Motorized RhoGAP myosin IXb (Myo9b) controls cell shape and motility

Roles of G proteins and their GTPase-activating proteins in platelets

Platelets are small anucleate blood cells supporting vascular function. They circulate in a quiescent state monitoring the vasculature for injuries. Platelets adhere to injury sites and can be rapidly activated to secrete granules and to form platelet/platelet aggregates. These responses are controlled by signalling networks that include G proteins and their regulatory guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Recent proteomics studies have revealed the complete spectrum of G proteins, GEFs, and GAPs present in platelets. Some of these proteins are specific for platelets and very few have been characterised in detail. GEFs and GAPs play a major role in setting local levels of active GTP-bound G proteins in response to activating and inhibitory signals encountered by platelets. Thus, GEFs and GAPs are highly regulated themselves and appear to integrate G protein regulation with other cellular processes. This review focuses on GAPs of small G proteins of the Arf, Rab, Ras, and Rho families, as well as of heterotrimeric G proteins found in platelets.

A liquid biomarker signature of inflammatory proteins accurately predicts early pancreatic cancer progression during FOLFIRINOX chemotherapy

BACKGROUND

Pancreatic ductal adenocarcinoma (PDAC) is often treated with FOLFIRINOX, a chemotherapy associated with high toxicity rates and variable efficacy. Therefore, it is crucial to identify patients at risk of early progression during treatment. This study aims to explore the potential of a multi-omics biomarker for predicting early PDAC progression by employing an in-depth mathematical modeling approach.

METHODS

Blood samples were collected from 58 PDAC patients undergoing FOLFIRINOX before and after the first cycle. These samples underwent gene (GEP) and inflammatory protein expression profiling (IPEP). We explored the predictive potential of exclusively IPEP through Stepwise (Backward) Multivariate Logistic Regression modeling. Additionally, we integrated GEP and IPEP using Bayesian Kernel Regression modeling, aiming to enhance predictive performance. Ultimately, the FOLFIRINOX IPEP (FFX-IPEP) signature was developed.

RESULTS

Our findings revealed that proteins exhibited superior predictive accuracy than genes. Consequently, the FFX-IPEP signature consisted of six proteins: AMN, BANK1, IL1RL2, ITGB6, MYO9B, and PRSS8. The signature effectively identified patients transitioning from disease control to progression early during FOLFIRINOX, achieving remarkable predictive accuracy with an AUC of 0.89 in an independent test set. Importantly, the FFX-IPEP signature outperformed the conventional CA19-9 tumor marker.

CONCLUSIONS

Our six-protein FFX-IPEP signature holds solid potential as a liquid biomarker for the early prediction of PDAC progression during toxic FOLFIRINOX chemotherapy. Further validation in an external cohort is crucial to confirm the utility of the FFX-IPEP signature. Future studies should expand to predict progression under different chemotherapies to enhance the guidance of personalized treatment selection in PDAC.

Recombinant Slit2 attenuates tracheal fibroblast activation in benign central airway obstruction by inhibiting the TGF-β1/Smad3 signaling pathway

Airway fibrosis is among the pathological manifestations of benign central airway obstruction noted in the absence of effective treatments and requires new drug targets to be developed. Slit guidance ligand 2-roundabout guidance receptor 1 (Slit2-Robo1) is involved in fibrosis and organ development. However, its significance in airway fibrosis has not yet been reported. The study explored how the recombinant protein Slit2 functions in transforming growth factor-β1 (TGF-β1)-mediated airway fibrosis in vivo and in vitro. In this study, Slit2 expression initially increased in the tracheal granulation tissues of patients with tracheobronchial stenosis but decreased in the fibrotic tissue. In primary rat tracheal fibroblasts (RTFs), recombinant Slit2 inhibited the expression of extracellular matrices such as Timp1, α-SMA, and COL1A2, whereas recombinant TGF-β1 promoted the expression of Robo1, α-SMA, and COL1A2. Slit2 and TGF-β1 played a mutual inhibitory role in RTFs. Slit2 supplementation and Robo1 downregulation inhibited excessive extracellular matrix (ECM) deposition induced by TGF-β1 in RTFs via the TGF-β1/Smad3 pathway. Ultimately, exogenous Slit2 and Robo1 knockdown-mediated attenuation of airway fibrosis were validated in a trauma-induced rat airway obstruction model. These findings demonstrate that recombinant Slit2 alleviated pathologic tracheobronchial healing by attenuating excessive ECM deposition. Slit2-Robo1 is an attractive target for further exploring the mechanisms and treatment of benign central airway obstruction.

Myo9b mutations are associated with altered dendritic cell functions and increased susceptibility to autoimmune diabetes onset

The regulation of autoimmunity against pancreatic islet β cells for type 1 diabetes (T1D) onset is still unclear. NOD/ShiLtJ (NOD) mice are prone to the onset of autoimmune diabetes, but its congenic strain, ALR/Lt (ALR), is not. Here we show that dendritic cells (DC) in ALR mice have impaired migratory and T-cell priming capability. Genomic comparative analysis maps a 33-bp deletion in the ALR Myosin IXb (Myo9b) gene when compared with NOD genome; meanwhile, data from knock-in models show that this ALR Myo9b allele impairs phenotypic and functional maturation of DCs, and prevents the development and progression of spontaneous autoimmune diabetes in NOD mice. In parallel, while the ALR 33-bp deletion of Myo9b is not conserved in human, we find a MYO9B R133Q polymorphism associating with increased risk of T1D and enhanced DC function in patients with T1D. Our results thus hint that alterations in Myo9b may contribute to altered DC function and autoimmune diabetes onset.

The Genetics of Coronary Artery Disease: A Vascular Perspective

Genome-wide association studies (GWAS) have identified a large number of genetic loci for coronary artery disease (CAD), with many located close to genes associated with traditional CAD risk pathways, such as lipid metabolism and inflammation. It is becoming evident with recent CAD GWAS meta-analyses that vascular pathways are also highly enriched and present an opportunity for novel therapeutics. This review examines GWAS-enriched vascular gene loci, the pathways involved and their potential role in CAD pathogenesis. The functionality of variants is explored from expression quantitative trait loci, massively parallel reporter assays and CRISPR-based gene-editing tools. We discuss how this research may lead to novel therapeutic tools to treat cardiovascular disorders.

The myosin and RhoGAP MYO9B influences osteocyte dendrite growth and responses to mechanical stimuli

Introduction: Myosin IXB (MYO9B) is an unconventional myosin with RhoGAP activity and thus is a regulator of actin cytoskeletal organization. MYO9B was previously shown to be necessary for skeletal growth and health and to play a role in actin-based functions of both osteoblasts and osteoclasts. However, its role in responses to mechanical stimulation of bone cells has not yet been described. Therefore, experiments were undertaken to determine the role of MYO9B in bone cell responses to mechanical stress both in vitro and in vivo. Methods: MYO9B expression was knocked down in osteoblast and osteocyte cell lines using RNA interference and the resulting cells were subjected to mechanical stresses including cyclic tensile strain, fluid shear stress, and plating on different substrates (no substrate vs. monomeric or polymerized collagen type I). Osteocytic cells were also subjected to MYO9B regulation through Slit-Robo signaling. Further, wild-type or Myo9b -/- mice were subjected to a regimen of whole-body vibration (WBV) and changes in bone quality were assessed by micro-CT. Results: Unlike control cells, MYO9B-deficient osteoblastic cells subjected to uniaxial cyclic tensile strain were unable to orient their actin stress fibers perpendicular to the strain. Osteocytic cells in which MYO9B was knocked down exhibited elongated dendrites but were unable to respond normally to treatments that increase dendrite length such as fluid shear stress and Slit-Robo signaling. Osteocytic responses to mechanical stimuli were also found to be dependent on the polymerization state of collagen type I substrates. Wild-type mice responded to WBV with increased bone tissue mineral density values while Myo9b -/- mice responded with bone loss. Discussion: These results demonstrate that MYO9B plays a key role in mechanical stress-induced responses of bone cells in vitro and in vivo.

