Three functionally distinct adhesions in filopodia: shaft adhesions control lamellar extension

Induction of filopodia formation by α-Actinin-2 via RelA with a feedforward activation loop promoting overt bone marrow metastasis of gastric cancer

BACKGROUND

Bone marrow metastasis (BMM) is underestimated in gastric cancer (GC). GC with BMM frequently complicate critical hematological abnormalities like diffused intravascular coagulation and microangiopathic hemolytic anemia, which constitute a highly aggressive GC (HAGC) subtype. HAGC present a very poor prognosis with peculiar clinical and pathological features when compared with not otherwise specified advanced GC (NAGC). But the molecular mechanisms underlying BMM from GC remain rudimentary.

METHODS

The transcriptomic difference between HAGC and NAGC were analyzed. Genes that were specifically upregulated in HAGC were identified, and their effect on cell migration and invasion was studied. The function of ACTN2 gene were confirmed by GC cell lines, bone-metastatic animal model and patients' tissues. Furthermore, the molecular mechanism of ACTN2 derived-BMM was explored by multiple immunofluorescence staining, western blot, chromatin immunoprecipitation, and luciferase reporter assays.

RESULTS

We elucidated the key mechanisms of BMM depending on the transcriptomic difference between HAGC and NAGC. Five genes specifically upregulated in HAGC were assessed their effect on cell migration and invasion. The ACTN2 gene encoding protein α-Actinin-2 was detected enhanced the metastatic capability and induced BMM of GC cells in mouse models. Mechanically, α-Actinin-2 was involved in filopodia formation where it promoted the Actin filament cross-linking by replacing α-Actinin-1 to form α-Actinin-2:α-Actinin-4 complexes in GC cells. Moreover, NF-κB subunit RelA and α-Actinin-2 formed heterotrimers in the nuclei of GC cells. As a direct target of RelA:α-Actinin-2 heterotrimers, the ACTN2 gene was a positive auto-regulatory loop for α-Actinin-2 expression.

CONCLUSIONS

We demonstrated a link between filopodia, BMM and ACTN2 activation, where a feedforward activation loop between ACTN2 and RelA is established via actin in response to distant metastasis. Given the novel filopodia formation function and the new mechanism of BMM in GC, we propose ACTN2 as a druggable molecular vulnerability that may provide potential therapeutic benefit against BMM of GC.

Activated I-BAR IRSp53 clustering controls the formation of VASP-actin-based membrane protrusions

Filopodia are actin-rich membrane protrusions essential for cell morphogenesis, motility, and cancer invasion. How cells control filopodium initiation on the plasma membrane remains elusive. We performed experiments in cellulo, in vitro, and in silico to unravel the mechanism of filopodium initiation driven by the membrane curvature sensor IRSp53 (insulin receptor substrate protein of 53 kDa). We showed that full-length IRSp53 self-assembles into clusters on membranes depending on PIP₂. Using well-controlled in vitro reconstitution systems, we demonstrated that IRSp53 clusters recruit the actin polymerase VASP (vasodilator-stimulated phosphoprotein) to assemble actin filaments locally on membranes, leading to the generation of actin-filled membrane protrusions reminiscent of filopodia. By pulling membrane nanotubes from live cells, we observed that IRSp53 can only be enriched and trigger actin assembly in nanotubes at highly dynamic membrane regions. Our work supports a regulation mechanism of IRSp53 in its attributes of curvature sensation and partner recruitment to ensure a precise spatial-temporal control of filopodium initiation.

Activated I-BAR IRSp53 clustering controls the formation of VASP-actin-based membrane protrusions

Filopodia are actin-rich membrane protrusions essential for cell morphogenesis, motility, and cancer invasion. How cells control filopodium initiation on the plasma membrane remains elusive. We performed experiments in cellulo, in vitro, and in silico to unravel the mechanism of filopodium initiation driven by the membrane curvature sensor IRSp53 (insulin receptor substrate protein of 53 kDa). We showed that full-length IRSp53 self-assembles into clusters on membranes depending on PIP₂. Using well-controlled in vitro reconstitution systems, we demonstrated that IRSp53 clusters recruit the actin polymerase VASP (vasodilator-stimulated phosphoprotein) to assemble actin filaments locally on membranes, leading to the generation of actin-filled membrane protrusions reminiscent of filopodia. By pulling membrane nanotubes from live cells, we observed that IRSp53 can only be enriched and trigger actin assembly in nanotubes at highly dynamic membrane regions. Our work supports a regulation mechanism of IRSp53 in its attributes of curvature sensation and partner recruitment to ensure a precise spatial-temporal control of filopodium initiation.

Mechanical processes underlying precise and robust cell matching

During the development of complicated multicellular organisms, the robust formation of specific cell-cell connections (cell matching) is required for the generation of precise tissue structures. Mismatches or misconnections can lead to various diseases. Diverse mechanical cues, including differential adhesion and temporally varying cell contractility, are involved in regulating the process of cell-cell recognition and contact formation. Cells often start the process of cell matching through contact via filopodia protrusions, mediated by specific adhesion interactions at the cell surface. These adhesion interactions give rise to differential mechanical signals that can be further perceived by the cells. In conjunction with contractions generated by the actomyosin networks within the cells, this differentially coded adhesion information can be translated to reposition and sort cells. Here, we review the role of these different cell matching components and suggest how these mechanical factors cooperate with each other to facilitate specificity in cell-cell contact formation.

Enhanced mechanosensing of cells in synthetic 3D matrix with controlled biophysical dynamics

3D culture of cells in designer biomaterial matrices provides a biomimetic cellular microenvironment and can yield critical insights into cellular behaviours not available from conventional 2D cultures. Hydrogels with dynamic properties, achieved by incorporating either degradable structural components or reversible dynamic crosslinks, enable efficient cell adaptation of the matrix and support associated cellular functions. Herein we demonstrate that given similar equilibrium binding constants, hydrogels containing dynamic crosslinks with a large dissociation rate constant enable cell force-induced network reorganization, which results in rapid stellate spreading, assembly, mechanosensing, and differentiation of encapsulated stem cells when compared to similar hydrogels containing dynamic crosslinks with a low dissociation rate constant. Furthermore, the static and precise conjugation of cell adhesive ligands to the hydrogel subnetwork connected by such fast-dissociating crosslinks is also required for ultra-rapid stellate spreading (within 18 h post-encapsulation) and enhanced mechanosensing of stem cells in 3D. This work reveals the correlation between microscopic cell behaviours and the molecular level binding kinetics in hydrogel networks. Our findings provide valuable guidance to the design and evaluation of supramolecular biomaterials with cell-adaptable properties for studying cells in 3D cultures.

