Non-neuronal induction of immunoproteasome subunits in an ALS model: Possible mediation by cytokines

Protein aggregation is a pathologic hallmark of familial amyotrophic lateral sclerosis caused by mutations in the Cu, Zn superoxide dismutase gene. Although SOD1-positive aggregates can be cleared by proteasomes, aggregates have been hypothesized to interfere with proteasome activity, leading to a vicious cycle that further enhances aggregate accumulation. To address this issue, we measured proteasome activity in transgenic mice expressing a G93A SOD1 mutation. We find that proteasome activity is induced in the spinal cord of such mice compared to controls but is not altered in uninvolved organs such as liver or spleen. This induction within spinal cord is not related to an overall increase in the total number of proteasome subunits, as evidenced by the steady expression levels of constitutive α7 and β5 subunits. In contrast, we found a marked increase of inducible beta proteasome subunits, LMP2, MECL-1 and LMP7. This induction of immunoproteasome subunits does not occur in all spinal cord cell types but appears limited to astrocytes and microglia. The induction of immunoproteasome subunits in G93A spinal cord organotypic slices treated with TNF-α and interferon-γ suggest that certain cytokines may mediate such responses in vivo. Our results indicate that there is an overall increase in proteasome function in the spinal cords of G93A SOD1 mice that correlates with an induction of immunoproteasomes subunits and a shift toward immunoproteasome composition. These results suggest that increased, rather than decreased, proteasome function is a response of certain cell types to mutant SOD1-induced disease within spinal cord.