Myosin at work: motor adaptations for a variety of cellular functions

Deciphering Mechanochemical Influences of Emergent Actomyosin Crosstalk using QCM-D

Purpose

Cytoskeletal protein ensembles exhibit emergent mechanics where behavior exhibited in teams is not necessarily the sum of the components' single molecule properties. In addition, filaments may act as force sensors that distribute feedback and influence motor protein behavior. To understand the design principles of such emergent mechanics, we developed an approach utilizing QCM-D to measure how actomyosin bundles respond mechanically to environmental variables that alter constituent myosin II motor behavior.

Methods

QCM-D is used for the first time to probe alterations in actin-myosin bundle viscoelasticity due to changes in skeletal myosin II concentration and motor nucleotide state. Actomyosin bundles were constructed on a gold QCM-D sensor using a microfluidic setup, and frequency and dissipation change measurements were recorded for each component addition to decipher which assay constituents lead to changes in bundle structural compliancy.

Results

Lowering myosin concentration is detected as lower shifts in frequency and dissipation, while the relative changes in frequency and dissipation shifts for both the first and second actin additions are relatively similar. Strikingly, buffer washes with different nucleotides (ATP vs. ADP) yielded unique signatures in frequency and dissipation shifts. As myosin II's ADP-bound state tightly binds actin filaments, we observe an increase in frequency and decrease in dissipation change, indicating a decrease in viscoelasticity, likely due to myosin's increased affinity for actin, conversion from an active motor to a static crosslinker, and ability to recruit additional actin filaments from the surface, making an overall more rigid sensor coating. However, lowering the ADP concentration results in increased system compliancy, indicating that transient crosslinking and retaining a balance of motor activity perhaps results in a more cooperative and productive force generating system.

Conclusions

QCM-D can detect changes in actomyosin viscoelasticity due to molecular-level alterations, such as motor concentration and nucleotide state. These results provide support for actin's role as a mechanical force-feedback sensor and demonstrate a new approach for deciphering the feedback mechanisms that drive emergent cytoskeletal ensemble crosstalk and intracellular mechanosensing. This approach can be adapted to investigate environmental influences on more complex cytoskeletal ensemble mechanics, including addition of other motors, crosslinkers, and filament types.

Exploring Protein Conformational Changes Using a Large-Scale Biophysical Sampling Augmented Deep Learning Strategy

Inspired by the success of deep learning in predicting static protein structures, researchers are now actively exploring other deep learning algorithms aimed at predicting the conformational changes of proteins. Currently, a major challenge in the development of such models lies in the limited training data characterizing different conformational transitions. To address this issue, molecular dynamics simulations is combined with enhanced sampling methods to create a large-scale database. To this end, the study simulates the conformational changes of 2635 proteins featuring two known stable states, and collects the structural information along each transition pathway. Utilizing this database, a general deep learning model capable of predicting the transition pathway for a given protein is developed. The model exhibits general robustness across proteins with varying sequence lengths (ranging from 44 to 704 amino acids) and accommodates different types of conformational changes. Great agreement is shown between predictions and experimental data in several systems and successfully apply this model to identify a novel allosteric regulation in an important biological system, the human β-cardiac myosin. These results demonstrate the effectiveness of the model in revealing the nature of protein conformational changes.

A novel kinetic model to demonstrate the independent effects of ATP and ADP/Pi concentrations on sarcomere function

Understanding muscle contraction mechanisms is a standing challenge, and one of the approaches has been to create models of the sarcomere-the basic contractile unit of striated muscle. While these models have been successful in elucidating many aspects of muscle contraction, they fall short in explaining the energetics of functional phenomena, such as rigor, and in particular, their dependence on the concentrations of the biomolecules involved in the cross-bridge cycle. Our hypothesis posits that the stochastic time delay between ATP adsorption and ADP/Pi release in the cross-bridge cycle necessitates a modeling approach where the rates of these two reaction steps are controlled by two independent parts of the total free energy change of the hydrolysis reaction. To test this hypothesis, we built a two-filament, stochastic-mechanical half-sarcomere model that separates the energetic roles of ATP and ADP/Pi in the cross-bridge cycle's free energy landscape. Our results clearly demonstrate that there is a nontrivial dependence of the cross-bridge cycle's kinetics on the independent concentrations of ATP, ADP, and Pi. The simplicity of the proposed model allows for analytical solutions of the more basic systems, which provide novel insight into the dominant mechanisms driving some of the experimentally observed contractile phenomena.

The F-actin bundler SWAP-70 promotes tumor metastasis

Dynamic rearrangements of the F-actin cytoskeleton are a hallmark of tumor metastasis. Thus, proteins that govern F-actin rearrangements are of major interest for understanding metastasis and potential therapies. We hypothesized that the unique F-actin binding and bundling protein SWAP-70 contributes importantly to metastasis. Orthotopic, ectopic, and short-term tail vein injection mouse breast and lung cancer models revealed a strong positive dependence of lung and bone metastasis on SWAP-70. Breast cancer cell growth, migration, adhesion, and invasion assays revealed SWAP-70's key role in these metastasis-related cell features and the requirement for SWAP-70 to bind F-actin. Biophysical experiments showed that tumor cell stiffness and deformability are negatively modulated by SWAP-70. Together, we present a hitherto undescribed, unique F-actin modulator as an important contributor to tumor metastasis.

Actin-membrane linkers: Insights from synthetic reconstituted systems

At the cell surface, the actin cytoskeleton and the plasma membrane interact reciprocally in a variety of processes related to the remodeling of the cell surface. The actin cytoskeleton has been known to modulate membrane organization and reshape the membrane. To this end, actin-membrane linking molecules play a major role in regulating actin assembly and spatially direct the interaction between the actin cytoskeleton and the membrane. While studies in cells have provided a wealth of knowledge on the molecular composition and interactions of the actin-membrane interface, the complex molecular interactions make it challenging to elucidate the precise actions of the actin-membrane linkers at the interface. Synthetic reconstituted systems, consisting of model membranes and purified proteins, have been a powerful approach to elucidate how actin-membrane linkers direct actin assembly to drive membrane shape changes. In this review, we will focus only on several actin-membrane linkers that have been studied by using reconstitution systems. We will discuss the design principles of these reconstitution systems and how they have contributed to the understanding of the cellular functions of actin-membrane linkers. Finally, we will provide a perspective on future research directions in understanding the intricate actin-membrane interaction.

Exploring the Super-Relaxed State of Human Cardiac β-Myosin by Molecular Dynamics Simulations

Human β-cardiac myosin plays a critical role in generating the mechanical forces necessary for cardiac muscle contraction. This process relies on a delicate dynamic equilibrium between the disordered relaxed state (DRX) and the super-relaxed state (SRX) of myosin. Disruptions in this equilibrium due to mutations can lead to heart diseases. However, the structural characteristics of SRX and the molecular mechanisms underlying pathogenic mutations have remained elusive. To bridge this gap, we conducted molecular dynamics simulations and free energy calculations to explore the conformational changes in myosin. Our findings indicate that the size of the phosphate-binding pocket can serve as a valuable metric for characterizing the transition from the DRX to SRX state. Importantly, we established a global dynamic coupling network within the myosin motor head at the residue level, elucidating how the pathogenic mutation E483K impacts the equilibrium between SRX and DRX through allosteric effects. Our work illuminates molecular details of SRX and offers valuable insights into disease treatment through the regulation of SRX.

