| Spinal cord injury results in paralysis and loss of sensation. Astrocytes, prominent glial cells in the central nervous system, respond by proliferating, migrating to the site of injury, and forming the glial scar, a physical and chemical barrier to regeneration. Although traditionally viewed as detrimental to recovery, astrocytes also perform tasks that are vital for repair. Because of this dual role after injury, we hypothesized that if astrocytes can be stimulated after injury to enhance their beneficial, rather than detrimental characteristics, then axonal growth can be achieved.;In order to manipulate astrocytes, we must better understand the normal astrocyte response to injury. Astrocytes of the glial scar can arise from endogenous adult spinal cord neural progenitor cells (ASCNPCs) and also mature astrocytes present near the injury. We determined that early proliferating cells accumulate at the lesion border in the first week after injury, and that approximately half of those progenitors chronically differentiate into astrocytes. We predicted that those progenitors destined to be astrocytes could be identified prior to differentiation using developmental markers of radial glia. Using the astrocyte-specific radial glial marker brain lipid binding protein (BLBP), we identified possible astrocyte precursors in the parenchyma surrounding the injury site and in the cells surrounding the central canal early after injury. These studies show that manipulation may be best induced in the first week after injury to expand upon this population of astrocyte progenitors.;To promote a growth-supportive astrocyte response to injury, we used transforming growth factor alpha (TGFalpha), a ligand for the epidermal growth factor receptor (EGFR). We found that an intrathecal infusion of TGFalpha into the injured mouse spinal cord results in increased astrocyte and axonal infiltration into the lesion, a reponse we predicted was due to a direct activation of the EGFR on astrocytes. Indeed, early after injury, we found that astrocytes and cells surrounding the central canal were the primary expressors of the EGFR. Using a battery of in vitro assays, we showed that TGFalpha has direct, potent effects on ASCNPCs and astrocytes, and that these effects result in an axonal growth-supportive phenotype. To better target TGFalpha to the site of injury, we used an adeno-associated virus (AAV) that increases TGFalpha. Using this technique, we have shown that a short-term administration of the virus results in increased astrocyte and axonal presence within the lesion core. However, when administered chroncially, mice that receive TGFalpha-AAV do not exhibit infiltration of astrocytes or axons into the lesion, possibly due to chronic EGFR activation on astrocytes and other non-astrocytic cells.;In completing these studies, we have shown that there are astrocyte-specific precursors present early after injury that may be manipulated to improve repair, and that EGFR activation via TGFalpha is a successful method for promoting a growth-supportive astrocytic phenotype. Thus, using EGFR activation, we may be able to target specific astrocytic populations to acquire a pro-reparative phenotype and improve regeneration after SCI. This dissertation provides necessary information that lays a foundation for future studies employing the growth-supportive aspects of astrocytes to promote repair. |
