The key feature for early migratory processes: Dependence of adhesion, actin bundles, force generation and transmission on filopodia

Peptide TK-HR from the Skin of Chinese Folk Medicine Frog Hoplobatrachus Rugulosus Accelerates Wound Healing via the Activation of the Neurokinin-1 Receptor

Wound healing is a complex process and remains a considerable challenge in clinical trials due to the lack of ideal therapeutic drugs. Here, a new peptide TK-HR identified from the skin of the frog Hoplobatrachus rugulosus was tested for its ability to heal cutaneous wounds in mice. Topical application of TK-HR at doses of 50-200 μg/mL significantly accelerated wound closure without causing any adverse effects in the animals. In vitro and in vivo investigations proved the regulatory role of the peptide on neutrophils, macrophages, keratinocytes, and vein endothelial cells involved in the inflammatory, proliferative, and remodeling phases of wound healing. Notably, TK-HR activated the MAPK and TGF-β-Smad signaling pathways by acting on NK1R in RAW264.7 cells and mice. The current work has identified that TK-HR is a potent wound healing regulator that can be applied for the treatment of wounds, including diabetic foot ulcers and infected wounds, in the future.

Cell Force-Driven Basement Membrane Disruption Fuels EGF- and Stiffness-Induced Invasive Cell Dissemination from Benign Breast Gland Acini

Local basement membrane (BM) disruption marks the initial step of breast cancer invasion. The activation mechanisms of force-driven BM-weakening remain elusive. We studied the mechanical response of MCF10A-derived human breast cell acini with BMs of tuneable maturation to physical and soluble tumour-like extracellular matrix (ECM) cues. Traction force microscopy (TFM) and elastic resonator interference stress microscopy (ERISM) were used to quantify pro-invasive BM stress and protrusive forces. Substrate stiffening and mechanically impaired BM scaffolds induced the invasive transition of benign acini synergistically. Robust BM scaffolds attenuated this invasive response. Additional oncogenic EGFR activation compromised the BMs’ barrier function, fuelling invasion speed and incidence. Mechanistically, EGFR-PI3-Kinase downstream signalling modulated both MMP- and force-driven BM-weakening processes. We show that breast acini form non-proteolytic and BM-piercing filopodia for continuous matrix mechanosensation, which significantly push and pull on the BM and ECM under pro-invasive conditions. Invasion-triggered acini further shear and compress their BM by contractility-based stresses that were significantly increased (3.7-fold) compared to non-invasive conditions. Overall, the highest amplitudes of protrusive and contractile forces accompanied the highest invasiveness. This work provides a mechanistic concept for tumour ECM-induced mechanically misbalanced breast glands fuelling force-driven BM disruption. Finally, this could facilitate early cell dissemination from pre-invasive lesions to metastasize eventually.

Regulation of keratin network dynamics by the mechanical properties of the environment in migrating cells

Keratin intermediate filaments provide mechanical resilience for epithelia. They are nevertheless highly dynamic and turn over continuously, even in sessile keratinocytes. The aim of this study was to characterize and understand how the dynamic behavior of the keratin cytoskeleton is integrated in migrating cells. By imaging human primary keratinocytes producing fluorescent reporters and by using standardized image analysis we detect inward-directed keratin flow with highest rates in the cell periphery. The keratin flow correlates with speed and trajectory of migration. Changes in fibronectin-coating density and substrate stiffness induces concordant changes in migration speed and keratin flow. When keratinocytes are pseudo-confined on stripes, migration speed and keratin flow are reduced affecting the latter disproportionately. The regulation of keratin flow is linked to the regulation of actin flow. Local speed and direction of keratin and actin flow are very similar in migrating keratinocytes with keratin flow lagging behind actin flow. Conversely, reduced actin flow in areas of high keratin density indicates an inhibitory function of keratins on actin dynamics. Together, we propose that keratins enhance persistence of migration by directing actin dynamics and that the interplay of keratin and actin dynamics is modulated by matrix adhesions.

Rab11a-Rab8a cascade regulates the formation of tunneling nanotubes through vesicle recycling

Tunneling nanotubes (TNTs) are actin-enriched membranous channels enabling cells to communicate over long distances. TNT-like structures form between various cell types and mediate the exchange of different cargos, such as ions, vesicles, organelles and pathogens; thus, they may play a role in physiological conditions and diseases (e.g. cancer and infection). TNTs also allow the intercellular passage of protein aggregates related to neurodegenerative diseases, thus propagating protein misfolding. Understanding the mechanism of TNT formation is mandatory in order to reveal the mechanism of disease propagation and to uncover their physiological function. Vesicular transport controlled by the small GTPases Rab11a and Rab8a can promote the formation of different plasma membrane protrusions (filopodia, cilia and neurites). Here, we report that inhibiting membrane recycling reduces the number of TNT-connected cells and that overexpression of Rab11a and Rab8a increases the number of TNT-connected cells and the propagation of vesicles between cells in co-culture. We demonstrate that these two Rab GTPases act in a cascade in which Rab11a activation of Rab8a is independent of Rabin8. We also show that VAMP3 acts downstream of Rab8a to regulate TNT formation.

