Ly49Q, an ITIM-bearing NK receptor, positively regulates osteoclast differentiation

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.

The combined effect of oleonuezhenide and wedelolactone on proliferation and osteoblastogenesis of bone marrow mesenchymal stem cells

BACKGROUND

Regulation of the survival and differentiation of bone marrow mesenchymal stem cells is an essential consideration in the development of targeted drugs for treatment of osteoporosis.

PURPOSE

The present study aimed to evaluate the combined effect of wedelolactone and oleonuezhenide, two compounds from Chinese formula Er-Zhi-Wan, on osteoblastogenesis and the underlying molecular mechanisms.

METHODS

MTT assay was taken to evaluate cell proliferation. The alkaline phosphatase (ALP) activity assay was used to determine the activity of ALP. Alizarin red S (ARS) staining was taken to indicate the intensity of the calcium deposits. Quantitative real-time PCR and Western blot were performed to the levels of Runx2, Osteocalcin, and Osterix expression in mouse bone marrow mesenchymal stem cells (BMSCs). Ovariectomized mouse model and bone histomorphometric analysis were also used to research the effects of wedelolactone and oleonuezhenide on bone loss caused by ovariectomy.

RESULTS

Wedelolactone combined with oleonuezhenide enhanced osteoblast differentiation and bone mineralization. Osteoblastogenesis-related marker genes including osteocalcin, Runx2, and osteorix were upregulated in the presence of wedelolactone and oleonuezhenide. At the molecular level, oleonuezhenide did not affect GSK-3β phosphorylation induced by wedelolactone, but elevated casein kinase 2-alpha (CK2α) expression, resulting in β-catenin and Runx2 nuclear translocation. In addition, 30 µM wedelolactone-induced cytotoxicity in bone marrow mesenchymal stem cells was relieved by 9 µM oleonuezhenide. These cells were protected by oleonuezhenide and maintained osteoblastic activity. Oleonuezhenide increased Wnt5a and CK2α expression. Wedelolactone-reduced extracellular signal-regulated kinase (ERK) phosphorylation was reversed by oleonuezhenide. In ovariectomized mice, administration of wedelolactone and oleonuezhenide prevented ovariectomy-induced bone loss by enhancing osteoblastic activity.

CONCLUSION

These results suggested that oleonuezhenide enhanced the effects of wedelolactone on osteoblastogenesis. These two compounds could be developed as a combined therapeutic agent for osteoporosis.

Cysteinyl leukotriene receptor 1 (cysLT1R) regulates osteoclast differentiation and bone resorption

Excessive bone resorption induced by abnormal osteoclast differentiation has been associated with bone microstructure damage and bone-associated disorders, including osteoporosis. Here, we investigated the physiological roles of the type 1 cysteinyl leukotriene receptor (cysLTR-1) and the pharmacological functions of the specific cysLTR-1 antagonist montelukast on M-CSF- and RANKL-induced osteoclast differentiation. We showed that cysLTR-1 but not cysLTR-2 is expressed in osteoclast precursor cells: mouse bone marrow-derived macrophages (BMMs). We also found that treatment with M-CSF and RANKL significantly increased expression of cysLTR-1. Overexpression of cysLTR-1 promoted osteoclast differentiation of BMMs by increasing NFATc1 and TRAP. In contrast, treatment with montelukast prevented M-CSF- and RANKL-induced osteoclast differentiation of BMMs. Mechanically, our findings demonstrate that montelukast treatment attenuated activation of the ERK1/2, p38, JNK and NF-κB signalling pathways. Additionally, we reported that montelukast treatment ameliorated the generation of ROS and calcium signalling. Importantly, the co-immunoprecipitation assay displayed that montelukast treatment prevented the interaction of RANK and TRAF6. Finally, in vivo experiments indicated that montelukast rescued the reduction of bone volume as well as trabecular number in an ovariectomy mouse model.

