para-Nitroblebbistatin, the non-cytotoxic and photostable myosin II inhibitor

Nucleocytoplasmic transport senses mechanical forces independently of cell density in cell monolayers

Cells sense and respond to mechanical forces through mechanotransduction, which regulates processes in health and disease. In single adhesive cells, mechanotransduction involves the transmission of force from the extracellular matrix to the cell nucleus, where it affects nucleocytoplasmic transport (NCT) and the subsequent nuclear localization of transcriptional regulators, such as YAP (also known as YAP1). However, if and how NCT is mechanosensitive in multicellular systems is unclear. Here, we characterize and use a fluorescent sensor of nucleocytoplasmic transport (Sencyt) and demonstrate that NCT responds to mechanical forces but not cell density in cell monolayers. Using monolayers of both epithelial and mesenchymal phenotype, we show that NCT is altered in response both to osmotic shocks and to the inhibition of cell contractility. Furthermore, NCT correlates with the degree of nuclear deformation measured through nuclear solidity, a shape parameter related to nuclear envelope tension. In contrast, YAP is sensitive to cell density, showing that the YAP response to cell-cell contacts is not via a mere mechanical effect of NCT. Our results demonstrate the generality of the mechanical regulation of NCT.

Actomyosin-II protects axons from degeneration induced by mild mechanical stress

Whether, to what extent, and how the axons in the central nervous system (CNS) can withstand sudden mechanical impacts remain unclear. By using a microfluidic device to apply controlled transverse mechanical stress to axons, we determined the stress levels that most axons can withstand and explored their instant responses at nanoscale resolution. We found mild stress triggers a highly reversible, rapid axon beading response, driven by actomyosin-II-dependent dynamic diameter modulations. This mechanism contributes to hindering the long-range spread of stress-induced Ca2+ elevations into non-stressed neuronal regions. Through pharmacological and molecular manipulations in vitro, we found that actomyosin-II inactivation diminishes the reversible beading process, fostering progressive Ca2+ spreading and thereby increasing acute axonal degeneration in stressed axons. Conversely, upregulating actomyosin-II activity prevents the progression of initial injury, protecting stressed axons from acute degeneration both in vitro and in vivo. Our study unveils the periodic actomyosin-II in axon shafts cortex as a novel protective mechanism, shielding neurons from detrimental effects caused by mechanical stress.

Structure, regulation, and mechanisms of nonmuscle myosin-2

Members of the myosin superfamily of molecular motors are large mechanochemical ATPases that are implicated in an ever-expanding array of cellular functions. This review focuses on mammalian nonmuscle myosin-2 (NM2) paralogs, ubiquitous members of the myosin-2 family of filament-forming motors. Through the conversion of chemical energy into mechanical work, NM2 paralogs remodel and shape cells and tissues. This process is tightly controlled in time and space by numerous synergetic regulation mechanisms to meet cellular demands. We review how recent advances in structural biology together with elegant biophysical and cell biological approaches have contributed to our understanding of the shared and unique mechanisms of NM2 paralogs as they relate to their kinetics, regulation, assembly, and cellular function.

Adhesion-induced cortical flows pattern E-cadherin-mediated cell contacts

Metazoan development relies on the formation and remodeling of cell-cell contacts. Dynamic reorganization of adhesion receptors and the actomyosin cell cortex in space and time plays a central role in cell-cell contact formation and maturation. Nevertheless, how this process is mechanistically achieved when new contacts are formed remains unclear. Here, by building a biomimetic assay composed of progenitor cells adhering to supported lipid bilayers functionalized with E-cadherin ectodomains, we show that cortical F-actin flows, driven by the depletion of myosin-2 at the cell contact center, mediate the dynamic reorganization of adhesion receptors and cell cortex at the contact. E-cadherin-dependent downregulation of the small GTPase RhoA at the forming contact leads to both a depletion of myosin-2 and a decrease of F-actin at the contact center. At the contact rim, in contrast, myosin-2 becomes enriched by the retraction of bleb-like protrusions, resulting in a cortical tension gradient from the contact rim to its center. This tension gradient, in turn, triggers centrifugal F-actin flows, leading to further accumulation of F-actin at the contact rim and the progressive redistribution of E-cadherin from the contact center to the rim. Eventually, this combination of actomyosin downregulation and flows at the contact determines the characteristic molecular organization, with E-cadherin and F-actin accumulating at the contact rim, where they are needed to mechanically link the contractile cortices of the adhering cells.

Boosting the Release of Leaving Group from Blebbistatin Derivative Photocages via Enhancing Intramolecular Charge Transfer and Stabilizing Cationic Intermediate

Blebbistatin (Bleb) derivatives are a visible light photocage platform. During the photocleavage process, intramolecular charge transfer (ICT) and cationic intermediates play a decisive role. However, slow photolysis rate and low photolysis quantum yield are the main problems for Bleb's derivatives. Herein, by introducing a substituted OCH3 group at the para-position of the D ring, Bleb and Bleb derivatives with various leaving groups were synthesized and studied, and the photolysis performance was unveiled by steady-state spectra, photolysis rate experiments, photolysis quantum yield, and density functional theory calculations. Substituted OCH3 derivatives of Bleb may enhance the photolysis rate and increase the photolysis quantum yield because the electron-donating group can promote the ICT process and stabilize the cationic intermediate during the photolytic reaction. More generally, the insights gained from this structure-reactivity relationship may provide theoretical guidance and aid in the development of new highly efficient photoreactions.

Adaptive pathfinding by nucleokinesis during amoeboid migration

Motile cells encounter microenvironments with locally heterogeneous mechanochemical composition. Individual compositional parameters, such as chemokines and extracellular matrix pore sizes, are well known to provide guidance cues for pathfinding. However, motile cells face diverse cues at the same time, raising the question of how they respond to multiple and potentially competing signals on their paths. Here, we reveal that amoeboid cells require nuclear repositioning, termed nucleokinesis, for adaptive pathfinding in heterogeneous mechanochemical micro-environments. Using mammalian immune cells and the amoeba Dictyostelium discoideum, we discover that frequent, rapid and long-distance nucleokinesis is a basic component of amoeboid pathfinding, enabling cells to reorientate quickly between locally competing cues. Amoeboid nucleokinesis comprises a two-step polarity switch and is driven by myosin-II forces that readjust the nuclear to the cellular path. Impaired nucleokinesis distorts path adaptions and causes cellular arrest in the microenvironment. Our findings establish that nucleokinesis is required for amoeboid cell navigation. Given that many immune cells, amoebae, and some cancer cells utilize an amoeboid migration strategy, these results suggest that nucleokinesis underlies cellular navigation during unicellular biology, immunity, and disease.

Miniaturized engineered heart tissues from hiPSC-derived triple cell type co-cultures to study human cardiac function

Human heart tissues grown as three-dimensional spheroids and consisting of different cardiac cell types derived from pluripotent stem cells (hiPSCs) recapitulate aspects of human physiology better than standard two-dimensional models in vitro. They typically consist of less than 5000 cells and are used to measure contraction kinetics although not contraction force. By contrast, engineered heart tissues (EHTs) formed around two flexible pillars, can measure contraction force but conventional EHTs often require between 0.5 and 2 million cells. This makes large-scale screening of many EHTs costly. Our goals here were (i) to create a physiologically relevant model that required fewer cells than standard EHTs making them less expensive, and (ii) to ensure that this miniaturized model retained correct functionality. We demonstrated that fully functional EHTs could be generated from physiologically relevant combinations of hiPSC-derived cardiomyocytes (70%), cardiac fibroblasts (15%) and cardiac endothelial cells (15%), using as few as 1.6 × 104 cells. Our results showed that these EHTs were viable and functional up to 14 days after formation. The EHTs could be electrically paced in the frequency range between 0.6 and 3 Hz, with the optimum between 0.6 and 2 Hz. This was consistent across three downscaled EHT sizes tested. These findings suggest that miniaturized EHTs could represent a cost-effective microphysiological system for disease modelling and examining drug responses particularly in secondary screens for drug discovery.

