Down regulation of Akt/mTOR signaling pathway proteins in hippocampus of Alzheimer’s disease mouse model

Abstract

AbstractBackgroundAkt and mechanistic target of rapamycin (mTOR) signaling pathways are implicated in AD pathology. Activation of these pathways by external cues facilitates new protein synthesis at the synapse which leads to the synaptic plasticity. We have demonstrated that activity dependent Akt1 mediated new protein synthesis is abolished in primary cortical neurons of APP/PS1 mice. However, the molecular underpinnings for the impairment of new protein synthesis at the synapse are still elusive. Our aim is to understand the molecular mechanisms prior to manifestation of pathological symptoms by examining the Akt1 and mTOR signaling cascades and new protein synthesis in hippocampus of WT and APP/PS1 male mice.MethodsWe isolated post nuclear supernatant (PNS) and synaptosomes from brain cortices of WT and APP/PS1 mice at different ages. Synaptosomes and PNS were resolved on SDS‐PAGE and immunoblotted against respective antibodies. Further, synaptoneurosomes were also prepared from hippocampus and performed 35S methionine incorporation assay without or with KCl stimulation. Statistical comparisons were performed using two‐way ANOVA followed by Tukey’s post‐hoc test or two‐tailed Mann‐Whitney U test.ResultsWe observed that pAkt1, pGSK3β, pmTOR, pS6 ribosomal protein, p4E‐BP1 levels are significantly decreased in both post nuclear supernatant (PNS) and synaptosomal fractions isolated from hippocampus of one month old (presymptomatic) APP/PS1 compared to WT male mice. Further, activity dependent new protein synthesis at the synapse was significantly impaired in synaptoneurosomes isolated from hippocampus of presymptomatic APP/PS1 mice, and this deficit is sustained at young adults (three months). However, no impairment was observed in basal protein synthesis at both ages in hippocampus of APP/PS1 compared to WT male mice. Down regulation of Akt1 precludes synaptic activity dependent protein translation at the dendrites but not in the soma in hippocampal neurons from APP/PS1 mice compared to WT mice.ConclusionsOur finding demonstrate that the dysregulation of Akt1/mTOR and its downstream signaling molecules in hippocampus contributes to synaptic dysfunction in AD through loss of activity dependent new protein synthesis at the synapse in hippocampus which is essential for synaptic plasticity and maintenance.