Osteocyte dysfunction promotes osteoarthritis through MMP13-dependent suppression of subchondral bone homeostasis

Injectable hydrogel encapsulating siMMP13 with anti-ROS and anti-apoptotic functions for osteoarthritis treatment

BACKGROUND

Osteoarthritis (OA) is a degenerative joint disease characterized by the progressive degeneration of articular cartilage, leading to pain, stiffness, and loss of joint function. The pathogenesis of OA involves multiple factors, including increased intracellular reactive oxygen species (ROS), enhanced chondrocyte apoptosis, and disturbances in cartilage matrix metabolism. These processes contribute to the breakdown of the extracellular matrix (ECM) and the loss of cartilage integrity, ultimately resulting in joint damage and dysfunction. RNA interference (RNAi) therapy has emerged as a promising approach for the treatment of various diseases, including hATTR and acute hepatic porphyria. By harnessing the natural cellular machinery for gene silencing, RNAi allows for the specific inhibition of target genes involved in disease pathogenesis. In the context of OA, targeting key molecules such as matrix metalloproteinase-13 (MMP13), which plays a critical role in cartilage degradation, holds great therapeutic potential.

RESULTS

In this study, we developed an innovative therapeutic approach for OA using a combination of liposome-encapsulated siMMP13 and NG-Monomethyl-L-arginine Acetate (L-NMMA) to form an injectable hydrogel. The hydrogel served as a delivery vehicle for the siMMP13, allowing for sustained release and targeted delivery to the affected joint. Experiments conducted on destabilization of the medial meniscus (DMM) model mice demonstrated the therapeutic efficacy of this composite hydrogel. Treatment with the hydrogel significantly inhibited the degradation of cartilage matrix, as evidenced by histological analysis showing preserved cartilage structure and reduced loss of proteoglycans. Moreover, the hydrogel effectively suppressed intracellular ROS accumulation in chondrocytes, indicating its anti-oxidative properties. Furthermore, it attenuated chondrocyte apoptosis, as demonstrated by decreased levels of apoptotic markers.

CONCLUSION

In summary, the injectable hydrogel containing siMMP13, endowed with anti-ROS and anti-apoptotic properties, may represent an effective therapeutic strategy for osteoarthritis in the future.

The osteocytic actions of glucocorticoids on bone mass, mechanical properties, or perilacunar remodeling outcomes are not rescued by PTH(1-34)

Glucocorticoids (GC) and parathyroid hormone (PTH) are widely used therapeutic endocrine hormones where their effects on bone and joint arise from actions on multiple skeletal cell types. In osteocytes, GC and PTH exert opposing effects on perilacunar canalicular remodeling (PLR). Suppressed PLR can impair bone quality and joint homeostasis, including in GC-induced osteonecrosis. However, combined effects of GC and PTH on PLR are unknown. Given the untapped potential to target osteocytes to improve skeletal health, this study sought to test the feasibility of therapeutically mitigating PLR suppression. Focusing on subchondral bone and joint homeostasis, we hypothesize that PTH(1-34), a PLR agonist, could rescue GC-suppressed PLR. The skeletal effects of GC and PTH(1-34), alone or combined, were examined in male and female mice by micro-computed tomography, mechanical testing, histology, and gene expression analysis. For each outcome, females were more responsive to GC and PTH(1-34) than males. GC and PTH(1-34) exerted regional differences, with GC increasing trabecular bone volume but reducing cortical bone thickness, stiffness, and ultimate force. Despite PTH(1-34)'s anabolic effects on trabecular bone, it did not rescue GC's catabolic effects on cortical bone. Likewise, cartilage integrity and subchondral bone apoptosis, tartrate-resistant acid phosphatase (TRAP) activity, and osteocyte lacunocanalicular networks showed no evidence that PTH(1-34) could offset GC-dependent effects. Rather, GC and PTH(1-34) each increased cortical bone gene expression implicated in bone resorption by osteoclasts and osteocytes, including Acp5, Mmp13, Atp6v0d2, Ctsk, differences maintained when GC and PTH(1-34) were combined. Since PTH(1-34) is insufficient to rescue GC's effects on young female mouse bone, future studies are needed to determine if osteocyte PLR suppression, due to GC, aging, or other factors, can be offset by a PLR agonist.

