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

Get a grip: Podosomes as potential players in phagocytosis

Podosomes have been known for several decades as micron-sized, F-actin-rich structures that play a pivotal role in cell migration and invasion, as they are able to mediate both cell-matrix attachment as well as extracellular matrix degradation. Particularly in monocytic cells, podosomes have been shown to fulfill a variety of additional functions such as sensing of substrate rigidity and topography, or cell-cell fusion. Increasing evidence now points to the involvement of podosome-like structures also during phagocytosis by immune cells such as macrophages, dendritic cells, and neutrophils. Here, we compare the different cell models and experimental set ups where "phagocytic podosomes" have been described. We also discuss the composition and architecture of these structures, their potential involvement in mechanosensing and particle disruption, as well as the pros and cons for addressing them as bona fide podosomes.

Guidelines for mouse and human DC functional assays

This article is part of the Dendritic Cell Guidelines article series, which provides a collection of state-of-the-art protocols for the preparation, phenotype analysis by flow cytometry, generation, fluorescence microscopy, and functional characterization of mouse and human dendritic cells (DC) from lymphoid organs and various non-lymphoid tissues. Recent studies have provided evidence for an increasing number of phenotypically distinct conventional DC (cDC) subsets that on one hand exhibit a certain functional plasticity, but on the other hand are characterized by their tissue- and context-dependent functional specialization. Here, we describe a selection of assays for the functional characterization of mouse and human cDC. The first two protocols illustrate analysis of cDC endocytosis and metabolism, followed by guidelines for transcriptomic and proteomic characterization of cDC populations. Then, a larger group of assays describes the characterization of cDC migration in vitro, ex vivo, and in vivo. The final guidelines measure cDC inflammasome and antigen (cross)-presentation activity. While all protocols were written by experienced scientists who routinely use them in their work, this article was also peer-reviewed by leading experts and approved by all co-authors, making it an essential resource for basic and clinical DC immunologists.

Physiological functions of podosomes: From structure and function to therapy implications in osteoclast biology of bone resorption

With increasing age, bone tissue undergoes significant alterations in composition, architecture, and metabolic functions, probably causing senile osteoporosis. Osteoporosis possess the vast majority of bone disease and associates with a reduction in bone mass and increased fracture risk. Bone loss is on account of the disorder in osteoblast-induced bone formation and osteoclast-induced bone resorption. As a unique bone resorptive cell type, mature bone-resorbing osteoclasts exhibit dynamic actin-based cytoskeletal structures called podosomes that participate in cell-matrix adhesions specialized in the degradation of mineralized bone matrix. Podosomes share many of the same molecular constitutions as focal adhesions, but they have a unique structural organization, with a central core abundant in F-actin and encircled by scaffolding proteins, kinases and integrins. Here, we conclude recent advancements in our knowledge of the architecture and the functions of podosomes. We also discuss the regulatory pathways in osteoclast podosomes, providing a reference for future research on the podosomes of osteoclasts and considering podosomes as a therapeutic target for inhibiting bone resorption.

Met-signaling Controls Dendritic Cell Migration in Skin by Regulating Podosome Formation and Function

Signaling through the HGF receptor/Met in skin-resident Langerhans cells (LC) and dermal dendritic cells (dDC) is essential for their emigration toward draining lymph nodes upon inflammation-induced activation. Here, we addressed the role of Met-signaling in distinct steps of LC/dDC emigration from the skin by employing a conditional Met-deficient mouse model (Metf^lox/flox). We found that Met deficiency severely impaired podosome formation in DC and concomitantly decreased the proteolytic degradation of gelatin. Accordingly, Met-deficient LC failed to efficiently cross the extracellular matrix (ECM)-rich basement membrane between the epidermis and dermis. We further observed that HGF-dependent Met activation reduced the adhesion of bone marrow-derived LC to various ECM factors and enhanced the motility of DC in 3D collagen matrices, which was not the case for Met-deficient LC/DC. We found no impact of Met-signaling on the integrin-independent amoeboid migration of DC in response to the c-c chemokine receptor 7 (CCR7) ligand CCL19. Collectively, our data show that the Met-signaling pathway regulates the migratory properties of DC in HGF-dependent and HGF-independent manners.

Mechanisms and roles of podosomes and invadopodia

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

Engineering physical microenvironments to study innate immune cell biophysics

Innate immunity forms the core of the human body's defense system against infection, injury, and foreign objects. It aims to maintain homeostasis by promoting inflammation and then initiating tissue repair, but it can also lead to disease when dysregulated. Although innate immune cells respond to their physical microenvironment and carry out intrinsically mechanical actions such as migration and phagocytosis, we still do not have a complete biophysical description of innate immunity. Here, we review how engineering tools can be used to study innate immune cell biophysics. We first provide an overview of innate immunity from a biophysical perspective, review the biophysical factors that affect the innate immune system, and then explore innate immune cell biophysics in the context of migration, phagocytosis, and phenotype polarization. Throughout the review, we highlight how physical microenvironments can be designed to probe the innate immune system, discuss how biophysical insight gained from these studies can be used to generate a more comprehensive description of innate immunity, and briefly comment on how this insight could be used to develop mechanical immune biomarkers and immunomodulatory therapies.

Actin nano-architecture of phagocytic podosomes

Podosomes are actin-enriched adhesion structures important for multiple cellular processes, including migration, bone remodeling, and phagocytosis. Here, we characterize the structure and organization of phagocytic podosomes using interferometric photoactivated localization microscopy, a super-resolution microscopy technique capable of 15-20 nm resolution, together with structured illumination microscopy and localization-based super-resolution microscopy. Phagocytic podosomes are observed during frustrated phagocytosis, a model in which cells attempt to engulf micropatterned IgG antibodies. For circular patterns, this results in regular arrays of podosomes with well-defined geometry. Using persistent homology, we develop a pipeline for semi-automatic identification and measurement of podosome features. These studies reveal an hourglass shape of the podosome actin core, a protruding knob at the bottom of the core, and two actin networks extending from the core. Additionally, the distributions of paxillin, talin, myosin II, α-actinin, cortactin, and microtubules relative to actin are characterized.

