Mesenchymal Migration on Adhesive-Nonadhesive Alternate Surfaces in Macrophages

Mesenchymal migration usually happens on adhesive substrates, while cells adopt amoeboid migration on low/nonadhesive surfaces. Protein-repelling reagents, e.g., poly(ethylene) glycol (PEG), are routinely employed to resist cell adhering and migrating. Contrary to these perceptions, this work discovers a unique locomotion of macrophages on adhesive-nonadhesive alternate substrates in vitro that they can overcome nonadhesive PEG gaps to reach adhesive regions in the mesenchymal mode. Adhering to extracellular matrix regions is a prerequisite for macrophages to perform further locomotion on the PEG regions. Podosomes are found highly enriched on the PEG region in macrophages and support their migration across the nonadhesive regions. Increasing podosome density through myosin IIA inhibition facilitates cell motility on adhesive-nonadhesive alternate substrates. Moreover, a developed cellular Potts model reproduces this mesenchymal migration. These findings together uncover a new migratory behavior on adhesive-nonadhesive alternate substrates in macrophages.

Molecular Force Imaging Reveals That Integrin-Dependent Mechanical Checkpoint Regulates Fcγ-Receptor-Mediated Phagocytosis in Macrophages

Macrophages are a type of immune cell that helps eliminate pathogens and diseased cells. Recent research has shown that macrophages can sense mechanical cues from potential targets to perform effective phagocytosis, but the mechanisms behind it remain unclear. In this study, we used DNA-based tension probes to study the role of integrin-mediated forces in FcγR-mediated phagocytosis. The results showed that when the phagocytic receptor FcγR is activated, the force-bearing integrins create a "mechanical barrier" that physically excludes the phosphatase CD45 and facilitates phagocytosis. However, if the integrin-mediated forces are physically restricted at lower levels or if the macrophage is on a soft matrix, CD45 exclusion is significantly reduced. Moreover, CD47-SIRPα "don't eat me" signaling can reduce CD45 segregation by inhibiting the mechanical stability of the integrin barrier. These findings demonstrate how macrophages use molecular forces to identify physical properties and combine them with biochemical signals from phagocytic receptors to guide phagocytosis.

Cytoskeleton network participates in the anti-infection responses of macrophage

During immune responses against invading pathogenic bacteria, the cytoskeleton network enables macrophages to implement multiple essential functions. To protect the host from infection, macrophages initially polarize to adopt different phenotypes in response to distinct signals from the microenvironment. The extracellular stimulus regulates the rearrangement of the cytoskeleton, thereby altering the morphology and migratory properties of macrophages. Subsequently, macrophages degrade the extracellular matrix (ECM) and migrate toward the sites of infection to directly contact invading pathogens, during which the involvement of cytoskeleton-based structures such as podosomes and lamellipodia is indispensable. Ultimately, macrophages execute the function of phagocytosis to engulf and eliminate the invading pathogens. Phagocytosis is a complex process that requires the cooperation of cytoskeleton-enriched super-structures, such as filopodia, lamellipodia, and phagocytic cup. This review presents an overview of cytoskeletal regulations in macrophage polarization, ECM degradation, migration, and phagocytosis, highlighting the pivotal role of the cytoskeleton in host defense against infection.

Expression of non-phosphorylatable S5A-L-plastin exerts phenotypes distinct from L-plastin deficiency during podosome formation and phagocytosis

Introduction: The actin cytoskeleton remodels to enable diverse processes essential to immunity, such as cell adhesion, migration and phagocytosis. A panoply of actin-binding proteins regulate these rapid rearrangements to induce actin-based shape changes and to generate force. L-plastin (LPL) is a leukocyte-specific, actin-bundling protein that is regulated in part by phosphorylation of the Ser-5 residue. LPL deficiency in macrophages impairs motility, but not phagocytosis; we recently found that expression of LPL in which the S5 residue is converted to a non-phosphorylatable alanine (S5A-LPL) resulted in diminished phagocytosis, but unimpaired motility. Methods: To provide mechanistic insight into these findings, we now compare the formation of podosomes (an adhesive structure) and phagosomes in alveolar macrophages derived from wild-type (WT), LPL-deficient, or S5A-LPL mice. Both podosomes and phagosomes require rapid remodeling of actin, and both are force-transmitting. Actin rearrangement, force generation, and signaling rely upon recruitment of many actin-binding proteins, including the adaptor protein vinculin and the integrin-associated kinase Pyk2. Prior work suggested that vinculin localization to podosomes was independent of LPL, while Pyk2 was displaced by LPL deficiency. We therefore chose to compare vinculin and Pyk2 co-localization with F-actin at sites of adhesion of phagocytosis in AMs derived from WT, S5A-LPL or LPL^-/- mice, using Airyscan confocal microscopy. Results: As described previously, podosome stability was significantly disrupted by LPL deficiency. In contrast, LPL was dispensable for phagocytosis and was not recruited to phagosomes. Recruitment of vinculin to sites of phagocytosis was significantly enhanced in cells lacking LPL. Expression of S5A-LPL impeded phagocytosis, with reduced appearance of ingested bacteria-vinculin aggregates. Discussion: Our systematic analysis of the regulation of LPL during podosome vs. phagosome formation illuminates essential remodeling of actin during key immune processes.

Potential mechanisms of osteoprotegerin-induced damage to osteoclast adhesion structures via P2X7R-mediated MAPK signaling

Osteoprotegerin (OPG) is a negative regulator of osteoclast formation by competing with receptor activator of the nuclear factor-κB (NF-κB) ligand (RANKL) for RANK. OPG is not only a soluble decoy receptor for RANKL, but is also considered as a direct effector of osteoclast functions. However, the mechanismsresponsible for OPG-induced changes to osteoclast bone resorption functionsremain unknown. P2X7R is involved in the process of multi-nucleation and cell fusion. Therefore, in the present study, mitogen-activated protein kinase (MAPK) inhibitors and the RNA interference of purinergic receptor P2X7 (P2X7R) were usedtoexamine the effects of P2X7R-mediated MAPK signaling on changes to osteoclast adhesion structure induced by OPG; for this purpose, western blot analysis and immunofluorescence staining were performed. The results revealed that OPG inhibited osteoclast adhesion-related protein expression, disrupted adhesion protein distribution, and destroyed osteoclast filopodia and lamellipodia structures. The inhibitors partially restored osteoclast adhesion structure, including protein expression, distribution and cell morphology. The absence of P2X7R markedly inhibited osteoclast formation, and subsequent OPG treatment accelerated the damage to adhesion structures. However, P2X7R activation significantly recosvered the phosphorylation of paxillin, vinculin, phosphorylated protein tyrosine kinase 2 and SRC proto-oncogene, non-receptor tyrosine kinase induced by OPG, and their distribution was uniform at the osteoclast periphery. P2X7R silencing suppressed the phosphorylation of MAPK. On the whole, the findings of the present study highlighta key role of P2X7R/MAPK signaling in osteoclast adhesion, and provide a novel therapeutic target for bone disease.

