Cytoskeletal functions of cytoplasmic contractile proteins

One must reconstitute the functions of interest from purified proteins

I am often asked by students and younger colleagues and now by the editors of this issue to tell the history of the development of the in vitro motility assay and the dual-beam single-molecule laser trap assay for myosin-driven actin filament movement, used widely as key assays for understanding how both muscle and nonmuscle myosin molecular motors work. As for all discoveries, the history of the development of the myosin assays involves many people who are not authors of the final publications, but without whom the assays would not have been developed as they are. Also, early experiences shape how one develops ideas and experiments, and influence future discoveries in major ways. I am pleased here to trace my own path and acknowledge the many individuals involved and my early science experiences that led to the work I and my students, postdoctoral fellows, and sabbatical visitors did to develop these assays. Mentors are too often overlooked in historical descriptions of discoveries, and my story starts with those who mentored me.

Towards a biomimetic cellular structure and physical morphology with liposome-encapsulated agarose sol systems

The cytoplasm, serving as the primary hub of cellular metabolism, stands as a pivotal cornerstone for the harmonious progression of life. The ideal artificial cell should not only have a biomembrane structure system similar to that of a cell and the function of carrying genetic information, but also should have an intracellular environment. In this pursuit, we employed a method involving the incorporation of glycerol into agarose, resulting in the formation of agarose-glycerol mixed sol (AGs). This dynamic sol exhibited fluidic properties at ambient temperature, closely mimicking the viscosity of authentic cytoplasm. Harnessing the electroformation technique, AGs was encapsulated within liposomes, enabling the efficient creation of artificial cells that closely resembled native cellular dimensions through meticulous parameter adjustments of the alternating current (AC) field. Subsequently, artificial cells harboring AGs were subjected to diverse electrolyte and non-electrolyte solutions, enabling a comprehensive exploration of their deformation phenomena, encompassing both inward and outward budding. This study represents a significant stride forward in addressing one of the most fundamental challenges in the construction of artificial cytoplasm. It is our fervent aspiration that this work shall offer invaluable insights and guidance for future endeavors in the realm of artificial cell construction.

Mechanical properties of subisostatic random networks composed of nonlinear fibers

Fibrous protein networks provide structural integrity to different biological materials such as soft tissues. These networks display an unusual exponential strain-stiffening behavior when subjected to mechanical loads. This nonlinear strain-stiffening behavior has so far been explained in terms of the network microstructure and the flexibility of constituting fibers. Here, we conduct a comprehensive computational study to characterize the importance of material properties of individual fibers in the overall nonlinear mechanical response of random fiber networks. To this end, we consider three nonlinear material models, ranging from an almost linear form to a highly nonlinear one, for the fibers of subisostatic disordered networks. We characterize the amount of strain-stiffening as a function of bending rigidity of the fibers, the amount of nonlinearity of the fibers, and the connectivity of random networks. We find that networks composed of highly nonlinear fibers exhibit much more strain-stiffening than networks made up of linear fibers. Furthermore, the local strain distribution becomes more homogenous as the amount of nonlinearity in the material models increases. Increasing the network connectivity signifies the importance of the nonlinear material response of individual fibers in the overall mechanical behavior of networks. The constitutive behavior of fibers plays an important role in defining the failure response of networks particularly in the damage initiation and evolution. These important findings for how the mechanical response of individual fibers affects the overall mechanical properties of random networks could find applications in designing new biomimetic materials and could help scientists better understand the mechanical properties of biological materials.

Cell Motility and Cytokinesis: From Mysteries to Molecular Mechanisms in Five Decades

This is the story of someone who has been fortunate to work in a field of research where essentially nothing was known at the outset but that blossomed with the discovery of profound insights about two basic biological processes: cell motility and cytokinesis. The field started with no molecules, just a few people, and primitive methods. Over time, technological advances in biophysics, biochemistry, and microscopy allowed the combined efforts of scientists in hundreds of laboratories to explain mysterious processes with molecular mechanisms that can be embodied in mathematical equations and simulated by computers. The success of this field is a tribute to the power of the reductionist strategy for understanding biology.