Functional HLA-C expressing trophoblast spheroids as a model to study placental-maternal immune interactions during human implantation

In healthy couples over half of the conceptions result in failed pregnancy and around 30% of them occur during implantation defining it as a rate-limiting step for the success of native and in vitro fertilization. The understanding of the factors regulating each step of implantation and immune recognition is critical for the pregnancy outcome. Creation of 3D-cell culture models, such as spheroids and organoids, is in the focus of placental tissue engineering in attempt to resemble the in vivo complexity of the maternal-fetal interface and to overcome the need of laboratory animals and human embryos. We constructed stable, reliable, and reproducible trophoblast Sw71 spheroids which are functional independently of the serum level in the culture media. These models resemble the hatched human blastocyst in size, shape and function and are useful for in vitro studies of the in vivo concealed human implantation. Since Sw71 spheroids produce HLA-C, the only classical MHC molecule indispensable for establishment of the immune tolerance and proper human implantation, they are applicable for the evaluation not only of implantation itself but also of maternal-trophoblasts immune interactions. In addition, Sw71-blastocyst-like spheroids are manipulable in low-volume platform, easy to monitor and analyze automatically under treatment with favorable/detrimental factors.

DNA Tension Probes Show that Cardiomyocyte Maturation Is Sensitive to the Piconewton Traction Forces Transmitted by Integrins

Cardiac muscle cells (CMCs) are the unit cells that comprise the heart. CMCs go through different stages of differentiation and maturation pathways to fully mature into beating cells. These cells can sense and respond to mechanical cues through receptors such as integrins which influence maturation pathways. For example, cell traction forces are important for the differentiation and development of functional CMCs, as CMCs cultured on varying substrate stiffness function differently. Most work in this area has focused on understanding the role of bulk extracellular matrix stiffness in mediating the functional fate of CMCs. Given that stiffness sensing mechanisms are mediated by individual integrin receptors, an important question in this area pertains to the specific magnitude of integrin piconewton (pN) forces that can trigger CMC functional maturation. To address this knowledge gap, we used DNA adhesion tethers that rupture at specific thresholds of force (∼12, ∼56, and ∼160 pN) to test whether capping peak integrin tension to specific magnitudes affects CMC function. We show that adhesion tethers with greater force tolerance lead to functionally mature CMCs as determined by morphology, twitching frequency, transient calcium flux measurements, and protein expression (F-actin, vinculin, α-actinin, YAP, and SERCA2a). Additionally, sarcomeric actinin alignment and multinucleation were significantly enhanced as the mechanical tolerance of integrin tethers was increased. Taken together, the results show that CMCs harness defined pN integrin forces to influence early stage development. This study represents an important step toward biophysical characterization of the contribution of pN forces in early stage cardiac differentiation.

Megakaryocytes form linear podosomes devoid of digestive properties to remodel medullar matrix

Bone marrow megakaryocytes (MKs) undergo a maturation involving contacts with the microenvironment before extending proplatelets through sinusoids to deliver platelets in the bloodstream. We demonstrated that MKs assemble linear F-actin-enriched podosomes on collagen I fibers. Microscopy analysis evidenced an inverse correlation between the number of dot-like versus linear podosomes over time. Confocal videomicroscopy confirmed that they derived from each-other. This dynamics was dependent on myosin IIA. Importantly, MKs progenitors expressed the Tks4/5 adaptors, displayed a strong gelatinolytic ability and did not form linear podosomes. While maturing, MKs lost Tks expression together with digestive ability. However, those MKs were still able to remodel the matrix by exerting traction on collagen I fibers through a collaboration between GPVI, ß1 integrin and linear podosomes. Our data demonstrated that a change in structure and composition of podosomes accounted for the shift of function during megakaryopoiesis. These data highlight the fact that members of the invadosome family could correspond to different maturation status of the same entity, to adapt to functional responses required by differentiation stages of the cell that bears them.

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

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

Biomaterial-based physical regulation of macrophage behaviour

Macrophages play a critical role in regulating immune reactions induced by implanted biomaterials. They are highly plastic and in response to diverse stimuli in the microenvironment can exhibit a spectrum of phenotypes and functions. In addition to biochemical signals, the physical properties of biomaterials are becoming increasingly appreciated for their significant impact on macrophage behaviour, and the underlying mechanisms deserve more in-depth investigations. This review first summarises the effects of key physical cues - including stiffness, topography, physical confinement and applied force - on macrophage behaviour. Then, it reviews the current knowledge of cellular sensing and transduction of physical cues into intracellular signals. Finally, it discusses the major challenges in understanding mechanical regulation that could provide insights for biomaterial design.

Probing invadosomes: technologies for the analysis of invadosomes

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

The role of fibroblasts in the modulation of dental pulp inflammation

BACKGROUND/PURPOSE

Dental pulp fibroblasts can protect dental pulp from microbial invasion. However, little is known about the interaction between pulp fibroblasts and the immune cells. In this study, the production of proinflammatory cytokines related to inflammatory cell recruitment was evaluated in tumor necrosis factor (TNF)-α-stimulated human dental pulp fibroblasts (HDPFs). The role of TNF-α-stimulated HDPFs in the cell fusion under inflammatory process was determined with the cell co-culture with peripheral blood mononuclear cells (PBMCs).

METHODS

HDPFs were stimulated with various concentrations of TNF-α, and the secretion of interleukin (IL)-6, IL-8 and monocyte chemoattractant protein (MCP)-1 was analyzed by the enzyme-linked immunosorbent assay. The mRNA expression levels of intercellular adhesion molecule-1 (ICAM-1), macrophage colony-stimulating factor (M-CSF), receptor activator of nuclear factor κB ligand (RANKL) and osteoprotegerin (OPG) were determined by real-time quantitative polymerase chain reaction. TNF-α-treated HDPFs were co-cultured with PBMCs for 21 days, and characteristics of cell differentiation were assessed.

