Abr, a Rho-regulating protein, modulates osteoclastogenesis by enhancing lamellipodia formation by interacting with poly(ADP-ribose) glycohydrolase

BACKGROUND

Osteoclasts are multinucleated bone-resorbing cells formed by the fusion of monocyte/macrophage lineage. During osteoclast differentiation, Rho GTPases are involved in various processes, including cell migration, adhesion, and polarity. However, the role of Rho-regulatory molecules in the regulation of osteoclast differentiation remains unclear. In this study, among these genes, we focused on active breakpoint cluster region-related (Abr) protein that is a multifunctional regulator of Rho GTPases.

METHODS AND RESULTS

We examined using knockdown and overexpression experiments in RANKL-stimulated RAW-D macrophages whether Abr regulates osteoclast differentiation and cell morphology. We observed an increase in Abr expression during osteoclast differentiation and identified expression of a variant of the Abr gene in osteoclasts. Knockdown of Abr suppressed osteoclast differentiation and resorption. Abr knockdown markedly inhibited the expression of osteoclast markers, such as Nfatc1, c-fos, Src, and Ctsk in osteoclasts. Conversely, overexpression of Abr enhanced the formation of multinucleated osteoclasts, bone resorption activity, and osteoclast marker gene expression. Moreover, Abr overexpression accelerated lamellipodia formation and induced the formation of well-developed actin in osteoclasts. Importantly, the Abr protein interacted with poly(ADP-ribose) glycohydrolase (PARG) and Rho GTPases, including RhoA, Rac1/2/3, and Cdc42 in osteoclasts.

CONCLUSIONS

Taken together, these results indicate that Abr modulates osteoclastogenesis by enhancing lamellipodia formation via its interaction with PARG.

Resolving the role of podoplanin in the motility of papillary thyroid carcinoma-derived cells using RNA sequencing

The intracellular level of podoplanin (PDPN), a transmembrane protein of still unclear function, is frequently altered in metastatic tumors. High expression of PDPN is frequently observed in papillary thyroid cancer (PTC) specimens. Similarly, PTC-derived cell lines (BCPAP and TPC1, harboring the BRAF V600E mutation and RET/PTC1 fusion, respectively), also present enhanced PDPN yield. We previously reported that depletion of PDPN impairs migration of TPC1 cells, but augments metastasis of BCPAP cells. Interestingly, this phenomenon stays in contrast to the migratory pattern observed for wild-type cells, where TPC1 exhibited higher motility than BCPAP cells. Here, we aimed to elucidate the potential role of PDPN in regulation of molecular mechanisms leading to the diverse metastatic features of the studied PTC-derived cells. We consider that this phenomenon may be caused by alternative regulation of signaling pathways due to the presence of the mutated BRAF allele or RET/PTC1 fusion. The high-throughput RNA sequencing (RNA-seq) technique was used to uncover the genes and signaling pathways affected in wild-type and PDPN-depleted TPC1 and BCPAP cells. We found that changes in the expression of various factors of signaling pathways, like RHOA and RAC1 GTPases and their regulators, are linked with both high PDPN levels and presence of the BRAF V600E mutation. We imply that the suppressed motility of wild-type BCPAP cells results from overactivation of RHOA through natively high PDPN expression. This process is accompanied by inhibition of the PI3K kinase and consequently RAC1, due to overactivation of RAS-mediated signaling and the PTEN regulator.

Regulation of actin filament assembly and disassembly in growth cone motility and axon guidance

Directed outgrowth of axons is fundamental for the establishment of neuronal networks. Axon outgrowth is guided by growth cones, highly motile structures enriched in filamentous actin (F-actin) located at the axons' distal tips. Growth cones exploit F-actin-based protrusions to scan the environment for guidance cues, and they contain the sensory apparatus to translate guidance cue information into intracellular signaling cascades. These cascades act upstream of actin-binding proteins (ABP) and thereby control assembly and disassembly of F-actin. Spatiotemporally controlled F-actin dis-/assembly in growth cones steers the axon towards attractants and away from repellents, and it thereby navigates the axon through the developing nervous system. Hence, ABP that control F-actin dynamics emerged as critical regulators of neuronal network formation. In the present review article, we will summarize and discuss current knowledge of the mechanisms that control remodeling of the actin cytoskeleton in growth cones, focusing on recent progress in the field. Further, we will introduce tools and techniques that allow to study actin regulatory mechanism in growth cones.

Caveolin-1 signaling-driven mitochondrial fission and cytoskeleton remodeling promotes breast cancer migration

Mitochondria are highly dynamic organelles that constantly divide and fuse to maintain their proper structure and function. Cancer cells are often accompanied by an imbalance of mitochondrial fusion and fission, cancer progression is greatly affected by this imbalance. Here, we found that high-metastatic breast cancer MDA-MB-231 cells possess higher caveolin-1 (Cav-1) expression compared with low-metastatic breast cancer MCF-7 cells or normal breast epithelial MCF-10A cells. Downregulation of Cav-1 decreases the migratory and invasive abilities of MDA-MB-231 cells. Our results further demonstrated that downregulation of Cav-1 facilitated DRP1 and MFN2 to translocate to mitochondria, increasing the inhibitory phosphorylation level of DRP1 at Ser637 by protein kinase A (PKA), resulting in mitochondria elongation. We also showed that downregulation of Cav-1 significantly reduced the Rac1 activity by affecting intracellular reactive oxygen species (ROS) generation, which then inhibited F-actin formation. Based on these findings, we proposed that Cav-1 mediated mitochondrial fission-affected intracellular ROS generation and activated Rho GTPases, leading to F-actin-dependent formation of lamellipodia and promotion of breast cancer motility.

Methods for assessment of membrane protrusion dynamics

Membrane protrusions are a critical facet of cell function. Mediating fundamental processes such as cell migration, cell-cell interactions, phagocytosis, as well as assessment and remodeling of the cell environment. Different protrusion types and morphologies can promote different cellular functions and occur downstream of distinct signaling pathways. As such, techniques to quantify and understand the inner workings of protrusion dynamics are critical for a comprehensive understanding of cell biology. In this chapter, we describe approaches to analyze cellular protrusions and correlate physical changes in cell morphology with biochemical signaling processes. We address methods to quantify and characterize protrusion types and velocity, mathematical approaches to predictive models of cytoskeletal changes, and implementation of protein engineering and biosensor design to dissect cell signaling driving protrusive activity. Combining these approaches allows cell biologists to develop a comprehensive understanding of the dynamics of membrane protrusions.

Computational toolbox for ultrastructural quantitative analysis of filament networks in cryo-ET data

A precise quantitative description of the ultrastructural characteristics underlying biological mechanisms is often key to their understanding. This is particularly true for dynamic extra- and intracellular filamentous assemblies, playing a role in cell motility, cell integrity, cytokinesis, tissue formation and maintenance. For example, genetic manipulation or modulation of actin regulatory proteins frequently manifests in changes of the morphology, dynamics, and ultrastructural architecture of actin filament-rich cell peripheral structures, such as lamellipodia or filopodia. However, the observed ultrastructural effects often remain subtle and require sufficiently large datasets for appropriate quantitative analysis. The acquisition of such large datasets has been enabled by recent advances in high-throughput cryo-electron tomography (cryo-ET) methods. This also necessitates the development of complementary approaches to maximize the extraction of relevant biological information. We have developed a computational toolbox for the semi-automatic quantification of segmented and vectorized filamentous networks from pre-processed cryo-electron tomograms, facilitating the analysis and cross-comparison of multiple experimental conditions. GUI-based components simplify the processing of data and allow users to obtain a large number of ultrastructural parameters describing filamentous assemblies. We demonstrate the feasibility of this workflow by analyzing cryo-ET data of untreated and chemically perturbed branched actin filament networks and that of parallel actin filament arrays. In principle, the computational toolbox presented here is applicable for data analysis comprising any type of filaments in regular (i.e. parallel) or random arrangement. We show that it can ease the identification of key differences between experimental groups and facilitate the in-depth analysis of ultrastructural data in a time-efficient manner.

