GMF promotes leading-edge dynamics and collective cell migration in vivo

Direct observation of cortactin protecting Arp2/3-actin filament branch junctions from GMF-mediated destabilization

How cells tightly control the formation and turnover of branched actin filament arrays to drive cell motility, endocytosis, and other cellular processes is still not well understood. Here, we investigated the mechanistic relationship between two binding partners of the Arp2/3 complex, glia maturation factor (GMF) and cortactin. Individually, GMF and cortactin have opposite effects on the stability of actin filament branches, but it is unknown how they work in concert with each other to govern branch turnover. Using TIRF microscopy, we observe that GMF's branch destabilizing activities are potently blocked by cortactin (IC50 = 1.3 nM) and that this inhibition requires direct interactions of cortactin with Arp2/3 complex. The simplest model that would explain these results is competition for binding Arp2/3 complex. However, we find that cortactin and GMF do not compete for free Arp2/3 complex in solution. Further, we use single molecule analysis to show that cortactin's on-rate (3 ×107 s-1 M-1) and off-rate (0.03 s-1) at branch junctions are minimally affected by excess GMF. Together, these results show that cortactin binds with high affinity to branch junctions, where it blocks the destabilizing effects of GMF, possibly by a mechanism that is allosteric in nature. In addition, the affinities we measure for cortactin at actin filament branch junctions (Kd = 0.9 nM) and filament sides (Kd = 206 nM) are approximately 20-fold stronger than previously reported. These observations contribute to an emerging view of molecular complexity in how Arp2/3 complex is regulated through the integration of multiple inputs.

The stabilization of Arp2/3 complex generated actin filaments

The Arp2/3 complex, which generates both branched but also linear actin filaments via activation of SPIN90, is evolutionarily conserved in eukaryotes. Several factors regulate the stability of filaments generated by the Arp2/3 complex to maintain the dynamics and architecture of actin networks. In this review, we summarise recent studies on the molecular mechanisms governing the tuning of Arp2/3 complex nucleated actin filaments, which includes investigations using microfluidics and single-molecule imaging to reveal the mechanosensitivity, dissociation and regeneration of actin branches. We also discuss the high-resolution cryo-EM structure of cortactin bound to actin branches, as well as the differences and similarities between the stability of Arp2/3 complex nucleated branches and linear filaments. These new studies provide a clearer picture of the stabilisation of Arp2/3 nucleated filaments at the molecular level. We also identified gaps in our understanding of how different factors collectively contribute to the stabilisation of Arp2/3 complex-generated actin networks.

Mechanisms of actin disassembly and turnover

Cellular actin networks exhibit a wide range of sizes, shapes, and architectures tailored to their biological roles. Once assembled, these filamentous networks are either maintained in a state of polarized turnover or induced to undergo net disassembly. Further, the rates at which the networks are turned over and/or dismantled can vary greatly, from seconds to minutes to hours or even days. Here, we review the molecular machinery and mechanisms employed in cells to drive the disassembly and turnover of actin networks. In particular, we highlight recent discoveries showing that specific combinations of conserved actin disassembly-promoting proteins (cofilin, GMF, twinfilin, Srv2/CAP, coronin, AIP1, capping protein, and profilin) work in concert to debranch, sever, cap, and depolymerize actin filaments, and to recharge actin monomers for new rounds of assembly.

CD9 negatively regulates collective electrotaxis of the epidermal monolayer by controlling and coordinating the polarization of leader cells

Background

Endogenous electric fields (EFs) play an essential role in guiding the coordinated collective migration of epidermal cells to the wound centre during wound healing. Although polarization of leadercells is essential for collective migration, the signal mechanisms responsible for the EF-induced polarization of leader cells under electrotactic collective migration remain unclear. This study aims to determine how the leader cells are polarized and coordinated during EF-guided collective migration of epidermal cell sheets.

Methods

Collective migration of the human epidermal monolayer (human immortalized keratinocytes HaCaT) under EFs was observed via time-lapse microscopy. The involvement of tetraspanin-29 (CD9) in EF-induced fibrous actin (F-actin) polarization of leader cells as well as electrotactic migration of the epidermal monolayer was evaluated by genetic manipulation. Blocking, rescue and co-culture experiments were conducted to explore the downstream signalling of CD9.

Results

EFs guided the coordinated collective migration of the epithelial monolayer to the anode, with dynamic formation of pseudopodia in leader cells at the front edge of the monolayer along the direction of migration. F-actin polarization, as expected, played an essential role in pseudopod formation in leader cells under EFs. By confocal microscopy, we found that CD9 was colocalized with F-actin on the cell surface and was particularly downregulated in leader cells by EFs. Interestingly, genetic overexpression of CD9 abolished EF-induced F-actin polarization in leader cells as well as collective migration in the epidermal monolayer. Mechanistically, CD9 determined the polarization of F-actin in leader cells by downregulating a disintegrin and metalloprotease 17/heparin-binding epidermal growth factor-like growth factor/epidermal growth factor receptor (ADAM17/HB-EGF/EGFR) signalling. The abolished polarization of leader cells due to CD9 overexpression could be restored in a co-culture monolayer where normal cells and CD9-overexpressing cells were mixed; however, this restoration was eliminated again by the addition of the HB-EGF-neutralizing antibody.