3D culture systems can provide critical insights into cellular behaviour. Here, the authors study the binding timescale of dynamic crosslinks and the conjugation stability of cell-adhesive ligands in cell–hydrogel network interactions to evaluate the impact on stem cell behaviour, mechanosensing and differentiation.

Active perception during angiogenesis: filopodia speed up Notch selection of tip cells in silico and in vivo

How do cells make efficient collective decisions during tissue morphogenesis? Humans and other organisms use feedback between movement and sensing known as 'sensorimotor coordination' or 'active perception' to inform behaviour, but active perception has not before been investigated at a cellular level within organs. Here we provide the first proof of concept in silico/in vivo study demonstrating that filopodia (actin-rich, dynamic, finger-like cell membrane protrusions) play an unexpected role in speeding up collective endothelial decisions during the time-constrained process of 'tip cell' selection during blood vessel formation (angiogenesis). We first validate simulation predictions in vivo with live imaging of zebrafish intersegmental vessel growth. Further simulation studies then indicate the effect is due to the coupled positive feedback between movement and sensing on filopodia conferring a bistable switch-like property to Notch lateral inhibition, ensuring tip selection is a rapid and robust process. We then employ measures from computational neuroscience to assess whether filopodia function as a primitive (basal) form of active perception and find evidence in support. By viewing cell behaviour through the 'basal cognitive lens' we acquire a fresh perspective on the tip cell selection process, revealing a hidden, yet vital time-keeping role for filopodia. Finally, we discuss a myriad of new and exciting research directions stemming from our conceptual approach to interpreting cell behaviour. This article is part of the theme issue 'Basal cognition: multicellularity, neurons and the cognitive lens'.

Cellular response of human apical papilla cells to calcium hydroxide and tricalcium silicate-based cements

Background

This study aimed to evaluate the biological response of human apical papilla cells to different calcium hydroxide formulations and three tricalcium silicate-based materials.

Methods

Primary cells were obtained from explants of young immature premolars. 20,000 cells adhered for 24 h over discs of Biodentine™, ProRoot®MTA, BioRoot®RCS and calcium hydroxide mixed either with sodium chloride 0.9%w/v or polyethylene glycol and UltraCal® were used to evaluate cell adhesion by scanning electron microscopy and cell viability by MTT assay.

Results

Cells adhered to ProRoot®MTA showed an increase of F-actin like protrusions, suggesting bioactivity. Cells adhered to UltraCal® show protrusion such as filopodia. On the contrary, cells adhered to BioRoot®RCS showed no signs of any cellular protrusion. Regarding viability between the materials, we found a higher percentage of viability in cells cultured over discs of Biodentine™ and ProRoot®MTA.

Conclusion

ProRoot®MTA and Biodentine™ exhibit a better cellular response of human apical papilla cells in vitro conditions compared to BioRoot® and calcium hydroxide diluted in sodium chloride.

Biomechanical Aspects of Actin Bundle Dynamics

Lamellipodial and filopodial protrusions are two of the main aggregate types of filamentous actin in living cells. Even though filopodia are essential to a range of vital cell functions, the mechanisms leading to their formation are still debated. Filopodia are relatively stiff and rod-like structures that are embedded in the highly dynamic framework of the backward flowing meshwork of the lamellipodium. Phenomena such as lateral filopodia drift and collision events suggest that mechanical aspects play a significant role in filopodia dynamics. In this paper, we systematically analyze the interplay between the backward flow of actin in the lamellipodium and the drift velocity of actin bundles, that we identify to be filopodia, in a quantitative manner in cells of given morphology and controlled myosin activity. Moreover, we study mechanical aspects of fusion of actin bundles drifting laterally in the lamellipodium. We find that the dynamics of actin bundles drift and fusion can be captured in a mechanical framework, which leads to a model of actin bundles orientation.

Topographic cues reveal filopodia-mediated cell locomotion in 3D microenvironment

In cell-material interactions, the formation and functioning of filopodia have been demonstrated to be very sensitive to topographic cues. However, substrate-exploring functions of filopodia in a 3D microenvironment remain elusive. In this study, the silk fibroin film with a micropillar structure was prepared to reveal a filopodial-mediated cell response to 3D topographic cues. The micropillars provided a confined space for cell spreading by a simplified 3D structure, allowing initial cells to settle on the bottom of substrates rather than on the top of micropillars. Shortly after cell adhesion, the authors describe how cells transform from a filopodia-rich spherical cell state to a lamellipodia-dominated state that enables cell to climb along micropillars and spread on the top of the micropillars. The authors found that filopodia not only served as sensors for pathfinding but also provided nucleation scaffolds for the formation and orientation of minilamellipodia on the micropillar substrate. On the route of long filopodial extension following micropillars, all three functional filopodial adhesions have the ability to form veil-like minilamellipodium, simply by tethering the filopodium to the micropillars. Stable filopodia contacts consistently stimulated the local protrusion of a lamellipodium, which ultimately steered cell migration. Their results suggest the filopodia-mediated cell locomotion in the 3D microenvironment using a filopodia-to-minilamellipodium transformation mechanism.

Polyplexes Are Endocytosed by and Trafficked within Filopodia

The improvement of nonviral gene therapies relies to a large extent on understanding many fundamental physical and biological properties of these systems. This includes interactions of synthetic delivery systems with the cell and mechanisms of trafficking delivery vehicles, which remain poorly understood on both the extra- and intracellular levels. In this study, the mechanisms of cellular internalization and trafficking of polymer-based nanoparticle complexes consisting of polycations and nucleic acids, termed polyplexes, have been observed in detail at the cellular level. For the first time evidence has been obtained that filopodia, actin projections that radiate out from the surface of cells, serve as a route for the direct endocytosis of polyplexes. Confocal microscopy images demonstrated that filopodia on HeLa cells detect external polyplexes and extend into the extracellular milieu to internalize these particles. Polyplexes are observed to be internalized into membrane-bound vesicles (i.e., clathrin-coated pits and caveolae) directly within filopodial projections and are subsequently transported along actin to the main cell body for potential delivery of the nucleic acids to the nucleus. The kinetics and speed of polyplex trafficking have also been measured. The polyplex-loaded vesicles were also discovered to traffic between two cells within filopodial bridges. These findings provide novel insight into the early events of cellular contact with polyplexes through filopodial-based interactions in addition to endocytic vesicle trafficking-an important fundamental discovery to enable advancement of nonviral gene editing, nucleic acid therapies, and biomedical materials.

