Deficiency of chemokine receptor CCR1 causes osteopenia due to impaired functions of osteoclasts and osteoblasts

Chemokine receptor 5 signaling in oral diseases and degenerative temporomandibular joint disease

BACKGROUND

Chemokine receptor 5 (CCR5)-mediated signals are involved in various biological responses and inflammatory diseases. Recent studies have revealed the roles of this signaling pathway in bone metabolism, metabolic bone diseases, and joint diseases.

HIGHLIGHT

Through preclinical and clinical studies, our research group has demonstrated that CCR5 signaling is deeply involved in degenerative changes in the temporomandibular joint (TMJ).

CONCLUSION

In this short review, we outline the diverse functions of CCR5 signaling in oral and degenerative TMJ diseases.

Exploring biological mechanisms in orthodontic tooth movement: Bridging the gap between basic research experiments and clinical applications - A comprehensive review

OBJECTIVES

The molecular mechanisms behind orthodontic tooth movements (OTM) were investigated by clarifying the role of chemical messengers released by cells.

METHODS

Using the Cochrane library, Google scholar, and PubMed databases, a literature search was conducted, and studies published from 1984 to 2024 were considered.

RESULTS

Both bone growth and remodeling may occur when a tooth is subjected to mechanical stress. These chemicals have a significant effect on the stimulation and regulation of osteoblasts, osteoclasts, and osteocytes during alveolar bone remodeling. This regulation can take place in pathological conditions, such as periodontal diseases, or during OTM alone. This comprehensive review outlines key molecular mechanisms underlying OTM and explores various clinical assumptions associated with specific molecules and their functional domains during this process. Furthermore, clinical applications of certain molecules such as relaxin, prostaglandin E (PGE), and interleukin-1β (IL-1β) in accelerating OTM have been reported. Our findings underscore the existing gap between OTM clinical applications and basic research investigations.

CONCLUSION

A comprehensive understanding of orthodontic treatment is enriched by insights into biological systems. We reported the activation of osteoblasts, osteoclast precursor cells, osteoclasts, and osteocytes in response to mechanical stress, leading to targeted cellular and molecular interventions and facilitating rapid and regulated alveolar bone remodeling during tooth movement. Despite the shortcomings of clinical studies in accelerating OTM, this review highlights the crucial role of biological agents in this process and advocates for prioritizing high-quality human studies in future research to gain further insights from clinical trials.

To Evaluate the Value of Vertebral Body Cortical Thickness in Predicting Osteoporosis by Opportunistic CT

BACKGROUND

In vertebrae, the amount of cortical bone has been estimated at 30-60%, but 45-75% of axial load on a vertebral body is borne by cortical bone (1).

RATIONALE AND OBJECTIVES

The purpose of this study is to investigate the accuracy, sensitivity, specificity, positive predictive value and negative predictive value of vertebral body cortical thickness in predicting osteoporosis (OP) by analyzing the relationship between vertebral body cortical thickness and bone mineral density (BMD) in different age and gender groups. The optimal diagnostic cut-off value of vertebral body cortical thickness in predicting OP was analyzed.

MATERIALS AND METHODS

The data of 150 patients (50-89 years old) who underwent chest or abdominal Quantitative computed tomography (QCT) scan (obtained in one scan) in our hospital from July 2021 to July 2022 were retrospectively analyzed. The average volume bone mineral density (vBMD) of L1-L2 vertebral bodies was obtained and grouped according to BMD, age, and gender. According to BMD, the patients were divided into three groups: osteoporosis, osteopenia and normal. According to age, the patients were divided into three groups: 50-59 years, 60-69 years and ≥70 years. The axial images of T11, T12 and L1 were reconstructed with 1.25 mm slice thickness by AW4.7 workstation provided by General Electric Co (GE) Company. The images were imported into the computed tomography (CT) Spine Bone Quantification System software for spine analysis, and the vertebral body cortical thickness values were obtained. CT Spine Bone Quantification System is a software for quantitative analysis and separation of cortical bone and cancellous bone.

RESULTS

A total of 150 patients were enrolled in this study, including 49 patients in the osteoporosis group, 51 patients in the osteopenia group, and 50 patients in the normal group. The cortical thickness values of T11, T12 and L1 were positively correlated with BMD, and the correlation coefficient was 0.750 at T11. According to the receiver operating characteristic (ROC) curve analysis of T11, T12, L1 cortical thickness value and BMD, OP was diagnosed when T11 < 2.75 mm, T12 < 3.06 mm, and L1 < 2.67 mm. The sensitivity was 83.67%, 87.76%, 75.51%, respectively. The specificity was 79.21%, 71.29% and 90.10%, respectively, and the difference was statistically significant.

CONCLUSION

Vertebral body cortical thickness is correlated with BMD and age. According to the cut-off value of different vertebral bodies, OP can be predicted when T11 < 2.75 mm or T12 < 3.06 mm or L1 < 2.67 mm.

JAK inhibition ameliorates bone destruction by simultaneously targeting mature osteoclasts and their precursors

BACKGROUND

Rheumatoid arthritis (RA) is characterized by chronic inflammation and resultant cartilage/bone destruction because of aberrantly activated osteoclasts. Recently, novel treatments with several Janus kinase (JAK) inhibitors have been shown to successfully ameliorate arthritis-related inflammation and bone erosion, although their mechanisms of action for limiting bone destruction remain unclear. Here, we examined the effects of a JAK inhibitor on mature osteoclasts and their precursors by intravital multiphoton imaging.

METHODS

Inflammatory bone destruction was induced by local injection of lipopolysaccharides into transgenic mice carrying reporters for mature osteoclasts or their precursors. Mice were treated with the JAK inhibitor, ABT-317, which selectively inhibits the activation of JAK1, and then subjected to intravital imaging with multiphoton microscopy. We also used RNA sequencing (RNA-Seq) analysis to investigate the molecular mechanism underlying the effects of the JAK inhibitor on osteoclasts.

RESULTS

The JAK inhibitor, ABT-317, suppressed bone resorption by blocking the function of mature osteoclasts and by targeting the migratory behaviors of osteoclast precursors to the bone surface. Further exhaustive RNA-Seq analysis demonstrated that Ccr1 expression on osteoclast precursors was suppressed in the JAK inhibitor-treated mice; the CCR1 antagonist, J-113863, altered the migratory behaviors of osteoclast precursors, which led to the inhibition of bone destruction under inflammatory conditions.

CONCLUSIONS

This is the first study to determine the pharmacological actions by which a JAK inhibitor blocks bone destruction under inflammatory conditions; this inhibition is beneficial because of its dual effects on both mature osteoclasts and immature osteoclast precursors.

Association between an Increased Serum CCL5 Level and Pathophysiology of Degenerative Joint Disease in the Temporomandibular Joint in Females

Degenerative joint disease of the temporomandibular joints (DJD-TMJ) clinically manifests with symptoms such as orofacial pain, joint sounds and limited jaw movements. Our research group previously reported the functional necessity of a chemokine-chemokine receptor axis of CCL5-CCR5 in osteoclasts. Accumulated studies reported that this axis was involved in the pathogenesis of bone and joint destructive diseases, suggesting CCL5 as a potent biomarker. This study investigated whether or not the serum level of CCL5 can be a biomarker of DJD-TMJ and concomitantly analyzed changes in the serum and urine levels of bone markers to see whether or not changes in the rate of bone metabolism were predisposing. We enrolled 17 female subjects with diagnosed DJD-TMJ and sexually and age-matched 17 controls. The serum CCL5 level in DJD-TMJ subjects was significantly higher than that in the control subjects. Multivariate analyses indicated an association between an augmented CCL5 level and the rate of bone metabolism, especially in relatively young DJD-TMJ subjects without other systemic symptoms. A principal component analysis of serum markers and our pharmacological experiment using a postmenopausal model of ovariectomized rats suggested that an augmented serum CCL5 level specifically reflected DJD-TMJ and that covert changes in the rate of bone metabolism predisposed individuals to DJD-TMJ.

