Identification and characterization of nonmuscle myosin II-C, a new member of the myosin II family

myh9b is a critical non-muscle myosin II encoding gene that interacts with myh9a and myh10 during zebrafish development in both compensatory and redundant pathways

Non-muscle myosin (NMII) motor proteins have diverse developmental functions due to their roles in cell shape changes, cell migration, and cell adhesion. Zebrafish are an ideal vertebrate model system to study the NMII encoding myh genes and proteins due to high sequence homology, established gene editing tools, and rapid ex utero development. In humans, mutations in the NMII encoding MYH genes can lead to abnormal developmental processes and disease. This study utilized zebrafish myh9a, myh9b, and myh10 null mutants to examine potential genetic interactions and roles for each gene in development. It was determined that the myh9b gene is the most critical NMII encoding gene, as myh9b mutants develop pericardial edema and have a partially penetrant lethal phenotype, which was not observed in the other myh mutants. This study also established that genetic interactions occur between the zebrafish myh9a, myh9b, and myh10 genes where myh9b is required for the expression of both myh9a and myh10, and myh10 is required for the expression of myh9b. Additionally, protein analyses suggested that enhanced NMII protein stability in some mutant backgrounds may play a role in compensation. Finally, double mutant studies revealed different and more severe phenotypes at earlier time points than single mutants, suggesting roles for tissue specific genetic redundancy, and in some genotypes, haploinsufficiency. These mutants are the first in vivo models allowing for the study of complete loss of the NMIIA and NMIIB proteins, establishing them as valuable tools to elucidate the role of NMII encoding myh genes in development and disease.

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.

Non-Muscle Myosin II A: Friend or Foe in Cancer?

Non-muscle myosin IIA (NM IIA) is a motor protein that belongs to the myosin II family. The myosin heavy chain 9 (MYH9) gene encodes the heavy chain of NM IIA. NM IIA is a hexamer and contains three pairs of peptides, which include the dimer of heavy chains, essential light chains, and regulatory light chains. NM IIA is a part of the actomyosin complex that generates mechanical force and tension to carry out essential cellular functions, including adhesion, cytokinesis, migration, and the maintenance of cell shape and polarity. These functions are regulated via light and heavy chain phosphorylation at different amino acid residues. Apart from physiological functions, NM IIA is also linked to the development of cancer and genetic and neurological disorders. MYH9 gene mutations result in the development of several autosomal dominant disorders, such as May-Hegglin anomaly (MHA) and Epstein syndrome (EPS). Multiple studies have reported NM IIA as a tumor suppressor in melanoma and head and neck squamous cell carcinoma; however, studies also indicate that NM IIA is a critical player in promoting tumorigenesis, chemoradiotherapy resistance, and stemness. The ROCK-NM IIA pathway regulates cellular movement and shape via the control of cytoskeletal dynamics. In addition, the ROCK-NM IIA pathway is dysregulated in various solid tumors and leukemia. Currently, there are very few compounds targeting NM IIA, and most of these compounds are still being studied in preclinical models. This review provides comprehensive evidence highlighting the dual role of NM IIA in multiple cancer types and summarizes the signaling networks involved in tumorigenesis. Furthermore, we also discuss the role of NM IIA as a potential therapeutic target with a focus on the ROCK-NM IIA pathway.

Deficiency in Prader-Willi syndrome gene necdin leads to attenuated cardiac contractility

Prader-Willi syndrome (PWS) is a genetic disorder characterized by behavioral disturbances, hyperphagia, and intellectual disability. Several surveys indicate that PWS is also associated with cardiac abnormalities, possibly contributing to a high incidence of sudden death. However, the pathological mechanisms underlying cardiac dysfunction in PWS remain unclear. In this study, we found that deficiency in necdin, an intronless gene within PWS region, led to heart systolic and diastolic dysfunction in mice. Through yeast two-hybrid screening, we identified an interaction between necdin and non-muscle myosin regulatory light chain 12a/b (MYL12 A/B). We further showed that necdin stabilized MYL12 A/B via SGT1-heat shock protein 90 (HSP90) chaperone machinery. The zebrafish lacking the MYL12 A/B analog, MYL12.1, exhibited impaired heart function, while cardiac-specific overexpression of MYL12A normalized the heart dysfunction in necdin-deficient mice. Our findings revealed necdin dysfunction as a contributing factor to cardiomyopathy in PWS patients and emphasized the importance of HSP90 chaperone machinery and non-muscle myosin in heart fitness.

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.

Myosin II mediates Shh signals to shape dental epithelia via control of cell adhesion and movement

The development of ectodermal organs begins with the formation of a stratified epithelial placode that progressively invaginates into the underlying mesenchyme as the organ takes its shape. Signaling by secreted molecules is critical for epithelial morphogenesis, but how that information leads to cell rearrangement and tissue shape changes remains an open question. Using the mouse dentition as a model, we first establish that non-muscle myosin II is essential for dental epithelial invagination and show that it functions by promoting cell-cell adhesion and persistent convergent cell movements in the suprabasal layer. Shh signaling controls these processes by inducing myosin II activation via AKT. Pharmacological induction of AKT and myosin II can also rescue defects caused by the inhibition of Shh. Together, our results support a model in which the Shh signal is transmitted through myosin II to power effective cellular rearrangement for proper dental epithelial invagination.

Engines of change: Nonmuscle myosin II in mechanobiology

The emergence of mechanobiology has unveiled complex mechanisms by which cells adjust intracellular force production to their needs. Most communicable intracellular forces are generated by myosin II, an actin-associated molecular motor that transforms adenosine triphosphate (ATP) hydrolysis into contraction in nonmuscle and muscle cells. Myosin II-dependent force generation is tightly regulated, and deregulation is associated with specific pathologies. Here, we focus on the role of myosin II (nonmuscle myosin II, NMII) in force generation and mechanobiology. We outline the regulation and molecular mechanism of force generation by NMII, focusing on the actual outcome of contraction, that is, force application to trigger mechanosensitive events or the building of dissipative structures. We describe how myosin II-generated forces drive two major types of events: modification of the cellular morphology and/or triggering of genetic programs, which enhance the ability of cells to adapt to, or modify, their microenvironment. Finally, we address whether targeting myosin II to impair or potentiate its activity at the motor level is a viable therapeutic strategy, as illustrated by recent examples aimed at modulating cardiac myosin II function in heart disease.

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.

