Mechanisms of contractile ring tension production and constriction

Subcellular context-specific tuning of actomyosin ring contractility within a common cytoplasm

The non-muscle actomyosin cytoskeleton generates contractile force through the dynamic rearrangement of its constituent parts. Actomyosin rings are a specialization of the non-muscle actomyosin cytoskeleton that drive cell shape changes during division, wound healing, and other events. Contractile rings throughout phylogeny and in a range of cellular contexts are built from conserved components including non-muscle myosin II (NMMII), actin filaments (F-actin), and crosslinking proteins. However, it is unknown whether diverse actomyosin rings close via a single unifying mechanism. To explore how contractile forces are generated by actomyosin rings, we studied three instances of ring closure within the common cytoplasm of the C. elegans oogenic germline: mitotic cytokinesis of germline stem cells (GSCs), apoptosis of meiotic compartments, and cellularization of oocytes. We found that each ring type closed with unique kinetics, protein density and abundance dynamics. These measurements suggested that the mechanism of contractile force generation varied across the subcellular contexts. Next, we formulated a physical model that related the forces generated by filament-filament interactions to the material properties of these rings that dictate the kinetics of their closure. Using this framework, we related the density of conserved cytoskeletal proteins anillin and NMMII to the kinematics of ring closure. We fitted model rings to in situ measurements to estimate parameters that are currently experimentally inaccessible, such as the asymmetric distribution of protein along the length of F-actin, which occurs naturally due to differences in the dimensions of the crosslinker and NMMII filaments. Our work predicted that the role of NMMII varies across these ring types, due in part to its distribution along F-actin and motoring. Our model also predicted that the degree of contractility and the impact of ring material properties on contractility differs among ring types.

Emergence of periodic circumferential actin cables from the anisotropic fusion of actin nanoclusters during tubulogenesis

The periodic circumferential cytoskeleton supports various tubular tissues. Radial expansion of the tube lumen causes anisotropic tensile stress, which can be exploited as a geometric cue. However, the molecular machinery linking anisotropy to robust circumferential patterning is poorly understood. Here, we aim to reveal the emergent process of circumferential actin cable formation in a Drosophila tracheal tube. During luminal expansion, sporadic actin nanoclusters emerge and exhibit circumferentially biased motion and fusion. RNAi screening reveals the formin family protein, DAAM, as an essential component responding to tissue anisotropy, and non-muscle myosin II as a component required for nanocluster fusion. An agent-based model simulation suggests that crosslinkers play a crucial role in nanocluster formation and cluster-to-cable transition occurs in response to mechanical anisotropy. Altogether, we propose that an actin nanocluster is an organizational unit that responds to stress in the cortical membrane and builds a higher-order cable structure.

Modelling contractile ring formation and division to daughter cells for simulating proliferative multicellular dynamics

Cell proliferation is a fundamental process underlying embryogenesis, homeostasis, wound healing, and cancer. The process involves multiple events during each cell cycle, such as cell growth, contractile ring formation, and division to daughter cells, which affect the surrounding cell population geometrically and mechanically. However, existing methods do not comprehensively describe the dynamics of multicellular structures involving cell proliferation at a subcellular resolution. In this study, we present a novel model for proliferative multicellular dynamics at the subcellular level by building upon the nonconservative fluid membrane (NCF) model that we developed in earlier research. The NCF model utilizes a dynamically-rearranging closed triangular mesh to depict the shape of each cell, enabling us to analyze cell dynamics over extended periods beyond each cell cycle, during which cell surface components undergo dynamic turnover. The proposed model represents the process of cell proliferation by incorporating cell volume growth and contractile ring formation through an energy function and topologically dividing each cell at the cleavage furrow formed by the ring. Numerical simulations demonstrated that the model recapitulated the process of cell proliferation at subcellular resolution, including cell volume growth, cleavage furrow formation, and division to daughter cells. Further analyses suggested that the orientation of actomyosin stress in the contractile ring plays a crucial role in the cleavage furrow formation, i.e., circumferential orientation can form a cleavage furrow but isotropic orientation cannot. Furthermore, the model replicated tissue-scale multicellular dynamics, where the successive proliferation of adhesive cells led to the formation of a cell sheet and stratification on the substrate. Overall, the proposed model provides a basis for analyzing proliferative multicellular dynamics at subcellular resolution.