Pathophysiology of human hearing loss associated with variants in myosins

Deleterious variants of more than one hundred genes are associated with hearing loss including MYO3A, MYO6, MYO7A and MYO15A and two conventional myosins MYH9 and MYH14. Variants of MYO7A also manifest as Usher syndrome associated with dysfunction of the retina and vestibule as well as hearing loss. While the functions of MYH9 and MYH14 in the inner ear are debated, MYO3A, MYO6, MYO7A and MYO15A are expressed in inner ear hair cells along with class-I myosin MYO1C and are essential for developing and maintaining functional stereocilia on the apical surface of hair cells. Stereocilia are large, cylindrical, actin-rich protrusions functioning as biological mechanosensors to detect sound, acceleration and posture. The rigidity of stereocilia is sustained by highly crosslinked unidirectionally-oriented F-actin, which also provides a scaffold for various proteins including unconventional myosins and their cargo. Typical myosin molecules consist of an ATPase head motor domain to transmit forces to F-actin, a neck containing IQ-motifs that bind regulatory light chains and a tail region with motifs recognizing partners. Instead of long coiled-coil domains characterizing conventional myosins, the tails of unconventional myosins have various motifs to anchor or transport proteins and phospholipids along the F-actin core of a stereocilium. For these myosins, decades of studies have elucidated their biochemical properties, interacting partners in hair cells and variants associated with hearing loss. However, less is known about how myosins traffic in a stereocilium using their motor function, and how each variant correlates with a clinical condition including the severity and onset of hearing loss, mode of inheritance and presence of symptoms other than hearing loss. Here, we cover the domain structures and functions of myosins associated with hearing loss together with advances, open questions about trafficking of myosins in stereocilia and correlations between hundreds of variants in myosins annotated in ClinVar and the corresponding deafness phenotypes.

Cytosolic actin isoforms form networks with different rheological properties that indicate specific biological function

The implications of the existence of different actins expressed in epithelial cells for network mechanics and dynamics is investigated by microrheology and confocal imaging. γ-actin predominately found in the apical cortex forms stiffer networks compared to β-actin, which is preferentially organized in stress fibers. We attribute this to selective interactions with Mg2+-ions interconnecting the filaments' N-termini. Bundling propensity of the isoforms is different in the presence of Mg2+-ions, while crosslinkers such as α-actinin, fascin, and heavy meromyosin alter the mechanical response independent of the isoform. In the presence of myosin, β-actin networks show a large number of small contraction foci, while γ-actin displays larger but fewer foci indicative of a stronger interaction with myosin motors. We infer that subtle changes in the amino acid sequence of actin isoforms lead to alterations of the mechanical properties on the network level with potential implications for specific biological functions.

Go with the flow - bulk transport by molecular motors

Cells are the smallest building blocks of all living eukaryotic organisms, usually ranging from a couple of micrometers (for example, platelets) to hundreds of micrometers (for example, neurons and oocytes) in size. In eukaryotic cells that are more than 100 µm in diameter, very often a self-organized large-scale movement of cytoplasmic contents, known as cytoplasmic streaming, occurs to compensate for the physical constraints of large cells. In this Review, we discuss cytoplasmic streaming in multiple cell types and the mechanisms driving this event. We particularly focus on the molecular motors responsible for cytoplasmic movements and the biological roles of cytoplasmic streaming in cells. Finally, we describe bulk intercellular flow that transports cytoplasmic materials to the oocyte from its sister germline cells to drive rapid oocyte growth.

Protrusion growth driven by myosin-generated force

Actin-based protrusions extend from the surface of all eukaryotic cells, where they support diverse activities essential for life. Models of protrusion growth hypothesize that actin filament assembly exerts force for pushing the plasma membrane outward. However, membrane-associated myosin motors are also abundant in protrusions, although their potential for contributing, growth-promoting force remains unexplored. Using an inducible system that docks myosin motor domains to membrane-binding modules with temporal control, we found that application of myosin-generated force to the membrane is sufficient for driving robust protrusion elongation in human, mouse, and pig cell culture models. Protrusion growth scaled with motor accumulation, required barbed-end-directed force, and was independent of cargo delivery or recruitment of canonical elongation factors. Application of growth-promoting force was also supported by structurally distinct myosin motors and membrane-binding modules. Thus, myosin-generated force can drive protrusion growth, and this mechanism is likely active in diverse biological contexts.

Myosin II Adjusts Motility Properties and Regulates Force Production Based on Motor Environment

Introduction

Myosin II has been investigated with optical trapping, but single motor-filament assay arrangements are not reflective of the complex cellular environment. To understand how myosin interactions propagate up in scale to accomplish system force generation, we devised a novel actomyosin ensemble optical trapping assay that reflects the hierarchy and compliancy of a physiological environment and is modular for interrogating force effectors.

Methods

Hierarchical actomyosin bundles were formed in vitro. Fluorescent template and cargo actin filaments (AF) were assembled in a flow cell and bundled by myosin. Beads were added in the presence of ATP to bind the cargo AF and activate myosin force generation to be measured by optical tweezers.

Results

Three force profiles resulted across a range of myosin concentrations: high force with a ramp-plateau, moderate force with sawtooth movement, and baseline. The three force profiles, as well as high force output, were recovered even at low solution concentration, suggesting that myosins self-optimize within AFs. Individual myosin steps were detected in the ensemble traces, indicating motors are taking one step at a time while others remain engaged in order to sustain productive force generation.

Conclusions

Motor communication and system compliancy are significant contributors to force output. Environmental conditions, motors taking individual steps to sustain force, the ability to backslip, and non-linear concentration dependence of force indicate that the actomyosin system contains a force-feedback mechanism that senses the local cytoskeletal environment and communicates to the individual motors whether to be in a high or low duty ratio mode.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-022-00731-1.

Small Molecule RPI-194 Stabilizes Activated Troponin to Increase the Calcium Sensitivity of Striated Muscle Contraction

Small molecule cardiac troponin activators could potentially enhance cardiac muscle contraction in the treatment of systolic heart failure. We designed a small molecule, RPI-194, to bind cardiac/slow skeletal muscle troponin (Cardiac muscle and slow skeletal muscle share a common isoform of the troponin C subunit.) Using solution NMR and stopped flow fluorescence spectroscopy, we determined that RPI-194 binds to cardiac troponin with a dissociation constant K_D of 6-24 μM, stabilizing the activated complex between troponin C and the switch region of troponin I. The interaction between RPI-194 and troponin C is weak (K_D 311 μM) in the absence of the switch region. RPI-194 acts as a calcium sensitizer, shifting the pCa₅₀ of isometric contraction from 6.28 to 6.99 in mouse slow skeletal muscle fibers and from 5.68 to 5.96 in skinned cardiac trabeculae at 100 μM concentration. There is also some cross-reactivity with fast skeletal muscle fibers (pCa₅₀ increases from 6.27 to 6.52). In the slack test performed on the same skinned skeletal muscle fibers, RPI-194 slowed the velocity of unloaded shortening at saturating calcium concentrations, suggesting that it slows the rate of actin-myosin cross-bridge cycling under these conditions. However, RPI-194 had no effect on the ATPase activity of purified actin-myosin. In isolated unloaded mouse cardiomyocytes, RPI-194 markedly decreased the velocity and amplitude of contractions. In contrast, cardiac function was preserved in mouse isolated perfused working hearts. In summary, the novel troponin activator RPI-194 acts as a calcium sensitizer in all striated muscle types. Surprisingly, it also slows the velocity of unloaded contraction, but the cause and significance of this is uncertain at this time. RPI-194 represents a new class of non-specific troponin activator that could potentially be used either to enhance cardiac muscle contractility in the setting of systolic heart failure or to enhance skeletal muscle contraction in neuromuscular disorders.