Conclusion

CD9 functions as a key regulator in the EF-guided collective migration of the epidermal monolayer by controlling and coordinating the polarization of leader cells through ADAM17/HB-EGF/EGFR signalling.

Regulation of branched versus linear Arp2/3-generated actin filaments

Activation of the Arp2/3 complex by VCA-motif-bearing actin nucleation-promoting factors results in the formation of "daughter" actin filaments branching off the sides of pre-existing "mother" filaments. Alternatively, when stimulated by SPIN90, Arp2/3 directly nucleates "linear" actin filaments. Uncovering the similarities and differences between these two mechanisms is fundamental to understanding how actin cytoskeleton dynamics are regulated. Here, analysis of individual filaments reveals that, unexpectedly, the VCA motifs of WASP, N-WASP, and WASH destabilize existing branches, as well as SPIN90-Arp2/3 at linear filament ends. Furthermore, branch stabilizer cortactin and destabilizer GMF each have a similar impact on SPIN90-activated Arp2/3. However, unlike branch junctions, SPIN90-Arp2/3 at the ends of linear filaments is not destabilized by piconewton forces and does not become less stable with time. It thus appears that linear and branched Arp2/3-generated filaments respond similarly to the regulatory proteins we have tested, albeit with some differences, but significantly differ in their responses to aging and mechanical stress. These kinetic differences likely reflect the small conformational differences recently reported between Arp2/3 in branch junctions and linear filaments and suggest that their turnover in cells may be differently regulated.

Biochemical and mechanical regulation of actin dynamics

Polymerization of actin filaments against membranes produces force for numerous cellular processes, such as migration, morphogenesis, endocytosis, phagocytosis and organelle dynamics. Consequently, aberrant actin cytoskeleton dynamics are linked to various diseases, including cancer, as well as immunological and neurological disorders. Understanding how actin filaments generate forces in cells, how force production is regulated by the interplay between actin-binding proteins and how the actin-regulatory machinery responds to mechanical load are at the heart of many cellular, developmental and pathological processes. During the past few years, our understanding of the mechanisms controlling actin filament assembly and disassembly has evolved substantially. It has also become evident that the activities of key actin-binding proteins are not regulated solely by biochemical signalling pathways, as mechanical regulation is critical for these proteins. Indeed, the architecture and dynamics of the actin cytoskeleton are directly tuned by mechanical load. Here we discuss the general mechanisms by which key actin regulators, often in synergy with each other, control actin filament assembly, disassembly, and monomer recycling. By using an updated view of actin dynamics as a framework, we discuss how the mechanics and geometry of actin networks control actin-binding proteins, and how this translates into force production in endocytosis and mesenchymal cell migration.

Single-cell transcriptomics identifies Keap1-Nrf2 regulated collective invasion in a Drosophila tumor model

Apicobasal cell polarity loss is a founding event in epithelial-mesenchymal transition and epithelial tumorigenesis, yet how pathological polarity loss links to plasticity remains largely unknown. To understand the mechanisms and mediators regulating plasticity upon polarity loss, we performed single-cell RNA sequencing of Drosophila ovaries, where inducing polarity-gene l(2)gl-knockdown (Lgl-KD) causes invasive multilayering of the follicular epithelia. Analyzing the integrated Lgl-KD and wildtype transcriptomes, we discovered the cells specific to the various discernible phenotypes and characterized the underlying gene expression. A genetic requirement of Keap1-Nrf2 signaling in promoting multilayer formation of Lgl-KD cells was further identified. Ectopic expression of Keap1 increased the volume of delaminated follicle cells that showed enhanced invasive behavior with significant changes to the cytoskeleton. Overall, our findings describe the comprehensive transcriptome of cells within the follicle cell tumor model at the single-cell resolution and identify a previously unappreciated link between Keap1-Nrf2 signaling and cell plasticity at early tumorigenesis.

Mind bomb 2 promotes cell migration and epithelial structure by regulating adhesion complexes and the actin cytoskeleton

Cell migration is essential in animal development and co-opted during metastasis and inflammatory diseases. Some cells migrate collectively, which requires them to balance epithelial characteristics such as stable cell-cell adhesions with features of motility like rapid turnover of adhesions and dynamic cytoskeletal structures. How this is regulated is not entirely clear but important to understand. While investigating Drosophila oogenesis, we found that the putative E3 ubiquitin ligase, Mind bomb 2 (Mib2), is required to promote epithelial stability and the collective cell migration of border cells. Through biochemical analysis, we identified components of Mib2 complexes, which include E-cadherin and α- and β-catenins, as well as actin regulators. We also found that three Mib2 interacting proteins, RhoGAP19D, Supervillin, and Myosin heavy chain-like, affect border cell migration. mib2 mutant main body follicle cells have drastically reduced E-cadherin-based adhesion complexes and diminished actin filaments. We conclude that Mib2 acts to stabilize E-cadherin-based adhesion complexes and promote a robust actin cytoskeletal network, which is important for maintenance of epithelial integrity. The interaction with cadherin adhesion complexes and other cytoskeletal regulators contribute to its role in collective cell migration. Since Mib2 is well conserved, it may have similar functional significance in other organisms.

