Actin and myosin and cell movement

Approaches to embryonic neurodevelopment: from neural cell to neural tube formation through mathematical models

The development of the human central nervous system initiates in the early embryonic period until long after delivery. It has been shown that several neurological and neuropsychiatric diseases originate from prenatal incidents. Mathematical models offer a direct way to understand neurodevelopmental processes better. Mathematical modelling of neurodevelopment during the embryonic period is challenging in terms of how to 'Approach', how to initiate modelling and how to propose the appropriate equations that fit the underlying dynamics of neurodevelopment during the embryonic period while including the variety of elements that are built-in naturally during the process of neurodevelopment. It is imperative to answer where and how to start modelling; in other words, what is the appropriate 'Approach'? Therefore, one objective of this study was to tackle the mathematical issue broadly from different aspects and approaches. The approaches were divided into three embryonic categories: cell division, neural tube growth and neural plate growth. We concluded that the neural plate growth approach provides a suitable platform for simulation of brain formation/neurodevelopment compared to cell division and neural tube growth. We devised a novel equation and designed algorithms that include geometrical and topological algorithms that could fit most of the necessary elements of the neurodevelopmental process during the embryonic period. Hence, the proposed equations and defined mathematical structure would be a platform to generate an artificial neural network that autonomously grows and develops.

Systematic Investigation of Dose-Dependent Protein Thermal Stability Changes to Uncover the Mechanisms of the Pleiotropic Effects of Metformin

Metformin is a widely used drug to treat type II diabetes. Beyond lowering blood sugar, it has been reported to have pleiotropic effects such as suppressing cancer growth and attenuating cell oxidative stress and inflammation. However, the underlying mechanisms of these effects remain to be explored. Here, we systematically study the thermal stability changes of proteins in liver cells (HepG2) induced by a wide dosage range of metformin by using the proteome integral solubility alteration (PISA) assay. The current results demonstrate that, besides the most accepted target of metformin (complex I), low concentrations of metformin (such as 0.2 μM) stabilize the complex IV subunits, suggesting its important role in the sugar-lowering effect. Low-dose metformin also results in stability alterations of ribosomal proteins, correlating with its inhibitive effect on cell proliferation. We further find that low-concentration metformin impacts mitochondrial cargo and vesicle transport, while high-concentration metformin affects cell redox responses and cell membrane protein sorting. This study provides mechanistic insights into the molecular mechanisms of lowering blood sugar and the pleiotropic effects of metformin.

Prognostic and immune correlation analysis of mitochondrial autophagy and aging-related genes in lung adenocarcinoma

PURPOSE

Mitophagy and aging (MiAg) are very important pathophysiological mechanisms contributing to tumorigenesis. MiAg-related genes have prognostic value in lung adenocarcinoma (LUAD). However, prognostic, and immune correlation studies of MiAg-related genes in LUAD are lacking.

METHODS

MiAg differentially expressed genes (DEGs) in LUAD were obtained from public sequencing datasets. A prognostic model including MiAg DEGs was constructed according to patients divided into low- and high-risk groups. Gene Ontology, gene set enrichment analysis, gene set variation analysis, CIBERSORT immune infiltration analysis, and clinical characteristic correlation analyses were performed for functional annotation and correlation of MiAgs with prognosis in patients with LUAD.

RESULTS

Seven MiAg DEGs of LUAD were identified: CAV1, DSG2, DSP, MYH11, NME1, PAICS, PLOD2, and the expression levels of these genes were significantly correlated (P < 0.05). The RiskScore of the MiAg DEG prognostic model demonstrated high predictive ability of overall survival of patients diagnosed with LUAD. Patients with high and low MiAg phenotypic scores exhibited significant differences in the infiltration levels of eight types of immune cells (P < 0.05). The multi-factor DEG regression model showed higher efficacy in predicting 5-year survival than 3- and 1-year survival of patients with LUAD.

CONCLUSIONS

Seven MiAg-related genes were identified to be significantly associated with the prognosis of patients diagnosed with LUAD. Moreover, the identified MiAg DEGs might affect the immunotherapy strategy of patients with LUAD.

A high-throughput 3D cantilever array to model airway smooth muscle hypercontractility in asthma

Asthma is often characterized by tissue-level mechanical phenotypes that include remodeling of the airway and an increase in airway tightening, driven by the underlying smooth muscle. Existing therapies only provide symptom relief and do not improve the baseline narrowing of the airway or halt progression of the disease. To investigate such targeted therapeutics, there is a need for models that can recapitulate the 3D environment present in this tissue, provide phenotypic readouts of contractility, and be easily integrated into existing assay plate designs and laboratory automation used in drug discovery campaigns. To address this, we have developed DEFLCT, a high-throughput plate insert that can be paired with standard labware to easily generate high quantities of microscale tissues in vitro for screening applications. Using this platform, we exposed primary human airway smooth muscle cell-derived microtissues to a panel of six inflammatory cytokines present in the asthmatic niche, identifying TGF-β1 and IL-13 as inducers of a hypercontractile phenotype. RNAseq analysis further demonstrated enrichment of contractile and remodeling-relevant pathways in TGF-β1 and IL-13 treated tissues as well as pathways generally associated with asthma. Screening of 78 kinase inhibitors on TGF-β1 treated tissues suggests that inhibition of protein kinase C and mTOR/Akt signaling can prevent this hypercontractile phenotype from emerging, while direct inhibition of myosin light chain kinase does not. Taken together, these data establish a disease-relevant 3D tissue model for the asthmatic airway, which combines niche specific inflammatory cues and complex mechanical readouts that can be utilized in drug discovery efforts.

