Moesin activation controls bone resorption and tunneling nanotube-dependent osteoclast fusion.

Osteoclasts are multinucleated cells unique in their ability to resorb bone. Osteoclastogenesis involves several steps of actin-driven rearrangements that participate not only in the cell-cell fusion process, but also in the formation of the sealing zone, the adhesive structure determining the resorption area. Despite the importance of these actin cytoskeleton-based processes, their precise mechanisms of regulation are still poorly characterized. Here, we found that moesin, a member of the Ezrin/Radixin/Moesin (ERM) protein family, is activated during osteoclast maturation and plays an instrumental role for both osteoclast fusion and function. In mouse and human osteoclast precursors, moesin is negatively regulated to potentiate their ability to fuse and degrade bone. Accordingly, we demonstrated that moesin depletion decreases membrane-to-cortex attachment and enhances formation of tunneling nanotubes (TNTs), F-actin-containing intercellular bridges that we revealed to trigger osteoclast fusion. In addition, via a ß3-integrin/RhoA/SLK pathway and independently of its role in fusion, moesin regulates the number and organization of sealing zones in mature osteoclast, and thus participates in the control of bone resorption. Supporting these findings, we found that moesin-deficient mice are osteopenic with a reduced density of trabecular bones and increased osteoclast abundance and activity. These findings provide a better understanding of the regulation of osteoclast biology, and open new opportunities to specifically target osteoclast activity in bone disease therapy.

Productive HIV-1 infection of tissue macrophages by fusion with infected CD4+ T cells.

Macrophages are essential for HIV-1 pathogenesis and represent major viral reservoirs. Therefore, it is critical to understand macrophage infection, especially in tissue macrophages, which are widely infected in vivo, but poorly permissive to cell-free infection. Although cell-to-cell transmission of HIV-1 is a determinant mode of macrophage infection in vivo, how HIV-1 transfers toward macrophages remains elusive. Here, we demonstrate that fusion of infected CD4+ T lymphocytes with human macrophages leads to their efficient and productive infection. Importantly, several tissue macrophage populations undergo this heterotypic cell fusion, including synovial, placental, lung alveolar, and tonsil macrophages. We also find that this mode of infection is modulated by the macrophage polarization state. This fusion process engages a specific short-lived adhesion structure and is controlled by the CD81 tetraspanin, which activates RhoA/ROCK-dependent actomyosin contractility in macrophages. Our study provides important insights into the mechanisms underlying infection of tissue-resident macrophages, and establishment of persistent cellular reservoirs in patients.

Nanoscale architecture and coordination of actin cores within the sealing zone of human osteoclasts.

Osteoclasts are unique in their capacity to degrade bone tissue. To achieve this process, osteoclasts form a specific structure called the sealing zone, which creates a close contact with bone and confines the release of protons and hydrolases for bone degradation. The sealing zone is composed of actin structures called podosomes nested in a dense actin network. The organization of these actin structures inside the sealing zone at the nano scale is still unknown. Here, we combine cutting-edge microscopy methods to reveal the nanoscale architecture and dynamics of the sealing zone formed by human osteoclasts on bone surface. Random illumination microscopy allowed the identification and live imaging of densely packed actin cores within the sealing zone. A cross-correlation analysis of the fluctuations of actin content at these cores indicates that they are locally synchronized. Further examination shows that the sealing zone is composed of groups of synchronized cores linked by α-actinin1 positive filaments, and encircled by adhesion complexes. Thus, we propose that the confinement of bone degradation mediators is achieved through the coordination of islets of actin cores and not by the global coordination of all podosomal subunits forming the sealing zone.

Cellular and molecular actors of myeloid cell fusion: podosomes and tunneling nanotubes call the tune.