Characterization of unique functionalities in c-Src domains required for osteoclast podosome belt formation

Deletion of c-Src, a ubiquitously expressed tyrosine kinase, results in osteoclast dysfunction and osteopetrosis, in which bones harden into "stone." In contrast, deletion of the genes encoding other members of the Src family kinase (SFK) fails to produce an osteopetrotic phenotype. This suggests that c-Src performs a unique function in the osteoclast that cannot be compensated for by other SFKs. We aimed to identify the molecular basis of this unique role in osteoclasts and bone resorption. We found that c-Src, Lyn, and Fyn were the most highly expressed SFKs in WT osteoclasts, whereas Hck, Lck, Blk, and Fgr displayed low levels of expression. Formation of the podosome belt, clusters of unique actin assemblies, was disrupted in src^-/- osteoclasts; introduction of constitutively activated SFKs revealed that only c-Src and Fyn could restore this process. To identify the key structural domains responsible, we constructed chimeric Src-Hck and Src-Lyn constructs in which the unique, SH3, SH2, or catalytic domains had been swapped. We found that the Src unique, SH3, and kinase domains were each crucial to establish Src functionality. The SH2 domain could however be substituted with Lyn or Hck SH2 domains. Furthermore, we demonstrate that c-Src's functionality is, in part, derived from an SH3-proximal proline-rich domain interaction with c-Cbl, leading to phosphorylation of c-Cbl Tyr700. These data help clarify Src's unique functionality in the organization of the cytoskeleton in osteoclasts, required for efficient bone resorption and explain why c-Src cannot be replaced, in osteoclasts, by other SFKs.

Podosome-Driven Defect Development in Lamellar Bone under the Conditions of Senile Osteoporosis Observed at the Nanometer Scale

The degradation mechanism of human trabecular bone harvested from the central part of the femoral head of a patient with a fragility fracture of the femoral neck under conditions of senile osteoporosis was investigated by high-resolution electron microscopy. As evidenced by light microscopy, there is a disturbance of bone metabolism leading to severe and irreparable damages to the bone structure. These defects are evoked by osteoclasts and thus podosome activity. Podosomes create typical pit marks and holes of about 300-400 nm in diameter on the bone surface. Detailed analysis of the stress field caused by the podosomes in the extracellular bone matrix was performed. The calculations yielded maximum stress in the range of few megapascals resulting in formation of microcracks around the podosomes. Disintegration of hydroxyapatite and free lying collagen fibrils were observed at the edges of the plywood structure of the bone lamella. At the ultimate state, the disintegration of the mineralized collagen fibrils to a gelatinous matrix comes along with a delamination of the apatite nanoplatelets resulting in a brittle, porous bone structure. The nanoplatelets aggregate to big hydroxyapatite plates with a size of up to 10 x 20 μm². The enhanced plate growth can be explained by the interaction of two mechanisms in the ruffled border zone: the accumulation of delaminated hydroxyapatite nanoplatelets near clusters of podosomes and the accelerated nucleation and random growth of HAP nanoplatelets due to a nonsufficient concentration of process-directing carboxylated osteocalcin cOC.

HIV-1 uses dynamic podosomes for entry into macrophages

Macrophages are one of the major targets of Human Immunodeficiency virus 1 (HIV-1) and play crucial roles in viral dissemination and persistence during AIDS progression. Here, we reveal the dynamic podosome-mediated entry of HIV-1 into macrophages. Inhibition of podosomes prevented HIV-1 entry into macrophages, while stimulation of podosome formation promoted viral entry. Single-virus tracking revealed the temporal and spatial mechanism of the dynamic podosome-mediated viral entry process. The core and ring structures of podosomes played complex roles in viral entry. The HIV coreceptor, CCR5, was recruited to form specific clusters at the podosome ring, where it participated in viral entry. The podosome facilitated HIV-1 entry with a rotation mode triggered by dynamic actin. Our discovery of this novel HIV-1 entry route into macrophages, mediated by podosomes critical for cell migration and tissue infiltration, provides a new view of HIV infection and pathogenesis, which may assist in the development of new antiviral strategies.IMPORTANCEMacrophages are motile leukocytes and play critical roles in HIV-1 infection and AIDS progression. Podosomes, as small dynamic adhesion microdomains driven by the dynamic actin cytoskeleton, are mainly involved in cell migration of macrophages. Herein, we found that HIV-1 uses dynamic podosomes to facilitate its entry into macrophages. Single-virus imaging coupled with drug assays revealed the mechanism underlying the podosome-mediated route of HIV-1 entry into macrophages, including the dynamic relationship between the viral particles and the podosome core and ring structures, the CCR5 coreceptor. The dynamic podosome-mediated entry of HIV-1 into macrophages will be very significant for HIV-1 pathogenesis, especially for viral dissemination via macrophage migration and tissue infiltration. Thus, we report a novel HIV-1 entry route into macrophages mediated by podosomes, which extends our understanding of HIV infection and pathogenesis.

Tissue remodeling by invadosomes

One of the strategies used by cells to degrade and remodel the extracellular matrix (ECM) is based on invadosomes, actin-based force-producing cell–ECM contacts that function in adhesion and migration and are characterized by their capacity to mediate pericellular proteolysis of ECM components. Invadosomes found in normal cells are called podosomes, whereas invadosomes of invading cancer cells are named invadopodia. Despite their broad involvement in cell migration and in protease-dependent ECM remodeling and their detection in living organisms and in fresh tumor tissue specimens, the specific composition and dynamic behavior of podosomes and invadopodia and their functional relevance in vivo remain poorly understood. Here, we discuss recent findings that underline commonalities and peculiarities of podosome and invadopodia in terms of organization and function and propose an updated definition of these cellular protrusions, which are increasingly relevant in patho-physiological tissue remodeling.

Characterization of the Signaling Modalities of Prostaglandin E2 Receptors EP2 and EP4 Reveals Crosstalk and a Role for Microtubules

Prostaglandin E2 (PGE2) is a lipid mediator that modulates the function of myeloid immune cells such as macrophages and dendritic cells (DCs) through the activation of the G protein-coupled receptors EP2 and EP4. While both EP2 and EP4 signaling leads to an elevation of intracellular cyclic adenosine monophosphate (cAMP) levels through the stimulating Gα_s protein, EP4 also couples to the inhibitory Gα_i protein to decrease the production of cAMP. The receptor-specific contributions to downstream immune modulatory functions are still poorly defined. Here, we employed quantitative imaging methods to characterize the early EP2 and EP4 signaling events in myeloid cells and their contribution to the dissolution of adhesion structures called podosomes, which is a first and essential step in DC maturation. We first show that podosome loss in DCs is primarily mediated by EP4. Next, we demonstrate that EP2 and EP4 signaling leads to distinct cAMP production profiles, with EP4 inducing a transient cAMP response and EP2 inducing a sustained cAMP response only at high PGE2 levels. We further find that simultaneous EP2 and EP4 stimulation attenuates cAMP production, suggesting a reciprocal control of EP2 and EP4 signaling. Finally, we demonstrate that efficient signaling of both EP2 and EP4 relies on an intact microtubule network. Together, these results enhance our understanding of early EP2 and EP4 signaling in myeloid cells. Considering that modulation of PGE2 signaling is regarded as an important therapeutic possibility in anti-tumor immunotherapy, our findings may facilitate the development of efficient and specific immune modulators of PGE2 receptors.

