Stimulation-dependent remodeling of the corticospinal tract requires reactivation of growth-promoting developmental signaling pathways

Abstract

The corticospinal tract (CST) can become damaged after spinal cord injury or stroke, resulting in weakness or paralysis. Repair of the damaged CST is limited because mature CST axons fail to regenerate, which is partly because the intrinsic axon growth capacity is downregulated in maturity. Whereas CST axons sprout after injury, this is insufficient to recover lost functions. Chronic motor cortex (MCX) electrical stimulation is a neuromodulatory strategy to promote CST axon sprouting, leading to functional recovery after CST lesion. Here we examine the molecular mechanisms of stimulation-dependent CST axonal sprouting and synapse formation. MCX stimulation rapidly upregulates mTOR and Jak/Stat signaling in the corticospinal system. Chronic stimulation, which leads to CST sprouting and increased CST presynaptic sites, further enhances mTOR and Jak/Stat activity. Importantly, chronic stimulation shifts the equilibrium of the mTOR repressor PTEN to the inactive phosphorylated form suggesting a molecular transition to an axon growth state. We blocked each signaling pathway selectively to determine potential differential contributions to axonal outgrowth and synapse formation. mTOR blockade prevented stimulation-dependent axon sprouting. Surprisingly, Jak/Stat blockade did not abrogate sprouting, but instead prevented the increase in CST presynaptic sites produced by chronic MCX stimulation. Chronic stimulation increased the number of spinal neurons expressing the neural activity marker cFos. Jak/Stat blockade prevented the increase in cFos-expressing neurons after chronic stimulation, confirming an important role for Jak/Stat signaling in activity-dependent CST synapse formation. MCX stimulation is a neuromodulatory repair strategy that reactivates distinct developmentally-regulated signaling pathways for axonal outgrowth and synapse formation.

Functional Electrical Stimulation and the Modulation of the Axon Regeneration Program

“…whereas, inhibiting Stat3 activation with AG490 reduced both bouton-like structures on corticospinal axons as well as levels of cFos expression in ipsilateral cervical spinal cord neurons without affecting corticospinal collateral formation (Zareen et al, 2018). These studies demonstrate that not only can electrical stimulation activate growth-promoting molecular pathways, it can strengthen the formation of novel connections needed to elicit functional recovery.…”

“…Chronic electrical stimulation of the motor cortex over ten consecutive days leads to activation of the mTOR pathway, inactivation of PTEN, and increased phosphorylation of ribosomal protein S6 ( Zareen et al, 2018 ). Additionally, this chronic stimulation drives increased levels of Janus kinase-signal transducer and activator of transcription (JAK/STAT) signaling ( Zareen et al, 2018 ), a critical mediator of cytokine signaling. Enhancing JAK/STAT signaling through deletion of suppressor of cytokine signaling 3 ( SOCS3 ) increases optic nerve regeneration ( Smith et al, 2009 ).…”

“…New corticospinal collaterals sprout and form synaptic connections in the spinal cord during 10 days of stimulation. Pharmacological studies implicate mTOR signaling in collateral formation as corticospinal collateral sprouting is blocked by rapamycin; whereas, inhibiting Stat3 activation with AG490 reduced both bouton-like structures on corticospinal axons as well as levels of cFos expression in ipsilateral cervical spinal cord neurons without affecting corticospinal collateral formation ( Zareen et al, 2018 ). These studies demonstrate that not only can electrical stimulation activate growth-promoting molecular pathways, it can strengthen the formation of novel connections needed to elicit functional recovery.…”

Chronic neuronal activation increases dynamic microtubules to enhance functional axon regeneration after dorsal root crush injury

“…mTOR is known to protect microtubules from microtubule stabilization 90 and to regulate the interaction of microtubules with CLIP170, a protein that associates with the tips of dynamic microtubules 91 , so it may be via these effects that neuronal activation alters microtubules. Moreover, previous studies showed that stimulation promotes corticospinal tract axonal sprouting and synapse formation via upregulation of mTOR and Jak/Stat signaling 92 and that inhibiting tubulin detyrosination to keep microtubules dynamic mimicked the axon growth-promoting effects of sustained GSK3 activity 51 , 93 . Collectively, these results support the view that various different pathways beneficial to nerve regeneration may converge on a common downstream effect, which is to shift the microtubule array of the axon, especially the distal axon, toward being less post-translationally modified and more dynamic.…”

Repetitive Transcranial Magnetic Stimulation (rTMS) Improves the Gait Disorders of Rats Under Simulated Microgravity Conditions Associated With the Regulation of Motor Cortex

“…The inconsistency may be due to the length of modeling time ( Nishijima et al, 2010 ; Mysoet et al, 2017 ). However, it has been reported that the activation of mTOR can promote axonal regeneration of corticospinal neurons, and then enhance neuronal activity ( Zareen et al, 2018 ). Our results suggest that rTMS may enhance the neuronal activity of the motor cortex through the activation of mTOR, leading to the improvement of gait disorders.…”