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Yongwon Choi

Publications and source records attributed to Yongwon Choi.

At least 19 recordsLinked to original sources
PubMed2007

Osteoblasts support B-lymphocyte commitment and differentiation from hematopoietic stem cells.

Early B lymphopoiesis in mammals is induced within the bone marrow (BM) microenvironment, but which cells constitute this niche is not known. Previous studies had shown that osteoblasts (OBs) support hematopoietic stem cell (HSC) proliferation and myeloid differentiation. We now find that purified primary murine OBs also support the differentiation of primitive hematopoietic stem cells through lymphoid commitment and subsequent differentiation to all stages of B-cell precursors and mature B cells. Lin(-)Sca-1(+)Rag-2(-) BM cell differentiation to B cells requires their attachment to OBs in vitro, and this developmental process is mediated via VCAM-1, SDF-1, and IL-7 signaling induced by parathyroid hormone (PTH). Addition of cytokines produced by nonosteoblastic stromal cells (c-Kit ligand, IL-6, and IL-3) shifted the cultures toward myelopoiesis. Confirming the role of OBs in B lymphopoiesis, we found that selective elimination of osteoblasts in Col2.3Delta-TK transgenic mice severely depleted pre-pro-B and pro-B cells from BM, preceding any decline in HSCs. Taken together, these results demonstrate that osteoblasts are both necessary and sufficient for murine B-cell commitment and maturation, and thereby constitute the cellular homolog of the avian bursa of Fabricius.

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PubMed2006

v-ATPase V0 subunit d2-deficient mice exhibit impaired osteoclast fusion and increased bone formation.

Matrix-producing osteoblasts and bone-resorbing osteoclasts maintain bone homeostasis. Osteoclasts are multinucleated, giant cells of hematopoietic origin formed by the fusion of mononuclear pre-osteoclasts derived from myeloid cells. Fusion-mediated giant cell formation is critical for osteoclast maturation; without it, bone resorption is inefficient. To understand how osteoclasts differ from other myeloid lineage cells, we previously compared global mRNA expression patterns in these cells and identified genes of unknown function predominantly expressed in osteoclasts, one of which is the d2 isoform of vacuolar (H(+)) ATPase (v-ATPase) V(0) domain (Atp6v0d2). Here we show that inactivation of Atp6v0d2 in mice results in markedly increased bone mass due to defective osteoclasts and enhanced bone formation. Atp6v0d2 deficiency did not affect differentiation or the v-ATPase activity of osteoclasts. Rather, Atp6v0d2 was required for efficient pre-osteoclast fusion. Increased bone formation was probably due to osteoblast-extrinsic factors, as Atp6v02 was not expressed in osteoblasts and their differentiation ex vivo was not altered in the absence of Atp6v02. Our results identify Atp6v0d2 as a regulator of osteoclast fusion and bone formation, and provide genetic data showing that it is possible to simultaneously inhibit osteoclast maturation and stimulate bone formation by therapeutically targeting the function of a single gene.

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PubMed2006

The adaptor molecule MyD88 activates PI-3 kinase signaling in CD4+ T cells and enables CpG oligodeoxynucleotide-mediated costimulation.

While T cells respond directly to toll-like receptor (TLR) agonists, TLR-signaling pathways in T cells are poorly characterized. Here we demonstrate in CD4(+) T cells that CpG DNA directly enhances proliferation, prevents anergy, and augments humoral responses to a T cell-dependent antigen by a Myeloid differentiation primary-response protein 88 (MyD88) and Phosphatidylinositol 3-kinase (PI-3 kinase)-dependent pathway. PI-3 kinase activation required a putative Src-homology domain (SH2) binding motif in the MyD88 Toll-Like or IL-1 Receptor (TIR) domain. Reconstitution of MyD88-deficient primary T cells with a MyD88 transgene mutated in this motif abrogated association of PI-3 kinase with MyD88, phosphorylation of protein kinase B (Akt) and Glycogen Synthetase Kinase-3 (GSK-3), and interleukin-2 (IL-2) production. The MyD88 death domain, on the other hand, was required for NF-kB activation and survival. These studies identify a MyD88-dependent PI-3 kinase-signaling pathway in T cells that differentiates CpG DNA-mediated proliferation from survival and is required for an in vivo T cell-dependent immune response.

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PubMed2006

Cutting edge: T cell requirement for CD28 costimulation is due to negative regulation of TCR signals by PTEN.

