Contribution of cytoplasm viscoelastic properties to mitotic spindle positioning

Abstract

Cells are filled with macromolecules and polymer networks that set scale-dependent viscous and elastic properties to the cytoplasm. Although the role of these parameters in molecular diffusion, reaction kinetics, and cellular biochemistry is being increasingly recognized, their contributions to the motion and positioning of larger organelles, such as mitotic spindles for cell division, remain unknown. Here, using magnetic tweezers to displace and rotate mitotic spindles in living embryos, we uncovered that the cytoplasm can impart viscoelastic reactive forces that move spindles, or passive objects with similar size, back to their original positions. These forces are independent of cytoskeletal force generators yet reach hundreds of piconewtons and scale with cytoplasm crowding. Spindle motion shears and fluidizes the cytoplasm, dissipating elastic energy and limiting spindle recoils with functional implications for asymmetric and oriented divisions. These findings suggest that bulk cytoplasm material properties may constitute important control elements for the regulation of division positioning and cellular organization.

Size and position dependent cytoplasm viscoelasticity through hydrodynamic interactions with the cell surface

“…Our data suggest that the impact of hydrodynamic coupling becomes pronounced for objects over ~20-30% of cell size, which is the typical size ratio for organelles such as nuclei, mitotic spindles or microtubule asters. Accordingly, in vivo direct or indirect force measurements reported spindle, nuclei and aster drags that largely exceeded Stokes drags calculated from the size of these organelles and cytoplasm viscosity (20,28,34). Other relevant parameters that could impact cytoplasm resistance include organelle shape and porosity especially when considering fibrous-like structures such as asters and spindles (21,35).…”

“…To reach component sizes closer to cell size, we injected a suspension of soya oil mixed with hydrophobic ~1 µm magnetic beads inside eggs. Upon injection, this suspension formed large magnetized oil droplets that ranged in size from 18 to 45 µm (corresponding to ~20-48% of the cell diameter) (20). We applied calibrated forces, by approaching a magnet tip close to the cell surface.…”

“…However, these studies were restricted to regimes of relatively small objects typically below the micron size, as well as low forces and displacement amplitudes. As objects reach closer to the cell size and move across longer distances, they are predicted to drag and shear large portions of the cytoplasm fluid (20)(21)(22). In such regime, cytoplasm resistance may be influenced by boundary conditions at the static cell surface, through so-called wall effects.…”

A nuclear jamming transition in vertebrate organogenesis

“…2c; Methods). To reproduce the experimentally observed nuclear positioning close to the cell center 44,45 , we introduced a restoring force for the nucleus to the geometric cell center (Fig. 2c).…”

Effective simulations of interacting droplets

“…Generally, the cytoplasm inside the cell can show visco-elastic properties; see Ref. [157], which might lead to a slow ripening of the condensates. Weiss et al [158] also demonstrated that condensates can be strongly influenced by the cytoskeleton, and anomalous diffusion can arise due to molecular crowding, thus adversely affecting larger condensates and in turn not affecting diffusion across smaller condensates as much.…”