Platelet rich plasma promotes skeletal muscle cell migration in association with up-regulation of FAK, paxillin, and F-Actin formation

Platelet rich plasma (PRP) contains various cytokines and growth factors which may be beneficial to the healing process of injured muscle. The aim of this study was to investigate the effect and molecular mechanism of PRP on migration of skeletal muscle cells. Skeletal muscle cells intrinsic to Sprague-Dawley rats were treated with PRP. The cell migration was evaluated by transwell filter migration assay and electric cell-substrate impedance sensing. The spreading of cells was evaluated microscopically. The formation of filamentous actin (F-actin) cytoskeleton was assessed by immunofluorescence staining. The protein expressions of paxillin and focal adhesion kinase (FAK) were assessed by Western blot analysis. Transfection of paxillin small-interfering RNA (siRNAs) to muscle cells was performed to validate the role of paxillin in PRP-mediated promotion of cell migration. Dose-dependently PRP promotes migration of and spreading and muscle cells. Protein expressions of paxillin and FAK were up-regulated dose-dependently. F-actin formation was also enhanced by PRP treatment. Furthermore, the knockdown of paxillin expression impaired the effect of PRP to promote cell migration. It was concluded that PRP promoting migration of muscle cells is associated with up-regulation of proteins expression of paxillin and FAK as well as increasing F-actin formation. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2506-2512, 2017.

The prehistory of the cytoskeleton concept

Here we discuss how the concept and the name of cytoskeleton were generated and started to evolve over the last two centuries into what is presently a basic topic of modern biology. We also attempt to describe some facets of the emergence of cytoskeleton component characterization in which our laboratory was in part involved.

Tropomyosin-1 protects endothelial cell-cell junctions against cigarette smoke extract through F-actin stabilization in EA.hy926 cell line

The aim of the study was to estimate the effect of cigarette smoke extract (CSE) on EA.hy926 endothelial cells in culture in the context of maintenance of cell-cell junctions through the structural stabilization of the actin cytoskeleton. In the present study, F-actin was stabilized by the overexpression of tropomyosin-1, which is known to stabilize actin filaments in muscle and non-muscle cells. Our study showed that the stabilization of F-actin significantly increased the survival of cells treated with 25% CSE. In addition, after stabilization of F-actin the migratory potential of EA.hy926 cells subjected to CSE treatment was increased. Our results also showed increased fluorescence intensity of alpha- and beta-catenin after CSE treatment in cells which had stabilized F-actin. Analysis of fluorescence intensity of Zonula occludens-1 did not reveal any significant differences when EA.hy926 cells overexpressing tropomyosin-1 were compared with those lacking overexpression. It would appear that overexpression of tropomyosin-1 preserved the structure of actin filaments in the cells treated with CSE. In conclusion, the present study demonstrates that stabilization of F-actin protects EA.hy926 cells against CSE-induced loss of both adherens and tight junctions. The data presented in this study suggest that overexpression of tropomyosin-1 stabilizes the organizational structure of actin filaments and helps preserve the endothelial barrier function under conditions of strong oxidative stress.

Actin assembly factors regulate the gelation kinetics and architecture of F-actin networks

Dynamic regulation of the actin cytoskeleton is required for diverse cellular processes. Proteins regulating the assembly kinetics of the cytoskeletal biopolymer F-actin are known to impact the architecture of actin cytoskeletal networks in vivo, but the underlying mechanisms are not well understood. Here, we demonstrate that changes to actin assembly kinetics with physiologically relevant proteins profilin and formin (mDia1 and Cdc12) have dramatic consequences on the architecture and gelation kinetics of otherwise biochemically identical cross-linked F-actin networks. Reduced F-actin nucleation rates promote the formation of a sparse network of thick bundles, whereas increased nucleation rates result in a denser network of thinner bundles. Changes to F-actin elongation rates also have marked consequences. At low elongation rates, gelation ceases and a solution of rigid bundles is formed. By contrast, rapid filament elongation accelerates dynamic arrest and promotes gelation with minimal F-actin density. These results are consistent with a recently developed model of how kinetic constraints regulate network architecture and underscore how molecular control of polymer assembly is exploited to modulate cytoskeletal architecture and material properties.

Effect of L-homocysteine on endothelial cell-cell junctions following F-actin stabilization through tropomyosin-1 overexpression