Cripto-1 localizes to dynamic and shed filopodia associated with cellular migration in glioblastoma cells

Cripto-1 is a protein participating in tissue orientation during embryogenesis but has also been implicated in a wide variety of cancers, such as colon, lung and breast cancer. Cripto-1 plays a role in the regulation of different pathways, including TGF-β/Smad and Wnt/β-catenin, which are highly associated with cell migration both during embryonal development and cancer progression. Little is known about the detailed subcellular localization of cripto-1 and how it participates in the directional movement of cells. In this study, the subcellular localization of cripto-1 in glioblastoma cells was investigated in vitro with high-resolution microscopy techniques. Cripto-1 was found to be localized to dynamic and shed filopodia and transported between cells through tunneling nanotubes. Our results connect the refined subcellular localization of cripto-1 to its functions in cellular orientation and migration.

Roles for Ena/VASP proteins in FMNL3-mediated filopodial assembly

Filopodia are actin-dependent finger-like structures that protrude from the plasma membrane. Actin filament barbed-end-binding proteins localized to filopodial tips are key to filopodial assembly. Two classes of barbed-end-binding proteins are formins and Ena/VASP proteins, and both classes have been localized to filopodial tips in specific cellular contexts. Here, we examine the filopodial roles of the FMNL formins and Ena/VASP proteins in U2OS cells. FMNL3 suppression reduces filopodial assembly by 90%, and FMNL3 is enriched at >95% of filopodial tips. Suppression of VASP or Mena (also known as ENAH) reduces filopodial assembly by >75%. However, VASP and Mena do not display consistent filopodial tip localization, but are enriched in focal adhesions (FAs). Interestingly, >85% of FMNL3-containing filopodia are associated with FAs. Two situations increase Ena/VASP filopodial localization: (1) expression of myosin-X, and (2) actively spreading cells. In spreading cells, filopodia often mark sites of nascent adhesions. Interestingly, VASP suppression in spreading cells causes a significant increase in adhesion assembly at filopodial tips. This work demonstrates that, in U2OS cells, Ena/VASP proteins play roles in filopodia beyond those at filopodial tips.This article has an associated First Person interview with the first author of the paper.

How does calcium interact with the cytoskeleton to regulate growth cone motility during axon pathfinding?

The precision with which neurons form connections is crucial for the normal development and function of the nervous system. The development of neuronal circuitry in the nervous system is accomplished by axon pathfinding: a process where growth cones guide axons through the embryonic environment to connect with their appropriate synaptic partners to form functional circuits. Despite intense efforts over many years to understand how this process is regulated, the complete repertoire of molecular mechanisms that govern the growth cone cytoskeleton and hence motility, remain unresolved. A central tenet in the axon guidance field is that calcium signals regulate growth cone behaviours such as extension, turning and pausing by regulating rearrangements of the growth cone cytoskeleton. Here, we provide evidence that not only the amplitude of a calcium signal is critical for growth cone motility but also the source of calcium mobilisation. We provide an example of this idea by demonstrating that manipulation of calcium signalling via L-type voltage gated calcium channels can perturb sensory neuron motility towards a source of netrin-1. Understanding how calcium signals can be transduced to initiate cytoskeletal changes represents a significant gap in our current knowledge of the mechanisms that govern axon guidance, and consequently the formation of functional neural circuits in the developing nervous system.

Urinary bladder extracellular matrix hydrogels and matrix-bound vesicles differentially regulate central nervous system neuron viability and axon growth and branching

Central nervous system neurons often degenerate after trauma due to the inflammatory innate immune response to injury, which can lead to neuronal cell death, scarring, and permanently lost neurologic function. Extracellular matrix bioscaffolds, derived by decellularizing healthy tissues, have been widely used in both preclinical and clinical studies to promote positive tissue remodeling, including neurogenesis, in numerous tissues, with extracellular matrix from homologous tissues often inducing more positive responses. Extracellular matrix hydrogels are liquid at room temperature and enable minimally invasive extracellular matrix injections into central nervous system tissues, before gelation at 37℃. However, few studies have analyzed how extracellular matrix hydrogels influence primary central nervous system neuron survival and growth, and whether central nervous system and non-central nervous system extracellular matrix specificity is critical to neuronal responses. Urinary bladder extracellular matrix hydrogels increase both primary hippocampal neuron survival and neurite growth to similar or even greater extents, suggesting extracellular matrix from non-homologous tissue sources, such as urinary bladder matrix-extracellular matrix, may be a more economical and safer alternative to developing central nervous system extracellular matrices for central nervous system applications. Additionally, we show matrix-bound vesicles derived from urinary bladder extracellular matrix are endocytosed by hippocampal neurons and positively regulate primary hippocampal neuron neurite growth. Matrix-bound vesicles carry protein and RNA cargos, including noncoding RNAs and miRNAs that map to the human genome and are known to regulate cellular processes. Thus, urinary bladder matrix-bound vesicles provide natural and transfectable cargoes which offer new experimental tools and therapeutic applications to study and treat central nervous system neuron injury.

Asymmetric division coordinates collective cell migration in angiogenesis

The asymmetric division of stem or progenitor cells generates daughters with distinct fates and regulates cell diversity during tissue morphogenesis. However, roles for asymmetric division in other more dynamic morphogenetic processes, such as cell migration, have not previously been described. Here we combine zebrafish in vivo experimental and computational approaches to reveal that heterogeneity introduced by asymmetric division generates multicellular polarity that drives coordinated collective cell migration in angiogenesis. We find that asymmetric positioning of the mitotic spindle during endothelial tip cell division generates daughters of distinct size with discrete 'tip' or 'stalk' thresholds of pro-migratory Vegfr signalling. Consequently, post-mitotic Vegfr asymmetry drives Dll4/Notch-independent self-organization of daughters into leading tip or trailing stalk cells, and disruption of asymmetry randomizes daughter tip/stalk selection. Thus, asymmetric division seamlessly integrates cell proliferation with collective migration, and, as such, may facilitate growth of other collectively migrating tissues during development, regeneration and cancer invasion.

