The architecture of the adhesive apparatus of cultured osteoclasts: from podosome formation to sealing zone assembly

Role of protein-tyrosine phosphatases in regulation of osteoclastic activity

Osteoclasts, the primary cell type mediating bone resorption, are multinucleated, giant cells derived from hematopoietic cells of monocyte-macrophage lineage. Osteoclast activity is, in a large part, regulated by protein-tyrosine phosphorylation. While information about functional roles of several protein-tyrosine kinases (PTK), including c-Src, in osteoclastic resorption has been accumulated, little is known about the roles of protein-tyrosine phosphatases (PTPs) in regulation of osteoclast activity. Recent evidence implicates important regulatory roles for four PTPs (SHP-1, cyt-PTP-epsilon, PTP-PEST, and PTPoc) in osteoclasts. Cyt-PTP-epsilon, PTP-PEST, and PTP-oc are positive regulators of osteoclast activity, while SHP-1 is a negative regulator. Of these PTPs in osteoclasts, only PTP-oc is a positive regulator of c-Src PTK through dephosphorylation of the inhibitory phosphotyrosine-527 residue. Although some information about mechanisms of action of these PTPs to regulate osteoclast activity is reviewed in this article, much additional work is required to provide more comprehensive details about their functions in osteoclasts.

[The endothelial podosome]

Podosomes are highly dynamic adhesion microdomains formed at the ventral membrane of some monocyte-derived cells. Structurally, their most distinguishing feature is their two-part architecture, consisting in a core of F-actin and actin-associated proteins, surrounded by a ring structure consisting of plaque proteins as well as signalling proteins. In addition to the presence of specific markers, they are distinguished from other adhesion structures by the presence of metalloproteases, endowing them with the ability to degrade the extracellular matrix. Invadopodia are related structures, of similar molecular composition but of distinct architecture, made by fibroblasts or epithelial cells transformed by the v-src oncogene or aggressive carcinoma cells. Such membrane-associated cellular devices, now named invadosomes, are thought to have a central role in mediating polarized migration in cells that cross anatomical boundaries. Podosomes have now been shown to form in endothelial cells, non monocytic and non tumoral cells, endowed with tissue invasive activities during vascular remodelling. Here, we summarize the recent advances and developments in this field, discuss how endothelial podosomes combine specificities of monocytic podosomes and invadopodia and provide our provisional outlook into the future understanding of endothelial podosomes.

Regulated proteolysis of nonmuscle myosin IIA stimulates osteoclast fusion

The nonmuscle myosin IIA heavy chain (Myh9) is strongly associated with adhesion structures of osteoclasts. In this study, we demonstrate that during osteoclastogenesis, myosin IIA heavy chain levels are temporarily suppressed, an event that stimulates the onset of cell fusion. This suppression is not mediated by changes in mRNA or translational levels but instead is due to a temporary increase in the rate of myosin IIA degradation. Intracellular activity of cathepsin B is significantly enhanced at the onset of osteoclast precursor fusion, and specific inhibition of its activity prevents myosin IIA degradation. Further, treatment of normal cells with cathepsin B inhibitors during the differentiation process reduces cell fusion and bone resorption capacity, whereas overexpression of cathepsin B enhances fusion. Ongoing suppression of the myosin IIA heavy chain via RNA interference results in formation of large osteoclasts with significantly increased numbers of nuclei, whereas overexpression of myosin IIA results in less osteoclast fusion. Increased multinucleation caused by myosin IIA suppression does not require RANKL. Further, knockdown of myosin IIA enhances cell spreading and lessens motility. These data taken together strongly suggest that base-line expression of nonmuscle myosin IIA inhibits osteoclast precursor fusion and that a temporary, cathepsin B-mediated decrease in myosin IIA levels triggers precursor fusion during osteoclastogenesis.

