Glial cell line-derived neurotrophic factor (GDNF), a distant member of the transforming growth factor-beta (TGF-beta) family, is widely expressed in the developing and adult central nervous system (CNS). Limited knowledge is available for its role in the repair of spinal cord injury (SCI). Preliminary results demonstrate that 1) distinct CNS regions that compose descending motor pathways expressed GFR(I and c-Ret, the two co-receptors that are required for GDNF signaling; 2) both GDNF and its co-receptors were increasingly expressed in these regions after SCI; 3) GDNF significantly enhanced axonal regeneration and myelination within mini-guidance channels implanted into a spinal hemisection gap when it was delivered either alone or in combination with Schwann cells (SCs); and 4) intrathecal delivery of GDNF demonstrated a protective/regenerative effect on distinct supraspinal axons following a contusive SCI. Thus, GDNF may play a novel therapeutic role in the regeneration and myelinogenesis of the injured spinal cord. We have previously also demonstrated that neurotrophins BDNF and NT-3 promoted robust axonal outgrowth from SC-bridge transplants into the distal host spinal cord; some of these axons formed bouton-like endings on host spinal neuronal cell bodies. The goal of the present proposal is to investigate whether a preformed growth-promoting pathway of GDNF (used individually or in combination with BDNF and NT-3), extending from the site of injury to the lumbar central pattern generator (CPG), promotes axons of responsive tracts to regenerate across the lesion and, more importantly, grow back into the host spinal cord to re-establish connections with appropriate distal neuronal targets. This may lead to improved recovery of hind limb locomotor function following SCI. Using a low-thoracic spinal cord hemisection and SC-seeded mini-channel implantation mode, we will test the central hypotheses that 1) regeneration of injured spinal and brainstem neurons across a SC-bridge transplant and into the distal spinal cord is regulated by the supply of neurotrophic factors (GDNF, BDNF, and NT-3) along the growth promoting pathway, and 2) recovery of hind limb locomotion is proportional to the number of spinal- and brainstem-derived axons that re-innervate neuronal targets within the central pattern generator. In addition, we will also investigate mechanisms underlying GDNF induced axonal regeneration and myelination by studying its direct or indirect effect on SCs both in vitro and in vivo. Finally, a combinatory repair strategy involving GDNF, SCs, guidance channels, neurotrophins, gene transfer, and dural repair will be employed to provide the best possible environment for observation of effective axonal regeneration and meaningful functional recovery.
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