Inflammatory osteolysis: a conspiracy against bone

Antibacterial and immunogenic behavior of silver coatings on additively manufactured porous titanium

Implant-associated infections (IAI) are often recurrent, expensive to treat, and associated with high rates of morbidity, if not mortality. We biofunctionalized the surface of additively manufactured volume-porous titanium implants using electrophoretic deposition (EPD) as a way to eliminate the peri-operative bacterial load and prevent IAI. Chitosan-based (Ch) coatings were incorporated with different concentrations of silver (Ag) nanoparticles or vancomycin. A full-scale in vitro and in vivo study was then performed to evaluate the antibacterial, immunogenic, and osteogenic activity of the developed implants. In vitro, Ch + vancomycin or Ch + Ag coatings completely eliminated, or reduced the number of planktonic and adherent Staphylococcus aureus by up to 4 orders of magnitude, respectively. In an in vivo tibia intramedullary implant model, Ch + Ag coatings caused no adverse immune or bone response under aseptic conditions. Following Staphylococcus aureus inoculation, Ch + vancomycin coatings reduced the implant infection rate as compared to chitosan-only coatings. Ch + Ag implants did not demonstrate antibacterial effects in vivo and even aggravated infection-mediated bone remodeling including increased osteoclast formation and inflammation-induced new bone formation. As an explanation for the poor antibacterial activity of Ch + Ag implants, it was found that antibacterial Ag concentrations were cytotoxic for neutrophils, and that non-toxic Ag concentrations diminished their phagocytic activity. This study shows the potential of EPD coating to biofunctionalize porous titanium implants with different antibacterial agents. Using this method, Ag-based coatings seem inferior to antibiotic coatings, as their adverse effects on the normal immune response could cancel the direct antibacterial effects of Ag nanoparticles. STATEMENT OF SIGNIFICANCE: Implant-associated infections (IAI) are a clinical, societal, and economical burden. Surface biofunctionalization approaches can render complex metal implants with strong local antibacterial action. The antibacterial effects of inorganic materials such as silver nanoparticles (Ag NPs) are often highlighted under very confined conditions in vitro. As a novelty, this study also reports the antibacterial, immunogenic, and osteogenic activity of Ag NP-coated additively-manufactured titanium in vivo. Importantly, it was found that the developed coatings could impair the normal function of neutrophils, the most important phagocytic cells protecting us from IAI. Not surprisingly, the Ag NP-based coatings were outperformed by an antibiotic-based coating. This emphasizes the importance of also targeting implant immune-modulatory functions in future coating strategies against IAI.

S100A12 facilitates osteoclast differentiation from human monocytes

Osteoclasts play a critical role not only in bone homeostasis but also in inflammatory osteolysis, such as that occurring in inflammatory arthritis and systemic inflammation. In both inflammation conditions, inflammatory cytokines like Interleukin (IL)-1, IL-6 and tumor necrosis factor (TNF)-α induce RANKL expression in osteoblasts, but the roles of these cytokines in osteoclast activation remain unclear. S100A12, an S100 family member, is a low-molecular-weight calcium-binding protein. Although it has a pro-inflammatory role, its effects on osteoclast differentiation have been unclear. Here we examined the direct effects of S100A12 on human osteoclasts in vitro. S100A12 facilitated osteoclast formation in the presence of RANKL, as judged by the cells’ morphology and elevated expression of osteoclast-related molecules, including NFATc1, ACP5, CALCR, and ITGβ3. In addition, S100A12 administration markedly enhanced the osteoclasts’ bone resorption ability, consistent with their increased expression levels of CTSK and CA2. Blocking RAGE and TLR4 cancelled the effects of S100A12. Our results indicate that S100A12 is a potential therapeutic target for inflammatory osteolysis.

