Dicing with dogma: de-branching the lamellipodium

High Mobility Group Box 1 Promotes Lung Cancer Cell Migration and Motility via Regulation of Dynamin-Related Protein 1

High mobility group box 1 (HMGB1) has been demonstrated to promote the migration and invasion of non-small cell lung cancer (NSCLC). However, the mechanism of action of HMGB1 in regulating tumor mobility remains unclear. Therefore, we aimed to investigate whether HMGB1 affects mitochondria distribution and regulates dynamin-related protein 1 (DRP1)-mediated lamellipodia/filopodia formation to promote NSCLC migration. The regulation of mitochondrial membrane tension, dynamics, polarization, fission process, and cytoskeletal rearrangements in lung cancer cells by HMGB1 was analyzed using confocal microscopy. The HMGB1-mediated regulation of DRP1 phosphorylation and colocalization was determined using immunostaining and co-immunoprecipitation assays. The tumorigenic potential of HMGB1 was assessed in vivo and further confirmed using NSCLC patient samples. Our results showed that HMGB1 increased the polarity and mobility of cells (mainly by regulating the cytoskeletal system actin and microtubule dynamics and distribution), promoted the formation of lamellipodia/filopodia, and enhanced the expression and phosphorylation of DRP1 in both the nucleus and cytoplasm. In addition, HMGB1 and DRP1 expressions were positively correlated and exhibited poor prognosis and survival in patients with lung cancer. Collectively, HMGB1 plays a key role in the formation of lamellipodia and filopodia by regulating cytoskeleton dynamics and DRP1 expression to promote lung cancer migration.

The Chemosensory Function of Primary Cilia Regulates Cholangiocyte Migration, Invasion, and Tumor Growth

Cholangiocytes, the epithelial cells lining the biliary tree in the liver, express primary cilia that can detect several kinds of environmental signals and then transmit this information into the cell. We have reported that cilia are significantly reduced in cholangiocarcinoma (CCA) and that the experimental deciliation of normal cells induces a malignant-like phenotype with increased proliferation, anchorage-independent growth, invasion, and migration. Here, we tested the hypothesis that the chemosensory function of cholangiocyte primary cilia acts as a mechanism for tumor suppression. We found that in the presence of extracellular nucleotides cilia-dependent chemosensation of the nucleotides inhibited migration and invasion in normal ciliated cholangiocytes through a P2Y11 receptor and liver kinase B1 (LKB1)-phosphatase and tensin homolog-AKT-dependent mechanism. In contrast, in normal deciliated cholangiocytes and CCA cells, the nucleotides induced the opposite effects, i.e., increased migration and invasion. As activation of LKB1 through a cilia-dependent mechanism was required for the nucleotide-mediated inhibitory effects on migration and invasion, we attempted to activate LKB1 directly, independent of ciliary expression, using the compound hesperidin methyl chalcone (HMC). We found that HMC induced activation of LKB1 in both ciliated and deciliated cells in vitro, resulting in the inhibition of migration and proliferation. Furthermore, using a rat syngeneic orthotopic CCA model, we found that HMC inhibited tumor growth in vivo. Conclusion: These findings highlight the importance of the chemosensory function of primary cilia for the control of migration and invasion and suggest that, by directly activating LKB1 and bypassing the need for primary cilia, it is possible to emulate this chemosensory function in CCA cells; these data warrant further studies evaluating the possibility of using HMC as therapy for CCA.

A fruitful tree: developing the dendritic nucleation model of actin-based cell motility

A fundamental question in cell biology concerns how cells move, and this has been the subject of intense research for decades. In the 1990s, a major leap forward was made in our understanding of cell motility, with the proposal of the dendritic nucleation model. This essay describes the events leading to the development of the model, including findings from many laboratories and scientific disciplines. The story is an excellent example of the scientific process in action, with the combination of multiple perspectives leading to robust conclusions.

