Antibody to myosin: the specific visualization of myosin-containing filaments in nonmuscle cells

Biomimetic Fibers Derived from an Equidistant Micropillar Platform Dictate Osteocyte Fate via Mechanoreception

It is a big challenge to design a biomimetic physical microenvironment with greater similarity to in vivo tissue to observe real cell behaviors. We established a novel cell culture platform based on patterned equidistant micropillars with stiff and soft stiffnesses to mimic the changes that happened in the transition from normal to osteoporotic disease. We first demonstrated that the soft micropillar substrate decreased osteocyte synaptogenesis through synaptogyrin 1 and that this decrease was accompanied by impairment of cell mechanoperception and a decrease in cellular cytoskeletal rearrangement. We then found that the soft equidistant micropillar substrate reduced the osteocyte synaptogenesis mainly via the inactivation of Erk/MAPK signaling. We finally found that soft micropillar substrate-mediated synaptogenesis impacted the cell-to-cell communication and matrix mineralization of osteocytes. Taken together, this study provides evidence of cellular mechanical responses that are much more similar to those of real osteocytes at the bone tissue level.

From tensegrity to human organs-on-chips: implications for mechanobiology and mechanotherapeutics

The field of mechanobiology, which focuses on the key role that physical forces play in control of biological systems, has grown enormously over the past few decades. Here, I provide a brief personal perspective on the development of the tensegrity theory that contributed to the emergence of the mechanobiology field, the key role that crossing disciplines has played in its development, and how it has matured over time. I also describe how pursuing questions relating to mechanochemical transduction and mechanoregulation can lead to the creation of novel technologies and open paths for development of new therapeutic strategies for a broad range of diseases and disorders.

A Short History of Plant Light Microscopy

When the microscope was first introduced to scientists in the 17^th century, it started a revolution. Suddenly, a whole new world, invisible to the naked eye, was opened to curious explorers. In response to this realization, Nehemiah Grew, an English plant anatomist and physiologist and one of the early microscopists, noted in 1682 "that Nothing hereof remains further to be known, is a Thought not well Calculated". Since Grew made his observations, the microscope has undergone numerous variations, developing from early compound microscopes-hollow metal tubes with a lens on each end-to the modern, sophisticated, out-of-the-box super-resolution microscopes available to researchers today. In this Overview article, I describe these developments and discuss how each new and improved variant of the microscope led to major breakthroughs in the life sciences, with a focus on the plant field. These advances start with Grew's simple and-at the time-surprising realization that plant cells are as complex as animals cells, and that the different parts of the plant body indeed qualify to be called "organs", then move on to the development of the groundbreaking "cell theory" in the mid-19^th century and the description of eu- and heterochromatin in the early 20^th century, and finish with the precise localization of individual proteins in intact, living cells that we can perform today. Indeed, Grew was right; with ever-increasing resolution, there really does not seem to be an end to what can be explored with a microscope. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC.

Reciprocal regulation of actin filaments and cellular metabolism

For cells to adhere, migrate and proliferate, remodeling of the actin cytoskeleton is required. This process consumes a large amount of ATP while having an intimate connection with cellular metabolism. Signaling pathways that regulate energy homeostasis can also affect actin dynamics, whereas a variety of actin binding proteins directly or indirectly interact with the anabolic and catabolic regulators in cells. Here, we discuss the inter-regulation between actin filaments and cellular metabolism, reviewing recent discoveries on key metabolic enzymes that respond to actin remodeling as well as historical findings on metabolic stress-induced cytoskeletal reorganization. We also address emerging techniques that would benefit the study of cytoskeletal dynamics and cellular metabolism in high spatial-temporal resolution.

Correlative cryo-ET identifies actin/tropomyosin filaments that mediate cell-substrate adhesion in cancer cells and mechanosensitivity of cell proliferation

The actin cytoskeleton is the primary driver of cellular adhesion and mechanosensing due to its ability to generate force and sense the stiffness of the environment. At the cell's leading edge, severing of the protruding Arp2/3 actin network generates a specific actin/tropomyosin (Tpm) filament population that controls lamellipodial persistence. The interaction between these filaments and adhesion to the environment is unknown. Using cellular cryo-electron tomography we resolve the ultrastructure of the Tpm/actin copolymers and show that they specifically anchor to nascent adhesions and are essential for focal adhesion assembly. Re-expression of Tpm1.8/1.9 in transformed and cancer cells is sufficient to restore cell-substrate adhesions. We demonstrate that knock-out of Tpm1.8/1.9 disrupts the formation of dorsal actin bundles, hindering the recruitment of α-actinin and non-muscle myosin IIa, critical mechanosensors. This loss causes a force-generation and proliferation defect that is notably reversed when cells are grown on soft surfaces. We conclude that Tpm1.8/1.9 suppress the metastatic phenotype, which may explain why transformed cells naturally downregulate this Tpm subset during malignant transformation.

Ventral stress fibers induce plasma membrane deformation in human fibroblasts

Interactions between the actin cytoskeleton and the plasma membrane are important in many eukaryotic cellular processes. During these processes, actin structures deform the cell membrane outward by applying forces parallel to the fiber's major axis (as in migration) or they deform the membrane inward by applying forces perpendicular to the fiber's major axis (as in the contractile ring during cytokinesis). Here we describe a novel actin-membrane interaction in human dermal myofibroblasts. When labeled with a cytosolic fluorophore, the myofibroblasts displayed prominent fluorescent structures on the ventral side of the cell. These structures are present in the cell membrane and colocalize with ventral actin stress fibers, suggesting that the stress fibers bend the membrane to form a "cytosolic pocket" that the fluorophores diffuse into, creating the observed structures. The existence of this pocket was confirmed by transmission electron microscopy. While dissolving the stress fibers, inhibiting fiber protein binding, or inhibiting myosin II binding of actin removed the observed pockets, modulating cellular contractility did not remove them. Taken together, our results illustrate a novel actin-membrane bending topology where the membrane is deformed outward rather than being pinched inward, resembling the topological inverse of the contractile ring found in cytokinesis.