PAK4 Kinase Activity Plays a Crucial Role in the Podosome Ring of Myeloid Cells

p21-Activated kinase 4 (PAK4), a serine/threonine kinase, is purported to localize to podosomes: transient adhesive structures that degrade the extracellular matrix to facilitate rapid myeloid cell migration. We find that treatment of transforming growth factor β (TGF-β)-differentiated monocytic (THP-1) cells with a PAK4-targeted inhibitor significantly reduces podosome formation and induces the formation of focal adhesions. This switch in adhesions confers a diminution of matrix degradation and reduced cell migration. Furthermore, reduced PAK4 expression causes a significant reduction in podosome number that cannot be rescued by kinase-dead PAK4, supporting a kinase-dependent role. Concomitant with PAK4 depletion, phosphorylation of Akt is perturbed, whereas a specific phospho-Akt signal is detected within the podosomes. Using superresolution analysis, we find that PAK4 specifically localizes in the podosome ring, nearer to the actin core than other ring proteins. We propose PAK4 kinase activity intersects with the Akt pathway at the podosome ring:core interface to drive regulation of macrophage podosome turnover.

Heterogeneity and Actin Cytoskeleton in Osteoclast and Macrophage Multinucleation

Osteoclast signatures are determined by two transcriptional programs, the lineage-determining transcription pathway and the receptor activator of nuclear factor kappa-B ligand (RANKL)-dependent differentiation pathways. During differentiation, mononuclear precursors become multinucleated by cell fusion. Recently, live-cell imaging has revealed a high level of heterogeneity in osteoclast multinucleation. This heterogeneity includes the difference in the differentiation states and the mobility of the fusion precursors, as well as the mode of fusion among the fusion precursors with different numbers of nuclei. In particular, fusion partners often form morphologically distinct actin-based linkages that allow two cells to exchange lipids and proteins before membrane fusion. However, the origin of this heterogeneity remains elusive. On the other hand, osteoclast multinucleation is sensitive to the environmental cues. Such cues promote the reorganization of the actin cytoskeleton, especially the formation and transformation of the podosome, an actin-rich punctate adhesion. This review covers the heterogeneity of osteoclast multinucleation at the pre-fusion stage with reference to the environment-dependent signaling pathway responsible for reorganizing the actin cytoskeleton. Furthermore, we compare osteoclast multinucleation with macrophage fusion, which results in multinucleated giant macrophages.

Site-directed MT1-MMP trafficking and surface insertion regulate AChR clustering and remodeling at developing NMJs

At vertebrate neuromuscular junctions (NMJs), the synaptic basal lamina contains different extracellular matrix (ECM) proteins and synaptogenic factors that induce and maintain synaptic specializations. Here, we report that podosome-like structures (PLSs) induced by ubiquitous ECM proteins regulate the formation and remodeling of acetylcholine receptor (AChR) clusters via focal ECM degradation. Mechanistically, ECM degradation is mediated by PLS-directed trafficking and surface insertion of membrane-type 1 matrix metalloproteinase (MT1-MMP) to AChR clusters through microtubule-capturing mechanisms. Upon synaptic induction, MT1-MMP plays a crucial role in the recruitment of aneural AChR clusters for the assembly of postsynaptic specializations. Lastly, the structural defects of NMJs in embryonic MT1-MMP^-/- mice further demonstrate the physiological role of MT1-MMP in normal NMJ development. Collectively, this study suggests that postsynaptic MT1-MMP serves as a molecular switch to synaptogenesis by modulating local ECM environment for the deposition of synaptogenic signals that regulate postsynaptic differentiation at developing NMJs.

Nck adapter proteins promote podosome biogenesis facilitating extracellular matrix degradation and cancer invasion

Background

Podosomes are membrane‐bound adhesive structures formed by actin remodeling. They are capable of extracellular matrix (ECM) degradation, which is a prerequisite for cancer cell invasion and metastasis. The signaling mechanism of podosome formation is still unknown in cancer. We previously reported that Nck adaptors regulate directional cell migration and endothelial lumen formation by actin remodeling, while deficiency of Nck reduces cancer metastasis. This study evaluated the role of Nck adaptors in podosome biogenesis and cancer invasion.

Methods

This study was conducted in vitro using both healthy cells (Human Umbilical Vein Endothelial Cell, 3T3 fibroblasts) and cancer cells (prostate cancer cell line; PC3, breast cancer cell line; MDA‐MB‐231). Confocal and TIRF imaging of cells expressing Green Fluorescence Protein (GFP)  mutant under altered levels of Nck or downstream of kinase 1 (Dok1) was used to evaluate the podosome formation and fluorescent gelatin matrix degradation. Levels of Nck in human breast carcinoma tissue sections were detected by immune histochemistry using Nck polyclonal antibody. Biochemical interaction of Nck/Dok1 was detected in podosome forming cells using immune precipitation and far‐western blotting.

Results

This study demonstrates that ectopic expression of Nck1 and Nck2 can induce the endothelial podosome formation in vitro. Nck silencing by short‐hairpin RNA blocked podosome biogenesis and ECM degradation in cSrc‐Y530F transformed endothelial cells in this study. Immunohistochemical analysis revealed the Nck overexpression in human breast carcinoma tissue sections. Immunoprecipitation and far‐western blotting revealed the biochemical interaction of Nck/p62Dok in podosome forming cells.

Conclusions

Nck adaptors in interaction with Dok1 induce podosome biogenesis and ECM degradation facilitating cancer cell invasion, and therefore a bona fide target of cancer therapy.

Scattered podosomes and podosomes associated with the sealing zone architecture in cultured osteoclasts revealed by cell shearing, quick freezing, and platinum-replica electron microscopy

Osteoclasts (OCs) can adhere to a variety of substrate surfaces by highly dynamic actin-based cytoskeletal structures termed podosomes. This tight attachment is established by a sealing zone (SZ), which is made of interconnected individual podosomes. Compared with scattered podosomes in various cell types, the architecture of the SZ is still unclear. Especially, ultrastructural studies on the details of the cytoskeletal structure of an OC have been challenging, because the high density of filaments in their podosomes obscure visualization of individual filaments. Therefore, to study this organization in more exact detail, we employed shearing open combined with replica electron microscopy. The present study provides several new details of the podosome and SZ structure, which were previously unrecognized: (a) the SZ consists of recognizable podosomes with a dense actin network of interpodosomal regions characterized by multiple layers of crossing, branching and anastomosing actin filament networks; (b) the Arp2/3 complex is distributed throughout the actin network of podosomes and SZ, indicating that actin polymerization is concentrated at these regions; (c) a close spatial relationship between the podosome and the dorsal membrane; and (d) a network of membranous organelles in close proximity to the podosomes in the SZ. Taken together, the present study reveals that a more complicated interpodosomal actin network among neighboring individual podosomes, which is more complicated than previously thought, appears to form the SZ. Indeed, individual podosomes are not an isolated structural unit from other organelles; and, in turn, their dynamism might affect the surrounding interpodosomal cytoskeletons, membranous organelles, and plasma membrane.

Nrf2 Sequesters Keap1 Preventing Podosome Disassembly: A Quintessential Duet Moonlights in Endothelium

AIMS

Nrf2 (nuclear factor erythroid 2-like 2) is a transcription factor known to modulate blood vessel formation. Various experimental settings, however, attribute to Nrf2 either stimulatory or repressive influence on angiogenesis. Our findings unveil the mechanism of Nrf2-dependent vessel formation, which reaches beyond transactivation of gene expression and reconciles previous discrepancies.

