Possible translocation of actin and alpha-actinin along stress fibers

Methods of myofibrillogenesis modeling

Organization in the heart is important on multiple length scales. Myofibrillogenesis processes control the assembly of this multi-scale architecture. Understanding myofibrillogenesis might allow us to better control self-assembly of cardiac tissues. One approach consists of creating phenomenological models and comparing these models to in vitro data from primary myocytes. In this chapter, we present a method for building these models to recapitulate different aspects of myofibrillogenesis. We present a specific example for a cardiomyocyte model, but the same procedure can be used to model fibrillogenesis with other mechanisms such as motility. In sum, the models allow for a better understanding of mechanisms behind self-assembly.

Self-organization of muscle cell structure and function

The organization of muscle is the product of functional adaptation over several length scales spanning from the sarcomere to the muscle bundle. One possible strategy for solving this multiscale coupling problem is to physically constrain the muscle cells in microenvironments that potentiate the organization of their intracellular space. We hypothesized that boundary conditions in the extracellular space potentiate the organization of cytoskeletal scaffolds for directed sarcomeregenesis. We developed a quantitative model of how the cytoskeleton of neonatal rat ventricular myocytes organizes with respect to geometric cues in the extracellular matrix. Numerical results and in vitro assays to control myocyte shape indicated that distinct cytoskeletal architectures arise from two temporally-ordered, organizational processes: the interaction between actin fibers, premyofibrils and focal adhesions, as well as cooperative alignment and parallel bundling of nascent myofibrils. Our results suggest that a hierarchy of mechanisms regulate the self-organization of the contractile cytoskeleton and that a positive feedback loop is responsible for initiating the break in symmetry, potentiated by extracellular boundary conditions, is required to polarize the contractile cytoskeleton.

How muscle is organized impacts its function. However, understanding how muscle organizes is challenging, as the process occurs over several length scales. We approach this multiscale coupling problem by constraining the overall shapes of muscle cells to indirectly control the organization of their intracellular space. We hypothesized the cellular boundary conditions direct the organization of cytoskeletal scaffolds. We developed a model of how the cytoskeleton of cardiomyocytes organizes with respect to boundary cues. Our computational and experimental results to control myocyte shape indicated that distinct muscle architectures arise from two main organizational mechanisms: the interaction between actin fibers, premyofibrils and focal adhesions, as well as cooperative alignment and parallel bundling of more mature myofibrils. We show that a hierarchy of processes regulate the self-organization of cardiomyocytes. Our results suggest that a symmetry break, due to the boundary conditions imposed on the cell, is responsible for polarization of the contractile cytoskeletal organization.

Movement of stress fibers away from focal adhesions identifies focal adhesions as sites of stress fiber assembly in stationary cells

Force generated in contractile actin filament bundles (stress fibers-SFs) is transmitted to the extracellular matrix (ECM) via linker proteins and transmembrane integrins at focal adhesions (FAs). Though it has long been known that actin is rapidly exchanged in FAs, the connection between SFs and FAs has not been studied in detail. We introduced fiduciary marks on SFs by expressing GFP-palladin or GFP-alpha-actinin-1, which are both FA and dense body proteins, and by pattern bleaching of GFP-actin. Following fiduciary marks on SFs over time by time-lapse fluorescence microscopy, we detected assembly of SFs at FAs in stationary cells resulting in movement of SFs away from FAs with a velocity of 0.2-0.4 microm/min. Visualization of FAs in GFP-palladin/DsRed-paxillin double transfected cells showed that SF elongation was not accompanied by a change in FA length. SF elongation at FAs depended on actin polymerization and force as demonstrated by inhibitors of actin polymerization (cytochalasin D, jasplakinolide) and inhibitors of myosin-dependent contraction (blebbistatin, Y-27632), respectively. Our finding of SF assembly at FAs has important implications for SF formation, force transmission, and tension distribution within the actin cytoskeletal network of stationary cells.

Viscoelastic retraction of single living stress fibers and its impact on cell shape, cytoskeletal organization, and extracellular matrix mechanics

Cells change their form and function by assembling actin stress fibers at their base and exerting traction forces on their extracellular matrix (ECM) adhesions. Individual stress fibers are thought to be actively tensed by the action of actomyosin motors and to function as elastic cables that structurally reinforce the basal portion of the cytoskeleton; however, these principles have not been directly tested in living cells, and their significance for overall cell shape control is poorly understood. Here we combine a laser nanoscissor, traction force microscopy, and fluorescence photobleaching methods to confirm that stress fibers in living cells behave as viscoelastic cables that are tensed through the action of actomyosin motors, to quantify their retraction kinetics in situ, and to explore their contribution to overall mechanical stability of the cell and interconnected ECM. These studies reveal that viscoelastic recoil of individual stress fibers after laser severing is partially slowed by inhibition of Rho-associated kinase and virtually abolished by direct inhibition of myosin light chain kinase. Importantly, cells cultured on stiff ECM substrates can tolerate disruption of multiple stress fibers with negligible overall change in cell shape, whereas disruption of a single stress fiber in cells anchored to compliant ECM substrates compromises the entire cellular force balance, induces cytoskeletal rearrangements, and produces ECM retraction many microns away from the site of incision; this results in large-scale changes of cell shape (> 5% elongation). In addition to revealing fundamental insight into the mechanical properties and cell shape contributions of individual stress fibers and confirming that the ECM is effectively a physical extension of the cell and cytoskeleton, the technologies described here offer a novel approach to spatially map the cytoskeletal mechanics of living cells on the nanoscale.