Mechanosensitive accumulation of non-muscle myosin IIB during mitosis requires its translocation activity

Non-muscle myosin II (NMII) is a force-generating mechanosensitive enzyme that responds to mechanical forces. NMIIs mechanoaccumulate at the cell cortex in response to mechanical forces. It is essential for cells to mechanically adapt to the physical environment, failure of which results in mitotic defects when dividing in confined environment. Much less is known about how NMII mechanoaccumulation is regulated during mitosis. We show that mitotic cells respond to compressive stress by promoting accumulation of active RhoA at the cell cortex as in interphase cells. RhoA mechanoresponse during mitosis activates and stabilizes NMIIB via ROCK signaling, leading to NMIIB mechanoaccumulation at the cell cortex. Using disease-related myosin II mutations, we found that NMIIB mechanoaccumulation requires its motor activity that translocates actin filaments, but not just its actin-binding function. Thus, the motor activity coordinates structural movement and nucleotide state changes to fine-tune actin-binding affinity optimal for NMIIs to generate and respond to forces.

A mechanosensing mechanism controls plasma membrane shape homeostasis at the nanoscale

As cells migrate and experience forces from their surroundings, they constantly undergo mechanical deformations which reshape their plasma membrane (PM). To maintain homeostasis, cells need to detect and restore such changes, not only in terms of overall PM area and tension as previously described, but also in terms of local, nanoscale topography. Here, we describe a novel phenomenon, by which cells sense and restore mechanically induced PM nanoscale deformations. We show that cell stretch and subsequent compression reshape the PM in a way that generates local membrane evaginations in the 100 nm scale. These evaginations are recognized by I-BAR proteins, which triggers a burst of actin polymerization mediated by Rac1 and Arp2/3. The actin polymerization burst subsequently re-flattens the evagination, completing the mechanochemical feedback loop. Our results demonstrate a new mechanosensing mechanism for PM shape homeostasis, with potential applicability in different physiological scenarios.

Lem2 is essential for cardiac development by maintaining nuclear integrity

AIMS

Nuclear envelope integrity is essential for the compartmentalization of the nucleus and cytoplasm. Importantly, mutations in genes encoding nuclear envelope (NE) and associated proteins are the second highest cause of familial dilated cardiomyopathy. One such NE protein that causes cardiomyopathy in humans and affects mouse heart development is Lem2. However, its role in the heart remains poorly understood.

METHODS AND RESULTS

We generated mice in which Lem2 was specifically ablated either in embryonic cardiomyocytes (Lem2 cKO) or in adult cardiomyocytes (Lem2 iCKO) and carried out detailed physiological, tissue, and cellular analyses. High-resolution episcopic microscopy was used for three-dimensional reconstructions and detailed morphological analyses. RNA-sequencing and immunofluorescence identified altered pathways and cellular phenotypes, and cardiomyocytes were isolated to interrogate nuclear integrity in more detail. In addition, echocardiography provided a physiological assessment of Lem2 iCKO adult mice. We found that Lem2 was essential for cardiac development, and hearts from Lem2 cKO mice were morphologically and transcriptionally underdeveloped. Lem2 cKO hearts displayed high levels of DNA damage, nuclear rupture, and apoptosis. Crucially, we found that these defects were driven by muscle contraction as they were ameliorated by inhibiting myosin contraction and L-type calcium channels. Conversely, reducing Lem2 levels to ∼45% in adult cardiomyocytes did not lead to overt cardiac dysfunction up to 18 months of age.

CONCLUSIONS

Our data suggest that Lem2 is critical for integrity at the nascent NE in foetal hearts, and protects the nucleus from the mechanical forces of muscle contraction. In contrast, the adult heart is not detectably affected by partial Lem2 depletion, perhaps owing to a more established NE and increased adaptation to mechanical stress. Taken together, these data provide insights into mechanisms underlying cardiomyopathy in patients with mutations in Lem2 and cardio-laminopathies in general.

Optogenetic manipulation of cardiac repolarization gradients using sub-threshold illumination

Introduction: Mechanisms underlying cardiac arrhythmias are typically driven by abnormalities in cardiac conduction and/or heterogeneities in repolarization time (RT) across the heart. While conduction slowing can be caused by either electrophysiological defects or physical blockade in cardiac tissue, RT heterogeneities are mainly related to action potential (AP) prolongation or abbreviation in specific areas of the heart. Importantly, the size of the area with altered RT and the difference between the short RT and long RT (RT gradient) have been identified as critical determinators of arrhythmogenicity. However, current experimental methods for manipulating RT gradient rely on the use of ion channel inhibitors, which lack spatial and temporal specificity and are commonly only partially reversible. Therefore, the conditions facilitating sustained arrhythmia upon the presence of RT heterogeneities and/or defects in cardiac conduction remain to be elucidated. Methods: We here employ an approach based on optogenetic stimulation in a low-intensity fashion (sub-threshold illumination), to selectively manipulate cardiac electrical activity in defined areas of the heart. Results: As previously described, subthreshold illumination is a robust tool able to prolong action potentials (AP), decrease upstroke velocity as well as slow cardiac conduction, in a fully reversible manner. By applying a patterned sub-threshold illumination in intact mouse hearts constitutively expressing the light-gated ion channel channelrhodopsin-2 (ChR2), we optically manipulate RT gradients and cardiac conduction across the heart in a spatially selective manner. Moreover, in a proof-of-concept assessment we found that in the presence of patterned sub-threshold illumination, mouse hearts were more susceptible to arrhythmias. Hence, this optogenetic-based approach may be able to mimic conduction slowing and RT heterogeneities present in pathophysiological conditions.

Dissecting the influence of cellular senescence on cell mechanics and extracellular matrix formation in vitro

Tissue formation and healing both require cell proliferation and migration, but also extracellular matrix production and tensioning. In addition to restricting proliferation of damaged cells, increasing evidence suggests that cellular senescence also has distinct modulatory effects during wound healing and fibrosis. Yet, a direct role of senescent cells during tissue formation beyond paracrine signaling remains unknown. We here report how individual modules of the senescence program differentially influence cell mechanics and ECM expression with relevance for tissue formation. We compared DNA damage-mediated and DNA damage-independent senescence which was achieved through over-expression of either p16^Ink4a or p21^Cip1 cyclin-dependent kinase inhibitors in primary human skin fibroblasts. Cellular senescence modulated focal adhesion size and composition. All senescent cells exhibited increased single cell forces which led to an increase in tissue stiffness and contraction in an in vitro 3D tissue formation model selectively for p16 and p21-overexpressing cells. The mechanical component was complemented by an altered expression profile of ECM-related genes including collagens, lysyl oxidases, and MMPs. We found that particularly the lack of collagen and lysyl oxidase expression in the case of DNA damage-mediated senescence foiled their intrinsic mechanical potential. These observations highlight the active mechanical role of cellular senescence during tissue formation as well as the need to synthesize a functional ECM network capable of transferring and storing cellular forces.

Non-muscle myosin 2 at a glance

Non-muscle myosin 2 (NM2) motors are the major contractile machines in most cell types. Unsurprisingly, these ubiquitously expressed actin-based motors power a plethora of subcellular, cellular and multicellular processes. In this Cell Science at a Glance article and the accompanying poster, we review the biochemical properties and mechanisms of regulation of this myosin. We highlight the central role of NM2 in multiple fundamental cellular processes, which include cell migration, cytokinesis, epithelial barrier function and tissue morphogenesis. In addition, we highlight recent studies using advanced imaging technologies that have revealed aspects of NM2 assembly hitherto inaccessible. This article will hopefully appeal to both cytoskeletal enthusiasts and investigators from outside the cytoskeleton field who have interests in one of the many basic cellular processes requiring actomyosin force production.

