Elasticity-based boosting of neuroepithelial nucleokinesis via indirect energy transfer from mother to daughter

Abstract

Neural progenitor cells (NPCs), which are apicobasally elongated and densely packed in the developing brain, systematically move their nuclei/somata in a cell cycle–dependent manner, called interkinetic nuclear migration (IKNM): apical during G2 and basal during G1. Although intracellular molecular mechanisms of individual IKNM have been explored, how heterogeneous IKNMs are collectively coordinated is unknown. Our quantitative cell-biological and in silico analyses revealed that tissue elasticity mechanically assists an initial step of basalward IKNM. When the soma of an M-phase progenitor cell rounds up using actomyosin within the subapical space, a microzone within 10 μm from the surface, which is compressed and elastic because of the apical surface’s contractility, laterally pushes the densely neighboring processes of non–M-phase cells. The pressed processes then recoil centripetally and basally to propel the nuclei/somata of the progenitor’s daughter cells. Thus, indirect neighbor-assisted transfer of mechanical energy from mother to daughter helps efficient brain development.

Two‐photon microscopic observation of cell‐production dynamics in the developing mammalian neocortex in utero

“…We compared the frequency of occurrence of such identified mitosis events per unit time period (60 min) and per unit apical-surface area (60 μm × 60 μm). As shown in Figure 3a, the frequency was 7.59 ± 1.14 in 3D culture (obtained from three separate fields of an E13 cerebral wall), reproducing the results obtained in previous studies that used E13 cerebral wall cultures (Okamoto et al, 2013;Shinoda et al, 2018). As shown in Figure 3b, the frequency of mitosis occurrence was 8.00 ± 1.00 in in utero 2PM (three separate fields from two independent E13 embryos), and no statistically significant difference was obtained between these two groups (p = .7, exact Wilcoxon rank-sum test; Figure 3c).…”

“…When IUE-labeled embryos were subjected to intravital 2PM, finding ideally labeled NPCs in the VZ was difficult, and excessive searching of appropriate fields damaged the embryos. Previous studies revealed that mitosis and IKNM in NPCs visualized in slices prepared from H2B-mCherry mouse embryos (Okamoto et al, 2013;Shinoda et al, 2018) were equivalent to those observed during sporadic visualization of NPCs (Konno et al, 2008;Miyata et al, 2001;Okamoto et al, 2013). Technically, monitoring at 5 μm from the apical surface is useful for capturing (a) apical-ward migration of a G2-phase NPC's nucleus, (b) mitosis of that NPC, and (c) basal-ward IKNM of G1-phase daughter cells generated by that NPC (Okamoto et al, 2013;Shinoda et al, 2018).…”

“…(e) A graph comparing the density of cells in metaphase or anaphase (identified in six 60 μm × 60 μm fields of snapshot pictures) is shown. (f) A graph comparing the time from the beginning of anaphase until giving rise to two daughter cells (which was roughly regarded as "birth" of the daughter cells) and the "departure" (disappearance from the 5 μm level from the apical surface, illustrated in g) of the nucleus of one (earlier-leaving) daughter cell (Shinoda et al, 2018) is shown. p = .5563 (exact Wilcoxon rank-sum test; n = 13 in in utero 2PM, n = 17 in cerebral wall culture) (Haubensak, Attardo, Denk, & Huttner, 2004;Miyata et al, 2004;Noctor, Martinez-Cerdeno, Ivic, & Kriegstein, 2004; Figure 1c), indicates that an NPC is successfully progressing into the late M phase ("event 2" in Figure 4a).…”

“…In three-dimensional environments, cell production via mitosis depends not only on the progression of the cell cycle but also on the movement of cells' nuclei/somata into place for mitosis, making the overall cell-production events highly temporally and spatially dynamic (Keller, Schmidt, Wittbrodt, & Stelzer., 2008;Kurotaki, Hatta, Nakao, Nabeshima, & Fujimori, 2007;Miyata, 2008;Norden, Young, Link, & Harris, 2009;Taverna & Huttner, 2010). Recent studies using slice culture-based imaging and mechanical assessment of developing mammalian brain walls have shown that such dynamic cytogenetic events occur at a very high density under physiologically crowded cellular conditions (Miyata, Okamoto, Shinoda, & Kawaguchi, 2015;Nagasaka et al, 2016;Okamoto et al, 2013;Okamoto, Shinoda, Kawaue, Nagasaka, & Miyata, 2014;Saito, Kawasoe, Sasaki, Kawaguchi, & Miyata, 2018;Shinoda et al, 2018); the studies also show that delays in the cell cycle or nuclear migration in a subpopulation of progenitors can easily lead, in a cell non-autonomous manner, to secondary (more widespread) disorganization of histogenesis (Okamoto et al, 2013;Watanabe, Kawaue, & Miyata, 2018).…”

Mechanical Regulation of Nuclear Translocation in Migratory Neurons

“…Further studies have demonstrated that the apical surface of the ventricular zone undergoes actomyosin-dependent contraction, further crowding the apical surface with elastic processes of surrounding progenitor cells. These dense processes exert a centripetal force on the dividing nuclei, thereby enhancing their dorsal displacement (Shinoda et al, 2018) (Figure 4). Recent studies have also demonstrated that apical-to-basal migration stops at the boundary of the ventricular zone and the SVZ by the physical fence made by differentiated SVZ neurons (Watanabe et al, 2018) (Figure 4).…”