The multi-domain structure of extracellular matrix molecules: implications for nervous system regeneration

Extracellular matrix alterations, accelerated leukocyte infiltration and enhanced axonal sprouting after spinal cord hemisection in tenascin-C-deficient mice

The extracellular matrix glycoprotein tenascin-C has been implicated in wound repair and axonal growth. Its role in mammalian spinal cord injury is largely unknown. In vitro it can be both neurite-outgrowth promoting and repellent. To assess its effects on glial reactions, extracellular matrix formation, and axonal regrowth/sprouting in vivo, 20 tenascin-C-deficient and 20 wild type control mice underwent lumbar spinal cord hemisection. One, three, seven and fourteen days post-surgery, cryostat sections of the spinal cord were examined by conventional histology and by immunohistochemistry using antibodies against F4/80 (microglia/macrophage), GFAP (astroglia), neurofilament, fibronectin, laminin and collagen type IV. Fibronectin immunoreactivity was significantly down-regulated in tenascin-C-deficient mice. Moreover, fourteen days after injury, immunodensity of neurofilament-positive fibers was two orders of magnitude higher along the incision edges of tenascin-C-deficient mice as compared to control mice. In addition, lymphocyte infiltration was seen two days earlier in tenascin-C-deficient mice than in control mice and neutrophil infiltration was increased seven days after injury. The increase in thin neurofilament positive fibers in tenascin-C-deficient mice indicates that lack of tenascin-C alters the inflammatory reaction and extracellular matrix composition in a way that penetration of axonal fibers into spinal cord scar tissue may be facilitated.

Granulocyte-macrophage colony-stimulating factor induces an expression program in neonatal microglia that primes them for antigen presentation

Neonatal microglial cells respond to GM-CSF and M-CSF by acquiring different morphologies and phenotypes. To investigate the extent and consequences of this process, a global gene expression analysis was performed, with significant changes in transcript levels confirmed by biochemical analyses. Primary murine microglial cells underwent substantial expression reprogramming after treatment with GM-CSF or M-CSF with many differentially expressed transcripts important in innate and adaptive immunity. In particular, many gene products involved in Ag presentation were induced by GM-CSF, but not M-CSF, thus potentially priming relatively quiescent microglia cells for Ag presentation. This function of GM-CSF is distinct from its primary function in cell proliferation and survival.

Tenascin-Y is concentrated in adult nerve roots and has barrier properties in vitro

Several molecules have been identified as potential sources of the barriers to glial cell mixing and sensory regeneration that exist at the boundary between the peripheral and central nervous systems, including tenascin-C, tenascin-R, chondroitin sulfate proteoglycans, and NG2. Here we show that tenascin-Y, the avian homologue of tenascin-X, is concentrated in the proximal portions of peripheral nerves in the chicken. In vitro analyses of cultures enriched for Schwann cells demonstrate that recombinant tenascin-Y has dose-dependent effects on glial cell attachment, spreading, and migration. In addition, nanomolar concentrations of tenascin-Y cause the rapid collapse of sensory growth cones cultured on fibronectin, and regenerating sensory neurites preferentially migrate on fibronectin and avoid tenascin-Y in microstripe assays. We conclude that the expression pattern of tenascin-Y and its properties in vitro are consistent with a role as an inhibitor of glial cell migration and sensory regeneration in nerve roots.