The osteoclast, a target cell for microorganisms

Inhibition of ferroptosis rescues M2 macrophages and alleviates arthritis by suppressing the HMGB1/TLR4/STAT3 axis in M1 macrophages

Ferroptosis is a type of programmed cell death driven by iron-dependent lipid peroxidation. The TNF-mediated biosynthesis of glutathione has been shown to protect synovial fibroblasts from ferroptosis in the hyperplastic synovium. Ferroptosis induction provides a novel therapeutic approach for rheumatoid arthritis (RA) by reducing the population of synovial fibroblasts. The beginning and maintenance of synovitis in RA are significantly influenced by macrophages, as they generate cytokines that promote inflammation and contribute to the destruction of cartilage and bone. However, the vulnerability of macrophages to ferroptosis in RA remains unclear. In this study, we found that M2 macrophages are more vulnerable to ferroptosis than M1 macrophages in the environment of the arthritis synovium with a high level of iron, leading to an imbalance in the M1/M2 ratio. During ferroptosis, HMGB1 released by M2 macrophages interacts with TLR4 on M1 macrophages, which in turn triggers the activation of STAT3 signaling in M1 macrophages and contributes to the inflammatory response. Knockdown of TLR4 decreased the level of cytokines induced by HMGB1 in M1 macrophages. The ferroptosis inhibitor liproxstatin-1 (Lip-1) started at the presymptomatic stage in collagen-induced arthritis (CIA) model mice, and GPX4 overexpression in M2 macrophages at the onset of collagen antibody-induced arthritis (CAIA) protected M2 macrophages from ferroptotic cell death and significantly prevented the development of joint inflammation and destruction. Thus, our study demonstrated that M2 macrophages are vulnerable to ferroptosis in the microenvironment of the hyperplastic synovium and revealed that the HMGB1/TLR4/STAT3 axis is critical for the ability of ferroptotic M2 macrophages to contribute to the exacerbation of synovial inflammation in RA. Our findings provide novel insight into the progression and treatment of RA.

Catalpol attenuates osteoporosis in ovariectomized rats through promoting osteoclast apoptosis via the Sirt6-ERα-FasL axis

BACKGROUND

Catalpol, a major active component of the Chinese herb Rehmannia glutinosa, possesses various pharmacological benefits, including anti-inflammatory, antidiabetic, and antitumor properties. Recent studies have reported that catalpol can attenuate bone loss and enhance bone formation. Nevertheless, the molecular mechanisms underlying its effects on osteoporosis pathogenesis remain unclear.

PURPOSE

We investigated whether catalpol had a protective effect against postmenopausal osteoporosis (PMOP) and explored its exact mechanism of action.

METHODS

Seventy-two rats were randomly divided into six groups: sham, model, low-dose catalpol (5 mg/kg/day), medium-dose catalpol (10 mg/kg/day), high-dose catalpol (20 mg/kg/day), and positive control (alendronate, 2.5 mg/kg). In this experiment, a ovariectomy was performed to establish a female rat model of PMOP. After 12 weeks of gavage, micro-computed tomography (micro-CT) and histochemical staining were performed to evaluate bone mass, bone microstructure and histological parameters. Furthermore, RAW 264.7 cells were induced by RANKL to form mature osteoclasts to investigate the effect of catalpol on osteoclast differentiation and apoptosis in vitro. Additionally, the osteoclast apoptosis-related proteins of Sirt6, ERα, FasL, NFATc1, cleaved-caspase 8, cleaved-caspase 3, and Bax were assessed using western blotting. The expressions of NFATc1, Ctsk, Oscar, and Trap were quantified using RT-qPCR. The apoptotic rate of the osteoclasts was determined using flow cytometry. Sirt6 knockdown was performed using siRNA gene silencing in experiments to investigate its role in catalpol-mediated osteoclast apoptosis. The deacetylation of ERα in osteoclasts was tested via co-immunoprecipitation.

