Hyperexcitability precedes motoneuron loss in the Smn 2B/- mouse model of spinal muscular atrophy

• Published in: Journal of neurophysiology 2019-10, Vol.122 (4), p.1297-1311
• Main Authors: Quinlan, K A, Reedich, E J, Arnold, W D, Puritz, A C, Cavarsan, C F, Heckman, C J, DiDonato, C J
• Format: Article
• Language: English
• Subjects: Action Potentials; Animals; Disease Models, Animal; Mice, Knockout; Motor Neurons - pathology; Motor Neurons - physiology; Muscle, Skeletal - innervation; Muscle, Skeletal - physiopathology; Muscular Atrophy, Spinal - pathology; Muscular Atrophy, Spinal - physiopathology; Survival of Motor Neuron 1 Protein - genetics; Survival of Motor Neuron 1 Protein - physiology
• ISSN: 0022-3077; 1522-1598
• DOI: 10.1152/jn.00652.2018

Spinal motoneuron dysfunction and loss are pathological hallmarks of the neuromuscular disease spinal muscular atrophy (SMA). Changes in motoneuron physiological function precede cell death, but how these alterations vary with disease severity and motoneuron maturational state is unknown. To address this question, we assessed the electrophysiology and morphology of spinal motoneurons of presymptomatic mice older than 1 wk of age and tracked the timing of motor unit loss in this model using motor unit number estimation (MUNE). In contrast to other commonly used SMA mouse models, mice exhibit more typical postnatal development until postnatal day (P)11 or 12 and have longer survival (~3 wk of age). We demonstrate that motoneuron hyperexcitability, marked by hyperpolarization of the threshold voltage for action potential firing, was present at P9-10 and preceded the loss of motor units. Using MUNE studies, we determined that motor unit loss in this mouse model occurred 2 wk after birth. motoneurons were also larger in size, which may reflect compensatory changes taking place during postnatal development. This work suggests that motoneuron hyperexcitability, marked by a reduced threshold for action potential firing, is a pathological change preceding motoneuron loss that is common to multiple models of severe SMA with different motoneuron maturational states. Our results indicate voltage-gated sodium channel activity may be altered in the disease process. Changes in spinal motoneuron physiologic function precede cell death in spinal muscular atrophy (SMA), but how they vary with maturational state and disease severity remains unknown. This study characterized motoneuron and neuromuscular electrophysiology from the model of SMA. Motoneurons were hyperexcitable at postnatal day (P)9-10, and specific electrophysiological changes in motoneurons preceded functional motor unit loss at P14, as determined by motor unit number estimation studies.