Regulation of RPTPβ/phosphacan expression and glycosaminoglycan epitopes in injured brain and cytokine-treated glia

Abstract

Several chondroitin sulfate proteoglycans (CSPGs) are upregulated after CNS injury and are thought to limit axonal regeneration in the adult mammalian CNS. Therefore, we examined the expression of the CSPG, receptor protein tyrosine phosphatase β (RPTPβ)/phosphacan, after a knife lesion to the cerebral cortex and after treatment of glial cultures with regulatory factors. The three splice variants of this CSPG gene, the secreted isoform, phosphacan, and the two transmembrane isoforms, the long and short RPTPβ, were examined. Western blot and immunostaining analysis of injured and uninjured tissue revealed a transient decrease of phosphacan protein levels, but not of short RPTPβ, in the injured tissue from 1 to 7 days postlesion (dpl). By real time RT-PCR, we show that phosphacan and long RPTPβ mRNA levels are transiently down-regulated at 2 dpl, unlike those of short RPTPβ which increased after 4 dpl. In contrast to the core glycoprotein, the phosphacan chondroitin sulfate (CS) glycosaminoglycan epitope DSD-1 was up-regulated after 7 dpl. Phosphacan was expressed by cultivated astrocytes and oligodendrocyte precursors but was more glycanated in oligodendrocyte precursors, which produce more of DSD-1 epitope than astrocytes. Epidermal growth factor/transforming growth factor α strongly increased the astrocytic expression of long RPTPβ and phosphacan and slightly the short RPTPβ protein levels, while interferon γ and tumor necrosis factor α reduced astrocytic levels of phosphacan, but not of the receptor forms. Examining the effects of phosphacan on axon growth from rat E17 cortical neurons, we found that phosphacan stimulates outgrowth in a largely CS dependent manner, while it blocks the outgrowth-promoting effects of laminin through an interaction that is not affected by removal of the CS chains. These results demonstrate complex injury-induced modifications in phosphacan expression and glycanation that may well influence axonal regeneration and repair processes in the damaged CNS.