Nonlinear Mechanical Properties of Prestressed Branched Fibrous Networks

Random fiber networks constitute the solid skeleton of many biological materials such as the cytoskeleton of cells and extracellular matrix of soft tissues. These random networks show unique mechanical properties such as nonlinear shear strain-stiffening and strain softening when subjected to preextension and precompression, respectively. In this study, we perform numerical simulations to characterize the influence of axial prestress on the nonlinear mechanical response of random network structures as a function of their micromechanical and geometrical properties. We build our numerical network models using the microstructure of disordered hexagonal lattices and quantify their nonlinear shear response as a function of uniaxial prestress strain. We consider three different material models for individual fibers and fully characterize their influence on the mechanical response of prestressed networks. Moreover, we investigate both the influence of geometric disorder keeping the network connectivity constant and the influence of the randomness in the stiffness of individual fibers keeping their mean stiffness constant. The effects of network connectivity and bending rigidity of fibers are also determined. Several important conclusions are made, including that the tensile and compressive prestress strains, respectively, increase and decrease the initial network shear stiffness but have no effect on the maximal shear modulus. We discuss the findings in terms of microstructural properties such as the local strain energy distribution.

Non-Muscle Myosin II in Axonal Cell Biology: From the Growth Cone to the Axon Initial Segment

By binding to actin filaments, non-muscle myosin II (NMII) generates actomyosin networks that hold unique contractile properties. Their dynamic nature is essential for neuronal biology including the establishment of polarity, growth cone formation and motility, axon growth during development (and axon regeneration in the adult), radial and longitudinal axonal tension, and synapse formation and function. In this review, we discuss the current knowledge on the spatial distribution and function of the actomyosin cytoskeleton in different axonal compartments. We highlight some of the apparent contradictions and open questions in the field, including the role of NMII in the regulation of axon growth and regeneration, the possibility that NMII structural arrangement along the axon shaft may control both radial and longitudinal contractility, and the mechanism and functional purpose underlying NMII enrichment in the axon initial segment. With the advances in live cell imaging and super resolution microscopy, it is expected that in the near future the spatial distribution of NMII in the axon, and the mechanisms by which it participates in axonal biology will be further untangled.

Clinical and prognostic significance of MYH11 in lung cancer

Myosin heavy chain 11 (MYH11), encoded by the MYH11 gene, is a protein that participates in muscle contraction through the hydrolysis of adenosine triphosphate. Although previous studies have demonstrated that MYH11 gene expression levels are downregulated in several types of cancer, its expression levels have rarely been investigated in lung cancer. The present study aimed to explore the clinical significance and prognostic value of MYH11 expression levels in lung cancer and to further study the underlying molecular mechanisms of the function of this gene. The Oncomine database showed that the MYH11 expression levels were decreased in lung cancer compared with those noted in the normal lung tissue (P<0.05). Kaplan-Meier plotter results revealed that the decreased MYH11 expression levels were correlated with poor prognosis in lung cancer patients. Among the lung cancer cases with gene alteration of MYH11, mutation was the most common of all alteration types. Coexpedia and Metascape analyses revealed that the target genes were primarily enriched in ‘muscle contraction’, ‘contractile fiber part’, ‘actin cytoskeleton’ and the ‘adherens junction’. These results indicated that MYH11 is a potential novel drug target and prognostic indicator of lung cancer.

Interplay between Confinement and Drag Forces Determine the Fate of Amyloid Fibrils

The fine interplay between the simultaneous stretching and confinement of amyloid fibrils is probed by combining a microcapillary setup with atomic force microscopy. Single-molecule statistics reveal how the stretching of fibrils changed from force to confinement dominated at different length scales. System order, however, is solely ruled by confinement. Coarse-grained simulations support the results and display the potential to tailor system properties by tuning the two effects. These findings may further help shed light on in vivo amyloid fibril growth and transport in highly confined environments such as blood vessels.

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.

Inner membrane complex 1l protein of Plasmodium falciparum links membrane lipids with cytoskeletal element 'actin' and its associated motor 'myosin'

The inner membrane complex (IMC) is a defining feature of apicomplexans comprising of lipid and protein components involved in gliding motility and host cell invasion. Motility of Plasmodium parasites is accomplished by an actin and myosin based glideosome machinery situated between the parasite plasma membrane (PPM) and IMC. Here, we have studied in vivo expression and localization of a Plasmodium falciparum (Pf) IMC protein 'PfIMC1l' and characterized it functionally by using biochemical assays. We have identified cytoskeletal protein 'actin' and motor protein 'myosin' as novel binding partners of PfIMC1l, alongside its interaction with the lipids 'cholesterol' and 'phosphatidyl-inositol 4, 5 bisphosphate' (PIP2). While actin and myosin compete for interaction with PfIMC1l, actin and either of the lipids (cholesterol or PIP2) simultaneously bind PfIMC1l. Interestingly, PfIMC1l showed enhanced binding with actin in the presence of calcium ions, and displayed direct binding with calcium. Based on our in silico analysis and experimental data showing PfIMC1l-actin/myosin and PfIMC1l-lipid interactions, we propose that this protein may anchor the IMC membrane with the parasite gliding apparatus. Considering its binding with key proteins involved in motility viz. myosin and actin (with calcium dependence), we suggest that PfIMC1l may have a role in the locomotion of Plasmodium.

Long Noncoding RNA CRYBG3 Blocks Cytokinesis by Directly Binding G-Actin

The dynamic interchange between monomeric globular actin (G-actin) and polymeric filamentous actin filaments (F-actin) is fundamental and essential to many cellular processes, including cytokinesis and maintenance of genomic stability. Here, we report that the long noncoding RNA LNC CRYBG3 directly binds G-actin to inhibit its polymerization and formation of contractile rings, resulting in M-phase cell arrest. Knockdown of LNC CRYBG3 in tumor cells enhanced their malignant phenotypes. Nucleotide sequence 228-237 of the full-length LNC CRYBG3 and the ser14 domain of β-actin is essential for their interaction, and mutation of either of these sites abrogated binding of LNC CRYBG3 to G-actin. Binding of LNC CRYBG3 to G-actin blocked nuclear localization of MAL, which consequently kept serum response factor (SRF) away from the promoter region of several immediate early genes, including JUNB and Arp3, which are necessary for cellular proliferation, tumor growth, adhesion, movement, and metastasis. These findings reveal a novel lncRNA-actin-MAL-SRF pathway and highlight LNC CRYBG3 as a means to block cytokinesis and to treat cancer by targeting the actin cytoskeleton.Significance: Identification of the long noncoding RNA LNC CRYBG3 as a mediator of microfilament disorganization marks it as a novel therapeutic antitumor strategy. Cancer Res; 78(16); 4563-72. ©2018 AACR.