Mutations in MYO9B are associated with Charcot-Marie-Tooth disease type 2 neuropathies and isolated optic atrophy

BACKGROUND AND PURPOSE

Charcot-Marie-Tooth disease (CMT) is a heterogeneous group of disorders caused by mutations in at least 100 genes. However, approximately 60% of cases with axonal neuropathies (CMT2) still remain without a genetic diagnosis. We aimed at identifying novel disease genes responsible for CMT2.

METHODS

We performed whole exome sequencing and targeted next generation sequencing panel analyses on a cohort of CMT2 families with evidence for autosomal recessive inheritance. We also performed functional studies to explore the pathogenetic role of selected variants.

RESULTS

We identified rare, recessive variants in the MYO9B (myosin IX) gene in two families with CMT2. MYO9B has not yet been associated with a human disease. MYO9B is an unconventional single-headed processive myosin motor protein with signaling properties, and, consistent with this, our results indicate that a variant occurring in the MYO9B motor domain impairs protein expression level and motor activity. Interestingly, a Myo9b-null mouse has degenerating axons in sciatic nerves and optic nerves, indicating that MYO9B plays an essential role in both peripheral nervous system and central nervous system axons, respectively. The degeneration observed in the optic nerve prompted us to screen for MYO9B mutations in a cohort of patients with optic atrophy (OA). Consistent with this, we found compound heterozygous variants in one case with isolated OA.

CONCLUSIONS

Novel or very rare variants in MYO9B are associated with CMT2 and isolated OA.

Unconventional Myosins from Caenorhabditis elegans as a Probe to Study Human Orthologues

Unconventional myosins are a superfamily of actin-based motor proteins that perform a number of roles in fundamental cellular processes, including (but not limited to) intracellular trafficking, cell motility, endocytosis, exocytosis and cytokinesis. 40 myosins genes have been identified in humans, which belong to different 12 classes based on their domain structure and organisation. These genes are widely expressed in different tissues, and mutations leading to loss of function are associated with a wide variety of pathologies while over-expression often results in cancer. Caenorhabditis elegans (C. elegans) is a small, free-living, non-parasitic nematode. ~38% of the genome of C. elegans has predicted orthologues in the human genome, making it a valuable tool to study the function of human counterparts and human diseases. To date, 8 unconventional myosin genes have been identified in the nematode, from 6 different classes with high homology to human paralogues. The hum-1 and hum-5 (heavy chain of an unconventional myosin) genes encode myosin of class I, hum-2 of class V, hum-3 and hum-8 of class VI, hum-6 of class VII and hum-7 of class IX. The hum-4 gene encodes a high molecular mass myosin (307 kDa) that is one of the most highly divergent myosins and is a member of class XII. Mutations in many of the human orthologues are lethal, indicating their essential properties. However, a functional characterisation for many of these genes in C. elegans has not yet been performed. This article reviews the current knowledge of unconventional myosin genes in C. elegans and explores the potential use of the nematode to study the function and regulation of myosin motors to provide valuable insights into their role in diseases.

Discovery and systematic characterization of risk variants and genes for coronary artery disease in over a million participants

The discovery of genetic loci associated with complex diseases has outpaced the elucidation of mechanisms of disease pathogenesis. Here we conducted a genome-wide association study (GWAS) for coronary artery disease (CAD) comprising 181,522 cases among 1,165,690 participants of predominantly European ancestry. We detected 241 associations, including 30 new loci. Cross-ancestry meta-analysis with a Japanese GWAS yielded 38 additional new loci. We prioritized likely causal variants using functionally informed fine-mapping, yielding 42 associations with less than five variants in the 95% credible set. Similarity-based clustering suggested roles for early developmental processes, cell cycle signaling and vascular cell migration and proliferation in the pathogenesis of CAD. We prioritized 220 candidate causal genes, combining eight complementary approaches, including 123 supported by three or more approaches. Using CRISPR-Cas9, we experimentally validated the effect of an enhancer in MYO9B, which appears to mediate CAD risk by regulating vascular cell motility. Our analysis identifies and systematically characterizes >250 risk loci for CAD to inform experimental interrogation of putative causal mechanisms for CAD.

The RhoGAP-myosin Myo9b regulates ocular lens pit morphogenesis

BACKGROUND

During eye development the lens placode invaginates to form the lens pit. Further bending of lens epithelium and separation from ectoderm leads eventually to a spherical lens vesicle with enclosed extracellular fluid. Changes in epithelial morphology involve the actin cytoskeleton and its regulators. The myosin Myo9b is simultaneously an actin-based motor and Rho GTPase-activating protein that regulates actin cytoskeleton organization. Myo9b-deficient adult mice and embryos were analyzed for eye malformations and alterations in lens development.

RESULTS

Myo9b-deficient mice showed a high incidence of microphthalmia and cataracts with occasional blepharitis. Formation of the lens vesicle during embryonic lens development was disordered in virtually all embryos. Lens placode invagination was less deep and gave rise to a conical structure instead of a spherical pit. At later stages either no lens vesicle was formed or a significantly smaller one that was not enclosed by the optic cup. Expression of the cell fate marker Pax6 was not altered. Staining of adherens junctions and F-actin was most intense at the tip of conical invaginations, suggesting that mechanical forces are not properly coordinated between epithelial cells that form the pit.

CONCLUSIONS

Myo9b is a critical regulator of ocular lens vesicle morphogenesis during eye development.

Myosin motors on the pathway of viral infections

Molecular motors are microscopic machines that use energy from adenosine triphosphate (ATP) hydrolysis to generate movement. While kinesins and dynein are molecular motors associated with microtubule tracks, myosins bind to and move on actin filaments. Mammalian cells express several myosin motors. They power cellular processes such as endo- and exocytosis, intracellular trafficking, transcription, migration, and cytokinesis. As viruses navigate through cells, they may take advantage or be hindered by host components and machinery, including the cytoskeleton. This review delves into myosins' cell roles and compares them to their reported functions in viral infections. In most cases, the previously described myosin functions align with their reported role in viral infections, although not in all cases. This opens the possibility that knowledge obtained from studying myosins in viral infections might shed light on new physiological roles for myosins in cells. However, given the high number of myosins expressed and the variety of viruses investigated in the different studies, it is challenging to infer whether the interactions found are specific to a single virus or can be applied to other viruses with the same characteristics. We conclude that the participation of myosins in viral cycles is still a largely unexplored area, especially concerning unconventional myosins.

Turning Platelets Off and On: Role of RhoGAPs and RhoGEFs in Platelet Activity

Platelet cytoskeletal reorganisation is a critical component of platelet activation and thrombus formation in haemostasis. The Rho GTPases RhoA, Rac1 and Cdc42 are the primary drivers in the dynamic reorganisation process, leading to the development of filopodia and lamellipodia which dramatically increase platelet surface area upon activation. Rho GTPases cycle between their active (GTP-bound) and inactive (GDP-bound) states through tightly regulated processes, central to which are the guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). GEFs catalyse the dissociation of GDP by inducing changes in the nucleotide binding site, facilitating GTP binding and activating Rho GTPases. By contrast, while all GTPases possess intrinsic hydrolysing activity, this reaction is extremely slow. Therefore, GAPs catalyse the hydrolysis of GTP to GDP, reverting Rho GTPases to their inactive state. Our current knowledge of these proteins is constantly being updated but there is considerably less known about the functionality of Rho GTPase specific GAPs and GEFs in platelets. In the present review, we discuss GAP and GEF proteins for Rho GTPases identified in platelets, their regulation, biological function and present a case for their further study in platelets.