RESULTS

We provide evidence that growth differentiation factor 15 (GDF-15)- and stromal cell-derived factor 1 (SDF-1)-induced angiogenesis strongly depends on the presence of Nrf2 protein but does not rely on its transcriptional activity. Instead, Nrf2 serves as a protein restraining Keap1 (Kelch-like ECH-associated protein 1), its known transcriptional repressor. Angiogenic response is abrogated in Nrf2-deficient endothelial cells but not in cells expressing dominant negative form or Keap1-binding fragment of Nrf2. Deficiency of Nrf2 protein available for Keap1 leads to the overabundance of RhoGAP1 (Rho GTPase-activating protein 1), the protein regulating cell division cycle 42 (Cdc42) activity. This impairs podosome assembly and disrupts actin rearrangements, thereby preventing angiogenesis. Effects of Nrf2 deficiency can be rescued by concomitant knockdown of RhoGAP1 or Keap1. Importantly, in the established murine model of Nrf2 deficiency, the N-terminal fragment of Nrf2 containing Keap1 binding domain is preserved. Thus, this model can be used to characterize Nrf2 as a transcription factor, but not as a Keap1-sequestering protein. Innovation and Conclusion: To date, the significance of Nrf2 in cell function has been ascribed solely to the regulation of transcription. We demonstrate that Nrf2 serves as a protein tethering Keap1 to allow podosome assembly and angiogenesis. Moreover, we emphasize that the new Nrf2 function of a Keap1 scavenger implies revisiting the interpretation of some of the previous data on the Nrf2-Keap1 system.

Integrins: Moonlighting Proteins in Invadosome Formation

Invadopodia are actin-rich protrusions developed by transformed cells in 2D/3D environments that are implicated in extracellular matrix (ECM) remodeling and degradation. These structures have an undoubted association with cancer invasion and metastasis because invadopodium formation in vivo is a key step for intra/extravasation of tumor cells. Invadopodia are closely related to other actin-rich structures known as podosomes, which are typical structures of normal cells necessary for different physiological processes during development and organogenesis. Invadopodia and podosomes are included in the general term 'invadosomes,' as they both appear as actin puncta on plasma membranes next to extracellular matrix metalloproteinases, although organization, regulation, and function are slightly different. Integrins are transmembrane proteins implicated in cell-cell and cell-matrix interactions and other important processes such as molecular signaling, mechano-transduction, and cell functions, e.g., adhesion, migration, or invasion. It is noteworthy that integrin expression is altered in many tumors, and other pathologies such as cardiovascular or immune dysfunctions. Over the last few years, growing evidence has suggested a role of integrins in the formation of invadopodia. However, their implication in invadopodia formation and adhesion to the ECM is still not well known. This review focuses on the role of integrins in invadopodium formation and provides a general overview of the involvement of these proteins in the mechanisms of metastasis, taking into account classic research through to the latest and most advanced work in the field.

Trophoblastic proliferation and invasion regulated by ACTN4 is impaired in early onset preeclampsia

Successful pregnancy requires normal placentation, which largely depends on the tight regulation of proliferation, invasion, and migration of trophoblast cells. Abnormal functioning of trophoblast cells may cause failure of uterine spiral artery remodeling, which may be related to pregnancy-related disorders, such as preeclampsia. Here, we reported that an actin-binding protein, α-actinin (ACTN)4, was dysregulated in placentas from early onset preeclampsia. Moreover, knockdown of ACTN4 markedly inhibited trophoblast cell proliferation by reducing AKT membrane translocation. Furthermore, E-cadherin regulated ACTN4 and β-catenin colocalization on trophoblast cell podosomes, and ACTN4 down-regulation suppressed the E-cadherin-induced cell invasion increase via depolymerizing actin filaments. Moreover, loss of ACTN4 recapitulated a number of the features of human preeclampsia. Therefore, our data indicate that ACNT4 plays a role in trophoblast function and is required for normal placental development.-Peng, W., Tong, C., Li, L., Huang, C., Ran, Y., Chen, X., Bai, Y., Liu, Y., Zhao, J., Tan, B., Luo, X., Wang, H., Wen, L., Zhang, C., Zhang, H., Ding, Y., Qi, H., Baker, P. N. Trophoblastic proliferation and invasion regulated by ACTN4 is impaired in early onset preeclampsia.

Arf GAPs as Regulators of the Actin Cytoskeleton-An Update

Arf GTPase-activating proteins (Arf GAPs) control the activity of ADP-ribosylation factors (Arfs) by inducing GTP hydrolysis and participate in a diverse array of cellular functions both through mechanisms that are dependent on and independent of their Arf GAP activity. A number of these functions hinge on the remodeling of actin filaments. Accordingly, some of the effects exerted by Arf GAPs involve proteins known to engage in regulation of the actin dynamics and architecture, such as Rho family proteins and nonmuscle myosin 2. Circular dorsal ruffles (CDRs), podosomes, invadopodia, lamellipodia, stress fibers and focal adhesions are among the actin-based structures regulated by Arf GAPs. Arf GAPs are thus important actors in broad functions like adhesion and motility, as well as the specialized functions of bone resorption, neurite outgrowth, and pathogen internalization by immune cells. Arf GAPs, with their multiple protein-protein interactions, membrane-binding domains and sites for post-translational modification, are good candidates for linking the changes in actin to the membrane. The findings discussed depict a family of proteins with a critical role in regulating actin dynamics to enable proper cell function.

Sphingosine-1-Phosphate Receptor 2 Controls Podosome Components Induced by RANKL Affecting Osteoclastogenesis and Bone Resorption

Proinflammatory cytokine production, cell chemotaxis, and osteoclastogenesis can lead to inflammatory bone loss. Previously, we showed that sphingosine-1-phosphate receptor 2 (S1PR2), a G protein coupled receptor, regulates inflammatory cytokine production and osteoclastogenesis. However, the signaling pathways regulated by S1PR2 in modulating inflammatory bone loss have not been elucidated. Herein, we demonstrated that inhibition of S1PR2 by a specific S1PR2 antagonist (JTE013) suppressed phosphoinositide 3-kinase (PI3K), mitogen-activated protein kinases (MAPKs), and nuclear factor kappa-B (NF-κB) induced by an oral bacterial pathogen, Aggregatibacter actinomycetemcomitans, and inhibited the release of IL-1β, IL-6, TNF-α, and S1P in murine bone marrow cells. In addition, shRNA knockdown of S1PR2 or treatment by JTE013 suppressed cell chemotaxis induced by bacteria-stimulated cell culture media. Furthermore, JTE013 suppressed osteoclastogenesis and bone resorption induced by RANKL in murine bone marrow cultures. ShRNA knockdown of S1PR2 or inhibition of S1PR2 by JTE013 suppressed podosome components, including PI3K, Src, Pyk2, integrin β_3, filamentous actin (F-actin), and paxillin levels induced by RANKL in murine bone marrow cells. We conclude that S1PR2 plays an essential role in modulating proinflammatory cytokine production, cell chemotaxis, osteoclastogenesis, and bone resorption. Inhibition of S1PR2 signaling could be a novel therapeutic strategy for bone loss associated with skeletal diseases.

Invadosome-Mediated Human Extracellular Matrix Degradation by Entamoeba histolytica

Entamoeba histolytica is a protozoan parasite that causes invasive amoebiasis when it invades the human colon. Tissue invasion requires a shift from an adhesive lifestyle in the colonic lumen to a motile and extracellular matrix (ECM) degradative lifestyle in the colonic tissue layers. How the parasite regulates these two lifestyles is largely unknown. Previously, we showed that silencing the E. histolytica surface metalloprotease EhMSP-1 results in parasites that are hyperadherent and less motile. To better understand the molecular mechanism of this phenotype, we now show that the parasites with EhMSP-1 silenced cannot efficiently form specialized dot-like polymerized actin (F actin) structures upon interaction with the human ECM component fibronectin. We characterized these F actin structures and found that they are very short-lived structures that are the sites of fibronectin degradation. Motile mammalian cells form F actin structures called invadosomes that are similar in stability and function to these amoebic actin dots. Therefore, we propose here that E. histolytica forms amoebic invadosomes to facilitate colonic tissue invasion.