Calcium inhibition as an intracellular signal for actin-myosin interaction

Intracellular signaling pathways include both the activation and the inhibition of biological processes. The activation of Ca2+ regulation of actin-myosin interactions was examined first, whereas it took 20 years for the author to clarify the inhibitory mode by using Physarum polycephalum, a lower eukaryote. This review describes the investigation of the inhibitory mode since 1980. The inhibitory effect of Ca2+ on myosin was detected chemically by ATPase assays and mechanically by in vitro motility assays. The Ca2+-binding ability of Physarum myosin is as high as that of scallop myosin. Ca2+ inhibits Physarum myosin, whereas it activates scallop myosin. We cloned cDNA of the myosin heavy chain and light chains to express a hybrid of Physarum and scallop myosin, and found that the Ca-binding light chain (CaLc), which belongs to an alkali light chain class, plays a major role in Ca inhibition. The role of CaLc was confirmed by mutating its EF-hand, Ca-binding structure and expressing Physarum myosin as a recombinant protein. Thus, the data obtained by classical protein purification were confirmed by the results obtained with the modern recombinant techniques. However, there are some discrepancies that remain to be solved as described in Section XII.

Continuous-Wave Stimulated Emission Depletion Microscope for Imaging Actin Cytoskeleton in Fixed and Live Cells

Stimulated emission depletion (STED) microscopy provides a new opportunity to study fine sub-cellular structures and highly dynamic cellular processes, which are challenging to observe using conventional optical microscopy. Using actin as an example, we explored the feasibility of using a continuous wave (CW)-STED microscope to study the fine structure and dynamics in fixed and live cells. Actin plays an important role in cellular processes, whose functioning involves dynamic formation and reorganization of fine structures of actin filaments. Frequently used confocal fluorescence and STED microscopy dyes were employed to image fixed PC-12 cells (dyed with phalloidin- fluorescein isothiocyante) and live rat chondrosarcoma cells (RCS) transfected with actin-green fluorescent protein (GFP). Compared to conventional confocal fluorescence microscopy, CW-STED microscopy shows improved spatial resolution in both fixed and live cells. We were able to monitor cell morphology changes continuously; however, the number of repetitive analyses were limited primarily by the dyes used in these experiments and could be improved with the use of dyes less susceptible to photobleaching. In conclusion, CW-STED may disclose new information for biological systems with a proper characteristic length scale. The challenges of using CW-STED microscopy to study cell structures are discussed.

The selective role of myosin VI in lymphoid leukemia cell migration

Several myosin isotypes are discussed to be involved in the migration of various cells ranging from tumor cells to leukocytes. We investigated the involvement of myosins II and VI in the lymphoid leukemia cells lines Jurkat, NB4, Dohh-2, and Molt-4 by a three-dimensional, collagen-based migration assay. Down-regulation of myosin VI by siRNA significantly reduced the migratory activity of all cells, whereas the pharmacological inhibition of non-muscle myosin II using blebbistatin had only marginal influence. Therefore, in contrast to differentiated leukocytes and cells from solid tumors, myosin VI plays a crucial role in the migration of leukemic cells.

Clinical diagnosis in patients with smooth muscle antibodies. A study of a one-year material

Out of 17 109 sera tested for autoantibodies by indirect immunofluorescence, 236 contained smooth muscle antibodies (SMA) with a titre of greater than or equal to 1/25. The majority of these sera, from 190 patients, reacted both with smooth muscle and renal glomeruli and the specificity of these SMA is against actin. 91% of high-titred sera (greater than or equal to 1/100) with IgG antibodies giving this staining pattern were derived from patients with chronic inflammatory liver disease, mainly chronic active hepatitis. In the group with a titre of 1/25, non-liver diseases such as joint diseases were more common and liver conditions occurred only in 55%. Sera with SMA of IgM class weremostly derived from patients with acute viral hepatitis.

Actin: its cumbersome pilgrimage through cellular compartments

In this article, we follow the history of one of the most abundant, most intensely studied proteins of the eukaryotic cells: actin. We report on hallmarks of its discovery, its structural and functional characterization and localization over time, and point to present days’ knowledge on its position as a member of a large family. We focus on the rather puzzling number of diverse functions as proposed for actin as a dual compartment protein. Finally, we venture on some speculations as to its origin.

Myosins: tails (and heads) of functional diversity

The myosin family of actin filament-based molecular motors consists of at least 20 structurally and functionally distinct classes. The human genome contains nearly 40 myosin genes, encoding 12 of these classes. Myosins have been implicated in a variety of intracellular functions, including cell migration and adhesion; intracellular transport and localization of organelles and macromolecules; signal transduction; and tumor suppression. In this review, recent insights into the remarkable diversity in the mechanochemical and functional properties associated with this family of molecular motors are discussed.

Human cytoplasmic actin proteins are encoded by a multigene family

We characterized nine human actin genes that we isolated (Engel et al., Proc. Natl. Acad. Sci. U.S.A. 78:4674-4678, 1981) from a library of cloned human DNA. Measurements of the thermal stability of hybrids formed between each cloned actin gene and alpha-, beta-, and gamma-actin mRNA demonstrated that only one of the clones is most homologous to sarcomeric actin mRNA, whereas the remaining eight clones are most homologous to cytoplasmic actin mRNA. By the following criteria we show that these nine clones represent nine different actin gene loci rather than different alleles or different parts of a single gene: (i) the restriction enzyme maps of the coding regions are dissimilar; (ii) each clone contains sufficient coding region to encode all or most of an entire actin gene; and (iii) each clone contains sequences homologous to both the 5' and 3' ends of the coding region of a cloned chicken beta-actin cDNA. We conclude, therefore, that the human cytoplasmic actin proteins are encoded by a multigene family.

