Asymmetry in the distribution of free versus cytoskeletal myosin II in locomoting microcapillary endothelial cells

Semi-retentive cytoskeletal fractionation (SERCYF): A novel method for the biochemical analysis of the organization of microtubule and actin cytoskeleton networks

A variety of biochemical fractionation methods are available for the quantification of cytoskeletal components. However, each method is designed to target only one cytoskeletal network, either the microtubule (MT) or actin cytoskeleton, and non-targeted cytoskeletal networks are ignored. Considering the importance of MT-actin crosstalk, the organization of both the targeted and non-targeted cytoskeletal networks must be retained intact during fractionation for the accurate analysis of cytoskeletal organization. In this study, we reveal that existing fractionation methods, represented by the MT-sedimentation-method for MTs and the Triton X-100 solubility assay-method for actin cytoskeletons, disrupt the organizations of the non-targeted cytoskeletons. We demonstrate a novel fractionation method for the accurate analysis of the cytoskeletal organizations using a taxol-containing PEM-based permeabilization buffer, which we name "semi-retentive cytoskeletal fractionation (SERCYF)-method". The organizations of both MTs and actin cytoskeletons were retained intact even after permeabilization with this buffer. By using the SERCYF-method, we analyzed the effects of nocodazole on the cytoskeletal organizations biochemically and showed promotion of the actin cytoskeletal organization by MT depolymerization.

Nonmuscle myosin II folds into a 10S form via two portions of tail for dynamic subcellular localization

Nonmuscle myosin II forms a folded conformation (10S form) in the inactivated state; however, the physiological importance of the 10S form is still unclear. To investigate the role of 10S form, we generated a chimeric mutant of nonmuscle myosin IIB (IIB-SK1·2), in which S1462-R1490 and L1551-E1577 were replaced with the corresponding portions of skeletal muscle myosin heavy chain. The IIB-SK1·2 mutant did not fold into a 10S form under physiological condition in vitro. IIB-SK1·2 was less dynamic by stabilizing the filamentous form and accumulated in the posterior region of migrating cells. IIB-SK1·2 functioned properly in cytokinesis but altered migratory properties; the rate and directional persistence were increased by IIB-SK1·2 expression. Surprisingly, endogenous nonmuscle myosin IIA was excluded from the posterior region of migrating cells expressing IIB-SK1·2, which may underlie the change of the cellular migratory properties. These results suggest that the 10S form is necessary for maintaining nonmuscle myosin II in an unassembled state and for recruitment of nonmuscle myosin II to a specific region of the cell.

Non-muscle myosin II takes centre stage in cell adhesion and migration

Non-muscle myosin II (NM II) is an actin-binding protein that has actin cross-linking and contractile properties and is regulated by the phosphorylation of its light and heavy chains. The three mammalian NM II isoforms have both overlapping and unique properties. Owing to its position downstream of convergent signalling pathways, NM II is central in the control of cell adhesion, cell migration and tissue architecture. Recent insight into the role of NM II in these processes has been gained from loss-of-function and mutant approaches, methods that quantitatively measure actin and adhesion dynamics and the discovery of NM II mutations that cause monogenic diseases.

Local cortical tension by myosin II guides 3D endothelial cell branching

A key feature of angiogenesis is directional control of endothelial cell (EC) morphogenesis and movement [1]. During angiogenic sprouting, endothelial "tip cells" directionally branch from existing vessels in response to biochemical cues such as VEGF or hypoxia and migrate and invade the surrounding extracellular matrix (ECM) in a process that requires ECM remodeling by matrix metalloproteases (MMPs) [2-4]. Tip EC branching is mediated by directional protrusion of subcellular pseudopodial branches [5, 6]. Here, we seek to understand how EC pseudopodial branching is locally regulated to directionally guide angiogenesis. We develop an in vitro 3D EC model system in which migrating ECs display branched pseudopodia morphodynamics similar to those in living zebrafish. Using this system, we find that ECM stiffness and ROCK-mediated myosin II activity inhibit EC pseudopodial branch initiation. Myosin II is dynamically localized to the EC cortex and is partially released under conditions that promote branching. Local depletion of cortical myosin II precedes branch initiation, and initiation can be induced by local inhibition of myosin II activity. Thus, local downregulation of myosin II cortical contraction allows pseudopodium initiation to mediate EC branching and hence guide directional migration and angiogenesis.

