Rho kinase-dependent apical constriction counteracts M-phase apical expansion to enable mouse neural tube closure

Abstract

Cellular generation of mechanical forces required to close the presumptive spinal neural tube, the “posterior neuropore” (PNP), involves interkinetic nuclear migration (INM) and apical constriction. Both processes change neuroepithelial apical dimensions, but how they are biomechanically integrated is unknown. Rho kinase (Rock) inhibition in mouse whole embryo culture progressively widens the PNP. PNP widening is not caused by increased mechanical tension opposing closure, as evidenced by diminished recoil following laser ablation. Rather, Rock inhibition diminishes neuroepithelial apical constriction, producing larger neuroepithelial apical dimensions despite diminished tension. Neuroepithelial apices are also dynamically related to INM progression, with the smallest dimensions achieved in cells positive for the pan-M phase marker pRB-S780. Brief (2 hr) Rock inhibition selectively increases apical dimensions of pRB-S780+, but not pre-anaphase pHH3+ cells. Longer inhibition (8 hrs, >1 cell cycle) increases apical areas of pHH3+ cells, suggesting cell cycle-dependent accumulation of cells with larger apical surfaces during PNP widening. Consequently, arresting cell cycle progression with hydroxyurea prevents PNP widening following Rock inhibition. Thus, Rock-dependent apical constriction compensates for PNP-widening effects of INM to enable progression of closure.