Targeted Restoration of Respiratory Neural Circuitry following Cervical Spinal Cord Injury
Damage to respiratory neural circuitry and consequent loss of diaphragm function is a major cause of morbidity and mortality following cervical spinal cord injury (SCI). Upon SCI, inspiratory signals originating in the rostral ventral respiratory group (rVRG) of the medulla become disrupted from their phrenic motor neuron (PhMN) targets, resulting in diaphragm paralysis. Using the rat model of C2 hemisection SCI, we investigated two mechanisms of axonal plasticity to drive respiratory revovery: promoting regeneration of injured rVRG axons and enhancing contralateral rVRG sprouting. We aimed to stimulate rVRG axon regeneration via induction of the mammalian target of rapamycin (mTOR) pathway, a signaling system that regulates neuronal-intrinsic axon growth potential. Specifically, we targeted two key components of this pathway via: viral vector-mediated expression of a constitutively-active form of ras homolog enriched in brain (Rheb); and systemic treatment with a small-molecule peptide inhibitor of phosphatase and tensin homolog (PTEN). Expression of cRheb selectively in rVRG neurons promoted significant regeneration of ipsilateral rVRG axons. PTEN peptide administration enabled injured rVRG axons to regrow through the lesion and back to the PhMN pool within C3-C5 spinal cord (i.e. several segments from the injury). This robust rVRG axon regrowth coincided with significant restoration of diaphragm activity, as assessed by in vivo electromyography (EMG) recordings. Furthermore, surgical “re-lesion” through the lesion ablated the functional improvement induced by PTEN inhibition. We also labeled contralateral rVRG axons in a separate cohort; we observed no increases in contralateral rVRG axon input to the PhMN pool ipsilateral to the hemisection, suggesting that increased drive from spared contralateral rVRG is likely not responsible for recovery. To address contralateral sprouting driving diaphragm recovery, we administered a PTPσ inhibitory peptide following C2 hemisection. We found no increase in ipsilateral regeneration following treatment; however, we found robust contralateral rVRG sprouting at all three regions of the spinal cord, C3, C4, and C5. Enhanced sprouting also coincided with diaphragm recovery, which was not ablated upon relesion suggesting that the diaphragm recovery was through contralateral axonal plasticity. Collectively these exciting results demonstrate that targeting both ipsilateral regeneration and contralateral sprouting promotes rVRG-PhMN circuit re-connectivity and recovery of diaphragm function following cervical SCI.
Urban, Mark William, "Targeted Restoration of Respiratory Neural Circuitry following Cervical Spinal Cord Injury" (2019). ETD Collection for Thomas Jefferson University. AAI13808659.