Cellular and molecular mechanisms of glial scarring and progressive cavitation: in vivo and in vitro analysis of inflammation-induced secondary injury after CNS trauma.

Post-traumatic cystic cavitation, in which the size and severity of a CNS injury progress from a small area of direct trauma to a greatly enlarged secondary injury surrounded by glial scar tissue, is a poorly understood complication of damage to the brain and spinal cord. Using minimally invasive techniques to avoid primary physical injury, this study demonstrates in vivo that inflammatory processes alone initiate a cascade of secondary tissue damage, progressive cavitation, and glial scarring in the CNS. An in vitro model allowed us to test the hypothesis that specific molecules that stimulate macrophage inflammatory activation are an important step in initiating secondary neuropathology. Time-lapse video analyses of inflammation-induced cavitation in our in vitro model revealed that this process occurs primarily via a previously undescribed cellular mechanism involving dramatic astrocyte morphological changes and rapid migration. The physical process of cavitation leads to astrocyte abandonment of neuronal processes, neurite stretching, and secondary injury. The macrophage mannose receptor and the complement receptor type 3 beta2-integrin are implicated in the cascade that induces cavity and scar formation. We also demonstrate that anti-inflammatory agents modulating transcription via the nuclear hormone receptor peroxisome proliferator-activated receptor-gamma may be therapeutic in preventing progressive cavitation by limiting inflammation and subsequent secondary damage after CNS injury.

Activated macrophages and the blood-brain barrier: inflammation after CNS injury leads to increases in putative inhibitory molecules.

The cellular responses to spinal cord or brain injury include the production of molecules that modulate wound healing. This study examined the upregulation of chondroitin sulfate proteoglycans, a family of molecules present in the wound healing matrix that may inhibit axon regeneration in the central nervous system (CNS) after trauma. We have demonstrated increases in these putative inhibitory molecules in brain and spinal cord injury models, and we observed a close correlation between the tissue distribution of their upregulation and the presence of inflammation and a compromised blood-brain barrier. We determined that the presence of degenerating and dying axons injured by direct trauma does not provide a sufficient signal to induce the increases in proteoglycans observed after injury. Activated macrophages, their products, or other serum components that cross a compromised blood-brain barrier may provide a stimulus for changes in extracellular matrix molecules after CNS injury. While gliosis is associated with increased levels of proteoglycans, not all reactive astrocytes are associated with augmented amounts of these extracellular matrix molecules, which suggests a heterogeneity among glial cells that exhibit a reactive phenotype. Chondroitin sulfate also demarcates developing cavities of secondary necrosis, implicating these types of boundary molecules in the protective response of the CNS to trauma.

Glial cell extracellular matrix: boundaries for axon growth in development and regeneration.

Astrocytes and other glia in the central nervous system are now thought to produce molecules that negatively modulate axon growth, thereby influencing axon pathfinding in both development and regeneration. The relevant evidence for glial cell boundaries and the inhibitory molecules present in these extracellular matrix structures is discussed in this minireview.

Spinal cord injury in the rat: treatment with bacterial lipopolysaccharide and indomethacin enhances cellular repair and locomotor function.

Trauma to the rat's spinal cord results in a lesion characterized by ingrowth of glial cells, accumulation of macrophages, and the progressive development of necrosis and cavitation. Since, when appropriately activated, both astrocytes and macrophages secrete growth-promoting cytokines, we examined whether treatment with drugs that stimulate the secretory activities of these cells might promote tissue repair and reduce necrosis in the traumatized spinal cord. The spinal cord of rats was crushed extradurally at T8 and the rats were injected intraperitoneally with (i) a lipopolysaccharide (LPS) or ImuVert to activate cytokine secretion, (ii) Indomethacin to reduce necrosis by inhibiting prostaglandin synthesis, (iii) a combination of LPS+Indomethacin, or (iv) vehicle. After 28 days the lesion site was examined quantitatively by light microscopical image analysis. The lesion of vehicle-treated control animals showed large cavities, extensive infiltration by debris-engorged macrophages, and relatively few axons. Treatment with LPS or ImuVert significantly reduced the degree of cavitation and increased the number of cells and axons in the lesion. Treatment with LPS+Indomethacin was significantly more effective than treatment with LPS alone, while treatment with Indomethacin alone was ineffective. To test whether the histopathological differences between treated and control rats might be reflected in functional improvement, rats were subjected to a contusion (weight-drop) injury and their walking ability was quantified by the Tarlov scale for 28 days postoperatively. Treatment with LPS+Indomethacin significantly improved locomotor function of animals subjected to a moderate (1.25 g x 20 cm) injury. We conclude that tissue repair and functional recovery after spinal cord injury are enhanced by combined treatment with agents that promote the secretory activities of the nonneuronal cells and that inhibit prostaglandin synthesis. These results indicate that the search for more effective treatments should include studies on combinations of drugs having different pharmacological specificities.

Regeneration of adult axons in white matter tracts of the central nervous system.

It is widely accepted that the adult mammalian central nervous system (CNS) is unable to regenerate axons. In addition to physical or molecular barriers presented by glial scarring at the lesion site, it has been suggested that the normal myelinated CNS environment contains potent growth inhibitors or lacks growth-promoting molecules. Here we investigate whether adult CNS white matter can support long-distance regeneration of adult axons in the absence of glial scarring, by using a microtransplantation technique that minimizes scarring to inject minute volumes of dissociated adult rat dorsal root ganglia directly into adult rat CNS pathways. This atraumatic injection procedure allowed considerable numbers of regenerating adult axons immediate access to the host glial terrain, where we found that they rapidly extended for long distances in white matter, eventually invading grey matter. Abortive regeneration correlated precisely with increased levels of proteoglycans within the extracellular matrix at the transplant interface, whereas successfully regenerating transplants were associated with minimal upregulation of these molecules. Our results demonstrate, to our knowledge for the first time, that reactive glial extracellular matrix at the lesion site is directly associated with failure of axon regrowth in vivo, and that adult myelinated white matter tracts beyond the glial scar can be highly permissive for regeneration.