RESULTS

TNF-α induced IL-6, IL-8 and MCP-1 production in HDPFs. Moreover, mRNA expression levels of ICAM-1, M-CSF and OPG were significantly increased in TNF-α-treated HDPFs. Co-culture of TNF-α-treated HDPFs and PBMCs stimulated formation of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells, and the F-actin rings were observed in these multinucleated cells.

CONCLUSION

Our results indicate that under the stimulation of TNF-α, HDPFs may amplify inflammatory response by cytokines production, which in turn can modulate the differentiation of immune cells.

Matrix metalloproteinase-14 regulates collagen degradation and migration of mononuclear cells during infection with genotype VII Newcastle disease virus

Upregulation of matrix metalloproteinase (MMP)-14, a major driven force of extracellular-matrix (ECM) remodelling and cell migration, correlates with ECM breakdown and pathologic manifestation of genotype VII Newcastle disease virus (NDV) in chickens. However, the functional relevance between MMP-14 and pathogenesis of genotype VII NDV remains to be investigated. In this study, expression, biofunction and regulation of MMP-14 induced by genotype VII NDV were analysed in chicken peripheral blood mononuclear cells (PBMCs). The results showed that JS5/05 significantly increased expression and membrane accumulation of MMP-14 in PBMCs, correlating to enhanced collagen degradation and cell migration. Specific MMP-14 inhibition significantly impaired collagen degradation and migration of JS5/05-infected cells, suggesting dependence of these features on MMP-14. In addition, MMP-14 upregulation correlated with activation of the extracellular signal-regulated kinase (ERK) pathway upon JS5/05 infection, and blockage of the ERK signalling significantly suppressed MMP-14-mediated collagen degradation and migration of JS5/05-infected cells. Using a panel of chimeric NDVs derived from gene exchange between genotype VII and IV NDV, the fusion and haemagglutinin-neuraminidase genes were identified as the major viral determinants for MMP-14 expression and activity. In conclusion, MMP-14 was defined as a critical regulator of collagen degradation and cell migration of chicken PBMCs infected with genotype VII NDV, which may contribute to pathology of the virus. Our findings add novel information to the body of knowledge regarding virus-host biology and NDV pathogenesis.

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.

Nucleobindin-1 regulates ECM degradation by promoting intra-Golgi trafficking of MMPs

Matrix metalloproteinases (MMPs) degrade several ECM components and are crucial modulators of cell invasion and tissue organization. Although much has been reported about their function in remodeling ECM in health and disease, their trafficking across the Golgi apparatus remains poorly understood. Here we report that the cis-Golgi protein nucleobindin-1 (NUCB1) is critical for MMP2 and MT1-MMP trafficking along the Golgi apparatus. This process is Ca2+-dependent and is required for invasive MDA-MB-231 cell migration as well as for gelatin degradation in primary human macrophages. Our findings emphasize the importance of NUCB1 as an essential component of MMP transport and its overall impact on ECM remodeling.

Pediatric Celiac Disease Patients Show Alterations of Dendritic Cell Shape and Actin Rearrangement

Celiac disease (CD) is a frequent intestinal inflammatory disease occurring in genetically susceptible individuals upon gluten ingestion. Recent studies point to a role in CD for genes involved in cell shape, adhesion and actin rearrangements, including a Rho family regulator, Rho GTPase-activating protein 31 (ARHGAP31). In this study, we investigated the morphology and actin cytoskeletons of peripheral monocyte-derived dendritic cells (DCs) from children with CD and controls when in contact with a physiological substrate, fibronectin. DCs were generated from peripheral blood monocytes of pediatric CD patients and controls. After adhesion on fibronectin, DCs showed a higher number of protrusions and a more elongated shape in CD patients compared with controls, as assessed by immunofluorescence actin staining, transmitted light staining and video time-lapse microscopy. These alterations did not depend on active intestinal inflammation associated with gluten consumption and were specific to CD, since they were not found in subjects affected by other intestinal inflammatory conditions. The elongated morphology was not a result of differences in DC activation or maturation status, and did not depend on the human leukocyte antigen (HLA)-DQ2 haplotype. Notably, we found that ARH-GAP31 mRNA levels were decreased while RhoA-GTP activity was increased in CD DCs, pointing to an impairment of the Rho pathway in CD cells. Accordingly, Rho inhibition was able to prevent the cytoskeleton rearrangements leading to the elongated morphology of celiac DCs upon adhesion on fibronectin, confirming the role of this pathway in the observed phenotype. In conclusion, adhesion on fibronectin discriminated CD from the controls' DCs, revealing a gluten-independent CD-specific cellular phenotype related to DC shape and regulated by RhoA activity.

Actin dynamics during tumor cell dissemination

The actin cytoskeleton is a dynamic network that regulates cellular behavior from development to disease. By rearranging the actin cytoskeleton, cells are capable of migrating and invading during developmental processes; however, many of these cellular properties are hijacked by cancer cells to escape primary tumors and disseminate to distant organs in the body. In this review article, we highlight recent work describing how cancer cells regulate the actin cytoskeleton to achieve efficient invasion and metastatic colonization. We also review new imaging technologies that are capable of revealing the complex architecture and regulation of the actin cytoskeleton during motility and invasion of tumor cells.

Advances in Understanding TKS4 and TKS5: Molecular Scaffolds Regulating Cellular Processes from Podosome and Invadopodium Formation to Differentiation and Tissue Homeostasis

Scaffold proteins are typically thought of as multi-domain "bridging molecules." They serve as crucial regulators of key signaling events by simultaneously binding multiple participants involved in specific signaling pathways. In the case of epidermal growth factor (EGF)-epidermal growth factor receptor (EGFR) binding, the activated EGFR contacts cytosolic SRC tyrosine-kinase, which then becomes activated. This process leads to the phosphorylation of SRC-substrates, including the tyrosine kinase substrates (TKS) scaffold proteins. The TKS proteins serve as a platform for the recruitment of key players in EGFR signal transduction, promoting cell spreading and migration. The TKS4 and the TKS5 scaffold proteins are tyrosine kinase substrates with four or five SH3 domains, respectively. Their structural features allow them to recruit and bind a variety of signaling proteins and to anchor them to the cytoplasmic surface of the cell membrane. Until recently, TKS4 and TKS5 had been recognized for their involvement in cellular motility, reactive oxygen species-dependent processes, and embryonic development, among others. However, a number of novel functions have been discovered for these molecules in recent years. In this review, we attempt to cover the diverse nature of the TKS molecules by discussing their structure, regulation by SRC kinase, relevant signaling pathways, and interaction partners, as well as their involvement in cellular processes, including migration, invasion, differentiation, and adipose tissue and bone homeostasis. We also describe related pathologies and the established mouse models.