Articular Chondrocyte Phenotype Regulation through the Cytoskeleton and the Signaling Processes That Originate from or Converge on the Cytoskeleton: Towards a Novel Understanding of the Intersection between Actin Dynamics and Chondrogenic Function

Numerous studies have assembled a complex picture, in which extracellular stimuli and intracellular signaling pathways modulate the chondrocyte phenotype. Because many diseases are mechanobiology-related, this review asked to what extent phenotype regulators control chondrocyte function through the cytoskeleton and cytoskeleton-regulating signaling processes. Such information would generate leverage for advanced articular cartilage repair. Serial passaging, pro-inflammatory cytokine signaling (TNF-α, IL-1α, IL-1β, IL-6, and IL-8), growth factors (TGF-α), and osteoarthritis not only induce dedifferentiation but also converge on RhoA/ROCK/Rac1/mDia1/mDia2/Cdc42 to promote actin polymerization/crosslinking for stress fiber (SF) formation. SF formation takes center stage in phenotype control, as both SF formation and SOX9 phosphorylation for COL2 expression are ROCK activity-dependent. Explaining how it is molecularly possible that dedifferentiation induces low COL2 expression but high SF formation, this review theorized that, in chondrocyte SOX9, phosphorylation by ROCK might effectively be sidelined in favor of other SF-promoting ROCK substrates, based on a differential ROCK affinity. In turn, actin depolymerization for redifferentiation would “free-up” ROCK to increase COL2 expression. Moreover, the actin cytoskeleton regulates COL1 expression, modulates COL2/aggrecan fragment generation, and mediates a fibrogenic/catabolic expression profile, highlighting that actin dynamics-regulating processes decisively control the chondrocyte phenotype. This suggests modulating the balance between actin polymerization/depolymerization for therapeutically controlling the chondrocyte phenotype.

Actomyosin and the MRTF-SRF pathway downregulate FGFR1 in mesenchymal stromal cells

Both biological and mechanical signals are known to influence cell proliferation. However, biological signals are mostly studied in two-dimensions (2D) and the interplay between these different pathways is largely unstudied. Here, we investigated the influence of the cell culture environment on the response to bFGF, a widely studied and important proliferation growth factor. We observed that human mesenchymal stromal cells (hMSCs), but not fibroblasts, lose the ability to respond to soluble or covalently bound bFGF when cultured on microfibrillar substrates. This behavior correlated with a downregulation of FGF receptor 1 (FGFR1) expression of hMSCs on microfibrillar substrates. Inhibition of actomyosin or the MRTF/SRF pathway decreased FGFR1 expression in hMSCs, fibroblasts and MG63 cells. To our knowledge, this is the first time FGFR1 expression is shown to be regulated through a mechanosensitive pathway in hMSCs. These results add to the sparse literature on FGFR1 regulation and potentially aid designing tissue engineering constructs that better control cell proliferation.

Qualitative analysis of contribution of intracellular skeletal changes to cellular elasticity

Cells are dynamic structures that continually generate and sustain mechanical forces within their environments. Cells respond to mechanical forces by changing their shape, moving, and differentiating. These reactions are caused by intracellular skeletal changes, which induce changes in cellular mechanical properties such as stiffness, elasticity, viscoelasticity, and adhesiveness. Interdisciplinary research combining molecular biology with physics and mechanical engineering has been conducted to characterize cellular mechanical properties and understand the fundamental mechanisms of mechanotransduction. In this review, we focus on the role of cytoskeletal proteins in cellular mechanics. The specific role of each cytoskeletal protein, including actin, intermediate filaments, and microtubules, on cellular elasticity is summarized along with the effects of interactions between the fibers.

Dimensionality changes actin network through lamin A/C and zyxin

Mechanosensing proteins have mainly been investigated in 2D culture platforms, while understanding their regulation in 3D enviroments is critical for tissue engineering. Among mechanosensing proteins, the actin cytoskeleton plays a key role in human mesenchymal stromal cells (hMSCs) activity, but its regulation in 3D tissue engineered scaffolds remains poorly studied. Here, we show that human mesenchymal stromal cells (hMSCs) cultured on 3D electrospun scaffolds made of a stiff material do not form actin stress fibers, contrary to hMSCs on 2D films of the same material. On 3D electrospun and additive manufactured scaffolds, hMSCs also displayed fewer focal adhesions, lower lamin A and C expression and less YAP1 nuclear localization and myosin light chain phosphorylation. Together, this strongly suggests that dimensionality prevents the build-up of cellular tension, even on stiff materials. Knock down of either lamin A and C or zyxin resulted in fewer stress fibers in the cell center. Zyxin knock down reduced lamin A and C expression, but not vice versa, showing that this signal chain starts from the outside of the cell. Lineage commitment was not affected by the lack of these important osteogenic proteins in 3D, as all cells committed to osteogenesis in bi-potential medium. Our study demonstrates that dimensionality changes the actin cytoskeleton through lamin A and C and zyxin, and highlights the difference in the regulation of lineage commitment in 3D enviroments. Together, these results can have important implications for future scaffold design for both stiff- and soft tissue engineering constructs.

The Endothelial Cytoskeleton: Organization in Normal and Regenerating Endothelium*

The components of the endothelial cell cytoskeleton that have been shown to be important in maintaining endothelial structural integrity and in regulating endothelial repair include F-actin microfilament bundles, including stress fibers, and microtubules, and centrosomes. Endothelial cells contain peripheral and central actin microfilaments. The dense peripheral band (DPB) consists of peripheral actin microfilament bundles which are associated with vinculin adhesion plaques and are most prominent in low or no hemodynamic shear stress conditions. The central microfilaments are very prominent in areas of elevated hemodynamic shear stress. There is a redistribution of actin microfilaments characterized by a decrease of peripheral actin and an increase in central microfilaments under a variety of conditions, including exposure to thrombin, phorbol-esters, and hemodynamic shear stress. During reendothelialization, there is a sequential series of cytoskeletal changes. The DPB remains intact during the rapid lamellipodia mediated repair of very small wounds except at the base of the lamellipodia where it is splayed. The DPB is reduced or absent when cell locomotion occurs to repair a wound. In addition, when cell locomotion is required, the centrosome, in the presence of intact microtubules, redistributes to the front of the cell to establish cell polarity and acts as a modulator of the directionality of migration. This occurs prior to the loss of the DPB but does not occur in very small wounds that close without migration. Thus, the cytoskeleton is a dynamic intracellular system which regulates endothelial integrity and repair and is modulated by external stimuli that are present at the vessel wall-blood interface.

Reciprocal regulation of actomyosin organization and contractility in nonmuscle cells by tropomyosins and alpha-actinins

Contractile arrays of actin and myosin II filaments drive many essential processes in nonmuscle cells, including migration and adhesion. Sequential organization of actin and myosin along one dimension is followed by expansion into a two-dimensional network of parallel actomyosin fibers, in which myosin filaments are aligned to form stacks. The process of stack formation has been studied in detail. However, factors that oppose myosin stack formation have not yet been described. Here, we show that tropomyosins act as negative regulators of myosin stack formation. Knockdown of any or all tropomyosin isoforms in rat embryonic fibroblasts resulted in longer and more numerous myosin stacks and a highly ordered actomyosin organization. The molecular basis for this, we found, is the competition between tropomyosin and alpha-actinin for binding actin. Surprisingly, excessive order in the actomyosin network resulted in smaller focal adhesions, lower tension within the network, and smaller traction forces. Conversely, disordered actomyosin bundles induced by alpha-actinin knockdown led to higher than normal tension and traction forces. Thus, tropomyosin acts as a check on alpha-actinin to achieve intermediate levels of myosin stacks matching the force requirements of the cell.