The non-muscle actinopathy-associated mutation E334Q in cytoskeletal γ-actin perturbs interaction of actin filaments with myosin and ADF/cofilin family proteins

Various heterozygous cytoskeletal γ-actin mutations have been shown to cause Baraitser-Winter cerebrofrontofacial syndrome, non-syndromic hearing loss, or isolated eye coloboma. Here, we report the biochemical characterization of human cytoskeletal γ-actin carrying mutation E334Q, a mutation that leads to a hitherto unspecified non-muscle actinopathy. Following expression, purification, and removal of linker and thymosin β4 tag sequences, the p.E334Q monomers show normal integration into linear and branched actin filaments. The mutation does not affect thermal stability, actin filament nucleation, elongation, and turnover. Model building and normal mode analysis predict significant differences in the interaction of p.E334Q filaments with myosin motors and members of the ADF/cofilin family of actin-binding proteins. Assays probing the interactions of p.E334Q filaments with human class 2 and class 5 myosin motor constructs show significant reductions in sliding velocity and actin affinity. E334Q differentially affects cofilin-mediated actin dynamics by increasing the rate of cofilin-mediated de novo nucleation of actin filaments and decreasing the efficiency of cofilin-mediated filament severing. Thus, it is likely that p.E334Q-mediated changes in myosin motor activity, as well as filament turnover, contribute to the observed disease phenotype.

MYH9: A key protein involved in tumor progression and virus-related diseases

The myosin heavy chain 9 (MYH9) gene encodes the heavy chain of non-muscle myosin IIA (NMIIA), which belongs to the myosin II subfamily of actin-based molecular motors. Previous studies have demonstrated that abnormal expression and mutations of MYH9 were correlated with MYH9-related diseases and tumors. Furthermore, earlier investigations identified MYH9 as a tumor suppressor. However, subsequent research revealed that MYH9 promoted tumorigenesis, progression and chemoradiotherapy resistance. Note-worthily, MYH9 has also been linked to viral infections, like severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), Epstein-Barr virus, and hepatitis B virus, as a receptor or co-receptor. In addition, MYH9 promotes the development of hepatocellular carcinoma by interacting with the hepatitis B virus-encoding X protein. Finally, various findings highlighted the role of MYH9 in the development of these illnesses, especially in tumors. This review summarizes the involvement of the MYH9-regulated signaling network in tumors and virus-related diseases and presents possible drug interventions on MYH9, providing insights for the use of MYH9 as a therapeutic target for tumors and virus-mediated diseases.

Expression of nonmuscle myosin IIC is regulated by non-canonical binding activity of miRNAs

The expression of mechanoresponsive nonmuscle myosin II (NMII)C is found to be inducible during tumor progression, but its mechanism is yet to be explored. Here, we report a group of microRNAs (mmu-miR-200a-5p, mmu-miR-532-3p, mmu-miR-680, and mmu-miR-1901) can significantly repress the expression of nonmuscle myosin IIC (NMIIC). Interestingly, these microRNAs have both canonical and non-canonical binding sites at 3/UTR and coding sequence (CDS) of NMIIC's heavy chain (HC) mRNA. Each of the miRNA downregulates NMHC-IIC to a different degree as assessed by dual-luciferase and immunoblot analyses. When we abolish the complementary base pairing at canonical binding site, mmu-miR-532-3p can still bind at non-canonical binding site and form Argonaute2 (AGO2)-miRNA complex to downregulate the expression of NMIIC. Modulating the expression of NMIIC by miR-532-3p in mouse mammary tumor cells, 4T1, increases its tumorigenic potential both in vitro and in vivo. Together, these studies provide the functional role of miRNA's non-canonical binding mediated NMIIC regulation in tumor cells.

Whole-exome sequencing analysis identifies novel variants associated with Kawasaki disease susceptibility

BACKGROUND

Kawasaki disease (KD) is an acute pediatric vasculitis affecting genetically susceptible infants and children. Although the pathogenesis of KD remains unclear, growing evidence links genetic susceptibility to the disease.

METHODS

To explore the genes associated with susceptibility in KD, we applied whole-exome sequencing to KD and control subjects from Yunnan province, China. We conducted association study analysis on the two groups.

RESULTS

In this study, we successfully identified 11 significant rare variants in two genes (MYH14 and RBP3) through the genotype/allele frequency analysis. A heterozygous variant (c.2650G > A, p.V884M) of the RBP3 gene was identified in 12 KD cases, while eight heterozygous variants (c.566G > A, p.R189H; c.1109 C > T, p.S370L; c.3917T > G, p.L1306R; c.4301G > A, p.R1434Q; c.5026 C > T, p.R1676W; c.5329 C > T, p.R1777C; c.5393 C > A, p.A1798D and c.5476 C > T, p.R1826C) of the MYH14 gene were identified in 8 KD cases respectively.

CONCLUSION

This study suggested that nine variants in MYH14 and RBP3 gene may be associated with KD susceptibility in the population from Yunnan province.

The Role of non-muscle actin paralogs in cell cycle progression and proliferation

Uncontrolled cell proliferation leads to several pathologies, including cancer. Thus, this process must be tightly regulated. The cell cycle accounts for cell proliferation, and its progression is coordinated with changes in cell shape, for which cytoskeleton reorganization is responsible. Rearrangement of the cytoskeleton allows for its participation in the precise division of genetic material and cytokinesis. One of the main cytoskeletal components is filamentous actin-based structures. Mammalian cells have at least six actin paralogs, four of which are muscle-specific, while two, named β- and γ-actin, are abundantly present in all types of cells. This review summarizes the findings that establish the role of non-muscle actin paralogs in regulating cell cycle progression and proliferation. We discuss studies showing that the level of a given non-muscle actin paralog in a cell influences the cell's ability to progress through the cell cycle and, thus, proliferation. Moreover, we elaborate on the non-muscle actins' role in regulating gene transcription, interactions of actin paralogs with proteins involved in controlling cell proliferation, and the contribution of non-muscle actins to different structures in a dividing cell. The data cited in this review show that non-muscle actins regulate the cell cycle and proliferation through varying mechanisms. We point to the need for further studies addressing these mechanisms.

