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

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.

Degrading devices: invadosomes in proteolytic cell invasion

Podosomes and invadopodia, collectively known as invadosomes, are cell-matrix contacts in a variety of cell types, such as monocytic cells or cancer cells, that have to cross tissue barriers. Both structures share an actin-rich core, which distinguishes them from other matrix contacts, and are regulated by a multitude of signaling pathways including RhoGTPases, kinases, actin-associated proteins, and microtubule-dependent transport. Invadosomes recruit and secrete proteinases and are thus able to lyse extracellular matrix components. They are therefore considered to be potential key structures in proteolytic cell invasion in both physiological and pathological settings. This review provides an overview of the field, with special focus on current developments such as intracellular transport processes, ultrastructural analysis, the possible involvement of invadosomes in disease, and the tentative identification of invadosomes in 3D environments and in vivo.

Podosome rings generate forces that drive saltatory osteoclast migration

Podosomes are dynamic, actin-containing adhesion structures that collectively self-organize as rings. In this study, we first show by observing osteoclasts plated on bead-seeded soft substrates that podosome assemblies, such as rings, are involved in tension forces. During the expansion of a podosome ring, substrate displacement is oriented outward, suggesting that podosomal structures push the substrate away. To further elucidate the function of forces generated by podosomes, we analyze osteoclast migration. Determining the centers of mass of the whole cell (G) and of actin (P), we demonstrate that osteoclasts migrate by "jumps" and that the trajectories of G and P are strongly correlated. The velocity of the center of mass as a function of time reveals that osteoclasts rapidly catch up with podosomal structures in a periodic pattern. We conclude that actin dynamics inside the cell are not only correlated with cell migration, but drive it.

Actin machinery and mechanosensitivity in invadopodia, podosomes and focal adhesions

The invasiveness of cells is correlated with the presence of dynamic actin-rich membrane structures called invadopodia, which are membrane protrusions that are associated with localized polymerization of sub-membrane actin filaments. Similar to focal adhesions and podosomes, invadopodia are cell-matrix adhesion sites. Indeed, invadopodia share several features with podosomes, but whether they are distinct structures is still a matter of debate. Invadopodia are built upon an N-WASP-dependent branched actin network, and the Rho GTPase Cdc42 is involved in inducing invadopodial-membrane protrusion, which is mediated by actin filaments that are organized in bundles to form an actin core. Actin-core formation is thought to be an early step in invadopodium assembly, and the actin core is perpendicular to the extracellular matrix and the plasma membrane; this contrasts with the tangential orientation of actin stress fibers anchored to focal adhesions. In this Commentary, we attempt to summarize recent insights into the actin dynamics of invadopodia and podosomes, and the forces that are transmitted through these invasive structures. Although the mechanisms underlying force-dependent regulation of invadopodia and podosomes are largely unknown compared with those of focal adhesions, these structures do exhibit mechanosensitivity. Actin dynamics and associated forces might be key elements in discriminating between invadopodia, podosomes and focal adhesions. Targeting actin-regulatory molecules that specifically promote invadopodium formation is an attractive strategy against cancer-cell invasion.

Mechanisms for transcellular diapedesis: probing and pathfinding by 'invadosome-like protrusions'

Immune-system functions require that blood leukocytes continuously traffic throughout the body and repeatedly cross endothelial barriers (i.e. diapedese) as they enter (intravasate) and exit (extravasate) the circulation. The very earliest studies to characterize diapedesis directly in vivo suggested the coexistence of two distinct migratory pathways of leukocytes: between (paracellular pathway) and directly through (transcellular pathway) individual endothelial cells. In vivo studies over the past 50 years have demonstrated significant use of the transcellular diapedesis pathway in bone marrow, thymus, secondary lymphoid organs, various lymphatic structures and peripheral tissues during inflammation and across the blood-brain barrier and blood-retinal barrier during inflammatory pathology. Recently, the first in vitro reports of transcellular diapedesis have emerged. Together, these in vitro and in vivo observations suggest a model of migratory pathfinding in which dynamic 'invadosome-like protrusions' formed by leukocytes have a central role in both identifying and exploiting endothelial locations that are permissive for transcellular diapedesis. Such 'probing' activity might have additional roles in this and other settings.

