C2 hemisection results in paralysis of the ipsilateral hemidiaphragm. The paralysis is a result of disruption of bulbospinal inputs from medullary respiratory centers to the phrenic nucleus. However, there exists a very small, latent pathway that descends contralateral to the hemisection and crosses the midline innervating phrenic neurons, essentially bypassing the lesion. Normal plasticity and activation of this so-called """"""""crossed phrenic pathway"""""""" (CPP) can slowly restore partial function to the initially paralyzed hemidiaphragm. Motor neurons, including phrenic motor neurons, are enveloped in a perineuronal net that is composed of chondroitin sulfate proteoglycans (CSPGs), extracellular matrix molecules whose glycosaminoglycan sugar side chains create an unfavorable environment for neuronal sprouting and synaptic plasticity. CSPGs in the forming scar also block overt regeneration through the lesion site. Recent studies have demonstrated that the enzyme chondroitinase ABC (ChABC) by cleaving the sugar side chains has the ability to abrogate axon growth inhibition of both the perineuronal net and scar-associated matrix resulting in regeneration and/or sprouting after spinal cord injury. We will test the hypothesis that, by enzymatically modifying inhibitory extracellular matrices in the perineuronal net surrounding phrenic motor neurons ipsilateral to a high cervical hemicordotomy, the sprouting capacity of such remaining fibers from the contralateral side, especially those of the serotonergic system, will be maximized. In additional experiments we will attempt to drive activity in the sprouted fibers using intermittent hypoxia or pharmacological manipulation of cAMP. Finally, we will construct a PNS bridge across the lesion to promote long distance regeneration but also allow regenerated axons to exit the bridge via degradation of inhibitory ECM at the PNS/CNS interface. This multipartite strategy has the potential to lead to an unprecedented amount of functional respiratory plasticity/regeneration and recovery after SCI.
Greater than 50% of all spinal cord injuries (SCI) occur at the cervical level. Respiratory complications following SCI are some of the leading causes of despair and death in the SCI population. In this proposal, we plan to utilize a C2 spinal cord hemisection model of SCI in adult rodents to investigate potential therapies to modulate inhibitory extracellular matrix molecules in an attempt to promote regeneration and plasticity of damaged respiratory pathways. Our strategy has the potential to restore breathing in animals lesioned at high cervical levels.
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