Since the identification of actin in non‑muscle cells, it has been suggested that the regulation of the mechanical behaviors of the actin cytoskeleton regulates cellular shape changes and the generation of forces during cell migration and division. The maintenance of cell shape and polarity are important in the formation of cell-cell junctions. The aim of the present study was to determine the effect of L‑homocysteine thiolactone hydrochloride on EA.hy926 endothelial cells in the context of the maintenance cell-cell junctions through the stabilization of filamentous actin cytoskeleton (F‑actin). The actin filaments were stabilized by the overexpression of tropomyosin-1, which has the ability to stabilize actin filaments in muscle and non-muscle cells. The stabilization of F-actin induced a significant decrease in the percentage of late apoptotic and necrotic cells following treatment with L-homocysteine. Moreover, the migratory potential of the endothelial cells was greater in the cells overexpressing tropomyosin-1 treated with L-homocysteine. Additionally, our results indicated that the stabilization of F-actin in the EA.hy926 cells significantly increased the expression of junctional β‑catenin, as compared to the cells not overexpressing tropomyosin‑1. Similarly, the fluorescence intensity of junctional α-catenin was also increased in the cells with stabilized F‑actin cytoskeleton. However, this increase was only slightly higher than that observed in the EA.hy926 cells not overexpressing tropomyosin-1. Furthermore, the analysis of Zonula occludens (ZO)‑1 relative fluorescence demonstrated a statistically significant decrease in the cell-cell junction areas among the cells with stabilized F-actin cytoskeleton in comparison to the cells not overexpressing tropomyosin-1. Our results indicate that the stabilization of F-actin does not affect the migratory potential of cells, and consequently protects the EA.hy926 cells against the L-homocysteine-induced decrease in cell mobility. Moreover, it is suggested that α‑catenin may participate in the suppression of actin polymerization in the area of cell-cell junctions. It can be hypothesized that the stabilization of F-actin strengthens endothelial adherens and tight junctions by increasing the number of cell-cell junctions due to the amplification of β-catenin and the ZO‑1 fluorescence signal. However, ZO-1 stabilizes the endothelial barrier function through the stabilization of F-actin and F-actin itself stabilizes the localization of ZO-1.

Interaction of profilin-1 and F-actin via a β-arrestin-1/JNK signaling pathway involved in prostaglandin E(2)-induced human mesenchymal stem cells migration and proliferation

Although many previous reports have examined the function of prostaglandin E(2) (PGE(2)) in the migration and proliferation of various cell types, the role of the actin cytoskeleton in human mesenchymal stem cells (hMSCs) migration and proliferation has not been reported. The present study examined the involvement of profilin-1 (Pfn-1) and filamentous-actin (F-actin) in PGE(2)-induced hMSC migration and proliferation and its related signal pathways. PGE(2) (10(-6) M) increased both cell migration and proliferation, and also increased E-type prostaglandin receptor 2 (EP2) mRNA expression, β-arrestin-1 phosphorylation, and c-Jun N-terminal kinase (JNK) phosphorylation. Small interfering RNA (siRNA)-mediated knockdown of β-arrestin-1 and JNK (-1, -2, -3) inhibited PGE(2)-induced growth of hMSCs. PGE(2) also activated Pfn-1, which was blocked by JNK siRNA, and induced F-actin level and organization. Downregulation of Pfn-1 by siRNA decreased the level and organization of F-actin. In addition, specific siRNA for TRIO and F-actin-binding protein (TRIOBP) reduced the PGE(2)-induced increase in hMSC migration and proliferation. Together, these experimental data demonstrate that PGE(2) partially stimulates hMSCs migration and proliferation by interaction of Pfn-1 and F-actin via EP2 receptor-dependent β-arrestin-1/JNK signaling pathways.

Mechanics of the F-actin cytoskeleton

Dynamic regulation of the filamentous actin (F-actin) cytoskeleton is critical to numerous physical cellular processes, including cell adhesion, migration and division. Each of these processes require precise regulation of cell shape and mechanical force generation which, to a large degree, is regulated by the dynamic mechanical behaviors of a diverse assortment of F-actin networks and bundles. In this review, we review the current understanding of the mechanics of F-actin networks and identify areas of further research needed to establish physical models. We first review our understanding of the mechanical behaviors of F-actin networks reconstituted in vitro, with a focus on the nonlinear mechanical response and behavior of "active" F-actin networks. We then explore the types of mechanical response measured of cytoskeletal F-actin networks and bundles formed in living cells and identify how these measurements correspond to those performed on reconstituted F-actin networks formed in vitro. Together, these approaches identify the challenges and opportunities in the study of living cytoskeletal matter.

Dynamic cross-linking by alpha-actinin determines the mechanical properties of actin filament networks

We used smooth muscle alpha-actinin to evaluate the contribution of cross-linker dynamics to the mechanical properties of actin filament networks. Recombinant actin-binding domain (residues 2-269) binds actin filaments with a Kd of 1 microM at 25 degrees C, 20 times stronger than actin-binding domain produced by thermolysin digestion of native alpha-actinin (residues 25-257). Between 8 and 25 degrees C the rate constants for recombinant actin-binding domain to bind to (0.8-2.7 microM-1 s-1) and dissociate from (0.2-2.4 s-1) actin filaments depend on temperature. At 8 degrees C actin filaments cross-linked with alpha-actinin are stiff and nearly solid, whereas at 25 degrees C the mechanical properties approach those of actin filaments alone. In these experiments, high actin concentrations kept most of the alpha-actinin bound to actin and temperature varied a single parameter, cross-linker dynamics, because the mechanical properties of pure actin filaments (a viscoelastic gel) or biotinylated actin filaments cross-linked irreversibly by avidin (a stiff viscoelastic solid) depend little on temperature. These results show that the rate of exchange of dynamic cross-links between actin filaments is an important determinant of the mechanical properties of the networks.