Expression and functional activity of myosin II in hyperplastic prostates of varying volumes

Prostate volume (PV) differs dramatically among benign prostatic hyperplasia (BPH) patients. Estimation of PV is important to guide the most appropriate pharmacologic or interventional treatment approach. However, the underlying pathophysiological mechanisms for the differences in PV remain unknown. We recently found that the myosin II system might participate in the etiology and development of BPH via static and dynamic factors. Our present study aims to explore the expression and functional activities of myosin II isoforms including smooth muscle (SM) myosin II (SMM II) and non-muscle myosin II (NMM II) in hyperplastic prostates with varied PV. Human hyperplastic prostates and the testosterone-induced rat BPH model were employed for this study. Hematoxylin and Eosin (H&E), Masson's trichrome, immunohistochemical staining, in vitro organ bath, RT-polymerase chain reaction (PCR) and Western-blotting were performed. Also, a BPH tissue microarray (TMA) was constructed to determine the correlations between myosin II isoforms with clinical parameters of BPH patients. With the increase of PV, the expression of NMMHC-A, NMMHC-C, SM-A and LC_17b isoforms were increased, and the contractility of prostate smooth muscle was enhanced but force developed more slowly. Consistently, NMMHC-A, NMMHC-C, SM-A and LC_17b were correlated positively with PV. Similar outcomes were also observed in the BPH rat model with different PVs. Alterations in the expression and function of myosin the II system may be involved in the pathophysiological mechanism of PV differences between BPH patients.

Non-muscle myosin 2 at a glance

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

Non-muscle myosins control radial glial basal endfeet to mediate interneuron organization

Radial glial cells (RGCs) are essential for the generation and organization of neurons in the cerebral cortex. RGCs have an elongated bipolar morphology with basal and apical endfeet that reside in distinct niches. Yet, how this subcellular compartmentalization of RGCs controls cortical development is largely unknown. Here, we employ in vivo proximity labeling, in the mouse, using unfused BirA to generate the first subcellular proteome of RGCs and uncover new principles governing local control of cortical development. We discover a cohort of proteins that are significantly enriched in RGC basal endfeet, with MYH9 and MYH10 among the most abundant. Myh9 and Myh10 transcripts also localize to endfeet with distinct temporal dynamics. Although they each encode isoforms of non-muscle myosin II heavy chain, Myh9 and Myh10 have drastically different requirements for RGC integrity. Myh9 loss from RGCs decreases branching complexity and causes endfoot protrusion through the basement membrane. In contrast, Myh10 controls endfoot adhesion, as mutants have unattached apical and basal endfeet. Finally, we show that Myh9- and Myh10-mediated regulation of RGC complexity and endfoot position non-cell autonomously controls interneuron number and organization in the marginal zone. Our study demonstrates the utility of in vivo proximity labeling for dissecting local control of complex systems and reveals new mechanisms for dictating RGC integrity and cortical architecture.

Proteomics Profiling of Osteoporosis and Osteopenia Patients and Associated Network Analysis

Bone mass reduction due to an imbalance in osteogenesis and osteolysis is characterized by low bone mineral density (LBMD) and is clinically classified as osteopenia (ON) or osteoporosis (OP), which is more severe. Multiple biomarkers for diagnosing OP and its progression have been reported; however, most of these lack specificity. This cohort study aimed to investigate sensitive and specific LBMD-associated protein biomarkers in patients diagnosed with ON and OP. A label-free liquid chromatography-mass spectrometry (LC-MS) proteomics approach was used to analyze serum samples. Patients’ proteomics profiles were filtered for potential confounding effects, such as age, sex, chronic diseases, and medication. A distinctive proteomics profile between the control, ON, and OP groups (Q² = 0.7295, R² = 0.9180) was identified, and significant dysregulation in a panel of proteins (n = 20) was common among the three groups. A comparison of these proteins showed that the levels of eight proteins were upregulated in ON, compared to those in the control and the OP groups, while the levels of eleven proteins were downregulated in the ON group compared to those in the control group. Interestingly, only one protein, myosin heavy chain 14 (MYH14), showed a linear increase from the control to the ON group, with the highest abundance in the OP group. A significant separation in the proteomics profile between the ON and OP groups (Q² = 0.8760, R² = 0.991) was also noted. Furthermore, a total of twenty-six proteins were found to be dysregulated between the ON and the OP groups, with fourteen upregulated and twelve downregulated proteins in the OP, compared to that in the ON group. Most of the identified dysregulated proteins were immunoglobulins, complement proteins, cytoskeletal proteins, coagulation factors, and various enzymes. Of these identified proteins, the highest area under the curve (AUC) in the receiver operating characteristic (ROC) analysis was related to three proteins (immunoglobulin Lambda constant 1 (IGLC1), RNA binding protein (MEX3B), and fibulin 1 (FBLN1)). Multiple reaction monitoring (MRM), LC-MS, was used to validate some of the identified proteins. A network pathway analysis of the differentially abundant proteins demonstrated dysregulation of inflammatory signaling pathways in the LBMD patients, including the tumor necrosis factor (TNF), toll-like receptor (TL4), and interferon-γ (IFNG) signaling pathways. These results reveal the existence of potentially sensitive protein biomarkers that could be used in further investigations of bone health and OP progression.

Unique and redundant functions of cytoplasmic actins and nonmuscle myosin II isoforms at epithelial junctions

The integrity and functions of epithelial barriers depend on the formation of adherens junctions (AJs) and tight junctions (TJs). A characteristic feature of AJs and TJs is their association with the cortical cytoskeleton composed of actin filaments and nonmuscle myosin II (NM-II) motors. Mechanical forces generated by the actomyosin cytoskeleton are essential for junctional assembly, stability, and remodeling. Epithelial cells express two different actin proteins and three NM-II isoforms, all known to be associated with AJs and TJs. Despite their structural similarity, different actin and NM-II isoforms have distinct biochemical properties, cellular distribution, and functions. The diversity of epithelial actins and myosin motors could be essential for the regulation of different steps of junctional formation, maturation, and disassembly. This review focuses on the roles of actin and NM-II isoforms in controlling the integrity and barrier properties of various epithelia. We discuss the effects of the depletion of individual actin isoforms and NM-II motors on the assembly and barrier function of AJs and TJs in model epithelial monolayers in vitro. We also describe the functional consequences of either total or tissue-specific gene knockout of different actins and NM-II motors, with a focus on the development and integrity of different epithelia in vivo.

Myosins and membrane trafficking in intestinal brush border assembly

Myosins are a class of motors that participate in a wide variety of cellular functions including organelle transport, cell adhesion, endocytosis and exocytosis, movement of RNA, and cell motility. Among the emerging roles for myosins is regulation of the assembly, morphology, and function of actin protrusions such as microvilli. The intestine harbors an elaborate apical membrane composed of highly organized microvilli. Microvilli assembly and function are intricately tied to several myosins including Myosin 1a, non-muscle Myosin 2c, Myosin 5b, Myosin 6, and Myosin 7b. Here, we review the research progress made in our understanding of myosin mediated apical assembly.