RESULTS

Catalpol (10 and 20 mg/kg) and alendronate (2.5 mg/kg) could significantly improve bone mineral density (BMD) and microstructure and decrease osteoclast density in ovariectomized (OVX) rats. In addition, catalpol (10 and 20 mg/kg) upregulated the expression of Sirt6, ERα, FasL, cleaved-caspase 8, cleaved-caspase 3, Bax, and downregulated the expression of NFATc1, Ctsk, Oscar, Trap both in vivo and in vitro. Catalpol also promoted ERα deacetylation and stabilized ERα protein to enhance the expression of FasL. In addition, Sirt6 knockdown by siRNA prevented ERα deacetylation and eliminated catalpol-mediated osteoclast apoptosis.

CONCLUSIONS

The present study demonstrated that catalpol prevents estrogen deficiency-induced osteoporosis by promoting osteoclast apoptosis via the Sirt6-ERα-FasL axis. These findings revealed a novel molecular mechanism underpinning the impact of catalpol in the progression of osteoporosis and provided novel insights into the treatment of osteoporosis.

Global Transcriptomic Study of Circular-RNA Expression Profile in Osteoclasts Infected by Intracellular Staphylococcus aureus

Osteomyelitis is difficult to cure, and the rapidly rising morbidity is a thorny problem accompanied by a large number of joint replacement applications. Staphylococcus aureus is the main pathogen of osteomyelitis. Circular RNAs (circRNAs), as emerging noncoding RNAs, play important roles in multiple physiopathological processes which could provide novel insights into osteomyelitis. However, little is known about the roles of circRNAs in the pathogenesis of osteomyelitis. Osteoclasts, considered bone sentinels, are the resident macrophages in bone and may play the immune defense roles in osteomyelitis. It has been reported that S. aureus can survive in osteoclasts, but the function of osteoclast circRNAs in response to intracellular S. aureus infection remains unclear. In this study, we investigated the profile of circRNAs in osteoclasts infected by intracellular S. aureus through high-throughput RNA sequencing. In total, 24 upregulated and 62 downregulated differentially expressed circRNAs were identified and subsequently analyzed to demonstrate their potential functions. On this basis, three circRNAs (chr4:130718154-130728164+, chr8:77409548-77413627-, and chr1:190871592-190899571-) were confirmed as potential novel biomarkers for the diagnosis of osteomyelitis through the murine model of osteomyelitis. Most importantly, we verified that the circRNA chr4:130718154-130728164+ named circPum1 could regulate the host autophagy to affect the intracellular infection of S. aureus through miR-767. In addition, circPum1 could serve as a promising serum biomarker in osteomyelitis patients caused by S. aureus infection. Taken together, this study provided the first global transcriptomic profile analysis of circRNAs in osteoclasts infected by intracellular S. aureus and first proposed a novel perspective for the pathogenesis and immunotherapy of S. aureus-induced osteomyelitis from the term of circRNAs.

Probucol suppresses osteoclastogenesis via activating Nrf2 signaling and ameliorates ovariectomy-induced bone loss

Osteoporosis is a systemic and endocrine bone disorder distinguished by declined bone mineral density, compromised bone strength, and destruction of trabecular structure. The abnormally excessive osteoclastogenesis and bone erosion play imperative roles in the progression of osteoporosis. However, treatment of osteoporosis is far from satisfactory due to poor adherence to existing medications and adverse reactions, there is an urgent to develop novel therapies for osteoporosis. Probucol, a synthetic compound with two characteristic phenolic rings, owns anti-inflammatory and antioxidant properties. Accumulating evidence have indicated that intracellular reactive oxygen species (ROS) is closely related to osteoclastogenesis. Hence, we investigated the potential effects of probucol on osteoclastogenesis in vivo and in vitro. In this study, TRAP staining and bone slice resorption assay showed that probucol suppressed RANKL-induced osteoclast formation and function. The mRNA and protein levels of osteoclastogenesis marker genes were reduced by probucol in a concentration-dependent manner. Besides, probucol suppressed osteoclast differentiation by inhibiting ROS production, MAPKs and NF-κB signaling pathways, while Nrf2 silencing reversed the inhibitory effect of probucol on osteoclast formation and function. Consistent with the above findings, in vivo experiments demonstrated that probucol visibly alleviated bone loss caused by estrogen deficiency. In brief, these results showed the potential of anti-oxidant compound probucol in the treatment of osteoporosis, highlighting Nrf2 as a promising target in osteoclast-related disease.