The Cytoskeleton-A Complex Interacting Meshwork

The cytoskeleton of animal cells is one of the most complicated and functionally versatile structures, involved in processes such as endocytosis, cell division, intra-cellular transport, motility, force transmission, reaction to external forces, adhesion and preservation, and adaptation of cell shape. These functions are mediated by three classical cytoskeletal filament types, as follows: Actin, microtubules, and intermediate filaments. The named filaments form a network that is highly structured and dynamic, responding to external and internal cues with a quick reorganization that is orchestrated on the time scale of minutes and has to be tightly regulated. Especially in brain tumors, the cytoskeleton plays an important role in spreading and migration of tumor cells. As the cytoskeletal organization and regulation is complex and many-faceted, this review aims to summarize the findings about cytoskeletal filament types, including substructures formed by them, such as lamellipodia, stress fibers, and interactions between intermediate filaments, microtubules and actin. Additionally, crucial regulatory aspects of the cytoskeletal filaments and the formed substructures are discussed and integrated into the concepts of cell motility. Even though little is known about the impact of cytoskeletal alterations on the progress of glioma, a final point discussed will be the impact of established cytoskeletal alterations in the cellular behavior and invasion of glioma.

Ordering of myosin II filaments driven by mechanical forces: experiments and theory

Myosin II filaments form ordered superstructures in both cross-striated muscle and non-muscle cells. In cross-striated muscle, myosin II (thick) filaments, actin (thin) filaments and elastic titin filaments comprise the stereotypical contractile units of muscles called sarcomeres. Linear chains of sarcomeres, called myofibrils, are aligned laterally in registry to form cross-striated muscle cells. The experimentally observed dependence of the registered organization of myofibrils on extracellular matrix elasticity has been proposed to arise from the interactions of sarcomeric contractile elements (considered as force dipoles) through the matrix. Non-muscle cells form small bipolar filaments built of less than 30 myosin II molecules. These filaments are associated in registry forming superstructures ('stacks') orthogonal to actin filament bundles. Formation of myosin II filament stacks requires the myosin II ATPase activity and function of the actin filament crosslinking, polymerizing and depolymerizing proteins. We propose that the myosin II filaments embedded into elastic, intervening actin network (IVN) function as force dipoles that interact attractively through the IVN. This is in analogy with the theoretical picture developed for myofibrils where the elastic medium is now the actin cytoskeleton itself. Myosin stack formation in non-muscle cells provides a novel mechanism for the self-organization of the actin cytoskeleton at the level of the entire cell.This article is part of the theme issue 'Self-organization in cell biology'.

The Actin Cytoskeleton and Actin-Based Motility

The actin cytoskeleton-a collection of actin filaments with their accessory and regulatory proteins-is the primary force-generating machinery in the cell. It can produce pushing (protrusive) forces through coordinated polymerization of multiple actin filaments or pulling (contractile) forces through sliding actin filaments along bipolar filaments of myosin II. Both force types are particularly important for whole-cell migration, but they also define and change the cell shape and mechanical properties of the cell surface, drive the intracellular motility and morphogenesis of membrane organelles, and allow cells to form adhesions with each other and with the extracellular matrix.

An integrated enhancement and reconstruction strategy for the quantitative extraction of actin stress fibers from fluorescence micrographs

BACKGROUND

The stress fibers are prominent organization of actin filaments that perform important functions in cellular processes such as migration, polarization, and traction force generation, and whose collective organization reflects the physiological and mechanical activities of the cells. Easily visualized by fluorescence microscopy, the stress fibers are widely used as qualitative descriptors of cell phenotypes. However, due to the complexity of the stress fibers and the presence of other actin-containing cellular features, images of stress fibers are relatively challenging to quantitatively analyze using previously developed approaches, requiring significant user intervention. This poses a challenge for the automation of their detection, segmentation, and quantitative analysis.

RESULT

Here we describe an open-source software package, SFEX (Stress Fiber Extractor), which is geared for efficient enhancement, segmentation, and analysis of actin stress fibers in adherent tissue culture cells. Our method made use of a carefully chosen image filtering technique to enhance filamentous structures, effectively facilitating the detection and segmentation of stress fibers by binary thresholding. We subdivided the skeletons of stress fiber traces into piecewise-linear fragments, and used a set of geometric criteria to reconstruct the stress fiber networks by pairing appropriate fiber fragments. Our strategy enables the trajectory of a majority of stress fibers within the cells to be comprehensively extracted. We also present a method for quantifying the dimensions of the stress fibers using an image gradient-based approach. We determine the optimal parameter space using sensitivity analysis, and demonstrate the utility of our approach by analyzing actin stress fibers in cells cultured on various micropattern substrates.

CONCLUSION

We present an open-source graphically-interfaced computational tool for the extraction and quantification of stress fibers in adherent cells with minimal user input. This facilitates the automated extraction of actin stress fibers from fluorescence images. We highlight their potential uses by analyzing images of cells with shapes constrained by fibronectin micropatterns. The method we reported here could serve as the first step in the detection and characterization of the spatial properties of actin stress fibers to enable further detailed morphological analysis.

Control of Myofibroblast Differentiation and Function by Cytoskeletal Signaling

The cytoskeleton consists of three distinct types of protein polymer structures - microfilaments, intermediate filaments, and microtubules; each serves distinct roles in controlling cell shape, division, contraction, migration, and other processes. In addition to mechanical functions, the cytoskeleton accepts signals from outside the cell and triggers additional signals to extracellular matrix, thus playing a key role in signal transduction from extracellular stimuli through dynamic recruitment of diverse intermediates of the intracellular signaling machinery. This review summarizes current knowledge about the role of cytoskeleton in the signaling mechanism of fibroblast-to-myofibroblast differentiation - a process characterized by accumulation of contractile proteins and secretion of extracellular matrix proteins, and being critical for normal wound healing in response to tissue injury as well as for aberrant tissue remodeling in fibrotic disorders. Specifically, we discuss control of serum response factor and Hippo signaling pathways by actin and microtubule dynamics as well as regulation of collagen synthesis by intermediate filaments.