Actin filament-microtubule interactions in axon initiation and branching

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

Human mesenchymal stem cell behavior on femtosecond laser-textured Ti-6Al-4V surfaces

AIM

The aim of the present work was to investigate ultrafast laser surface texturing as a surface treatment of Ti-6Al-4V alloy dental and orthopedic implants to improve osteoblastic commitment of human mesenchymal stem cells (hMSCs).

MATERIALS & METHODS

Surface texturing was carried out by direct writing with an Yb:KYW chirped-pulse regenerative amplification laser system with a central wavelength of 1030 nm and a pulse duration of 500 fs. The surface topography and chemical composition were investigated by scanning electron microscopy and x-ray photoelectron spectroscopy, respectively. Three types of surface textures with potential interest to improve implant osseointegration can be produced by this method: laser-induced periodic surface structures (LIPSSs); nanopillars (NPs); and microcolumns covered with LIPSSs, forming a bimodal roughness distribution. The potential of the laser treatment in improving hMSC differentiation was assessed by in vitro study of hMSCs spreading, adhesion, elongation and differentiation using epifluorescence microscopy at different times after cell seeding, after specific stainings and immunostainings.

RESULTS

Cell area and focal adhesion area were lower on the laser-textured surfaces than on a polished reference surface. Obviously, the laser-textured surfaces have an impact on cell shape. Osteoblastic commitment was observed independently of the surface topography after 2 weeks of cell seeding. When the cells were cultured (after 4 weeks of seeding) in osteogenic medium, LIPSS- and NP- textured surfaces enhanced matrix mineralization and bone-like nodule formation as compared with polished and microcolumn-textured surfaces.

CONCLUSION

The present work shows that surface nanotextures consisting of LIPSSs and NPs can, potentially, improve hMSC differentiation into an osteoblastic lineage.

PCP Signaling between Migrating Neurons and their Planar-Polarized Neuroepithelial Environment Controls Filopodial Dynamics and Directional Migration

The planar cell polarity (PCP) pathway is a cell-contact mediated mechanism for transmitting polarity information between neighboring cells. PCP “core components” (Vangl, Fz, Pk, Dsh, and Celsr) are essential for a number of cell migratory events including the posterior migration of facial branchiomotor neurons (FBMNs) in the plane of the hindbrain neuroepithelium in zebrafish and mice. While the mechanism by which PCP signaling polarizes static epithelial cells is well understood, how PCP signaling controls highly dynamic processes like neuronal migration remains an important outstanding question given that PCP components have been implicated in a range of directed cell movements, particularly during vertebrate development. Here, by systematically disrupting PCP signaling in a rhombomere-restricted manner we show that PCP signaling is required both within FBMNs and the hindbrain rhombomere 4 environment at the time when they initiate their migration. Correspondingly, we demonstrate planar polarized localization of PCP core components Vangl2 and Fzd3a in the hindbrain neuroepithelium, and transient localization of Vangl2 at the tips of retracting FBMN filopodia. Using high-resolution timelapse imaging of FBMNs in genetic chimeras we uncover opposing cell-autonomous and non-cell-autonomous functions for Fzd3a and Vangl2 in regulating FBMN protrusive activity. Within FBMNs, Fzd3a is required to stabilize filopodia while Vangl2 has an antagonistic, destabilizing role. However, in the migratory environment Fzd3a acts to destabilize FBMN filopodia while Vangl2 has a stabilizing role. Together, our findings suggest a model in which PCP signaling between the planar polarized neuroepithelial environment and FBMNs directs migration by the selective stabilization of FBMN filopodia.

Planar cell polarity (PCP) is a common feature of many animal tissues. This type of polarity is most obvious in cells that are organized into epithelial sheets, where PCP signaling components act to orient cells in the plane of the tissue. Although, PCP is best understood for its function in polarizing stable epithelia, PCP is also required for the dynamic process of cell migration in animal development and disease. The goal of this study was to determine how PCP functions to control cell migration. We used the migration of facial branchiomotor neurons in the zebrafish hindbrain, which requires almost the entire suite of PCP core components, to address this question. We present evidence that PCP signaling within migrating neurons, and between migrating neurons and cells of their migratory environment promote migration by regulating filopodial dynamics. Our results suggest that broadly conserved interactions between PCP components control the cytoskeleton in motile cells and non-motile epithelia alike.

Cell Invasion Dynamics into a Three Dimensional Extracellular Matrix Fibre Network

The dynamics of filopodia interacting with the surrounding extracellular matrix (ECM) play a key role in various cell-ECM interactions, but their mechanisms of interaction with the ECM in 3D environment remain poorly understood. Based on first principles, here we construct an individual-based, force-based computational model integrating four modules of 1) filopodia penetration dynamics; 2) intracellular mechanics of cellular and nuclear membranes, contractile actin stress fibers, and focal adhesion dynamics; 3) structural mechanics of ECM fiber networks; and 4) reaction-diffusion mass transfers of seven biochemical concentrations in related with chemotaxis, proteolysis, haptotaxis, and degradation in ECM to predict dynamic behaviors of filopodia that penetrate into a 3D ECM fiber network. The tip of each filopodium crawls along ECM fibers, tugs the surrounding fibers, and contracts or retracts depending on the strength of the binding and the ECM stiffness and pore size. This filopodium-ECM interaction is modeled as a stochastic process based on binding kinetics between integrins along the filopodial shaft and the ligands on the surrounding ECM fibers. This filopodia stochastic model is integrated into migratory dynamics of a whole cell in order to predict the cell invasion into 3D ECM in response to chemotaxis, haptotaxis, and durotaxis cues. Predicted average filopodia speed and that of the cell membrane advance agreed with experiments of 3D HUVEC migration at r(2) > 0.95 for diverse ECMs with different pore sizes and stiffness.

Filopodia in cell adhesion, 3D migration and cancer cell invasion

This review discusses recent advances in our understanding of the role filopodia and filopodia-like structures in cell adhesion and three dimensional (3D) cell migration both in vitro and in vivo. In particular, we focus on recent advances demonstrating that filopodia are involved in substrate tethering and environment sensing in vivo. We further discuss the emerging role of filopodia and filopodial proteins in tumor dissemination as mounting in vitro, in vivo and clinical evidence suggest that filopodia drive cancer cell invasion and highlight filopodia proteins as attractive therapeutic targets. Finally, we outline outstanding questions that remain to be addressed to elucidate the role of filopodia during 3D cell migration.

