A step-by-step guide to fragmenting bundled actin filaments.

There has long been conflicting evidence as to how bundled actin filaments, found in cellular structures such as filopodia, are disassembled. In this issue, Chikireddy et al. (https://doi.org/10.1083/jcb.202312106) provide a detailed in vitro analysis of the steps involved in fragmentation of fascin-bundled actin filaments and propose a novel mechanism for severing two-filament bundles.

SMAD4 promotes somatic-germline contact during murine oocyte growth.

Development of the mammalian oocyte requires physical contact with the surrounding granulosa cells of the follicle, which provide it with essential nutrients and regulatory signals. This contact is achieved through specialized filopodia, termed transzonal projections (TZPs), that extend from the granulosa cells to the oocyte surface. Transforming growth factor (TGFß) family ligands produced by the oocyte increase the number of TZPs, but how they do so is unknown. Using an inducible Cre recombinase strategy together with expression of green fluorescent protein to verify Cre activity in individual cells, we examined the effect of depleting the canonical TGFß mediator, SMAD4, in mouse granulosa cells. We observed a 20-50% decrease in the total number of TZPs in SMAD4-depleted granulosa cell-oocyte complexes, and a 50% decrease in the number of newly generated TZPs when the granulosa cells were reaggregated with wild-type oocytes. Three-dimensional image analysis revealed that TZPs of SMAD4-depleted cells were longer than controls and more frequently oriented towards the oocyte. Strikingly, the transmembrane proteins, N-cadherin and Notch2, were reduced by 50% in SMAD4-depleted cells. SMAD4 may thus modulate a network of cell adhesion proteins that stabilize the attachment of TZPs to the oocyte, thereby amplifying signalling between the two cell types.

BIN1 regulates actin-membrane interactions during IRSp53-dependent filopodia formation.

Amphiphysin 2 (BIN1) is a membrane and actin remodeling protein mutated in congenital and adult centronuclear myopathies. Here, we report an unexpected function of this N-BAR domain protein BIN1 in filopodia formation. We demonstrated that BIN1 expression is necessary and sufficient to induce filopodia formation. BIN1 is present at the base of forming filopodia and all along filopodia, where it colocalizes with F-actin. We identify that BIN1-mediated filopodia formation requires IRSp53, which allows its localization at negatively-curved membrane topologies. Our results show that BIN1 bundles actin in vitro. Finally, we identify that BIN1 regulates the membrane-to-cortex architecture and functions as a molecular platform to recruit actin-binding proteins, dynamin and ezrin, to promote filopodia formation.

The concerted action of SEPT9 and EPLIN modulates the adhesion and migration of human fibroblasts.

Septins are cytoskeletal proteins that participate in cell adhesion, migration, and polarity establishment. The septin subunit SEPT9 directly interacts with the single LIM domain of epithelial protein lost in neoplasm (EPLIN), an actin-bundling protein. Using a human SEPT9 KO fibroblast cell line, we show that cell adhesion and migration are regulated by the interplay between both proteins. The low motility of SEPT9-depleted cells could be partly rescued by increased levels of EPLIN. The normal organization of actin-related filopodia and stress fibers was directly dependent on the expression level of SEPT9 and EPLIN. Increased levels of SEPT9 and EPLIN enhanced the size of focal adhesions in cell protrusions, correlating with stabilization of actin bundles. Conversely, decreased levels had the opposite effect. Our work thus establishes the interaction between SEPT9 and EPLIN as an important link between the septin and the actin cytoskeleton, influencing cell adhesion, motility, and migration.

SOX4 induces cytoskeleton remodeling and promotes cell motility via N-wasp/ARP2/3 pathway in colorectal cancer cells.