Differential identity of Filopodia and Tunneling Nanotubes revealed by the opposite functions of actin regulatory complexes

Tunneling Nanotubes (TNTs) are actin enriched filopodia-like protrusions that play a pivotal role in long-range intercellular communication. Different pathogens use TNT-like structures as "freeways" to propagate across cells. TNTs are also implicated in cancer and neurodegenerative diseases, making them promising therapeutic targets. Understanding the mechanism of their formation, and their relation with filopodia is of fundamental importance to uncover their physiological function, particularly since filopodia, differently from TNTs, are not able to mediate transfer of cargo between distant cells. Here we studied different regulatory complexes of actin, which play a role in the formation of both these structures. We demonstrate that the filopodia-promoting CDC42/IRSp53/VASP network negatively regulates TNT formation and impairs TNT-mediated intercellular vesicle transfer. Conversely, elevation of Eps8, an actin regulatory protein that inhibits the extension of filopodia in neurons, increases TNT formation. Notably, Eps8-mediated TNT induction requires Eps8 bundling but not its capping activity. Thus, despite their structural similarities, filopodia and TNTs form through distinct molecular mechanisms. Our results further suggest that a switch in the molecular composition in common actin regulatory complexes is critical in driving the formation of either type of membrane protrusion.

Traction microscopy to identify force modulation in subresolution adhesions

We present a reconstruction algorithm that resolves cellular tractions in diffraction-limited nascent adhesions (NAs). The enabling method is the introduction of sparsity regularization to the solution of the inverse problem, which suppresses noise without underestimating traction magnitude. We show that NAs transmit a distinguishable amount of traction and that NA maturation depends on traction growth rate. A software package implementing this numerical approach is provided.

The Frog Skin-Derived Antimicrobial Peptide Esculentin-1a(1-21)NH2 Promotes the Migration of Human HaCaT Keratinocytes in an EGF Receptor-Dependent Manner: A Novel Promoter of Human Skin Wound Healing?

One of the many functions of skin is to protect the organism against a wide range of pathogens. Antimicrobial peptides (AMPs) produced by the skin epithelium provide an effective chemical shield against microbial pathogens. However, whereas antibacterial/antifungal activities of AMPs have been extensively characterized, much less is known regarding their wound healing-modulatory properties. By using an in vitro re-epithelialisation assay employing special cell-culture inserts, we detected that a derivative of the frog-skin AMP esculentin-1a, named esculentin-1a(1-21)NH₂, significantly stimulates migration of immortalized human keratinocytes (HaCaT cells) over a wide range of peptide concentrations (0.025–4 μM), and this notably more efficiently than human cathelicidin (LL-37). This activity is preserved in primary human epidermal keratinocytes. By using appropriate inhibitors and an enzyme-linked immunosorbent assay we found that the peptide-induced cell migration involves activation of the epidermal growth factor receptor and STAT3 protein. These results suggest that esculentin-1a(1-21)NH₂ now deserves to be tested in standard wound healing assays as a novel candidate promoter of skin re-epithelialisation. The established ability of esculentin-1a(1-21)NH₂ to kill microbes without harming mammalian cells, namely its high anti-Pseudomonal activity, makes this AMP a particularly attractive candidate wound healing promoter, especially in the management of chronic, often Pseudomonas-infected, skin ulcers.

F-actin bundles direct the initiation and orientation of lamellipodia through adhesion-based signaling

Mesenchymal cells such as fibroblasts are weakly polarized and reorient directionality by a lamellipodial branching mechanism that is stabilized by phosphoinositide 3-kinase (PI3K) signaling. However, the mechanisms by which new lamellipodia are initiated and directed are unknown. Using total internal reflection fluorescence microscopy to monitor cytoskeletal and signaling dynamics in migrating cells, we show that peripheral F-actin bundles/filopodia containing fascin-1 serve as templates for formation and orientation of lamellipodia. Accordingly, modulation of fascin-1 expression tunes cell shape, quantified as the number of morphological extensions. Ratiometric imaging reveals that F-actin bundles/filopodia play both structural and signaling roles, as they prime the activation of PI3K signaling mediated by integrins and focal adhesion kinase. Depletion of fascin-1 ablated fibroblast haptotaxis on fibronectin but not platelet-derived growth factor chemotaxis. Based on these findings, we conceptualize haptotactic sensing as an exploration, with F-actin bundles directing and lamellipodia propagating the process and with signaling mediated by adhesions playing the role of integrator.