Deterministic control of mean alignment and elongation of neuron-like cells by grating geometry: a computational approach

Neuron-like cells are driven by their surrounding environment through local topography. A causal mechanotransductive web of topography-force relationships influences and controls complex cellular phenomena such as growth and alignment. This work aimed to provide a computational framework able to model the behaviour of neuron-like (PC12) cells on gratings, accounting for the twofold ability of topographical cues to simultaneously align and enhance the growth of cells. In particular, starting from the mechanical behaviour of the growth cone and filopodia, the effect of grating geometry (e.g., the periodicity and the size of grooves and ridges) on the neuritic mean alignment angle and on the outgrowth rate of cells was explored through theoretical tools and combinatorial simulations, which were able to predict (R(2) > 0.9) experimental data in a time range of 72-120 hours.

Gelatin modified ultrathin silk fibroin films for enhanced proliferation of cells

Silk fibroin (SF) films were modified with gelatin (G) to explore if such SF/G films could enhance the surface biocompatibility of silk as cell growth biomaterials. Ultrathin films were coated from aqueous SF solutions pre-mixed with different amounts of G. It was found that the SF/G blended films after methanol treatment were highly stable in physiological conditions. The incorporation of G smoothed the surface morphology of the SF/G films formed. Surface-exposed RGD sequences were successfully identified on the SF/G films through specific recognition of an integrin-mimicking peptide (bearing the sequence of CWDDGWLC). Cell culture experiments with 3T3 fibroblasts demonstrated that SF/G films with 1.2-20% (w/w) G gave clear improvement in promoting cell attachment and proliferation over pure SF films. Films containing 10-20% (w/w) of G showed cell attachment and growth even superior to the pure G films. The differences as observed from this study suggest that due to the lack of mechanical strength associated with its high solubility, G could not work alone as a cell growth scaffold. The enhanced cellular responses from the blended SF/G films must result from improvement in film stability arising from SF and in cytocompatibility arising from G. The results thus indicate the potential of the SF/G blends in tissue engineering and biomedical engineering where physical and biological properties could be manipulated via mixing either as bulk biomaterials or for coating purposes.

Promoting filopodial elongation in neurons by membrane-bound magnetic nanoparticles

Filopodia are 5-10 μm long processes that elongate by actin polymerization, and promote axon growth and guidance by exerting mechanical tension and by molecular signaling. Although axons elongate in response to mechanical tension, the structural and functional effects of tension specifically applied to growth cone filopodia are unknown. Here we developed a strategy to apply tension specifically to retinal ganglion cell (RGC) growth cone filopodia through surface-functionalized, membrane-targeted superparamagnetic iron oxide nanoparticles (SPIONs). When magnetic fields were applied to surface-bound SPIONs, RGC filopodia elongated directionally, contained polymerized actin filaments, and generated retrograde forces, behaving as bona fide filopodia. Data presented here support the premise that mechanical tension induces filopodia growth but counter the hypothesis that filopodial tension directly promotes growth cone advance. Future applications of these approaches may be used to induce sustained forces on multiple filopodia or other subcellular microstructures to study axon growth or cell migration. From the clinical editor: Mechanical tension to the tip of filopodia is known to promote axonal growth. In this article, the authors used superparamagnetic iron oxide nanoparticles (SPIONs) targeted specifically to membrane molecules, then applied external magnetic field to elicit filopodial elongation, which provided a tool to study the role of mechanical forces in filopodia dynamics and function.

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.

The effects of confinement on neuronal growth cone morphology and velocity

Optimizing growth cone guidance through the use of patterned substrates is important for designing regenerative substrates to aid in recovery from neuronal injury. Using laser ablation, we designed micron-scale patterns capable of confining dissociated mouse cerebellar granule neuron growth cones to channels of different widths ranging from 1.5 to 12 μm. Growth cone dynamics in these channels were observed using time-lapse microscopy. Growth cone area was decreased in channels between 1.5 and 6 μm as compared to that in 12 μm and unpatterned substrates. Growth cone aspect ratio was also affected as narrower channels forced growth cones into a narrow, elongated shape. There was no difference in the overall rate of growth cone advance in uniform channels between 1.5 and 12 μm as compared to growth on unpatterned substrates. The percentage of time growth cones advanced, paused, and retracted was also similar. However, growth cones did respond to changes in confinement: growth cones in narrow lanes rapidly sped up when encountering a wide region and then slowed down as they entered another narrow region. Our results suggest that the rate of neurite extension is not affected by the degree of confinement, but does respond to changes in confinement.

Macrophages lift off surface-bound bacteria using a filopodium-lamellipodium hook-and-shovel mechanism

To clear pathogens from host tissues or biomaterial surfaces, phagocytes have to break the adhesive bacteria-substrate interactions. Here we analysed the mechanobiological process that enables macrophages to lift-off and phagocytose surface-bound Escherichia coli (E. coli). In this opsonin-independent process, macrophage filopodia hold on to the E. coli fimbriae long enough to induce a local protrusion of a lamellipodium. Specific contacts between the macrophage and E. coli are formed via the glycoprotein CD48 on filopodia and the adhesin FimH on type 1 fimbriae (hook). We show that bacterial detachment from surfaces occurrs after a lamellipodium has protruded underneath the bacterium (shovel), thereby breaking the multiple bacterium-surface interactions. After lift-off, the bacterium is engulfed by a phagocytic cup. Force activated catch bonds enable the long-term survival of the filopodium-fimbrium interactions while soluble mannose inhibitors and CD48 antibodies suppress the contact formation and thereby inhibit subsequent E. coli phagocytosis.

