Role of Rac in controlling the actin cytoskeleton and chemotaxis in motile cells

PTEN and the PTEN-like phosphatase CnrN have both distinct and overlapping roles in a Dictyostelium chemorepulsion pathway

Little is known about eukaryotic chemorepulsion. The enzymes phosphatase and tensin homolog (PTEN) and CnrN dephosphorylate phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P3] to phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. Dictyostelium discoideum cells require both PTEN and CnrN to induce chemorepulsion of cells away from the secreted chemorepellent protein AprA. How D. discoideum cells utilize two proteins with redundant phosphatase activities in response to AprA is unclear. Here, we show that D. discoideum cells require both PTEN and CnrN to locally inhibit Ras activation, decrease basal levels of PI(3,4,5)P3 and increase basal numbers of macropinosomes, and AprA prevents this increase. AprA requires both PTEN and CnrN to increase PI(4,5)P2 levels, decrease PI(3,4,5)P3 levels, inhibit proliferation, decrease myosin II phosphorylation and increase filopod sizes. PTEN, but not CnrN, decreases basal levels of PI(4,5)P2, and AprA requires PTEN, but not CnrN, to induce cell roundness. Together, our results suggest that CnrN and PTEN play unique roles in AprA-induced chemorepulsion.

PTEN and the PTEN-like phosphatase CnrN have both distinct and overlapping roles in a Dictyostelium chemorepulsion pathway

The directed movement of eukaryotic cells is crucial for processes such as embryogenesis and immune cell trafficking. The enzyme Phosphatase and tensin homolog (PTEN) dephosphorylates phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P 3 ] to phosphatidylinositol 4,5-bisphosphate [PI(4,5)P 2 ]. Dictyostelium discoideum cells require both PTEN and the PTEN-like phosphatase CnrN to locally inhibit Ras activation to induce biased movement of cells away from the secreted chemorepellent protein AprA. Both PTEN and CnrN decrease basal levels of PI(3,4,5)P 3 and increase basal numbers of macropinosomes, and AprA prevents this increase. AprA requires both PTEN and CnrN to increase PI(4,5)P 2 levels, decrease PI(3,4,5)P 3 levels, inhibit proliferation, decrease myosin II phosphorylation, and increase filopod sizes. AprA causes PTEN, similar to CnrN, to localize to the side of the cell towards AprA in an AprA gradient. However, PTEN and CnrN also have distinct roles in some signaling pathways. PTEN, but not CnrN, decreases basal levels of PI(4,5)P 2 , AprA requires PTEN, but not CnrN, to induce cell roundness, and CnrN and PTEN have different effects on the number of filopods and pseudopods, and the sizes of filopods. Together, our results suggest that CnrN and PTEN play unique roles in D. discoideum signaling pathways, and possibly dephosphorylate PI(3,4,5)P 3 in different membrane domains, to mediate chemorepulsion away from AprA.

Endolysosomal TRPML1 channel regulates cancer cell migration by altering intracellular trafficking of E-cadherin and β1-integrin

Metastasis still accounts for 90% of all cancer-related death cases. An increase of cellular mobility and invasive traits of cancer cells mark two crucial prerequisites of metastasis. Recent studies highlight the involvement of the endolysosomal cation channel TRPML1 in cell migration. Our results identified a widely antimigratory effect upon loss of TRPML1 function in a panel of cell lines in vitro and reduced dissemination in vivo. As mode-of-action, we established TRPML1 as a crucial regulator of cytosolic calcium levels, actin polymerization, and intracellular trafficking of two promigratory proteins: E-cadherin and β1-integrin. Interestingly, KO of TRPML1 differentially interferes with the recycling process of E-cadherin and β1-integrin in a cell line-dependant manner, while resulting in the same phenotype of decreased migratory and adhesive capacities in vitro. Additionally, we observed a coherence between reduction of E-cadherin levels at membrane site and phosphorylation of NF-κB in a β-catenin/p38-mediated manner. As a result, an E-cadherin/NF-κB feedback loop is generated, regulating E-cadherin expression on a transcriptional level. Consequently, our findings highlight the role of TRPML1 as a regulator in migratory processes and suggest the ion channel as a suitable target for the inhibition of migration and invasion.

Phosphoinositide Signaling in Immune Cell Migration

In response to different immune challenges, immune cells migrate to specific sites in the body, where they perform their functions such as defense against infection, inflammation regulation, antigen recognition, and immune surveillance. Therefore, the migration ability is a fundamental aspect of immune cell function. Phosphoinositide signaling plays critical roles in modulating immune cell migration by controlling cell polarization, cytoskeletal rearrangement, protrusion formation, and uropod contraction. Upon chemoattractant stimulation, specific phosphoinositide kinases and phosphatases control the local phosphoinositide levels to establish polarized phosphoinositide distribution, which recruits phosphoinositide effectors to distinct subcellular locations to facilitate cell migration. In this Special Issue of "Molecular Mechanisms Underlying Cell Adhesion and Migration", we discuss the significance of phosphoinositide production and conversion by phosphoinositide kinases and phosphatases in the migration of different types of immune cells.

The therapeutic potential of neurofibromin signaling pathways and binding partners

Neurofibromin controls many cell processes, such as growth, learning, and memory. If neurofibromin is not working properly, it can lead to health problems, including issues with the nervous, skeletal, and cardiovascular systems and cancer. This review examines neurofibromin's binding partners, signaling pathways and potential therapeutic targets. In addition, it summarizes the different post-translational modifications that can affect neurofibromin's interactions with other molecules. It is essential to investigate the molecular mechanisms that underlie neurofibromin variants in order to provide with functional connections between neurofibromin and its associated proteins for possible therapeutic targets based on its biological function.

Intelectin enhances the phagocytosis of macrophages via CDC42-WASF2-ARPC2 signaling axis in Megalobrama amblycephala

Intelectin has been identified in various vertebrates and plays an important role in the host immune system. In our previous studies, recombinant Megalobrama amblycephala intelectin (rMaINTL) protein with excellent bacterial binding and agglutination activities enhances the phagocytic and killing activities of macrophages in M. amblycephala; however, the underlying regulatory mechanisms remain unclear. The present study showed that treatment with Aeromonas hydrophila and LPS induced the expression of rMaINTL in macrophages, and its level and distribution in macrophages or kidney tissue markedly increased after incubation or injection with rMaINTL. The cellular structure of macrophages was significantly affected after incubation with rMaINTL, resulting in an increased surface area and pseudopodia extension, which might contribute to enhancing the phagocytic ability of macrophages. Then, digital gene expression profiling analysis of the kidneys from rMaINTL-treated juvenile M. amblycephala identified some phagocytosis-related signaling factors that were enriched in pathways involved in the regulation of the actin cytoskeleton. In addition, qRT-PCR and western blotting verified that rMaINTL upregulated the expression of CDC42, WASF2, and ARPC2 in vitro and in vivo; however, the expression of these proteins was inhibited by a CDC42 inhibitor in macrophages. Moreover, CDC42 mediated the promotion of rMaINTL on actin polymerization by increasing the F-actin/G-actin ratio, which led to the extension of pseudopodia and remodeling of the macrophage cytoskeleton. Furthermore, the enhancement of macrophage phagocytosis by rMaINTL was blocked by the CDC42 inhibitor. These results suggested that rMaINTL induced the expression of CDC42 as well as the downstream signaling molecules WASF2 and ARPC2, thereby facilitating actin polymerization to promote cytoskeletal remodeling and phagocytosis. Overall, MaINTL enhanced the phagocytosis activity of macrophages in M. amblycephala via activation of the CDC42-WASF2-ARPC2 signaling axis.

