CFL1-dependent dynamicity of surface ectoderm filopodia-like protrusions enhances neurulation zippering speed in mice

Epithelial fusion is critical for formation of many embryonic tissues. The developing mammalian spinal cord fuses by zippering, through cell behaviours dependent on F-actin turnover. We propose a caudal-to-rostral sequence of surface ectoderm cell behaviours which drive zippering progression in mice, and test the requirement for the F-actin severing protein CFL1 in this process. Key zipper-advancing behaviours are i. constriction of supracellular actomyosin cables; ii. extension of short-lived (<2 min) filopodial or more persistent lamellipodial protrusions, which we live-image establishing ‘pioneering’ contacts across the midline; remodelling of iii. cell-cell and iv. cell-ECM adhesions; v. zipper advancement through constriction of leading-edge cell borders. In wildtype embryos CFL1 is enriched at the leading edge of surface ectoderm cells adjacent to the zippering point and localised within some protrusions. Conditional Cfl1 deletion in the surface ectoderm produces more prominent F-actin stress fibres, makes filopodial protrusions excessively stable, diminishes their exploratory movements which may establish nascent contacts, and impairs junctional constriction of cells at the point of fusion. Zippering speed is reduced ∼30% in conditional Cfl1 -deleted embryos and ∼30% develop spina bifida. We therefore propose that impaired filopodial dynamicity limits the sequence of cell behaviours driving spinal zippering in mice, predisposing to spina bifida.