Transplants of fibroblasts genetically modified to express BDNF promote axonal regeneration from supraspinal neurons following chronic spinal cord injury.

Transplants of fibroblasts genetically modified to express BDNF (Fb/BDNF) have been shown to promote regeneration of rubrospinal axons and recovery of forelimb function when placed acutely into the injured cervical spinal cord of adult rats. Here we investigated whether Fb/BDNF cells could stimulate supraspinal axon regeneration and recovery after chronic (4 week) injury. Adult female Sprague-Dawley rats received a complete unilateral hemisection injury at the third cervical spinal cord segment (C3). Four-five weeks later the injury site was exposed and rats received transplants of unmodified fibroblasts (Fb/UM) or Fb/BDNF. Four-five weeks after transplantation, locomotor recovery was examined on a test of forelimb usage and regeneration of supraspinal axons was studied following injection of the anterograde tracer biotin dextran amine (BDA). Rubrospinal tract (RST), reticulospinal tract (ReST), and vestibulospinal tract (VST) axons regenerated into transplants of either Fb/UM or Fb/BDNF but the length of axonal growth was significantly different in the two groups. The absolute distance of ReST growth was 1.8-fold greater in Fb/BDNF than in Fb/UM and the absolute distance of growth of RST and VST axons showed a statistically significant 4-fold increase. All three types of regenerated axons occupied a greater proportional length of Fb/BDNF transplants than of Fb/UM transplants. Only VST axons extended into the host spinal cord caudal to the Fb/BDNF grafts, but these axons were sparse. Rats receiving Fb/BDNF used both forelimbs together to explore walls of a cylinder more often than rats receiving Fb/UM, indicating partial recovery of forelimb usage. These results demonstrate that fibroblasts genetically modified to express BDNF promote axon regeneration from supraspinal neurons in the chronically injured spinal cord with accompanying partial recovery of locomotor performance.