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Copolymerization of nonmuscle myosins IIA and IIB followed by their differential turnover in stress fibers leads to self-sorting of IIA and IIB along the front–rear axis of the cell, thus producing a polarized actin cytoskeleton., Nonmuscle myosin II (NMII) is uniquely responsible for cell contractility and thus defines multiple aspects of cell behavior. To generate contraction, NMII molecules polymerize into bipolar minifilaments. Different NMII paralogs are often coexpressed in cells and can copolymerize, suggesting that they may cooperate to facilitate cell motility. However, whether such cooperation exists and how it may work remain unknown. We show that copolymerization of NMIIA and NMIIB followed by their differential turnover leads to self-sorting of NMIIA and NMIIB along the front–rear axis, thus producing a polarized actin–NMII cytoskeleton. Stress fibers newly formed near the leading edge are enriched in NMIIA, but over time, they become progressively enriched with NMIIB because of faster NMIIA turnover. In combination with retrograde flow, this process results in posterior accumulation of more stable NMIIB-rich stress fibers, thus strengthening cell polarity. By copolymerizing with NMIIB, NMIIA accelerates the intrinsically slow NMIIB dynamics, thus increasing cell motility and traction and enabling chemotaxis.

SUMMARYLysophophatidic acid (LPA), a biologically active phospholipid that is ubiquitously present in tissues and organs, provokes cellular responses such as proliferation, apoptosis, differentiation and migration via activation of G-protein coupled receptors. These receptors activate a broad range of intracellular signaling cascades to mediate these responses. Using microfluidic chambers that generate and maintain stable gradients, we observed that chemotaxis of fibroblasts to LPA has higher directional fidelity than chemotaxis provoked by the receptor tyrosine kinase (RTK) ligand platelet-derived growth factor (PDGF). Unlike fast moving amoeboid cells, mesenchymal cells such as fibroblasts do not require PI3K for chemotaxis to a GPCR ligand. In addition, the Arp2/3 complex is not required for fibroblast GPCR-based chemotaxis in either 2D or 3D environments. Our data indicate that combinatorial regulation of myosin II involving global activation by RhoA/ROCK and local inhibition of myosin II at the leading edge by PKC results in highly efficient chemotaxis of fibroblasts to LPA. Based on these observations, we develop a simple mathematical model to explain how dual regulation of myosin II is responsible for enhanced chemotaxis in LPA gradients relative to PDGF. Using pharmacological approaches, we test predictions of this model and modulate the fidelity of LPA and PDGF chemotaxis.

The primary activity of GMFβ in vivo is actin branch disassembly (and not inhibition of Arp2/3 activation), and this activity plays an important role in lamellipodial dynamics and directional migration toward ECM cues., The lamellipodium is an important structure for cell migration containing branched actin nucleated via the Arp2/3 complex. The formation of branched actin is relatively well studied, but less is known about its disassembly and how this influences migration. GMF is implicated in both Arp2/3 debranching and inhibition of Arp2/3 activation. Modulation of GMFβ, a ubiquitous GMF isoform, by depletion or overexpression resulted in changes in lamellipodial dynamics, branched actin content, and migration. Acute pharmacological inhibition of Arp2/3 by CK-666, coupled to quantitative live-cell imaging of the complex, showed that depletion of GMFβ decreased the rate of branched actin disassembly. These data, along with mutagenesis studies, suggest that debranching (not inhibition of Arp2/3 activation) is a primary activity of GMFβ in vivo. Furthermore, depletion or overexpression of GMFβ disrupted the ability of cells to directionally migrate to a gradient of fibronectin (haptotaxis). These data suggest that debranching by GMFβ plays an important role in branched actin regulation, lamellipodial dynamics, and directional migration.