Matrix architecture dictates three-dimensional migration modes of human macrophages: differential involvement of proteases and podosome-like structures

Tissue infiltration of macrophages, although critical for innate immunity, is also involved in pathologies, such as chronic inflammation and cancer. In vivo, macrophages migrate mostly in a constrained three-dimensional (3D) environment. However, in vitro studies, mainly focused on two dimensions, do not provide meaningful clues about the mechanisms involved in 3D macrophage migration. In contrast, tumor cell 3D migration is well documented. It comprises a protease-independent and Rho kinase (ROCK)-dependent amoeboid migration mode and a protease-dependent and ROCK-independent mesenchymal migration mode. In this study, we examined the influence of extracellular matrix (composition, architecture, and stiffness) on 3D migration of human macrophages derived from blood monocytes (MDMs). We show that: 1) MDMs use either the amoeboid migration mode in fibrillar collagen I or the mesenchymal migration mode in Matrigel and gelled collagen I, whereas HT1080 tumor cells only perform mesenchymal migration; 2) when MDMs use the mesenchymal migratory mode, they form 3D collagenolytic structures at the tips of cell protrusions that share several markers with podosomes as described in two dimensions; 3) in contrast to tumor cells, matrix metalloproteinase inhibitors do not impair protease-dependent macrophage 3D migration, suggesting the involvement of other proteolytic systems; and 4) MDMs infiltrating matrices of similar composition but with variable stiffness adapt their migration mode primarily to the matrix architecture. In conclusion, although it is admitted that leukocytes 3D migration is restricted to the amoeboid mode, we show that human macrophages also perform the mesenchymal mode but in a distinct manner than tumor cells, and they naturally adapt their migration mode to the environmental constraints.

Propagating waves separate two states of actin organization in living cells

Propagating actin waves are dynamic supramolecular structures formed by the self-assembly of proteins within living cells. They are built from actin filaments together with single-headed myosin, the Arp23 complex, and coronin in a defined three-dimensional order. The function of these waves in structuring the cell cortex is studied on the substrate-attached surface of Dictyostelium cells by the use of total internal reflection fluorescence (TIRF) microscopy. Actin waves separate two areas of the cell cortex from each other, which are distinguished by the arrangement of actin filaments. The Arp23 complex dominates in the area enclosed by a wave, where it has the capacity of building dendritic structures, while the proteins prevailing in the external area, cortexillin I and myosin-II, bundle actin filaments and arrange them in antiparallel direction. Wave propagation is accompanied by transitions in the state of actin with a preferential period of 5 min. Wave generation is preceded by local fluctuations in actin assembly, some of the nuclei of polymerized actin emanating from clathrin-coated structures, others emerging independently. The dynamics of phase transitions has been analyzed to provide a basis for modeling the nonlinear interactions that produce spatio-temporal patterns in the actin system of living cells.

Synthetic materials in the study of cell response to substrate rigidity

While it has long been understood that cells can sense and respond to a variety of stimuli, including soluble and insoluble factors, light, and externally applied mechanical stresses, the extent to which cells can sense and respond to the mechanical properties of their environment has only recently begun to be studied. Cell response to substrate stiffness has been suggested to play an important role in processes ranging from developmental morphogenesis to the pathogenesis of disease states and may have profound implications for cell and tissue culture and tissue engineering. Given the importance of this phenomenon, there is a clear need for systems for cell study in which substrate mechanics can be carefully defined and varied independently of biochemical and other signals. This review will highlight past work in the field of cell response to substrate rigidity as well as areas for future study.

