Cortical hyperexcitability drives dying forward amyotrophic lateral sclerosis symptoms and pathology in mice

Degeneration of both upper motor neurons in the brain and lower motor neurons in the spinal cord defines amyotrophic lateral sclerosis (ALS), but how they are linked in ALS pathophysiology is unclear. Here, we uncover a cortical origin of neurodegeneration in ALS mediated by upper motor neuron hyperexcitability. Chronic hyperexcitability of upper motor neurons induced by excitatory chemogenetics in healthy adult mice induced progressive motor deficits, weakness and core pathological hallmarks of ALS, including upper motor neurons loss, synaptic pathology, corticospinal tract degeneration and reactive gliosis. Importantly, upper motor neuron hyperexcitability and loss were sufficient to drive degeneration of lower motor neurons and their distal axons and neuromuscular junctions, associated with astrocyte and microglial activation in spinal cord. Cortical hyperexcitability also triggered cytoplasmic TAR DNA binding protein 43 (TDP-43) aggregation in upper motor neurons and lower motor neurons, placing hyperexcitability upstream of TDP-43 proteinopathy in ALS. These findings establish a cortical origin of ALS mediated by upper motor neurons, consistent with an anterograde mechanism of neurodegeneration throughout the central and peripheral nervous systems.