Human cytoplasmic actin proteins are encoded by a multigene family

We characterized nine human actin genes that we isolated (Engel et al., Proc. Natl. Acad. Sci. U.S.A. 78:4674-4678, 1981) from a library of cloned human DNA. Measurements of the thermal stability of hybrids formed between each cloned actin gene and alpha-, beta-, and gamma-actin mRNA demonstrated that only one of the clones is most homologous to sarcomeric actin mRNA, whereas the remaining eight clones are most homologous to cytoplasmic actin mRNA. By the following criteria we show that these nine clones represent nine different actin gene loci rather than different alleles or different parts of a single gene: (i) the restriction enzyme maps of the coding regions are dissimilar; (ii) each clone contains sufficient coding region to encode all or most of an entire actin gene; and (iii) each clone contains sequences homologous to both the 5' and 3' ends of the coding region of a cloned chicken beta-actin cDNA. We conclude, therefore, that the human cytoplasmic actin proteins are encoded by a multigene family.

Ectoplasmic specializations in the Sertoli cell: new vistas based on genetic defects and testicular toxicology

Abstract The ectoplasmic specialization is a unique junctional complex formed in two cortical areas of the Sertoli cell in the mammalian testis: one near the base of the seminiferous epithelium forming the blood-testis barrier, and the other near the lumen of the seminiferous tubule embracing the acrosome region of the developing spermatids. The specialization consists of the Sertoli cell plasma membrane, a subsurface cistern of the endoplasmic reticulum and a layer of closely packed actin filaments that is sandwiched between the plasma membrane and the subsurface cistern. No functions of the specializations other than the blood-testis barrier have been established. However, over the past decade, knowledge about the ectoplasmic specialization has been steadily accumulating and, in particular, there has been a tremendous increase in knowledge based on molecular biological approaches to specialization-associated proteins, including tight junction-associated proteins, based on phenotype analyses of gene-knockout mice and mutant animals, and based on analyses of the effects of exogenous estrogens, so-called endocrine disruptors. Progress in studies on the ectoplasmic specialization will facilitate the elucidation of numerous important questions regarding spermatogenesis, including the pathogenesis of azoospermia and the mechanisms of action of endocrine disruptors.

Effect of telmisartan on angiotensin II-mediated collagen gel contraction by adult rat cardiac fibroblasts

The possible contributions of the angiotensin receptor subtypes 1 and 2 on the angiotensin II-induced collagen gel contraction by adult rat cardiac fibroblasts were studied using the specific angiotensin receptor type 1 and 2 antagonists telmisartan and P-186, respectively. Cardiac fibroblasts (from normal male adult rats) from passage 2 were cultured to confluency and added to a hydrated collagen gel, with or without angiotensin II, angiotensin II plus telmisartan, or angiotensin II plus P-186 in Dulbecco's Modified Eagle's Medium containing 5% fetal bovine serum for 1, 2, or 3 days. Control gels containing adult rat cardiac fibroblasts showed a significant amount of contraction after 3 days of incubation, causing a contraction to 67.9 +/- 7.1% of the area after 1 day. Angiotensin II (10(-7) M) stimulated (p < or = 0.05) the contraction of collagen mediated by cardiac fibroblasts after 1, 2, or 3 days. Telmisartan (10(-7) M) completely blocked the angiotensin II-induced collagen contraction by cardiac fibroblasts. P-186 (10(-7) M) had no effect on the angiotensin II-induced collagen contraction by cardiac fibroblasts. Addition of telmisartan and P-186 alone did not affect the collagen gel contraction by cardiac fibroblasts. Our data demonstrate that the effects of angiotensin II on the collagen gel contraction by adult rat cardiac fibroblasts are angiotensin II type 1 receptor mediated because they were abolished by the specific angiotensin II type 1 receptor antagonist telmisartan but not by the specific angiotensin II type 2 receptor antagonist P-186.

Polymerization, three-dimensional structure and mechanical properties of Ddictyostelium versus rabbit muscle actin filaments

To assess more systematically functional differences among non-muscle and muscle actins and the effect of specific mutations on their function, we compared actin from Dictyostelium discoideum (D-actin) with actin from rabbit skeletal muscle (R-actin) with respect to the formation of filaments, their three-dimensional structure and mechanical properties. With Mg(2+) occupying the single high-affinity divalent cation-binding site, the course of polymerization is very similar for the two types of actin. In contrast, when Ca(2+ )is bound, D-actin exhibits a significantly longer lag phase at the onset of polymerization than R-actin. Crossover spacing and helical screw angle of negatively stained filaments are similar for D and R-F-actin filaments, irrespective of the tightly bound divalent cation. However, three-dimensional helical reconstructions reveal that the intersubunit contacts along the two long-pitch helical strands of D-(Ca)F-actin filaments are more tenuous compared to those in R-(Ca)F-actin filaments. D-(Mg)F-actin filaments on the other hand exhibit more massive contacts between the two long-pitch helical strands than R-(Mg)F-actin filaments. Moreover, in contrast to the structure of R-F-actin filaments which is not significantly modulated by the divalent cation, the intersubunit contacts both along and between the two long-pitch helical strands are weaker in D-(Ca)F-actin compared to D-(Mg)F-actin filaments. Consistent with these structural differences, D-(Ca)F-actin filaments were significantly more flexible than D-(Mg)F-actin. Taken together, this work documents that despite being highly conserved, muscle and non-muscle actins exhibit subtle differences in terms of their polymerization behavior, and the three-dimensional structure and mechanical properties of their F-actin filaments which, in turn, may account for their functional diversity.