Myo5b Transports Fibronectin-Containing Vesicles and Facilitates FN1 Secretion from Human Pleural Mesothelial Cells

Pleural mesothelial cells (PMCs) play a central role in the progression of pleural fibrosis. As pleural injury progresses to fibrosis, PMCs transition to mesenchymal myofibroblast via mesothelial mesenchymal transition (MesoMT), and produce extracellular matrix (ECM) proteins including collagen and fibronectin (FN1). FN1 plays an important role in ECM maturation and facilitates ECM-myofibroblast interaction, thus facilitating fibrosis. However, the mechanism of FN1 secretion is poorly understood. We report here that myosin 5b (Myo5b) plays a critical role in the transportation and secretion of FN1 from human pleural mesothelial cells (HPMCs). TGF-β significantly increased the expression and secretion of FN1 from HPMCs and facilitates the close association of Myo5B with FN1 and Rab11b. Moreover, Myo5b directly binds to GTP bound Rab11b (Rab11b-GTP) but not GDP bound Rab11b. Myo5b or Rab11b knockdown via siRNA significantly attenuated the secretion of FN1 without changing FN1 expression. TGF-β also induced Rab11b-GTP formation, and Rab11b-GTP but not Rab11b-GDP significantly activated the actin-activated ATPase activity of Myo5B. Live cell imaging revealed that Myo5b- and FN1-containing vesicles continuously moved together in a single direction. These results support that Myo5b and Rab11b play an important role in FN1 transportation and secretion from HPMCs, and consequently may contribute to the development of pleural fibrosis.

Detection of Myosin 1g Overexpression in Pediatric Leukemia by Novel Monoclonal Antibodies

Myosin 1g (Myo1g) is a mechanoenzyme associated with actin filaments, expressed exclusively in hematopoietic cells, and involved in various cellular functions, including cell migration, adhesion, and membrane trafficking. Despite the importance of Myo1g in distinct functions, there is currently no monoclonal antibody (mAb) against Myo1g. mAbs are helpful tools for the detection of specific antigens in tumor cells and other tissues. The development of mAbs against targeted dysregulated molecules in cancer cells remains a crucial tool for aiding in the diagnosis and the treatment of patients. Using hybridoma technology, we generated a panel of hybridomas specific for Myo1g. ELISA, immunofluorescence, and Western blot assay results revealed the recognition of Myo1g by these novel monoclonal antibodies in normal and transformed T and B cells. Here, we report the development and application of new monoclonal antibodies against Myo1g for their potential use to detect its overexpression in acute lymphoblastic leukemia (ALL) patients.

Polymer-Carrying Ability of Actin Filaments Interacting with Myosin Motors in a Biological Motility System In Vitro

The reconstituted motility system of actin-myosin is expected to be used in bioinspired transport devices, in which carried materials are attached to either moving actin filaments or walking myosin molecules. However, the dependence of the ability to transport on the size of the attached materials is still inadequately understood. Here, as carried materials, polyethylene glycols (PEGs) of various sizes are covalently bound to actin filaments, and the motility of PEG-attached filaments on a heavy meromyosin immobilized on a glass surface is observed via fluorescence microscopy. Full attachment of 2 kDa PEG, with an approximately 2 nm gyration radius, decreases the velocity and fraction of moving actin filaments by approximately 10% relative to unattached filaments. For the 5 kDa PEG, the fraction of moving filaments is decreased by approximately 70% even when the filaments contain only 20% PEG-attached actin. The attachment of both sizes of PEGs suppresses the actin-activated ATPase activity at the same level. These results suggest that actin filaments can carry PEGs up to 2 kDa having the same size as actin monomers, while the rate of ATP hydrolysis is limited. The size-dependence may provide a criterion for material delivery via actin filaments in nanotransport applications. This article is protected by copyright. All rights reserved.

Deconstructing the role of myosin contractility in force fluctuations within focal adhesions

Force fluctuations exhibited in focal adhesions that connect a cell to its extracellular environment point to the complex role of the underlying machinery that controls cell migration. To elucidate the explicit role of myosin motors in the temporal traction force oscillations, we vary the contractility of these motors in a dynamical model based on the molecular clutch hypothesis. As the contractility is lowered, effected both by changing the motor velocity and the rate of attachment/detachment, we show analytically in an experimentally relevant parameter space, that the system goes from decaying oscillations to stable limit cycle oscillations through a supercritical Hopf bifurcation. As a function of the motor activity and the number of clutches, the system exhibits a rich array of dynamical states. We corroborate our analytical results with stochastic simulations of the motor-clutch system. We obtain limit cycle oscillations in the parameter regime as predicted by our model. The frequency range of oscillations in the average clutch and motor deformation compares well with experimental results.

Nonmuscle Myosin II in cancer cell migration and mechanotransduction

Cell migration is a key step of cancer metastasis, immune-cell navigation, homing of stem cells and development. What adds complexity to it is the heterogeneity of the tissue environment that gives rise to a vast diversity of migratory mechanisms utilized by cells. A majority of cell motility mechanisms reported elsewhere largely converge in depicting the importance of the activity and complexity of actomyosin networks in the cell. In this review, we highlight the less discussed functional diversity of these actomyosin complexes and describe in detail how the major cellular actin-binding molecular motor proteins, nonmuscle myosin IIs are regulated and how they participate and mechanically reciprocate to changes in the microenvironment during cancer cell migration and tumor progression. Understanding the role of nonmuscle myosin IIs in the cancer cell is important for designing efficient therapeutic strategies to prevent cancer metastasis.

Proteomes, kinases and signalling pathways in virus-induced filopodia, as potential antiviral therapeutics targets

Filopodia are thin finger-like protrusions at the surface of cells that are internally occupied with bundles of tightly parallel actin filaments. They play significant roles in cellular physiological processes, such as adhesion to extracellular matrix, guidance towards chemo-attractants and in wound healing. Filopodia were recently reported to play important roles in viral infection including initial viral attachment to host cells, cell surfing, viral trafficking, internalization, budding, virus release and spread to other cells in a form that would avoid the host immune system. The detailed virus-host protein interactions underlying most of these processes remain to be elucidated. This review will describe some reported virus-host protein interactions on filopodia with the aim of identifying potential new anti-virus therapeutic targets. Exploring this research area may lead to the development of novel classes of anti-viral therapeutics that can block signalling pathways used by the virus to trigger filopodia formation. Successful compounds would inhibit initial virus attachment, formation of filopodia, expression of putative virus binding protein, extracellular virus trafficking, and budding.

Functional Role of Class III Myosins in Hair Cells

Cytoskeletal motors produce force and motion using the energy from ATP hydrolysis and function in a variety of mechanical roles in cells including muscle contraction, cargo transport, and cell division. Actin-based myosin motors have been shown to play crucial roles in the development and function of the stereocilia of auditory and vestibular inner ear hair cells. Hair cells can contain hundreds of stereocilia, which rely on myosin motors to elongate, organize, and stabilize their structure. Mutations in many stereocilia-associated myosins have been shown to cause hearing loss in both humans and animal models suggesting that each myosin isoform has a specific function in these unique parallel actin bundle-based protrusions. Here we review what is known about the classes of myosins that function in the stereocilia, with a special focus on class III myosins that harbor point mutations associated with delayed onset hearing loss. Much has been learned about the role of the two class III myosin isoforms, MYO3A and MYO3B, in maintaining the precise stereocilia lengths required for normal hearing. We propose a model for how class III myosins play a key role in regulating stereocilia lengths and demonstrate how their motor and regulatory properties are particularly well suited for this function. We conclude that ongoing studies on class III myosins and other stereocilia-associated myosins are extremely important and may lead to novel therapeutic strategies for the treatment of hearing loss due to stereocilia degeneration.