Coronin 2B Regulates Neuronal Migration via Rac1-Dependent Multipolar-Bipolar Transition

In the developing cortex, excitatory neurons migrate along the radial fibers to their final destinations and build up synaptic connection with each other to form functional circuitry. The shaping of neuronal morphologies by actin cytoskeleton dynamics is crucial for neuronal migration. However, it is largely unknown how the distribution and assembly of the F-actin cytoskeleton are coordinated. In the present study, we found that an actin regulatory protein, coronin 2B, is indispensable for the transition from a multipolar to bipolar morphology during neuronal migration in ICR mice of either sex. Loss of coronin 2B led to heterotopic accumulation of migrating neurons in the intermediate zone along with reduced dendritic complexity and aberrant neuronal activity in the cortical plate. This was accompanied by increased seizure susceptibility, suggesting the malfunction of cortical development in coronin 2B-deficient brains. Coronin 2B knockdown disrupted the distribution of the F-actin cytoskeleton at the leading processes, while the migration defect in coronin 2B-deficient neurons was partially rescued by overexpression of Rac1 and its downstream actin-severing protein, cofilin. Our results collectively reveal the physiological function of coronin 2B during neuronal migration whereby it maintains the proper distribution of activated Rac1 and the F-actin cytoskeleton.SIGNIFICANCE STATEMENT Deficits in neuronal migration during cortical development result in various neurodevelopmental disorders (e.g., focal cortical dysplasia, periventricular heterotopia, epilepsy, etc.). Most signaling pathways that control neuronal migration process converge to regulate actin cytoskeleton dynamics. Therefore, it is important to understand how actin dynamics is coordinated in the critical processes of neuronal migration. Herein, we report that coronin 2B is a key protein that regulates neuronal migration through its ability to control the distribution of the actin cytoskeleton and its regulatory signaling protein Rac1 during the multipolar-bipolar transition in the intermediate zone, providing insights into the molecular machinery that drives the migration process of newborn neurons.

MicroRNAs as the critical regulators of cell migration and invasion in thyroid cancer

Thyroid cancer (TC) is one of the most frequent endocrine malignancies that is more common among females. Tumor recurrence is one of the most important clinical manifestations in differentiated TC which is associated with different factors including age, tumor size, and histological features. Various molecular processes such as genetic or epigenetic modifications and non-coding RNAs are also involved in TC progression and metastasis. The epithelial-to-mesenchymal transition (EMT) is an important biological process during tumor invasion and migration that affects the initiation and transformation of early-stage tumors into invasive malignancies. A combination of transcription factors, growth factors, signaling pathways, and epigenetic regulations affect the thyroid cell migration and EMT process. MicroRNAs (miRNAs) are important molecular factors involved in tumor metastasis by regulation of EMT-activating signaling pathways. Various miRNAs are involved in the signaling pathways associated with TC metastasis which can be used as diagnostic and therapeutic biomarkers. Since, the miRNAs are sensitive, specific, and non-invasive, they can be suggested as efficient and optimal biomarkers of tumor invasion and metastasis. In the present review, we have summarized all of the miRNAs which have been significantly involved in thyroid tumor cells migration and invasion. We also categorized all of the reported miRNAs based on their cellular processes to clarify the molecular role of miRNAs during thyroid tumor cell migration and invasion. This review paves the way of introducing a non-invasive diagnostic and prognostic panel of miRNAs in aggressive and metastatic TC patients.

Myosin II and Arp2/3 cross-talk governs intracellular hydraulic pressure and lamellipodia formation

Human fibroblasts can switch between lamellipodia-dependent and -independent migration mechanisms on two-dimensional surfaces and in three-dimensional (3D) matrices. RhoA GTPase activity governs the switch from low-pressure lamellipodia to high-pressure lobopodia in response to the physical structure of the 3D matrix. Inhibiting actomyosin contractility in these cells reduces intracellular pressure and reverts lobopodia to lamellipodial protrusions via an unknown mechanism. To test the hypothesis that high pressure physically prevents lamellipodia formation, we manipulated pressure by activating RhoA or changing the osmolarity of the extracellular environment and imaged cell protrusions. We find RhoA activity inhibits Rac1-mediated lamellipodia formation through two distinct pathways. First, RhoA boosts intracellular pressure by increasing actomyosin contractility and water influx but acts upstream of Rac1 to inhibit lamellipodia formation. Increasing osmotic pressure revealed a second RhoA pathway, which acts through nonmuscle myosin II (NMII) to disrupt lamellipodia downstream from Rac1 and elevate pressure. Interestingly, Arp2/3 inhibition triggered a NMII-dependent increase in intracellular pressure, along with lamellipodia disruption. Together, these results suggest that actomyosin contractility and water influx are coordinated to increase intracellular pressure, and RhoA signaling can inhibit lamellipodia formation via two distinct pathways in high-pressure cells.

Rac activation is key to cell motility and directionality: An experimental and modelling investigation

Cell migration is a tightly-regulated process that involves protein gradients formed by the Rho family of GTPases, including Rho and Rac. The front (rear) of cells is generally characterized by higher active Rac (Rho) and lower active Rho (Rac) concentrations. Protein clusters, called adhesions, that anchor cells to their external environment have been shown to be dynamic and small (stable and large) at the cell front (rear), forming the force-transmission points necessary for persistent movement. Differences in adhesion sizes and dynamics have been linked to gradients in Rac and Rho activity. Here, we study the effects of Rac activation and gradients in Rac and Rho concentrations and activities on cellular polarity and adhesion size using mathematical and experimental approaches. The former is accomplished by expanding an existing reaction-diffusion model to a 2D domain utilizing stochastic dynamics. The model revealed that a hysteresis between the induced/uninduced states (corresponding to higher/lower Rac concentrations, respectively) along with Rac and Rho activation gradients, generated by chemical cues, were vital for forming polarity. Experimentally, the induced state was generated by increasing the cellular βPIX (a Rac-GEF) level and/or decreasing ROCK (a Rac-GAP effector protein) activity with Y-27632 (a ROCK-inhibitor). In agreement with the simulations, our results showed that cells with elevated RacGTP migrated faster, indicating more robust cellular polarization. However, the directionality of cells was not changed significantly, suggesting that external and/or internal physical or chemical cues were needed. Complementing the faster migration observed, adhesions were smaller, generating the phenotype expected with the induced state.

Proteomic and Transcriptomic Profiling Identifies Early Developmentally Regulated Proteins in Dictyostelium Discoideum

Cyclic AMP acts as a secondary messenger involving different cellular functions in eukaryotes. Here, proteomic and transcriptomic profiling has been combined to identify novel early developmentally regulated proteins in eukaryote cells. These proteomic and transcriptomic experiments were performed in Dictyostelium discoideum given the unique advantages that this organism offers as a eukaryotic model for cell motility and as a nonmammalian model of human disease. By comparing whole-cell proteome analysis of developed (cAMP-pulsed) wild-type AX2 cells and an independent transcriptomic analysis of developed wild-type AX4 cells, our results show that up to 70% of the identified proteins overlap in the two independent studies. Among them, we have found 26 proteins previously related to cAMP signaling and identified 110 novel proteins involved in calcium signaling, adhesion, actin cytoskeleton, the ubiquitin-proteasome pathway, metabolism, and proteins that previously lacked any annotation. Our study validates previous findings, mostly for the canonical cAMP-pathway, and also generates further insight into the complexity of the transcriptomic changes during early development. This article also compares proteomic data between parental and cells lacking glkA, a GSK-3 kinase implicated in substrate adhesion and chemotaxis in Dictyostelium. This analysis reveals a set of proteins that show differences in expression in the two strains as well as overlapping protein level changes independent of GlkA.

Coordination between Rac1 and Rab Proteins: Functional Implications in Health and Disease

The small GTPases of the Rho family regulate many aspects of actin dynamics, but are functionally connected to many other cellular processes. Rac1, a member of this family, besides its known function in the regulation of actin cytoskeleton, plays a key role in the production of reactive oxygen species, in gene transcription, in DNA repair, and also has been proven to have specific roles in neurons. This review focuses on the cooperation between Rac1 and Rab proteins, analyzing how the coordination between these GTPases impact on cells and how alterations of their functions lead to disease.