High-speed 2D light-sheet fluorescence microscopy enables quantification of spatially varying calcium dynamics in ventricular cardiomyocytes

Introduction: Reduced synchrony of calcium release and t-tubule structure organization in individual cardiomyocytes has been linked to loss of contractile strength and arrhythmia. Compared to confocal scanning techniques widely used for imaging calcium dynamics in cardiac muscle cells, light-sheet fluorescence microscopy enables fast acquisition of a 2D plane in the sample with low phototoxicity. Methods: A custom light-sheet fluorescence microscope was used to achieve dual-channel 2D timelapse imaging of calcium and the sarcolemma, enabling calcium sparks and transients in left and right ventricle cardiomyocytes to be correlated with the cell microstructure. Imaging electrically stimulated dual-labelled cardiomyocytes immobilized with para-nitroblebbistatin, a non-phototoxic, low fluorescence contraction uncoupler, with sub-micron resolution at 395 fps over a 38 μm × 170 µm FOV allowed characterization of calcium spark morphology and 2D mapping of the calcium transient time-to-half-maximum across the cell. Results: Blinded analysis of the data revealed sparks with greater amplitude in left ventricle myocytes. The time for the calcium transient to reach half-maximum amplitude in the central part of the cell was found to be, on average, 2 ms shorter than at the cell ends. Sparks co-localized with t-tubules were found to have significantly longer duration, larger area and spark mass than those further away from t-tubules. Conclusion: The high spatiotemporal resolution of the microscope and automated image-analysis enabled detailed 2D mapping and quantification of calcium dynamics of n = 60 myocytes, with the findings demonstrating multi-level spatial variation of calcium dynamics across the cell, supporting the dependence of synchrony and characteristics of calcium release on the underlying t-tubule structure.

Membrane compartmentalization of Ect2/Cyk4/Mklp1 and NuMA/dynein regulates cleavage furrow formation

In animal cells, spindle elongation during anaphase is temporally coupled with cleavage furrow formation. Spindle elongation during anaphase is regulated by NuMA/dynein/dynactin complexes that occupy the polar region of the cell membrane and are excluded from the equatorial membrane. How NuMA/dynein/dynactin are excluded from the equatorial membrane and the biological significance of this exclusion remains unknown. Here, we show that the centralspindlin (Cyk4/Mklp1) and its interacting partner RhoGEF Ect2 are required for NuMA/dynein/dynactin exclusion from the equatorial cell membrane. The Ect2-based (Ect2/Cyk4/Mklp1) and NuMA-based (NuMA/dynein/dynactin) complexes occupy mutually exclusive membrane surfaces during anaphase. The equatorial membrane enrichment of Ect2-based complexes is essential for NuMA/dynein/dynactin exclusion and proper spindle elongation. Conversely, NuMA-based complexes at the polar region of the cell membrane ensure spatially confined localization of Ect2-based complexes and thus RhoA. Overall, our work establishes that membrane compartmentalization of NuMA-based and Ect2-based complexes at the two distinct cell surfaces restricts dynein/dynactin and RhoA for coordinating spindle elongation with cleavage furrow formation.

Cortical tension regulates desmosomal morphogenesis

Mechanical stability is a fundamental and essential property of epithelial cell sheets. It is in large part determined by cell-cell adhesion sites that are tightly integrated by the cortical cytoskeleton. An intimate crosstalk between the adherens junction-associated contractile actomyosin system and the desmosome-anchored keratin intermediate filament system is decisive for dynamic regulation of epithelial mechanics. A major question in the field is whether and in which way mechanical stress affects junctional plasticity. This is especially true for the desmosome-keratin scaffold whose role in force-sensing is virtually unknown. To examine this question, we inactivated the actomyosin system in human keratinocytes (HaCaT) and canine kidney cells (MDCK) and monitored changes in desmosomal protein turnover.

Partial inhibition of myosin II by para-nitro-blebbistatin led to a decrease of the cells' elastic modulus and to reduced desmosomal protein turnover in regions where nascent desmosomes are formed and, to a lower degree, in regions where larger, more mature desmosomes are present. Interestingly, desmosomal proteins are affected differently: a significant decrease in turnover was observed for the desmosomal plaque protein desmoplakin I (DspI), which links keratin filaments to the desmosomal core, and the transmembrane cadherin desmoglein 2 (Dsg2). On the other hand, the turnover of another type of desmosomal cadherin, desmocollin 2 (Dsc2), was not significantly altered under the tested conditions. Similarly, the turnover of the adherens junction-associated E-cadherin was not affected by the low doses of para-nitro-blebbistatin. Inhibition of actin polymerization by low dose latrunculin B treatment and of ROCK-driven actomyosin contractility by Y-27632 treatment also induced a significant decrease in desmosomal DspI turnover. Taken together, we conclude that changes in the cortical force balance affect desmosome formation and growth. Furthermore, they differentially modulate desmosomal protein turnover resulting in changes of desmosome composition. We take the observations as evidence for a hitherto unknown desmosomal mechanosensing and mechanoresponse pathway responding to an altered force balance.

Polymerization power: effectors of actin polymerization as regulators of T lymphocyte migration through complex environments

During their life span, T cells are tasked with patrolling the body for potential pathogens. To do so, T cells migrate through numerous distinct anatomical sites and tissue environments with different biophysical characteristics. To migrate through these different environments, T cells use various motility strategies that rely on actin network remodeling to generate shape changes and mechanical forces. In this review, we initially discuss the migratory journey of T cells and then cover the actin polymerization effectors at play in T cells, and finally, we focus on the function of these effectors of actin cytoskeleton remodeling in mediating T-cell migration through diverse tissue environments. Specifically, we will discuss the current state of the field pertaining to our understanding of the roles in T-cell migration played by members of the three main families of actin polymerization machinery: the Arp2/3 complex; formin proteins; and Ena/VASP proteins.

Binding Networks Identify Targetable Protein Pockets for Mechanism-Based Drug Design

The human genome codes only a few thousand druggable proteins, mainly receptors and enzymes. While this pool of available drug targets is limited, there is an untapped potential for discovering new drug-binding mechanisms and modes. For example, enzymes with long binding cavities offer numerous prerequisite binding sites that may be visited by an inhibitor during migration from a bulk solution to the destination site. Drug design can use these prerequisite sites as new structural targets. However, identifying these ephemeral sites is challenging. Here, we introduce a new method called NetBinder for the systematic identification and classification of prerequisite binding sites at atomic resolution. NetBinder is based on atomistic simulations of the full inhibitor binding process and provides a networking framework on which to select the most important binding modes and uncover the entire binding mechanism, including previously undiscovered events. NetBinder was validated by a study of the binding mechanism of blebbistatin (a potent inhibitor) to myosin 2 (a promising target for cancer chemotherapy). Myosin 2 is a good test enzyme because, like other potential targets, it has a long internal binding cavity that provides blebbistatin with numerous potential prerequisite binding sites. The mechanism proposed by NetBinder of myosin 2 structural changes during blebbistatin binding shows excellent agreement with experimentally determined binding sites and structural changes. While NetBinder was tested on myosin 2, it may easily be adopted to other proteins with long internal cavities, such as G-protein-coupled receptors or ion channels, the most popular current drug targets. NetBinder provides a new paradigm for drug design by a network-based elucidation of binding mechanisms at an atomic resolution.

Blue or Near-Infrared Light-Triggered Release of Halogens via Blebbistatin Photocage

Photocages can provide spatial and temporal control to accurately release the various chemicals and bioactive groups when excited by light. Although the absorption spectra of most photocages are in the ultraviolet absorption region, only a few absorb in the visible or near-infrared region. Blebbistatin (Bleb) would release a hydroxyl radical under blue one-photon or two-photon near-infrared light (800 nm) irradiation. In this work, typical chlorine and bromine as leaving groups substituted hydroxyl compounds (Bleb-Cl, Bleb-Br) are synthesized to evaluate the photocage's capability of Bleb's platform. Driven by the excited-state charge transfer, Bleb-Cl and Bleb-Br show good photolysis quantum yield to uncage the halogen anion and the uncaging process would be accelerated in water solution. The photochemical reaction, final product's analysis, and femtosecond transient absorption studies on Bleb-Cl/Bleb-Br demonstrate that Bleb can act as a photocage platform to release the halogen ion via heterolytic reaction when irradiated by blue or near-infrared light. Therefore, Bleb can be a new generation of visible or near-infrared light-triggered photocage.