Myosin II proteins are required for organization of calcium-induced actin networks upstream of mitochondrial division

The formin INF2 polymerizes a calcium-activated cytoplasmic network of actin filaments, which we refer to as calcium-induced actin polymerization (CIA). CIA plays important roles in multiple cellular processes, including mitochondrial dynamics and vesicle transport. Here, we show that nonmuscle myosin II (NMII) is activated within 60 s of calcium stimulation and rapidly recruited to the CIA network. Knockout of any individual NMII in U2OS cells affects the organization of the CIA network, as well as three downstream effects: endoplasmic-reticulum-to-mitochondrial calcium transfer, mitochondrial Drp1 recruitment, and mitochondrial division. Interestingly, while NMIIC is the least abundant NMII in U2OS cells (>200-fold less than NMIIA and >10-fold less than NMIIB), its knockout is equally deleterious to CIA. On the basis of these results, we propose that myosin II filaments containing all three NMII heavy chains exert organizational and contractile roles in the CIA network. In addition, NMIIA knockout causes a significant decrease in myosin regulatory light chain levels, which might have additional effects.

Discovery of ultrafast myosin, its amino acid sequence, and structural features

Cytoplasmic streaming with extremely high velocity (∼70 μm s^-1) occurs in cells of the characean algae (Chara). Because cytoplasmic streaming is caused by myosin XI, it has been suggested that a myosin XI with a velocity of 70 μm s^-1, the fastest myosin measured so far, exists in Chara cells. However, the velocity of the previously cloned Chara corallina myosin XI (CcXI) was about 20 μm s^-1, one-third of the cytoplasmic streaming velocity in Chara Recently, the genome sequence of Chara braunii has been published, revealing that this alga has four myosin XI genes. We cloned these four myosin XI (CbXI-1, 2, 3, and 4) and measured their velocities. While the velocities of CbXI-3 and CbXI-4 motor domains (MDs) were similar to that of CcXI MD, the velocities of CbXI-1 and CbXI-2 MDs were 3.2 times and 2.8 times faster than that of CcXI MD, respectively. The velocity of chimeric CbXI-1, a functional, full-length CbXI-1 construct, was 60 μm s^-1 These results suggest that CbXI-1 and CbXI-2 would be the main contributors to cytoplasmic streaming in Chara cells and show that these myosins are ultrafast myosins with a velocity 10 times faster than fast skeletal muscle myosins in animals. We also report an atomic structure (2.8-Å resolution) of myosin XI using X-ray crystallography. Based on this crystal structure and the recently published cryo-electron microscopy structure of acto-myosin XI at low resolution (4.3-Å), it appears that the actin-binding region contributes to the fast movement of Chara myosin XI. Mutation experiments of actin-binding surface loops support this hypothesis.

HMGA1 stimulates MYH9-dependent ubiquitination of GSK-3β via PI3K/Akt/c-Jun signaling to promote malignant progression and chemoresistance in gliomas

Myosin heavy chain 9 (MYH9) plays an essential role in human diseases, including multiple cancers; however, little is known about its role in gliomas. In the present study, we revealed that HMGA1 and MYH9 were upregulated in gliomas and their expression correlated with WHO grade, and HMGA1 promoted the acquisition of malignant phenotypes and chemoresistance of glioma cells by regulating the expression of MYH9 through c-Jun-mediated transcription. Moreover, MYH9 interacted with GSK-3β to inhibit the expression of GSK-3β protein by promoting its ubiquitination; the downregulation of GSK-3β subsequently promoted the nuclear translocation of β-catenin, enhancing growth, invasion, migration, and temozolomide resistance in glioma cells. Expression levels of HMGA1 and MYH9 were significantly correlated with patient survival and should be considered as independent prognostic factors. Our findings provide new insights into the role of HMGA1 and MYH9 in gliomagenesis and suggest the potential application of HMGA1 and MYH9 in cancer therapy in the future.

Immunolocalization of protease-activated receptors in endothelial cells of splenic sinuses

The immunolocalization of protease-activated receptors (PARs) and related proteins in splenic sinus endothelial cells was examined using immunofluorescence and electron microscopy. Immunofluorescence microscopy showed that PAR1 colocalized with PAR2, PAR3, and PAR4. PAR4 colocalized with PAR3 and P2Y12. Myosin heavy chain IIA localized to the outer shape and at the base of cells, but did not colocalize with α-catenin. The localization of di-phosphorylated myosin regulatory light chains (ppMLC) was partially detected on the outer circumference and conspicuously at the base of cells. Macrophage migration inhibitory factor (MIF) also localized in cells. Immunogold electron microscopy revealed the localization of PAR1 on the caveolar membrane, plasma membrane, and junctional membrane of cells. PAR2 and PAR3 localized to the plasma membrane of cells. PAR4 localized to the plasma membrane, depressions in the plasma membrane, and cytoplasmic vesicles. PpMLC was detected in stress fibers, but rarely near the adherens junctions of neighboring cells. MIF localized in vesicles on the apical and basal sides of the Golgi apparatus. Electron microscopy of endothelial cells with saponin extraction showed the depression of many coated pits formed by clathrin from the plasma membrane. Stress fibers developed at the base of cells; however, few actin filaments were observed near adherens junctions. These results indicate that PARs play important roles in splenic sinus endothelial cells, such as in endothelial barrier protection and the maintenance of firm adhesion to ring fibers.

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

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

Aurora-B phosphorylates the myosin II heavy chain to promote cytokinesis

Cytokinesis, the final step of mitosis, is mediated by an actomyosin contractile ring, the formation of which is temporally and spatially regulated following anaphase onset. Aurora-B is a member of the chromosomal passenger complex, which regulates various processes during mitosis; it is not understood, however, how Aurora-B is involved in cytokinesis. Here, we show that Aurora-B and myosin-IIB form a complex in vivo during telophase. Aurora-B phosphorylates the myosin-IIB rod domain at threonine 1847 (T¹⁸⁴⁷), abrogating the ability of myosin-IIB monomers to form filaments. Furthermore, phosphorylation of myosin-IIB filaments by Aurora-B also promotes filament disassembly. We show that myosin-IIB possessing a phosphomimetic mutation at T¹⁸⁴⁷ was unable to rescue cytokinesis failure caused by myosin-IIB depletion. Cells expressing a phosphoresistant mutation at T¹⁸⁴⁷ had significantly longer intercellular bridges, implying that Aurora-B-mediated phosphorylation of myosin-IIB is important for abscission. We propose that myosin-IIB is a substrate of Aurora-B and reveal a new mechanism of myosin-IIB regulation by Aurora-B in the late stages of mitosis.