The Sealing Zone in Osteoclasts: A Self-Organized Structure on the Bone

Osteoclasts form a specialized cell-matrix adhesion structure, known as the "sealing zone", during bone resorption. The sealing zone is a dynamic actin-rich structure that defines the resorption area of the bone. The detailed dynamics and fine structure of the sealing zone have been elusive. Osteoclasts plated on glass do not form a sealing zone, but generate a separate supra-molecular structure called the "podosome belt". Podosomes are integrin-based adhesion complexes involved in matrix adhesion, cell migration, matrix degradation, and mechanosensing. Invadopodia, podosome-like protrusions in cancer cells, are involved in cell invasion into other tissues by promoting matrix degradation. Both podosomes and invadopodia exhibit actin pattern transitions during maturation. We previously found that Arp2/3-dependent actin flow occurs in all observed assembly patterns of podosomes in osteoclasts on glass. It is known that the actin wave in Dictyostelium cells exhibits a similar pattern transition in its evolution. Because of significant advances in our understanding regarding the mechanism of podosomes/invadopodia formation over the last decade, we revisited the structure and function of the sealing zone in this review, highlighting the possible involvement of self-organized actin waves in the organogenesis of the sealing zone.

Effect of l-caldesmon on osteoclastogenesis in RANKL-induced RAW264.7 cells

Non-muscle caldesmon (l-CaD) is involved in the regulation of actin cytoskeletal remodeling in the podosome formation, but its function in osteoclastogenesis remains to be determined. In this study, RANKL-induced differentiation of RAW264.7 murine macrophages to osteoclast-like cells (OCs) was used as a model to determine the physiological role of l-CaD and its phosphorylation in osteoclastogenesis. Upon RANKL treatment, RAW264.7 cells undergo cell-cell fusion into multinucleate, and TRAP-positive large OCs with a concomitant increase of l-CaD expression. Using gain- and loss-of-function in OC precursor cells followed by RANKL induction, we showed that the expression of l-CaD in response to RANKL activation is an important event for osteoclastogenesis, and bone resorption. To determine the effect of l-CaD phosphorylation in osteoclastogenesis, three decoy peptides of l-CaD were used with, respectively, Ser-to-Ala mutations at the Erk- and Pak1-mediated phosphorylation sites, and Ser-to-Asp mutation at the Erk-mediated phosphorylation sites. Both the former two peptides competed with the C-terminal segment of l-CaD for F-actin binding and accelerated formation of podosome-like structures in RANKL-induced OCs, while the third peptide did not significantly affect the F-actin binding of l-CaD, and decreased the formation of podosome-like structures in OCs. With the experiments using dephosphorylated and phosphorylated l-CaD mutants, we further showed that dephosphorylated l-CaD mutant facilitated RANKL-induced TRAP activity with an increased cell fusion index, whereas phosphorylated l-CaD decreased the TRAP activity and cell fusion. Our findings suggested that both the level of l-CaD expression and the extent of l-CaD phosphorylation play a role in RANKL-induced osteoclast differentiation.

Bone degradation machinery of osteoclasts: An HIV-1 target that contributes to bone loss

Bone deficits are frequent in HIV-1-infected patients. We report here that osteoclasts, the cells specialized in bone resorption, are infected by HIV-1 in vivo in humanized mice and ex vivo in human joint biopsies. In vitro, infection of human osteoclasts occurs at different stages of osteoclastogenesis via cell-free viruses and, more efficiently, by transfer from infected T cells. HIV-1 infection markedly enhances adhesion and osteolytic activity of human osteoclasts by modifying the structure and function of the sealing zone, the osteoclast-specific bone degradation machinery. Indeed, the sealing zone is broader due to F-actin enrichment of its basal units (i.e., the podosomes). The viral protein Nef is involved in all HIV-1-induced effects partly through the activation of Src, a regulator of podosomes and of their assembly as a sealing zone. Supporting these results, Nef-transgenic mice exhibit an increased osteoclast density and bone defects, and osteoclasts derived from these animals display high osteolytic activity. Altogether, our study evidences osteoclasts as host cells for HIV-1 and their pathological contribution to bone disorders induced by this virus, in part via Nef.

Cortactin function in invadopodia

Actin remodeling plays an essential role in diverse cellular processes such as cell motility, vesicle trafficking or cytokinesis. The scaffold protein and actin nucleation promoting factor Cortactin is present in virtually all actin-based structures, participating in the formation of branched actin networks. It has been involved in the control of endocytosis, and vesicle trafficking, axon guidance and organization, as well as adhesion, migration and invasion. To migrate and invade through three-dimensional environments, cells have developed specialized actin-based structures called invadosomes, a generic term to designate invadopodia and podosomes. Cortactin has emerged as a critical regulator of invadosome formation, function and disassembly. Underscoring this role, Cortactin is frequently overexpressed in several types of invasive cancers. Herein we will review the roles played by Cortactin in these specific invasive structures.

Smart mechanosensing machineries enable migration of vascular smooth muscle cells in atherosclerosis-relevant 3D matrices

At the early stage of atherosclerosis, neointima is formed due to the migration of vascular smooth muscle cells (VSMCs) from the media to the intima. VSMCs are surrounded by highly adhesive 3D matrices. They take specific strategies to cross various 3D matrices in the media, including heterogeneous collagen and mechanically strong basement membrane. Migration of VSMCs is potentially caused by biomechanical mechanism. Most in vitro studies focus on cell migration on 2D substrates in response to biochemical factors. How the cells move through 3D matrices under the action of mechanosensing machineries remains unexplored. In this review, we propose that several interesting tension-dependent machineries act as "tractor"-posterior myosin II accumulation, and "wrecker"-anterior podosome maintaining, to power VSMCs ahead. VSMCs embedded in 3D matrices may accumulate a minor myosin II isoform, myosin IIB, at the cell rear. Anisotropic myosin IIB distribution creates cell rear, polarizes cell body, pushes the nucleus and reshapes the cell body, and cooperates with a uniformly distributed myosin IIA to propel the cell forward. On the other hand, matrix digestion by podosome further promote the migration when the matrix becomes denser. Actomyosin tension activates Src to induce podosome in soft 3D matrices and retain the podosome integrity to steadily digest the matrix.

Inhibition of KLF5-Myo9b-RhoA Pathway-Mediated Podosome Formation in Macrophages Ameliorates Abdominal Aortic Aneurysm

RATIONALE

Abdominal aortic aneurysms (AAAs) are characterized by pathological remodeling of the aortic wall. Although both increased Krüppel-like factor 5 (KLF5) expression and macrophage infiltration have been implicated in vascular remodeling, the role of KLF5 in macrophage infiltration and AAA formation remains unclear.

OBJECTIVE

To determine the role of KLF5 in AAA formation and macrophage infiltration into AAAs.