Inhibition of microRNA suppression of Dishevelled results in Wnt pathway-associated developmental defects in sea urchin

MicroRNAs (miRNAs) are highly conserved, small non-coding RNAs that regulate gene expressions by binding to the 3' untranslated region of target mRNAs thereby silencing translation. Some miRNAs are key regulators of the Wnt signaling pathways, which impact developmental processes. This study investigates miRNA regulation of different isoforms of Dishevelled (Dvl/Dsh), which encode a key component in the Wnt signaling pathway. The sea urchin Dvl mRNA isoforms have similar spatial distribution in early development, but one isoform is distinctively expressed in the larval ciliary band. We demonstrated that Dvl isoforms are directly suppressed by miRNAs. By blocking miRNA suppression of Dvl isoforms, we observed dose-dependent defects in spicule length, patterning of the primary mesenchyme cells, gut morphology, and cilia. These defects likely result from increased Dvl protein levels, leading to perturbation of Wnt-dependent signaling pathways and additional Dvl-mediated processes. We further demonstrated that overexpression of Dvl isoforms recapitulated some of the Dvl miRNATP-induced phenotypes. Overall, our results indicate that miRNA suppression of Dvl isoforms plays an important role in ensuring proper development and function of primary mesenchyme cells and cilia.

Actomyosin contractility and RhoGTPases affect cell-polarity and directional migration during haptotaxis

Although much is known about chemotaxis- induced by gradients of soluble chemical cues - the molecular mechanisms involved in haptotaxis (migration induced by substrate-bound protein gradients) are largely unknown. We used micropatterning to produce discontinuous gradients consisting of μm-sized fibronectin-dots arranged at constant lateral but continuously decreasing axial spacing. Parameters like gradient slope, protein concentration and size or shape of the fibronectin dots were modified to determine optimal conditions for directional cell migration in gradient patterns. We demonstrate that fibroblasts predominantly migrate uphill towards a higher fibronectin density in gradients with a dot size of 2 × 2 μm, a 2% and 6% slope, and a low fibronectin concentration of 1 μg ml^-1. Increasing dot size to 3.5 × 3.5 μm resulted in stationary cells, whereas rectangular dots (2 × 3 μm) orientated perpendicular to the gradient axis preferentially induce lateral migration. During haptotaxis, the Golgi apparatus reorients to a posterior position between the nucleus and the trailing edge. Using pharmacological inhibitors, we demonstrate that actomyosin contractility and microtubule dynamics are a prerequisite for gradient recognition indicating that asymmetric intracellular forces are necessary to read the axis of adhesive gradients. In the haptotaxis signalling cascade, RhoA and Cdc42, and the atypical protein kinase C zeta (aPKCζ), but not Rac, are located upstream of actomyosin contractility.

WASP and SCAR are evolutionarily conserved in actin-filled pseudopod-based motility

Eukaryotic cells use diverse cellular mechanisms to crawl through complex environments. Fritz-Laylin et al. define α-motility as a mode of migration associated with dynamic, actin-filled pseudopods and show that WASP and SCAR constitute an evolutionarily conserved genetic signature of α-motility.

Diverse eukaryotic cells crawl through complex environments using distinct modes of migration. To understand the underlying mechanisms and their evolutionary relationships, we must define each mode and identify its phenotypic and molecular markers. In this study, we focus on a widely dispersed migration mode characterized by dynamic actin-filled pseudopods that we call “α-motility.” Mining genomic data reveals a clear trend: only organisms with both WASP and SCAR/WAVE—activators of branched actin assembly—make actin-filled pseudopods. Although SCAR has been shown to drive pseudopod formation, WASP’s role in this process is controversial. We hypothesize that these genes collectively represent a genetic signature of α-motility because both are used for pseudopod formation. WASP depletion from human neutrophils confirms that both proteins are involved in explosive actin polymerization, pseudopod formation, and cell migration. WASP and WAVE also colocalize to dynamic signaling structures. Moreover, retention of WASP together with SCAR correctly predicts α-motility in disease-causing chytrid fungi, which we show crawl at >30 µm/min with actin-filled pseudopods. By focusing on one migration mode in many eukaryotes, we identify a genetic marker of pseudopod formation, the morphological feature of α-motility, providing evidence for a widely distributed mode of cell crawling with a single evolutionary origin.

Aspirin-triggered resolvin D1 attenuates PDGF-induced vascular smooth muscle cell migration via the cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) pathway

BACKGROUND AND OBJECTIVES

Resolvin D1 (RvD1) is a specialized pro-resolving lipid mediator that has been previously shown to attenuate vascular smooth muscle cell (VSMC) migration, a key process in the development of intimal hyperplasia. We sought to investigate the role of the cAMP/PKA pathway in mediating the effects of the aspirin-triggered epimer 17R-RvD1 (AT-RvD1) on VSMC migration.

METHODS

VSMCs were harvested from human saphenous veins. VSMCs were analyzed for intracellular cAMP levels and PKA activity after exposure to AT-RvD1. Platelet-derived growth factor (PDGF)-induced migration and cytoskeletal changes in VSMCs were observed through scratch, Transwell, and cell shape assays in the presence or absence of a PKA inhibitor (Rp-8-Br-cAMP). Further investigation of the pathways involved in AT-RvD1 signaling was performed by measuring Rac1 activity, vasodilator stimulated phosphoprotein (VASP) phosphorylation and paxillin translocation. Finally, we examined the role of RvD1 receptors (GPR32 and ALX/FPR2) in AT-RvD1 induced effects on VSMC migration and PKA activity.

RESULTS

Treatment with AT-RvD1 induced a significant increase in cAMP levels and PKA activity in VSMCs at 5 minutes and 30 minutes, respectively. AT-RvD1 attenuated PDGF-induced VSMC migration and cytoskeletal rearrangements. These effects were attenuated by the PKA inhibitor Rp-8-Br-cAMP, suggesting cAMP/PKA involvement. Treatment of VSMC with AT-RvD1 inhibited PDGF-stimulated Rac1 activity, increased VASP phosphorylation, and attenuated paxillin localization to focal adhesions; these effects were negated by the addition of Rp-8-Br-cAMP. The effects of AT-RvD1 on VSMC migration and PKA activity were attenuated by blocking ALX/FPR2, suggesting an important role of this G-protein coupled receptor.

CONCLUSIONS

Our results suggest that AT-RvD1 attenuates PDGF-induced VSMC migration via ALX/FPR2 and cAMP/PKA. Interference with Rac1, VASP and paxillin function appear to mediate the downstream effects of AT-RvD1 on VSMC migration.

Biophysical Tools to Study Cellular Mechanotransduction

The cell membrane is the interface that volumetrically isolates cellular components from the cell's environment. Proteins embedded within and on the membrane have varied biological functions: reception of external biochemical signals, as membrane channels, amplification and regulation of chemical signals through secondary messenger molecules, controlled exocytosis, endocytosis, phagocytosis, organized recruitment and sequestration of cytosolic complex proteins, cell division processes, organization of the cytoskeleton and more. The membrane's bioelectrical role is enabled by the physiologically controlled release and accumulation of electrochemical potential modulating molecules across the membrane through specialized ion channels (e.g., Na⁺, Ca2+, K⁺ channels). The membrane's biomechanical functions include sensing external forces and/or the rigidity of the external environment through force transmission, specific conformational changes and/or signaling through mechanoreceptors (e.g., platelet endothelial cell adhesion molecule (PECAM), vascular endothelial (VE)-cadherin, epithelial (E)-cadherin, integrin) embedded in the membrane. Certain mechanical stimulations through specific receptor complexes induce electrical and/or chemical impulses in cells and propagate across cells and tissues. These biomechanical sensory and biochemical responses have profound implications in normal physiology and disease. Here, we discuss the tools that facilitate the understanding of mechanosensitive adhesion receptors. This article is structured to provide a broad biochemical and mechanobiology background to introduce a freshman mechano-biologist to the field of mechanotransduction, with deeper study enabled by many of the references cited herein.