Plasmodium parasitophorous vacuole membrane-resident protein UIS4 manipulates host cell actin to avoid parasite elimination

Parasite-derived PVM-resident proteins are critical for complete parasite development inside hepatocytes, although the function of most of these proteins remains unknown. Here, we show that the upregulated in infectious sporozoites 4 (UIS4) protein, resident at the PVM, interacts with the host cell actin. By suppressing filamentous actin formation, UIS4 avoids parasite elimination. Host cell actin dynamics increases around UIS4-deficient parasites, which is associated with subsequent parasite elimination. Notably, parasite elimination is impaired significantly by the inhibition of host myosin-II, possibly through relieving the compression generated by actomyosin complexes at the host-parasite interface. Together, these data reveal that UIS4 has a critical role in the evasion of host defensive mechanisms, enabling hence EEF survival and development.

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Plasmodium PVM-resident protein UIS4 interacts with host cell actin

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Host actin dynamics is altered around exoerythocytic forms (EEFs) lacking UIS4

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Actin activity around EEFs lacking UIS4 is associated with parasite elimination

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Parasite elimination depends on actomyosin complexes formed around the PVM

A B-cell actomyosin arc network couples integrin co-stimulation to mechanical force-dependent immune synapse formation

B-cell activation and immune synapse (IS) formation with membrane-bound antigens are actin-dependent processes that scale positively with the strength of antigen-induced signals. Importantly, ligating the B-cell integrin, LFA-1, with ICAM-1 promotes IS formation when antigen is limiting. Whether the actin cytoskeleton plays a specific role in integrin-dependent IS formation is unknown. Here, we show using super-resolution imaging of mouse primary B cells that LFA-1:ICAM-1 interactions promote the formation of an actomyosin network that dominates the B-cell IS. This network is created by the formin mDia1, organized into concentric, contractile arcs by myosin 2A, and flows inward at the same rate as B-cell receptor (BCR):antigen clusters. Consistently, individual BCR microclusters are swept inward by individual actomyosin arcs. Under conditions where integrin is required for synapse formation, inhibiting myosin impairs synapse formation, as evidenced by reduced antigen centralization, diminished BCR signaling, and defective signaling protein distribution at the synapse. Together, these results argue that a contractile actomyosin arc network plays a key role in the mechanism by which LFA-1 co-stimulation promotes B-cell activation and IS formation.

Deciphering cellular signals in adult mouse sinoatrial node cells

Sinoatrial node (SAN) cells are the pacemakers of the heart. This study describes a method for culturing and infection of adult mouse SAN cells with FRET-based biosensors that can be exploited to examine signaling events. SAN cells cultured in media with blebbistatin or (S)-nitro-blebbistatin retain their morphology, protein distribution, action potential (AP) waveform, and cAMP dynamics for at least 40 h. SAN cells expressing targeted cAMP sensors show distinct β-adrenergic-mediated cAMP pools. Cyclic GMP, protein kinase A, Ca2+/CaM kinase II, and protein kinase D in SAN cells also show unique dynamics to different stimuli. Heart failure SAN cells show a decrease in cAMP and cGMP levels. In summary, a reliable method for maintaining adult mouse SAN cells in culture is presented, which facilitates studies of signaling networks and regulatory mechanisms during physiological and pathological conditions.

Tachycardiomyopathy entails a dysfunctional pattern of interrelated mitochondrial functions

Tachycardiomyopathy is characterised by reversible left ventricular dysfunction, provoked by rapid ventricular rate. While the knowledge of mitochondria advanced in most cardiomyopathies, mitochondrial functions await elucidation in tachycardiomyopathy. Pacemakers were implanted in 61 rabbits. Tachypacing was performed with 330 bpm for 10 days (n = 11, early left ventricular dysfunction) or with up to 380 bpm over 30 days (n = 24, tachycardiomyopathy, TCM). In n = 26, pacemakers remained inactive (SHAM). Left ventricular tissue was subjected to respirometry, metabolomics and acetylomics. Results were assessed for translational relevance using a human-based model: induced pluripotent stem cell derived cardiomyocytes underwent field stimulation for 7 days (TACH-iPSC-CM). TCM animals showed systolic dysfunction compared to SHAM (fractional shortening 37.8 ± 1.0% vs. 21.9 ± 1.2%, SHAM vs. TCM, p < 0.0001). Histology revealed cardiomyocyte hypertrophy (cross-sectional area 393.2 ± 14.5 µm² vs. 538.9 ± 23.8 µm², p < 0.001) without fibrosis. Mitochondria were shifted to the intercalated discs and enlarged. Mitochondrial membrane potential remained stable in TCM. The metabolite profiles of ELVD and TCM were characterised by profound depletion of tricarboxylic acid cycle intermediates. Redox balance was shifted towards a more oxidised state (ratio of reduced to oxidised nicotinamide adenine dinucleotide 10.5 ± 2.1 vs. 4.0 ± 0.8, p < 0.01). The mitochondrial acetylome remained largely unchanged. Neither TCM nor TACH-iPSC-CM showed relevantly increased levels of reactive oxygen species. Oxidative phosphorylation capacity of TCM decreased modestly in skinned fibres (168.9 ± 11.2 vs. 124.6 ± 11.45 pmol·O₂·s^-1·mg^-1 tissue, p < 0.05), but it did not in isolated mitochondria. The pattern of mitochondrial dysfunctions detected in two models of tachycardiomyopathy diverges from previously published characteristic signs of other heart failure aetiologies.

SynActJ: Easy-to-Use Automated Analysis of Synaptic Activity

Neuronal synapses are highly dynamic communication hubs that mediate chemical neurotransmission via the exocytic fusion and subsequent endocytic recycling of neurotransmitter-containing synaptic vesicles (SVs). Functional imaging tools allow for the direct visualization of synaptic activity by detecting action potentials, pre- or postsynaptic calcium influx, SV exo- and endocytosis, and glutamate release. Fluorescent organic dyes or synapse-targeted genetic molecular reporters, such as calcium, voltage or neurotransmitter sensors and synapto-pHluorins reveal synaptic activity by undergoing rapid changes in their fluorescence intensity upon neuronal activity on timescales of milliseconds to seconds, which typically are recorded by fast and sensitive widefield live cell microscopy. The analysis of the resulting time-lapse movies in the past has been performed by either manually picking individual structures, custom scripts that have not been made widely available to the scientific community, or advanced software toolboxes that are complicated to use. For the precise, unbiased and reproducible measurement of synaptic activity, it is key that the research community has access to bio-image analysis tools that are easy-to-apply and allow the automated detection of fluorescent intensity changes in active synapses. Here we present SynActJ (Synaptic Activity in ImageJ), an easy-to-use fully open-source workflow that enables automated image and data analysis of synaptic activity. The workflow consists of a Fiji plugin performing the automated image analysis of active synapses in time-lapse movies via an interactive seeded watershed segmentation that can be easily adjusted and applied to a dataset in batch mode. The extracted intensity traces of each synaptic bouton are automatically processed, analyzed, and plotted using an R Shiny workflow. We validate the workflow on time-lapse images of stimulated synapses expressing the SV exo-/endocytosis reporter Synaptophysin-pHluorin or a synapse-targeted calcium sensor, Synaptophysin-RGECO. We compare the automatic workflow to manual analysis and compute calcium-influx and SV exo-/endocytosis kinetics and other parameters for synaptic vesicle recycling under different conditions. We predict SynActJ to become an important tool for the analysis of synaptic activity and synapse properties.

Discovery of Selective Inhibitors for In Vitro and In Vivo Interrogation of Skeletal Myosin II

Myosin IIs, actin-based motors that utilize the chemical energy of adenosine 5'-triphosphate (ATP) to generate force, have potential as therapeutic targets. Their heavy chains differentiate the family into muscle (skeletal [SkMII], cardiac, smooth) and nonmuscle myosin IIs. Despite the therapeutic potential for muscle disorders, SkMII-specific inhibitors have not been reported and characterized. Here, we present the discovery, synthesis, and characterization of "skeletostatins," novel derivatives of the pan-myosin II inhibitor blebbistatin, with selectivity 40- to 170-fold for SkMII over all other myosin II family members. In addition, the skeletostatins bear improved potency, solubility, and photostability, without cytotoxicity. Based on its optimal in vitro profile, MT-134's in vivo tolerability, efficacy, and pharmacokinetics were determined. MT-134 was well-tolerated in mice, impaired motor performance, and had excellent exposure in muscles. Skeletostatins are useful probes for basic research and a strong starting point for drug development.