Immunobiology of periprosthetic inflammation and pain following ultra-high-molecular-weight-polyethylene wear debris in the lumbar spine

INTRODUCTION

Wear debris-induced osteolysis is a common cause of arthroplasty failure in several joints including the knee, hip and intervertebral disc. Debris from the prosthesis can trigger an inflammatory response that leads to aseptic loosening and prosthesis failure. In the spine, periprosthetic pain also occurs following accumulation of wear debris through neovascularization of the disc. The role of the immune system in the pathobiology of periprosthetic osteolysis of joint replacements is debatable. Areas covered: We discussed the stimulation of pro-inflammatory and pro-protective and pro-regenerative pathways due to debris from the prosthetics. The balance between the two pathways may determine the outcome results. Also, the role of cytokines and immune cells in periprosthetic inflammation in the etiology of osteolysis is critically reviewed. Expert commentary: Therapies targeting the inflammatory process associated with ultra-high-molecular-weight polyethylene wear debris could reduce implant failure. Additionally, therapies targeting neovascularization of discs following arthroplasty could mitigate periprosthetic pain.

Modulating calcium-mediated NFATc1 and mitogen-activated protein kinase deactivation underlies the inhibitory effects of kavain on osteoclastogenesis and bone resorption

Osteoclasts are responsible for bone resorption during the process of bone remodeling. Increased osteoclast numbers and bone resorption activity are the main factors contributing to bone loss-related diseases such as osteoporosis. Therefore, modulating the formation and function of osteoclasts is critical for the effective treatment of osteolysis and osteoporosis. Kavain is the active ingredient extracted from the root of the kava plant, which possesses known anti-inflammatory properties. However, the effects of kavain on osteoclastogenesis and bone resorption remain unclear. In this study, we found that kavain inhibits receptor activator of nuclear factor-κB ligand (RANKL)-induced osteoclast differentiation and fusion using tartrate-resistant acid phosphatase staining and immunofluorescence. Furthermore, kavain inhibited bone resorption performed by osteoclasts. Using reverse transcription-polymerase chain reaction and western blot analysis, we found that kavain downregulates the expression of osteoclast marker genes, such as nuclear factor of activated T cells, cytoplasmic 1 (Nfatc1), v-atpase d2 (Atp6v0d2), dendrocyte expressed seven transmembrane protein (Dcstamp), matrix metallopeptidase 9 (Mmp9), cathepsin K (Ctsk), and Acp5. Additionally, kavain repressed RANKL-induced calcium oscillations, nuclear factor of activated T cells activation, and mitogen-activated protein kinase phosphorylation, while leaving NF-κB unaffected. We found no effects of kavain on either osteoblast proliferation or differentiation. Besides, kavain inhibited bone loss in ovariectomized mice by suppressing osteoclastogenesis. Collectively, these data suggest a potential use for kavain as a candidate drug for the treatment of osteolytic diseases.

Association of gout with osteoporotic fractures

PURPOSE

Previous studies have shown that serum uric acid levels and inflammation are associated with bone mineral density. Gout, a disease characterized by hyperuricemia and inflammation, contributes to the risk of osteoporotic fractures. However, this association is controversial. Therefore, this study investigated whether gout in older people (age > 55 years) is associated with osteoporotic fracture risk.

METHODS

This population-based, cross-sectional study included 2674 participants (147 cases of gout and 388 fractures). Standardized and self-administered questionnaires were employed and physical examinations, blood tests, and bone mineral density examinations were performed; multivariate-adjusted logistic regression models were used to evaluate associations between gout and osteoporotic fracture risk.

RESULTS

The data were adjusted for age; smoking status; alcohol status; physical activity; body mass index; waist circumference; hypertension; cardiovascular events; diabetes mellitus; rheumatoid arthritis; serum levels of total cholesterol (TC), triglycerides, and high- and low-density lipids; and T-scores. We found a significant association between gout and osteoporotic fracture risk in women (odds ratio [OR], 2.00; 95% confidence interval [CI], 1.12-3.56; P = 0.019), but no such association in men (OR, 1.30; 95% CI, 0.58-2.88; P = 0.525). Further stratified analyses showed a significant association between gout and osteoporotic fracture risk in women without rheumatic arthritis and in those with high TC levels or with osteoporosis (all, P < 0.05).

