Loss of ninein interferes with osteoclast formation and causes premature ossification.

Ninein is a centrosome protein that has been implicated in microtubule anchorage and centrosome cohesion. Mutations in the human NINEIN gene have been linked to Seckel syndrome and to a rare form of skeletal dysplasia. However, the role of ninein in skeletal development remains unknown. Here, we describe a ninein knockout mouse with advanced endochondral ossification during embryonic development. Although the long bones maintain a regular size, the absence of ninein delays the formation of the bone marrow cavity in the prenatal tibia. Likewise, intramembranous ossification in the skull is more developed, leading to a premature closure of the interfrontal suture. We demonstrate that ninein is strongly expressed in osteoclasts of control mice, and that its absence reduces the fusion of precursor cells into syncytial osteoclasts, whereas the number of osteoblasts remains unaffected. As a consequence, ninein-deficient osteoclasts have a reduced capacity to resorb bone. At the cellular level, the absence of ninein interferes with centrosomal microtubule organization, reduces centrosome cohesion, and provokes the loss of centrosome clustering in multinucleated mature osteoclasts. We propose that centrosomal ninein is important for osteoclast fusion, to enable a functional balance between bone-forming osteoblasts and bone-resorbing osteoclasts during skeletal development.

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.

Tunneling nanotubes and tumor microtubes-Emerging data on their roles in intercellular communication and pathophysiology: Summary of an International FASEB Catalyst Conference October 2023.

In the past decade, there has been a steady rise in interest in studying novel cellular extensions and their potential roles in facilitating human diseases, including neurologic diseases, viral infectious diseases, cancer, and others. One of the exciting new aspects of this field is improved characterization and understanding of the functions and potential mechanisms of tunneling nanotubes (TNTs), which are actin-based filamentous protrusions that are structurally distinct from filopodia. TNTs form and connect cells at long distance and serve as direct conduits for intercellular communication in a wide range of cell types in vitro and in vivo. More researchers are entering this field and investigating the role of TNTs in mediating cancer cell invasion and drug resistance, cellular transfer of proteins, RNA or organelles, and intercellular spread of infectious agents, such as viruses, bacteria, and prions. Even further, the elucidation of highly functional membrane tubes called "tumor microtubes" (TMs) in incurable gliomas has further paved a new path for understanding how and why the tumor type is highly invasive at the cellular level and also resistant to standard therapies. Due to the wide-ranging and rapidly growing applicability of TNTs and TMs in pathophysiology across the spectrum of biology, it has become vital to bring researchers in the field together to discuss advances and the future of research in this important niche of protrusion biology.

The chemokine receptor CCR5: multi-faceted hook for HIV-1.

Chemokines are cytokines whose primary role is cellular activation and stimulation of leukocyte migration. They perform their various functions by interacting with G protein-coupled cell surface receptors (GPCRs) and are involved in the regulation of many biological processes such as apoptosis, proliferation, angiogenesis, hematopoiesis or organogenesis. They contribute to the maintenance of the homeostasis of lymphocytes and coordinate the function of the immune system. However, chemokines and their receptors are sometimes hijacked by some pathogens to infect the host organism. For a given chemokine receptor, there is a wide structural, organizational and conformational diversity. In this review, we describe the evidence for structural variety reported for the chemokine receptor CCR5, how this variability can be exploited by HIV-1 to infect its target cells and what therapeutic solutions are currently being developed to overcome this problem.

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.

Dysregulation of the IFN-I signaling pathway by Mycobacterium tuberculosis leads to exacerbation of HIV-1 infection of macrophages.

While tuberculosis (TB) is a risk factor in HIV-1-infected individuals, the mechanisms by which Mycobacterium tuberculosis (Mtb), the agent of TB in humans, worsens HIV-1 pathogenesis still need to be fully elucidated. Recently, we showed that HIV-1 infection and spread are exacerbated in macrophages exposed to TB-associated microenvironments. Transcriptomic analysis of macrophages conditioned with medium of Mtb-infected human macrophages (cmMTB) revealed an up-regulation of the typeI interferon (IFN-I) pathway, characterized by the overexpression of IFN-inducible genes. Historically, IFN-I are well known for their antiviral functions, but our previous work showed that this is not the case in the context of coinfection with HIV-1. Here, we show that the IFN-I response signature in cmMTB-treated macrophages matches the one observed in the blood of active TB patients, and depends on the timing of incubation with cmMTB. This suggests that the timing of macrophage's exposure to IFN-I can impact their capacity to control HIV-1 infection. Strikingly, we found that cmMTB-treated macrophages are hyporesponsive to extrastimulation with exogenous IFN-I, used to mimic HIV-1 infection. Yet, depleting STAT1 by gene silencing to block the IFN-I signaling pathway reduced TB-induced exacerbation of HIV-1 infection. Altogether, by aiming to understand why TB-derived IFN-I preexposure of macrophages did not induce antiviral immunity against HIV-1, we demonstrated that these cells are hyporesponsive to exogenous IFN-I, a phenomenon that prevents macrophage activation against HIV-1.

