Traumatic brain injury results in disparate regions of chondroitin sulfate proteoglycan expression that are temporally limited.

Traumatic brain injury (TBI) is associated with significant neurological deficits that in most cases do not completely resolve with time. Because axonal injury produced by shearing impact forces is a major hallmark of TBI (Adams et al., 1989), it seems likely that therapies directed toward enhancing axon repair and connectivity have the potential to improve functional outcomes after TBI. However, there is a large amount of literature that describes a significant axon growth inhibitory environment after central nervous system (CNS) lesions (Asher et al., 2000, 2002; Levine et al., 2001; Chen et al., 2003; Dobbertin et al., 2003; Sandvig et al., 2004; Silver and Miller, 2004; Dubreuil et al., 2006; Marklund et al., 2006; Thompson et al., 2006). This environment consists of increases in axon growth inhibitory extracellular matrix and myelin-associated molecules (growth-IMs) as well as increases in reactive astrocytes, microglia, and oligodendrocytes, all of which are associated with glial scar formation and can contribute to prevention of axon plasticity or synapse formation (Fawcett and Asher, 1999). Unfortunately, there has been little research to determine how the brain responds to TBI in terms of the glial scar and the local brain environment for growth inhibition. By addressing this gap in our knowledge, it should be possible to determine whether axon sprouting could underlie some spontaneous recovery of function after CNS injury, as some studies suggest (Castro-Alamancos and Borrell, 1995; Christman et al., 1997; Hulsebosch et al., 1998; Emery et al., 2000), and then proceed to determine whether this might be enhanced through therapeutic measures. The major growth-IMs so far identified consist of myelin-associated glycoprotein, oligodendrocyte myelin glycoprotein, chondroitin sulfate proteoglycans (CSPGs), and NOGO, and these growth-IMs are generally, but not always, up-regulated after CNS injury (Fawcett and Asher, 1999; Sandvig et al., 2004; Silver and Miller, 2004). The CSPGs are extracellular molecules composed of secreted and transmembrane forms. They have a central protein core attached to glycosaminoglycan side chains containing repeating dissacharide units. The members of the CSPG family, versican, neurocan, phosphacan, brevican, neurocan, and NG2, differ in the protein core and the number and length of sulfation of the side chains (Morgenstern et al., 2002), which confers binding properties and hence function (Properzi et al., 2003). CSPGs undergo proteolytic cleavage to create additional forms (Morgenstern et al., 2002), and most are normally expressed by astrocytes, although there is now good evidence that immature oligodendrocytes either produce and/or associate with neurocan, versican, and NG2 (Ong and Levine, 1999; Asher et al., 2000, 2002) and that neurons produce at least some species of aggrecan, brevican, and neurocan (Oohira et al., 1994; Lander et al., 1998; Seidenbecher et al., 2002). The CSPGs are present in normal brain, either within the extracellular space or aggregated as perineuronal nets that encase or surround neurons (Lander et al., 1998). They have been shown to be up-regulated after spinal cord injury (Lemons et al., 1999; Jones et al., 2003; Tang et al., 2003), stroke (Carmichael et al., 2005), hypoxic-ischemic injury (Aya-ay et al., 2005), or nigrostriatal lesion (Moon et al., 2002), and they are major inhibitory components of the resulting glial scar (Fawcett and Asher, 1999). A CSPG receptor site has not been identified, but Rho GTPase, which is increased after TBI (Dubreuil et al., 2006), appears to play a role (Jain et al., 2004) as does the epidermal growth factor receptor (Koprivica et al., 2005). The idea that all CSPGs are largely up-regulated after CNS injury and that they are all inhibitory to axon outgrowth is an oversimplification. Individual CSPGs have differential and even opposing effects on outgrowth, depending on the specific neuronal population and the developmental state of the neuron (Maeda and Noda, 1996). Previous reports suggest that the CSPG response to injury is quite heterogenous and may vary as a function of injury model or clinical disease state, temporal factors, or the specific species of growth-IMs (Levine, 1994; Jaworski et al., 1999; Asher et al., 2000, 2002; Lemons et al., 2001; Garwood et al., 2003; Dobbertin et al., 2003; Okamoto et al., 2003; Miyata et al., 2004). Although on the face of it one might anticipate similar pathophysiology after TBI and other CNS injuries such as stroke, the initial insult very clearly differs between these two injuries, especially with regard to the mechanism of primary axonal damage (Bramlett and Dietrich, 2004). Differences might therefore be expected in the temporal and/or spatial expression of molecules that affect axonal repair as a function of injury model. We reasoned that a study designed to investigate CSPGs associated with the glial scar after TBI would be an important first step for future efforts to design therapies that might enhance axon repair and functional outcome following TBI. The current study was designed to determine temporal changes in mRNA and protein expression levels of the CSPGs aggrecan, phosphacan, versican, and neurocan and to determine their regional localization and cellular association by using immunohistochemistry, in a controlled cortical impact (CCI) model of mild-to-moderate TBI.