Moderate-Intensity Treadmill Exercise Promotes mTOR-Dependent Motor Cortical Neurotrophic Factor Expression and Functional Recovery in a Murine Model of Crush Spinal Cord Injury (SCI).

Abstract

Treadmill exercise is widely considered an effective strategy for restoration of skilled motor function after spinal cord injury (SCI). However, the specific exercise intensity that optimizes recovery and the underlying mechanistic basis of this recovery remain unclear. To that end, we sought to investigate the effect of different treadmill exercise intensities on cortical mTOR activity, a key regulator of functional recovery following CNS trauma, in an animal model of C5 crush spinal cord injury (SCI). Following injury, animals were subjected to treadmill exercise for 4 consecutive weeks at three different intensities (low intensity [LEI]; moderate intensity [MEI]; and high intensity [HEI]). Motor function recovery was assessed by horizontal ladder test, cylinder rearing test, and electrophysiology, while neurotrophic factors and cortical mechanistic target of rapamycin (mTOR) pathway-related proteins were assessed by Western blotting. The activation of the cortical mTOR pathway and axonal sprouting was evaluated by immunofluorescence and the changes of plasticity in motor cortex neurons were assessed by Golgi staining. In keeping with previous studies, we found that 4weeks of treadmill training resulted in improved skilled motor function, enhanced nerve conduction capability, increased neuroplasticity, and axonal sprouting. Importantly, we also demonstrated that when compared with the LEI group, MEI and HEI groups demonstrated elevated expression of brain-derived neurotrophic factor (BDNF), insulin-like growth factor 1 (IGF-1), phosphorylated ribosomal S6 protein (p-S6), and protein kinase B (p-Akt), consistent with an intensity-dependent activation of the mTOR pathway and neurotrophic factor expression in the motor cortex. We also observed impaired exercise endurance and higher mortality during training in the HEI group than in the LEI and MEI groups. Collectively, our findings suggest that treadmill exercise following SCI is an effective means of promoting recovery and highlight the importance of the cortical mTOR pathway and neurotrophic factors as mediators of this effect. Importantly, our findings also demonstrate that excessive exercise can be detrimental, suggesting that moderation may be the optimal strategy. These findings provide an important foundation for further investigation of treadmill training as a modality for recovery following spinal cord injury and of the underlying mechanisms.