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 mSOD1G93A 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 mSOD1G93A mice to mSOD1G93A mice with genetically silenced C-boutons [mSOD1G93A ; Dbx1::cre; ChATfl/fl (mSOD1G93A/Coff )], we show that the C-boutons do not influence the humane end point of mSOD1G93A 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 mSOD1G93A 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 mSOD1G93A 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.

Cholinergic modulation of motor neurons through the C-boutons are necessary for the locomotor compensation for severe motor neuron loss during amyotrophic lateral sclerosis disease progression.

Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease characterized by progressive motor neuron (MN) death that leads to muscle weakness, paralysis, and eventually death. When symptoms become clinically evident, patients and ALS model animals (mSod1G93A mice) have already lost a large portion of motor units, suggesting the existence of a compensatory mechanism that allows for reactively normal movement despite denervation. Furthermore, it has been shown that specialized cholinergic synapses, the C-boutons, regulate activity strength of motor output in a task dependent manner. We hypothesized that the cholinergic modulation of motor neurons through C-boutons increases motor neuron excitability, and that this C-bouton associated activity increase in surviving motor neurons could compensate for motor unit loss during ALS disease progression. We first provide a thorough analysis of the muscle denervation and behavioral changes in the mSod1G93A mice using immunohistology, electrophysiology, and quantitative analysis of locomotor behavior. Then, in support of our hypothesis, we show that task dependent modulation of hindlimb muscle activation that relies on C-bouton activation diminishes as the disease progresses. Furthermore, the capability of mSod1G93A mice to walk at higher speeds on a treadmill decreases significantly at younger ages when C-boutons are silenced. Our observations that C-bouton modulation of motor neurons is involved in compensation during ALS disease progression can have significant therapeutic implications for sustaining mobility and preserving the quality of life in human ALS patients.

Modular organization of murine locomotor pattern in the presence and absence of sensory feedback from muscle spindles.

KEY POINTS: Locomotion on land and in water requires the coordination of a great number of muscle activations and joint movements. Constant feedback about the position of own body parts in relation to the surrounding environment and the body itself (proprioception) is required to maintain stability and avoid failure. The central nervous system may follow a modular type of organization by controlling muscles in orchestrated groups (muscle synergies) rather than individually. We used this concept on genetically modified mice lacking muscle spindles, one of the two main classes of proprioceptors. We provide evidence that proprioceptive feedback is required by the central nervous system to accurately tune the modular organization of locomotion. ABSTRACT: For exploiting terrestrial and aquatic locomotion, vertebrates must build their locomotor patterns based on an enormous amount of variables. The great number of muscles and joints, together with the constant need for sensory feedback information (e.g. proprioception), make the task of controlling movement a problem with overabundant degrees of freedom. It is widely accepted that the central nervous system may simplify the creation and control of movement by generating activation patterns common to muscle groups, rather than specific to individual muscles. These activation patterns, called muscle synergies, describe the modular organization of movement. We extracted synergies through electromyography from the hind limb muscle activities of wild-type and genetically modified mice lacking sensory feedback from muscle spindles. Muscle spindle-deficient mice underwent a modification of the temporal structure (motor primitives) of muscle synergies that resulted in diminished functionality during walking. In addition, both the temporal and spatial (motor modules) components of synergies were severely affected when external perturbations were introduced or when animals were immersed in water. These findings show that sensory feedback from group Ia/II muscle spindles regulates motor function in normal and perturbed walking. Moreover, when group Ib Golgi tendon organ feedback is lacking due to enhanced buoyancy, the modular organization of swimming is almost completely compromised.