The Rho1 GTPase controls anillo-septin assembly to facilitate contractile ring closure during cytokinesis

Animal cell cytokinesis requires activation of the GTPase RhoA (Rho1 in Drosophila), which assembles an F-actin- and myosin II-dependent contractile ring (CR) at the equatorial plasma membrane. CR closure is poorly understood, but involves the multidomain scaffold protein, Anillin. Anillin binds many CR components including F-actin and myosin II (collectively actomyosin), RhoA and the septins. Anillin recruits septins to the CR but the mechanism is unclear. Live imaging of Drosophila S2 cells and HeLa cells revealed that the Anillin N-terminus, which scaffolds actomyosin, cannot recruit septins to the CR. Rather, septin recruitment required the ability of the Anillin C-terminus to bind Rho1-GTP and the presence of the Anillin PH domain, in a sequential mechanism occurring at the plasma membrane, independently of F-actin. Anillin mutations that blocked septin recruitment, but not actomyosin scaffolding, slowed CR closure and disrupted cytokinesis. Thus, CR closure requires coordination of two Rho1-dependent networks: actomyosin and anillo-septin.

Actin turnover protects the cytokinetic contractile ring from structural instability

In common with other actomyosin contractile cellular machineries, actin turnover is required for normal function of the cytokinetic contractile ring. Cofilin is an actin-binding protein contributing to turnover by severing actin filaments, required for cytokinesis by many organisms. In fission yeast cofilin mutants, contractile rings suffer bridging instabilities in which segments of the ring peel away from the plasma membrane, forming straight bridges whose ends remain attached to the membrane. The origin of bridging instability is unclear. Here, we used molecularly explicit simulations of contractile rings to examine the role of cofilin. Simulations reproduced the experimentally observed cycles of bridging and reassembly during constriction, and the occurrence of bridging in ring segments with low density of the myosin II protein Myo2. The lack of cofilin severing produced ∼2-fold longer filaments and, consequently, ∼2-fold higher ring tensions. Simulations identified bridging as originating in the boosted ring tension, which increased centripetal forces that detached actin from Myo2, which was anchoring actin to the membrane. Thus, cofilin serves a critical role in cytokinesis by providing protection from bridging, the principal structural threat to contractile rings.

Actomyosin-Driven Division of a Synthetic Cell

One of the major challenges of bottom-up synthetic biology is rebuilding a minimal cell division machinery. From a reconstitution perspective, the animal cell division apparatus is mechanically the simplest and therefore attractive to rebuild. An actin-based ring produces contractile force to constrict the membrane. By contrast, microbes and plant cells have a cell wall, so division requires concerted membrane constriction and cell wall synthesis. Furthermore, reconstitution of the actin division machinery helps in understanding the physical and molecular mechanisms of cytokinesis in animal cells and thus our own cells. In this review, we describe the state-of-the-art research on reconstitution of minimal actin-mediated cytokinetic machineries. Based on the conceptual requirements that we obtained from the physics of the shape changes involved in cell division, we propose two major routes for building a minimal actin apparatus capable of division. Importantly, we acknowledge both the passive and active roles that the confining lipid membrane can play in synthetic cytokinesis. We conclude this review by identifying the most pressing challenges for future reconstitution work, thereby laying out a roadmap for building a synthetic cell equipped with a minimal actin division machinery.