Inhibition of invadopodia formation by diosgenin in tumor cells

Diosgenin is a type of steroid extracted from the rhizome of Dioscorea plants. In traditional Chinese medicine, Dioscorea has the effect of ‘eliminating phlegm, promoting digestion, relaxing tendons, promoting blood circulation and inhibiting malaria’. Recent studies have confirmed that diosgenin exhibits a number of pharmacological effects, including antitumor activities. Through its antitumor effect, diosgenin is able to block tumor progression and increase the survival rate of patients with cancer; ultimately improving their quality of life. However, the mechanism underlying its pharmacological action remains unclear. Once tumor cells reach a metastatic phase, it can be fatal. Increased migration and invasiveness are the hallmarks of metastatic tumor cells. Invadopodia formation is key to maintaining the high migration and invasive ability of tumor cells. Invadopodia are a type of membrane structure process rich in filamentous-actin and are common in highly invasive tumor cells. In addition to actin, numerous actin regulators, including cortical actin-binding protein (Cortactin), accumulate in invadopodia. Cortactin is a microfilament actin-binding protein with special repetitive domains that are directly involved in the formation of the cortical microfilament actin cell skeleton. Cortactin is also one of the main substrates of intracellular Src-type tyrosine protein kinases and represents a highly conserved family of intracellular cortical signaling proteins. In recent years, great progress has been made in understanding the role of Cortactin and its molecular mechanism in cell motility. However, the diosgenin-Cortactin-invadopodia mechanism is still under investigation. Therefore, the present review focused on the current research on the regulation of invadopodia by diosgenin via Cortactin.

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.

The osteoclast cytoskeleton - current understanding and therapeutic perspectives for osteoporosis

Osteoclasts are giant multinucleated myeloid cells specialized for bone resorption, which is essential for the preservation of bone health throughout life. The activity of osteoclasts relies on the typical organization of osteoclast cytoskeleton components into a highly complex structure comprising actin, microtubules and other cytoskeletal proteins that constitutes the backbone of the bone resorption apparatus. The development of methods to differentiate osteoclasts in culture and manipulate them genetically, as well as improvements in cell imaging technologies, has shed light onto the molecular mechanisms that control the structure and dynamics of the osteoclast cytoskeleton, and thus the mechanism of bone resorption. Although essential for normal bone physiology, abnormal osteoclast activity can cause bone defects, in particular their hyper-activation is commonly associated with many pathologies, hormonal imbalance and medical treatments. Increased bone degradation by osteoclasts provokes progressive bone loss, leading to osteoporosis, with the resulting bone frailty leading to fractures, loss of autonomy and premature death. In this context, the osteoclast cytoskeleton has recently proven to be a relevant therapeutic target for controlling pathological bone resorption levels. Here, we review the present knowledge on the regulatory mechanisms of the osteoclast cytoskeleton that control their bone resorption activity in normal and pathological conditions.

The differential role of CR3 (CD11b/CD18) and CR4 (CD11c/CD18) in the adherence, migration and podosome formation of human macrophages and dendritic cells under inflammatory conditions

CR3 and CR4, the leukocyte specific β2-integrins, involved in cellular adherence, migration and phagocytosis, are often assumed to have similar functions. Previously however, we proved that under physiological conditions CR4 is dominant in the adhesion to fibrinogen of human monocyte-derived macrophages (MDMs) and dendritic cells (MDDCs). Here, using inflammatory conditions, we provide further evidence that the expression and function of CR3 and CR4 are not identical in these cell types. We found that LPS treatment changes their expression differently on MDMs and MDDCs, suggesting a cell type specific regulation. Using mAb24, specific for the high affinity conformation of CD18, we proved that the activation and recycling of β2-integrins is significantly enhanced upon LPS treatment. Adherence to fibrinogen was assessed by two fundamentally different approaches: a classical adhesion assay and a computer-controlled micropipette, capable of measuring adhesion strength. While both receptors participated in adhesion, we demonstrated that CR4 exerts a dominant role in the strong attachment of MDDCs. Studying the formation of podosomes we found that MDMs retain podosome formation after LPS activation, whereas MDDCs lose this ability, resulting in a significantly reduced adhesion force and an altered cellular distribution of CR3 and CR4. Our results suggest that inflammatory conditions reshape differentially the expression and role of CR3 and CR4 in macrophages and dendritic cells.

Poji: a Fiji-based tool for analysis of podosomes and associated proteins

Podosomes are actin-based adhesion and invasion structures in a variety of cell types, with podosome-forming cells displaying up to several hundreds of these structures. Podosome number, distribution and composition can be affected by experimental treatments or during regular turnover, necessitating a tool that is able to detect even subtle differences in podosomal properties. Here, we present a Fiji-based macro code termed 'Poji' ('podosome analysis by Fiji'), which serves as an easy-to-use tool to characterize a variety of cellular and podosomal parameters, including area, fluorescence intensity, relative enrichment of associated proteins and radial podosome intensity profiles. This tool should be useful to gain more detailed insight into the regulation, architecture and functions of podosomes. Moreover, we show that Poji is easily adaptable for the analysis of invadopodia and associated extracellular matrix degradation, and likely also of other micron-size punctate structures. This article describes the workflow of the Poji macro, presents several examples of its applications, and also points out limitations, as well as respective solutions, and adaptable features to streamline the analysis.This article has an associated First Person interview with the first author of the paper.