The dynamic and motile properties of intermediate filaments

For many years, cytoplasmic intermediate filaments (IFs) were considered to be stable cytoskeletal elements contributing primarily to the maintenance of the structural and mechanical integrity of cells. However, recent studies of living cells have revealed that IFs and their precursors possess a remarkably wide array of dynamic and motile properties. These properties are in large part due to interactions with molecular motors such as conventional kinesin, cytoplasmic dynein, and myosin. The association between IFs and motors appears to account for much of the well-documented molecular cross talk between IFs and the other major cytoskeletal elements, microtubules, and actin-containing microfilaments. Furthermore, the associations with molecular motors are also responsible for the high-speed, targeted delivery of nonfilamentous IF protein cargo to specific regions of the cytoplasm where they polymerize into IFs. This review considers the functional implications of the motile properties of IFs and discusses the potential relationships between malfunctions in these motile activities and human diseases.

Actin-filled nuclear invaginations indicate degree of cell de-differentiation

For years the existence of nuclear actin has been heavily debated, but recent data have clearly demonstrated that actin, as well as actin-binding proteins (ABPs), are located in the nucleus. We examined live EGFP-actin-expressing cells using confocal microscopy and saw the presence of structures strongly resembling actin filaments in the nuclei of MDA-MB-231 human mammary epithelial tumor cells. Many nuclei had more than one of these filamentous structures, some of which appeared to cross the entire nucleus. Extensive analysis, including fluorescence recovery after photobleaching (FRAP), showed that all EGFP-actin in the nucleus is monomeric (G-actin) rather than filamentous (F-actin) and that the apparent filaments seen in the nucleus are invaginations of cytoplasmic monomeric actin. Immunolocalization of nuclear pore complex proteins shows that similar invaginations are seen in cells that are not overexpressing EGFP-actin. To determine whether there is a correlation between increased levels of invagination in the cell nuclei and the state of de-differentiation of the cell, we examined a variety of cell types, including live Xenopus embryonic cells. Cells that were highly de-differentiated, or cancerous, had an increased incidence of invagination, while cells that were differentiated had few nuclear invaginations. The nuclei of embryonic cells that were not yet differentiated underwent multiple shape changes throughout interphase, and demonstrated numerous transient invaginations of varying sizes and shapes. Although the function of these actin-filled invaginations remains speculative, their presence correlates with cells that have increased levels of nuclear activity.

Motile properties of vimentin intermediate filament networks in living cells

The motile properties of intermediate filament (IF) networks have been studied in living cells expressing vimentin tagged with green fluorescent protein (GFP-vimentin). In interphase and mitotic cells, GFP-vimentin is incorporated into the endogenous IF network, and accurately reports the behavior of IF. Time-lapse observations of interphase arrays of vimentin fibrils demonstrate that they are constantly changing their configurations in the absence of alterations in cell shape. Intersecting points of vimentin fibrils, or foci, frequently move towards or away from each other, indicating that the fibrils can lengthen or shorten. Fluorescence recovery after photobleaching shows that bleach zones across fibrils rapidly recover their fluorescence. During this recovery, bleached zones frequently move, indicating translocation of fibrils. Intriguingly, neighboring fibrils within a cell can exhibit different rates and directions of movement, and they often appear to extend or elongate into the peripheral regions of the cytoplasm. In these same regions, short filamentous structures are also seen actively translocating. All of these motile properties require energy, and the majority appear to be mediated by interactions of IF with microtubules and microfilaments.

Formation and movement of myosin-containing structures in living fibroblasts

Gizzard myosin, fluorescently labeled with tetramethylrhodamine iodoacetamide, was microinjected into living 3T3 fibroblasts to label myosin-containing structures. The fluorophore was located predominantly on the heavy chain near the COOH terminus of the S1 head and on the 17-kD light chain. After microinjection of a tracer amount into living 3T3 cells, the fluorescent myosin showed a distribution identical to that revealed by immunofluorescence with antimyosin antibodies. Injected myosin became localized in small beads, which were found along large stress fibers, along fine fibers, and in a poorly organized form near the lamellipodia. De novo assembly of beads was observed continuously within or near the lamellipodia, suggesting that myosin molecules may undergo a constant cycling between polymerized and unpolymerized states. The nascent structures then moved away from lamellipodia and became organized into linear arrays. Similar movement was also observed for beads already associated with linear structures, and may represent a continuous flux of myosin structures. The dynamic reorganization of myosin may play an important role in cell movement and polarity.