Protease-Activated Receptor 1 Deletion Causes Enhanced Osteoclastogenesis in Response to Inflammatory Signals through a Notch2-Dependent Mechanism.

We found that protease-activated receptor 1 (PAR1) was transiently induced in cultured osteoclast precursor cells. Therefore, we examined the bone phenotype and response to resorptive stimuli of PAR1-deficient (knockout [KO]) mice. Bones and bone marrow-derived cells from PAR1 KO and wild-type (WT) mice were assessed using microcomputed tomography, histomorphometry, in vitro cultures, and RT-PCR. Osteoclastic responses to TNF-α (TNF) challenge in calvaria were analyzed with and without a specific neutralizing Ab to the Notch2-negative regulatory region (N2-NRR Ab). In vivo under homeostatic conditions, there were minimal differences in bone mass or bone cells between PAR1 KO and WT mice. However, PAR1 KO myeloid cells demonstrated enhanced osteoclastogenesis in response to receptor activator of NF-κB ligand (RANKL) or the combination of RANKL and TNF. Strikingly, in vivo osteoclastogenic responses of PAR1 KO mice to TNF were markedly enhanced. We found that N2-NRR Ab reduced TNF-induced osteoclastogenesis in PAR1 KO mice to WT levels without affecting WT responses. Similarly, in vitro N2-NRR Ab reduced RANKL-induced osteoclastogenesis in PAR1 KO cells to WT levels without altering WT responses. We conclude that PAR1 functions to limit Notch2 signaling in responses to RANKL and TNF and moderates osteoclastogenic response to these cytokines. This effect appears, at least in part, to be cell autonomous because enhanced osteoclastogenesis was seen in highly purified PAR1 KO osteoclast precursor cells. It is likely that this pathway is involved in regulating the response of bone to diseases associated with inflammatory signals.

Necrotic Bone Stimulates Proinflammatory Responses in Macrophages through the Activation of Toll-Like Receptor 4.

In Legg-Calvé-Perthes disease, loss of blood supply results in ischemic osteonecrosis of the femoral head (ONFH). Generally, macrophages play important roles in inflammatory responses to tissue necrosis, but their role in ONFH is not known. The purpose of this study was to determine the macrophage-inflammatory responses after ONFH and the receptor mechanisms involved in sensing the necrotic bone, using a piglet model of Legg-Calvé-Perthes disease. Induction of ONFH resulted in increased numbers of CD14+ macrophages in the fibrovascular repair tissue compared with normal, as determined by immunohistochemistry. Quantitative real-time PCR analysis of macrophages isolated by laser capture microdissection showed significantly increased expression of proinflammatory cytokines IL-1ß, tumor necrosis factor-α, and IL-6 in ONFH compared with normal. Because Toll-like receptors (TLRs) mediate macrophage-inflammatory responses in other inflammatory conditions, we determined their gene expression in macrophages and found significantly increased levels of TLR4 but not TLR2 and TLR9 in ONFH. Mechanistically, in vitro, bone marrow-derived macrophages treated with necrotic bone showed increased extracellular signal-regulated kinases 1/2 and Iκ kinase-α phosphorylation, increased proliferation, migration, and inflammatory cytokine expression, which were blocked by TLR4 inhibitor, TAK242, and by TLR4 ablation in macrophages using the clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein-9 nuclease method. In conclusion, necrotic bone stimulates macrophage-inflammatory responses through TLR4 activation.

Development of a mouse model of ischemic osteonecrosis.