HgCl2-induced alteration of actin filaments in cultured primary rat proximal tubule epithelial cells labelled with fluorescein phalloidin

When proximal tubule epithelial cells are exposed to HgCl2, cytoplasmic blebs are formed. These represent an early, potentially reversible response to injury. These blebs are accompanied by reorganization of cytoskeletal proteins, and presumably by alternations in cytoskeletal-plasma membrane interactions. Ca(2+)-activated proteinases, such as calpain, are known to affect cytoskeletal proteins and to be involved in diverse cellular processes. However, the role of calpains in cytotoxicity due to HgCl2 is unknown. To determine the relationship between F-actin, calpain, and HgCl2 toxicity, cells were stained with fluorescein phalloidin before and after treatment with HgCl2. Cells were grown on coverslips and exposed to HgCl2 (10 or 25 microM) in the presence or absence of the calpain inhibitor, leupeptin. Untreated cells were flat, polygonal, and contained many fluorescent-stained cables of actin filaments. Generally, cells exposed to HgCl2 became pleomorphic and contracted as the blebs formed. These cells showed fewer actin cables and fluorescence was seen mostly as either compact areas of dense stain or as peripheral rings. In many cells, actin cables and filaments were completely absent. Disappearance of F-actin was initially seen by 2 min after exposure to HgCl2. Thus, disruption of the actin cytoskeleton and blebbing were found to be early events in HgCl2 toxicity. When leupeptin was used with HgCl2 treatment, the actin staining appeared similar to that of untreated cells.

Skeletal muscle rhabdomyosarcomas in inbred laboratory mice

A total of 14 well differentiated rhabdomyosarcomas were diagnosed at necropsy in 10,000 mice. Of the 14 affected mice, ten were BALB/cJ, and there was one case each of A/HeJ, BALB/cByJ, C58/J, and C.B-17-scid/scid strains. Most often (10/14) tumors originated in the quadriceps muscles and metastases occurred in six cases. When submitted, affected mice were 2 to 8 months of age, with a mean age of 4 months. Tumor frequency for BALB/cJ mice was calculated to be 2.4/100,000 mice retained as breeders. No sexual dimorphisms were determined when data were correlated to actual numbers of each sex in the colony. All 14 primary tumors and metastases were positive by immunohistochemistry for the proteins pan myosin, sarcomeric actin, desmin, actin, and myosin, but were negative for smooth muscle actin, thus confirming the diagnosis. Using cell free homogenates of primary tumors, inoculated by intraperitoneal or intramuscular injection, tumors were not induced in either BALB/cJ or C58/J mice observed over a 22-week period. Southern blot analysis of DNA prepared from tumors and hybridized with a murine leukemia virus probe that recognizes both ecotropic and dualtropic viruses did not demonstrate viral genomic fragments in addition to those known to occur in each strain.

An in vitro study of the effects of androgens on the cytoskeleton of ovarian granulosa cells with special reference to actin

Ovarian granulosa cells from small antral follicles from immature rats were cultured in a serum-free medium for 1-6 days with or without the presence of 10(-5) M dehydroepiandrosterone (DHEA) or 10(-5) M-androstenedione (delta 4-A). Control cultures reveal that the cells are flattened and contain many filamentous bundles organized as stress fibers, numerous scattered cytoplasmic actin filaments, microtubules and vimentin. Alpha actinin and myosin were shown by immunocytochemistry to have a punctate pattern along the stress fibers. For the most part, cells exposed to androgens did not flatten; however, they assumed a varied shape and contained fewer stress fibers and actin filaments. Many of these cells did not develop stress fibers and those that did develop were fewer in number and displayed--actinin and myosin in a punctate pattern. Microtubules and vimentin filaments remained unaltered when compared to controls. It is believed that the deficiency of actin filaments, coupled with certain other degenerative changes which express themselves in other cellular compartments, leads to an early atresia of the granulosa cell cultured in high concentrations of androgens.

Hindered diffusion of inert tracer particles in the cytoplasm of mouse 3T3 cells

Using fluorescence recovery after photobleaching, we have studied the diffusion of fluorescein-labeled, size-fractionated Ficoll in the cytoplasmic space of living Swiss 3T3 cells as a probe of the physical chemical properties of cytoplasm. The results reported here corroborate and extend the results of earlier experiments with fluorescein-labeled, size-fractionated dextran: diffusion of nonbinding particles in cytoplasm is hindered in a size-dependent manner. Extrapolation of the data suggests that particles larger than 260 A in radius may be completely nondiffusible in the cytoplasmic space. In contrast, diffusion of Ficoll in protein solutions of concentration comparable to the range reported for cytoplasm is not hindered in a size-dependent manner. Although we cannot at present distinguish among several physical chemical models for the organization of cytoplasm, these results make it clear that cytoplasm possesses some sort of higher-order intermolecular interactions (structure) not found in simple aqueous protein solutions, even at high concentration. These results also suggest that, for native cytoplasmic particles whose smallest radial dimension approaches 260 A, size may be as important a determinant of cytoplasmic diffusibility as binding specificity. This would include most endosomes, polyribosomes, and the larger multienzyme complexes.