Different types of multinucleated giant cells (MGCs) of myeloid origin have been described; osteoclasts are the most extensively studied because of their importance in bone homeostasis. MGCs are formed by cell-to-cell fusion, and most types have been observed in pathological conditions, especially in infectious and non-infectious chronic inflammatory contexts. The precise role of the different MGCs and the mechanisms that govern their formation remain poorly understood, likely due to their heterogeneity. First, we will introduce the main populations of MGCs derived from the monocyte/macrophage lineage. We will then discuss the known molecular actors mediating the early stages of fusion, focusing on cell-surface receptors involved in the cell-to-cell adhesion steps that ultimately lead to multinucleation. Given that cell-to-cell fusion is a complex and well-coordinated process, we will also describe what is currently known about the evolution of F-actin-based structures involved in macrophage fusion, i.e., podosomes, zipper-like structures, and tunneling nanotubes (TNT). Finally, the localization and potential role of the key fusion mediators related to the formation of these F-actin structures will be discussed. This review intends to present the current status of knowledge of the molecular and cellular mechanisms supporting multinucleation of myeloid cells, highlighting the gaps still existing, and contributing to the proposition of potential disease-specific MGC markers and/or therapeutic targets.

HIV-1-Infected Human Macrophages, by Secreting RANK-L, Contribute to Enhanced Osteoclast Recruitment.

HIV-1 infection is frequently associated with low bone density, which can progress to osteoporosis leading to a high risk of fractures. Only a few mechanisms have been proposed to explain the enhanced osteolysis in the context of HIV-1 infection. As macrophages are involved in bone homeostasis and are critical host cells for HIV-1, we asked whether HIV-1-infected macrophages could participate in bone degradation. Upon infection, human macrophages acquired some osteoclast features: they became multinucleated, upregulated the osteoclast markers RhoE and ß3 integrin, and organized their podosomes as ring superstructures resembling osteoclast sealing zones. However, HIV-1-infected macrophages were not fully differentiated in osteoclasts as they did not upregulate NFATc-1 transcription factor and were unable to degrade bone. Investigating whether infected macrophages participate indirectly to virus-induced osteolysis, we showed that they produce RANK-L, the key osteoclastogenic cytokine. RANK-L secreted by HIV-1-infected macrophages was not sufficient to stimulate multinucleation, but promoted the protease-dependent migration of osteoclast precursors. In conclusion, we propose that, by stimulating RANK-L secretion, HIV-1-infected macrophages contribute to create a microenvironment that favors the recruitment of osteoclasts, participating in bone disorders observed in HIV-1 infected patients.

Primary myeloid cell proteomics and transcriptomics: importance of ß-tubulin isotypes for osteoclast function.

Among hematopoietic cells, osteoclasts (OCs) and immature dendritic cells (DCs) are closely related myeloid cells with distinct functions: OCs participate skeleton maintenance while DCs sample the environment for foreign antigens. Such specificities rely on profound modifications of gene and protein expression during OC and DC differentiation. We provide global proteomic and transcriptomic analyses of primary mouse OCs and DCs, based on original stable isotope labeling with amino acids in cell culture (SILAC) and RNAseq data. We established specific signatures for OCs and DCs, including genes and proteins of unknown functions. In particular, we showed that OCs and DCs have the same α- and ß-tubulin isotype repertoire but that OCs express much more of the ß tubulin isotype Tubb6 (also known as TBB6). In both mouse and human OCs, we demonstrate that elevated expression of Tubb6 in OCs is necessary for correct podosomes organization and thus for the structure of the sealing zone, which sustains the bone resorption apparatus. Hence, lowering Tubb6 expression hinders OC resorption activity. Overall, we highlight here potential new regulators of OC and DC biology, and illustrate the functional importance of the tubulin isotype repertoire in the biology of differentiated cells.

Cell-to-Cell Spreading of HIV-1 in Myeloid Target Cells Escapes SAMHD1 Restriction.