Long-range self-organization of cytoskeletal myosin II filament stacks

Although myosin II filaments are known to exist in non-muscle cells, their dynamics and organization are incompletely understood. Here, we combined structured illumination microscopy with pharmacological and genetic perturbations, to study the process of actomyosin cytoskeleton self-organization into arcs and stress fibres. A striking feature of the myosin II filament organization was their 'registered' alignment into stacks, spanning up to several micrometres in the direction orthogonal to the parallel actin bundles. While turnover of individual myosin II filaments was fast (characteristic half-life time 60 s) and independent of actin filament turnover, the process of stack formation lasted a longer time (in the range of several minutes) and required myosin II contractility, as well as actin filament assembly/disassembly and crosslinking (dependent on formin Fmnl3, cofilin1 and α-actinin-4). Furthermore, myosin filament stack formation involved long-range movements of individual myosin filaments towards each other suggesting the existence of attractive forces between myosin II filaments. These forces, possibly transmitted via mechanical deformations of the intervening actin filament network, may in turn remodel the actomyosin cytoskeleton and drive its self-organization.

Mathematical modeling of the dynamic mechanical behavior of neighboring sarcomeres in actin stress fibers

Actin stress fibers (SFs) in live cells consist of series of dynamic individual sarcomeric units. Within a group of consecutive SF sarcomeres, individual sarcomeres can spontaneously shorten or lengthen without changing the overall length of this group, but the underlying mechanism is unclear. We used a computational model to test our hypothesis that this dynamic behavior is inherent to the heterogeneous mechanical properties of the sarcomeres and the cytoplasmic viscosity. Each sarcomere was modeled as a discrete element consisting of an elastic spring, a viscous dashpot and an active contractile unit all connected in parallel, and experiences forces as a result of actin filament elastic stiffness, myosin II contractility, internal viscoelasticity, or cytoplasmic drag. When all four types of forces are considered, the simulated dynamic behavior closely resembles the experimental observations, which include a low-frequency fluctuation in individual sarcomere length and compensatory lengthening and shortening of adjacent sarcomeres. Our results suggest that heterogeneous stiffness and viscoelasticity of actin fibers, heterogeneous myosin II contractility, and the cytoplasmic drag are sufficient to cause spontaneous fluctuations in SF sarcomere length. Our results shed new light to the dynamic behavior of SF and help design experiments to further our understanding of SF dynamics.

Actin stress fibre subtypes in mesenchymal-migrating cells

Mesenchymal cell migration is important for embryogenesis and tissue regeneration. In addition, it has been implicated in pathological conditions such as the dissemination of cancer cells. A characteristic of mesenchymal-migrating cells is the presence of actin stress fibres, which are thought to mediate myosin II-based contractility in close cooperation with associated focal adhesions. Myosin II-based contractility regulates various cellular activities, which occur in a spatial and temporal manner to achieve directional cell migration. These myosin II-based activities involve the maturation of integrin-based adhesions, generation of traction forces, establishment of the front-to-back polarity axis, retraction of the trailing edge, extracellular matrix remodelling and mechanotransduction. Growing evidence suggests that actin stress fibre subtypes, namely dorsal stress fibres, transverse arcs and ventral stress fibres, could provide this spatial and temporal myosin II-based activity. Consistent with their functional differences, recent studies have demonstrated that the molecular composition of actin stress fibre subtypes differ significantly. This present review focuses on the current view of the molecular composition of actin stress fibre subtypes and how these fibre subtypes regulate mesenchymal cell migration.

Lateral communication between stress fiber sarcomeres facilitates a local remodeling response

Actin stress fibers (SFs) are load-bearing and mechanosensitive structures. To our knowledge, the mechanisms that enable SFs to sense and respond to strain have not been fully defined. Acute local strain events can involve a twofold extension of a single SF sarcomere, but how these dramatic local events affect the overall SF architecture is not believed to be understood. Here we have investigated how SF architecture adjusts to episodes of local strain that occur in the cell center. Using fluorescently tagged zyxin to track the borders of sarcomeres, we characterize the dynamics of resting sarcomeres and strain-site sarcomeres. We find that sarcomeres flanking a strain site undergo rapid shortening that directly compensates for the strain-site extension, illustrating lateral communication of mechanical information along the length of a stress fiber. When a strain-site sarcomere extends asymmetrically, its adjacent sarcomeres exhibit a parallel asymmetric shortening response, illustrating that flanking sarcomeres respond to strain magnitude. After extension, strain-site sarcomeres become locations of new sarcomere addition, highlighting mechanical strain as a trigger of sarcomere addition and revealing a, to our knowledge, novel type of SF remodeling. Our findings provide evidence to suggest SF sarcomeres act as strain sensors and are interconnected to support communication of mechanical information.

Actin stress fibers--assembly, dynamics and biological roles

Actin filaments assemble into diverse protrusive and contractile structures to provide force for a number of vital cellular processes. Stress fibers are contractile actomyosin bundles found in many cultured non-muscle cells, where they have a central role in cell adhesion and morphogenesis. Focal-adhesion-anchored stress fibers also have an important role in mechanotransduction. In animal tissues, stress fibers are especially abundant in endothelial cells, myofibroblasts and epithelial cells. Importantly, recent live-cell imaging studies have provided new information regarding the mechanisms of stress fiber assembly and how their contractility is regulated in cells. In addition, these studies might elucidate the general mechanisms by which contractile actomyosin arrays, including muscle cell myofibrils and cytokinetic contractile ring, can be generated in cells. In this Commentary, we discuss recent findings concerning the physiological roles of stress fibers and the mechanism by which these structures are generated in cells.

Investigation of in vivo microtubule and stress fiber mechanics with laser ablation

Laser ablation has emerged as a useful technique to study the mechanical properties of the cytoskeleton in living cells. Laser ablation perturbs the force balance in the cytoskeleton, resulting in a dynamic response which can be imaged. Quantitative measurement of the dynamic response allows the testing of mechanical theories of the cytoskeleton in living cells. This review discusses recent work in applying laser ablation to study stress fiber and microtubule mechanics in living endothelial cells. These studies reveal that molecular motors are major determinants of the mechanical properties of the cytoskeleton in cells.

Actin cytoskeleton in myofibroblast differentiation: ultrastructure defining form and driving function

Myofibroblasts are modified fibroblasts characterized by the presence of a well-developed contractile apparatus and the formation of robust actin stress fibers. These mechanically active cells are thought to orchestrate extracellular matrix remodeling during normal wound healing in response to tissue injury; these cells are found also in aberrant tissue remodeling in fibrosing disorders. This review surveys the understanding of the role of actin stress fibers in myofibroblast biology. Actin stress fibers are discussed as a defining ultrastructural and morphologic feature and well-accepted observations demonstrating its participation in contraction, focal adhesion maturation, and extracellular matrix reorganization are presented. Finally, more recent observations are reviewed, demonstrating its role in transducing mechanical force into biochemical signals, transcriptional control of genes involved in locomotion, contraction, and matrix reorganization, as well as the localized regulation of messenger RNA (mRNA) translation. This breadth of functionality of the actin stress fiber serves to reinforce and amplify its mechanical function, via induced expression of proteins that themselves augment contraction, focal adhesion formation, and matrix remodeling. In composite, the functions of the actin cytoskeleton are most often aligned, allowing for the integration and amplification of signals promoting both myofibroblast differentiation and matrix remodeling during fibrogenesis.