Osteoclasts: Other functions

Osteoclasts are the only cells that can efficiently resorb bone. They do so by sealing themselves on to bone and removing the mineral and organic components. Osteoclasts are essential for bone homeostasis and are involved in the development of diseases associated with decreased bone mass, like osteoporosis, or abnormal bone turnover, like Paget's disease of bone. In addition, compromise of their development or resorbing machinery is pathogenic in multiple types of osteopetrosis. However, osteoclasts also have functions other than bone resorption. Like cells of the innate immune system, they are derived from myeloid precursors and retain multiple immune cell properties. In addition, there is now strong evidence that osteoclasts regulate osteoblasts through a process known as coupling, which coordinates rates of bone resorption and bone formation during bone remodeling. In this article we review the non-resorbing functions of osteoclasts and highlight their importance in health and disease.

Zika virus infects human osteoclasts and blocks differentiation and bone resorption

Bone-related complications are commonly reported following arbovirus infection. These arboviruses are known to disturb bone-remodeling and induce inflammatory bone loss via increased activity of bone resorbing osteoclasts (OCs). We previously showed that Zika virus (ZIKV) could disturb the function of bone forming osteoblasts, but the susceptibility of OCs to ZIKV infection is not known. Here, we investigated the effect of ZIKV infection on osteoclastogenesis and report that infection of pre- and early OCs with ZIKV significantly reduced the osteoclast formation and bone resorption. Interestingly, infection of pre-OCs with a low dose ZIKV infection in the presence of flavivirus cross-reacting antibodies recapitulated the phenotype observed with a high viral dose, suggesting a role for antibody-dependent enhancement in ZIKV-associated bone pathology. In conclusion, we have characterized a primary in vitro model to study the role of osteoclastogenesis in ZIKV pathogenesis, which will help to identify possible new targets for developing therapeutic and preventive measures.

α-Mangostin inhibits LPS-induced bone resorption by restricting osteoclastogenesis via NF-κB and MAPK signaling

Background

Excessive osteoclast activation is an important cause of imbalanced bone remodeling that leads to pathological bone destruction. This is a clear feature of many osteolytic diseases such as rheumatoid arthritis, osteoporosis, and osteolysis around prostheses. Because many natural compounds have therapeutic potential for treating these diseases by suppressing osteoclast formation and function, we hypothesized that α-mangostin, a natural compound isolated from mangosteen, might be a promising treatment as it exhibits anti‐inflammatory, anticancer, and cardioprotective effects.

Methods

We evaluated the therapeutic effect of α-mangostin on the processes of osteoclast formation and bone resorption. The receptor activator of nuclear factor-κB (NF-κB) ligand (RANKL) induces osteoclast formation in vitro, and potential pathways of α-mangostin to inhibit osteoclast differentiation and function were explored. A mouse model of lipopolysaccharide‐induced calvarial osteolysis was established. Subsequently, micro-computed tomography and histological assays were used to evaluate the effect of α-mangostin in preventing inflammatory osteolysis.

Results

We found that α-mangostin could inhibit RANKL-induced osteoclastogenesis and reduced osteoclast‐related gene expression in vitro. F-actin ring immunofluorescence and resorption pit assays indicated that α-mangostin also inhibited osteoclast functions. It achieved these effects by disrupting the activation of NF-κB/mitogen-activated protein kinase signaling pathways. Our in vivo data revealed that α-mangostin could protect mouse calvarial bone from osteolysis.

Conclusions

Our findings demonstrate that α-mangostin can inhibit osteoclastogenesis both in vitro and in vivo and may be a potential option for treating osteoclast-related diseases.

