Quantitative regulation of the dynamic steady state of actin networks

Fascin-induced bundling protects actin filaments from disassembly by cofilin

Actin filament turnover plays a central role in shaping actin networks, yet the feedback mechanism between network architecture and filament assembly dynamics remains unclear. The activity of ADF/cofilin, the main protein family responsible for filament disassembly, has been mainly studied at the single filament level. This study unveils that fascin, by crosslinking filaments into bundles, strongly slows down filament disassembly by cofilin. We show that this is due to a markedly slower initiation of the first cofilin clusters, which occurs up to 100-fold slower on large bundles compared with single filaments. In contrast, severing at cofilin cluster boundaries is unaffected by fascin bundling. After the formation of an initial cofilin cluster on a filament within a bundle, we observed the local removal of fascin. Notably, the formation of cofilin clusters on adjacent filaments is highly enhanced, locally. We propose that this interfilament cooperativity arises from the local propagation of the cofilin-induced change in helicity from one filament to the other filaments of the bundle. Overall, taking into account all the above reactions, we reveal that fascin crosslinking slows down the disassembly of actin filaments by cofilin. These findings highlight the important role played by crosslinkers in tuning actin network turnover by modulating the activity of other regulatory proteins.

Reconstitution of actin-based cellular processes: Why encapsulation changes the rules

While in vitro reconstitution of cellular processes is progressing rapidly, the encapsulation of biomimetic systems to reproduce the cellular environment is a major challenge. Here we review the difficulties, using reconstitution of processes dependent on actin polymerization as an example. Some of the problems are purely technical, due to the need for engineering strategies to encapsulate concentrated solutions in micrometer-sized compartments. However, other significant issues arise from the reduction of experimental volumes, which alters the chemical evolution of these non-equilibrium systems. Important parameters to consider for successful reconstitutions are the amount of each component, their consumption and renewal rates to guarantee their continuous availability.

Length control emerges from cytoskeletal network geometry

Many cytoskeletal networks consist of individual filaments that are organized into elaborate higher order structures. While it is appreciated that the size and architecture of these networks are critical for their biological functions, much of the work investigating control over their assembly has focused on mechanisms that regulate the turnover of individual filaments through size-dependent feedback. Here, we propose a very different, feedback-independent mechanism to explain how yeast cells control the length of their actin cables. Our findings, supported by quantitative cell imaging and mathematical modeling, indicate that actin cable length control is an emergent property that arises from the cross-linked and bundled organization of the filaments within the cable. Using this model, we further dissect the mechanisms that allow cables to grow longer in larger cells, and propose that cell length-dependent tuning of formin activity allows cells to scale cable length with cell length. This mechanism is a significant departure from prior models of cytoskeletal filament length control and presents a new paradigm to consider how cells control the size, shape, and dynamics of higher order cytoskeletal structures.

Recycling of the actin monomer pool limits the lifetime of network turnover

Intracellular organization is largely mediated by actin turnover. Cellular actin networks continuously assemble and disassemble, while maintaining their overall appearance. This behavior, called "dynamic steady state," allows cells to sense and adapt to their environment. However, how structural stability can be maintained during the constant turnover of a limited actin monomer pool is poorly understood. To answer this question, we developed an experimental system where polystyrene beads are propelled by an actin comet in a microwell containing a limited amount of components. We used the speed and the size of the actin comet tails to evaluate the system's monomer consumption and its lifetime. We established the relative contribution of actin assembly, disassembly, and recycling for a bead movement over tens of hours. Recycling mediated by cyclase-associated protein (CAP) is the key step in allowing the reuse of monomers for multiple assembly cycles. ATP supply and protein aging are also factors that limit the lifetime of actin turnover. This work reveals the balancing mechanism for long-term network assembly with a limited amount of building blocks.

A mechanism with severing near barbed ends andannealing explains structure and dynamics of dendriticactin networks

Single molecule imaging has shown that part of actin disassembles within a few seconds after incorporation into the dendritic filament network in lamellipodia, suggestive of frequent destabilization near barbed ends. To investigate the mechanisms behind network remodeling, we created a stochastic model with polymerization, depolymerization, branching, capping, uncapping, severing, oligomer diffusion, annealing, and debranching. We find that filament severing, enhanced near barbed ends, can explain the single molecule actin lifetime distribution, if oligomer fragments reanneal to free ends with rate constants comparable to in vitro measurements. The same mechanism leads to actin networks consistent with measured filament, end, and branch concentrations. These networks undergo structural remodeling, leading to longer filaments away from the leading edge, at the +/- 35𝑜 orientation pattern. Imaging of actin speckle lifetimes at sub-second resolution verifies frequent disassembly of newly-assembled actin. We thus propose a unified mechanism that fits a diverse set of basic lamellipodia phenomenology.

F-Actin Cytoskeleton Network Self-Organization Through Competition and Cooperation

Many fundamental cellular processes such as division, polarization, endocytosis, and motility require the assembly, maintenance, and disassembly of filamentous actin (F-actin) networks at specific locations and times within the cell. The particular function of each network is governed by F-actin organization, size, and density as well as by its dynamics. The distinct characteristics of different F-actin networks are determined through the coordinated actions of specific sets of actin-binding proteins (ABPs). Furthermore, a cell typically assembles and uses multiple F-actin networks simultaneously within a common cytoplasm, so these networks must self-organize from a common pool of shared globular actin (G-actin) monomers and overlapping sets of ABPs. Recent advances in multicolor imaging and analysis of ABPs and their associated F-actin networks in cells, as well as the development of sophisticated in vitro reconstitutions of networks with ensembles of ABPs, have allowed the field to start uncovering the underlying principles by which cells self-organize diverse F-actin networks to execute basic cellular functions.

