Increase of HCN current in SOD1-associated amyotrophic lateral sclerosis

The clinical manifestations of sporadic amyotrophic lateral sclerosis (ALS) vary widely. However, the current classification of ALS is based mainly on clinical presentations, and the roles of electrophysiological and biomedical biomarkers remain limited. Herein, we investigated a group of patients with sporadic ALS and an ALS mouse model with superoxide dismutase 1 (SOD1)/G93A transgenes using nerve excitability tests (NETs) to investigate axonal membrane properties and chemical precipitation, followed by ELISA analysis to measure plasma misfolded protein levels. Six of 19 patients (31.6%) with sporadic ALS had elevated plasma misfolded SOD1 protein levels. In sporadic ALS patients, only those with elevated misfolded SOD1 protein levels showed an increased inward rectification in the current-voltage threshold curve and an increased threshold reduction in the hyperpolarizing threshold electrotonus in the NET study. Two familial ALS patients with SOD1 mutations also exhibited similar electrophysiological patterns of NET. For patients with sporadic ALS showing significantly increased inward rectification in the current-voltage threshold curve, we noted an elevation in plasma misfolded SOD1 level, but not in total SOD1, misfolded C9orf72 or misfolded phosphorylated TDP43 levels. Computer simulations demonstrated that the aforementioned axonal excitability changes are likely to be associated with an increase in hyperpolarization-activated cyclic nucleotide-gated (HCN) current. In SOD1/G93A mice, NET also showed an increased inward rectification in the current-voltage threshold curve, which could be reversed by a single injection of the HCN channel blocker, ZD7288. Daily treatment of SOD1/G93A mice with ZD7288 partly prevented the early motor function decline and spinal motor neuron death. In summary, sporadic ALS patients with elevated plasma misfolded SOD1 exhibited similar patterns of motor axonal excitability changes to familial ALS patients and ALS mice with mutant SOD1, suggesting the existence of SOD1-associated sporadic ALS. The observed NET pattern of increased inward rectification in the current-voltage threshold curve was attributable to an elevation in the HCN current in SOD1-associated ALS.