During cell migration, F-actin bundles/filopodia serve as templates for formation and orientation of lamellipodia and prime their stabilization by adhesion-based PI3K signaling., Mesenchymal cells such as fibroblasts are weakly polarized and reorient directionality by a lamellipodial branching mechanism that is stabilized by phosphoinositide 3-kinase (PI3K) signaling. However, the mechanisms by which new lamellipodia are initiated and directed are unknown. Using total internal reflection fluorescence microscopy to monitor cytoskeletal and signaling dynamics in migrating cells, we show that peripheral F-actin bundles/filopodia containing fascin-1 serve as templates for formation and orientation of lamellipodia. Accordingly, modulation of fascin-1 expression tunes cell shape, quantified as the number of morphological extensions. Ratiometric imaging reveals that F-actin bundles/filopodia play both structural and signaling roles, as they prime the activation of PI3K signaling mediated by integrins and focal adhesion kinase. Depletion of fascin-1 ablated fibroblast haptotaxis on fibronectin but not platelet-derived growth factor chemotaxis. Based on these findings, we conceptualize haptotactic sensing as an exploration, with F-actin bundles directing and lamellipodia propagating the process and with signaling mediated by adhesions playing the role of integrator.

Somatic inactivation of the serine/threonine kinase gene STK11/LKB1/PAR-4 occurs in a variety of cancers, including ∼10% of melanoma. However, how the loss of LKB1 activity facilitates melanoma invasion and metastasis remains poorly understood. In LKB1-null cells derived from an autochthonous murine model of melanoma with activated Kras and Lkb1 loss and matched reconstituted controls, we have investigated the mechanism by which LKB1 loss increases melanoma invasive motility. Using a microfluidic gradient chamber system and time-lapse microscopy, in this paper, we uncover a new function for LKB1 as a directional migration sensor of gradients of extracellular matrix (haptotaxis) but not soluble growth factor cues (chemotaxis). Systematic perturbation of known LKB1 effectors demonstrated that this response does not require canonical adenosine monophosphate–activated protein kinase (AMPK) activity but instead requires the activity of the AMPK-related microtubule affinity-regulating kinase (MARK)/PAR-1 family kinases. Inhibition of the LKB1–MARK pathway facilitated invasive motility, suggesting that loss of the ability to sense inhibitory matrix cues may promote melanoma invasion.

Haptotaxis is the process by which cells respond to gradients of substrate-bound cues, such as extracellular matrix proteins (ECM); however, the cellular mechanism of this response remains poorly understood and has mainly been studied by comparing cell behavior on uniform ECMs with different concentrations of components. To study haptotaxis in response to gradients, we utilized microfluidic chambers to generate gradients of the ECM protein fibronectin, and imaged the cell migration response. Lamellipodia are fan-shaped protrusions that are common in migrating cells. Here, we define a new function for lamellipodia and the cellular mechanism required for haptotaxis – differential actin and lamellipodial protrusion dynamics lead to biased cell migration. Modest differences in lamellipodial dynamics occurring over time periods of seconds to minutes are summed over hours to produce differential whole cell movement towards higher concentrations of fibronectin. We identify a specific subset of lamellipodia regulators as being crucial for haptotaxis. Numerous studies have linked components of this pathway to cancer metastasis and, consistent with this, we find that expression of the oncogenic Rac1 P29S mutation abrogates haptotaxis. Finally, we show that haptotaxis also operates through this pathway in 3D environments.

Summary The Arp2/3 complex nucleates branched actin, forming networks involved in lamellipodial protrusion, phagocytosis, and cell adhesion. We derived primary bone marrow macrophages lacking Arp2/3 complex ( Arpc2 −/− ) and directly tested its role in macrophage functions. Despite protrusion and actin assembly defects, Arpc2 −/− macrophages competently phagocytose via FcR and chemotax toward CSF and CX3CL1. However, CR3 phagocytosis and fibronectin haptotaxis, both integrin-dependent processes, are disrupted. Integrin-responsive actin assembly and αM/β2 integrin localization are compromised in Arpc2 −/− cells. Using an in vivo system to observe endogenous monocytes migrating toward full-thickness ear wounds we found that Arpc2 −/− monocytes maintain cell speeds and directionality similar to control. Our work reveals that the Arp2/3 complex is not a general requirement for phagocytosis or chemotaxis but is a critical driver of integrin-dependent processes. We demonstrate further that cells lacking Arp2/3 complex function in vivo remain capable of executing important physiological responses that require rapid directional motility.