Adhesion, motility and matrix-degrading gene expression changes in CSF-1-induced mouse macrophage differentiation

Migratory macrophages play critical roles in tissue development, homeostasis and disease, so it is important to understand how their migration machinery is regulated. Whole-transcriptome sequencing revealed that CSF-1-stimulated differentiation of bone marrow-derived precursors into mature macrophages is accompanied by widespread, profound changes in the expression of genes regulating adhesion, actin cytoskeletal remodeling and extracellular matrix degradation. Significantly altered expression of almost 40% of adhesion genes, 60-86% of Rho family GTPases, their regulators and effectors and over 70% of extracellular proteases occurred. The gene expression changes were mirrored by changes in macrophage adhesion associated with increases in motility and matrix-degrading capacity. IL-4 further increased motility and matrix-degrading capacity in mature macrophages, with additional changes in migration machinery gene expression. Finally, siRNA-induced reductions in the expression of the core adhesion proteins paxillin and leupaxin decreased macrophage spreading and the number of adhesions, with distinct effects on adhesion and their distribution, and on matrix degradation. Together, the datasets provide an important resource to increase our understanding of the regulation of migration in macrophages and to develop therapies targeting disease-enhancing macrophages.

C-phycocyanin attenuates RANKL-induced osteoclastogenesis and bone resorption in vitro through inhibiting ROS levels, NFATc1 and NF-κB activation

Excessive bone loss occurs in inflammatory disorders such as periodontitis and osteoporosis. The underlying mechanism is related to the differentiation of macrophages into multinucleated giant osteoclasts and their bone resorptive activity. C-Phycocyanin (C-PC) is a phycobiliprotein extracted from the blue-green algae, which has been shown to have various pharmacological effects. The role of C-PC on bone metabolism needs revelation. In this study, we determined the effectiveness of C-PC as an inhibitor of osteoclast differentiation, activity, and survival in vitro. We found that C-PC strongly inhibited the differentiation of macrophages to TRAP-positive osteoclasts, distinctive osteoclast specific podosomal organization, and dentine matrix resorption without any cytotoxicity. Also, it suppressed the expression of osteoclast specific markers, such as cathepsin K and integrin β3 at mRNA and protein levels. RANKL mediated signaling utilizes reactive oxygen species (ROS) for the differentiation of osteoclasts. C-PC attenuated RANKL stimulated ROS. Mechanistic studies indicate that C-PC has the potential to reduce osteoclast formation via blocking the degradation of cytosolic IκB-α and hence, the activation of downstream markers such as c-Fos and NFATc1. However, it does not have any effect on osteoblast-mediated bone formation in vitro. Collectively, our data suggest that C-PC may be utilized as a therapeutic agent that can target bone loss mediated by excessive osteoclastic bone resorption without affecting osteoblastic activity in bone.

Integrin-associated molecules and signalling cross talking in osteoclast cytoskeleton regulation

In the ageing skeleton, the balance of bone reconstruction could commonly be broken by the increasing of bone resorption and decreasing of bone formation. Consequently, the bone resorption gradually occupies a dominant status. During this imbalance process, osteoclast is unique cell linage act the bone resorptive biological activity, which is a highly differentiated ultimate cell derived from monocyte/macrophage. The erosive function of osteoclasts is that they have to adhere the bone matrix and migrate along it, in which adhesive cytoskeleton recombination of osteoclast is essential. In that, the podosome is a membrane binding microdomain organelle, based on dynamic actin, which forms a cytoskeleton superstructure connected with the plasma membrane. Otherwise, as the main adhesive protein, integrin regulates the formation of podosome and cytoskeleton, which collaborates with the various molecules including: c-Cbl, p130^Cas , c-Src and Pyk2, through several signalling cascades cross talking, including: M-CSF and RANKL. In our current study, we discuss the role of integrin and associated molecules in osteoclastogenesis cytoskeletal, especially podosomes, regulation and relevant signalling cascades cross talking.

L-Plastin deficiency produces increased trabecular bone due to attenuation of sealing ring formation and osteoclast dysfunction

Bone resorption requires the formation of complex, actin-rich cytoskeletal structures. During the early phase of sealing ring formation by osteoclasts, L-plastin regulates actin-bundling to form the nascent sealing zones (NSZ). Here, we show that L-plastin knockout mice produce osteoclasts that are deficient in the formation of NSZs, are hyporesorptive, and make superficial resorption pits in vitro. Transduction of TAT-fused full-length L-plastin peptide into osteoclasts from L-plastin knockout mice rescued the formation of nascent sealing zones and sealing rings in a time-dependent manner. This response was not observed with mutated full-length L-plastin (Ser-5 and -7 to Ala-5 and -7) peptide. In contrast to the observed defect in the NSZ, L-plastin deficiency did not affect podosome formation or adhesion of osteoclasts in vitro or in vivo. Histomorphometry analyses in 8- and 12-week-old female L-plastin knockout mice demonstrated a decrease in eroded perimeters and an increase in trabecular bone density, without a change in bone formation by osteoblasts. This decrease in eroded perimeters supports that osteoclast function is attenuated in L-plastin knockouts. Micro-CT analyses confirmed a marked increase in trabecular bone mass. In conclusion, female L-plastin knockout mice had increased trabecular bone density due to impaired bone resorption by osteoclasts. L-plastin could be a potential target for therapeutic interventions to treat trabecular bone loss.

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

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

Modular actin nano-architecture enables podosome protrusion and mechanosensing

Basement membrane transmigration during embryonal development, tissue homeostasis and tumor invasion relies on invadosomes, a collective term for invadopodia and podosomes. An adequate structural framework for this process is still missing. Here, we reveal the modular actin nano-architecture that enables podosome protrusion and mechanosensing. The podosome protrusive core contains a central branched actin module encased by a linear actin module, each harboring specific actin interactors and actin isoforms. From the core, two actin modules radiate: ventral filaments bound by vinculin and connected to the plasma membrane and dorsal interpodosomal filaments crosslinked by myosin IIA. On stiff substrates, the actin modules mediate long-range substrate exploration, associated with degradative behavior. On compliant substrates, the vinculin-bound ventral actin filaments shorten, resulting in short-range connectivity and a focally protrusive, non-degradative state. Our findings redefine podosome nanoscale architecture and reveal a paradigm for how actin modularity drives invadosome mechanosensing in cells that breach tissue boundaries.

