Scar-Busting Chondroitinase With Peripheral Nerve Grafting Promotes Axonal Regeneration in Chronic Spinal Cord Injury (2010)

Houlé et al at Drexel College of Medicine, report in the Journal of Neuroscience (2009, 29(47):14881–14890) that glial scar-busting chondroitinase ABC in combination with peripheral nerve grafting allows for functional axonal regeneration in rats injured with a chronic (8 weeks) cervical hemicontusion. After spinal cord injury, numerous barriers to successful axon regeneration exist, including glial scarring which produce signals inhibiting axonal recovery via chondroitin sulfate proteoglycan upregulation (CSPGs). Digestion of CSPGs with the bacterial chonroitinase ABC enzyme, cleaves the inhibitory sugar moiety from the protein core, improving regrowth of acutely injured axons in vivo. Treatment with chondroitinase ABC in acutely injured animals has recently shown to improve axonal regeneration and functional outcome, but its role in chronically injured spinal cords has not been studied. This recent report hypothesizes that even chronically injured spinal cords can regenerate long-injured axons with the support of growth-promoting peripheral nerve graft, CSPG digestion and GDNF.FIGURE: Peripheral nerve graft regenerated axons (in green) after treating with ChABC. Axons were found several millimeters away from the distal graft interface with the injured spinal cord (inset box with asterix magnified). Control specimens did not show any axon staining at corresponding distances (not shown).Investigators used a rat model of cervical hemicontusion to produce the spinal cord injury at the C5 level. Animals were untreated for 8 weeks to produce a chronic spinal cord injury site. The injured site was then treated with either a spanning peripheral nerve graft in the aspirated necrotic area or a bridging peripheral nerve graft. For the bridging graft, the proximal end of the nerve was placed at the chronic injury site, while the distal end was placed 3 weeks later at a separate acutely injured site caudal at C7. Prior to grafting, sites were treated with GDNF or GDNF plus ChABC. Animals were then tested for open field and forced locomotion to evaluate stride length and time. Biotinylated dextran amine was used by tracer diffusion to label regenerating axons. Histologic evaluation included stains for CSPG, GFAP, and c-Fos. Within this experimental design, treatment with chondroitinase showed only modest improvement in locomotion scores. Treatment in the setting of a bridging graft was more beneficial than a spanning graft. However, chondroitinase treatment of a chronically injured site allowed for regenerating axons from a nerve graft to extend into the injury site as well as allow for chronically injured axons regenerating out of a peripheral nerve graft bridge to do the same. Trans-synaptic neuronal activation as marked by c-Fos expression was significantly elevated in sections of chronically injured spinal cord treated with bridging nerve graft and ChABC plus GDNF. Previous studies have demonstrated the efficacy of glial-scar busting treatment on functional improvement following acute spinal cord injury. This study shows that glial-scar busting ChABC can also promote axonal regeneration in chronic spinal cord injury in the setting of a bridging peripheral nerve graft. Despite the lack of robust functional improvement in this study, the authors' findings are compelling as they suggest that axonal regeneration is possible even in long standing spinal cord injury. JOHN H. CHI