CONCLUSIONS

In older Chinese adults, gout is significantly associated with the risk of osteoporotic fractures in women, especially those without rheumatic arthritis and in those with high TC levels or with osteoporosis.

From macrophage to osteoclast - How metabolism determines function and activity

Osteoclasts are specialised cells that resorb bone and develop from the monocyte/macrophage lineage. While there is a wealth of information on the regulation of macrophage function through metabolic activity, the connection between osteoclast differentiation and metabolism is less well understood. Recent data show that mitochondria participate in switching macrophages from an inflammatory phenotype towards differentiation into osteoclasts. Additionally, it was found that reactive oxygen species (ROS) actively take place in osteoclast differentiation by acting as secondary signalling molecules. Bone resorption is an energy demanding process and differentiating osteoclasts triggers the biogenesis of mitochondria. In addition, the activity of specific OXPHOS components of macrophages and osteoclasts is differentially regulated. This review summarises our knowledge on macrophage-mediated inflammation, its impact on a cell's metabolic activity and its effect on osteoclast differentiation.

Innate Immunity to Staphylococcus aureus: Evolving Paradigms in Soft Tissue and Invasive Infections

Staphylococcus aureus causes a wide range of diseases that together embody a significant public health burden. Aided by metabolic flexibility and a large virulence repertoire, S. aureus has the remarkable ability to hematogenously disseminate and infect various tissues, including skin, lung, heart, and bone, among others. The hallmark lesions of invasive staphylococcal infections, abscesses, simultaneously denote the powerful innate immune responses to tissue invasion as well as the ability of staphylococci to persist within these lesions. In this article, we review the innate immune responses to S. aureus during infection of skin and bone, which serve as paradigms for soft tissue and bone disease, respectively.

Role of nutritional vitamin D in osteoporosis treatment

Osteoporosis is a systemic skeletal disorder characterized by a decrease in bone mass and microarchitectural deterioration of bone tissue. The World Health Organization has defined osteoporosis as a decrease in bone mass (50%) and bony quality (50%). Vitamin D, a steroid hormone, is crucial for skeletal health and in mineral metabolism. Its direct action on osteoblasts and osteoclasts and interaction with nonskeletal tissues help in maintaining a balance between bone turnover and bone growth. Vitamin D affects the activity of osteoblasts, osteoclasts, and osteocytes, suggesting that it affects bone formation, bone resorption, and bone quality. At physiological concentrations, active vitamin D maintains a normal rate of bone resorption and formation through the RANKL/OPG signal. However, active vitamin D at pharmacological concentration inhibits bone resorption at a higher rate than that of bone formation, which influences the bone quality and quantity. Nutritional vitamin D rather than active vitamin D activates osteoblasts and maintains serum 25(OH)D₃ concentration. Despite many unanswered questions, much data support nutritional vitamin D use in osteoporosis patients. This article emphasizes the role of nutritional vitamin D replacement in different turnover status (high or low bone turnover disorders) of osteoporosis together with either anti-resorptive (Bisphosphonate, Denosumab et.) or anabolic (Teriparatide) agents when osteoporosis persists.

Ostm1 Bifunctional Roles in Osteoclast Maturation: Insights From a Mouse Model Mimicking a Human OSTM1 Mutation