Recurring views on the structure and function of the cytoskeleton: a 300-year epic

Some unnoticed or seldom remembered precedents of current views on biological motion and its structural bases are briefly outlined, followed by a concise recapitulation of how the present theory has been constructed in the last few decades. It is shown that the evolution of the concept of fibers as main constituents of living matter led to hypothesizing microscopic structures closely resembling microtubules in the 18th century. At the beginning of this period, fibers sliding over each other and driven by interposed moving elements were envisioned as the cause of muscle contraction. In the following century, an account of the mechanism of myofibril contraction visualized longitudinal displacements of myosin-containing submicroscopic rodlets. The existence of fibrils in the protoplasm of non-muscle cells, a subject of long debate in the second half of the 19th century, was virtually discarded as irrelevant or fallacious 100 years ago. The issue resurfaced in the early 1930s as a theoretical notion--the cytosquelette--nearly two decades before intracellular filamentous structures were first observed with electron microscopy. The role originally assumed for such fibrils as signal conductors is nowadays being reappraised, although under new interpretations with a much wider significance including modulation of gene expression, morphogenesis, and even consciousness. Since all of the above ancestral conceptions were eventually abandoned, the corresponding current views are, to a certain extent, recurrent.

Calcium regulation of the actin-myosin interaction of Physarum polycephalum

Plasmodia of Physarum polycephalum show vigorous cytoplasmic streaming, the motive force of which is supported by the actin-myosin interaction. Calcium is not required for the interaction but inhibits it. This calcium inhibition, a regulatory mode first discovered in Physarum, is the overwhelming mode of regulation of cytoplasmic streaming of plant cells and lower eukaryotes, and it is diametrically opposite to calcium activation of the interaction found in muscle and nonmuscle cells of the animal kingdom. Myosin, myosin II in myosin superfamily, is the most important protein for Ca2+ action. Its essential light chain, called calcium-binding light chain, is the sole protein that binds Ca2+. Although phosphorylation and dephosphorylation of myosin modify its properties, regulation of physiological significance is shown to be Ca-binding to myosin. The actin-binding protein of Physarum amplifies calcium inhibition when Ca2+ binds to calmodulin and other calcium-binding proteins. This review also includes characterization of this and other calcium-binding proteins of Physarum.

Troponin I is present in human cartilage and inhibits angiogenesis

Cartilage is an avascular and relatively tumor-resistant tissue. Work from a number of laboratories, including our own, has demonstrated that cartilage is an enriched source of endogenous inhibitors of angiogenesis. In the course of a study designed to identify novel cartilage-derived inhibitors of new capillary growth, we have purified an inhibitory protein that was identified by peptide microsequencing and protein database analysis as troponin I (TnI). TnI is a subunit of the troponin complex (troponin-C and troponin-T being the other two), which, along with tropomyosin, is responsible for the calcium-dependent regulation of striated muscle contraction; independently, TnI is capable of inhibiting actomyosin ATPase. Because troponin has never previously been reported to be present in cartilage, we have cloned and expressed the cDNA of human cartilage TnI, purified this protein to apparent homogeneity, and demonstrated that it is a potent and specific inhibitor of angiogenesis in vivo and in vitro, as well as of tumor metastasis in vivo.

Actin gene expression in developing sea urchin embryos

We show that the synthesis of actin is regulated developmentally during early sea urchin embryogenesis and that the level of synthesis of this protein parallels the steady-state amounts of the actin messenger ribonucleic acids (RNA). An in vitro translation and RNA blotting analysis of embryo RNA from several stages of early development indicated that during the first 8 h after fertilization there was a low and relatively constant level of actin messenger RNA in the embryo. Between 8 and 13 h of development, the amount of actin messenger RNA began to increase both in the cytoplasm and on polysomes, and by 18 h the amounts of actin message per embryo had risen between approximately 10- and 25-fold in the cytoplasm and between 15- and 40-fold on polysomes. Two size classes of actin messenger RNA (2.2 and 1.8 kilobases) were identified in unfertilized eggs and in all of the developmental stages examined. The amount of each actin message class increased over a similar time interval during early development. However, the amounts of these size classes in the cytoplasm relative to each other shifted between the earliest stages examined (2 to 5 h) and the hatching blastula stage (18 h), with the ratio of the 1.8-kilobase actin messenger RNA to the 2.2-kilobase actin messenger RNA increasing almost threefold during this period.

Abundance of alpha 1(I) and alpha 1(III) procollagen and p21 mRNAs in fibroblasts cultured from fetal and postnatal dermis

The steady-state abundance of alpha 1(I) and alpha 1(III) procollagen mRNAs, p21Sdi1 mRNA, and beta-actin mRNA was determined in 29 skin fibroblast lines established from fetal, young and old donors. Donor ages ranged from 12 gestational weeks to nonagenarian. Adult donors were members of the Baltimore Longitudinal Study of Aging. The abundance of alpha 1(I) procollagen mRNA was decreased in cell lines from both young and old donors compared with fetal lines. Additionally, one alpha 1(I) transcript observed in the fetal lines was not detected in postnatal lines. The abundance of alpha 1(III) procollagen mRNA was decreased in postnatal lines from old donors compared with fetal lines. The abundance of beta-actin mRNA was lower in postnatal lines from both young and old donors compared to fetal lines. These results suggest that cultures of fetal skin fibroblasts exhibit a greater capacity for synthesis of procollagens and beta-actin than postnatal lines. In contrast, the abundance of p21Sdi1 mRNA was elevated in lines established from postnatal donors. These results are consistent with developmental changes in amounts of procollagen, beta-actin and p21.