Glia Maturation Factor-β Supports Liver Regeneration by Remodeling Actin Network to Enhance STAT3 Proliferative Signals

BACKGROUND & AIMS

Glia maturation factor-β (GMFB) is a bona fide member of the actin depolymerizing factor homology family. Recently, emerging evidence suggested its implication in liver diseases, but data on its role in liver remain limited.

METHODS

Assessment of GMFB in liver histology, impact on liver regeneration and hepatocyte proliferation, and the underlying molecular pathways were conducted using mouse models with acute liver injury.

RESULTS

GMFB is widely distributed in normal liver. Its expression increases within 24 hours after partial hepatectomy (PHx). Adult Gmfb knockout mice and wild-type littermates are similar in gross appearance, body weight, liver function, and histology. However, compared with wild-type control, Gmfb knockout mice post-PHx develop more serious liver damage and steatosis and have delayed liver regeneration; the dominant change in liver transcriptome at 24 hours after PHx is the significantly suppressed acute inflammation pathways; the top down-regulated gene sets relate to interleukin (IL)6/Janus kinase/signal transducer and activator of transcription 3 (STAT3) signaling. Another mouse model intoxicated with carbon tetrachloride replicated these findings. Furthermore, Gmfb knockout and wild-type groups have the similar numbers of Kupffer cells, but Gmfb knockout Kupffer cells once stimulated produce less IL6, tumor necrosis factor, and IL1β. In hepatocytes treated with IL6, GMFB associates positively with cell proliferation and STAT3/cyclin D1 activation, but without any direct interaction with STAT3. In Gmfb knockout hepatocytes, cytoskeleton-related gene expression was changed significantly, with an abnormal-appearing morphology of actin networks. In hepatocyte modeling, actin-filament turnover, STAT3 activation, and metabolite excretion show a strong reliance on the status of actin-filament organization.

CONCLUSIONS

GMFB plays a significant role in liver regeneration by promoting acute inflammatory response in Kupffer cells and by intracellularly coordinating the responsive hepatocyte proliferation.

Single-molecule analysis of actin filament debranching by cofilin and GMF

Eukaryotic cells contain branched actin networks that are essential for endocytosis, motility, and other key cellular processes. These networks, which are formed by filamentous actin and the Arp2/3 complex, must subsequently be debranched to allow network remodeling and to recycle the Arp2/3 complex. Debranching appears to be catalyzed by two different members of the actin depolymerizing factor homology protein family: cofilin and glial maturation factor (GMF). However, their mechanisms of debranching are only partially understood. Here, we used single-molecule fluorescence imaging of Arp2/3 complex and actin filaments under physiological ionic conditions to observe debranching by GMF and cofilin. We demonstrate that cofilin, like GMF, is an authentic debrancher independent of its filament-severing activity and that the debranching activities of the two proteins are additive. While GMF binds directly to the Arp2/3 complex, cofilin selectively accumulates on branch-junction daughter filaments in tropomyosin-decorated networks just prior to debranching events. Quantitative comparison of debranching rates with the known kinetics of cofilin-actin binding suggests that cofilin occupancy of a particular single actin site at the branch junction is sufficient to trigger debranching. In rare cases in which the order of departure could be resolved during GMF- or cofilin-induced debranching, the Arp2/3 complex left the branch junction bound to the pointed end of the daughter filament, suggesting that both GMF and cofilin can work by destabilizing the mother filament-Arp2/3 complex interface. Taken together, these observations suggest that GMF and cofilin promote debranching by distinct yet complementary mechanisms.

Deletion of gmfA induces keratocyte-like migration in Dictyostelium

Glia maturation factor (GMF) has been established as an inactivating factor of the actin‐related protein 2/3 (Arp2/3) complex, which regulates actin assembly. Regulation of actin assembly and reorganization is crucial for various cellular events, such as cell migration, cell division, and development. Here, to examine the roles of ADF‐H domain‐containing protein (also known as glia maturation factor; GmfA), the product of a single GMF homologous gene in Dictyostelium, gmfA‐null cells were generated. They had moderate defects in cell growth and cytokinesis. Interestingly, they showed a keratocyte‐like fan shape with a broader pseudopod, where Arp3 accumulated at higher levels than in wild‐type cells. They migrated with higher persistence, but their velocities were comparable to those of wild‐type cells. The polar pseudopods during cell division were also broader than those in wild‐type cells. However, GmfA did not localize at the pseudopods in migrating cells or the polar pseudopods in dividing cells. Adhesions of mutant cells to the substratum were much stronger than that of wild‐type cells. Although the mutant cells showed chemotaxis comparable to that of wild‐type cells, they formed disconnected streams during the aggregation stage; however, they finally formed normal fruiting bodies. These results suggest that GmfA plays a crucial role in cell migration.

