Spatiotemporal dynamics of actin-rich adhesion microdomains: influence of substrate flexibility

The influence of biophysical niche on tumor-associated macrophages in liver cancer

HCC, the most common type of primary liver cancer, is a leading cause of cancer-related mortality worldwide. Although the advancement of immunotherapies by immune checkpoint inhibitors (ICIs) that target programmed cell death 1 or programmed cell death 1-ligand 1 has revolutionized the treatment for HCC, the majority is still not beneficial. Accumulating evidence has pointed out that the potent immunosuppressive tumor microenvironment in HCC poses a great challenge to ICI therapeutic efficacy. As a key component in tumor microenvironment, tumor-associated macrophages (TAMs) play vital roles in HCC development, progression, and ICI low responsiveness. Mechanistically, TAM can promote cancer invasion and metastasis, angiogenesis, epithelial-mesenchymal transition, maintenance of stemness, and most importantly, immunosuppression. Targeting TAMs, therefore, represents an opportunity to enhance the ICI therapeutic efficacy in patients with HCC. While previous research has primarily focused on biochemical cues influencing macrophages, emerging evidence highlights the critical role of biophysical signals, such as substrate stiffness, topography, and external forces. In this review, we summarize the influence of biophysical characteristics within the tumor microenvironment that regulate the phenotype and function of TAMs in HCC pathogenesis and progression. We also explore the possible mechanisms and discuss the potential of manipulating biophysical cues in regulating TAM for HCC therapy. By gaining a deeper understanding of how macrophages sense and respond to mechanical forces, we may potentially usher in a path toward a curative approach for combinatory cancer immunotherapies.

Chemo-mechanical diffusion waves explain collective dynamics of immune cell podosomes

Immune cells, such as macrophages and dendritic cells, can utilize podosomes, mechanosensitive actin-rich protrusions, to generate forces, migrate, and patrol for foreign antigens. Individual podosomes probe their microenvironment through periodic protrusion and retraction cycles (height oscillations), while oscillations of multiple podosomes in a cluster are coordinated in a wave-like fashion. However, the mechanisms governing both the individual oscillations and the collective wave-like dynamics remain unclear. Here, by integrating actin polymerization, myosin contractility, actin diffusion, and mechanosensitive signaling, we develop a chemo-mechanical model for podosome dynamics in clusters. Our model reveals that podosomes show oscillatory growth when actin polymerization-driven protrusion and signaling-associated myosin contraction occur at similar rates, while the diffusion of actin monomers drives wave-like coordination of podosome oscillations. Our theoretical predictions are validated by different pharmacological treatments and the impact of microenvironment stiffness on chemo-mechanical waves. Our proposed framework can shed light on the role of podosomes in immune cell mechanosensing within the context of wound healing and cancer immunotherapy.

Mechanisms and roles of podosomes and invadopodia

Cell invasion into the surrounding extracellular matrix or across tissue boundaries and endothelial barriers occurs in both physiological and pathological scenarios such as immune surveillance or cancer metastasis. Podosomes and invadopodia, collectively called 'invadosomes', are actin-based structures that drive the proteolytic invasion of cells, by forming highly regulated platforms for the localized release of lytic enzymes that degrade the matrix. Recent advances in high-resolution microscopy techniques, in vivo imaging and high-throughput analyses have led to considerable progress in understanding mechanisms of invadosomes, revealing the intricate inner architecture of these structures, as well as their growing repertoire of functions that extends well beyond matrix degradation. In this Review, we discuss the known functions, architecture and regulatory mechanisms of podosomes and invadopodia. In particular, we describe the molecular mechanisms of localized actin turnover and microtubule-based cargo delivery, with a special focus on matrix-lytic enzymes that enable proteolytic invasion. Finally, we point out topics that should become important in the invadosome field in the future.

Self-supervised machine learning for live cell imagery segmentation

Segmenting single cells is a necessary process for extracting quantitative data from biological microscopy imagery. The past decade has seen the advent of machine learning (ML) methods to aid in this process, the overwhelming majority of which fall under supervised learning (SL) which requires vast libraries of pre-processed, human-annotated labels to train the ML algorithms. Such SL pre-processing is labor intensive, can introduce bias, varies between end-users, and has yet to be shown capable of robust models to be effectively utilized throughout the greater cell biology community. Here, to address this pre-processing problem, we offer a self-supervised learning (SSL) approach that utilizes cellular motion between consecutive images to self-train a ML classifier, enabling cell and background segmentation without the need for adjustable parameters or curated imagery. By leveraging motion, we achieve accurate segmentation that trains itself directly on end-user data, is independent of optical modality, outperforms contemporary SL methods, and does so in a completely automated fashion-thus eliminating end-user variability and bias. To the best of our knowledge, this SSL algorithm represents a first of its kind effort and has appealing features that make it an ideal segmentation tool candidate for the broader cell biology research community.

The circle of life: Phases of podosome formation, turnover and reemergence

Podosomes are highly dynamic actin-rich structures in a variety of cell types, especially monocytic cells. They fulfill multiple functions such as adhesion, mechanosensing, or extracellular matrix degradation, thus allowing cells to detect and respond to a changing environment. These abilities are based on an intricate architecture that enables podosomes to sense mechanical properties of their substratum and to transduce them intracellularly in order to generate an appropriate cellular response. These processes are enabled through the tightly orchestrated interplay of more than 300 different components that are dynamically recruited during podosome formation and turnover. In this review, we discuss the different phases of the podosome life cycle and the current knowledge on regulatory factors that impact on the genesis, activity, dissolution and reemergence of podosomes. We also highlight mechanoregulatory processes that become important during these different stages, on the level of individual podosomes, and also at podosome sub- and superstructures.

