Neurotrophin-3 gradients established by lentiviral gene delivery promote short-distance axonal bridging beyond cellular grafts in the injured spinal cord.

Neurotrophic factor delivery to sites of spinal cord injury (SCI) promotes axon growth into but not beyond lesion sites. We tested the hypothesis that sustained growth factor gradients beyond regions of SCI will promote significant axonal bridging into and beyond lesions. Adult rats underwent C3 lesions to transect ascending dorsal column sensory axons, and autologous bone marrow stromal cells were grafted into the lesion to provide a cellular bridge for growth into the injured region. Concurrently, lentiviral vectors expressing neurotrophin-3 (NT-3) or green fluorescent protein (GFP) (controls) were injected into the host cord rostral to the lesion to promote axon extension beyond the graft/lesion. Four weeks later, NT-3 gradients beyond the lesion were detectable by ELISA in animals that received NT-3-expressing lentiviral vectors, with highest average NT-3 levels located near the rostral vector injection site. Significantly more ascending sensory axons extended into tissue rostral to the lesion site in animals injected with NT-3 vectors compared with GFP vectors, but only if the zone of NT-3 vector transduction extended continuously from the injection site to the graft; any "gap" in NT-3 expression from the graft to rostral tissue resulted in axon bridging failure. Despite axon bridging beyond the lesion, regenerating axons did not continue to grow over very long distances, even in the presence of a continuing growth factor gradient beyond the lesion. These findings indicate that a localized and continuous gradient of NT-3 can achieve axonal bridging beyond the glial scar, but growth for longer distances is not sustainable simply with a trophic stimulus.