The Role of Mammalian Target of Rapamycin (mTOR) in a Mouse Model of Cerebral Palsy

Abstract

Background and Purpose –The mammalian target of rapamycin (mTOR) pathway has been implicated in cellular responses to hypoxia and inflammation. Cerebral palsy (CP) is a neurodevelopmental disorder often linked to hypoxic and inflammatory injury to the brain, however, a role for mTOR modulation in CP has not been investigated. We hypothesized that mTOR inhibition would prevent neuronal death and diminish inflammation in a mouse model of CP. Methods – Post-natal day 6 mouse pups were subjected to hypoxia-ischemia and lipopolysaccharide-induced inflammation (HIL), a model of CP causing injury to several brain areas. Mice received rapamycin (5mg/kg) following HIL, and then daily for 3 subsequent days. The phospho-activation of the mTOR effector mTOR effector proteins S6, S6K and 4EBP as well as upstream negative regulators, TSC1 and Redd1, were assessed as an in vivo measure of the mTOR signaling cascade. Expression of hypoxia inducible factor 1 (HIF-1 alpha) was assayed as an indicator of hypoxia-mediated cellular injury. Neuronal cell death was defined with Fluoro-Jade C (FJC) and cleaved-caspase 3 (CC3), a marker of apoptosis. Autophagy was measured using Beclin-1 and LC3II expression. Lastly, neuroninflammation following HIL was evaluated by examining Iba-1 labeled microglia number and morphology, as well as P-STAT3 expression. Results – Neuronal death, HIF-1alpha expression, and numerous Iba-1 labeled microglia were evident at 24 and 48 hours following HIL. Basal mTOR signaling was unchanged by HIL. Coincident with persistent mTOR signaling, a decreased in Redd1 expression but not TSC1 was observed in HIL. Increased P-STAT3 expression was observed at 24 and 48 hours post-HIL. Rapamycin treatment following HIL significantly reduced neuronal death, decreased HIF-1 alpha and P-STAT3 expression, and microglial activation, coincident with enhanced expression of Beclin-1 and LC3II, markers of autophagy induction. Increase in neuronal death was observed with concomitant administration of rapamycin and chloroquine, an autophagy inhibitor. Administration of a S6K inhibitor, PF-4708671, following HIL also decreased FJC staining further supporting an mTOR-dependent effect of HIL. Conclusions – mTOR inhibition prevented neuronal cell death and diminished neuroinflammation in this model of CP. Persistent mTOR signaling following HIL suggests a failure of autophagy induction, which may contribute to neuronal death in CP. These results suggest that mTOR signaling may be a novel therapeutic target to reduce neuronal cell death in CP.