Matrix Metalloproteinases Shape the Tumor Microenvironment in Cancer Progression

Cancer progression with uncontrolled tumor growth, local invasion, and metastasis depends largely on the proteolytic activity of numerous matrix metalloproteinases (MMPs), which affect tissue integrity, immune cell recruitment, and tissue turnover by degrading extracellular matrix (ECM) components and by releasing matrikines, cell surface-bound cytokines, growth factors, or their receptors. Among the MMPs, MMP-14 is the driving force behind extracellular matrix and tissue destruction during cancer invasion and metastasis. MMP-14 also influences both intercellular as well as cell-matrix communication by regulating the activity of many plasma membrane-anchored and extracellular proteins. Cancer cells and other cells of the tumor stroma, embedded in a common extracellular matrix, interact with their matrix by means of various adhesive structures, of which particularly invadopodia are capable to remodel the matrix through spatially and temporally finely tuned proteolysis. As a deeper understanding of the underlying functional mechanisms is beneficial for the development of new prognostic and predictive markers and for targeted therapies, this review examined the current knowledge of the interplay of the various MMPs in the cancer context on the protein, subcellular, and cellular level with a focus on MMP14.

Low kindlin-3 levels in osteoclasts of kindlin-3 hypomorphic mice result in osteopetrosis due to leaky sealing zones

Osteoclasts form special integrin-mediated adhesion structures called sealing zones that enable them to adhere to and resorb bone. Sealing zones consist of densely packed podosomes tightly interconnected by actin fibers. Their formation requires the presence of the hematopoietic integrin regulator kindlin-3 (also known as Fermt3). In this study, we investigated osteoclasts and their adhesion structures in kindlin-3 hypomorphic mice expressing only 5-10% of the kindlin-3 level of wild-type mice. Low kindlin-3 expression reduces integrin activity, results in impaired osteoclast adhesion and signaling, and delays cell spreading. Despite these defects, in vitro-generated kindlin-3-hypomorphic osteoclast-like cells arrange their podosomes into adhesion patches and belts, but their podosome and actin organization is abnormal. Remarkably, kindlin-3-hypomorphic osteoclasts form sealing zones when cultured on calcified matrix in vitro and on bone surface in vivo. However, functional assays, immunohistochemical staining and electron micrographs of bone sections showed that they fail to seal the resorption lacunae properly, which is required for secreted proteinases to digest bone matrix. This results in mild osteopetrosis. Our study reveals a new, hitherto understudied function of kindlin-3 as an essential organizer of integrin-mediated adhesion structures, such as sealing zones.

Small GTPases all over invadosomes

Cell invasion is associated with numerous patho-physiologic states including cell development and metastatic dissemination. This process couples the activation of cell motility with the capacity to degrade the extracellular matrix, thereby permitting cells to pass through basal membranes. Invasion is sustained by the actions of invadosomes, an ensemble of subcellular structures with high functional homology. Invadosomes are 3D acto-adhesive structures that can also mediate local extracellular matrix degradation through the controlled delivery of proteases. Intracellular RHO GTPases play a central role in the regulation of invadosomes where their complex interplay regulates multiple invadosome functions. This review aims to provide an overview of the synergistic activities of the small GTPases in invadosome biology. This broad-based review also reinforces the importance of the spatiotemporal regulation of small GTPases and the impact of this process on invadosome dynamics.

Probing invadosomes: technologies for the analysis of invadosomes

Invadosomes are protrusive and mechanosensitive actin devices critical for cell migration, invasion, and extracellular matrix remodeling. The dynamic, proteolytic, and protrusive natures of invadosomes have made these structures fascinating and attracted many scientists to develop new technologies for their analysis. With these exciting methodologies, many biochemical and biophysical properties of invadosomes have been well characterized and appreciated, and those discoveries elegantly explained the biological and pathological effects of invadosomes in human health and diseases. In this review, we focus on these commonly used or newly developed methods for invadosome analysis and effort to reason some discrepancies among those assays. Finally, we explore the opposite regulatory mechanisms among invadosomes and focal adhesions, another actin-rich adhesive structures, and speculate a potential rule for their switch.

Cell-Substrate Patterns Driven by Curvature-Sensitive Actin Polymerization: Waves and Podosomes

Cells adhered to an external solid substrate are observed to exhibit rich dynamics of actin structures on the basal membrane, which are distinct from those observed on the dorsal (free) membrane. Here we explore the dynamics of curved membrane proteins, or protein complexes, that recruit actin polymerization when the membrane is confined by the solid substrate. Such curved proteins can induce the spontaneous formation of membrane protrusions on the dorsal side of cells. However, on the basal side of the cells, such protrusions can only extend as far as the solid substrate and this constraint can convert such protrusions into propagating wave-like structures. We also demonstrate that adhesion molecules can stabilize localized protrusions that resemble some features of podosomes. This coupling of curvature and actin forces may underlie the differences in the observed actin-membrane dynamics between the basal and dorsal sides of adhered cells.