The Fractalkine‐CX3CR1 axis regulates non‐inflammatory osteoclastogenesis by enhancing precursor cell survival

The chemokine fractalkine (FKN) is produced by various cell types, including osteoblasts and endothelial cells in bone tissue, and signals through a sole receptor, CX3CR1, which is expressed on monocytes/macrophages, including osteoclast precursors (OCPs). However, the direct effects of FKN signaling on osteoclast lineage cells under homeostatic noninflammatory conditions remain unclear. Here, we report that FKN regulates mouse OCP survival and primes OCPs for subsequent osteoclast differentiation. Wild‐type but not CX3CR1‐deficient OCPs grown on immobilized FKN showed enhanced osteoclast formation following receptor activator of NF‐κB ligand (RANKL) stimulation, with increased expression of osteoclast differentiation markers. Interestingly, the growth of OCPs on immobilized FKN increased the expression of Cx3cr1 and Tnfrsf11a (Rank) transcripts, but following RANKL stimulation, OCPs rapidly downregulated Cx3cr1 expression. Consistently, anti‐FKN monoclonal antibody (mAb) treatment attenuated RANKL‐induced osteoclast formation on immobilized FKN before, but not during, RANKL stimulation. CX3CR1 and RANK proteins were highly expressed on bone marrow‐derived CD11b^high CD115⁺ OCPs. Growth on immobilized FKN prior to RANKL stimulation also increased CD11b^high CD115⁺ OCP number and their survival and differentiation potential. In a RANKL‐based mouse model of bone loss, anti‐FKN mAb pretreatment significantly inhibited RANKL‐dependent bone loss. Thus, blocking the FKN‐CX3CR1 axis could represent a therapeutic option in noninflammatory bone loss diseases. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Interspecies Single-Cell RNA-Seq Analysis Reveals the Novel Trajectory of Osteoclast Differentiation and Therapeutic Targets

Bone turnover is finely tuned by cells in the bone milieu, including osteoblasts, osteoclasts, and osteocytes. Osteoclasts are multinucleated giant cells with a bone-resorbing function that play a critical role in regulating skeletal homeostasis. Osteoclast differentiation is characterized by dramatic changes in morphology and gene expression following receptor activator of nuclear factor-kappa-Β ligand (RANKL) stimulation. We performed single-cell RNA-sequencing analyses of human and murine osteoclast-lineage cells (OLCs) and found that OLCs in the mitotic phase do not differentiate into mature osteoclasts. We also identified a guanosine triphosphatase (GTPase) family member, RAB38, as a highly expressed molecule in both human and murine osteoclast clusters; RAB38 gene expression is associated with dynamic changes in histone modification and transcriptional regulation. Silencing Rab38 expression by using short hairpin RNA (shRNA) inhibited osteoclast differentiation and maturation. In summary, we established an integrated fate map of human and murine osteoclastogenesis; this will help identify therapeutic targets in bone diseases. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Targeting Chemokine Receptor CCR1 as a Potential Therapeutic Approach for Multiple Myeloma

Multiple myeloma is an incurable plasma B-cell malignancy with 5-year survival rates approximately 10-30% lower than other hematologic cancers. Treatment options include combination chemotherapy followed by autologous stem cell transplantation. However, not all patients are eligible for autologous stem cell transplantation, and current pharmacological agents are limited in their ability to reduce tumor burden and extend multiple myeloma remission times. The "chemokine network" is comprised of chemokines and their cognate receptors, and is a critical component of the normal bone microenvironment as well as the tumor microenvironment of multiple myeloma. Antagonists targeting chemokine-receptor 1 (CCR1) may provide a novel approach for treating multiple myeloma. In vitro CCR1 antagonists display a high degree of specificity, and in some cases signaling bias. In vivo studies have shown they can reduce tumor burden, minimize osteolytic bone damage, deter metastasis, and limit disease progression in multiple myeloma models. While multiple CCR1 antagonists have entered the drug pipeline, none have entered clinical trials for treatment of multiple myeloma. This review will discuss whether current CCR1 antagonists are a viable treatment option for multiple myeloma, and studies aimed at identifying which CCR1 antagonist(s) are most appropriate for this disease.

Chemokine ligand 28 (CCL28) negatively regulates trabecular bone mass by suppressing osteoblast and osteoclast activities

INTRODUCTION

Bone metabolism imbalances cause bone metabolism diseases, like osteoporosis, through aging. Although some chemokines are known to be involved in bone mass regulation, many have not been investigated. Thus, the present study aimed to investigate the role of chemokine ligand 28 (CCL28) on bone metabolism.

MATERIALS AND METHODS

To investigate the role of CCL28 on bone metabolism, 10-week-old male wild-type and Ccl28 knockout (Ccl28 KO) mice were analyzed. Microcomputed tomography analysis and bone tissue morphometry were used to investigate the effect of Ccl28 deficiency on the bone. CCL28 localization in bone tissue was assumed by immunohistochemistry. Osteoblast and osteoclast markers were evaluated by enzyme-linked immunosorbent assay and quantitative reverse transcription-polymerase chain reaction. Finally, in vitro experiments using MC3T3-E1 and bone marrow macrophages revealed the direct effect of CCL28 on osteoblast and osteoclast.

RESULTS

This study showed that Ccl28 deficiency significantly increased bone mass and the number of mature osteoblasts. Immunoreactivity for CCL28 was observed in osteoblasts and osteoclasts on bone tissue. Additionally, Ccl28 deficiency promoted osteoblast and osteoclast maturation. Moreover, CCL28 treatment decreased osteoblast and osteoclast activities but did not affect differentiation.

CONCLUSION

In summary, this study indicated that CCL28 is one of the negative regulators of bone mass by suppressing osteoblast and osteoclast activities. These results provide important insights into bone immunology and the selection of new osteoporosis treatments.

Involvement of Eotaxins (CCL11, CCL24, CCL26) in Pathogenesis of Osteopenia and Osteoporosis

BACKGROUND

The purpose of this study was to investigate the role of eotaxin family members including C-C motif chemokine 11 (CCL11), C-C motif chemokine 24 (CCL24), and C-C motif chemokine 26 (CCL26) as the subgroups of CC-chemokine in patients affected with osteoporosis and osteopenia.

METHODS

Overall, 19 osteoporotic patients, 18 osteopenic individuals, and 20 healthy subjects were recruited in this study. The bone mineral density (BMD) was then measured at the lumbar spine (L1-L4) and the hip (femoral neck and total hip) using dual-energy X-ray absorptiometry for diagnosis of bone density and related disorders. Additionally, enzyme-linked immunosorbent assay (ELISA) technique was employed to measure the serum levels of CCL11, CCL24, and CCL26.

RESULTS

The circulating levels of CCL11, CCL24, and CCL26 had been increased in both groups of patients with osteopenia and osteoporosis compared to those in healthy subjects (P<0.05); while no significant difference was observed between serum levels of these chemokines in such patients.

CONCLUSION

Eotaxins can play a role in the pathogenesis of osteoporosis and osteopenia; however, further studies are needed to clarify various roles of eotaxins in the pathophysiology of osteoporosis and osteopenia.

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.

Osteoclasts Derive Predominantly from Bone Marrow-Resident CX3CR1+ Precursor Cells in Homeostasis, whereas Circulating CX3CR1+ Cells Contribute to Osteoclast Development during Fracture Repair

Osteoclasts (OC) originate from either bone marrow (BM)-resident or circulating myeloid OC progenitors (OCP) expressing the receptor CX₃CR1. Multiple lines of evidence argue that OCP in homeostasis and inflammation differ. We investigated the relative contributions of BM-resident and circulating OCP to osteoclastogenesis during homeostasis and fracture repair. Using CX₃CR1-EGFP/TRAP tdTomato mice, we found CX₃CR1 expression in mononuclear cells, but not in multinucleated TRAP⁺ OC. However, CX₃CR1-expressing cells generated TRAP⁺ OC on bone within 5 d in CX₃CR1CreERT2/Ai14 tdTomato reporter mice. To define the role that circulating cells play in osteoclastogenesis during homeostasis, we parabiosed TRAP tdTomato mice (CD45.2) on a C57BL/6 background with wild-type (WT) mice (CD45.1). Flow cytometry (CD45.1/45.2) demonstrated abundant blood cell mixing between parabionts after 2 wk. At 4 wk, there were numerous tdTomato⁺ OC in the femurs of TRAP tdTomato mice but almost none in WT mice. Similarly, cultured BM stimulated to form OC demonstrated multiple fluorescent OC in cell cultures from TRAP tdTomato mice, but not from WT mice. Finally, flow cytometry confirmed low-level engraftment of BM cells between parabionts but significant engraftment in the spleens. In contrast, during fracture repair, we found that circulating CX₃CR1⁺ cells migrated to bone, lost expression of CX₃CR1, and became OC. These data demonstrate that OCP, but not mature OC, express CX₃CR1 during both homeostasis and fracture repair. We conclude that, in homeostasis mature OC derive predominantly from BM-resident OCP, whereas during fracture repair, circulating CX₃CR1⁺ cells can become OC.