BACKGROUND: Availability of a reliable mouse model of ischemic osteonecrosis could accelerate the development of novel therapeutic strategies to stimulate bone healing after ischemic osteonecrosis; however, no mouse model of ischemic osteonecrosis is currently available. QUESTIONS/PURPOSES: To develop a surgical mouse model of ischemic osteonecrosis, we asked, (1) if the blood vessels that contribute to the blood supply of the distal femoral epiphysis are cauterized, can we generate an osteonecrosis mouse model; (2) what are the histologic changes observed in this mouse model, and (3) what are the morphologic changes in the model. METHODS: We performed microangiography to identify blood vessels supplying the distal femoral epiphysis in mice, and four vessels were cauterized using microsurgical techniques to induce ischemic osteonecrosis. Histologic assessment of cell death in the trabecular bone was performed using terminal deoxynucleotidyl transferase mediated dUTP nick-end labeling (TUNEL) and counting the empty lacunae in three serial sections. Quantitation of osteoclast and osteoblast numbers was performed using image analysis software. Morphologic assessments of the distal femoral epiphysis for deformity and for trabecular bone parameters were performed using micro-CT. RESULTS: We identified four blood vessels about the knee that had to be cauterized to induce total ischemic osteonecrosis of the distal femoral epiphysis. Qualitative assessment of histologic sections of the epiphysis showed a loss of nuclear staining of marrow cells, disorganized marrow structure, and necrotic blood vessels at 1 week. By 2 weeks, vascular tissue invasion of the necrotic marrow space was observed with a progressive increase in infiltration of the necrotic marrow space with the vascular tissue at 4 and 6 weeks. TUNEL staining showed extensive cell death in the marrow and trabecular bone 24 hours after the induction of ischemia. The mean percent of TUNEL-positive osteocytes in the trabecular bone increased from 2% ± 1% in the control group to a peak of 98% ± 3% in the ischemic group 1 week after induction of ischemia (mean difference, 96%; 95% CI, 81%-111%; p < 0.0001). The mean percent of empty lacunae increased from 1% ± 1% in the control group to a peak of 78% ± 15% in the ischemic group at 4 weeks (mean difference, 77%; 95% CI, 56%-97%; p < 0.0001). Quantitative analysis showed that the mean number of osteoclasts per bone surface was decreased in the ischemic group at 1, 2, and 4 weeks (p < 0.0001, < 0.0001, and p = 0.02, respectively) compared with the control group. The mean number of osteoclasts increased to a level similar to that of the control group at 6 weeks (p = 0.23). The numbers of osteoblasts per bone surface were decreased in the ischemic group at 1, 2 and 4 weeks (p < 0.0001 for each) compared with the numbers in the control group. The mean number of osteoblasts also increased to a level similar to that of the control group at 6 weeks (p = 0.91). Mean bone volume percent assessed by micro-CT was lower in the ischemic group compared with the control group from 2 to 6 weeks. The mean differences in the percent bone volume between the control and ischemic groups at 2, 4, and 6 weeks were 5.5% (95% CI, 0.9%-10.2%; p = 0.01), 5.3% (95% CI, 0.6%-9.9%; p = 0.02), and 6.0% (95% CI, 1.1%-10.9%; p = 0.008), respectively. A deformity of the distal femoral epiphysis was observed at 6 weeks with the mean epiphyseal height to width ratio of 0.74 ± 0.03 in the control group compared with 0.66 ± 0.06 in the ischemic group (mean difference, 0.08; 95% CI, 0.00-0.16; p = 0.03). CONCLUSION: We developed a novel mouse model of ischemic osteonecrosis that produced extensive cell death in the distal femoral epiphysis which developed a deformity with time. CLINICAL RELEVANCE: The new mouse model may be a useful tool to test potential therapeutic strategies to improve bone healing after ischemic osteonecrosis.

Loss of Cbl-PI3K interaction enhances osteoclast survival due to p21-Ras mediated PI3K activation independent of Cbl-b.

Cbl family proteins, Cbl and Cbl-b, are E3 ubiquitin ligases and adaptor proteins, which play important roles in bone-resorbing osteoclasts. Loss of Cbl in mice decreases osteoclast migration, resulting in delayed bone development where as absence of Cbl-b decreases bone volume due to hyper-resorptive osteoclasts. A major structural difference between Cbl and Cbl-b is tyrosine 737 (in YEAM motif) only on Cbl, which upon phosphorylation interacts with the p85 subunit of phosphatidylinositol-3 Kinase (PI3K). In contrast to Cbl(-/-) and Cbl-b(-/-) , mice lacking Cbl-PI3K interaction due to a Y737F (tyrosine to phenylalanine, YF) mutation showed enhanced osteoclast survival, but defective bone resorption. To investigate whether Cbl-PI3K interaction contributes to distinct roles of Cbl and Cbl-b in osteoclasts, mice bearing CblY737F mutation in the Cbl-b(-/-) background (YF/YF;Cbl-b(-/-) ) were generated. The differentiation and survival were augmented similarly in YF/YF and YF/YF;Cbl-b(-/-) osteoclasts, associated with enhanced PI3K signaling suggesting an exclusive role of Cbl-PI3K interaction, independent of Cbl-b. In addition to PI3K, the small GTPase Ras also regulates osteoclast survival. In the absence of Cbl-PI3K interaction, increased Ras GTPase activity and Ras-PI3K binding were observed and inhibition of Ras activation attenuated PI3K mediated osteoclast survival. In contrast to differentiation and survival, increased osteoclast activity observed in Cbl-b(-/-) mice persisted even after introduction of the resorption-defective YF mutation in YF/YF;Cbl-b(-/-) mice. Hence, Cbl and Cbl-b play mutually exclusive roles in osteoclasts. Whereas Cbl-PI3K interaction regulates differentiation and survival, bone resorption is predominantly regulated by Cbl-b in osteoclasts.

Altered hematopoietic stem cell and osteoclast precursor frequency in cathepsin K null mice.

Cathepsin K (CatK) is a lysosomal cysteine protease necessary for bone resorption by osteoclasts (OCs), which originate from myeloid hematopoietic precursors. CatK-deficient (CatK(-/-) ) mice show osteopetrosis due to defective resorption by OCs, which are increased in number in these mice. We investigated whether genetic ablation of CatK altered the number of hematopoietic stem cells (HSCs) and OC precursor cells (OCPs) using two mouse models: CatK(-/-) mice and a knock-in mouse model in which the CatK gene (ctsk) is replaced by cre recombinase. We found that CatK deletion in mice significantly increased the number of HSCs in the spleen and decreased their number in bone marrow. In contrast, the number of early OCPs was unchanged in the bone marrow. However, the number of committed CD11b(+) OCPs was increased in the bone marrow of CatK(-/-) compared to wild-type (WT) mice. In addition, the percentage but not the number of OCPs was decreased in the spleen of CatK(-/-) mice relative to WT. To understand whether increased commitment to OC lineage in CatK(-/-) mice is influenced by the bone marrow microenvironment, CatK(Cre/+) or CatK(Cre/Cre) red fluorescently labeled OCPs were injected into WT mice, which were also subjected to a mid-diaphyseal femoral fracture. The number of OCs derived from the intravenously injected CatK(Cre/Cre) OCPs was lower in the fracture callus compared to mice injected with CatK(+/Cre) OCPs. Hence, in addition to its other effects, the absence of CatK in OCP limits their ability to engraft in a repairing fracture callus compared to WT OCP.