Probing the mechanical landscape – new insights into podosome architecture and mechanics

Podosomes are dynamic adhesion structures formed constitutively by macrophages, dendritic cells and osteoclasts and transiently in a wide variety of cells, such as endothelial cells and megakaryocytes. They mediate numerous functions, including cell–matrix adhesion, extracellular matrix degradation, mechanosensing and cell migration. Podosomes present as micron-sized F-actin cores surrounded by an adhesive ring of integrins and integrin–actin linkers, such as talin and vinculin. In this Review, we highlight recent research that has considerably advanced our understanding of the complex architecture–function relationship of podosomes by demonstrating that the podosome ring actually consists of discontinuous nano-clusters and that the actin network in between podosomes comprises two subsets of unbranched actin filaments, lateral and dorsal podosome-connecting filaments. These lateral and dorsal podosome-connecting filaments connect the core and ring of individual podosomes and adjacent podosomes, respectively. We also highlight recent insights into the podosome cap as a novel regulatory module of actomyosin-based contractility. We propose that these newly identified features are instrumental for the ability of podosomes to generate protrusion forces and to mechanically probe their environment. Furthermore, these new results point to an increasing complexity of podosome architecture and have led to our current view of podosomes as autonomous force generators that drive cell migration.

Costameres, dense plaques and podosomes: the cell matrix adhesions in cardiovascular mechanosensing

The stiffness of the cardiovascular environment changes during ageing and in disease and contributes to disease incidence and progression. For instance, increased arterial stiffness can lead to atherosclerosis, while stiffening of the heart due to fibrosis can increase the chances of heart failure. Cells can sense the stiffness of the extracellular matrix through integrin adhesions and other mechanosensitive structures and in response to this initiate mechanosignalling pathways that ultimately change the cellular behaviour. Over the past decades, interest in mechanobiology has steadily increased and with this also our understanding of the molecular basis of mechanosensing and transduction. However, much of our knowledge about the mechanisms is derived from studies investigating focal adhesions in non-muscle cells, which are distinct in several regards from the cell-matrix adhesions in cardiomyocytes (costameres) or vascular smooth muscle cells (dense plaques or podosomes). Therefore, we will look here first at the evidence for mechanical sensing in the cardiovascular system, before comparing the different cytoskeletal arrangements and adhesion sites in cardiomyocytes and vascular smooth muscle cells and what is known about mechanical sensing through the various structures.

MMP Secretion Rate and Inter-invadopodia Spacing Collectively Govern Cancer Invasiveness

Invadopodia are micron-sized invasive structures that mediate extracellular matrix (ECM) degradation through a combination of membrane-bound and soluble matrix metalloproteinases (MMPs). However, how such localized degradation is converted into pores big enough for cancer cells to invade, and the relative contributions of membrane-bound versus soluble MMPs to this process remain unclear. In this article, we address these questions by combining experiments and simulations. We show that in MDA-MB-231 cells, an increase in ECM density enhances invadopodia-mediated ECM degradation and decreases inter-invadopodia spacing. ECM degradation is mostly mediated by soluble MMPs, which are activated by membrane-bound MT1-MMP. We present a computational model of invadopodia-mediated ECM degradation, which recapitulates the above observations and identifies MMP secretion rate as an important regulator of invadopodia stability. Simulations with multiple invadopodia suggest that inter-invadopodia spacing and MMP secretion rate collectively dictate the size of the degraded zones. Taken together, our results suggest that for creating pores conducive for cancer invasion, cells must tune inter-invadopodia spacing and MMP secretion rate in an ECM density-dependent manner, thereby striking a balance between invadopodia penetration and ECM degradation.

Cell Migration in Microfluidic Devices: Invadosomes Formation in Confined Environments

The last 20 years have seen the blooming of microfluidics technologies applied to biological sciences. Microfluidics provides effective tools for biological analysis, allowing the experimentalists to extend their playground to single cells and single molecules, with high throughput and resolution which were inconceivable few decades ago. In particular, microfluidic devices are profoundly changing the conventional way of studying the cell motility and cell migratory dynamics. In this chapter we will furnish a comprehensive view of the advancements made in the research domain of confinement-induced cell migration, thanks to the use of microfluidic devices. The chapter is subdivided in three parts. Each section will be addressing one of the fundamental questions that the microfluidic technology is contributing to unravel: (i) where cell migration takes place, (ii) why cells migrate and, (iii) how the cells migrate. The first introductory part is devoted to a thumbnail, and partially historical, description of microfluidics and its impact in biological sciences. Stress will be put on two aspects of the devices fabrication process, which are crucial for biological applications: materials used and coating methods. The second paragraph concerns the cell migration induced by environmental cues: chemical, leading to chemotaxis, mechanical, at the basis of mechanotaxis, and electrical, which induces electrotaxis. Each of them will be addressed separately, highlighting the fundamental role of microfluidics in providing the well-controlled experimental conditions where cell migration can be induced, investigated and ultimately understood. The third part of the chapter is entirely dedicated to how the cells move in confined environments. Invadosomes (the joint name for podosomes and invadopodia) are cell protrusion that contribute actively to cell migration or invasion. The formation of invadosomes under confinement is a research topic that only recently has caught the attention of the scientific community: microfluidic design is helping shaping the future direction of this emerging field of research.

Neotuberostemonine inhibits osteoclastogenesis via blockade of NF-κB pathway

Osteoporosis has been attributed to low bone mass arising from cellular communications between bone formation and bone resorption. Osteoclastogenesis is induced by M-CSF and RANKL in hematopoietic lineage cells. Once RANK/RANKL complex is formed, TRAF6 is recruited and triggers the activation of NF-κB pathway and the expression of osteoclast-related genes including NFATc1. Neotuberostemonine (NTS) is an active compound isolated from Stemona tuberosa Lour. Pharmacologically, NTS has been known to possess antitussive, anti-fibrotic and anti-inflammatory activities through regulation of macrophage. However, the influence of NTS to osteoclastogenesis has not been reported. The purpose of this study is to investigate whether NTS can modulate the osteoclastogenesis induced by RANKL or cancer cells. We found that NTS inhibits RANKL- or cancer cell-mediated osteoclastogenesis via blockade of TRAF6 and NF-κB activation. NTS also impairs the formation of F-actin ring structure, an important feature of osteoclast differentiation and function. These results indicate that NTS can be a preventive and therapeutic candidate for bone-related disease and that NTS provides insights underlying molecular mechanisms that influence osteoclastogenesis.