Polyelectrolyte-coated gold nanorods and their interactions with type I collagen

Gold nanorods (AuNRs) have unique optical properties for numerous biomedical applications, but the interactions between AuNRs and proteins, particularly those of the extracellular matrix (ECM), are poorly understood. Here the effects of AuNRs on the self-assembly, mechanics, and remodeling of type I collagen gels were examined in vitro. AuNRs were modified with polyelectrolyte multilayers (PEMs) to minimize cytotoxicity, and AuNRs with different terminal polymer chemistries were examined for their interactions with collagen by turbidity assays, rheological tests, and microscopy. Gel contraction assays were used to examine the effects of the PEM-coated AuNRs on cell-mediated collagen remodeling. Polyanion-terminated AuNRs significantly reduced the lag (nucleation) phase of collagen self-assembly and significantly increased the dynamic shear modulus of the polymerized gels, whereas polycation-terminated AuNRs had no effect on the mechanical properties of the collagen. Both polyanion- and polycation-terminated AuNRs significantly inhibited collagen gel contraction by cardiac fibroblasts, and the nanoparticles were localized in intra-, peri-, and extracellular compartments, suggesting that PEM-coated AuNRs influence cell behavior via multiple mechanisms. These results demonstrate the significance of nanoparticle-ECM interactions in determining the bioactivity of nanoparticles.

Prestress and adhesion site dynamics control cell sensitivity to extracellular stiffness

This study aims at improving the understanding of mechanisms responsible for cell sensitivity to extracellular environment. We explain how substrate mechanical properties can modulate the force regulation of cell sensitive elements primarily adhesion sites. We present a theoretical and experimental comparison between two radically different approaches of the force regulation of adhesion sites that depends on their either stationary or dynamic behavior. The most classical stationary model fails to predict cell sensitivity to substrate stiffness whereas the dynamic model predicts extracellular stiffness dependence. This is due to a time dependent reaction force in response to actomyosin traction force exerted on cell sensitive elements. We purposely used two cellular models, i.e., alveolar epithelial cells and alveolar macrophages exhibiting respectively stationary and dynamic adhesion sites, and compared their sensitivity to theoretical predictions. Mechanical and structural results show that alveolar epithelial cells exhibit significant prestress supported by evident stress fibers and lacks sensitivity to substrate stiffness. On the other hand, alveolar macrophages exhibit low prestress and exhibit sensitivity to substrate stiffness. Altogether, theory and experiments consistently show that adhesion site dynamics and cytoskeleton prestress control cell sensitivity to extracellular environment with an optimal sensitivity expected in the intermediate range.

Nonlinear elastic properties of polyacrylamide gels: implications for quantification of cellular forces

Because of their tunable mechanical properties, polyacrylamide gels (PAG) are frequently used for studying cell adhesion and migratory responses to extracellular substrate stiffness. Since these responses are known to heavily depend on the tensional balance between cell contractility and substrate mechanical resistance, a precise knowledge of PAG's mechanical properties becomes quite crucial. Using the micropipette aspiration technique, we first exhibited the nonlinear elastic behavior of PAG and then successfully modeled it by an original strain-energy function. This function depends on the Poisson's ratio and on two material parameters, which have been explicitly related to acrylamide and bis-acrylamide concentrations. Implications of these results have been highlighted with regard to traction force microscopy experiments where cellular force quantification is derived from displacements of beads embedded in PAG. We found that considering PAG as a linear elastic medium tends to significantly underestimate traction forces for substrate displacements larger than 2 mum. Interestingly, we also showed that in the range of cellular force amplitude and PAG stiffness currently used in cell traction force experiments, finite size effects become critical for PAG substrate thickness below 60 mum. Thus, our improved characterization of PAG nonlinear mechanical properties through a new constitutive law could have significant impact onto biological experimentations where such extracellular substrates experience large strains.

Fluorescence proteins, live-cell imaging, and mechanobiology: seeing is believing

Fluorescence proteins (FPs) have been widely used for live-cell imaging in the past decade. This review summarizes the recent advances in FP development and imaging technologies using FPs to monitor molecular localization and activities and gene expressions in live cells. We also discuss the utilization of FPs to develop molecular biosensors and the principles and application of advanced technologies such as fluorescence resonance energy transfer (FRET), fluorescence recovery after photobleaching (FRAP), fluorescence lifetime imaging microscopy (FLIM), and chromophore-assisted light inactivation (CALI). We present examples of the application of FPs and biosensors to visualize mechanotransduction events with high spatiotemporal resolutions in live cells. These live-cell imaging technologies, which represent a frontier area in biomedical engineering, can shed new light on the mechanisms regulating mechanobiology at cellular and molecular levels in normal and pathophysiological conditions.