Chemokines and Bone

Chemokines are a family of small proteins, subdivided by their conserved cysteine residues and common structural features. Chemokines interact with their cognate G-protein-coupled receptors to elicit downstream signals that result in cell migration, proliferation, and survival. This review presents evidence for how the various CXC and CC subfamily chemokines influence bone hemostasis by acting on osteoclasts, osteoblasts, and progenitor cells. Also discussed are the ways in which chemokines contribute to bone loss as a result of inflammatory diseases such as rheumatoid arthritis, HIV infection, and periodontal infection. Both positive and negative effects of chemokines on bone formation and bone loss are presented. In addition, the role of chemokines in altering the bone microenvironment through effects on angiogenesis and tumor invasion is discussed. Very few therapeutic agents that influence bone formation by targeting chemokines or chemokine receptors are available, although a few are currently being evaluated.

Chemokines in Physiological and Pathological Bone Remodeling

The bone matrix is constantly remodeled by bone-resorbing osteoclasts and bone-forming osteoblasts. These two cell types are fundamentally different in terms of progenitor cells, mode of action and regulation by specific molecules, acting either systemically or locally. Importantly, there is increasing evidence for an impact of cell types or molecules of the adaptive and innate immune system on bone remodeling. Understanding these influences is the major goal of a novel research area termed osteoimmunology, which is of key relevance in the context of inflammation-induced bone loss, skeletal metastases, and diseases of impaired bone remodeling, such as osteoporosis. This review article aims at summarizing the current knowledge on one particular aspect of osteoimmunology, namely the impact of chemokines on skeletal cells in order to regulate bone remodeling under physiological and pathological conditions. Chemokines have key roles in the adaptive immune system by controlling migration, localization, and function of immune cells during inflammation. The vast majority of chemokines are divided into two subgroups based on the pattern of cysteine residues. More specifically, there are 27 known C-C-chemokines, binding to 10 different C-C receptors, and 17 known C-X-C-chemokines binding to seven different C-X-C receptors. Three additional chemokines do not fall into this category, and only one of them, i.e., CX3CL1, has been shown to influence bone remodeling cell types. There is a large amount of published studies demonstrating specific effects of certain chemokines on differentiation and function of osteoclasts and/or osteoblasts. Chemokine signaling by skeletal cells or by other cells of the bone marrow niche regulates bone formation and resorption through autocrine and paracrine mechanisms. In vivo evidence from mouse deficiency models strongly supports the role of certain chemokine signaling pathways in bone remodeling. We will summarize these data in the present review with a special focus on the most established subsets of chemokines. In combination with the other review articles of this issue, the knowledge presented here confirms that there is a physiologically relevant crosstalk between the innate immune system and bone remodeling cell types, whose molecular understanding is of high clinical relevance.

The role of GPCRs in bone diseases and dysfunctions

The superfamily of G protein-coupled receptors (GPCRs) contains immense structural and functional diversity and mediates a myriad of biological processes upon activation by various extracellular signals. Critical roles of GPCRs have been established in bone development, remodeling, and disease. Multiple human GPCR mutations impair bone development or metabolism, resulting in osteopathologies. Here we summarize the disease phenotypes and dysfunctions caused by GPCR gene mutations in humans as well as by deletion in animals. To date, 92 receptors (5 glutamate family, 67 rhodopsin family, 5 adhesion, 4 frizzled/taste2 family, 5 secretin family, and 6 other 7TM receptors) have been associated with bone diseases and dysfunctions (36 in humans and 72 in animals). By analyzing data from these 92 GPCRs, we found that mutation or deletion of different individual GPCRs could induce similar bone diseases or dysfunctions, and the same individual GPCR mutation or deletion could induce different bone diseases or dysfunctions in different populations or animal models. Data from human diseases or dysfunctions identified 19 genes whose mutation was associated with human BMD: 9 genes each for human height and osteoporosis; 4 genes each for human osteoarthritis (OA) and fracture risk; and 2 genes each for adolescent idiopathic scoliosis (AIS), periodontitis, osteosarcoma growth, and tooth development. Reports from gene knockout animals found 40 GPCRs whose deficiency reduced bone mass, while deficiency of 22 GPCRs increased bone mass and BMD; deficiency of 8 GPCRs reduced body length, while 5 mice had reduced femur size upon GPCR deletion. Furthermore, deficiency in 6 GPCRs induced osteoporosis; 4 induced osteoarthritis; 3 delayed fracture healing; 3 reduced arthritis severity; and reduced bone strength, increased bone strength, and increased cortical thickness were each observed in 2 GPCR-deficiency models. The ever-expanding number of GPCR mutation-associated diseases warrants accelerated molecular analysis, population studies, and investigation of phenotype correlation with SNPs to elucidate GPCR function in human diseases.

Influence of Chemokine N-Terminal Modification on Biased Agonism at the Chemokine Receptor CCR1

Leukocyte migration, a hallmark of the inflammatory response, is stimulated by the interactions between chemokines, which are expressed in injured or infected tissues, and chemokine receptors, which are G protein-coupled receptors (GPCRs) expressed in the leukocyte plasma membrane. One mechanism for the regulation of chemokine receptor signaling is biased agonism, the ability of different chemokine ligands to preferentially activate different intracellular signaling pathways via the same receptor. To identify features of chemokines that give rise to biased agonism, we studied the activation of the receptor CCR1 by the chemokines CCL7, CCL8, and CCL15(Δ26). We found that, compared to CCL15(Δ26), CCL7 and CCL8 exhibited biased agonism towards cAMP inhibition and away from β-Arrestin 2 recruitment. Moreover, N-terminal substitution of the CCL15(Δ26) N-terminus with that of CCL7 resulted in a chimera with similar biased agonism to CCL7. Similarly, N-terminal truncation of CCL15(Δ26) also resulted in signaling bias between cAMP inhibition and β-Arrestin 2 recruitment signals. These results show that the interactions of the chemokine N-terminal region with the receptor transmembrane region play a key role in selecting receptor conformations coupled to specific signaling pathways.

Chemokine receptor 3 is a negative regulator of trabecular bone mass in female mice

Chemokines are secreted by a wide variety of cells; their functions are dependent on the binding to their chemokine receptors (CCRs) which induce directed chemotaxis in nearby responsive cells. Chemokines and their receptors can be induced under several different conditions. Based on data from clinical studies showing an increased expression of chemokine receptor 3 (CCR3) in circulating monocytes of human subjects with lower bone mineral density (BMD) as compared to those with high BMD, we predicted a role for CCR3 in the development of peak bone mass. We, therefore, first evaluated the expression pattern of Ccr3 in bone cells, in comparison to other CCRs, that have common ligands with CCR3. While Ccr1 and Ccr3 messenger RNA (mRNA) levels increased during both RANKL-induced osteoclast differentiation and AA-induced osteoblast differentiation, the levels of Ccr5 mRNA only increased during osteoblast differentiation. To examine if CCR3 influences osteoclast and/or osteoblast differentiation, we evaluated the consequence of blocking CCR3 function using neutralizing antibody on the expression of osteoclast and osteoblast differentiation markers. Treatment with CCR3 neutralizing antibody increased mRNA levels of Trap and cathepsin K in osteoclasts and osteocalcin in osteoblasts compared to cells treated with control IgG. Based on these in vitro findings, we next assessed the role of CCR3 in vivo by evaluating the skeletal phenotypes of Ccr3 knockout and corresponding control littermate mice. Disruption of CCR3 resulted in a significant increase in femur areal BMD at 5 and 8 weeks of age by dual-energy X-ray absorptiometry. Micro-CT analysis revealed a 25% increase in trabecular bone mass at 10 weeks of age caused by corresponding changes in trabecular number and thickness compared to wild type mice. Based on our findings, we conclude that disruption of CCR3 function favors bone mass accumulation, in part via enhancement of bone metabolism. Understanding the molecular pathways through which CCR3 acts to regulate osteoclast and osteoblast functions could lead to new therapeutic approaches to prevent inflammation-induced bone loss.

