Combination Therapy Targeting Akt and Mammalian Target of Rapamycin Improves Functional Outcome after Controlled Cortical Impact in Mice

Abstract

Akt and mammalian target of rapamycin (mTOR) are both activated after traumatic brain injury (TBI), however complex interplay between the two hampers deciphering their functional implications in vivo. We examined the effects of single and combination inhibitors of Akt/mTOR in a mouse controlled cortical impact (CCI) model. Following CCI, phospho-Akt-473 (p-Akt) and -S6 ribosomal protein (p-S6RP), a downstream substrate of mTOR, were increased in cortical and hippocampal brain homogenates (P<0.05 versus sham). At 24 hours, p-S6RP was detected in neurons and was robustly induced in microglia and astrocytes in injured hippocampus. In vivo activity of Akt and mTOR inhibitors administered separately was confirmed by reduced expression of p-GSK3β (P<0.01) or p-S6RP (P<0.05), respectively, after CCI. Importantly, administration of Akt and mTOR inhibitors together (but not of either alone) improved postinjury motor (P=0.02) and cognitive deficits (hidden platform trials, P=0.001; probe trials, P<0.05), decreased propidium iodide-positive cells in CA1 and CA3 (P<0.005), and unexpectedly increased p-GSK3β in hippocampus. Although the roles of Akt and mTOR in the pathogenesis of TBI remain to be fully elucidated, dual inhibition of Akt and mTOR may have therapeutic potential for TBI.

Role of Akt and Mammalian Target of Rapamycin in Functional Outcome after Concussive Brain Injury in Mice

“…The beneficial effects of Nec-1 on post-injury cognitive recovery were associated with increased phosphorylation of Akt and S6RP, and were abrogated by co-administration of Akt inhibitor viii or rapamycin. In striking contrast to our previous findings in a CCI model, 13 the data suggest a protective role for Akt and mTOR pathways after concussive TBI. Closed head injury used herein models specific features of human concussion, including impact and rotational accelerationdeceleration forces, loss of consciousness, neuroinflammation, and cognitive deficits; 24,25 however, this model does not induce detectable acute cell death or chronic brain tissue loss.…”

“…Closed head injury used herein models specific features of human concussion, including impact and rotational accelerationdeceleration forces, loss of consciousness, neuroinflammation, and cognitive deficits; 24,25 however, this model does not induce detectable acute cell death or chronic brain tissue loss. 21 Although Akt and mTOR activation have been noted in a number of TBI models, 13,15,16 the current study is the first to our knowledge to examine Akt/mTOR in a concussion model devoid of neuronal cell death. Akt activation in the brain following TBI is mainly considered in the context of limiting apoptosis.…”

“…[9][10][11][12] However, phosphorylation of Akt and mTOR and their direct substrates occurs in contused mouse brain, and pharmacological inhibition of Akt and mTOR together improves post-injury cognitive outcome after controlled cortical impact (CCI) in mice. 13 The aforementioned observations, along with other studies showing beneficial effects of rapamycin, suggest a detrimental role for acute induction of Akt and mTOR in cerebral contusion models. [13][14][15] However, whether the same is true for concussion TBI is unknown.…”

“…We chose doses of inhibitors based on our prior experience in a CCI model 13 and their ability to inhibit substrate phosphorylation at 24 hours after CHI ( Figure 4). As expected, treatment with rapamycin (12.5 mmol/L) reduced phosphorylation of S6RP but not Akt ( Figures 4A and 4B), whereas treatment with Akt inhibitor viii Figures 4C and 4D).…”

“…We hypothesized that concussive TBI would lead to phosphorylation of Akt and mTOR and their respective downstream substrates, and that pharmacological inhibition of Akt and mTOR would improve cognitive outcome as previously reported after CCI in mice. 13 …”