Cytoskeletal organization affects cellular responses to cytochalasins: comparison of a normal line and its transformant

The relationships between cytoskeletal network organization and cellular response to cytochalasin D (CD) in a normal rat fibroblast cell line (Hmf-n) and its spontaneous transformant (tHmf-e), with markedly different cytoskeletal phenotypes, were compared (using immunofluorescence, electron microscopy, and DNAse I assay for actin content). Hmf-n have prominent, polar stress fiber (SF) arrays terminating in vinculin adhesion plaques whereas tHmf-e, which are apolar, epithelioid cells with dense plasma membrane-associated actin networks, lack SF and adhesion plaques. Hmf-n exposed to CD become markedly retracted and dendritic, SF-derived actin aggregates form large endoplasmic masses, and discrete tabular aggregates at the distal ends of retraction processes. Prolonged exposure leads to recession of process, cellular rounding, and development of large cystic vacuoles. tHmf-e cells exposed to similar doses of CD display a diagnostically different response; retraction is less drastic, cells retain broad processes containing scattered actin aggregates in discrete foci often associated with plasma membrane, large tabular aggregates are never found and processes persist throughout long exposure, vacuolation is uncommon. The CD-induced microfilamentous aggregates in Hmf-n are composed of short, kinky filament fragments forming a felt-like skein, often aggregates contain a more ordered array of roughly parallel fragments, while those of tHmf-e are very short, kinky, randomly orientated filaments imparting a distinctly granular nature to the mass. Total actin content and the amount of actin associated with detergent-resistant cytoskeletons increase following CD exposure in both cell types. Throughout exposure to CD, the actin-associated contractile proteins tropomyosin, myosin, and alpha-actinin co-localize within the actin aggregates in both cell types. Fodrin, the protein linking cortical actin to membrane, co-localizes with actin aggregates in tHmf-e cells and most, but not all, such aggregates in Hmf-n cells, consistent with their stress fiber derivation. Vinculin is lost from the tabular aggregates at the distal ends of retraction processes in Hmf-n cells concomitant with the fragmentation and contraction of SF. The aborized processes in both cells types contain strikingly similar axial cores of bundled vimentin filaments associated with passively compressed microtubules. The characteristic CD-induced distribution of actin filament aggregates and redistribution of vimentin in these cell types also occur when cells are allowed to respread from the rounded state in the presence of CD.

An abundant ubiquitous glycoprotein (GP100) in nucleated mammalian cells

Two-dimensional gel electrophoresis with the 125I-Con A overlay and affinity purification with Con A-agarose revealed the presence of an abundant ubiquitous 100-kDa glycoprotein (GP100) in nucleated mammalin cells. The amount in cultured human and murine cells varies from 3 to 20 X 10(6) molecules per cell making GP100 the most abundant glycoprotein in nucleated cells. Peptide mapping shows that it is different from erythrocyte Band III protein. Several properties of GP100 suggest that it could play a structural role in nucleated cell membranes.

An hypothesis on the role of cellular colloid osmotic pressure in determining behavior of cells in vitro including anchorage dependency and maintenance of the differentiated state

The osmotic problems involved when cells are isolated from tissues are analyzed. Evidence is considered which indicates that in vivo the Na pump is operating at maximal or near maximal rates and that this depends on low leak rates for salts and water due to various aspects of the tissues structure. Dispersion of the tissue results in breakdown of these barriers on free diffusion and the isolated cell is subjected to an enormous increase in passive influx due to colloid osmotic pressure without being able to increase its pumping rate to the extent needed to maintain volume control. It is proposed that the primary problem the cell faces in vitro is to compensate for the effective increase in its colloid pressure, e.g. the colloid osmotic pressure excess, emerging with the breakdown of the tissue structure. The finding that most normal cells have to adhere to a surface in order to grow or "anchorage dependency" is analyzed in terms of the way adhesion and spreading result in changes in ion and water movements into cells enabling them to achieve fluid balance in the face of the colloid pressure excess. It is also proposed that the differentiated state is more dependent on colloid osmotic balance than proliferation. The failure of conditions used in tissue culture to compensate adequately for the colloid pressure excess results in limiting the amount of protein which can be synthesized, dissipation of cellular energy, and changes in orientation of cellular components which contribute directly to the loss of differentiation which occurs during growth in vitro.