Contributions of the lower dimer to supramolecular actin patterning revealed by TIRF microscopy

Two distinct dimers are formed during the initial steps of actin polymerization. The first one, referred to as the 'lower dimer' (LD) was discovered many years ago by means of chemical crosslinking. Owing to its transient nature, a biological relevance had long been precluded when, using LD-specific antibodies, we detected LD-like contacts in actin assemblies that are associated with the endolysosomal compartment in a number of different cell lines. Moreover, immunofluorescence showed the presence of LD-related structures at the cell periphery of migrating fibroblasts, in the nucleus, and in association with the centrosome of interphase cells. Here, we explore contributions of the LD to the assembly of supramolecular actin structures in real time by total internal reflection fluorescence (TIRF) microscopy. Our data shows that while LD on its own cannot polymerize under filament forming conditions, it is able to incorporate into growing F-actin filaments. This incorporation of LD triggers the formation of X-shaped filament assemblies with barbed ends that are pointing in the same direction in the majority of cases. Similarly, an increased frequency of junction sites was observed when filaments were assembled in the presence of oxidized actin. This data suggests that a disulfide bridge between Cys374 residues might stabilize LD-contacts. Based on our findings, we propose two possible models for the molecular mechanism underlying the supramolecular actin patterning in LD-related structures.

Plasma membrane--cortical cytoskeleton interactions: a cell biology approach with biophysical considerations

From a biophysical standpoint, the interface between the cell membrane and the cytoskeleton is an intriguing site where a "two-dimensional fluid" interacts with an exceedingly complex three-dimensional protein meshwork. The membrane is a key regulator of the cytoskeleton, which not only provides docking sites for cytoskeletal elements through transmembrane proteins, lipid binding-based, and electrostatic interactions, but also serves as the source of the signaling events and molecules that control cytoskeletal organization and remolding. Conversely, the cytoskeleton is a key determinant of the biophysical and biochemical properties of the membrane, including its shape, tension, movement, composition, as well as the mobility, partitioning, and recycling of its constituents. From a cell biological standpoint, the membrane-cytoskeleton interplay underlies--as a central executor and/or regulator--a multitude of complex processes including chemical and mechanical signal transduction, motility/migration, endo-/exo-/phagocytosis, and other forms of membrane traffic, cell-cell, and cell-matrix adhesion. The aim of this article is to provide an overview of the tight structural and functional coupling between the membrane and the cytoskeleton. As biophysical approaches, both theoretical and experimental, proved to be instrumental for our understanding of the membrane/cytoskeleton interplay, this review will "oscillate" between the cell biological phenomena and the corresponding biophysical principles and considerations. After describing the types of connections between the membrane and the cytoskeleton, we will focus on a few key physical parameters and processes (force generation, curvature, tension, and surface charge) and will discuss how these contribute to a variety of fundamental cell biological functions.

FilGAP and its close relatives: a mediator of Rho-Rac antagonism that regulates cell morphology and migration

Cell migration, phagocytosis and cytokinesis are mechanically intensive cellular processes that are mediated by the dynamic assembly and contractility of the actin cytoskeleton. GAPs (GTPase-activating proteins) control activities of the Rho family proteins including Cdc42, Rac1 and RhoA, which are prominent upstream regulators of the actin cytoskeleton. The present review concerns a class of Rho GAPs, FilGAP (ARHGAP24 gene product) and its close relatives (ARHGAP22 and AHRGAP25 gene products). FilGAP is a GAP for Rac1 and a binding partner of FLNa (filamin A), a widely expressed F-actin (filamentous actin)-cross-linking protein that binds many different proteins that are important in cell regulation. Phosphorylation of FilGAP serine/threonine residues and binding to FLNa modulate FilGAP's GAP activity and, as a result, its ability to regulate cell protrusion and spreading. FLNa binds to FilGAP at F-actin-enriched sites, such as at the leading edge of the cell where Rac1 activity is controlled to inhibit actin assembly. FilGAP then dissociates from FLNa in actin networks by myosin-dependent mechanical deformation of FLNa's FilGAP-binding site to relocate at the plasma membrane by binding to polyphosphoinositides. Since actomyosin contraction is activated downstream of RhoA-ROCK (Rho-kinase), RhoA activity regulates Rac1 through FilGAP by signalling to the force-generating system. FilGAP and the ARHGAP22 gene product also act as mediators between RhoA and Rac1 pathways, which lead to amoeboid and mesenchymal modes of cell movements respectively. Therefore FilGAP and its close relatives are key regulators that promote the reciprocal inhibitory relationship between RhoA and Rac1 in cell shape changes and the mesenchymal-amoeboid transition in tumour cells.

