Non-muscle myosin 2 at a glance

Waves of change: Dynamic actomyosin networks in embryonic development

As animals develop, molecules, cells, and cell ensembles move in beautifully orchestrated choreographies. Movement at each of these scales requires generation of mechanical force. In eukaryotic cells, the actomyosin cytoskeleton generates mechanical forces. Continuous advances in in vivo microscopy have enabled visualization and quantitative assessment of actomyosin dynamics and force generation, within and across cells, in living embryos. Recent studies reveal that actomyosin networks can form periodic waves in vivo. Here, we highlight contributions of actomyosin waves to molecular transport, cell movement, and cell coordination in developing embryos.

Actomyosin forces in cell migration: Moving beyond cell body retraction

In textbook illustrations of migrating cells, actomyosin contractility is typically depicted as the contraction force necessary for cell body retraction. This dogma has been transformed by the molecular clutch model, which acknowledges that actomyosin traction forces also generate and transmit biomechanical signals at the leading edge, enabling cells to sense and shape their migratory path in mechanically complex environments. To fulfill these complementary functions, the actomyosin system assembles a gradient of contractile energy along the front-rear axis of migratory cells. Here, we highlight the hierarchic assembly and self-regulatory network structure of the actomyosin system and explain how the kinetics of different nonmuscle myosin II (NM II) paralogs synergize during contractile force generation. Our aim is to emphasize how protrusion formation, cell adhesion, contraction, and retraction are spatiotemporally integrated during different modes of migration, including chemotaxis and durotaxis. Finally, we hypothesize how different NM II paralogs might tune aspects of migration in vivo, highlighting future research directions.

Principles and regulation of mechanosensing

Research over the past two decades has highlighted that mechanical signaling is a crucial component in regulating biological processes. Although many processes and proteins are termed 'mechanosensitive', the underlying mechanisms involved in mechanosensing can vary greatly. Recent studies have also identified mechanosensing behaviors that can be regulated independently of applied force. This important finding has major implications for our understanding of downstream mechanotransduction, the process by which mechanical signals are converted into biochemical signals, as it offers another layer of biochemical regulatory control for these crucial signaling pathways. In this Review, we discuss the different molecular and cellular mechanisms of mechanosensing, how these processes are regulated and their effects on downstream mechanotransduction. Together, these discussions provide an important perspective on how cells and tissues control the ways in which they sense and interpret mechanical signals.

Myosin-1C augments endothelial secretion of von Willebrand factor by linking contractile actomyosin machinery to the plasma membrane

ABSTRACT

Blood endothelial cells control the hemostatic and inflammatory response by secreting von Willebrand factor (VWF) and P-selectin from storage organelles called Weibel-Palade bodies (WPBs). Actin-associated motor proteins regulate this secretory pathway at multiple points. Before fusion, myosin Va forms a complex that anchors WPBs to peripheral actin structures, allowing for the maturation of content. After fusion, an actomyosin ring/coat is recruited and compresses the WPB to forcibly expel the largest VWF multimers. Here, we provide, to our knowledge, the first evidence for the involvement of class I myosins during regulated VWF secretion. We show that the unconventional myosin-1C (Myo1c) is recruited after fusion via its pleckstrin homology domain in an actin-independent process. This provides a link between the actin ring and phosphatidylinositol 4,5-bisphosphate (PIP2) at the membrane of the fused organelle and is necessary to ensure maximal VWF secretion. This is an active process requiring Myo1c ATPase activity because inhibition of class I myosins using the inhibitor pentachloropseudilin or expression of an ATPase-deficient Myo1c rigor mutant perturbs the expulsion of VWF and alters the kinetics of the exocytic actin ring. These data offer a novel insight into the control of an essential physiological process and provide a new way in which it can be regulated.

