647. Expression of Neurotrophin-3 Promotes Axonal Plasticity in the Acute but Not Chronic Injured Spinal Cord

Abstract

There is little spontaneous recovery in the adult spinal cord after injury because of its limited ability to regenerate and sprout axons. We reported recently that over-expression of neurotrophin-3 (NT-3) by motoneurons in the spinal cord of rats promoted sprouting of axons after acute spinal cord injury. However, NT-3 did not induce sprouting when the spinal cord was not injured. This suggests that one or more factors produced by the injury may act with NT-3 to elicit axonal plasticity. One possible source of the factors associated with acute injury is the process of Wallerian degeneration (WD). WD is a complex cascade of events caused by an axotomy that include the breakdown of the cut axons, opening of the blood-brain barrier, phagocytosis and clearance of myelin debris by microglial, and death of oligodendrocytes. To determine whether WD plays a role in axonal sprouting after injury, we treated the rats with a replication-defective adenoviral vector carrying the NT-3 gene (Adv.NT-3) 2 weeks or 4 months after a producing discrete injury in a tract that innervates the spinal cord. The corticospinal tract (CST) was cut unilaterally at the level of the medulla and the Adv.NT-3 was delivered to the spinal motoneurons by retrograde transport through the sciatic nerve. Adv.LacZ was used as a control vector. Four days later after delivery of vectors, biotinylated dextran amine (BDA) was injected into the area of the sensorimotor cortex that innervated the unlesioned CST to label its axons in the spinal cord. Morphometric analysis of axonal sprouting 3 weeks after BDA injection was performed to measure the number of CST axons that arise from the intact CST, traverse the midline, and grow into the gray matter of the lesioned side of the spinal cord where the NT-3 was over-expressed. In the animals that were treated 2 weeks after injury, the number of axons that grew across the midline was significantly greater in the rats treated with Adv.NT-3 than in the rats treated with Adv.LacZ. These axons persisted for up to 6 months after Adv.NT-3 delivery. In contrast, when Adv.NT-3 was delivered 4 months after lesion, there was not a significant difference in the number of CST axons that crossed the midline compared to controls. Since the processes of WD have resolved within 4 months after injury, these data demonstrate that WD is a likely source of the signals that support axonal plasticity. Identification of these co-inducing factors that are produced by WD may provide a basis for an effective gene therapy for chronic spinal cord injury.