CK1α protects WAVE from degradation to regulate cell shape and motility in immune response

The WAVE regulatory complex (WRC) is the main activator of the Arp2/3 complex, promoting lamellipodial protrusions in migrating cells. The WRC is basally inactive but can be activated by Rac1 and phospholipids, and through phosphorylation. However, the in vivo relevance of the phosphorylation of WAVE proteins remains largely unknown. Here, we identified casein kinase I alpha (CK1α) as a regulator of WAVE, thereby controlling cell shape and cell motility in Drosophila macrophages. CK1α binds and phosphorylates WAVE in vitro. Phosphorylation of WAVE by CK1α appears not to be required for activation but, rather, regulates its stability. Pharmacologic inhibition of CK1α promotes ubiquitin-dependent degradation of WAVE. Consistently, loss of Ck1α but not ck2 function phenocopies the depletion of WAVE. Phosphorylation-deficient mutations in the CK1α consensus sequences within the VCA domain of WAVE can neither rescue mutant lethality nor lamellipodium defects. By contrast, phosphomimetic mutations rescue all cellular and developmental defects. Finally, RNAi-mediated suppression of 26S proteasome or E3 ligase complexes substantially rescues lamellipodia defects in CK1α-depleted macrophages. Therefore, we conclude that basal phosphorylation of WAVE by CK1α protects it from premature ubiquitin-dependent degradation, thus promoting WAVE function in vivo.

This article has an associated First Person interview with the first author of the paper.

Summary: We identified CK1α as a novel regulator of WAVE controlling cell shape and motility in immune response. Basal phosphorylation of WAVE by CK1α protects it from premature proteasomal degradation.

Twinfilin1 controls lamellipodial protrusive activity and actin turnover during vertebrate gastrulation

The dynamic control of the actin cytoskeleton is a key aspect of essentially all animal cell movements. Experiments in single migrating cells and in vitro systems have provided an exceptionally deep understanding of actin dynamics. However, we still know relatively little of how these systems are tuned in cell-type-specific ways, for example in the context of collective cell movements that sculpt the early embryo. Here, we provide an analysis of the actin-severing and depolymerization machinery during vertebrate gastrulation, with a focus on Twinfilin1 (Twf1) in Xenopus. We find that Twf1 is essential for convergent extension, and loss of Twf1 results in a disruption of lamellipodial dynamics and polarity. Moreover, Twf1 loss results in a failure to assemble polarized cytoplasmic actin cables, which are essential for convergent extension. These data provide an in vivo complement to our more-extensive understanding of Twf1 action in vitro and provide new links between the core machinery of actin regulation and the specialized cell behaviors of embryonic morphogenesis.

Differential cellular responses to adhesive interactions with galectin-8- and fibronectin-coated substrates

The mechanisms underlying the cellular response to extracellular matrices (ECMs) that consist of multiple adhesive ligands are still poorly understood. Here, we address this topic by monitoring specific cellular responses to two different extracellular adhesion molecules - the main integrin ligand fibronectin and galectin-8, a lectin that binds β-galactoside residues  - as well as to mixtures of the two proteins. Compared with cell spreading on fibronectin, cell spreading on galectin-8-coated substrates resulted in increased projected cell area, more-pronounced extension of filopodia and, yet, the inability to form focal adhesions and stress fibers. These differences can be partially reversed by experimental manipulations of small G-proteins of the Rho family and their downstream targets, such as formins, the Arp2/3 complex and Rho kinase. We also show that the physical adhesion of cells to galectin-8 was stronger than adhesion to fibronectin. Notably, galectin-8 and fibronectin differently regulate cell spreading and focal adhesion formation, yet act synergistically to upregulate the number and length of filopodia. The physiological significance of the coherent cellular response to a molecularly complex matrix is discussed. This article has an associated First Person interview with the first author of the paper.

Rac1 activation can generate untemplated, lamellar membrane ruffles

BACKGROUND

Membrane protrusions that occur on the dorsal surface of a cell are an excellent experimental system to study actin machinery at work in a living cell. Small GTPase Rac1 controls the membrane protrusions that form and encapsulate extracellular volumes to perform pinocytic or phagocytic functions.

RESULTS

Here, capitalizing on rapid volumetric imaging capabilities of lattice light-sheet microscopy (LLSM), we describe optogenetic approaches using photoactivable Rac1 (PA-Rac1) for controlled ruffle generation. We demonstrate that PA-Rac1 activation needs to be continuous, suggesting a threshold local concentration for sustained actin polymerization leading to ruffling. We show that Rac1 activation leads to actin assembly at the dorsal surface of the cell membrane that result in sheet-like protrusion formation without any requirement of a template. Further, this approach can be used to study the complex morpho-dynamics of the protrusions or to investigate specific proteins that may be enriched in the ruffles. Deactivating PA-Rac1 leads to complex contractile processes resulting in formation of macropinosomes. Using multicolour imaging in combination with these approaches, we find that Myo1e specifically is enriched in the ruffles.

CONCLUSIONS

Combining LLSM and optogenetics enables superior spatial and temporal control for studying such dynamic mechanisms. Demonstrated here, the techniques implemented provide insight into the complex nature of the molecular interplay involved in dynamic actin machinery, revealing that Rac1 activation can generate untemplated, lamellar protrusions.

α-Linolenic acid induces clearance of Tau seeds via Actin-remodeling in Microglia

Alzheimer's disease (AD) is known by characteristic features, extracellular burden of amyloid-β and intracellular neuronal Tau. Microglia, the innate immune cell of the brain has the ability to clear the burden of accumulated proteins via phagocytosis. But the excessive proinflammatory cytokine production, altered cellular signaling and actin remodeling hampers the process of migration and phagocytosis by microglia. Actin remodeling is necessary to initiate the chemotactic migration of microglia towards the target and engulf it. The formation of lamellipodia, filopodia, membrane ruffling and rapid turnover of F-actin is necessary to sense the extracellular target by the cells. Omega-3 fatty acids, are known to impose anti-inflammatory phenotype of microglia by enhancing its ability for migration and phagocytosis. But the role of omega-3 fatty acids in cellular actin remodeling, which is the basis of cellular functions such as migration and phagocytosis, is not well understood. Here, we have focused on the effect of dietary supplement of α-linolenic acid (ALA) on extracellular Tau internalization and assisted actin polymerization for the process. ALA is found to induce membrane ruffling and phagocytic cup formation along with cytoskeletal rearrangement. ALA also enhances the localization of Arp2/3 complex at the leading edge and its colocalization with F-actin to induce the actin polymerization. The excessive actin polymerization might help the cell to protrude forward and perform its migration. The results suggest that dietary supplement of ALA could play a neuroprotective role and slow down the AD pathology.

Switching between blebbing and lamellipodia depends on the degree of non-muscle myosin II activity

Cells can adopt both mesenchymal and amoeboid modes of migration through membrane protrusive activities, namely formation of lamellipodia and blebbing. How the molecular players control the transition between lamellipodia and blebs is yet to be explored. Here, we show that addition of the ROCK inhibitor Y27632 or low doses of blebbistatin, an inhibitor of non-muscle myosin II (NMII) ATPase activity and filament partitioning, induces blebbing to lamellipodia conversion (BLC), whereas addition of low doses of ML7, an inhibitor of myosin light chain kinase (MLCK), induces lamellipodia to blebbing conversion (LBC) in human MDA-MB-231 cells. Similarly, siRNA-mediated knockdown of ROCK and MLCK induces BLC and LBC, respectively. Interestingly, both blebs and lamellipodia membrane protrusions are able to maintain the ratio of phosphorylated to unphosphorylated regulatory light chain at cortices when MLCK and ROCK, respectively, are inhibited either pharmacologically or genetically, suggesting that MLCK and ROCK activities are interlinked in BLC and LBC. Such BLCs and LBCs are also inducible in other cell lines, including MCF7 and MCF10A. These studies reveal that the relative activity of ROCK and MLCK, which controls both the ATPase activity and filament-forming property of NMII, is a determining factor in whether a cell exhibits blebbing or lamellipodia.

