Active Condensation of Filaments Under Spatial Confinement

Abstract

Living systems exhibit self-organization, a phenomenon that enables organisms to perform functions essential for life. The interior of living cells is a crowded environment in which the self-assembly of cytoskeletal networks is spatially constrained by membranes and organelles. Cytoskeletal filaments undergo active condensation in the presence of crosslinking motor proteins. In past studies, confinement has been shown to alter the morphology of active condensates. Here, we perform simulations to explore systems of filaments and crosslinking motors in a variety of confining geometries. We simulate spatial confinement imposed by hard spherical, cylindrical, and planar boundaries. These systems exhibit non-equilibrium condensation behavior where crosslinking motors condense a fraction of the overall filament population, leading to coexistence of vapor and condensed states. We find that the confinement lengthscale modifies the dynamics and condensate morphology. With end-pausing crosslinking motors, filaments self-organize into half asters and fully-symmetric asters under spherical confinement, polarity-sorted bilayers and bottle-brush-like states under cylindrical confinement, and flattened asters under planar confinement. The number of crosslinking motors controls the size and shape of condensates, with flattened asters becoming hollow and ring-like for larger motor number. End pausing plays a key role affecting condensate morphology: systems with end-pausing motors evolve into aster-like condensates while those with non-end-pausing crosslinking motor proteins evolve into disordered clusters and polarity-sorted bundles.

VASP phase separation with priming proteins of fast endophilin mediated endocytosis modulates actin polymerization

“…Interestingly, simulations have shown that the spatial confinement of actin facilitates symmetry breaking allowing for the formation of organised structures. In our system, the quasi-2D and 3D condensates initiate polymerization within the confined volumes of the condensates 62 . In conjunction with recent observations 35 , we believe that our findings experimentally demonstrate that the confined polymerization of actin within LPD 850-1250 -VASP condensates is an example of a series of symmetry breaking events (Fig 7).…”

“…Ring morphologies have been observed in micron sized contractile rings 68 and nano-scale structures at the base of endocytic buds [69][70][71] . Interestingly, simulations and experiments have shown that the spatial confinement of actin facilitates symmetry breaking, allowing for the formation of organised structures 72,73 . In our system, actin polymerization is initiated within the confined volumes of quasi-2D and 3D condensates, suggesting a possible role for symmetry breaking.…”

Polarity sorting of actin filaments by motor-driven cargo transport

“…In plant cells, the cell wall provides rigid external confinement, and the large vacuole can occupy up to 90% of the cytoplasm, leading to quasi-2D environments between the vacuole and plasma membrane (42). Recent experimental and computational studies have demonstrated that confinement impacts the organization of motor-filament systems, promoting the formation of polar and nematic flows (43)(44)(45), as well as aster-like structures (22).…”

“…Sorting by filament polarity has been associated with polar flows, vortices, and asters in kinesin-microtubule systems (18, 19) and asters in myosin II-actin systems (20, 21). Polarity sorting in these systems has been attributed to the end-dwelling nature of kinesins and myosin II (20, 22). Recently, Memarian et al (23) studied microtubules propelled by membrane-bound kinesin motors.…”

“…A successful class of models uses dynamical, coarse-grained representations of filaments and motors, with filaments modeled as semiflexible polymers and motors modeled either explicitly with heads that bind to and move along filaments (24, 25) or implicitly as tangential forces acting along the contour of filaments (26, 27). Simulation packages such as CytoSim (28), MEDYAN (29), AFINES (30), and aLENS (22) have been developed and utilized to study systems of myosin II motors and actin filaments. Simulations using these approaches have elucidated collective behavior of filaments in response to motor forces, including the formation of bundles (31, 32), asters (24, 31), vortices, and rings (27, 33).…”