Maturation of filopodia shaft adhesions is upregulated by local cycles of lamellipodia advancements and retractions

While cell-substrate adhesions that form between the protruding edge of a spreading cell and flat surfaces have been studied extensively, processes that regulate the maturation of filopodia adhesions are far less characterized. Since little is known about how the kinetics of formation or disassembly of filopodia adhesions is regulated upon integration into the lamellum, a kinetic analysis of the formation and disassembly of filopodia adhesions was conducted at the leading edge of β3-integrin-EGFP-expressing rat embryonic fibroblasts spreading on fibronectin-coated glass or on soft polyacrylamide gels. Filopodia β3-integrin adhesions matured only if the lamellipodium in their immediate vicinity showed cyclic protrusions and retractions. Filopodia β3-integrin shaft adhesions elongated rapidly when they were overrun by the advancing lamellipodium. Subsequently and once the lamellipodium stopped its advancement at the distal end of the filopodia β3-integrin adhesion, these β3-integrin shaft adhesions started to grow sidewise and colocalize with the newly assembled circumferential actin stress fibers. In contrast, the suppression of the cyclic protrusions and retractions of the lamellipodium by blocking myosin light chain kinase suppressed the growth of filopodia adhesion and resulted in the premature disassembly of filopodia adhesions. The same failure to stabilize those adhesions was found for the advancing lamellipodium that rapidly overran filopodia shaft adhesions without pausing as seen often during fast cell spreading. In turn, plating cells on soft polyacrylamide gels resulted in a reduction of lamellipodia activity, which was partially restored locally by the presence of filopodia adhesions. Thus filopodia adhesions could also mature and be integrated into the lamellum for fibroblasts on soft polyacrylamide substrates.

Ena/VASP regulates mDia2-initiated filopodial length, dynamics, and function

Filopodia are long plasma membrane extensions involved in the formation of adhesive, contractile, and protrusive actin-based structures in spreading and migrating cells. Whether filopodia formed by different molecular mechanisms equally support these cellular functions is unresolved. We used Enabled/vasodilator-stimulated phosphoprotein (Ena/VASP)-deficient MV(D7) fibroblasts, which are also devoid of endogenous mDia2, as a model system to investigate how these different actin regulatory proteins affect filopodia morphology and dynamics independently of one another. Filopodia initiated by either Ena/VASP or mDia2 contained similar molecular inventory but differed significantly in parameters such as number, length, F-actin organization, lifetime, and protrusive persistence. Moreover, in the absence of Ena/VASP, filopodia generated by mDia2 did not support initiation of integrin-dependent signaling cascades required for adhesion and subsequent lamellipodial extension, thereby causing a defect in early cell spreading. Coexpression of VASP with constitutively active mDia2(M/A) rescued these early adhesion defects. We conclude that Ena/VASP and mDia2 support the formation of filopodia with significantly distinct properties and that Ena/VASP regulates mDia2-initiated filopodial morphology, dynamics, and function.

How filopodia pull: what we know about the mechanics and dynamics of filopodia

In recent years, the dynamic, hair-like cell protrusions called filopodia have attracted considerable attention. They have been found in a multitude of different cell types and are often called "sensory organelles," since they seem to sense the mechanical and chemical environment of a cell. Once formed, filopodia can exhibit complex behavior, they can grow and retract, push or pull, and transform into distinct structures. They are often found to make first adhesive contact with the extracellular matrix, pathogens or with adjacent cells, and to subsequently exert pulling forces. Much is known about the cytoskeletal players involved in filopodia formation, but only recently have we started to explore the mechanics of filopodia together with the related cytoskeletal dynamics. This review summarizes current advancements in our understanding of the mechanics and dynamics of filopodia, with a focus on the molecular mechanisms behind filopodial force exertion.

Insights into the origin of metazoan filopodia and microvilli

Filopodia are fine actin-based cellular projections used for both environmental sensing and cell motility, and they are essential organelles for metazoan cells. In this study, we reconstruct the origin of metazoan filopodia and microvilli. We first report on the evolutionary assembly of the filopodial molecular toolkit and show that homologs of many metazoan filopodial components, including fascin and myosin X, were already present in the unicellular or colonial progenitors of metazoans. Furthermore, we find that the actin crosslinking protein fascin localizes to filopodia-like structures and microvilli in the choanoflagellate Salpingoeca rosetta. In addition, homologs of filopodial genes in the holozoan Capsaspora owczarzaki are upregulated in filopodia-bearing cells relative to those that lack them. Therefore, our findings suggest that proteins essential for metazoan filopodia and microvilli are functionally conserved in unicellular and colonial holozoans and that the last common ancestor of metazoans bore a complex and specific filopodial machinery.