Macrophage in vitro Response on Hybrid Coatings Obtained by Matrix Assisted Pulsed Laser Evaporation

The improvement in the research area of the implant by surface functionalization when correlated with the biological response is of major interest in the biomedical field. Based on the fact that the inflammatory response is directly involved in the ultimate response of the implant within the body, it is essential to study the macrophage-material interactions. Within this context, we have investigated the composite material-macrophage cell interactions and the inflammatory response to these composites with amorphous hydroxyapatite (HA), Lactoferrin (Lf), and polyethylene glycol-polycaprolactone (PEG-PCL) copolymer. All materials are obtained by Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique and characterized by Atomic Force Microscopy and Scanning Electron Microscopy. Macrophage-differentiated THP-1 cells proliferation and metabolic activity were assessed by qualitative and quantitative methods. The secretion of tumor necrosis factor alpha (TNF-α) and interleukin 10 (IL-10) cytokine, in the presence and absence of the inflammatory stimuli (bacterial endotoxin; lipopolysaccharide (LPS)), was measured using an ELISA assay. Our results revealed that the cellular response depended on the physical-chemical characteristics of the coatings. Copolymer-HA-Lf coatings led to low level of pro-inflammatory TNF-α, the increased level of anti-inflammatory IL-10, and the polarization of THP-1 cells towards an M2 pro-reparative phenotype in the presence of LPS. These findings could have important potential for the development of composite coatings in implant applications.

Tks5 and Dynamin-2 enhance actin bundle rigidity in invadosomes to promote myoblast fusion

Skeletal muscle development requires the cell-cell fusion of differentiated myoblasts to form muscle fibers. The actin cytoskeleton is known to be the main driving force for myoblast fusion; however, how actin is organized to direct intercellular fusion remains unclear. Here we show that an actin- and dynamin-2-enriched protrusive structure, the invadosome, is required for the fusion process of myogenesis. Upon differentiation, myoblasts acquire the ability to form invadosomes through isoform switching of a critical invadosome scaffold protein, Tks5. Tks5 directly interacts with and recruits dynamin-2 to the invadosome and regulates its assembly around actin filaments to strengthen the stiffness of dynamin-actin bundles and invadosomes. These findings provide a mechanistic framework for the acquisition of myogenic fusion machinery during myogenesis and reveal a novel structural function for Tks5 and dynamin-2 in organizing actin filaments in the invadosome to drive membrane fusion.

Cell Migration in Microfluidic Devices: Invadosomes Formation in Confined Environments

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

Macrophage functions in wound healing

Macrophages play a crucial role in regeneration and consecutive phases of wound healing. In this review, we summarise current knowledge on the ontogeny, origin, phenotypical heterogeneity, and functional exchangeability of macrophages participating in these processes. We also describe the genetic, pharmacologic, and bioengineering methods for manipulation of macrophage phenotype and functions and their potential for development of the novel, clinically applicable therapies.

Myosins in Osteoclast Formation and Function

Skeletal quantity and quality are determined by processes of bone modeling and remodeling, which are undertaken by cells that build and resorb bone as they respond to mechanical, hormonal, and other external and internal signals. As the sole bone resorptive cell type, osteoclasts possess a remarkably dynamic actin cytoskeleton that drives their function in this enterprise. Actin rearrangements guide osteoclasts’ capacity for precursor fusion during differentiation, for migration across bone surfaces and sensing of their composition, and for generation of unique actin superstructures required for the resorptive process. In this regard, it is not surprising that myosins, the superfamily of actin-based motor proteins, play key roles in osteoclast physiology. This review briefly summarizes current knowledge of the osteoclast actin cytoskeleton and describes myosins’ roles in osteoclast differentiation, migration, and actin superstructure patterning.

Anti-osteoclastic effects of C-glucosidic ellagitannins mediated by actin perturbation

Actin subunits assemble into actin filaments whose dynamics and three-dimensional architectures are further regulated by a variety of cellular factors to establish the functional actin cytoskeleton. The C-glucosidic ellagitannin vescalagin and its simpler analogue vescalin, affect both the dynamics and the ultrastructure of the actin cytoskeleton by directly binding to F-actin. Herein, we show that in vitro, the two compounds induce the formation of distinct F-actin networks characterized by different superstructures and dynamics. In living mature osteoclasts, highly specialized bone-degrading cells that constantly remodel their cytoskeleton, vescalagin and vescalin alter actin dynamics at podosomes and compromise the integrity of the podosome belt that forms the bone-degrading apparatus. Both compounds target the bone-resorbing activity at concentrations that preserve osteoclastic maturation and survival and with no detectable impact on the behaviour of bone-forming osteoblastic cells. This anti-osteoclastic activity of vescalagin and vescalin reveals the potential of targeting actin dynamics as a new therapeutic opportunity and, in this case, as a plausible approach for the local treatment of osteoporosis.

Non-canonical Wnt signals regulate cytoskeletal remodeling in osteoclasts

Osteoclasts are multinucleated cells responsible for bone resorption. Osteoclasts adhere to the bone surface through integrins and polarize to form actin rings, which are formed by the assembly of podosomes. The area contained within actin rings (also called sealing zones) has an acidic pH, which causes dissolution of bone minerals including hydroxyapatite and the degradation of matrix proteins including type I collagen by the protease cathepsin K. Osteoclasts resorb bone matrices while moving on bone surfaces. Osteoclasts change their cell shapes and exhibit three modes for bone resorption: motile resorbing mode for digging trenches, static resorbing mode for digging pits, and motile non-resorbing mode. Therefore, the actin cytoskeleton is actively remodeled in osteoclasts. Recent studies have revealed that many molecules, such as Rac, Cdc42, Rho, and small GTPase regulators and effectors, are involved in actin cytoskeletal remodeling during the formation of actin rings and resorption cavities on bone slices. In this review, we introduce how these molecules and non-canonical Wnt signaling regulate the bone-resorbing activity of osteoclasts.