Physical model for membrane protrusions during spreading

During cell spreading onto a substrate, the kinetics of the contact area is an observable quantity. This paper is concerned with a physical approach to modeling this process in the case of ameboid motility where the membrane detaches itself from the underlying cytoskeleton at the leading edge. The physical model we propose is based on previous reports which highlight that membrane tension regulates cell spreading. Using a phenomenological feedback loop to mimic stress-dependent biochemistry, we show that the actin polymerization rate can be coupled to the stress which builds up at the margin of the contact area between the cell and the substrate. In the limit of small variation of membrane tension, we show that the actin polymerization rate can be written in a closed form. Our analysis defines characteristic lengths which depend on elastic properties of the membrane-cytoskeleton complex, such as the membrane-cytoskeleton interaction, and on molecular parameters, the rate of actin polymerization. We discuss our model in the case of axi-symmetric and non-axi-symmetric spreading and we compute the characteristic time scales as a function of fundamental elastic constants such as the strength of membrane-cytoskeleton adherence.

Self-organized podosomes are dynamic mechanosensors

Podosomes are self-organized, dynamic, actin-containing structures that adhere to the extracellular matrix via integrins [1-5]. Yet, it is not clear what regulates podosome dynamics and whether podosomes can function as direct mechanosensors, like focal adhesions [6-9]. We show here that myosin-II proteins form circular structures outside and at the podosome actin ring to regulate podosome dynamics. Inhibiting myosin-II-dependent tension dissipated podosome actin rings before dissipating the myosin-ring structure. As podosome rings changed size or shape, tractions underneath the podosomes were exerted onto the substrate and were abolished when myosin-light-chain activity was inhibited. The magnitudes of tractions were comparable to those generated underneath focal adhesions, and they increased with substrate stiffness. The dynamics of podosomes and of focal adhesions were different. Torsional tractions underneath the podosome rings were generated with rotations of podosome rings in a nonmotile, nonrotating cell, suggesting a unique feature of these circular structures. Stresses applied via integrins at the apical surface directly displaced podosomes near the basal surface. Stress-induced podosome displacements increased nonlinearly with applied stresses. Our results suggest that podosomes are dynamic mechanosensors in which interactions of myosin tension and actin dynamics are crucial for regulating these self-organized structures in living cells.

Rho GTPases in osteoclasts: orchestrators of podosome arrangement

Cells from the myeloid lineage, namely macrophages, dendritic cells and osteoclasts, develop podosomes instead of stress fibers and focal adhesions to adhere and migrate. Podosomes share many components with focal adhesions but differ in their molecular organization, with a dense core of polymerized actin surrounded by scaffolding proteins, kinases and integrins. Podosomes are found either isolated both in macrophages and dendritic cells or arranged into superstructures in osteoclasts. When osteoclasts resorb bone, they form an F-actin rich sealing zone, which is a dense array of connected podosomes that firmly anchors osteoclasts to bone. It delineates a compartment in which protons and proteases are secreted to dissolve and degrade the mineralized matrix. Since Rho GTPases have been shown to control F-actin stress fibers and focal adhesions in mesenchymal cells, the question of whether they could also control podosome formation and arrangement in cells from the myeloid lineage, and particularly in osteoclasts, rapidly emerged. This article considers recent advances made in our understanding of podosome arrangements in osteoclasts and how Rho GTPases may control it.

Podosome-type adhesions and focal adhesions, so alike yet so different

Cell-matrix adhesions are essential for cell migration, tissue organization and differentiation, therefore playing central roles in embryonic development, remodeling and homeostasis of tissues and organs. Matrix adhesion-dependent signals cooperate with other pathways to regulate biological functions such as cell survival, cell proliferation, wound healing, and tumorigenesis. Cell migration and invasion are integrated processes requiring the continuous, coordinated assembly and disassembly of integrin-mediated adhesions. An understanding of how integrins regulate cell migration and invasiveness through the dynamic regulation of adhesions is fundamental to both physiological and pathological situations. A variety of cell-matrix adhesions has been identified, namely, focal complexes, focal adhesions, fibrillar adhesions, podosomes, and invadopodia (podosome-type adhesions). These adhesion sites contain integrin clusters able to develop specialized structures, which are different in their architecture and dynamics although they share almost the same proteins. Here we compare recent advances and developments in the elucidation of the organization and dynamics of focal adhesions and podosome-type adhesions, in order to understand how such subcellular sites - though closely related in their composition - can be structurally and functionally different. The underlying question is how their respective physiological or pathological roles are related to their distinct organization.

