Axonal dying back of upper motor neurons in human ALS

Patients with amyotrophic lateral sclerosis (ALS) typically present with arm, leg, or bulbar weakness. While genetics plays a clear role, it cannot explain why symptoms start focally or how upper (UMN) and lower motor neuron (LMN) systems are linked. In this clinicopathological case series, we examined the relationships between UMN/LMN disease in ten ALS patients. Detailed clinical assessments and motor cortex, brainstem, and spinal cord tissues were collected via rapid autopsy. Tissues were stained for UMN/LMN, myelin, axons, microglia, and pTDP43, and RNA-sequencing was performed. None of the patients had symptoms of frontotemporal dementia (FTD), but all had focal sites of clinical onset and both UMN/LMN involvement. LMN degeneration and microglial activation were highest at disease onset sites. UMN degeneration was present at all spinal cord levels through the medulla, regardless of onset site. Surprisingly, there was no evidence of UMN axonal degeneration above the brainstem. While extensive pTDP43 aggregates were seen in degenerating LMNs, no pTDP43 aggregates were seen in UMN cell bodies or their axons. RNA-sequencing implicated inflammatory pathways at sites of disease onset. Our findings suggest that some ALS patients without FTD have a dying back of UMN axons rather than a primary upper neuronopathy of neurons.