A Comprehensive Review of Immunoreceptor Regulation of Osteoclasts

Osteoclasts require coordinated co-stimulation by several signaling pathways to initiate and regulate their cellular differentiation. Receptor activator for NF-κB ligand (RANKL or TNFSF11), a tumor necrosis factor (TNF) superfamily member, is the master cytokine required for osteoclastogenesis with essential co-stimulatory signals mediated by immunoreceptor tyrosine-based activation motif (ITAM)-signaling adaptors, DNAX-associated protein 12 kDa size (DAP12) and FcεRI gamma chain (FcRγ). The ITAM-signaling adaptors do not have an extracellular ligand-binding domain and, therefore, must pair with ligand-binding immunoreceptors to interact with their extracellular environment. DAP12 pairs with a number of different immunoreceptors including triggering receptor expressed on myeloid cells 2 (TREM2), myeloid DAP12-associated lectin (MDL-1), and sialic acid-binding immunoglobulin-type lectin 15 (Siglec-15); while FcRγ pairs with a different set of receptors including osteoclast-specific activating receptor (OSCAR), paired immunoglobulin receptor A (PIR-A), and Fc receptors. The ligands for many of these receptors in the bone microenvironment remain unknown. Here, we will review immunoreceptors known to pair with either DAP12 or FcRγ that have been shown to regulate osteoclastogenesis. Co-stimulation and the effects of ITAM-signaling have turned out to be complex, and now include paradoxical findings that ITAM-signaling adaptor-associated receptors can inhibit osteoclastogenesis and immunoreceptor tyrosine-based inhibitory motif (ITIM) receptors can promote osteoclastogenesis. Thus, co-stimulation of osteoclastogenesis continues to reveal additional complexities that are important in the regulatory mechanisms that seek to maintain bone homeostasis.

Wedelolactone enhances osteoblastogenesis by regulating Wnt/β-catenin signaling pathway but suppresses osteoclastogenesis by NF-κB/c-fos/NFATc1 pathway

Bone homeostasis is maintained by formation and destruction of bone, which are two processes tightly coupled and controlled. Targeting both stimulation on bone formation and suppression on bone resorption becomes a promising strategy for treating osteoporosis. In this study, we examined the effect of wedelolactone, a natural product from Ecliptae herba, on osteoblastogenesis as well as osteoclastogenesis. In mouse bone marrow mesenchymal stem cells (BMSC), wedelolactone stimulated osteoblast differentiation and bone mineralization. At the molecular level, wedelolactone directly inhibited GSK3β activity and enhanced the phosphorylation of GSK3β, thereafter stimulated the nuclear translocation of β-catenin and runx2. The expression of osteoblastogenesis-related marker gene including osteorix, osteocalcin and runx2 increased. At the same concentration range, wedelolactone inhibited RANKL-induced preosteoclastic RAW264.7 actin-ring formation and bone resorption pits. Further, wedelolactone blocked NF-kB/p65 phosphorylation and abrogated the NFATc1 nuclear translocation. As a result, osteoclastogenesis-related marker gene expression decreased, including c-src, c-fos, and cathepsin K. In ovariectomized mice, administration of wedelolactone prevented ovariectomy-induced bone loss by enhancing osteoblast activity and inhibiting osteoclast activity. Together, these data demonstrated that wedelolactone facilitated osteoblastogenesis through Wnt/GSK3β/β-catenin signaling pathway and suppressed RANKL-induced osteoclastogenesis through NF-κB/c-fos/NFATc1 pathway. These results suggested that wedelolacone could be a novel dual functional therapeutic agent for osteoporosis.

Aryl hydrocarbon receptors in osteoclast lineage cells are a negative regulator of bone mass

Aryl hydrocarbon receptors (AhRs) play a critical role in various pathological and physiological processes. Although recent research has identified AhRs as a key contributor to bone metabolism following studies in systemic AhR knockout (KO) or transgenic mice, the cellular and molecular mechanism(s) in this process remain unclear. In this study, we explored the function of AhR in bone metabolism using AhR^{RANKΔOc/ΔOc} (RANK^Cre/+;AhR^flox/flox) mice. We observed enhanced bone mass together with decreased resorption in both male and female 12 and 24-week-old AhR^{RANKΔOc/ΔOc} mice. Control mice treated with 3-methylcholanthrene (3MC), an AhR agonist, exhibited decreased bone mass and increased bone resorption, whereas AhR^{CtskΔOc/ΔOc} (Ctsk^Cre/+;AhR^flox/flox) mice injected with 3MC appeared to have a normal bone phenotype. In vitro, bone marrow-derived macrophages (BMDMs) from AhR^{RANKΔOc/ΔOc} mice exhibited impaired osteoclastogenesis and repressed differentiation with downregulated expression of B lymphocyte-induced maturation protein 1 (Blimp1), and cytochrome P450 genes Cyp1b1 and Cyp1a2. Collectively, our results not only demonstrated that AhR in osteoclast lineage cells is a physiologically relevant regulator of bone resorption, but also highlighted the need for further studies on the skeletal actions of AhR inhibitors in osteoclast lineage cells commonly associated with bone diseases, especially diseases linked to environmental pollutants known to induce bone loss.