Actin filament localization in normal and migrating rabbit corneal epithelium

The molecular probe NBD phallacidin (7-nitro-benz-2-oxa-1, 3-diazolylphallacidin), which reacts specifically with filamentous actin (f-actin), was used to study the distribution of polymerized actin oligmers in normal and migrating rabbit corneal epithelial cells. In the normal cornea, the majority of the NBD phallacidin fluorescence was localized to the cortical or sub-plasma membrane area of the superficial and wing epithelial cells. Following full thickness corneal trephination injury, migrating corneal epithelial cell exhibited a marked increase in the cortical NBD phallacidin fluorescence. Transmission electron microscopy, using fixation techniques which revealed bundles of fine filaments (6nm underlying the plasma membrane of migrating epithelial cells, thus supporting the fluorescent results. These findings suggest that corneal re-epithelialization is characterized by a marked increase in the amount of filamentous actin within the migrating epithelial cells. We conclude that NBD phallacidin may be of value in analyzing changes in actin polymerization during wound healing.

Reexamination of the reality or artifacts of the microtrabeculae

The polyethylene glycol (PEG) method revealed that model systems such as erythrocytes and protein solutions, which are supposed to lack structured components, exhibit lattice structures not unlike the microtrabeculae. The compactness of the lattice was dependent on the concentration of proteins. The gelated state of gelatin exhibited lattices more compact than those of the solated state at any given concentration. Comparison of images by PEG and rapid-freezing, deep-etching replica methods showed no basic differences in the ultrastructure of the intestinal epithelial cell. This indicates that the PEG method, including chemical fixation, produces little, if any, disorganization of the cytoskeleton. All of the present findings suggest that cytoplasmic protein, nonstructure-bound or structure-forming, might be present in intact cells which could form microtrabecular structures when specimens are fixed by chemical fixatives without any extractions. Therefore, the microtrabeculae should generally be regarded as a simple marker for the presence of proteinaceous macromolecules. It is also suggested that the microtrabecular lattice, as a whole, might represent a gelated state in a given compartment when another, looser lattice is simultaneously present in the same compartment, i.e., within a single cell.

The influence of poly(ethylene glycol) 6000 on the properties of skeletal-muscle actin

Poly(ethylene glycol) 6000 affected many of the properties of skeletal-muscle actin. It accelerated the rate and increased the extent of actin polymerization as measured by light-scattering and sedimentation studies respectively. Moreover, intrinsic-fluorescence measurements showed that addition of poly(ethylene glycol) 6000 decreased the rate of EDTA-induced denaturation of actin monomer and increased the temperature at which irreversible conformational changes occur in actin monomer. These effects occurred without any apparent direct binding interaction and are postulated to be a consequence of the effect of excluded volume on the thermodynamic activity of actin. A relationship based on spherical geometry was formulated which described the co-volume increment that occurs upon addition of a monomer to a long linear polymer in the presence of a space-filling macromolecule. The application of this relationship to the poly(ethylene glycol) 6000-actin system was not without assumption, but it permitted quantitative estimation of the co-volume increment which proved to be of the sign and magnitude required to explain the increased extent of actin polymerization found experimentally in the presence of various concentrations of poly(ethylene glycol) 6000. It is suggested that, in vivo, excluded volume may play a role in actin-filament formation and in the maintenance of the native G-actin structure.

Interaction of embryonic surface and cytoskeleton with extracellular matrix

Evidence for cell-matrix in vitro and in the embryo is briefly reviewed, and more detailed observations are presented on the reactions of corneal epithelium and mesenchyme to extracellular matrix (ECM). The basal surface of embryonic corneal epithelium blebs when the underlying ECM is removed. If the epithelium is cultured on top of the lens capsule or collagen gel, the basal surface flattens and the cortical cytoskeleton reorganizes to resemble that present in vivo. The basal surface also responds to soluble matrix molecules (types I-IV collagens, laminin, fibronectin), and the cells step up synthesis of corneal stroma as measured by incorporation of proline into collagen. When embryonic corneal fibroblasts are placed on top of hydrated gels they tend to burrow into the gel rather than sitting on top as does epithelium. When grown inside collagen gels, these mesenchymal cells elongate and the entire cell surface and cytoskeleton organize in response to matrix. Stress fibers and ruffling membranes characterize the cells grown on glass. When embryonic lens or corneal epithelial are place within, instead of on top of, collagen gels, they give rise to mesenchyme-like cells from their apical surfaces. In vivo, these epithelia do not give rise to mesenchyme. The rules for epithelial-mesenchymal transformation in vivo are discussed in relation to these observations on cell-matrix interaction.