Effect of silica-coated magnetic nanoparticles on rigidity sensing of human embryonic kidney cells

BACKGROUND

Nanoparticles (NPs) can enter cells and cause cellular dysfunction. For example, reactive oxygen species generated by NPs can damage the cytoskeleton and impair cellular adhesion properties. Previously, we reported that cell spreading and protrusion structures such as lamellipodia and filopodia was reduced when cells are treated with silica-coated magnetic nanoparticles incorporating rhodamine B isothiocyanate (MNPs@SiO2(RITC)), even at 0.1 μg/μL. These protruded structures are involved in a cell's rigidity sensing, but how these NPs affect rigidity sensing is unknown.

RESULTS

Here, we report that the rigidity sensing of human embryonic kidney (HEK293) cells was impaired even at 0.1 μg/μL of MNPs@SiO2(RITC). At this concentration, cells were unable to discern the stiffness difference between soft (5 kPa) and rigid (2 MPa) flat surfaces. The impairment of rigidity sensing was further supported by observing the disappearance of locally contracted elastomeric submicron pillars (900 nm in diameter, 2 μm in height, 24.21 nN/μm in stiffness k) under MNPs@SiO2(RITC) treated cells. A decrease in the phosphorylation of paxillin, which is involved in focal adhesion dynamics, may cause cells to be insensitive to stiffness differences when they are treated with MNPs@SiO2(RITC).

CONCLUSIONS

Our results suggest that NPs may impair the rigidity sensing of cells even at low concentrations, thereby affecting cell adhesion and spreading.

Lamellipodia-like protrusions and focal adhesions contribute to collective cell migration in zebrafish

Collective cell migration is a process where cohorts of cells exhibit coordinated migratory behavior. During individual and collective cellular migration, cells must extend protrusions to interact with the extracellular environment, sense chemotactic cues, and act as points of attachment. The mechanisms and regulators of protrusive behavior have been widely studied in individually migrating cells; however, how this behavior is regulated throughout collectives is not well understood. To address this, we used the zebrafish posterior lateral line primordium (pLLP) as a model. The pLLP is a cluster of ~150 ​cells that migrates along the zebrafish trunk, depositing groups of cells that will become sensory organs. To define protrusive behavior, we performed mosaic analysis to sparsely label pLLP cells with a transgene marking filamentous actin. This approach revealed an abundance of brush-like protrusions throughout the pLLP that orient in the direction of migration. Formation of these protrusions depends on the Arp2/3 complex, a regulator of dendritic actin. This argues that these brush-like protrusions are an in vivo example of lamellipodia. Mosaic analysis demonstrated that these lamellipodia-like protrusions are located in a close proximity to the overlying skin. Immunostaining revealed an abundance of focal adhesion complexes surrounding the pLLP. Disruption of these complexes specifically in pLLP cells led to impaired pLLP migration. Finally, we show that Erk signaling, a known regulator of focal adhesions, is required for proper formation of lamellipodia-like protrusions and pLLP migration. Altogether, our results suggest a model where the coordinated dynamics of lamellipodia-like protrusions, making contact with either the overlying skin or the extracellular matrix through focal adhesions, promotes migration of pLLP cells.

Effects of a local auxiliary protein on the two-dimensional affinity of a TCR-peptide MHC interaction

The affinity of T-cell receptors (TCRs) for major histocompatibility complex molecules (MHCs) presenting cognate antigens likely determines whether T cells initiate immune responses, or not. There exist few measurements of two-dimensional (2D) TCR-MHC interactions, and the effect of auxiliary proteins on binding is unexplored. Here, Jurkat T-cells expressing the MHC molecule HLA-DQ8-glia-α1 and the ligand of an adhesion protein (rat CD2) were allowed to bind supported lipid bilayers (SLBs) presenting fluorescently labelled L3-12 TCR and rat CD2, allowing measurements of binding unconfounded by cell signaling effects or co-receptor binding. The 2D Kd for L3-12 TCR binding to HLA-DQ8-glia-α1, of 14±5 molecules/μm2 (mean±s.d.), was only marginally influenced by including CD2 up to ∼200 bound molecules/μm2 but higher CD2 densities reduced the affinity up to 1.9-fold. Cell-SLB contact size increased steadily with ligand density without affecting binding for contacts at up to ∼20% of total cell area, but beyond this lamellipodia appeared, giving an apparent increase in bound receptors of up to 50%. Our findings show how parameters other than the specific protein-protein interaction can influence binding behavior at cell-cell contacts.

Actin cytoskeleton in mesenchymal-to-amoeboid transition of cancer cells

During development of metastasis, tumor cells migrate through different tissues and encounter different extracellular matrices. An ability of cells to adapt mechanisms of their migration to these diverse environmental conditions, called migration plasticity, gives tumor cells an advantage over normal cells for long distant dissemination. Different modes of individual cell motility-mesenchymal and amoeboid-are driven by different molecular mechanisms, which largely depend on functions of the actin cytoskeleton that can be modulated in a wide range by cellular signaling mechanisms in response to environmental conditions. Various triggers can switch one motility mode to another, but regulations of these transitions are incompletely understood. However, understanding of the mechanisms driving migration plasticity is instrumental for finding anti-cancer treatment capable to stop cancer metastasis. In this review, we discuss cytoskeletal features, which allow the individually migrating cells to switch between mesenchymal and amoeboid migrating modes, called mesenchymal-to-amoeboid transition (MAT). We briefly describe main characteristics of different cell migration modes, and then discuss the triggering factors that initiate MAT with special attention to cytoskeletal features essential for migration plasticity.

Antibacterial AZT derivative regulates metastasis of breast cancer cells

Antimicrobial peptides (AMP) with anticancer activity have drawn remarkable attention in modern treatments. However, long peptide length and protease instability are the most addressing factors, which hampers their further development as therapeutic agents. In view of this, herein, we designed and synthesized a series of AZT-based cationic small molecule incorporating a variety of hydrophobic groups and cationic charges, including amine and guanidine groups to mimic the amphipathic structure of AMPs. These compounds were evaluated for their antibacterial activity against Gram-positive and Gram-negative bacteria. Through an extensive structure activity relationship study (SAR), we identified ADG-2e as the most potent antibacterial agent, which exhibited remarkable potency against drug resistant bacterial strains such as MRSA and MDRPA. Further, ADG-2e was examined for their anti-metastatic ability by investigating the cancer cell migration and invasiveness through scratch wound-healing assay and transwell invasive assay, respectively. In addition, time-lapse cell tracking analysis also performed for analyzing the cell movement pattern. Treatment of ADG-2e against metastatic breast cancer cells (MDA-MB-231) suppressed tumor cell migration by multi-directional lamellipodium formation, indicating their anti-metastatic potential. Thus, our cationic AZT based small molecules may evolve as an appealing class of antibacterial agents with anti-metastasis potential.