Haploid male germ cells-the Grand Central Station of protein transport

BACKGROUND

The precise movement of proteins and vesicles is an essential ability for all eukaryotic cells. Nowhere is this more evident than during the remarkable transformation that occurs in spermiogenesis-the transformation of haploid round spermatids into sperm. These transformations are critically dependent upon both the microtubule and the actin cytoskeleton, and defects in these processes are thought to underpin a significant percentage of human male infertility.

OBJECTIVE AND RATIONALE

This review is aimed at summarising and synthesising the current state of knowledge around protein/vesicle transport during haploid male germ cell development and identifying knowledge gaps and challenges for future research. To achieve this, we summarise the key discoveries related to protein transport using the mouse as a model system. Where relevant, we anchored these insights to knowledge in the field of human spermiogenesis and the causality of human male infertility.

SEARCH METHODS

Relevant studies published in English were identified using PubMed using a range of search terms related to the core focus of the review-protein/vesicle transport, intra-flagellar transport, intra-manchette transport, Golgi, acrosome, manchette, axoneme, outer dense fibres and fibrous sheath. Searches were not restricted to a particular time frame or species although the emphasis within the review is on mammalian spermiogenesis.

OUTCOMES

Spermiogenesis is the final phase of sperm development. It results in the transformation of a round cell into a highly polarised sperm with the capacity for fertility. It is critically dependent on the cytoskeleton and its ability to transport protein complexes and vesicles over long distances and often between distinct cytoplasmic compartments. The development of the acrosome covering the sperm head, the sperm tail within the ciliary lobe, the manchette and its role in sperm head shaping and protein transport into the tail, and the assembly of mitochondria into the mid-piece of sperm, may all be viewed as a series of overlapping and interconnected train tracks. Defects in this redistribution network lead to male infertility characterised by abnormal sperm morphology (teratozoospermia) and/or abnormal sperm motility (asthenozoospermia) and are likely to be causal of, or contribute to, a significant percentage of human male infertility.

WIDER IMPLICATIONS

A greater understanding of the mechanisms of protein transport in spermiogenesis offers the potential to precisely diagnose cases of male infertility and to forecast implications for children conceived using gametes containing these mutations. The manipulation of these processes will offer opportunities for male-based contraceptive development. Further, as increasingly evidenced in the literature, we believe that the continuous and spatiotemporally restrained nature of spermiogenesis provides an outstanding model system to identify, and de-code, cytoskeletal elements and transport mechanisms of relevance to multiple tissues.

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

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

TORC2-Gad8-dependent myosin phosphorylation modulates regulation by calcium

Cells respond to changes in their environment through signaling networks that modulate cytoskeleton and membrane organization to coordinate cell-cycle progression, polarized cell growth and multicellular development. Here, we define a novel regulatory mechanism by which the motor activity and function of the fission yeast type one myosin, Myo1, is modulated by TORC2-signalling-dependent phosphorylation. Phosphorylation of the conserved serine at position 742 (S742) within the neck region changes both the conformation of the neck region and the interactions between Myo1 and its associating calmodulin light chains. S742 phosphorylation thereby couples the calcium and TOR signaling networks that are involved in the modulation of myosin-1 dynamics to co-ordinate actin polymerization and membrane reorganization at sites of endocytosis and polarised cell growth in response to environmental and cell-cycle cues.

Arf6, JIP3, and dynein shape and mediate macropinocytosis

Macropinocytosis is an actin-driven form of clathrin-independent endocytosis that generates an enlarged structure, the macropinosome. Although many studies focus on signaling molecules and phosphoinositides involved in initiating macropinocytosis, the commitment to forming a macropinosome and the handling of that membrane have not been studied in detail. Here we show in HT1080 cells, a human fibrosarcoma cell line, a requirement for microtubules, dynein, the JIP3 microtubule motor scaffold protein, and Arf6, a JIP3 interacting protein, for the formation and inward movement of the macropinosome. While actin and myosin II also play critical roles in the formation of ruffling membrane, microtubules provide an important tract for initiation, sealing, and transport of the macropinosome through the actin- and myosin-rich lamellar region.

Class I myosins: Highly versatile proteins with specific functions in the immune system

Connections established between cytoskeleton and plasma membrane are essential in cellular processes such as cell migration, vesicular trafficking, and cytokinesis. Class I myosins are motor proteins linking the actin-cytoskeleton with membrane phospholipids. Previous studies have implicated these molecules in cell functions including endocytosis, exocytosis, release of extracellular vesicles and the regulation of cell shape and membrane elasticity. In immune cells, those proteins also are involved in the formation and maintenance of immunological synapse-related signaling. Thus, these proteins are master regulators of actin cytoskeleton dynamics in different scenarios. Although the localization of class I myosins has been described in vertebrates, their functions, regulation, and mechanical properties are not very well understood. In this review, we focused on and summarized the current understanding of class I myosins in vertebrates with particular emphasis in leukocytes.

Force percolation of contractile active gels

Living systems provide a paradigmatic example of active soft matter. Cells and tissues comprise viscoelastic materials that exert forces and can actively change shape. This strikingly autonomous behavior is powered by the cytoskeleton, an active gel of semiflexible filaments, crosslinks, and molecular motors inside cells. Although individual motors are only a few nm in size and exert minute forces of a few pN, cells spatially integrate the activity of an ensemble of motors to produce larger contractile forces (∼nN and greater) on cellular, tissue, and organismal length scales. Here we review experimental and theoretical studies on contractile active gels composed of actin filaments and myosin motors. Unlike other active soft matter systems, which tend to form ordered patterns, actin-myosin systems exhibit a generic tendency to contract. Experimental studies of reconstituted actin-myosin model systems have long suggested that a mechanical interplay between motor activity and the network's connectivity governs this contractile behavior. Recent theoretical models indicate that this interplay can be understood in terms of percolation models, extended to include effects of motor activity on the network connectivity. Based on concepts from percolation theory, we propose a state diagram that unites a large body of experimental observations. This framework provides valuable insights into the mechanisms that drive cellular shape changes and also provides design principles for synthetic active materials.

Variable phenotypic expression and onset in MYH14 distal hereditary motor neuropathy phenotype in a large, multigenerational North American family

INTRODUCTION

Distal hereditary motor neuropathy (dHMN) causes distal-predominant weakness without prominent sensory loss. Myosin heavy chain disorders most commonly result in distal myopathy and cardiomyopathy with or without hearing loss, but a complex phenotype with dHMN, myopathy, hoarseness, and hearing loss was reported in a Korean family with a c.2822G>T mutation in MYH14. In this study we report phenotypic features in a North American family with the c.2822G>T in MYH14.

METHODS

Clinical and molecular characterization was performed in a large, 6-generation, Caucasian family with MYH14 dHMN.

RESULTS

A total of 11 affected and 7 unaffected individuals were evaluated and showed varying age of onset and severity of weakness. Genotypic concordance was confirmed with molecular analysis. Electrophysiological studies demonstrated distal motor axonal degeneration without myopathy in all affected subjects tested.