Ultrastructure of protrusive actin filament arrays

The actin cytoskeleton is the major force-generating machinery in the cell, which can produce pushing, pulling, and resistance forces. To accomplish these diverse functions, actin filaments, with help of numerous accessory proteins, form higher order ensembles, networks and bundles, adapted to specific tasks. Moreover, dynamic properties of the actin cytoskeleton allow a cell to constantly build, renew, and redesign actin structures according to its changing needs. High resolution architecture of actin filament arrays provides key information for understanding mechanisms of force generation. To generate pushing force, cells use coordinated polymerization of multiple actin filaments organized into branched (dendritic) networks or parallel bundles. This review summarizes our current knowledge of the structural organization of these two actin filament arrays.

Physical model for self-organization of actin cytoskeleton and adhesion complexes at the cell front

Cell motion is driven by interplay between the actin cytoskeleton and the cell adhesions in the front part of the cell. The actin network segregates into lamellipodium and lamellum, whereas the adhesion complexes are characteristically distributed underneath the actin system. Here, we suggest a computational model for this characteristic organization of the actin-adhesion system. The model is based on the ability of the adhesion complexes to sense mechanical forces, the stick-slip character of the interaction between the adhesions and the moving actin network, and a hypothetical propensity of the actin network to disintegrate upon sufficiently strong stretching stresses. We identify numerically three possible types of system organization, all observed in living cells: two states in which the actin network exhibits segregation into lamellipodium and lamellum, whereas the cell edge either remains stationary or moves, and a state where the actin network does not undergo segregation. The model recovers the asynchronous fluctuations and outward bulging of the cell edge, and the dependence of the edge protrusion velocity on the rate of the nascent adhesion generation, the membrane tension, and the substrate rigidity.

Role of molecular determinants of store-operated Ca(2+) entry (Orai1, phospholipase A2 group 6, and STIM1) in focal adhesion formation and cell migration

BACKGROUND

Store-operated Ca(2+) entry is important for cell migration.

RESULTS

This study presents characterization of localization and roles of Orai1, STIM1, and PLA2g6 in adhesion dynamics during cell migration.

CONCLUSION

Orai1 and PLA2g6 are involved in adhesion formation at the front, whereas STIM1 participates in both adhesion formation and disassembly.

SIGNIFICANCE

Results uncovered new parameters of Orai1, STIM1, and PLA2g6 involvement in cell migration. Store-operated Ca(2+) entry and its major determinants are known to be important for cell migration, but the mechanism of their involvement in this complex process is unknown. This study presents a detailed characterization of distinct roles of Orai1, STIM1, and PLA2g6 in focal adhesion (FA) formation and migration. Using HEK293 cells, we discovered that although molecular knockdown of Orai1, STIM1, or PLA2g6 resulted in a similar reduction in migration velocity, there were profound differences in their effects on number, localization, and lifetime of FAs. Knockdown of STIM1 caused an increase in lifetime and number of FAs, their redistribution toward lamellae region, and an increase in cell tail length. In contrast, the number of FAs in Orai1- or PLA2g6-deficient cells was significantly reduced, and FAs accumulated closer to the leading edge. Assembly rate and Vinculin phosphorylation of FAs was similarly reduced in Orai1, PLA2g6, or STIM1-deficient cells. Although Orai1 and PLA2g6 accumulated and co-localized at the leading edge, STIM1 distribution was more complex. We found STIM1 protrusions in lamellipodia, which co-localized with FAs, whereas major accumulation could be seen in central and retracting parts of the cell. Interestingly, knockdown of Orai1 and PLA2g6 produced similar and non-additive effect on migration, whereas knockdown of STIM1 simultaneously with either Orai1 or PLA2g6 produced additional inhibition. Together these data suggest that although Orai1, PLA2g6, and STIM1 play major roles in formation of new FAs at the leading edge, STIM1 may also be involved in Orai1- and PLA2g6-independent disassembly of FAs in the back of cells.