Mitochondrial Dynamics in Retinal Ganglion Cell Axon Regeneration and Growth Cone Guidance

Failed axon regeneration and retinal ganglion cell (RGC) death after trauma or disease, including glaucomatous and mitochondrial optic neuropathies, are increasingly linked to mitochondrial dysfunction. Mitochondria are highly dynamic organelles whose size, organization, and function are regulated by a balance between mitochondrial fission and fusion. Mitochondria are ubiquitous in axonal growth cones both in vitro and in vivo and during development and regeneration. However, the roles that mitochondrial fission and fusion dynamics play in the growth cone during axon regeneration are largely unstudied. Here we discuss recent data suggesting mitochondria in the distal axon and growth cone play a central role in axon growth by integrating intrinsic axon growth states with signaling from extrinsic cues. Mitochondrial fission and fusion are intrinsically regulated in the distal axon in the growth cones of acutely purified embryonic and postnatal RGCs with differing intrinsic axon growth potentials. These differences in fission and fusion correlate with differences in mitochondrial bioenergetics; embryonic RGCs with high intrinsic axon growth potential rely more on glycolysis whereas RGCs with low intrinsic axon growth potential rely more on oxidative phosphorylation. Mitochondrial fission and fusion are also differentially modulated by KLFs that either promote or suppress intrinsic axon growth, and altering the balance between mitochondrial fission and fusion can differentially regulate axon growth rate and growth cone guidance responses to both inhibitory and permissive guidance cues.

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.

More than one ring to bind them all: recent insights into the structure of the axon

This brief review outlines recent developments in the understanding of the ultrastructural organization of the axonal and growth cone actin filament cytoskeleton. A novel form of structural organization has arisen as a regulator of the actin cytoskeleton: ring-like structures. Rings may represent a conserved functional theme exhibited by diverse molecular systems and have implications for the understanding of the axon in development, maturity, and disease.

Cell guidance on nanogratings: a computational model of the interplay between PC12 growth cones and nanostructures

Background

Recently, the effects of nanogratings have been investigated on PC12 with respect to cell polarity, neuronal differentiation, migration, maturation of focal adhesions and alignment of neurites.

Methodology/Principal Findings

A synergistic procedure was used to study the mechanism of alignment of PC12 neurites with respect to the main direction of nanogratings. Finite Element simulations were used to qualitatively assess the distribution of stresses at the interface between non-spread growth cones and filopodia, and to study their dependence on filopodial length and orientation. After modelling all adhesions under non-spread growth cone and filopodial protrusions, the values of local stress maxima resulted from the length of filopodia. Since the stress was assumed to be the main triggering cause leading to the increase and stabilization of filopodia, the position of the local maxima was directly related to the orientation of neurites. An analytic closed form equation was then written to quantitatively assess the average ridge width needed to achieve a given neuritic alignment (R² = 0.96), and the alignment course, when the ridge depth varied (R² = 0.97). A computational framework was implemented within an improved free Java environment (CX3D) and in silico simulations were carried out to reproduce and predict biological experiments. No significant differences were found between biological experiments and in silico simulations (alignment, p = 0.3571; tortuosity, p = 0.2236) with a standard level of confidence (95%).

Conclusions/Significance

A mechanism involved in filopodial sensing of nanogratings is proposed and modelled through a synergistic use of FE models, theoretical equations and in silico simulations. This approach shows the importance of the neuritic terminal geometry, and the key role of the distribution of the adhesion constraints for the cell/substrate coupling process. Finally, the effects of the geometry of nanogratings were explicitly considered in cell/surface interactions thanks to the analytic framework presented in this work.

Filopodia as sensors

Filopodia are sensors on both excitable and non-excitable cells. The sensing function is well documented in neurons and blood vessels of adult animals and is obvious during dorsal closure in embryonic development. Nerve cells extend neurites in a bidirectional fashion with growth cones at the tips where filopodia are concentrated. Their sensing of environmental cues underpins the axon's ability to "guide," bypassing non-target cells and moving toward the target to be innervated. This review focuses on the role of filopodia structure and dynamics in the detection of environmental cues, including both the extracellular matrix (ECM) and the surfaces of neighboring cells. Other protrusions including the stereocilia of the inner ear and epididymus, the invertebrate Type I mechanosensors, and the elongated processes connecting osteocytes, share certain principles of organization with the filopodia. Actin bundles, which may be inside or outside of the excitable cell, function to transduce stress from physical perturbations into ion signals. There are different ways of detecting such perturbations. Osteocyte processes contain an actin core and are physically anchored on an extracellular structure by integrins. Some Type I mechanosensors have bridge proteins that anchor microtubules to the membrane, but bundles of actin in accessory cells exert stress on this complex. Hair cells of the inner ear rely on attachments between the actin-based protrusions to activate ion channels, which then transduce signals to afferent neurons. In adherent filopodia, the focal contacts (FCs) integrated with ECM proteins through integrins may regulate integrin-coupled ion channels to achieve signal transduction. Issues that are not understood include the role of Ca(2+) influx in filopodia dynamics and how integrins coordinate or gate signals arising from perturbation of channels by environmental cues.

Mechanisms underlying the initiation and dynamics of neuronal filopodia: from neurite formation to synaptogenesis

Filopodia are finger-like cellular protrusions found throughout the metazoan kingdom and perform fundamental cellular functions during development and cell migration. Neurons exhibit a wide variety of extremely complex morphologies. In the nervous system, filopodia underlie many major morphogenetic events. Filopodia have roles spanning the initiation and guidance of neuronal processes, axons and dendrites to the formation of synaptic connections. This chapter addresses the mechanisms of the formation and dynamics of neuronal filopodia. Some of the major lessons learned from the study of neuronal filopodia are (1) there are multiple mechanisms that can regulate filopodia in a context-dependent manner, (2) that filopodia are specialized subcellular domains, (3) that filopodia exhibit dynamic membrane recycling which also controls aspects of filopodial dynamics, (4) that neuronal filopodia contain machinery for the orchestration of the actin and microtubule cytoskeleton, and (5) localized protein synthesis contributes to neuronal filopodial dynamics.

Focal adhesion kinase regulates actin nucleation and neuronal filopodia formation during axonal growth

The establishment of neural circuits depends on the ability of axonal growth cones to sense their surrounding environment en route to their target. To achieve this, a coordinated rearrangement of cytoskeleton in response to extracellular cues is essential. Although previous studies have identified different chemotropic and adhesion molecules that influence axonal development, the molecular mechanism by which these signals control the cytoskeleton remains poorly understood. Here, we show that in vivo conditional ablation of the focal adhesion kinase gene (Fak) from mouse hippocampal pyramidal cells impairs axon outgrowth and growth cone morphology during development, which leads to functional defects in neuronal connectivity. Time-lapse recordings and in vitro FRAP analysis indicate that filopodia motility is altered in growth cones lacking FAK, probably owing to deficient actin turnover. We reveal the intracellular pathway that underlies this process and describe how phosphorylation of the actin nucleation-promoting factor N-WASP is required for FAK-dependent filopodia formation. Our study reveals a novel mechanism through which FAK controls filopodia formation and actin nucleation during axonal development.