MultiBac System-based Purification and Biophysical Characterization of Human Myosin-7a

Myosin-7a is an actin-based motor protein vital for auditory and visual processes. Mutations in myosin-7a lead to Usher syndrome type 1, the most common and severe form of deaf-blindness in humans. It is hypothesized that myosin-7a forms a transmembrane adhesion complex with other Usher proteins, essential for the structural-functional integrity of photoreceptor and cochlear hair cells. However, due to the challenges in obtaining pure, intact protein, the exact functional mechanisms of human myosin-7a remain elusive, with limited structural and biomechanical studies available. Recent studies have shown that mammalian myosin-7a is a multimeric motor complex consisting of a heavy chain and three types of light chains: regulatory light chain (RLC), calmodulin, and calmodulin-like protein 4 (CALML4). Unlike calmodulin, CALML4 does not bind to calcium ions. Both the calcium-sensitive, and insensitive calmodulins are critical for mammalian myosin-7a for proper fine-tuning of its mechanical properties. Here, we describe a detailed method to produce recombinant human myosin-7a holoenzyme using the MultiBac Baculovirus protein expression system. This yields milligram quantities of high-purity full-length protein, allowing for its biochemical and biophysical characterization. We further present a protocol for assessing its mechanical and motile properties using tailored in vitro motility assays and fluorescence microscopy. The availability of the intact human myosin-7a protein, along with the detailed functional characterization protocol described here, paves the way for further investigations into the molecular aspects of myosin-7a in vision and hearing.

Non-muscle myosin 2 can incorporate into established filaments in cells without an assembly competence domain

Myosin 2 dynamically assembles into filaments that exert force on the actin cytoskeleton. To form filaments, myosin 2 monomers transition between folded and unfolded states. Monomer unfolding exposes an extended coiled-coil that interacts with other monomers in parallel and antiparallel fashions, enabling bipolar filament formation. A C-terminal domain of the coiled-coil, termed assembly competence domain (ACD), has been repeatedly identified as necessary for filament assembly. Here, we revisit ACD contribution when full-length filaments are present. Non-muscle myosin 2A lacking the ACD (ΔACD) initially appears diffuse, but triton extraction of cytosolic fraction reveals cytoskeletal association. Disruption of the folded monomer enhances the cytoskeletal fraction, while inhibition of endogenous filament assembly appears to reduce it. Finally, high resolution imaging of endogenous and exogenous bipolar filamentous structures reveals highly coincident signal, suggesting ΔACD constructs co-assemble with endogenous myosin 2A filaments. Our data demonstrate that while the ACD is required for de novo filament assembly, it is not required for monomers to recognize and associate with established filaments in cells. More broadly, this highlights the existence of distinct mechanisms governing myosin 2 monomer assembly into nascent filaments, and monomer recognition and association with established filaments to maintain steady-state contractile networks.

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.

MT-125 Inhibits Non-Muscle Myosin IIA and IIB, Synergizes with Oncogenic Kinase Inhibitors, and Prolongs Survival in Glioblastoma

We have identified a NMIIA and IIB-specific small molecule inhibitor, MT-125, and have studied its effects in GBM. MT-125 has high brain penetrance and retention and an excellent safety profile; blocks GBM invasion and cytokinesis, consistent with the known roles of NMII; and prolongs survival as a single agent in murine GBM models. MT-125 increases signaling along both the PDGFR- and MAPK-driven pathways through a mechanism that involves the upregulation of reactive oxygen species, and it synergizes with FDA-approved PDGFR and mTOR inhibitors in vitro . Combining MT-125 with sunitinib, a PDGFR inhibitor, or paxalisib, a combined PI3 Kinase/mTOR inhibitor significantly improves survival in orthotopic GBM models over either drug alone, and in the case of sunitinib, markedly prolongs survival in ∼40% of mice. Our results provide a powerful rationale for developing NMII targeting strategies to treat cancer and demonstrate that MT-125 has strong clinical potential for the treatment of GBM.