Significance of the Tks4 scaffold protein in bone tissue homeostasis

The main driver of osteoporosis is an imbalance between bone resorption and formation. The pathogenesis of osteoporosis has also been connected to genetic alterations in key osteogenic factors and dysfunction of bone marrow mesenchymal stem/stromal cells (BM-MSCs). Tks4 (encoded by the Sh3pxd2b gene) is a scaffold protein involved in podosome organization. Homozygous mutational inactivation of Sh3pxd2b causes Frank-ter Haar syndrome (FTHS), a genetic disease that affects bone tissue as well as eye, ear, and heart functions. To date, the role of Tks4 in adult bone homeostasis has not been investigated. Therefore, the aim of this study was to analyze the facial and femoral bone phenotypes of Sh3pxd2b knock-out (KO) mice using micro-CT methods. In addition to the analysis of the Sh3pxd2b-KO mice, the bone microstructure of an FTHS patient was also examined. Macro-examination of skulls from Tks4-deficient mice revealed craniofacial malformations that were very similar to symptoms of the FTHS patient. The femurs of the Sh3pxd2b-KO mice had alterations in the trabecular system and showed signs of osteoporosis, and, similarly, the FTHS patient also showed increased trabecular separation/porosity. The expression levels of the Runx2 and osteocalcin bone formation markers were reduced in the bone and bone marrow of the Sh3pxd2b-KO femurs, respectively. Our recent study demonstrated that Sh3pxd2b-KO BM-MSCs have a reduced ability to differentiate into osteoblast lineage cells; therefore, we concluded that the Tks4 scaffold protein is important for osteoblast formation, and that it likely plays a role in bone cell homeostasis.

Evaluation and extension of the two-site, two-step model for binding and activation of the chemokine receptor CCR1

Interactions between secreted immune proteins called chemokines and their cognate G protein-coupled receptors regulate the trafficking of leukocytes in inflammatory responses. The two-site, two-step model describes these interactions. It involves initial binding of the chemokine N-loop/β3 region to the receptor's N-terminal region and subsequent insertion of the chemokine N-terminal region into the transmembrane helical bundle of the receptor concurrent with receptor activation. Here, we test aspects of this model with C-C motif chemokine receptor 1 (CCR1) and several chemokine ligands. First, we compared the chemokine-binding affinities of CCR1 with those of peptides corresponding to the CCR1 N-terminal region. Relatively low affinities of the peptides and poor correlations between CCR1 and peptide affinities indicated that other regions of the receptor may contribute to binding affinity. Second, we evaluated the contributions of the two CCR1-interacting regions of the cognate chemokine ligand CCL7 (formerly monocyte chemoattractant protein-3 (MCP-3)) using chimeras between CCL7 and the non-cognate ligand CCL2 (formerly MCP-1). The results revealed that the chemokine N-terminal region contributes significantly to binding affinity but that differences in binding affinity do not completely account for differences in receptor activation. On the basis of these observations, we propose an elaboration of the two-site, two-step model-the "three-step" model-in which initial interactions of the first site result in low-affinity, nonspecific binding; rate-limiting engagement of the second site enables high-affinity, specific binding; and subsequent conformational rearrangement gives rise to receptor activation.

Chemokine analysis as a novel diagnostic modality in the early prediction of the outcome of non-union therapy: a matched pair analysis

BACKGROUND

Despite the regenerative capability of skeletal tissue fracture, non-union is common. Treatment of non-unions remains challenging, and early determination of the outcome is impossible. Chemokines play an important role in promoting the formation of new bone and remodeling existing bone. Despite their importance regarding the regulation of bone biology, the potential of chemokines as biological markers reflecting osseous regeneration is unknown. The purpose of this study was to determine (1) if serum chemokine expression levels correlate with the outcome of non-union surgery and (2) if chemokine expression analysis can be used to identify patients at risk for treatment failure.

METHODS

Non-union patients receiving surgical therapy in our institution between March 2012 and March 2014 were prospectively enrolled in a clinical observer study. Regular clinical and radiological follow-up was conducted for 12 months including collection of blood during the first 12 weeks. Based on the outcome, patients were declared as responders or non-responders to the therapy. To minimize biases, patients were matched (age, sex, body mass index (BMI)) and two groups of patients could be formed: responders (R, n = 10) and non-responders (NR, n = 10). Serum chemokine expression (CCL-2, CCL-3, CCL-4, CXCL-10, CCL-11, and interferon gamma (IFN-γ)) was analyzed using Luminex assays. Data was compared and correlated to the outcome.

RESULTS

CCL-3 expression in NR was significantly higher during the course of the study compared to R (p = 0.002), and the expression pattern of CCL-4 correlated with CCL-3 in both groups (NR: p < 0.001 and r = 0.63). IFN-γ expression in NR was continuously higher than in R (p < 0.001), and utilization of CCL-3 and IFN-γ serum expression levels 2 weeks after the treatment resulted in a predictive model that had an AUC of 0.92 (CI 0.74-1.00).

CONCLUSION

Serum chemokine expression analysis over time is a valid and promising diagnostic tool. The chemokine expression pattern correlates with the outcome of the Masquelet therapy of lower limb non-unions. Utilization of the serum analysis of CCL-3 and IFN-γ 2 weeks after the treatment resulted in an early predictive value regarding the differentiation between patients that are likely to heal and those that are prone to high risk of treatment failure.

The HIV co-receptor CCR5 regulates osteoclast function

C–C chemokine receptor 5 (CCR5) is a co-receptor of HIV. Epidemiological findings suggest that the functional loss of CCR5 is correlated with a lower incidence of bone-destructive diseases as well as of HIV transmission. However, it is not clear whether CCR5 is involved in regulation of the function of bone cells, in addition to that of immune cells. Here we show that blockade of CCR5 using specific antibodies impairs human osteoclast function in vitro. Ccr5-deficient (Ccr5^−/−) mice presented with dysfunctional osteoclasts and were resistant to osteoporosis induced by receptor activator of nuclear factor kappa-B ligand (RANKL), which triggers osteoporosis independently of inflammatory and immunomodulatory pathways. Furthermore, Ccr5 deficiency impairs the cellular locomotion and bone-resorption activity of osteoclasts, which is associated with the disarrangement of podosomes and adhesion complex molecules including Pyk2. Overall, the data provides evidence that CCR5 has an essential role in bone-destructive conditions through the functional regulation of osteoclasts.

CCR5 is a co-receptor for HIV, and loss of function is associated with lower incidence of HIV but also with bone-destructive diseases. Here the authors show that ablation of CCR5 impairs osteoclast function and improves resistance to osteoporosis in mouse models.

CCL11, a novel mediator of inflammatory bone resorption

Normal bone homeostasis, which is regulated by bone-resorbing osteoclasts and bone-forming osteoblasts is perturbed by inflammation. In chronic inflammatory disease with disturbed bone remodelling, e.g. rheumatoid arthritis, patients show increased serum levels of the chemokine eotaxin-1 (CCL11). Herein, we demonstrate an inflammatory driven expression of CCL11 in bone tissue and a novel role of CCL11 in osteoclast migration and resorption. Using an inflammatory bone lesion model and primary cell cultures, we discovered that osteoblasts express CCL11 in vivo and in vitro and that expression increased during inflammatory conditions. Osteoclasts did not express CCL11, but the high affinity receptor CCR3 was significantly upregulated during osteoclast differentiation and found to colocalise with CCL11. Exogenous CCL11 was internalised in osteoclast and stimulated the migration of pre-osteoclast and concomitant increase in bone resorption. Our data pinpoints that the CCL11/CCR3 pathway could be a new target for treatment of inflammatory bone resorption.

Annulus fibrosus cells express and utilize C-C chemokine receptor 5 (CCR5) for migration

BACKGROUND CONTEXT

Disc degeneration is associated with the progressive loss of the proteoglycan content of the intervertebral disc, decreased matrix synthesis, higher concentrations of proteolytic enzymes, and increased levels of proinflammatory cytokines. In previous studies, we have shown that C-C chemokine ligand (CCL)2, CCL3, and CCL5 are highly expressed by cultured nucleus pulposus (NP) and annulus fibrosus (AF) cells that have been treated by interleukin-1. The major function of these chemokines is to recruit immune cells into the disc. It is unclear if disc cells can respond to these chemokines. Recent studies by Phillips et al. (2015) showed that NP cells express a number of cytokines and chemokine receptors.