PGE2-mediated podosome loss in dendritic cells is dependent on actomyosin contraction downstream of the RhoA-Rho-kinase axis

Podosomes are dynamic adhesion structures found in dendritic cells (DCs) and other cells of the myeloid lineage. We previously showed that prostaglandin E2 (PGE2), an important proinflammatory mediator produced during DC maturation, induces podosome disassembly within minutes after stimulation. Here, we demonstrate that this response is mediated by cAMP elevation, occurs downstream of Rho kinase and is dependent on myosin II. Whereas PGE2 stimulation leads to activation of the small GTPase RhoA, decreased levels of Rac1-GTP and Cdc42-GTP are observed. These results show that PGE2 stimulation leads to activation of the RhoA-Rho-kinase axis to promote actomyosin-based contraction and subsequent podosome dissolution. Because podosome disassembly is accompanied by de novo formation of focal adhesions, we propose that the disassembly/formation of these two different adhesion structures is oppositely regulated by actomyosin contractility and relative activities of RhoA, Rac1 and Cdc42.

Invadopodia and matrix degradation, a new property of prostate cancer cells during migration and invasion

The present study demonstrated that invadopodia are associated with invasion by degradation of matrix in prostate cancer cells PC3. To find out the presence of invadopodia in PC3 cells, we performed a few comparative analyses with osteoclasts, which utilize podosomes for migration. Our investigations indeed demonstrated that invadopodia are comparable to podosomes in the localization of Wiskott-Aldrich syndrome protein (WASP)/matrix metalloproteinase-9 and the degradation of matrix. Invadopodia are different from podosomes in the localization of actin/vinculin, distribution during migration, and the mode of degradation of extracellular matrix. Invadopodia enable polarized invasion of PC3 cells into the gelatin matrix in a time-dependent manner. Gelatin degradation was confined within the periphery of the cell. Osteoclasts demonstrated directional migration with extensive degradation of matrix underneath and around the osteoclasts. A pathway of degradation of matrix representing a migratory track was observed due to the rearrangement of podosomes as rosettes or clusters at the leading edge. Reducing the matrix metalloproteinase-9 levels by RNA interference inhibited the degradation of matrix but not the formation of podosomes or invadopodia. Competition experiments with TAT-fused WASP peptides suggest that actin polymerization and formation of invadopodia involve the WASP-Arp2/3 complex pathway. Moreover, PC3 cells overexpressing osteopontin (OPN) displayed an increase in the number of invadopodia and gelatinolytic activity as compared with PC3 cells and PC3 cells expressing mutant OPN in integrin-binding domain and null for OPN. Thus, we conclude that OPN/integrin alphavbeta3 signaling participates in the process of migration and invasion of PC3 cells through regulating processes essential for the formation and function of invadopodia.

Paxillin phosphorylation controls invadopodia/podosomes spatiotemporal organization

In Rous sarcoma virus (RSV)-transformed baby hamster kidney (BHK) cells, invadopodia can self-organize into rings and belts, similarly to podosome distribution during osteoclast differentiation. The composition of individual invadopodia is spatiotemporally regulated and depends on invadopodia localization along the ring section: the actin core assembly precedes the recruitment of surrounding integrins and integrin-linked proteins, whereas the loss of the actin core was a prerequisite to invadopodia disassembly. We have shown that invadopodia ring expansion is controlled by paxillin phosphorylations on tyrosine 31 and 118, which allows invadopodia disassembly. In BHK-RSV cells, ectopic expression of the paxillin mutant Y31F-Y118F induces a delay in invadopodia disassembly and impairs their self-organization. A similar mechanism is unraveled in osteoclasts by using paxillin knockdown. Lack of paxillin phosphorylation, calpain or extracellular signal-regulated kinase inhibition, resulted in similar phenotype, suggesting that these proteins belong to the same regulatory pathways. Indeed, we have shown that paxillin phosphorylation promotes Erk activation that in turn activates calpain. Finally, we observed that invadopodia/podosomes ring expansion is required for efficient extracellular matrix degradation both in BHK-RSV cells and primary osteoclasts, and for transmigration through a cell monolayer.