Functional Role of Non-Muscle Myosin II in Microglia: An Updated Review

Myosins are a remarkable superfamily of actin-based motor proteins that use the energy derived from ATP hydrolysis to translocate actin filaments and to produce force. Myosins are abundant in different types of tissues and involved in a large variety of cellular functions. Several classes of the myosin superfamily are expressed in the nervous system; among them, non-muscle myosin II (NM II) is expressed in both neurons and non-neuronal brain cells, such as astrocytes, oligodendrocytes, endothelial cells, and microglia. In the nervous system, NM II modulates a variety of functions, such as vesicle transport, phagocytosis, cell migration, cell adhesion and morphology, secretion, transcription, and cytokinesis, as well as playing key roles during brain development, inflammation, repair, and myelination functions. In this review, we will provide a brief overview of recent emerging roles of NM II in resting and activated microglia cells, the principal regulators of immune processes in the central nervous system (CNS) in both physiological and pathological conditions. When stimulated, microglial cells react and produce a number of mediators, such as pro-inflammatory cytokines, free radicals, and nitric oxide, that enhance inflammation and contribute to neurodegenerative diseases. Inhibition of NM II could be a new therapeutic target to treat or to prevent CNS diseases.

Cardiac myosin super relaxation (SRX): a perspective on fundamental biology, human disease and therapeutics

The fundamental basis of muscle contraction 'the sliding filament model' (Huxley and Niedergerke, 1954; Huxley and Hanson, 1954) and the 'swinging, tilting crossbridge-sliding filament mechanism' (Huxley, 1969; Huxley and Brown, 1967) nucleated a field of research that has unearthed the complex and fascinating role of myosin structure in the regulation of contraction. A recently discovered energy conserving state of myosin termed the super relaxed state (SRX) has been observed in filamentous myosins and is central to modulating force production and energy use within the sarcomere. Modulation of myosin function through SRX is a rapidly developing theme in therapeutic development for both cardiovascular disease and infectious disease. Some 70 years after the first discoveries concerning muscular function, modulation of myosin SRX may bring the first myosin targeted small molecule to the clinic, for treating hypertrophic cardiomyopathy (Olivotto et al., 2020). An often monogenic disease HCM afflicts 1 in 500 individuals, and can cause heart failure and sudden cardiac death. Even as we near therapeutic translation, there remain many questions about the governance of muscle function in human health and disease. With this review, we provide a broad overview of contemporary understanding of myosin SRX, and explore the complexities of targeting this myosin state in human disease.This article has an associated Future Leaders to Watch interview with the authors of the paper.

Non-muscle myosin II regulates aortic stiffness through effects on specific focal adhesion proteins and the non-muscle cortical cytoskeleton

Non‐muscle myosin II (NMII) plays a role in many fundamental cellular processes including cell adhesion, migration, and cytokinesis. However, its role in mammalian vascular function is not well understood. Here, we investigated the function of NMII in the biomechanical and signalling properties of mouse aorta. We found that blebbistatin, an inhibitor of NMII, decreases agonist‐induced aortic stress and stiffness in a dose‐dependent manner. We also specifically demonstrate that in freshly isolated, contractile, aortic smooth muscle cells, the non‐muscle myosin IIA (NMIIA) isoform is associated with contractile filaments in the core of the cell as well as those in the non‐muscle cell cortex. However, the non‐muscle myosin IIB (NMIIB) isoform is excluded from the cell cortex and colocalizes only with contractile filaments. Furthermore, both siRNA knockdown of NMIIA and NMIIB isoforms in the differentiated A7r5 smooth muscle cell line and blebbistatin‐mediated inhibition of NM myosin II suppress agonist‐activated increases in phosphorylation of the focal adhesion proteins FAK Y925 and paxillin Y118. Thus, we show in the present study, for the first time that NMII regulates aortic stiffness and stress and that this regulation is mediated through the tension‐dependent phosphorylation of the focal adhesion proteins FAK and paxillin.

Switching between blebbing and lamellipodia depends on the degree of non-muscle myosin II activity

Cells can adopt both mesenchymal and amoeboid modes of migration through membrane protrusive activities, namely formation of lamellipodia and blebbing. How the molecular players control the transition between lamellipodia and blebs is yet to be explored. Here, we show that addition of the ROCK inhibitor Y27632 or low doses of blebbistatin, an inhibitor of non-muscle myosin II (NMII) ATPase activity and filament partitioning, induces blebbing to lamellipodia conversion (BLC), whereas addition of low doses of ML7, an inhibitor of myosin light chain kinase (MLCK), induces lamellipodia to blebbing conversion (LBC) in human MDA-MB-231 cells. Similarly, siRNA-mediated knockdown of ROCK and MLCK induces BLC and LBC, respectively. Interestingly, both blebs and lamellipodia membrane protrusions are able to maintain the ratio of phosphorylated to unphosphorylated regulatory light chain at cortices when MLCK and ROCK, respectively, are inhibited either pharmacologically or genetically, suggesting that MLCK and ROCK activities are interlinked in BLC and LBC. Such BLCs and LBCs are also inducible in other cell lines, including MCF7 and MCF10A. These studies reveal that the relative activity of ROCK and MLCK, which controls both the ATPase activity and filament-forming property of NMII, is a determining factor in whether a cell exhibits blebbing or lamellipodia.

Alpha and beta myosin isoforms and human atrial and ventricular contraction

Human atrial and ventricular contractions have distinct mechanical characteristics including speed of contraction, volume of blood delivered and the range of pressure generated. Notably, the ventricle expresses predominantly β-cardiac myosin while the atrium expresses mostly the α-isoform. In recent years exploration of the properties of pure α- & β-myosin isoforms have been possible in solution, in isolated myocytes and myofibrils. This allows us to consider the extent to which the atrial vs ventricular mechanical characteristics are defined by the myosin isoform expressed, and how the isoform properties are matched to their physiological roles. To do this we Outline the essential feature of atrial and ventricular contraction; Explore the molecular structural and functional characteristics of the two myosin isoforms; Describe the contractile behaviour of myocytes and myofibrils expressing a single myosin isoform; Finally we outline the outstanding problems in defining the differences between the atria and ventricles. This allowed us consider what features of contraction can and cannot be ascribed to the myosin isoforms present in the atria and ventricles.