The migration of olfactory ensheathing cells during development and regeneration

The primary olfactory nervous system is unique in that it continuously renews itself and regenerates after injury. These properties are attributed to the presence of olfactory glia, termed olfactory ensheathing cells (OECs). Evidence is now emerging that individual OEC populations exist with distinct anatomical localisations and physiological properties, but their differential roles have not been determined. Unlike other glia, OECs can migrate from the periphery into the central nervous system, and organised OEC migration can enhance axonal extension after injury. Despite this, the mechanisms regulating OEC migration are largely unknown. Here, we provide an overview of the roles of OECs in development and adulthood. We review the latest research describing the differences between individual OEC subpopulations and discuss potential regulatory mechanisms for OEC guidance and migration. Using advanced time lapse techniques, we have obtained novel insights into how OECs behave in a complex multicellular environment which we discuss here with particular focus on cell-cell interactions. Significantly, transplantation of OECs constitutes a promising novel therapy for nerve injuries, but results are highly variable and the method needs improvement. We here review the roles of transplanted OECs in neural repair of damaged neuronal tracts distinct from the primary olfactory nervous system.

Unraveling the determinants of protrusion formation

A COMPUTERIZED MORPHOMETRIC CLASSIFICATION TECHNIQUE BASED ON LATENT FACTORS REVEALS MAJOR PROTRUSION CLASSES: factors 4, 5, and 7. Previous work showed that factor 4 represented filopodia, 5 the distribution of lamellar cytoplasm, and 7 a blunt protrusion. We explore the relationship of focal contact (FC) characteristics and their integrated actin cables to factors values. The results show that FC maturation/cytoskeletal integration affects factor 5, because FC elongation/integration was correlated with its values. On the contrary, 7 values decreased with maturation, so cable or FC size or their integration must be restricted to form these protrusions. Where integration did occur, the cables showed distinctive size and orientation, as indicated by correlation of 7 values with FC shape. Results obtained with myosin inhibitors support the interpretation that a central, isometric, contractile network puts constraints on both factor 5 and 7 protrusions. We conclude that cells establish functional domains by rearranging the cytoskeleton.

Signaling endosomes and growth cone motility in axon regeneration

During development and regeneration, growth cones guide neurites to their targets by altering their motility in response to extracellular guidance cues. One class of cues critical to nervous system development is the neurotrophins. Neurotrophin binding to their cognate receptors stimulates their endocytosis into signaling endosomes. Current data indicate that the spatiotemporal localization of signaling endosomes can direct diverse processes regulating cell motility, including membrane trafficking, cytoskeletal remodeling, adhesion dynamics, and local translation. Recent experiments manipulating signaling endosome localization in neuronal growth cones support these views and place the neurotrophin signaling endosome in a central role regulating growth cone motility during axon growth and regeneration.

The role of actin bundling proteins in the assembly of filopodia in epithelial cells

The goal of this review is to highlight how emerging new models of filopodia assembly, which include tissue specific actin-bundling proteins, could provide more comprehensive representations of filopodia assembly that would describe more adequately and effectively the complexity and plasticity of epithelial cells.  This review also describes how the true diversity of actin bundling proteins must be considered to predict the far-reaching significance and versatile functions of filopodia in epithelial cells.

Reflectivity and topography of cells grown on glass-coverslips measured with phase-shifted laser feedback interference microscopy

In spite of the advantages associated with the molecular specificity of fluorescence imaging, there is still a significant need to augment these approaches with label-free imaging. Therefore, we have implemented a form of interference microscopy based upon phase-shifted, laser-feedback interferometry and developed an algorithm that can be used to separate the contribution of the elastically scattered light by sub-cellular structures from the reflection at the coverslip-buffer interface. The method offers an opportunity to probe protein aggregation, index of refraction variations and structure. We measure the topography and reflection from calibration spheres and from stress fibers and adhesions in both fixed and motile cells. Unlike the data acquired with reflection interference contrast microscopy, where the reflection from adhesions can appear dark, our approach demonstrates that these regions have high reflectivity. The data acquired from fixed and live cells show the presence of a dense actin layer located ≈ 100 nm above the coverslip interface. Finally, the measured dynamics of filopodia and the lamella in a live cell supports retrograde flow as the dominate mechanism responsible for filopodia retraction.

Robust patterns in the stochastic organization of filopodia

Background

Filopodia are actin-based cellular projections that have a critical role in initiating and sustaining directional migration in vertebrate cells. Filopodia are highly dynamic structures that show a rich diversity in appearance and behavior. While there are several mathematical models of filopodia initiation and growth, testing the capacity of these theoretical models in predicting empirical behavior has been hampered by a surprising shortage of quantitative data related to filopodia. Neither is it clear how quantitatively robust the cellular filopodial network is and how perturbations alter it.

Results

We have measured the length and interfilopodial separation distances of several thousand filopodia in the rodent cell line Rat2 and measured these parameters in response to genetic, chemical and physical perturbation. Our work shows that length and separation distance have a lognormal pattern distribution over their entire detection range (0.4 μm to 50 μm).

Conclusions

We find that the lognormal distribution of length and separation is robust and highly resistant to perturbation. We also find that length and separation are independent variables. Most importantly, our empirical data is not entirely in agreement with predictions made based on existing theoretical models and that filopodial size and separation are an order of magnitude larger than what existing models suggest.

