In silico and cell-biological findings support an actomyosin-dependent bouncing-like mechanism for daughter cells’ initial nucleokinesis.
(A) Mathematical simulation showing a bouncing-like basal displacement of a G1-phase cell’s nucleus/soma following apical and lateral pushing of the subapical space by the mother cell (G2–M) (see also S13 Movie and S3A Fig). Two strings, fixed to the apical surface, behave elastically, mimicking the densely process-filled subapical space of VZ, with force storage (red in color code [during M phase]) and subsequent release (during early G1). (B) Graph showing the relationship between the spring constant (k) of the elastic strings and the trajectory of early G1-phase cells’ nuclei/somata. (C) Three-dimensionally projected view of a clonally Lifeact-EGFP–labeled progenitor cell (panels 1–3) and its daughter cells (panels 4 and 5) (see also S14 Movie). (D) Graphs showing the time-dependent changes in the intensity of Lifeact-EGFP and the horizontal sectional area at the basal pole or the soma of a single progenitor cell from G2 phase to the end of M phase (n = 3 clones). Reflecting that most progenitors divide horizontally [39], the horizontal section of the observed cell’s soma became ellipsoidal with the long axis parallel to the apical surface (130–140 min). The thickness of the basal process and the degree of F-actin accumulation (both measured at the portion indicated by the arrowhead) changed in an inversely correlated manner. (E) Accumulation of phospho-myosin-light-chain (green) in the basal process of a phospho-Vimentin+ (magenta) M-phase cell, shown in a cross-sectional view and a view obtained by orthogonally sectioning at the dotted line. (F) Myosin II inhibition resulted in basal (up to 20 μm) bouncing of the M-phase cell’s soma (prior to cytokinesis, which occurred at 150–190 min) (see also S3C for in vivo immunohistochemical results and S16 Movie). Scale, 10 μm in C, E, and F. Underlying data can be found in S1 Data. AU, arbitrary unit; p-MLC, phosphorylated myosin light chain; VZ, ventricular zone.