TULA-2, a novel histidine phosphatase, regulates bone remodeling by modulating osteoclast function.

Bone is a dynamic tissue that depends on the intricate relationship between protein tyrosine kinases (PTK) and protein tyrosine phosphatases (PTP) for maintaining homeostasis. PTKs and PTPs act like molecular on and off switches and help modulate differentiation and the attachment of osteoclasts to bone matrix regulating bone resorption. The protein T cell ubiquitin ligand-2 (TULA-2), which is abundantly expressed in osteoclasts, is a novel histidine phosphatase. Our results show that of the two family members, only TULA-2 is expressed in osteoclasts and that its expression is sustained throughout the course of osteoclast differentiation, suggesting that TULA-2 may play a role during early as well late stages of osteoclast differentiation. Skeletal analysis of mice that do not express TULA or TULA-2 proteins (DKO mice) revealed that there was a decrease in bone volume due to increased osteoclast numbers and function. Furthermore, in vitro experiments indicated that bone marrow precursor cells from DKO mice have an increased potential to form osteoclasts. At the molecular level, the absence of TULA-2 in osteoclasts results in increased Syk phosphorylation at the Y352 and Y525/526 residues and activation of phospholipase C gamma 2 (PLCγ2) upon engagement of immune-receptor-tyrosine-based-activation-motif (ITAM)-mediated signaling. Furthermore, expression of a phosphatase-dead TULA-2 leads to increased osteoclast function. Taken together, these results suggest that TULA-2 negatively regulates osteoclast differentiation and function.

Glutamine metabolism modulates chondrocyte inflammatory response.

Osteoarthritis is the most common joint disease in the world with significant societal consequences but lacks effective disease-modifying interventions. The pathophysiology consists of a prominent inflammatory component that can be targeted to prevent cartilage degradation and structural defects. Intracellular metabolism has emerged as a culprit of the inflammatory response in chondrocytes, with both processes co-regulating each other. The role of glutamine metabolism in chondrocytes, especially in the context of inflammation, lacks a thorough understanding and is the focus of this work. We display that mouse chondrocytes utilize glutamine for energy production and anabolic processes. Furthermore, we show that glutamine deprivation itself causes metabolic reprogramming and decreases the inflammatory response of chondrocytes through inhibition of NF-κB activity. Finally, we display that glutamine deprivation promotes autophagy and that ammonia is an inhibitor of autophagy. Overall, we identify a relationship between glutamine metabolism and inflammatory signaling and display the need for increased study of chondrocyte metabolic systems.

The loss of Cbl-phosphatidylinositol 3-kinase interaction perturbs RANKL-mediated signaling, inhibiting bone resorption and promoting osteoclast survival.

Cbl is an adaptor protein and an E3 ligase that plays both positive and negative roles in several signaling pathways that affect various cellular functions. Tyrosine 737 is unique to Cbl and is phosphorylated by Syk and Src family kinases. Phosphorylated Cbl Tyr(737) creates a binding site for the p85 regulatory subunit of PI3K, which also plays an important role in the regulation of bone resorption by osteoclasts. To investigate the role of Cbl-PI3K interaction in bone homeostasis, we examined the knock-in mice (Cbl(YF/YF)) in which the PI3K binding site in Cbl is ablated due to the mutation in the regulatory tyrosine. We report that in Cbl(YF/YF) mice, despite increased numbers of osteoclasts, bone volume is increased due to defective osteoclast function. Additionally, in ex vivo cultures, mature Cbl(YF/YF) osteoclasts showed an increased ability to survive in the presence of RANKL due to delayed onset of apoptosis. RANKL-mediated signaling is perturbed in Cbl(YF/YF) osteoclasts, and most interestingly, AKT phosphorylation is up-regulated, suggesting that the lack of PI3K sequestration by Cbl results in increased survival and decreased bone resorption. Cumulatively, these in vivo and in vitro results show that, on one hand, binding of Cbl to PI3K negatively regulates osteoclast differentiation, survival, and signaling events (e.g. AKT phosphorylation), whereas on the other hand it positively influences osteoclast function.

Abrogation of Cbl-PI3K interaction increases bone formation and osteoblast proliferation.