METHODS AND RESULTS

KLF5 expression was significantly increased in human AAA tissues and in 2 mouse models of experimental AAA. Moreover, in myeloid-specific Klf5 knockout mice (myeKlf5^-/- mice), macrophage infiltration, medial smooth muscle cell loss, elastin degradation, and AAA formation were markedly decreased. In cell migration and time-lapse imaging analyses, the migration of murine myeKlf5^-/- macrophages was impaired, and in luciferase reporter assays, KLF5 activated Myo9b (myosin IXB) transcription by direct binding to the Myo9b promoter. In subsequent coimmunostaining studies, Myo9b was colocalized with filamentous actin, cortactin, vinculin, and Tks5 in the podosomes of phorbol 12,13-dibutyrate-treated macrophages, indicating that Myo9b participates in podosome formation. Gain- and loss-of-function experiments showed that KLF5 promoted podosome formation in macrophages by upregulating Myo9b expression. Furthermore, RhoA-GTP levels increased after KLF5 knockdown in macrophages, suggesting that KLF5 lies upstream of RhoA signaling. Finally, Myo9b expression was increased in human AAA tissues, located in macrophages, and positively correlated with AAA size.

CONCLUSIONS

These data are the first to indicate that KLF5-dependent regulation of Myo9b/RhoA is required for podosome formation and macrophage migration during AAA formation, warranting consideration of the KLF5-Myo9b-RhoA pathway as a therapeutic target for AAA treatment.

Podosome dynamics and location in vascular smooth muscle cells require CLASP-dependent microtubule bending

Extracellular matrix (ECM) remodeling during physiological processes is mediated by invasive protrusions called podosomes. Positioning and dynamics of podosomes define the extent of ECM degradation. Microtubules are known to be involved in podosome regulation, but the role of microtubule (MT) network configuration in podosome dynamics and positioning is not well understood. Here, we show that the arrangement of the microtubule network defines the pattern of podosome formation and relocation in vascular smooth muscle cells (VSMCs). We show that microtubule plus-end targeting facilitates de novo formation of podosomes, in addition to podosome remodeling. Moreover, specialized bent microtubules with plus ends reversed towards the cell center promote relocation of podosomes from the cell edge to the cell center, resulting in an evenly distributed podosome pattern. Microtubule bending is induced downstream of protein kinase C (PKC) activation and requires microtubule-stabilizing proteins known as cytoplasmic linker associated proteins (CLASPs) and retrograde actin flow. Similar to microtubule depolymerization, CLASP depletion by siRNA blocks microtubule bending and eliminates centripetal relocation of podosomes. Podosome relocation also coincides with translocation of podosome-stimulating kinesin KIF1C, which is known to move preferentially along CLASP-associated microtubules. These findings indicate that CLASP-dependent microtubule network configuration is critical to the cellular location and distribution of KIF1C-dependent podosomes. © 2016 Wiley Periodicals, Inc.

Podosome Formation and Development in Monocytes Restricted by the Nanoscale Spatial Distribution of ICAM1

We studied podosome formation and development in activated monocytes (THP1) at ICAM1 (intercellular adhesion molecule 1) nanopatterns of circular and ring-shaped domains and show that cellular binding to a preclustered ICAM1 nanopattern requires ligand patches of at least 200 nm (corresponding to 14 or more integrins). Podosome-like adhesion formation depends on the structure of the ligand pattern under the developing podosome with larger single domains promoting adhesion in a single patch and multiple smaller domains allowing podosome formation by integration of at least 2 smaller domains on either side of the podosome core. Maturation to rosette structures and recruitment of proteases were only observed with macroscopic ICAM1 presentation.

Advanced glycation end products biphasically modulate bone resorption in osteoclast-like cells

Advanced glycation end products (AGEs) disturb bone remodeling during aging, and this process is accelerated in diabetes. However, their role in modulation of osteoclast-induced bone resorption is controversial, with some studies indicating that AGEs enhance bone resorption and others showing the opposite effect. We determined whether AGEs present at different stages of osteoclast differentiation affect bone resorption differently. Based on increased levels of tartrate-resistant acid phosphatase (TRAP) and cathepsin K (CTSK), we identified day 4 of induction as the dividing time of cell fusion stage and mature stage in RAW264.7 cell-derived osteoclast-like cells (OCLs). AGE-modified BSA (50-400 μg/ml) or control BSA (100 μg/ml) was then added at the beginning of each stage. Results showed that the presence of AGEs at the cell fusion stage reduced pit numbers, resorption area, and CTSK expression. Moreover, expression of receptor activator of nuclear factor-κB (RANK) as well as the number of TRAP-positive cells, nuclei per OCL, actin rings, and podosomes also decreased. However, the presence of AGEs at the mature stage enlarged the resorption area markedly and increased pit numbers slightly. Intriguingly, only the number of nuclei per OCL and podosomes increased. These data indicate that AGEs biphasically modulate bone resorption activity of OCLs in a differentiation stage-dependent manner. AGEs at the cell fusion stage reduce bone resorption dramatically, mainly via suppression of RANK expression in osteoclast precursors, whereas AGEs at the mature stage enhance bone resorption slightly, most likely by increasing the number of podosomes in mature OCLs.

Podosomes and invadopodia: tools to breach vascular basement membrane

The vascular basement membrane (BM) is a thin and dense cross-linked extracellular matrix layer that covers and protects blood vessels. Understanding how cells cross the physical barrier of the vascular BM will provide greater insight into the potentially critical role of vascular BM breaching in cancer extravasation, leukocyte trafficking and angiogenic sprouting. In the last year, new evidence has mechanistically linked the breaching of vascular BM with the formation of specific cellular micro-domains known as podosomes and invadopodia. These structures are specialized cell-matrix contacts with an inherent ability to degrade the extracellular matrix. Specifically, the formation of podosomes or invadopodia was shown as an important step in vascular sprouting and tumor cell extravasation, respectively. Here, we review and comment on these recent findings and explore the functions of podosomes and invadopodia within the context of pathological processes such as tumor dissemination and tumor angiogenesis.

Osteoprotegerin disrupts peripheral adhesive structures of osteoclasts by modulating Pyk2 and Src activities

Osteoprotegerin has previously been shown to modulate bone mass by blocking osteoclast maturation and function. The detailed mechanisms of osteoprotegerin-induced disassembly of podosomes, disruption of adhesive structures and modulation of adhesion-related proteins in osteoclasts, however, are not well characterized. In this study, tartrate-resistant acidic phosphatase staining demonstrated that osteoprotegerin inhibited differentiation of osteoclasts. The use of scanning electron microscopy, real-time cell monitoring and confocal microscopy indicated that osteoclasts responded in a time and dose-dependent manner to osteoprotegerin treatments with retraction of peripheral adhesive structures and detachment from the extracellular substrate. Combined imaging and Western blot studies showed that osteoprotegerin induced dephosphorylation of Tyr 402 in Pyk2 and decreased its labeling in peripheral adhesion regions. osteoprotegerin induced increased intracellular labeling of Tyr 402 in Pyk2, Tyr 416 in Src, increased dephosphorylation of Tyr 527 in Src, and increased Pyk2/Src association in the central region of osteoclasts. This evidence suggests that Src may function as an adaptor protein that competes for Pyk2 and relocates it from the peripheral adhesive zone to the central region of osteoclasts in response to osteoprotegerin treatment. Osteoprotegerin may induce podosome reassembly and peripheral adhesive structure detachment by modulating phosphorylation of Pyk2 and Src and their intracellular distribution in osteoclasts.

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.