CONCLUSION

Mutation of MYH14 can result in a range of neuromuscular phenotypes that includes a dHMN and hearing loss phenotype with variable age of onset. Muscle Nerve 56: 341-345, 2017.

The microtubule end-binding protein FgEB1 regulates polar growth and fungicide sensitivity via different interactors in Fusarium graminearum

In yeasts, the end-binding protein 1 (EB1) homologs regulate microtubule dynamics, cell polarization, and chromosome stability. However, functions of EB1 orthologs in plant pathogenic fungi have not been characterized yet. Here, we observed that the FgEB1 deletion mutant (ΔFgEB1) of Fusarium graminearum exhibits twisted hyphae, increased hyphal branching and curved conidia, indicating that FgEB1 is involved in the regulation of cellular polarity. Microscopic examination further showed that the microtubules of ΔFgEB1 exhibited less organized in comparison with those of the wild type. In addition, the lack of FgEB1 also altered the distribution of polarity-related class I myosin via the interaction with the actin. On the other hand, we identified four core septins as FgEB1-interacting proteins, and found that FgEB1 and septins regulated conidial polar growth in the opposite orientation. Interestingly, FgEB1 and FgKar9 constituted another complex that modulated the response to carbendazim, a microtubule-damaging agent specifically. In addition, the deletion of FgEB1 led to dramatically decreased deoxynivalenol (DON) biosynthesis. Taken together, results of this study indicate that FgEB1 regulates cellular polarity, fungicide sensitivity and DON biosynthesis via different interactors in F. graminarum, which provides a novel insight into understanding of the biological functions of EB1 in filamentous fungi.

Activity of nonmuscle myosin II isoforms determines localization at the cleavage furrow of megakaryocytes

Megakaryocyte polyploidy is characterized by cytokinesis failure resulting from defects in contractile forces at the cleavage furrow. Although immature megakaryocytes express 2 nonmuscle myosin II isoforms (MYH9 [NMIIA] and MYH10 [NMIIB]), only NMIIB localizes at the cleavage furrow, and its subsequent absence contributes to polyploidy. In this study, we tried to understand why the abundant NMIIA does not localize at the furrow by focusing on the RhoA/ROCK pathway that has a low activity in polyploid megakaryocytes. We observed that under low RhoA activity, NMII isoforms presented different activity that determined their localization. Inhibition of RhoA/ROCK signaling abolished the localization of NMIIB, whereas constitutively active RhoA induced NMIIA at the cleavage furrow. Thus, although high RhoA activity favored the localization of both the isoforms, only NMIIB could localize at the furrow at low RhoA activity. This was further confirmed in erythroblasts that have a higher basal RhoA activity than megakaryocytes and express both NMIIA and NMIIB at the cleavage furrow. Decreased RhoA activity in erythroblasts abolished localization of NMIIA but not of NMIIB from the furrow. This differential localization was related to differences in actin turnover. Megakaryocytes had a higher actin turnover compared with erythroblasts. Strikingly, inhibition of actin polymerization was found to be sufficient to recapitulate the effects of inhibition of RhoA/ROCK pathway on NMII isoform localization; thus, cytokinesis failure in megakaryocytes is the consequence of both the absence of NMIIB and a low RhoA activity that impairs NMIIA localization at the cleavage furrow through increased actin turnover.

Overexpression of myosin VI regulates gastric cancer cell progression

Myosin VI (MYO6) is a unique member of the myosin superfamily. Although it has been reported to participate in human cancer progression, the role of MYO6 in gastric cancer remains unclear. In this study, we found the expression of MYO6 gene was higher in gastric cancer tissues than in the normal tissues by Oncomine database mining and affects patient overall survival using the Kaplan-Meier plotter online analysis. Additionally, the expression levels of MYO6 were widely expressed in gastric cancer cells by quantitative real-time Polymerase Chain Reaction (qRT-PCR) and western blot assay. Then knockdown of MYO6 significantly suppressed the proliferation and colony formation abilities of AGS and MGC80-3 cells. Moreover, cell cycle analysis showed that inhibition of MYO6 induced cell cycle arrested in G0/G1 phase in AGS and MGC80-3 cells. Further analysis showed knockdown of MYO6 downregulated cell-cycle activators cyclin A and cyclin D1 and upregulated cell-cycle inhibitor p21, as determined by qRT-PCR and western blot analysis in MGC80-3 cells. Meanwhile, MYO6 inhibition significantly induced apoptosis in AGS and MGC80-3 cells. Also, knockdown of MYO6 increased the expression of apoptosis-related proteins Bax and cleaved Caspase-3, and decreased Bcl-2 expression by western blot analysis in MGC80-3 cells. In addition, MYO6 knockdown also inhibited cell migration ability in MGC80-3 cells. Taken together, our study indicates that MYO6 may play an important role in gastric cancer tumorigenesis and may serve as a potential therapeutic target in human gastric cancer.

Structural Basis of Cargo Recognition by Unconventional Myosins in Cellular Trafficking

Unconventional myosins are a superfamily of actin-based molecular motors playing diverse roles including cellular trafficking, mechanical supports, force sensing and transmission, etc. The variable neck and tail domains of unconventional myosins function to bind to specific cargoes including proteins and lipid vesicles and thus are largely responsible for the diverse cellular functions of myosins in vivo. In addition, the tail regions, together with their cognate cargoes, can regulate activities of the motor heads. This review outlines the advances made in recent years on cargo recognition and cargo binding-induced regulation of the activity of several unconventional myosins including myosin-I, V, VI and X in cellular trafficking. We approach this topic by describing a series of high-resolution structures of the neck and tail domains of these unconventional myosins either alone or in complex with their specific cargoes, and by discussing potential implications of these structural studies on cellular trafficking of these myosin motors.

The Relay/Converter Interface Influences Hydrolysis of ATP by Skeletal Muscle Myosin II

The interface between relay and converter domain of muscle myosin is critical for optimal myosin performance. Using Drosophila melanogaster indirect flight muscle S1, we performed a kinetic analysis of the effect of mutations in the converter and relay domain. Introduction of a mutation (R759E) in the converter domain inhibits the steady-state ATPase of myosin S1, whereas an additional mutation in the relay domain (N509K) is able to restore the ATPase toward wild-type values. The R759E S1 construct showed little effect on most steps of the actomyosin ATPase cycle. The exception was a 25-30% reduction in the rate constant of the hydrolysis step, the step coupled to the cross-bridge recovery stroke that involves a change in conformation at the relay/converter domain interface. Significantly, the double mutant restored the hydrolysis step to values similar to the wild-type myosin. Modeling the relay/converter interface suggests a possible interaction between converter residue 759 and relay residue 509 in the actin-detached conformation, which is lost in R759E but is restored in N509K/R759E. This detailed kinetic analysis of Drosophila myosin carrying the R759E mutation shows that the interface between the relay loop and converter domain is important for fine-tuning myosin kinetics, in particular ATP binding and hydrolysis.

