An essential role for the association of CD47 to SHPS-1 in skeletal remodeling

Exploring the impact of osteoprotegerin on osteoclast and precursor fusion: Mechanisms and modulation by ATP

Osteoclast (OC) differentiation, vital for bone resorption, depends on osteoclast and precursor fusion. Osteoprotegerin (OPG) inhibits osteoclast differentiation. OPG's influence on fusion and mechanisms is unclear. Osteoclasts and precursors were treated with OPG alone or with ATP. OPG significantly reduced OC number, area and motility and ATP mitigated OPG's inhibition. However, OPG hardly affected the motility of precusors. OPG downregulated fusion-related molecules (CD44, CD47, DC-STAMP, ATP6V0D2) in osteoclasts, reducing only CD47 in precursors. OPG reduced Connexin43 phosphorylated forms (P1 and P2) in osteoclasts, affecting only P2 in precursors. OPG disrupted subcellular localization of CD44, CD47, DC-STAMP, ATP6V0D2, and Connexin43 in both cell types. Findings underscore OPG's multifaceted impact, inhibiting multinucleated osteoclast and mononuclear precursor fusion through distinct molecular mechanisms. Notably, ATP mitigates OPG's inhibitory effect, suggesting a potential regulatory role for the ATP signaling pathway. This study enhances understanding of intricate processes in osteoclast differentiation and fusion, offering insights into potential therapeutic targets for abnormal bone metabolism.

Progress in cancer research on the regulator of phagocytosis CD47, which determines the fate of tumor cells (Review)

Cluster of differentiation 47 (CD47) is a transmembrane protein that is widely and moderately expressed on the surface of various cells and can have an essential role in mediating cell proliferation, migration, phagocytosis, apoptosis, immune homeostasis and other related responses by binding to its ligands, integrins, thrombospondin-1 and signal regulatory protein α. The poor prognosis of cancer patients is closely associated with high expression of CD47 in glioblastoma, ovarian cancer, breast cancer, bladder cancer, colon cancer and hepatocellular carcinoma. Upregulation of CD47 expression facilitates the growth of numerous types of tumor cells, while downregulation of its expression promotes phagocytosis of tumor cells by macrophages, thereby limiting tumor growth. In addition, blocking CD47 activates the cyclic GMP-AMP (cGAMP) synthase/cGAMP/interferon gene stimulating factor signaling pathway and initiates an adaptive immune response that kills tumor cells. The present review describes the structure, function and interactions of CD47 with its ligands, as well as its regulation of phagocytosis and tumor cell fate. It summarizes the therapeutics, mechanisms of action, research advances and challenges of targeting CD47. In addition, this paper provides an overview of the latest therapeutic options for targeting CD47, such as chimeric antigen receptor (CAR) T-cells, CAR macrophages and nanotechnology-based delivery systems, which are essential for future clinical research on targeting CD47.

CD47 is Required for Mesenchymal Progenitor Proliferation and Fracture Repair

CD47 is a ubiquitous and pleiotropic cell-surface receptor. Disrupting CD47 enhances injury repair in various tissues but the role of CD47 has not been studied in bone injuries. In a murine closed-fracture model, CD47-null mice showed decreased callus bone volume, bone mineral content, and tissue mineral content as assessed by microcomputed tomography 10 days post-fracture, and increased fibrous volume as determined by histology. To understand the cellular basis for this phenotype, mesenchymal progenitors (MSC) were harvested from bone marrow. CD47-null MSC showed decreased large fibroblast colony formation (CFU-F), significantly less proliferation, and fewer cells in S-phase, although osteoblast differentiation was unaffected. However, consistent with prior research, CD47-null endothelial cells showed increased proliferation relative to WT cells. Similarly, in a murine ischemic fracture model, CD47-null mice showed reduced fracture callus bone volume and bone mineral content relative to WT. Consistent with our In vitro results, in vivo EdU labeling showed decreased cell proliferation in the callus of CD47-null mice, while staining for CD31 and endomucin demonstrated increased endothelial cell mass. Finally, WT mice administered a CD47 morpholino, which blocks CD47 protein production, showed a callus phenotype similar to that of non-ischemic and ischemic fractures in CD47-null mice, suggesting the phenotype was not due to developmental changes in the knockout mice. Thus, inhibition of CD47 during bone healing reduces both non-ischemic and ischemic fracture healing, in part, by decreasing MSC proliferation. Furthermore, the increase in endothelial cell proliferation and early blood vessel density caused by CD47 disruption is not sufficient to overcome MSC dysfunction.

