SOD1 Protein Content in Human Central Nervous System and Peripheral Tissues

Gene silencing therapy is an effective treatment for amyotrophic lateral sclerosis (ALS) patients carrying mutations in the superoxide dismutase-1 (SOD1) gene aiming to reduce noxious forms of SOD1 in the central nervous system (CNS). The normal steady-state level of SOD1 protein in human CNS is therefore of interest but is contested. In this work we have analyzed SOD1 protein content, total protein content, and SOD1 enzymatic activity in six areas of the CNS as well as in four peripheral tissues from sporadic and familial ALS patients and non-ALS controls. Our results show that SOD1 in the human CNS constitutes around 100 μg/g wet weight corresponding to about 0.16% of the total protein in the studied areas. Of the peripheral tissues analyzed, kidney and erythrocytes contain roughly equal amounts, liver higher, and skeletal muscle lower levels of SOD1 compared to the CNS. This data shows SOD1 protein levels around 10 times lower compared to previously published figures. However, SOD1 can still be considered an abundant protein considering that > 12 000 proteins are expressed in human cells. There was no difference in SOD1 protein content between sporadic or familial ALS patients and control individuals. The level and activity of SOD1 are not deviating in the areas of the CNS that are most vulnerable to ALS. Instead, insufficient control of SOD1 structure and aggregation could be important factors behind the vulnerability of motor areas to SOD1 proteotoxicity.