Recent studies suggest that the phosphatase and tensin homolog deleted on chromosome 10 (PTEN) plays a critical role in the maintenance of self-tolerance. Using T cell-specific PTEN knockout mice (PTENDeltaT), we have identified a novel mechanism by which PTEN regulates T cell tolerance. We found that TCR stimulation alone, without CD28 costimulation, is sufficient to induce hyperactivation of the PI3K pathway, which leads to enhanced IL-2 production by naive PTENDeltaT T cells. Importantly, as a result of this increased response to TCR stimulation, PTENDeltaT CD4(+) T cells no longer require CD28 costimulation for in vitro or in vivo expansion. In fact, unlike wild-type T cells, PTENDeltaT CD4(+) T cells are not anergized by delivery of TCR stimulation alone. These data suggest that by negatively regulating TCR signals, PTEN imposes a requirement for CD28 costimulation, thus defining a novel mechanism for its role in self-tolerance.

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PubMed2006

Identification and characterization of the precursors committed to osteoclasts induced by TNF-related activation-induced cytokine/receptor activator of NF-kappa B ligand.

Osteoclasts are terminally differentiated from cells of monocyte/macrophage lineage by stimulation with TNF-related activation-induced cytokine (TRANCE) (receptor activator of NF-kappaB ligand/osteoprotegerin ligand/osteoclast differentiation factor/TNFSF11/CD254). In the present study, we attempted to determine when and how the cell fate of precursors becomes committed to osteoclasts following TRANCE stimulation. Although mouse bone marrow-derived macrophages (BMMs) were able to differentiate into either osteoclasts or dendritic cells, the cells no longer differentiated into dendritic cells after treatment with TRANCE for 24 h, indicating that their cell fate was committed to osteoclasts. Committed cells as well as BMMs were still quite weak in tartrate-resistant acid phosphatase activity, an osteoclast marker, and incorporated zymosan particles by phagocytosis. Interestingly, committed cells, but not BMMs, could still differentiate into osteoclasts even after incorporation of the zymosan particles. Furthermore, IL-4 and IFN-gamma, potent inhibitors of osteoclast differentiation, failed to inhibit osteoclast differentiation from committed cells, and blocking of TRANCE stimulation by osteoprotegerin resulted in cell death. Adhesion to culture plates was believed to be essential for osteoclast differentiation; however, committed cells, but not BMMs, differentiated into multinucleated osteoclasts without adhesion to culture plates. Although LPS activated the NF-kappaB-mediated pathway in BMMs as well as in committed cells, the mRNA expression level of TNF-alpha in the committed cells was significantly lower than that in BMMs. These results suggest that characteristics of the committed cells induced by TRANCE are distinctively different from that of BMMs and osteoclasts.

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PubMed2006

TRAF6 is a T cell-intrinsic negative regulator required for the maintenance of immune homeostasis.

TRAF6 has a key role in the regulation of innate immune responses by mediating signals from both TNF receptor and interleukin-1 receptor/Toll-like receptor superfamilies. Here we show that T cell-specific deletion of TRAF6 unexpectedly results in multiorgan inflammatory disease. TRAF6-deficient T cells exhibit hyperactivation of the phosphatidylinositol 3-kinase (PI3K)-Akt pathway compared with wild-type T cells and, as a result, become resistant to suppression by CD4+ CD25+ regulatory T cells. These data identify a previously unrecognized role for TRAF6 in the maintenance of peripheral tolerance, and suggest the presence of a T cell-intrinsic control mechanism to render responder T cells susceptible to tolerizing signals.

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PubMed2006

Osteoimmunology: interplay between the immune system and bone metabolism.

Studies of bone and the immune system have converged in recent years under the banner of osteoimmunology. The immune system is spawned in the bone marrow reservoir, and investigators now recognize that important niches also exist there for memory lymphocytes. At the same time, various factors produced during immune responses are capable of profoundly affecting regulation of bone. Mechanisms have evolved to prevent excessive interference by the immune system with bone homeostasis, yet pathologic bone loss is a common sequela associated with autoimmunity and cancer. There are also developmental links, or parallels, between bone and the immune system. Cells that regulate bone turnover share a common precursor with inflammatory immune cells and may restrict themselves anatomically, in part by utilizing a signaling network analogous to lymphocyte costimulation. Efforts are currently under way to further characterize how these two organ systems overlap and to develop therapeutic strategies that benefit from this understanding.