CD47 is Required for Mesenchymal Progenitor Proliferation and Fracture Repair

CD47 is a ubiquitous and pleiotropic cell-surface receptor. Disrupting CD47 enhances injury repair in various tissues but the role of CD47 has not been studied in bone injuries. In a murine closed-fracture model, CD47-null mice showed decreased callus bone volume, bone mineral content, and tissue mineral content as assessed by microcomputed tomography 10 days post-fracture, and increased fibrous volume as determined by histology. To understand the cellular basis for this phenotype, mesenchymal progenitors (MSC) were harvested from bone marrow. CD47-null MSC showed decreased large fibroblast colony formation (CFU-F), significantly less proliferation, and fewer cells in S-phase, although osteoblast differentiation was unaffected. However, consistent with prior research, CD47-null endothelial cells showed increased proliferation relative to WT cells. Similarly, in a murine ischemic fracture model, CD47-null mice showed reduced fracture callus bone volume and bone mineral content relative to WT. Consistent with our in vitro results, in vivo EdU labeling showed decreased cell proliferation in the callus of CD47-null mice, while staining for CD31 and endomucin demonstrated increased endothelial cell mass. Finally, WT mice administered a CD47 morpholino, which blocks CD47 protein production, showed a callus phenotype similar to that of non-ischemic and ischemic fractures in CD47-null mice, suggesting the phenotype was not due to developmental changes in the knockout mice. Thus, inhibition of CD47 during bone healing reduces both non-ischemic and ischemic fracture healing, in part, by decreasing MSC proliferation. Furthermore, the increase in endothelial cell proliferation and early blood vessel density caused by CD47 disruption is not sufficient to overcome MSC dysfunction.

Thrombospondins modulate cell function and tissue structure in the skeleton

Thrombospondins (TSPs) belong to a functional class of ECM proteins called matricellular proteins that are not primarily structural, but instead influence cellular interactions within the local extracellular environment. The 3D arrangement of TSPs allow interactions with other ECM proteins, sequestered growth factors, and cell surface receptors. They are expressed in mesenchymal condensations and limb buds during skeletal development, but they are not required for patterning. Instead, when absent, there are alterations in musculoskeletal connective tissue ECM structure, organization, and function, as well as altered skeletal cell phenotypes. Both functional redundancies and unique contributions to musculoskeletal tissue structure and physiology are revealed in mouse models with compound TSP deletions. Crucial roles of individual TSPs are revealed during musculoskeletal injury and regeneration. The interaction of TSPs with mesenchymal stem cells (MSC), and their influence on cell fate, function, and ultimately, musculoskeletal phenotype, suggest that TSPs play integral, but as yet poorly understood roles in musculoskeletal health. Here, unique and overlapping contributions of trimeric TSP1/2 and pentameric TSP3/4/5 to musculoskeletal cell and matrix physiology are reviewed. Opportunities for new research are also noted.

The origins and formation of bone-resorbing osteoclasts

Osteoclasts (OCLs) are hematopoietic cells whose physiological function is to degrade bone. OCLs are key players in the processes that determine and maintain the mass, shape, and physical properties of bone. OCLs adhere to bone tightly and degrade its matrix by secreting protons and proteases onto the underlying surface. The combination of low pH and proteases degrades the mineral and protein components of the matrix and forms a resorption pit; the degraded material is internalized by the cell and then secreted into the circulation. Insufficient or excessive activity of OCLs can lead to significant changes in bone and either cause or exacerbate symptoms of diseases, as in osteoporosis, osteopetrosis, and cancer-induced bone lysis. OCLs are derived from monocyte-macrophage precursor cells whose origins are in two distinct embryonic cell lineages - erythromyeloid progenitor cells of the yolk sac, and hematopoietic stem cells. OCLs are formed in a multi-stage process that is induced by the cytokines M-CSF and RANKL, during which the cells differentiate, fuse to form multi-nucleated cells, and then differentiate further to become mature, bone-resorbing OCLs. Recent studies indicate that OCLs can undergo fission in vivo to generate smaller cells, called "osteomorphs", that can be "re-cycled" by fusing with other cells to form new OCLs. In this review we describe OCLs and discuss their cellular origins and the cellular and molecular events that drive osteoclastogenesis.

Role of CD47-SIRPα Checkpoint in Nanomedicine-Based Anti-Cancer Treatment

Many cancers have evolved various mechanisms to evade immunological surveillance, such as the inhibitory immune checkpoint of the CD47-SIRPα signaling pathway. By targeting this signaling pathway, researchers have developed diverse nanovehicles with different loaded drugs and modifications in anticancer treatment. In this review, we present a brief overview of CD47-SIRPα interaction and nanomedicine. Then, we delve into recent applications of the CD47-SIRPα interaction as a target for nanomedicine-based antitumor treatment and its combination with other targeting pathway drugs and/or therapeutic approaches.