Rho GTPases and related signaling complexes in cell migration and invasion

Cell migration and invasion play an important role in the development of cancer. Cell migration is associated with several specific actin filament-based structures, including lamellipodia, filopodia, invadopodia and blebs, and with cell-cell adhesion, cell-extracellular matrix adhesion. Migration occurs via different modes, human epithelial cancer cells mainly migrate collectively, while in vivo imaging studies in laboratory animals have found that most cells migrate as single cells. Rho GTPases play an important role in the process of cell migration, and several Rho GTPase-related signaling complexes are also involved. However, the exact mechanism by which these signaling complexes act remains unclear. This paper reviews how Rho GTPases and related signaling complexes interact with other proteins, how their expression is regulated, how tumor microenvironment-related factors play a role in invasion and metastasis, and the mechanism of these complex signaling networks in cell migration and invasion.

Plasma membrane localization of MLC1 regulates cellular morphology and motility

Background

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare form of infantile-onset leukodystrophy. The disorder is caused primarily by mutations of MLC1 that leads to a series of phenotypic outcomes including vacuolation of myelin and astrocytes, subcortical cysts, brain edema, and macrocephaly. Recent studies have indicated that functional interactions among MLC1, GlialCAM, and ClC-2 channels play key roles in the regulation of neuronal, glial and vascular homeostasis. However, the physiological role of MLC1 in cellular homeostatic communication remains poorly understood. In the present study, we investigated the cellular function of MLC1 and its effects on cell–cell interactions.

Methods

MLC1-dependent cellular morphology and motility were analyzed by using confocal and live cell imaging technique. Biochemical approaches such as immunoblotting, co-immunoprecipitation, and surface biotinylation were conducted to support data.

Results

We found that the altered MLC1 expression and localization led to a great alteration in cellular morphology and motility through actin remodeling. MLC1 overexpression induced filopodia formation and suppressed motility. And, MLC1 proteins expressed in patient-derived MLC1 mutants resulted in trapping in the ER although no changes in morphology or motility were observed. Interestingly knockdown of Mlc1 induced Arp3-Cortactin interaction, lamellipodia formation, and increased the membrane ruffling of the astrocytes. These data indicate that subcellular localization of expressed MLC1 at the plasma membrane is critical for changes in actin dynamics through ARP2/3 complex. Thus, our results suggest that misallocation of pathogenic mutant MLC1 may disturbs the stable cell-cell communication and the homeostatic regulation of astrocytes in patients with MLC.

Metformin Inhibit Cervical Cancer Migration by Suppressing the FAK/Akt Signaling Pathway

Background:

Metformin, an antidiabetic drug, has been previously reported to have anti-cancer activities. However, its role in the control of cancer cell migration remains elusive.

Methods:

To examine the possible effect of metformin on migration of cervical cancer cells. The related mechanisms were further determined by immunocytochemistry and Western’s blotting assay.

Results:

The results showed that metformin treatment substantially inhibited the migration ability of cervical cancer cells. Consistently, the filopodia and lamellipodia formation were depleted after exposure to metformin. The suppression of migration mediated through the regulatory proteins such as focal adhesion kinase (FAK), ATP-dependent tyrosine kinase (Akt), Rac1 and RhoA after metformin treatment.

Conclusion:

Metformin displays antimigration effects in cervical cancer cells by inhibiting filopodia and lamellipodia formation through the suppression of FAK, Akt and its downstream Rac1 and RhoA protein. We propose that metformin could be a novel potential candidate as an antimetastatic cancer drug in the cervical cancer management.

CROCC-mutated rhabdoid colorectal carcinoma showing in intercellular spaces lamellipodia and cellular projections revealed by electron microscopy

BACKGROUND

Rhabdoid colorectal carcinoma (RC) is a rare lesion localized to the proximal colon of patients with a mean age at diagnosis of around 70 years. This tumor shows an aggressive behavior with an overall survival period shorter than 12 months. The diagnostic hallmark is the presence of rhabdoid cells. Alterations in chromatin remodeling (SMARCB1) and in the centrosome structure (CROCC) are reported in RC usually BRAF^mut and MSI-H. RKO intestinal neoplastic cells culture (BRAF^mut, SMARCB1^wt, MSI-H) with CROCC knockdown exhibit rhabdoid features and develop prominent projections from the edge of the cell.

METHODS

Here, we investigated two cases of CROCC^mutSMARCB1^wt RC by scanning and transmission electron microscopy (SEM, TEM).

RESULTS

TEM confirmed the diagnostic presence of intermediate cytoplasmic filaments and nucleolar margination. SEM showed cellular protrusions (lamellipodia) in the intercellular spaces not evident at light microscopy.

CONCLUSIONS

These protrusions CROCC-related might represent the pathogenetic mechanism underlying the rhabdoid aggressive behavior, independently of tumor staging. To our knowledge, the SEM technique was applied in the study of this neoplasm for the first time.

Targeting non-muscle myosin II promotes corneal endothelial migration through regulating lamellipodial dynamics

Corneal endothelial cell (CEC) dysfunction causes corneal edema that may lead to blindness. In addition to corneal transplantation, simple descemetorhexis has been proposed to treat centrally located disease with adequate peripheral cell reserve, but promoting the centripetal migration of CECs is pivotal to this strategy. Here, we show that targeting non-muscle myosin II (NMII) activity by Y27632, a ROCK inhibitor, or blebbistatin, a selective NMII inhibitor, promotes directional migration of CECs and accelerates in vitro wound healing. The lamellipodial protrusion persistence is increased, and actin retrograde flow is decreased after NMII inhibition. Counteracting lamellipodial protrusion by actin-related protein 2/3 (ARP2/3) inhibitor abolishes this migration-promoting effect. Although both Y27632 and blebbistatin accelerate wound healing, cell junctional integrity and barrier function are better preserved after blebbistatin treatment, leading to more rapid corneal deturgescence in rabbit corneal endothelial wounding model. Our findings indicate that NMII is a promising therapeutic target in the treatment of CEC dysfunction. KEY MESSAGES: NMII inhibition promotes directional migration and wound healing of CECs in vitro. Lamellipodial protrusion persistence is increased after NMII inhibition. Selective NMII inhibitor preserves junctional integrity better than ROCK inhibitor. Selective NMII inhibitor accelerates corneal deturgescence after wounding in vivo.

P-Rex1 Overexpression Results in Aberrant Neuronal Polarity and Psychosis-Related Behaviors

Neuronal polarity is involved in multiple developmental stages, including cortical neuron migration, multipolar-to-bipolar transition, axon initiation, apical/basal dendrite differentiation, and spine formation. All of these processes are associated with the cytoskeleton and are regulated by precise timing and by controlling gene expression. The P-Rex1 (phosphatidylinositol-3,4,5-trisphosphate dependent Rac exchange factor 1) gene for example, is known to be important for cytoskeletal reorganization, cell motility, and migration. Deficiency of P-Rex1 protein leads to abnormal neuronal migration and synaptic plasticity, as well as autism-related behaviors. Nonetheless, the effects of P-Rex1 overexpression on neuronal development and higher brain functions remain unclear. In the present study, we explored the effect of P-Rex1 overexpression on cerebral development and psychosis-related behaviors in mice. In utero electroporation at embryonic day 14.5 was used to assess the influence of P-Rex1 overexpression on cell polarity and migration. Primary neuron culture was used to explore the effects of P-Rex1 overexpression on neuritogenesis and spine morphology. In addition, P-Rex1 overexpression in the medial prefrontal cortex (mPFC) of mice was used to assess psychosis-related behaviors. We found that P-Rex1 overexpression led to aberrant polarity and inhibited the multipolar-to-bipolar transition, leading to abnormal neuronal migration. In addition, P-Rex1 overexpression affected the early development of neurons, manifested as abnormal neurite initiation with cytoskeleton change, reduced the axon length and dendritic complexity, and caused excessive lamellipodia in primary neuronal culture. Moreover, P-Rex1 overexpression decreased the density of spines with increased height, width, and head area in vitro and in vivo. Behavioral tests showed that P-Rex1 overexpression in the mouse mPFC caused anxiety-like behaviors and a sensorimotor gating deficit. The appropriate P-Rex1 level plays a critical role in the developing cerebral cortex and excessive P-Rex1 might be related to psychosis-related behaviors.