Dendritic cells (DCs) and macrophages as well as osteoclasts (OCs) are emerging as target cells of HIV-1 involved in virus transmission, dissemination, and establishment of persistent tissue virus reservoirs. While these myeloid cells are poorly infected by cell-free viruses because of the high expression levels of cellular restriction factors such as SAMHD1, we show here that HIV-1 uses a specific and common cell-to-cell fusion mechanism for virus transfer and dissemination from infected T lymphocytes to the target cells of the myeloid lineage, including immature DCs (iDCs), OCs, and macrophages, but not monocytes and mature DCs. The establishment of contacts with infected T cells leads to heterotypic cell fusion for the fast and massive transfer of viral material into OC and iDC targets, which subsequently triggers homotypic fusion with noninfected neighboring OCs and iDCs for virus dissemination. These two cell-to-cell fusion processes are not restricted by SAMHD1 and allow very efficient spreading of virus in myeloid cells, resulting in the formation of highly virus-productive multinucleated giant cells. These results reveal the cellular mechanism for SAMHD1-independent cell-to-cell spreading of HIV-1 in myeloid cell targets through the formation of the infected multinucleated giant cells observed in vivo in lymphoid and nonlymphoid tissues of HIV-1-infected patients.IMPORTANCE We demonstrate that HIV-1 uses a common two-step cell-to-cell fusion mechanism for massive virus transfer from infected T lymphocytes and dissemination to myeloid target cells, including dendritic cells and macrophages as well as osteoclasts. This cell-to-cell infection process bypasses the restriction imposed by the SAMHD1 host cell restriction factor for HIV-1 replication, leading to the formation of highly virus-productive multinucleated giant cells as observed in vivo in lymphoid and nonlymphoid tissues of HIV-1-infected patients. Since myeloid cells are emerging as important target cells of HIV-1, these results contribute to a better understanding of the role of these myeloid cells in pathogenesis, including cell-associated virus sexual transmission, cell-to-cell virus spreading, and establishment of long-lived viral tissue reservoirs.

The osteoclast, a target cell for microorganisms.

Bone is a highly adaptive tissue with regenerative properties that is subject to numerous diseases. Infection is one of the causes of altered bone homeostasis. Bone infection happens subsequently to bone surgery or to systemic spreading of microorganisms. In addition to osteoblasts, osteoclasts (OCs) also constitute cell targets for pathogens. OCs are multinucleated cells that have the exclusive ability to resorb bone mineral tissue. However, the OC is much more than a bone eater. Beyond its role in the control of bone turnover, the OC is an immune cell that produces and senses inflammatory cytokines, ingests microorganisms and presents antigens. Today, increasing evidence shows that several pathogens use OC as a host cell to grow, generating debilitating bone defects. In this review, we exhaustively inventory the bacteria and viruses that infect OC and report the present knowledge in this topic. We point out that most of the microorganisms enhance the bone resorption activity of OC. We notice that pathogen interactions with the OC require further investigation, in particular to validate the OC as a host cell in vivo and to identify the cellular mechanisms involved in altered bone resorption. Thus, we conclude that the OC is a new cell target for pathogens; this new research area paves the way for new therapeutic strategies in the infections causing bone defects.

Tuberculosis Exacerbates HIV-1 Infection through IL-10/STAT3-Dependent Tunneling Nanotube Formation in Macrophages.

The tuberculosis (TB) bacillus, Mycobacterium tuberculosis (Mtb), and HIV-1 act synergistically; however, the mechanisms by which Mtb exacerbates HIV-1 pathogenesis are not well known. Using in vitro and ex vivo cell culture systems, we show that human M(IL-10) anti-inflammatory macrophages, present in TB-associated microenvironment, produce high levels of HIV-1. In vivo, M(IL-10) macrophages are expanded in lungs of co-infected non-human primates, which correlates with disease severity. Furthermore, HIV-1/Mtb co-infected patients display an accumulation of M(IL-10) macrophage markers (soluble CD163 and MerTK). These M(IL-10) macrophages form direct cell-to-cell bridges, which we identified as tunneling nanotubes (TNTs) involved in viral transfer. TNT formation requires the IL-10/STAT3 signaling pathway, and targeted inhibition of TNTs substantially reduces the enhancement of HIV-1 cell-to-cell transfer and overproduction in M(IL-10) macrophages. Our study reveals that TNTs facilitate viral transfer and amplification, thereby promoting TNT formation as a mechanism to be explored in TB/AIDS potential therapeutics.

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.

HIV-1 reprograms the migration of macrophages.