Assembly and dynamics of myofibrils

We review some of the problems in determining how myofibrils may be assembled and just as importantly how this contractile structure may be renewed by sarcomeric proteins moving between the sarcomere and the cytoplasm. We also address in this personal review the recent evidence that indicates that the assembly and dynamics of myofibrils are conserved whether the cells are analyzed in situ or in tissue culture conditions. We suggest that myofibrillogenesis is a fundamentally conserved process, comparable to protein synthesis, mitosis, or cytokinesis, whether examined in situ or in vitro.

Sarcomere mechanics in capillary endothelial cells

Tension generation in endothelial cells of the aorta, spleen, and eye occurs in actin stress fibers, and is necessary for normal cell function. Sarcomeres are the tension-generating units of actin stress fibers in endothelial cells. How sarcomeres generate and maintain tension in stress fibers is not well understood. Using femtosecond laser ablation, we severed living stress fibers and measured sarcomere contraction under zero tension. The length of the sarcomere decreased in two phases: an instantaneous initial response, followed by a slower change in length attributed to myosin activity. The latter phase ceased abruptly after a minimum sarcomere length was reached, suggesting a rigid resistance that prevents further contraction. Furthermore, severed, contracted stress fibers did not relax when treated with myosin inhibitors, indicating that contracted stress fibers do not store elastic potential energy. These novel measurements combined with modeling suggest that myosin-generated forces in adjacent sarcomeres are directly in balance, and argue against sarcomere models with springlike elements in parallel with myosin contractile elements. We propose a new model for tension generation in the sarcomere, which provides a mechanistic interpretation for our observations and previous observations of inhomogeneous sarcomere contraction and apparent stress fiber viscoelastic behavior.

Pulsation and stabilization: contractile forces that underlie morphogenesis

Embryonic development involves global changes in tissue shape and architecture that are driven by cell shape changes and rearrangements within cohesive cell sheets. Morphogenetic changes at the cell and tissue level require that cells generate forces and that these forces are transmitted between the cells of a coherent tissue. Contractile forces generated by the actin-myosin cytoskeleton are critical for morphogenesis, but the cellular and molecular mechanisms of contraction have been elusive for many cell shape changes and movements. Recent studies that have combined live imaging with computational and biophysical approaches have provided new insights into how contractile forces are generated and coordinated between cells and tissues. In this review, we discuss our current understanding of the mechanical forces that shape cells, tissues, and embryos, emphasizing the different modes of actomyosin contraction that generate various temporal and spatial patterns of force generation.

Actin stress fibres

Animal cell movement is effected through a combination of protrusive and contractile events. Non-muscle cells contain stress fibres - bundles of actomyosin that are the major mediators of cell contraction and that can be compared to the highly organised actomyosin arrays of muscle cells. Recent studies have defined regulatory mechanisms that control stress fibre formation, placing the ROCK protein kinase at the centre of a complex signalling network controlling actomyosin contractility and stress fibre assembly. As we uncover the details of stress fibre construction, it is becoming clear that different categories of stress fibres exist. Some of these structures are less suited for cell motility and more suited to static contraction. In keeping with this, many specialised contractile cell types use stress fibres to remodel tissues and extracellular matrix.

Hypertonicity triggers RhoA-dependent assembly of myosin-containing striated polygonal actin networks in endothelial cells

Endothelial cells respond to mechanical stresses of the circulation with cytoskeletal rearrangements such as F-actin stress fiber alignment along the axis of fluid flow. Endothelial cells are exposed to hypertonic stress in the renal medulla or during mannitol treatment of cerebral edema. We report here that arterial endothelial cells exposed to hypertonic stress rearranged F-actin into novel actin-myosin II fibers with regular 0.5-microm striations, in which alpha-actinin colocalizes with actin. These striated fibers assembled over hours into three-dimensional, irregular, polygonal actin networks most prominent at the cell base, and occasionally surrounding the nucleus in a geodesic-like structure. Hypertonicity-induced assembly of striated polygonal actin networks was inhibited by cytochalasin D, blebbistatin, cell ATP depletion, and intracellular Ca(2+) chelation but did not require intact microtubules, regulatory volume increase, or de novo RNA or protein synthesis. Striated polygonal actin network assembly was insensitive to inhibitors of MAP kinases, tyrosine kinases, or phosphatidylinositol 3-kinase, but was prevented by C3 exotoxin, by the RhoA kinase inhibitor Y-27632, and by overexpressed dominant-negative RhoA. In contrast, overexpression of dominant-negative Rac or of dominant-negative cdc42 cDNAs did not prevent striated polygonal actin network assembly. The actin networks described here are novel in structure, as striated actin-myosin structures in nonmuscle cells, as a cellular response to hypertonicity, and as a cytoskeletal regulatory function of RhoA. Endothelial cells may use RhoA-dependent striated polygonal actin networks, possibly in concert with cytoskeletal load-bearing elements, as a contractile, tension-generating component of their defense against isotropic compressive forces.

Simultaneous stretching and contraction of stress fibers in vivo

To study the dynamics of stress fiber components in cultured fibroblasts, we expressed alpha-actinin and the myosin II regulatory myosin light chain (MLC) as fusion proteins with green fluorescent protein. Myosin activation was stimulated by treatment with calyculin A, a serine/threonine phosphatase inhibitor that elevates MLC phosphorylation, or with LPA, another agent that ultimately stimulates phosphorylation of MLC via a RhoA-mediated pathway. The resulting contraction caused stress fiber shortening and allowed observation of changes in the spacing of stress fiber components. We have observed that stress fibers, unlike muscle myofibrils, do not contract uniformly along their lengths. Although peripheral regions shortened, more central regions stretched. We detected higher levels of MLC and phosphorylated MLC in the peripheral region of stress fibers. Fluorescence recovery after photobleaching revealed more rapid exchange of myosin and alpha-actinin in the middle of stress fibers, compared with the periphery. Surprisingly, the widths of the myosin and alpha-actinin bands in stress fibers also varied in different regions. In the periphery, the banding patterns for both proteins were shorter, whereas in central regions, where stretching occurred, the bands were wider.

