Transplants of cells genetically modified to express neurotrophin-3 rescueaxotomized Clarke's nucleus neurons after spinal cord hemisection in adultrats

To test the idea that genetically engineered cells can rescue axotomized ne urons, we transplanted fibroblasts and immortalized neural stem cells (NSCs ) modified to express neurotrophic factors into the injured spinal cord. Th e neurotrophin-3 (NT-3) or nerve growth factor (NGF) transgene was introduc ed into these cells using recombinant retroviral vectors containing an inte rnal ribosome entry site (IRES) sequence and the beta -galactosidase or alk aline phosphatase reporter gene. Bioassay confirmed biological activity of the secreted neurotrophic factors. Clarke's nucleus (CN) axons, which proje ct to the rostra] spinal cord and cerebellum, were cut unilaterally in adul t rats by T8 hemisection. Rats received transplants of fibroblasts or NSCs genetically modified to express NT-3 or NGF and a reporter gene, only a rep orter gene, or no transplant. Two months postoperatively, grafted cells sur vived at the hemisection site. Grafted fibroblasts and NSCs expressed a rep orter gene and immunoreactivity for the NGF or NT-3 transgene. Rats receivi ng no transplant or a transplant expressing only a reporter gene showed a 3 0% loss of CN neurons in the L1 segment on the lesioned side. NGF-expressin g transplants produced partial rescue compared with hemisection alone. Ther e was no significant neuron loss in rats receiving grafts of either fibrobl asts or NSCs engineered to express NT-3. We postulate that NT-3 mediates su rvival of CN neurons through interaction with trkC receptors, which are exp ressed on CN neurons. These results support the idea that NT-3 contributes to long-term survival of axotomized CN neurons and show that genetically mo dified cells rescue axotomized neurons as efficiently as fetal CNS transpla nts. J. Neurosci. Res. 65:549-564, 2001. (C) 2001 Wiley-Liss, Inc.