SummaryChemotaxis, migration toward soluble chemical cues, is critical for processes such as wound healing and immune surveillance and is exhibited by various cell types, from rapidly migrating leukocytes to slow-moving mesenchymal cells. To study mesenchymal chemotaxis, we observed cell migration in microfluidic chambers that generate stable gradients of platelet-derived growth factor (PDGF). Surprisingly, we found that pathways implicated in amoeboid chemotaxis, such as PI3K and mammalian target of rapamycin signaling, are dispensable for PDGF chemotaxis. Instead, we find that local inactivation of Myosin IIA, through a noncanonical Ser1/2 phosphorylation of the regulatory light chain, is essential. This site is phosphorylated by PKCα, which is activated by an intracellular gradient of diacylglycerol generated by PLCγ. Using a combination of live imaging and gradients of activators/inhibitors in the microfluidic chambers, we demonstrate that this signaling pathway and subsequent inhibition of Myosin II activity at the leading edge are required for mesenchymal chemotaxis.

A decrease in Arp2/3 levels results in an NF-κB–dependent increase in the expression of several secreted factors, resulting in nonautonomous effects on chemotaxis., Arp2/3-branched actin is critical for cytoskeletal dynamics and cell migration. However, perturbations and diseases affecting this network have phenotypes that cannot be fully explained by cell-autonomous effects. In this paper, we report nonautonomous effects of Arp2/3 depletion. We show that, upon Arp2/3 depletion, the expression of numerous genes encoding secreted factors, including chemokines, growth factors, and matrix metalloproteases, was increased, a signature resembling the senescence-associated secretory phenotype. These factors affected epidermal growth factor chemotaxis in a nonautonomous way, resolving the recent contradictions about the role of Arp2/3 in chemotaxis. We demonstrate that these genes were activated by nuclear factor κB via a CCM2–MEKK3 pathway that has been implicated in hyperosmotic stress signaling. Consistent with this, Arp2/3-depleted cells showed misregulation of volume control and reduced actin in the submembranous cortex. The defects in osmotic signaling in the Arp2/3-depleted cells can be rescued by hypoosmotic treatment. Thus, perturbations of Arp2/3 have nonautonomous effects that should be considered when evaluating experimental manipulations and diseases affecting the Arp2/3-actin cytoskeleton.

SummaryLamellipodia are sheet-like, leading edge protrusions in firmly adherent cells that contain Arp2/3-generated dendritic actin networks. Although lamellipodia are widely believed to be critical for directional cell motility, this notion has not been rigorously tested. Using fibroblasts derived from Ink4a/Arf-deficient mice, we generated a stable line depleted of Arp2/3 complex that lacks lamellipodia. This line shows defective random cell motility and relies on a filopodia-based protrusion system. Utilizing a microfluidic gradient generation system, we tested the role of Arp2/3 complex and lamellipodia in directional cell migration. Surprisingly, Arp2/3-depleted cells respond normally to shallow gradients of PDGF, indicating that lamellipodia are not required for fibroblast chemotaxis. Conversely, these cells cannot respond to a surface-bound gradient of extracellular matrix (haptotaxis). Consistent with this finding, cells depleted of Arp2/3 fail to globally align focal adhesions, suggesting that one principle function of lamellipodia is to organize cell-matrix adhesions in a spatially coherent manner.