Macrophages are motile leukocytes, targeted by HIV-1, thought to play a critical role in host dissemination of the virus. However, whether infection impacts their migration capacity remains unknown. We show that 2-dimensional migration and the 3-dimensional (3D) amoeboid migration mode of HIV-1-infected human monocyte-derived macrophages were inhibited, whereas the 3D mesenchymal migration was enhanced. The viral protein Nef was necessary and sufficient for all HIV-1-mediated effects on migration. In Nef transgenic mice, tissue infiltration of macrophages was increased in a tumor model and in several tissues at steady state, suggesting a dominant role for mesenchymal migration in vivo. The mesenchymal motility involves matrix proteolysis and podosomes, cell structures constitutive of monocyte-derived cells. Focusing on the mechanisms used by HIV-1 Nef to control the mesenchymal migration, we show that the stability, size, and proteolytic function of podosomes are increased via the phagocyte-specific kinase Hck and Wiskott-Aldrich syndrome protein (WASP), 2 major regulators of podosomes. In conclusion, HIV-1 reprograms macrophage migration, which likely explains macrophage accumulation in several patient tissues, which is a key step for virus spreading and pathogenesis. Moreover, Nef points out podosomes and the Hck/WASP signaling pathway as good candidates to control tissue infiltration of macrophages, a detrimental phenomenon in several diseases.

γ-Tubulin Ring Complexes and EB1 play antagonistic roles in microtubule dynamics and spindle positioning.

γ-Tubulin is critical for microtubule (MT) assembly and organization. In metazoa, this protein acts in multiprotein complexes called γ-Tubulin Ring Complexes (γ-TuRCs). While the subunits that constitute γ-Tubulin Small Complexes (γ-TuSCs), the core of the MT nucleation machinery, are essential, mutation of γ-TuRC-specific proteins in Drosophila causes sterility and morphological abnormalities via hitherto unidentified mechanisms. Here, we demonstrate a role of γ-TuRCs in controlling spindle orientation independent of MT nucleation activity, both in cultured cells and in vivo, and examine a potential function for γ-TuRCs on astral MTs. γ-TuRCs locate along the length of astral MTs, and depletion of γ-TuRC-specific proteins increases MT dynamics and causes the plus-end tracking protein EB1 to redistribute along MTs. Moreover, suppression of MT dynamics through drug treatment or EB1 down-regulation rescues spindle orientation defects induced by γ-TuRC depletion. Therefore, we propose a role for γ-TuRCs in regulating spindle positioning by controlling the stability of astral MTs.

Establishment and mitotic characterization of new Drosophila acentriolar cell lines from DSas-4 mutant.

In animal cells the centrosome is commonly viewed as the main cellular structure driving microtubule (MT) assembly into the mitotic spindle apparatus. However, additional pathways, such as those mediated by chromatin and augmin, are involved in the establishment of functional spindles. The molecular mechanisms involved in these pathways remain poorly understood, mostly due to limitations inherent to current experimental systems available. To overcome these limitations we have developed six new Drosophila cell lines derived from Drosophila homozygous mutants for DSas-4, a protein essential for centriole biogenesis. These cells lack detectable centrosomal structures, astral MT, with dispersed pericentriolar proteins D-PLP, Centrosomin and γ-tubulin. They show poorly focused spindle poles that reach the plasma membrane. Despite being compromised for functional centrosome, these cells could successfully undergo mitosis. Live-cell imaging analysis of acentriolar spindle assembly revealed that nascent MTs are nucleated from multiple points in the vicinity of chromosomes. These nascent MTs then grow away from kinetochores allowing the expansion of fibers that will be part of the future acentriolar spindle. MT repolymerization assays illustrate that acentriolar spindle assembly occurs "inside-out" from the chromosomes. Colchicine-mediated depolymerization of MTs further revealed the presence of a functional Spindle Assembly Checkpoint (SAC) in the acentriolar cells. Finally, pilot RNAi experiments open the potential use of these cell lines for the molecular dissection of anastral pathways in spindle and centrosome assembly.

Crystal structure of γ-tubulin complex protein GCP4 provides insight into microtubule nucleation.

Microtubule nucleation in all eukaryotes involves γ-tubulin small complexes (γTuSCs) that comprise two molecules of γ-tubulin bound to γ-tubulin complex proteins (GCPs) GCP2 and GCP3. In many eukaryotes, multiple γTuSCs associate with GCP4, GCP5 and GCP6 into large γ-tubulin ring complexes (γTuRCs). Recent cryo-EM studies indicate that a scaffold similar to γTuRCs is formed by lateral association of γTuSCs, with the C-terminal regions of GCP2 and GCP3 binding γ-tubulin molecules. However, the exact role of GCPs in microtubule nucleation remains unknown. Here we report the crystal structure of human GCP4 and show that its C-terminal domain binds directly to γ-tubulin. The human GCP4 structure is the prototype for all GCPs, as it can be precisely positioned within the γTuSC envelope, revealing the nature of protein-protein interactions and conformational changes regulating nucleation activity.

