Axonal dying back of upper motor neurons in human ALS

Patients with amyotrophic lateral sclerosis (ALS) present with arm, leg, or bulbar weakness with or without spasticity. While genetics plays an important role, it cannot explain why symptoms start focally or how upper (UMN) and lower motor neuron (LMN) systems are linked. Here, we examined the clinicopathological relationships between UMN and LMN disease in ten ALS patients. Detailed clinical assessments were collected with tissues from the motor cortex, brainstem, and spinal cord via a rapid autopsy protocol. Tissues were stained for UMN/LMN, myelin, axons, microglia, and pTDP43. Total RNA-sequencing was performed in the medulla, cervical, and lumbar spinal cords from each patient to identify pathways enriched at sites of disease onset. None of the patients had symptoms of frontotemporal dementia (FTD), but all had focal sites of clinical onset and spasticity, indicating both UMN and LMN involvement. Postmortem examination showed LMN degeneration and microglial activation were highest at sites of disease onset. In contrast, UMN degeneration of the corticospinal tract (CST) was present equally at all levels of the spinal cord up through the medulla, regardless of the site of disease onset. Surprisingly, there was no UMN degeneration of cortical motor neurons and their proximal axons down to the medulla. Similarly, while extensive pTDP43 aggregates were seen in degenerating LMNs, no pTDP43 aggregates were seen in UMN cell bodies or their axons. Mechanistically, RNA-sequencing implicated inflammatory pathways, especially at sites of disease onset. Our findings suggest that many ALS patients without FTD have a dying back of UMN axons, independent of the site of disease onset, which stops in the brainstem with preservation of cortical motor neurons and their proximal axons. UMN axonal degeneration could be directly triggered by LMN degeneration and inflammation.