Mycobacteria Manipulate G-Protein-Coupled Receptors to Increase Mucosal Rac1 Expression in the Lungs

Mycobacterium bovis bacille Calmette-Guérin (BCG) is currently the only approved vaccine against tuberculosis (TB). BCG mimics M. tuberculosis (Mtb) in its persistence in the body and is used as a benchmark to compare new vaccine candidates. BCG was originally designed for mucosal vaccination, but comprehensive knowledge about its interaction with epithelium is currently lacking. We used primary airway epithelial cells (AECs) and a murine model to investigate the initial events of mucosal BCG interactions. Furthermore, we analysed the impact of the G-protein-coupled receptors (GPCRs), CXCR1 and CXCR2, in this process, as these receptors were previously shown to be important during TB infection. BCG infection of AECs induced GPCR-dependent Rac1 up-regulation, resulting in actin redistribution. The altered distribution of the actin cytoskeleton involved the MAPK signalling pathway. Blocking of the CXCR1 or CXCR2 prior to infection decreased Rac1 expression, and increased epithelial transcriptional activity and epithelial cytokine production. BCG infection did not result in epithelial cell death as measured by p53 phosphorylation and annexin. This study demonstrated that BCG infection of AECs manipulated the GPCRs to suppress epithelial signalling pathways. Future vaccine strategies could thus be improved by targeting GPCRs.

Regulation of Adipose Tissue Stem Cells Angiogenic Potential by Tumor Necrosis Factor-Alpha

Tissue regeneration requires coordinated "teamwork" of growth factors, proteases, progenitor and immune cells producing inflammatory cytokines. Mesenchymal stem cells (MSC) might play a pivotal role by substituting cells or by secretion of growth factors or cytokines, and attraction of progenitor and inflammatory cells, which participate in initial stages of tissue repair. Due to obvious impact of inflammation on regeneration it seems promising to explore whether inflammatory factors could influence proangiogenic abilities of MSC. In this study we investigated effects of TNF-α on activity of adipose-derived stem cells (ADSC). We found that treatment with TNF-α enhances ADSC proliferation, F-actin microfilament assembly, increases cell motility and migration through extracellular matrix. Exposure of ADSC to TNF-α led to increased mRNA expression of proangiogenic factors (FGF-2, VEGF, IL-8, and MCP-1), inflammatory cytokines (IL-1β, IL-6), proteases (MMPs, uPA) and adhesion molecule ICAM-1. At the protein level, VEGF, IL-8, MCP-1, and ICAM-1 production was also up-regulated. Pre-incubation of ADSC with TNF-α-enhanced adhesion of monocytes to ADSC but suppressed adherence of ADSC to endothelial cells (HUVEC). Stimulation with TNF-α triggers ROS generation and activates a number of key intracellular signaling mediators known to positively regulate angiogenesis (Akt, small GTPase Rac1, ERK1/2, and p38 MAP-kinases). Pre-treatment with TNF-α-enhanced ADSC ability to promote growth of microvessels in a fibrin gel assay and accelerate blood flow recovery, which was accompanied by increased arteriole density and reduction of necrosis in mouse hind limb ischemia model. These findings indicate that TNF-α plays a role in activation of ADSC angiogenic and regenerative potential.

Involvement of Rho GAP GRAF1 in maintenance of epithelial phenotype

Adhesion of epithelial cell to each other and to extracellular matrix, as well as cell migration ability and cytoskeleton organization undergo significant alterations in the course of neoplastic transformation, but regulatory mechanisms involved in these processes are not fully understood. Here, we studied the role of a Rho GAP protein GRAF1 (GTPase Regulator Associated with Focal adhesion kinase-1) in the regulation of the epithelial phenotype in cells of breast derived, non-malignant, MCF10A cell line. GRAF1 was shown to be localized to cell-cell junctions, and its depletion resulted in accelerated cell migration velocity, elongation of the cells and cell colonies, impaired monolayer integrity and significant disruption of desmosomes with a loss of associated keratin filaments. These processes were accompanied by formation of larger focal adhesions, an increased number of contractile actin stress fibers, reduction in epithelial markers and increase in mesenchymal markers such as epithelial-mesenchymal transition (EMT)-specific transcription factors Snail-1 and Snail-2, as well as N-cadherin, and vimentin. Moreover, unlike control cells, GRAF1 knocked-down cells demonstrated anchorage-independent growth in soft agar. GRAF1 expression in several highly invasive breast cancer cell lines was low, as compared to the non-malignant MCF10A cells, while overexpressing of GRAF1 in the malignant BT-549 cell line led to a decrease of mesenchymal markers, especially the Snail-1 and 2. Altogether, our analysis suggests that GRAF1 plays a role in the maintenance of normal epithelial phenotype and its depletion leads to an EMT-like process that might be involved in neoplastic transformation.

Endoplasmic Reticulum-resident Heat Shock Protein 90 (HSP90) Isoform Glucose-regulated Protein 94 (GRP94) Regulates Cell Polarity and Cancer Cell Migration by Affecting Intracellular Transport

Heat shock protein 90 (HSP90) is a molecular chaperone that is up-regulated in cancer and is required for the folding of numerous signaling proteins. Consequently, HSP90 represents an ideal target for the development of new anti-cancer agents. The human HSP90 isoform, glucose-regulated protein 94 (GRP94), resides in the endoplasmic reticulum and regulates secretory pathways, integrins, and Toll-like receptors, which contribute to regulating immunity and metastasis. However, the cellular function of GRP94 remains underinvestigated. We report that GRP94 knockdown cells are defective in intracellular transport and, consequently, negatively impact the trafficking of F-actin toward the cellular cortex, integrin α2 and integrin αL toward the cell membrane and filopodia, and secretory vesicles containing the HSP90α-AHA1-survivin complex toward the leading edge. As a result, GRP94 knockdown cells form a multipolar spindle instead of bipolar morphology and consequently manifest a defect in cell migration and adhesion.

Quantitative Motion Analysis in Two and Three Dimensions

This chapter describes 2D quantitative methods for motion analysis as well as 3D motion analysis and reconstruction methods. Emphasis is placed on the analysis of dynamic cell shape changes that occur through extension and retraction of force generating structures such as pseudopodia and lamellipodia. Quantitative analysis of these structures is an underutilized tool in the field of cell migration. Our intent, therefore, is to present methods that we developed in an effort to elucidate mechanisms of basic cell motility, directed cell motion during chemotaxis, and metastasis. We hope to demonstrate how application of these methods can more clearly define alterations in motility that arise due to specific mutations or disease and hence, suggest mechanisms or pathways involved in normal cell crawling and treatment strategies in the case of disease. In addition, we present a 4D tumorigenesis model for high-resolution analysis of cancer cells from cell lines and human cancer tissue in a 3D matrix. Use of this model led to the discovery of the coalescence of cancer cell aggregates and unique cell behaviors not seen in normal cells or normal tissue. Graphic illustrations to visually display and quantify cell shape are presented along with algorithms and formulae for calculating select 2D and 3D motion analysis parameters.

VEGF Enhances the Migration of MSCs in Neural Differentiation by Regulating Focal Adhesion Turnover

Mesenchymal stem cells (MSCs) hold great promise in neural regeneration, due to their intrinsic neuronal potential and migratory tropism to damaged nervous tissues. However, the chemotactic signals mediating the migration of MSCs remain poorly understood. Here, we investigated the regulatory roles for focal adhesion kinase (FAK) and Rac1 in vascular endothelial growth factor (VEGF)-stimulated migration of MSCs in neural differentiation. We found that MSCs in various differentiation states show significant different chemotactic responses to VEGF and cells in 24-h preinduction state possess the highest migration speed and efficiency. FAK, as the downstream signaling molecule, is involved in the VEGF-induced migration by regulating the assembly and distribution of focal adhesions (FAs) and reorganization of F-actin. The features of FAs and cytoskeletons and the ability of lamellipodia formation are closely related to the neural differentiation states of MSCs. VEGF promotes FA formation with an asymmetric distribution of FAs and induces the activation of Y397-FAK and Y31/118-paxillin of undifferentiated and 24-h preinduced MSCs in a time-dependent manner. Inhibition of FAK by PF-228 or expressing FAK-Y397F mutant impairs the dynamics of FAs in MSCs during VEGF-induced migration. Furthermore, Rac1 regulates FA formation in a FAK-dependent manner. Overexpression of constitutive activated mutants of Rac1 increases the number of FAs in undifferentiated and 24-h preinduced MSCs, while VEGF-induced increase of FA formation is decreased by inhibiting FAK by PF-228. Collectively, these results demonstrate that FAK and Rac1 signalings coordinately regulate the dynamics of FAs during VEGF-induced migration of MSCs in varying neural differentiation states.