Protein tyrosine phosphatases in skeletal development and diseases

Skeletal development and homeostasis in mammals are modulated by finely coordinated processes of migration, proliferation, differentiation, and death of skeletogenic cells originating from the mesoderm and neural crest. Numerous molecular mechanisms are involved in these regulatory processes, one of which is protein posttranslational modifications, particularly protein tyrosine phosphorylation (PYP). PYP occurs mainly through the action of protein tyrosine kinases (PTKs), modifying protein enzymatic activity, changing its cellular localization, and aiding in the assembly or disassembly of protein signaling complexes. Under physiological conditions, PYP is balanced by the coordinated action of PTKs and protein tyrosine phosphatases (PTPs). Dysregulation of PYP can cause genetic, metabolic, developmental, and oncogenic skeletal diseases. Although PYP is a reversible biochemical process, in contrast to PTKs, little is known about how this equilibrium is modulated by PTPs in the skeletal system. Whole-genome sequencing has revealed a large and diverse superfamily of PTP genes (over 100 members) in humans, which can be further divided into cysteine (Cys)-, aspartic acid (Asp)-, and histidine (His)-based PTPs. Here, we review current knowledge about the functions and regulatory mechanisms of 28 PTPs involved in skeletal development and diseases; 27 of them belong to class I and II Cys-based PTPs, and the other is an Asp-based PTP. Recent progress in analyzing animal models that harbor various mutations in these PTPs and future research directions are also discussed. Our literature review indicates that PTPs are as crucial as PTKs in supporting skeletal development and homeostasis.

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.

Putting the brakes on phagocytosis: "don't-eat-me" signaling in physiology and disease

Timely removal of dying or pathogenic cells by phagocytes is essential to maintaining host homeostasis. Phagocytes execute the clearance process with high fidelity while sparing healthy neighboring cells, and this process is at least partially regulated by the balance of "eat-me" and "don't-eat-me" signals expressed on the surface of host cells. Upon contact, eat-me signals activate "pro-phagocytic" receptors expressed on the phagocyte membrane and signal to promote phagocytosis. Conversely, don't-eat-me signals engage "anti-phagocytic" receptors to suppress phagocytosis. We review the current knowledge of don't-eat-me signaling in normal physiology and disease contexts where aberrant don't-eat-me signaling contributes to pathology.

CD47 as a Potential Target to Therapy for Infectious Diseases

The integrin associated protein (CD47) is a widely and moderately expressed glycoprotein in all healthy cells. Cancer cells are known to induce increased CD47 expression. Similar to cancer cells, all immune cells can upregulate their CD47 surface expression during infection. The CD47-SIRPa interaction induces an inhibitory effect on macrophages and dendritic cells (dendritic cells) while CD47-thrombospondin-signaling inhibits T cells. Therefore, the disruption of the CD47 interaction can mediate several biologic functions. Upon the blockade and knockout of CD47 reveals an immunosuppressive effect of CD47 during LCMV, influenza virus, HIV-1, mycobacterium tuberculosis, plasmodium and other bacterial pneumonia infections. In our recent study we shows that the blockade of CD47 using the anti-CD47 antibody increases the activation and effector function of macrophages, dendritic cells and T cells during viral infection. By enhancing both innate and adaptive immunity, CD47 blocking antibody promotes antiviral effect. Due to its broad mode of action, the immune-stimulatory effect derived from this antibody could be applicable in nonresolving and (re)emerging infections. The anti-CD47 antibody is currently under clinical trial for the treatment of cancer and could also have amenable therapeutic potential against infectious diseases. This review highlights the immunotherapeutic targeted role of CD47 in the infectious disease realm.

Learning from Monocyte-Macrophage Fusion and Multinucleation: Potential Therapeutic Targets for Osteoporosis and Rheumatoid Arthritis

Excessive bone resorption by osteoclasts (OCs) covers an essential role in developing bone diseases, such as osteoporosis (OP) and rheumatoid arthritis (RA). Monocytes or macrophages fusion and multinucleation (M-FM) are key processes for generating multinucleated mature cells with essential roles in bone remodelling. Depending on the phenotypic heterogeneity of monocyte/macrophage precursors and the extracellular milieu, two distinct morphological and functional cell types can arise mature OCs and giant cells (GCs). Despite their biological relevance in several physiological and pathological responses, many gaps exist in our understanding of their formation and role in bone, including the molecular determinants of cell fusion and multinucleation. Here, we outline fusogenic molecules during M-FM involved in OCs and GCs formation in healthy conditions and during OP and RA. Moreover, we discuss the impact of the inflammatory milieu on modulating macrophages phenotype and their differentiation towards mature cells. Methodological approach envisaged searches on Scopus, Web of Science Core Collection, and EMBASE databases to select relevant studies on M-FM, osteoclastogenesis, inflammation, OP, and RA. This review intends to give a state-of-the-art description of mechanisms beyond osteoclastogenesis and M-FM, with a focus on OP and RA, and to highlight potential biological therapeutic targets to prevent extreme bone loss.