N-Terminus of Cardiac Myosin Essential Light Chain Modulates Myosin Step-Size

Muscle myosin cyclically hydrolyzes ATP to translate actin. Ventricular cardiac myosin (βmys) moves actin with three distinct unitary step-sizes resulting from its lever-arm rotation and with step-frequencies that are modulated in a myosin regulation mechanism. The lever-arm associated essential light chain (vELC) binds actin by its 43 residue N-terminal extension. Unitary steps were proposed to involve the vELC N-terminal extension with the 8 nm step engaging the vELC/actin bond facilitating an extra ∼19 degrees of lever-arm rotation while the predominant 5 nm step forgoes vELC/actin binding. A minor 3 nm step is the unlikely conversion of the completed 5 to the 8 nm step. This hypothesis was tested using a 17 residue N-terminal truncated vELC in porcine βmys (Δ17βmys) and a 43 residue N-terminal truncated human vELC expressed in transgenic mouse heart (Δ43αmys). Step-size and step-frequency were measured using the Qdot motility assay. Both Δ17βmys and Δ43αmys had significantly increased 5 nm step-frequency and coincident loss in the 8 nm step-frequency compared to native proteins suggesting the vELC/actin interaction drives step-size preference. Step-size and step-frequency probability densities depend on the relative fraction of truncated vELC and relate linearly to pure myosin species concentrations in a mixture containing native vELC homodimer, two truncated vELCs in the modified homodimer, and one native and one truncated vELC in the heterodimer. Step-size and step-frequency, measured for native homodimer and at two or more known relative fractions of truncated vELC, are surmised for each pure species by using a new analytical method.

In vitro and in vivo single myosin step-sizes in striated muscle

Myosin in muscle transduces ATP free energy into the mechanical work of moving actin. It has a motor domain transducer containing ATP and actin binding sites, and, mechanical elements coupling motor impulse to the myosin filament backbone providing transduction/mechanical-coupling. The mechanical coupler is a lever-arm stabilized by bound essential and regulatory light chains. The lever-arm rotates cyclically to impel bound filamentous actin. Linear actin displacement due to lever-arm rotation is the myosin step-size. A high-throughput quantum dot labeled actin in vitro motility assay (Qdot assay) measures motor step-size in the context of an ensemble of actomyosin interactions. The ensemble context imposes a constant velocity constraint for myosins interacting with one actin filament. In a cardiac myosin producing multiple step-sizes, a "second characterization" is step-frequency that adjusts longer step-size to lower frequency maintaining a linear actin velocity identical to that from a shorter step-size and higher frequency actomyosin cycle. The step-frequency characteristic involves and integrates myosin enzyme kinetics, mechanical strain, and other ensemble affected characteristics. The high-throughput Qdot assay suits a new paradigm calling for wide surveillance of the vast number of disease or aging relevant myosin isoforms that contrasts with the alternative model calling for exhaustive research on a tiny subset myosin forms. The zebrafish embryo assay (Z assay) performs single myosin step-size and step-frequency assaying in vivo combining single myosin mechanical and whole muscle physiological characterizations in one model organism. The Qdot and Z assays cover "bottom-up" and "top-down" assaying of myosin characteristics.

Stresses at the cell surface during animal cell morphogenesis

Cell shape is determined by cellular mechanics. Cell deformations in animal cells, such as those required for cell migration, division or epithelial morphogenesis, are largely controlled by changes in mechanical stress and tension at the cell surface. The plasma membrane and the actomyosin cortex control surface mechanics and determine cell surface tension. Tension in the actomyosin cortex primarily arises from myosin-generated stresses and depends strongly on the ultrastructural architecture of the network. Plasma membrane tension is controlled mainly by the surface area of the membrane relative to cell volume and can be modulated by changing membrane composition, shape and the organization of membrane-associated proteins. We review here our current understanding of the control of cortex and membrane tension by molecular processes. We particularly highlight the need for studies that bridge the scales between microscopic events and emergent properties at the cellular level. Finally, we discuss how the mechanical interplay between membrane dynamics and cortex contractility is key to understanding the biomechanical control of cell morphogenesis.

Cargo recognition and cargo-mediated regulation of unconventional myosins

Organized motions are hallmarks of living organisms. Such motions range from collective cell movements during development and muscle contractions at the macroscopic scale all the way down to cellular cargo (e.g., various biomolecules and organelles) transportation and mechanoforce sensing at more microscopic scales. Energy required for these biological motions is almost invariably provided by cellular chemical fuels in the form of nucleotide triphosphate. Biological systems have designed a group of nanoscale engines, known as molecular motors, to convert cellular chemical fuels into mechanical energy. Molecular motors come in various forms including cytoskeleton motors (myosin, kinesin, and dynein), nucleic-acid-based motors, cellular membrane-based rotary motors, and so on. The main focus of this Account is one subfamily of actin filament-based motors called unconventional myosins (other than muscle myosin II, the remaining myosins are collectively referred to as unconventional myosins). In general, myosins can use ATP to fuel two types of mechanomotions: dynamic tethering actin filaments with various cellular compartments or structures and actin filament-based intracellular transport. In contrast to rich knowledge accumulated over many decades on ATP hydrolyzing motor heads and their interactions with actin filaments, how various myosins recognize their specific cargoes and whether and how cargoes can in return regulate functions of motors are less understood. Nonetheless, a series of biochemical and structural investigations in the past few years, including works from our own laboratory, begin to shed lights on these latter questions. Some myosins (e.g., myosin-VI) can function both as cellular transporters and as mechanical tethers. To function as a processive transporter, myosins need to form dimers or multimers. To be a mechanical tether, a monomeric myosin is sufficient. It has been shown for myosin-VI that its cellular cargo proteins can play critical roles in determining the motor properties. Dab2, an adaptor protein linking endocytic vesicles with actin-filament-bound myosin-VI, can induce the motor to form a transport competent dimer. Such a cargo-mediated dimerization mechanism has also been observed in other myosins including myosin-V and myosin-VIIa. The tail domains of myosins are very diverse both in their lengths and protein domain compositions and thus enable motors to engage a broad range of different cellular cargoes. Remarkably, the cargo binding tail of one myosin alone often can bind to multiple distinct target proteins. A series of atomic structures of myosin-V/cargo complexes solved recently reveals that the globular cargo binding tail of the motor contains a number of nonoverlapping target recognition sites for binding to its cargoes including melanophilin, vesicle adaptors RILPL2, and vesicle-bound GTPase Rab11. The structures of the MyTH4-FERM tandems from myosin-VIIa and myosin-X in complex with their respective targets reveal that MyTH4 and FERM domains extensively interact with each other forming structural and functional supramodules in both motors and demonstrate that the structurally similar MyTH4-FERM tandems of the two motors display totally different target binding modes. These structural studies have also shed light on why numerous mutations found in these myosins can cause devastating human diseases such as deafness and blindness, intellectual disabilities, immune disorders, and diabetes.

Non-muscle myosins in tumor progression, cancer cell invasion, and metastasis

The actin cytoskeleton, which regulates cell polarity, adhesion, and migration, can influence cancer progression, including initial acquisition of malignant properties by normal cells, invasion of adjacent tissues, and metastasis to distant sites. Actin-dependent molecular motors, myosins, play key roles in regulating tumor progression and metastasis. In this review, we examine how non-muscle myosins regulate neoplastic transformation and cancer cell migration and invasion. Members of the myosin superfamily can act as either enhancers or suppressors of tumor progression. This review summarizes the current state of knowledge on how mutations or epigenetic changes in myosin genes and changes in myosin expression may affect tumor progression and patient outcomes and discusses the proposed mechanisms linking myosin inactivation or upregulation to malignant phenotype, cancer cell migration, and metastasis.