The Actin-Disassembly Protein Glia Maturation Factor γ Enhances Actin Remodeling and B Cell Antigen Receptor Signaling at the Immune Synapse

Signaling by the B cell antigen receptor (BCR) initiates actin remodeling. The assembly of branched actin networks that are nucleated by the Arp2/3 complex exert outward force on the plasma membrane, allowing B cells to form membrane protrusions that can scan the surface of antigen-presenting cells (APCs). The resulting Arp2/3 complex-dependent actin retrograde flow promotes the centripetal movement and progressive coalescence of BCR microclusters, which amplifies BCR signaling. Glia maturation factor γ (GMFγ) is an actin disassembly-protein that releases Arp2/3 complex-nucleated actin filaments from actin networks. By doing so, GMFγ could either oppose the actions of the Arp2/3 complex or support Arp2/3 complex-nucleated actin polymerization by contributing to the recycling of actin monomers and Arp2/3 complexes. We now show that reducing the levels of GMFγ in human B cell lines via transfection with a specific siRNA impairs the ability of B cells to spread on antigen-coated surfaces, decreases the velocity of actin retrograde flow, diminishes the coalescence of BCR microclusters into a central cluster at the B cell-APC contact site, and decreases APC-induced BCR signaling. These effects of depleting GMFγ are similar to what occurs when the Arp2/3 complex is inhibited. This suggests that GMFγ cooperates with the Arp2/3 complex to support BCR-induced actin remodeling and amplify BCR signaling at the immune synapse.

Twinfilin uncaps filament barbed ends to promote turnover of lamellipodial actin networks

Coordinated polymerization of actin filaments provides force for cell migration, morphogenesis and endocytosis. Capping protein (CP) is a central regulator of actin dynamics in all eukaryotes. It binds to actin filament (F-actin) barbed ends with high affinity and slow dissociation kinetics to prevent filament polymerization and depolymerization. However, in cells, CP displays remarkably rapid dynamics within F-actin networks, but the underlying mechanism remains unclear. Here, we report that the conserved cytoskeletal regulator twinfilin is responsible for CP's rapid dynamics and specific localization in cells. Depletion of twinfilin led to stable association between CP and cellular F-actin arrays, as well as to its retrograde movement throughout leading-edge lamellipodia. These were accompanied by diminished F-actin turnover rates. In vitro single-filament imaging approaches revealed that twinfilin directly promotes dissociation of CP from filament barbed ends, while enabling subsequent filament depolymerization. These results uncover a bipartite mechanism that controls how actin cytoskeleton-mediated forces are generated in cells.

The Role of ADF/Cofilin in Synaptic Physiology and Alzheimer's Disease

Actin-depolymerization factor (ADF)/cofilin, a family of actin-binding proteins, are critical for the regulation of actin reorganization in response to various signals. Accumulating evidence indicates that ADF/cofilin also play important roles in neuronal structure and function, including long-term potentiation and depression. These are the most extensively studied forms of long-lasting synaptic plasticity and are widely regarded as cellular mechanisms underlying learning and memory. ADF/cofilin regulate synaptic function through their effects on dendritic spines and the trafficking of glutamate receptors, the principal mediator of excitatory synaptic transmission in vertebrates. Regulation of ADF/cofilin involves various signaling pathways converging on LIM domain kinases and slingshot phosphatases, which phosphorylate/inactivate and dephosphorylate/activate ADF/cofilin, respectively. Actin-depolymerization factor/cofilin activity is also regulated by other actin-binding proteins, activity-dependent subcellular distribution and protein translation. Abnormalities in ADF/cofilin have been associated with several neurodegenerative disorders such as Alzheimer’s disease. Therefore, investigating the roles of ADF/cofilin in the brain is not only important for understanding the fundamental processes governing neuronal structure and function, but also may provide potential therapeutic strategies to treat brain disorders.

Phosphorylation of GMFγ by c-Abl Coordinates Lamellipodial and Focal Adhesion Dynamics to Regulate Airway Smooth Muscle Cell Migration

Airway smooth muscle cells require coordinated protrusion and focal adhesion dynamics to migrate properly. However, the signaling cascades that connect these two processes remain incompletely understood. Glia maturation factor (GMF)-γ has been implicated in inducing actin debranching and inhibiting nucleation. In this study, we discovered that GMFγ phosphorylation at Y104 regulates human airway smooth muscle cell migration. Using high-resolution microscopy coupled with three-dimensional object-based quantitative image analysis software, Imaris 9.2.0, phosphomimetic mutant, Y104D-GMFγ, was enriched at nascent adhesions along the leading edge where it recruited activated neural Wiskott-Aldrich syndrome protein (N-WASP; pY256) to promote actin-branch formation, which enhanced lamellipodial dynamics and limited the growth of focal adhesions. Unexpectedly, we found that nonphosphorylated mutant, Y104F-GMFγ, was enriched in growing adhesions where it promoted a linear branch organization and focal adhesion clustering, and recruited zyxin to increase maturation, thus inhibiting lamellipodial dynamics and cell migration. The localization of GMFγ between the leading edge and focal adhesions was dependent upon myosin activity. Furthermore, c-Abl tyrosine kinase regulated the GMFγ phosphorylation-dependent processes. Together, these results unveil the importance of GMFγ phosphorylation in coordinating lamellipodial and focal adhesion dynamics to regulate cell migration.

Rap1 Negatively Regulates the Hippo Pathway to Polarize Directional Protrusions in Collective Cell Migration

In collective cell migration, directional protrusions orient cells in response to external cues, which requires coordinated polarity among the migrating cohort. However, the molecular mechanism has not been well defined. Drosophila border cells (BCs) migrate collectively and invade via the confined space between nurse cells, offering an in vivo model to examine how group polarity is organized. Here, we show that the front/back polarity of BCs requires Rap1, hyperactivation of which disrupts cluster polarity and induces misoriented protrusions and loss of asymmetry in the actin network. Conversely, hypoactive Rap1 causes fewer protrusions and cluster spinning during migration. A forward genetic screen revealed that downregulation of the Hippo (Hpo) pathway core components hpo or mats enhances the Rap1^V12-induced migration defect and misdirected protrusions. Mechanistically, association of Rap1^V12 with the kinase domain of Hpo suppresses its activity, which releases Hpo signaling-mediated suppression of F-actin elongation, promoting cellular protrusions in collective cell migration.