A 3D printed Ga containing scaffold with both anti-infection and bone homeostasis-regulating properties for the treatment of infected bone defects

Large bone defects face a high risk of infection, which can also lead to bone homeostasis disorders. This seriously hinders the bone healing process; therefore, the help of a dual-functional scaffold that has both anti-infection and bone-homeostasis-regulating capacities is needed in the treatment of infected bone defects. In this study, a 3D printed dual-functional scaffold composed of poly-ε-caprolactone (PCL), mesoporous bioactive glasses (MBG), and gallium (Ga) was produced. In vitro experiments demonstrated the excellent antibacterial ability of the PCL/MBG/Ga scaffold against methicillin-resistant Staphylococcus aureus (MRSA) and Escherichia coli (E. coli). The scaffold also significantly inhibited osteoclastic activity and promoted osteogenic differentiation. Furthermore, a rabbit model with an infected bone defect in the radius was used to evaluate the in vivo bone healing capability of PCL/MBG/Ga. The results demonstrate that the PCL/MBG/Ga scaffold can significantly accelerate bone healing and prevent bone resorption, suggesting its potential for application in repairing infected bone defects.

Cellular and molecular actors of myeloid cell fusion: podosomes and tunneling nanotubes call the tune

Different types of multinucleated giant cells (MGCs) of myeloid origin have been described; osteoclasts are the most extensively studied because of their importance in bone homeostasis. MGCs are formed by cell-to-cell fusion, and most types have been observed in pathological conditions, especially in infectious and non-infectious chronic inflammatory contexts. The precise role of the different MGCs and the mechanisms that govern their formation remain poorly understood, likely due to their heterogeneity. First, we will introduce the main populations of MGCs derived from the monocyte/macrophage lineage. We will then discuss the known molecular actors mediating the early stages of fusion, focusing on cell-surface receptors involved in the cell-to-cell adhesion steps that ultimately lead to multinucleation. Given that cell-to-cell fusion is a complex and well-coordinated process, we will also describe what is currently known about the evolution of F-actin-based structures involved in macrophage fusion, i.e., podosomes, zipper-like structures, and tunneling nanotubes (TNT). Finally, the localization and potential role of the key fusion mediators related to the formation of these F-actin structures will be discussed. This review intends to present the current status of knowledge of the molecular and cellular mechanisms supporting multinucleation of myeloid cells, highlighting the gaps still existing, and contributing to the proposition of potential disease-specific MGC markers and/or therapeutic targets.

Gamabufotalin Inhibits Osteoclastgenesis and Counteracts Estrogen-Deficient Bone Loss in Mice by Suppressing RANKL-Induced NF-κB and ERK/MAPK Pathways

Osteolytic bone disease is a condition of imbalanced bone homeostasis, characterized mainly by excessive bone-resorptive activity, which could predispose these populations, such as the old and postmenopausal women, to developing high risk of skeletal fragility and fracture. The nature of bone homeostasis is the coordination between the osteoblasts (OBs) and osteoclasts (OCs). Abnormal activation of osteoclasts (OCs) could compromise the bone homeostasis, constantly followed by a clutch of osteolytic diseases, including postmenopausal osteoporosis, osteoarthritis, and rheumatoid arthritis. Thus, it is imperatively urgent to explore effective medical interventions for patients. The traditional Chinese medicine (TCM) gamabufotalin (CS-6) is a newly identified natural product from Chansu and has been utilized for oncologic therapies owing to its good clinical efficacy with less adverse events. Previous study suggested that CS-6 could be a novel anti-osteoporotic agent. Nevertheless, whether CS-6 suppresses RANK-(receptor activator of nuclear factor-κ B ligand)/TRAF6 (TNF receptor-associated factor 6)-mediated downstream signaling activation in OCs, as well as the effects of CS-6 on OC differentiation in vivo, remains elusive. Therefore, in this present study, we aimed to explore the biological effects of CS-6 on osteoclastogenesis and RANKL-induced activation of related signaling pathways, and further to examine the potential therapeutic application in estrogen-deficient bone loss in the mice model. The results of in vitro experiment showed that CS-6 can inhibit RANKL-induced OC formation and the ability of bone resorption in a dose-dependent manner at both the early and late stages of osteoclastogenesis. The gene expression of OC-related key genes such as tartrate-resistant acid phosphatase (TRAP), CTSK, DC-STAMP, MMP9, and β3 integrin was evidently reduced. In addition, CS-6 could mitigate the systemic estrogen-dependent bone loss and pro-inframammary cytokines in mice in vivo. The molecular mechanism analysis suggested that CS-6 can suppress RANKL/TRAF6-induced early activation of NF-κB and ERK/MAPK signaling pathways, which consequently suppressed the transcription activity of c-Fos and NFATc1. Taken together, this present study provided ample evidence that CS-6 has the promise to become a therapeutic candidate in treating osteolytic conditions mediated by elevated OC formation and bone resorption.