Rho-Kinase as a Target for Cancer Therapy and Its Immunotherapeutic Potential

Cancer immunotherapy is fast rising as a prominent new pillar of cancer treatment, harnessing the immune system to fight against numerous types of cancer. Rho-kinase (ROCK) pathway is involved in diverse cellular activities, and is therefore the target of interest in various diseases at the cellular level including cancer. Indeed, ROCK is well-known for its involvement in the tumor cell and tumor microenvironment, especially in its ability to enhance tumor cell progression, migration, metastasis, and extracellular matrix remodeling. Importantly, ROCK is also considered to be a novel and effective modulator of immune cells, although further studies are needed. In this review article, we describe the various activities of ROCK and its potential to be utilized in cancer treatment, particularly in cancer immunotherapy, by shining a light on its activities in the immune system.

GCN2 adapts protein synthesis to scavenging-dependent growth

Pancreatic cancer cells with limited access to free amino acids can grow by scavenging extracellular protein. In a murine model of pancreatic cancer, we performed a genome-wide CRISPR screen for genes required for scavenging-dependent growth. The screen identified key mediators of macropinocytosis, peripheral lysosome positioning, endosome-lysosome fusion, lysosomal protein catabolism, and translational control. The top hit was GCN2, a kinase that suppresses translation initiation upon amino acid depletion. Using isotope tracers, we show that GCN2 is not required for protein scavenging. Instead, GCN2 prevents ribosome stalling but without slowing protein synthesis; cells still use all of the limiting amino acids as they emerge from lysosomes. GCN2 also adapts gene expression to the nutrient-poor environment, reorienting protein synthesis away from ribosomes and toward lysosomal hydrolases, such as cathepsin L. GCN2, cathepsin L, and the other genes identified in the screen are potential therapeutic targets in pancreatic cancer.

The Actin Cytoskeleton at the Immunological Synapse of Dendritic Cells

Dendritic cells (DCs) are considered the most potent antigen-presenting cells. DCs control the activation of T cells (TCs) in the lymph nodes. This process involves forming a specialized superstructure at the DC-TC contact zone called the immunological synapse (IS). For the sake of clarity, we call IS(DC) and IS(TC) the DC and TC sides of the IS, respectively. The IS(DC) and IS(TC) seem to organize as multicentric signaling hubs consisting of surface proteins, including adhesion and costimulatory molecules, associated with cytoplasmic components, which comprise cytoskeletal proteins and signaling molecules. Most of the studies on the IS have focused on the IS(TC), and the information on the IS(DC) is still sparse. However, the data available suggest that both IS sides are involved in the control of TC activation. The IS(DC) may govern activities of DCs that confer them the ability to activate the TCs. One key component of the IS(DC) is the actin cytoskeleton. Herein, we discuss experimental data that support the concept that actin polarized at the IS(DC) is essential to maintaining IS stability necessary to induce TC activation.

RhoA Signaling in Immune Cell Response and Cardiac Disease

Chronic inflammation, the activation of immune cells and their cross-talk with cardiomyocytes in the pathogenesis and progression of heart diseases has long been overlooked. However, with the latest research developments, it is increasingly accepted that a vicious cycle exists where cardiomyocytes release cardiocrine signaling molecules that spiral down to immune cell activation and chronic state of low-level inflammation. For example, cardiocrine molecules released from injured or stressed cardiomyocytes can stimulate macrophages, dendritic cells, neutrophils and even T-cells, which then subsequently increase cardiac inflammation by co-stimulation and positive feedback loops. One of the key proteins involved in stress-mediated cardiomyocyte signal transduction is a small GTPase RhoA. Importantly, the regulation of RhoA activation is critical for effective immune cell response and is being considered as one of the potential therapeutic targets in many immune-cell-mediated inflammatory diseases. In this review we provide an update on the role of RhoA at the juncture of immune cell activation, inflammation and cardiac disease.

Microexon alternative splicing of small GTPase regulators: Implication in central nervous system diseases

Microexons are small sized (≤51 bp) exons which undergo extensive alternative splicing in neurons, microglia, embryonic stem cells, and cancer cells, giving rise to cell type specific protein isoforms. Due to their small sizes, microexons provide a unique challenge for the splicing machinery. They frequently lack exon splicer enhancers/repressors and require specialized neighboring trans-regulatory and cis-regulatory elements bound by RNA binding proteins (RBPs) for their inclusion. The functional consequences of including microexons within mRNAs have been extensively documented in the central nervous system (CNS) and aberrations in their inclusion have been observed to lead to abnormal processes. Despite the increasing evidence for microexons impacting cellular physiology within CNS, mechanistic details illustrating their functional importance in diseases of the CNS is still limited. In this review, we discuss the unique characteristics of microexons, and how RBPs participate in regulating their inclusion and exclusion during splicing. We consider recent findings of microexon alternative splicing and their implication for regulating the function of small GTPases in the context of the microglia, and we extrapolate these findings to what is known in neurons. We further discuss the emerging evidence for dysregulation of the Rho GTPase pathway in CNS diseases and the consequences contributed by the mis-splicing of microexons. This article is categorized under: RNA Processing > Splicing Mechanisms RNA Processing > Splicing Regulation/Alternative Splicing RNA in Disease and Development > RNA in Disease.

Statistical profiling reveals correlations between the cell response to and the primary structure of Rho-GAPs

Rho-GTPase-activating proteins (Rho-GAPs) are essential upstream regulators of the Rho family of GTPases. Currently, it remains unclear if the phenotypic change caused by perturbations to a Rho-GAP is predictable from its amino acid sequence. Here we analyze the relationship between the morphological response of cells to the silencing of Rho-GAPs and their primary structure. For all possible pairs of 57 different Rho-GAPs expressed in MCF10A epithelial cells, the similarity in the Rho-GAP silencing-induced morphological change was quantified and compared to the similarity in the primary structure of the corresponding pairs. We found a distinct correlation between the morphological and sequence similarities in a specific group of RhoA-targeting Rho-GAPs. Thus, the family-wide analysis revealed a common feature shared by the specific Rho-GAPs.

Local Myo9b RhoGAP activity regulates cell motility

To migrate, cells assume a polarized morphology, extending forward with a leading edge with their trailing edge retracting back toward the cell body. Both cell extension and retraction critically depend on the organization and dynamics of the actin cytoskeleton, and the small, monomeric GTPases Rac and Rho are important regulators of actin. Activation of Rac induces actin polymerization and cell extension, whereas activation of Rho enhances acto-myosin II contractility and cell retraction. To coordinate migration, these processes must be carefully regulated. The myosin Myo9b, a Rho GTPase-activating protein (GAP), negatively regulates Rho activity and deletion of Myo9b in leukocytes impairs cell migration through increased Rho activity. However, it is not known whether cell motility is regulated by global or local inhibition of Rho activity by Myo9b. Here, we addressed this question by using Myo9b-deficient macrophage-like cells that expressed different recombinant Myo9b constructs. We found that Myo9b accumulates in lamellipodial extensions generated by Rac-induced actin polymerization as a function of its motor activity. Deletion of Myo9b in HL-60-derived macrophages altered cell morphology and impaired cell migration. Reintroduction of Myo9b or Myo9b motor and GAP mutants revealed that local GAP activity rescues cell morphology and migration. In summary, Rac activation leads to actin polymerization and recruitment of Myo9b, which locally inhibits Rho activity to enhance directional cell migration.

