Structural and functional alterations of neuromuscular synapses in a mouse model of ACTA1 congenital myopathy

Mutations in skeletal muscle α-actin (Acta1) cause a variety of myopathies. In a mouse model of congenital myopathy, heterozygous Acta1 (H40Y) knock-in ( Acta1 + /Ki ) mice exhibit features of human nemaline myopathy, including premature lethality, severe muscle weakness, reduced mobility, and the presence of nemaline rods in muscle fibers. In this study, we investigated the structure and function of the neuromuscular junction (NMJ) in the Acta1 + /Ki mice. We found marked impairments in NMJ structure in the mutant mice, including fragmented endplates and nerve terminals, reduced density of acetylcholine receptors (AChRs) on endplate membranes, reduced nerve occupancy at endplates, and increased numbers of muscle fiber subsynaptic nuclei. We compared the NMJs in three different types of muscles – the extensor digitorum longus (EDL, composed of fast-twitch muscle fibers), soleus (Sol, enriched in slow-twitch fibers) and the triangularis sterni muscle (TS, a mixed fiber type muscle). Among these three types of muscles, EDL was affected to the greatest extent, suggesting that fast-twitch fibers may be most susceptible to NMJ fragmentation in Acta1 + /Ki nemaline myopathy. Electrophysiological analysis of mutant NMJs showed a reduced quantal size (reduced mEPP amplitude), increased mEPP frequency, and increased quantal content, but normal EPP amplitude compared to wild type ( WT ) NMJs. The results suggest that affected synapses may have undergone homeostatic compensation to maintain normal levels of neurotransmitter release. In addition, paired-pulse facilitation was reduced and synaptic depression under repetitive nerve stimulation was enhanced, indicating shortterm synaptic plasticity was compromised in the mutant mice. Key points Mice heterozygous for an Acta1 (H40Y) knock-in mutation exhibit clinical features of human nemaline myopathy. We report structural and functional alterations of neuromuscular synapses in these mutant mice. The NMJ impairments include endplate fragmentation, reduced endplate nerve occupancy, and increased numbers of subsynaptic nuclei in muscle fibers. Neuromuscular synaptic transmission was compromised – demonstrating both increased quantal content and changes in short-term synaptic plasticity. Increases in spontaneous neurotransmitter release and quantal content suggest homeostatic compensation of synapses to maintain normal transmitter release in the mutant NMJs.