signaling pathways. In this respect, the APP intracellular domain (AICD) was reported to act as a transcriptional regulator. Here, we used a transcriptome-based approach to identify the genes transcriptionally regulated by APP in the rodent embryonic cortex and upon maturation of primary cortical neurons. Surprisingly, the overall transcriptional changes were subtle, but a more detailed analysis pointed to genes clustered in neuronal-activity dependent pathways. In particular, we observed a decreased transcription of Neuronal PAS domain protein 4 (NPAS4) in APP-/- neurons. NPAS4 is an inducible transcription factor (ITF) regulated by neuronal depolarization. The down-regulation of NPAS4 co-occurs with an increased production of the inhibitory neurotransmitter GABA and a reduced expression of the GABAA receptors alpha1. CRISPR-Cas-mediated silencing of NPAS4 in neurons led to similar observations. Patch-clamp investigation did not reveal any functional decrease of GABAA receptors activity, but LTP measurement supported an increased GABA component in synaptic transmission of APP-/- mice. Together, NPAS4 appears to be a downstream target involved in APP-dependent regulation of inhibitory synaptic transmission.Significance Statement The Amyloid Precursor Protein (APP) is a key player in Alzheimer's disease (AD) pathogenesis. We report the down-regulation of the activity-dependent transcription factor Neuronal PAS domain protein 4 (NPAS4) in APP-deficient neurons, along with an increase in GABAergic neuron markers and GABA release, but not in excitatory glutamatergic markers. We identified NPAS4 as an APP target gene by a transcriptome analysis of APP+/+ versus APP-/- primary cortical neurons at different stages of differentiation. The downregulation of NPAS4 observed in APP-/- neurons was confirmed by APP silencing with a CRISPR/Cas9 approach. CRISPR/Cas9-silencing of NPAS4 mimicked APP deficiency and increased GABAergic markers. The activity-dependent transcription factor NPAS4 is therefore a key downstream target in the synaptic functions regulated by APP.