Functional association between nicotinic acetylcholine receptor and sarcomeric proteins via actin and desmin filaments

By affinity chromatography utilizing alpha-cobrotoxin from digitonin-solubilized fractions of rabbit skeletal muscle, we found that many proteins are associated with the nicotinic acetylcholine receptor (AChR). In addition to the proteins we previously reported to bind to AChR (including dystrophin-dystrophin-associated protein (DAP) complex, utrophin, rapsyn, and actin; Mitsui et al. [1996] Biochem. Biophys. Res. Commun.224:802-807), alpha-actinin, desmin, myosin, tropomyosin, troponin T, and titin are also identified to be associated with AChR. Alkaline treatment or Triton X-100 solubilization released dystrophin-DAP complex, utrophin, and rapsyn from the AChR fraction, while actin and desmin remained associated. These findings demonstrate that AChR is supported primarily by a submembranous organization of actin and desmin filaments, and is linked to sarcomeric proteins via these filaments. To further investigate whether the association has any functional role, we studied the effect of acetylcoline on ATPase activity of the AChR fraction. Acetylcholine (0.5-4 microM) significantly activated Mg(2+)-ATPase activity of digitonin-solubilized AChR fraction (P < 0.05). Furthermore, we found that desmin as well as actin activated myosin Mg(2+)-ATPase activity. From these findings, it is suggested that desmin and actin form a submembranous organization in the postsynaptic region, and function as mediators of excitation of AChR to the sarcomeric contraction system.

A role for gelsolin in stress fiber-dependent cell contraction

Gelsolin is an abundant actin binding protein that mediates the rapid remodeling of cortical actin filaments through severing, capping, and nucleating activities. Most of the attention on the intracellular function of gelsolin has focused on the remodeling of the cortical actin meshwork but the localization of gelsolin to other regions of the cell suggests that it may have other important functions elsewhere. In cultured fibroblasts, gelsolin is heavily enriched in stress fibers, where its function in these contractile organelles is unknown. To study gelsolin function during stress fiber formation and cell contraction, we first assessed gelsolin levels in stress fiber preparations from lysophosphatidic acid (LPA)-treated human fibroblasts. LPA induced a large, time-dependent, calcium-independent increase of actin, gelsolin, alpha-actinin, and tropomyosin in stress fiber preparations. A microinjected gelsolin antibody that inhibits severing by gelsolin reduced stress fibers. Anti-sense-transfected gelsolin-depleted 3T3 cell lines treated with LPA after serum starvation required approximately 6 h to form stress fibers and focal adhesions, in contrast to control lines transfected with vector only, which formed stress fibers 15 min after addition of LPA. In cells microinjected with the gelsolin antibody that inhibits severing, Mg-ATP-induced cell contraction was greatly reduced in approximately 90% of injected cells compared to cells injected with an irrelevant antibody. Gelsolin-depleted cells were incapable of collagen gel contraction and showed no apparent Mg-ATP-induced cell contraction compared to cell lines transfected with vector only. The involvement of gelsolin in cell contraction and remodeling of collagen gels suggests a novel role for gelsolin in stress fiber-dependent cell function.

Identification of novel graded polarity actin filament bundles in locomoting heart fibroblasts: implications for the generation of motile force

We have determined the structural organization and dynamic behavior of actin filaments in entire primary locomoting heart fibroblasts by S1 decoration, serial section EM, and photoactivation of fluorescence. As expected, actin filaments in the lamellipodium of these cells have uniform polarity with barbed ends facing forward. In the lamella, cell body, and tail there are two observable types of actin filament organization. A less abundant type is located on the inner surface of the plasma membrane and is composed of short, overlapping actin bundles (0.25-2.5 microm) that repeatedly alternate in polarity from uniform barbed ends forward to uniform pointed ends forward. This type of organization is similar to the organization we show for actin filament bundles (stress fibers) in nonlocomoting cells (PtK2 cells) and to the known organization of muscle sarcomeres. The more abundant type of actin filament organization in locomoting heart fibroblasts is mostly ventrally located and is composed of long, overlapping bundles (average 13 microm, but can reach up to about 30 microm) which span the length of the cell. This more abundant type has a novel graded polarity organization. In each actin bundle, polarity gradually changes along the length of the bundle. Actual actin filament polarity at any given point in the bundle is determined by position in the cell; the closer to the front of the cell the more barbed ends of actin filaments face forward. By photoactivation marking in locomoting heart fibroblasts, as expected in the lamellipodium, actin filaments flow rearward with respect to substrate. In the lamella, all marked and observed actin filaments remain stationary with respect to substrate as the fibroblast locomotes. In the cell body of locomoting fibroblasts there are two dynamic populations of actin filaments: one remains stationary and the other moves forward with respect to substrate at the rate of the cell body. This is the first time that the structural organization and dynamics of actin filaments have been determined in an entire locomoting cell. The organization, dynamics, and relative abundance of graded polarity actin filament bundles have important implications for the generation of motile force during primary heart fibroblast locomotion.

The Croonian Lecture, 1978 - The crawling movement of metazoan cells

A great variety of metazoan cells have the ability to crawl across a surface. Students of inflammation, of wound healing, of developing embryos or of cell cultures have for many years investigated accessible aspects of the phenomenon, but largely in isolation. A general physiology of the movement hardly seemed possible. But the scene is beginning to change, and this lecture is an attempt to see what sort of a general picture may now be drawn. Present knowledge of the machinery of movement, which is undergoing a revolution as the muscle-like proteins of the cytoplasm emerge more clearly, is considered. The lecture then outlines the circumstances in which cell movement occursin vivo, and suggests the functions that it performs. Finally, attention is turned to what may be called the behavioural aspects, to how the movement is guided in particular directions and started or stopped by the conditions around the cell.

Focal adhesions, contractility, and signaling

Focal adhesions are sites of tight adhesion to the underlying extracellular matrix developed by cells in culture. They provided a structural link between the actin cytoskeleton and the extracellular matrix and are regions of signal transduction that relate to growth control. The assembly of focal adhesions is regulated by the GTP-binding protein Rho. Rho stimulates contractility which, in cells that are tightly adherent to the substrate, generates isometric tension. In turn, this leads to the bundling of actin filaments and the aggregation of integrins (extracellular matrix receptors) in the plane of the membrane. The aggregation of integrins activates the focal adhesion kinase and leads to the assembly of a multicomponent signaling complex.