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PubMed2006

T lymphocyte-deficient mice lose trabecular bone mass with ovariectomy.

UNLABELLED: We examined OVX-induced bone loss in three TLD mouse models. In TLD mice, OVX caused trabecular bone loss equivalent to that of WT. In contrast, cortical bone loss with OVX was variable. We conclude that T lymphocytes do not influence OVX-induced trabecular bone loss. INTRODUCTION: We examined ovariectomy (OVX)-induced bone loss in three T lymphocyte-deficient (TLD) mouse models: nude mice, recombination activating gene 2-deficient (RAG2 KO) mice, and T cell receptor alpha chain-deficient (TCRalpha KO) mice. MATERIALS AND METHODS: Bone mass was examined by DXA, microCT, and histomorphometry. We also examined the effect of OVX on T lymphocytes in the bone marrow and spleens of wildtype (WT) mice and on in vitro osteoclastogenesis and colony forming unit-granulocyte macrophage (CFU-GM) activity in the bone marrow of WT and nude mice. RESULTS: In WT mice, OVX did not alter T lymphocyte number in the bone marrow but did increase T lymphocytes in the spleen. Comparison of bone mass in nude, RAG2 KO, and TCRalpha KO mice with WT as measured by DXA showed decreased femoral bone mass in nude mice and increased vertebral bone mass in RAG2 KO mice. In TCRalpha KO mice, femoral, tibial, and vertebral bone mass were decreased. In vertebrae and long bones, bone loss with OVX was consistently present in WT mice but variably present in TLD mice as measured by DXA. In contrast, microCT and histomorphometry showed similar trabecular bone loss after OVX in all mice. However, femoral cortical bone loss occurred only in WT and RAG2 KO mice. OVX produced similar trabecular bone loss in WT and TCRalpha KO mice and also induced cortical bone loss in both. Histomorphometry showed that TRACP(+) area in bones was increased by OVX in femurs from both WT and nude mice as was in vitro osteoclast-like cell formation and CFU-GM activity. CONCLUSIONS: These results show that OVX caused similar trabecular bone loss in both WT and TLD mice. The ability of DXA and measurement of cortical bone loss to show OVX-induced effects on bone mass was variable. It seems that T lymphocytes are not critical for OVX-induced trabecular bone loss in these mouse models.

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PubMed2005

Id helix-loop-helix proteins negatively regulate TRANCE-mediated osteoclast differentiation.

Tumor necrosis factor (TNF)-related activation-induced cytokine (TRANCE) induces osteoclast formation from monocyte/macrophage lineage cells via various transcription factors, including the Mi transcription factor (Mitf). Here, we show that inhibitors of differentiation/DNA binding (Ids), helix-loop-helix (HLH) transcription factors, negatively regulate TRANCE-induced osteoclast differentiation. Expression levels of Id1, Id2, and Id3 genes are significantly reduced by TRANCE during osteoclastogenesis. Interestingly, overexpression of the 3 Id genes in bone marrow-derived monocyte/macrophage lineage cells (BMMs) inhibits the formation of tartrate-resistant acid phosphatase (TRAP)-positive multinuclear osteoclasts, but it does not alter the ability of BMMs to either phagocytose or differentiate into dendritic cells (DCs). Overexpression of Id2 in BMMs attenuates the gene induction of nuclear factor of activated T cells c1 (NFATc1) and osteoclast-associated receptor (OSCAR) during TRANCE-mediated osteoclastogenesis. Furthermore, Id proteins interact with Mitf, a basic HLH (bHLH) transcription factor, and inhibit its transactivation of OSCAR, which is a costimulatory receptor expressed by osteoclast precursors, by attenuating the DNA binding ability of Mitf to the E-box site of the OSCAR promoter. Taken together, our results reveal both a new facet of negative regulation, mediated by Id proteins, as well as the mechanism whereby TRANCE signaling overcomes it, allowing osteoclastogenesis to proceed.

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PubMed2005

Protection against osteoporosis by active immunization with TRANCE/RANKL displayed on virus-like particles.