Surface-induced regulation of podosome organization and dynamics in cultured osteoclasts

Bone is continuously repaired and remodeled through the well-coordinated activity of osteoblasts, which form new bone, and osteoclasts, which resorb it. How osteoclasts sense the properties of the bone surface remains unclear. By combining light and electron microscopy, we compared osteoclast behavior on three distinct surfaces: glass, calcite single crystals, and bone. Podosomes, the basic units of the adhesion structure, and their organization into superstructures were found to be common to cells that were attached to all three substrates, whereas the structure of the resorption organelle, the so-called "ruffled border," markedly differed. Moreover, the integrity, stability, and dynamic behavior of the adhesion superstructures also fundamentally differed, depending on the substrate. We conclude that osteoclasts sense the local properties of the underlying substrate and respond to these signals, both locally and globally.

Src-dependent repression of ARF6 is required to maintain podosome-rich sealing zones in bone-digesting osteoclasts

Bone digestion occurs when osteoclasts adhere onto bone surfaces and polarize to form acidic, hydrolase-rich resorption lacunae. For this process, they condense their actin-rich podosomes in tight belts to establish sealing zones, which segregate their basal membranes from those facing resorption lacunae. This polarization process remains poorly understood. Here, we combined quantitative proteomics and gene silencing to identify new substrates of the Src tyrosine kinase, a key regulator of osteoclast function. We now report that a depletion of the ARF GTPase-activating protein GIT2, which localizes to sealing zones upon Src phosphorylation, or a lack of GTP hydrolysis on ARF6 impairs sealing zone formation and polarized membrane traffic. Surprisingly, the Rho guanine nucleotide exchange factors alpha and beta PIX, which usually coordinate ARF and Rho signaling, were found to be dispensable. We conclude that the Src-dependent localization of GIT2 is essential for down-regulating ARF6 activity at sealing zones, and thus for maintaining osteoclast polarity.

Cell biology: watching the first steps of podosome formation

Podosomes and invadopodia are actin-rich structures that have come under intense scrutiny over the past several years due to their critical roles in cell migration and invasion. Examination of the initial stages of podosome formation has revealed an important role for the phosphoinositide PI(3,4)P(2) in anchoring the scaffold protein Tks5 to the plasma membrane.

Plectin deposition at podosome rings requires myosin contractility

Metalloproteinase-dependent tissue invasion requires the formation of podosomes and invadopodia for localized matrix degradation. Actin cytoskeleton remodeling via Arp2/3-mediated actin polymerization is essential for podosome formation, and dynamic microtubules have an important role in maintaining podosome turnover in macrophages and osteoclasts. Little is known, however, about the involvement of the intermediate filament cytoskeleton in formation, stabilization, and turnover of podosomes. Here we show that vimentin intermediate filaments colocalize with the early sites of podosome formation at the stress fiber - focal adhesion interface in cultured vascular smooth muscle cells, but do not directly contribute to podosome formation, or stabilization. In unstimulated A7r5 cells the cytolinker protein plectin poorly colocalized with vimentin and the microdomains, but following induction by phorbol ester accumulated in the rings that surround the podosomes. In plectin-deficient A7r5 cells actin stress fiber remodelling is reduced in response to PDBu, and small podosomes remain localized at stable actin stress fibres. Pharmacological inhibition of actomyosin contractility by blebbistatin leads to an aberrant localization of podosomes away from the cell periphery and induces failure of plectin to surround the outer perimeter of these invasive adhesions. Taken together, we conclude that plectin is involved in growth and maturation of podosomes by reducing focal adhesion and stress fiber turnover, and that actomyosin-dependent contractility is required for the peripheral localization and specific deposition of plectin at the podosome rings.

Rho GTPases in osteoclasts: orchestrators of podosome arrangement

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

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

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

Protein tyrosine phosphatases in osteoclast differentiation, adhesion, and bone resorption

Osteoclasts are large cells derived from the monocyte-macrophage hematopoietic cell lineage. Their primary function is to degrade bone in various physiological contexts. Osteoclasts adhere to bone via podosomes, specialized adhesion structures whose structure and subcellular organization are affected by mechanical contact of the cell with bone matrix. Ample evidence indicates that reversible tyrosine phosphorylation of podosomal proteins plays a major role in determining the organization and dynamics of podosomes. Although roles of several tyrosine kinases are known in detail in this respect, little is known concerning the roles of protein tyrosine phosphatases (PTPs) in regulating osteoclast adhesion. Here we summarize available information concerning the known and hypothesized roles of the best-researched PTPs in osteoclasts - PTPRO, PTP epsilon, SHP-1, and PTP-PEST. Of these, PTPRO, PTP epsilon, and PTP-PEST appear to support osteoclast activity while SHP-1 inhibits it. Additional studies are required to provide full molecular details of the roles of these PTPs in regulating osteoclast adhesion, and to uncover additional PTPs that participate in this process.

