Combining Peripheral Nerve Grafts and Chondroitinase Promotes Functional Axonal Regeneration in the Chronically Injured Spinal Cord

Abstract

Because there currently is no treatment for spinal cord injury, most patients are living with long-standing injuries. Therefore, strategies aimed at promoting restoration of function to the chronically injured spinal cord have high therapeutic value. For successful regeneration, long-injured axons must overcome their poor intrinsic growth potential as well as the inhibitory environment of the glial scar established around the lesion site. Acutely injured axons that regenerate into growth-permissive peripheral nerve grafts (PNGs) reenter host tissue to mediate functional recovery if the distal graft-host interface is treated with chondroitinase ABC (ChABC) to cleave inhibitory chondroitin sulfate proteoglycans in the scar matrix. To determine whether a similar strategy is effective for a chronic injury, we combined grafting of a peripheral nerve into a highly relevant, chronic, cervical contusion site with ChABC treatment of the glial scar and glial cell line-derived neurotrophic factor (GDNF) stimulation of long-injured axons. We tested this combination in two grafting paradigms: (1) a peripheral nerve that was grafted to span a chronic injury site or (2) a PNG that bridged a chronic contusion site with a second, more distal injury site. Unlike GDNF-PBS treatment, GDNF-ChABC treatment facilitated axons to exit the PNG into host tissue and promoted some functional recovery. Electrical stimulation of axons in the peripheral nerve bridge induced c-Fos expression in host neurons, indicative of synaptic contact by regenerating fibers. Thus, our data demonstrate, for the first time, that administering ChABC to a distal graft interface allows for functional axonal regeneration by chronically injured neurons.

Peripheral Nerve Grafts and Chondroitinase ABC Application Improves Functional Recovery After Complete Spinal Cord Transection

“…These results are consistent with previous studies, which have shown axons growing through the distal interface of the PNG [16][17][18].…”

“…It is well documented that regeneration of injured axons after an acute or chronic SCI can be facilitated by the application of ChABC [8,18]. In this study, we did have more functional recovery in the ChABC treated rats, although it was not always significant.…”

“…There have been several promising results when using PNGs in incomplete spinal cord injuries [16][17][18]. However, when using an incomplete injury model, it can be difficult to inflict the exact same injury, so that all rats start the recovery process with the same level of function.…”

“…For all surgical procedures, rats were anesthetized with 4% isofluorane, and then maintained on 2-3% isofluorane for the duration of the surgery. To prevent graft rejection, rats were given 10 mg/kg/24 hr of cyclosporine A via subcutaneous injection for 2 days pre-operatively and for 14 days afterwards, then 100 µg/mL of CsA was administered through drinking water for the remainder of the study [16,18,28].…”

“…This creates an anatomical gap that axons are unable to cross. There have been several approaches used to bridge this anatomical gap, including poly(lactide-co-glycolide) (PLGA) scaffolds [9,10], Schwann cells [11], poly(ethylene glycol) hydrogels [12,13], agarose scaffolds [14,15], and peripheral nerve grafts (PNGs) [16][17][18][19][20][21][22]. PNGs are a living tissue containing several growth factors that have been shown to promote axonal growth, and cytokines that modulate inflammation [23,24].…”