UC San Diego

In order for axon regeneration to result in functional recovery after spinal cord injury (SCI), axons must grow beyond lesion sites and form synapses with neuronal targets in distal host tissue. We tested the hypothesis that neurotrophic factor gradients established distal to lesion sites in adult rats would promote growth of sensory axons into and beyond a site of injury and support synapse formation in denervated targets. First, gradients of neurotrophin expression were established rostral to cell- filled C3 dorsal column lesion sites using delivery of NT- 3-expressing lentiviral vectors. Sensory axons regenerated for short distances beyond lesion sites in animals treated with NT-3, but not control vectors. Long distance axon growth along neurotrophin gradients was not observed. However, when conditioning lesions, which are thought to initiate an overall growth program in neuronal cell bodies, were combined with NT-3 gradients beyond lesion sites, significantly more axons regenerated over greater distances into distal host tissue than when either treatment alone was given. We next tested the hypothesis that combinatorial treatment would promote growth of sensory axons originating from hindlimb dorsal root ganglia across C1 lesion sites into the nucleus gracilis, the termination site for these axons. Animals subjected to high cervical lesions and combinatorially treated with conditioning lesions and NT-3 exhibited significantly more axons within the nucleus gracilis than animals receiving control lentiviral vectors. Ultrastructural examination of nucleus gracilis tissue indicated the presence of possible synaptic contacts formed by regenerated axons in the denervated target. These data demonstrate that combined stimulation of a neuronal growth program (conditioning lesions) and provision of a positive stimulus at the level of the growth cone (NT-3) can promote long distance axon growth in the inhibitory environment beyond a spinal cord lesion site. In addition, combinatorial treatment can be used to guide axons to appropriate target regions and may support formation of synaptic contacts