Cbl is an adaptor protein and E3 ligase that plays both positive and negative roles in several signaling pathways that affect various cellular functions. Tyrosine 737 is unique to Cbl and phosphorylated by Src family kinases. Phosphorylated CblY737 creates a binding site for the p85 regulatory subunit of phosphatidylinositol 3 kinase (PI3K) that also plays an important role in the regulation of bone homeostasis. To investigate the role of Cbl-PI3K interaction in bone homeostasis, we examined knock-in mice in which the PI3K binding site on Cbl was ablated due to the substitution of tyrosine 737 to phenylalanine (Cbl(YF/YF), YF mice). We previously reported that bone volume in these mice is increased due to decreased osteoclast function (Adapala et al., J Biol Chem 285:36745-36758, 19). Here, we report that YF mice also have increased bone formation and osteoblast numbers. In ex vivo cultures bone marrow-derived YF osteoblasts showed increased Col1A expression and their proliferation was also significantly augmented. Moreover, proliferation of MC3T3-E1 cells was increased after treatment with conditioned medium generated by culturing YF bone marrow stromal cells. Expression of stromal derived factor-1 (SDF-1) was increased in YF bone marrow stromal cells compared to wild type. Increased immunostaining of SDF-1 and CXCR4 was observed in YF bone marrow stromal cells compared to wild type. Treatment of YF condition medium with neutralizing anti-SDF-1 and anti-CXCR4 antibodies attenuated MC3T3-E1 cell proliferation. Cumulatively, these results show that abrogation of Cbl-PI3K interaction perturbs bone homeostasis, affecting both osteoclast function and osteoblast proliferation.

Anti-Interleukin-6 Therapy Decreases Hip Synovitis and Bone Resorption and Increases Bone Formation Following Ischemic Osteonecrosis of the Femoral Head.

Legg-Calvé-Perthes disease (LCPD) is a juvenile form of ischemic femoral head osteonecrosis, which produces chronic hip synovitis, permanent femoral head deformity, and premature osteoarthritis. Currently, there is no medical therapy for LCPD. Interleukin-6 (IL-6) is significantly elevated in the synovial fluid of patients with LCPD. We hypothesize that IL-6 elevation promotes chronic hip synovitis and impairs bone healing after ischemic osteonecrosis. We set out to test if anti-IL-6 therapy using tocilizumab can decrease hip synovitis and improve bone healing in the piglet model of LCPD. Fourteen piglets were surgically induced with ischemic osteonecrosis and assigned to two groups: the no treatment group (n = 7) and the tocilizumab group (15 to 20 mg/kg, biweekly intravenous injection, n = 7). All animals were euthanized 8 weeks after the induction of osteonecrosis. Hip synovium and femoral heads were assessed for hip synovitis and bone healing using histology, micro-CT, and histomorphometry. The mean hip synovitis score and the number of synovial macrophages and vessels were significantly lower in the tocilizumab group compared with the no treatment group (p < .0001, p = .01, and p < .01, respectively). Micro-CT analysis of the femoral heads showed a significantly higher bone volume in the tocilizumab group compared with the no treatment group (p = .02). The histologic assessment revealed a significantly lower number of osteoclasts per bone surface (p < .001) in the tocilizumab group compared with the no treatment group. Moreover, fluorochrome labeling showed a significantly higher percent of mineralizing bone surface (p < .01), bone formation rate per bone surface (p < .01), and mineral apposition rate (p = .04) in the tocilizumab group. Taken together, tocilizumab therapy decreased hip synovitis and osteoclastic bone resorption and increased new bone formation after ischemic osteonecrosis. This study provides preclinical evidence that tocilizumab decreases synovitis and improves bone healing in a large animal model of LCPD. © 2020 American Society for Bone and Mineral Research (ASBMR).

Inflammatory osteolysis is regulated by site-specific ISGylation of the scaffold protein NEMO.

Inflammatory osteolysis is governed by exacerbated osteoclastogenesis. Ample evidence points to central role of NF-κB in such pathologic responses, yet the precise mechanisms underpinning specificity of these responses remain unclear. We propose that motifs of the scaffold protein IKKγ/NEMO partly facilitate such functions. As proof-of-principle, we used site-specific mutagenesis to examine the role of NEMO in mediating RANKL-induced signaling in mouse bone marrow macrophages, known as osteoclast precursors. We identified lysine (K)270 as a target regulating RANKL signaling as K270A substitution results in exuberant osteoclastogenesis in vitro and murine inflammatory osteolysis in vivo. Mechanistically, we discovered that K270A mutation disrupts autophagy, stabilizes NEMO, and elevates inflammatory burden. Specifically, K270A directly or indirectly hinders binding of NEMO to ISG15, a ubiquitin-like protein, which we show targets the modified proteins to autophagy-mediated lysosomal degradation. Taken together, our findings suggest that NEMO serves as a toolkit to fine-tune specific signals in physiologic and pathologic conditions.

The human skeleton contains over 200 bones that together act as an internal framework for the body. Over our lifetime, the body constantly removes older bone tissue from the skeleton and replaces it with new bone tissue. This "bone remodeling" also controls how bones are repaired after being damaged by injuries, disease or normal wear and tear. Cells known as osteoclasts are responsible for breaking down old bone tissue and participate in repairing damaged bone. A cellular pathway known as NF-kB signaling stimulates other cells called "bone marrow macrophages" to become osteoclasts. A certain level of NF-kB signaling is required to maintain a healthy skeleton. However, under certain inflammatory conditions, the level of NF-kB signaling becomes too high causing hyperactive osteoclasts to accumulate and inflict severe bone breakdown. This abnormal osteoclast activity leads to eroded and fragile bones and joints, as is the case in diseases such as rheumatoid arthritis and osteoporosis. Previous studies have shown that a protein called NEMO is a core component of the NF-kB signal pathway, but the precise role of NEMO in the diseased response remained unclear. Adapala, Swarnkar, Arra et al. have now used site-directed mutagenesis approach to study the role of NEMO in bone marrow macrophages in mice. The experiments showed that one specific site within the NEMO protein, referred to as lysine 270, is crucial for its role in controlling osteoclasts and the breakdown of bone tissue. Mutating NEMO at lysine 270 led to uncontrolled NF-kB signaling in the bone marrow macrophages. Further experiments showed that lysine 270 served as a sensor to allow NEMO to bind another protein called ISG15, which in turn helped to decrease NF-kB signaling and slow down the erosion of the bone. These findings suggest that site-specific targeting of NEMO, rather than inhibiting the whole NF-kB pathway, may help to reduce the symptoms of bone disease while maintaining the beneficial roles of this essential pathway. However, additional research is required to identify NEMO sites responsible for controlling the inflammatory component.