Ostm1 mutations are responsible for the most severe form of osteopetrosis in human and mice. To gain insight into Ostm1 cellular functions, we engineered a conditional in-frame deletion of the Ostm1 transmembrane domain and generated the first Ostm1 mouse model with a human mutation. Systemic targeting of Ostm1 loss of transmembrane domain produced osteopetrosis, as in the null Ostm1 gl/gl mouse. Significantly, conditional osteoclast targeting of Ostm1 resulted in similar osteopetrosis, thereby demonstrating that the intrinsic Ostm1 osteoclast deficiency is solely responsible for the mouse phenotype. Our analysis showed oversized osteoclasts with enhanced multinucleation associated with stimulation of intracellular calcium levels, of Nfatc1 nuclear re-localization, and of specific downstream Nfatc1 target genes, providing compelling evidence that Ostm1 is a negative regulator of preosteoclast fusion. Moreover, mature OCs with Ostm1 loss of transmembrane domain show appropriate levels of intracellular acidification but an altered distribution pattern, highlighting misregulation of endolysosome localization and dispersion. Consistently, the hydrolases tartrate-resistant acid phosphatase (TRAP) and cathepsin K (Ctsk) normally produced are sequestered within the osteoclasts and are not extracellularly secreted. These studies defined bifunctional roles for Ostm1 as a major regulator of preosteoclast cytoskeletal rearrangements toward cell multinucleation and of mature osteoclast intracellular lysosomal trafficking and exocytosis mechanism, both of which are essential for bone resorption. Importantly, these Ostm1 molecular and regulatory functions could serve as preclinical targets in this mouse model toward osteoclastogenic pathologies as osteoporosis and inflammation-induced bone loss. © 2018 American Society for Bone and Mineral Research.

From Osteoimmunology to Osteomicrobiology: How the Microbiota and the Immune System Regulate Bone

Osteomicrobiology refers to the role of microbiota in bone health and the mechanisms by which the microbiota regulates post-natal skeletal development, bone aging, and pathologic bone loss. Here, we review recent reports linking gut microbiota to changes in bone phenotype. A pro-inflammatory cytokine milieu drives bone resorption in conditions such as sex steroid hormone deficiency. The response of the immune system to activation by the microbiome results in increased circulating osteoclastogenic cytokines in a T cell-dependent mechanism. Additionally, gut microbiota affect bone homeostasis through nutrient absorption, mediation of the IGF-1 pathway, and short chain fatty acid and metabolic products. Manipulation of microbiota through prebiotics or probiotics reduces inflammatory cytokine production, leading to changes in bone density. One mechanism of probiotic action is through upregulating tight junction proteins, increasing the strength of the gut epithelial layer, and leading to less antigen presentation and less activation of intestinal immune cells. Thus, prebiotics or probiotics may represent a future therapeutic avenue for ameliorating the risk of postmenopausal bone loss in humans.

Selective inhibition of the p38α MAPK-MK2 axis inhibits inflammatory cues including inflammasome priming signals

A unique p38α MAPK–MK2 pathway inhibitor, CDD-450, is used to uncover the function of this protein complex in inflammasome priming signals. Importantly, CDD-450 is as efficacious as global p38α MAPK inhibitors in decreasing inflammation in disease models.

p38α activation of multiple effectors may underlie the failure of global p38α inhibitors in clinical trials. A unique inhibitor (CDD-450) was developed that selectively blocked p38α activation of the proinflammatory kinase MK2 while sparing p38α activation of PRAK and ATF2. Next, the hypothesis that the p38α–MK2 complex mediates inflammasome priming cues was tested. CDD-450 had no effect on NLRP3 expression, but it decreased IL-1β expression by promoting IL-1β mRNA degradation. Thus, IL-1β is regulated not only transcriptionally by NF-κB and posttranslationally by the inflammasomes but also posttranscriptionally by p38α–MK2. CDD-450 also accelerated TNF-α and IL-6 mRNA decay, inhibited inflammation in mice with cryopyrinopathy, and was as efficacious as global p38α inhibitors in attenuating arthritis in rats and cytokine expression by cells from patients with cryopyrinopathy and rheumatoid arthritis. These findings have clinical translation implications as CDD-450 offers the potential to avoid tachyphylaxis associated with global p38α inhibitors that may result from their inhibition of non-MK2 substrates involved in antiinflammatory and housekeeping responses.

