Investigating Neurophysiological Dysfunction in C9ORF72 Patient-derived Striatal and Lower Motor Neurons

Frontotemporal dementia and related amyotrophic lateral sclerosis (FTD/ALS) are fatal neurodegenerative diseases that sit on opposite ends of a clinical disease spectrum typically characterised by progressive degeneration of cortical and motor neurons. However, it is becoming increasingly evident that FTD/ALS pathologically impacts other brain regions. Recently, the striatum, a crucial integrative hub within the brain, has been shown to have a major role in cognitive functions impacted in FTD/ALS, displays classical TDP-43 pathology and neuronal atrophy. Altered neuronal excitability is a hallmark feature of neurodegenerative disease and contributes to disease onset and progression. Here, I have investigated whether electrophysiological function relating to intrinsic excitability and responsiveness to major ionotropic receptors are pathologically relevant in FTD/ALS. To this end, enriched cultures of induced pluripotent stem cell-derived striatal medium spiny neurons (MSNs) and lower motor neurons (LMNs) harbouring C9ORF72 repeat expansion mutations, the most common genetic impairment within the FTD/ALS spectrum, were generated. Using whole-cell patch-clamp electrophysiology my data provides novel evidence of reduced excitability in C9 MSNs, driven by deficits in AP waveform components and delayed outwardly-rectifying K+ (Ik) channel dysfunction. Pharmacological targeting of big conductance Ca2+-activated potassium (BK) channels and voltage-gated K+ channels (Kv3) restored IK channel function and rescued AP waveform deficits, but not intrinsic hypoexcitability, suggesting an interplay between ion channel dysfunction and structural AIS abnormalities. In contrast, C9 LMNs displayed stable excitability under basal and acute KCl stress conditions but exhibited increased excitability upon chronic KCl depolarising stress, implicating homeostatic plasticity mechanisms. Furthermore, C9 LMNs showed potentiated excitatory and inhibitory neurotransmission, with a shift toward NMDAr-mediated signalling, highlighting potential excitotoxic vulnerability. These data implicate complex, regionally specific pathophysiological impairments in C9FTD/ALS that varies between neuronal populations, which has significant implications for future therapeutic strategies.