Two Classes of Myosin Inhibitors, Para-nitroblebbistatin and Mavacamten, Stabilize β-Cardiac Myosin in Different Structural and Functional States

In addition to a conventional relaxed state, a fraction of myosins in the cardiac muscle exists in a low-energy consuming super-relaxed (SRX) state, which is kept as a reserve pool that may be engaged under sustained increased cardiac demand. The conventional relaxed and the super-relaxed states are widely assumed to correspond to a structure where myosin heads are in an open configuration, free to interact with actin, and a closed configuration, inhibiting binding to actin, respectively. Disruption of the myosin SRX population is an emerging model in different heart diseases, such as hypertrophic cardiomyopathy, which results in excessive muscle contraction, and stabilizing them using myosin inhibitors is budding as an attractive therapeutic strategy. Here we examined the structure-function relationships of two myosin ATPase inhibitors, mavacamten and para-nitroblebbistatin, and found that binding of mavacamten at a site different than para-nitroblebbistatin populates myosin into the SRX state. Para-nitroblebbistatin, binding to a distal pocket to the myosin lever arm near the nucleotide-binding site, does not affect the usual myosin SRX state but instead appears to render myosin into a new, perhaps much more inhibited, 'ultra-relaxed' state. X-ray scattering-based rigid body modeling shows that both mavacamten and para-nitroblebbistatin induce novel conformations in human β-cardiac heavy meromyosin that diverge significantly from the hypothetical open and closed states, and furthermore, mavacamten treatment causes greater compaction than para-nitroblebbistatin. Taken together, we conclude that mavacamten and para-nitroblebbistatin stabilize myosin in different structural states, and such states may give rise to different functional energy-sparing states.

Novel Optics-Based Approaches for Cardiac Electrophysiology: A Review

Optical techniques for recording and manipulating cellular electrophysiology have advanced rapidly in just a few decades. These developments allow for the analysis of cardiac cellular dynamics at multiple scales while largely overcoming the drawbacks associated with the use of electrodes. The recent advent of optogenetics opens up new possibilities for regional and tissue-level electrophysiological control and hold promise for future novel clinical applications. This article, which emerged from the international NOTICE workshop in 2018, reviews the state-of-the-art optical techniques used for cardiac electrophysiological research and the underlying biophysics. The design and performance of optical reporters and optogenetic actuators are reviewed along with limitations of current probes. The physics of light interaction with cardiac tissue is detailed and associated challenges with the use of optical sensors and actuators are presented. Case studies include the use of fluorescence recovery after photobleaching and super-resolution microscopy to explore the micro-structure of cardiac cells and a review of two photon and light sheet technologies applied to cardiac tissue. The emergence of cardiac optogenetics is reviewed and the current work exploring the potential clinical use of optogenetics is also described. Approaches which combine optogenetic manipulation and optical voltage measurement are discussed, in terms of platforms that allow real-time manipulation of whole heart electrophysiology in open and closed-loop systems to study optimal ways to terminate spiral arrhythmias. The design and operation of optics-based approaches that allow high-throughput cardiac electrophysiological assays is presented. Finally, emerging techniques of photo-acoustic imaging and stress sensors are described along with strategies for future development and establishment of these techniques in mainstream electrophysiological research.

Pharmacological Modulation of Neurite Outgrowth in Human Neural Progenitor Cells by Inhibiting Non-muscle Myosin II

Studies on neural development and neuronal regeneration after injury are mainly based on animal models. The establishment of pluripotent stem cell (PSC) technology, however, opened new perspectives for better understanding these processes in human models by providing unlimited cell source for hard-to-obtain human tissues. Here, we aimed at identifying the molecular factors that confine and modulate an early step of neural regeneration, the formation of neurites in human neural progenitor cells (NPCs). Enhanced green fluorescent protein (eGFP) was stably expressed in NPCs differentiated from human embryonic and induced PSC lines, and the neurite outgrowth was investigated under normal and injury-related conditions using a high-content screening system. We found that inhibitors of the non-muscle myosin II (NMII), blebbistatin and its novel, non-toxic derivatives, initiated extensive neurite outgrowth in human NPCs. The extracellular matrix components strongly influenced the rate of neurite formation but NMII inhibitors were able to override the inhibitory effect of a restrictive environment. Non-additive stimulatory effect on neurite generation was also detected by the inhibition of Rho-associated, coiled-coil-containing protein kinase 1 (ROCK1), the upstream regulator of NMII. In contrast, inhibition of c-Jun N-terminal kinases (JNKs) had only a negligible effect, suggesting that the ROCK1 signal is dominantly manifested by actomyosin activity. In addition to providing a reliable cell-based in vitro model for identifying intrinsic mechanisms and environmental factors responsible for impeded axonal regeneration in humans, our results demonstrate that NMII and ROCK1 are important pharmacological targets for the augmentation of neural regeneration at the progenitor level. These studies may open novel perspectives for development of more effective pharmacological treatments and cell therapies for various neurodegenerative disorders.

Changes in the expression and functional activities of Myosin II isoforms in human hyperplastic prostate

Benign prostatic hyperplasia (BPH) is a common disease among aging males with the etiology remaining unclear. We recently found myosin II was abundantly expressed in rat and cultured human prostate cells with permissive roles in the dynamic and static components. The present study aimed to explore the expression and functional activities of myosin II isoforms including smooth muscle (SM) myosin II (SMM II) and non-muscle myosin II (NMM II) in the hyperplastic prostate. Human prostate cell lines and tissues from normal human and BPH patients were used. Hematoxylin and Eosin (H&E), Masson's trichrome, immunohistochemical staining, in vitro organ bath, RT-polymerase chain reaction (PCR) and Western-blotting were performed. We further created cell models with NMM II isoforms silenced and proliferation, cycle, and apoptosis of prostate cells were determined by cell counting kit-8 (CCK-8) assay and flow cytometry. Hyperplastic prostate SM expressed more SM1 and LC17b isoforms compared with their alternatively spliced counterparts, favoring a slower more tonic-type contraction and greater force generation. For BPH group, blebbistatin (BLEB, a selective myosin II inhibitor), exhibited a stronger effect on relaxing phenylephrine (PE) pre-contracted prostate strips and inhibiting PE-induced contraction. Additionally, NMMHC-A and NMMHC-B were up-regulated in hyperplastic prostate with no change in NMMHC-C. Knockdown of NMMHC-A or NMMHC-B inhibited prostate cell proliferation and induced apoptosis, with no changes in cell cycle. Our novel data demonstrate that expression and functional activities of myosin II isoforms are altered in human hyperplastic prostate, suggesting a new pathological mechanism for BPH. Thus, the myosin II system may provide potential new therapeutic targets for BPH/lower urinary tract symptoms (LUTS).

Chiral HPLC separation of enantiomeric blebbistatin derivatives and racemization analysis in vertebrate tissues

Simple and consistent chiral HPLC methods for the efficient separation of enantiomeric blebbistatin derivatives, namely parent compound blebbistatin and derivatives 4-nitroblebbistatin, 4-aminoblebbistatin, 4-dimethylaminoblebbistatin, and 4-t-butylblebbistatin were developed using cellulose tris(3,5-dimethylphenylcarbamate) as a stationary phase (Lux cellulose-1 column). Blebbistatin, 4-aminoblebbistatin, and 4-dimethylaminoblebbistatin racemates were well-separated in normal-phase HPLC conditions while 4-nitroblebbistatin and 4-t-butylblebbistatin were effectively separated in both normal- and reversed-phase HPLC conditions. Furthermore, the order of elution of enantiopure compounds was found to be independent of mobile phase compositions and conditions used, and solely depends on the interaction between the enantiomer and the chiral stationary phase. We found that despite the chiral center being present far from the D-ring in the blebbistatin structure, the D-ring substitutions prominently affect the chiral separation. Ex vivo racemization studies of the most popular blebbistatin derivative (S)-(-)-4-aminoblebbistatin in rat blood and brain tissues revealed that the compound does not convert into the inactive enantiomer. This confirms that (S)-(-)-4-aminoblebbistatin is a useful tool compound in cellular and molecular biology studies without the risks of racemization and degradation effects.

