Mechanical regulation of cell size, fate, and behavior during asymmetric cell division

Abstract

Asymmetric cell division (ACD) is an evolutionary conserved mechanism used by prokaryotes and eukaryotes alike to generate cell diversity. ACD can be manifested in biased segregation of macromolecules or differential partitioning of cell organelles. Cells are also constantly subject to extrinsic or intrinsic mechanical forces, influencing cell behavior and fate. During ACD, cell intrinsic forces generated through the spatiotemporal regulation of the actomyosin cytoskeleton can influence sibling cell size. External mechanical stresses are further translated by transcriptional coactivators or mechanically gated ion channels. Here, we will discuss recent literature, exploring how mechanical cues influence various aspects of ACD and stem cell behavior, and how these mechanical cues contribute to cell fate decisions.

Keywords: Asymmetric cell division; Mechanobiology; Sibling cell size asymmetry; Spindle positioning; Stem cells.

Figure 1:. Spindle-dependent mechanisms inducing sibling cell size asymmetry

(a) After the establishment of the 8-cell embryo, AGS and Galphai are localized to the vegetal pole within the vegetal blastomeres. Vasa is localized in the nucleus. During metaphase, the spindle is pulled towards the vegetal cortex. This is achieved through cortical AGS and Galphai, interacting with Dynein via NuMA. Vasa forms a complex with Dynein, NuMA, Galphai and AGS on the vegetal cortex but is excluded from the animal spindle pole through Plk1, which dissociates NuMA, Dynein and Vasa. This disparity between the two spindle poles reinforces asymmetric spindle localization, resulting in the formation of large macromeres and small, Vasa-containing micromeres. (b) During meiosis I in mouse oocytes, Formin-2 is present around the spindle allowing for the nucleation of F-actin on vesicles surrounding the spindle. This nucleation is essential for initial spindle movement. Additionally, these vesicles are surrounded by mitochondria which may be utilized as a method of force to push the spindle towards the cell cortex further escalating asymmetry. (c) After chromosome segregation during meiosis II in mouse oocytes, each pole has a separate actomyosin domain of influence resulting in symmetric hydrodynamic forces. However, these forces become unbalanced, due to the presence of RanGTP on the chromosomes. The RanGTP signaling results in the activation of Arp2/3 while simultaneously inhibiting Myosin at the cell cortex. Together these asymmetric hydrodynamic forces drive spindle rotation.

Figure 2:. Spindle-independent symmetry breaking events, inducing sibling cell size asymmetry

(a) The interplay of biased Myosin relocalization and build-up of hydrostatic pressure initiates the establishment of physical asymmetry in fly neural stem cells (neuroblasts). Active cortical tension is isotropically distributed in prophase. In early anaphase, apical cortical tension is downregulated; Pins recruits Pkn apically, which is necessary for local Myosin inactivation. The resulting anisotropy in active tension results in a basally directed Myosin flow, relieving the apical cell cortex of Myosin contraction. This built-up hydrostatic pressure dissipates apically, the cell cortex and membrane outwardly expand. Subsequently, spindle-dependent cues direct an apical Myosin flow towards the cleavage furrow. The mechanisms resulting in basal Myosin inactivation are unclear. Myosin contraction at the contractile ring allows for continued apical and basal cortical expansion. (b) In scallops, Myosin enriches at the future cleavage furrow site as well as the future polar lobe construction site. Additionally, Arp2/3 becomes enriched on the polar lobe cortex. Arp2/3 subsequently also enriches on the animal cortex but retains its presence on the polar lobe cortex. Towards the end of cytokinesis, the polar lobe reintegrates into one sibling, giving rise to the larger CD cell.

Figure 3:. The contribution of mechanical forces to asymmetric cell division in the mammalian embryo

(a) Cell geometry and cell polarity both influence oriented cell division in the early mouse embryo. Surface tension stretches cells on the embryo surface, overriding intrinsic polarity cues (blue cortex). Stretched cells predominantly divide symmetrically, giving rise to two trophoectodermal cells. These symmetric divisions can alter the geometry of neighboring cells, so that polarity cues become the main determinant of spindle orientation. Cells oriented along the apico-basal cell axis result in asymmetric divisions, which place one sibling cell towards the inside of the blastocyst. Asymmetrically dividing cells can also stretch surface cells, inducing symmetric divisions. (b) Later during mouse development, a fluid-filled cavity is creating luminal pressure, which alters the cell geometry of blastocyst cells, triggering symmetric cell divisions.