Osteoclast Multinucleation: Review of Current Literature

Multinucleation is a hallmark of osteoclast maturation. The unique and dynamic multinucleation process not only increases cell size but causes functional alterations through reconstruction of the cytoskeleton, creating the actin ring and ruffled border that enable bone resorption. Our understanding of the molecular mechanisms underlying osteoclast multinucleation has advanced considerably in this century, especially since the identification of DC-STAMP and OC-STAMP as “master fusogens”. Regarding the molecules and pathways surrounding these STAMPs, however, only limited progress has been made due to the absence of their ligands. Various molecules and mechanisms other than the STAMPs are involved in osteoclast multinucleation. In addition, several preclinical studies have explored chemicals that may be able to target osteoclast multinucleation, which could enable us to control pathogenic bone metabolism more precisely. In this review, we will focus on recent discoveries regarding the STAMPs and other molecules involved in osteoclast multinucleation.

Thrombospondin-1 in maladaptive aging responses: a concept whose time has come

Numerous age-dependent alterations at the molecular, cellular, tissue and organ systems levels underlie the pathophysiology of aging. Herein, the focus is upon the secreted protein thrombospondin-1 (TSP1) as a promoter of aging and age-related diseases. TSP1 has several physiological functions in youth, including promoting neural synapse formation, mediating responses to ischemic and genotoxic stress, minimizing hemorrhage, limiting angiogenesis, and supporting wound healing. These acute functions of TSP1 generally require only transient expression of the protein. However, accumulating basic and clinical data reinforce the view that chronic diseases of aging are associated with accumulation of TSP1 in the extracellular matrix, which is a significant maladaptive contributor to the aging process. Identification of the relevant cell types that chronically produce and respond to TSP1 and the molecular mechanisms that mediate the resulting maladaptive responses could direct the development of therapeutic agents to delay or revert age-associated maladies.

Ageing characteristics of bone indicated by transcriptomic and exosomal proteomic analysis of cortical bone cells

BACKGROUND

Degenerative changes in the skeleton play an important role in ageing. As the foremost sensors and orchestrators of bone remodelling, osteocytes contribute significantly to the health of the skeleton. Embedded in a mineralized bone matrix, the osteocyte network and the surrounding lacunar canaliculae work together as a functional syncytium-the osteocytic lacunar-canalicular system (OLCS). However, changes in the OLCS during ageing and related mechanisms cannot be fully understood by using traditional histological analysis.

METHODS

To link the phenotypes of aged osteocytes and their functional changes during ageing, we analysed the changes in the gene expression profiles of bone cells and the proteomic profiles of OLCS exosomes derived from aged and young cortical bone.

RESULTS

Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis of differentially expressed genes (DEGs) suggested that a decline in cell energy metabolism and an increased level of the proinflammatory state are major characteristics of bone ageing. Moreover, some DEGs were key regulators of bone mechanical sensation and bone remodelling, which are indicative of reduced bone-specific function with age. Further, the identified proteins in OLCS exosomes showed potential changes in the secretory function bone. Compared with young controls, the decreased functional proteins in aged OLCS exosomes were enriched mainly in GO terms that included regulating bone development and remodelling, cell-matrix adhesion, and cell clearance and homeostasis. Notably, several functions of exosomal proteins of the aged group revealed potential new roles, such as regulating innate and adaptive immunity, wound healing, and angiogenesis and eliminating oxidative stress.

CONCLUSION

The information obtained from bone cells and OLCS exosomes will help us discover new features of bone ageing.

An Overview of the Derivation and Function of Multinucleated Giant Cells and Their Role in Pathologic Processes

Monocyte lineage cells play important roles in health and disease. Their differentiation into macrophages is crucial for a broad array of immunologic processes that regulate inflammation, neoplasia, and infection. In certain pathologic conditions, such as foreign body reactions and peripheral inflammatory lesions, monocytes fuse to form large, multinucleated giant cells (MGCs). Currently, our knowledge of the fusion mechanisms of monocytes and the regulation of MGC formation and function in discrete pathologies is limited. Herein, we consider the types and function of MGCs in disease and assess the mechanisms by which monocyte fusion contributes to the formation of MGCs. An improved understanding of the cellular origins and metabolic functions of MGCs will facilitate their identification and ultimately the treatment of diseases and disorders that involve MGCs.

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.

The contribution of cross-talk between the cell-surface proteins CD36 and CD47-TSP-1 in osteoclast formation and function

