Striatal neuron dysfunction in C9ORF72-FTD/ALS is driven by AIS and potassium channel dysregulation.

Frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) form a neurodegenerative spectrum characterized by progressive cognitive, behavioral, and motor decline, yet the contribution of the striatum to disease pathophysiology remains poorly understood. Here, we generate inhibitory striatal medium spiny neurons (MSNs) from human induced pluripotent stem cells carrying the C9ORF72 repeat expansion, the most common genetic cause of FTD/ALS, and compare them with isogenic-corrected, control, and patient-derived motor neurons. Using whole-cell electrophysiology, pharmacological manipulation, and high-resolution imaging, we identify a vulnerability of C9ORF72 MSNs to develop intrinsic hypoexcitability with linked synaptic dysfunction. These abnormalities are associated with axon initial segment shortening and altered voltage-gated potassium channel function relative to control and isogenic-corrected neurons. Pharmacological modulation partially restores action potential waveform properties, indicating that key electrophysiological abnormalities are reversible. These findings identify the striatum as a critical site of dysfunction in FTD/ALS and highlight striatal excitability as a potential therapeutic target.