Nonmuscle myosin-2 contractility-dependent actin turnover limits the length of epithelial microvilli

Brush border microvilli enable functions that are critical for epithelial homeostasis, including solute uptake and host defense. However, the mechanisms that regulate the assembly and morphology of these protrusions are poorly understood. The parallel actin bundles that support microvilli have their pointed-end rootlets anchored in a filamentous meshwork referred to as the "terminal web." Although classic electron microscopy studies revealed complex ultrastructure, the composition and function of the terminal web remain unclear. Here we identify nonmuscle myosin-2C (NM2C) as a component of the terminal web. NM2C is found in a dense, isotropic layer of puncta across the subapical domain, which transects the rootlets of microvillar actin bundles. Puncta are separated by ∼210 nm, the expected size of filaments formed by NM2C. In intestinal organoid cultures, the terminal web NM2C network is highly dynamic and exhibits continuous remodeling. Using pharmacological and genetic perturbations in cultured intestinal epithelial cells, we found that NM2C controls the length of growing microvilli by regulating actin turnover in a manner that requires a fully active motor domain. Our findings answer a decades-old question on the function of terminal web myosin and hold broad implications for understanding apical morphogenesis in diverse epithelial systems.

NMIIA promotes tumorigenesis and prevents chemosensitivity in colorectal cancer by activating AMPK/mTOR pathway

Non-muscle myosin IIA (NMIIA) has been reported to be involved in the carcinogenesis and malignant progression of various human tumors. However, the role and potential mechanism of NMIIA in the biological functions and apoptosis in colorectal cancer (CRC) remain elusive. In this study, we found that NMIIA was overexpressed in CRC tissues and significantly associated with poor survival in CRC patients. In addition, NMIIA promoted CRC cell proliferation and invasion via activating the AMPK/mTOR pathway in vitro, and NMIIA knockdown inhibited CRC growth in vivo. Meanwhile, NMIIA knockdown downregulated the CSCs markers (CD44 and CD133) expression in CRC cells. Furthermore, AMPK/mTOR pathway activation effectively reversed the NMIIA knockdown-induced inhibition of proliferation, invasion and stemness in CRC cells. Finally, NMIIA protects CRC cells from 5-FU-induced apoptosis and proliferation inhibition through the AMPK/mTOR pathway. Taken together, these results indicate that NMIIA plays a pivotal role in CRC growth and progression by regulating AMPK/mTOR pathway activation, and it may act as a novel therapeutic target prognostic factor in CRC.

Comparative Analysis of the Roles of Non-muscle Myosin-IIs in Cytokinesis in Budding Yeast, Fission Yeast, and Mammalian Cells

The contractile ring, which plays critical roles in cytokinesis in fungal and animal cells, has fascinated biologists for decades. However, the basic question of how the non-muscle myosin-II and actin filaments are assembled into a ring structure to drive cytokinesis remains poorly understood. It is even more mysterious why and how the budding yeast Saccharomyces cerevisiae, the fission yeast Schizosaccharomyces pombe, and humans construct the ring structure with one, two, and three myosin-II isoforms, respectively. Here, we provide a comparative analysis of the roles of the non-muscle myosin-IIs in cytokinesis in these three model systems, with the goal of defining the common and unique features and highlighting the major questions regarding this family of proteins.

Mutations in non-muscle myosin 2A disrupt the actomyosin cytoskeleton in Sertoli cells and cause male infertility

Mutations in non-muscle myosin 2A (NM2A) encompass a wide spectrum of anomalies collectively known as MYH9-Related Disease (MYH9-RD) in humans that can include macrothrombocytopenia, glomerulosclerosis, deafness, and cataracts. We previously created mouse models of the three mutations most frequently found in humans: R702C, D1424N, and E1841K. While homozygous R702C and D1424N mutations are embryonic lethal, we found homozygous mutant E1841K mice to be viable. However the homozygous male, but not female, mice were infertile. Here, we report that these mice have reduced testis size and defects in actin-associated junctions in Sertoli cells, resulting in inability to form the blood-testis barrier and premature germ cell loss. Moreover, compound double heterozygous (R702C/E1841K and D1424/E1841K) males show the same abnormalities in testes as E1841K homozygous males. Conditional ablation of either NM2A or NM2B alone in Sertoli cells has no effect on fertility and testis size, however deletion of both NM2A and NM2B in Sertoli cells results in infertility. Isolation of mutant E1841K Sertoli cells reveals decreased NM2A and F-actin colocalization and thicker NM2A filaments. Furthermore, A^E1841K/A^E1841K and double knockout Sertoli cells demonstrate microtubule disorganization and increased tubulin acetylation, suggesting defects in the microtubule cytoskeleton. Together, these results demonstrate that NM2A and 2B paralogs play redundant roles in Sertoli cells and are essential for testes development and normal fertility.

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.

Conventional and Non-Conventional Roles of Non-Muscle Myosin II-Actin in Neuronal Development and Degeneration

Myosins are motor proteins that use chemical energy to produce mechanical forces driving actin cytoskeletal dynamics. In the brain, the conventional non-muscle myosin II (NMII) regulates actin filament cytoskeletal assembly and contractile forces during structural remodeling of axons and dendrites, contributing to morphology, polarization, and migration of neurons during brain development. NMII isoforms also participate in neurotransmission and synaptic plasticity by driving actin cytoskeletal dynamics during synaptic vesicle release and retrieval, and formation, maturation, and remodeling of dendritic spines. NMIIs are expressed differentially in cerebral non-neuronal cells, such as microglia, astrocytes, and endothelial cells, wherein they play key functions in inflammation, myelination, and repair. Besides major efforts to understand the physiological functions and regulatory mechanisms of NMIIs in the nervous system, their contributions to brain pathologies are still largely unclear. Nonetheless, genetic mutations or deregulation of NMII and its regulatory effectors are linked to autism, schizophrenia, intellectual disability, and neurodegeneration, indicating non-conventional roles of NMIIs in cellular mechanisms underlying neurodevelopmental and neurodegenerative disorders. Here, we summarize the emerging biological roles of NMIIs in the brain, and discuss how actomyosin signaling contributes to dysfunction of neurons and glial cells in the context of neurological disorders. This knowledge is relevant for a deep understanding of NMIIs on the pathogenesis and therapeutics of neuropsychiatric and neurodegenerative diseases.

Myosins and Disease

Myosins constitute a superfamily of actin-based molecular motor proteins that mediates a variety of cellular activities including muscle contraction, cell migration, intracellular transport, the formation of membrane projections, cell adhesion, and cell signaling. The 12 myosin classes that are expressed in humans share sequence similarities especially in the N-terminal motor domain; however, their enzymatic activities, regulation, ability to dimerize, binding partners, and cellular functions differ. It is becoming increasingly apparent that defects in myosins are associated with diseases including cardiomyopathies, colitis, glomerulosclerosis, neurological defects, cancer, blindness, and deafness. Here, we review the current state of knowledge regarding myosins and disease.