Working together: spatial synchrony in the force and actin dynamics of podosome first neighbors

Podosomes are mechanosensitive adhesion cell structures that are capable of applying protrusive forces onto the extracellular environment. We have recently developed a method dedicated to the evaluation of the nanoscale forces that podosomes generate to protrude into the extracellular matrix. It consists in measuring by atomic force microscopy (AFM) the nanometer deformations produced by macrophages on a compliant Formvar membrane and has been called protrusion force microscopy (PFM). Here we perform time-lapse PFM experiments and investigate spatial correlations of force dynamics between podosome pairs. We use an automated procedure based on finite element simulations that extends the analysis of PFM experimental data to take into account podosome architecture and organization. We show that protrusion force varies in a synchronous manner for podosome first neighbors, a result that correlates with phase synchrony of core F-actin temporal oscillations. This dynamic spatial coordination between podosomes suggests a short-range interaction that regulates their mechanical activity.

Two-dimensional motility of a macrophage cell line on microcontact-printed fibronectin

The ability of macrophages to migrate to sites of infection and inflammation is critical for their role in the innate immune response. Macrophage cell lines have made it possible to study the roles of individual proteins responsible for migration using molecular biology, but it has not been possible to reliably elicit the motility of macrophage cell lines in two dimensions. In the past, measurements of the motility of macrophage cell lines have been largely limited to transwell assays which provide limited quantitative information on motility and limited ability to visualize cell morphology. We used microcontact printing to create polydimethylsiloxane (PDMS) surfaces functionalized with fibronectin that otherwise support little macrophage adhesion. We used these surfaces to measure macrophage migration in two dimensions and found that these cells migrate efficiently in a uniform field of colony-stimulating factor-1, CSF-1. Knockdown of Cdc42 led to a nonstatistically significant reduction in motility, whereas chemical inhibition of PI3K activity led to a complete loss of motility. Inhibition of the RhoA kinase, ROCK, did not abolish the motility of these cells but caused a quantitative change in motility, reducing motility significantly on high concentrations of fibronectin but not on low concentrations. This study illustrates the importance of studying cell motility on well controlled materials to better understand the exact roles of specific proteins on cell migration. © 2014 Wiley Periodicals, Inc.

Invadopodia, regulation, and assembly in cancer cell invasion

The occurrence of invadopodia has been, since its characterization, a hallmark of cancerous cell invasion and metastasis. These structures are now the subject of a controversy concerning their cellular function, molecular regulation, and assembly. The terms invadopodia and podosomes have been used interchangeably since their discovery back in 1980. Since then, these phenotypes are now more established and accepted by the scientific community as vital structures for 3D cancer cell motility. Many characteristics relating to invadopodia and podosomes have been elucidated, which might prove these structures as good targets for metastasis treatment. In this review, we briefly review the actin reorganization process needed in most types of cancer cell motility. We also review the important characteristics of invadopodia, including molecular components, assembly, markers, and the signaling pathways, providing a comprehensive model for invadopodia regulation.

Modulation of osteoclast differentiation and bone resorption by Rho GTPases

Bone is a dynamic tissue constantly renewed through a regulated balance between bone formation and resorption. Excessive bone degradation by osteoclasts leads to pathological decreased bone density characteristic of osteolytic diseases such as post-menopausal osteoporosis or bone metastasis. Osteoclasts are multinucleated cells derived from hematopoietic stem cells via a complex differentiation process. Their unique ability to resorb bone is dependent on the formation of the actin-rich sealing zone. Within this adhesion structure, the plasma membrane differentiates into the ruffled border where protons and proteases are secreted to demineralize and degrade bone, respectively. On the bone surface, mature osteoclasts alternate between stationary resorptive and migratory phases. These are associated with profound actin cytoskeleton reorganization, until osteoclasts die of apoptosis. In this review, we highlight the role of Rho GTPases in all the steps of osteoclasts differentiation, function, and death and conclude on their interest as targets for treatment of osteolytic pathologies.

Src-dependent Tks5 phosphorylation regulates invadopodia-associated invasion in prostate cancer cells

BACKGROUND

The Src tyrosine kinase substrate and adaptor protein Tks5 had previously been implicated in the invasive phenotype of normal and transformed cell types via regulation of cytoskeletal structures called podosomes/invadopodia. The role of Src-Tks5 signaling in invasive prostate cancer, however, had not been previously evaluated.

METHODS

We measured the relative expression of Tks5 in normal (n = 20) and cancerous (n = 184, from 92 patients) prostate tissue specimens by immunohistochemistry using a commercially available tumor microarray. We also manipulated the expression and activity of wild-type and mutant Src and Tks5 constructs in the LNCaP and PC-3 prostate cancer cell lines in order to ascertain the role of Src-Tks5 signaling in invadopodia development, matrix-remodeling activity, motility, and invasion.

RESULTS

Our studies demonstrated that Src was activated and Tks5 upregulated in high Gleason score prostate tumor specimens and in invasive prostate cancer cell lines. Remarkably, overexpression of Tks5 in LNCaP cells was sufficient to induce invadopodia formation and associated matrix degradation. This Tks5-dependent increase in invasive behavior further depended on Src tyrosine kinase activity and the phosphorylation of Tks5 at tyrosine residues 557 and 619. In PC-3 cells we demonstrated that Tks5 phosphorylation at these sites was necessary and sufficient for invadopodia-associated matrix degradation and invasion.

CONCLUSIONS

Our results suggest a general role for Src-Tks5 signaling in prostate tumor progression and the utility of Tks5 as a marker protein for the staging of this disease.

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.

Atypical protein kinase C in cell motility

Cell motility is defined as cell movement in the three-dimensional space leading to repositioning of the cell. Atypical protein kinase C (aPKC, including ζ and λ/ι) are a subfamily of PKC. Different from classic PKC and novel PKC, the activation of atypical PKC is not dependent on diacylglycerol or calcium. PKCζ can be activated by lipid components, such as phosphatidylinositols, phosphatidic acid, arachidonic acid, and ceramide. Both phosphatidylinositol (3,4,5)-trisphosphate and PDK1 are necessary for the complete and stable activation of PKCζ. Atypical PKC is involved in the regulation of cell polarization, directional sensing, formation of filopodia, and cell motility. It is essential for migration and invasion of multiple cancer cell types. Particularly, atypical PKC has been found in the regulation of the motility of hematopoietic cells. It also participates in the regulation of proteolytic activity of podosomes and invadopodia. It has been found that atypical PKC can work coordinately with other PKC subfamily members and other signaling pathways. Research on the roles of atypical PKC in cell motility may lead to new therapeutic strategies for cancer and other diseases.

PGI₂ signaling inhibits antigen uptake and increases migration of immature dendritic cells

PGI₂ signaling through IP inhibits allergen-induced inflammatory responses in mice. We reported previously that PGI₂ analogs decreased proinflammatory cytokine and chemokine production by mature BMDCs. However, whether PGI₂ modulates the function of immature DCs has not been investigated. We hypothesized that PGI2 negatively regulates immature DC function and investigated the effect of PGI2 analogs on immature BMDC antigen uptake and migration in vitro and in vivo. Immature BMDCs were obtained from WT and IPKO mice, both on a C57BL/6 background. The PGI2 analog cicaprost decreased FITC-OVA uptake by immature BMDCs. In addition, cicaprost increased immature BMDC podosome dissolution, pro-MMP-9 production, cell surface CCR7 expression, and chemotactic migration toward CCL19 and CCL21, as well as chemokinesis, in an IP-specific fashion. These in vitro results suggested that cicaprost promotes migration of immature DCs from mucosal surface to draining LNs. This concept was supported by the finding that migration of immature GFP⁺ BMDCs to draining LNs was enhanced by pretreatment with cicaprost. Further, migration of immature lung DCs labeled with PKH26 was enhanced by intranasal cicaprost administration. Our results suggest PGI2-IP signaling increases immature DC migration to the draining LNs and may represent a novel mechanism by which this eicosanoid inhibits immune responses.

