Contribution of actin to the structure of the cytoplasmic matrix

Myosin-5a facilitates stress granule formation by interacting with G3BP1

Stress granules (SGs) are non-membranous organelles composed of mRNA and proteins that assemble in the cytosol when the cell is under stress. Although the composition of mammalian SGs is both cell-type and stress-dependent, they consistently contain core components, such as Ras GTPase activating protein SH3 domain binding protein 1 (G3BP1). Upon stress, living cells rapidly assemble micrometric SGs, sometimes within a few minutes, suggesting that SG components may be actively transported by the microtubule and/or actin cytoskeleton. Indeed, SG assembly has been shown to depend on the microtubule cytoskeleton and the associated motor proteins. However, the role of the actin cytoskeleton and associated myosin motor proteins remains controversial. Here, we identified G3BP1 as a novel binding protein of unconventional myosin-5a (Myo5a). G3BP1 uses its C-terminal RNA-binding domain to interact with the middle portion of Myo5a tail domain (Myo5a-MTD). Suppressing Myo5a function in mammalian cells, either by overexpressing Myo5a-MTD, eliminating Myo5a gene expression, or treatment with myosin-5 inhibitor, inhibits the arsenite-induced formation of both small and large SGs. This is different from the effect of microtubule disruption, which abolishes the formation of large SGs but enhances the formation of small SGs under stress conditions. We therefore propose that, under stress conditions, Myo5a facilitates the formation of SGs at an earlier stage than the microtubule-dependent process.

Network topology enables efficient response to environment inPhysarum polycephalum

The network-shaped body plan distinguishes the unicellular slime mouldPhysarum polycephalumin body architecture from other unicellular organisms. Yet, network-shaped body plans dominate branches of multi-cellular life such as in fungi. What survival advantage does a network structure provide when facing a dynamic environment with adverse conditions? Here, we probe how network topology impactsP. polycephalum's avoidance response to an adverse blue light. We stimulate either an elongated, I-shaped amoeboid or a Y-shaped networked specimen and subsequently quantify the evacuation process of the light-exposed body part. The result shows that Y-shaped specimen complete the avoidance retraction in a comparable time frame, even slightly faster than I-shaped organisms, yet, at a lower almost negligible increase in migration velocity. Contraction amplitude driving mass motion is further only locally increased in Y-shaped specimen compared to I-shaped-providing further evidence that Y-shaped's avoidance reaction is energetically more efficient than in I-shaped amoeboid organisms. The difference in the retraction behaviour suggests that the complexity of network topology provides a key advantage when encountering adverse environments. Our findings could lead to a better understanding of the transition from unicellular to multicellularity.

On the significance of membrane unfolding in mechanosensitive cell spreading: Its individual and synergistic effects

Mechanosensitivity of cell spread area to substrate stiffness has been established both through experiments and different types of mathematical models of varying complexity including both the mechanics and biochemical reactions in the cell. What has not been addressed in previous mathematical models is the role of cell membrane dynamics on cell spreading, and an investigation of this issue is the goal of this work. We start with a simple mechanical model of cell spreading on a deformable substrate and progressively layer mechanisms to account for the traction dependent growth of focal adhesions, focal adhesion induced actin polymerization, membrane unfolding/exocytosis and contractility. This layering approach is intended to progressively help in understanding the role each mechanism plays in reproducing experimentally observed cell spread areas. To model membrane unfolding we introduce a novel approach based on defining an active rate of membrane deformation that is dependent on membrane tension. Our modeling approach allows us to show that tension-dependent membrane unfolding plays a critical role in achieving the large cell spread areas experimentally observed on stiff substrates. We also demonstrate that coupling between membrane unfolding and focal adhesion induced polymerization works synergistically to further enhance cell spread area sensitivity to substrate stiffness. This enhancement has to do with the fact that the peripheral velocity of spreading cells is associated with contributions from the different mechanisms by either enhancing the polymerization velocity at the leading edge or slowing down of the retrograde flow of actin within the cell. The temporal evolution of this balance in the model corresponds to the three-phase behavior observed experimentally during spreading. In the initial phase membrane unfolding is found to be particularly important.

Optical tweezer measurements of asymptotic nonlinearities in complex fluids

This article presents micro-medium-amplitude oscillatory shear (μMAOS), a method to measure the frequency-dependent micromechanical properties of soft materials in the asymptotically nonlinear regime using optical tweezers. We have developed a theoretical framework to extract these nonlinear mechanical properties of the material from experimental measurements and also proposed a physical interpretation of the third-order nonlinearities measured in single-tone oscillatory tests. We validate the method using a well-characterized surfactant solution of wormlike micelles, and subsequently employ this technique to demonstrate that the cytoplasm of a living cell undergoes strain softening and shear thinning when locally subjected to weakly nonlinear oscillatory deformations.

Living System Adapts Harmonics of Peristaltic Wave for Cost-Efficient Optimization of Pumping Performance

Wavelike patterns driving transport are ubiquitous in life. Peristaltic pumps are a paradigm of efficient mass transport by contraction driven flows-often limited by energetic constraints. We show that a cost-efficient increase in pumping performance can be achieved by modulating the phase difference between harmonics to increase occlusion. In experiments we find a phase difference shift in the living peristalsis model P. polycephalum as dynamic response to forced mass transport. Our findings provide a novel metric for wavelike patterns and demonstrate the crucial role of nonlinearities in life.

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.