Pushing myelination - developmental regulation of myosin expression drives oligodendrocyte morphological differentiation

Oligodendrocytes are the central nervous system myelin-forming cells providing axonal electrical insulation and higher-order neuronal circuitry. The mechanical forces driving the differentiation of oligodendrocyte precursor cells into myelinating oligodendrocytes are largely unknown, but likely require the spatiotemporal regulation of the architecture and dynamics of the actin and actomyosin cytoskeletons. In this study, we analyzed the expression pattern of myosin motors during oligodendrocyte development. We report that oligodendrocyte differentiation is regulated by the synchronized expression and non-uniform distribution of several members of the myosin network, particularly non-muscle myosins 2B and 2C, which potentially operate as nanomechanical modulators of cell tension and myelin membrane expansion at different cell stages.This article has an associated First Person interview with the first author of the paper.

A novel MYH14 mutation in a Chinese family with autosomal dominant nonsyndromic hearing loss

Background

MYH14 gene mutations have been suggested to be associated with nonsyndromic/syndromic sensorineural hearing loss. It has been reported that mutations in MYH14 can result in autosomal dominant nonsyndromic deafness-4A (DFNA4).

Methods

In this study, we examined a four-generation Han Chinese family with nonsyndromic hearing loss. Targeted next-generation sequencing of deafness genes was employed to identify the pathogenic variant. Sanger sequencing and PCR-RFLP analysis were performed in affected members of this family and 200 normal controls to further confirm the mutation.

Results

Four members of this family were diagnosed as nonsyndromic bilateral sensorineural hearing loss with postlingual onset and progressive impairment. A novel missense variant, c.5417C > A (p.A1806D), in MYH14 in the tail domain of NMH II C was successfully identified as the pathogenic cause in three affected individuals. The family member II-5 was suggested to have noise-induced deafness.

Conclusion

In this study, a novel missense mutation, c.5417C > A (p.A1806D), in MYH14 that led to postlingual nonsyndromic autosomal dominant SNHL were identified. The findings broadened the phenotype spectrum of MYH14 and highlighted the combined application of gene capture and Sanger sequencing is an efficient approach to screen pathogenic variants associated with genetic diseases.

Scribble, Lgl1, and myosin II form a complex in vivo to promote directed cell migration

Scribble (Scrib) and Lethal giant larvae 1 (Lgl1) are conserved polarity proteins that play important roles in different forms of cell polarity. The roles of Scrib and Lgl1 in apical-basal cell polarity have been studied extensively, but little is known about their roles in the cell polarity of migrating cells. Furthermore, the effect of Scrib and Lgl1 interaction on cell polarity is largely unknown. In this study, we show that Scrib, through its leucine-rich repeat domain, forms a complex in vivo with Lgl1. Scrib also forms a complex with myosin II, and Scrib, Lgl1, and myosin II colocalize at the leading edge of migrating cells. The cellular localization and the cytoskeletal association of Scrib and Lgl1 are interdependent, as depletion of either protein affects its counterpart. In addition, depletion of either Scrib or Lgl1 disrupts the cellular localization of myosin II. We show that depletion of either Scrib or Lgl1 affects cell adhesion through the inhibition of focal adhesion disassembly. Finally, we show that Scrib and Lgl1 are required for proper cell polarity of migrating cells. These results provide new insights into the mechanism regulating the cell polarity of migrating cells by Scrib, Lgl1, and myosin II.

Non-Muscle Myosin 2A (NM2A): Structure, Regulation and Function

Non-muscle myosin 2A (NM2A) is a motor cytoskeletal enzyme with crucial importance from the early stages of development until adulthood. Due to its capacity to convert chemical energy into force, NM2A powers the contraction of the actomyosin cytoskeleton, required for proper cell division, adhesion and migration, among other cellular functions. Although NM2A has been extensively studied, new findings revealed that a lot remains to be discovered concerning its spatiotemporal regulation in the intracellular environment. In recent years, new functions were attributed to NM2A and its activity was associated to a plethora of illnesses, including neurological disorders and infectious diseases. Here, we provide a concise overview on the current knowledge regarding the structure, the function and the regulation of NM2A. In addition, we recapitulate NM2A-associated diseases and discuss its potential as a therapeutic target.

Nonmuscle myosin 2 regulates cortical stability during sprouting angiogenesis

Among the three nonmuscle myosin 2 (NM2) paralogs, NM 2A and 2B, but not 2C, are detected in endothelial cells. To study the role of NM2 in vascular formation, we ablate NM2 in endothelial cells in mice. Ablating NM2A, but not NM2B, results in reduced blood vessel coverage and increased vascular branching in the developing mouse skin and coronary vasculature. NM2B becomes essential for vascular formation when NM2A expression is limited. Mice ablated for NM2B and one allele of NM2A develop vascular abnormalities similar to those in NM2A ablated mice. Using the embryoid body angiogenic sprouting assay in collagen gels reveals that NM2A is required for persistent angiogenic sprouting by stabilizing the endothelial cell cortex, and thereby preventing excessive branching and ensuring persistent migration of the endothelial sprouts. Mechanistically, NM2 promotes focal adhesion formation and cortical protrusion retraction during angiogenic sprouting. Further studies demonstrate the critical role of Rho kinase–activated NM2 signaling in the regulation of angiogenic sprouting in vitro and in vivo.

Linking the Landscape of MYH9-Related Diseases to the Molecular Mechanisms that Control Non-Muscle Myosin II-A Function in Cells

The MYH9 gene encodes the heavy chain (MHCII) of non-muscle myosin II A (NMII-A). This is an actin-binding molecular motor essential for development that participates in many crucial cellular processes such as adhesion, cell migration, cytokinesis and polarization, maintenance of cell shape and signal transduction. Several types of mutations in the MYH9 gene cause an array of autosomal dominant disorders, globally known as MYH9-related diseases (MYH9-RD). These include May-Hegglin anomaly (MHA), Epstein syndrome (EPS), Fechtner syndrome (FTS) and Sebastian platelet syndrome (SPS). Although caused by different MYH9 mutations, all patients present macrothrombocytopenia, but may later display other pathologies, including loss of hearing, renal failure and presenile cataracts. The correlation between the molecular and cellular effects of the different mutations and clinical presentation are beginning to be established. In this review, we correlate the defects that MYH9 mutations cause at a molecular and cellular level (for example, deficient filament formation, altered ATPase activity or actin-binding) with the clinical presentation of the syndromes in human patients. We address why these syndromes are tissue restricted, and the existence of possible compensatory mechanisms, including residual activity of mutant NMII-A and/ or the formation of heteropolymers or co-polymers with other NMII isoforms.