Coronin7 regulates WASP and SCAR through CRIB mediated interaction with Rac proteins

Coronin7 (CRN7) stabilizes F-actin and is a regulator of processes associated with the actin cytoskeleton. Its loss leads to defects in phagocytosis, motility and development. It harbors a CRIB (Cdc42- and Rac-interactive binding) domain in each of its WD repeat domains which bind to Rac GTPases preferably in their GDP-loaded forms. Expression of wild type CRN7 in CRN7 deficient cells rescued these defects, whereas proteins with mutations in the CRIB motifs which were associated with altered Rac binding were effective to varying degrees. The presence of one functional CRIB was sufficient to reestablish phagocytosis, cell motility and development. Furthermore, by molecular modeling and mutational analysis we identified the contact regions between CRN7 and the GTPases. We also identified WASP, SCAR and PAKa as downstream effectors in phagocytosis, development and cell surface adhesion, respectively, since ectopic expression rescued these functions.

Shlnc-EC6 regulates murine erythroid enucleation by Rac1-PIP5K pathway

Long noncoding RNAs (LncRNAs) are longer than 200 nucleotide noncoding RNAs without apparent functional coding capacity that function as regulators of cell growth and development. In recent years, increasing evidence implicates the involvement of LncRNAs in erythropoiesis. shlnc-EC6 is a LncRNA associated with erythroid differentiation but the mechanism remains undefined. In this study, we found that knockdown of shlnc-EC6 in purified mouse fetal liver erythroid progenitor and hematopoietic stem cells (FLEPHSCs) significantly blocked erythroid enucleation. We also showed that Rac1 was negatively regulated by shlnc-EC6 at the posttranscriptional level via specific binding to sites within the 3'UTR of Rac1 mRNA. Moreover, we found that knockdown of shlnc-EC6 led to upregulation of Rac1, followed by the activation of the downstream protein PIP5K, and subsequently resulted in the inhibition of enucleation in cultured mouse fetal erythroblasts. Thus, our findings suggest that shlnc-EC6 acts as a novel modulator to regulate mouse erythropoiesis via Rac1/PIP5K signaling pathway.

Absence of catalytic domain in a putative protein kinase C (PkcA) suppresses tip dominance in Dictyostelium discoideum

A number of organisms possess several isoforms of protein kinase C but little is known about the significance of any specific isoform during embryogenesis and development. To address this we characterized a PKC ortholog (PkcA; DDB_G0288147) in Dictyostelium discoideum. pkcA expression switches from prestalk in mound to prespore in slug, indicating a dynamic expression pattern. Mutants lacking the catalytic domain of PkcA (pkcA(-)) did not exhibit tip dominance. A striking phenotype of pkcA- was the formation of an aggregate with a central hollow, and aggregates later fragmented to form small mounds, each becoming a fruiting body. Optical density wave patterns of cAMP in the late aggregates showed several cAMP wave generation centers. We attribute these defects in pkcA(-) to impaired cAMP signaling, altered cell motility and decreased expression of the cell adhesion molecules - CadA and CsaA. pkcA(-) slugs showed ectopic expression of ecmA in the prespore region. Further, the use of a PKC-specific inhibitor, GF109203X that inhibits the activity of catalytic domain phenocopied pkcA(-).

Homer3 regulates the establishment of neutrophil polarity

Most chemoattractants rely on activation of the heterotrimeric G-protein Gαi to regulate directional cell migration, but few links from Gαi to chemotactic effectors are known. Through affinity chromatography using primary neutrophil lysate, we identify Homer3 as a novel Gαi2-binding protein. RNA interference-mediated knockdown of Homer3 in neutrophil-like HL-60 cells impairs chemotaxis and the establishment of polarity of phosphatidylinositol 3,4,5-triphosphate (PIP3) and the actin cytoskeleton, as well as the persistence of the WAVE2 complex. Most previously characterized proteins that are required for cell polarity are needed for actin assembly or activation of core chemotactic effectors such as the Rac GTPase. In contrast, Homer3-knockdown cells show normal magnitude and kinetics of chemoattractant-induced activation of phosphoinositide 3-kinase and Rac effectors. Chemoattractant-stimulated Homer3-knockdown cells also exhibit a normal initial magnitude of actin polymerization but fail to polarize actin assembly and intracellular PIP3 and are defective in the initiation of cell polarity and motility. Our data suggest that Homer3 acts as a scaffold that spatially organizes actin assembly to support neutrophil polarity and motility downstream of GPCR activation.

Hax-1 is required for Rac1-Cortactin interaction and ovarian carcinoma cell migration

Hax-1 is a multifunctional protein, which is involved in diverse cellular signaling pathways including tumor cell survival and migration. We have shown previously that cell migration stimulated by the oncogenic G protein, G_13, requires Hax-1 for the formation of a functional complex involving Gα₁₃, Rac1, and cortactin. However, the role of Hax-1 in cancer cell migration or its role in Rac1-cortactin complex formation, which is known to be required for such migration remains to be characterized. Results focused on resolving the role of Hax-1 in ovarian cancer pathophysiology indicate that Hax-1 is overexpressed in ovarian cancer cells and the silencing of Hax-1 inhibits lysophosphatidic acid (LPA)- or fetal bovine serum-stimulated migration of these cells. In addition, silencing of Hax-1 greatly reduces Rac1-cortactin interaction and their colocalization in SKOV3 cells. Mapping the structural domains of Hax-1 indicates that it interacts with cortactin via domains spanning amino acids 1 to 56 (Hax-D1) and amino acids 113 to 168 (Hax-D3). Much weaker interaction with cortactin was also observed with the region of Hax-1 spanning amino acids 169 – 224 (Hax-D4). Similar mapping of Hax-1 domains involved in Rac1 interaction indicates that it associates with Rac1 via two primary domains spanning amino acids 57 to 112 (Hax-D2) and 169 to 224 (Hax-D4). Furthermore, expression of either of these domains inhibits LPA-mediated migration of SKOV3 cells, possibly through their ability to exert competitive inhibition on endogenous Hax-1-Rac1 and/or Hax-1-cortactin interaction. More significantly, expression of Hax-D4 drastically reduces Rac1-cortactin colocalization in SKOV3 cells along with an attenuation of LPA-stimulated migration. Thus our results presented here describe for the first time that Hax-1 interaction is required for the association between Rac1 and cortactin and that these multiple interactions are required for the LPA-stimulated migration of SKOV3 ovarian cancer cells.

Hsa-microRNA-101 suppresses migration and invasion by targeting Rac1 in thyroid cancer cells

MicroRNAs (miRNAs) are 22- to 25-nucleotide non-coding RNA molecules that function as negative regulators of gene expression. In previous years, increasing evidence has arisen implicating the involvement of miRNAs in carcinogenesis. In previous studies, the role of miRNA-101 (miR-101) in tumors has been identified as a tumor suppressor and, until now, the role of miR-101 and Rac1 in thyroid cancer has remained undefined. This study revealed that miR-101 is significantly downregulated in papillary thyroid carcinoma (PTC) tissue and thyroid cancer cell lines, and that the downregulated miR-101 is associated with lymph node metastasis. Infection with the miR-101 murine stem cell virus may markedly inhibit cell migration and invasion in TPC-1 and HTH83 thyroid cancer cell lines. Rac1 was demonstrated to be negatively regulated by miR-101 at the post-transcriptional level, via a specific target site within the 3' untranslated region by dual-luciferase reporter assay. The expression of Rac1 was also observed to inversely correlate with miR-101 expression in PTC tissues; knockdown of Rac1 by shRNA inhibited thyroid cancer cell migration and invasion, resembling that of miR-101 overexpression. Thus, these findings suggested that miR-101 acts as a novel suppressor by targeting the Rac1 gene and inhibiting thyroid cancer cell migration and invasion.