Myosin II filament assemblies in the active lamella of fibroblasts: their morphogenesis and role in the formation of actin filament bundles

The morphogenesis of myosin II structures in active lamella undergoing net protrusion was analyzed by correlative fluorescence and electron microscopy. In rat embryo fibroblasts (REF 52) microinjected with tetramethylrhodamine-myosin II, nascent myosin spots formed close to the active edge during periods of retraction and then elongated into wavy ribbons of uniform width. The spots and ribbons initially behaved as distinct structural entities but subsequently aligned with each other in a sarcomeric-like pattern. Electron microscopy established that the spots and ribbons consisted of bipolar minifilaments associated with each other at their head-containing ends and arranged in a single row in an "open" zig-zag conformation or as a "closed" parallel stack. Ribbons also contacted each other in a nonsarcomeric, network-like arrangement as described previously (Verkhovsky and Borisy, 1993. J. Cell Biol. 123:637-652). Myosin ribbons were particularly pronounced in REF 52 cells, but small ribbons and networks were found also in a range of other mammalian cells. At the edge of the cell, individual spots and open ribbons were associated with relatively disordered actin filaments. Further from the edge, myosin filament alignment increased in parallel with the development of actin bundles. In actin bundles, the actin cross-linking protein, alpha-actinin, was excluded from sites of myosin localization but concentrated in paired sites flanking each myosin ribbon, suggesting that myosin filament association may initiate a pathway for the formation of actin filament bundles. We propose that zig-zag assemblies of myosin II filaments induce the formation of actin bundles by pulling on an actin filament network and that co-alignment of actin and myosin filaments proceeds via folding of myosin II filament assemblies in an accordion-like fashion.

Differential localization of myosin-II isozymes in human cultured cells and blood cells

We used purified polyclonal antibodies to human cytoplasmic myosin-IIA and myosin-IIB directly labeled with fluorescent dyes to localize these myosin-II isozymes in HeLa cells, melanoma cells and blood cells. Both antibodies react strongly with myosin-II isozymes in HeLa cells, melanoma cells and blood eosinophils, but only anti-myosin-IIA antibodies stain platelets, lymphocytes, neutrophils and monocytes in smears of human blood. Both antibodies stain small spots along the stress fibers of interphase HeLa cells and melanoma cells, but double staining revealed that the detailed distributions of myosin-IIA and myosin-IIB differ. A low concentration of diffuse myosin-IIB is present in the cortex, both in lamellar regions around the periphery of the cell and over the free surface. Myosin-IIB is also concentrated in spots along perinuclear stress fibers. Myosin-IIA is absent from the cortex but is concentrated in spots along stress fibers located near the basal surface of cultured cells. This population of peripheral stress fibers is highly enriched in myosin-IIA relative to myosin-IIB, but both are found together in centrally located stress fibers. In prophase and metaphase both isozymes are concentrated in the cortex in small spots less than 04.micron in size, similar to those in stress fibers. As the chromosomes begin the separate at anaphase, most of the myosin-II spots become concentrated in the outer 0.7 micron of the equatorial cortex in 100% of cells. This concentration of myosin-II isozymes in the cleavage furrow is maintained until the daughter cells separate. The superimposition of these small spots concentrated in the cleavage furrow produces the intense, uniform staining observed in conventional micrographs of whole cells.

Imaging subcellular structures of rat mammary carcinoma cells by scanning force microscopy

Scanning force microscopy (SFM) was used for imaging subcellular structures of cultured rat mammary carcinoma cells dried in air. Identification of cellular substructures was achieved by immunofluorescence and specific fluorescence probes. Cells grown attached to a glass support exhibited submicrometer thickness in the dried state. Inside the nuclear domain the nucleoli appeared as prominent conical protrusions. Membrane extensions, microspikes and microvilli were well preserved at the cell periphery after fixation in glutaraldehyde vapor and air-drying and were distinguishable either as isolated elements or intercellular communications. The plasma membrane and soluble proteins were selectively removed with nonionic detergent in a buffer system. The mitochondria were concentrated primarily in the perinuclear space and exhibited a well defined filamentous shape. Their identity was confirmed by specific fluorescence staining with rhodamine 123. In the membrane-free system achieved by dry-cleaving of the sample surface, the cytoskeletal network was resolved as a complex mesh of actin-containing fiber bundles interwoven with a filigree arrangement of thinner filaments. The smallest fibrous substructures revealed by SFM with the scanning tips used to date were approximately 8 to 10 nm in height and 80 nm in width.

Heterogeneity of myofibroblast phenotypic features: an example of fibroblastic cell plasticity

Granulation tissue fibroblasts (myofibroblasts) develop several ultrastructural and biochemical features of smooth muscle (SM) cells, including the presence of microfilament bundles and the expression of alpha-SM actin, the actin isoform present in SM cells and myoepithelial cells and particularly abundant in vascular SM cells. Myofibroblasts have been suggested to play a role in wound contraction and in retractile phenomena observed during fibrotic diseases. When contraction stops and the wound is fully epithelialized, myofibroblasts containing alpha-SM actin disappear, probably as a result of apoptosis, and the scar classically becomes less cellular and composed of typical fibroblasts with well-developed rough endoplasmic reticulum but with no more microfilaments. In contrast, alpha-SM actin expressing myofibroblasts persist in hypertrophic scars and in fibrotic lesions of many organs, including stroma reaction to epithelial tumours, where they are allegedly involved in retractile phenomena as well as in extracellular matrix accumulation. The mechanisms leading to the development of myofibroblastic features remain to be investigated. In vivo and in vitro investigations have shown that gamma-interferon exerts an antifibrotic activity at least in part by decreasing alpha-SM actin expression whereas heparin increases the proportion of alpha-SM actin positive cells. Recently, we have observed that the subcutaneous administration of transforming growth factor-beta 1 to rats results in the formation of a granulation tissue in which alpha-SM actin expressing myofibroblasts are particularly abundant. Other cytokines and growth factors, such as platelet-derived growth factor, basic fibroblast growth factor and tumour necrosis factor-alpha, despite their profibrotic activity, do not induce alpha-SM actin in myofibroblasts. In conclusion, fibroblastic cells are relatively undifferentiated and can assume a particular phenotype according to the physiological needs and/or the microenvironmental stimuli. Further studies on fibroblast adaptation phenomena appear to be useful for the understanding of the mechanisms of development and regression of pathological processes such as wound healing and fibrocontractive diseases.

Calcium control of actin-myosin-based contraction in Triton-treated murine bladder tumor cells

Using mouse bladder tumor cells (MBT-2 cells) and epithelial cells, the present study evaluated the functional characteristics of the actomyosin system in bladder cancer cells. An immunofluorescence study demonstrated the presence of contractile proteins (actin and myosin) in MBT-2 cells as well as bladder epithelial cells. Triton-treated MBT-2 cells and epithelial cells showed a Ca(2+)-dependent contraction. This was inhibited by N-ethylmaleimide-modified myosin subfragment 1 (NEM-S1), demonstrating that the interaction between actin and myosin is responsible for the contraction of Triton-treated cells. The extent of Ca(2+)-dependent contraction was much greater in MBT-2 cells than in epithelial cells. These results suggest that MBT-2 cells possess a locomotive apparatus consisting of actin and myosin, and that Ca2+ can activate this actomyosin system, leading to the contraction or active locomotory movement of tumor cells.

