Locomotor Compensation for Severe Motor Neuron Loss during Amyotrophic Lateral Sclerosis Disease Progression

Abstract

Amyotrophic lateral sclerosis (ALS) causes progressive motor neuron (MN) degeneration leading to muscle weakness, paralysis, and eventually death. When symptoms are detected in ALS patients and model animals (SOD1G93A mice) large motor unit loss has occurred, indicating existence of a compensatory mechanism that masks denervation. Although significant denervation has occurred by P90 in SOD1G93A mice, motor disturbances are minimal. Cholinergic modulation of MNs through the C-boutons was hypothesized to compensate for motor unit loss by increasing the excitability of surviving MNs. After P100, V0C interneurons, the neuronal source of C-boutons, became active during walking in SOD1G93A mice and the ability to upregulate gastrocnemius activity during swimming became impaired. Silencing of the C-boutons in SOD1G93A mice caused earlier changes in gait and deficits in ankle range of motion. Based on these observations, targeting C-bouton activation using exercise or pharmacological therapies could improve quality of life in ALS patients by preserving mobility.