Progesterone inhibits the migration and invasion of A549 lung cancer cells through membrane progesterone receptor α-mediated mechanisms

Lung cancer is the leading cause of cancer morbidity and mortality in the world. The incidence of lung cancer, particularly lung adenocarcinoma, is increasing in women compared to men. The role of sex hormones in the development of lung cancer has attracted substantial interest, but remains largely unknown. In this study, we demonstrated that membrane progesterone receptor α (mPRα) was expressed in a lung adenocarcinoma cell line, A549, and was located on the cell membrane. In additional experiments, we found that mPRα functioned as an essential mediator for progesterone (P4)-induced inhibitory effects on cell migration and invasion of A549 cells. Furthermore, PP1 (an Src pathway inhibitor), when co-incubated with P4, synchronously enhanced the inhibitory effects of P4 on cell migration and invasion. To explore the mechanisms of inhibition, we found that P4 and PP1 induced a cascade of molecular signaling events, such as dephosphorylation of focal adhesion kinase (FAK) and downregulation of matrix metalloproteinase 9 (MMP-9). Our study provides a mechanistic view on the effects of P4 through mPRα→Src/FAK relevant pathways in human lung adenocarcinoma cells and may aid in the development of novel therapeutic tools for the treatment of lung cancer.

Actinin-4 in keratinocytes regulates motility via an effect on lamellipodia stability and matrix adhesions

During wound repair, epidermal cells at the edge of an injury establish front-rear polarity through orchestrated changes in their cytoskeleton and adhesion structures. The polarity and directed migration of such cells is determined by the assembly, extension, and stabilization of a lamellipodium. Actinin-4 associates with lamellipodia and has been implicated in regulating lamellipodial structure, function and assembly. To study the functions of actinin-4 in human keratinocytes, we used shRNA to generate knockdown cells and compared their motility behavior and matrix adhesion assembly to scrambled shRNA treated control keratinocytes. Actinin-4 knockdown keratinocytes lack polarity, assemble multiple lamellipodia with a 2× increased area over controls, display reduced activity of the actin remodeling protein cofilin, and fail to migrate in a directional manner. This motility defect is rescued by plating knockdown cells on preformed laminin-332 matrix. In actinin-4-knockdown keratinocytes, focal contact area is increased by 25%, and hemidesmosome proteins are mislocalized. Specifically, α6β4 integrin localizes to large lamellipodial extensions, displays reduced dynamics, and fails to recruit its bullous pemphigoid antigen binding partners. Together, our data indicate a role for actinin-4 in regulating the steering mechanism of keratinocytes via profound effects on their matrix adhesion sites.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

SIGNIFICANCE

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

Fascin promotes filopodia formation independent of its role in actin bundling

Mutation of a critical residue of fascin eliminates the protein’s actin-bundling activity but maintains its positive role in filopodia formation

Fascin is an evolutionarily conserved actin-binding protein that plays a key role in forming filopodia. It is widely thought that this function involves fascin directly bundling actin filaments, which is controlled by an N-terminal regulatory serine residue. In this paper, by studying cellular processes in Drosophila melanogaster that require fascin activity, we identify a regulatory residue within the C-terminal region of the protein (S289). Unexpectedly, although mutation (S289A) of this residue disrupted the actin-bundling capacity of fascin, fascin S289A fully rescued filopodia formation in fascin mutant flies. Live imaging of migrating macrophages in vivo revealed that this mutation restricted the localization of fascin to the distal ends of filopodia. The corresponding mutation of human fascin (S274) similarly affected its interaction with actin and altered filopodia dynamics within carcinoma cells. These data reveal an evolutionarily conserved role for this regulatory region and unveil a function for fascin, uncoupled from actin bundling, at the distal end of filopodia.

