Stimulation of axonal sprouting by trophic factors immobilized within the wound core

Traumatic injury to the CNS results in peri-wound sprouting without significant axonal growth beyond the lesion edge. We have previously demonstrated that dopaminergic sprouting in the injured striatum follows an increasing gradient of BDNF and GDNF expression, with sprouting ceasing at the point of maximal factor expression. Progressively more complicated associations of sprouting fibers with increasingly activated microglia and macrophages suggest these factors are localized to the cell surface. To establish whether an increased concentration of immobilized BDNF and GDNF could stimulate axonal growth beyond the lesion edge, both factors were covalently attached to 10 µm polycarbonate microspheres. These spheres were implanted into the site of striatal injury 1 week after lesioning. A profusion of axons grew from the region of the lesion edge across the surface of the spheres. Some axons traversed the entire site of injury. Ultrastructural examination demonstrated the juxtaposition of regenerating axons to the surface of implanted spheres. CSPG immunostaining demonstrated that, in animals implanted with neurotrophin-microspheres, axonal growth was induced beyond the area of maximal CSPG reactivity. Surprisingly however, CSPG production at the wound edge was greater in control animals than those implanted with neurotrophin-microspheres. Overall, we show that axonal growth can be encouraged beyond the wound edge by an elevated concentration of immobilized trophic factors. This growth occurs despite the presence of inhibitory CSPGs at the lesion edge. Axonal growth appears to be stimulated mainly via the direct effects of neurotrophins. However, there also appears to be an indirect mechanism whereby neurotrophins reduce the synthesis of CSPG at the wound edge, making the peri-wound environment more permissive.