Antibody-mediated blockade of cluster of differentiation 47 (CD47)-thrombospondin-1 (TSP-1) interactions blocks osteoclast formation in vitro and attenuates parathyroid hormone (PTH)-induced hypercalcemia in vivo in mice. Hypercalcemia in this model reflects increased bone resorption. TSP-1 has two cell-associated binding partners, CD47 and CD36. The roles of these two molecules in mediating the effects of TSP1 in osteoclasts are unclear. Osteoclast formation was attenuated but not absent when preosteoclasts isolated from CD47^-/- mice were cocultured with WT osteoblasts. Suppressing CD36 in osteoclast progenitors also attenuated osteoclast formation. The hypercalcemic response to a PTH infusion was blunted in CD47^-/-/CD36^-/- (double knockout (DKO)) female mice but not CD47^-/- mice or CD36^-/- animals, supporting a role for both CD47 and CD36 in mediating this effect. Consistent with this, DKO osteoclasts had impaired resorptive activity when analyzed in vitro Inhibition of nitric oxide (NO) signaling is known to promote osteoclastogenesis, and TSP-1 suppresses NO production and signaling. An anti-TSP-1 antibody increased NO production in osteoclasts, and the inhibitory effect of anti-TSP-1 on osteoclastogenesis was completely rescued by l-nitroarginine methyl ester (l-NAME), a competitive NO synthase inhibitor. Supportive of an important role for CD36 in mediating the pro-osteoclastogenic effects of TSP-1, engaging CD36 with a synthetic agonist, p907, suppressed NO production in anti-TSP-1-treated cultures, allowing osteoclast maturation to occur. These results establish that CD36 and CD47 both participate in mediating the actions of TSP-1 in osteoclasts and establish a physiologically relevant cross-talk in bone tissue between these two molecules.

Osteoclast Fusion: Time-Lapse Reveals Involvement of CD47 and Syncytin-1 at Different Stages of Nuclearity

Investigations addressing the molecular keys of osteoclast fusion are primarily based on end-point analyses. No matter if investigations are performed in vivo or in vitro the impact of a given factor is predominantly analyzed by counting the number of multi-nucleated cells, the number of nuclei per multinucleated cell or TRAcP activity. But end-point analyses do not show how the fusion came about. This would not be a problem if fusion of osteoclasts was a random process and occurred by the same molecular mechanism from beginning to end. However, we and others have in the recent period published data suggesting that fusion partners may specifically select each other and that heterogeneity between the partners seems to play a role. Therefore, we set out to directly test the hypothesis that fusion factors have a heterogenic involvement at different stages of nuclearity. Therefore, we have analyzed individual fusion events using time-lapse and antagonists of CD47 and syncytin-1. All time-lapse recordings have been studied by two independent observers. A total of 1808 fusion events were analyzed. The present study shows that CD47 and syncytin-1 have different roles in osteoclast fusion depending on the nuclearity of fusion partners. While CD47 promotes cell fusions involving mono-nucleated pre-osteoclasts, syncytin-1 promotes fusion of two multi-nucleated osteoclasts, but also reduces the number of fusions between mono-nucleated pre-osteoclasts. Furthermore, CD47 seems to mediate fusion mostly through broad contact surfaces between the partners' cell membrane while syncytin-1 mediate fusion through phagocytic-cup like structure. J. Cell. Physiol. 232: 1396-1403, 2017. © 2016 Wiley Periodicals, Inc.

Lack of SIRPα phosphorylation and concomitantly reduced SHP-2-PI3K-Akt2 signaling decrease osteoblast differentiation

Normal differentiation of bone forming osteoblasts is a prerequisite for maintenance of skeletal health and is dependent on intricate cellular signaling pathways, including the essential transcription factor Runx2. The cell surface glycoprotein CD47 and its receptor signal regulatory protein alpha (SIRPα) have both been suggested to regulate bone cell differentiation. Here we investigated osteoblastic differentiation of bone marrow stromal cells from SIRPα mutant mice lacking the cytoplasmic signaling domain of SIRPα. An impaired osteoblastogenesis in SIRPα-mutant cell cultures was demonstrated by lower alkaline phosphatase activity and less mineral formation compared to wild-type cultures. This reduced osteoblastic differentiation potential in SIRPα-mutant stromal cells was associated with a significantly reduced expression of Runx2, osterix, osteocalcin, and alkaline phosphatase mRNA, as well as a reduced phosphorylation of SHP-2 and Akt2, as compared with that in wild-type stromal cells. Addition of a PI3K-inhibitor to wild-type stromal cells could mimic the impaired osteoblastogenesis seen in SIRPα-mutant cells. In conclusion, our data suggest that SIRPα signaling through SHP-2-PI3K-Akt2 strongly influences osteoblast differentiation from bone marrow stromal cells.

Studies of OC-STAMP in Osteoclast Fusion: A New Knockout Mouse Model, Rescue of Cell Fusion, and Transmembrane Topology