Controlling the cortical actin motor

Actin is the essential force-generating component of the microfilament system, which powers numerous motile processes in eukaryotic cells and undergoes dynamic remodeling in response to different internal and external signaling. The ability of actin to polymerize into asymmetric filaments is the inherent property behind the site-directed force-generating capacity that operates during various intracellular movements and in surface protrusions. Not surprisingly, a broad variety of signaling pathways and components are involved in controlling and coordinating the activities of the actin microfilament system in a myriad of different interactions. The characterization of these processes has stimulated cell biologists for decades and has, as a consequence, resulted in a huge body of data. The purpose here is to present a cellular perspective on recent advances in our understanding of the microfilament system with respect to actin polymerization, filament structure and specific folding requirements.

Differences in adhesion and protrusion properties correlate with differences in migration speed under EGF stimulation

Background

Cell migration plays an essential role in many biological processes, such as cancer metastasis, wound healing and immune response. Cell migration is mediated through protrusion and focal adhesion (FA) assembly, maturation and disassembly. Epidermal growth factor (EGF) is known to enhance migration rate in many cell types; however it is not known how FA maturation, FA dynamics and protrusion dynamics are regulated during EGF-induced migration. Here we use total internal reflection fluorescence (TIRF) microscopy and image analysis to quantify FA properties and protrusion dynamics under different doses of EGF stimulation.

Results

EGF was found to broaden the distribution of cell migration rates, generating more fast and slow cells. Furthermore, groups based on EGF stimulation condition or cell migration speed were marked by characteristic signatures. When data was binned based on EGF stimulation conditions, FA intensity and FA number per cell showed the largest difference among stimulation groups. FA intensity decreased with increasing EGF concentration and FA number per cell was highest under intermediate stimulation conditions. No difference in protrusion behavior was observed. However, when data was binned based on cell migration speed, FA intensity and not FA number per cell showed the largest difference among groups. FA intensity was lower for fast migrating cells. Additionally, waves of protrusion tended to correlate with fast migrating cells.

Conclusions

Only a portion of the FA properties and protrusion dynamics that correlate with migration speed, correlate with EGF stimulation condition. Those that do not correlate with EGF stimulation condition constitute the most sensitive output for identifying why cells respond differently to EGF. The idea that EGF can both increase and decrease the migration speed of individual cells in a population has particular relevance to cancer metastasis where the microenvironment can select subpopulations based on some adhesion and protrusion characteristics, leading to a more invasive phenotype as would be seen if all cells responded like an “average” cell.

Impact of the carbazole derivative wiskostatin on mechanical stability and dynamics of motile cells

Many essential functions in eukaryotic cells like phagocytosis, division, and motility rely on the dynamical properties of the actin cytoskeleton. A central player in the actin system is the Arp2/3 complex. Its activity is controlled by members of the WASP (Wiskott-Aldrich syndrome protein) family. In this work, we investigated the effect of the carbazole derivative wiskostatin, a recently identified N-WASP inhibitor, on actin-driven processes in motile cells of the social ameba Dictyostelium discoideum. Drug-treated cells exhibited an altered morphology and strongly reduced pseudopod formation. However, TIRF microscopy images revealed that the overall cortical network structure remained intact. We probed the mechanical stability of wiskostatin-treated cells using a microfluidic device. While the total amount of F-actin in the cells remained constant, their stiffness was strongly reduced. Furthermore, wiskostatin treatment enhanced the resistance to fluid shear stress, while spontaneous motility as well as chemotactic motion in gradients of cAMP were reduced. Our results suggest that wiskostatin affects the mechanical integrity of the actin cortex so that its rigidity is reduced and actin-driven force generation is impaired.