Three main patterns in the expression of six actin genes in the plerocercoid and adult Diphyllobothrium dendriticum tapeworm (Cestoda)

The expression of six actin genes was examined in adult and plerocercoid Diphyllobothrium dendriticum tapeworms using in situ hybridization. On the basis of their structures, these genes are divided into three groups, the cestoda-I, -II and -III actins. Current studies show that the expression of actins belonging to different groups vary to a great extent. The three cestoda-I actins are expressed primarily in muscle cells of both adult and plerocercoid tapeworms, the expression being restricted to fewer cells in the plerocercoid larva. The two cestoda-II actins are cytoplasmic actin isoforms, expressed in a variety of cells, i.e. in cells dividing, differentiating and migrating. Expression of the cestoda-III actin gene is detected merely in the peripheral part of the outer parenchyma, mainly in the tegument cell bodies. This pattern is very weak in plerocercoids. The results indicate that actins also in D. dendriticum can be divided into cytoplasmic and muscle-specific isoforms. In this organism, one major pattern of muscle actin gene expression (cestoda-I) and two major patterns of non-muscle actin gene expression (cestoda-II and -III) were found.

Roles of microfilaments and intermediate filaments in adrenal steroidogenesis

The problem for the steroidogenic cell if it is to accelerate steroid synthesis in response to trophic stimulation, consists in moving cholesterol from the sites of synthesis and storage to mitochondria at an accelerated rate. The most intensely studied situation is that in which the sterol is stored as ester in lipid droplets. Cholesterol ester must be de-esterified and transported to mitochondria where steroid synthesis begins. Since droplets and mitochondria are now known to be attached to intermediate filaments and since these structures are not contractile, it appears to be necessary to invoke the actions of other cytoskeletal elements. Actin microfilaments are involved in cholesterol transport so that it is tempting to propose that the contractile properties of actomyosin are used in this process. It is known that an energy-dependent contractile process involving actin is capable of disrupting intermediate filaments. Since the intermediate filaments appear to act by keeping lipid droplets and mitochondria apart, disruption of the filaments accompanied by a contractile process would be expected to allow these two structures to come together. This would open the way for the transfer of cholesterol to the steroidogenic pathway. This should be regarded as a first step. The events necessary for entry of cholesterol from droplets into the mitochondria remain to be clarified. In addition, the transport process for newly synthesized cholesterol that is not stored in droplets, is still not understood. At least four protein kinase enzymes have been identified in the cytoskeletons of adrenal cells, namely, Ca2+/calmodulin-dependent kinase, protein kinase (Ca2+ and phospholipid-dependent), myosin light chain kinase, and protein kinase A (cyclic AMP-dependent). The Ca2+/calmodulin kinase promotes transport of cholesterol to mitochondria and does so under conditions in which phosphorylation of vimentin and myosin light chain occurs. Phosphorylation of vimentin results in disruption of intermediate filaments while phosphorylation of light chain promotes contraction of the actomyosin ring. It now appears that intermediate filaments are cross-linked by actin filaments so that such contraction would be expected to produce significant structural changes in the cytoskeleton and the attached organelles. Although the details of the changes taking place in the organ in vivo are not known, the potential for interaction between droplets and mitochondria as the result of these changes in intermediate filaments and actomyosin, is clear. Protein kinase C is activated by ACTH and cyclic AMP, although this activation does not appear to be directly involved in the regulation of steroid synthesis. Nevertheless, vimentin is a substrate for this enzyme, and changes in the organisation of vimentin filaments and the attached organelles under the influence of protein kinase C have been reported in other cells. Presumably these changes represent part of the response to ACTH because when protein kinase C is activated by phorbol ester, the cytoskeletal changes necessary for rounding up take place but such changes are not accompanied by increased steroid synthesis. Protein kinase A causes rounding of adrenal cells. and cytoskeletons. This kinase also causes increased cholesterol transport and, hence, stimulation of steroid synthesis. The enzyme also causes phosphorylation of vimentin but with a different cytoskeletal reorganisation from that seen with the other three kinase enzymes. Clearly phosphorylation plays a major role in these responses. Phosphorylation alters the morphology and the functions of the cytoskeleton and this, in turn, is associated with accelerated cholesterol transport. It is now necessary to define the details of the specific phosphorylation reactions that occur during the response to ACTH, that is, which amino acids are phosphorylated and to what extent by each of the kinase enzymes.

Slow troponin C gene expression in chicken heart and liver is regulated by similar enhancers

Two isoforms of troponin C (TnC) are encoded by distinct single copy genes. Expression of fast TnC is restricted to the skeletal muscle, whereas the slow isoform is expressed in both skeletal and cardiac muscle. Chicken slow TnC (cTnC) gene is also expressed in some non-muscle tissues like the liver and the brain. Expression of cTnC gene is regulated by two distinct enhancers in cardiac and skeletal muscles. The cardiac specific enhancer is located in the immediate 5' flanking region (bp-124 to -79) of the murine cTnC gene whereas the skeletal enhancer is located within the first intron (bp 997 to 1141). In the present study we have examined how cTnC gene expression is regulated in the chicken liver. Transient transfection of liver cells with CTnC-CAT reporter constructs containing various regions of the murine cTnC gene showed that its expression in chicken liver is regulated by the cardiac specific enhancer. Furthermore, electrophoretic mobility shift assays using synthetic oligonucleotides corresponding to both CEF-1 and CEF-2 regions of the murine cardiac enhancer revealed formation of specific DNA-protein complexes. Ultraviolet light induced covalent linking of nuclear proteins to CEF-1 and CEF-2 oligomers were used to examine the nature of the cardiac enhancer binding polypeptides; one polypeptide of 48 kDa appeared to bind to both CEF-1 and CEF-2 sequences.

Non-muscle myosin as target antigen for human autoantibodies in patients with hepatitis C virus-associated chronic liver diseases

Three patients with hepatitis C virus (HCV)-related chronic liver disease were shown to have autoantibodies strongly reacting with cytoskeletal fibres of non-muscle cells. The heavy chain of non-muscle myosin microfilament was the main target for those autoantibodies, as determined by (i) cell and tissue immunofluorescence studies showing colocalization with an anti-myosin antibody prototype; (ii) primary reactivity in immunoblotting with a 200-kD protein, using either MOLT-4 cells, human platelets, or affinity-purified non-muscle myosin as antigen extract; and (iii) immunoblotting of similar immunoreactive fragments in papain-digested MOLT-4 cell extracts, by using those human sera and antibody prototype. Autoantibodies to non-muscle myosin heavy chain were not previously reported in patients with chronic liver diseases, especially in those associated with HCV infection.

