The Role of Calpain and Proteasomes in the Degradation of Carbonylated Neuronal Cytoskeletal Proteins in Acute Experimental Autoimmune Encephalomyelitis

The present study was designed to investigate the role of calpain and the proteasome in the removal of oxidized neuronal cytoskeletal proteins in myelin basic protein-induced experimental autoimmune encephalomyelitis (EAE). To this end, EAE rats received a single intrathecal injection of calpeptin or epoxomicin at the first sign of clinical disease. Forty-eight hours later, animals were sacrificed and lumbar spinal cord segments were dissected and used for biochemical analyses. The results show that calpain and proteasome activity is specifically, but partially, inhibited with calpeptin and epoxomicin, respectively. Calpain inhibition causes an increase in total protein carbonylation and in the amount of neurofilament proteins (NFPs), β-tubulin and β-actin that were spared from degradation, but no changes are seen in the oxidation of any of three NFPs. By contrast, proteasome inhibition has no effect on total protein carbonylation or cytoskeletal protein degradation but increases the amount of oxidized NFH and NFM. These results suggest that while the proteasome may contribute to removal of oxidized NFPs, calpain is the main protease involved in degradation of neuronal cytoskeleton and does not preferentially targets oxidized NFPs species in acute EAE. Different results were obtained in a cell-free system, where calpain inhibition rises the amount of oxidized NFH, and proteasome inhibition fails to change the oxidation state of the NFPs. The later finding suggests that the preferential degradation of oxidized NFH and NFM in vivo by the proteasome occurs via the 26S and not the 20S particle.