WD40 Repeat Proteins: Signalling Scaffold with Diverse Functions

The WD40 domain is one of the most abundant and interacting domains in the eukaryotic genome. In proteins the WD domain folds into a β-propeller structure, providing a platform for the interaction and assembly of several proteins into a signalosome. WD40 repeats containing proteins, in lower eukaryotes, are mainly involved in growth, cell cycle, development and virulence, while in higher organisms, they play an important role in diverse cellular functions like signal transduction, cell cycle control, intracellular transport, chromatin remodelling, cytoskeletal organization, apoptosis, development, transcriptional regulation, immune responses. To play the regulatory role in various processes, they act as a scaffold for protein-protein or protein-DNA interaction. So far, no WD40 domain has been identified with intrinsic enzymatic activity. Several WD40 domain-containing proteins have been recently characterized in prokaryotes as well. The review summarizes the vast array of functions performed by different WD40 domain containing proteins, their domain organization and functional conservation during the course of evolution.

Species-Specific Functions of Twinfilin in Actin Filament Depolymerization

Twinfilin is a highly conserved member of the actin depolymerization factor homology (ADF-H) protein superfamily, which also includes ADF/Cofilin, Abp1/Drebrin, GMF, and Coactosin. Twinfilin has a unique molecular architecture consisting of two ADF-H domains joined by a linker and followed by a C-terminal tail. Yeast Twinfilin, in conjunction with yeast cyclase-associated protein (Srv2/CAP), increases the rate of depolymerization at both the barbed and pointed ends of actin filaments. However, it has remained unclear whether these activities extend to Twinfilin homologs in other species. To address this, we purified the three mouse Twinfilin isoforms (mTwf1, mTwf2a, mTwf2b) and mouse CAP1, and used total internal reflection fluorescence microscopy assays to study their effects on filament disassembly. Our results show that all three mouse Twinfilin isoforms accelerate barbed end depolymerization similar to yeast Twinfilin, suggesting that this activity is evolutionarily conserved. In striking contrast, mouse Twinfilin isoforms and CAP1 failed to induce rapid pointed end depolymerization. Using chimeras, we show that the yeast-specific pointed end depolymerization activity is specified by the C-terminal ADF-H domain of yeast Twinfilin. In addition, Tropomyosin decoration of filaments failed to impede depolymerization by yeast and mouse Twinfilin and Srv2/CAP, but inhibited Cofilin severing. Together, our results indicate that Twinfilin has conserved functions in regulating barbed end dynamics, although its ability to drive rapid pointed end depolymerization appears to be species-specific. We discuss the implications of this work, including that pointed end depolymerization may be catalyzed by different ADF-H family members in different species.

GMF as an Actin Network Remodeling Factor

Glia maturation factor (GMF) has recently been established as a regulator of the actin cytoskeleton with a unique role in remodeling actin network architecture. Conserved from yeast to mammals, GMF is one of five members of the ADF-H family of actin regulatory proteins, which includes ADF/cofilin, Abp1/Drebrin, Twinfilin, and Coactosin. GMF does not bind actin, but instead binds the Arp2/3 complex with high affinity. Through this association, GMF catalyzes the debranching of actin filament networks and inhibits actin nucleation by Arp2/3 complex. Here, we discuss GMF's emerging role in controlling actin filament spatial organization and dynamics underlying cell motility, endocytosis, and other biological processes. Further, we attempt to reconcile these functions with its earlier characterization as a cell differentiation factor.

The Arp2/3 Regulatory System and Its Deregulation in Cancer

The Arp2/3 complex is an evolutionary conserved molecular machine that generates branched actin networks. When activated, the Arp2/3 complex contributes the actin branched junction and thus cross-links the polymerizing actin filaments in a network that exerts a pushing force. The different activators initiate branched actin networks at the cytosolic surface of different cellular membranes to promote their protrusion, movement, or scission in cell migration and membrane traffic. Here we review the structure, function, and regulation of all the direct regulators of the Arp2/3 complex that induce or inhibit the initiation of a branched actin network and that controls the stability of its branched junctions. Our goal is to present recent findings concerning novel inhibitory proteins or the regulation of the actin branched junction and place these in the context of what was previously known to provide a global overview of how the Arp2/3 complex is regulated in human cells. We focus on the human set of Arp2/3 regulators to compare normal Arp2/3 regulation in untransformed cells to the deregulation of the Arp2/3 system observed in patients affected by various cancers. In many cases, these deregulations promote cancer progression and have a direct impact on patient survival.