Myosin Motors: Novel Regulators and Therapeutic Targets in Colorectal Cancer

Simple Summary

Colorectal cancer (CRC) is a deadly disease that may go undiagnosed until it presents at an advanced metastatic stage for which few interventions are available. The development and metastatic spread of CRC is driven by remodeling of the actin cytoskeleton in cancer cells. Myosins represent a large family of actin motor proteins that play key roles in regulating actin cytoskeleton architecture and dynamics. Different myosins can move and cross-link actin filaments, attach them to the membrane organelles and translocate vesicles along the actin filaments. These diverse activities determine the key roles of myosins in regulating cell proliferation, differentiation and motility. Either mutations or the altered expression of different myosins have been well-documented in CRC; however, the roles of these actin motors in colon cancer development remain poorly understood. The present review aims at summarizing the evidence that implicate myosin motors in regulating CRC growth and metastasis and discusses the mechanisms underlying the oncogenic and tumor-suppressing activities of myosins.

Abstract

Colorectal cancer (CRC) remains the third most common cause of cancer and the second most common cause of cancer deaths worldwide. Clinicians are largely faced with advanced and metastatic disease for which few interventions are available. One poorly understood aspect of CRC involves altered organization of the actin cytoskeleton, especially at the metastatic stage of the disease. Myosin motors are crucial regulators of actin cytoskeletal architecture and remodeling. They act as mechanosensors of the tumor environments and control key cellular processes linked to oncogenesis, including cell division, extracellular matrix adhesion and tissue invasion. Different myosins play either oncogenic or tumor suppressor roles in breast, lung and prostate cancer; however, little is known about their functions in CRC. This review focuses on the functional roles of myosins in colon cancer development. We discuss the most studied class of myosins, class II (conventional) myosins, as well as several classes (I, V, VI, X and XVIII) of unconventional myosins that have been linked to CRC development. Altered expression and mutations of these motors in clinical tumor samples and their roles in CRC growth and metastasis are described. We also evaluate the potential of using small molecular modulators of myosin activity to develop novel anticancer therapies.

Filopodia-based contact stimulation of cell migration drives tissue morphogenesis

Cells migrate collectively to form tissues and organs during morphogenesis. Contact inhibition of locomotion (CIL) drives collective migration by inhibiting lamellipodial protrusions at cell-cell contacts and promoting polarization at the leading edge. Here, we report a CIL-related collective cell behavior of myotubes that lack lamellipodial protrusions, but instead use filopodia to move as a cohesive cluster in a formin-dependent manner. We perform genetic, pharmacological and mechanical perturbation analyses to reveal the essential roles of Rac2, Cdc42 and Rho1 in myotube migration. These factors differentially control protrusion dynamics and cell-matrix adhesion formation. We also show that active Rho1 GTPase localizes at retracting free edge filopodia and that Rok-dependent actomyosin contractility does not mediate a contraction of protrusions at cell-cell contacts, but likely plays an important role in the constriction of supracellular actin cables. Based on these findings, we propose that contact-dependent asymmetry of cell-matrix adhesion drives directional movement, whereas contractile actin cables contribute to the integrity of the migrating cell cluster.

Growth, lifetime, directional movement and myosin-dependent motility of mutant keratin granules in cultured cells

Intermediate filament polypeptides (IFPs) are prominent components of cytoplasmic aggregates, which are pathognomonic for multiple diseases. Recent observations in cultured cells suggest that they are dynamic and subject to regulated turnover. The emerging concept is that multiple factors contribute to motility and turnover of IFP-containing aggregates. To understand their relative contribution, quantitative tools are needed. The current study addresses this need using epithelial cells producing mutant keratin IFPs that have been identified as the cause of the hereditary blister-forming skin disease epidermolysis bullosa simplex. Digital image analysis of individual granules allowed mapping of their complete life cycle, with information on multiple characteristics at any given time-point. The deduced signet features revealed rapid granule fusion and directed transport from the periphery towards the cell centre, and a limited, ~ 30 min lifetime with a slow, continuous growth phase followed by fast disassembly. As paradigmatic proof-of-principle, we demonstrate that inhibition of myosin II selectively reduces granule movement, linking keratin granule motility to retrograde cortical acto-myosin flow. The newly developed methods and established parameters will help in the characterization of known and the identification of novel regulators of IFP-containing aggregates.

Improved Inhibitory and Absorption, Distribution, Metabolism, Excretion, and Toxicology (ADMET) Properties of Blebbistatin Derivatives Indicate That Blebbistatin Scaffold Is Ideal for drug Development Targeting Myosin-2

Blebbistatin, para-nitroblebbistatin (NBleb), and para-aminoblebbistatin (AmBleb) are highly useful tool compounds as they selectively inhibit the ATPase activity of myosin-2 family proteins. Despite the medical importance of the myosin-2 family as drug targets, chemical optimization has not yet provided a promising lead for drug development because previous structure-activity-relationship studies were limited to a single myosin-2 isoform. Here we evaluated the potential of blebbistatin scaffold for drug development and found that D-ring substitutions can fine-tune isoform specificity, absorption-distribution-metabolism-excretion, and toxicological properties. We defined the inhibitory properties of NBleb and AmBleb on seven different myosin-2 isoforms, which revealed an unexpected potential for isoform specific inhibition. We also found that NBleb metabolizes six times slower than blebbistatin and AmBleb in rats, whereas AmBleb metabolizes two times slower than blebbistatin and NBleb in human, and that AmBleb accumulates in muscle tissues. Moreover, mutagenicity was also greatly reduced in case of AmBleb. These results demonstrate that small substitutions have beneficial functional and pharmacological consequences, which highlight the potential of the blebbistatin scaffold for drug development targeting myosin-2 family proteins and delineate a route for defining the chemical properties of further derivatives to be developed. SIGNIFICANCE STATEMENT: Small substitutions on the blebbistatin scaffold have beneficial functional and pharmacological consequences, highlighting their potential in drug development targeting myosin-2 family proteins.

The Planar Polarity Component VANGL2 Is a Key Regulator of Mechanosignaling

VANGL2 is a component of the planar cell polarity (PCP) pathway, which regulates tissue polarity and patterning. The Vangl2 Lp mutation causes lung branching defects due to dysfunctional actomyosin-driven morphogenesis. Since the actomyosin network regulates cell mechanics, we speculated that mechanosignaling could be impaired when VANGL2 is disrupted. Here, we used live-imaging of precision-cut lung slices (PCLS) from Vangl2 Lp/+ mice to determine that alveologenesis is attenuated as a result of impaired epithelial cell migration. Vangl2 Lp/+ tracheal epithelial cells (TECs) and alveolar epithelial cells (AECs) exhibited highly disrupted actomyosin networks and focal adhesions (FAs). Functional assessment of cellular forces confirmed impaired traction force generation in Vangl2 Lp/+ TECs. YAP signaling in Vangl2 Lp airway epithelium was reduced, consistent with a role for VANGL2 in mechanotransduction. Furthermore, activation of RhoA signaling restored actomyosin organization in Vangl2 Lp/+ , confirming RhoA as an effector of VANGL2. This study identifies a pivotal role for VANGL2 in mechanosignaling, which underlies the key role of the PCP pathway in tissue morphogenesis.