The fusion of monocyte/macrophage lineage cells into fully active, multinucleated, bone resorbing osteoclasts is a complex cell biological phenomenon that utilizes specialized proteins. OC-STAMP, a multi-pass transmembrane protein, has been shown to be required for pre-osteoclast fusion and for optimal bone resorption activity. A previously reported knockout mouse model had only mononuclear osteoclasts with markedly reduced resorption activity in vitro, but with paradoxically normal skeletal micro-CT parameters. To further explore this and related questions, we used mouse ES cells carrying a gene trap allele to generate a second OC-STAMP null mouse strain. Bone histology showed overall normal bone form with large numbers of TRAP-positive, mononuclear osteoclasts. Micro-CT parameters were not significantly different between knockout and wild type mice at 2 or 6 weeks old. At 6 weeks, metaphyseal TRAP-positive areas were lower and mean size of the areas were smaller in knockout femora, but bone turnover markers in serum were normal. Bone marrow mononuclear cells became TRAP-positive when cultured with CSF-1 and RANKL, but they did not fuse. Expression levels of other osteoclast markers, such as cathepsin K, carbonic anhydrase II, and NFATc1, were not significantly different compared to wild type. Actin rings were present, but small, and pit assays showed a 3.5-fold decrease in area resorbed. Restoring OC-STAMP in knockout cells by lentiviral transduction rescued fusion and resorption. N- and C-termini of OC-STAMP were intracellular, and a predicted glycosylation site was shown to be utilized and to lie on an extracellular loop. The site is conserved in all terrestrial vertebrates and appears to be required for protein stability, but not for fusion. Based on this and other results, we present a topological model of OC-STAMP as a 6-transmembrane domain protein. We also contrast the osteoclast-specific roles of OC- and DC-STAMP with more generalized cell fusion mechanisms.

The elementary fusion modalities of osteoclasts

The last step of the osteoclast differentiation process is cell fusion. Most efforts to understand the fusion mechanism have focused on the identification of molecules involved in the fusion process. Surprisingly, the basic fusion modalities, which are well known for fusion of other cell types, are not known for the osteoclast. Here we show that osteoclast fusion partners are characterized by differences in mobility, nuclearity, and differentiation level. Our demonstration was based on time-laps videos of human osteoclast preparations from three donors where 656 fusion events were analyzed. Fusions between a mobile and an immobile partner were most frequent (62%), while fusion between two mobile (26%) or two immobile partners (12%) was less frequent (p<0.001). In general, the immobile fusion partner contained more nuclei than the mobile one (p<0.01). Furthermore, enrichment in nuclei of an osteoclast with three or more nuclei resulted from fusion with a mono-nucleated cell in 67% of the cases (p<0.001), while mono-nucleated cells fused with a multinucleated cell in 61% of the cases (p<0.05). This observation suggested that a more mature osteoclast prefers to fuse with a less mature pre-osteoclast. This hypothesis was supported by a nucleus-tracing approach in a co-culture of more and less differentiated pre-osteoclasts/osteoclasts. Furthermore, we found that osteoclast fusion proceeds through primarily two different types of cell contacts: phagocytic-cup and broad-contact-surfaces (>80% of all fusions). We conclude that osteoclasts most often gain nuclei by addition of one nucleus at a time, and that this nucleus is most often delivered by a moving cell to an immobile cell. These characteristics fit the in vivo observations where mono-nucleated precursors migrating from the bone marrow fuse with more mature osteoclasts sitting on the bone surface. They also fit the fusion modalities of other cell types.

Osteoclast fusion is based on heterogeneity between fusion partners

Bone-resorbing osteoclasts are formed through fusion of mononucleated precursors. Their choice of partners during the fusion process remains unclear. We hypothesized that osteoclasts are selective in their choice of fusion partner and that this selectivity is based on heterogeneity among the cells with respect to their maturation stage and their expression and cellular organization of fusion factors. Support for this hypothesis was found from immunofluorescence staining of the osteoclast fusion factors CD47, dendritic cell-specific transmembrane protein (DC-STAMP), and syncytin-1. These stainings revealed heterogeneous localization patterns of all three factors within a given culture of osteoclasts. CD47 was found to be localized primarily in small osteoclasts and preosteoclasts, which were also positive for DC-STAMP but negative for cathepsin K expression. A role of CD47 in the early osteoclast fusion steps was also suggested from experiments with a CD47 blocking antibody, which resulted in an inhibition of the fusion of small osteoclasts. Conversely, blocking of connexin 43 affected the fusion of larger osteoclasts with four or more nuclei. The suggestion that different fusion factors function at different stages of osteoclast fusion supports the idea of heterogeneity in the osteoclast population; our results suggest that osteoclast fusion is indeed based on heterogeneity. Considering the in vivo environment in which osteoclasts develop and fuse, our findings seem very applicable and provide novel, important insight into key issues in bone and fusion research.