Global epigenomic analysis indicates protocadherin-7 activates osteoclastogenesis by promoting cell-cell fusion

Gene expression is dependent not only on genomic sequences, but also epigenetic control, in which the regulation of chromatin by histone modification plays a crucial role. Histone H3 lysine 4 trimethylation (H3K4me3) and histone H3 lysine 27 trimethylation (H3K27me3) are related to transcriptionally activated and silenced sequences, respectively. Osteoclasts, the multinucleated cells that resorb bone, are generated by the fusion of precursor cells of monocyte/macrophage lineage. To elucidate the molecular and epigenetic regulation of osteoclast differentiation, we performed a chromatin immunoprecipitation sequencing (ChIP-seq) analysis for H3K4me3 and H3K27me3 in combination with RNA sequencing. We focused on the histone modification change from H3K4me3(+)H3K27me3(+) to H3K4me3(+)H3K27me3(-) and identified the protocadherin-7 gene (Pcdh7) to be among the genes epigenetically regulated during osteoclastogenesis. Pcdh7 was induced by RANKL stimulation in an NFAT-dependent manner. The knockdown of Pcdh7 inhibited RANKL-induced osteoclast differentiation due to the impairment of cell-cell fusion, accompanied by a decreased expression of the fusion-related genes Dcstamp, Ocstamp and Atp6v0d2. This study demonstrates that Pcdh7 plays a key role in osteoclastogenesis by promoting cell-cell fusion.

Ly49 receptors: innate and adaptive immune paradigms

The Ly49 receptors are type II C-type lectin-like membrane glycoproteins encoded by a family of highly polymorphic and polygenic genes within the mouse natural killer (NK) gene complex. This gene family is designated Klra, and includes genes that encode both inhibitory and activating Ly49 receptors in mice. Ly49 receptors recognize class I major histocompatibility complex-I (MHC-I) and MHC-I-like proteins on normal as well as altered cells. Their functional homologs in humans are the killer cell immunoglobulin-like receptors, which recognize HLA class I molecules as ligands. Classically, Ly49 receptors are described as being expressed on both the developing and mature NK cells. The inhibitory Ly49 receptors are involved in NK cell education, a process in which NK cells acquire function and tolerance toward cells that express “self-MHC-I.” On the other hand, the activating Ly49 receptors recognize altered cells expressing activating ligands. New evidence shows a broader Ly49 expression pattern on both innate and adaptive immune cells. Ly49 receptors have been described on multiple NK cell subsets, such as uterine NK and memory NK cells, as well as NKT cells, dendritic cells, plasmacytoid dendritic cells, macrophages, neutrophils, and cells of the adaptive immune system, such as activated T cells and regulatory CD8⁺ T cells. In this review, we discuss the expression pattern and proposed functions of Ly49 receptors on various immune cells and their contribution to immunity.

The ly49 gene family. A brief guide to the nomenclature, genetics, and role in intracellular infection

Understanding the Ly49 gene family can be challenging in terms of nomenclature and genetic organization. The Ly49 gene family has two major gene nomenclature systems, Ly49 and Killer Cell Lectin-like Receptor subfamily A (klra). Mice from different strains have varying numbers of these genes with strain specific allelic variants, duplications, deletions, and pseudogene sequences. Some members activate NK lymphocytes, invariant NKT (iNKT) lymphocytes and γδ T lymphocytes while others inhibit killing activity. One family member, Ly49Q, is expressed only on myeloid cells and is not found on NK, iNKT, or γδ T cells. There is growing evidence that these receptors may regulate not just the immune response to viruses, but other intracellular pathogens as well. Thus, this review’s primary goal is to provide a guide for researchers first encountering the Ly49 gene family and a foundation for future studies on the role that these gene products play in the immune response, particularly the response to intracellular viral and bacterial pathogens.