TNF-related activation-induced cytokine (TRANCE), also known as receptor activator of NF-kappaB ligand (RANKL), is the key molecule responsible for the bone loss observed in osteoporosis. Passive administration of osteoprotegerin, the soluble decoy receptor of TRANCE/RANKL, is efficient in blocking disease progression, but may not find widespread clinical use due to patient compliance problems and the expected high costs. In this study, we describe an efficient, safe, and potentially cost-effective active immunization strategy against TRANCE/RANKL. We show in mice that immunization with TRANCE/RANKL covalently linked to virus-like particles can overcome the natural tolerance of the immune system toward self proteins and produce high levels of specific Abs without the addition of any adjuvant. Serum Abs of immunized mice neutralized TRANCE/RANKL activity in vitro and were highly active in preventing bone loss in a mouse model of osteoporosis. Active immunization against TRANCE/RANKL was essentially reversible and did not produce any measurable immunosuppressive side effects, underscoring its potential as a new therapeutic approach to the treatment of human bone-degenerative disorders.

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PubMed2005

Osteoclast differentiation independent of the TRANCE-RANK-TRAF6 axis.

Osteoclasts are derived from myeloid lineage cells, and their differentiation is supported by various osteotropic factors, including the tumor necrosis factor (TNF) family member TNF-related activation-induced cytokine (TRANCE). Genetic deletion of TRANCE or its receptor, receptor activator of nuclear factor kappaB (RANK), results in severely osteopetrotic mice with no osteoclasts in their bones. TNF receptor-associated factor (TRAF) 6 is a key signaling adaptor for RANK, and its deficiency leads to similar osteopetrosis. Hence, the current paradigm holds that TRANCE-RANK interaction and subsequent signaling via TRAF6 are essential for the generation of functional osteoclasts. Surprisingly, we show that hematopoietic precursors from TRANCE-, RANK-, or TRAF6-null mice can become osteoclasts in vitro when they are stimulated with TNF-alpha in the presence of cofactors such as TGF-beta. We provide direct evidence against the current paradigm that the TRANCE-RANK-TRAF6 pathway is essential for osteoclast differentiation and suggest the potential existence of alternative routes for osteoclast differentiation.

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PubMed2005

Selenite-induced apoptosis of osteoclasts mediated by the mitochondrial pathway.

The possible effects of sodium selenite on mature osteoclasts were investigated. Incubation of osteoclast-like cells differentiated from RAW 264.7 cells with sodium selenite induced apoptosis as revealed by morphological changes, internucleosomal DNA fragmentation, and activation of caspase-3. Selenite also induced generation of the superoxide anion and reduced the number of free thiol groups in the osteoclast-like cells, suggestive of a shift to a more oxidizing intracellular environment. In addition, selenite induced protein aggregation by thiol cross-linking, loss of the mitochondrial membrane potential, and cytochrome c release in mitochondria isolated from the osteoclast-like cells. Finally, selenite-induced DNA fragmentation in osteoclasts was inhibited both by cyclosporin A, a blocker of the mitochondrial permeability transition pore, and by DEVD-CHO, a cell-permeable inhibitor of caspase-3. These results thus suggest that selenite induces apoptosis mediated by the mitochondrial pathway in mature osteoclasts.

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PubMed2005

Nuclear factor of activated T cells c1 induces osteoclast-associated receptor gene expression during tumor necrosis factor-related activation-induced cytokine-mediated osteoclastogenesis.

Osteoclast differentiation from hematopoietic precursors is controlled by the tumor necrosis factor family member tumor necrosis factor-related activation-induced cytokine (TRANCE) via induction of various transcription factors, including nuclear factor of activated T cells (NFAT) c1. During osteoclast differentiation, NFATc1 is further activated via calcium signaling when costimulatory receptors expressed on osteoclast precursors, such as osteoclast-associated receptor (OSCAR), are stimulated. Here we show that NFATc1 expression precedes that of OSCAR during TRANCE-mediated osteoclastogenesis and that inhibition of NFATc1 by cyclosporin A abolishes TRANCE-induced OSCAR expression and subsequent osteoclast differentiation. Moreover, we show that the 1.0-kb promoter region of the OSCAR gene contains three potential NFATc1-binding sites. Induction of an OSCAR promoter-luciferase reporter is significantly increased when transiently transfected into 293T cells in combination with NFATc1 expression plasmid. Deletion and site-directed mutant constructs confirmed that NFATc1-binding sites are both functional and NFATc1-specific. Furthermore, NFATc1 synergistically activates an OSCAR reporter construct together with microphthalmia transcription factor and PU.1, transcription factors previously shown to be critical for osteoclast differentiation. In addition, a plasmid expressing constitutively active MAP kinase kinase 6 enhances the transactivation activity of NFATc1/microphthalmia transcription factor/PU.1 on the OSCAR promoter. Taken together, our results indicate that NFATc1 is an important transcription factor in the induction of OSCAR during osteoclastogenesis. Elucidation of NFATc1 as a transcription factor for OSCAR expression implies the presence of a positive feedback circuit of TRANCE-induced activation of NFATc1, involving NFATc1-mediated OSCAR expression and its subsequent activation of NFATc1, necessary for efficient differentiation of osteoclasts.