Filopodia are thin, actin-rich projection from the plasma membrane that promote cancer cell invasion and migration. Sex-determining region Y-related high-mobility group-box 4 (SOX4) is a crucial transcription factor that plays a role in the development and metastasis of colorectal cancer (CRC). However, the involvement of SOX4 in cytoskeleton remodeling in CRC remains unknown. For the first time, we demonstrate that SOX4 is a potent regulator of filopodia formation in CRC cells. Overexpression of SOX4 protein enhances both migration and invasion ability of HCT116, and CACO2 cells, which is relevant to the metastasis. Furthermore, through phalloidin staining, cytoskeleton re-assembly was observed in SOX4-modified cell lines. Enhanced expression of SOX4 increased the number and length of filopodia on cell surface. In contrast, silencing SOX4 in SW620 cells with higher endogenous expression of SOX4, impeded the filopodia formation. Moreover, SOX4 was found to be positively regulating the expression of central regulators of actin cytoskeleton - N-Wiskott-Aldrich syndrome protein (N-WASP); WAVE2; Actin related proteins, ARP2 and ARP3. Inhibiting the N-WASP/ARP2/3 pathway diminishes the filopodia formation and the migration of CRC cells. These results indicate the crucial role of SOX4 in the regulation of filopodia formation mediated by N-WASP/ARP2/3 pathway in CRC cells.

KLF5 regulates actin remodeling to enhance the metastasis of nasopharyngeal carcinoma.

Transcription factors (TFs) engage in various cellular essential processes including differentiation, growth and migration. However, the master TF involved in distant metastasis of nasopharyngeal carcinoma (NPC) remains largely unclear. Here we show that KLF5 regulates actin remodeling to enhance NPC metastasis. We analyzed the msVIPER algorithm-generated transcriptional regulatory networks and identified KLF5 as a master TF of metastatic NPC linked to poor clinical outcomes. KLF5 regulates actin remodeling and lamellipodia formation to promote the metastasis of NPC cells in vitro and in vivo. Mechanistically, KLF5 preferentially occupies distal enhancer regions of ACTN4 to activate its transcription, whereby decoding the informative DNA sequences. ACTN4, extensively localized within actin cytoskeleton, facilitates dense and branched actin networks and lamellipodia formation at the cell leading edge, empowering cells to migrate faster. Collectively, our findings reveal that KLF5 controls robust transcription program of ACTN4 to modulate actin remodeling and augment cell motility which enhances NPC metastasis, and provide new potential biomarkers and therapeutic interventions for NPC.

The scaffolding protein AKAP12 regulates mRNA localization and translation.

Regulation of subcellular messenger (m)RNA localization is a fundamental biological mechanism, which adds a spatial dimension to the diverse layers of post-transcriptional control of gene expression. The cellular compartment in which mRNAs are located may define distinct aspects of the encoded proteins, ranging from production rate and complex formation to localized activity. Despite the detailed roles of localized mRNAs that have emerged over the past decades, the identity of factors anchoring mRNAs to subcellular domains remains ill-defined. Here, we used an unbiased method to profile the RNA-bound proteome in migrating endothelial cells (ECs) and discovered that the plasma membrane (PM)-associated scaffolding protein A-kinase anchor protein (AKAP)12 interacts with various mRNAs, including transcripts encoding kinases with Actin remodeling activity. In particular, AKAP12 targets a transcript coding for the kinase Abelson Tyrosine-Protein Kinase 2 (ABL2), which we found to be necessary for adequate filopodia formation and angiogenic sprouting. Moreover, we demonstrate that AKAP12 is necessary for anchoring ABL2 mRNA to the PM and show that in the absence of AKAP12, the translation efficiency of ABL2 mRNA is reduced. Altogether, our work identified a unique post-transcriptional function for AKAP12 and sheds light into mechanisms of spatial control of gene expression.

A nanobody inhibitor of Fascin-1 actin-bundling activity and filopodia formation.