Identification and characterization of Myosin from rat testicular peritubular myoid cells

In the mammalian testis, peritubular myoid cells (PMCs) surround seminiferous tubules. These cells are contractile, express the cytoskeletal markers of true smooth muscle-alpha-isoactin and F-actin-and participate in the contraction of seminiferous tubules during the transport of spermatozoa and testicular fluid to the rete testis. Myosin from PMCs (PMC-myosin) was isolated from adult rat testis and purified by cycles of assembly-disassembly and sucrose gradient centrifugation. PMC-myosin was recognized by a monoclonal anti-smooth muscle myosin antibody, and the peptide sequence shared partial homology with rat smooth muscle myosin-II, MYH11 (also known as SMM-II). Most PMC-myosin (95%) was soluble in the PMC cytosol, and purified PMC-myosin did not assemble into filaments in the in vitro salt dialysis assay at 4 degrees C, but did at 20 degrees C. PMC-myosin filaments are stable to ionic strength to the same degree as gizzard MYH11 filaments, but PMC-myosin filaments were more unstable in the presence of ATP. When PMCs were induced to contract by endothelin 1, a fraction of the PMC-myosin was found to be involved in the contraction. From these results we infer that PMCs express an isoform of smooth muscle myosin-II that is characterized by solubility at physiological ionic strength, a requirement for high temperature to assemble into filaments in vitro, and instability at low ATP concentrations. PMC-myosin is part of the PMC contraction apparatus when PMCs are stimulated with endothelin 1.

The role of myosin II motor activity in distributing myosin asymmetrically and coupling protrusive activity to cell translocation

Nonmuscle myosin IIA and IIB distribute preferentially toward opposite ends of migrating endothelial cells. To understand the mechanism and function of this behavior, myosin II was examined in cells treated with the motor inhibitor, blebbistatin. Blebbistatin at > or = 30 microM inhibited anterior redistribution of myosin IIA, with 100 microM blebbistatin causing posterior accumulation. Posterior accumulation of myosin IIB was unaffected. Time-lapse cinemicrography showed myosin IIA entering lamellipodia shortly after their formation, but failing to move into lamellipodia in blebbistatin. Thus, myosin II requires motor activity to move forward onto F-actin in protrusions. However, this movement is inhibited by myosin filament assembly, because whole myosin was delayed relative to a tailless fragment. Inhibiting myosin's forward movement reduced coupling between protrusive activity and translocation of the cell body: In untreated cells, body movement followed advancing lamellipodia, whereas blebbistatin-treated cells extended protrusions without displacement of the body or with a longer delay before movement. Anterior cytoplasm of blebbistatin-treated cells contained disorganized bundles of parallel microfilaments, but anterior F-actin bundles in untreated cells were mostly oriented perpendicular to movement. Myosin II may ordinarily move anteriorly on actin filaments and pull crossed filaments into antiparallel bundles, with the resulting realignment pulling the cell body forward.

A novel role for myosin II in insulin-stimulated glucose uptake in 3T3-L1 adipocytes

Insulin-stimulated glucose uptake requires the activation of several signaling pathways to mediate the translocation and fusion of GLUT4 vesicles from an intracellular pool to the plasma membrane. The studies presented here show that inhibition of myosin II activity impairs GLUT4-mediated glucose uptake but not GLUT4 translocation to the plasma membrane. We also show that adipocytes express both myosin IIA and IIB isoforms, and that myosin IIA is recruited to the plasma membrane upon insulin stimulation. Taken together, the data presented here represent the first demonstration that GLUT4-mediated glucose uptake is a myosin II-dependent process in adipocytes. Based on our findings, we hypothesize that myosin II is activated upon insulin stimulation and recruited to the cell cortex to facilitate GLUT4 fusion with the plasma membrane. The identification of myosin II as a key component of GLUT4-mediated glucose uptake represents an important advance in our understanding of the mechanisms regulating glucose homeostasis.