Synthesis and Evaluation of 4-Hydroxycoumarin Imines as Inhibitors of Class II Myosins

Inhibitors of muscle myosin ATPases are needed to treat conditions that could be improved by promoting muscle relaxation. The lead compound for this study ((3-(N-butylethanimidoyl)ethyl)-4-hydroxy-2H-chromen-2-one; BHC) was previously discovered to inhibit skeletal myosin II. BHC and 34 analogues were synthesized to explore structure-activity relationships. The properties of analogues, including solubility, stability, and toxicity, suggest that the BHC scaffold may be useful for developing therapeutics. Inhibition of actin-activated ATPase activity of fast skeletal and cardiac muscle myosin II, inhibition of skeletal muscle contractility ex vivo, and slowing of in vitro actin-sliding velocity were measured. Several analogues with aromatic side arms showed improved potency (half-maximal inhibitory concentration (IC50) <1 μM) and selectivity (≥12-fold) for skeletal myosin versus cardiac myosin compared to BHC. Several analogues blocked neurotransmission, suggesting that they are selective for nonmuscle myosin II over skeletal myosin. Competition and molecular docking studies suggest that BHC and blebbistatin bind to the same site on myosin.

A simple and robust cell-based assay for the discovery of novel cytokinesis inhibitors

Cytokinesis is the last step of mitotic cell division that separates the cytoplasm of dividing cells. Small molecule inhibitors targeting either the elements of the regulatory pathways controlling cytokinesis, or the terminal effectors have been of interest as potential drug candidates for the treatment of various diseases. Here we present a detailed protocol for a cell-based cytokinesis assay that can be used for the discovery of novel cytokinesis inhibitors. The assay is performed in a 96-well plate format in 48 h. Living cells, nuclei and nuclei of dead cells are identified by a single staining step using three fluorescent dyes, followed by rapid live cell imaging. The primary signal is the nuclei-to-cell ratio (NCR). In the presence of cytokinesis inhibitors, this ratio increases over time, as the ratio of multinucleated cells increases in the population. The ratio of dead nuclei to total nuclei provides a simultaneous measure of cytotoxicity. A screening window coefficient (Z`) of 0.65 indicates that the assay is suitable for screening purposes, as the positive and negative controls are well-separated. EC₅₀ values can be reliably determined in a single 96-well plate by using only six different compound concentrations, enabling the testing of 4 compounds per plate. An excellent test-retest reliability (R² = 0.998) was found for EC₅₀ values covering a ~1500-fold range of potencies. Established small molecule inhibitors of cytokinesis operating via direct action on actin dynamics or nonmuscle myosin II are used to demonstrate the robustness, simplicity and flexibility of the assay.

Sarcomere integrated biosensor detects myofilament-activating ligands in real time during twitch contractions in live cardiac muscle

The sarcomere is the functional unit of cardiac muscle, essential for normal heart function. To date, it has not been possible to study, in real time, thin filament-based activation dynamics in live cardiac muscle. We report here results from a cardiac troponin C (TnC) FRET-based biosensor integrated into the cardiac sarcomere via stoichiometric replacement of endogenous TnC. The TnC biosensor provides, for the first time, evidence of multiple thin filament activating ligands, including troponin I interfacing with TnC and cycling myosin, during a cardiac twitch. Results show that the TnC FRET biosensor transient significantly precedes that of peak twitch force. Using small molecules and genetic modifiers known to alter sarcomere activation, independently of the intracellular Ca²⁺ transient, the data show that the TnC biosensor detects significant effects of the troponin I switch domain as a sarcomere-activating ligand. Interestingly, the TnC biosensor also detected the effects of load-dependent altered myosin cycling, as shown by a significant delay in TnC biosensor transient inactivation during the isometric twitch. In addition, the TnC biosensor detected the effects of myosin as an activating ligand during the twitch by using a small molecule that directly alters cross-bridge cycling, independently of the intracellular Ca²⁺ transient. Collectively, these results aid in illuminating the basis of cardiac muscle contractile activation with implications for gene, protein, and small molecule-based strategies designed to target the sarcomere in regulating beat-to-beat heart performance in health and disease.

Effect of allosteric inhibition of non-muscle myosin 2 on its intracellular diffusion

Subcellular dynamics of non-muscle myosin 2 (NM2) is crucial for a broad-array of cellular functions. To unveil mechanisms of NM2 pharmacological control, we determined how the dynamics of NM2 diffusion is affected by NM2′s allosteric inhibitors, i.e. blebbistatin derivatives, as compared to Y-27632 inhibiting ROCK, NM2′s upstream regulator. We found that NM2 diffusion is markedly faster in central fibers than in peripheral stress fibers. Y-27632 accelerated NM2 diffusion in both peripheral and central fibers, whereas in peripheral fibers blebbistatin derivatives slightly accelerated NM2 diffusion at low, but markedly slowed it at high inhibitor concentrations. In contrast, rapid NM2 diffusion in central fibers was unaffected by direct NM2 inhibition. Using our optopharmacological tool, Molecular Tattoo, sub-effective concentrations of a photo-crosslinkable blebbistatin derivative were increased to effective levels in a small, irradiated area of peripheral fibers. These findings suggest that direct allosteric inhibition affects the diffusion profile of NM2 in a markedly different manner compared to the disruption of the upstream control of NM2. The pharmacological action of myosin inhibitors is channeled through autonomous molecular processes and might be affected by the load acting on the NM2 proteins.

Cellular locomotion using environmental topography

Eukaryotic cells migrate by coupling the intracellular force of the actin cytoskeleton to the environment. While force coupling is usually mediated by transmembrane adhesion receptors, especially those of the integrin family, amoeboid cells such as leukocytes can migrate extremely fast despite very low adhesive forces¹. Here we show that leukocytes cannot only migrate under low adhesion but can also transmit forces in the complete absence of transmembrane force coupling. When confined within three-dimensional environments, they use the topographical features of the substrate to propel themselves. Here the retrograde flow of the actin cytoskeleton follows the texture of the substrate, creating retrograde shear forces that are sufficient to drive the cell body forwards. Notably, adhesion-dependent and adhesion-independent migration are not mutually exclusive, but rather are variants of the same principle of coupling retrograde actin flow to the environment and thus can potentially operate interchangeably and simultaneously. As adhesion-free migration is independent of the chemical composition of the environment, it renders cells completely autonomous in their locomotive behaviour.

Force and Calcium Transients Analysis in Human Engineered Heart Tissues Reveals Positive Force-Frequency Relation at Physiological Frequency

Force measurements in ex vivo and engineered heart tissues are well established. Analysis of calcium transients (CaT) is complementary to force, and the combined analysis is meaningful to the study of cardiomyocyte biology and disease. This article describes a model of human induced pluripotent stem cell cardiomyocyte-derived engineered heart tissues (hiPSC-CM EHTs) transduced with the calcium sensor GCaMP6f followed by sequential analysis of force and CaT. Average peak analysis demonstrated the temporal sequence of the CaT preceding the contraction twitch. The pharmacological relevance of the test system was demonstrated with inotropic indicator compounds. Force-frequency relationship was analyzed in the presence of ivabradine (300 nM), which reduced spontaneous frequency and unmasked a positive correlation of force and CaT at physiological human heart beating frequency with stimulation frequency between 0.75 and 2.5 Hz (force +96%; CaT +102%). This work demonstrates the usefulness of combined force/CaT analysis and demonstrates a positive force-frequency relationship in hiPSC-CM EHTs.

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Analysis of calcium transients and force in engineered heart tissues

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Accurate replications of drug effects on calcium transients and force analysis

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Positive force- and calcium transients-frequency relationship

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Reverse correlation between omecamtiv mecarbil's inotropic effect and frequency

The role of actin and myosin in antigen extraction by B lymphocytes

B cells must extract antigens attached to the surface of antigen presenting cells to generate high-affinity antibodies. Antigen extraction requires force, and recent studies have implicated actomyosin-dependent pulling forces generated within the B cell as the major driver of antigen extraction. These actomyosin-dependent pulling forces also serve to test the affinity of the B cell antigen receptor for antigen prior to antigen extraction. Such affinity discrimination is central to the process of antibody affinity maturation. Here we review the evidence that actomyosin-dependent pulling forces generated within the B cell promote affinity discrimination and power antigen extraction. Our take on these critical B cell functions is influenced significantly by the recent identification of formin-generated, myosin-rich, concentric actin arcs in the medial portion of the T cell immune synapse, as B cells appear to contain a similar contractile actomyosin structure.