Ly49Q positively regulates type I IFN production by plasmacytoid dendritic cells in an immunoreceptor tyrosine-based inhibitory motif-dependent manner

Plasmacytoid dendritic cells (pDC) are the major producers of type I IFN during the initial immune response to viral infection. Ly49Q, a C-type lectin-like receptor specific for MHC-I, possesses a cytoplasmic ITIM and is highly expressed on murine pDC. Using Ly49Q-deficient mice, we show that, regardless of strain background, this receptor is required for maximum IFN-α production by pDC. Furthermore, Ly49Q expression on pDC, but not myeloid dendritic cells, is necessary for optimal IL-12 secretion, MHC-II expression, activation of CD4(+) T cell proliferation, and nuclear translocation of the master IFN-α regulator IFN regulatory factor 7 in response to TLR9 agonists. In contrast, the absence of Ly49Q did not affect plasmacytoid dendritic cell-triggering receptor expressed on myeloid cells expression or pDC viability. Genetic complementation revealed that IFN-α production by pDC is dependent on an intact tyrosine residue in the Ly49Q cytoplasmic ITIM. However, pharmacological inhibitors and phosphatase-deficient mice indicate that Src homology 2 domain-containing phosphatase 1 (SHP)-1, SHP-2, and SHIP phosphatase activity is dispensable for this function. Finally, we observed that Ly49Q itself is downregulated on pDC in response to CpG exposure in an ITIM-independent manner. In conclusion, Ly49Q enhances TLR9-mediated signaling events, leading to IFN regulatory factor 7 nuclear translocation and expression of IFN-I genes in an ITIM-dependent manner that can proceed without the involvement of SHP-1, SHP-2, and SHIP.

TLT-1s, alternative transcripts of triggering receptor expressed on myeloid cell-like transcript-1 (TLT-1), Inhibits the triggering receptor expressed on myeloid cell-2 (TREM-2)-mediated signaling pathway during osteoclastogenesis

Triggering receptor expressed on myeloid cells (TREM)-like transcript-1 (TLT-1) is an immunoreceptor tyrosine-based inhibitory motif (ITIM)-baring TREM family protein. In this study, we identified an alternative transcript form of TLT-1, namely TLT-1s, which has very short extracellular immunoglobulin domain consisting of only 202 amino acids. TLT-1s was mainly expressed in macrophages and osteoclast precursor cells. Upon receptor activator of nuclear factor-κB ligand stimulation, TLT-1s mRNA and protein levels were gradually decreased in BMMs. We also showed the TLT-1s is localized to the cytoplasmic membrane in osteoclast precursor cells. TLT-1s silencing strongly enhanced the formation and resorption activity of osteoclast. In addition, forced expression of TLT-1s showed reduced formation of osteoclast. Because ITIM-baring proteins inhibit immunoreceptor tyrosine-based activation motif (ITAM)-mediated receptor signaling, we tested whether TLT-1s physically interacted with TREM-2, the ITAM-associated co-stimulatory receptor essential for osteoclast differentiation. We showed that TLT-1s is associated with TREM-2 in osteoclast precursor cells. TLT-1s is also associated with tyrosine Src homology 2 domain-containing phosphatase-1 and SH2 domain-containing inositol phosphatase-1 and recruited them to the TREM2-ITAM signaling complex. In addition, knockdown of TLT-1s markedly elevated the intracellular calcium concentration and oscillation in osteoclast precursor cells. In addition, calcium-mediated induction of nuclear factor of activated T cells was also increased by TLT-1s silencing. Furthermore, TREM-2-mediated Akt activation and proliferation of osteoclast precursor cells were also enhanced in TLT-1s silenced cells. In this paper, we found the noble ITIM-baring inhibitory membrane protein; TLT-1s, which regulates ITAM-mediated signaling on osteoclastogenesis.

Osteoprotection by semaphorin 3A

The bony skeleton is maintained by local factors that regulate bone-forming osteoblasts and bone-resorbing osteoclasts, in addition to hormonal activity. Osteoprotegerin protects bone by inhibiting osteoclastic bone resorption, but no factor has yet been identified as a local determinant of bone mass that regulates both osteoclasts and osteoblasts. Here we show that semaphorin 3A (Sema3A) exerts an osteoprotective effect by both suppressing osteoclastic bone resorption and increasing osteoblastic bone formation. The binding of Sema3A to neuropilin-1 (Nrp1) inhibited receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast differentiation by inhibiting the immunoreceptor tyrosine-based activation motif (ITAM) and RhoA signalling pathways. In addition, Sema3A and Nrp1 binding stimulated osteoblast and inhibited adipocyte differentiation through the canonical Wnt/β-catenin signalling pathway. The osteopenic phenotype in Sema3a−/− mice was recapitulated by mice in which the Sema3A-binding site of Nrp1 had been genetically disrupted. Intravenous Sema3A administration in mice increased bone volume and expedited bone regeneration. Thus, Sema3A is a promising new therapeutic agent in bone and joint diseases.

