Paget's Disease of Bone: Osteoimmunology and Osteoclast Pathology

The Effects and Mechanisms of Chrysosplenetin in Targeting RANKL-Induced NF-κB Signaling and NFATc1 Activation to Protect Bone Density in Osteolytic Diseases

Chrysosplenetin (CHR), an O-methylated flavonol from Chamomilla recutita and Laggera pterodonta, has previously demonstrated efficacy in enhancing osteoblast differentiation for treating postmenopausal osteoporosis. This study aims to evaluate CHR's potential to inhibit osteoclastogenesis and prevent bone deterioration in both in vitro and in vivo models. Using tartaric acid-resistant acid phosphatase staining and hydroxyapatite resorption assays, we examined the impact of CHR on RANKL-induced osteoclasts derived from mouse bone marrow monocytes. Additionally, Western blot analysis and qRT-PCR were utilized to assess the protein and gene expressions within the MAPK and NF-κB signaling pathways, as well as the NFATc1 pathway. In vivo, CHR's effects were validated using micro-CT and histomorphometry in an ovariectomized mouse model, showing significant reduction in osteoclast activity and bone loss. The study confirms CHR's inhibition of osteoclastogenesis through interference with RANKL-mediated signaling pathways, suggesting its potential as a novel therapeutic agent for osteolytic conditions related to osteoclast-osteoblast dysregulation.

Proton-activated chloride channel increases endplate porosity and pain in a mouse spine degeneration model

Chronic low back pain (LBP) can severely affect daily physical activity. Aberrant osteoclast-mediated resorption leads to porous endplates, which allow the sensory innervation that causes LBP. Here, we report that expression of the proton-activated chloride (PAC) channel was induced during osteoclast differentiation in the porous endplates via a RANKL/NFATc1 signaling pathway. Extracellular acidosis evoked robust PAC currents in osteoclasts. An acidic environment of porous endplates and elevated PAC activation-enhanced osteoclast fusion provoked LBP. Furthermore, we found that genetic knockout of the PAC gene Pacc1 significantly reduced endplate porosity and spinal pain in a mouse LBP model, but it did not affect bone development or homeostasis of bone mass in adult mice. Moreover, both the osteoclast bone-resorptive compartment environment and PAC traffic from the plasma membrane to endosomes to form an intracellular organelle Cl channel had a low pH of approximately 5.0. The low pH environment activated the PAC channel to increase sialyltransferase St3gal1 expression and sialylation of TLR2 in the initiation of osteoclast fusion. Aberrant osteoclast-mediated resorption is also found in most skeletal disorders, including osteoarthritis, ankylosing spondylitis, rheumatoid arthritis, heterotopic ossification, and enthesopathy. Thus, elevated Pacc1 expression and PAC activity could be a potential therapeutic target for the treatment of LBP and osteoclast-associated pain.

Ctdnep1 phosphatase is required for negative regulation of RANKL-induced osteoclast differentiation in RAW264.7 cells

Osteoclasts are multinucleated cells with bone resorption activity. Excessive osteoclast activity has been implicated in osteoporosis, rheumatoid arthritis, and bone destruction due to bone metastases from cancer, making osteoclasts essential target cells in bone and joint diseases. C-terminal domain nuclear envelope phosphatase 1 (Ctdnep1, formerly Dullard) is a negative regulator of transforming growth factor (TGF)-β superfamily signaling and regulates endochondral ossification in mesenchymal cells during skeletal development. In this study, we investigated the role of Ctdnep1 in the Receptor activator of nuclear factor-kappa B ligand (RANKL)-induced RAW264.7 osteoclast differentiation. Expression of Ctdnep1 did not change during osteoclast differentiation; Ctdnep1 protein localized to the cytoplasm before and after osteoclast differentiation. Small interfering RNA-mediated knockdown of Ctdnep1 increased tartrate-resistant acid phosphatase-positive multinucleated osteoclasts and the expression of osteoclast marker genes, including Acp5, Ctsk, and Nfatc1. Interestingly, the knockdown of Ctdnep1 increased the protein level of Nfatc1 in cells unstimulated with RANKL. Knockdown of Ctdnep1 also enhanced calcium-resorbing activity. Mechanistically, the knockdown of Ctdnep1 increased the phosphorylation of RANKL signaling components. These results suggest that Ctdnep1 negatively regulates osteoclast differentiation by suppressing the RANKL signaling pathway.

