Myomaker and Myomixer are required for craniofacial myoblast fusion in zebrafish.

Background: Craniofacial and trunk skeletal muscles are derived from different progenitor populations during development. Trunk skeletal muscles contain mostly multinucleated myofibers that are formed through myoblast fusion. However, myoblast fusion in craniofacial muscles and its molecular regulation are not well understood. Recent studies revealed that genetic mutations in MYOMAKER and MYOMIXER fusogens in humans cause Carey-Fineman-Ziter Syndrome (CFZS), characterized by facial weakness and lower jaw deformity.

Results: Previous studies in zebrafish revealed that knockout of myomaker and myomixer resulted in deformed craniofacial formation. To establish the causal connection between loss of fusogen function and craniofacial deformities, we characterized myoblast fusion in zebrafish craniofacial muscles. Our results demonstrate that myomaker and myomixer are expressed in both slow and fast craniofacial muscles, and loss of these fusogens results in defects in craniofacial myoblast fusion. Interestingly, unlike trunk muscles of early embryos and larvae that show fast-fiber-specific fusogen expression and fusion while slow muscle fusion only occurs at 3 weeks post-fertilization, both slow and fast craniofacial muscles fuse as early as 3 days post-fertilization.

Conclusions: Collectively, this study demonstrates that myomaker and myomixer are expressed in both slow and fast-twitch craniofacial muscles and are essential for myoblast fusion and the development of craniofacial muscles.