Mycobacterium tuberculosis induces hyporesponsiveness of the IFN-I signaling pathway in macrophages, leading to the exacerbation of HIV-1 replication.

Mechanisms of HIV-1 cell-to-cell transfer to myeloid cells.

In addition to CD4+ T lymphocytes, cells of the myeloid lineage such as macrophages, dendritic cells (DCs), and osteoclasts (OCs) are emerging as important target cells for HIV-1, as they likely participate in all steps of pathogenesis, including sexual transmission and early virus dissemination in both lymphoid and nonlymphoid tissues where they can constitute persistent virus reservoirs. At least in vitro, these myeloid cells are poorly infected by cell-free viral particles. In contrast, intercellular virus transmission through direct cell-to-cell contacts may be a predominant mode of virus propagation in vivo leading to productive infection of these myeloid target cells. HIV-1 cell-to-cell transfer between CD4+ T cells mainly through the formation of the virologic synapse, or from infected macrophages or dendritic cells to CD4+ T cell targets, have been extensively described in vitro. Recent reports demonstrate that myeloid cells can be also productively infected through virus homotypic or heterotypic cell-to-cell transfer between macrophages or from virus-donor-infected CD4+ T cells, respectively. These modes of infection of myeloid target cells lead to very efficient spreading in these poorly susceptible cell types. Thus, the goal of this review is to give an overview of the different mechanisms reported in the literature for cell-to-cell transfer and spreading of HIV-1 in myeloid cells.

Modulation of Cystatin C in Human Macrophages Improves Anti-Mycobacterial Immune Responses to Mycobacterium tuberculosis Infection and Coinfection With HIV.

Tuberculosis owes its resurgence as a major global health threat mostly to the emergence of drug resistance and coinfection with HIV. The synergy between HIV and Mycobacterium tuberculosis (Mtb) modifies the host immune environment to enhance both viral and bacterial replication and spread. In the lung immune context, both pathogens infect macrophages, establishing favorable intracellular niches. Both manipulate the endocytic pathway in order to avoid destruction. Relevant players of the endocytic pathway to control pathogens include endolysosomal proteases, cathepsins, and their natural inhibitors, cystatins. Here, a mapping of the human macrophage transcriptome for type I and II cystatins during Mtb, HIV, or Mtb-HIV infection displayed different profiles of gene expression, revealing cystatin C as a potential target to control mycobacterial infection as well as HIV coinfection. We found that cystatin C silencing in macrophages significantly improves the intracellular killing of Mtb, which was concomitant with an increased general proteolytic activity of cathepsins. In addition, downmodulation of cystatin C led to an improved expression of the human leukocyte antigen (HLA) class II in macrophages and an increased CD4+ T-lymphocyte proliferation along with enhanced IFN-γ secretion. Overall, our results suggest that the targeting of cystatin C in human macrophages represents a promising approach to improve the control of mycobacterial infections including multidrug-resistant (MDR) TB.

A Pulmonary Lactobacillus murinus Strain Induces Th17 and RORγt+ Regulatory T Cells and Reduces Lung Inflammation in Tuberculosis.

The lungs harbor multiple resident microbial communities, otherwise known as the microbiota. There is an emerging interest in deciphering whether the pulmonary microbiota modulate local immunity, and whether this knowledge could shed light on mechanisms operating in the response to respiratory pathogens. In this study, we investigate the capacity of a pulmonary Lactobacillus strain to modulate the lung T cell compartment and assess its prophylactic potential upon infection with Mycobacterium tuberculosis, the etiological agent of tuberculosis. In naive mice, we report that a Lactobacillus murinus (Lagilactobacillus murinus) strain (CNCM I-5314) increases the presence of lung Th17 cells and of a regulatory T cell (Treg) subset known as RORγt+ Tregs. In particular, intranasal but not intragastric administration of CNCM I-5314 increases the expansion of these lung leukocytes, suggesting a local rather than systemic effect. Resident Th17 and RORγt+ Tregs display an immunosuppressive phenotype that is accentuated by CNCM I-5314. Despite the well-known ability of M. tuberculosis to modulate lung immunity, the immunomodulatory effect by CNCM I-5314 is dominant, as Th17 and RORγt+ Tregs are still highly increased in the lung at 42-d postinfection. Importantly, CNCM I-5314 administration in M. tuberculosis-infected mice results in reduction of pulmonary inflammation, without increasing M. tuberculosis burden. Collectively, our findings provide evidence for an immunomodulatory capacity of CNCM I-5314 at steady state and in a model of chronic inflammation in which it can display a protective role, suggesting that L. murinus strains found in the lung may shape local T cells in mice and, perhaps, in humans.

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.

Dissemination of Mycobacterium tuberculosis is associated to a SIGLEC1 null variant that limits antigen exchange via trafficking extracellular vesicles.