Endothelial Basement Membrane Laminins as an Environmental Cue in Monocyte Differentiation to Macrophages

Monocyte differentiation to macrophages is triggered by migration across the endothelial barrier, which is constituted by both endothelial cells and their underlying basement membrane. We address here the role of the endothelial basement membrane laminins (laminins 411 and 511) in this monocyte to macrophage switch. Chimeric mice carrying CX3CR1-GFP bone marrow were employed to track CCL2-induced monocyte extravasation in a cremaster muscle model using intravital microscopy, revealing faster extravasation in mice lacking endothelial laminin 511 (Tek-cre::Lama5^−/−) and slower extravasation in mice lacking laminin 411 (Lama4^−/−). CX3CR1-GFP^low extravasating monocytes were found to have a higher motility at laminin 511 low sites and to preferentially exit vessels at these sites. However, in vitro experiments reveal that this is not due to effects of laminin 511 on monocyte migration mode nor on the tightness of the endothelial barrier. Rather, using an intestinal macrophage replenishment model and in vitro differentiation studies, we demonstrate that laminin 511, together with the attached endothelium, promote monocyte differentiation to macrophages. Macrophage differentiation is associated with a change in integrin profile, permitting differentiating macrophages to distinguish between laminin 511 high and low areas and to preferentially migrate across laminin 511 low sites. These studies highlight the endothelial basement membrane as a critical site for monocyte differentiation to macrophages, which may be relevant to the differentiation of other cells at vascular niches.

The RhoA regulators Myo9b and GEF-H1 are targets of cyclic nucleotide-dependent kinases in platelets

BACKGROUND

Circulating platelets are maintained in an inactive state by the endothelial lining of the vasculature. Endothelium-derived prostacyclin and nitric oxide stimulate cAMP- and cGMP-dependent kinases, PKA and PKG, to inhibit platelets. PKA and PKG effects include the inhibition of the GTPase RhoA, which has been suggested to involve the direct phosphorylation of RhoA on serine 188.

OBJECTIVES

We wanted to confirm RhoA S188 phosphorylation by cyclic nucleotide-dependent kinases and to identify possible alternative mechanisms of RhoA regulation in platelets.

METHODS

Phosphoproteomics data of human platelets were used to identify candidate PKA and PKG substrates. Phosphorylation of individual proteins was studied by Western blotting and Phos-tag gel electrophoresis in human platelets and transfected HEK293T cells. Pull-down assays were performed to analyze protein interaction and function.

RESULTS

Our data indicate that RhoA is not phosphorylated by PKA in platelets. Instead, we provide evidence that cyclic nucleotide effects are mediated through the phosphorylation of the RhoA-specific GTPase-activating protein Myo9b and the guanine nucleotide exchange factor GEF-H1. We identify Myo9b S1354 and guanine nucleotide exchange factor-H1 (GEF-H1) S886 as PKA and PKG phosphorylation sites. Myo9b S1354 phosphorylation enhances its GTPase activating protein function leading to reduced RhoA-GTP levels. GEF-H1 S886 phosphorylation stimulates binding of 14-3-3β and has been shown to inhibit GEF function by facilitating binding of GEF-H1 to microtubules. Microtubule disruption increases RhoA-GTP levels confirming the importance of GEF-H1 in platelets.

CONCLUSION

Phosphorylation of RhoA regulatory proteins Myo9b and GEF-H1, but not RhoA itself, is involved in cyclic nucleotide-mediated control of RhoA in human platelets.

TSAd Plays a Major Role in Myo9b-Mediated Suppression of Malignant Pleural Effusion by Regulating TH1/TH17 Cell Response

Emerging evidence indicates that Myo9b is a cancer metastasis-related protein and functions in a variety of immune-related diseases. However, it is not clear whether and how Myo9b functions in malignant pleural effusion (MPE). In this study, our data showed that Myo9b expression levels correlated with lung cancer pleural metastasis, and nucleated cells in MPE from either patients or mice expressed a lower level of Myo9b than those in the corresponding blood. Myo9b deficiency in cancer cells suppressed MPE development via inhibition of migration. Myo9b deficiency in mice suppressed MPE development by decreasing T_H1 cells and increasing T_H17 cells. CD4⁺ naive T cells isolated from Myo9b^-/- mouse spleens exhibited less T_H1 cell differentiation and more T_H17 cell differentiation in vitro. mRNA sequencing of nucleated cells showed that T cell-specific adaptor protein (TSAd) was downregulated in Myo9b^-/- mouse MPE, and enrichment of the H3K27me3 mark in the TSAd promoter region was found in the Myo9b^-/- group. Naive T cells purified from wild type mouse spleens transfected with TSAd-specific small interfering RNAs (siRNAs) also showed less T_H1 cell differentiation and more T_H17 cell differentiation than those from the siRNA control group. Furthermore, downregulation of TSAd in mice using cholesterol-conjugated TSAd-specific siRNA suppressed MPE development, decreased T_H1 cells, and increased T_H17 cells in MPE in vivo. Taken together, Myo9b deficiency suppresses MPE development not only by suppressing pleural cancer metastasis but also by regulating T_H1/T_H17 cell response via a TSAd-dependent pathway. This work suggests Myo9b and TSAd as novel candidates for future basic and clinical investigations of cancer.

SLIT2/ROBO1-signaling inhibits macropinocytosis by opposing cortical cytoskeletal remodeling

Macropinocytosis is essential for myeloid cells to survey their environment and for growth of RAS-transformed cancer cells. Several growth factors and inflammatory stimuli are known to induce macropinocytosis, but its endogenous inhibitors have remained elusive. Stimulation of Roundabout receptors by Slit ligands inhibits directional migration of many cell types, including immune cells and cancer cells. We report that SLIT2 inhibits macropinocytosis in vitro and in vivo by inducing cytoskeletal changes in macrophages. In mice, SLIT2 attenuates the uptake of muramyl dipeptide, thereby preventing NOD2-dependent activation of NF-κB and consequent secretion of pro-inflammatory chemokine, CXCL1. Conversely, blocking the action of endogenous SLIT2 enhances CXCL1 secretion. SLIT2 also inhibits macropinocytosis in RAS-transformed cancer cells, thereby decreasing their survival in nutrient-deficient conditions which resemble tumor microenvironment. Our results identify SLIT2 as a physiological inhibitor of macropinocytosis and challenge the conventional notion that signals that enhance macropinocytosis negatively regulate cell migration, and vice versa.

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.