Highlights

MT-125 is a highly specific small molecule inhibitor of non-muscle myosin IIA and IIB, is well-tolerated, and achieves therapeutic concentrations in the brain with systemic dosing.Treating preclinical models of glioblastoma with MT-125 produces durable improvements in survival.MT-125 stimulates PDGFR- and MAPK-driven signaling in glioblastoma and increases dependency on these pathways.Combining MT-125 with an FDA-approved PDGFR inhibitor in a mouse GBM model synergizes to improve median survival over either drug alone, and produces tumor free, prolonged survival in over 40% of mice.

Motor proteins, spermatogenesis and testis function

The role of motor proteins in supporting intracellular transports of vesicles and organelles in mammalian cells has been known for decades. On the other hand, the function of motor proteins that support spermatogenesis is also well established since the deletion of motor protein genes leads to subfertility and/or infertility. Furthermore, mutations and genetic variations of motor protein genes affect fertility in men, but also a wide range of developmental defects in humans including multiple organs besides the testis. In this review, we seek to provide a summary of microtubule and actin-dependent motor proteins based on earlier and recent findings in the field. Since these two cytoskeletons are polarized structures, different motor proteins are being used to transport cargoes to different ends of these cytoskeletons. However, their involvement in germ cell transport across the blood-testis barrier (BTB) and the epithelium of the seminiferous tubules remains relatively unknown. It is based on recent findings in the field, we have provided a hypothetical model by which motor proteins are being used to support germ cell transport across the BTB and the seminiferous epithelium during the epithelial cycle of spermatogenesis. In our discussion, we have highlighted the areas of research that deserve attention to bridge the gap of research in relating the function of motor proteins to spermatogenesis.

Moonwalking molecular machines: Unraveling the choreography of myosin filament assembly

We have made tremendous progress in identifying the machines that shape the architecture of actin filaments. However, we know less about the mechanisms mediating myosin assembly at the supramolecular level. In this issue, Quintanilla et al. (https://doi.org/10.1083/jcb.202305023) provide important new insights into this process.

Local monomer levels and established filaments potentiate non-muscle myosin 2 assembly

The ability to dynamically assemble contractile networks is required throughout cell physiology, yet direct biophysical mechanisms regulating non-muscle myosin 2 filament assembly in living cells are lacking. Here, we use a suite of dynamic, quantitative imaging approaches to identify deterministic factors that drive myosin filament appearance and amplification. We find that actin dynamics regulate myosin assembly, but that the static actin architecture plays a less clear role. Instead, remodeling of actin networks modulates the local myosin monomer levels and facilitates assembly through myosin:myosin-driven interactions. Using optogenetically controlled myosin, we demonstrate that locally concentrating myosin is sufficient to both form filaments and jump-start filament amplification and partitioning. By counting myosin monomers within filaments, we demonstrate a myosin-facilitated assembly process that establishes filament stacks prior to partitioning into clusters that feed higher-order networks. Together, these findings establish the biophysical mechanisms regulating the assembly of non-muscle contractile structures that are ubiquitous throughout cell biology.

Suggesting Dictyostelium as a Model for Disease-Related Protein Studies through Myosin II Polymerization Pathway

Dictyostelium myosin II displays remarkable dynamism within the cell, continually undergoing polymerization and depolymerization processes. Under low-ion conditions, it assumes a folded structure like muscle myosins and forms thick filaments through polymerization. In our study, we presented intermediate structures observed during the early stages of polymerization of purified myosin via negative staining electron microscopy, immediately crosslinked with glutaraldehyde at the onset of polymerization. We identified folded monomers, dimers, and tetramers in the process. Our findings suggest that Dictyostelium myosin II follows a polymerization pathway in vitro akin to muscle myosin, with folded monomers forming folded parallel and antiparallel dimers that subsequently associate to create folded tetramers. These folded tetramers eventually unfold and associate with other tetramers to produce long filaments. Furthermore, our research revealed that ATP influences filament size, reducing it regardless of the status of RLC phosphorylation while significantly increasing the critical polymerization concentrations from 0.2 to 9 nM. In addition, we demonstrate the morphology of fully matured Dictyostelium myosin II filaments.