Asymmetric distribution of myosin IIB in migrating endothelial cells is regulated by a rho-dependent kinase and contributes to tail retraction

All vertebrates contain two nonmuscle myosin II heavy chains, A and B, which differ in tissue expression and subcellular distributions. To understand how these distinct distributions are controlled and what role they play in cell migration, myosin IIA and IIB were examined during wound healing by bovine aortic endothelial cells. Immunofluorescence showed that myosin IIA skewed toward the front of migrating cells, coincident with actin assembly at the leading edge, whereas myosin IIB accumulated in the rear 15-30 min later. Inhibition of myosin light-chain kinase, protein kinases A, C, and G, tyrosine kinase, MAP kinase, and PIP3 kinase did not affect this asymmetric redistribution of myosin isoforms. However, posterior accumulation of myosin IIB, but not anterior distribution of myosin IIA, was inhibited by dominant-negative rhoA and by the rho-kinase inhibitor, Y-27632, which also inhibited myosin light-chain phosphorylation. This inhibition was overcome by transfecting cells with constitutively active myosin light-chain kinase. These observations indicate that asymmetry of myosin IIB, but not IIA, is regulated by light-chain phosphorylation mediated by rho-dependent kinase. Blocking this pathway inhibited tail constriction and retraction, but did not affect protrusion, suggesting that myosin IIB functions in pulling the rear of the cell forward.

Myofibrillogenesis in the first cardiomyocytes formed from isolated quail precardiac mesoderm

De novo assembly of myofibrils was investigated in explants of precardiac mesoderm from quail embryos to address a controversy about different models of myofibrillogenesis. The sequential expression of sarcomeric components was visualized in double- and triple-stained explants before, during, and just after the first cardiomyocytes began to beat. In explants from stage 6 embryos, cultured for 10 h, ectoderm, endoderm, and the precardiac mesoderm displayed arrays of stress fibers with alternating bands of the nonmuscle isoforms of alpha-actinin and myosin IIB. With increasing time in culture, mesoderm cells contained fibrils composed of actin, nonmuscle myosin IIB, and sarcomeric alpha-actinin. Several hours later, before beating occurred, both nonmuscle and muscle myosin II localized in some of the fibrils in the cells. Concentrations of muscle myosin began as thin bundles, dispersed in the cytoplasm, often overlapping one another, and progressed to small, aligned A-band-sized aggregates. The amount of nonmuscle myosin decreased dramatically when Z-bands formed, the muscle myosin became organized into A-bands, and the cells began beating. The sequential changes in protein composition of the fibrils in the developing muscle cells supports the model of myofibrillogenesis in which assembly begins with premyofibrils and progresses through nascent myofibrils to mature myofibrils.

Alterations of Ca2+ mobilizing properties in migrating endothelial cells

Endothelial migration is one of the major events of pathological neovascularization. We compared the characteristics of Ca2+ mobilization in nonconfluent, confluent, and migrating endothelial cells. Migration of endothelial cells was induced by wounding the confluent cell monolayer. The basal intracellular Ca2+ concentration was lower in migrating cells and higher in confluent cells than in nonconfluent cells. Thapsigargin (TG)-induced Ca2+ leak and TG-evoked Ca2+ entry were accelerated in migrating cells, whereas the latter was suppressed in confluent cells. The ATP-induced Ca2+ transient was also much larger in migrating cells than in confluent cells. These alterations were also observed in a cell as an intracellular polarization, i.e., the leading edge showed an acceleration of TG-evoked Ca2+ entry and an augmentation of the ATP-induced Ca2+ transient. Endothelial migration was significantly suppressed by TG or cyclopiazonic acid. These observations suggest that the alterations of Ca2+ store site-related Ca2+ mobilizations, i.e., Ca2+ sequestration, release, and TG-evoked Ca2+ entry, may be involved in the cellular mechanisms of endothelial migration.

