AXON INITIAL SEGMENT REMODELLING IN CORTICOSPINAL TRACT NEURONS ACROSS ALS MOUSE MODELS

Altered intrinsic excitability of both corticospinal tract neurons (CSTNs) and spinal motoneurons is a consistent feature of amyotrophic lateral sclerosis (ALS). In spinal motoneurons, these excitability changes are associated with axon initial segment (AIS) remodelling, however, whether CSTNs undergo similar structural AIS changes remains unknown.
To address this question, we quantified AIS morphology in CSTNs across two ALS mouse models. SOD1G93A mice, modelling familial ALS, were analysed at pre-symptomatic (65 days), symptomatic (130 days), and end-stage disease (160 days). An inducible TDP-43 ΔNLS mouse model, recapitulating cytoplasmic TDP-43 pathology relevant to sporadic ALS, was analysed at the symptomatic stage (4 weeks post-transgene induction). AIS morphology was quantified in layer V Ctip2+ neurons of the primary motor cortex using Ankyrin-G immunolabelling and confocal microscopy.
In both models, CSTN AIS length was significantly reduced, with shortening already evident at the pre-symptomatic stage in SOD1G93A mice. This shortening is the opposite of the AIS elongation observed in spinal motoneurons. A significant reduction in proximal AIS diameter was also observed in both models but only at end stage in SOD1G93A mice. Thus, CSTNs undergo a consistent pattern of AIS remodelling in ALS across models, confirming that AIS constriction appears to be a consistent feature of ALS. Overall, our results indicate that AIS length remodelling is dependent on neuronal locationwithin the motor network rather than reflecting a uniform, cell-intrinsic pathological response. Whether this remodelling is a homeostatic response to sustained excitation or is directly driven by ALS pathology remains unclear.