PURPOSE

The purpose of this study is to determine the gene and protein expression of C-C chemokine receptor (CCR)1, CCR2, and CCR5 in NP and AF cells, and to test if these receptors can respond to their ligands in these cells by cell signaling and migration.

STUDY DESIGN/SETTING

This is an in vitro study.

METHODS

For RNA, surface expression, and cell signaling studies, human cells were isolated from the NP and AF tissues collected after spine surgery or from donated spine segments (Gift of Hope Human Donor & Tissue Network of Illinois) and cultured in monolayer. The gene expression of human CCR1, CCR2, and CCR5 was analyzed using real-time polymerase chain reaction. The surface expression of CCR1, CCR2, and CCR5 was analyzed using flow cytometry and fluorescently tagged antibodies specific for these proteins. Extracellular signal-regulated kinase (ERK) phosphorylation was analyzed from the cell lysates of NP and AF cells treated with CCL2 and CCL5 for 1 hour using enzyme-linked immunosorbent assay. Migration of primary rabbit AF cells was assayed using 8-µm Corning Transwell inserts in the presence or absence of CCL5. This study was partially funded by a North American Spine Society 2014 Basic Research Grant Award ($50,000).

RESULTS

RNA analysis showed that gene expression of CCR1, CCR2, and CCR5 was evident in human NP and AF cells (n=6). Only a small population of NP and AF cells expressed CCR1 (1.9% and 1.2%, respectively) and CCR2 (0.8% and 1.4%, respectively) on the cell surface, whereas a larger percentage expressed CCR5 (12.7% and 11.6%, respectively). Significantly higher levels of ERK phosphorylation were detected in AF cells after treatment with CCL5 and not CCL2. Treatment with either chemokine did not cause significantly higher ERK phosphorylation in NP cells. There was an increase in average AF cell migration in the presence of CCL5. The increase was significant when the migration was induced with CCL5 (500 ng/mL) at both 2- and 6-hour time points.

CONCLUSIONS

CCR5 is expressed at the RNA level and on the cell surface of NP and AF cells. In the presence of CCL5, we detected increased levels of ERK phosphorylation and AF cell migration, suggesting that the CCR5 receptors in AF cells are functional. These data suggest that AF cells may have the ability to migrate in response to disc damage or inflammation.

Inflammatory Cell Migration in Rheumatoid Arthritis: A Comprehensive Review

Rheumatoid arthritis (RA) is a chronic inflammatory autoimmune disease that primarily affects the joints. Self-reactive B and T lymphocytes cooperate to promote antibody responses against self proteins and are major drivers of disease. T lymphocytes also promote RA independently of B lymphocytes mainly through the production of key inflammatory cytokines, such as IL-17, that promote pathology. While the innate signals that initiate self-reactive adaptive immune responses are poorly understood, the disease is predominantly caused by inflammatory cellular infiltration and accumulation in articular tissues, and by bone erosions driven by bone-resorbing osteoclasts. Osteoclasts are giant multinucleated cells formed by the fusion of multiple myeloid cells that require short-range signals, such as the cytokines MCSF and RANKL, for undergoing differentiation. The recruitment and positioning of osteoclast precursors to sites of osteoclast differentiation by chemoattractants is an important point of control for osteoclastogenesis and bone resorption. Recently, the GPCR EBI2 and its oxysterol ligand 7a, 25 dihydroxycholesterol, were identified as important regulators of osteoclast precursor positioning in proximity to bone surfaces and of osteoclast differentiation under homeostasis. In chronic inflammatory diseases like RA, osteoclast differentiation is also driven by inflammatory cytokines such as TNFa and IL-1, and can occur independently of RANKL. Finally, there is growing evidence that the chemotactic signals guiding osteoclast precursors to inflamed articular sites contribute to disease and are of great interest. Furthering our understanding of the complex osteoimmune cell interactions should provide new avenues of therapeutic intervention for RA.

CCL2 and CCR2 are Essential for the Formation of Osteoclasts and Foreign Body Giant Cells

Osteoclasts are multinucleated cells responsible for bone resorption. They are derived from the fusion of cells in the monocyte/macrophage lineage. Monocytes and macrophages can also fuse to form foreign body giant cells (FBGC). Foreign body giant cells are observed at the interface between a host and a foreign body such as implants during a foreign body reaction. Macrophages are attracted to the site of bone resorption and foreign body reactions by different cytokines. Chemokine (C-C) ligand-2 (CCL2) is an important chemotactic factor and binds to a receptor CCR2. In this study we investigated the importance of CCL2 and the receptor CCR2 in the formation of osteoclasts and FBGC. CCL2 mRNA was more highly expressed in giant cell culture than macrophages, being 9-fold and 16-fold more abundant in osteoclasts and FBGC respectively. Significantly fewer osteoclasts and FBGC were cultured from the bone marrow of CCL2 and CCR2 knockout mice, when compared to wild type. Not only were the number of giant cells reduced but there was a significant reduction in the number of nuclei and the size of these cells in the cultures of CCL2 and CCR2 knockout mice. Formation of osteoclasts and FBGC were recovered in cultures by addition of exogenous CCL2 to the media containing marrow cells from CCL2-/- mice. We conclude that CCL2 and its receptor CCR2 are important for the formation of osteoclasts and FBGC and absence of these genes causes inhibition of osteoclast and FBGC formation.

Effects of Long-Term Odanacatib Treatment on Bone Gene Expression in Ovariectomized Adult Rhesus Monkeys: Differentiation From Alendronate

Similar efficacy of the cathepsin K inhibitor odanacatib (ODN) and the bisphosphonate alendronate (ALN) in reducing bone turnover markers and increasing bone mineral density in spine and hip were previously demonstrated in ovariectomized (OVX)-monkeys treated for 20 months in prevention mode. Here, we profiled RNA from tibial metaphysis and diaphysis of the same study using Affymetrix microarrays, and selected 204 probe sets (p < 0.001, three-group ANOVA) that were differentially regulated by ODN or ALN versus vehicle. Both drugs produced strikingly different effects on known bone-related genes and pathways at the transcriptional level. Although ALN either reduced or had neutral effects on bone resorption-related genes, ODN significantly increased the expression of osteoclast genes (eg, APC5, TNFRSF11A, CTSK, ITGB3, and CALCR), consistent with previous findings on the effects of this agent in enhancing the number of nonresorbing osteoclasts. Conversely, ALN reduced the expression of known bone formation-related genes (eg, TGFBR1, SPP1, RUNX2, and PTH1R), whereas ODN either increased or had neutral effects on their expression. These differential effects of ODN versus ALN on bone resorption and formation were highly correlative to the changes in bone turnover markers, cathepsin K (Catk) target engagement marker serum C-terminal cross-linked telopeptide (1-CTP) and osteoclast marker tartrate resistant acid phosphatase isoform 5b (TRAP5b) in the same monkeys. Overall, the molecular profiling results are consistent with the known pharmacological actions of these agents on bone remodeling and clearly differentiate the molecular mechanisms of ODN from the bisphosphonates.

Characterization and application of two RANK-specific antibodies with different biological activities

Antibodies play an important role in therapy and investigative biomedical research. The TNF-family member Receptor Activator of NF-κB (RANK) is known for its role in bone homeostasis and is increasingly recognized as a central player in immune regulation and epithelial cell activation. However, the study of RANK biology has been hampered by missing or insufficient characterization of high affinity tools that recognize RANK. Here, we present a careful description and comparison of two antibodies, RANK-02 obtained by phage display (Newa, 2014 [1]) and R12-31 generated by immunization (Kamijo, 2006 [2]). We found that both antibodies recognized mouse RANK with high affinity, while RANK-02 and R12-31 recognized human RANK with high and lower affinities, respectively. Using a cell apoptosis assay based on stimulation of a RANK:Fas fusion protein, and a cellular NF-κB signaling assay, we showed that R12-31 was agonist for both species. R12-31 interfered little or not at all with the binding of RANKL to RANK, in contrast to RANK-02 that efficiently prevented this interaction. Depending on the assay and species, RANK-02 was either a weak agonist or a partial antagonist of RANK. Both antibodies recognized human Langerhans cells, previously shown to express RANK, while dermal dendritic cells were poorly labeled. In vivo R12-31 agonist activity was demonstrated by its ability to induce the formation of intestinal villous microfold cells in mice. This characterization of two monoclonal antibodies should now allow better evaluation of their application as therapeutic reagents and investigative tools.