To metastasize and disseminate, carcinoma cells must invade the surrounding extracellular matrix (ECM) and migrate to reach blood or lymphatic vessels. Collagen (CN) and fibronectin (FN), two components of the ECM surrounding tumors, are highly expressed in metastatic tumors and their patterns of expression can correlate with metastatic outcome in patient samples. Mena, an actin regulatory protein, is upregulated in breast cancer and is alternatively spliced to produce protein isoforms with distinct functions during tumor progression. The invasion-specific MenaINV isoform increases metastasis by potentiating tumor cell motility and invasion responses to various growth factors. All Mena isoforms bind directly to the α5 subunit of the α5β1 integrin, an FN receptor. We hypothesized that the interaction between MenaINV and α5β1 plays an important role in integrating signals from the ECM as well as growth factors to drive invasion. MDA-MB-231 breast cancer cells expressing MenaINV showed increased adhesion to FN alone as well as mixture of FN and CN, compared to control cells. MenaINV cells also had an increased number of α5-positive adhesions as well as increased signalling at focal adhesions as measured by phospho-Paxilin, compared to control. In 3D invasion assays, addition of FN to a CN gel drove invasion of MenaINV-expressing cells even in the absence of any growth factor ligand, an effect driven by its interaction with α5 and dependent on signalling via EGFR and Met. Furthermore, addition of FN caused an even greater potentiation of EGF-induced invasion by MenaINV, an effect that was dependent upon its interaction with α5. Next, we examined how MenaINV affects tumor cell responses to a gradient of FN within a 3D collagen gel. Control cells failed to respond to an FN gradient, while cells expressing MenaINV migrated preferentially towards FN and significantly reorganized both the collagen and FN surrounding them. Deletion of the α5 binding site or treatment with an α5 function blocking antibody ablated the effects MenaINV on movement towards a FN gradient as well ECM reorganization. Overall, these results suggest that some aspects of the pro-metastatic effects of MenaINV involve its ability to bind α5 and regulate bidirectional signaling with FN and growth factors in the tumor microenvironment. Citation Format: Madeleine J. Oudin, Liliane C. Broye, Alisha Lussiez, Sreeja B. Asokan, Miles A. Miller, Douglas A. Lauffenburger, James E. Bear, Frank B. Gertler. MenaINV interaction with α5β1 promotes tumor cell invasion in response to gradients of growth factors and fibronectin. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1988. doi:10.1158/1538-7445.AM2014-1988

Mena, an actin regulatory protein, is upregulated in human breast cancer and its expression is associated with increased metastatic progression. An invasion-specific isoform, MenaINV, is upregulated selectively in invasive tumor cells and increases invasion and metastasis by potentiating tumor cell responses to EGF. MenaINV causes dysregulation of the tyrosine phosphatase PTP1B, leading to increased phosphorylation of EGFR and other downstream targets. We investigated whether MenaINV could similarly promote invasion through dysregulation of signaling downstream of other receptor tyrosine kinases (RTKs). The behavior of MDA-MB-231 triple negative breast cancer cells stably expressing MenaINV or the other cannonical, broadly expressed Mena lacking the “INV” exon was measured after growth factor treatment in two migration assays: a 2D protrusion assay on matrigel and collagen coated coverslips and a 3D invasion assay into collagen gels. We found that expression of MenaINV sensitized responses to other EGFR ligands including HB-EGF and amphiregulin. In addition, MenaINV expression enhanced responses mediated by two other RTKs associated with tumor progression, Met and IGFR, both known PTP1B substrates. Cells expressing MenaINV respond to 20-fold lower concentrations HGF and IGF than control cells. However, there was no effect of MenaINV on responses to the ERBB3 ligand neuregulin and the chemokine SDF-1. Recently we reported that Mena can regulate cell motility in fibroblasts via its interaction with α5β1 integrin, which is known to co-traffic with EGFR and other RTKs to regulate invasion in tumor cells expressing mutant p53 or upon inhibition of αvβ3. We asked whether the direct interaction of Mena with α5β1 also contributes to invasion. Disrupting the interaction between the Mena isoforms and α5β1 in MDA-MB-231 cells affected how cells responded to EGF in a 3D collagen gel. In addition, fibroblasts that were null for Mena were unable to respond to a fibronectin gradient in a haptotaxis assay. Overall, these results suggest that the effects of MenaINV on invasion and metastasis could be mediated via different RTKs, as well as through direct interaction with integrins and the extracellular matrix. Experiments are currently underway to investigate how MenaINV regulates signalling downstream of these RTKs and integrins, to understand the molecular mechanism underlying the pro-metastatic effect of MenaINV. Citation Format: Madeleine J. Oudin, Shannon K. Hughes-Alford, Miles Miller, Sreeja B. Asokan, James E. Bear, Doug A. Lauffenburger, Frank B. Gertler. MenaINV dysregulation of receptor tyrosine kinase signaling. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr C41.