Poly-dipeptides produced from C9orf72 hexanucleotide repeats cause selective motor neuron hyperexcitability in ALS

Expansion of the GGGGCC hexanucleotide repeat in the chromosome 9 open reading frame 72 (C9orf72) gene is the most common genetic cause of amyotrophic lateral sclerosis (ALS). As in other forms of ALS, selective hyperexcitability of the motor cortex has been implicated as a cause of the motor neuron death in C9orf72-associated ALS. Here, we show that proline–arginine (PR) poly-dipeptides generated from C9orf72 repeat expansions increase the intrinsic excitability in pyramidal neurons of the motor cortex but not in the principal neurons of the visual cortex, somatosensory cortex, or hippocampus. We further show that this effect is attributable to PR-induced enhancement of the persistent sodium current primarily through an Nav1.2-β1-β4 complex. Reconstitution assays reveal that an auxiliary subunit, β4, plays a crucial role in the PR-mediated modulation of human Nav1.2 channel activity. Moreover, compared with the visual cortex, binding of PR poly-dipeptide to Nav1.2 is stronger in the motor cortex, where β4 is highly expressed. Taken together, these studies suggest a cellular mechanism underlying cortical hyperexcitability in C9orf72 ALS by providing evidence that PR poly-dipeptides induce hyperexcitability in cortical motor neurons by modulating the Nav1.2 channel complex.