Oxysterols and EBI2 promote osteoclast precursor migration to bone surfaces and regulate bone mass homeostasis

The mechanisms guiding cells toward bone surfaces are generally unknown. Here, Nevius et al. show that the Gαi protein–coupled receptor EBI2 is expressed in mouse osteoclast precursors to guide these cells toward bone surfaces. Defective EBI2 signaling increased bone mass in male mice and protected female mice from age- and estrogen deficiency–induced osteoporosis.

Bone surfaces attract hematopoietic and nonhematopoietic cells, such as osteoclasts (OCs) and osteoblasts (OBs), and are targeted by bone metastatic cancers. However, the mechanisms guiding cells toward bone surfaces are essentially unknown. Here, we show that the Gαi protein–coupled receptor (GPCR) EBI2 is expressed in mouse monocyte/OC precursors (OCPs) and its oxysterol ligand 7α,25-dihydroxycholesterol (7α,25-OHC) is secreted abundantly by OBs. Using in vitro time-lapse microscopy and intravital two-photon microscopy, we show that EBI2 enhances the development of large OCs by promoting OCP motility, thus facilitating cell–cell interactions and fusion in vitro and in vivo. EBI2 is also necessary and sufficient for guiding OCPs toward bone surfaces. Interestingly, OCPs also secrete 7α,25-OHC, which promotes autocrine EBI2 signaling and reduces OCP migration toward bone surfaces in vivo. Defective EBI2 signaling led to increased bone mass in male mice and protected female mice from age- and estrogen deficiency–induced osteoporosis. This study identifies a novel pathway involved in OCP homing to the bone surface that may have significant therapeutic potential.

CCL5/RANTES is important for inducing osteogenesis of human mesenchymal stem cells and is regulated by dexamethasone

In this study, we examine the effect of chemokine (C-C motif) ligand 5 (CCL5)/Regulated on Activation Normal T cell Expressed and Secreted (RANTES), a pro-inflammatory cytokine on osteogenic differentiation of human mesenchymal stem cells (hMSCs). We found CCL5 expression was increased during osteogenic differentiation of hMSCs and CCL5 expression is dependent on the presence of dexamethasone. Knocking down endogenous CCL5 expression blocked osteogenesis, as revealed by decreasing alkaline phosphatase (ALP) activity and a reduction in the expression levels of ALP, bone sialoprotein (BSP), and osteopontin (OPN). Of note, the overexpression of CCL5 was sufficient to increase ALP expression and activity. Moreover, the down-regulation of chemokine (C-C motif) receptor 1 (CCR1), one of the CCL5 receptors, significantly decreased the osteogenesis of hMSCs. Interestingly, the down-regulation of CCR1, but not CCL5, was sufficient to affect the cell numbers during the process of osteogenesis. Our findings reveal that both CCL5 and CCR1 are required for osteogenesis of human MSCs, CCL5 is sufficient for the osteogenesis, and provide a novel link between dexamethasone and CCL5 in human osteogenesis.

NF-κB-direct activation of microRNAs with repressive effects on monocyte-specific genes is critical for osteoclast differentiation

Background

Monocyte-to-osteoclast conversion is a unique terminal differentiation process that is exacerbated in rheumatoid arthritis and bone metastasis. The mechanisms implicated in upregulating osteoclast-specific genes involve transcription factors, epigenetic regulators and microRNAs (miRNAs). It is less well known how downregulation of osteoclast-inappropriate genes is achieved.

Results

In this study, analysis of miRNA expression changes in osteoclast differentiation from human primary monocytes revealed the rapid upregulation of two miRNA clusters, miR-212/132 and miR-99b/let-7e/125a. We demonstrate that they negatively target monocyte-specific and immunomodulatory genes like TNFAIP3, IGF1R and IL15. Depletion of these miRNAs inhibits osteoclast differentiation and upregulates their targets. These miRNAs are also upregulated in other inflammatory monocytic differentiation processes. Most importantly, we demonstrate for the first time the direct involvement of Nuclear Factor kappa B (NF-κB) in the regulation of these miRNAs, as well as with their targets, whereby NF-κB p65 binds the promoters of these two miRNA clusters and NF-κB inhibition or depletion results in impaired upregulation of their expression.

Conclusions

Our results reveal the direct involvement of NF-κB in shutting down certain monocyte-specific genes, including some anti-inflammatory activities, through a miRNA-dependent mechanism for proper osteoclast differentiation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-014-0561-5) contains supplementary material, which is available to authorized users.

Identification of co-expression gene networks, regulatory genes and pathways for obesity based on adipose tissue RNA Sequencing in a porcine model

BACKGROUND

Obesity is a complex metabolic condition in strong association with various diseases, like type 2 diabetes, resulting in major public health and economic implications. Obesity is the result of environmental and genetic factors and their interactions, including genome-wide genetic interactions. Identification of co-expressed and regulatory genes in RNA extracted from relevant tissues representing lean and obese individuals provides an entry point for the identification of genes and pathways of importance to the development of obesity. The pig, an omnivorous animal, is an excellent model for human obesity, offering the possibility to study in-depth organ-level transcriptomic regulations of obesity, unfeasible in humans. Our aim was to reveal adipose tissue co-expression networks, pathways and transcriptional regulations of obesity using RNA Sequencing based systems biology approaches in a porcine model.

METHODS

We selected 36 animals for RNA Sequencing from a previously created F2 pig population representing three extreme groups based on their predicted genetic risks for obesity. We applied Weighted Gene Co-expression Network Analysis (WGCNA) to detect clusters of highly co-expressed genes (modules). Additionally, regulator genes were detected using Lemon-Tree algorithms.

RESULTS

WGCNA revealed five modules which were strongly correlated with at least one obesity-related phenotype (correlations ranging from -0.54 to 0.72, P < 0.001). Functional annotation identified pathways enlightening the association between obesity and other diseases, like osteoporosis (osteoclast differentiation, P = 1.4E-7), and immune-related complications (e.g. Natural killer cell mediated cytotoxity, P = 3.8E-5; B cell receptor signaling pathway, P = 7.2E-5). Lemon-Tree identified three potential regulator genes, using confident scores, for the WGCNA module which was associated with osteoclast differentiation: CCR1, MSR1 and SI1 (probability scores respectively 95.30, 62.28, and 34.58). Moreover, detection of differentially connected genes identified various genes previously identified to be associated with obesity in humans and rodents, e.g. CSF1R and MARC2.

CONCLUSIONS

To our knowledge, this is the first study to apply systems biology approaches using porcine adipose tissue RNA-Sequencing data in a genetically characterized porcine model for obesity. We revealed complex networks, pathways, candidate and regulatory genes related to obesity, confirming the complexity of obesity and its association with immune-related disorders and osteoporosis.

Chondrocytes play a major role in the stimulation of bone growth by thyroid hormone

Thyroid hormone (T3) is required for postnatal skeletal growth. It exerts its effect by binding to nuclear receptors, TRs including TRα1 and TRβ1, which are present in most cell types. These cell types include chondrocytes and osteoblasts, the interactions of which are known to regulate endochondral bone formation. In order to analyze the respective functions of T3 stimulation in chondrocytes and osteoblasts during postnatal growth, we use Cre/loxP recombination to express a dominant-negative TRα1(L400R) mutant receptor in a cell-specific manner. Phenotype analysis revealed that inhibiting T3 response in chondrocytes is sufficient to reproduce the defects observed in hypothyroid mice, not only for cartilage maturation, but also for ossification and mineralization. TRα1(L400R) in chondrocytes also results in skull deformation. In the meantime, TRα1(L400R) expression in mature osteoblasts has no visible effect. Transcriptome analysis identifies a number of changes in gene expression induced by TRα1(L400R) in cartilage. These changes suggest that T3 normally cross talks with several other signaling pathways to promote chondrocytes proliferation, differentiation, and skeletal growth.