Cytoskeletal remodeling induced by substrate rigidity regulates rheological behaviors in endothelial cells

Altered microenvrionmental mechanical cues induce cytoskeletal remodeling in cells and have a profound impact on their functions as well as rheological properties. This article is aimed to characterize the viscoelastic behavior of endothelial cells, cultivated on variably compliant substrates. Synthetic tunable poly(dimethylsyloxane) substrates, with elastic moduli ranging from 1.5 MPa to 3 kPa, were used to trigger cytoskeletal remodeling of endothelial cells, verified by morphological analysis and actin fluorescent labeling. Elasticity and stress relaxation tests were conducted using an AFM, resulting in a wide range of data. To account for this heterogeneity, fuzzy c-means clustering algorithm was applied to partition elastic data into biologically meaningful groups, representative of different regions in cells. Nanocharacterization of biomechanical properties, along with cytoskeletal studies, proved a significant correlation between substrate flexibility and viscoelasticity of the cells. Regardless of the viscoelastic model applied, increasing substrate rigidity was related to an overall increase in cell stiffness and apparent viscosity (2.95 ± 1.56 kPa and 921.45 ± 102.46 Pa.s for the stiff substrate; 2.17 ± 1.30 kPa and 557.37 ± 494.11 Pa.s for the intermediate substrate), associated with an organized actin cytoskeleton. Conversely, cells on soft substrate were more deformable (1.84 ± 1.3 kPa) and less viscous (327.13 ± 124.25 Pa.s), exhibiting an increased actin disorganization. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 71-80, 2019.

Cytoplasmic transport and nuclear import of plasmid DNA

Productive transfection and gene transfer require not simply the entry of DNA into cells and subsequent transcription from an appropriate promoter, but also a number of intracellular events that allow the DNA to move from the extracellular surface of the cell into and through the cytoplasm, and ultimately across the nuclear envelope and into the nucleus before any transcription can initiate. Immediately upon entry into the cytoplasm, naked DNA, either delivered through physical techniques or after disassembly of DNA-carrier complexes, associates with a large number of cellular proteins that mediate subsequent interactions with the microtubule network for movement toward the microtubule organizing center and the nuclear envelope. Plasmids then enter the nucleus either upon the mitotic disassembly of the nuclear envelope or through nuclear pore complexes in the absence of cell division, using a different set of proteins. This review will discuss our current understanding of these pathways used by naked DNA during the transfection process. While much has been elucidated on these processes, much remains to be discerned, but with the development of a number of model systems and approaches, great progress is being made.

A combined experimental and theoretical approach towards mechanophenotyping of biological cells using a constricted microchannel

We report a combined experimental and theoretical technique that enables the characterization of various mechanical properties of biological cells. The cells were infused into a microfluidic device that comprises multiple parallel micro-constrictions to eliminate device clogging and facilitate characterization of cells of different sizes and types on a single device. The extension ratio λ and transit velocity U_c of the cells were measured using high-speed and high-resolution imaging which were then used in a theoretical model to predict the Young's modulus E_c = f(λ, U_c) of the cells. The predicted Young's modulus E_c values for three different cell lines (182 ± 34.74 Pa for MDA MB 231, 360 ± 75 Pa for MCF 10A and, 763 ± 93 Pa for HeLa) compare well with those reported in the literature from micropipette measurements and atomic force microscopy measurement within 10% and 15%, respectively. Also, the Young's modulus of MDA-MB-231 cells treated with 50 μM 4-hyrdroxyacetophenone (for localization of myosin II) for 30 min was found out to be 260 ± 52 Pa. The entry time t_e of cells into the micro-constrictions was predicted using the model and validated using experimentally measured data. The entry and transit behaviors of cells in the micro-constriction including cell deformation (extension ratio λ) and velocity U_c were experimentally measured and used to predict various cell properties such as the Young's modulus, cytoplasmic viscosity and induced hydrodynamic resistance of different types of cells. The proposed combined experimental and theoretical approach leads to a new paradigm for mechanophenotyping of biological cells.

Mechanics of Vascular Smooth Muscle

Vascular smooth muscle (VSM; see Table 1 for a list of abbreviations) is a heterogeneous biomaterial comprised of cells and extracellular matrix. By surrounding tubes of endothelial cells, VSM forms a regulated network, the vasculature, through which oxygenated blood supplies specialized organs, permitting the development of large multicellular organisms. VSM cells, the engine of the vasculature, house a set of regulated nanomotors that permit rapid stress-development, sustained stress-maintenance and vessel constriction. Viscoelastic materials within, surrounding and attached to VSM cells, comprised largely of polymeric proteins with complex mechanical characteristics, assist the engine with countering loads imposed by the heart pump, and with control of relengthening after constriction. The complexity of this smart material can be reduced by classical mechanical studies combined with circuit modeling using spring and dashpot elements. Evaluation of the mechanical characteristics of VSM requires a more complete understanding of the mechanics and regulation of its biochemical parts, and ultimately, an understanding of how these parts work together to form the machinery of the vascular tree. Current molecular studies provide detailed mechanical data about single polymeric molecules, revealing viscoelasticity and plasticity at the protein domain level, the unique biological slip-catch bond, and a regulated two-step actomyosin power stroke. At the tissue level, new insight into acutely dynamic stress-strain behavior reveals smooth muscle to exhibit adaptive plasticity. At its core, physiology aims to describe the complex interactions of molecular systems, clarifying structure-function relationships and regulation of biological machines. The intent of this review is to provide a comprehensive presentation of one biomachine, VSM.

High-Throughput Assessment of Cellular Mechanical Properties

Traditionally, cell analysis has focused on using molecular biomarkers for basic research, cell preparation, and clinical diagnostics; however, new microtechnologies are enabling evaluation of the mechanical properties of cells at throughputs that make them amenable to widespread use. We review the current understanding of how the mechanical characteristics of cells relate to underlying molecular and architectural changes, describe how these changes evolve with cell-state and disease processes, and propose promising biomedical applications that will be facilitated by the increased throughput of mechanical testing: from diagnosing cancer and monitoring immune states to preparing cells for regenerative medicine. We provide background about techniques that laid the groundwork for the quantitative understanding of cell mechanics and discuss current efforts to develop robust techniques for rapid analysis that aim to implement mechanophenotyping as a routine tool in biomedicine. Looking forward, we describe additional milestones that will facilitate broad adoption, as well as new directions not only in mechanically assessing cells but also in perturbing them to passively engineer cell state.