Osteoimmunology: the expanding role of immunoreceptors in osteoclasts and bone remodeling

The study of bone and immunology (termed osteoimmunology) has led to the discovery of many important similarities between the two systems including shared niches, mechanisms, cytokines and receptors. The bone marrow provides a niche for hematopoietic cells including those of the lymphoid and myeloid lineage. Osteoclasts, specialized polykarons arising from myeloid precursors, bind to bone and resorb the organic and inorganic components through secretion of acid and proteases. Osteoclasts are differentiated and activated by cytokines that can be produced by immune cells and osteoclast activity can be dysregulated in states of autoimmunity or high inflammation. Similar to B and T cells, osteoclasts require coordinated co-stimulation of signaling pathways provided in the form of receptor-associated immunoreceptor tyrosine-based activation motif adaptor proteins, DAP12 and FcRγ, to drive differentiation and activation. In this review, we will cover the differentiation process of osteoclasts from the earliest precursors shown to have differentiation potential and the signals needed to drive these cells into osteoclast commitment and activation.

Regulation of human osteoclast development by dendritic cell-specific transmembrane protein (DC-STAMP)

Osteoclasts (OC) are bone-resorbing, multinucleated cells that are generated via fusion of OC precursors (OCP). The frequency of OCP is elevated in patients with erosive inflammatory arthritis and metabolic bone diseases. Although many cytokines and cell surface receptors are known to participate in osteoclastogenesis, the molecular mechanisms underlying the regulation of this cellular transformation are poorly understood. Herein, we focused our studies on the dendritic cell-specific transmembrane protein (DC-STAMP), a seven-pass transmembrane receptor-like protein known to be essential for cell-to-cell fusion during osteoclastogenesis. We identified an immunoreceptor tyrosine-based inhibitory motif (ITIM) in the cytoplasmic tail of DC-STAMP, and developed an anti-DC-STAMP monoclonal antibody 1A2 that detected DC-STAMP expression on human tumor giant cells, blocked OC formation in vitro, and distinguished four patterns of human PBMC with a positive correlation to OC potential. In freshly isolated monocytes, DC-STAMP(high) cells produced a higher number of OC in culture than DC-STAMP(low) cells and the surface expression of DC-STAMP gradually declined during osteoclastogenesis. Importantly, we showed that DC-STAMP is phosphorylated on its tyrosine residues and physically interacts with SHP-1 and CD16, an SH2-domain-containing tyrosine phosphatase and an ITAM-associated protein, respectively. Taken together, these data show that DC-STAMP is a potential OCP biomarker in inflammatory arthritis. Moreover, in addition to its effect on cell fusion, DC-STAMP dynamically regulates cell signaling during osteoclastogenesis.

The role of the BH3-only protein Noxa in bone homeostasis

Bone homeostasis is maintained by a dynamic balance between bone resorption by osteoclasts and bone formation by osteoblasts. Since excessive osteoclast activity is implicated in pathological bone resorption, understanding the mechanism underlying osteoclast differentiation, function and survival is of both scientific and clinical importance. Osteoclasts are monocyte/macrophage lineage cells with a short life span that undergo rapid apoptosis, the rate of which critically determines the level of bone resorption in vivo. However, the molecular basis of rapid osteoclast apoptosis remains obscure. Here we report the role of a BH3-only protein, Noxa (encoded by the Pmaip1 gene), in bone homeostasis using Noxa-deficient mice. Among the Bcl-2 family members, Noxa was selectively induced during osteoclastogenesis. Mice lacking Noxa exhibit a severe osteoporotic phenotype due to an increased number of osteoclasts. Noxa deficiency did not have any effect on the number of osteoclast precursor cells or the expression of osteoclast-specific genes, but led to a prolonged survival of osteoclasts. Furthermore, adenovirus-mediated Noxa overexpression remarkably reduced bone loss in a model of inflammation-induced bone destruction. This study reveals Noxa to be a crucial regulator of osteoclast apoptosis, and may provide a molecular basis for a new therapeutic approach to bone diseases.

