Spatiotemporal coordination of actin regulators generates invasive protrusions in cell-cell fusion.

Invasive membrane protrusions play a central role in a variety of cellular processes. Unlike filopodia, invasive protrusions are mechanically stiff and propelled by branched actin polymerization. However, how branched actin filaments are organized to create finger-like invasive protrusions is unclear.

Plasma Membrane Blebbing Is Controlled by Subcellular Distribution of Vimentin Intermediate Filaments.

The formation of specific cellular protrusions, plasma membrane blebs, underlies the amoeboid mode of cell motility, which is characteristic for free-living amoebae and leukocytes, and can also be adopted by stem and tumor cells to bypass unfavorable migration conditions and thus facilitate their long-distance migration. Not all cells are equally prone to bleb formation. We have previously shown that membrane blebbing can be experimentally induced in a subset of HT1080 fibrosarcoma cells, whereas other cells in the same culture under the same conditions retain non-blebbing mesenchymal morphology.

Septins mediate a microtubule-actin crosstalk that enables actin growth on microtubules.

Cellular morphogenesis and processes such as cell division and migration require the coordination of the microtubule and actin cytoskeletons. Microtubule-actin crosstalk is poorly understood and largely regarded as the capture and regulation of microtubules by actin. Septins are filamentous guanosine-5'-triphosphate (GTP) binding proteins, which comprise the fourth component of the cytoskeleton along microtubules, actin, and intermediate filaments.

Feedback between mechanosensitive signaling and active forces governs endothelial junction integrity.

The formation and recovery of gaps in the vascular endothelium governs a wide range of physiological and pathological phenomena, from angiogenesis to tumor cell extravasation. However, the interplay between the mechanical and signaling processes that drive dynamic behavior in vascular endothelial cells is not well understood. In this study, we propose a chemo-mechanical model to investigate the regulation of endothelial junctions as dependent on the feedback between actomyosin contractility, VE-cadherin bond turnover, and actin polymerization, which mediate the forces exerted on the cell-cell interface.

INF2-mediated actin filament reorganization confers intrinsic resilience to neuronal ischemic injury.

During early ischemic brain injury, glutamate receptor hyperactivation mediates neuronal death via osmotic cell swelling. Here we show that ischemia and excess NMDA receptor activation cause actin to rapidly and extensively reorganize within the somatodendritic compartment. Normally, F-actin is concentrated within dendritic spines.

HRS phosphorylation drives immunosuppressive exosome secretion and restricts CD8 T-cell infiltration into tumors.

The lack of tumor infiltration by CD8 T cells is associated with poor patient response to anti-PD-1 therapy. Understanding how tumor infiltration is regulated is key to improving treatment efficacy. Here, we report that phosphorylation of HRS, a pivotal component of the ESCRT complex involved in exosome biogenesis, restricts tumor infiltration of cytolytic CD8 T cells.

Adenomatous Polyposis Coli (APC) in cell migration.

Adenomatous Polyposis Coli (APC) protein is mostly known as a tumor suppressor that regulates Wnt signaling, but is also an important cytoskeletal protein. Mutations in the APC gene are linked to colorectal cancer and various neurological disorders and intellectual disabilities. Cytoskeletal functions of APC appear to have significant contributions to both types of these disorders.

Myosin II and Arp2/3 cross-talk governs intracellular hydraulic pressure and lamellipodia formation.

Human fibroblasts can switch between lamellipodia-dependent and -independent migration mechanisms on two-dimensional surfaces and in three-dimensional (3D) matrices. RhoA GTPase activity governs the switch from low-pressure lamellipodia to high-pressure lobopodia in response to the physical structure of the 3D matrix. Inhibiting actomyosin contractility in these cells reduces intracellular pressure and reverts lobopodia to lamellipodial protrusions via an unknown mechanism.

Actin cytoskeleton in mesenchymal-to-amoeboid transition of cancer cells.

During development of metastasis, tumor cells migrate through different tissues and encounter different extracellular matrices. An ability of cells to adapt mechanisms of their migration to these diverse environmental conditions, called migration plasticity, gives tumor cells an advantage over normal cells for long distant dissemination. Different modes of individual cell motility-mesenchymal and amoeboid-are driven by different molecular mechanisms, which largely depend on functions of the actin cytoskeleton that can be modulated in a wide range by cellular signaling mechanisms in response to environmental conditions.

T-Plastin reinforces membrane protrusions to bridge matrix gaps during cell migration.

Migrating cells move across diverse assemblies of extracellular matrix (ECM) that can be separated by micron-scale gaps. For membranes to protrude and reattach across a gap, actin filaments, which are relatively weak as single filaments, must polymerize outward from adhesion sites to push membranes towards distant sites of new adhesion. Here, using micropatterned ECMs, we identify T-Plastin, one of the most ancient actin bundling proteins, as an actin stabilizer that promotes membrane protrusions and enables bridging of ECM gaps.

Pseudo-obstruction-inducing ACTG2R257C alters actin organization and function.

Actin γ 2, smooth muscle (ACTG2) R257C mutation is the most common genetic cause of visceral myopathy. Individuals with ACTG2 mutations endure prolonged hospitalizations and surgical interventions, become dependent on intravenous nutrition and bladder catheterization, and often die in childhood. Currently, we understand little about how ACTG2 mutations cause disease, and there are no mechanism-based treatments.

Branched actin networks are assembled on microtubules by adenomatous polyposis coli for targeted membrane protrusion.

Cell migration is driven by pushing and pulling activities of the actin cytoskeleton, but migration directionality is largely controlled by microtubules. This function of microtubules is especially critical for neuron navigation. However, the underlying mechanisms are poorly understood.

