The amyloid precursor protein (APP) modulates synaptic activity, resulting from the fine tuning of excitatory and inhibitory neurotransmission. GABAergic inhibitory neurotransmission is affected by modifications in intracellular chloride concentrations regulated by Na+-K+-2Cl− cotransporter 1 (NKCC1) and neuronal K+-Cl− cotransporter 2 (KCC2), allowing entrance and efflux of chloride, respectively. Modifications in NKCC1 and KCC2 expression during maturation of cortical cells induce a shift in GABAergic signaling. Here, we demonstrated that APP affects this GABA shift. Expression of APP in cortical cells decreased the expression of KCC2, without modifying NKCC1, eliciting a less inhibitory GABA response. Downregulation of KCC2 expression by APP was independent of the APP intracellular domain, but correlated with decreased expression of upstream stimulating factor 1 (USF1), a potent regulator of Slc12a5 gene expression (encoding KCC2). KCC2 was also downregulated in vivo following APP expression in neonatal mouse brain. These results argue for a key role of APP in the regulation of GABAergic neurotransmission.

The Amyloid Precursor Protein (APP) has been extensively studied as the precursor of the βamyloid peptide (Aβ) 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 GABA A receptors alpha1. CRISPR-Cas-mediated silencing of NPAS4 in neurons led to similar observations. Patch-clamp investigation did not reveal any functional decrease of GABA A 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.

Mechanisms driving cognitive improvements following nuclear receptor activation are poorly understood. The peroxisome proliferator–activated nuclear receptor alpha (PPARα) forms heterodimers with the nuclear retinoid X receptor (RXR). We report that PPARα mediates the improvement of hippocampal synaptic plasticity upon RXR activation in a transgenic mouse model with cognitive deficits. This improvement results from an increase in GluA1 subunit expression of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor, eliciting an AMPA response at the excitatory synapses. Associated with a two times higher PPARα expression in males than in females, we show that male, but not female, PPARα null mutants display impaired hippocampal long-term potentiation. Moreover, PPARα knockdown in the hippocampus of cognition-impaired mice compromises the beneficial effects of RXR activation on synaptic plasticity only in males. Furthermore, selective PPARα activation with pemafibrate improves synaptic plasticity in male cognition-impaired mice, but not in females. We conclude that striking sex differences in hippocampal synaptic plasticity are observed in mice, related to differences in PPARα expression levels.

No abstract

33Sequential proteolysis of the amyloid precursor protein (APP) and amyloid-β peptide (Aβ) release 34is an upstream event in Alzheimer's disease (AD) pathogenesis. The function of APP in neuronal 35 physiology is still, however, poorly understood. Along with its paralog APP-like Proteins 1 and 2 36 (APLP1-2), APP is involved in neurite formation and synaptic function by mechanisms that are 37 not elucidated. APP is a single-pass transmembrane protein expressed at high levels in the brain 38 that resembles a cell adhesion molecule or a membrane receptor, suggesting that its function relies 39 on cell interaction processes and/or activation of intracellular pathways of signal transduction. 40Along this line, the APP intracellular domain (AICD) was reported to act as a transcriptional factor 41 for targeted gene activation that mediates physiological APP functions. Here, we used an unbiased 42 transcriptome-based approach to identify the genes transcriptionally regulated by APP in the rodent 43 embryonic cortex and upon maturation of primary cortical neurons. The transcriptome analysis did 44 not detect any significant differences in expression of previously proposed AICD target genes. The 45 overall transcriptional changes were subtle, but we found that genes clustered in neuronal-activity 46 dependent pathways are dysregulated in the absence of APP. Among these genes, we found the 47 activity-dependent Neuronal PAS domain protein 4 (NPAS4) Immediate Early Gene to be 48 downregulated in the absence of APP. Down-regulation of NPAS4 in APP knock-out (KO) neurons 49is not related to AICD but to the APP ectodomain. We studied the effect of APP deficiency on 50GABAergic and glutamatergic transmission, and found an increased production of the inhibitory 51 neurotransmitter GABA in APP KO neurons, along with a reduced expression of the GABA (A) 52 receptors alpha1, suggesting an impaired GABAergic neurotransmission in the absence of APP. 53 CRISPR-Cas-mediated silencing of NPAS4 in neurons led to similar observations. Altogether, our 54 results point out a new role for APP in the regulation of excitatory/inhibitory neurotransmission 55 through the regulation of the activity-dependent NPAS4 gene. 56 4 excitation onto the same neurons (Spiegel et al., 2014). NPAS4 is therefore a key player in the 96 maintenance of excitatory/inhibitory balance in neuronal network. 97The precise mechanisms underlying APP synaptic functions are still elusive. One could suspect 98 APP to regulate the expression of genes involved in synaptic activity, or to shape the structure of 99 the synapse. APP was shown to control gene expression through its intracellular domain called 100AICD. An increasing list of AICD candidate genes has emerged from various models (reviewed in 101Pardossi-Piquard and Checler, 2012). Some of these candidate genes failed to be confirmed by 102 transcription analysis in APP-deficient cell lines (Hebert et al., 2006; Waldron et al., 2008), and 103 APP was also reported to regulate gene transcription independently of AICD...