Encapsulated actomyosin patterns drive cell-like membrane shape changes

Cell shape changes from locomotion to cytokinesis are, to a large extent, driven by myosin-driven remodeling of cortical actin patterns. Passive crosslinkers such as α-actinin and fascin as well as actin nucleator Arp2/3 complex largely determine actin network architecture and, consequently, membrane shape changes. Here we reconstitute actomyosin networks inside cell-sized lipid bilayer vesicles and show that depending on vesicle size and concentrations of α-actinin and fascin actomyosin networks assemble into ring and aster-like patterns. Anchoring actin to the membrane does not change actin network architecture yet exerts forces and deforms the membrane when assembled in the form of a contractile ring. In the presence of α-actinin and fascin, an Arp2/3 complex-mediated actomyosin cortex is shown to assemble a ring-like pattern at the equatorial cortex followed by myosin-driven clustering and consequently blebbing. An active gel theory unifies a model for the observed membrane shape changes induced by the contractile cortex.

Contractile ring constriction and septation in fission yeast are integrated mutually stabilizing processes.. [DOI: 10.1101/2021.06.25.449700] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

In common with other cellular machineries, the actomyosin contractile ring that divides cells during cytokinesis does not operate in isolation. Contractile rings in animal cells interact with contiguous actomyosin cortex, while ring constriction in many cell-walled organisms couples tightly to cell wall growth. In fission yeast, a septum grows in the wake of the constricting ring, ensuring cytokinesis leaves two daughter cells fully enclosed by cell wall. Here we mathematical modeled the integrated constriction-septation system in fission yeast, with a kinetic growth model evolving the 3D septum shape coupled to a molecularly explicit simulation of the contractile ring highly constrained by experimental data. Simulations revealed influences in both directions, stabilizing the ring-septum system as a whole. By providing a smooth circular anchoring surface for the ring, the inner septum leading edge stabilized ring organization and tension production; by mechanically regulating septum circularity and in-plane growth, ring tension stabilized septum growth and shape. Genetic or pharmacological perturbation of either subsystem destabilized this delicate balance, precipitating uncontrolled positive feedback with disastrous morphological and functional consequences. Thus, high curvature septum irregularities triggered bridging instabilities, in which contractile ring segments became unanchored. Bridging abolished the local tension-mediated septum shape regulation, exacerbating the irregularity in a mutually destabilizing runaway process. Our model explains a number of previously mysterious experimental observations, including unanchoring of ring segments observed in cells with mutations in the septum-growing β-glucan synthases, and irregular septa in cells with mutations in the contractile ring myosin-II Myo2. Thus, the contractile ring and cell wall growth cellular machineries operate as a single integrated system, whose stability relies on mutual regulation by the two subsystems.

Animal Cell Cytokinesis: The Rho-Dependent Actomyosin-Anilloseptin Contractile Ring as a Membrane Microdomain Gathering, Compressing, and Sorting Machine

Cytokinesis is the last step of cell division that partitions the cellular organelles and cytoplasm of one cell into two. In animal cells, cytokinesis requires Rho-GTPase-dependent assembly of F-actin and myosin II (actomyosin) to form an equatorial contractile ring (CR) that bisects the cell. Despite 50 years of research, the precise mechanisms of CR assembly, tension generation and closure remain elusive. This hypothesis article considers a holistic view of the CR that, in addition to actomyosin, includes another Rho-dependent cytoskeletal sub-network containing the scaffold protein, Anillin, and septin filaments (collectively termed anillo-septin). We synthesize evidence from our prior work in Drosophila S2 cells that actomyosin and anillo-septin form separable networks that are independently anchored to the plasma membrane. This latter realization leads to a simple conceptual model in which CR assembly and closure depend upon the micro-management of the membrane microdomains to which actomyosin and anillo-septin sub-networks are attached. During CR assembly, actomyosin contractility gathers and compresses its underlying membrane microdomain attachment sites. These microdomains resist this compression, which builds tension. During CR closure, membrane microdomains are transferred from the actomyosin sub-network to the anillo-septin sub-network, with which they flow out of the CR as it advances. This relative outflow of membrane microdomains regulates tension, reduces the circumference of the CR and promotes actomyosin disassembly all at the same time. According to this hypothesis, the metazoan CR can be viewed as a membrane microdomain gathering, compressing and sorting machine that intrinsically buffers its own tension through coordination of actomyosin contractility and anillo-septin-membrane relative outflow, all controlled by Rho. Central to this model is the abandonment of the dogmatic view that the plasma membrane is always readily deformable by the underlying cytoskeleton. Rather, the membrane resists compression to build tension. The notion that the CR might generate tension through resistance to compression of its own membrane microdomain attachment sites, can account for numerous otherwise puzzling observations and warrants further investigation using multiple systems and methods.

