Gupton, S.L. et al. Cell migration without a lamellipodium: translation of actin dynamics into cell movement mediated by tropomyosin. J. Cell Biol. 168, 619-631

Department of Cell Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.
The Journal of Cell Biology (Impact Factor: 9.83). 03/2005; 168(4):619-31. DOI: 10.1083/jcb.200406063
Source: PubMed


The actin cytoskeleton is locally regulated for functional specializations for cell motility. Using quantitative fluorescent speckle microscopy (qFSM) of migrating epithelial cells, we previously defined two distinct F-actin networks based on their F-actin-binding proteins and distinct patterns of F-actin turnover and movement. The lamellipodium consists of a treadmilling F-actin array with rapid polymerization-dependent retrograde flow and contains high concentrations of Arp2/3 and ADF/cofilin, whereas the lamella exhibits spatially random punctae of F-actin assembly and disassembly with slow myosin-mediated retrograde flow and contains myosin II and tropomyosin (TM). In this paper, we microinjected skeletal muscle alphaTM into epithelial cells, and using qFSM, electron microscopy, and immunolocalization show that this inhibits functional lamellipodium formation. Cells with inhibited lamellipodia exhibit persistent leading edge protrusion and rapid cell migration. Inhibition of endogenous long TM isoforms alters protrusion persistence. Thus, cells can migrate with inhibited lamellipodia, and we suggest that TM is a major regulator of F-actin functional specialization in migrating cells.

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    • "Indeed, altered focal adhesion morphology has been observed upon reduction of Arp2/3 activity [12]. In the absence of lamellipodia, alternate actin-polymerization machinery within the lamella [13] or in filopodia [14]–[17] can facilitate cell edge protrusion and adhesion. "
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    ABSTRACT: Here we demonstrate that Arp2/3 regulates a transition between mesenchymal and amoeboid protrusions in MCF10A epithelial cells. Using genetic and pharmacological means, we first show Arp2/3 inhibition impairs directed cell migration. Arp2/3 inhibition results in a dramatically impaired cell adhesion, causing deficient cell attachment and spreading to ECM as well as an 8-fold decrease in nascent adhesion assembly at the leading edge. While Arp2/3 does not play a significant role in myosin-dependent adhesion growth, mature focal adhesions undergo large scale movements against the ECM suggesting reduced coupling to the ECM. Cell edge protrusions occur at similar rates when Arp2/3 is inhibited but their morphology is dramatically altered. Persistent lamellipodia are abrogated and we observe a markedly increased incidence of blebbing and unstable pseuodopods. Micropipette-aspiration assays indicate that Arp2/3-inhibited cells have a weak coupling between the cell cortex and the plasma membrane, and suggest a potential mechanism for increased pseudopod and bleb formation. Pseudopods are not sensitive to reduced in formin or myosin II activity. Collectively, these results indicate that Arp2/3 is not necessary for rapid protrusion of the cell edge but plays a crucial role in assembling focal adhesions required for its stabilization.
    Full-text · Article · Jun 2014 · PLoS ONE
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    • "FSM study: correlation of actin assembly with a green fluorescent protein (GFP)–p34 signal In biological systems, Arp2/3 is a protein complex that is thought to promote polymerization of networking filaments by aiding new filament creation from preexisting ones. This complex has been described in the literature as a mediator of actin, helping to regulate processes of polymerization and depolymerization in tandem [15] [16] [17]. It has been shown that Arp2/3 is present along the lamellipodium, the cytoskeletal actin at the mobile edge of cells. "
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    ABSTRACT: Actin is a multifunctional eukaryotic protein with a globular monomer form that polymerizes into a thin, linear microfilament in cells. Through interactions with various actin-binding proteins (ABPs), actin plays an active role in many cellular processes, such as cell motility and structure. Microscopy techniques are powerful tools for determining the role and mechanism of actin-ABP interactions in these processes. In this article, we describe the basic concepts of fluorescence speckle microscopy, total internal reflection fluorescence microscopy, atomic force microscopy, and cryo-electron microscopy and review recent studies that utilize these techniques to visualize the binding of actin with ABPs.
    Full-text · Article · Sep 2013 · Analytical Biochemistry
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    • "Because engagement of F-actin flow at FA is thought to limit the width of the lamellipodium and to establish a border between lamellipodium and lamellum (Ponti et al., 2004; Alexandrova et al., 2008; Shemesh et al., 2009), we sought to test whether vinculin affected the spatial organization of lamellipodium and lamellum. We localized F-actin and the lamellipodial protein cortactin or the lamellum protein phosphoserine19 myosin regulatory light chain-2 (pS19MLC2) in control and Vcl-KO MEF (Wu and Parsons, 1993; Ponti et al., 2004; Gupton et al., 2005; Lai et al., 2008). Line scans of staining intensity across the leading edge of control MEF revealed a sharply defined, narrow band of cortactin that colocalized with dense lamellipodial F-actin (Fig. 2, A [arrow], D, and E). "
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    ABSTRACT: In migrating cells, integrin-based focal adhesions (FAs) assemble in protruding lamellipodia in association with rapid filamentous actin (F-actin) assembly and retrograde flow. How dynamic F-actin is coupled to FA is not known. We analyzed the role of vinculin in integrating F-actin and FA dynamics by vinculin gene disruption in primary fibroblasts. Vinculin slowed F-actin flow in maturing FA to establish a lamellipodium-lamellum border and generate high extracellular matrix (ECM) traction forces. In addition, vinculin promoted nascent FA formation and turnover in lamellipodia and inhibited the frequency and rate of FA maturation. Characterization of a vinculin point mutant that specifically disrupts F-actin binding showed that vinculin-F-actin interaction is critical for these functions. However, FA growth rate correlated with F-actin flow speed independently of vinculin. Thus, vinculin functions as a molecular clutch, organizing leading edge F-actin, generating ECM traction, and promoting FA formation and turnover, but vinculin is dispensible for FA growth.
    Full-text · Article · Jul 2013 · The Journal of Cell Biology
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