The identification of individuals with null alleles enables studying how the loss of gene function affects infection. We previously described a non-functional variant in SIGLEC1, which encodes the myeloid-cell receptor Siglec-1/CD169 implicated in HIV-1 cell-to-cell transmission. Here we report a significant association between the SIGLEC1 null variant and extrapulmonary dissemination of Mycobacterium tuberculosis (Mtb) in two clinical cohorts comprising 6,256 individuals. Local spread of bacteria within the lung is apparent in Mtb-infected Siglec-1 knockout mice which, despite having similar bacterial load, developed more extensive lesions compared to wild type mice. We find that Siglec-1 is necessary to induce antigen presentation through extracellular vesicle uptake. We postulate that lack of Siglec-1 delays the onset of protective immunity against Mtb by limiting antigen exchange via extracellular vesicles, allowing for an early local spread of mycobacteria that increases the risk for extrapulmonary dissemination.

Host-Derived Lipids from Tuberculous Pleurisy Impair Macrophage Microbicidal-Associated Metabolic Activity.

Mycobacterium tuberculosis (Mtb) regulates the macrophage metabolic state to thrive in the host, yet the responsible mechanisms remain elusive. Macrophage activation toward the microbicidal (M1) program depends on the HIF-1α-mediated metabolic shift from oxidative phosphorylation (OXPHOS) toward glycolysis. Here, we ask whether a tuberculosis (TB) microenvironment changes the M1 macrophage metabolic state. We expose M1 macrophages to the acellular fraction of tuberculous pleural effusions (TB-PEs) and find lower glycolytic activity, accompanied by elevated levels of OXPHOS and bacillary load, compared to controls. The eicosanoid fraction of TB-PE drives these metabolic alterations. HIF-1α stabilization reverts the effect of TB-PE by restoring M1 metabolism. Furthermore, Mtb-infected mice with stabilized HIF-1α display lower bacillary loads and a pronounced M1-like metabolic profile in alveolar macrophages (AMs). Collectively, we demonstrate that lipids from a TB-associated microenvironment alter the M1 macrophage metabolic reprogramming by hampering HIF-1α functions, thereby impairing control of Mtb infection.

Fatty acid oxidation of alternatively activated macrophages prevents foam cell formation, but Mycobacterium tuberculosis counteracts this process via HIF-1α activation.

The ability of Mycobacterium tuberculosis (Mtb) to persist inside host cells relies on metabolic adaptation, like the accumulation of lipid bodies (LBs) in the so-called foamy macrophages (FM), which are favorable to Mtb. The activation state of macrophages is tightly associated to different metabolic pathways, such as lipid metabolism, but whether differentiation towards FM differs between the macrophage activation profiles remains unclear. Here, we aimed to elucidate whether distinct macrophage activation states exposed to a tuberculosis-associated microenvironment or directly infected with Mtb can form FM. We showed that the triggering of signal transducer and activator of transcription 6 (STAT6) in interleukin (IL)-4-activated human macrophages (M(IL-4)) prevents FM formation induced by pleural effusion from patients with tuberculosis. In these cells, LBs are disrupted by lipolysis, and the released fatty acids enter the ß-oxidation (FAO) pathway fueling the generation of ATP in mitochondria. Accordingly, murine alveolar macrophages, which exhibit a predominant FAO metabolism, are less prone to become FM than bone marrow derived-macrophages. Interestingly, direct infection of M(IL-4) macrophages with Mtb results in the establishment of aerobic glycolytic pathway and FM formation, which could be prevented by FAO activation or inhibition of the hypoxia-inducible factor 1-alpha (HIF-1α)-induced glycolytic pathway. In conclusion, our results demonstrate that Mtb has a remarkable capacity to induce FM formation through the rewiring of metabolic pathways in human macrophages, including the STAT6-driven alternatively activated program. This study provides key insights into macrophage metabolism and pathogen subversion strategies.

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.

Tuberculosis-associated IFN-I induces Siglec-1 on tunneling nanotubes and favors HIV-1 spread in macrophages.

While tuberculosis (TB) is a risk factor in HIV-1-infected individuals, the mechanisms by which Mycobacterium tuberculosis (Mtb) worsens HIV-1 pathogenesis remain scarce. We showed that HIV-1 infection is exacerbated in macrophages exposed to TB-associated microenvironments due to tunneling nanotube (TNT) formation. To identify molecular factors associated with TNT function, we performed a transcriptomic analysis in these macrophages, and revealed the up-regulation of Siglec-1 receptor. Siglec-1 expression depends on Mtb-induced production of type I interferon (IFN-I). In co-infected non-human primates, Siglec-1 is highly expressed by alveolar macrophages, whose abundance correlates with pathology and activation of IFN-I/STAT1 pathway. Siglec-1 localizes mainly on microtubule-containing TNT that are long and carry HIV-1 cargo. Siglec-1 depletion decreases TNT length, diminishes HIV-1 capture and cell-to-cell transfer, and abrogates the exacerbation of HIV-1 infection induced by Mtb. Altogether, we uncover a deleterious role for Siglec-1 in TB-HIV-1 co-infection and open new avenues to understand TNT biology.