Intra-bundle contractions enable extensile properties of active actin networks

The cellular cortex is a dynamic and contractile actomyosin network modulated by actin-binding proteins. We reconstituted a minimal cortex adhered to a model cell membrane mimicking two processes mediated by the motor protein myosin: contractility and high turnover of actin monomers. Myosin reorganized these networks by extensile intra‑bundle contractions leading to an altered growth mechanism. Hereby, stress within tethered bundles induced nicking of filaments followed by repair via incorporation of free monomers. This mechanism was able to break the symmetry of the previously disordered network resulting in the generation of extensile clusters, reminiscent of structures found within cells.

The dynamics of actin network turnover is self-organized by a growth-depletion feedback

The dynamics of actin networks is modulated by a machinery consisting of actin binding proteins that control the turnover of filaments in space and time. To study this complex orchestration, in vitro reconstitution approaches strive to project actin dynamics in ideal, minimal systems. To this extent we reconstitute a self-supplying, dense network of globally treadmilling filaments. In this system we analyze growth and intrinsic turnover by means of FRAP measurements and thereby demonstrate how the depletion of monomers and actin binding partners modulate the dynamics in active actin networks. The described effects occur only in dense networks, as single filament dynamics are unable to produce depletion effects to this extent. Furthermore, we demonstrate a synergistic relationship between the nucleators formin and Arp2/3 when branched networks and formin-induced networks are colocalized. As a result, the formin-enhanced filament turnover depletes cofilin at the surface and thus protects the dense, Arp2/3 polymerized network from debranching. Ultimately, these results may be key for understanding the maintenance of the two contradicting requirements of network stability and dynamics in cells.

The Actin Cytoskeleton as an Active Adaptive Material

Actin is the main protein used by biological cells to adapt their structure and mechanics to their needs. Cellular adaptation is made possible by molecular processes that strongly depend on mechanics. The actin cytoskeleton is also an active material that continuously consumes energy. This allows for dynamical processes that are possible only out of equilibrium and opens up the possibility for multiple layers of control that have evolved around this single protein.Here we discuss the actin cytoskeleton from the viewpoint of physics as an active adaptive material that can build structures superior to man-made soft matter systems. Not only can actin be used to build different network architectures on demand and in an adaptive manner, but it also exhibits the dynamical properties of feedback systems, like excitability, bistability, or oscillations. Therefore, it is a prime example of how biology couples physical structure and information flow and a role model for biology-inspired metamaterials.

Encapsulation of the cytoskeleton: towards mimicking the mechanics of a cell

The cytoskeleton of a cell controls all the aspects of cell shape changes and motility from its physiological functions for survival to reproduction to death. The structure and dynamics of the cytoskeletal components: actin, microtubules, intermediate filaments, and septins - recently regarded as the fourth member of the cytoskeleton family - are conserved during evolution. Such conserved and effective control over the mechanics of the cell makes the cytoskeletal components great candidates for in vitro reconstitution and bottom-up synthetic biology studies. Here, we review the recent efforts in reconstitution of the cytoskeleton in and on membrane-enclosed biomimetic systems and argue that co-reconstitution and synergistic interplay between cytoskeletal filaments might be indispensable for efficient mechanical functionality of active minimal cells. Further, mechanical equilibrium in adherent eukaryotic cells is achieved by the formation of integrin-based focal contacts with extracellular matrix (ECM) and the transmission of stresses generated by actomyosin contraction to ECM. Therefore, a minimal mimic of such balance of forces and quasi-static kinetics of the cell by bottom-up reconstitution requires a careful construction of contractile machineries and their link with adhesive contacts. In this review, in addition to cytoskeletal crosstalk, we provide a perspective on reconstruction of cell mechanical equilibrium by reconstitution of cortical actomyosin networks in lipid membrane vesicles adhered on compliant substrates and also discuss future perspectives of this active research area.

A numerical mechanical model integrating actin treadmilling and receptor recycling to explain selective disengagement of immune cells

T-killer cells of the immune system eliminate virus-infected cells by releasing toxic granules through a direct cell to cell surface interface (synapse). The release of toxic granules only through the cell surface interface assures the specificity of the immune response. The toxic granule releasing apparatus, however, may not be aligned toward the synapse at the moment of the synapse formation. Therefore, mechanisms for reorienting the killing apparatus inside the T-killer cell to the interface with the target is required. Numerous research works were reported to suggest the mechanisms with direct and indirect evidence, but the most adversary situation, that is when the cell's initial orientation is the complete opposite of the desired direction, either remained answered or brought skepticism toward the suggested mechanisms. To address this issue, a computational mechanical model of T-killer cell synapse formation is constructed to test previously suggested models in a more realistic setting and at the same time to test previously neglected component in the model, namely the actin network formation on the synapse. In this model, by capturing the mechanical interaction between T-killer cell surface receptor dynamics and mechanical properties of synapse formation, it is shown that T-killer cells can selectively engage or disengage from the target cell depending on the killing apparatus alignment with the target. The mechanism works as a safeguard measure ensuring target-cell killing and specificity, so it will be effective when T-killer cells are stranded in situations where the alignment of the killing apparatus is challenging.