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Synaptic silencing of fast muscle is compensated by rewired innervation of slow muscle
Buntaro Zempo1,2 (Fumihito Ono1,3)

For decades, numerous studies have proposed that fast muscles contribute to quick movement, while slow muscles underlie locomotion requiring endurance. By generating mutant zebrafish whose fast muscles are synaptically silenced, we examined the contribution of fast muscles in both larval and adult zebrafish. In the larval stage, mutants lacked the characteristic startle response to tactile stimuli: bending of the trunk (C-bend) followed by robust forward propulsion. Surprisingly, adult mutants with silenced fast muscles showed robust C-bends and forward propulsion upon stimulation. Retrograde labeling revealed that motor neurons genetically programmed to form synapses on fast muscles are instead re-routed and innervate slow muscles, which led to partial conversion of slow and intermediate muscles to fast muscles. Thus, extended silencing of fast muscle synapses changed motor neuron innervation and caused muscle cell type conversion, revealing an unexpected mechanism of locomotary adaptation.

Zempo B, Yamamoto, Y., Williams, T., Ono, F. Synaptic silencing of fast muscle is compensated by rewired innervation of slow muscle. Science Advances 6: eaax8382 (2020)

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In adult zebrafish lacking acetylcholine receptors in fast muscles (epsilon KO), synaptic silencing of fast muscles led to the innervation of fast muscle–specific pMNs on slow muscle. This reinnervation caused conversion of slow to fast muscles.

1Department of Physiology, Osaka Medical College, Japan 3National Institutes of Health (2Current address: Department of Physiology, Jichi Medical University, Japan)