{gamma}-Tubulin ring complexes regulate microtubule plus end dynamics.

gamma-Tubulin is critical for the initiation and regulation of microtubule (MT) assembly. In Drosophila melanogaster, it acts within two main complexes: the gamma-tubulin small complex (gamma-TuSC) and the gamma-tubulin ring complex (gamma-TuRC). Proteins specific of the gamma-TuRC, although nonessential for viability, are required for efficient mitotic progression. Until now, their role during interphase remained poorly understood. Using RNA interference in Drosophila S2 cells, we show that the gamma-TuRC is not critical for overall MT organization. However, depletion of any component of this complex results in an increase of MT dynamics. Combined immunofluorescence and live imaging analysis allows us to reveal that the gamma-TuRC localizes along interphase MTs and that distal gamma-tubulin spots match with sites of pause or rescue events. We propose that, in addition to its role in nucleation, the gamma-TuRC associated to MTs may regulate their dynamics by limiting catastrophes.

Spindle assembly defects leading to the formation of a monopolar mitotic apparatus.

Mitotic spindle formation in animal cells involves microtubule nucleation from two centrosomes that are positioned at opposite sides of the nucleus. Microtubules are captured by the kinetochores and stabilized. In addition, microtubules can be nucleated independently of the centrosome and stabilized by a gradient of Ran-GTP, surrounding the mitotic chromatin. Complex regulation ensures the formation of a bipolar apparatus, involving motor proteins and controlled polymerization and depolymerization of microtubule ends. The bipolar apparatus is, in turn, responsible for faithful chromosome segregation. During recent years, a variety of experiments has indicated that defects in specific motor proteins, centrosome proteins, kinases and other proteins can induce the assembly of aberrant spindles with a monopolar morphology or with poorly separated poles. Induction of monopolar spindles may be a useful strategy for cancer therapy, since ensuing aberrant mitotic exit will usually lead to cell death. In this review, we will discuss the various underlying molecular mechanisms that may be responsible for monopolar spindle formation.

Gamma-tubulin complexes and microtubule organization.

Microtubule nucleation requires gamma-tubulin, which exists in two main protein complexes: the gamma-tubulin small complex, and the gamma-tubulin ring complex. During mitosis, these complexes accumulate at the centrosome to support spindle formation. Gamma-tubulin complexes are also present at non-centrosomal microtubule nucleation sites, both in interphase and in mitosis. In interphase, non-centrosomal nucleation enables the formation of microtubule bundles or networks of branched microtubules. Gamma-tubulin complexes may be involved not only in microtubule nucleation, but also in regulating microtubule dynamics. Recent findings indicate that the dynamics of microtubule plus-ends are altered, depending on the expression of gamma-tubulin complex proteins.

Drosophila melanogaster gamma-TuRC is dispensable for targeting gamma-tubulin to the centrosome and microtubule nucleation.

In metazoans, gamma-tubulin acts within two main complexes, gamma-tubulin small complexes (gamma-TuSCs) and gamma-tubulin ring complexes (gamma-TuRCs). In higher eukaryotes, it is assumed that microtubule nucleation at the centrosome depends on gamma-TuRCs, but the role of gamma-TuRC components remains undefined. For the first time, we analyzed the function of all four gamma-TuRC-specific subunits in Drosophila melanogaster: Dgrip75, Dgrip128, Dgrip163, and Dgp71WD. Grip-motif proteins, but not Dgp71WD, appear to be required for gamma-TuRC assembly. Individual depletion of gamma-TuRC components, in cultured cells and in vivo, induces mitotic delay and abnormal spindles. Surprisingly, gamma-TuSCs are recruited to the centrosomes. These defects are less severe than those resulting from the inhibition of gamma-TuSC components and do not appear critical for viability. Simultaneous cosilencing of all gamma-TuRC proteins leads to stronger phenotypes and partial recruitment of gamma-TuSC. In conclusion, gamma-TuRCs are required for assembly of fully functional spindles, but we suggest that gamma-TuSC could be targeted to the centrosomes, which is where basic microtubule assembly activities are maintained.