The interactions between pristine graphene and macrophages and the production of cytokines/chemokines via TLR- and NF-κB-related signaling pathways

Graphene may have attractive properties for some biomedical applications, but its potential adverse biological effects, in particular, possible modulation of immune responses, require further investigation. Macrophages are one of the most important effector cells of the innate immune system, and play pivotal roles in the response to graphene exposure. We have previously reported that exposure of macrophages to high concentrations of graphene triggers cell death via MAPK- and TGF-related pathways. However, little is known about the influence of exposure to low concentrations of graphene on the function of macrophages. In the present investigation, we demonstrate the biological effects of sub-cytotoxic concentrations of commercial pristine graphene on both primary murine macrophages and immortalized macrophages. Graphene significantly stimulates the secretion of Th1/Th2 cytokines including IL-1α, IL-6, IL-10, TNF-α and GM-CSF as well as chemokines such as MCP-1, MIP-1α, MIP-1β and RANTES, probably by activating TLR-mediated and NF-κB-dependent transcription. Furthermore, these graphene-induced factors alter the morphology of naïve macrophages by remodeling their actin assembly, decreasing their ability to adhere to the extracellular matrix, and attenuating their phagocytosis. This negative feedback of the immune response of macrophages by graphene-induced factors may play an important role in the prevention of their over-activation after graphene exposure. These findings shed light on the interaction of graphene and macrophages in vitro. Further research is needed to systematically assess the biological responses of graphene, both to improve its safety and to contribute to the design of new biological applications.

Toxic effect on tissues and differentially expressed genes in hepatopancreas identified by suppression subtractive hybridization of freshwater pearl mussel (Hyriopsis cumingii) following microcystin-LR challenge

Microcystins are a family of potent hepatotoxins produced by freshwater cyanobacteria and can cause animal intoxications and human diseases. In this study, the effect of microcystin-LR (MC-LR) on the tissues of freshwater pearl mussel (Hyriopsis cumingii) was evaluated and differentially expressed genes in the hepatopancreas of the mussel exposed to MC-LR were identified. HPLC analysis of cell extracts from various tissues of the mussel indicated that the hepatopancreas had the highest MC-LR levels (55.78 ± 6.73 μg g⁻¹ DW) after 15-day exposure. The MC-LR concentration in gill or muscle was an order of magnitude less than in hepatopancreas or gonad. Subtractive cDNA library was constructed by suppression subtractive hybridization (SSH), and ∼400 positive clones were sequenced, from which 98 high quality sequences were obtained by BLAST analysis. The screening identified numerous genes involved in apoptosis, signal transduction, cytoskeletal remodel, innate immunity, material and energy metabolism, translation and transcription which were extensively discussed. The results of this study add large amount of information to the mussel genome data, and for the first time present the basic data on toxicity effect of MC-LR on mussel.

From water and ions to crowded biomacromolecules: in vivo structuring of a prokaryotic cell

The interactions and processes which structure prokaryotic cytoplasm (water, ions, metabolites, and biomacromolecules) and ensure the fidelity of the cell cycle are reviewed from a physicochemical perspective. Recent spectroscopic and biological evidence shows that water has no active structuring role in the cytoplasm, an unnecessary notion still entertained in the literature; water acts only as a normal solvent and biochemical reactant. Subcellular structuring arises from localizations and interactions of biomacromolecules and from the growth and modifications of their surfaces by catalytic reactions. Biomacromolecular crowding is a fundamental physicochemical characteristic of cells in vivo. Though some biochemical and physiological effects of crowding (excluded volume effect) have been documented, crowding assays with polyglycols, dextrans, etc., do not properly mimic the compositional variety of biomacromolecules in vivo. In vitro crowding assays are now being designed with proteins, which better reflect biomacromolecular environments in vivo, allowing for hydrophobic bonding and screened electrostatic interactions. I elaborate further the concept of complex vectorial biochemistry, where crowded biomacromolecules structure the cytosol into electrolyte pathways and nanopools that electrochemically "wire" the cell. Noncovalent attractions between biomacromolecules transiently supercrowd biomacromolecules into vectorial, semiconducting multiplexes with a high (35 to 95%)-volume fraction of biomacromolecules; consequently, reservoirs of less crowded cytosol appear in order to maintain the experimental average crowding of ∼25% volume fraction. This nonuniform crowding model allows for fast diffusion of biomacromolecules in the uncrowded cytosolic reservoirs, while the supercrowded vectorial multiplexes conserve the remarkable repeatability of the cell cycle by preventing confusing cross talk of concurrent biochemical reactions.

Transcriptional alteration of cytoskeletal genes induced by microcystins in three organs of rats

This study explored the mechanisms of toxicity of microcystins by measuring the transcription levels of nine cytoskeletal genes (actin, tubulin, vimentin, ezrin, radixin, moesin, MAP1b, tau, stathmin) in the liver, kidney and spleen of male Wistar rats treated with microcystins at a dose of 80 microg MC-LReq kg(-1) bw. Microcystins disrupted the transcriptional homeostasis of cytoskeletal genes in these organs. Changes in the transcription of four genes (beta-actin, ezrin, radixin and tau) in liver, one gene (stathmin) in kidney, and one gene (radixin) in spleen were significantly correlated with the tissue concentration of microcystins. However, the influences on the transcription of most genes we studied were greater in the liver than in the kidney or spleen. The effects of microcystins on the transcription of cytoskeletal genes may explain some of the morphological and pathological changes observed in these organs and provide new information on the hepatotoxicity of these compounds. Additionally, transcriptional changes in tumor-associated cytoskeletal genes (ezrin, moesin and stathmin) that were observed in the present study provide a possible clue to the tumor-promoting potential of microcystins and their influences on the transcription of MAP1b and tau imply possible neurological toxicity of microcystins in vertebrates.