Injectable and tunable hyaluronic acid hydrogels releasing chemotactic and angiogenic growth factors for endodontic regeneration

Bioengineered soft tissues on any meaningful scale or complexity must incorporate aspects of the functional tissue, namely a vasculature, providing cells oxygen and nutrients critical for their survival. However, the ability of tissue engineering strategies to promote a fast revascularization is critically limited. Particularly in endodontic regenerative therapies, the complicated anatomy of the root canal system, and the narrow apical access limit the supply of new blood vessels and pulp tissue ingrowth. Here we characterize the viscoelastic and microstructural properties of a class of injectable hyaluronic acid (HA) hydrogels formed in situ, reinforced with cellulose nanocrystals (CNCs) and enriched with platelet lysate (PL), and test its ability to promote cells recruitment and proangiogenic activity in vitro. The incorporation of CNCs enhanced the stability of the materials against hydrolytic and enzymatic degradation. Moreover, the release of the chemotactic and pro-angiogenic growth factors (GFs) (PDGF and VEGF) from the PL-laden hydrogels showed an improved sustained profile proportional to the amount of incorporated CNCs. The PL-laden hydrogels exhibited preferential supportive properties of encapsulated human dental pulp cells (hDPCs) in in vitro culture conditions. Finally, PL-laden hydrogels stimulated chemotactic and pro-angiogenic activity by promoting hDPCs recruitment and cell sprouting in hDPCs/human umbilical vein endothelial cell co-cultures in vitro, and in an ex vivo model. These results support the use of the combined system as a scaffold for GFs delivery and cells recruitment, thereby exhibiting great clinical potential in treating injuries in vascularized tissues.

STATEMENT OF SIGNIFICANCE

Innovative strategies for improved chemotactic and pro-angiogenic features of TE constructs are needed. In this study, we developed an injectable HA/CNC/PL hydrogel with improved structural and biologic properties, that not only provide a sustained release of chemotactic and proangiogenic GFs from PL but also enhance the cells' viability and angiogenic activity. As a result of their unique traits, the developed hydrogels are ideally suited to simultaneously act as a GFs controlled delivery system and as a supportive matrix for cell culture, recruitment, and revascularization induction, holding great potential for the regeneration of vascularized soft tissues, such as the dentin-pulp complex.

S100A4 regulates macrophage invasion by distinct myosin-dependent and myosin-independent mechanisms

S100A4, a member of the S100 family of Ca²⁺-binding proteins, is a key regulator of cell migration and invasion. Our previous studies showed that bone marrow–derived macrophages from S100A4^−/− mice exhibit defects in directional motility and chemotaxis in vitro and reduced recruitment to sites of inflammation in vivo. We now show that the loss of S100A4 produces two mechanistically distinct phenotypes with regard to macrophage invasion: a defect in matrix degradation, due to a disruption of podosome rosettes caused by myosin-IIA overassembly, and a myosin-independent increase in microtubule acetylation, which increases podosome rosette stability and is sufficient to inhibit macrophage invasion. Our studies point to S100A4 as a critical regulator of matrix degradation, whose actions converge on the dynamics and degradative functions of podosome rosettes.

Invadosomes are coming: new insights into function and disease relevance

Invadopodia and podosomes are discrete, actin-based molecular protrusions that form in cancer cells and normal cells, respectively, in response to diverse signaling pathways and extracellular matrix cues. Although they participate in a host of different cellular processes, they share a common functional theme of controlling pericellular proteolytic activity, which sets them apart from other structures that function in migration and adhesion, including focal adhesions, lamellipodia, and filopodia. In this review, we highlight research that explores the function of these complex structures, including roles for podosomes in embryonic and postnatal development, in angiogenesis and remodeling of the vasculature, in maturation of the postsynaptic membrane, in antigen sampling and recognition, and in cell-cell fusion mechanisms, as well as the involvement of invadopodia at multiple steps of the metastatic cascade, and how all of this may apply in the treatment of human disease states. Finally, we explore recent research that implicates a novel role for exosomes and microvesicles in invadopodia-dependent and invadopodia-independent mechanisms of invasion, respectively.

Expression of typical osteoclast markers by PBMCs after PEG-induced fusion as a model for studying osteoclast differentiation

Bone is a metabolically active organ subjected to continuous remodeling process that involves resorption by osteoclast and subsequent formation by osteoblasts. Osteoclast involvement in this physiological event is regulated by macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor κB ligand (RANKL). Fusion of mono-nuclear pre-osteoclasts is a critical event for osteoclast differentiation and for bone resorption. Here we show that PBMCs can be successfully fused with polyethylenglicol (PEG) in order to generated viable osteoclast-like cells that exhibit tartrate-resistant acid phosphatase (TRAP) and bone resorptive activities. PEG-fused PBMCs expressed additional markers compatible with osteoclastogenic differentiation such as carbonic anhydrase II (CAII), calcitonin receptor (CR), cathepsin K (Cat K), vacuolar ATPase (V-ATPase) subunit C1 (V-ATPase), integrin β3, RANK and cell surface aminopeptidase N/CD13. Actin redistribution in PEG-fused cells was found to be affected by cell cycle synchronization at G0/G1 or G2/M phases. PEG-induced fusion also led to expression of tyrosine kinases c-Src and Syk in their phosphorylated state. Scanning electron microscopy images showed morphological features typical of osteoclast-like cells. The results here shown allow concluding that PEG-induced fusion of PBMCs provides a suitable model system for understanding the mechanisms involved in osteoclastogenesis and for assaying new therapeutic strategies.