The CD47-SIRPα signalling system: its physiological roles and therapeutic application

Signal regulatory protein α (SIRPα), also known as SHPS-1/BIT/ CD172a, is an immunoglobulin superfamily protein that binds to the protein tyrosine phosphatases SHP-1 and SHP-2 through its cytoplasmic region. CD47, another immunoglobulin superfamily protein, is a ligand for SIRPα, with the two proteins constituting a cell-cell communication system (the CD47-SIRPα signalling system). SIRPα is particularly abundant in the myeloid-lineage hematopoietic cells such as macrophages or dendritic cells (DCs), whereas CD47 is expressed ubiquitously. Interaction of CD47 (on red blood cells) with SIRPα (on macrophages) is thought to prevent the phagocytosis by the latter cells of the former cells, determining the lifespan of red blood cells. Recent studies further indicate that this signalling system plays important roles in engraftment of hematopoietic stem cells as well as in tumour immune surveillance through regulation of the phagocytic activity of macrophages. In the immune system, the CD47-SIRPα interaction is also important for the development of a subset of CD11c(+)DCs as well as organization of secondary lymphoid organs. Finally, the CD47-SIRPα signalling system likely regulates bone homeostasis by osteoclast development. Newly emerged functions of the CD47-SIRPα signalling system thus provide multiple therapeutic strategies for cancer, autoimmune diseases and bone disorders.

Lack of CD47 impairs bone cell differentiation and results in an osteopenic phenotype in vivo due to impaired signal regulatory protein α (SIRPα) signaling

Here, we investigated whether the cell surface glycoprotein CD47 was required for normal formation of osteoblasts and osteoclasts and to maintain normal bone formation activity in vitro and in vivo. In parathyroid hormone or 1α,25(OH)2-vitamin D3 (D3)-stimulated bone marrow cultures (BMC) from CD47(-/-) mice, we found a strongly reduced formation of multinuclear tartrate-resistant acid phosphatase (TRAP)(+) osteoclasts, associated with reduced expression of osteoclastogenic genes (nfatc1, Oscar, Trap/Acp, ctr, catK, and dc-stamp). The production of M-CSF and RANKL (receptor activator of nuclear factor κβ ligand) was reduced in CD47(-/-) BMC, as compared with CD47(+/+) BMC. The stromal cell phenotype in CD47(-/-) BMC involved a blunted expression of the osteoblast-associated genes osterix, Alp/Akp1, and α-1-collagen, and reduced mineral deposition, as compared with that in CD47(+/+) BMC. CD47 is a ligand for SIRPα (signal regulatory protein α), which showed strongly reduced tyrosine phosphorylation in CD47(-/-) bone marrow stromal cells. In addition, stromal cells lacking the signaling SIRPα cytoplasmic domain also had a defect in osteogenic differentiation, and both CD47(-/-) and non-signaling SIRPα mutant stromal cells showed a markedly reduced ability to support osteoclastogenesis in wild-type bone marrow macrophages, demonstrating that CD47-induced SIRPα signaling is critical for stromal cell support of osteoclast formation. In vivo, femoral bones of 18- or 28-week-old CD47(-/-) mice showed significantly reduced osteoclast and osteoblast numbers and exhibited an osteopenic bone phenotype. In conclusion, lack of CD47 strongly impairs SIRPα-dependent osteoblast differentiation, deteriorate bone formation, and cause reduced formation of osteoclasts.

Disruption of thrombospondin-2 accelerates ischemic fracture healing

Thrombospondin-2 (TSP2) is a matricellular protein that is highly up-regulated during fracture healing. TSP2 negatively regulates vascularity, vascular reperfusion following ischemia, and cutaneous wound healing. As well, TSP2-null mice show increased endocortical bone formation due to an enhanced number of mesenchymal progenitor cells and show increased cortical thickness. Mice deficient in TSP2 (TSP2-null) show an alteration in fracture healing, that is unrelated to their cortical bone phenotype, which is characterized by enhanced vascularization with a shift towards an intramembranous healing phenotype; thus, we hypothesized that there would be enhanced ischemic fracture healing in the absence of TSP2. We investigated whether an absence of TSP2 would enhance ischemic fracture healing utilizing Laser doppler, µCT and histological analysis. Ischemic tibial fractures were created in wildtype (WT) and TSP2-null mice and harvested 10, 20, or 40 days post-fracture. TSP2-null mice show enhanced vascular perfusion following ischemic fracture. At day 10 post-fracture, TSP2-null mice have 115% greater bone volume than WT mice. This is associated with a 122% increase in vessel density, 20% increase in cell proliferation, and 15% decrease in apoptosis compared to WT. At day 20, TSP2-null mice have 34% more bone volume, 51% greater bone volume fraction, and 37% more bone tissue mineral density than WT. By 40 days after fracture the TSP2-null mice have a 24% increase in bone volume fraction, but other parameters show no significant differences. These findings indicate TSP2 is a negative regulator of ischemic fracture healing and that in the absence of TSP2 bone regeneration is enhanced.