Assembly and biological role of podosomes and invadopodia

Regulated tissue invasion via motile and lytic events is critical for physiological processes such as immune system function and inflammatory responses, wound healing, and organ development, but pathological subversion of this process drives tumour cell invasion and metastasis. Cell migration and invasion require the integration of several processes that include: first, the local modulation of cytoskeleton structure and contractile forces; second, the turnover of substrate adhesions and their associated microfilaments; and third, the generation of specialised, transient domains that mediate the protease-dependent focal degradation of the extracellular matrix. Recent work has re-discovered prominent actin-based cellular structures, termed invadopodia and podosomes, as unique structural and functional modules through which major invasive mechanisms are regulated. The stage is now set to unravel their roles in the physiology and pathology of tissue plasticity and repair.

Cofilin activity downstream of Pak1 regulates cell protrusion efficiency by organizing lamellipodium and lamella actin networks

Protrusion of the leading edge of migrating epithelial cells requires precise regulation of two actin filament (F-actin) networks, the lamellipodium and the lamella. Cofilin is a downstream target of Rho GTPase signaling that promotes F-actin cycling through its F-actin-nucleating, -severing, and -depolymerizing activity. However, its function in modulating lamellipodium and lamella dynamics, and the implications of these dynamics for protrusion efficiency, has been unclear. Using quantitative fluorescent speckle microscopy, immunofluorescence, and electron microscopy, we establish that the Rac1/Pak1/LIMK1 signaling pathway controls cofilin activity within the lamellipodium. Enhancement of cofilin activity accelerates F-actin turnover and retrograde flow, resulting in widening of the lamellipodium. This is accompanied by increased spatial overlap of the lamellipodium and lamella networks and reduced cell-edge protrusion efficiency. We propose that cofilin functions as a regulator of cell protrusion by modulating the spatial interaction of the lamellipodium and lamella in response to upstream signals.

Defective microtubule-dependent podosome organization in osteoclasts leads to increased bone density in Pyk2(-/-) mice

The protein tyrosine kinase Pyk2 is highly expressed in osteoclasts, where it is primarily localized in podosomes. Deletion of Pyk2 in mice leads to mild osteopetrosis due to impairment in osteoclast function. Pyk2-null osteoclasts were unable to transform podosome clusters into a podosome belt at the cell periphery; instead of a sealing zone only small actin rings were formed, resulting in impaired bone resorption. Furthermore, in Pyk2-null osteoclasts, Rho activity was enhanced while microtubule acetylation and stability were significantly reduced. Rescue experiments by ectopic expression of wild-type or a variety of Pyk2 mutants in osteoclasts from Pyk2^−/− mice have shown that the FAT domain of Pyk2 is essential for podosome belt and sealing zone formation as well as for bone resorption. These experiments underscore an important role of Pyk2 in microtubule-dependent podosome organization, bone resorption, and other osteoclast functions.

Tropomyosin 4 regulates adhesion structures and resorptive capacity in osteoclasts

Tropomyosins (Tms) are alpha-helical dimers that bind and stabilize actin microfilaments while regulating their accessibility to other actin-associated proteins. Four genes encode expression of over forty Tms, most of which are expressed in nonmuscle cells. In recent years, it has become clear that individual Tm isoforms may regulate specific actin pools within cells. In this study, we examined how osteoclast function may be regulated by the tropomyosin isoform Tm-4, which we previously showed to be highly localized to podosomes and sealing zones of osteoclasts. RNAi-mediated knockdown of Tm-4, both in RAW264.7- and mouse marrow-derived osteoclasts, resulted in thinning of the actin ring of the sealing zone. Knockdown of Tm-4 also resulted in diminished bone resorptive capacity and altered resorption pit shape. In contrast, osteoclasts overexpressing Tm-4 demonstrated thickened podosomes on glass as well as thickened, aberrant actin structures on bone, and diminished motility and resorptive capacity. These results indicate that Tm-4 plays a role in regulating adhesion structures of osteoclasts, most likely by stabilizing the actin microfilaments present in podosomes and the sealing zone.