Moving towards a paradigm: common mechanisms of chemotactic signaling in Dictyostelium and mammalian leukocytes

Chemotaxis, or directed migration of cells along a chemical gradient, is a highly coordinated process that involves gradient sensing, motility, and polarity. Most of our understanding of chemotaxis comes from studies of cells undergoing amoeboid-type migration, in particular the social amoeba Dictyostelium discoideum and leukocytes. In these amoeboid cells the molecular events leading to directed migration can be conceptually divided into four interacting networks: receptor/G protein, signal transduction, cytoskeleton, and polarity. The signal transduction network occupies a central position in this scheme as it receives direct input from the receptor/G protein network, as well as feedback from the cytoskeletal and polarity networks. Multiple overlapping modules within the signal transduction network transmit the signals to the actin cytoskeleton network leading to biased pseudopod protrusion in the direction of the gradient. The overall architecture of the networks, as well as the individual signaling modules, is remarkably conserved between Dictyostelium and mammalian leukocytes, and the similarities and differences between the two systems are the subject of this review.

Recombinant Protein Hydrogels for Cell Injection and Transplantation

As found in nature, full-length proteins consist of a genetically specified sequence of the 20 canonical amino acids, of a defined length. This sequence of chemically diverse functional groups enables the many highly controlled interactions with other molecules found in natural proteins. Recombinant proteins can be engineered to incorporate some of nature's palette of protein functionality into hydrogels for cell delivery. Current work demonstrates how this level of molecular precision can be used to address challenges in cell therapies, such as post-implantation viability, localization, and control, via specified gelation mechanics and tailored bioactive domains. Leveraging recombinant technology, including protein engineering, gene synthesis, expression, and purification, materials scientists have appropriated and modified naturally occurring proteins to achieve hydrogels that combine defined gelation mechanics with specified bioactive protein chemistries. Here, we specifically review recent developments in recombinant protein hydrogels that are either inspired by native extracellular matrix proteins (e.g. elastin, collagen, and resilin) or designed from non-matrix peptides (e.g. mixing-induced two-component hydrogels). In many of these case studies, domain- and sequence-level engineering enables a broad range of biochemical activity and mechanical control via gelation. Despite the remaining challenges of scalability and forward-designed predictability, hydrogels made of recombinant proteins offer exciting possibilities for sophisticated delivery of therapeutic cells, including multifactorial control, native-like mechanics, and sensitivity to signals from delivered cells or host tissues.

Tight coupling between nucleus and cell migration through the perinuclear actin cap

Although eukaryotic cells are known to alternate between 'advancing' episodes of fast and persistent movement and 'hesitation' episodes of low speed and low persistence, the molecular mechanism that controls the dynamic changes in morphology, speed and persistence of eukaryotic migratory cells remains unclear. Here, we show that the movement of the interphase nucleus during random cell migration switches intermittently between two distinct modes - rotation and translocation - that follow with high fidelity the sequential rounded and elongated morphologies of the nucleus and cell body, respectively. Nuclear rotation and translocation mediate the stop-and-go motion of the cell through the dynamic formation and dissolution, respectively, of the contractile perinuclear actin cap, which is dynamically coupled to the nuclear lamina and the nuclear envelope through LINC complexes. A persistent cell movement and nuclear translocation driven by the actin cap are halted following the disruption of the actin cap, which in turn allows the cell to repolarize for its next persistent move owing to nuclear rotation mediated by cytoplasmic dynein light intermediate chain 2.

Roles of ion transport in control of cell motility

Cell motility is an essential feature of life. It is essential for reproduction, propagation, embryonic development, and healing processes such as wound closure and a successful immune defense. If out of control, cell motility can become life-threatening as, for example, in metastasis or autoimmune diseases. Regardless of whether ciliary/flagellar or amoeboid movement, controlled motility always requires a concerted action of ion channels and transporters, cytoskeletal elements, and signaling cascades. Ion transport across the plasma membrane contributes to cell motility by affecting the membrane potential and voltage-sensitive ion channels, by inducing local volume changes with the help of aquaporins and by modulating cytosolic Ca(2+) and H(+) concentrations. Voltage-sensitive ion channels serve as voltage detectors in electric fields thus enabling galvanotaxis; local swelling facilitates the outgrowth of protrusions at the leading edge while local shrinkage accompanies the retraction of the cell rear; the cytosolic Ca(2+) concentration exerts its main effect on cytoskeletal dynamics via motor proteins such as myosin or dynein; and both, the intracellular and the extracellular H(+) concentration modulate cell migration and adhesion by tuning the activity of enzymes and signaling molecules in the cytosol as well as the activation state of adhesion molecules at the cell surface. In addition to the actual process of ion transport, both, channels and transporters contribute to cell migration by being part of focal adhesion complexes and/or physically interacting with components of the cytoskeleton. The present article provides an overview of how the numerous ion-transport mechanisms contribute to the various modes of cell motility.

Differential TIMP3 expression affects tumor progression and angiogenesis in melanomas through regulation of directionally persistent endothelial cell migration

The angiogenic potential of solid tumors, or the ability to initiate neovasculature development from pre-existing host vessels, is facilitated by soluble factors secreted by tumor cells and involves breaching of extracellular matrix barriers, endothelial cell (EC) proliferation, migration and reassembly. We evaluated the angiogenic potential of human melanoma cell lines differing in their degree of aggressiveness, based on their ability to regulate directionally persistent EC migration. We observed that conditioned-medium (CM) of the aggressive melanoma cell line BLM induced a high effective migratory response in ECs, while CMs of Mel57 and 1F6 had an inhibitory effect. Further, the melanoma cell lines exhibited a varied expression profile of tissue inhibitor of metalloproteinase-3 (TIMP3), detectable in the CM. TIMP3 expression inversely correlated with aggressiveness of the melanoma cell line, and ability of the respective CMs to induce directed EC migration. Interestingly, TIMP3 expression was found to be silenced in the BLM cell line, concurrent with its role as a tumor suppressor. Treatment with recombinant human TIMP3 and CM of modified, TIMP3 expressing, BLM cells mitigated directional EC migration, while CM of TIMP3 silenced 1F6 cells induced directed EC migration. The functional implication of TIMP3 expression on tumor growth and angiogenic potential in melanoma was evaluated in vivo. We observed that TIMP3 expression reduced tumor growth, angiogenesis and macrophage infiltration of BLM tumors while silencing TIMP3 increased tumor growth and angiogenesis of 1F6 tumors. Taken together, our results demonstrate that TIMP3 expression correlates with inhibition of directionally persistent EC migration and adversely affects the angiogenic potential and growth of melanomas.

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) promotes glioblastoma cell chemotaxis via Lyn activation

The long-term survival of patients with glioblastoma is compromised by the proclivity for local invasion into the surrounding normal brain, escaping surgical resection and contributing to therapeutic resistance. Tumor necrosis factor-like weak inducer of apoptosis (TWEAK), a member of the tumor necrosis factor superfamily, can stimulate glioma cell invasion via binding to fibroblast growth factor-inducible 14 (Fn14) and subsequent activation of the Rho guanosine triphosphatase family member Rac1. Here, we demonstrate that TWEAK acts as a chemotactic factor for glioma cells, a potential process for driving cell invasion into the surrounding brain tissue. TWEAK exposure induced the activation of Src family kinases (SFKs), and pharmacologic suppression of SFK activity inhibited TWEAK-induced chemotactic migration. We employed a multiplexed Luminex assay and identified Lyn as a candidate SFK activated by TWEAK. Depletion of Lyn suppressed TWEAK-induced chemotaxis and Rac1 activity. Furthermore, Lyn gene expression levels increase with primary glioma tumor grade and inversely correlate with patient survival. These results show that TWEAK-induced glioma cell chemotaxis is dependent upon Lyn kinase function and, thus, provides opportunities for therapeutic targeting of this deadly disease.

