CFL1-dependency of discrete cell behaviours driving mouse spinal zippering

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 filopodial/lamellipodial protrusions at periodic intervals; (iii) constriction of leading-edge cell borders bringing together protrusions which establish 'pioneering' contacts across the midline; (iv) cell-cell and v. cell-ECM adhesion remodelling; (vi) zipper advancement through cell partner exchange. In wild-type 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 denser 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 and junctional constriction limits the sequence of behaviours driving spinal zippering, predisposing to spina bifida.