Inhibition of RhoA and mTORC2/Rictor by Fingolimod (FTY720) induces p21-activated kinase 1, PAK-1 and amplifies podosomes in mouse peritoneal macrophages

Macrophage functions in the immune response depend on their ability to infiltrate tissues and organs. The penetration between and within the tissues requires degradation of extracellular matrix (ECM), a function performed by the specialized, endopeptidase- and actin filament- rich organelles located at the ventral surface of macrophage, called the podosomes. Podosome formation requires local inhibition of small GTPase RhoA activity, and depends on Rac 1/Rho guanine nucleotide exchange factor 7, β-PIX and its binding partner the p21-activated kinase (PAK-1). The activity of RhoA and Rac 1 is in turn regulated by mTOR/mTORC2 pathway. Here we showed that a fungus metabolite Fingolimod (FTY720, Gilenya), which is clinically approved for the treatment of multiple sclerosis, down-regulates Rictor, which is a signature molecule of mTORC2 and dictates its substrate (actin cytoskeleton) specificity, down-regulates RhoA, up-regulates PAK-1, and causes amplification of podosomes in mouse peritoneal macrophages.

"Immuno-Transient Receptor Potential Ion Channels": The Role in Monocyte- and Macrophage-Mediated Inflammatory Responses

Monocytes and macrophages play important roles in health and disease. They have a central role in protecting the host, as they clear pathogens and modulate other immune cell functions through the production of regulatory molecules. Their functions include immune surveillance, bacterial killing, tissue remodeling and repair, clearance of cell debris and more. Macrophages can have beneficial and detrimental effects on the outcome of several diseases depending on the microenvironment and the activation state of cells. Over the past few years, there has been an increasing interest in the expression and functions of ion channels, in particular of transient receptor potential (TRP) channel family in immune cells. The 30 members of mammalian TRP channels are subdivided into TRPC, TRPV, TRPM, TRPML, TRPP, and TRPA superfamily, and several members of TRP subfamily have been found to be functionally expressed in monocytes and macrophages. TRP are cation-selective channels that are weakly voltage-sensitive and diversely gated by temperature, mechanical force, electrophiles, ligands, and internal cues, such as membrane composition and pH, contributing to immune and inflammatory responses. The TRP channels play major roles in controlling several monocyte and macrophage functions such as phagocytosis, production of chemokines and cytokines, cell survival, polarization and so forth. In addition, they can also be potential therapeutic targets in a variety of inflammatory diseases. Thus, the goal of this review is to describe the role of TRP channels in the control of monocyte–macrophage functions in inflammatory and immune-mediated diseases.

Podosomes, But Not the Maturation Status, Determine the Protease-Dependent 3D Migration in Human Dendritic Cells

Dendritic cells (DC) are professional Antigen-Presenting Cells scattered throughout antigen-exposed tissues and draining lymph nodes, and survey the body for pathogens. Their ability to migrate through tissues, a 3D environment, is essential for an effective immune response. Upon infection, recognition of Pathogen-Associated Molecular Patterns (PAMP) by Toll-like receptors (TLR) triggers DC maturation. Mature DC (mDC) essentially use the protease-independent, ROCK-dependent amoeboid mode in vivo, or in collagen matrices in vitro. However, the mechanisms of 3D migration used by human immature DC (iDC) are still poorly characterized. Here, we reveal that human monocyte-derived DC are able to use two migration modes in 3D. In porous matrices of fibrillar collagen I, iDC adopted the amoeboid migration mode. In dense matrices of gelled collagen I or Matrigel, iDC used the protease-dependent, ROCK-independent mesenchymal migration mode. Upon TLR4 activation by LPS, mDC-LPS lose the capacity to form podosomes and degrade the matrix along with impaired mesenchymal migration. TLR2 activation by Pam₃CSK₄ resulted in DC maturation, podosome maintenance, and efficient mesenchymal migration. Under all these conditions, when DC used the mesenchymal mode in dense matrices, they formed 3D podosomes at the tip of cell protrusions. Using PGE₂, known to disrupt podosomes in DC, we observed that the cells remained in an immature status and the mesenchymal migration mode was abolished. We also observed that, while CCL5 (attractant of iDC) enhanced both amoeboid and mesenchymal migration of iDC, CCL19 and CCL21 (attractants of mDC) only enhanced mDC-LPS amoeboid migration without triggering mesenchymal migration. Finally, we examined the migration of iDC in tumor cell spheroids, a tissue-like 3D environment. We observed that iDC infiltrated spheroids of tumor cells using both migration modes. Altogether, these results demonstrate that human DC adopt the mesenchymal mode to migrate in 3D dense environments, which relies on their capacity to form podosomes independent of their maturation status, paving the way of further investigations on in vivo DC migration in dense tissues and its regulation during infections.

Lymphocyte-specific protein 1 regulates mechanosensory oscillation of podosomes and actin isoform-based actomyosin symmetry breaking

Subcellular fine-tuning of the actomyosin cytoskeleton is a prerequisite for polarized cell migration. We identify LSP (lymphocyte-specific protein) 1 as a critical regulator of actomyosin contractility in primary macrophages. LSP1 regulates adhesion and migration, including the parameters cell area and speed, and also podosome turnover, oscillation and protrusive force. LSP1 recruits myosin IIA and its regulators, including myosin light chain kinase and calmodulin, and competes with supervillin, a myosin hyperactivator, for myosin regulators, and for actin isoforms, notably β-actin. Actin isoforms are anisotropically distributed in myosin IIA-expressing macrophages, and contribute to the differential recruitment of LSP1 and supervillin, thus enabling an actomyosin symmetry break, analogous to the situation in cells expressing two myosin II isoforms. Collectively, these results show that the cellular pattern of actin isoforms builds the basis for the differential distribution of two actomyosin machineries with distinct properties, leading to the establishment of discrete zones of actomyosin contractility.

The actomyosin cytoskeleton plays an important role in polarised cell migration. Here the authors identify lymphocyte-specific protein (LSP)-1 as a regulator of actomyosin contractility in macrophages, by competing with supervillin for myosin IIA activators acting specifically on the β-actin isoform.