Multiple levels of chemokine receptor regulation in the control of mouse natural killer cell development

Chemokines play a fundamental role in lymphocyte development, mainly attributable to the control of the correct localization in the proper microenvironments of cells undergoing maturation. Natural killer (NK) cell development occurs in the bone marrow (BM) where their localization is regulated by the balance of chemokine function in cell retention into tissues and mobilization into circulation. In addition, NK cells from several extra-medullary tissues are phenotypically and functionally different from their circulating counterpart suggesting that maturation can be completed in organs other than BM. Indeed, a role of chemokines in NK cell localization into tissues during homeostatic conditions is also documented. In this review, we summarize the current notion related to the relevance of several chemokine/chemokine receptor axes in NK cell development with a focus on the regulation of their expression and function.

The expression of Fn14 via mechanical stress-activated JNK contributes to apoptosis induction in osteoblasts

Bone mass is maintained by the balance between the activities of bone-forming osteoblasts and bone-resorbing osteoclasts. It is well known that adequate mechanical stress is essential for the maintenance of bone mass, whereas excess mechanical stress induces bone resorption. However, it has not been clarified how osteoblasts respond to different magnitudes of mechanical stress. Here we report that large-magnitude (12%) cyclic stretch induced Ca(2+) influx, which activated reactive oxygen species generation in MC3T3-E1 osteoblasts. Reactive oxygen species then activated the ASK1-JNK/p38 pathways. The activated JNK led to transiently enhanced expression of FGF-inducible 14 (Fn14, a member of the TNF receptor superfamily) gene. Cells with enhanced expression of Fn14 subsequently acquired sensitivity to the ligand of Fn14, TNF-related weak inducer of apoptosis, and underwent apoptosis. On the other hand, the ASK1-p38 pathway induced expression of the monocyte chemoattractant protein 3 (MCP-3) gene, which promoted chemotaxis of preosteoclasts. In contrast, the ERK pathway was activated by small-magnitude stretching (1%) and induced expression of two osteogenic genes, collagen Ia (Col1a) and osteopontin (OPN). Moreover, activated JNK suppressed Col1a and OPN induction in large-magnitude mechanical stretch-loaded cells. The enhanced expression of Fn14 and MCP-3 by 12% stretch and the enhanced expression of Col1a and OPN by 1% stretch were also observed in mouse primary osteoblasts. These results suggest that differences in the response of osteoblasts to varying magnitudes of mechanical stress play a key role in switching the mode of bone metabolism between formation and resorption.

The chemokine receptor CCR1 is constitutively active, which leads to G protein-independent, β-arrestin-mediated internalization

Activation of G protein-coupled receptors by their associated ligands has been extensively studied, and increasing structural information about the molecular mechanisms underlying ligand-dependent receptor activation is beginning to emerge with the recent expansion in GPCR crystal structures. However, some GPCRs are also able to adopt active conformations in the absence of agonist binding that result in the initiation of signal transduction and receptor down-modulation. In this report, we show that the CC-type chemokine receptor 1 (CCR1) exhibits significant constitutive activity leading to a variety of cellular responses. CCR1 expression is sufficient to induce inhibition of cAMP formation, increased F-actin content, and basal migration of human and murine leukocytes. The constitutive activity leads to basal phosphorylation of the receptor, recruitment of β-arrestin-2, and subsequent receptor internalization. CCR1 concurrently engages Gαi and β-arrestin-2 in a multiprotein complex, which may be accommodated by homo-oligomerization or receptor clustering. The data suggest the presence of two functional states for CCR1; whereas receptor coupled to Gαi functions as a canonical GPCR, albeit with high constitutive activity, the CCR1·β-arrestin-2 complex is required for G protein-independent constitutive receptor internalization. The pertussis toxin-insensitive uptake of chemokine by the receptor suggests that the CCR1·β-arrestin-2 complex may be related to a potential scavenging function of the receptor, which may be important for maintenance of chemokine gradients and receptor responsiveness in complex fields of chemokines during inflammation.

Mycobacterium tuberculosis escapes from the phagosomes of infected human osteoclasts reprograms osteoclast development via dysregulation of cytokines and chemokines

Spinal tuberculosis is a condition characterized by massive resorption of the spinal vertebrae due to the infection with Mycobacterium tuberculosis (Mtb). However, the pathogenesis of spinal tuberculosis has not been established because it was almost completely eradicated by the establishment of antibiotic treatment in the mid-20th century. In this study, we investigated the inflammatory responses of human multinucleated osteoclasts infected with virulent Mtb strain. We found that the intracellular Mtb infection of multinuclear osteoclasts resulted in the rapid growth of Mtb and an osteolytic response, rather than inflammation. In response to Mtb infection, the mononuclear osteoclast precursors produced proinflammatory cytokines including tumor necrosis factor (TNF)-α, an intrinsic characteristic they share with macrophages. In contrast, highly fused multinucleated osteoclasts incapacitated the production of these cytokines. Instead, the intracellular Mtb inside multinuclear osteoclasts escaped from the endosome/phagosome, leading to a different pattern of osteoclast activation, with the production of chemokines such as CCL5, CCL17, CCL20, CCL22, CCL24, and CCL25. Moreover, intracellular infection with an avirulent Mtb strain resulted in diminished production of these chemokines. These findings indicate that intracellular Mtb infection in multinuclear osteoclasts reprograms osteoclast development via the dysregulation of cytokines and chemokines.

CC chemokine receptors and chronic inflammation--therapeutic opportunities and pharmacological challenges

Chemokines are a family of low molecular weight proteins with an essential role in leukocyte trafficking during both homeostasis and inflammation. The CC class of chemokines consists of at least 28 members (CCL1-28) that signal through 10 known chemokine receptors (CCR1-10). CC chemokine receptors are expressed predominantly by T cells and monocyte-macrophages, cell types associated predominantly with chronic inflammation occurring over weeks or years. Chronic inflammatory diseases including rheumatoid arthritis, atherosclerosis, and metabolic syndrome are characterized by continued leukocyte infiltration into the inflammatory site, driven in large part by excessive chemokine production. Over years or decades, persistent inflammation may lead to loss of tissue architecture and function, causing severe disability or, in the case of atherosclerosis, fatal outcomes such as myocardial infarction or stroke. Despite the existence of several clinical strategies for targeting chronic inflammation, these diseases remain significant causes of morbidity and mortality globally, with a concomitant economic impact. Thus, the development of novel therapeutic agents for the treatment of chronic inflammatory disease continues to be a priority. In this review we introduce CC chemokine receptors as critical mediators of chronic inflammatory responses and explore their potential role as pharmacological targets. We discuss functions of individual CC chemokine receptors based on in vitro pharmacological data as well as transgenic animal studies. Focusing on three key forms of chronic inflammation--rheumatoid arthritis, atherosclerosis, and metabolic syndrome--we describe the pathologic function of CC chemokine receptors and their possible relevance as therapeutic targets.

The effect of CCL3 and CCR1 in bone remodeling induced by mechanical loading during orthodontic tooth movement in mice

Bone remodeling is affected by mechanical loading and inflammatory mediators, including chemokines. The chemokine (C-C motif) ligand 3 (CCL3) is involved in bone remodeling by binding to C-C chemokine receptors 1 and 5 (CCR1 and CCR5) expressed on osteoclasts and osteoblasts. Our group has previously demonstrated that CCR5 down-regulates mechanical loading-induced bone resorption. Thus, the present study aimed to investigate the role of CCR1 and CCL3 in bone remodeling induced by mechanical loading during orthodontic tooth movement in mice. Our results showed that bone remodeling was significantly decreased in CCL3(-/-) and CCR1(-/-) mice and in animals treated with Met-RANTES (an antagonist of CCR5 and CCR1). mRNA levels of receptor activator of nuclear factor kappa-B (RANK), its ligand RANKL, tumor necrosis factor alpha (TNF-α) and RANKL/osteoprotegerin (OPG) ratio were diminished in the periodontium of CCL3(-/-) mice and in the group treated with Met-RANTES. Met-RANTES treatment also reduced the levels of cathepsin K and metalloproteinase 13 (MMP13). The expression of the osteoblast markers runt-related transcription factor 2 (RUNX2) and periostin was decreased, while osteocalcin (OCN) was augmented in CCL3(-/-) and Met-RANTES-treated mice. Altogether, these findings show that CCR1 is pivotal for bone remodeling induced by mechanical loading during orthodontic tooth movement and these actions depend, at least in part, on CCL3.