Cytoskeletal role in differential adhesion patterns of normal fibroblasts and breast cancer cells inside silicon microenvironments

In this paper we studied differential adhesion of normal human fibroblast cells and human breast cancer cells to three dimensional (3-D) isotropic silicon microstructures and investigated whether cell cytoskeleton in healthy and diseased state results in differential adhesion. The 3-D silicon microstructures were formed by a single-mask single-isotropic-etch process. The interaction of these two cell lines with the presented microstructures was studied under static cell culture conditions. The results show that there is not a significant elongation of both cell types attached inside etched microstructures compared to flat surfaces. With respect to adhesion, the cancer cells adopt the curved shape of 3-D microenvironments while fibroblasts stretch to avoid the curved sidewalls. Treatment of fibroblast cells with cytochalasin D changed their adhesion, spreading and morphology and caused them act similar to cancer cells inside the 3-D microstructures. Statistical analysis confirmed that there is a significant alteration (P < 0.001) in fibroblast cell morphology and adhesion property after adding cytochalasin D. Adding cytochalasin D to cancer cells made these cells more rounded while there was not a significant alteration in their adhesion properties. The distinct geometry-dependent cell-surface interactions of fibroblasts and breast cancer cells are attributed to their different cytoskeletal structure; fibroblasts have an organized cytoskeletal structure and less deformable while cancer cells deform easily due to their impaired cytoskeleton. These 3-D silicon microstructures can be used as a tool to investigate cellular activities in a 3-D architecture and compare cytoskeletal properties of various cell lines.

Cell responses regulated by early reorganization of actin cytoskeleton

Microfilaments exist in a dynamic equilibrium between monomeric and polymerized actin and the ratio of monomers to polymeric forms is influenced by a variety of extracellular stimuli. The polymerization, depolymerization and redistribution of actin filaments are modulated by several actin-binding proteins, which are regulated by upstream signalling molecules. Actin cytoskeleton is involved in diverse cellular functions including migration, ion channels activity, secretion, apoptosis and cell survival. In this review we have outlined the role of actin dynamics in representative cell functions induced by the early response to extracellular stimuli.

Regulated motion of glycoproteins revealed by direct visualization of a single cargo in the endoplasmic reticulum

The quality of cargo proteins in the endoplasmic reticulum (ER) is affected by their motion during folding. To understand how the diffusion of secretory cargo proteins is regulated in the ER, we directly analyze the motion of a single cargo molecule using fluorescence imaging/fluctuation analyses. We find that the addition of two N-glycans onto the cargo dramatically alters their diffusion by transient binding to membrane components that are confined by hyperosmolarity. Via simultaneous observation of a single cargo and ER exit sites (ERESs), we could exclude ERESs as the binding sites. Remarkably, actin cytoskeleton was required for the transient binding. These results provide a molecular basis for hypertonicity-induced immobilization of cargo, which is dependent on glycosylation at multiple sites but not the completion of proper folding. We propose that diffusion of secretory glycoproteins in the ER lumen is controlled from the cytoplasm to reduce the chances of aggregation.

The Cytoskeletal Connection to Ion Channels as a Potential Mechanosensory Mechanism: Lessons from Polycystin-2 (TRPP2)

Mechanosensitivity of ion channels, or the ability to transfer mechanical forces into a gating mechanism of channel regulation, is split into two main working (not mutually exclusive) hypotheses. One is that elastic and/or structural changes in membrane properties act as a transducing mechanism of channel regulation. The other hypothesis involves tertiary elements, such as the cytoskeleton which, itself by dynamic interactions with the ion channel, may convey conformational changes, including those ascribed to mechanical forces. This hypothesis is supported by numerous instances of regulatory changes in channel behavior by alterations in cytoskeletal structures/interactions. However, only recently, the molecular nature of these interactions has slowly emerged. Recently, a surge of evidence has emerged to indicate that transient receptor potential (TRP) channels are key elements in the transduction of a variety of environmental signals. This chapter describes the molecular linkage and regulatory elements of polycystin-2 (PC2), a TRP-type (TRPP2) nonselective cation channel whose mutations cause autosomal dominant polycystic kidney disease (ADPKD). The chapter focuses on the involvement of cytoskeletal structures in the regulation of PC2 and discusses how these connections are the transducing mechanism of environmental signals to its channel function.

Acto-myosin reorganization and PAR polarity in C. elegans

The symmetry-breaking event during polarization of C. elegansembryos is an asymmetric rearrangement of the acto-myosin network, which dictates cell polarity through the differential recruitment of PAR proteins. The sperm-supplied centrosomes are required to initiate this cortical reorganization. Several questions about this event remain unanswered: how is the acto-myosin network regulated during polarization and how does acto-myosin reorganization lead to asymmetric PAR protein distribution? As we discuss,recent studies show that C. elegans embryos use two GTPases, RHO-1 and CDC-42, to regulate these two steps in polarity establishment. Although RHO-1 and CDC-42 control distinct aspects of polarization, they function interdependently to regulate polarity establishment in C. elegansembryos.

Intracellular trafficking of plasmids for gene therapy: mechanisms of cytoplasmic movement and nuclear import

Under physiologically relevant conditions, the levels of non-viral gene transfer are low at best. The reason for this is that many barriers exist for the efficient transfer of genes to cells, even before any gene expression can occur. While many transfection strategies focus on DNA condensation and overcoming the plasma membrane, events associated with the intracellular trafficking of the DNA complexes have not been as extensively studied. Once internalized, plasmids must travel potentially long distances through the cytoplasm to reach their next barrier, the nuclear envelope. This review summarizes the current progress on the cytoplasmic trafficking and nuclear transport of plasmids used for gene therapy applications. Both of these processes utilize specific and defined mechanisms to facilitate movement of DNA complexes through the cell. The continued elucidation and exploitation of these mechanisms will lead to improved strategies for transfection and successful gene therapy.