Fascin-1-mediated actin-bundling activity is central to the generation of plasma membrane protrusions required for cell migration. Dysregulated formation of cellular protrusions is observed in metastatic cancers, where they are required for increased invasiveness, and is often correlated with increased Fascin-1 abundance. Therefore, there is interest in generating therapeutic Fascin-1 inhibitors. We present the identification of Nb 3E11, a nanobody inhibitor of Fascin-1 actin-bundling activity and filopodia formation. The crystal structure of the Fascin-1/Nb 3E11 complex reveals the structural mechanism of inhibition. Nb 3E11 occludes an actin-binding site on the third ß-trefoil domain of Fascin-1 that is currently not targeted by chemical inhibitors. Binding of Nb 3E11 to Fascin-1 induces a conformational change in the adjacent domains to stabilize Fascin-1 in an inhibitory state similar to that adopted in the presence of small-molecule inhibitors. Nb 3E11 could be used as a tool inhibitor molecule to aid in the development of Fascin-1 targeted therapeutics.

Two-component macrophage model for active phagocytosis with pseudopod formation.

Macrophage phagocytosis is critical for the immune response, homeostasis regulation, and tissue repair. This intricate process involves complex changes in cell morphology, cytoskeletal reorganization, and various receptor-ligand interactions controlled by mechanical constraints. However, there is a lack of comprehensive theoretical and computational models that investigate the mechanical process of phagocytosis in the context of cytoskeletal rearrangement. To address this issue, we propose a novel coarse-grained mesoscopic model that integrates a fluid-like cell membrane and a cytoskeletal network to study the dynamic phagocytosis process. The growth of actin filaments results in the formation of long and thin pseudopods, and the initial cytoskeleton can be disassembled upon target entry and reconstructed after phagocytosis. Through dynamic changes in the cytoskeleton, our macrophage model achieves active phagocytosis by forming a phagocytic cup utilizing pseudopods in two distinct ways. We have developed a new algorithm for modifying membrane area to prevent membrane rupture and ensure sufficient surface area during phagocytosis. In addition, the bending modulus, shear stiffness, and cortical tension of the macrophage model are investigated through computation of the axial force for the tubular structure and micropipette aspiration. With this model, we simulate active phagocytosis at the cytoskeletal level and investigate the mechanical process during the dynamic interplay between macrophage and target particles.

Filopodial protrusion driven by density-dependent Ena-TOCA-1 interactions.

Filopodia are narrow actin-rich protrusions with important roles in neuronal development where membrane-binding adaptor proteins, such as I-BAR- and F-BAR-domain-containing proteins, have emerged as upstream regulators that link membrane interactions to actin regulators such as formins and proteins of the Ena/VASP family. Both the adaptors and their binding partners are part of diverse and redundant protein networks that can functionally compensate for each other. To explore the significance of the F-BAR domain-containing neuronal membrane adaptor TOCA-1 (also known as FNBP1L) in filopodia we performed a quantitative analysis of TOCA-1 and filopodial dynamics in Xenopus retinal ganglion cells, where Ena/VASP proteins have a native role in filopodial extension. Increasing the density of TOCA-1 enhances Ena/VASP protein binding in vitro, and an accumulation of TOCA-1, as well as its coincidence with Ena, correlates with filopodial protrusion in vivo. Two-colour single-molecule localisation microscopy of TOCA-1 and Ena supports their nanoscale association. TOCA-1 clusters promote filopodial protrusion and this depends on a functional TOCA-1 SH3 domain and activation of Cdc42, which we perturbed using the small-molecule inhibitor CASIN. We propose that TOCA-1 clusters act independently of membrane curvature to recruit and promote Ena activity for filopodial protrusion.

Cytoneme signaling provides essential contributions to mammalian tissue patterning.

During development, morphogens pattern tissues by instructing cell fate across long distances. Directly visualizing morphogen transport in situ has been inaccessible, so the molecular mechanisms ensuring successful morphogen delivery remain unclear. To tackle this longstanding problem, we developed a mouse model for compromised sonic hedgehog (SHH) morphogen delivery and discovered that endocytic recycling promotes SHH loading into signaling filopodia called cytonemes. We optimized methods to preserve in vivo cytonemes for advanced microscopy and show endogenous SHH localized to cytonemes in developing mouse neural tubes. Depletion of SHH from neural tube cytonemes alters neuronal cell fates and compromises neurodevelopment. Mutation of the filopodial motor myosin 10 (MYO10) reduces cytoneme length and density, which corrupts neuronal signaling activity of both SHH and WNT. Combined, these results demonstrate that cytoneme-based signal transport provides essential contributions to morphogen dispersion during mammalian tissue development and suggest MYO10 is a key regulator of cytoneme function.