Limiting pool and actin architecture controls myosin cluster sizes in adherent cells

The actomyosin cytoskeleton generates mechanical forces that power important cellular processes, such as cell migration, cell division, and mechanosensing. Actomyosin self-assembles into contractile networks and bundles that underlie force generation and transmission in cells. A central step is the assembly of the myosin II filament from myosin monomers, regulation of which has been extensively studied. However, myosin filaments are almost always found as clusters within the cell cortex. While recent studies characterized cluster nucleation dynamics at the cell periphery, how myosin clusters grow on stress fibers remains poorly characterized. Here, we utilize a U2OS osteosarcoma cell line with endogenously tagged myosin II to measure the myosin cluster size distribution in the lamella of adherent cells. We find that myosin clusters can grow with Rho-kinase (ROCK) activity alone in the absence of myosin motor activity. Time-lapse imaging reveals that myosin clusters grow via increased myosin association to existing clusters, which is potentiated by ROCK-dependent myosin filament assembly. Enabling myosin motor activity allows further myosin cluster growth through myosin association that is dependent on F-actin architecture. Using a toy model, we show that myosin self-affinity is sufficient to recapitulate the experimentally observed myosin cluster size distribution, and that myosin cluster sizes are determined by the pool of myosin available for cluster growth. Together, our findings provide new insights into the regulation of myosin cluster sizes within the lamellar actomyosin cytoskeleton.

TMEM25 is a Par3-binding protein that attenuates claudin assembly during tight junction development

The tight junction (TJ) in epithelial cells is formed by integral membrane proteins and cytoplasmic scaffolding proteins. The former contains the claudin family proteins with four transmembrane segments, while the latter includes Par3, a PDZ domain-containing adaptor that organizes TJ formation. Here we show the single membrane-spanning protein TMEM25 localizes to TJs in epithelial cells and binds to Par3 via a PDZ-mediated interaction with its C-terminal cytoplasmic tail. TJ development during epithelial cell polarization is accelerated by depletion of TMEM25, and delayed by overexpression of TMEM25 but not by that of a C-terminally deleted protein, indicating a regulatory role of TMEM25. TMEM25 associates via its N-terminal extracellular domain with claudin-1 and claudin-2 to suppress their cis- and trans-oligomerizations, both of which participate in TJ strand formation. Furthermore, Par3 attenuates TMEM25-claudin association via binding to TMEM25, implying its ability to affect claudin oligomerization. Thus, the TJ protein TMEM25 appears to negatively regulate claudin assembly in TJ formation, which regulation is modulated by its interaction with Par3.

Non-muscle myosin II regulates presynaptic actin assemblies and neuronal mechanobiology

Neuromuscular junctions (NMJs) are evolutionarily ancient, specialized contacts between neurons and muscles. Axons and NMJs must endure mechanical strain through a lifetime of muscle contraction, making them vulnerable to aging and neurodegenerative conditions. However, cellular strategies for mitigating this mechanical stress remain unknown. In this study, we used Drosophila larval NMJs to investigate the role of actin and myosin (actomyosin)-mediated contractility in generating and responding to cellular forces at the neuron-muscle interface. We identified a new long-lived, low-turnover presynaptic actin core traversing the NMJ, which partly co-localizes with non-muscle myosin II (NMII). Neuronal RNAi of NMII induced disorganization of this core, suggesting that this structure might have contractile properties. Interestingly, neuronal RNAi of NMII also decreased NMII levels in the postsynaptic muscle proximal to neurons, suggesting that neuronal actomyosin rearrangements propagate their effects trans-synaptically. We also observed reduced Integrin levels upon NMII knockdown, indicating that neuronal actomyosin disruption triggers rearrangements of Integrin-mediated connections between neurons and surrounding muscle tissue. In summary, our study identifies a previously uncharacterized presynaptic actomyosin subpopulation that upholds the neuronal mechanical continuum, transmits signals to adjacent muscle tissue, and collaborates with Integrin receptors to govern the mechanobiology of the neuromuscular junction.