An improved confocal FRAP technique for the measurement of long-term actin dynamics in individual stress fibers

The present study describes an improved fluorescent recovery after photobleaching (FRAP) technique, which has been successfully used to quantify actin dynamics within individual fibers. Chondrocytes were transfected with an eGFP-actin plasmid and cultured on glass coverslips. In cells expressing eGFP-actin, confocal microscopy was used to bleach 3 x 1 microm regions accurately positioned along individual stress fibers. The subsequent fluorescent recovery over a 10-min imaging period was assessed from a series of intensity profiles, positioned along the length of the stress fibers and spanning the bleach region. From these profiles, the normalized fluorescent intensity values were plotted against time. In this way, the technique provided sufficient spatial precision to describe the long-term behavior within individual stress fibers while accounting for the inherent movement. An identical procedure was used to examine FRAP for eGFP-actin within the interfiber region. The FRAP curves for stress fibers were accurately modeled by two phase exponentials which indicated only partial recovery with a mobile fraction of 46%. This suggests that some of the F-actin molecules were in a tightly bound configuration with negligible turnover. The interfiber region exhibited similar two phase exponential FRAP with a mobile fraction of 68%. This partial recovery may be due to the presence, within the interfiber region, of both G-actin and fine F-actin fibers beneath the resolution of the confocal microscope. In conclusion, the present FRAP methodology overcomes many of the limitations of previous studies in order to provide new data describing long-term actin dynamics within individual stress fibers.

Cell Mechanics: FilaminA Leads the Way

Actin filaments are thought to be the major structural components of most eukaryotic cells, but reconstituted actin networks have yet to account for the remarkable strength exhibited by cellular networks. A new study has found that reconstituted networks that include the cross-linker filaminA can replicate many of the mechanical properties of cells if they are stressed prior to mechanical measurement.

Optical rheology of biological cells

A step stress deforming suspended cells causes a passive relaxation, due to a transiently cross-linked isotropic actin cortex underlying the cellular membrane. The fluid-to-solid transition occurs at a relaxation time coinciding with unbinding times of actin cross-linking proteins. Elastic contributions from slowly relaxing entangled filaments are negligible. The symmetric geometry of suspended cells ensures minimal statistical variability in their viscoelastic properties in contrast with adherent cells and thus is defining for different cell types. Mechanical stimuli on time scales of minutes trigger active structural responses.

Nano- to microscale dynamics of P-selectin detachment from leukocyte interfaces. II. Tether flow terminated by P-selectin dissociation from PSGL-1

We have used a biomembrane force probe decorated with P-selectin to form point attachments with PSGL-1 receptors on a human neutrophil (PMN) in a calcium-containing medium and then to quantify the forces experienced by the attachment during retraction of the PMN at fixed speed. From first touch to final detachment, the typical force history exhibited the following sequence of events: i), an initial linear-elastic displacement of the PMN surface, ii), an abrupt crossover to viscoplastic flow that signaled membrane separation from the interior cytoskeleton and the beginning of a membrane tether, and iii), the final detachment from the probe tip most often by one precipitous step of P-selectin:PSGL-1 dissociation. Analyzing the initial elastic response and membrane unbinding from the cytoskeleton in our companion article I, we focus in this article on the regime of tether extrusion that nearly always occurred before release of the extracellular adhesion bond at pulling speeds > or =1 microm/s. The force during tether growth appeared to approach a plateau at long times. Examined over a large range of pulling speeds up to 150 microm/s, the plateau force exhibited a significant shear thinning as indicated by a weak power-law dependence on pulling speed, f(infinity) = 60 pN(nu(pull)/microm/s)(0.25). Using this shear-thinning response to describe the viscous element in a nonlinear Maxwell-like fluid model, we show that a weak serial-elastic component with a stiffness of approximately 0.07 pN/nm provides good agreement with the time course of the tether force approach to the plateau under constant pulling speed.

Actin cytoskeleton as the principal determinant of size-dependent DNA mobility in cytoplasm: a new barrier for non-viral gene delivery

The cytosol of mammalian cells is a crowded environment containing soluble proteins and a network of cytoskeletal filaments. Gene delivery by synthetic vectors involves the endocytosis of DNA-polycation complexes, escape from endosomes, and diffusion of non-complexed DNA through the cytosol to reach the nucleus. We found previously that the translational diffusion of large DNAs (>250 bp) in cytoplasm was greatly slowed compared with that of smaller DNAs (Lukacs, G. L., Haggie, P., Seksek, O., Lechardeur, D., Freedman, N., and Verkman, A. S. (2000) J. Biol. Chem. 275, 1625-1629). To determine the mechanisms responsible for size-dependent DNA diffusion, we used fluorescence correlation spectroscopy to measure the diffusion of single fluorophore-labeled DNAs in crowded solutions, cytosol extracts, actin network, and living cells. DNA diffusion (D) in solutions made crowded with Ficoll-70 (up to 40 weight percentage) or soluble cytosol extracts (up to 100 mg/ml) relative to diffusion of the same sized DNAs in saline (D/D(o)) was approximately independent of DNA size (20-4500 bp), quite different from the strong reduction in D/D(o) in the cytoplasm of living cells. However, the reduced D/D(o) with increasing DNA size was closely reproduced in solutions containing cross-linked actin filaments assembled with gelsolin, whereas soluble macromolecules of the same size and concentration did not reduce D/D(o). In intact cells microinjected with fluorescent DNAs and studied by fluorescence correlation spectroscopy or photobleaching methods, D/D(o) was reduced by 5-150-fold (20-6000 bp); however, the size-dependent reduction in D/D(o) was abolished after actin cytoskeleton disruption. Our results identify the actin cytoskeleton as a major barrier restricting cytoplasmic transport of non-complexed DNA in non-viral gene transfer.