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PubMed2005

IL-1 receptor-associated kinase M is a central regulator of osteoclast differentiation and activation.

Osteoporosis is a serious problem worldwide; it is characterized by bone fractures in response to relatively mild trauma. Osteoclasts originate from the fusion of macrophages and they play a central role in bone development and remodeling via the resorption of bone. Therefore, osteoclasts are important mediators of bone loss that leads, for example, to osteoporosis. Interleukin (IL)-1 receptor (IL-1R)-associated kinase M (IRAK-M) is only expressed in cells of the myeloid lineage and it inhibits signaling downstream of IL-1R and Toll-like receptors (TLRs). However, it lacks a functional catalytic site and, thus, cannot function as a kinase. IRAK-M associates with, and prevents the dissociation of, IRAK-IRAK-4-TNF receptor-associated factor 6 from the TLR signaling complex, with resultant disruption of downstream signaling. Thus, IRAK-M acts as a dominant negative IRAK. We show here that mice that lack IRAK-M develop severe osteoporosis, which is associated with the accelerated differentiation of osteoclasts, an increase in the half-life of osteoclasts, and their activation. Ligation of IL-1R or TLRs results in hyperactivation of NF-kappaB and mitogen-activated protein kinase signaling pathways, which are essential for osteoclast differentiation. Thus, IRAK-M is a key regulator of the bone loss that is due to osteoclastic resorption of bone.

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PubMed2005

TLR ligands can activate dendritic cells to provide a MyD88-dependent negative signal for Th2 cell development.

During infection, CD4(+) Th cell responses polarize to become primarily Th1 or Th2. Th1 cells, which make IFN-gamma, are crucial for immunity to many bacterial and protozoal infections, whereas Th2 cells, which make IL-4, IL-5, and IL-13, are important for resistance to helminth infections. Polarized Th1 responses are induced by dendritic cells (DCs), which respond to pathogen-derived TLR ligands to produce IL-12 and related cytokines that are instrumental in Th1 cell outgrowth, and coordinately process and present Ag in the context of MHC class II to activate naive Th cells. In this study we show that in addition to providing positive signals for Th1 cell development, mouse DCs activated by TLR engagement can also provide a potent negative signal that prevents the development of Th2 cells. Production of this signal, which is not IL-12, IL-18, IL-23, IL-27, or IFN-gamma and is not provided via Th1 cells, is dependent upon a MyD88-dependent, TNF receptor-associated factor-6-independent signaling pathway in DCs. The signal is released from DCs in response to activation via TLR ligands and exerts an effect directly on Th cells rather than through a third-party cell. Our findings indicate that DCs can provide potent negative as well as positive instruction for Th response polarization, and that these instructional signals are distinct and independent.

Adaptor Proteins, Signal Transducing↗
PubMed2005

Strength of TRAF6 signalling determines osteoclastogenesis.

TRANCE/TRAF6 signalling governs osteoclastogenesis in vivo. Only the TRANCE receptor (TRANCE-R) has been shown to induce osteoclastogenesis, even though other immune receptors, including CD40 and IL-1R/Toll-like receptor, use TRAF6 to activate overlapping signalling cascades. These observations led us to question whether qualitative or quantitative differences exist between the TRAF6-mediated signals induced by TRANCE and by other ligand-receptor pairs. Here we show that stimulation by overexpressed wild-type CD40 can induce osteoclastogenesis. Stimulation through modified CD40 containing increased numbers of TRAF6-binding sites in the cytoplasmic tails showed a dose-dependent increase in the activation of p38 kinase and more pronounced osteoclastogenesis. Moreover, precursors overexpressing TRAF6 differentiate into osteoclasts in the absence of additional signals from TRANCE. Our results suggest that differences in the osteoclastogenesis-inducing capacity of TRANCE-R versus other TRAF6-associated receptors may in part stem from a quantitative difference in the TRAF6-mediated signals.

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PubMed2005

Osteoimmunology.

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