Biophysical Regulation of Cell Behavior-Cross Talk between Substrate Stiffness and Nanotopography

The stiffness and nanotopographical characteristics of the extracellular matrix (ECM) influence numerous developmental, physiological, and pathological processes in vivo. These biophysical cues have therefore been applied to modulate almost all aspects of cell behavior, from cell adhesion and spreading to proliferation and differentiation. Delineation of the biophysical modulation of cell behavior is critical to the rational design of new biomaterials, implants, and medical devices. The effects of stiffness and topographical cues on cell behavior have previously been reviewed, respectively; however, the interwoven effects of stiffness and nanotopographical cues on cell behavior have not been well described, despite similarities in phenotypic manifestations. Herein, we first review the effects of substrate stiffness and nanotopography on cell behavior, and then focus on intracellular transmission of the biophysical signals from integrins to nucleus. Attempts are made to connect extracellular regulation of cell behavior with the biophysical cues. We then discuss the challenges in dissecting the biophysical regulation of cell behavior and in translating the mechanistic understanding of these cues to tissue engineering and regenerative medicine.

Force-dependent breaching of the basement membrane

Clinically, non-invasive carcinomas are confined to the epithelial side of the basement membrane and are classified as benign, whereas invasive cancers invade through the basement membrane and thereby acquire the potential to metastasize. Recent findings suggest that, in addition to protease-mediated degradation and chemotaxis-stimulated migration, basement membrane invasion by malignant cells is significantly influenced by the stiffness of the associated interstitial extracellular matrix and the contractility of the tumor cells that is dictated in part by their oncogenic genotype. In this review, we highlight recent findings that illustrate unifying molecular mechanisms whereby these physical cues contribute to tissue fibrosis and malignancy in three epithelial organs: breast, pancreas, and liver. We also discuss the clinical implications of these findings and the biological properties and clinical challenges linked to the unique biology of each of these organs.

Mechanotransduction pulls the strings of matrix degradation at invadosome

Degradation of the extracellular matrix is a critical step of tumor cell invasion. Both protease-dependent and -independent mechanisms have been described as alternate processes in cancer cell motility. Interestingly, some effectors of protease-dependent degradation are focalized at invadosomes and are directly coupled with contractile and adhesive machineries composed of multiple mechanosensitive proteins. This review presents recent findings in protease-dependent mechanisms elucidating the ways the force affects extracellular matrix degradation by targeting protease expression and activity at invadosome. The aim is to highlight mechanosensing and mechanotransduction processes to direct the degradative activity at invadosomes, with the focus on membrane tension, proteases and mechanosensitive ion channels.

Force Matters: Biomechanical Regulation of Cell Invasion and Migration in Disease

Atherosclerosis, cancer, and various chronic fibrotic conditions are characterized by an increase in the migratory behavior of resident cells and the enhanced invasion of assorted exogenous cells across a stiffened extracellular matrix (ECM). This stiffened scaffold aberrantly engages cellular mechanosignaling networks in cells, which promotes the assembly of invadosomes and lamellae for cell invasion and migration. Accordingly, deciphering the conserved molecular mechanisms whereby matrix stiffness fosters invadosome and lamella formation could identify therapeutic targets to treat fibrotic conditions, and reducing ECM stiffness could ameliorate disease progression.

The microenvironment controls invadosome plasticity

Invadosomes are actin-based structures involved in extracellular matrix degradation. Invadosomes is a term that includes podosomes and invadopodia, which decorate normal and tumour cells, respectively. They are mainly organised into dots or rosettes, and podosomes and invadopodia are often compared and contrasted. Various internal or external stimuli have been shown to induce their formation and/or activity. In this Commentary, we address the impact of the microenvironment and the role of matrix receptors on the formation, and dynamic and degradative activities of invadosomes. In particular, we highlight recent findings regarding the role of type I collagen fibrils in inducing the formation of a new linear organisation of invadosomes. We will also discuss invadosome plasticity more generally and emphasise its physio-pathological relevance.

Feel the force: Podosomes in mechanosensing

Cells interact with their environment through highly localized contact structures. Podosomes represent a subgroup of cell-matrix contacts, which is especially prominent in cells of the monocytic lineage such as monocytes, macrophages and dendritic cells, but also in a variety of other cell types. Comparable to other adhesion structures, podosomes feature a complex architecture, which forms the basis for their extensive repertoire of sensory and effector functions. These functions are mainly linked to interactions with the extracellular matrix and comprise well known properties such as cell-matrix adhesion and extracellular matrix degradation. A more recent discovery is the ability of podosomes to act as mechanosensory devices, by detecting rigidity and topography of the substratum. In this review, we focus especially on the molecular events involved in mechanosensing by podosomes, the structural elements of podosomes that enable this function, as well as the intra- and extracellular signals generated downstream of podosome mechanosensing.