Dynamics of Bio-Polymeric Brushes Growing from a Cellular Membrane: Tentative Modelling of the Actin Turnover within an Adhesion Unit; the Podosome

Podosomes are involved in the adhesion process of various cells to a solid substrate. They have been proven to consist of a dense actin core surrounded by an actin cloud. The podosomes, which nucleate when the cell comes in the vicinity of a substrate, contribute to link the membrane to the solid surface, but rather than frozen links, collective dynamical behaviors are experimentally observed. Depending on the differentiation stage, podosomes assemble and form clusters, rings or belts. Considering the dynamics of a polymeric brush, we design a simple model aiming at the description of a single podosome, the basic unit of these complex adhesion-structures and compare our theoretical conclusions to recent experimental results. Particularly, we explain, by solving the diffusion problem around the podosome, why the structure is likely to have a finite life-span.

Palladin regulation of the actin structures needed for cancer invasion

Cell migration and invasion involve the formation of cell adhesion structures as well as the dynamic and spatial regulation of the cytoskeleton. The adhesive structures known as podosomes and invadopodia share a common role in cell motility, adhesion, and invasion, and form when the plasma membrane of motile cells undergoes highly regulated protrusions. Palladin, a molecular scaffold, co-localizes with actin-rich structures where it plays a role in their assembly and maintenance in a wide variety of cell lines. Palladin regulates actin cytoskeleton organization as well as cell adhesion formation. Moreover, palladin contributes to the invasive nature of cancer metastatic cells by regulating invadopodia formation. Palladin seems to regulate podosome and invodopodia formation through Rho GTPases, which are known as key players in coordinating the cellular responses required for cell migration and metastasis.

Precursors linked via the zipper-like structure or the filopodium during the secondary fusion of osteoclasts

We previously reported the transient appearance of an actin superstructure, called the zipper-like structure, during the primary fusion (fusion of mononuclear precursors) and the secondary fusion (fusion of multinucleated cells) of osteoclasts. Here, we focus on the actin-based superstructures that link two precursor cells during the secondary fusion event. In one type of secondary fusion, the osteoclasts transformed the podosome belts into the zipper-like structure at the site of cell contact and the apposed plasma membranes in the zipper-like structure attached to each other via a discontinuous interface. In another type of secondary fusion, the osteoclasts used a filopodium-like protrusion that linked the two cells. Both types of cell fusion required a lag period between the adhesion of the cells and the fusion of cell bodies. Thus, the secondary fusion of osteoclasts uses actin-based superstructures for cell-cell interactions before the definitive fusion of the plasma membranes.

Osteoclast motility: putting the brakes on bone resorption

As the skeleton ages, the balanced formation and resorption of normal bone remodeling is lost, and bone loss predominates. The osteoclast is the specialized cell that is responsible for bone resorption. It is a highly polarized cell that must adhere to the bone surface and migrate along it while resorbing, and cytoskeletal reorganization is critical. Podosomes, highly dynamic actin structures, mediate osteoclast motility. Resorbing osteoclasts form a related actin complex, the sealing zone, which provides the boundary for the resorptive microenvironment. Similar to podosomes, the sealing zone rearranges itself to allow continuous resorption while the cell is moving. The major adhesive protein controlling the cytoskeleton is αvβ3 integrin, which collaborates with the growth factor M-CSF and the ITAM receptor DAP12. In this review, we discuss the signaling complexes assembled by these molecules at the membrane, and their downstream mediators that control OC motility and function via the cytoskeleton.

Membrane lipids in invadopodia and podosomes: key structures for cancer invasion and metastasis

Invadopodia are extracellular matrix (ECM)-degrading protrusions formed by invasive cancer cells. Podosomes are structures functionally similar to invadopodia that are found in oncogene-transformed fibroblasts and monocyte-derived cells, including macrophages and osteoclasts. These structures are thought to play important roles in the pericellular remodeling of ECM during cancer invasion and metastasis. Much effort has been directed toward identification of the molecular components and regulators of invadopodia/podosomes, which could be therapeutic targets in the treatment of malignant cancers. However, it remains largely unknown how these components are assembled into invadopodia/podosomes and how the assembly process is spatially and temporally regulated. This review will summarize recent progress on the molecular mechanisms of invadopodia/podosome formation, with strong emphasis on the roles of lipid rafts and phosphoinositides.

Nck adaptor proteins link Tks5 to invadopodia actin regulation and ECM degradation

Invadopodia are actin-based projections enriched with proteases, which invasive cancer cells use to degrade the extracellular matrix (ECM). The Phox homology (PX)-Src homology (SH)3 domain adaptor protein Tks5 (also known as SH3PXD2A) cooperates with Src tyrosine kinase to promote invadopodia formation but the underlying pathway is not clear. Here we show that Src phosphorylates Tks5 at Y557, inducing it to associate directly with the SH3-SH2 domain adaptor proteins Nck1 and Nck2 in invadopodia. Tks5 mutants unable to bind Nck show reduced matrix degradation-promoting activity and recruit actin to invadopodia inefficiently. Conversely, Src- and Tks5-driven matrix proteolysis and actin assembly in invadopodia are enhanced by Nck1 or Nck2 overexpression and inhibited by Nck1 depletion. We show that clustering at the plasma membrane of the Tks5 inter-SH3 region containing Y557 triggers phosphorylation at this site, facilitating Nck recruitment and F-actin assembly. These results identify a Src-Tks5-Nck pathway in ECM-degrading invadopodia that shows parallels with pathways linking several mammalian and pathogen-derived proteins to local actin regulation.

Adhesions ring: a structural comparison between podosomes and the immune synapse

Podosomes and the immune synapse are integrin-mediated adhesive structures that share a common ring-like morphology. Both podosomes and immune synapses have a central core surrounded by a peripheral ring containing talin, vinculin and paxillin. Recent progress suggests significant parallels between the regulatory mechanisms that contribute to the formation of these adhesive structures. In this review, we compare the structures, functions and regulation of podosomes and the immune synapse.

Distribution of inositol 1,4,5-trisphosphate receptors in rat osteoclasts

Inositol 1,4,5-trisphosphate (IP₃) receptors (IP₃Rs) are Ca²⁺ channels that localize to intracellular Ca²⁺ stores such as the endoplasmic reticulum (ER). Recently, IP₃Rs were found to participate in the formation of the cytoskeleton and cellular adhesions. In this study, we examined the cellular localization of type I, II, and III IP₃Rs to assess their role in cellular adhesion in rat osteoclasts. Rat bone marrow cells were cultured in α-MEM with 10% fetal bovine serum, M-CSF, RANKL, and 1,25(OH)₂D₃ for 1 week to promote osteoclast formation. Type I, II, and III IP₃R expression in the osteoclasts was then examined by RT-PCR. Double-staining was performed using antibodies against type I, II, and III IP₃Rs and DiOC₆, an ER marker, or TRITC-phalloidin, an actin filament marker. Expression of all three IP₃Rs was detected in the newly formed osteoclasts; however, the localization of the type I and II IP₃Rs was predominantly close to nuclear, and possibly colocalized with the ER, while the type III IP₃Rs were localized to the ER and podosomes, actin-rich adhesion structures in osteoclasts. These findings suggest that type III IP₃Rs are associated with osteoclast adhesion.