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

Migration of cells is one of the most essential prerequisites to form higher organisms and depends on a strongly coordinated sequence of processes. Early migratory events include substrate sensing, adhesion formation, actin bundle assembly and force generation. While substrate sensing was ascribed to filopodia, all other processes were believed to depend mainly on lamellipodia of migrating cells. In this work we show for motile keratinocytes that all processes from substrate sensing to force generation strongly depend on filopodial focal complexes as well as on filopodial actin bundles. In a coordinated step by step process, filopodial focal complexes have to be tightly adhered to the substrate and to filopodial actin bundles to enlarge upon lamellipodial contact forming classical focal adhesions. Lamellipodial actin filaments attached to those focal adhesions originate from filopodia. Upon cell progression, the incorporation of filopodial actin bundles into the lamellipodium goes along with a complete change in actin cross-linker composition from filopodial fascin to lamellipodial alpha-actinin. alpha-Actinin in turn is replaced by myosin II and becomes incorporated directly behind the leading edge. Myosin II activity makes this class of actin bundles with their attached FAs the major source of force generation and transmission at the cell front. Furthermore, connection of FAs to force generating actin bundles leads to their stabilization and further enlargement. Consequently, adhesion sites formed independently of filopodia are not connected to detectable actin bundles, transmit weak forces to the substrate and disassemble within a few minutes without having been increased in size.

Tipping the balance: robustness of tip cell selection, migration and fusion in angiogenesis

Vascular abnormalities contribute to many diseases such as cancer and diabetic retinopathy. In angiogenesis new blood vessels, headed by a migrating tip cell, sprout from pre-existing vessels in response to signals, e.g., vascular endothelial growth factor (VEGF). Tip cells meet and fuse (anastomosis) to form blood-flow supporting loops. Tip cell selection is achieved by Dll4-Notch mediated lateral inhibition resulting, under normal conditions, in an interleaved arrangement of tip and non-migrating stalk cells. Previously, we showed that the increased VEGF levels found in many diseases can cause the delayed negative feedback of lateral inhibition to produce abnormal oscillations of tip/stalk cell fates. Here we describe the development and implementation of a novel physics-based hierarchical agent model, tightly coupled to in vivo data, to explore the system dynamics as perpetual lateral inhibition combines with tip cell migration and fusion. We explore the tipping point between normal and abnormal sprouting as VEGF increases. A novel filopodia-adhesion driven migration mechanism is presented and validated against in vivo data. Due to the unique feature of ongoing lateral inhibition, 'stabilised' tip/stalk cell patterns show sensitivity to the formation of new cell-cell junctions during fusion: we predict cell fates can reverse. The fusing tip cells become inhibited and neighbouring stalk cells flip fate, recursively providing new tip cells. Junction size emerges as a key factor in establishing a stable tip/stalk pattern. Cell-cell junctions elongate as tip cells migrate, which is shown to provide positive feedback to lateral inhibition, causing it to be more susceptible to pathological oscillations. Importantly, down-regulation of the migratory pathway alone is shown to be sufficient to rescue the sprouting system from oscillation and restore stability. Thus we suggest the use of migration inhibitors as therapeutic agents for vascular normalisation in cancer.

Relationship of p21-activated kinase (PAK) and filopodia to persistence and oncogenic transformation

Previously, we found that oncogenically transformed cells had fewer filopodia and more large, p21-activated kinase (PAK)-dependent features than normal cells. These large protrusions (LPs) were increased in cells expressing RhoA(N19) with Cdc42-associated kinase (ACK). Here, we determine how GTPase-mediated mechanisms of focal contact (FC) regulation affect these protrusions. Constructs encoding various proteins were introduced into cells which were then studied by microscopy and computerized image processing and analysis. Constructs that prevented PAK recruitment by PAK-interacting exchange factor (PIX) or restricted PAK residence time on FCs decreased both protrusions. Thus, filopodia were also PAK-dependent. A comparison of FC distribution in cells expressing PAK in the presence or absence of PAK kinase inhibitor domain (KID) suggested that PAK enlarged FCs without affecting the prevalence of either protrusion. KID or Nck expression increased LPs but not filopodia. Nck failed to synergize with KID or ACK and RhoA(N19) in enhancing LPs. Nck and KID synergistically enhanced filopodia, possibly because Nck recruited PAK to FCs while KID prevented their dissociation by PAK-mediated autophosphorylation. Coexpression of Nck, ACK, and RhoA(N19) abrogated filopodia and replicated the transformed phenotype. Since Nck recruitment of PAK is implicated in persistence of directional movement, we studied the PAK-Nck interface. Filopodia were eliminated by the Nck PAK-binding domain and LPs by the PAK Nck-binding domain. The results suggested that filopodia formation has more stringent requirements than LP formation, and Nck and PAK are used differently in the protrusions. Loss of filopodia in transformed cells may reflect defective regulation of GTPase mechanisms.

Contact inhibition revisited

Contact inhibition of cell movement was originally defined in the 1950s as a way of interpreting studies that were ethological and statistical in nature. Research done in succeeding decades provided a more detailed study of the initial contact and its consequences for the cell. The behavior called contact inhibition is characterized by the cessation of ruffling and forward movement in the lamellipodium of the cell making the contact. A new ruffling membrane then arises elsewhere on the cell perimeter. A comparison between the contact behavior described in the early literature and that of the nerve growth cone, described recently by Steketee and Tosney, suggests that filopodia mediate the sensing function in both cases. Since transformed cells have fewer filopodia than normal cells, the contact behavior may decline in direct response to the degraded function of filopodia. This new "filopodia focal signal transduction" hypothesis of contact inhibition elevates the filopodia sensing function and the cessation of lamellipodial advance to the highest importance as phenomena underlying the altered behavior of cancer cells.

One step ahead: role of filopodia in adhesion formation during cell migration of keratinocytes

Cell adhesion is an essential prerequisite for cell function and movement. It depends strongly on focal adhesion complexes connecting the extracellular matrix to the actin cytoskeleton. Especially in moving cells focal adhesions are highly dynamic and believed to be formed closely behind the leading edge. Filopodia were thought to act mainly as guiding cues using their tip complexes for elongation. Here we show for keratinocytes a strong dependence of lamellipodial adhesion sites on filopodia. Upon stable contact of the VASP-containing tip spot to the substrate, a filopodial focal complex (filopodial FX) is formed right behind along the filopodia axis. These filopodial FXs are fully assembled, yet small adhesions containing all adhesion markers tested. Filopodial FXs when reached by the lamellipodium are just increased in size resulting in classical focal adhesions. At the same time most filopodia regain their elongation ability. Blocking filopodia inhibits development of new focal adhesions in the lamellipodium, while focal adhesion maturation in terms of vinculin exchange dynamics remains active. Our data therefore argue for a strong spatial and temporal dependence of focal adhesions on filopodial focal complexes in keratinocytes with filopodia not permanently initiated via new clustering of actin filaments to induce elongation.