Intrinsic and extrinsic factors for the regeneration of brainstem axons after spinal cord injury

After traumatic injury, the successful regeneration of axons in the mammalian central nervous system (CNS) that leads to functional recovery will require a combination of therapeutic strategies. Following complete transection of the thoracic spinal cord, previous studies found the implantation of a bridge containing Schwann cells (SCs) and Matrigel promotes intraspinal but not supraspinal axon regeneration. Therefore, work in this dissertation sought to elucidate both intrinsic and extrinsic factors that enhance the regeneration of brainstem axons in a SC bridge. Adeno-associated viral (AAV) vector-mediated expression of green fluorescent protein (GFP) was utilized as a novel method for anterogradely tracing the regeneration of brainstem axons. AAV vectors also were used to determine if expression of a developmentally regulated transcription factor, MASH-1, allows mature brainstem neurons to regenerate axons more effectively. Compared to control animals, the expression of MASH-1 in brainstem neurons enhanced the regeneration of noradrenergic axons in a SC bridge and improved hindlimb joint movements. Surprisingly, in contrast to previous studies control animals also exhibited some brainstem axon regeneration. Analyses of the host spinal cord/SC bridge interfaces of control animals revealed variation from sharp to irregular boundaries. At the irregular boundaries of the rostral interface in those animals, the regeneration of axons from noradrenergic neurons and other brainstem populations associated with the presence of long astrocyte processes entering the bridge. Furthermore, the total number of astrocyte processes that entered from the rostral and caudal interfaces associated with improvements in hindlimb joint movements. Previous studies of the SC bridge model implanted pre-gelled mixtures of SCs and Matrigel, in contrast to initially fluid mixtures implanted in this dissertation. Therefore, a direct comparison of these two bridge preparations was performed and demonstrated that initially fluid bridges exhibit increases in both brainstem axon regeneration and the number of astrocyte processes in the bridge. In summary, this work is the first to demonstrate that overexpression of MASH-1 can enhance CNS axon regeneration and that astrocyte processes are important for axon regeneration across a SC bridge. The combination of these intrinsic and extrinsic factors offers a new therapeutic strategy for promoting functional recovery after CNS injury.