An influenza-derived membrane tension-modulating peptide regulates cell movement and morphology via actin remodeling

Tension in cell membranes is closely related to various cellular events, including cell movement and morphogenesis. Therefore, modulation of membrane tension can be a new approach for manipulating cellular events. Here, we show that an amphipathic peptide derived from the influenza M2 protein (M2[45-62]) yields lamellipodia at multiple sites in the cell. Effect of M2[45-62] on cell membrane tension was evaluated by optical tweezer. The membrane tension sensor protein FBP17 was involved in M2[45-62]-driven lamellipodium formation. Lysine-to-arginine substitution in M2[45-62] further enhanced its activity of lamellipodium formation. M2[45-62] had an ability to reduce cell motility, evaluated by scratch wound migration and transwell migration assays. An increase in neurite outgrowth was also observed after treatment with M2[45-62]. The above results suggest the potential of M2[45-62] to modulate cell movement and morphology by modulating cell membrane tension.

Identification of regulatory elements recapitulating early expression of L-plastin in the zebrafish enveloping layer and embryonic periderm

We have cloned and characterized an intronic fragment of zebrafish lymphocyte cytosolic protein 1 (lcp1, also called L-plastin) that drives expression to the zebrafish enveloping layer (EVL). L-plastin is a calcium-dependent actin-bundling protein belonging to the plastin/fimbrin family of proteins, and is necessary for the proper migration and attachment of several adult cell types, including leukocytes and osteoclasts. However, in zebrafish lcp1 is abundantly expressed much earlier, during differentiation of the EVL. The cells of this epithelial layer migrate collectively, spreading vegetally over the yolk. L-plastin expression persists into the larval periderm, a transient epithelial tissue that forms the first larval skin. This finding establishes that L-plastin is activated in two different embryonic waves, with a distinct regulatory switch between the early EVL and the later leukocyte. To better study L-plastin expressing cells we attempted CRISPR/Cas9 homology-driven recombination (HDR) to insert a self-cleaving peptide (Cre-P2A-EGFP-CAAX) downstream of the native lcp1 promoter. This produced a stable zebrafish line expressing Cre recombinase in EVL nuclei and green fluorescence in EVL cell membranes. In vivo tracking of these labeled cells provided enhanced views of EVL migration behavior, membrane extensions, and mitotic events. Finally, we experimentally dissected key elements of the targeted lcp1 locus, discovering a ∼300 bp intronic sequence sufficient to drive EVL expression. The lcp1: Cre-P2A-EGFP-CAAX zebrafish should be useful for studying enveloping layer specification, gastrulation movements and periderm development in this widely used vertebrate model. In addition, the conserved regulatory sequences we have isolated predict that L-plastin orthologs may have a similar early expression pattern in other vertebrate embryos.

Does self-organized criticality drive leading edge protrusion?

Arp2/3 complex nucleates dendritic actin networks and plays a pivotal role in the formation of lamellipodia at the leading edge of motile cells. Mouse fibroblasts lacking functional Arp2/3 complex have the characteristic smooth, veil-like lamellipodial leading edge of wild-type cells replaced by a massive, bifurcating filopodia-like protrusions (FLPs) with fractal geometry. The nanometer-scale actin-network organization of these FLPs can be linked to the fractal geometry of the cell boundary by a self-organized criticality through the bifurcation behavior of cross-linked actin bundles. Despite the pivotal role of the Arp2/3 complex in cell migration, the cells lacking functional Arp2/3 complex migrate at rates similar to wild-type cells. However, these cells display defects in the persistence of a directional movement. We suggest that Arp2/3 complex suppresses the formation of FLPs by locally fine-tuning actin networks and favoring dendritic geometry over bifurcating bundles, giving cells a distinct evolutionary edge by providing the means for a directed movement.

Cortical Actin Dynamics in Endothelial Permeability

The pulmonary endothelial cell forms a critical semi-permeable barrier between the vascular and interstitial space. As part of the blood-gas barrier in the lung, the endothelium plays a key role in normal physiologic function and pathologic disease. Changes in endothelial cell shape, defined by its plasma membrane, determine barrier integrity. A number of key cytoskeletal regulatory and effector proteins including non-muscle myosin light chain kinase, cortactin, and Arp 2/3 mediate actin rearrangements to form cortical and membrane associated structures in response to barrier enhancing stimuli. These actin formations support and interact with junctional complexes and exert forces to protrude the lipid membrane to and close gaps between individual cells. The current knowledge of these cytoskeletal processes and regulatory proteins are the subject of this review. In addition, we explore novel advancements in cellular imaging that are poised to shed light on the complex nature of pulmonary endothelial permeability.

Sphingolipids Signaling in Lamellipodia Formation and Enhancement of Endothelial Barrier Function

Sphingolipids, first described in the brain in 1884, are important structural components of biological membranes of all eukaryotic cells. In recent years, several lines of evidence support the critical role of sphingolipids such as sphingosine, sphingosine-1-phosphate (S1P), and ceramide as anti- or pro-inflammatory bioactive lipid mediators in a variety of human pathologies including pulmonary and vascular disorders. Among the sphingolipids, S1P is a naturally occurring agonist that exhibits potent barrier enhancing property in the endothelium by signaling via G protein-coupled S1P1 receptor. S1P, S1P analogs, and other barrier enhancing agents such as HGF, oxidized phospholipids, and statins also utilize the S1P/S1P1 signaling pathway to generate membrane protrusions or lamellipodia, which have been implicated in resealing of endothelial gaps and maintenance of barrier integrity. A better understanding of sphingolipids mediated regulation of lamellipodia formation and barrier enhancement of the endothelium will be critical for the development of sphingolipid-based therapies to alleviate pulmonary disorders such as sepsis-, radiation-, and mechanical ventilation-induced acute lung injury.

SOD1 stimulates lamellipodial protrusions in Neuro 2A cell lines

We here investigated the effects of overexpressed superoxide dismutase (SOD)1 and amyotrophic lateral sclerosis (ALS)-linked SOD1 mutants G93A and G147S in Neuro 2A (N2A) cell lines, and found a three-fold increase in lamellipodia either in cells cultured under differentiated or undifferentiated growth conditions. In undifferentiated N2A cells, SOD1 constructs promoted lamellipodial protrusions to similar extent as the overexpression of Rac1, and SOD1-mediated lamellipodia were prevented by coexpression of the N17 dominant-negative form of Rac1, or shRNA for a downstream effector of Rac1, the insulin receptor tyrosine kinase substrate p53 (IRSp53) or its binding partner LIN7. Moreover, no additive effect was measured by coexpression of the SOD1 constructs with Rac1, IRSp53 or LIN7. Collectively these data support a role for SOD1 in the regulation of Rac1-mediated lamellipodia pathway, a property fully retained by the two SOD1 mutants.

Transforming growth factor-β modulates pancreatic cancer associated fibroblasts cell shape, stiffness and invasion

BACKGROUND

Tumor microenvironment consists of the extracellular matrix (ECM), stromal cells, such as fibroblasts (FBs) and cancer associated fibroblasts (CAFs), and a myriad of soluble factors. In many tumor types, including pancreatic tumors, the interplay between stromal cells and the other tumor microenvironment components leads to desmoplasia, a cancer-specific type of fibrosis that hinders treatment. Transforming growth factor beta (TGF-β) and CAFs are thought to play a crucial role in this tumor desmoplastic reaction, although the involved mechanisms are unknown.

METHODS

Optical/fluorescence microscopy, atomic force microscopy, image processing techniques, invasion assay in 3D collagen I gels and real-time PCR were employed to investigate the effect of TGF-β on normal pancreatic FBs and CAFs with regard to crucial cellular morphodynamic characteristics and relevant gene expression involved in tumor progression and metastasis.