Nucleophosmin1 is a negative regulator of the small GTPase Rac1

The Rac1 GTPase is a critical regulator of cytoskeletal dynamics and controls many biological processes, such as cell migration, cell-cell contacts, cellular growth and cell division. These complex processes are controlled by Rac1 signaling through effector proteins. We have previously identified several effector proteins of Rac1 that also act as Rac1 regulatory proteins, including caveolin-1 and PACSIN2. Here, we report that Rac1 interacts through its C-terminus with nucleophosmin1 (NPM1), a multifunctional nucleo-cytoplasmic shuttling protein with oncogenic properties. We show that Rac1 controls NPM1 subcellular localization. In cells expressing active Rac1, NPM1 translocates from the nucleus to the cytoplasm. In addition, Rac1 regulates the localization of the phosphorylated pool of NPM1 as this pool translocated from the nucleus to the cytosol in cells expressing activated Rac1. Conversely, we found that expression of NPM1 limits Rac1 GTP loading and cell spreading. In conclusion, this study identifies NPM1 as a novel, negative regulator of Rac1.

Synergistic induction of cancer cell migration regulated by Gβγ and phosphatidylinositol 3-kinase

Phosphatidylinositol 3-kinase (PI3K) is essential for both G protein-coupled receptor (GPCR)- and receptor tyrosine kinase (RTK)-mediated cancer cell migration. Here, we have shown that maximum migration is achieved by full activation of phosphatidylinositol 3,4,5-trisphosphate-dependent Rac exchanger 1 (P-Rex1) in the presence of Gβγ and PI3K signaling pathways. Lysophosphatidic acid (LPA)- induced migration was higher than that of epidermal growth factor (EGF)-induced migration; however, LPA-induced activation of Akt was lower than that stimulated by EGF. LPA-induced migration was partially blocked by either Gβγ or RTK inhibitor and completely blocked by both inhibitors. LPA-induced migration was synergistically increased in the presence of EGF and vice versa. In correlation with these results, sphingosine-1-phosphate (S1P)-induced migration was also synergistically induced in the presence of insulin-like growth factor-1 (IGF-1). Finally, silencing of P-Rex1 abolished the synergism in migration as well as in Rac activation. Moreover, synergistic activation of MMP-2 and cancer cell invasion was attenuated by silencing of P-Rex1. Given these results, we suggest that P-Rex1 requires both Gβγ and PI3K signaling pathways for synergistic activation of Rac, thereby inducing maximum cancer cell migration and invasion.

Daydreamer, a Ras effector and GSK-3 substrate, is important for directional sensing and cell motility

Daydreamer (DydA), a new Mig10/RIAM/lamellipodin family adaptor protein, is a Ras effector required for cell polarization and directional movement during chemotaxis. DydA is phosphorylated by glycogen synthase kinase-3, which is required for some, but not all, of DydA's functions. gskA⁻ cells exhibit very strong chemotactic phenotypes, a subset of which are exhibited by dydA⁻ cells.

How independent signaling pathways are integrated to holistically control a biological process is not well understood. We have identified Daydreamer (DydA), a new member of the Mig10/RIAM/lamellipodin (MRL) family of adaptor proteins that localizes to the leading edge of the cell. DydA is a putative Ras effector that is required for cell polarization and directional movement during chemotaxis. dydA⁻ cells exhibit elevated F-actin and assembled myosin II (MyoII), increased and extended phosphoinositide-3-kinase (PI3K) activity, and extended phosphorylation of the activation loop of PKB and PKBR1, suggesting that DydA is involved in the negative regulation of these pathways. DydA is phosphorylated by glycogen synthase kinase-3 (GSK-3), which is required for some, but not all, of DydA's functions, including the proper regulation of PKB and PKBR1 and MyoII assembly. gskA⁻ cells exhibit very strong chemotactic phenotypes, as previously described, but exhibit an increased rate of random motility. gskA⁻ cells have a reduced MyoII response and a reduced level of phosphatidylinositol (3,4,5)-triphosphate production, but a highly extended recruitment of PI3K to the plasma membrane and highly extended kinetics of PKB and PKBR1 activation. Our results demonstrate that GSK-3 function is essential for chemotaxis, regulating multiple substrates, and that one of these effectors, DydA, plays a key function in the dynamic regulation of chemotaxis.

Involvement of Rab6 in the regulation of phagocytosis against virus infection in invertebrates

Phagocytosis, which is of fundamental importance for innate and adaptive immunity in animals, is driven by organization of the actin cytoskeleton. To date, however, the molecular events involved in the regulation of phagocytosis through reorganization of actin by small G proteins remains to be elucidated. To address this issue, the molecular mechanism of Rab6 in phagocytosis against virus infection in invertebrates was characterized in this study. The results showed that the Rab6 obtained from shrimp could interact with actin to regulate shrimp hemocyte phagocytosis through induction of the rearrangement of actin to protect against white spot syndrome virus (WSSV) infection. The Rab6 protein in Drosophila melanogaster shared the same mechanism of action as that of Rab6 in shrimp, indicating that the function of Rab6 in phagocytosis was conserved in invertebrates. By comparison with the early marker (Rab5) and late marker (LAMP1) of phagosomes, Rab6 was critically involved in the regulation of actin organization throughout the entire phagocytosis process. The presence of the evolutionarily conserved amino acid sequences of Rab6 in invertebrates and vertebrates indicated a conserved mechanism of Rab6 function in phagocytosis of animals. Therefore, our findings presented novel molecular events in the regulation of phagocytosis by small G proteins.

Oxidized LDL/CD36 interaction induces loss of cell polarity and inhibits macrophage locomotion

Cell polarization is essential for migration and the exploratory function of leukocytes. However, the mechanism by which cells maintain polarity or how cells revert to the immobilized state by gaining cellular symmetry is not clear. Previously we showed that interaction between oxidized low-density lipoprotein (oxLDL) and CD36 inhibits macrophage migration; in the current study we tested the hypothesis that oxLDL/CD36-induced inhibition of migration is the result of intracellular signals that regulate cell polarity. Live cell imaging of macrophages showed that oxLDL actuated retraction of macrophage front end lamellipodia and induced loss of cell polarity. Cd36 null and macrophages null for Vav, a guanine nucleotide exchange factor (GEF), did not show this effect. These findings were caused by Rac-mediated inhibition of nonmuscle myosin II, a cell polarity determinant. OxLDL induced dephosphorylation of myosin regulatory light chain (MRLC) by increasing the activity of Rac. Six-thioguanine triphosphate (6-thio-GTP), which inhibits Vav-mediated activation of Rac, abrogated the effect of oxLDL. Activation of the Vav-Rac-myosin II pathway by oxidant stress may induce trapping of macrophages at sites of chronic inflammation such as atherosclerotic plaque.

Rab8a regulates the exocyst-mediated kiss-and-run discharge of the Dictyostelium contractile vacuole

A molecular dissection of contractile vacuole (CV) discharge shows that Rab8a is recruited to the CV a few seconds before the exocyst. Together they tether it to the plasma membrane and commit it to fusion. GTP hydrolysis is necessary for vacuole detethering, a process in which LvsA, a protein of the Chédiak–Higashi family, plays a crucial role.