The tyrosine kinase activity of c-Src regulates actin dynamics and organization of podosomes in osteoclasts

Podosomes are dynamic actin-rich structures composed of a dense F-actin core surrounded by a cloud of more diffuse F-actin. Src performs one or more unique functions in osteoclasts (OCLs), and podosome belts and bone resorption are impaired in the absence of Src. Using Src(-/-) OCLs, we investigated the specific functions of Src in the organization and dynamics of podosomes. We found that podosome number and the podosome-associated actin cloud were decreased in Src(-/-) OCLs. Videomicroscopy and fluorescence recovery after photobleaching analysis revealed that the life span of Src(-/-) podosomes was increased fourfold and that the rate of actin flux in the core was decreased by 40%. Thus, Src regulates the formation, structure, life span, and rate of actin polymerization in podosomes and in the actin cloud. Rescue of Src(-/-) OCLs with Src mutants showed that both the kinase activity and either the SH2 or the SH3 binding domain are required for Src to restore normal podosome organization and dynamics. Moreover, inhibition of Src family kinase activities in Src(-/-) OCLs by Src inhibitors or by expressing dominant-negative Src(K295M) induced the formation of abnormal podosomes. Thus, Src is an essential regulator of podosome structure, dynamics and organization.

Erk1/2 MAPK and caldesmon differentially regulate podosome dynamics in A7r5 vascular smooth muscle cells

We tested the hypothesis that the MEK/Erk/caldesmon phosphorylation cascade regulates PKC-mediated podosome dynamics in A7r5 cells. We observed the phosphorylation of MEK, Erk and caldesmon, and their translocation to the podosomes upon phorbol dibutyrate (PDBu) stimulation, together with the nuclear translocation of phospho-MEK and phospho-Erk. After MEK inhibition by U0126, Erk translocated to the interconnected actin-rich columns but failed to translocate to the nucleus, suggesting that podosomes served as a site for Erk phosphorylation. The interconnected actin-rich columns in U0126-treated, PDBu-stimulated cells contained alpha-actinin, caldesmon, vinculin, and metalloproteinase-2. Caldesmon and vinculin became integrated with F-actin at the columns, in contrast to their typical location at the ring of podosomes. Live-imaging experiments suggested the growth of these columns from podosomes that were slow to disassemble. The observed modulation of podosome size and life time in A7r5 cells overexpressing wild-type and phosphorylation-deficient caldesmon-GFP mutants in comparison to untransfected cells suggests that caldesmon and caldesmon phosphorylation modulate podosome dynamics in A7r5 cells. These results suggest that Erk1/2 and caldesmon differentially modulate PKC-mediated formation and/or dynamics of podosomes in A7r5 vascular smooth muscle cells.

Adhesion-mediated mechanosensitivity: a time to experiment, and a time to theorize

Adhesion-mediated signaling provides cells with information about multiple parameters of their microenvironment, including mechanical characteristics. Often, such signaling is based on a unique feature of adhesion structures: their ability to grow and strengthen when force is applied to them, either from within the cell or from the outside. Such adhesion reinforcement is characteristic of integrin-mediated cell-matrix adhesions, but may also operate in other types of adhesion structures. Though the amount of knowledge about adhesion-mediated signaling is growing rapidly, the mechanisms underlying force-dependent regulation of junction assembly are largely unknown. Experiments have been carried out that have started to uncover the major signaling pathways involved in the response of adhesion sites to force. Theoretical models have also been used to address the physical mechanisms underlying adhesion-mediated mechanosensing.