A Semi-High-Throughput Adaptation of the NADH-Coupled ATPase Assay for Screening Small Molecule Inhibitors

ATPase enzymes utilize the free energy stored in adenosine triphosphate to catalyze a wide variety of endergonic biochemical processes in vivo that would not occur spontaneously. These proteins are crucial for essentially all aspects of cellular life, including metabolism, cell division, responses to environmental changes and movement. The protocol presented here describes a nicotinamide adenine dinucleotide (NADH)-coupled ATPase assay that has been adapted to semi-high throughput screening of small molecule ATPase inhibitors. The assay has been applied to cardiac and skeletal muscle myosin II's, two actin-based molecular motor ATPases, as a proof of principle. The hydrolysis of ATP is coupled to the oxidation of NADH by enzymatic reactions in the assay. First, the ADP generated by the ATPase is regenerated to ATP by pyruvate kinase (PK). PK catalyzes the transition of phosphoenolpyruvate (PEP) to pyruvate in parallel. Subsequently, pyruvate is reduced to lactate by lactate dehydrogenase (LDH), which catalyzes the oxidation of NADH in parallel. Thus, the decrease in ATP concentration is directly correlated to the decrease in NADH concentration, which is followed by change to the intrinsic fluorescence of NADH. As long as PEP is available in the reaction system, the ADP concentration remains very low, avoiding inhibition of the ATPase enzyme by its own product. Moreover, the ATP concentration remains nearly constant, yielding linear time courses. The fluorescence is monitored continuously, which allows for easy estimation of the quality of data and helps to filter out potential artifacts (e.g., arising from compound precipitation or thermal changes).

Targeting non-muscle myosin II promotes corneal endothelial migration through regulating lamellipodial dynamics

Corneal endothelial cell (CEC) dysfunction causes corneal edema that may lead to blindness. In addition to corneal transplantation, simple descemetorhexis has been proposed to treat centrally located disease with adequate peripheral cell reserve, but promoting the centripetal migration of CECs is pivotal to this strategy. Here, we show that targeting non-muscle myosin II (NMII) activity by Y27632, a ROCK inhibitor, or blebbistatin, a selective NMII inhibitor, promotes directional migration of CECs and accelerates in vitro wound healing. The lamellipodial protrusion persistence is increased, and actin retrograde flow is decreased after NMII inhibition. Counteracting lamellipodial protrusion by actin-related protein 2/3 (ARP2/3) inhibitor abolishes this migration-promoting effect. Although both Y27632 and blebbistatin accelerate wound healing, cell junctional integrity and barrier function are better preserved after blebbistatin treatment, leading to more rapid corneal deturgescence in rabbit corneal endothelial wounding model. Our findings indicate that NMII is a promising therapeutic target in the treatment of CEC dysfunction. KEY MESSAGES: NMII inhibition promotes directional migration and wound healing of CECs in vitro. Lamellipodial protrusion persistence is increased after NMII inhibition. Selective NMII inhibitor preserves junctional integrity better than ROCK inhibitor. Selective NMII inhibitor accelerates corneal deturgescence after wounding in vivo.

Contractile acto-myosin network on nuclear envelope remnants positions human chromosomes for mitosis

To ensure proper segregation during mitosis, chromosomes must be efficiently captured by spindle microtubules and subsequently aligned on the mitotic spindle. The efficacy of chromosome interaction with the spindle can be influenced by how widely chromosomes are scattered in space. Here, we quantify chromosome-scattering volume (CSV) and find that it is reduced soon after nuclear envelope breakdown (NEBD) in human cells. The CSV reduction occurs primarily independently of microtubules and is therefore not an outcome of interactions between chromosomes and the spindle. We find that, prior to NEBD, an acto-myosin network is assembled in a LINC complex-dependent manner on the cytoplasmic surface of the nuclear envelope. This acto-myosin network remains on nuclear envelope remnants soon after NEBD, and its myosin-II-mediated contraction reduces CSV and facilitates timely chromosome congression and correct segregation. Thus, we find a novel mechanism that positions chromosomes in early mitosis to ensure efficient and correct chromosome-spindle interactions.

Origin, Organization, Dynamics, and Function of Actin and Actomyosin Networks at the T Cell Immunological Synapse

The engagement of a T cell with an antigen-presenting cell (APC) or activating surface results in the formation within the T cell of several distinct actin and actomyosin networks. These networks reside largely within a narrow zone immediately under the T cell's plasma membrane at its site of contact with the APC or activating surface, i.e., at the immunological synapse. Here we review the origin, organization, dynamics, and function of these synapse-associated actin and actomyosin networks. Importantly, recent insights into the nature of these actin-based cytoskeletal structures were made possible in several cases by advances in light microscopy.

Nuclear positioning facilitates amoeboid migration along the path of least resistance

During metazoan development, immune surveillance and cancer dissemination, cells migrate in complex three-dimensional microenvironments^1-3. These spaces are crowded by cells and extracellular matrix, generating mazes with differently sized gaps that are typically smaller than the diameter of the migrating cell^4,5. Most mesenchymal and epithelial cells and some-but not all-cancer cells actively generate their migratory path using pericellular tissue proteolysis⁶. By contrast, amoeboid cells such as leukocytes use non-destructive strategies of locomotion⁷, raising the question how these extremely fast cells navigate through dense tissues. Here we reveal that leukocytes sample their immediate vicinity for large pore sizes, and are thereby able to choose the path of least resistance. This allows them to circumnavigate local obstacles while effectively following global directional cues such as chemotactic gradients. Pore-size discrimination is facilitated by frontward positioning of the nucleus, which enables the cells to use their bulkiest compartment as a mechanical gauge. Once the nucleus and the closely associated microtubule organizing centre pass the largest pore, cytoplasmic protrusions still lingering in smaller pores are retracted. These retractions are coordinated by dynamic microtubules; when microtubules are disrupted, migrating cells lose coherence and frequently fragment into migratory cytoplasmic pieces. As nuclear positioning in front of the microtubule organizing centre is a typical feature of amoeboid migration, our findings link the fundamental organization of cellular polarity to the strategy of locomotion.

Live imaging of alveologenesis in precision-cut lung slices reveals dynamic epithelial cell behaviour

Damage to alveoli, the gas-exchanging region of the lungs, is a component of many chronic and acute lung diseases. In addition, insufficient generation of alveoli results in bronchopulmonary dysplasia, a disease of prematurity. Therefore visualising the process of alveolar development (alveologenesis) is critical for our understanding of lung homeostasis and for the development of treatments to repair and regenerate lung tissue. Here we show live alveologenesis, using long-term, time-lapse imaging of precision-cut lung slices. We reveal that during this process, epithelial cells are highly mobile and we identify specific cell behaviours that contribute to alveologenesis: cell clustering, hollowing and cell extension. Using the cytoskeleton inhibitors blebbistatin and cytochalasin D, we show that cell migration is a key driver of alveologenesis. This study reveals important novel information about lung biology and provides a new system in which to manipulate alveologenesis genetically and pharmacologically.

The process of alveologenesis is incompletely understood, partly due to the lack of applicable real-time imaging methods. Here the authors describe the process of alveologenesis and the behaviour of epithelial cells in real-time, using widefield microscopy and image deconvolution in precision-cut lung slices, revealing the dominant role of epithelial cell migration.

Simultaneous voltage and calcium imaging and optogenetic stimulation with high sensitivity and a wide field of view

Transmembrane voltage and intracellular calcium concentration are coupled parameters essential to the function of neurons, cardiomyocytes, and other excitable cells. Here we introduce the Firefly-HR microscope for simultaneous optogenetic stimulation and voltage and calcium imaging with fluorescent proteins using three spectrally distinct visible color bands. Firefly-HR combines patterned stimulation, near-total internal reflection laser excitation through a prism located between the sample and a water-immersion objective, and concurrent imaging of three color channels. The microscope has efficient light collection, low fluorescent background, and a large field of view (0.24 x 1.2 mm @ 1000 frames/sec). We characterize optical crosstalk and demonstrate capabilities with three applications: (1) probing synaptically connected neuronal microcircuits, (2) examining the coupling between neuronal action potentials and calcium influx, and (3) studying the pharmacology of paced human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) via simultaneous recordings of voltage, calcium, and contraction.