Filopodia and adhesion in cancer cell motility

Slender bundled actin containing plasma membrane protrusions, called filopodia, are important for many essential cellular processes like cell adhesion, migration, angiogenesis and the formation of cell-cell contacts. In migrating cells, filopodia are the pioneers at the leading edge which probe the environment for cues. Integrins are cell surface adhesion receptors critically implicated in cell migration and they are transported actively to filopodia tips by an unconventional myosin, myosin-X. Integrin mediated adhesion stabilizes filopodia and promotes cell migration even though integrins are not essential for filopodia initiation. Myosin-X binds also PIP3 and this regulates its activation and localization to filopodia. Filopodia stimulate cell migration in many cell types and increased filopodia density has been described in cancer. Furthermore, several proteins implicated in filopodia formation, like fascin, are also relevant for cancer progression. To investigate this further, we performed a meta-analysis of the expression profiles of 10 filopodia-linked genes in human breast cancer. These data implicated that several different filopodia inducing genes may contribute in a collective manner to cancer progression and the high metastasis rates associated with basal-type breast carcinomas.

The structure of cell-matrix adhesions: the new frontier

Adhesions between the cell and the extracellular matrix (ECM) are mechanosensitive multi-protein assemblies that transmit force across the cell membrane and regulate biochemical signals in response to the chemical and mechanical environment. These combined functions in force transduction, signaling and mechanosensing contribute to cellular phenotypes that span development, homeostasis and disease. These adhesions form, mature and disassemble in response to actin organization and physical forces that originate from endogenous myosin activity or external forces by the extracellular matrix. Despite advances in our understanding of the protein composition, interactions and regulation, our understanding of matrix adhesion structure and organization, how forces affect this organization, and how these changes dictate specific signaling events is limited. Insights across multiple structural levels are acutely needed to elucidate adhesion structure and ultimately the molecular basis of signaling and mechanotransduction. Here we describe the challenges and recent advances and prospects for unraveling the structure of cell-matrix adhesions and their response to force.

Walking to work: roles for class V myosins as cargo transporters

Cells use molecular motors, such as myosins, to move, position and segregate their organelles. Class V myosins possess biochemical and structural properties that should make them ideal actin-based cargo transporters. Indeed, studies show that class V myosins function as cargo transporters in yeast, moving a range of organelles, such as the vacuole, peroxisomes and secretory vesicles. There is also increasing evidence in vertebrate cells that class V myosins not only tether organelles to actin but also can serve as short-range, point-to-point organelle transporters, usually following long-range, microtubule-dependent organelle transport.

Robust organizational principles of protrusive biopolymer networks in migrating living cells

Cell migration is associated with the dynamic protrusion of a thin actin-based cytoskeletal extension at the cell front, which has been shown to consist of two different substructures, the leading lamellipodium and the subsequent lamellum. While the formation of the lamellipodium is increasingly well understood, organizational principles underlying the emergence of the lamellum are just beginning to be unraveled. We report here on a 1D mathematical model which describes the reaction-diffusion processes of a polarized actin network in steady state, and reproduces essential characteristics of the lamellipodium-lamellum system. We observe a steep gradient in filament lengths at the protruding edge, a local depolymerization maximum a few microns behind the edge, as well as a differential dominance of the network destabilizer ADF/cofilin and the stabilizer tropomyosin. We identify simple and robust organizational principles giving rise to the derived network characteristics, uncoupled from the specifics of any molecular implementation, and thus plausibly valid across cell types. An analysis of network length dependence on physico-chemical system parameters implies that to limit array treadmilling to cellular dimensions, network growth has to be truncated by mechanisms other than aging-induced depolymerization, e.g., by myosin-associated network dissociation at the transition to the cell body. Our work contributes to the analytical understanding of the cytoskeletal extension's bisection into lamellipodium and lamellum and sheds light on how cells organize their molecular machinery to achieve motility.