ERM family members as molecular linkers between the cell surface glycoprotein CD44 and actin-based cytoskeletons

The ERM family members, ezrin, radixin, and moesin, localizing just beneath the plasma membranes, are thought to be involved in the actin filament/plasma membrane association. To identify the integral membrane protein directly associated with ERM family members, we performed immunoprecipitation studies using antimoesin mAb and cultured baby hamster kidney (BHK) cells metabolically labeled with [35S]methionine or surface-labeled with biotin. The results indicated that moesin is directly associated with a 140-kD integral membrane protein. Using BHK cells as antigens, we obtained a mAb that recognized the 140-kD membrane protein. We next cloned a cDNA encoding the 140-kD membrane protein and identified it as CD44, a broadly distributed cell surface glycoprotein. Immunoprecipitation with various anti-CD44 mAbs showed that ezrin and radixin, as well as moesin, are associated with CD44, not only in BHK cells, but also in mouse L fibroblasts. Furthermore, immunofluorescence microscopy revealed that in both BHK and L cells, the Triton X-100-insoluble CD44 is precisely colocalized with ERM family members. We concluded that ERM family members work as molecular linkers between the cytoplasmic domain of CD44 and actin-based cytoskeletons.

A novel actin cDNA is expressed in the neurons of Aplysia californica

A novel actin cDNA has been isolated from an abdominal ganglion cDNA library of Aplysia californica by differential screening. This cDNA of 1596 nucleotides in length encodes a putative actin protein of 41.8 kDa. This protein shows 95.2% identity with another Aplysia actin gene previously shown to be expressed in the muscular sheath of the ganglion (DesGroseillers et al. (1990) Nucleic Acids Res. 18, 3654). However, the 5' and 3' untranslated regions of these cDNAs are completely different. PCR experiments performed with mRNA isolated from dissected neurons, ovotestis or kidney reveal that the gene is expressed in the neurons of the ganglia and in other tissues as well. Southern blot analysis reveals that the neuronal actin gene is a member of a large gene family.

Cytoskeleton and motility: an immunohistological and computer simulation analysis of melanocytic skin tumors

Tumor cell motility and tumor cell proliferation are supposed to be essential for tumor invasion. The cytoskeleton, which consists of different components, is considered to be important for maintaining cell shape and facilitating cell movement. Numerous data are available about tumor cell motility in vitro, but the behavior of tumor cells in vivo is as yet poorly understood. In the present study, estimates of tumor cell motility and proliferation were statistically derived from morphological tumor patterns in human melanocytic skin tumors, and their relationship to expression of certain cytoskeletal components was evaluated. Overexpression of vimentin within tumor cells correlated with low actual tumor cell motility and proliferation, indicating a structurally stabilizing function of these filaments. An overexpression of actin was found within tumor cells of high motility and proliferation, suggesting the contribution of cytocontractile elements to active tumor cell locomotion in situ. Concerning the cytoskeleton of the stromal cells, expression of actin, myosin and tubulin correlated with a high number of motile tumor cells and high mitotic counts. Thus increased tumor cell motility seems to be associated with cytoskeletal changes not only of the tumor cells themselves but also of the surrounding stromal cells.

Contraction of reconstituted Dictyostelium cytoskeletons: an apparent role for higher order associations among myosin filaments

A large number of cellular functions require assembly of actin and myosin and coordinated interactions between the resulting filaments. To better understand the structure and function of one such contractile assembly, we have begun fractionation and reconstitution studies of Dictyostelium cytoskeletons. Isolated cytoskeletons rapidly contracted when mixed with Mg-ATP, and myosin II was essential for this since myosin-depleted (stripped) cytoskeletons failed to contract. Dictyostelium, Acanthamoeba, or skeletal muscle myosins bound to stripped cytoskeletons with equal efficiency, and the Mg-ATPase of all three myosins was stimulated by the cytoskeleton-associated actin. Near neutral pH, however, only the homologous system reconstituted with Dictyostelium myosin contracted, despite the fact that under the same conditions all three myosins bound to myosin-depleted (ghost) muscle myofibrils and restored contractility. Individual Dictyostelium myosin thick filaments have a strong tendency to aggregate and associate end-to-end, and this may be important for functional contraction of cytoskeletons. This suggestion is supported by the observation that under conditions where individual Acanthamoeba myosin filaments aggregated, reconstituted cytoskeletons contracted. None of the solution conditions tested caused rabbit muscle myosin filaments to aggregate or to contract cytoskeletons. Thus higher order associations among individual myosin filaments may be essential for some types of cell motility.

Assembly dynamics of actin in adherent human neutrophils

We have extended our previous studies of adherent neutrophils and compared actin depolymerization and intracellular calcium changes induced by adherence to laminin and fibronectin. In order to accurately assess cellular actin changes, F-actin depolymerization in the cell lysates must be inhibited. We found that phalloidin or 3.7% formaldehyde treatment effectively inhibited the depolymerization of F-actin fragments following cell lysis. Formaldehyde and phalloidin treatment reduced G-actin levels 75-80% in suspended cells, 35-73% in cells adherent for 1 min, and about 50% for cells adherent for 3 min. When the actin was fixed, there were highly significant differences in G-actin levels between the suspended and adherent cells as compared with unfixed cells. Adhesion to both laminin and fibronectin initiated a rapid rise in G-actin with a corresponding decrease in F-actin. However, the changes were more pronounced in cells adherent to laminin. The peak of depolymerization occurred by 1 min and, thereafter, G-actin decreased and F-actin increased reaching a steady state at 5 min. Adhesion to both laminin- and fibronectin-coated surfaces was accompanied by an increase of [Ca2+]i with a peak at 3 min, followed by a decrease from 3-5 min and a steady state attained between 5 and 10 min. The rise of [Ca2+]i in laminin-adherent cells was about twice that in fibronectin-adherent cells at 3 min (P < 0.02). Pertussis toxin, H-7, and staurosporin treatments did not alter the dynamic changes of actin in adherent cells, suggesting that these metabolic events are transduced by a G-protein and Protein Kinase C independent mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of near-UV radiation on elasmobranch lens cytoskeletal actin