Non-sarcomeric mode of myosin II organization in the fibroblast lamellum

The organization of myosin in the fibroblast lamellum was studied by correlative fluorescence and electron microscopy after a novel procedure to reveal its underlying morphology. An X-rhodamine analog of conventional smooth muscle myosin (myosin II) that colocalized after microinjection with endogenous myosin was used to trace myosin distribution in living fibroblasts. Then, the same cells were examined by EM of platinum replicas. To visualize the structural arrangement of myosin, other cytoskeletal fibrillar structures had to be removed: microtubules were depolymerized by nocodazole treatment of the living cells before injection of myosin; continued nocodazole treatment also induced the intermediate filaments to concentrate near the nucleus, thus removing them from the lamellar region; actin filaments were removed after lysis of the cells by incubation of the cytoskeletons with recombinant gelsolin. Possible changes in myosin organization caused by this treatment were examined by fluorescence microscopy. No significant differences in myosin distribution patterns between nocodazole-treated and control cells were observed. Cell lysis and depletion of actin also did not induce reorganization of myosin as was shown by direct comparison of myosin distribution in the same cells in the living state and after gelsolin treatment. EM of the well-spread, peripheral regions of actin-depleted cytoskeletons revealed a network of bipolar myosin mini-filaments, contracting each other at their terminal, globular regions. The morphology of this network corresponded well to the myosin distribution observed by fluorescence microscopy. A novel mechanism of cell contraction by folding of the myosin filament network is proposed.

Endothelial cell connecting filaments anchor endothelial cells to the subjacent elastic lamina in the developing aortic intima of the mouse

The ultrastructural association of endothelial cells with the subjacent elastic lamina was investigated in the developing mouse aorta by electron microscopy. In the 5-day postnatal aorta, extensive filament bundles extend along the subendothelial matrix connecting the endothelial cells to the underlying elastic lamina. The connecting filaments form lateral associations with the abluminal surface of the endothelial cells in regions of membrane occupied by membrane-associated dense plaques. On the intracellular face of each plaque, the termini of stress fibers penetrate and anchor to the cell membrane in alignment with the extracellular connecting filaments. Both the stress fibers and the connecting filaments are oriented parallel to the longitudinal axis of the vessel. High magnification electron micrographs of individual endothelial cell connecting filaments reveal features similar to those of elastin-associated microfibrils. Each connecting filament consists of a 9-10 nm linear core with an electron-lucent center and peripheral spike-like projections. From the filaments, small thread-like extensions span laterally, linking the filaments into a loose bundle and anchoring them to the endothelial cell membrane and the surface of the elastic lamina. The filaments also appear heavily coated with electron-dense material; often with some degree of periodicity along the filament length. During development, the number of endothelial cell connecting filaments decreases as the elastic lamina expands and the subendothelial matrix is reduced. In the aortic intima of mature mice, the elastic lamina is closely apposed to the abluminal surface of the endothelial cell and no connecting filaments are seen.(ABSTRACT TRUNCATED AT 250 WORDS)

Gas2, a growth arrest-specific protein, is a component of the microfilament network system

In this report we analyze the protein product of a growth arrest-specific gene, gas2, by means of an affinity-purified antibody raised against the protein produced in bacteria. The regulation of Gas2 biosynthesis reflects the pattern of mRNA expression (Schneider, C., R. King, and L. Philipson. 1988. Cell. 54:787-793): its relative level is tightly associated with growth arrest. Gas2 seems to be regulated also at the posttranslational level via a phosphorylation mechanism. Gas2 is well conserved during the evolution with the same apparent molecular mass (36 kD) between mouse and human. We also demonstrate that Gas2 is a component of the microfilament system. It colocalizes with actin fiber, at the cell border and also along the stress fiber, in growth-arrested NIH 3T3 cells. The pattern of distribution, detected in arrested cells, can also be observed in growing cells when they are microinjected with the purified GST-Gas2 protein. In none of the analyzed oncogene-transformed NIH 3T3 cell lines was Gas2 expression induced under serum starvation.

Organization of non-muscle myosin during early murine embryonic differentiation

A monoclonal antibody (3D10) recognizing myosin heavy chain was isolated following immunization with a synthetic peptide sequence of eight amino acids. The antibody reacted with purified rabbit skeletal myosin and light mero-myosin in enzyme-linked immunosorbent assays and Western immunoblotting. A band of approximately 200 kDa was detected in cell extracts of an embryonal carcinoma (EC) cell line (P19EC) and one of its cloned differentiated derivatives, suggesting reactivity against non-muscle myosin. By indirect immunofluorescence, typical myosin banding patterns were observed in cryostat sections of human skeletal and cardiac muscle tissue. In undifferentiated P19EC cells, speckled immunofluorescent staining was observed in the cytoplasm that became organized in cortical rings where the cells made direct contact with each other. These rings consisted of circular bundles of F-actin decorated by myosin. Undifferentiated embryonic stem (ES) cells derived directly from mouse embryos shared the same features, although the pattern was less pronounced. Human testicular primary germ cell tumours showed cortical staining in the embryonal carcinoma component reminiscent of the staining of EC cells in vitro while cytoplasmic staining was observed in tumour cells with a differentiated morphology. In preimplantation embryos, the immunofluorescent staining was observed at cell apices of blastomeres of morula stage embryos. In blastocysts, staining of inner cell mass cells was not detectable. By contrast, various differentiated derivatives of P19EC contained extensive F-actin microfilament bundles throughout the cytoplasm decorated with myosin. Thick stress fibers in filopodious extensions of cells were particularly highly decorated by myosin. Over the nucleus, linear arrays of myosin containing speckled patterns of immunofluorescence were observed that were not associated with F-actin. The same pattern of staining could be observed in trophectoderm cells of the blastocyst. We conclude that embryonic non-muscle myosin is organized in specific patterns depending on the state of differentiation. As the myosin is primarily associated with F-actin we suspect that it forms part of a contractile apparatus that may have significance during embryonic development.

The yeast type II myosin heavy chain: analysis of its predicted polypeptide sequence

We have completed the nucleotide sequence of the yeast MYO1 gene and deduced its amino acid sequence. The gene is 5553 bp long and contains no introns. Analysis of the sequence, as well as its comparison with other myosins, demonstrate that the yeast protein is a type II myosin heavy chain with characteristic head and tail regions. The latter domain contains six proline residues in two clusters of three, at approximately two thirds from the start of the gene.