Increased migration in late G(1) phase in cultured smooth muscle cells

Migration and proliferation of smooth muscle cells (SMC) contribute to neointimal formation after arterial injury. However, the relation between migration and proliferation in these cells is obscure. To discriminate between migration and proliferation, we employed a migration assay of SMC at different phases of the cell cycle. Serum-deprived SMC were synchronized in different phases of the cell cycle by addition of serum for various periods of time. Migration induced by platelet-derived growth factor B-chain homodimer was maximal in SMC that were predominantly in the late G(1) (G(1b)) phase. In addition, in nonsynchronized SMC, 65-75% of SMC that had migrated were in the G(1b) phase. Phosphorylated myosin light chain was enriched around the cell periphery in SMC in the G(1b) phase compared with SMC in the other cell cycle phases. Interestingly, the Triton X-100-insoluble fraction of myosin was remarkably decreased in G(1b)-enriched SMC. These findings suggest that migratory activity of SMC may be coupled with the G(1b) phase. The phosphorylation and retention of myosin might explain some of the properties responsible for increased migration.

Regulatory light chain phosphorylation and the assembly of myosin II into the cytoskeleton of microcapillary endothelial cells

During the crawling movements of non-muscle cells, myosin II-containing structures assemble and disassemble with a high degree of spatial and temporal heterogeneity. In order to understand how this is controlled, we examined factors that influence the association of myosin II with detergent-resistant cytoskeletons of cultured endothelial cells. Treatment of cells with 0.05% Triton X-100 in an actin-stabilizing buffer released approximately 42% of the myosin II from the cytoplasm. Most remaining myosin II was dissociated from the cytoskeleton by treatment with ATP or AMPPNP, but not ADP, suggesting that myosin II is retained as ATP-sensitive filaments or via rigor-like binding to F-actin. Disruption of actin filaments with cytochalasin or latrunculin prior to detergent permeabilization sharply decreased the amount of myosin II retained, suggesting the latter type of association. Because phosphorylation of myosin II affects filament assembly and actin binding in vitro, phosphorylation levels in soluble and cytoskeletal myosin II were measured. Phosphorylation of myosin heavy chains was not significantly different between the two fractions, but regulatory light chains of cytoskeletal myosin II were 5 times more phosphorylated than in soluble myosin II. Tryptic-peptide mapping showed that cytoskeletal light chains were phosphorylated predominantly at serine 19/threonine 18, which regulates myosin II assembly in vitro, whereas soluble light chains were not phosphorylated or were phosphorylated at threonine 9. Treating cells with the kinase inhibitor, staurosporine, prior to permeabilization decreased light-chain phosphorylation with concomitant reduction in myosin retention. These observations suggest that assembly of myosin II into cytoskeletal structures, where it can generate and resist forces, is regulated in vivo by phosphorylation of myosin light chains at serine 19/threonine 18.

Expression, subcellular localization, and cloning of the 130-kDa regulatory subunit of myosin phosphatase in porcine aortic endothelial cells

In endothelial cells in situ and in primary culture, immunoblot analysis revealed an expression of the 130-kDa subunit of myosin phosphatase, similar to the myosin phosphatase targeting subunit (MYPT) of smooth muscle. Screening of an endothelial cell cDNA library yielded a clone encoding an NH2-terminal fragment of 89.6 kDa, closely related to smooth muscle MYPT1. Two isoforms differing by a central insert of 56 residues were detected. In growing cells, MYPT1 was localized on stress fiber, but at confluence the localization pattern changed and MYPT1 was distributed close to the cell membrane and at cell-cell contacts. The membrane localization of MYPT1 suggested a target other than myosin and raised the possibility that MYPT1 may be involved in dephosphorylation of alternative substrate(s). These distinct mechanisms would also be dependent on the growth state of the endothelial cells, i.e., regulation of actin-myosin interactions in growing cells and an unknown function in cells at confluence.