Spatiotemporal organization and mechanosensory function of podosomes

Podosomes are small, circular adhesions formed by cells such as osteoclasts, macrophages, dendritic cells, and endothelial cells. They comprise a protrusive actin core module and an adhesive ring module composed of integrins and cytoskeletal adaptor proteins such as vinculin and talin. Furthermore, podosomes are associated with an actin network and often organize into large clusters. Recent results from our laboratory and others have shed new light on podosome structure and dynamics, suggesting a revision of the classical "core-ring" model. Also, these studies demonstrate that the adhesive and protrusive module are functionally linked by the actin network likely facilitating mechanotransduction as well as providing feedback between these two modules. In this commentary, we briefly summarize these recent advances with respect to the knowledge on podosome structure and discuss force distribution mechanisms within podosomes and their emerging role in mechanotransduction.

Tools of the trade: podosomes as multipurpose organelles of monocytic cells

Podosomes are adhesion and invasion structures that are particularly prominent in cells of the monocytic lineage such as macrophages, dendritic cells, and osteoclasts. They are multifunctional organelles that combine several key abilities required for cell migration and invasion. The podosome repertoire includes well-established functions such as cell-substrate adhesion, and extracellular matrix degradation, recently discovered abilities such as rigidity and topology sensing as well as antigen sampling, and also more speculative functions such as cell protrusion stabilization and transmigration. Collectively, podosomes not only enable dynamic interactions of cells with their surroundings, they also gather information about the pericellular environment, and are actively involved in its reshaping. This review presents an overview of the current knowledge on podosome composition, architecture, and regulation. We focus in particular on the growing list of podosome functions and discuss the specific properties of podosomes in macrophages, dendritic cells, and osteoclasts. Moreover, this article highlights podosome-related intracellular transport processes, the formation of podosomes in 3D environments as well as potentially podosome-associated diseases involving monocytic cells.

A growth factor-induced, spatially organizing cytoskeletal module enables rapid and persistent fibroblast migration

Directional migration requires robust front/back polarity. We find that fibroblasts treated with platelet-derived growth factor (PDGF) and prepolarized by plating on a fibronectin line substrate exhibit persistent migration for hours. This does not occur in the absence of PDGF or on uniformly coated fibronectin substrates. Persistent migration arises from establishment of two functional modules at cell front and back. At the front, formation of a zone containing podosome-like structures (PLS) dynamically correlates with low RhoA and myosin activity and absence of a contractile lamella. At the back, myosin contractility specifically controls tail retraction with minimal crosstalk to the front module. The PLS zone is maintained in a dynamic steady state that preserves size and position relative to the cell front, allowing for long-term coordination of front and back modules. We propose that front/back uncoupling achieved by the PLS zone is crucial for persistent migration in the absence of directional cues.

Protrusion force microscopy reveals oscillatory force generation and mechanosensing activity of human macrophage podosomes

Podosomes are adhesion structures formed in monocyte-derived cells. They are F-actin-rich columns perpendicular to the substrate surrounded by a ring of integrins. Here, to measure podosome protrusive forces, we designed an innovative experimental setup named protrusion force microscopy (PFM), which consists in measuring by atomic force microscopy the deformation induced by living cells onto a compliant Formvar sheet. By quantifying the heights of protrusions made by podosomes onto Formvar sheets, we estimate that a single podosome generates a protrusion force that increases with the stiffness of the substratum, which is a hallmark of mechanosensing activity. We show that the protrusive force generated at podosomes oscillates with a constant period and requires combined actomyosin contraction and actin polymerization. Finally, we elaborate a model to explain the mechanical and oscillatory activities of podosomes. Thus, PFM shows that podosomes are mechanosensing cell structures exerting a protrusive force.

Importance of RhoGTPases in formation, characteristics, and functions of invadosomes

Podosomes and invadopodia (collectively known as invadosomes) are specialized plasma-membrane actin-based microdomains that combine adhesive properties with matrix degrading and/or mechanosensor activities. These organelles have been extensively studied in vitro and current concerted efforts aim at establishing their physiological relevance and subsequent association with human diseases. Proper functioning of the bone, immune, and vascular systems is likely to depend on these structures while their occurrence in cancer cells appears to be linked to tumor metastasis. The elucidation of the mechanisms driving invadosome assembly is a prerequisite to understanding their role in vivo and ultimately to controlling their functions. Adhesive and soluble ligands act via transmembrane receptors that propagate signals to the cytoskeleton via small G proteins of the Rho family, assisted by tyrosine kinases and scaffold proteins to induce invadosome formation and rearrangements. Oncogene expression and cell-cell interactions may also trigger their assembly. Manipulation of the signals that regulate invadosome formation and dynamics could therefore be a strategy to interfere with their functions in a multitude of pathological settings, such as excessive bone breakdown, infections, vascular remodeling, transendothelial diapedesis, and metastasis.

Cellular traction stresses mediate extracellular matrix degradation by invadopodia

During tumorigenesis, matrix rigidity can drive oncogenic transformation via altered cellular proliferation and migration. Cells sense extracellular matrix (ECM) mechanical properties with intracellular tensile forces generated by actomyosin contractility. These contractile forces are transmitted to the matrix surface as traction stresses, which mediate mechanical interactions with the ECM. Matrix rigidity has been shown to increase proteolytic ECM degradation by cytoskeletal structures known as invadopodia that are critical for cancer progression, suggesting that cellular contractility promotes invasive behavior. However, both increases and decreases in traction stresses have been associated with metastatic behavior. Therefore, the role of cellular contractility in invasive migration leading to metastasis is unclear. To determine the relationship between cellular traction stresses and invadopodia activity, we characterized the invasive and contractile properties of an aggressive carcinoma cell line utilizing polyacrylamide gels of different rigidities. We found that ECM degradation and traction stresses were linear functions of matrix rigidity. Using calyculin A to augment myosin contractility, we also found that traction stresses were strongly predictive of ECM degradation. Overall, our data suggest that cellular force generation may play an important part in invasion and metastasis by mediating invadopodia activity in response to the mechanical properties of the tumor microenvironment.

