Osteoarthritis (OA) is the most common degenerative joint disease that causes painful swelling and permanent damage to the joints in the body. The molecular mechanisms of OA are currently unknown. OA is a heterogeneous disease that affects the entire joint, and multiple tissues are altered during OA development. To better understand the pathological mechanisms of OA, new approaches, methods, and techniques need to be used to understand OA pathogenesis. In this review, we first focus on the epigenetic regulation of OA, with a particular focus on DNA methylation, histone modification, and microRNA regulation, followed by a summary of several key mediators in OA-associated pain. We then introduce several innovative techniques that have been and will continue to be used in the fields of OA and OA-associated pain, such as CRISPR, scRNA sequencing, and lineage tracing. Next, we discuss the timely updates concerning cell death regulation in OA pathology, including pyroptosis, ferroptosis, and autophagy, as well as their individual roles in OA and potential molecular targets in treating OA. Finally, our review highlights new directions on the role of the synovial lymphatic system in OA. An improved understanding of OA pathogenesis will aid in the development of more specific and effective therapeutic interventions for OA.

Tumor necrosis factor (TNF) plays an important role in the pathogenesis of inflammatory bone loss through stimulation of osteoclastic bone resorption and inhibition of osteoblastic bone formation. Compared with the well established role of TNF in osteoclastogenesis, mechanisms by which TNF inhibits osteoblast function have not been fully determined. Runx2 is an osteoblast-specific transcription factor whose steady-state protein levels are regulated by proteasomal degradation, mediated by the E3 ubiquitin ligases, Smurf1 and Smurf2. We hypothesized that TNF inhibits osteoblast function through Smurf-mediated Runx2 degradation. We treated C2C12 and 2T3 osteoblast precursor cell lines and primary osteoblasts with TNF and found that TNF, but not interleukin-1, significantly increased Smurf1 and Smurf2 expression. TNF increased the degradation of endogenous or transfected Runx2 protein, which was blocked by treating cells with a proteasomal inhibitor or by infecting cells with small interfering (si)RNA against Smurf1 or Smurf2. TNF inhibited the expression of bone morphogenetic protein and transforming growth factor-β signaling reporter constructs, and the inhibition of each was blocked by Smurf1 siRNA and Smurf2 siRNA, respectively. Overexpression of Smurf1 and/or Smurf2 siRNAs prevented the inhibitory effect of TNF on Runx2 reporter. Consistent with these in vitro findings, bones from TNF transgenic mice or TNF-injected wild type mice had increased Smurf1 and decreased Runx2 protein levels. We propose that one of the mechanisms by which TNF inhibits bone formation in inflammatory bone disorders is by promoting Runx2 proteasomal degradation through up-regulation of Smurf1 and Smurf2 expression.

Extravasated erythrocytes in cerebrospinal fluid (CSF) critically contribute to the pathogenesis of subarachnoid hemorrhage (SAH). Meningeal lymphatics have been reported to drain macromolecules and immune cells from CSF into cervical lymph nodes (CLNs). However, whether meningeal lymphatics are involved in clearing extravasated erythrocytes in CSF after SAH remains unclear. Here we show that a markedly higher number of erythrocytes are accumulated in the lymphatics of CLNs and meningeal lymphatics after SAH. When the meningeal lymphatics are depleted in a mouse model of SAH, the degree of erythrocyte aggregation in CLNs is significantly lower, while the associated neuroinflammation and the neurologic deficits are dramatically exacerbated. In addition, during SAH lymph flow is increased but without significant lymphangiogenesis and lymphangiectasia. Taken together, this work demonstrates that the meningeal lymphatics drain extravasated erythrocytes from CSF into CLNs after SAH, while suggesting that modulating this draining may offer therapeutic approaches to alleviate SAH severity.

To address the controversy of whether TNFα can compensate for RANKL in osteoclastogenesis in vivo, we used a TNFα-induced animal model of inflammatory arthritis and blocked RANKL/RANK signaling. TNFα increased osteoclast precursors available for RANK-dependent osteoclastogenesis. RANK signaling is not required for the TNFα-stimulated increase in CD11bhi osteoclast precursors but is essential for mature osteoclast formation. Introduction: Although critical roles of TNFα in inflammatory arthritis and RANKL in bone resorption have been firmly established, a central controversy remains about the extent to which TNFα can compensate for RANKL during osteoclastogenesis and the stage at which RANK signaling is required for osteoclastogenesis. Here, we used the human TNFα transgenic mouse model (TNF-Tg) of erosive arthritis to determine if there are both RANK-dependent and -independent stages of osteoclastogenesis in TNFα-induced erosive arthritis. Materials and Methods: Osteoclastogenesis and osteoclast precursor (OCP) frequency were analyzed using histology, fluorescence-activated cell sorting (FACS), and cell culture from (1) TNF-Tg mice treated with the RANKL antagonist, RANK:Fc, or (2) TNF-Tg × RANK−/− mice generated by crossing TNF-Tg mice with RANK−/− mice. Results: Treatment of TNF-Tg mice, which have increased OCPs in their spleens, with RANK:Fc dramatically reduced osteoclast numbers on the surface of their arthritic joints and within their bones, but did not decrease CD11bhi OCP numbers in their spleens. Long-term RANK:Fc administration alleviated joint erosion. Furthermore, TNF-Tg × RANK−/− mice had severe osteopetrosis, no osteoclasts, and no joint erosion, but increased CD11bhi precursor numbers that failed to form mature osteoclasts in vitro. Conclusion: RANK signaling is essential for mature osteoclast formation in TNFα-mediated inflammatory arthritis but not for the TNFα-induced increase in CD11bhi OCP that subsequently can differentiate into osteoclasts in inflamed joints.

Objective: To investigate the mechanisms whereby tumor necrosis factor α (TNFα) increases osteoclastogenesis in vivo.Methods: TNFα‐transgenic (TNF‐Tg) and wild‐type mice injected with TNFα were studied. In vitro osteoclastogenesis assays, monocyte colony‐forming assays, and fluorescence‐activated cell sorting were performed using splenocytes, peripheral blood mononuclear cells (PBMCs), and bone marrow cells to quantify and characterize osteoclast precursors (OCPs). Etanercept, a TNFα antagonist, was used to block TNFα activity in vivo. The effects of TNFα on proliferation, apoptosis, and differentiation of OCPs were assessed using 5‐bromo‐2′‐deoxyuridine labeling, annexin V staining, and reverse transcriptase–polymerase chain reaction.Results: OCP numbers were increased 4–7‐fold in PBMCs and spleen, but not in bone marrow of TNF‐Tg mice. The OCPs in spleen were in the CD11b^high population and contained both c‐Fms− and c‐Fms+ cells. The increased number of OCPs correlated with the initiation of detectable TNFα in serum and the onset of inflammatory arthritis in TNF‐Tg mice. Etanercept eliminated the increase in peripheral OCPs. TNFα did not affect proliferation, survival, or differentiation of CD11b^high splenocytes in vivo or in vitro, but caused a rapid increase in CD11b+ cells in blood within 4 hours of a single injection and an accumulation of CD11b^high OCPs in spleen after 3 days of multiple injections.Conclusion: Systemic TNFα induces a marked increase in circulating OCPs that is reversible by anti‐TNF therapy and may result from their mobilization from bone marrow. Our findings provide a new mechanism whereby TNFα stimulates osteoclastogenesis in patients with inflammatory arthritis, suggesting that CD11b+ PBMCs could be used to evaluate a patient's potential for erosive disease and the efficacy of anti‐TNF therapy.