RESULTS

CAFs present specific myofibroblast-like characteristics, such as α-smooth muscle actin expression and cell elongation, they also form more lamellipodia and are softer than FBs. TGF-β treatment increases cell stiffness (Young's modulus) of both FBs and CAFs and increases CAF's (but not FB's) elongation, cell spreading, lamellipodia formation and spheroid invasion. Gene expression analysis shows that these morphodynamic characteristics are mediated by Rac, RhoA and ROCK expression in CAFs treated with TGF-β.

CONCLUSIONS

TGF-β modulates CAFs', but not FBs', cell shape, stiffness and invasion.

GENERAL SIGNIFICANCE

Our findings elucidate on the effects of TGF-β on CAFs' behavior and stiffness providing new insights into the mechanisms involved.

An In Vitro System to Study the Mesenchymal-to-Amoeboid Transition

During the last few years, significant attention has been given to the plasticity of cell migration, i.e., the ability of individual cell to switch between different motility modes, in particular between mesenchymal and amoeboid motilities. This phenomenon is called the mesenchymal-to-amoeboid transition (MAT). Such a plasticity of cell migration is a mechanism, by which cancer cells can adapt their migration mode to different microenvironments and thus it may promote tumor dissemination. It was shown that interventions at certain regulatory points of mesenchymal motility as well as alterations of environmental conditions can trigger MAT. One of the approaches to induce MAT is to mechanically confine cells and one of the simplest ways to achieve this is to cultivate cells under agarose. This method does not require any special tool, is easily reproducible and allows cell tracking by videomicroscopy. We describe here a protocol, where MAT is associated with chemotaxis.

NCAM affects directional lamellipodia formation of BMSCs via β1 integrin signal-mediated cofilin activity

The neural cell adhesion molecule (NCAM), a key member of the immunoglobulin-like CAM family, was reported to regulate the migration of bone marrow-derived mesenchymal stem cells (BMSCs). However, the detailed cellular behaviors including lamellipodia formation in the initial step of directional migration remain largely unknown. In the present study, we reported that NCAM affects the lamellipodia formation of BMSCs. Using BMSCs from Ncam knockout mice we found that Ncam deficiency significantly impaired the migration and the directional lamellipodia formation of BMSCs. Further studies revealed that Ncam knockout decreased the activity of cofilin, an actin-cleaving protein, which was involved in directional protrusions. To explore the molecular mechanisms involved, we examined protein tyrosine phosphorylation levels in Ncam knockout BMSCs by phosphotyrosine peptide array analyses, and found that the tyrosine phosphorylation level of β1 integrin, a protein upstream of cofilin, was greatly upregulated in Ncam-deficient BMSCs. Notably, by blocking the function of β1 integrin with RGD peptide or ROCK inhibitor, the cofilin activity and directional lamellipodia formation of Ncam knockout BMSCs could be rescued. Finally, we found that the effect of NCAM on tyrosine phosphorylation of β1 integrin was independent of the fibroblast growth factor receptor. These results indicated that NCAM regulates directional lamellipodia formation of BMSCs through β1 integrin signal-mediated cofilin activity.

Localization of dynamin-related protein 1 and its potential role in lamellipodia formation

Dynamin-related protein1 (Drp1) plays an essential role in mitochondrial fission: Cytosolic Drp1 is translocated to the mitochondria upon stimulus, and oligomerized Drp1 constricts mitochondria by aid of actin filaments. Drp1 completes the fission process with GTP hydrolysis by its own GTPase activity. The importance of actin filament and its interaction with Drp1 in the mitochondrial fission process have been demonstrated. In this study, we found that Drp1 is enriched in the actin-rich leading edge of lamellipodia of mouse embryonic fibroblasts (MEFs) wherein mitochondria or peroxisomes are absent. Mff-binding mutant (A395D) of Drp1, which cannot be recruited to mitochondria, was also localized in lamellipodia, indicating that Drp1 in lamellipodia is not related to mitochondria. When lamellipodia formation was induced by platelet-derived growth factor (PDGF) in MEFs, S616 phosphorylated form of Drp1 was accumulated to the lamellipodia. Inhibition of Drp1 with Mdivi-1 or a specific shRNA significantly decreased PDGF-induced lamellipodia formation or initial cell spreading during re-plating of the cells, respectively. Interestingly, defective lamellipodia formation and cell adhesion caused by Drp1 inhibition were not rescued by supplementing L-carnitine, although it restored mitochondrial energy loss caused by Drp1 inhibition. Collectively, these results favor the idea that Drp1 might play a significant role in lamellipodia formation and cell spreading through a different mechanism from that used for regulating mitochondrial dynamics/function.

Deficiency in ubiquitin-like protein Ubl4A impairs migration of fibroblasts and macrophages

Ubiquitin-like protein Ubl4A is a small, multi-functional protein with no ubiquitination activity. We have previously demonstrated that Ubl4A directly interacts with actin-related protein 2/3 complex (Arp2/3) and promotes Arp2/3-dependent actin branching, thereby accelerating plasma membrane translocation of protein kinase Akt upon insulin stimulation. Here, we show that Ubl4A is critical for plasma membrane protrusion and cell migration. Ubl4A, F-actin and Arp2/3 are co-localized at the cell leading edges during wound closure. Knockout of Ubl4A significantly reduces actin-mediated membrane protrusion and delays wound healing by primary mouse embryonic fibroblasts. Consistently, the ability of fibroblasts to migrate out of corneal tissue ex vivo is also impaired in Ubl4A-deficient mice. Furthermore, cell motility, but not phagocytosis, is significantly decreased in Ubl4A-deficient macrophages compared with wild-type controls. These results imply an important role for Ubl4A in cell migration-associated pathophysiological processes.

Actin filament-microtubule interactions in axon initiation and branching

Neurons begin life as spherical cells. A major hallmark of neuronal development is the formation of elongating processes from the cell body which subsequently differentiate into dendrites and the axon. The formation and later development of neuronal processes is achieved through the concerted organization of actin filaments and microtubules. Here, we review the literature regarding recent advances in the understanding of cytoskeletal interactions in neurons focusing on the initiation of processes from neuronal cell bodies and the collateral branching of axons. The complex crosstalk between cytoskeletal elements is mediated by a cohort of proteins that either bind both cytoskeletal systems or allow one to regulate the other. Recent studies have highlighted the importance of microtubule plus-tip proteins in the regulation of the dynamics and organization of actin filaments, while also providing a mechanism for the subcellular capture and guidance of microtubule tips by actin filaments. Although the understanding of cytoskeletal crosstalk and interactions in neuronal morphogenesis has advanced significantly in recent years the appreciation of the neuron as an integrated cytoskeletal system remains a frontier.

Lamellipodia are crucial for haptotactic sensing and response

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.

Screening and target identification of bioactive compounds that modulate cell migration and autophagy

Cell migration is a fundamental step for embryonic development, wound repair, immune responses, and tumor cell invasion and metastasis. It is well known that protrusive structures, namely filopodia and lamellipodia, can be observed at the leading edge of migrating cells. The formation of these structures is necessary for cell migration; however, the molecular mechanisms behind the formation of these structures remain largely unclear. Therefore, bioactive compounds that modulate protrusive structures are extremely powerful tools for studying the mechanisms behind the formation of these structures and subsequent cell migration. Therefore, we have screened for bioactive compounds that inhibit the formation of filopodia, lamellipodia, or cell migration from natural products, and attempted to identify the target molecules of our isolated compounds. Additionally, autophagy is a bulk, non-specific protein degradation system that is involved in the pathogenesis of cancer and neurodegenerative disorders. Recent extensive studies have revealed the molecular mechanisms of autophagy, however, they also remain largely unclear. Thus, we also have screened for bioactive compounds that modulate autophagy, and identified the target molecules. In the present article, we introduce the phenotypic screening system and target identification of four bioactive compounds.