Podosomes in adhesion, migration, mechanosensing and matrix remodeling

Cells use various actin-based motile structures to allow them to move across and through matrix of varying density and composition. Podosomes are actin cytoskeletal structures that form in motile cells and that mediate adhesion to substrate, migration, and other specialized functions such as transmigration through cell and matrix barriers. The podosome is a unique and interesting entity, which appears in the light microscope as an individual punctum, but is linked to other podosomes like a node on a network of the underlying cytoskeleton. Here, we discuss the signals that control podosome assembly and dynamics in different cell types and the actin organising proteins that regulate both the inner actin core and integrin-rich surrounding ring structures. We review the structure and composition of podosomes and also their functions in various cell types of both myeloid and endothelial lineage. We also discuss the emerging idea that podosomes can sense matrix stiffness and enable cells to respond to their environment.

Regulation of bone resorption and sealing zone formation in osteoclasts occurs through protein kinase B-mediated microtubule stabilization

We investigated the role of protein kinase B (Akt), a downstream effector of phosphatidylinositol 3-kinase, in bone-resorbing activity of mature osteoclasts. Treatment with a specific Akt inhibitor disrupted sealing zone formation and decreased the bone-resorbing activity of osteoclasts. The normal microtubule structures were lost and the Akt inhibitor reduced the amount of acetylated tubulin, which reflects stabilized microtubules, whereas forced Akt activation by adenovirus vectors resulted in the opposite effect. Forced Akt activation increased the binding of the microtubule-associated protein adenomatous polyposis coli (APC), the APC-binding protein end-binding protein 1 (EB1) and dynactin, a dynein activator complex, with microtubules. Depletion of Akt1 and Akt2 resulted in a disconnection of APC/EB1 and a decrease in bone-resorbing activity along with reduced sealing zone formation, both of which were recovered upon the addition of LiCl, a glycogen synthase kinase-3β (GSK-3β) inhibitor. The Akt1 and Akt2 double-knockout mice exhibited osteosclerosis due to reduced bone resorption. These findings indicate that Akt controls the bone-resorbing activity of osteoclasts by stabilizing microtubules via a regulation of the binding of microtubule associated proteins.

Megakaryocytes assemble podosomes that degrade matrix and protrude through basement membrane

Megakaryocytes give rise to platelets via extension of proplatelet arms, which are released through the vascular sinusoids into the bloodstream. Megakaryocytes and their precursors undergo varying interactions with the extracellular environment in the bone marrow during their maturation and positioning in the vascular niche. We demonstrate that podosomes are abundant in primary murine megakaryocytes adherent on multiple extracellular matrix substrates, including native basement membrane. Megakaryocyte podosome lifetime and density, but not podosome size, are dependent on the type of matrix, with podosome lifetime dramatically increased on collagen fibers compared with fibrinogen. Podosome stability and dynamics depend on actin cytoskeletal dynamics but not matrix metalloproteases. However, podosomes degrade matrix and appear to be important for megakaryocytes to extend protrusions across a native basement membrane. We thus demonstrate for the first time a fundamental requirement for podosomes in megakaryocyte process extension across a basement membrane, and our results suggest that podosomes may have a role in proplatelet arm extension or penetration of basement membrane.

Extracellular matrix rigidity controls podosome induction in microvascular endothelial cells

BACKGROUND INFORMATION

Podosomes are actin-based structures involved in cell adhesion, migration, invasion and extracellular matrix degradation. They have been described in large vessel endothelial cells, but nothing is known concerning microvascular endothelial cells. Here, we focussed on liver sinusoidal endothelial cells (LSECs), fenestrated microvascular cells that play major roles in liver physiology. Liver fibrosis induces a dedifferentiation of LSECs leading notably to a loss of fenestrae. Because liver fibrosis is associated with increased matrix stiffness, and because substrate stiffness is known to regulate the actin cytoskeleton, we investigated the impact of matrix rigidity on podosome structures in LSECs.

RESULTS

Using primary LSECs, we demonstrated that microvascular endothelial cells are able to form constitutive podosomes. Podosome presence in LSECs was independent of cytokines such as transforming growth factor-β or vascular endothelial growth factor, but could be modulated by matrix stiffness. As expected, LSECs lost their differentiated phenotype during cell culture, which was paralleled by a loss of podosomes. LSECs however retained the capacity to form active podosomes following detachment/reseeding or actin-destabilising drug treatments. Finally, constitutive podosomes were also found in primary microvascular endothelial cells from other organs.

CONCLUSIONS

Our results show that microvascular endothelial cells are able to form podosomes without specific stimulation. Our data suggest that the major determinant of podosome induction in these cells is substrate rigidity.