Pathologic conditions of hard tissue: role of osteoclasts in osteolytic lesion

Hard tissue homeostasis is regulated by the balance between bone formation by osteoblasts and bone resorption by osteoclasts. This physiologic process allows adaptation to mechanical loading and calcium homeostasis. Under pathologic conditions, however, this process is ill-balanced resulting in either over-resorption or over-formation of hard tissue. Local over-resorption by osteoclasts is typically observed in osteolytic metastases of malignancies, autoimmune arthritis, and giant cell tumor of bone (GCTB). In tumor-related local osteolysis, tumor-derived osteoclast-activating factors induce bone resorption not by directly acting on osteoclasts but by indirectly upregulating receptor activator of NFκB ligand (RANKL) on osteoblastic cells. Similarly, synovial tissue in the autoimmune arthritis model does overexpress RANKL and contains numerous osteoclast precursors, and like a landing craft, when it comes in contact with eroded bone surfaces, osteoclast precursors are immediately polarized to become mature osteoclasts, inducing rapidly progressive bone destruction at a late stage of the disease. GCTB, on the other hand, is a common primary bone tumor, usually arising at the metaphysis of the long bone in young adults. After the discovery of RANKL, the concept of GCTB as a tumor of RANKL-expressing stromal cells was established, and comprehensive exosome studies finally disclosed the causative single-point mutation at histone H3.3 (H3F3A) in stromal cells. Thus, osteolytic lesions under various pathological conditions are ultimately attributable to the overexpression of RANKL, which opens up a common, practical and useful therapeutic target for diverse osteolytic conditions.

Lithium-containing biomaterials inhibit osteoclastogenesis of macrophagesin vitroand osteolysisin vivo

Li-containing bioceramics were promising biomaterials for inhibiting osteoclastogenesis of macrophages and osteolysisin vivo, potentially using for treating osteoporosis.

[Changes in laboratory parameters of peripheral blood reflect cell and protein content of the immune system in bone resorption]

The aim of this study was to investigate dynamic changes in the laboratory parameters of peripheral blood, reflecting cellular and protein composition of the immune system in bone resorption. The study involved 108 patients with degenerative-dystrophic and posttraumatic disorders of the hip joint before and after joint replacement surgery. Half of the cases resulted in bone resorption. Dynamic monitoring was performed up to 7.5 years. It included flow cytometry (Coulter Epics XL, USA) and enzyme immunoassay to determine the amount of immunocompetent cells, immunoglobulin class M, A, G, E, cytokines, acute phase proteins (C-reactive protein, fibrinogen, albumin, ceruloplasmin, haptoglobin), parameters of neutrophil functional activity (lactoferrin cationic protein, myeloperoxidase, superoxide anion production). The results of the study revealed the leading role of haptoglobin, albumin, and IL-1b in bone resorption. The use of multiple regression analysis made it possible to propose criteria for prediction of bone resorption. In particular, the Ig G concentration one month after operation at a value of 13 or less g/l with a probability of 86.8% suggests a high risk of bone tissue destruction in the operated area of the joint (diagnostic sensitivity and diagnostic specificity of 85.7% and 86.9%, respectively). Determination of the IL-1b level also has a good predictive power: its concentration exceeding 191.2 pg/ml six month after surgery with the probability of 87.4% suggests destruction of bone tissue in the operated area of the joint (diagnostic sensitivity and diagnostic specificity of 87.2% and 88.1%, respectively).

The brain and immune system prompt energy shortage in chronic inflammation and ageing

Sequelae frequently seen in patients with chronic inflammatory diseases, such as fatigue, depressed mood, sleep alterations, loss of appetite, muscle wasting, cachectic obesity, bone loss and hypertension, can be the result of energy shortages caused by an overactive immune system. These sequelae can also be found in patients with chronic inflammatory diseases that are in remission and in ageing individuals, despite the immune system being less active in these situations. This Perspectives article proposes a new way of understanding situations of chronic inflammation (such as rheumatic diseases) and ageing based on the principles of evolutionary medicine, energy regulation and neuroendocrine-immune crosstalk. A conceptual framework is provided to enable physicians and scientists to better understand the signs and symptoms of chronic inflammatory diseases and long-term disease consequences resulting from physical and mental inactivity.