Knockout mouse models reveal the contributions of G protein subunits to complement C5a receptor-mediated chemotaxis

G protein-coupled receptor signaling is required for the navigation of immune cells along chemoattractant gradients. However, chemoattractant receptors may couple to more than one type of heterotrimeric G protein, each of which consists of a Gα, Gβ, and Gγ subunit, making it difficult to delineate the critical signaling pathways. Here, we used knockout mouse models and time-lapse microscopy to elucidate Gα and Gβ subunits contributing to complement C5a receptor-mediated chemotaxis. Complement C5a-mediated chemokinesis and chemotaxis were almost completely abolished in macrophages lacking Gnai2 (encoding Gα_i2), consistent with a reduced leukocyte recruitment previously observed in Gnai2 ^-/- mice, whereas cells lacking Gnai3 (Gα_i3) exhibited only a slight decrease in cell velocity. Surprisingly, C5a-induced Ca²⁺ transients and lamellipodial membrane spreading were persistent in Gnai2 ^-/- macrophages. Macrophages lacking both Gnaq (Gα_q) and Gna11 (Gα₁₁) or both Gna12 (Gα₁₂) and Gna13 (Gα₁₃) had essentially normal chemotaxis, Ca²⁺ signaling, and cell spreading, except Gna12/Gna13-deficient macrophages had increased cell velocity and elongated trailing ends. Moreover, Gnaq/Gna11-deficient cells did not respond to purinergic receptor P2Y₂ stimulation. Genetic deletion of Gna15 (Gα₁₅) virtually abolished C5a-induced Ca²⁺ transients, but chemotaxis and cell spreading were preserved. Homozygous Gnb1 (Gβ₁) deletion was lethal, but mice lacking Gnb2 (Gβ₂) were viable. Gnb2 ^-/- macrophages exhibited robust Ca²⁺ transients and cell spreading, albeit decreased cell velocity and impaired chemotaxis. In summary, complement C5a-mediated chemotaxis requires Gα_i2 and Gβ₂, but not Ca²⁺ signaling, and membrane protrusive activity is promoted by G proteins that deplete phosphatidylinositol 4,5-bisphosphate.

Rho GTPase Regulators and Effectors in Autism Spectrum Disorders: Animal Models and Insights for Therapeutics

The Rho family GTPases are small G proteins that act as molecular switches shuttling between active and inactive forms. Rho GTPases are regulated by two classes of regulatory proteins, guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Rho GTPases transduce the upstream signals to downstream effectors, thus regulating diverse cellular processes, such as growth, migration, adhesion, and differentiation. In particular, Rho GTPases play essential roles in regulating neuronal morphology and function. Recent evidence suggests that dysfunction of Rho GTPase signaling contributes substantially to the pathogenesis of autism spectrum disorder (ASD). It has been found that 20 genes encoding Rho GTPase regulators and effectors are listed as ASD risk genes by Simons foundation autism research initiative (SFARI). This review summarizes the clinical evidence, protein structure, and protein expression pattern of these 20 genes. Moreover, ASD-related behavioral phenotypes in animal models of these genes are reviewed, and the therapeutic approaches that show successful treatment effects in these animal models are discussed.

Imaging of the immune system - towards a subcellular and molecular understanding

Immune responses involve many types of leukocytes that traffic to the site of injury, recognize the insult and respond appropriately. Imaging of the immune system involves a set of methods and analytical tools that are used to visualize immune responses at the cellular and molecular level as they occur in real time. We will review recent and emerging technological advances in optical imaging, and their application to understanding the molecular and cellular responses of neutrophils, macrophages and lymphocytes. Optical live-cell imaging provides deep mechanistic insights at the molecular, cellular, tissue and organism levels. Live-cell imaging can capture quantitative information in real time at subcellular resolution with minimal phototoxicity and repeatedly in the same living cells or in accessible tissues of the living organism. Advanced FRET probes allow tracking signaling events in live cells. Light-sheet microscopy allows for deeper tissue penetration in optically clear samples, enriching our understanding of the higher-level organization of the immune response. Super-resolution microscopy offers insights into compartmentalized signaling at a resolution beyond the diffraction limit, approaching single-molecule resolution. This Review provides a current perspective on live-cell imaging in vitro and in vivo with a focus on the assessment of the immune system.

Regulation of global CD8+ T-cell positioning by the actomyosin cytoskeleton

CD8⁺ T cells have evolved as one of the most motile mammalian cell types, designed to continuously scan peptide-major histocompatibility complexes class I on the surfaces of other cells. Chemoattractants and adhesion molecules direct CD8⁺ T-cell homing to and migration within secondary lymphoid organs, where these cells colocalize with antigen-presenting dendritic cells in confined tissue volumes. CD8⁺ T-cell activation induces a switch to infiltration of non-lymphoid tissue (NLT), which differ in their topology and biophysical properties from lymphoid tissue. Here, we provide a short overview on regulation of organism-wide trafficking patterns during naive T-cell recirculation and their switch to non-lymphoid tissue homing during activation. The migratory lifestyle of CD8⁺ T cells is regulated by their actomyosin cytoskeleton, which translates chemical signals from surface receptors into mechanical work. We explore how properties of the actomyosin cytoskeleton and its regulators affect CD8⁺ T cell function in lymphoid and non-lymphoid tissue, combining recent findings in the field of cell migration and actin network regulation with tissue anatomy. Finally, we hypothesize that under certain conditions, intrinsic regulation of actomyosin dynamics may render NLT CD8⁺ T-cell populations less dependent on input from extrinsic signals during tissue scanning.

Spatial proteomics revealed a CX3CL1-dependent crosstalk between the urothelium and relocated macrophages through IL-6 during an acute bacterial infection in the urinary bladder

The urothelium of the urinary bladder represents the first line of defense. However, uropathogenic E. coli (UPEC) damage the urothelium and cause acute bacterial infection. Here, we demonstrate the crosstalk between macrophages and the urothelium stimulating macrophage migration into the urothelium. Using spatial proteomics by MALDI-MSI and LC-MS/MS, a novel algorithm revealed the spatial activation and migration of macrophages. Analysis of the spatial proteome unravelled the coexpression of Myo9b and F4/80 in the infected urothelium, indicating that macrophages have entered the urothelium upon infection. Immunofluorescence microscopy additionally indicated that intraurothelial macrophages phagocytosed UPEC and eliminated neutrophils. Further analysis of the spatial proteome by MALDI-MSI showed strong expression of IL-6 in the urothelium and local inhibition of this molecule reduced macrophage migration into the urothelium and aggravated the infection. After IL-6 inhibition, the expression of matrix metalloproteinases and chemokines, such as CX₃CL1 was reduced in the urothelium. Accordingly, macrophage migration into the urothelium was diminished in the absence of CX₃CL1 signaling in Cx₃cr1^gfp/gfp mice. Conclusively, this study describes the crosstalk between the infected urothelium and macrophages through IL-6-induced CX₃CL1 expression. Such crosstalk facilitates the relocation of macrophages into the urothelium and reduces bacterial burden in the urinary bladder.

Class IX Myosins: Motorized RhoGAP Signaling Molecules

Class IX myosins are simultaneously motor and signaling molecules. In addition to myosin class-specific functions of the tail region, they feature unique motor properties. Within their motor region they contain a long insertion with a calmodulin- and a F-actin-binding site. The rate-limiting step in the ATPase cycle is ATP hydrolysis rather than, typical for other myosins, the release of either product. This means that class IX myosins spend a large fraction of their cycle time in the ATP-bound state, which is typically a low F-actin affinity state. Nevertheless, class IX myosins in the ATP-bound state stochastically switch between a low and a high F-actin affinity state. Single motor domains even show characteristics of processive movement towards the plus end of actin filaments. The insertion thereby acts as an actin tether. The motor domain transports as intramolecular cargo a signaling Rho GTPase-activating protein domain located in the tail region. Rho GTPase-activating proteins catalyze the conversion of active GTP-bound Rho to inactive GDP-bound Rho by stimulating GTP hydrolysis. In cells, Rho activity regulates actin cytoskeleton organization and actomyosin II contractility. Thus, class IX myosins regulate cell morphology, cell migration, cell-cell junctions and membrane trafficking. These cellular functions affect embryonic development, adult organ homeostasis and immune responses. Human diseases associated with mutations in the two class IX myosins, Myo9a and Myo9b, have been identified, including hydrocephalus and congenital myasthenic syndrome in connection with Myo9a and autoimmune diseases in connection with Myo9b.