S100A4 downregulates filopodia formation through increased dynamic instability

Cell migration requires the initial formation of cell protrusions, lamellipodia and/or filopodia, the attachment of the leading lamella to extracellular cues and the formation and efficient recycling of focal contacts at the leading edge. The small calcium binding EF-hand protein S100A4 has been shown to promote cell motility but the direct molecular mechanisms responsible remain to be elucidated. In this work, we provide new evidences indicating that elevated levels of S100A4 affect the stability of filopodia and prevent the maturation of focal complexes. Increasing the levels of S100A4 in a rat mammary benign tumor derived cell line results in acquired cellular migration on the wound healing scratch assay. At the cellular levels, we found that high levels of S100A4 induce the formation of many nascent filopodia, but that only a very small and limited number of those can stably adhere and mature, as opposed to control cells, which generate fewer protrusions but are able to maintain these into more mature projections. This observation was paralleled by the fact that S100A4 overexpressing cells were unable to establish stable focal adhesions. Using different truncated forms of the S100A4 proteins that are unable to bind to myosin IIA, our data suggests that this newly identified functions of S100A4 is myosin-dependent, providing new understanding on the regulatory functions of S100A4 on cellular migration.

The filopodium: a stable structure with highly regulated repetitive cycles of elongation and persistence depending on the actin cross-linker fascin

The ability of mammalian cells to adhere and to migrate is an essential prerequisite to form higher organisms. Early migratory events include substrate sensing, adhesion formation, actin bundle assembly and force generation. Latest research revealed that filopodia are important not only for sensing the substrate but for all of the aforementioned highly regulated processes. However, the exact regulatory mechanisms are still barely understood. Here, we demonstrate that filopodia of human keratinocytes exhibit distinct cycles of repetitive elongation and persistence. A single filopodium thereby is able to initiate the formation of several stable adhesions. Every single filopodial cycle is characterized by an elongation phase, followed by a stabilization time and in many cases a persistence phase. The whole process is strongly connected to the velocity of the lamellipodial leading edge, characterized by a similar phase behavior with a slight time shift compared to filopodia and a different velocity. Most importantly, re-growth of existing filopodia is induced at a sharply defined distance between the filopodial tip and the lamellipodial leading edge. On the molecular level this re-growth is preceded by a strong filopodial reduction of the actin bundling protein fascin. This reduction is achieved by a switch to actin polymerization without fascin incorporation at the filopodial tip and therefore subsequent out-transport of the cross-linker by actin retrograde flow.

Quantitative mapping of averaged focal adhesion dynamics in migrating cells by shape normalization

The spatially ordered formation and disassembly of focal adhesions is a basic requirement for effective cell locomotion. Because focal adhesions couple the contractile actin-myosin network to the substrate, their distribution determines the pattern of traction forces propelling the cell in a certain direction. In the present study, we quantitatively analyzed the spatial patterning of cell-substrate adhesion in migrating cells by mapping averaged focal adhesion growth dynamics to a standardized cell coordinate system. These maps revealed distinct zones of focal adhesion assembly, disassembly and stability and were strongly interrelated with corresponding actin flow and traction force patterns. Moreover, the mapping technique enables precise detection of even minute responses of adhesion dynamics upon targeted signaling perturbations. For example, the partial inhibition of vinculin phosphorylation was followed by the reduced number of newly formed adhesions, whereas growth dynamics of existing adhesions remained unaffected.

Protein kinase A activity and anchoring are required for ovarian cancer cell migration and invasion

Epithelial ovarian cancer (EOC) is the deadliest of the gynecological malignancies, due in part to its clinically occult metastasis. Therefore, understanding the mechanisms governing EOC dissemination and invasion may provide new targets for antimetastatic therapies or new methods for detection of metastatic disease. The cAMP-dependent protein kinase (PKA) is often dysregulated in EOC. Furthermore, PKA activity and subcellular localization by A-kinase anchoring proteins (AKAPs) are important regulators of cytoskeletal dynamics and cell migration. Thus, we sought to study the role of PKA and AKAP function in both EOC cell migration and invasion. Using the plasma membrane-directed PKA biosensor, pmAKAR3, and an improved migration/invasion assay, we show that PKA is activated at the leading edge of migrating SKOV-3 EOC cells, and that inhibition of PKA activity blocks SKOV-3 cell migration. Furthermore, we show that while the PKA activity within the leading edge of these cells is mediated by anchoring of type-II regulatory PKA subunits (RII), inhibition of anchoring of either RI or RII PKA subunits blocks cell migration. Importantly, we also show--for the first time--that PKA activity is up-regulated at the leading edge of SKOV-3 cells during invasion of a three-dimensional extracellular matrix and, as seen for migration, inhibition of either PKA activity or AKAP-mediated PKA anchoring blocks matrix invasion. These data are the first to demonstrate that the invasion of extracellular matrix by cancer cells elicits activation of PKA within the invasive leading edge and that both PKA activity and anchoring are required for matrix invasion. These observations suggest a role for PKA and AKAP activity in EOC metastasis.