Michael T. Fitch

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.

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.

Abstract.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.

Beyond the Glial Scar: Cellular and Molecular Mechanisms by which Glial Cells Contribute to CNS Regenerative Failure

Publisher Summary This chapter discusses the cellular as well as molecular responses of oligodendrocytes, astrocytes, and microglial cells to traumatic injury and their proposed roles in the failure of functional regeneration of the adult mammalian central nervous system (CNS). Astrocyte cellular hypertrophy, hyperplasia, and increased production of intermediate filaments characterize astrocyte gliosis, and cells responding in these ways to injury are often referred to as “reactive astrocytes.” Astocytes are easily identified by immunocytochemical methods directed toward the astrocyte specific glial fibrillary acidic protein (GFAP), and astrocyte hypertrophy and increased GFAP following injury have been demonstrated using these techniques to label reactive astrocytes. The inflammatory response in the CNS following injury is composed primarily of two components: activation of intrinsic microglial cells and recruitment of bone marrow-derived inflammatory cells from the peripheral bloodstream. Chemical injuries to the brain lead to a predominantly microglial cell inflammatory response, whereas direct stab wounds and injections are composed mostly of peripheral monocytes. Microglial cytokines are the possible sources of nervous system impairment following injury, and neutrophilic leukocytes may augment necrosis and inflammation following a CNS wound. Microgial cells are capable of releasing cytotoxic factors that can kill neurons, and play a role in disconnecting existing neuronal connections and destroying neurons surrounding areas of injury.