Nonmuscle myosin II in cardiac and skeletal muscle cells

De novo assembly of contractile myofibrils begins with the formation of premyofibrils where filaments of non-muscle myosin (NM II), and actin organize in sarcomeric patterns with Z-Bodies containing muscle-specific alpha-actinin. Interactions of muscle specific myosin (MM II) with NM II occur in a nascent myofibril stage that precedes the assembly of mature myofibrils. By the final stage of myofibrillogenesis, the only myosin II present in the mature myofibrils is MM II. In this current study of myofibril assembly, the three vertebrate isoforms of NM II (A, B, and C) and sarcomeric alpha-actinin, ligated to GFP family proteins, were coexpressed in avian embryonic skeletal and cardiac muscle cells. Each isoform of NM II localized only in the mini-A-Bands of premyofibrils and nascent myofibrils. There was no evidence of localization of NM II in Z-Bodies of premyofibrils and nascent myofibrils or in Z-Bands of mature myofibrils. Fluorescence Recovery After Photobleaching (FRAP) experiments indicated similar exchange rates in premyofibrils for NM II isoforms A and B, whereas the IIC isoform was significantly less dynamic. Fluorescence Resonance Energy Transfer (FRET) measurements of colocalized fluorescent pairs of different NM II isoforms yielded signals similar to identical pairs, indicating copolymerization of the different NM II pairs. The role of NM II may reside in establishing the future sarcomere pattern in mature myofibrils by binding to the oppositely polarized actin filaments that extend between pairs of Z-Bodies along premyofibrils prior to their transformation into mature myofibrils.

The R941L mutation in MYH14 disrupts mitochondrial fission and associates with peripheral neuropathy

Background

Peripheral neuropathies are often caused by disruption of genes responsible for myelination or axonal transport. In particular, impairment in mitochondrial fission and fusion are known causes of peripheral neuropathies. However, the causal mechanisms for peripheral neuropathy gene mutations are not always known. While loss of function mutations in MYH14 typically cause non-syndromic hearing loss, the recently described R941L mutation in MYH14, encoding the non-muscle myosin protein isoform NMIIC, leads to a complex clinical presentation with an unexplained peripheral neuropathy phenotype.

Methods

Confocal microscopy was used to examine mitochondrial dynamics in MYH14 patient fibroblast cells, as well as U2OS and M17 cells overexpressing NMIIC. The consequence of the R941L mutation on myosin activity was modeled in C. elegans.

Findings

We describe the third family carrying the R941L mutation in MYH14, and demonstrate that the R941L mutation impairs non-muscle myosin protein function. To better understand the molecular basis of the peripheral neuropathy phenotype associated with the R941L mutation, which has been hindered by the fact that NMIIC is largely uncharacterized, we have established a previously unrecognized biological role for NMIIC in mediating mitochondrial fission in human cells. Notably, the R941L mutation acts in a dominant-negative fashion to inhibit mitochondrial fission, especially in the cell periphery. In addition, we observed alterations to the organization of the mitochondrial genome.

Interpretation

As impairments in mitochondrial fission cause peripheral neuropathy, this insight into the function of NMIIC likely explains the peripheral neuropathy phenotype associated with the R941L mutation.

Fund

This study was supported by the Alberta Children's Hospital Research Institute, the Canadian Institutes of Health Research and the Care4Rare Canada Consortium.

MYH9-related disease mutations cause abnormal red blood cell morphology through increased myosin-actin binding at the membrane

MYH9-related disease (MYH9-RD) is a rare, autosomal dominant disorder caused by mutations in MYH9, the gene encoding the actin-activated motor protein non-muscle myosin IIA (NMIIA). MYH9-RD patients suffer from bleeding syndromes, progressive kidney disease, deafness, and/or cataracts, but the impact of MYH9 mutations on other NMIIA-expressing tissues remains unknown. In human red blood cells (RBCs), NMIIA assembles into bipolar filaments and binds to actin filaments (F-actin) in the spectrin-F-actin membrane skeleton to control RBC biconcave disk shape and deformability. Here, we tested the effects of MYH9 mutations in different NMIIA domains (motor, coiled-coil rod, or non-helical tail) on RBC NMIIA function. We found that MYH9-RD does not cause clinically significant anemia and that patient RBCs have normal osmotic deformability as well as normal membrane skeleton composition and micron-scale distribution. However, analysis of complete blood count data and peripheral blood smears revealed reduced hemoglobin content and elongated shapes, respectively, of MYH9-RD RBCs. Patients with mutations in the NMIIA motor domain had the highest numbers of elongated RBCs. Patients with mutations in the motor domain also had elevated association of NMIIA with F-actin at the RBC membrane. Our findings support a central role for motor domain activity in NMIIA regulation of RBC shape and define a new sub-clinical phenotype of MYH9-RD.

Angiotensin II-mediated MYH9 downregulation causes structural and functional podocyte injury in diabetic kidney disease

MYH9, a widely expressed gene encoding nonmuscle myosin heavy chain, is also expressed in podocytes and is associated with glomerular pathophysiology. However, the mechanisms underlying MYH9-related glomerular diseases associated with proteinuria are poorly understood. Therefore, we investigated the role and mechanism of MYH9 in diabetic kidney injury. MYH9 expression was decreased in glomeruli from diabetic patients and animals and in podocytes treated with Ang II in vitro. Ang II treatment and siRNA-mediated MYH9 knockdown in podocytes resulted in actin cytoskeleton reorganization, reduced cell adhesion, actin-associated protein downregulation, and increased albumin permeability. Ang II treatment increased NOX4 expression and ROS generation. The Ang II receptor blocker losartan and the ROS scavenger NAC restored MYH9 expression in Ang II-treated podocytes, attenuated disrupted actin cytoskeleton and decreased albumin permeability. Furthermore, MYH9 overexpression in podocytes restored the effects of Ang II on the actin cytoskeleton and actin-associated proteins. Ang II-mediated TRPC6 activation reduced MYH9 expression. These results suggest that Ang II-mediated MYH9 depletion in diabetic nephropathy may increase filtration barrier permeability by inducing structural and functional podocyte injury through TRPC6-mediated Ca²⁺ influx by NOX4-mediated ROS generation. These findings reveal a novel MYH9 function in maintaining urinary filtration barrier integrity. MYH9 may be a potential target for treating diabetic nephropathy.