Salubrinal Alleviates Collagen-Induced Arthritis through Promoting P65 Degradation in Osteoclastogenesis

Rheumatoid arthritis (RA) is a complex systemic autoimmune disorder that primarily involves joints, further affects the life quality of patients, and has increased mortality. The pathogenesis of RA involves multiple pathways, resulting in some patients showing resistance to the existing drugs. Salubrinal is a small molecule compound that has recently been shown to exert multiple beneficial effects on bone tissue. However, the effect of Salubrinal in RA has not been clearly confirmed. Hence, we induced collagen-induced arthritis (CIA) in DBA/1J mice and found that Salubrinal treatment decreased the clinical score of CIA mice, inhibiting joint damage and bone destruction. Furthermore, Salubrinal treatment downregulated osteoclast number in knee joint of CIA in mice, and suppressed bone marrow-derived osteoclast formation and function, downregulated osteoclast-related gene expression. Moreover, Salubrinal treatment inhibited RANKL-induced NF-κB signaling pathway, and promoted P65 degradation through the ubiquitin-proteasome system, further restrained RANKL-induced osteoclastogenesis. This study explains the mechanism by which Salubrinal ameliorates arthritis of CIA in mice, indicating that Salubrinal may be a potential drug for RA, and expands the potential uses of Salubrinal in the treatment of bone destruction-related diseases.

Antibacterial biomaterials in bone tissue engineering

Bone infection is a devastating disease characterized by recurrence, drug-resistance, and high morbidity, that has prompted clinicians and scientists to develop novel approaches to combat it. Currently, although numerous biomaterials that possess excellent biocompatibility, biodegradability, porosity, and mechanical strength have been developed, their lack of effective antibacterial ability substantially limits bone-defect treatment efficacy. There is, accordingly, a pressing need to design antibacterial biomaterials for effective bone-infection prevention and treatment. This review focuses on antibacterial biomaterials and strategies; it presents recently reported biomaterials, including antibacterial implants, antibacterial scaffolds, antibacterial hydrogels, and antibacterial bone cement types, and aims to provide an overview of these antibacterial materials for application in biomedicine. The antibacterial mechanisms of these materials are discussed as well.

Osteosarcoma-derived exosomal miR-501-3p promotes osteoclastogenesis and aggravates bone loss

Emerging evidence indicates that osteoclasts from osteosarcoma patients have higher tartrate resistant acid phosphatase (TRAP) activity. Exosomes are important mediators of the cell-to-cell communication. However, whether osteosarcoma cell-derived exosomes mediate the osteoclastogenesis of bone marrow-derived monocytes (BMDMs) and its mechanisms are largely unknown. In this research, we validated the communication between osteosarcoma cells and BMDMs. Here, we found that osteosarcoma cell-derived exosomes can be transfered to BMDMs to promote osteoclast differentiation. The miR-501-3p is highly expressed in exosomes derived from osteosarcoma and could be transferred to BMDMs through the exosomes. Moreover, osteosarcoma-derived exosomal miR-501-3p mediate its role in promoting osteoclast differentiation and aggravates bone loss in vitro and in vivo. Mechanistically, osteosarcoma cell-derived exosomal miR-501-3p could promote osteoclast differentiation via PTEN/PI3K/Akt signaling pathway. Collectively, our results suggest that osteosarcoma-derived exosomal miR-501-3p promotes osteoclastogenesis and aggravates bone loss. Therefore, our study reveals a novel mechanism of osteoclastogenesis in osteosarcoma patients and provides a novel target for diagnosis or treatment.