2,3-butanedione monoxime (BDM), a potent inhibitor of actin-myosin interaction, induces ion and fluid transport in MDCK monolayers

Membrane-cytoskeleton interactions have been shown to be crucial to modulate polarity, cell shape and the paracellular pathway in epithelial MDCK cell monolayers. In particular, actin organization and myosin-dependent contractility play an important role in the regulation of these functions. Participation of myosin in vectorial transport, expressed as formation of domes, was investigated in confluent monolayers of high transepithelial electrical resistance (TER) plated on non-permeable supports. Cells exposed to 2,3-butanedione monoxime, a selective inhibitor of myosin ATPase, showed a remarkable increase in the number of domes. Replacement of extracellular Na+ and Cl- and inhibition of Na+-K+-ATPase blocked the induction of domes. The monoxime also caused a reduction of the TER leading to an increase in the paracellular flux of small molecular weight dextran. However, immunofluorescence microscopy of drug-treated cells showed that the localization and staining pattern of tight junction proteins ZO-1, occludin, and claudin 1, or the actin-myosin ring at the zonula adherens, were not modified. Treatment with the drug produced striking re-arrangements of actin filaments at the microvilli and at the basal level of the cells. Our data show that disruption of actin-myosin interaction at several cellular sites contributed importantly to the increased transport activity and the formation of the domes. These results point to the relevant role or actin-myosin dynamics and actin organization in the regulation of ion and water channel activity in these cells.

Interaction of Heliothis armigera nuclear polyhedrosis viral capsid protein with its host actin

In order to find the cellular interaction factors of the Heliothis armigera nuclear polyhedrosis virus capsid protein VP39, a Heliothis armigera cell cDNA library was constructed. Then VP39 was used as bait. The host actin gene was isolated from the cDNA library with the yeast two-hybrid system. This demonstrated that VP39 could interact with its host actin in yeast. In order to corroborate this interaction in vivo, the vp39 gene was fused with the green fluorescent protein gene in plasmid pEGFP39. The fusion protein was expressed in the Hz-AM1 cells under the control of the Autographa californica multiple nucleopolyhedrovirus immediate early gene promoter. The host actin was labeled specifically by the red fluorescence substance, tetramethy rhodamine isothicyanete-phalloidin. Observation under a fluorescence microscopy showed that VP39, which was indicated by green fluorescence, began to appear in the cells 6 h after being transfected with pEGFP39. Red actin cables were also formed in the cytoplasm at the same time. Actin was aggregated in the nucleus 9 h after the transfection. The green and red fluorescence always appeared in the same location of the cells, which demonstrated that VP39 could combine with the host actin. Such a combination would result in the actin skeleton rearrangement.

WDR1 colocalizes with ADF and actin in the normal and noise-damaged chick cochlea

Auditory hair cells of birds, unlike hair cells in the mammalian organ of Corti, can regenerate following sound-induced loss. We have identified several genes that are upregulated following such an insult. One gene, WDR1, encodes the vertebrate homologue of actin-interacting protein 1, which interacts with actin depolymerization factor (ADF) to enhance the rate of actin filament cleavage. We examined WDR1 expression in the developing, mature, and noise-damaged chick cochlea by in situ hybridization and immunocytochemistry. In the mature cochlea, WDR1 mRNA was detected in hair cells, homogene cells, and cuboidal cells, all of which contain high levels of F-actin. In the developing inner ear, WDR1 mRNA was detected in homogene cells and cuboidal cells by embryonic day 7, in the undifferentiated sensory epithelium by day 9, and in hair cells at embryonic day 16. We also demonstrated colocalization of WDR1, ADF, and F-actin in all three cell types in the normal and noise-damaged cochlea. Immediately after acoustic overstimulation, WDR1 mRNA was seen in supporting cells. These cells contribute to the structural integrity of the basilar papilla, the maintenance of the ionic barrier at the reticular lamina, and the generation of new hair cells. These results indicate that one of the immediate responses of the supporting cell after noise exposure is to induce WDR1 gene expression and thus to increase the rate of actin filament turnover. These results suggest that WDR1 may play a role either in restoring cytoskeletal integrity in supporting cells or in a cell signaling pathway required for regeneration.

The optical stretcher: a novel laser tool to micromanipulate cells

When a dielectric object is placed between two opposed, nonfocused laser beams, the total force acting on the object is zero but the surface forces are additive, thus leading to a stretching of the object along the axis of the beams. Using this principle, we have constructed a device, called an optical stretcher, that can be used to measure the viscoelastic properties of dielectric materials, including biologic materials such as cells, with the sensitivity necessary to distinguish even between different individual cytoskeletal phenotypes. We have successfully used the optical stretcher to deform human erythrocytes and mouse fibroblasts. In the optical stretcher, no focusing is required, thus radiation damage is minimized and the surface forces are not limited by the light power. The magnitude of the deforming forces in the optical stretcher thus bridges the gap between optical tweezers and atomic force microscopy for the study of biologic materials.

Role of gelsolin in the actin filament regulation of cardiac L-type calcium channels

The actin cytoskeleton is an important contributor to the modulation of the cell function. However, little is known about the regulatory role of this supermolecular structure in the membrane events that take place in the heart. In this report, the regulation of cardiac myocyte function by actin filament organization was investigated in neonatal mouse cardiac myocytes (NMCM) from both wild-type mice and mice genetically devoid of the actin filament severing protein gelsolin (Gsn-/-). Cardiac L-type calcium channel currents (I(Ca)) were assessed using the whole cell voltage-clamp technique. Addition of the actin filament stabilizer phalloidin to wild-type NMCM increased I(Ca) by 227% over control conditions. The basal I(Ca) of Gsn-/- NMCM was 300% higher than wild-type controls. This increase was completely reversed by intracellular perfusion of the Gsn-/- NMCM with exogenous gelsolin. Further, cytoskeletal disruption of either Gsn-/- or phalloidin-dialyzed wild-type NMCM with cytochalasin D (CD) decreased the enhanced I(Ca) by 84% and 87%, respectively. The data indicate that actin filament stabilization by either a lack of gelsolin or intracellular dialysis with phalloidin increase I(Ca), whereas actin filament disruption with CD or dialysis of Gsn-/- NMCM with gelsolin decrease I(Ca). We conclude that cardiac L-type calcium channel regulation is tightly controlled by actin filament organization. Actin filament rearrangement mediated by gelsolin may contribute to calcium channel inactivation.