The role of near-UV radiation as a cytoskeletal actin-damaging agent was investigated. Two procedures were used to analyse fresh smooth dogfish (Mustelus canis) eye lenses that were incubated for up to 22 hr in vitro, with elasmobranch Ringer's medium, and with or without exposure to a near-UV lamp (emission principally at 365 nm; irradiance of 2.5 mW cm-2). These were observed histologically using phalloidin-rhodamine specific staining and by transmission electron microscopy. In addition, solutions of purified polymerized rabbit muscle actin were exposed to the same UV conditions and depolymerization was assayed by ultracentrifugation and high-pressure liquid chromatography. While the two actins studied do differ very slightly in some amino acid sequences, they would react physically nearly identically. The results showed that dogfish lenses developed superficial opacities due to near-UV exposure. Whole mounts of lens epithelium exhibited breakdown of actin filaments in the basal region of the cells within 18 hr of UV exposure. TEM confirmed the breakdown of actin filaments due to UV exposure. SDS-PAGE and immunoblotting positively identified actin in these cells. Direct exposure of purified polymerized muscle actin in polymerizing buffer led to an increase in actin monomer of approximately 25% in the UV-exposed solutions within 3-18 hr, whether assayed by ultracentrifugation or HPLC. The above indicates that elasmobranch lens epithelial cells contain UV-labile actin filaments, and that near-UV radiation, as is present in the sunlit environment, can break down the actin structure in these cells. Furthermore, breakdown of purified polymerized muscle actin does occur due to near-UV light exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluorescent staining of the actin cytoskeleton in human lymphocytes, monocytes and polymorphonuclear cells using a DNAse 1/anti-DNAse 1 immunoglobulin fluorescein conjugated system

The actin associated with membrane-enriched extracts of leukocytes can be quantitated by DNAse 1 inhibition. Using this assay, we previously demonstrated that the actin level in monocytes was significantly higher than that in polymorphonuclear, T and B cells respectively. However, the extracellular location of the actin fraction detected by DNAse 1 inhibition (monomeric "G") remained unclear. This study using the DNAse 1/anti DNAse 1 immunoglobulin fluorescein conjugated system demonstrated that G-actin is present primarily in the cortical cell cytoplasm of leukocytes, in confirmation of our previous biochemical findings. Since the solubilized G-actin activities of membrane-rich lymphoid cell fractions, measured by DNAse 1 inhibition, are a reflection of the migratory potential, this immunofluorescent system may permit identification of the leukocytic cell subpopulations that have a potential for active circulation.

Reconstitution of the filamentous backbone of lens beaded-chain filaments from a purified 49kD polypeptide

The beaded-chain filaments unique to the fiber cells of the crystalline lens are composed of a linear array of spheroidal particles which appear to be connected by a filamentous backbone. In order to determine the existence of the putative backbone and to characterize its constituents, one of the major proteins associated with beaded-chains in the chicken lens was investigated. 49kD was isolated in an enriched fraction derived from the 8M urea extract of the lens cell water-insoluble residue. The polypeptide (which exists in several charge isoforms, the major at pI 5.2) was purified sequentially by gel filtration on Sephacryl S-200, hydrophobic interaction chromatography on phenyl-Sepharose, and anionic exchange chromatography on Mono Q, all under denaturing conditions. Immunoblot analyses established that 49kD was immunologically distinct from vimentin, actin, and tubulin/MAPs (representing the three classes of cytoplasmic filaments), as well as from the crystallins. Amino acid analyses demonstrated compositional differences for 49kD compared with lens actin and vimentin, and one- and two-dimensional peptide mapping of 49kD and vimentin revealed no homology. Electron microscopy demonstrated that short, contorted filaments were produced upon removal of purified 49kD from urea to low-salt buffers. In the presence of physiological salt concentrations 49kD assembled into extensive 4-6nm diameter, straight filaments similar to the backbone seen in native beaded-chain filaments, but morphologically distinct from the other cytoplasmic filament classes.

Stopped-flow measurement of cytoskeletal contraction: Dictyostelium myosin II is specifically required for contraction of amoeba cytoskeletons

Cytoskeletons provide valuable information on the composition and organization of the cell's contractile machinery, and in many cases these cell models retain the ability to contract. To quantitate contraction rates, we developed a novel stopped-flow assay permitting simultaneous analysis of thousands of Dictyostelium cytoskeletons within milliseconds of mixing with Mg-ATP. Cytoskeletons were placed in one syringe of the stopped flow apparatus and the appropriate buffer was placed in the second syringe. Mixing with Mg-ATP caused an immediate increase in the absorbance at 310 nm. Rapid fixation of the cytoskeletons during the reaction confirmed that this change in absorbance was highly correlated with contraction of the cytoskeletons. This spectroscopic change was used to measure the effects of temperature, pH, ionic strength, and nucleotides on contraction rate. Treatment with high salt and ATP removed most of the myosin, some actin, and small amounts of minor proteins. These extracted cytoskeletons lost the ability to contract, but after the addition of purified Dictyostelium myosin they regained full function. In contrast, rabbit skeletal muscle myosin was unable to restore contractility, even though it bound to the extracted cytoskeletons. Cytoskeletons prepared from a myosin-null mutant did not contract. Upon the addition of purified ameba myosin, however, they became contractile. These results suggest that filamentous Dictyostelium myosin II is essential for contraction, and that the actin cytoskeleton and associated proteins retain their functional organization in the absence of myosin.