Functions of actin-interacting protein 1 (AIP1)/WD repeat protein 1 (WDR1) in actin filament dynamics and cytoskeletal regulation

Actin-depolymerizing factor (ADF)/cofilin and actin-interacting protein 1 (AIP1), also known as WD-repeat protein 1 (WDR1), are conserved among eukaryotes and play critical roles in dynamic reorganization of the actin cytoskeleton. AIP1 preferentially promotes disassembly of ADF/cofilin-decorated actin filaments but exhibits minimal effects on bare actin filaments. Therefore, AIP1 has been often considered to be an ancillary co-factor of ADF/cofilin that merely boosts ADF/cofilin activity level. However, genetic and cell biological studies show that AIP1 deficiency often causes lethality or severe abnormalities in multiple tissues and organs including muscle, epithelia, and blood, suggesting that AIP1 is a major regulator of many biological processes that depend on actin dynamics. This review summarizes recent progress in studies on the biochemical mechanism of actin filament severing by AIP1 and in vivo functions of AIP1 in model organisms and human diseases.

A novel function of twins, B subunit of protein phosphatase 2A, in regulating actin polymerization

Actin is an important component of the cytoskeleton and its polymerization is delicately regulated by several kinases and phosphatases. Heterotrimeric protein phosphatase 2A (PP2A) is a potent phosphatase that is crucial for cell proliferation, apoptosis, tumorigenesis, signal transduction, cytoskeleton arrangement, and neurodegeneration. To facilitate these varied functions, different regulators determine the different targets of PP2A. Among these regulators of PP2A, the B subunits in particular may be involved in cytoskeleton arrangement. However, little is known about the role of PP2A in actin polymerization in vivo. Using sophisticated fly genetics, we demonstrated a novel function for the fly B subunit, twins, to promote actin polymerization in varied tissue types, suggesting a broad and conserved effect. Furthermore, our genetic data suggest that twins may act upstream of the actin-polymerized-proteins, Moesin and Myosin-light-chain, and downstream of Rho to promote actin polymerization. This work opens a new avenue for exploring the biological functions of a PP2A regulator, twins, in cytoskeleton regulation.

Peptide regulation of cofilin activity in the CNS: A novel therapeutic approach for treatment of multiple neurological disorders

Cofilin is a ubiquitous protein which cooperates with many other actin-binding proteins in regulating actin dynamics. Cofilin has essential functions in nervous system development including neuritogenesis, neurite elongation, growth cone pathfinding, dendritic spine formation, and the regulation of neurotransmission and spine function, components of synaptic plasticity essential for learning and memory. Cofilin's phosphoregulation is a downstream target of many transmembrane signaling processes, and its misregulation in neurons has been linked in rodent models to many different neurodegenerative and neurological disorders including Alzheimer disease (AD), aggression due to neonatal isolation, autism, manic/bipolar disorder, and sleep deprivation. Cognitive and behavioral deficits of these rodent models have been largely abrogated by modulation of cofilin activity using viral-mediated, genetic, and/or small molecule or peptide therapeutic approaches. Neuropathic pain in rats from sciatic nerve compression has also been reduced by modulating the cofilin pathway within neurons of the dorsal root ganglia. Neuroinflammation, which occurs following cerebral ischemia/reperfusion, but which also accompanies many other neurodegenerative syndromes, is markedly reduced by peptides targeting specific chemokine receptors, which also modulate cofilin activity. Thus, peptide therapeutics offer potential for cost-effective treatment of a wide variety of neurological disorders. Here we discuss some recent results from rodent models using therapeutic peptides with a surprising ability to cross the rodent blood brain barrier and alter cofilin activity in brain. We also offer suggestions as to how neuronal-specific cofilin regulation might be achieved.

Global treadmilling coordinates actin turnover and controls the size of actin networks

Various cellular processes (including cell motility) are driven by the regulated, polarized assembly of actin filaments into distinct force-producing arrays of defined size and architecture. Branched, linear, contractile and cytosolic arrays coexist in vivo, and cells intricately control the number, length and assembly rate of filaments in these arrays. Recent in vitro and in vivo studies have revealed novel molecular mechanisms that regulate the number of filament barbed and pointed ends and their respective assembly and disassembly rates, thus defining classes of dynamically different filaments, which coexist in the same cell. We propose that a global treadmilling process, in which a steady-state amount of polymerizable actin monomers is established by the dynamics of each network, is responsible for defining the size and turnover of coexisting actin networks. Furthermore, signal-induced changes in the partitioning of actin to distinct arrays (mediated by RHO GTPases) result in the establishment of various steady-state concentrations of polymerizable monomers, thereby globally influencing the growth rate of actin filaments.

The roles and regulation of the actin cytoskeleton, intermediate filaments and microtubules in smooth muscle cell migration

Smooth muscle cell migration has been implicated in the development of respiratory and cardiovascular systems; and airway/vascular remodeling. Cell migration is a polarized cellular process involving a protrusive cell front and a retracting trailing rear. There are three cytoskeletal systems in mammalian cells: the actin cytoskeleton, the intermediate filament network, and microtubules; all of which regulate all or part of the migrated process. The dynamic actin cytoskeleton spatially and temporally regulates protrusion, adhesions, contraction, and retraction from the cell front to the rear. c-Abl tyrosine kinase plays a critical role in regulating actin dynamics and migration of airway smooth muscle cells and nonmuscle cells. Recent studies suggest that intermediate filaments undergo reorganization during migration, which coordinates focal adhesion dynamics, cell contraction, and nucleus rigidity. In particular, vimentin intermediate filaments undergo phosphorylation and reorientation in smooth muscle cells, which may regulate cell contraction and focal adhesion assembly/disassembly. Motile cells are characterized by a front-rear polarization of the microtubule framework, which regulates all essential processes leading to cell migration through its role in cell mechanics, intracellular trafficking, and signaling. This review recapitulates our current knowledge how the three cytoskeletal systems spatially and temporally modulate the migratory properties of cells. We also summarize the potential role of migration-associated biomolecules in lung and vascular diseases.