Squeezing in a Meal: Myosin Functions in Phagocytosis

Phagocytosis is a receptor-mediated, actin-dependent process of internalization of large extracellular particles, such as pathogens or apoptotic cells. Engulfment of phagocytic targets requires the activity of myosins, actin-dependent molecular motors, which perform a variety of functions at distinct steps during phagocytosis. By applying force to actin filaments, the plasma membrane, and intracellular proteins and organelles, myosins can generate contractility, directly regulate actin assembly to ensure proper phagocytic internalization, and translocate phagosomes or other cargo to appropriate cellular locations. Recent studies using engineered microenvironments and phagocytic targets have demonstrated how altering the actomyosin cytoskeleton affects phagocytic behavior. Here, we discuss how studies using genetic and biochemical manipulation of myosins, force measurement techniques, and live-cell imaging have advanced our understanding of how specific myosins function at individual steps of phagocytosis.

RhoA as a Key Regulator of Innate and Adaptive Immunity

RhoA is a ubiquitously expressed cytoplasmic protein that belongs to the family of small GTPases. RhoA acts as a molecular switch that is activated in response to binding of chemokines, cytokines, and growth factors, and via mDia and the ROCK signaling cascade regulates the activation of cytoskeletal proteins, and other factors. This review aims to summarize our current knowledge on the role of RhoA as a general key regulator of immune cell differentiation and function. The contribution of RhoA for the primary functions of innate immune cell types, namely neutrophils, macrophages, and conventional dendritic cells (DC) to (i) get activated by pathogen-derived and endogenous danger signals, (ii) migrate to sites of infection and inflammation, and (iii) internalize pathogens has been fairly established. In activated DC, which constitute the most potent antigen-presenting cells of the immune system, RhoA is also important for the presentation of pathogen-derived antigen and the formation of an immunological synapse between DC and antigen-specific T cells as a prerequisite to induce adaptive T cell responses. In T cells and B cells as the effector cells of the adaptive immune system Rho signaling is pivotal for activation and migration. More recently, mutations of Rho and Rho-modulating factors have been identified to predispose for autoimmune diseases and as causative for hematopoietic malignancies.

The RhoGAP HUM-7/Myo9 integrates signals to modulate RHO-1/RhoA during embryonic morphogenesis in Caenorhabditiselegans

During embryonic morphogenesis, cells and tissues undergo dramatic movements under the control of F-actin regulators. Our studies of epidermal cell migrations in developing Caenorhabditiselegans embryos have identified multiple plasma membrane signals that regulate the Rac GTPase, thus regulating WAVE and Arp2/3 complexes, to promote branched F-actin formation and polarized enrichment. Here, we describe a pathway that acts in parallel to Rac to transduce membrane signals to control epidermal F-actin through the GTPase RHO-1/RhoA. RHO-1 contributes to epidermal migration through effects on underlying neuroblasts. We identify signals to regulate RHO-1-dependent events in the epidermis. HUM-7, the C. elegans homolog of human MYO9A and MYO9B, regulates F-actin dynamics during epidermal migration. Genetics and biochemistry support that HUM-7 behaves as a GTPase-activating protein (GAP) for the RHO-1/RhoA and CDC-42 GTPases. Loss of HUM-7 enhances RHO-1-dependent epidermal cell behaviors. We identify SAX-3/ROBO as an upstream signal that contributes to attenuated RHO-1 activation through its regulation of HUM-7/Myo9. These studies identify a new role for RHO-1 during epidermal cell migration, and suggest that RHO-1 activity is regulated by SAX-3/ROBO acting on the RhoGAP HUM-7.

Myosins in Osteoclast Formation and Function

Skeletal quantity and quality are determined by processes of bone modeling and remodeling, which are undertaken by cells that build and resorb bone as they respond to mechanical, hormonal, and other external and internal signals. As the sole bone resorptive cell type, osteoclasts possess a remarkably dynamic actin cytoskeleton that drives their function in this enterprise. Actin rearrangements guide osteoclasts’ capacity for precursor fusion during differentiation, for migration across bone surfaces and sensing of their composition, and for generation of unique actin superstructures required for the resorptive process. In this regard, it is not surprising that myosins, the superfamily of actin-based motor proteins, play key roles in osteoclast physiology. This review briefly summarizes current knowledge of the osteoclast actin cytoskeleton and describes myosins’ roles in osteoclast differentiation, migration, and actin superstructure patterning.

Emerin modulates spatial organization of chromosome territories in cells on softer matrices

Cells perceive and relay external mechanical forces into the nucleus through the nuclear envelope. Here we examined the effect of lowering substrate stiffness as a paradigm to address the impact of altered mechanical forces on nuclear structure-function relationships. RNA sequencing of cells on softer matrices revealed significant transcriptional imbalances, predominantly in chromatin associated processes and transcriptional deregulation of human Chromosome 1. Furthermore, 3-Dimensional fluorescence in situ hybridization (3D-FISH) analyses showed a significant mislocalization of Chromosome 1 and 19 Territories (CT) into the nuclear interior, consistent with their transcriptional deregulation. However, CT18 with relatively lower transcriptional dysregulation, also mislocalized into the nuclear interior. Furthermore, nuclear Lamins that regulate chromosome positioning, were mislocalized into the nuclear interior in response to lowered matrix stiffness. Notably, Lamin B2 overexpression retained CT18 near the nuclear periphery in cells on softer matrices. While, cells on softer matrices also activated emerin phosphorylation at a novel Tyr99 residue, the inhibition of which in a phospho-deficient mutant (emerinY99F), selectively retained chromosome 18 and 19 but not chromosome 1 territories at their conserved nuclear locations. Taken together, emerin functions as a key mechanosensor, that modulates the spatial organization of chromosome territories in the interphase nucleus.

The Rho regulator Myosin IXb enables nonlymphoid tissue seeding of protective CD8+ T cells

Moalli et al. combine in vitro CD8⁺ T cell motility analysis with intravital imaging of mouse tissues to identify the actomyosin regulator Myo9b as a central player for nonlymphoid tissue infiltration during adaptive immune responses by facilitating crossing of tissue barriers.

T cells are actively scanning pMHC-presenting cells in lymphoid organs and nonlymphoid tissues (NLTs) with divergent topologies and confinement. How the T cell actomyosin cytoskeleton facilitates this task in distinct environments is incompletely understood. Here, we show that lack of Myosin IXb (Myo9b), a negative regulator of the small GTPase Rho, led to increased Rho-GTP levels and cell surface stiffness in primary T cells. Nonetheless, intravital imaging revealed robust motility of Myo9b^−/− CD8⁺ T cells in lymphoid tissue and similar expansion and differentiation during immune responses. In contrast, accumulation of Myo9b^−/− CD8⁺ T cells in NLTs was strongly impaired. Specifically, Myo9b was required for T cell crossing of basement membranes, such as those which are present between dermis and epidermis. As consequence, Myo9b^−/− CD8⁺ T cells showed impaired control of skin infections. In sum, we show that Myo9b is critical for the CD8⁺ T cell adaptation from lymphoid to NLT surveillance and the establishment of protective tissue–resident T cell populations.