Mobility of cytoplasmic and membrane-associated actin in living cells

We have combined fluorescent analogue cytochemistry with fluorescence photobleaching recovery to measure the mobility of fluorescently labeled actin and other labeled test proteins microinjected into living amoebae. Bovine serum albumin, ovalbumin, and ribonuclease A have a cytoplasmic mobility, expressed as a diffusion coefficient, that is 1/2 to 1/3 of that observed in aqueous solution; 90% of the actin has a mobility 1/2 to 1/8 of that of G-actin in aqueous solution, and approximately equal to 10% of the actin has a mobility comparable to that of F-actin in aqueous solution. Therefore, no more than 10% of the actin in the cytoplasm of amoebae can exist as static filaments. Microinjection of phalloidin decreases the diffusion coefficient of the mobile component of cytoplasmic actin, and it also increases the low-mobility fraction to 50% but has no effect on the mobility of labeled ovalbumin. By comparing the mobility of actin in different parts of amoebae and by separating cytoplasm from plasmalemma-ectoplasm, we found the low-mobility fraction of actin to be enriched in the tail, along the plasmalemma-ectoplasm, and in contracted cytoplasm.

On the dynamics of the microfilament system in HeLa cells

We measured the pools of unpolymerized and filamentous actin in homogenates of HeLa cells made in several different lysis buffers, as well as after treatment of cells with a variety of chemicals or trypsin, and after adenovirus (type 2) infection. This was possible when a series of factors concerning the basic culture conditions were kept constant: e.g., serum type used, serum batch, cell density, time after subcultivation of cells, and buffering substance in the medium. Homogenates from untreated cells usually contain 35-45 percent of the total actin in an unpolymerized form. With some batches of cells this number can be as high as 50 percent. In sparse cultures (3 x 10(4) cell/cm(2)), HeLa cells contain approximately 10 pg actin/cell, while the corresponding number is only 5 pg in dense cultures (3 x 10(5) cells/cm(2)). Treatment of cells with cytochalasin B increases the pool of unpolymerized actin by approximately 30-40 percent, while colchicine decreases the fraction of unpolymerized actin by 20 percent. The oxidant diamide increases the filamentous actin pool 25-50 percent. Glucose, sodium azide, dinitrophenol, serum starvation, or thymidine treatment does not affect the distribution between unpolymerized and filamentous actin to any significant extent. Trypsin and EDTA induced rounding up of cells but did not change the actin distribution. The distribution of actin between G- and F-forms was unchanged after adenovirus infection. These results show that significant changes in the actin pools can be induced in nucleated cells. However, several treatments which alter the morphology and motility of cells are not accompanied by an alteration in the G-/F-actin ratio.

Effects of cytochalasin B on membrane-associated microfilaments in a cell-free system

The mode of action of cytochalasin B was examined in vitro using bile canaliculus-enriched plasma membrane fractions isolated from rat liver. The pericanalicular microfilaments, which are mainly actin filaments and which are normally attached to the canalicular membranes, were dissociated from the membranes by cytochalasin B treatment. A microfilamentous network was found in the supernate of the cytochalasin B treatment. A microfilamentous network was found in the supernate of the cytochalasin-treated specimens and a number of polypeptides, of which a polypeptide corresponding in molecular weight to actin was a notable member. These results suggest that actin filaments become detached from the canaliculus membranes by cytochalasin B.

Actin filament bundles in vaccinia virus infected fibroblasts

When monolayer cultures of mouse 3T3 cells were infected with vaccinia virus, a rapid decrease of microfilament bundles was observed. The major disappearance of the cellular microfilaments occurred within the first hour after vaccinia virus infection, which is the period of uncoating of the virus core and early viral protein synthesis. The microfilament bundles of infected cells were significantly preserved in the presence of puromycin which inhibits viral protein synthesis.

A membrane cytoskeleton from Dictyostelium discoideum. I. Identification and partial characterization of an actin-binding activity

Dictyostelium discoideum plasma membranes isolated by each of three procedures bind F-actin. The interactions between these membranes and actin are examined by a novel application of falling ball viscometry. Treating the membranes as multivalent actin-binding particles analogous to divalent actin-gelation factors, we observe large increases in viscosity (actin cross-linking) when membranes of depleted actin and myosin are incubated with rabbit skeletal muscle F-actin. Pre-extraction of peripheral membrane proteins with chaotropes or the inclusion of Triton X-100 during the assay does not appreciably diminish this actin cross-linking activity. Lipid vesicles, heat-denatured membranes, proteolyzed membranes, or membranes containing endogenous actin show minimal actin cross-linking activity. Heat-denatured, but not proteolyzed, membranes regain activity when assayed in the presence of Triton X-100. Thus, integral membrane proteins appear to be responsible for some or all of the actin cross-linking activity of D. discoideum membranes. In the absence of MgATP, Triton X-100 extraction of isolated D. discoideum membranes results in a Triton-insoluble residue composed of actin, myosin, and associated membrane proteins. The inclusion of MgATP before and during Triton extraction greatly diminishes the amount of protein in the Triton-insoluble residue without appreciably altering its composition. Our results suggest the existence of a protein complex stabilized by actin and/or myosin (membrane cytoskeleton) associated with the D. discoideum plasma membrane.