Nonmuscle myosin 2 filaments are processive in cells

Directed transport of cellular components is often dependent on the processive movements of cytoskeletal motors. Myosin 2 motors predominantly engage actin filaments of opposing orientation to drive contractile events and are therefore not traditionally viewed as processive. However, recent in vitro experiments with purified nonmuscle myosin 2 (NM2) demonstrated myosin 2 filaments could move processively. Here, we establish processivity as a cellular property of NM2. Processive runs in central nervous system-derived CAD cells are most apparent on bundled actin in protrusions that terminate at the leading edge. We find that processive velocities in vivo are consistent with in vitro measurements. NM2 makes these processive runs in its filamentous form against lamellipodia retrograde flow, though anterograde movement can still occur in the absence of actin dynamics. Comparing the processivity of NM2 isoforms, we find that NM2A moves slightly faster than NM2B. Finally, we demonstrate that this is not a cell-specific property, as we observe processive-like movements of NM2 in the lamella and subnuclear stress fibers of fibroblasts. Collectively, these observations further broaden NM2 functionality and the biological processes in which the already ubiquitous motor can contribute.

Limiting Pool and Actin Architecture Controls Myosin Cluster Sizes in Adherent Cells

The actomyosin cytoskeleton generates mechanical forces that power important cellular processes, such as cell migration, cell division, and mechanosensing. Actomyosin self-assembles into contractile networks and bundles that underlie force generation and transmission in cells. A central step is the assembly of the myosin II filament from myosin monomers, regulation of which has been extensively studied. However, myosin filaments are almost always found as clusters within the cell cortex. While recent studies characterized cluster nucleation dynamics at the cell periphery, how myosin clusters grow on stress fibers remains poorly characterized. Here, we utilize a U2OS osteosarcoma cell line with endogenously tagged myosin II to measure the myosin cluster size distribution in the lamella of adherent cells. We find that myosin clusters can grow with Rho-kinase (ROCK) activity alone in the absence of myosin motor activity. Time-lapse imaging reveals that myosin clusters grow via increased myosin association to existing clusters, which is potentiated by ROCK-dependent myosin filament assembly. Enabling myosin motor activity allows further myosin cluster growth through myosin association that is dependent on F-actin architecture. Using a toy model, we show that myosin self-affinity is sufficient to recapitulate the experimentally observed myosin cluster size distribution, and that myosin cluster sizes are determined by the pool of myosin available for cluster growth. Together, our findings provide new insights into the regulation of myosin cluster sizes within the lamellar actomyosin cytoskeleton.

Local Monomer Levels and Established Filaments Potentiate Non-Muscle Myosin 2 Assembly

The ability to dynamically assemble contractile networks is required throughout cell physiology, yet the biophysical mechanisms regulating non-muscle myosin 2 filament assembly in living cells are lacking. Here we use a suite of dynamic, quantitative imaging approaches to identify deterministic factors that drive myosin filament appearance and amplification. We find that actin dynamics regulate myosin assembly, but that the actin architecture plays a minimal direct role. Instead, remodeling of actin networks modulates the local myosin monomer levels and facilitates assembly through myosin:myosin driven interactions. Using optogenetically controlled myosin, we demonstrate that locally concentrating myosin is sufficient to both form filaments and jump-start filament amplification and partitioning. By counting myosin monomers within filaments, we demonstrate a myosin-facilitated assembly process that establishes sub-resolution filament stacks prior to partitioning into clusters that feed higher-order networks. Together these findings establish the biophysical mechanisms regulating the assembly of non-muscle contractile structures that are ubiquitous throughout cell biology.