Differential roles for Akt and mTORC1 in the hypertrophy of Pten mutant neurons, a cellular model of brain overgrowth disorders.

Mutations in the PI3K/Akt/mTOR signaling pathway or in the upstream negative regulator Pten cause human brain overgrowth disorders, such as focal cortical dysplasia and megalencephaly, and are characterized by the presence of hypertrophic neurons. These disorders often have a pediatric onset and a high comorbidity with drug-resistant epilepsy; however, effective pharmacological treatments are lacking. We established forebrain excitatory neuron-specific Pten-deficient cultures as an in vitro model of brain overgrowth disorders, and investigated the effects of this Pten mutation on PI3K/Akt/mTOR signaling and neuronal growth. Mutant neurons exhibit excessive PI3K/Akt/mTOR signaling activity, enlarged somas and increased dendritic arborization. To understand the contributions of Akt and mTORC1 kinases to the hypertrophy phenotype, we evaluated the effects of short-term treatment with the Akt inhibitor MK-2206, and the mTORC1 inhibitor RAD001, which have shown safety and efficacy in human cancer clinical trials. We found that RAD001 treatment only partially reversed the morphological abnormalities of Pten mutant neurons, whereas MK-2206 treatment completely rescued the phenotype. Interestingly, neither treatment altered the size or morphology of normal neurons. Our results suggest that Akt is a major determinant of neuronal growth, and that Akt inhibition may be an effective strategy for pharmacological intervention in brain overgrowth disorders.