ALTERING THE ECM AFTER TBI TO ENHANCE REGENERATION AND PLASTICITY

TBI often leads to permanent disability because injured axons fail to regenerate. This regenerative failure is due, in part, to the inhibitory environment of the glial scar that forms after injury and acts as a molecular and physical barrier to axon regeneration. The sequence of events following TBI that leads to the formation of the glial scar is not understood. Neither are the differences in the glial response to different types of TBI. Scar formation is strongest after penetrating brain injury, but the glial response to other types of TBI is poorly described. As the glial response to injury results in prolonged changes in the environment in which attempts to promote regeneration will occur, it is vital that we understand the potentially harmful alterations in glial function caused by injury and develop ways to counteract them.Glial cells, specifically astrocytes and oligodendrocyte progenitor cells (OPCs), deposit an injury-induced extracellular matrix (ECM) that contributes to the post-lesion environment preventing neuronal reconnection and plasticity. It is thought that this limited regeneration and plasticity is due to the properties of the central nervous system ECM, especially the role of chondroitin sulfate proteoglycans (CSPGs). Much of this inhibitory biological activity is due to the sulfated glycosaminoglyan (GAG) sugar side chains on CSPGs. Interventions that degrade the GAG chains of the CSPGs with the bacterial enzyme, chondroitinase ABC (cABC), have led to increased functional recovery in rodent models of brain and spinal cord injury. Understanding the mechanisms through which glial cells respond to injury to upregulate synthesis and modification of the CSPGs, may provide a mechanism to intervene to reduce the inhibitory environment post-lesion. Our hypothesis therefore is that the glial response to TBI creates a hostile environment for differentiation and regeneration and thus reduced plasticity. Alteration of the post-lesion ECM, and specifically the level or composition of the CSPGs will promote a more supportive regenerative environment and enhanced plasticity. This project therefore seeks to understand the pathology surrounding the glial response to different types of TBI. We will then, through manipulation of production or levels of CSPGs, seek to determine whether these alterations lead to changes in plasticity or regeneration after TBI. Specifically we will1) Characterize the glial response and ECM deposition after controlled cortical impact (CCI) in mice. We will compare the activation and proliferation of different glial cells together with changes in the expression pattern of CSPGs produced after CCI.2) Manipulate the glial response and composition of the ECM after CCI through the use of a mutant mouse with an astrocytic-specific expression of a dominant negative TGF-¿ receptor. We will determine the effect of these manipulations on the cell fate of endogenous neural stem cells in the subventricular zone of the lateral ventricle through retroviral labeling.3) Evaluate changes in axonal structure and cortical barrel-field plasticity following siRNA or pharmacological manipulations that affect the composition of the ECM after TBI.These experiments will elucidate many aspects of the glial response to different types of TBI, determine whether manipulation of the glial-produced ECM may enhance plasticity and regeneration and lay the groundwork for further experiments seeking to manipulate glial responses to achieve greater regeneration after injury.