Actin Filament Structures in Migrating Cells

Cell migration is necessary for several developmental processes in multicellular organisms. Furthermore, many physiological processes such as wound healing and immunological events in adult animals are dependent on cell migration. Consequently, defects in cell migration are linked to various diseases including immunological disorders as well as cancer progression and metastasis formation. Cell migration is driven by specific protrusive and contractile actin filament structures, but the types and relative contributions of these actin filament arrays vary depending on the cell type and the environment of the cell. In this chapter, we introduce the most important actin filament structures that contribute to mesenchymal and amoeboid cell migration modes and discuss the mechanisms by which the assembly and turnover of these structures are controlled by various actin-binding proteins.

Depletion or over-expression of Sh3px1 results in dramatic changes in cell morphology

The mammalian Sorting Nexin 9 (Snx9) family consists of three paralogs: Snx9, Snx18 and Snx33. Most of the published literature to date has centered on the role of Snx9 in clathrin-mediated endocytosis (CME). Snx9 contains an Sh3 domain at its N-terminus and has been shown to interact with Dynamin and actin nucleation factors via this domain. In addition to the Sh3 domain, Snx9 also contains a C-terminal BAR domain. BAR domains are known to sense and/or induce membrane curvature. In addition to endocytosis, recent studies have implicated the Snx9 family in diverse processes such as autophagy, macropinocytosis, phagocytosis and mitosis. The Snx9 family is encoded by a single gene in Drosophila called sh3px1. In this report, we present our initial characterization of sh3px1. We found that depletion of Sh3px1 from Drosophila Schneider 2 (S2) cells resulted in defective lamellipodia formation. A similar phenotype has been reported upon depletion of Scar, the actin nucleation factor implicated in forming lamellipodia. In addition, we demonstrate that over-expression of Sh3px1 in S2 cells results in the formation of tubules as well as long protrusions. Formation of these structures required the C-terminal BAR domain as well as the adjacent Phox homology (PX) domain of Sh3px1. Furthermore, efficient protrusion formation by Sh3px1 required the actin nucleation factor Wasp. Tubules and protrusions were also generated upon over-expressing the mammalian orthologs Snx18 and Snx33 in S2 cells. By contrast, over-expressing Snx9 mostly induced long tubules.

Summary: Proteins containing BAR domains are known to generate membrane curvature. Some BAR domains generate tubules upon over-expression in cells, whereas others generate membrane protrusions. We demonstrate that Sh3px1, the Drosophila ortholog of the Snx9 family, is capable of inducing both tubules and protrusions.

Initiation of lamellipodia and ruffles involves cooperation between mDia1 and the Arp2/3 complex

Protrusion of lamellipodia and ruffles requires polymerization of branched actin filaments by the Arp2/3 complex. Although regulation of Arp2/3 complex activity has been extensively investigated, the mechanism of initiation of lamellipodia and ruffles remains poorly understood. Here, we show that mDia1 acts in concert with the Arp2/3 complex to promote initiation of lamellipodia and ruffles. We find that mDia1 is an epidermal growth factor (EGF)-regulated actin nucleator involved in membrane ruffling using a combination of knockdown and rescue experiments. At the molecular level, mDia1 polymerizes linear actin filaments, activating the Arp2/3 complex, and localizes within nascent and mature membrane ruffles. We employ functional complementation experiments and optogenetics to show that mDia1 cooperates with the Arp2/3 complex in initiating lamellipodia and ruffles. Finally, we show that genetic and pharmacological interference with this cooperation hampers ruffling and cell migration. Thus, we propose that the lamellipodium- and ruffle-initiating machinery consists of two actin nucleators that act sequentially to regulate membrane protrusion and cell migration.

Chemistry and biology of the compounds that modulate cell migration

Cell migration is a fundamental step for embryonic development, wound repair, immune responses, and tumor cell invasion and metastasis. Extensive studies have attempted to reveal the molecular mechanisms behind cell migration; however, they remain largely unclear. Bioactive compounds that modulate cell migration show promise as not only extremely powerful tools for studying the mechanisms behind cell migration but also as drug seeds for chemotherapy against tumor metastasis. Therefore, we have screened cell migration inhibitors and analyzed their mechanisms for the inhibition of cell migration. In this mini-review, we introduce our chemical and biological studies of three cell migration inhibitors: moverastin, UTKO1, and BU-4664L.

Rho-associated kinase signaling is required for osteopontin-induced cell invasion through inactivating cofilin in human non-small cell lung cancer cell lines

Osteopontin (OPN) is involved in tumor progression such as invasion and metastasis, and poor prognosis of lung cancer. However, how OPN affects the invasive behavior of lung cancer is not well defined. Here, we examined the underlying molecular mechanism of OPN-induced invasion in human non-small cell lung cancer (NSCLC) cell lines including A549 cells. OPN markedly increased the phosphorylation of LIM kinase 1 and 2 (LIMK1/2), and cofilin without affecting their total forms. The expression of Rho-associated kinase 1 (ROCK1), but not p21-activated kinase 1 and 2 (PAK1/2), was increased by OPN treatment as an upstream effector of LIMK/cofilin. The phosphorylation of cofilin by OPN was suppressed when cells were pretreated with ROCK inhibitor Y27632 by Western blot. Moreover, it verified that OPN inactivated cofilin through ROCK signaling in other NSCLC cell lines. OPN induced the phosphorylation of FAK and AKT. FAK inhibitor FAKi-14 and PI3K inhibitor wortmannin decreased the expressions of ROCK1, and phosphorylation of LIMK1/2 and cofilin. OPN caused a significant increase in the lamellipodia formation and cell invasion, and these are suppressed by FAK inhibitor FAKi-14, PI3K inhibitor wortmannin and ROCK inhibitor Y27632. Taken together, these results suggest that OPN triggers ROCK signaling mediated by FAK/PI3K/AKT pathway, which in turn induces the lamellipodia formation to allow the invasion of lung cancer cells through inactivating cofilin.

Emerging roles of protocadherins: from self-avoidance to enhancement of motility

Protocadherins are a group of transmembrane proteins belonging to the cadherin superfamily that are subgrouped into 'clustered' and 'non-clustered' protocadherins. Although cadherin superfamily members are known to regulate various forms of cell-cell interactions, including cell-cell adhesion, the functions of protocadherins have long been elusive. Recent studies are, however, uncovering their unique roles. The clustered protocadherins regulate neuronal survival, as well as dendrite self-avoidance. Combinatorial expression of clustered protocadherin isoforms creates a great diversity of adhesive specificity for cells, and this process is likely to underlie the dendritic self-avoidance. Non-clustered protocadherins promote cell motility rather than the stabilization of cell adhesion, unlike the classic cadherins, and mediate dynamic cellular processes, such as growth cone migration. Protocadherin dysfunction in humans is implicated in neurological disorders, such as epilepsy and mental retardation. This Commentary provides an overview of recent findings regarding protocadherin functions, as well as a discussion of the molecular basis underlying these functions.

