The actin cytoskeleton drives formation of membrane protrusions that promote cell–cell fusion. Chuang et al. now find that the invadosome scaffold protein Tks5 is required for myoblast fusion. Tks5 regulates the assembly of dynamin-2 around actin bundles, thus strengthening the stiffness of the invadosome to propel cell–cell fusion.

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Capsulin and Musculin are basic helix-loop-helix transcription factors, but their biophysiological roles in zebrafish cranial myogenesis are unclear. Expressions of endogenous capsulin transcripts are detected at the central- (~24-hpf) and at dorsal- and ventral-mesoderm cores (~30-72 hpf) of branchial arches. In contrast, musculin transcripts are expressed as a two-phase manner: early phase (20-22 hpf) expressions of musculin are detected at the head mesoderm, whereas late-phase (36-72 hpf) are detected at all presumptive head-muscle precursors. Knockdown of either capsulin or musculin leads to loss of all cranial muscles without affecting trunk muscle development. The defective phenotypes of Capsulin- and Musculin-morphant can be rescued by co-injection of mRNA of each other. Both myf5 and myod transcripts are down-regulated in the Capsulin-morphant while myod transcripts are up-regulated in the Musculin-morphant. Therefore, we propose a putative regulatory network to understand how capsulin/musculin regulate distinctly either myf5 or myod during zebrafish craniofacial myogenesis.