Novel 2,7-Diazaspiro[4,4]nonane Derivatives to Inhibit Mouse and Human Osteoclast Activities and Prevent Bone Loss in Ovariectomized Mice without Affecting Bone Formation

Osteoporosis is currently treated with drugs targeting the differentiation or viability osteoclasts, the cells responsible for physiological and pathological bone resorption. Nevertheless, osteoporosis drugs that target only osteoclast activity are expected to preserve bone formation by osteoblasts in contrast to current treatments. We report here the design, synthesis, and biological characterization of a series of novel N-arylsufonamides featuring a diazaspiro[4,4]nonane nucleus to target the guanine nucleotide exchange activity of DOCK5, which is essential for bone resorption by osteoclasts. These compounds can inhibit both mouse and human osteoclast activity. In particular, 4-chlorobenzyl-4-hydroxy-2-phenyl-1-thia-2,7-diazaspiro[4,4]nonane 1,1-dioxide (compound E197) prevented pathological bone loss in mice. Most interestingly, treatment with E197 did not affect osteoclast and osteoblast numbers and hence did not impair bone formation. E197 could represent a lead molecule to develop new antiosteoporotic drugs targeting the mechanism of osteoclast adhesion onto the bone.

PERK controls bone homeostasis through the regulation of osteoclast differentiation and function

Osteoclasts are multinucleated giant cells with the ability to degrade bone tissue, and are closely related to abnormal bone metabolic diseases. Endoplasmic reticulum (ER) is an organelle responsible for protein modification, quality control, and transportation. The accumulation of unfolded or misfolded proteins in ER cavity induces ER stress. Double-stranded RNA-dependent protein kinase-like ER kinase (PERK) is an ER stress-sensing protein, which is ubiquitous in eukaryotic cells. Systemic PERK knockout mice show severe bone loss, suggesting that PERK is of great significance for maintaining the normal growth and development of bone tissue, but the role of PERK in osteoclastogenesis is still unclear. In this study, we found that PERK was significantly activated during RANKL-induced osteoclast differentiation; knockdown of PERK by siRNA and inhibition of PERK by GSK2606414, respectively, had significant negative regulatory effects on the formation and bone resorption of osteoclasts. PERK inhibitor GSK2606414 down-regulated the mRNA levels and protein expression of osteoclast differentiation marker genes, and inhibited RANKL-induced activation of Mitogen-activated protein kinase (MAPK) and nuclear factor κB (NF-κB) pathways. Treatment with PERK inhibitor GSK2606414 in ovariectomized mouse model significantly suppressed bone loss and osteoclast formation. Thapsigargin activated ER stress to enhance autophagy, while GSK2606414 had a significant inhibitory effect on autophagy flux and autophagosome formation. Antioxidant N-acetylcysteine (NAC) could inhibit the expression of PERK phosphorylation, osteoclast-related proteins and autophagy-related proteins, but the use of PERK activator CCT020312 can reverse inhibition effect of NAC. Our findings demonstrate a key role for PERK in osteoclast differentiation and suggest its therapeutic potential.

People living with HIV and fracture risk

PLHIV have an increased risk of osteoporosis and fractures when compared with people of the same age and sex. In this review, we address the epidemiology and the pathophysiology of bone disease and fractures in PLHIV. The assessment of fracture risk and fracture prevention in these subjects is also discussed. The spectrum of HIV-associated disease has changed dramatically since the introduction of potent antiretroviral drugs. Today, the survival of people living with HIV (PLHIV) is close to that of the general population. However, the longer life-span in PLHIV is accompanied by an increased prevalence of chronic diseases. Detrimental effects on bone health are well recognised, with an increased risk of osteoporosis and fractures, including vertebral fractures, compared to the general population. The causes of bone disease in PLHIV are not fully understood, but include HIV-specific risk factors such as use of antiretrovirals and the presence of chronic inflammation, as well as traditional risk factors for fracture. Current guidelines recommend the use of FRAX to assess fracture probability in PLHIV age ≥ 40 years and measurement of bone mineral density in those at increased fracture risk. Vitamin D deficiency, if present, should be treated. Bisphosphonates have been shown to increase bone density in PLHIV although fracture outcomes are not available.