Thiol oxidation of actin produces dimers that enhance the elasticity of the F-actin network

Slow oxidation of sulfhydryls, forming covalently linked actin dimers and higher oligomers, accounts for increases in the shear elasticity of purified actin observed after aging. Disulfide-bonded actin dimers are incorporated into F-actin during polymerization and generate cross-links between actin filaments. The large gel strength of oxidized actin (>100 Pa for 1 mg/ml) in the absence of cross-linking proteins falls to within the theoretically predicted order of magnitude for uncross-linked actin filament networks (1 Pa) with the addition of sufficient concentrations of reducing agents such as 5 mM dithiothreitol or 10 mM beta-mercaptoethanol. As little as 1 gelsolin/1000 actin subunits also lowers the high storage modulus of oxidized actin. The effects of gelsolin may be both to increase filament number as it severs F-actin and to cover the barbed end of an actin filament, which otherwise might cross-link to the side of another filament via an actin dimer. These new findings may explain why previous studies of actin rheology report a wide range of values when purified actin is polymerized without added regulatory proteins.

Role of actin filament organization in cell volume and ion channel regulation

The actin cytoskeleton is an intracellular structure, which is involved in the onset and control of cell shape and function. In order for this relevant network to control its own and thus cell volume, specific interactions between the actin cytoskeleton and ion channel regulation controlling intracellular salt and water homeostasis may be invoked. The hypotonic shock-induced, cell volume regulatory decrease (RVD) of most eukaryotic cells is a particularly useful example, as it is initiated and regulated by concerted processes involving both adaptive changes in actin filament organization and bulk fluid extrusion triggered by saline movement and the consequent decrease in cell water. The onset of RVD is linked to the selective activation of osmotically-sensitive ion channels and other relevant ion transport mechanisms involved in the net ionic movement from the cytosol. Such regulatory processes, entailing effector changes in actin filament organization which target the plasma membrane, are largely unknown. In this report, recent studies are summarized implicating dynamic changes in gel properties of the actin cytoskeleton as the effector mechanism in the regulation of ion channel activity, and thus cell volume, in human melanoma cells. Based on the characterization of the hypotonic cell volume regulatory response of human melanoma cells devoid of a functional actin-binding protein (ABP-280, a filamin homolog) and their genetically rescued counterpart transfected with a functional ABP, a hypothesis is raised which is consistent with a regulatory "sensory" mechanism based on the ability of actin networks to respond to changes in the intracellular water-salt homeostasis, which in turn effects signals controlling membrane function, including ion channel activity.

The role of axonal cytoskeleton in diabetic neuropathy

The neuropathy associated with diabetes includes well documented impairment of axonal transport, a reduction in axon calibre and a reduced capacity for nerve regeneration. All of those aspects of nerve function rely on the integrity of the axonal cytoskeleton. Alterations in the axonal cytoskeleton in experimental diabetes include an insulin-dependent non-enzymatic glycation of actin that is reflected in increased glycation of platelet actin in the clinical situation. There is a reduced synthesis of mRNA for the isoforms of tubulin that are associated with nerve growth and regeneration and an elevated non-enzymatic glycation of peripheral nerve tubulin in both diabetic patients and diabetic animals. mRNAs for neurofilament proteins are selectively reduced in the diabetic rat and post-translational modification of at least one of the neurofilament proteins is altered. There is some evidence that altered expression of isoforms of protein kinases may contribute to these changes.

Cloning and sequencing of a full-length sea bream (Sparus aurata) beta-actin cDNA

A full-length cDNA clone encoding beta-actin (beta-actin) was isolated from a sea bream (Sparus aurata) liver cDNA library. Sequencing of this clone reveals an open reading frame encoding a 375 amino acid protein that shares a high degree of conservation to other known actins. The sea bream beta-actin sequence showed 98% identity to carp and human beta-actin and 95% and 94% identity to sea squirt and Dictyostelium cytoplasmic actins, respectively.

The compartmentalization of protein synthesis: importance of cytoskeleton and role in mRNA targeting

Following the synthesis of mRNA molecules in eukaryotic cells, the transcripts are processed in the nucleus and subsequently transported through the nuclear membrane into the cytoplasm before being sequestered into polysomes where the information contained in the RNA molecule is translated into an amino acid sequence. Recent evidence suggests that an association of mRNAs with the cytoskeleton might be important in targeting mechanisms and, furthermore, in the transport of mRNA from the nucleus to its correct location in the cytoplasm. Until recently, polysomes have been considered to exist in two classes, namely free or membrane-bound. There is now compelling evidence, however, that ribosomes, in addition to being associated with endoplasmic reticulum membranes, also are associated with components of the cytoskeleton. Thus, a large number of morphological and biochemical studies have shown that mRNA, polysomes and translational factors are associated with cytoskeletal structures. Although the actual nature and significance of the interaction between components of the translational apparatus and the cytoskeleton is not yet understood in detail, it would seem evident that such interactions are important in both the spatial organization and control of protein synthesis. Recent work has shown that a subcellular fraction, enriched in cytoskeletal components, contains polysomes and these (cytoskeletal-bound) polysomes have been shown to contain specific mRNA species. Thus, a population of cytoskeletal-bound polysomes may provide a specialized mechanism for the sorting, targeting and topographical segregation of mRNAs. In this review, current knowledge of the subcellular compartmentalization of mRNAs is discussed.

Structural and phylogenetic analysis of the actin gene from the yeast Phaffia rhodozyma

The gene coding for actin from Phaffia rhodozyma was cloned and sequenced. The Phaffia actin gene contains four intervening sequences and the predicted protein consists of 375 amino acids. The structural features of the Phaffia actin introns were studied and compared with actin introns from seven fungi and yeasts with ascomycetous and basidiomycetous affinity. It was shown that the architecture of the Phaffia introns most resembles that of the basidiomycete Filobasidiella neoformans (perfect stage of Cryptococcus neoformans), whereas least resemblance occurs with the ascomycetous yeasts. Based on the intron structure, the ascomycetous yeasts can be accommodated in one group in that their splice site sequences are very similar and show less homology with the other fungi investigated, including Phaffia. It was demonstrated that the Phaffia actin introns cannot be spliced in Saccharomyces cerevisiae, which shows that the differences found in intron structure are significant. Alignment of the Phaffia actin gene with the actin sequences from the yeasts and fungi investigated showed a high level of homology both on the DNA level and on the protein level. Based on these alignments Phaffia showed highest homology with F. neoformans and both organisms were accommodated in the same cluster. In addition, the actin gene comparisons also supported the distant relationship of Phaffia with the ascomycetous yeasts. These results supported the usefulness of actin sequences for phylogenetic studies.