Mice lacking Ly49E show normal NK cell development and provide evidence for probabilistic expression of Ly49E in NK cells and T cells

Ly49E is an unusual member of the Ly49 family that is expressed on fetal NK cells, epithelial T cells, and NKT cells, but not on resting adult NK cells. Ly49E(bgeo/bgeo) mice in which the Ly49E gene was disrupted by inserting a β-geo transgene were healthy, fertile, and had normal numbers of NK and T cells in all organs examined. Their NK cells displayed normal expression of Ly49 and other NK cell receptors, killed tumor and MHC class I-deficient cells efficiently, and produced normal levels of IFN-γ. In heterozygous Ly49E(+/bgeo) mice, the proportion of epidermal T cells, NKT cells, and IL-2-activated NK cells that expressed Ly49E was about half that found in wild-type mice. Surprisingly, although splenic T cells rarely expressed Ly49E, IL-2-activated splenic T cells from Ly49E(bgeo/bgeo) mice were as resistant to growth in G418 as NK cells and expressed similar levels of β-geo transcripts, suggesting that disruption of the Ly49E locus had increased its expression in these cells to the same level as that in NK cells. Importantly, however, the proportion of G418-resistant heterozygous Ly49E(+/bgeo) cells that expressed Ly49E from the wild-type allele was similar to that observed in control cells. Collectively, these findings demonstrate that Ly49E is not required for the development or homeostasis of NK and T cell populations or for the acquisition of functional competence in NK cells and provide compelling evidence that Ly49E is expressed in a probabilistic manner in adult NK cells and T cells.

Inhibitory C-type lectin receptors in myeloid cells

C-type lectin receptors encoded by the natural killer gene complex play critical roles in enabling NK cell discrimination between self and non-self. In recent years, additional genes at this locus have been identified with patterns of expression that extend to cells of the myeloid lineage where many of the encoded inhibitory receptors have equally important functions as regulators of immune homeostasis. In the present review we highlight the roles of some of these receptors including recent insights gained with regard to the identification of exogenous and endogenous ligands, mechanisms of cellular inhibition and activation, regulated expression within different cellular and immune contexts, as well as functions that include the regulation of bone homeostasis and involvement in autoimmunity.

Osteoclast inhibitory peptide-1 binding to the Fc gammaRIIB inhibits osteoclast differentiation

Osteoclast inhibitory peptide-1 (OIP) is an autocrine/paracrine inhibitor of osteoclast differentiation, and mice that overexpress OIP-1 in osteoclast lineage cells develop an osteopetrosis bone phenotype. In this study, we show that OIP-1 binding to the Fc gamma receptor IIB (Fc gammaRIIB) inhibits osteoclast differentiation. Confocal microscopy revealed colocalization of OIP-1 with Fc gammaRIIB in osteoclasts, and we observed that OIP-1 carboxy-terminal GPI-linked peptide forms a 1:1 complex with recombinant Fc gammaRIIB protein with an affinity binding of a dissociation constant of approximately 4 microm. Immunoreceptor tyrosine-based activation motif (ITAM)-bearing adapter proteins (FcR gamma and DNAX-activating protein of molecular mass 12 kDa) are critical for osteoclast development, and OIP-1 transgenic mouse-derived preosteoclast cells demonstrated suppression (6-fold) of ITAM phosphorylation of FcR gamma but not DNAX-activating protein of molecular mass 12 kDa. Interestingly, these preosteoclast cells demonstrated increased levels (4-fold) of immunoreceptor tyrosine-based inhibitory motif phosphorylation of Fc gammaRIIB and Src homology 2-domain-containing proteins tyrosine phosphatase 1 activation. Further, OIP-1 mouse-derived preosteoclasts cells demonstrated inhibition of spleen tyrosine kinase activation (4.5-fold), compared with wild-type mice. These results suggest that cross-regulation of immunoreceptor tyrosine-based inhibitory motif and ITAM bearing Fc receptors may play a role in OIP-1 suppression of spleen tyrosine kinase activation and inhibition of osteoclast differentiation. Thus, OIP-1 is an important physiologic regulator of osteoclast development and may have therapeutic utility for bone diseases with high bone turnover.