Cbl-PI3K interaction regulates Cathepsin K secretion in osteoclasts.

Effective bone resorption by osteoclasts is critical for balanced bone remodeling. We have previously reported that mice harboring a substitution mutation of tyrosine 737 to phenylalanine in the adapter protein Cbl (CblY737F, YF) have increased bone volume partly due to decreased osteoclast-mediated bone resorption. The CblY737F mutation abrogates interaction between Cbl and the p85 subunit of PI3K. Here, we studied the mechanism for defective resorptive function of YF mutant osteoclasts. The YF osteoclasts had intact actin cytoskeletons and sealing zones. Expression and localization of proteins needed for acidification of the resorptive lacunae were also comparable between the WT and YF osteoclasts. In contrast, secretion of Cathepsin K, a major protease needed to degrade collagen, was diminished in the conditioned media derived from YF osteoclasts. The targeting of Cathepsin K into LAMP2-positive vesicles was also compromised due to decreased number of LAMP2-positive vesicles in YF osteoclasts. Further, we found that in contrast to WT, conditioned media derived from YF osteoclasts promoted increased numbers of alkaline phosphatase positive colonies, and increased expression of osteogenic markers in WT calvarial cultures. Cumulatively, our results suggest that the Cbl-PI3K interaction regulates Cathepsin K secretion required for proper bone resorption, and secretion of factors which promote osteogenesis.

PI3K activation increases SDF-1 production and number of osteoclast precursors, and enhances SDF-1-mediated osteoclast precursor migration.

Our previous studies showed that in a mouse model in which PI3K-AKT activation was increased (YF mice), osteoclast numbers and levels of SDF-1, a chemokine, were augmented. The purpose of this study was to delineate the role of PI3K activation in regulating SDF-1 production and examine whether SDF-1 can stimulate differentiation and/or migration of osteoclast precursors. Using flow cytometric analysis, we demonstrated that compared to wild type mice, bone marrow of YF mice had increased numbers of CXCL12 abundant reticular (CAR) cells, that are a major cell type responsible for producing SDF-1. At the molecular level, transcription factor specificity protein 1 (Sp1) induced an increased transcription of SDF-1 that was dependent on PI3K/AKT activation. YF mice also contained an increased number of osteoclast precursors, in which expression of CXCR4, a major receptor for SDF-1, was increased. SDF-1 did not induce differentiation of osteoclast precursors into mature osteoclasts; compared to cells derived from WT mice, cells obtained from YF mice were more responsive to SDF-1. In conclusion, we demonstrate that PI3K activation resulted in increased SDF-1, increased the number of osteoclast precursors, and enhanced osteoclast precursor migration in response to SDF-1.

Interleukin-6 deletion stimulates revascularization and new bone formation following ischemic osteonecrosis in a murine model.

