Development of motor neurons and motor activity in zebrafish requires F-actin nucleation by Fmn2b

BackgroundCytoskeletal remodelling plays a pivotal role in the establishment of neuronal connectivity during development and in plasticity in adults. Mutations in the cytoskeleton regulatory protein Formin-2 (Fmn2) are associated with neurodevelopmental disorders like intellectual disability, though its function in neuronal morphogenesis has not been characterised in vivo. ResultsHere we develop a loss-of-function model for fmn2b, the zebrafish orthologue of Fmn2, using CRISPR/Cas9-mediated gene editing. fmn2b mutants display motor deficits starting from the earliest motor responses in the embryo. We find that fmn2b is expressed in spinal motor neurons and its loss reduces motor neuron innervation of the axial muscles without affecting myotome integrity. The translocation of caudal primary (CaP) motor neuron outgrowth is compromised in fmn2b mutants, while rostral primary (RoP) motor neurons have missing soma or stall at the horizontal myoseptum. Strikingly, axon collateral branching of the motor neurons is severely compromised and results in reduced synaptic coverage of the myotome. Rescue experiments identify the requirement for Fmn2-mediated actin nucleation for motor neuron outgrowth and arborisation. ConclusionsThe zebrafish loss-of-function model of Fmn2 reveals the specific requirement of F-actin polymerisation by Fmn2 in neuromuscular development. It also underscores the role of Fmn2 in motor neuropathies, especially as a proportion of individuals harbouring mutations in Fmn2 present with hypotonia.