Protein kinase C-delta deficiency perturbs bone homeostasis by selective uncoupling of cathepsin K secretion and ruffled border formation in osteoclasts

Bone homeostasis requires stringent regulation of osteoclasts, which secrete proteolytic enzymes to degrade the bone matrix. Despite recent progress in understanding how bone resorption occurs, the mechanisms regulating osteoclast secretion, and in particular the trafficking route of cathepsin K vesicles, remain elusive. Using a genetic approach, we describe the requirement for protein kinase C-delta (PKCδ) in regulating bone resorption by affecting cathepsin K exocytosis. Importantly, PKCδ deficiency does not perturb formation of the ruffled border or trafficking of lysosomal vesicles containing the vacuolar-ATPase (v-ATPase). Mechanistically, we find that cathepsin K exocytosis is controlled by PKCδ through modulation of the actin bundling protein myristoylated alanine-rich C-kinase substrate (MARCKS). The relevance of our finding is emphasized in vivo because PKCδ-/- mice exhibit increased bone mass and are protected from pathological bone loss in a model of experimental postmenopausal osteoporosis. Collectively, our data provide novel mechanistic insights into the pathways that selectively promote secretion of cathepsin K lysosomes independently of ruffled border formation, providing evidence of the presence of multiple mechanisms that regulate lysosomal exocytosis in osteoclasts.

Spatiotemporal regulation of Src and its substrates at invadosomes

In the past decade, substantial progress has been made in understanding how Src family kinases regulate the formation and function of invadosomes. Invadosomes are organized actin-rich structures that contain an F-actin core surrounded by an adhesive ring and mediate invasive migration. Src kinases orchestrate, either directly or indirectly, each phase of the invadosome life cycle including invadosome assembly, maturation and matrix degradation and disassembly. Complex arrays of Src effector proteins are involved at different stages of invadosome maturation and their spatiotemporal activity must be tightly regulated to achieve effective invasive migration. In this review, we highlight some recent progress and the challenges of understanding how Src is regulated temporally and spatially to orchestrate the dynamics of invadosomes and mediate cell invasion.

pH regulators in invadosomal functioning: proton delivery for matrix tasting

Invadosomes are actin-rich finger-like cellular structures sensing and interacting with the surrounding extracellular matrix (ECM) and involved in its proteolytic remodeling. Invadosomes are structures distinct from other adhesion complexes, and have been identified in normal cells that have to cross tissue barriers to fulfill their function such as leukocytes, osteoclasts and endothelial cells. They also represent features of highly aggressive cancer cells, allowing them to escape from the primary tumor, to invade surrounding tissues and to reach systemic circulation. They are localized to the ventral membrane of cells grown under 2-dimensional conditions and are supposed to be present all around cells grown in 3-dimensional matrices. Indeed invadosomes are key structures in physiological processes such as inflammation and the immune response, bone remodeling, tissue repair, but also in pathological conditions such as osteopetrosis and the development of metastases. Invadosomes are subdivided into podosomes, found in normal cells, and into invadopodia specific for cancer cells. While these two structures exhibit differences in organization, size, number and half-life, they share similarities in molecular composition, participation in cell-matrix adhesion and promoting matrix degradation. A key determinant in invadosomal function is the recruitment and release of proteases, such as matrix metalloproteinases (MMPs), serine proteases and cysteine cathepsins, together with their activation in a tightly controlled and highly acidic microenvironment. Therefore numerous pH regulators such as V-ATPases and Na(+)/H(+) exchangers, are found in invadosomes and are directly involved in their constitution as well as their functioning. This review focuses on the participation of pH regulators in invadosome function in physiological and pathological conditions, with a particular emphasis on ECM remodeling by osteoclasts during bone resorption and by cancer cells.

Geometry sensing by dendritic cells dictates spatial organization and PGE(2)-induced dissolution of podosomes

Assembly and disassembly of adhesion structures such as focal adhesions (FAs) and podosomes regulate cell adhesion and differentiation. On antigen-presenting dendritic cells (DCs), acquisition of a migratory and immunostimulatory phenotype depends on podosome dissolution by prostaglandin E₂ (PGE₂). Whereas the effects of physico-chemical and topographical cues have been extensively studied on FAs, little is known about how podosomes respond to these signals. Here, we show that, unlike for FAs, podosome formation is not controlled by substrate physico-chemical properties. We demonstrate that cell adhesion is the only prerequisite for podosome formation and that substrate availability dictates podosome density. Interestingly, we show that DCs sense 3-dimensional (3-D) geometry by aligning podosomes along the edges of 3-D micropatterned surfaces. Finally, whereas on a 2-dimensional (2-D) surface PGE₂ causes a rapid increase in activated RhoA levels leading to fast podosome dissolution, 3-D geometric cues prevent PGE₂-mediated RhoA activation resulting in impaired podosome dissolution even after prolonged stimulation. Our findings indicate that 2-D and 3-D geometric cues control the spatial organization of podosomes. More importantly, our studies demonstrate the importance of substrate dimensionality in regulating podosome dissolution and suggest that substrate dimensionality plays an important role in controlling DC activation, a key process in initiating immune responses.

Influence of substrate rigidity on primary nucleation of cell adhesion: a thermal fluctuation model

Experimental investigations have demonstrated that cells can actively sense and respond to physical aspects of their environments, such as substrate stiffness of biomaterials, via integrin receptors with the help of stochastic thermal undulations of cell membranes. This paper develops a physical model for the mechanism of integrin-dependent cell-substrate adhesion nucleation in order to investigate the influence of substrate stiffness on primary adhesion formation. In this model, a series of so-called energy potential wells are established to quantitatively describe force-driven conformational changes of integrins on elastic substrates with different rigidities. A concept of nucleation domain is proposed to characterize the necessary condition of integrin-mediated cell-substrate primary adhesion formation. In the framework of classical statistical mechanics, the competitive relationship is investigated between the local thermal undulations of plasma membranes and the conformational conversions of substrate-binding integrins. The quantitative dependence of integrin-mediated adhesion nucleation on substrate rigidities is systematically explored, which shows a reasonable agreement with existing experimental results.