Polymorphism of F-actin assembly. 1. A quantitative phase diagram of F-actin

We have made the first quantitative phase diagram of actin filament (F-actin) assembly represented by the concentration of F-actin and the chi parameter which characterizes solvent-solute interaction energy. We manipulated the chi value of F-actin by adding a high molecular weight poly- (ethylene glycol) with average molecular weight 6000 (PEG 6K). The preferential exclusion of PEG 6K from the region adjacent to F-actin increases the chi value of F-actin. We qualified the PEG 6K-induced increase of the chi value through analysis of the PEG-induced solubility change of protein. The phase diagram shows that F-actin changes its assembly structure from isotropic disordered distribution to anisotropic ordered phase of a lyotropic liquid crystalline with an increase in the concentration and to concentrated anisotropic ordered phase of a crystalline-like bundle with a small increase in chi respectively, in the physiological concentration range. The formation of the crystalline- like bundle suggests that some specific force may act between F-actin. The present results demonstrate that F-actin can take various assembly structures as observed in cytoplasm by itself, indicating that the versatility of F-actin assembly in cytoplasm may be based on the thermodynamic properties of F-actin as a rod-like molecule.

Polymorphism of F-actin assembly. 2. Effects of barbed end capping on F-actin assembly

In the accompanying paper [Suzuki, A., Yamazaki, M., & Ito, T. (1996) Biochemistry 35, 5238-5244], we presented a quantitative phase diagram of actin filament ( (F-actin) described with the F-actin concentration and delta chi value which characterizes the affinity of F-actin with solvent. The phase diagram shows that F-actin changes its assembly structure from an isotropic disordered distribution to a dilute ordered assembly of a lyotropic crystalline with an increase in the concentration an to a concentrated ordered assembly of a crystalline-like bundle with an increase in the delta chi value (i.e., with a decrease in the affinity with the solvent), respectively, in the physiological concentration range. We report here that capping the barbed end of F-actin significantly affects the phase diagram. The F-actin capped by gelsolin (capped F-actin) decreased the delta chi value required for the formation of the concentrated ordered assembly. The time taken for the decrease in the delta chi value to reach a stationary state after the barbed end capping was proportional to the filament length (approximately 1 h/microm length). the electron microscopic morphology of the concentrated ordered assembly of the capped F-actin was a wide and loose bundle, which was distinctly different from the crystalline-like bundle of the uncapped F-actin. Fragmin from the acellular slime mould, which has similar functions to gelsolin, showed the same effects. These results suggest that the barbed end capping of F-actin gradually changes the nature of the whole filament so as to make the interaction with the solvent more unstable, and the F-actin loses the ability to make a crystalline-like bundle.

Functional linkage of the cardiac ATP-sensitive K+ channel to the actin cytoskeleton

The role of the cytoskeleton in the rundown and reactivation of adenosine triphosphate (ATP) sensitive K+ channels (KATP channels) was examined by perturbing selectively the intracellular surface of inside-out membrane patches excised from guinea-pig ventricular myocytes. Actin filament-depolymerizing agents (cytochalasins and desoxyribonuclease I) accelerated channel rundown, while actin filament stabilizer (phalloidin) or phosphatidylinositol biphosphate (PIP2; inhibitor of F-actin-severing proteins) inhibited spontaneous and/or Ca2+-induced rundown. When rundown was induced by cytochalasin D or by long exposure to high Ca2+, channel activity could not be restored by exposure to MgATP, but application of F-actin with MgATP could reinstitute channel activity. The processes of rundown and reactivation of cardiac KATP channels may thus be influenced by the assembly and disassembly of the actin cytoskeletal network, which provides a novel regulatory mechanism of this channel.

Role of the actin cytoskeleton on epithelial Na+ channel regulation

The regulatory role of actin filament organization on epithelial Na+ channel activity is reviewed in this report. The actin cytoskeleton, consisting of actin filaments and associated actin-binding proteins, is essential to various cellular events including the maintenance of cell shape, the onset of cell motility, and the distribution and stability of integral membrane proteins. Functional interactions between the actin cytoskeleton and specific membrane transport proteins are, however, not as well understood. Recent studies from our laboratory have determined that dynamic changes in the actin cytoskeletal organization may represent a novel signaling mechanism in the regulation of ion transport in epithelia. This report summarizes work conducted in our laboratory leading to an understanding of the molecular steps associated with the regulatory role of the actin-based cytoskeleton on epithelial Na+ channel function. The basis of this interaction lies on the regulation by actin-binding proteins and adjacent structures, of actin filament organization which in turn, modulates ion channel activity. The scope of this interaction may extend to such relevant cellular events as the vasopressin response in the kidney.

Structural features and phylogeny of the actin gene of Chondrus crispus (Gigartinales, Rhodophyta)

We have characterized the cDNA and genomic sequences that encode actin from the multicellular red alga Chondrus crispus. Southern-blot analysis indicates that the C. crispus actin gene (ChAc) is present as a single copy. Northern analysis shows that, like the GapA gene, the actin gene is well expressed in gametophytes but weakly in protoplasts. Compared to actin genes of animals, fungi, green plants and oomycetes, that of C. crispus displays a higher evolutionary rate and does not show any of the amino-acid signatures characteristic of the other lineages. As previously described for GapA, ChAc is interrupted by a single intron at the beginning of the coding region. The site of initiation of transcription was characterized by RNAse protection. The promoter region displays a CAAT box but lacks a canonical TATA motif. Other noticeable features, such as a high content of pyrimidines as well as a 14-nt motif found in both the 5'-untranslated region and the intron, were observed.