Cofilin-1 and Other ADF/Cofilin Superfamily Members in Human Malignant Cells

Identification of actin-depolymerizing factor homology (ADF-H) domains in the structures of several related proteins led first to the formation of the ADF/cofilin family, which then expanded to the ADF/cofilin superfamily. This superfamily includes the well-studied cofilin-1 (Cfl-1) and about a dozen different human proteins that interact directly or indirectly with the actin cytoskeleton, provide its remodeling, and alter cell motility. According to some data, Cfl-1 is contained in various human malignant cells (HMCs) and is involved in the formation of malignant properties, including invasiveness, metastatic potential, and resistance to chemotherapeutic drugs. The presence of other ADF/cofilin superfamily proteins in HMCs and their involvement in the regulation of cell motility were discovered with the use of various OMICS technologies. In our review, we discuss the results of the study of Cfl-1 and other ADF/cofilin superfamily proteins, which may be of interest for solving different problems of molecular oncology, as well as for the prospects of further investigations of these proteins in HMCs.

Structural Basis of Arp2/3 Complex Inhibition by GMF, Coronin, and Arpin

The evolutionarily conserved Arp2/3 complex plays a central role in nucleating the branched actin filament arrays that drive cell migration, endocytosis, and other processes. To better understand Arp2/3 complex regulation, we used single-particle electron microscopy to compare the structures of Arp2/3 complex bound to three different inhibitory ligands: glia maturation factor (GMF), Coronin, and Arpin. Although the three inhibitors have distinct binding sites on Arp2/3 complex, they each induced an "open" nucleation-inactive conformation. Coronin promoted a standard (previously described) open conformation of Arp2/3 complex, with the N-terminal β-propeller domain of Coronin positioned near the p35/ARPC2 subunit of Arp2/3 complex. GMF induced two distinct open conformations of Arp2/3 complex, which correlated with the two suggested binding sites for GMF. Furthermore, GMF synergized with Coronin in inhibiting actin nucleation by Arp2/3 complex. Arpin, which uses VCA-related acidic (A) motifs to interact with the Arp2/3 complex, induced the standard open conformation, and two new masses appeared at positions near Arp2 and Arp3. Furthermore, Arpin showed additive inhibitory effects on Arp2/3 complex with Coronin and GMF. Together, these data suggest that Arp2/3 complex conformation is highly polymorphic and that its activities can be controlled combinatorially by different inhibitory ligands.

Circuitous Genetic Regulation Governs a Straightforward Cell Migration

Drosophila border cells undergo a straightforward and stereotypical collective migration during egg development. However, a complex genetic program underlies this process. A variety of approaches, including biochemical, genetic, and imaging strategies have identified many regulatory components, revealing layers of control. This complexity suggests that the active processes of evaluating the environment, remodeling the cytoskeleton, and coordinating movements among cells, demand rapid systems for modulating cell behaviors. Multiple signaling inputs, nodes of integration, and feedback loops act as molecular rheostats to fine-tune gene expression levels and physical responses. Since key genetic regulators of border cell migration have been shown to be required in other types of cell migration, this model system continues to provide an important avenue for genetic discovery.

Cellular functions of the ADF/cofilin family at a glance

The actin depolymerizing factor (ADF)/cofilin family comprises small actin-binding proteins with crucial roles in development, tissue homeostasis and disease. They are best known for their roles in regulating actin dynamics by promoting actin treadmilling and thereby driving membrane protrusion and cell motility. However, recent discoveries have increased our understanding of the functions of these proteins beyond their well-characterized roles. This Cell Science at a Glance article and the accompanying poster serve as an introduction to the diverse roles of the ADF/cofilin family in cells. The first part of the article summarizes their actions in actin treadmilling and the main mechanisms for their intracellular regulation; the second part aims to provide an outline of the emerging cellular roles attributed to the ADF/cofilin family, besides their actions in actin turnover. The latter part discusses an array of diverse processes, which include regulation of intracellular contractility, maintenance of nuclear integrity, transcriptional regulation, nuclear actin monomer transfer, apoptosis and lipid metabolism. Some of these could, of course, be indirect consequences of actin treadmilling functions, and this is discussed.

