Treadmill exercise supplemented by OPN promote axon regeneration through the IGF-1R/Akt/mTOR signaling pathway

Regeneration of the corticospinal tract (CST) is considered a therapeutic target to achieve improved recovery of motor function after spinal cord injury (SCI), which is an incurable CNS damage that affects millions of people. Exercise training is effective in improving multiple functions in spinal cord-injured patients. However, the effects of exercise training on axon regeneration have not been sufficiently reported. Osteopontin (OPN) has great potential application as a neuroprotective agent for the repair of the nervous system. Studies have shown that the extent of axon regeneration strongly correlates with the expression of OPN. Our previous studies demonstrated that treadmill exercise supplemented by OPN enhances motor function recovery, but axon regeneration is still limited. Extending the treadmill exercise for 12 weeks, we observed promoted axon regeneration, motor function improvement, and signaling pathway activation in mice with SCI after supplementing OPN. Axon regeneration was observed with an anterograde tracer, motor function recovery was evaluated by animal ethology and electrophysiology, and the levels of IGF-1R/Akt/mTOR signaling pathway were evaluated. The results showed that the CST of C5 crushed mice regenerated and formed synaptic connections with neurons after treadmill exercise supplemented by OPN, the horizontal ladder and cylinder rearing test of injured limbs were improved, motor evoked potential also suggested enhanced nerve conduction, and the expression of p-IR, p-Akt, and p-S6 were increased. And the improvements were more obvious than that of the exercise group. Collectively, our study found that treadmill exercise supplemented by OPN promote axon regeneration and motor function through the IGF-1R/Akt/mTOR signaling pathways, and these improvements can be inhibited by rapamycin and Methyl-β-CD(M-B-CD).