Relationships of bone characteristics in MYO9B deficient femurs

The objective of this study was to examine relationships among a variety of bone characteristics, including volumetric, mineral density, geometric, dynamic mechanical analysis, and static fracture mechanical properties. As MYO9B is an unconventional myosin in bone cells responsible for normal skeletal growth, bone characteristics of wild-type (WT), heterozygous (HET), and MYO9B knockout (KO) mice groups were compared as an animal model to express different bone quantity and quality. Forty-five sex-matched 12-week-old mice were used in this study. After euthanization, femurs were isolated and scanned using microcomputed tomography (micro-CT) to assess bone volumetric, tissue mineral density (TMD), and geometric parameters. Then, a non-destructive dynamic mechanical analysis (DMA) was performed by applying oscillatory bending displacement on the femur. Finally, the same femur was subject to static fracture testing. KO group had significantly lower length, bone mineral density (BMD), bone mass and volume, dynamic and static stiffness, and strength than WT and HET groups (p < 0.019). On the other hand, TMD parameters of KO group were comparable with those of WT group. HET group showed volumetric, geometric, and mechanical properties similar to WT group, but had lower TMD (p < 0.014). Non-destructive micro-CT and DMA parameters had significant positive correlations with strength (p < 0.015) without combined effect of groups and sex on the correlations (p > 0.077). This comprehensive characterization provides a better understanding of interactive behavior between the tissue- and organ-level of the same femur. The current findings elucidate that MYO9B is responsible for controlling bone volume to determine the growth rate and fracture risk of bone.

The MYO6 interactome reveals adaptor complexes coordinating early endosome and cytoskeletal dynamics

The intracellular functions of myosin motors requires a number of adaptor molecules, which control cargo attachment, but also fine-tune motor activity in time and space. These motor-adaptor-cargo interactions are often weak, transient or highly regulated. To overcome these problems, we use a proximity labelling-based proteomics strategy to map the interactome of the unique minus end-directed actin motor MYO6. Detailed biochemical and functional analysis identified several distinct MYO6-adaptor modules including two complexes containing RhoGEFs: the LIFT (LARG-Induced F-actin for Tethering) complex that controls endosome positioning and motility through RHO-driven actin polymerisation; and the DISP (DOCK7-Induced Septin disPlacement) complex, a novel regulator of the septin cytoskeleton. These complexes emphasise the role of MYO6 in coordinating endosome dynamics and cytoskeletal architecture. This study provides the first in vivo interactome of a myosin motor protein and highlights the power of this approach in uncovering dynamic and functionally diverse myosin motor complexes.

Keap1 Inhibits Metastatic Properties of NSCLC Cells by Stabilizing Architectures of F-Actin and Focal Adhesions

Low expression of the tumor suppressor Kelch-like ECH-associated protein 1 (KEAP1) in non-small cell lung cancer (NSCLC) often results in higher malignant biological behavior and poor prognosis; however, the underlying mechanism remains unclear. The present study demonstrates that overexpression of Keap1 significantly suppresses migration and invasion of three different lung cancer cells (A549, H460, and H1299). Highly expressed Keap1, compared with the control, promotes formation of multiple stress fibers with larger mature focal adhesion complexes in the cytoplasm where only fine focal adhesions were observed in the membrane under control conditions. RhoA activity significantly increased when Keap1 was overexpressed, whereas Myosin 9b expression was reduced but could be rescued by proteasome inhibition. Noticeably, mouse tumor xenografts with Keap1 overexpression were smaller in size and less metastatic relative to the control group. Taken together, these results demonstrate that Keap1 stabilizes F-actin cytoskeleton structures and inhibits focal adhesion turnover, thereby restraining the migration and invasion of NSCLC. Therefore, increasing Keap1 or targeting its downstream molecules might provide potential therapeutic benefits for the treatment of patients with NSCLC.Implications: This study provides mechanistic insight on the metastatic process in NSCLC and suggests that Keap1 and its downstream molecules may be valuable drug targets for NSCLC patients. Mol Cancer Res; 16(3); 508-16. ©2018 AACR.

Genome-wide survey of single-nucleotide polymorphisms reveals fine-scale population structure and signs of selection in the threatened Caribbean elkhorn coral, Acropora palmata

The advent of next-generation sequencing tools has made it possible to conduct fine-scale surveys of population differentiation and genome-wide scans for signatures of selection in non-model organisms. Such surveys are of particular importance in sharply declining coral species, since knowledge of population boundaries and signs of local adaptation can inform restoration and conservation efforts. Here, we use genome-wide surveys of single-nucleotide polymorphisms in the threatened Caribbean elkhorn coral, Acropora palmata, to reveal fine-scale population structure and infer the major barrier to gene flow that separates the eastern and western Caribbean populations between the Bahamas and Puerto Rico. The exact location of this break had been subject to discussion because two previous studies based on microsatellite data had come to differing conclusions. We investigate this contradiction by analyzing an extended set of 11 microsatellite markers including the five previously employed and discovered that one of the original microsatellite loci is apparently under selection. Exclusion of this locus reconciles the results from the SNP and the microsatellite datasets. Scans for outlier loci in the SNP data detected 13 candidate loci under positive selection, however there was no correlation between available environmental parameters and genetic distance. Together, these results suggest that reef restoration efforts should use local sources and utilize existing functional variation among geographic regions in ex situ crossing experiments to improve stress resistance of this species.

The RhoGAP Myo9b Promotes Bone Growth by Mediating Osteoblastic Responsiveness to IGF-1

The Ras homolog A (RhoA) subfamily of Rho guanosine triphosphatases (GTPases) regulates actin-based cellular functions in bone such as differentiation, migration, and mechanotransduction. Polymorphisms or genetic ablation of RHOA and some of its regulatory guanine exchange factors (GEFs) have been linked to poor bone health in humans and mice, but the effects of RhoA-specific GTPase-activating proteins (GAPs) on bone quality have not yet been identified. Therefore, we examined the consequences of RhoGAP Myo9b gene knockout on bone growth, phenotype, and cellular activity. Male and female mice lacking both alleles demonstrated growth retardation and decreased bone formation rates during early puberty. These mice had smaller, weaker bones by 4 weeks of age, but only female KOs had altered cellular numbers, with fewer osteoblasts and more osteoclasts. By 12 weeks of age, bone quality in KOs worsened. In contrast, 4-week-old heterozygotes demonstrated bone defects that resolved by 12 weeks of age. Throughout, Myo9b ablation affected females more than males. Osteoclast activity appeared unaffected. In primary osteogenic cells, Myo9b was distributed in stress fibers and focal adhesions, and its absence resulted in poor spreading and eventual detachment from culture dishes. Similarly, MC3T3-E1 preosteoblasts with transiently suppressed Myo9b levels spread poorly and contained decreased numbers of focal adhesions. These cells also demonstrated reduced ability to undergo IGF-1-induced spreading or chemotaxis toward IGF-1, though responses to PDGF and BMP-2 were unaffected. IGF-1 receptor (IGF1R) activation was normal in cells with diminished Myo9b levels, but the activated receptor was redistributed from stress fibers and focal adhesions into nuclei, potentially affecting receptor accessibility and gene expression. These results demonstrate that Myo9b regulates a subset of RhoA-activated processes necessary for IGF-1 responsiveness in osteogenic cells, and is critical for normal bone formation in growing mice. © 2017 American Society for Bone and Mineral Research.