Actin ‘purse string’ filaments are anchored by E-cadherin-mediated adherens junctions at the leading edge of the epithelial wound, providing coordinated cell movement

At the leading edge of healing embryonic epithelium, cables of actin filaments appear to extend from cell to cell, forming a ring around the wound circumference. It has been hypothesized that this actin filament cable functions as a contractile ‘purse string’ to facilitate wound closure. We have observed this cable in large, circular healing epithelial wounds in corneas of adult mice. To elucidate the role of the actin filament cable, we characterized the molecular components associated with the cell-cell junction where the actin filament cable inserts and with the actin filament cable itself, and we studied the effect of disruption of the cable using an E-cadherin function-blocking antibody, ECCD-1. Localization of E-cadherin and the direct association of catenins with actin filament cable at the cell-cell interface of the actin cable confirmed that the cell-cell junction associated with the actin filament cable is an adherens junction. The E-cadherin function-blocking antibody caused disruption of the actin filament cable and induction of prominent lamellipodial extensions on cells at the leading edge, leading to a ragged uneven epithelial wound margin. These data demonstrate that cell-to-cell associated E-cadherin molecules link the actin filament cable, forming a functional adherens junction, and that the actin filament cable plays a role in coordinating cell movement.

Cytoplasmic dynamics of myosin IIA and IIB: spatial ‘sorting’ of isoforms in locomoting cells

Different isoforms of non-muscle myosin II have different distributions in vivo, even within individual cells. In order to understand how these different distributions arise, the distribution and dynamics of non-muscle myosins IIA and myosin IIB were examined in cultured cells using immunofluorescence staining and time-lapse imaging of fluorescent analogs. Cultured bovine aortic endothelia contained both myosins IIA and IIB. Both isoforms distributed along stress fibers, in linear or punctate aggregates within lamellipodia, and diffusely around the nucleus. However, the A isoform was preferentially located toward the leading edge of migrating cells when compared with myosin IIB by double immunofluorescence staining. Conversely, the B isoform was enriched in structures at the cells' trailing edges. When fluorescent analogs of the two isoforms were co-injected into living cells, the injected myosins distributed with the same disparate localizations as endogenous myosins IIA and IIB. This indicated that the ability of the myosins to ‘sort’ within the cytoplasm is intrinsic to the proteins themselves, and not a result of localized synthesis or degradation. Furthermore, time-lapse imaging of injected analogs in living cells revealed differences in the rates at which the two isoforms rearranged during cell movement. The A isoform appeared in newly formed structures more rapidly than the B isoform, and was also lost more rapidly when structures disassembled. These observations suggest that the different localizations of myosins IIA and IIB reflect different rates at which the isoforms transit through assembly, movement and disassembly within the cell. The relative proportions of different myosin II isoforms within a particular cell type may determine the lifetimes of various myosin II-based structures in that cell.

Fluorescent analogues of myosin II for tracking the behavior of different myosin isoforms in living cells

Fluorescently labeled smooth muscle myosin II is often used to study myosin II dynamics in non-muscle cells. In order to provide more specific tools for tracking non-muscle myosin II in living cytoplasm, fluorescent analogues of non-muscle myosin IIA and IIB were prepared and characterized. In addition, smooth and non-muscle myosin II were labeled with both cy5 and rhodamine so that comparative, dynamic studies may be performed. Non-muscle myosin IIA was purified from bovine platelets, non-muscle myosin IIB from bovine brain, and smooth muscle myosin II from turkey gizzards. After being fluorescently labeled with tetramethylrhodamine-5-iodoacetamide or with a succinimidyl ester of cy5, they retained the following properties: (1) reversible assembly into thick filaments, (2) actin-activatable MgATPase, (3) phosphorylation by myosin light chain kinase, (4) increased MgATPase upon light-chain phosphorylation, (5) interconversion between 6S and 10S conformations, and (6) distribution into endogenous myosin II-containing structures when microinjected into cultured cells. These fluorescent analogues can be used to visualize isoform-specific dynamics of myosin II in living cells.