Impaired synaptic vesicle release and immaturity of neuromuscular junctions in spinal muscular atrophy mice

L Kong, X Wang, DW Choe, M Polley… - Journal of …, 2009 - Soc Neuroscience
L Kong, X Wang, DW Choe, M Polley, BG Burnett, M Bosch-Marcé, JW Griffin, MM Rich…
Journal of Neuroscience, 2009Soc Neuroscience
The motor neuron disease spinal muscular atrophy (SMA) causes profound muscle
weakness that most often leads to early death. At autopsy, SMA is characterized by loss of
motor neurons and muscle atrophy, but the initial cellular events that precipitate motor unit
dysfunction and loss remain poorly characterized. Here, we examined the function and
corresponding structure of neuromuscular junction (NMJ) synapses in a mouse model of
severe SMA (hSMN2/delta7SMN/mSmn−/−). Surprisingly, most SMA NMJs remained …
The motor neuron disease spinal muscular atrophy (SMA) causes profound muscle weakness that most often leads to early death. At autopsy, SMA is characterized by loss of motor neurons and muscle atrophy, but the initial cellular events that precipitate motor unit dysfunction and loss remain poorly characterized. Here, we examined the function and corresponding structure of neuromuscular junction (NMJ) synapses in a mouse model of severe SMA (hSMN2/delta7SMN/mSmn−/−). Surprisingly, most SMA NMJs remained innervated even late in the disease course; however they showed abnormal synaptic transmission. There was a two-fold reduction in the amplitudes of the evoked endplate currents (EPCs), but normal spontaneous miniature EPC (MEPC) amplitudes. These features in combination indicate reduced quantal content. SMA NMJs also demonstrated increased facilitation suggesting a reduced probability of vesicle release. By electron microscopy, we found a decreased density of synaptic vesicles that is likely to contribute to the reduced release probability. In addition to presynaptic defects, there were postsynaptic abnormalities. EPC and MEPC decay time constants were prolonged because of a slowed switch from the fetal acetylcholine receptor (AChR) γ-subunit to the adult ε-subunit. There was also reduced size of AChR clusters and small myofibers, which expressed an immature pattern of myosin heavy chains. Together these results indicate that impaired synaptic vesicle release at NMJs in severe SMA is likely to contribute to failed postnatal maturation of motor units and muscle weakness.
Soc Neuroscience