Roles of chemokine receptor CX3CR1 in maintaining murine bone homeostasis through the regulation of both osteoblasts and osteoclasts

Chemokines have recently been reported to be involved in pathological bone destruction. However, the physiological roles of chemokines in bone metabolism in vivo have not been well documented. We analyzed the bone phenotypes in Cx3cr1-deficient mice. The mice exhibited slight but significant increases in trabecular and cortical thickness, reduced numbers of osteoclasts and increased rates of osteoid formation. Although the morphometric parameters showed marginal differences, the Cx3cr1-deficient bones showed an elevated expression of Osterix/SP7, which encodes an essential transcriptional factor for osteoblasts, whereas the gene Osteocalcin/Bglap, which encodes a late marker, was downregulated. The levels of transcripts for various osteoclastic markers, such as receptor activator of NF-κB (RANK)/TNFRSF11A, receptor activator of NF-κB ligand (RANKL)/TNFSF11, tartrate-resistant acid phosphatase 5b (TRAP5B)/ACP5B, Cathepsin K(CTSK), MMP3 and MMP13, were significantly decreased in the Cx3cr1-deficient bones. Cultured Cx3cr1-deficient osteoblastic cells showed inverse temporal patterns of osteoblastic marker expression and reduced calcium deposition. Furthermore, in vitro studies and immunofluorescence staining against CX3CR1 and CX3CL1 suggested a role for the CX3CR1-CX3CL1 axis in an early stage of osteoblast differentiation, possibly through their trans and cis interactions. Cultured Cx3cr1-deficient pre-osteoclasts showed impaired differentiation, mainly due to a deficiency of the CD115(+)CD11b(lo) osteoclastogenic population of myeloid-lineage precursors. The treatment of bone-marrow-derived osteoclastic cultures with recombinant CX3CL1 at different time points suggested that the CX3CR1-CX3CL1 axis favors the maintenance of osteoclastic precursors, but not differentiated osteoclasts. These observations uncovered novel roles of the CX3CR1-CX3CL1 axis in the differentiation of both osteoblasts and osteoclasts.

Deletion of Orai1 alters expression of multiple genes during osteoclast and osteoblast maturation

Store-operated Ca(2+) entry (SOCE) is a major Ca(2+) influx pathway in most non-excitable cell types and Orai1 was recently identified as an essential pore-subunit of SOCE channels. Here we investigate the physiological role of Orai1 in bone homeostasis using Orai1-deficient mice (Orai1(-/-)). Orai1(-/-) mice developed osteopenia with decreased bone mineral density and trabecular bone volume. To identify the nature and origin of the bone defect, bone-resorbing osteoclasts and bone-forming osteoblasts from Orai1(-/-) mice were examined. Orai1-mediated SOCE was completely abolished in Orai1(-/-) osteoclast precursor cells and osteoclastogenesis in vitro from Orai1(-/-) mice was impaired due to a defect in cell fusion of pre-osteoclasts. Also, resorption activity in vitro was comparable but the size of pits formed by Orai1(-/-) osteoclasts was smaller. We next assessed the role of Orai1 in osteoblast differentiation and function by using a pre-osteoblast cell line, as well as primary osteoblasts from wild-type and Orai1(-/-) mice. SOCE in MC3T3-E1 pre-osteoblastic cells was inactivated by lentiviral overexpression of a pore-dead Orai1 mutant. Lack of SOCE in MC3T3-E1 had no effect on alkaline phosphatase staining and expression but substantially inhibited mineralized nodule formation. Consistent with this finding, Orai1-mediated SOCE was markedly reduced in Orai1(-/-) osteoblast precursor cells and osteoblastogenesis in vitro from Orai1(-/-) stromal cells showed impaired mineral deposition but no change in differentiation. This indicates that Orai1 is involved in the function but not in the differentiation of osteoblasts. Together, these results suggest that Orai1 plays a critical role in bone homeostasis by regulating both osteoblasts and osteoclasts.

Characterization and identification of subpopulations of mononuclear preosteoclasts induced by TNF-α in combination with TGF-β in rats

Osteoclasts are unique multinucleated cells formed by fusion of preosteoclasts derived from cells of the monocyte/macrophage lineage, which are induced by RANKL. However, characteristics and subpopulations of osteoclast precursor cells are poorly understood. We show here that a combination of TNF-α, TGF-β, and M-CSF efficiently generates mononuclear preosteoclasts but not multinucleated osteoclasts (MNCs) in rat bone marrow cultures depleted of stromal cells. Using a rat osteoclast-specific mAb, Kat1, we found that TNF-α and TGF-β specifically increased Kat1⁺c-fms⁺ and Kat1⁺c-fms⁻ cells but not Kat1⁻c-fms⁺ cells. Kat1⁻c-fms⁺ cells appeared in early stages of culture, but Kat1⁺c-fms⁺ and Kat1⁺c-fms⁻ cells increased later. Preosteoclasts induced by TNF-α, TGF-β, and M-CSF rapidly differentiated into osteoclasts in the presence of RANKL and hydroxyurea, an inhibitor of DNA synthesis, suggesting that preosteoclasts are terminally differentiated cells. We further analyzed the expression levels of genes encoding surface proteins in bone marrow macrophages (BMM), preosteoclasts, and MNCs. Preosteoclasts expressed itgam (CD11b) and chemokine receptors CCR1 and CCR2; however, in preosteoclasts the expression of chemokine receptors CCR1 and CCR2 was not up-regulated compared to their expression in BMM. However, addition of RANKL to preosteoclasts markedly increased the expression of CCR1. In contrast, expression of macrophage antigen emr-1 (F4/80) and chemokine receptor CCR5 was down-regulated in preosteoclasts. The combination of TNF-α, TGF-β, and M-CSF induced Kat1⁺CD11b⁺ cells, but these cells were also induced by TNF-α alone. In addition, MIP-1α and MCP-1, which are ligands for CCR1 and CCR2, were chemotactic for preosteoclasts, and promoted multinucleation of preosteoclasts. Finally, we found that Kat1⁺c-fms⁺ cells were present in bone tissues of rats with adjuvant arthritis. These data demonstrate that TNF-α in combination with TGF-β efficiently generates preosteoclasts in vitro. We delineated characteristics that are useful for identifying and isolating rat preosteoclasts, and found that CCR1 expression was regulated in the fusion step in osteoclastogenesis.

CCR1 blockade reduces tumor burden and osteolysis in vivo in a mouse model of myeloma bone disease

The chemokine CCL3/MIP-1α is a risk factor in the outcome of multiple myeloma (MM), particularly in the development of osteolytic bone disease. This chemokine, highly overexpressed by MM cells, can signal mainly through 2 receptors, CCR1 and CCR5, only 1 of which (CCR1) is responsive to CCL3 in human and mouse osteoclast precursors. CCR1 activation leads to the formation of osteolytic lesions and facilitates tumor growth. Here we show that formation of mature osteoclasts is blocked by the highly potent and selective CCR1 antagonist CCX721, an analog of the clinical compound CCX354. We also show that doses of CCX721 selected to completely inhibit CCR1 produce a profound decrease in tumor burden and osteolytic damage in the murine 5TGM1 model of MM bone disease. Similar effects were observed when the antagonist was used prophylactically or therapeutically, with comparable efficacy to that of zoledronic acid. 5TGM1 cells were shown to express minimal levels of CCR1 while secreting high levels of CCL3, suggesting that the therapeutic effects of CCX721 result from CCR1 inhibition on non-MM cells, most likely osteoclasts and osteoclast precursors. These results provide a strong rationale for further development of CCR1 antagonists for the treatment of MM and associated osteolytic bone disease.

Therapeutic potential of CCR1 antagonists for multiple myeloma

The chemokine receptor CCR1 has been the target of intensive research for nearly two decades. Small-molecule antagonists were first reported in 1998 and, since then, many inhibitors for CCR1 have been brought forth. Yet, with all the money and time spent, to date, no small-molecule antagonists have successfully moved past Phase II clinical trials. With the current advancement of CCR1 antagonists by Bristol-Myers Squibb and Chemocentrix, there has been renewed interest. In this review, we present an overview of CCR1, its activating ligands, methods of signaling, and downstream response. We discuss studies that indicate CCR1 plays an important role in multiple myeloma and the underlying molecular mechanisms. Finally, we present an overview of the clinical and preclinical compounds for CCR1. We address individual structures, discuss their pharmacological précis, and summarize the published evidence to assess their value for use in multiple myeloma.