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

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

CD44 and beta3 integrin organize two functionally distinct actin-based domains in osteoclasts

The actin cytoskeleton of mature osteoclasts (OCs) adhering to nonmineralized substrates is organized in a belt of podosomes reminiscent of the sealing zone (SZ) found in bone resorbing OCs. In this study, we demonstrate that the belt is composed of two functionally different actin-based domains: podosome cores linked with CD44, which are involved in cell adhesion, and a diffuse cloud associated with beta3 integrin, which is involved in cell adhesion and contraction. Wiskott Aldrich Syndrome Protein (WASp) Interacting Protein (WIP)-/- OCs were devoid of podosomes, but they still exhibited actin clouds. Indeed, WIP-/- OCs show diminished expression of WASp, which is required for podosome formation. CD44 is a novel marker of OC podosome cores and the first nonintegrin receptor detected in these structures. The importance of CD44 is revealed by showing that its clustering restores podosome cores and WASp expression in WIP-/- OCs. However, although CD44 signals are sufficient to form a SZ, the presence of WIP is indispensable for the formation of a fully functional SZ.

Reorganisation of the dendritic actin network during cancer cell migration and invasion

Invasion of cancer cells into surrounding tissues has a causal role in tumour progression and is an initial step in tumour metastasis. It requires cell migration, which is driven by the polymerisation of actin within two distinct structures, lamellipodia and filopodia, and attachment to the extracellular matrix through actin-rich adhesive structures. Podosomes and invadopodia are modified adhesive structures that not only establish contact with the substratum, but are also involved in matrix degradation leading to invasion. Actin dynamics and organisation are tightly regulated processes responsible for the range of different and specific cellular functions in response to various stimuli. This review explores the mechanistic basis of tumour cell invasion by focusing on the reorganisation of the dendritic actin network. Actin filaments are flexible structures that are poorly able to resist bending forces, causing them to bend rather than push when encountering obstacles. During migration, cells overcome this problem either by creating a dense array of short-branched filaments as found in lamellipodia, or by bundling filaments as found in filopodia. Here we discuss the possible switch mechanism for the two modes of actin organisation and the advantages of each in the perspective of cell migration and invasion during tumour metastasis.

Regulation of osteoclast polarization

Osteoclast function consists of several processes: recognition of mineralized tissues, development of ruffled borders and sealing zones, secretion of acids and proteolytic enzymes into the space beneath the ruffled border, and incorporation and secretion of bone degradation products using the transcytosis system. One of the most important questions concerning osteoclast function is how osteoclasts recognize bone and polarize. During the past decade, new approaches have been taken to investigate the regulation of osteoclast polarization. Attachment of osteoclasts to some proteins containing the Arg-Gly-Asp sequence motif through vitronectin receptors is the first step in inducing the polarization of osteoclasts. Physical properties of bone such as hardness or roughness are also required to induce osteoclast polarity. Osteoclasts cultured even on plastic dishes secrete protons toward the dish surface, suggesting that osteoclasts recognize plastic as a mineralized matrix and secrete protons. This notion was supported by the recent findings that bisphosphonates and reveromycin A were specifically incorporated into polarized osteoclasts cultured even on plastic dishes. On the other hand, a sealing zone, defined as a thick band of actin, is induced in osteoclasts adherent only on an apatite-containing mineralized matrix. These results suggest that osteoclasts recognize physical properties of the mineralized tissue to secrete protons, and also sense apatite itself or components of apatite to form the sealing zone. Here, we review recent findings on the regulation of osteoclast polarization. We also discuss how osteoclasts recognize mineralized tissues to form the sealing zone.