Dynamin regulates the dynamics and mechanical strength of the actin cytoskeleton as a multifilament actin-bundling protein.

The dynamin GTPase is known to bundle actin filaments, but the underlying molecular mechanism and physiological relevance remain unclear. Our genetic analyses revealed a function of dynamin in propelling invasive membrane protrusions during myoblast fusion in vivo. Using biochemistry, total internal reflection fluorescence microscopy, electron microscopy and cryo-electron tomography, we show that dynamin bundles actin while forming a helical structure.

Actin Cell Cortex: Structure and Molecular Organization.

The actin cytoskeleton consists of structurally and biochemically different actin filament arrays. Among them, the actin cortex is thought to have key roles in cell mechanics, but remains a poorly characterized part of the actin cytoskeleton. The cell cortex is typically defined as a thin layer of actin meshwork that uniformly underlies the plasma membrane of the entire cell.

Filamin A mediates isotropic distribution of applied force across the actin network.

Cell sensing of externally applied mechanical strain through integrin-mediated adhesions is critical in development and physiology of muscle, lung, tendon, and arteries, among others. We examined the effects of strain on force transmission through the essential cytoskeletal linker talin. Using a fluorescence-based talin tension sensor (TS), we found that uniaxial stretch of cells on elastic substrates increased tension on talin, which was unexpectedly independent of the orientation of the focal adhesions relative to the direction of strain.

Ultrastructure and dynamics of the actin-myosin II cytoskeleton during mitochondrial fission.

Mitochondrial fission involves the preconstriction of an organelle followed by scission by dynamin-related protein Drp1. Preconstriction is facilitated by actin and non-muscle myosin II through a mechanism that remains unclear, largely due to the unknown cytoskeletal ultrastructure at mitochondrial constrictions. Here, using platinum replica electron microscopy, we show that mitochondria in cells are embedded in an interstitial cytoskeletal network that contains abundant unbranched actin filaments.

Time-resolved ultrastructure of the cortical actin cytoskeleton in dynamic membrane blebs.

Membrane blebbing accompanies various cellular processes, including cytokinesis, apoptosis, and cell migration, especially invasive migration of cancer cells. Blebs are extruded by intracellular pressure and are initially cytoskeleton-free, but they subsequently assemble the cytoskeleton, which can drive bleb retraction. Despite increasing appreciation of physiological significance of blebbing, the molecular and, especially, structural mechanisms controlling bleb dynamics are incompletely understood.

Mammalian nonmuscle myosin II comes in three flavors.

Nonmuscle myosin II is an actin-based motor that executes numerous mechanical tasks in cells including spatiotemporal organization of the actin cytoskeleton, adhesion, migration, cytokinesis, tissue remodeling, and membrane trafficking. Nonmuscle myosin II is ubiquitously expressed in mammalian cells as a tissue-specific combination of three paralogs. Recent studies reveal novel specific aspects of their kinetics, intracellular regulation and functions.

Branched actin networks push against each other at adherens junctions to maintain cell-cell adhesion.

Adherens junctions (AJs) are mechanosensitive cadherin-based intercellular adhesions that interact with the actin cytoskeleton and carry most of the mechanical load at cell-cell junctions. Both Arp2/3 complex-dependent actin polymerization generating pushing force and nonmuscle myosin II (NMII)-dependent contraction producing pulling force are necessary for AJ morphogenesis. Which actin system directly interacts with AJs is unknown.

Ultrastructure of the actin cytoskeleton.

The actin cytoskeleton is the primary force-generating machinery in the cell, which can produce pushing (protrusive) forces using energy of actin polymerization and pulling (contractile) forces via sliding of bipolar filaments of myosin II along actin filaments, as well as perform other key functions. These functions are essential for whole cell migration, cell interaction with the environment, mechanical properties of the cell surface and other key aspects of cell physiology. The actin cytoskeleton is a highly complex and dynamic system of actin filaments organized into various superstructures by multiple accessory proteins.

Self-sorting of nonmuscle myosins IIA and IIB polarizes the cytoskeleton and modulates cell motility.

Nonmuscle myosin II (NMII) is uniquely responsible for cell contractility and thus defines multiple aspects of cell behavior. To generate contraction, NMII molecules polymerize into bipolar minifilaments. Different NMII paralogs are often coexpressed in cells and can copolymerize, suggesting that they may cooperate to facilitate cell motility.

Platinum replica electron microscopy: Imaging the cytoskeleton globally and locally.

Structural studies reveal how smaller components of a system work together as a whole. However, combining high resolution of details with full coverage of the whole is challenging. In cell biology, light microscopy can image many cells in their entirety, but at a lower resolution, whereas electron microscopy affords very high resolution, but usually at the expense of the sample size and coverage.

Neurite outgrowth is driven by actin polymerization even in the presence of actin polymerization inhibitors.

Actin polymerization is a universal mechanism to drive plasma membrane protrusion in motile cells. One apparent exception to this rule is continuing, or even accelerated outgrowth of neuronal processes in the presence of actin polymerization inhibitors. This fact together with a key role of microtubule dynamics in neurite outgrowth led to the concept that microtubules directly drive plasma membrane protrusion, either in the course of polymerization or motor-driven sliding.

Axon Initial Segment Cytoskeleton: Architecture, Development, and Role in Neuron Polarity.

The axon initial segment (AIS) is a specialized structure in neurons that resides in between axonal and somatodendritic domains. The localization of the AIS in neurons is ideal for its two major functions: it serves as the site of action potential firing and helps to maintain neuron polarity. It has become increasingly clear that the AIS cytoskeleton is fundamental to AIS functions.