A continuum mechanical model of cell motion driven by a biphasic traction stress.

The aim of this paper is to place the cell locomotion problem within the general framework of classical continuum mechanics, and while doing so, to account for the deformation of the actin network in the cytoskeleton; the myosin activity on the lamellum including its effect on depolymerization at the trailing edge; model the stress-dependent driving forces and kinetic laws controlling polymerization at the leading edge, depolymerization at the trailing edge and ATP hydrolysis consistently with the dissipation inequality; and, based on the observations in Gardel et al. (Gardel et al. 2008 J. Cell Biol. 183, 999-1005 (doi:10.1083/jcb.200810060)), include a biphasic velocity-dependent traction stress acting on the actin network. While we chose certain specific models for each of these, in part to allow for an analytical solution, the generality of the framework allows one to readily introduce different constitutive laws to describe these phenomena as might be needed, for example, to study some different type of cells. As described in §5, the predictions of the model compare well with observations such as the magnitude of the very different actin retrograde speeds in the lamellum and lamellipodium including their jump at the interface, the magnitude of the cell speed, and the relative lengths of the lamellipodium and lamellum.

filoVision - using deep learning and tip markers to automate filopodia analysis.

Filopodia are slender, actin-filled membrane projections used by various cell types for environment exploration. Analyzing filopodia often involves visualizing them using actin, filopodia tip or membrane markers. Due to the diversity of cell types that extend filopodia, from amoeboid to mammalian, it can be challenging for some to find a reliable filopodia analysis workflow suited for their cell type and preferred visualization method. The lack of an automated workflow capable of analyzing amoeboid filopodia with only a filopodia tip label prompted the development of filoVision. filoVision is an adaptable deep learning platform featuring the tools filoTips and filoSkeleton. filoTips labels filopodia tips and the cytosol using a single tip marker, allowing information extraction without actin or membrane markers. In contrast, filoSkeleton combines tip marker signals with actin labeling for a more comprehensive analysis of filopodia shafts in addition to tip protein analysis. The ZeroCostDL4Mic deep learning framework facilitates accessibility and customization for different datasets and cell types, making filoVision a flexible tool for automated analysis of tip-marked filopodia across various cell types and user data.

Cooperative function of oncogenic MAPK signaling and the loss of Pten for melanoma migration through the formation of lamellipodia.

The combination of oncogenes and tumor suppressors is involved in cancer development; however, it is still unknown whether their combination plays a critical role in cancer metastasis. We herein investigated whether genetic combinations affected cell migration ability by establishing the immortalized melanocytes, melan-a cells, with an oncogene, either BRAFV600E or GNA11Q209L, and the loss of mouse Pten. The loss of mouse Pten or human PTEN increased the cell migration ability of our established cells and human melanoma cell lines with oncogenic MAPK signaling and the BRAFV600E or NRASQ61R background, but not with the GNA11Q209L background or no oncogenes. Although increased migration was not related to PI3K-AKT activation, those migration is regulated by the induction of some components in the WAVE regulatory complex, resulting in a higher rate of the formation of lamellipodia. On the other hand, BRAFV600E induced EphA2 phosphorylation at serine 897 through RSK and was also required for cell migration and the formation of lamellipodia. Therefore, the oncogenic MAPK pathway and loss of Pten in melanoma were important for cell migration through the formation of lamellipodia, suggesting the significance of an appropriate combination of genetic alterations not only in cancer development, but also cancer metastasis.

Myo10 tail is crucial for promoting long filopodia.