Analytical comparison of natural and pharmaceutical ventricular myosin activators

Ventricular myosin (βMys) is the motor protein in cardiac muscle generating force using ATP hydrolysis free energy to translate actin. In the cardiac muscle sarcomere, myosin and actin filaments interact cyclically and undergo rapid relative translation facilitated by the low duty cycle motor. It contrasts with high duty cycle processive myosins for which persistent actin association is the priority. The only pharmaceutical βMys activator, omecamtive mecarbil (OM), upregulates cardiac contractility in vivo and is undergoing testing for heart failure therapy. In vitro βMys step-size, motility velocity, and actin-activated myosin ATPase were measured to determine duty cycle in the absence and presence of OM. A new parameter, the relative step-frequency, was introduced and measured to characterize βMys motility due to the involvement of its three unitary step-sizes. Step-size and relative step-frequency were measured using the Qdot assay. OM decreases motility velocity 10-fold without affecting step-size, indicating a large increase in duty cycle converting βMys to a near processive myosin. The OM conversion dramatically increases force and modestly increases power over the native βMys. Contrasting motility modification due to OM with that from the natural myosin activator, specific βMys phosphorylation, provides insight into their respective activation mechanisms and indicates the boilerplate screening characteristics desired for pharmaceutical βMys activators. New analytics introduced here for the fast and efficient Qdot motility assay create a promising method for high-throughput screening of motor proteins and their modulators.

Ventricular myosin modifies in vitro step-size when phosphorylated

Cardiac and skeletal muscle myosins have the central role in contraction transducing ATP free energy into the mechanical work of moving actin. Myosin has a motor domain containing ATP and actin binding sites and a lever-arm that undergoes rotation impelling bound actin. The lever-arm converts torque generated in the motor into the linear displacement known as step-size. The myosin lever-arm is stabilized by bound essential and regulatory light chains (ELC and RLC). RLC phosphorylation at S15 is linked to modified lever-arm mechanical characteristics contributing to myosin filament based contraction regulation and to the response of the muscle to disease. Myosin step-size was measured using a novel quantum dot (Qdot) assay that previously confirmed a 5nm step-size for fast skeletal myosin and multiple unitary steps, most frequently 5 and 8nm, and a rare 3nm displacement for β cardiac myosin (βMys). S15 phosphorylation in βMys is now shown to change step-size distribution by advancing the 8nm step frequency. After phosphorylation, the 8nm step is the dominant myosin step-size resulting in significant gain in the average step-size. An increase in myosin step-size will increase the amount of work produced per ATPase cycle. The results indicate that RLC phosphorylation modulates work production per ATPase cycle suggesting the mechanism for contraction regulation by the myosin filament.

Myosin IIA is critical for organelle distribution and F-actin organization in megakaryocytes and platelets

During proplatelet formation, a relatively homogeneous content of organelles is transported from the megakaryocyte (MK) to the nascent platelets along microtubule tracks. We found that platelets from Myh9(-/-) mice and a MYH9-RD patient were heterogeneous in their organelle content (granules and mitochondria). In addition, Myh9(-/-) MKs have an abnormal cytoplasmic clustering of organelles, suggesting that the platelet defect originates in the MKs. Myosin is not involved in the latest stage of organelle traffic along microtubular tracks in the proplatelet shafts as shown by confocal observations of proplatelet buds. By contrast, it is required for the earlier distribution of organelles within the large MK preplatelet fragments shed into the sinusoid circulation before terminal proplatelet remodeling. We show here that F-actin is abnormally clustered in the cytoplasm of Myh9(-/-) MKs and actin polymerization is impaired in platelets. Myosin IIA is required for normal granule motility and positioning within MKs, mechanisms that may be dependent on organelle traveling and tethering onto F-actin cytoskeleton tracks. Altogether, our results indicate that the distribution of organelles within platelets critically depends on a homogeneous organelle distribution within MKs and preplatelet fragments, which requires myosin IIA.

Live-cell imaging of Marburg virus-infected cells uncovers actin-dependent transport of nucleocapsids over long distances

Transport of large viral nucleocapsids from replication centers to assembly sites requires contributions from the host cytoskeleton via cellular adaptor and motor proteins. For the Marburg and Ebola viruses, related viruses that cause severe hemorrhagic fevers, the mechanism of nucleocapsid transport remains poorly understood. Here we developed and used live-cell imaging of fluorescently labeled viral and host proteins to characterize the dynamics and molecular requirements of nucleocapsid transport in Marburg virus-infected cells under biosafety level 4 conditions. The study showed a complex actin-based transport of nucleocapsids over long distances from the viral replication centers to the budding sites. Only after the nucleocapsids had associated with the matrix viral protein VP40 at the plasma membrane were they recruited into filopodia and cotransported with host motor myosin 10 toward the budding sites at the tip or side of the long cellular protrusions. Three different transport modes and velocities were identified: (i) Along actin filaments in the cytosol, nucleocapsids were transported at ∼200 nm/s; (ii) nucleocapsids migrated from one actin filament to another at ∼400 nm/s; and (iii) VP40-associated nucleocapsids moved inside filopodia at 100 nm/s. Unique insights into the spatiotemporal dynamics of nucleocapsids and their interaction with the cytoskeleton and motor proteins can lead to novel classes of antivirals that interfere with the trafficking and subsequent release of the Marburg virus from infected cells.

The Qdot-labeled actin super-resolution motility assay measures low-duty cycle muscle myosin step size

Myosin powers contraction in heart and skeletal muscle and is a leading target for mutations implicated in inheritable muscle diseases. During contraction, myosin transduces ATP free energy into the work of muscle shortening against resisting force. Muscle shortening involves relative sliding of myosin and actin filaments. Skeletal actin filaments were fluorescently labeled with a streptavidin conjugate quantum dot (Qdot) binding biotin-phalloidin on actin. Single Qdots were imaged in time with total internal reflection fluorescence microscopy and then spatially localized to 1-3 nm using a super-resolution algorithm as they translated with actin over a surface coated with skeletal heavy meromyosin (sHMM) or full-length β-cardiac myosin (MYH7). The average Qdot-actin velocity matches measurements with rhodamine-phalloidin-labeled actin. The sHMM Qdot-actin velocity histogram contains low-velocity events corresponding to actin translation in quantized steps of ~5 nm. The MYH7 velocity histogram has quantized steps at 3 and 8 nm in addition to 5 nm and larger compliance compared to that of sHMM depending on the MYH7 surface concentration. Low-duty cycle skeletal and cardiac myosin present challenges for a single-molecule assay because actomyosin dissociates quickly and the freely moving element diffuses away. The in vitro motility assay has modestly more actomyosin interactions, and methylcellulose inhibited diffusion to sustain the complex while preserving a subset of encounters that do not overlap in time on a single actin filament. A single myosin step is isolated in time and space and then characterized using super-resolution. The approach provides a quick, quantitative, and inexpensive step size measurement for low-duty cycle muscle myosin.

Myo1c facilitates G-actin transport to the leading edge of migrating endothelial cells

Addition of actin monomer (G-actin) to growing actin filaments (F-actin) at the leading edge generates force for cell locomotion. The polymerization reaction and its regulation have been studied in depth. However, the mechanism responsible for transport of G-actin substrate to the cell front is largely unknown; random diffusion, facilitated transport via myosin II contraction, local synthesis as a result of messenger ribonucleic acid localization, or F-actin turnover all might contribute. By tracking a photoactivatable, nonpolymerizable actin mutant, we show vectorial transport of G-actin in live migrating endothelial cells (ECs). Mass spectrometric analysis identified Myo1c, an unconventional F-actin-binding motor protein, as a major G-actin-interacting protein. The cargo-binding tail domain of Myo1c interacted with G-actin, and the motor domain was required for the transport. Local microinjection of Myo1c promoted G-actin accumulation and plasma membrane ruffling, and Myo1c knockdown confirmed its contribution to G-actin delivery to the leading edge and for cell motility. In addition, there is no obvious requirement for myosin II contractile-based transport of G-actin in ECs. Thus, Myo1c-facilitated G-actin transport might be a critical node for control of cell polarity and motility.

