C-Boutons and Their Influence on Amyotrophic Lateral Sclerosis Disease Progression

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease with progressive motor neuron death, where patients usually die within 5 years of diagnosis. Previously, we showed that the C-boutons, which are large cholinergic synapses to motor neurons that modulate motor neuron activity, are necessary for behavioral compensation in mSOD1^G93A mice, a mouse model for ALS. We reasoned that, since the C-boutons likely increase the excitability of surviving motor neurons to compensate for motor neuron loss during ALS disease progression, then amplitude modulation through the C-boutons likely increases motor neuron stress and worsens disease progression. By comparing male and female mSOD1^G93A mice to mSOD1^G93A mice with genetically silenced C-boutons [mSOD1^G93A ; Dbx1::cre; ChAT^fl/fl (mSOD1^G93A/C^off )], we show that the C-boutons do not influence the humane end point of mSOD1^G93A mice; however, our histologic analysis shows that C-bouton silencing significantly improves fast-twitch muscle innervation over time. Using immunohistology, we also show that the C-boutons are active in a task-dependent manner, and that symptomatic mSOD1^G93A mice show significantly higher C-bouton activity than wild-type mice during low-intensity walking. Last, by using behavioral analysis, we provide evidence that C-bouton silencing in combination with swimming is beneficial for the behavioral capabilities of mSOD1^G93A mice. Our observations suggest that manipulating the C-boutons in combination with a modulatory-targeted training program may therefore be beneficial for ALS patients and could result in improved mobility and quality of life.SIGNIFICANCE STATEMENT Despite decades of research on amyotrophic lateral sclerosis (ALS), there have been little improvements in treatments and therapies. We sought to better understand how the activation of C-boutons, which are large cholinergic modulatory synapses on motor neurons, change and affect the disease as it progresses. When these C-boutons are genetically silenced and exercises designed to otherwise activate the C-boutons are frequently performed in ALS model mice, the mice perform better than their untreated counterparts over time. C-bouton-targeted therapies could therefore be beneficial for ALS patients and could result in improved mobility and quality of life.