Legg-Calvé-Perthes disease (LCPD) is a childhood form of ischemic osteonecrosis of the femoral head which can produce a permanent femoral head deformity and early osteoarthritis. The femoral head deformity results from increased bone resorption and decreased bone formation during repair and remodeling of the necrotic femoral head. A recent study showed that a pro-inflammatory cytokine, interleukin-6 (IL-6), is significantly elevated in the synovial fluid of patients with LCPD. We hypothesized that IL-6 elevation decreases bone formation during the repair process following ischemic osteonecrosis and that IL-6 depletion will increase new bone formation. To test this hypothesis, we surgically induced ischemic osteonecrosis in the wild-type (n = 29) and IL-6 knockout (KO) mice (n = 25). The animals were assessed at 48 h, 2 weeks and 4 weeks following the induction of ischemic osteonecrosis using histologic, histomorphometric and micro-CT methods. IL-6 immunohistochemistry showed high expression of IL-6 in the osteonecrotic side of the wild-type mice at 48 h and 4 weeks following ischemic osteonecrosis, but not in the IL-6 KO mice. We also confirmed an undetectable level of IL-6 expression in the primary osteoblasts of the IL-6 KO mice compared to the readily detectable level in the wild-type mice. Furthermore, we confirmed that IL-6 deletion did not affect the extent of bone necrosis in the IL-6 KO mice compared to the wild-type mice by performing histologic and terminal deoxynucleotidyl transferase mediated dUTP nick-end labeling (TUNEL) assessments at 2 weeks following the induction of ischemia. Both groups had the same extent of ischemic osteonecrosis and absence of repair at 2 weeks. At 4 weeks, the necrotic epiphyses showed a significant increase in the extent of revascularization in the IL-6 KO mice compared to the wild-type mice (p = 0.001). In addition, a significantly greater recovery of the hematopoietic bone marrow was observed in the osteonecrotic side of the IL-6 KO mice compared to the wild-type mice (p < 0.01). Vascular endothelial growth factor (VEGF) immunohistochemistry showed regionally increased staining in the areas of repair in the osteonecrosis side of IL-6 KO mice compared to the wild-type mice at 4 weeks following ischemic osteonecrosis. Micro-CT assessment of the wild-type mice at 4 weeks showed a significant decrease in the percent bone volume (p < 0.01) in the osteonecrotic side compared to the control side. In contrast, IL-6 KO mice showed significantly increased bone volume in the osteonecrotic side compared to the osteonecrotic side of WT mice (p < 0.001). No significant difference in the bone volume percentage was found between the control side of the wild-type and the IL-6 KO mice. Histomorphometric analysis at 4 weeks revealed increased osteoblast number/bone surface (p < 0.001), bone formation rate (BFR) (p = 0.0001), and mineral apposition rate (MAR) (p < 0.0001) in the osteonecrotic side of the IL-6 KO mice compared to the wild-type mice. The number of osteoclast/bone surface was also increased in the IL-6 KO mice compared to the wild-type mice (p < 0.0001). No significant difference was observed between the control side of the wild-type and IL-6 KO mice with regards to the number of osteoblast or osteoclast/bone surface, BFR, and MAR. We next obtained primary osteoblasts from IL-6 KO mice and showed they expressed a significantly higher level of RANKL/OPG than wild-type mice (p = 0.001) in hypoxia culture condition. Taken together, the findings indicate that IL-6 deletion stimulates revascularization and new bone formation following ischemic osteonecrosis. This study provides new evidence that therapeutic strategies to block IL-6 may be beneficial for bone healing following ischemic osteonecrosis.

A genome-wide transcriptomic analysis of articular cartilage during normal maturation in pigs.

OBJECTIVE: The articular cartilage undergoes dramatic changes in structure and composition during post-natal maturation, but the associated transcriptional changes are not well characterized. Compared to a mature stage, the immature articular cartilage shows developmental features such as increased thickness, presence of blood vessels, and the presence of a deep layer of growth cartilage which undergoes endochondral ossification. These features decrease during normal development. Following maturation, the articular cartilage is known to undergo few minor modifications. Since mature articular cartilage has poor regenerative and repair capacity compared to the immature articular cartilage, a better understanding of the molecular changes during the normal postnatal articular cartilage development might reveal insights on the molecular adaptation. It may also provide new therapeutic strategies. The purpose of this study was to determine the differential expression of genes in the femoral head articular cartilage of 6-weeks old and 6-months old pigs using a genome-wide transcriptomic analysis. METHODS: The articular cartilage of the femoral head of 6-weeks and 6-months old normal pigs was assessed for thickness and vascularity (number of vascular canals) using Safranin O/Fast Green staining of paraffin sections (n=4 pigs/age group). The measurements were determined using Image J software. RNA was isolated from the femoral head articular cartilage from 6-weeks and 6-months old pigs (n=8 pigs/age group). A microarray analysis was performed using an Affymetrix Porcine GeneChip Array. A gene enrichment analysis and a functional clustering analysis were performed by DAVID and STRING software, respectively. The differential expression of selected genes was confirmed by a quantitative RTPCR analysis. RESULTS: The femoral head articular cartilage showed a significant decrease in thickness and number of vascular canals in 6-months old compared to 6-weeks old pigs. A microarray analysis revealed a differential gene expression of 576 genes, with 206 genes that were significantly upregulated and 370 genes that were significantly downregulated (>2-fold change, p<0.05) at 6-months compared to 6-weeks of age. Among the upregulated genes, DAVID analysis revealed that a significant number of genes represented the biological processes of responses to external stimuli, and wounding and inflammation at 6-months of age. These processes involved genes representing secretory and signaling proteins such as MMP-1, MMP-3, IL-8 and STAT3 suggesting increased inflammatory activity. In addition, an assessment of the downregulated genes indicated a decrease in the expression of genes representing the biological processes of developmental processes (e.g. BMPR1A, BMPR2, ACVR2, periostin, SFRP2, COL5A3) and regulation of blood vessel size (e.g. alpha adrenergic receptor 1B, alpha-SMA) at 6-months of age. A real-time qRTPCR analysis of selected upregulated genes, fibronectin, MMP-3, IL-8 and downregulated genes, BMPR2, PECAM, CCL2, TLR4 confirmed the differential gene expression in the microarray analysis. CONCLUSION: During the process of articular cartilage maturation from 6-weeks to 6-months of age in normal pigs, genes associated with inflammatory responses to injury were upregulated and genes involved in the development and vascular responses were downregulated. These findings suggest that during articular cartilage maturation, the transcriptional changes might increase the susceptibility of cartilage to inflammatory damage and decrease the regenerative capacity.

Targeted Disruption of NF1 in Osteocytes Increases FGF23 and Osteoid With Osteomalacia-like Bone Phenotype.