Allosteric communication in Dictyostelium myosin II

Myosin's affinities for nucleotides and actin are reciprocal. Actin-binding substantially reduces the affinity of ATP for myosin, but the effect of actin on myosin's ADP affinity is quite variable among myosin isoforms, serving as the principal mechanism for tuning the actomyosin system to specific physiological purposes. To understand the structural basis of this variable relationship between actin and ADP binding, we studied several constructs of the catalytic domain of Dictyostelium myosin II, varying their length (from the N-terminal origin) and cysteine content. The constructs varied considerably in their actin-activated ATPase activity and in the effect of actin on ADP affinity. Actin had no significant effect on ADP affinity for a single-cysteine catalytic domain construct, a double-cysteine construct partially restored the actin-dependence of ADP binding, and restoration of all native Cys restored it further, but full restoration of function (similar to that of skeletal muscle myosin II) was obtained only by adding all native Cys and an artificial lever arm extension. Pyrene-actin fluorescence confirmed these effects on ADP binding to actomyosin. We conclude that myosin's Cys content and lever arm both allosterically modulate the reciprocal affinities of myosin for ADP and actin, a key determinant of the biological functions of myosin isoforms.

A role for myosin IXb, a motor-RhoGAP chimera, in epithelial wound healing and tight junction regulation

Myo9b is a motor–RhoGAP chimera that has been implicated in inflammatory bowel disease. Findings suggest that Myo9b is essential during both collective and individual wound-induced cell migration. It is also important for maintaining tight junction barrier integrity.

Polymorphisms in the gene encoding the heavy chain of myosin IXb (Myo9b) have been linked to several forms of inflammatory bowel disease (IBD). Given that Myo9b contains a RhoGTPase-activating protein domain within its tail, it may play key roles in Rho-mediated actin cytoskeletal modifications critical to intestinal barrier function. In wounded monolayers of the intestinal epithelial cell line Caco2_BBe (BBe), Myo9b localizes to the extreme leading edge of lamellipodia of migrating cells. BBe cells exhibiting loss of Myo9b expression with RNA interference or Myo9b C-terminal dominant-negative (DN) tail-tip expression lack lamellipodia, fail to migrate into the wound, and form stress fiber–like arrays of actin at the free edges of cells facing the wound. These cells also exhibit disruption of tight junction (TJ) protein localization, including ZO-1, occludin, and claudin-1. Torsional motility and junctional permeability to dextran are greatly increased in cells expressing DN-tail-tip. Of interest, this effect is propagated to neighboring cells. Consistent with a role for Myo9b in regulating levels of active Rho, localization of both RhoGTP and myosin light chain phosphorylation corresponds to Myo9b-knockdown regions of BBe monolayers. These data reveal critical roles for Myo9b during epithelial wound healing and maintenance of TJ integrity—key functions that may be altered in patients with Myo9b-linked IBD.

Myosin VI is required for maintenance of brush border structure, composition, and membrane trafficking functions in the intestinal epithelial cell

Characterization of the intestinal epithelium of the Snell's waltzer (sv/sv) mouse revealed that myosin VI (Myo6) is required for proper brush border (BB) ultrastructure, composition and membrane traffic. The defects observed were distinct from that observed in the myosin Ia KO, even though Myo6 is lost from the BB in this KO. Myo6 is expressed throughout the length of the small and large intestine; it is localized to the subapical inter-microvillar (MV) domain and basolateral membrane. Defects in the BB include apparent lifting of the plasma membrane off of the actin cytoskeleton in the inter-MV region, fusion of MV, and disorganized morphology of the terminal web. The molecular composition of the sv/sv BB is altered. This includes increased expression of myosin Va, myosin Ie and the MV actin binding proteins espin and phosphorylated-ezrin; myosin Id is reduced. Changes in endocytic components include reduced clathrin and adaptin β, and increased disabled-2. Endocytic uptake of lumenal lactoferrin is inhibited in adult, but not neonatal intestinal epithelial cells. There is increased BB membrane-associated expression of both the Na(+)/H(+) exchanger, NHE3 and the Na(+)/phosphate transporter, NaPi2b. These results suggest that Myo6 is involved in the regulated trafficking of NHE3 and NaPi2b between the BB membrane and endosome.

Friction-controlled traction force in cell adhesion

The force balance between the extracellular microenvironment and the intracellular cytoskeleton controls the cell fate. We report a new (to our knowledge) mechanism of receptor force control in cell adhesion originating from friction between cell adhesion ligands and the supporting substrate. Adherent human endothelial cells have been studied experimentally on polymer substrates noncovalently coated with fluorescent-labeled fibronectin (FN). The cellular traction force correlated with the mobility of FN during cell-driven FN fibrillogenesis. The experimental findings have been explained within a mechanistic two-dimensional model of the load transfer at focal adhesion sites. Myosin motor activity in conjunction with sliding of FN ligands noncovalently coupled to the surface of the polymer substrates is shown to result in a controlled traction force of adherent cells. We conclude that the friction of adhesion ligands on the supporting substrate is important for mechanotransduction and cell development of adherent cells in vitro and in vivo.

Plus-end directed myosins accelerate actin filament sliding by single-headed myosin VI

Myosin VI (Myo6) is unique among myosins in that it moves toward the minus (pointed) end of the actin filament. Thus to exert tension on, or move cargo along an actin filament, Myo6 is working against potentially multiple plus (barbed)-end myosins. To test the effect of plus-end motors on Myo6, the gliding actin filament assay was used to assess the motility of single-headed Myo6 in the absence and presence of cardiac myosin II (Myo2) and myosin Va (Myo5a). Myo6 alone exhibited a filament gliding velocities of 60.34 ± 13.68 nm/s. Addition of either Myo2 or Myo5a, at densities below that required to promote plus-end movement resulted in an increase in Myo6 velocity (~100-150% increase). Movement in the presence of these plus-end myosins was minus-end directed as determined using polarity tagged filaments. High densities of Myo2 or Myo5a were required to convert to plus-end directed motility indicating that Myo6 is a potent inhibitor of Myo2 and Myo5a. Previous studies have shown that two-headed Myo6 slows and then stalls in an anchored state under load. Consistent with these studies, velocity of a two headed heavy mero myosin form of Myo6 was unaffected by Myo5a at low densities, and was inhibited at high Myo5a densities.

Alphaherpesviruses and the cytoskeleton in neuronal infections

Following infection of exposed peripheral tissues, neurotropic alphaherpesviruses invade nerve endings and deposit their DNA genomes into the nuclei of neurons resident in ganglia of the peripheral nervous system. The end result of these events is the establishment of a life-long latent infection. Neuroinvasion typically requires efficient viral transmission through a polarized epithelium followed by long-distance transport through the viscous axoplasm. These events are mediated by the recruitment of the cellular microtubule motor proteins to the intracellular viral particle and by alterations to the cytoskeletal architecture. The focus of this review is the interplay between neurotropic herpesviruses and the cytoskeleton.