Sliding filament and fixed filament mechanisms contribute to ring tension in the cytokinetic contractile ring

A fundamental challenge in cell biology is to understand how cells generate actomyosin-based contractile force. Here we study the actomyosin contractile ring that divides cells during cytokinesis and generates tension by a mechanism that remains poorly understood. Long ago a muscle-like sliding filament mechanism was proposed, but evidence for sarcomeric organization in contractile rings is lacking. We develop a coarse-grained model of the fission yeast cytokinetic ring, incorporating the two myosin-II isoforms Myo2 and Myp2 and severely constrained by experimental data. The model predicts that ring tension is indeed generated by a sliding filament mechanism, but a spatially and temporally homogeneous version of that in muscle. In this mechanism all pairs of oppositely oriented actin filaments are rendered tense as they are pulled toward one another and slide through clusters of myosin-II. The mechanism relies on anchoring of actin filament barbed ends to the plasma membrane, which resists lateral motion and enables filaments to become tense when pulled by myosin-II. A second fixed filament component is independent of lateral anchoring, generated by chains of like-oriented actin filaments. Myo2 contributes to both components, while Myp2 contributes to the sliding filament component only. In the face of instabilities inherent to actomyosin contractility, organizational homeostasis is maintained by rapid turnover of Myo2 and Myp2, and by drag forces that resist lateral motion of actin, Myo2 and Myp2. Thus, sliding and fixed filament mechanisms contribute to tension in the disordered contractile ring without the need for the sarcomeric architecture of muscle.

Anchoring of actin to the plasma membrane enables tension production in the fission yeast cytokinetic ring

The cytokinetic ring generates tensile force that drives cell division, but how tension emerges from the relatively disordered ring organization remains unclear. Long ago, a musclelike sliding filament mechanism was proposed, but evidence for sarcomeric order is lacking. Here we present quantitative evidence that in fission yeast, ring tension originates from barbed-end anchoring of actin filaments to the plasma membrane, providing resistance to myosin forces that enables filaments to develop tension. The role of anchoring was highlighted by experiments on isolated fission yeast rings, where sections of ring became unanchored from the membrane and shortened ∼30-fold faster than normal. The dramatically elevated constriction rates are unexplained. Here we present a molecularly explicit simulation of constricting partially anchored rings as studied in these experiments. Simulations accurately reproduced the experimental constriction rates and showed that following anchor release, a segment becomes tensionless and shortens via a novel noncontractile reeling-in mechanism at about the velocity of load-free myosin II. The ends are reeled in by barbed end-anchored actin filaments in adjacent segments. Other actin anchoring schemes failed to constrict rings. Our results quantitatively support a specific organization and anchoring scheme that generate tension in the cytokinetic ring.

Network Contractility During Cytokinesis-from Molecular to Global Views

Cytokinesis is the last stage of cell division, which partitions the mother cell into two daughter cells. It requires the assembly and constriction of a contractile ring that consists of a filamentous contractile network of actin and myosin. Network contractility depends on network architecture, level of connectivity and myosin motor activity, but how exactly is the contractile ring network organized or interconnected and how much it depends on motor activity remains unclear. Moreover, the contractile ring is not an isolated entity; rather, it is integrated into the surrounding cortex. Therefore, the mechanical properties of the cell cortex and cortical behaviors are expected to impact contractile ring functioning. Due to the complexity of the process, experimental approaches have been coupled to theoretical modeling in order to advance its global understanding. While earlier coarse-grained descriptions attempted to provide an integrated view of the process, recent models have mostly focused on understanding the behavior of an isolated contractile ring. Here we provide an overview of the organization and dynamics of the actomyosin network during cytokinesis and discuss existing theoretical models in light of cortical behaviors and experimental evidence from several systems. Our view on what is missing in current models and should be tested in the future is provided.