Filopodia are slender cellular protrusions containing parallel actin bundles involved in environmental sensing and signaling, cell adhesion and migration, and growth cone guidance and extension. Myosin 10 (Myo10), an unconventional actin-based motor protein, was reported to induce filopodial initiation with its motor domain. However, the roles of the multifunctional tail domain of Myo10 in filopodial formation and elongation remain elusive. Herein, we generated several constructs of Myo10-full-length Myo10, Myo10 with a truncated tail (Myo10 HMM), and Myo10 containing four mutations to disrupt its coiled-coil domain (Myo10 CC mutant). We found that the truncation of the tail domain decreased filopodial formation and filopodial length, while four mutations in the coiled-coil domain disrupted the motion of Myo10 toward filopodial tips and the elongation of filopodia. Furthermore, we found that filopodia elongated through multiple elongation cycles, which was supported by the Myo10 tail. These findings suggest that Myo10 tail is crucial for promoting long filopodia.

Myosin XV is a negative regulator of signaling filopodia during long-range lateral inhibition.

The self-organization of cells during development is essential for the formation of healthy tissues and requires the coordination of cell activities at local scales. Cytonemes, or signaling filopodia, are dynamic actin-based cellular protrusions that allow cells to engage in contact mediated signaling at a distance. While signaling filopodia have been shown to support several signaling paradigms during development, less is understood about how these protrusions are regulated. We investigated the role of the plus-end directed, unconventional MyTH4-FERM myosins in regulating signaling filopodia during sensory bristle patterning on the dorsal thorax of the fruit fly Drosophila melanogaster. We found that Myosin XV is required for regulating signaling filopodia dynamics and, as a consequence, lateral inhibition more broadly throughout the patterning epithelium. We found that Myosin XV is required for limiting the length and number of signaling filopodia generated by bristle precursor cells. Cells with additional and longer signaling filopodia due to loss of Myosin XV are not signaling competent, due to altered levels of Delta ligand and Notch receptor along their lengths. We conclude that Myosin XV acts to negatively regulate signaling filopodia, as well as promote the ability of signaling filopodia to engage in long-range Notch signaling. Since Myosin XV isoforms are present across several vertebrate and invertebrate systems, this may have significance for other long-range signaling mechanisms.

Estrogen receptors differentially modifies lamellipodial and focal adhesion dynamics in airway smooth muscle cell migration.

Sex-steroid signaling, especially estrogen, has a paradoxical impact on regulating airway remodeling. In our previous studies, we demonstrated differential effects of 17ß-estradiol (E2) towards estrogen receptors (ERs: α and ß) in regulating airway smooth muscle (ASM) cell proliferation and extracellular matrix (ECM) production. However, the role of ERs and their signaling on ASM migration is still unexplored. In this study, we examined how ERα versus ERß affects the mitogen (Platelet-derived growth factor, PDGF)-induced human ASM cell migration as well as the underlying mechanisms involved. We used Lionheart-FX automated microscopy and transwell assays to measure cell migration and found that activating specific ERs had differential effects on PDGF-induced ASM cell migration. Pharmacological activation of ERß or shRNA mediated knockdown of ERα and specific activation of ERß blunted PDGF-induced cell migration. Furthermore, specific ERß activation showed inhibition of actin polymerization by reducing the F/G-actin ratio. Using Zeiss confocal microscopy coupled with three-dimensional algorithmic ZEN-image analysis showed an ERß-mediated reduction in PDGF-induced expressions of neural Wiskott-Aldrich syndrome protein (N-WASP) and actin-related proteins-2/3 (Arp2/3) complex, thereby inhibiting actin-branching and lamellipodia. In addition, ERß activation also reduces the clustering of actin-binding proteins (vinculin and paxillin) at the leading edge of ASM cells. However, cells treated with E2 or ERα agonists do not show significant changes in actin/lamellipodial dynamics. Overall, these findings unveil the significance of ERß activation in regulating lamellipodial and focal adhesion dynamics to regulate ASM cell migration and could be a novel target to blunt airway remodeling.