Tropomyosin Promotes Lamellipodial Persistence by Collaborating with Arp2/3 at the Leading Edge

At the leading edge of migrating cells, protrusion of the lamellipodium is driven by Arp2/3-mediated polymerization of actin filaments [1]. This dense, branched actin network is promoted and stabilized by cortactin [2, 3]. In order to drive filament turnover, Arp2/3 networks are remodeled by proteins such as GMF, which blocks the actin-Arp2/3 interaction [4, 5], and coronin 1B, which acts by directing SSH1L to the lamellipodium where it activates the actin-severing protein cofilin [6, 7]. It has been shown in vitro that cofilin-mediated severing of Arp2/3 actin networks results in the generation of new pointed ends to which the actin-stabilizing protein tropomyosin (Tpm) can bind [8]. The presence of Tpm in lamellipodia, however, is disputed in the literature [9-19]. Here, we report that the Tpm isoforms 1.8/9 are enriched in the lamellipodium of fibroblasts as detected with a novel isoform-specific monoclonal antibody. RNAi-mediated silencing of Tpm1.8/9 led to an increase of Arp2/3 accumulation at the cell periphery and a decrease in the persistence of lamellipodia and cell motility, a phenotype consistent with cortactin- and coronin 1B-deficient cells [2, 7]. In the absence of coronin 1B or cofilin, Tpm1.8/9 protein levels are reduced while, conversely, inhibition of Arp2/3 with CK666 leads to an increase in Tpm1.8/9 protein. These findings establish a novel regulatory mechanism within the lamellipodium whereby Tpm collaborates with Arp2/3 to promote lamellipodial-based cell migration.

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.

GMFβ controls branched actin content and lamellipodial retraction in fibroblasts

The primary activity of GMFβ in vivo is actin branch disassembly (and not inhibition of Arp2/3 activation), and this activity plays an important role in lamellipodial dynamics and directional migration toward ECM cues.

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

Combinatorial genetic analysis of a network of actin disassembly-promoting factors

The patterning of actin cytoskeleton structures in vivo is a product of spatially and temporally regulated polymer assembly balanced by polymer disassembly. While in recent years our understanding of actin assembly mechanisms has grown immensely, our knowledge of actin disassembly machinery and mechanisms has remained comparatively sparse. Saccharomyces cerevisiae is an ideal system to tackle this problem, both because of its amenabilities to genetic manipulation and live‐cell imaging and because only a single gene encodes each of the core disassembly factors: cofilin (COF1), Srv2/CAP (SRV2), Aip1 (AIP1), GMF (GMF1/AIM7), coronin (CRN1), and twinfilin (TWF1). Among these six factors, only the functions of cofilin are essential and have been well defined. Here, we investigated the functions of the nonessential actin disassembly factors by performing genetic and live‐cell imaging analyses on a combinatorial set of isogenic single, double, triple, and quadruple mutants in S. cerevisiae. Our results show that each disassembly factor makes an important contribution to cell viability, actin organization, and endocytosis. Further, our data reveal new relationships among these factors, providing insights into how they work together to orchestrate actin turnover. Finally, we observe specific combinations of mutations that are lethal, e.g., srv2Δ aip1Δ and srv2Δ crn1Δ twf1Δ, demonstrating that while cofilin is essential, it is not sufficient in vivo, and that combinations of the other disassembly factors perform vital functions. © 2015 The Authors. Cytoskeleton Published by Wiley Periodicals, Inc.

Actin-interacting protein 1 controls assembly and permeability of intestinal epithelial apical junctions

Adherens junctions (AJs) and tight junctions (TJs) are crucial regulators of the integrity and restitution of the intestinal epithelial barrier. The structure and function of epithelial junctions depend on their association with the cortical actin cytoskeleton that, in polarized epithelial cells, is represented by a prominent perijunctional actomyosin belt. The assembly and stability of the perijunctional cytoskeleton is controlled by constant turnover (disassembly and reassembly) of actin filaments. Actin-interacting protein (Aip) 1 is an emerging regulator of the actin cytoskeleton, playing a critical role in filament disassembly. In this study, we examined the roles of Aip1 in regulating the structure and remodeling of AJs and TJs in human intestinal epithelium. Aip1 was enriched at apical junctions in polarized human intestinal epithelial cells and normal mouse colonic mucosa. Knockdown of Aip1 by RNA interference increased the paracellular permeability of epithelial cell monolayers, decreased recruitment of AJ/TJ proteins to steady-state intercellular contacts, and attenuated junctional reassembly in a calcium-switch model. The observed defects of AJ/TJ structure and functions were accompanied by abnormal organization and dynamics of the perijunctional F-actin cytoskeleton. Moreover, loss of Aip1 impaired the apico-basal polarity of intestinal epithelial cell monolayers and inhibited formation of polarized epithelial cysts in 3-D Matrigel. Our findings demonstrate a previously unanticipated role of Aip1 in regulating the structure and remodeling of intestinal epithelial junctions and early steps of epithelial morphogenesis.

Actin and endocytosis in budding yeast

Endocytosis, the process whereby the plasma membrane invaginates to form vesicles, is essential for bringing many substances into the cell and for membrane turnover. The mechanism driving clathrin-mediated endocytosis (CME) involves > 50 different protein components assembling at a single location on the plasma membrane in a temporally ordered and hierarchal pathway. These proteins perform precisely choreographed steps that promote receptor recognition and clustering, membrane remodeling, and force-generating actin-filament assembly and turnover to drive membrane invagination and vesicle scission. Many critical aspects of the CME mechanism are conserved from yeast to mammals and were first elucidated in yeast, demonstrating that it is a powerful system for studying endocytosis. In this review, we describe our current mechanistic understanding of each step in the process of yeast CME, and the essential roles played by actin polymerization at these sites, while providing a historical perspective of how the landscape has changed since the preceding version of the YeastBook was published 17 years ago (1997). Finally, we discuss the key unresolved issues and where future studies might be headed.