Subchondral osteoclasts and osteoarthritis: new insights and potential therapeutic avenues

Osteoarthritis (OA) is the most common joint disease, and good therapeutic results are often difficult to obtain due to its complex pathogenesis and diverse causative factors. After decades of research and exploration of OA, it has been progressively found that subchondral bone is essential for its pathogenesis, and pathological changes in subchondral bone can be observed even before cartilage lesions develop. Osteoclasts, the main cells regulating bone resorption, play a crucial role in the pathogenesis of subchondral bone. Subchondral osteoclasts regulate the homeostasis of subchondral bone through the secretion of degradative enzymes, immunomodulation, and cell signaling pathways. In OA, osteoclasts are overactivated by autophagy, ncRNAs, and Rankl/Rank/OPG signaling pathways. Excessive bone resorption disrupts the balance of bone remodeling, leading to increased subchondral bone loss, decreased bone mineral density and consequent structural damage to articular cartilage and joint pain. With increased understanding of bone biology and targeted therapies, researchers have found that the activity and function of subchondral osteoclasts are affected by multiple pathways. In this review, we summarize the roles and mechanisms of subchondral osteoclasts in OA, enumerate the latest advances in subchondral osteoclast-targeted therapy for OA, and look forward to the future trends of subchondral osteoclast-targeted therapies in clinical applications to fill the gaps in the current knowledge of OA treatment and to develop new therapeutic strategies.

An injectable and photocurable methacrylate-silk fibroin/nano-hydroxyapatite hydrogel for bone regeneration through osteoimmunomodulation

Tissue engineering has emerged as a promising approach for addressing bone defects. Most of the traditional 3D printing materials predominantly relying on polymers and ceramics. Although these materials exhibit superior osteogenic effects, their gradual degradation poses a limitation. Digital light processing (DLP) 3D bioprinting that uses natural biomaterials as bioinks has become one of the promising strategies for bone regeneration. In this study, we introduce a hydrogel biomaterial derived from silk fibroin (SF). Notably, we present the novel integration of nano-hydroxyapatite (nHA) into the hydrogel, forming a composite hydrogel that rapidly cross-links upon initiation. Moreover, we demonstrate the loading of nHA through non-covalent bonds in SilMA. In vitro experiments reveal that composite hydrogel scaffolds with 10 % nHA exhibit enhanced osteogenic effects. Transcriptomic analysis indicates that the composite hydrogel promotes bone regeneration by inducing M2 macrophage polarization. Furthermore, rat femoral defect experiments validate the efficacy of SilMA/nHA10 in bone regeneration. This study synthesis of a simple and effective composite hydrogel bioink for bone regeneration, presenting a novel strategy for the future implementation of digital 3D printing technology in bone tissue engineering.

Metabolic bone disorders and the promise of marine osteoactive compounds

Metabolic bone disorders and associated fragility fractures are major causes of disability and mortality worldwide and place an important financial burden on the global health systems. These disorders result from an unbalance between bone anabolic and resorptive processes and are characterized by different pathophysiological mechanisms. Drugs are available to treat bone metabolic pathologies, but they are either poorly effective or associated with undesired side effects that limit their use. The molecular mechanism underlying the most common metabolic bone disorders, and the availability, efficacy, and limitations of therapeutic options currently available are discussed here. A source for the unmet need of novel drugs to treat metabolic bone disorders is marine organisms, which produce natural osteoactive compounds of high pharmaceutical potential. In this review, we have inventoried the marine osteoactive compounds (MOCs) currently identified and spotted the groups of marine organisms with potential for MOC production. Finally, we briefly examine the availability of in vivo screening and validation tools for the study of MOCs.

Aged G Protein-Coupled Receptor Kinase 3 (Grk3)-Deficient Mice Exhibit Enhanced Osteoclastogenesis and Develop Bone Lesions Analogous to Human Paget's Disease of Bone

Paget's Disease of Bone (PDB) is a metabolic bone disease that is characterized by dysregulated osteoclast function leading to focal abnormalities of bone remodeling. It can lead to pain, fracture, and bone deformity. G protein-coupled receptor kinase 3 (GRK3) is an important negative regulator of G protein-coupled receptor (GPCR) signaling. GRK3 is known to regulate GPCR function in osteoblasts and preosteoblasts, but its regulatory function in osteoclasts is not well defined. Here, we report that Grk3 expression increases during osteoclast differentiation in both human and mouse primary cells and established cell lines. We also show that aged mice deficient in Grk3 develop bone lesions similar to those seen in human PDB and other Paget's Disease mouse models. We show that a deficiency in Grk3 expression enhances osteoclastogenesis in vitro and proliferation of hematopoietic osteoclast precursors in vivo but does not affect the osteoclast-mediated bone resorption function or cellular senescence pathway. Notably, we also observe decreased Grk3 expression in peripheral blood mononuclear cells of patients with PDB compared with age- and gender-matched healthy controls. Our data suggest that GRK3 has relevance to the regulation of osteoclast differentiation and that it may have relevance to the pathogenesis of PDB and other metabolic bone diseases associated with osteoclast activation.