The osteoclast cytoskeleton - current understanding and therapeutic perspectives for osteoporosis

Osteoclasts are giant multinucleated myeloid cells specialized for bone resorption, which is essential for the preservation of bone health throughout life. The activity of osteoclasts relies on the typical organization of osteoclast cytoskeleton components into a highly complex structure comprising actin, microtubules and other cytoskeletal proteins that constitutes the backbone of the bone resorption apparatus. The development of methods to differentiate osteoclasts in culture and manipulate them genetically, as well as improvements in cell imaging technologies, has shed light onto the molecular mechanisms that control the structure and dynamics of the osteoclast cytoskeleton, and thus the mechanism of bone resorption. Although essential for normal bone physiology, abnormal osteoclast activity can cause bone defects, in particular their hyper-activation is commonly associated with many pathologies, hormonal imbalance and medical treatments. Increased bone degradation by osteoclasts provokes progressive bone loss, leading to osteoporosis, with the resulting bone frailty leading to fractures, loss of autonomy and premature death. In this context, the osteoclast cytoskeleton has recently proven to be a relevant therapeutic target for controlling pathological bone resorption levels. Here, we review the present knowledge on the regulatory mechanisms of the osteoclast cytoskeleton that control their bone resorption activity in normal and pathological conditions.

Knockdown of Lrp1 in RAW264 cells inhibits osteoclast differentiation and osteoclast-osteoblast interactions in vitro

Low density lipoprotein receptor-related protein 1 (LRP1), a multifunctional cell surface protein, is expressed in bone marrow-derived macrophages. While LRP1 is thought to be a suppressor of osteoclast differentiation at late stages, its function at early stages remains unclear. Here we demonstrate that Lrp1 stable knockdown by lentiviral short hairpin RNA in macrophage cell line RAW264 cells inhibited RANKL-induced osteoclast formation and osteoclastic master transcription factor Nfatc1 mRNA expression as assessed by quantitative RT-PCR. Furthermore, knockdown of the Lrp1 gene suppressed not only differentiation, but also proliferation, and inhibitory effects on osteoblastic ALP activity by osteoclast-derived humoral factors. Thus, we propose that LRP1 in macrophages is required for both differentiation into osteoclasts and osteoclast-osteoblast interactions.

Periprosthetic Joint Infection Caused by Gram-Positive Versus Gram-Negative Bacteria: Lipopolysaccharide, but not Lipoteichoic Acid, Exerts Adverse Osteoclast-Mediated Effects on the Bone

Periprosthetic joint infection (PJI)-the most common cause of knee arthroplasty failure-may result from Gram-positive (GP) or Gram-negative (GN) bacterial infections. The question as to whether PJI due to GP or GN bacteria can lead to different rates of aseptic loosening after reimplantation remains open. We have investigated this issue through a retrospective review of clinical records obtained from 320 patients with bacterial PJI. The results revealed that, compared with GP infections, GN infections were associated with an increased risk of aseptic loosening. In animal studies, mice underwent intrafemoral injection of lipopolysaccharide (LPS) from GN bacteria or lipoteichoic acid (LTA) from GP bacteria. We demonstrate that LPS-but not LTA-reduced both the number of trabeculae and the bone mineral density in mice. In addition, LPS-treated mice exhibited a reduced body weight, higher serum osteocalcin levels, and an increased number of osteoclasts. LPS accelerated monocyte differentiation into osteoclast-like cells, whereas LTA did not. Finally, ibudilast-a toll-like receptor (TLR)-4 antagonist-was found to inhibit LPS-induced bone loss and osteoclast activation in mice. Taken together, our data indicate that PJI caused by GN bacteria portends a higher risk of aseptic loosening after reimplantation, mainly because of LPS-mediated effects on osteoclast differentiation.