Reduced cortico-muscular output is associated with intrinsic hypoexcitability and reduced persistent inward currents in motor cortex neurons of TDP-43Q331K ALS mice

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by spinal and cortical motor neuron loss and progressive neuromuscular decline. When ALS pathology involves the primary motor cortex (PMC), cortical excitability is often disrupted, yet how these alterations map onto motor deficits during symptomatic ALS remains unclear. To investigate this, we examined the neuromuscular function, cortico-muscular output, and neuronal excitability of symptomatic 4-month-old TDP-43^Q331K mice. TDP-43 mice exhibited reduced neuromuscular excitability and impaired strength compared to WT mice. Cranial motor evoked potentials were significantly reduced in TDP-43 mice, indicating decreased cortical output to muscle. Compared to WT mice, whole-cell patch-clamp recordings from TDP-43 PMC layer V pyramidal neurons revealed intrinsic hypoexcitability, diminished persistent inward currents (PICs), and decreased excitatory synaptic activity. Corroborating PIC findings, immunohistochemical analysis showed that PMC layer V neurons exhibited reduced signal intensity of the PIC-associated proteins Nav1.6 and 5-HT2C. Bulk RNA-seq of the cortex showed distinct transcriptional profiles in TDP-43 mice, with enrichment analysis indicating altered pathways relating to ion transport, synaptic signaling, and neuronal excitability. These results suggest that cortex-wide transcriptional changes may reflect broader and additional molecular mechanisms underlying cortical hypoexcitability in ALS. Together, our results demonstrate that symptomatic TDP-43^Q331K mice exhibit a reduction in cortico-muscular output and PMC neuron excitability, accompanied by reduced PICs and PIC-associated proteins within these neurons. These findings identify cortical hypoexcitability as a defining feature of the TDP-43^Q331k ALS mouse model and establish multi-level associations between cortical cellular-level dysfunction and impaired motor systems output.