Cytoneme-mediated transport of active Wnt5b-Ror2 complexes in zebrafish.

Chemical signalling is the primary means by which cells communicate in the embryo. The underlying principle refers to a group of ligand-producing cells and a group of cells that respond to this signal because they express the appropriate receptors1,2. In the zebrafish embryo, Wnt5b binds to the receptor Ror2 to trigger the Wnt-planar cell polarity (PCP) signalling pathway to regulate tissue polarity and cell migration3,4. However, it remains unclear how this lipophilic ligand is transported from the source cells through the aqueous extracellular space to the target tissue. In this study, we provide evidence that Wnt5b, together with Ror2, is loaded on long protrusions called cytonemes. Our data further suggest that the active Wnt5b-Ror2 complexes form in the producing cell and are handed over from these cytonemes to the receiving cell. Then, the receiving cell has the capacity to initiate Wnt-PCP signalling, irrespective of its functional Ror2 receptor status. On the tissue level, we further show that cytoneme-dependent spreading of active Wnt5b-Ror2 affects convergence and extension in the zebrafish gastrula. We suggest that cytoneme-mediated transfer of ligand-receptor complexes is a vital mechanism for paracrine signalling. This may prompt a reevaluation of the conventional concept of characterizing responsive and non-responsive tissues solely on the basis of the expression of receptors.

Actin-based protrusions at a glance.

Actin-based protrusions are at the base of many fundamental cellular processes, such as cell adhesion, migration and intercellular communication. In recent decades, the discovery of new types of actin-based protrusions with unique functions has enriched our comprehension of cellular processes. However, as the repertoire of protrusions continues to expand, the rationale behind the classification of newly identified and previously known structures becomes unclear. Although current nomenclature allows good categorization of protrusions based on their functions, it struggles to distinguish them when it comes to structure, composition or formation mechanisms. In this Cell Science at a Glance article, we discuss the different types of actin-based protrusions, focusing on filopodia, cytonemes and tunneling nanotubes, to help better distinguish and categorize them based on their structural and functional differences and similarities.

Lamellipodia dynamics and microrheology in endothelial cell paracellular gap closure.

Vascular endothelial cells (ECs) form a semipermeable barrier separating vascular contents from the interstitium, thereby regulating the movement of water and molecular solutes across small intercellular gaps, which are continuously forming and closing. Under inflammatory conditions, however, larger EC gaps form resulting in increased vascular leakiness to circulating fluid, proteins, and cells, which results in organ edema and dysfunction responsible for key pathophysiologic findings in numerous inflammatory disorders. In this study, we extend our earlier work examining the biophysical properties of EC gap formation and now address the role of lamellipodia, thin sheet-like membrane projections from the leading edge, in modulating EC spatial-specific contractile properties and gap closure. Micropillars, fabricated by soft lithography, were utilized to form reproducible paracellular gaps in human lung ECs. Using time-lapse imaging via optical microscopy, rates of EC gap closure and motility were measured with and without EC stimulation with the barrier-enhancing sphingolipid, sphingosine-1-phosphate. Peripheral ruffle formation was ubiquitous during gap closure. Kymographs were generated to quantitatively compare the lamellipodia dynamics of sphingosine-1-phosphate-stimulated and -unstimulated ECs. Utilizing atomic force microscopy, we characterized the viscoelastic behavior of EC lamellipodia. Our results indicate decreased stiffness and increased liquid-like behavior of expanding lamellipodia compared with regions away from the cellular edge (lamella and cell body) during EC gap closure, results in sync with the rapid kinetics of protrusion/retraction motion. We hypothesize this dissipative EC behavior during gap closure is linked to actomyosin cytoskeletal rearrangement and decreased cross-linking during lamellipodia expansion. In summary, these studies of the kinetic and mechanical properties of EC lamellipodia and ruffles at gap boundaries yield insights into the mechanisms of vascular barrier restoration and potentially a model system for examining the druggability of lamellipodial protein targets to enhance vascular barrier integrity.