Cytoplasmic free calcium and amoeboid movement

Measurements with the Ca2+ -sensitive photoprotein aequorin show that locomotion in the amoeba Chaos carolinense occurs without changes in the aequorin signal and that not more than 0.025% of the cytoplasm can exist at the micromolar threshold concentration for contraction. The results do not support the hypothesis that cytoplasmic streaming is under the control of changes in the cytoplasmic free Ca2+ concentration.

Viscometric analysis of the gelation of Acanthamoeba extracts and purification of two gelation factors

We have studied the kinetics of the gelation process that occurs upon warming cold extracts of Acanthamoeba using a low-shear falling ball assay. We find that the reaction has at least two steps, requires 0.5 mM ATP and 1.5 mM MgCl2, and is inhibited by micromolar Ca++. The optimum pH is 7.0 and temperature, 25 degrees-30 degrees C. The rate of the reaction is increased by cold preincubation with both MgCl2 and ATP. Nonhydrolyzable analogues of ATP will not substitute for ATP either in this "potentiation reaction" or in the gelation process. Either of two purified or any one of four partially purified Acanthamoeba proteins will cross-link purified actin to form a gel, but none can account for the dependence of the reaction in the crude extract on Mg-ATP or its regulation by Ca++. This suggests that the extract contains, in addition to actin-cross-linking proteins, factors dependent on Mg-ATP and Ca++ that regulate the gelation process.

Palate morphogenesis. I. Immunological and ultrastructural analyses of mouse palate

Midpalate was analyzed for the presence of nonmuscle contractile systems. The results indicate that increased amounts of actin and myosin are present in cells of regions 2 and 3. A localization of the contractile proteins in cellular projections (filopodia) and in the peripheral cytoplasm of the cell body was confirmed by indirect immunofluorescence studies, using antibodies directed against smooth muscle myosin and against skeletal muscle actin. Specificity of the immunofluorescence reactions was ascertained by immunoabsorption studies using purified myosin and actin. Electron microscopic observations of the mesenchymal cells in region 2 revealed 70A microfilaments along the cell periphery and packed in fliopodia-like projections which course between the cells. These cells, which surround a small ossification center, show no orientation, but extend up to the cranial base perichondrium and down into the shelf between the tongue side epithelium and the ossification center. The cells and projections are attached to each other by adherens and tight-like junctions, forming a putative cohesive contractile network. Putative contractile cells in region 3 are strikingly aligned perpendicular to the oral epithelium and extend one-third of the distance into the shelf. Projections from region 3 cells are contiguous with basement membrane material of the oral epithelium. Axonal bundles and single axons were commonly observed coursing through regions 2 and 3, often seen in close association with the mesenchymal cells. Both clear and dense-core vesicles were found in the axons and cells of these regions. The possible role of these putative nonmuscle contractile cells in palate morphogenesis is discussed.

Palate morphogenesis: II. Contraction of cytoplasmic processes in ATP-induced palate rotation in glycerinated mouse heads

It has been previously shown that non-muscle contractile system(s) exist in mouse palate mesenchyme underlying the palatal epithelium before shelf rotation. In order to obtain evidence that the non-muscle contractile system(s) function to elevate the palate, glycerinated heads have been incubated with ATP. It was shown that 5 mM ATP and a 30 min incubation at 25 degrees C stimulated palate rotation optimally. Elevation of the anterior end of the palate was nearly complete (PSI = 3.90, p less than 10(-6)). Although rotation of the posterior end was significant (p less than 0.02), movement was limited (PSI = 1.70). Light microscopy of the palate revealed that ATP caused a marked condensation of the cytoplasmic processes of the mesenchymal cells. The contraction of the processes of the mesenchymal cells induced by ATP increased roughly with increased palate shelf rotation and was greater at the peripheral than at the internal mesenchyme. Cytochalasin B pretreatment at 40 microM completely blocked the ATP-induced rotation at the anterior end. The effect of other nucleotides on palate rotation was tested. GTP caused a significant stimulation of anterior shelf rotation (p less than 0.005), which was less than ATP, while ADP and CTP were ineffective. Low temperature (6 degrees C) prevented the ATP-induced shelf rotation. These results suggest that the non-muscle contractile cells in the mesenchyme play a role in palate elevation and that contraction of the actomyosin containing microfilaments supplies the motive force.