Supervillin binds the Rac/Rho-GEF Trio and increases Trio-mediated Rac1 activation

We investigated cross-talk between the membrane-associated, myosin II-regulatory protein supervillin and the actin-regulatory small GTPases Rac1, RhoA, and Cdc42. Supervillin knockdown reduced Rac1-GTP loading, but not the GTP loading of RhoA or Cdc42, in HeLa cells with normal levels of the Rac1-activating protein Trio. No reduction in Rac1-GTP loading was observed when supervillin levels were reduced in Trio-depleted cells. Conversely, overexpression of supervillin isoform 1 (SV1) or, especially, isoform 4 (SV4) increased Rac1 activation. Inhibition of the Trio-mediated Rac1 guanine nucleotide exchange activity with ITX3 partially blocked the SV4-mediated increase in Rac1-GTP. Both SV4 and SV1 co-localized with Trio at or near the plasma membrane in ruffles and cell surface projections. Two sequences within supervillin bound directly to Trio spectrin repeats 4-7: SV1-171, which contains N-terminal residues found in both SV1 and SV4 and the SV4-specific differentially spliced coding exons 3, 4, and 5 within SV4 (SV4-E345; SV4 amino acids 276-669). In addition, SV4-E345 interacted with the homologous sequence in rat kalirin (repeats 4-7, amino acids 531-1101). Overexpressed SV1-174 and SV4-E345 affected Rac1-GTP loading, but only in cells with endogenous levels of Trio. Trio residues 771-1057, which contain both supervillin-interaction sites, exerted a dominant-negative effect on cell spreading. Supervillin and Trio knockdowns, separately or together, inhibited cell spreading, suggesting that supervillin regulates the Rac1 guanine nucleotide exchange activity of Trio, and potentially also kalirin, during cell spreading and lamellipodia extension.

Epithelial morphogenesis: the mouse eye as a model system

Morphogenesis is the developmental process by which tissues and organs acquire the shape that is critical to their function. Here, we review recent advances in our understanding of the mechanisms that drive morphogenesis in the developing eye. These investigations have shown that regulation of the actin cytoskeleton is central to shaping the presumptive lens and retinal epithelia that are the major components of the eye. Regulation of the actin cytoskeleton is mediated by Rho family GTPases, by signaling pathways and indirectly, by transcription factors that govern the expression of critical genes. Changes in the actin cytoskeleton can shape cells through the generation of filopodia (that, in the eye, connect adjacent epithelia) or through apical constriction, a process that produces a wedge-shaped cell. We have also learned that one tissue can influence the shape of an adjacent one, probably by direct force transmission, in a process we term inductive morphogenesis. Though these mechanisms of morphogenesis have been identified using the eye as a model system, they are likely to apply broadly where epithelia influence the shape of organs during development.

Caveolin-1 induces lamellipodia formation via an Akt-dependent pathway

BACKGROUND

The enhancement of migration is critical for facilitating cancer cell metastasis.

METHOD

Lung cancer H23 cells were transfected with either a caveolin-1 (Cav-1) overexpression or shCav-1 plasmid and further subjected to cell migration assays and lamellipodia characterization. The regulation of Cav-1 via an ATP-dependent tyrosine kinase (Akt) pathway was further examined by Akt knockdown in Cav-1 overexpressing cells and migratory behavior investigations.

RESULTS

Here, we demonstrate for the first time that overexpression of Cav-1 in human lung cancer H23 cells significantly increased the formation of lamellipodia, whereas the suppression of Cav-1 using shRNA transfection had the opposite effect. Consistent with an increase in lamellipodia, Cav-1 overexpressing cells exhibited increased migratory activity in comparison to their parental, control-transfected, H23 cells. The induction of lamellipodia was demonstrated to occur via the Akt pathway because the addition of the Akt inhibitor LY294002 inhibited lamellipodia in both Cav-1-overexpressing and H23 cells. Additionally, transient transfection with Akt-siRNA significantly inhibited the formation of lamellipodia and the migration of Cav-1-overexpressing H23 cells. In addition, Cav-1 levels and the migratory action of other lung cancer cells, namely, H460 and A549, were assessed, and the migration of these cells was found to be correlated with the basal Cav-1 level.

CONCLUSION

These data showed that Cav-1 enhances cancer cell migration through Akt-mediated lamellipodia formation. Our results provide novel insights regarding the molecular mechanism controlling cancer cell migration, leading to a better understanding of cancer cell biology.

Preferential targeting of p39-activated Cdk5 to Rac1-induced lamellipodia

Cdk5 is a member of the cyclin-dependent kinase (Cdk) family that plays a role in various neuronal activities including brain development, synaptic regulation, and neurodegeneration. Cdk5 requires the neuronal specific activators, p35 and p39 for subcellular compartmentalization. However, it is not known how active Cdk5 is recruited to F-actin cytoskeleton, which is a Cdk5 target. Here we found p35 and p39 localized to F-actin rich regions of the plasma membrane and investigated the underlying targeting mechanism in vitro by expressing them with Rho family GTPases in Neuro2A cells. Both p35 and p39 accumulated at the cell peripheral lamellipodia and perinuclear regions, where active Rac1 is localized. Interestingly, p35 and p39 displayed different localization patterns as p35 was found more at the perinuclear region and p39 was found more in peripheral lamellipodia. We then confirmed this distinct localization in primary hippocampal neurons. We also determined that the localization of p39 to lamellipodia requires myristoylation and Lys clusters within the N-terminal p10 region. Additionally, we found that p39-Cdk5, but not p35-Cdk5 suppressed lamellipodia formation by reducing Rac1 activity. These results suggest that p39-Cdk5 has a dominant role in Rac1-dependent lamellipodial activity.

Denbinobin, a phenanthrene from Dendrobium nobile, impairs prostate cancer migration by inhibiting Rac1 activity

Prostate cancer is the most prevalent type of cancer in the United States. The most common site of prostate cancer metastasis is bone. CXCL12 is preferentially expressed in bone and is targeted by prostate cancer cells, which over-express the receptor for CXCL12, CXCR4. In response to CXCL12 stimulation, Rac1, a GTPase, along with its effectors, regulates actin polymerization to form lamellipodia, which is a critical event for cell migration. Cortactin, an actin-binding protein, is recruited to the lamellipodia and is phosphorylated at tyrosine residues. The phosphorylated cortactin is also involved in cell migration. The inhibition of Rac1 activity using a dominant negative Rac1 impairs lamellipodial protrusion as well as cortactin translocation and cortactin phosphorylation. Denbinobin, a substance extracted from Dendrobium nobile, has anticancer effects in many cancer cell lines. Whether denbinobin can inhibit prostate cancer cell migration is not clear. Here, we report that denbinobin inhibited Rac1 activity. The inhibition of Rac1 activity prevented lamellipodial formation. Cortactin phosphorylation and translocation to the lamellipodia were also impaired, and PC3 cells were unable to migrate. These results indicate that denbinobin prevents CXCL12-induced PC3 cell migration by inhibiting Rac1 activity.

Systematic single cell analysis of migration and morphological changes of human neutrophils over stimulus concentration gradients

To compare the responses of individual neutrophils to chemoattractants, migration pathway data were obtained using TAXIScan, an optically accessible/horizontal apparatus in which a concentration gradient is established reproducibly for a given stimulus. The observed linear-mode trajectory pattern of neutrophils toward N-formyl-methionyl-leucyl-phenylalanine (fMLP) or Interleukin (IL)-8/CXCL8 was distinguished from random migration patterns toward leukotriene (LT) B4 or platelet activating factor (PAF). The median values of velocity and directionality calculated for individual cells toward fMLP and IL-8 were both relatively similar and high, whereas the values toward LTB4 and PAF were widely dispersed over a lower range of directionality and from low to high ranges of velocity. The different patterns between the groups may be explained by unique morphology with single polarity toward fMLP and IL-8, and unstable morphology with multiple polarities toward LTB4 and PAF. Unique morphologies toward fMLP and IL-8 were not affected by coexisting LTB4 or PAF. On the other hand, the addition of suboptimum concentrations of fMLP or IL-8 to LTB4 or PAF induced a nearly maximum chemotactic response in most cells. These data suggest that exogenous formyl peptides and endogenous chemokines augment neutrophil accumulation at inflammation sites, whereas lipid mediators may play a role in supporting activation of the inflammatory cells for recruitment.