Neurofibromatosis type 1 (NF1, OMIM 162200), caused by NF1 gene mutations, exhibits multi-system abnormalities, including skeletal deformities in humans. Osteocytes play critical roles in controlling bone modeling and remodeling. However, the role of neurofibromin, the protein product of the NF1 gene, in osteocytes is largely unknown. This study investigated the role of neurofibromin in osteocytes by disrupting Nf1 under the Dmp1-promoter. The conditional knockout (Nf1 cKO) mice displayed serum profile of a metabolic bone disorder with an osteomalacia-like bone phenotype. Serum FGF23 levels were 4 times increased in cKO mice compared with age-matched controls. In addition, calcium-phosphorus metabolism was significantly altered (calcium reduced; phosphorus reduced; parathyroid hormone [PTH] increased; 1,25(OH)2 D decreased). Bone histomorphometry showed dramatically increased osteoid parameters, including osteoid volume, surface, and thickness. Dynamic bone histomorphometry revealed reduced bone formation rate and mineral apposition rate in the cKO mice. TRAP staining showed a reduced osteoclast number. Micro-CT demonstrated thinner and porous cortical bones in the cKO mice, in which osteocyte dendrites were disorganized as assessed by electron microscopy. Interestingly, the cKO mice exhibited spontaneous fractures in long bones, as found in NF1 patients. Mechanical testing of femora revealed significantly reduced maximum force and stiffness. Immunohistochemistry showed significantly increased FGF23 protein in the cKO bones. Moreover, primary osteocytes from cKO femora showed about eightfold increase in FGF23 mRNA levels compared with control cells. The upregulation of FGF23 was specifically and significantly inhibited by PI3K inhibitor Ly294002, indicating upregulation of FGF23 through PI3K in Nf1-deficient osteocytes. Taken together, these results indicate that Nf1 deficiency in osteocytes dramatically increases FGF23 production and causes a mineralization defect (ie, hyperosteoidosis) via the alteration of calcium-phosphorus metabolism. This study demonstrates critical roles of neurofibromin in osteocytes for osteoid mineralization. © 2017 American Society for Bone and Mineral Research.

Comprehensive Genome-Wide Transcriptomic Analysis of Immature Articular Cartilage following Ischemic Osteonecrosis of the Femoral Head in Piglets.

OBJECTIVE: Ischemic osteonecrosis of the femoral head (ONFH) in piglets results in an ischemic injury to the immature articular cartilage. The molecular changes in the articular cartilage in response to ONFH have not been investigated using a transcriptomic approach. The purpose of this study was to perform a genome-wide transcriptomic analysis to identify genes that are upregulated in the immature articular cartilage following ONFH. METHODS: ONFH was induced in the right femoral head of 6-week old piglets. The unoperated femoral head was used as the normal control. At 24 hours (acute ischemic-hypoxic injury), 2 weeks (avascular necrosis in the femoral head) and 4 weeks (early repair) after surgery (n = 4 piglets/time point), RNA was isolated from the articular cartilage of the femoral head. A microarray analysis was performed using Affymetrix Porcine GeneChip Array. An enrichment analysis and functional clustering of the genes upregulated due to ONFH were performed using DAVID and STRING software, respectively. The increased expression of selected genes was confirmed by a real-time qRTPCR analysis. RESULTS: Induction of ONFH resulted in the upregulation of 383 genes at 24 hours, 122 genes at 2 weeks and 124 genes at 4 weeks compared to the normal controls. At 24 hours, the genes involved in oxidoreductive, cell-survival, and angiogenic responses were significantly enriched among the upregulated genes. These genes were involved in HIF-1, PI3K-Akt, and MAPK signaling pathways. At 2 weeks, secretory and signaling proteins involved in angiogenic and inflammatory responses, PI3K-Akt and matrix-remodeling pathways were significantly enriched. At 4 weeks, genes that represent inflammatory cytokines and chemokine signaling pathways were significantly enriched. Several index genes (genes that are upregulated at more than one time point following ONFH and are known to be important in various biological processes) including HIF-1A, VEGFA, IL-6, IL6R, IL-8, CCL2, FGF2, TGFB2, MMP1, MMP3, ITGA5, FN and Col6A1 were upregulated in the immature articular cartilage following ONFH. A qRTPCR analysis of selected genes confirmed the upregulated expression observed in the microarray analysis. CONCLUSION: Immature articular cartilage responds to ONFH by the upregulation of genes involved in hypoxic stress response, angiogenesis, matrix remodeling and inflammation. This study provides novel insights into the multi-faceted role of immature articular cartilage, with inflammation as a key component, following ONFH in piglets.

HIF-1-Dependent IL-6 Activation in Articular Chondrocytes Initiating Synovitis in Femoral Head Ischemic Osteonecrosis.