CD47: A Cell Surface Glycoprotein Which Regulates Multiple Functions of Hematopoietic Cells in Health and Disease

Interactions between cells and their surroundings are important for proper function and homeostasis in a multicellular organism. These interactions can either be established between the cells and molecules in their extracellular milieu, but also involve interactions between cells. In all these situations, proteins in the plasma membranes are critically involved to relay information obtained from the exterior of the cell. The cell surface glycoprotein CD47 (integrin-associated protein (IAP)) was first identified as an important regulator of integrin function, but later also was shown to function in ways that do not necessarily involve integrins. Ligation of CD47 can induce intracellular signaling resulting in cell activation or cell death depending on the exact context. By binding to another cell surface glycoprotein, signal regulatory protein alpha (SIRPα), CD47 can regulate the function of cells in the monocyte/macrophage lineage. In this spotlight paper, several functions of CD47 will be reviewed, although some functions may be more briefly mentioned. Focus will be on the ways CD47 regulates hematopoietic cells and functions such as CD47 signaling, induction of apoptosis, and regulation of phagocytosis or cell-cell fusion.

New insights into adhesion signaling in bone formation

Mineralized tissues that are protective scaffolds in the most primitive species have evolved and acquired more specific functions in modern animals. These are as diverse as support in locomotion, ion homeostasis, and precise hormonal regulation. Bone formation is tightly controlled by a balance between anabolism, in which osteoblasts are the main players, and catabolism mediated by the osteoclasts. The bone matrix is deposited in a cyclic fashion during homeostasis and integrates several environmental cues. These include diffusible elements that would include estrogen or growth factors and physicochemical parameters such as bone matrix composition, stiffness, and mechanical stress. Therefore, the microenvironment is of paramount importance for controlling this delicate equilibrium. Here, we provide an overview of the most recent data highlighting the role of cell-adhesion molecules during bone formation. Due to the very large scope of the topic, we focus mainly on the role of the integrin receptor family during osteogenesis. Bone phenotypes of some deficient mice as well as diseases of human bones involving cell adhesion during this process are discussed in the context of bone physiology.

Osteoclast fusion and regulation by RANKL-dependent and independent factors

Osteoclasts are the bone resorbing cells essential for bone remodeling. Osteoclasts are formed from hematopoietic progenitors in the monocyte/macrophage lineage. Osteoclastogenesis is composed of several steps including progenitor survival, differentiation to mono-nuclear pre-osteoclasts, fusion to multi-nuclear mature osteoclasts, and activation to bone resorbing osteoclasts. The regulation of osteoclastogenesis has been extensively studied, in which the receptor activator of NF-κB ligand (RANKL)-mediated signaling pathway and downstream transcription factors play essential roles. However, less is known about osteoclast fusion, which is a property of mature osteoclasts and is required for osteoclasts to resorb bone. Several proteins that affect cell fusion have been identified. Among them, dendritic cell-specific transmembrane protein (DC-STAMP) is directly associated to osteoclast fusion in vivo. Cytokines and factors influence osteoclast fusion through regulation of DC-STAMP. Here we review the recently discovered new factors that regulate osteoclast fusion with specific focus on DC-STAMP. A better understanding of the mechanistic basis of osteoclast fusion will lead to the development of a new therapeutic strategy for bone disorders due to elevated osteoclast bone resorption. Cell-cell fusion is essential for a variety of cellular biological processes. In mammals, there is a limited number of cell types that fuse to form multinucleated cells, such as the fusion of myoblasts for the formation of skeletal muscle and the fusion of cells of the monocyte/macrophage lineage for the formation of multinucleated osteoclasts and giant cells. In most cases, cell-cell fusion is beneficial for cells by enhancing function. Myoblast fusion increases myofiber size and diameter and thereby increases contractile strength. Multinucleated osteoclasts have far more bone resorbing activity than their mono-nuclear counterparts. Multinucleated giant cells are much more efficient in the removal of implanted materials and bacteria due to chronic infection than macrophages. Therefore, they are also called foreign-body giant cells. Cell fusion is a complicated process involving cell migration, chemotaxis, cell-cell recognition and attachment, as well as changes into a fusion-competent status. All of these steps are regulated by multiple factors. In this review, we will discuss osteoclast fusion and regulation.

Role of CD47 and Signal Regulatory Protein Alpha (SIRPα) in Regulating the Clearance of Viable or Aged Blood Cells

SUMMARY

The ubiquitously expressed cell surface glycoprotein CD47 is expressed by virtually all cells in the host, where it can function to regulate integrin-mediated responses, or constitute an important part of the erythrocyte band 3/Rh multi-protein complex. In addition, CD47 can protect viable cells from being phagocytosed by macrophages or dendritic cells. The latter mechanism is dependent on the interaction between target cell CD47 and SIRPα on the phagocyte. In this context, SIRPα functions to inhibit prophagocytic signaling from Fcγ receptors, complement receptors, and LDL receptor-related protein-1 (LRP-1), but not scavenger receptors. The expression level and/or distribution of CD47 may be altered on the surface of apoptotic/senescent cells, rendering the phagocytosis inhibitory function of the CD47/SIRPα interaction reduced or eliminated. Instead, the interaction between these 2 proteins may serve to enhance the binding of apoptotic/senescent target cells to the phagocyte to promote phagocytosis.