Decreased Akt/mTOR Pathway Is Associated with Reduced Excitatory Synaptic Marker PSD-95 and Autistic-like Behavior in Valproic Acid-Exposed Mice

Background:  The molecular mechanisms underlying autistic behaviors remain to be elucidated. Mutations in Autism Spectrum Disorder (ASD)-linked genes encode molecules regulating dendritic spine formation, function and plasticity and de-regulate the Akt/mTOR pathway which controls protein synthesis at dendritic spines. Previously, we demonstrated reduced TrkB/Akt/mTOR signaling via p70 S6 kinase/eIF4B in human idiopathic autism. We also determined that this down-regulation was associated with decreased excitatory synaptic marker PSD-95 in patients with idiopathic autism, suggesting that these molecular changes might have adverse consequences for excitatory synapses and contribute to autistic behavior.

Objectives:  In the current study, we aimed to investigate whether maternal exposure to the anticonvulsant valproic acid (VPA), which has been associated with autism-like phenotypes in both humans and rodents, affects TrkB, mTOR and their downstream effectors including PSD-95.

Methods:  Pregnant CD1 mice received a single intraperitoneal injection of 500mg/kg VPA on gestational day 12, while controls were injected with saline. Dams were weaned on postnatal day (PND) 21, and offspring’s behavior and somatosensory function were evaluated on PND29-30 in tests of nest-seeking, negative geotaxis and social interaction preference. Litters were then sacrificed and brain tissue harvested on PND30. Protein expression of TrkB, mTOR, p70S6K and PSD-95 were measured by Western blotting in the temporal/parietal neocortices of 14 VPA-exposed mice and 11 saline controls. 

Results:  Offspring of VPA-injected mothers had significantly decreased TrkB, mTOR, p70S6K and PSD-95 protein compared to controls.  VPA-exposed mice exhibited autistic-like behavior in tests of social interaction preference, nest-seeking and negative geotaxis. 

Conclusions:  Our molecular results demonstrate that, similarly to human idiopathic autism, offspring of VPA-treated mothers had significantly decreased TrkB, mTOR, p70S6K and PSD-95, supporting the hypothesis that defective TrkB and mTOR via p70S6K may have adverse consequences for excitatory synapses and contribute to autistic behavior. Indeed, we determined that down-regulation of these molecules was associated with deficits in somatosensory function and social approach behaviors in VPA-exposed mice compared to saline controls. In conclusion, our findings show that the use of valproic acid in mice induces autistic-like behavior and successfully models the molecular changes we observe in human idiopathic autism, supporting the hypothesis that defective TrkB/mTOR signaling contributes to autistic traits.