BACKGROUND: Ischemic osteonecrosis of the femoral head in children is associated with chronic hip synovitis and increased levels of the pro-inflammatory cytokine interleukin-6 (IL-6) in the synovial fluid due to unknown mechanisms. The purpose of this study was to investigate hypoxia-inducible factor-1 (HIF-1) activation as a molecular mechanism linking the induction of ischemic osteonecrosis to IL-6 production and the initiation of hip synovitis. METHODS: Ischemic osteonecrosis was surgically induced in the right femoral head of 6 piglets. A histologic score, synovial fluid volume, and IL-6 level were used to assess hip synovitis. IL-6 immunostaining of articular cartilage and synovial tissue was performed as well. To study the role of HIF-1 in IL-6 activation, in vitro experiments using an HIF-1α activator (deferoxamine) and inhibitor (HIF-1 small interfering-RNA [siRNA]) were carried out. Synovial cell responses to hypoxic chondrocyte-conditioned media with and without an IL-6 receptor blocker (tocilizumab) were assessed on the basis of IL-1ß and tumor necrosis factor-alpha (TNF-α) gene expressions and with a synovial cell-proliferation assay. RESULTS: Induction of ischemic osteonecrosis produced hip synovitis and increased IL-6 levels in the synovial fluid. Immunostaining and protein analysis demonstrated articular chondrocytes as a source of increased IL-6 production. When articular chondrocytes were cultured under hypoxic conditions, significantly increased HIF-1α and IL-6 expressions were observed. Under hypoxic culture conditions, IL-6 gene expression was significantly increased by HIF-1α activation using deferoxamine and inhibited by HIF-1α inhibition using HIF-1 siRNA. Synovial cells exposed to hypoxic chondrocyte-conditioned medium showed significant increases in IL-1ß and TNF-α gene expressions and cell proliferation, which were inhibited by the IL-6 receptor blocker tocilizumab. CONCLUSIONS: Induction of ischemic osteonecrosis results in IL-6 production in the articular cartilage through an HIF-1-dependent pathway. IL-6 produced by hypoxic articular chondrocytes stimulates inflammatory cytokine responses in synovial cells, which were significantly decreased by tocilizumab. CLINICAL RELEVANCE: This study provides new insight into the inherent relationship between the induction of ischemia and the initiation of hip synovitis following ischemic osteonecrosis and suggests a potential therapeutic target in the treatment of the synovitis.

Targeted disruption of BMP signaling through type IA receptor (BMPR1A) in osteocyte suppresses SOST and RANKL, leading to dramatic increase in bone mass, bone mineral density and mechanical strength.

Recent studies suggest a critical role of osteocytes in controlling skeletal development and bone remodeling although the molecular mechanism is largely unknown. This study investigated BMP signaling in osteocytes by disrupting Bmpr1a under the Dmp1-promoter. The conditional knockout (cKO) mice displayed a striking osteosclerotic phenotype with increased trabecular bone volume, thickness, number, and mineral density as assessed by X-ray and micro-CT. The bone histomorphometry, H&E, and TRAP staining revealed a dramatic increase in trabecular and cortical bone masses but a sharp reduction in osteoclast number. Moreover, there was an increase in BrdU positive osteocytes (2-5-fold) and osteoid volume (~4-fold) but a decrease in the bone formation rate (~85%) in the cKO bones, indicating a defective mineralization. The SEM analysis revealed poorly formed osteocytes: a sharp increase in cell numbers, a great reduction in cell dendrites, and a remarkable change in the cell distribution pattern. Molecular studies demonstrated a significant decrease in the Sost mRNA levels in bone (>95%), and the SOST protein levels in serum (~85%) and bone matrices. There was a significant increase in the ß-catenin (>3-fold) mRNA levels as well as its target genes Tcf1 (>6-fold) and Tcf3 (~2-fold) in the cKO bones. We also showed a significant decrease in the RANKL levels of serum proteins (~65%) and bone mRNA (~57%), and a significant increase in the Opg mRNA levels (>20-fold) together with a significant reduction in the Rankl/Opg ratio (>95%), which are responsible for a sharp reduction in the cKO osteoclasts. The values of mechanical strength were higher in cKO femora (i.e. max force, displacement, and work failure). These results suggest that loss of BMP signaling specifically in osteocytes dramatically increases bone mass presumably through simultaneous inhibition of RANKL and SOST, leading to osteoclast inhibition and Wnt activation together. Finally, a working hypothesis is proposed to explain how BMPR1A controls bone remodeling by inhibiting cell proliferation and stimulating differentiation. It is reported that RANKL and SOST are abundantly expressed by osteocytes. Thus, BMP signaling through BMPR1A plays important roles in osteocytes.

Role of Cbl-PI3K Interaction during Skeletal Remodeling in a Murine Model of Bone Repair.

Mice in which Cbl is unable to bind PI3K (YF mice) display increased bone volume due to enhanced bone formation and repressed bone resorption during normal bone homeostasis. We investigated the effects of disrupted Cbl-PI3K interaction on fracture healing to determine whether this interaction has an effect on bone repair. Mid-diaphyseal femoral fractures induced in wild type (WT) and YF mice were temporally evaluated via micro-computed tomography scans, biomechanical testing, histological and histomorphometric analyses. Imaging analyses revealed no change in soft callus formation, increased bony callus formation, and delayed callus remodeling in YF mice compared to WT mice. Histomorphometric analyses showed significantly increased osteoblast surface per bone surface and osteoclast numbers in the calluses of YF fractured mice, as well as increased incorporation of dynamic bone labels. Furthermore, using laser capture micro-dissection of the fracture callus we found that cells lacking Cbl-PI3K interaction have higher expression of Osterix, TRAP, and Cathepsin K. We also found increased expression of genes involved in propagating PI3K signaling in cells isolated from the YF fracture callus, suggesting that the lack of Cbl-PI3K interaction perhaps results in enhanced PI3K signaling, leading to increased bone formation, but delayed remodeling in the healing femora.