CNS injury, glial scars, and inflammation: Inhibitory extracellular matrices and regeneration failure

Abstract

Spinal cord and brain injuries lead to complex cellular and molecular interactions within the central nervous system in an attempt to repair the initial tissue damage. Many studies have illustrated the importance of the glial cell response to injury, and the influences of inflammation and wound healing processes on the overall morbidity and permanent disability that result. The abortive attempts of neuronal regeneration after spinal cord injury are influenced by inflammatory cell activation, reactive astrogliosis and the production of both growth promoting and inhibitory extracellular molecules. Despite the historical perspective that the glial scar was a mechanical barrier to regeneration, inhibitory molecules in the forming scar and methods to overcome them have suggested molecular modification strategies to allow neuronal growth and functional regeneration. Unlike myelin associated inhibitory molecules, which remain at largely static levels before and after central nervous system trauma, inhibitory extracellular matrix molecules are dramatically upregulated during the inflammatory stages after injury providing a window of opportunity for the delivery of candidate therapeutic interventions. While high dose methylprednisolone steroid therapy alone has not proved to be the solution to this difficult clinical problem, other strategies for modulating inflammation and changing the make up of inhibitory molecules in the extracellular matrix are providing robust evidence that rehabilitation after spinal cord and brain injury has the potential to significantly change the outcome for what was once thought to be permanent disability.

Figure 1. Wound healing, secondary damage, and abortive regeneration in the central nervous system

The lesion cavity of a central nervous system injury expands as inflammatory cells interact with the surrounding reactive astrocytes and other reactive glial cells. This region of glial scarring is associated with upregulation of inhibitory extracellular matrix molecules, such as proteoglycans, that are distributed in an increasing concentration gradient from the lesion penumbra to the lesion center. This intense inflammatory response leads to a cascade of secondary damage to axons initially spared from direct trauma, and demyelination of adjacent axons that are not readily re-myelinated by adult oligodendrocytes and precursor cells. The gradient of inhibitory molecules upregulated in the areas of intense inflammation provides an environment that is nonpermissive for regeneration, and dystrophic neurons develop the classically-described sterile end-balls with clubbed endings that are characteristic of abortive attempts at regeneration.