How myosin organization of the actin cytoskeleton contributes to the cancer phenotype

The human genome contains 39 genes that encode myosin heavy chains, classified on the basis of their sequence similarity into 12 classes. Most cells express at least 12 different genes, from at least 8 different classes, which are typically composed of several class 1 genes, at least one class 2 gene and classes 5, 6, 9, 10, 18 and 19. Although the different myosin isoforms all have specific and non-overlapping roles in the cell, in combination they all contribute to the organization of the actin cytoskeleton, and the shape and phenotype of the cell. Over (or under) expression of these different myosin isoforms can have strong effects on actin organization, cell shape and contribute to the cancer phenotype as discussed in this review.

Multiple roles of filopodial dynamics in particle capture and phagocytosis and phenotypes of Cdc42 and Myo10 deletion

Macrophage filopodia, finger-like membrane protrusions, were first implicated in phagocytosis more than 100 years ago, but little is still known about the involvement of these actin-dependent structures in particle clearance. Using spinning disk confocal microscopy to image filopodial dynamics in mouse resident Lifeact-EGFP macrophages, we show that filopodia, or filopodia-like structures, support pathogen clearance by multiple means. Filopodia supported the phagocytic uptake of bacterial (Escherichia coli) particles by (i) capturing along the filopodial shaft and surfing toward the cell body, the most common mode of capture; (ii) capturing via the tip followed by retraction; (iii) combinations of surfing and retraction; or (iv) sweeping actions. In addition, filopodia supported the uptake of zymosan (Saccharomyces cerevisiae) particles by (i) providing fixation, (ii) capturing at the tip and filopodia-guided actin anterograde flow with phagocytic cup formation, and (iii) the rapid growth of new protrusions. To explore the role of filopodia-inducing Cdc42, we generated myeloid-restricted Cdc42 knock-out mice. Cdc42-deficient macrophages exhibited rapid phagocytic cup kinetics, but reduced particle clearance, which could be explained by the marked rounded-up morphology of these cells. Macrophages lacking Myo10, thought to act downstream of Cdc42, had normal morphology, motility, and phagocytic cup formation, but displayed markedly reduced filopodia formation. In conclusion, live-cell imaging revealed multiple mechanisms involving macrophage filopodia in particle capture and engulfment. Cdc42 is not critical for filopodia or phagocytic cup formation, but plays a key role in driving macrophage lamellipodial spreading.

Inhibition of KLF5-Myo9b-RhoA Pathway-Mediated Podosome Formation in Macrophages Ameliorates Abdominal Aortic Aneurysm

RATIONALE

Abdominal aortic aneurysms (AAAs) are characterized by pathological remodeling of the aortic wall. Although both increased Krüppel-like factor 5 (KLF5) expression and macrophage infiltration have been implicated in vascular remodeling, the role of KLF5 in macrophage infiltration and AAA formation remains unclear.

OBJECTIVE

To determine the role of KLF5 in AAA formation and macrophage infiltration into AAAs.

METHODS AND RESULTS

KLF5 expression was significantly increased in human AAA tissues and in 2 mouse models of experimental AAA. Moreover, in myeloid-specific Klf5 knockout mice (myeKlf5^-/- mice), macrophage infiltration, medial smooth muscle cell loss, elastin degradation, and AAA formation were markedly decreased. In cell migration and time-lapse imaging analyses, the migration of murine myeKlf5^-/- macrophages was impaired, and in luciferase reporter assays, KLF5 activated Myo9b (myosin IXB) transcription by direct binding to the Myo9b promoter. In subsequent coimmunostaining studies, Myo9b was colocalized with filamentous actin, cortactin, vinculin, and Tks5 in the podosomes of phorbol 12,13-dibutyrate-treated macrophages, indicating that Myo9b participates in podosome formation. Gain- and loss-of-function experiments showed that KLF5 promoted podosome formation in macrophages by upregulating Myo9b expression. Furthermore, RhoA-GTP levels increased after KLF5 knockdown in macrophages, suggesting that KLF5 lies upstream of RhoA signaling. Finally, Myo9b expression was increased in human AAA tissues, located in macrophages, and positively correlated with AAA size.

CONCLUSIONS

These data are the first to indicate that KLF5-dependent regulation of Myo9b/RhoA is required for podosome formation and macrophage migration during AAA formation, warranting consideration of the KLF5-Myo9b-RhoA pathway as a therapeutic target for AAA treatment.

Myosins: Domain Organisation, Motor Properties, Physiological Roles and Cellular Functions

Myosins are cytoskeletal motor proteins that use energy derived from ATP hydrolysis to generate force and movement along actin filaments. Humans express 38 myosin genes belonging to 12 classes that participate in a diverse range of crucial activities, including muscle contraction, intracellular trafficking, cell division, motility, actin cytoskeletal organisation and cell signalling. Myosin malfunction has been implicated a variety of disorders including deafness, hypertrophic cardiomyopathy, Usher syndrome, Griscelli syndrome and cancer. In this chapter, we will first discuss the key structural and kinetic features that are conserved across the myosin family. Thereafter, we summarise for each member in turn its unique functional and structural adaptations, cellular roles and associated pathologies. Finally, we address the broad therapeutic potential for pharmacological interventions that target myosin family members.

Modular transcriptional repertoire and MicroRNA target analyses characterize genomic dysregulation in the thymus of Down syndrome infants

Trisomy 21-driven transcriptional alterations in human thymus were characterized through gene coexpression network (GCN) and miRNA-target analyses. We used whole thymic tissue--obtained at heart surgery from Down syndrome (DS) and karyotipically normal subjects (CT)--and a network-based approach for GCN analysis that allows the identification of modular transcriptional repertoires (communities) and the interactions between all the system's constituents through community detection. Changes in the degree of connections observed for hierarchically important hubs/genes in CT and DS networks corresponded to community changes. Distinct communities of highly interconnected genes were topologically identified in these networks. The role of miRNAs in modulating the expression of highly connected genes in CT and DS was revealed through miRNA-target analysis. Trisomy 21 gene dysregulation in thymus may be depicted as the breakdown and altered reorganization of transcriptional modules. Leading networks acting in normal or disease states were identified. CT networks would depict the "canonical" way of thymus functioning. Conversely, DS networks represent a "non-canonical" way, i.e., thymic tissue adaptation under trisomy 21 genomic dysregulation. This adaptation is probably driven by epigenetic mechanisms acting at chromatin level and through the miRNA control of transcriptional programs involving the networks' high-hierarchy genes.

Self-organization of actin networks by a monomeric myosin

The organization of actomyosin networks lies at the center of many types of cellular motility, including cell polarization and collective cell migration during development and morphogenesis. Myosin-IXa is critically involved in these processes. Using total internal reflection fluorescence microscopy, we resolved actin bundles assembled by myosin-IXa. Electron microscopic data revealed that the bundles consisted of highly ordered lattices with parallel actin polarity. The myosin-IXa motor domains aligned across the network, forming cross-links at a repeat distance of precisely 36 nm, matching the helical repeat of actin. Single-particle image processing resolved three distinct conformations of myosin-IXa in the absence of nucleotide. Using cross-correlation of a modeled actomyosin crystal structure, we identified sites of additional mass, which can only be accounted for by the large insert in loop 2 exclusively found in the motor domain of class IX myosins. We show that the large insert in loop 2 binds calmodulin and creates two coordinated actin-binding sites that constrain the actomyosin interactions generating the actin lattices. The actin lattices introduce orientated tracks at specific sites in the cell, which might install platforms allowing Rho-GTPase-activating protein (RhoGAP) activity to be focused at a definite locus. In addition, the lattices might introduce a myosin-related, force-sensing mechanism into the cytoskeleton in cell polarization and collective cell migration.