Osteoclasts, Master Sculptors of Bone

Osteoclasts are multinucleated cells with the unique ability to resorb bone matrix. Excessive production or activation of osteoclasts leads to skeletal pathologies that affect a significant portion of the population. Although therapies that effectively target osteoclasts have been developed, they are associated with sometimes severe side effects, and a fuller understanding of osteoclast biology may lead to more specific treatments. Along those lines, a rich body of work has defined essential signaling pathways required for osteoclast formation, function, and survival. Nonetheless, recent studies have cast new light on long-held views regarding the origin of these cells during development and homeostasis, their life span, and the cellular sources of factors that drive their production and activity during homeostasis and disease. In this review, we discuss these new findings in the context of existing work and highlight areas of ongoing and future investigation.

FTY720 Attenuates LPS-Induced Inflammatory Bone Loss by Inhibiting Osteoclastogenesis via the NF-κB and HDAC4/ATF Pathways

Osteoclast (OC) abnormalities lead to many osteolytic diseases, such as osteoporosis, inflammatory bone erosion, and tumor-induced osteolysis. Exploring effective strategies to remediate OCs dysregulation is essential. FTY720, also known as fingolimod, has been approved for the treatment of multiple sclerosis and has anti-inflammatory and immunosuppressive effects. Here, we found that FTY720 inhibited osteoclastogenesis and OC function by inhibiting nuclear factor kappa-B (NF-κB) signaling. Interestingly, we also found that FTY720 inhibited osteoclastogenesis by upregulating histone deacetylase 4 (HDAC4) expression levels and downregulating activating transcription factor 4 (ATF4) expression levels. In vivo, FTY720 treatment prevented lipopolysaccharide- (LPS-) induced calvarial osteolysis and significantly reduced the number of tartrate-resistant acid phosphatase- (TRAP-) positive OCs. Taken together, these results demonstrate that FTY720 can inhibit osteoclastogenesis and ameliorate inflammation-induced bone loss. Which may provide evidence of a new therapeutic target for skeletal diseases caused by OC abnormalities.

Hereditary Metabolic Bone Diseases: A Review of Pathogenesis, Diagnosis and Management

Hereditary metabolic bone diseases are characterized by genetic abnormalities in skeletal homeostasis and encompass one of the most diverse groups among rare diseases. In this review, we examine 25 selected hereditary metabolic bone diseases and recognized genetic variations of 78 genes that represent each of the three groups, including sclerosing bone disorders, disorders of defective bone mineralization and disorder of bone matrix and cartilage formation. We also review pathophysiology, manifestation and treatment for each disease. Advances in molecular genetics and basic sciences has led to accurate genetic diagnosis and novel effective therapeutic strategies for some diseases. For other diseases, the genetic basis and pathophysiology remain unclear. Further researches are therefore crucial to innovate ways to overcome diagnostic challenges and develop effective treatment options for these orphan diseases.

Paget's disease of bone and megaloblastic anemia in a 72‑year‑old patient: A case report and systematic literature review

Paget's disease of bone (PDB) is characterized by abnormal osteoclastic bone resorption with disorganized bone neo-formation, primarily affecting elderly (>55 years) patients. Although the majority of patients are asymptomatic, some patients may experience bone pain due to local periosteal involvement or osteoarthritic lesions in the spine; in addition, limb deformities may lead to secondary gait problems or degenerative joint changes. Anemia has an overall prevalence of 12-17% in elderly adults (>65 years old), with macrocytic anemia being the less common type. Megaloblastic anemia is a macrocytic anemia characterized by the presence of large, immature, nucleated cells (megaloblasts) in the blood, with the most common cause being a deficiency of folate and/or vitamin B12. We herein report the rare case of a 72-year-old male patient exhibiting both these conditions, with the aim of discussing the possible association between the two and, most importantly, the clinical management of the patient in a real-life setting over a period of 10 years. The patient was diagnosed based on clinical symptoms (bone pain), radiological imaging and specific laboratory tests, and received discontinuous courses of bisphosphonates and cyanocobalamin supplementation therapy, based mainly on aggravated symptomatology. A systematic literature review was also performed and revealed not only the scarcity of reports on similar cases, but also the mechanisms that may underlie the possible association of PDB with macrocytic anemia due to vitamin B12 deficiency in elderly patients.