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Long-distance axon regeneration promotes recovery of diaphragmatic respiratory function after spinal cord injury

Urban, MW;Ghosh, B;Block, CG;Strojny, LR;Charsar, BA;Goulo, M;Komaravolu, SS;Smith, GM;Wright, MC;Li, S;Lepore, AC;

Compromise in inspiratory breathing following cervical spinal cord injury (SCI) is caused by damage to descending bulbospinal axons originating in the rostral Ventral Respiratory Group (rVRG) and consequent denervation and silencing of phrenic motor neuron (PhMNs) that directly control diaphragm activation. In a rat model of high-cervical hemisection SCI, we performed systemic administration of an antagonist peptide directed against phosphatase and tensin homolog (PTEN), a central inhibitor of neuron-intrinsic axon growth potential. PTEN antagonist peptide (PAP4) robustly restored diaphragm function, as determined with electromyography recordings in living SCI animals. PAP4 promoted substantial, long-distance regeneration of injured rVRG axons through the lesion and back toward PhMNs located throughout the C3-C5 spinal cord. These regrowing rVRG axons also formed putative excitatory synaptic connections with PhMNs, demonstrating reconnection of rVRG-PhMN-diaphragm circuitry. Lastly, re-lesion through the hemisection site completely ablated functional recovery induced by PAP4. Collectively, our findings demonstrate that axon regeneration in response to systemic PAP4 administration promoted recovery of diaphragmatic respiratory function after cervical SCI.SIGNIFICANCE STATEMENT Using non-invasive and only transient systemic delivery of an antagonist peptide that targets PTEN, we promoted significant regeneration of bulbospinal respiratory axons into and through a large lesion site in a rat model of cervical SCI, as well as growth of these axons back into the intact distal spinal cord for several spinal segments. We also found that these regenerating axons synaptically reconnected with their appropriate post-synaptic respiratory motor neuron targets, resulting in reconnection of the damaged neural circuit. Furthermore, by showing that re-lesion of regenerating axons resulted in complete loss of PTEN peptide-induced diaphragm recovery, we demonstrate that long-distance axon regeneration can drive recovery of respiratory function following cervical SCI. Copyright 2019 Urban et al.