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

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

Non-identical twins: Different faces of CR3 and CR4 in myeloid and lymphoid cells of mice and men

Integrins are cell membrane receptors that are involved in essential physiological and serious pathological processes. Their main role is to ensure a closely regulated link between the extracellular matrix and the intracellular cytoskeletal network enabling cells to react to environmental stimuli. Complement receptor type 3 (CR3, αMβ₂, CD11b/CD18) and type 4 (CR4, αXβ₂, CD11c/CD18) are members of the β₂-integrin family expressed on most white blood cells. Both receptors bind multiple ligands like iC3b, ICAM, fibrinogen or LPS. β₂-integrins are accepted to play important roles in cellular adhesion, migration, phagocytosis, ECM rearrangement and inflammation. Several pathological conditions are linked to the impaired functions of these receptors. CR3 and CR4 are generally thought to mediate overlapping functions in monocytes, macrophages and dendritic cells, therefore the potential distinctive role of these receptors has not been investigated so far in satisfactory details. Lately it has become clear that a functional segregation has evolved between the two receptors regarding phagocytosis, cellular adhesion and podosome formation. In addition to their tasks on myeloid cells, the expression and function of CR3 and CR4 on lymphocytes have also gained interest recently. The picture is further complicated by the fact that while these β₂-integrins are expressed by immune cells both in mice and humans, there are significant differences in their expression level, functions and the pathological consequences of genetic defects. Here we aim to summarize our current knowledge on CR3 and CR4 and highlight the functional differences between these receptors, involving their expression in myeloid and lymphoid cells of both men and mice.

EphA receptors and ephrin-A ligands are upregulated by monocytic differentiation/maturation and promote cell adhesion and protrusion formation in HL60 monocytes

Background

Eph signaling is known to induce contrasting cell behaviors such as promoting and inhibiting cell adhesion/spreading by altering F-actin organization and influencing integrin activities. We have previously demonstrated that EphA2 stimulation by ephrin-A1 promotes cell adhesion through interaction with integrins and integrin ligands in two monocyte/macrophage cell lines. Although mature mononuclear leukocytes express several members of the EphA/ephrin-A subclass, their expression has not been examined in monocytes undergoing during differentiation and maturation.

Results

Using RT-PCR, we have shown that EphA2, ephrin-A1, and ephrin-A2 expression was upregulated in murine bone marrow mononuclear cells during monocyte maturation. Moreover, EphA2 and EphA4 expression was induced, and ephrin-A4 expression was upregulated, in a human promyelocytic leukemia cell line, HL60, along with monocyte differentiation toward the classical CD14⁺⁺CD16⁻ monocyte subset. Using RT-PCR and flow cytometry, we have also shown that expression levels of αL, αM, αX, and β2 integrin subunits were upregulated in HL60 cells along with monocyte differentiation while those of α4, α5, α6, and β1 subunits were unchanged. Using a cell attachment stripe assay, we have shown that stimulation by EphA as well as ephrin-A, likely promoted adhesion to an integrin ligand-coated surface in HL60 monocytes. Moreover, EphA and ephrin-A stimulation likely promoted the formation of protrusions in HL60 monocytes.

Conclusions

Notably, this study is the first analysis of EphA/ephrin-A expression during monocytic differentiation/maturation and of ephrin-A stimulation affecting monocyte adhesion to an integrin ligand-coated surface. Thus, we propose that monocyte adhesion via integrin activation and the formation of protrusions is likely promoted by stimulation of EphA as well as of ephrin-A.

Electronic supplementary material

The online version of this article (doi:10.1186/s12860-017-0144-x) contains supplementary material, which is available to authorized users.

Responses of rat and mouse primary microglia to pro- and anti-inflammatory stimuli: molecular profiles, K+ channels and migration

BACKGROUND

Acute CNS damage is commonly studied using rat and mouse models, but increasingly, molecular analysis is finding species differences that might affect the ability to translate findings to humans. Microglia can undergo complex molecular and functional changes, often studied by in vitro responses to discrete activating stimuli. There is considerable evidence that pro-inflammatory (M1) activation can exacerbate tissue damage, while anti-inflammatory (M2) states help resolve inflammation and promote tissue repair. However, in assessing potential therapeutic targets for controlling inflammation, it is crucial to determine whether rat and mouse microglia respond the same.

METHODS

Primary microglia from Sprague-Dawley rats and C57BL/6 mice were cultured, then stimulated with interferon-γ + tumor necrosis factor-α (I + T; M1 activation), interleukin (IL)-4 (M2a, alternative activation), or IL-10 (M2c, acquired deactivation). To profile their activation responses, NanoString was used to monitor messenger RNA (mRNA) expression of numerous pro- and anti-inflammatory mediators, microglial markers, immunomodulators, and other molecules. Western analysis was used to measure selected proteins. Two potential targets for controlling inflammation-inward- and outward-rectifier K⁺ channels (Kir2.1, Kv1.3)-were examined (mRNA, currents) and specific channel blockers were applied to determine their contributions to microglial migration in the different activation states.

RESULTS

Pro-inflammatory molecules increased after I + T treatment but there were several qualitative and quantitative differences between the species (e.g., iNOS and nitric oxide, COX-2). Several molecules commonly associated with an M2a state differed between species or they were induced in additional activation states (e.g., CD206, ARG1). Resting levels and/or responses of several microglial markers (Iba1, CD11b, CD68) differed with the activation state, species, or both. Transcripts for several Kir2 and Kv1 family members were detected in both species. However, the current amplitudes (mainly Kir2.1 and Kv1.3) depended on activation state and species. Treatment-induced changes in morphology and migratory capacity were similar between the species (migration reduced by I + T, increased by IL-4 or IL-10). In both species, Kir2.1 block reduced migration and Kv1.3 block increased it, regardless of activation state; thus, these channels might affect microglial migration to damage sites.

CONCLUSIONS

Caution is recommended in generalizing molecular and functional responses of microglia to activating stimuli between species.