Water expulsion by the contractile vacuole (CV) in Dictyostelium is carried out by a giant kiss-and-run focal exocytic event during which the two membranes are only transiently connected but do not completely merge. We present a molecular dissection of the GTPase Rab8a and the exocyst complex in tethering of the contractile vacuole to the plasma membrane, fusion, and final detachment. Right before discharge, the contractile vacuole bladder sequentially recruits Drainin, a Rab11a effector, Rab8a, the exocyst complex, and LvsA, a protein of the Chédiak–Higashi family. Rab8a recruitment precedes the nucleotide-dependent arrival of the exocyst to the bladder by a few seconds. A dominant-negative mutant of Rab8a strongly binds to the exocyst and prevents recruitment to the bladder, suggesting that a Rab8a guanine nucleotide exchange factor activity is associated with the complex. Absence of Drainin leads to overtethering and blocks fusion, whereas expression of constitutively active Rab8a allows fusion but blocks vacuole detachment from the plasma membrane, inducing complete fragmentation of tethered vacuoles. An indistinguishable phenotype is generated in cells lacking LvsA, implicating this protein in postfusion detethering. Of interest, overexpression of a constitutively active Rab8a mutant reverses the lvsA-null CV phenotype.

The F-BAR domain protein PACSIN2 associates with Rac1 and regulates cell spreading and migration

The Rac1 GTPase controls cytoskeletal dynamics and is a key regulator of cell spreading and migration mediated by signaling through effector proteins, such as the PAK kinases and the Scar and WAVE proteins. We previously identified a series of regulatory proteins that associate with Rac1 through its hypervariable C-terminal domain, including the Rac1 activator β-Pix (also known as Rho guanine-nucleotide-exchange factor 7) and the membrane adapter caveolin-1. Here, we show that Rac1 associates, through its C-terminus, with the F-BAR domain protein PACSIN2, an inducer of membrane tubulation and a regulator of endocytosis. We show that Rac1 localizes with PACSIN2 at intracellular tubular structures and on early endosomes. Active Rac1 induces a loss of PACSIN2-positive tubular structures. By contrast, Rac1 inhibition results in an accumulation of PACSIN2-positive tubules. In addition, PACSIN2 appears to regulate Rac1 signaling; siRNA-mediated loss of PACSIN2 increases the levels of Rac1-GTP and promotes cell spreading and migration in a wound healing assay. Moreover, ectopic expression of PACSIN2 reduces Rac1-GTP levels in a fashion that is dependent on the PACSIN2-Rac1 interaction, on the membrane-tubulating capacity of PACSIN2 and on dynamin. These data identify the BAR-domain protein PACSIN2 as a Rac1 interactor that regulates Rac1-mediated cell spreading and migration.

CXCL13 mediates prostate cancer cell proliferation through JNK signalling and invasion through ERK activation

OBJECTIVES

  The focus of this study was to determine the dedicator of cytokinesis 2 (DOCK2), extracellular signal-regulated kinase 1/2 (ERK1/2), c-Jun N-terminal kinase-1 (JNK) and Akt signals involved in CXCL13-mediated prostate cancer (PCa) cell invasion and proliferation.

MATERIALS AND METHODS

  Androgen-sensitive (LNCaP), hormone-refractory (PC3) cells and normal cells (RWPE-1) were used to determine CXCL13-mediated PCa cell invasion and proliferation. Immuno-blotting, fast activated cell-based (FACE) ELISA, caspase activity, cell invasion and proliferation assays were performed to ascertain some of the signalling events involved in PCa cell proliferation and invasion.

RESULTS

  Unlike androgen-sensitive LNCaP cells, we report for the first time that the hormone-refractory cell line, PC3, expresses DOCK2. CXCL13-mediated LNCaP and PC3 cell invasion was regulated by Akt and ERK1/2 activation in a DOCK2-independent fashion. CXCL13 also promoted LNCaP cell proliferation in a JNK-dependent fashion even in the absence of DOCK2. In contrast, CXCL13 induced PC3 cell proliferation through JNK activation, which required DOCK2.

CONCLUSIONS

  Our results show CXCL13-mediated PCa cell invasion requires Akt and ERK1/2 activation and suggests a new role for DOCK2 in proliferation of hormone-refractory CXCR5-positive PCa cells.

Selective activation of Akt1 by mammalian target of rapamycin complex 2 regulates cancer cell migration, invasion, and metastasis

Mammalian target of rapamycin complex (mTORC) regulates a variety of cellular responses including proliferation, growth, differentiation and cell migration. In this study, we show that mammalian target of rapamycin complex 2 (mTORC2) regulates invasive cancer cell migration through selective activation of Akt1. Insulin-like growth factor-1 (IGF-1)-induced SKOV-3 cell migration was completely abolished by phosphatidylinositol 3-kinase (PI3K) (LY294002, 10 μM) or Akt inhibitors (SH-5, 50 μM), whereas inhibition of extracellular-regulated kinase by an ERK inhibitor (PD98059, 10 μM) or inhibition of mammalian target of rapamycin complex 1 (mTORC1) by an mTORC1 inhibitor (Rapamycin, 100 nM) did not affect IGF-1-induced SKOV-3 cell migration. Inactivation of mTORC2 by silencing Rapamycin-insensitive companion of mTOR (Rictor), abolished IGF-1-induced SKOV-3 cell migration as well as activation of Akt. However, inactivation of mTORC1 by silencing of Raptor had no effect. Silencing of Akt1 but not Akt2 attenuated IGF-1-induced SKOV-3 cell migration. Rictor was preferentially associated with Akt1 rather than Akt2, and over-expression of Rictor facilitated IGF-1-induced Akt1 activation. Expression of PIP3-dependent Rac exchanger1 (P-Rex1), a Rac guanosine exchange factor and a component of the mTOR complex, strongly stimulated activation of Akt1. Furthermore, knockdown of P-Rex1 attenuated Akt activation as well as IGF-1-induced SKOV-3 cell migration. Silencing of Akt1 or P-Rex1 abolished IGF-1-induced SKOV-3 cell invasion. Finally, silencing of Akt1 blocked in vivo metastasis, whereas silencing of Akt2 did not. Given these results, we suggest that selective activation of Akt1 through mTORC2 and P-Rex1 regulates cancer cell migration, invasion and metastasis.

Costars, a Dictyostelium protein similar to the C-terminal domain of STARS, regulates the actin cytoskeleton and motility

Through analysis of a chemotaxis mutant obtained from a genetic screen in Dictyostelium discoideum, we have identified a new gene involved in regulating cell migration and have named it costars (cosA). The 82 amino acid Costars protein sequence appears highly conserved among diverse species, and significantly resembles the C-terminal region of the striated muscle activator of Rho signaling (STARS), a mammalian protein that regulates the serum response factor transcriptional activity through actin binding and Rho GTPase activation. The cosA-null (cosA(-)) cells formed smooth plaques on bacterial lawns, produced abnormally small fruiting bodies when developed on the non-nutrient agar and displayed reduced migration towards the cAMP source in chemotactic assays. Analysis of cell motion in cAMP gradients revealed decreased speed but wild-type-like directional persistence of cosA(-) cells, suggesting a defect in the cellular machinery for motility rather than for chemotactic orientation. Consistent with this notion, cosA(-) cells exhibited changes in the actin cytoskeleton, showing aberrant distribution of F-actin in fluorescence cell staining and an increased amount of cytoskeleton-associated actin. Excessive pseudopod formation was also noted in cosA(-) cells facing chemoattractant gradients. Expressing cosA or its human counterpart mCostars eliminated abnormalities of cosA(-) cells. Together, our results highlight a role for Costars in modulating actin dynamics and cell motility.

Eukaryotic chemotaxis: a network of signaling pathways controls motility, directional sensing, and polarity

Chemotaxis, the directed migration of cells in chemical gradients, is a vital process in normal physiology and in the pathogenesis of many diseases. Chemotactic cells display motility, directional sensing, and polarity. Motility refers to the random extension of pseudopodia, which may be driven by spontaneous actin waves that propagate through the cytoskeleton. Directional sensing is mediated by a system that detects temporal and spatial stimuli and biases motility toward the gradient. Polarity gives cells morphologically and functionally distinct leading and lagging edges by relocating proteins or their activities selectively to the poles. By exploiting the genetic advantages of Dictyostelium, investigators are working out the complex network of interactions between the proteins that have been implicated in the chemotactic processes of motility, directional sensing, and polarity.