Subchondral Bone Osteocyte Lacunae Morphology in End-Stage Osteoarthritis of the Human Tibial Plateau

Subchondral bone remodeling, mediated by osteocytes within the lacuno-canalicular network, plays a crucial role in osteoarthritis (OA) progression. Following cell death, lacunae preserve integrity, offering insights into bone remodeling mechanisms. Limited and controversial data on osteocyte lacuna morphology in OA result from small sample sizes and two-dimensional (2D) techniques that have been used thus far. This study aimed to quantify three-dimensional (3D) osteocyte lacunar characteristics at well-defined tibial plateau locations, known to be differently affected by OA. Specifically, 11 tibial plateaus were obtained from end-stage knee-OA patients with varus deformity. Each plateau provided one sample from the less affected lateral compartment and two samples from the medial compartment, at minimum and maximum bone volume fraction (BV/TV) locations. High-resolution desktop micro-computed tomography (micro-CT) at 0.7 μm voxel resolution imaged the 33 samples. Lacuna number density (Lc.N/BV) and lacuna volume density (Lc.TV/BV) were significantly lower (p < 0.02) in samples from the medial side with maximum BV/TV compared to lateral side samples. In the medial compartment at maximum local BV/TV, mean lacuna volume (Lc.V), total lacuna volume (Lc.TV), and Lc.TV/BV were significantly (p < 0.001) lower than in the region with minimum BV/TV. Lc.N/BV was also significantly lower (p < 0.02) at the maximum local BV/TV location compared to the region with minimum BV/TV. Our findings suggest that subchondral bone lacunae adapt to the changing loads in end-stage OA.

Spatial control of perilacunar canalicular remodeling during lactation

Osteocytes locally remodel their surrounding tissue through perilacunar canalicular remodeling (PLR). During lactation, osteocytes remove minerals to satisfy the metabolic demand, resulting in increased lacunar volume, quantifiable with synchrotron X-ray radiation micro-tomography (SRµCT). Although the effects of lactation on PLR are well-studied, it remains unclear whether PLR occurs uniformly throughout the bone and what mechanisms prevent PLR from undermining bone quality. We used SRµCT imaging to conduct an in-depth spatial analysis of the impact of lactation and osteocyte-intrinsic MMP13 deletion on PLR in murine bone. We found larger lacunae undergoing PLR are located near canals in the mid-cortex or endosteum. We show lactation-induced hypomineralization occurs 14 µm away from lacunar edges, past a hypermineralized barrier. Our findings reveal that osteocyte-intrinsic MMP13 is crucial for lactation-induced PLR near lacunae in the mid-cortex but not for whole-bone resorption. This research highlights the spatial control of PLR on mineral distribution during lactation.

Dietary intervention rescues a bone porosity phenotype in a murine model of Neurofibromatosis Type 1 (NF1)

Neurofibromatosis type 1 (NF1) is a complex genetic disorder that affects a range of tissues including muscle and bone. Recent preclinical and clinical studies have shown that Nf1 deficiency in muscle causes metabolic changes resulting in intramyocellular lipid accumulation and muscle weakness. These can be subsequently rescued by dietary interventions aimed at modulating lipid availability and metabolism. It was speculated that the modified diet may rescue defects in cortical bone as NF1 deficiency has been reported to affect genes involved with lipid metabolism. Bone specimens were analyzed from wild type control mice as well as Nf1Prx1-/- (limb-targeted Nf1 knockout mice) fed standard chow versus a range of modified chows hypothesized to influence lipid metabolism. Mice were fed from 4 weeks to 12 weeks of age. MicroCT analysis was performed on the cortical bone to examine standard parameters (bone volume, tissue mineral density, cortical thickness) and specific porosity measures (closed pores corresponding to osteocyte lacunae, and larger open pores). Nf1Prx1-/- bones were found to have inferior bone properties to wild type bones, with a 4-fold increase in the porosity attributed to open pores. These measures were rescued by dietary interventions including a L-carnitine + medium-chain fatty acid supplemented chow previously shown to improve muscle histology function. Histological staining visualized these changes in bone porosity. These data support the concept that lipid metabolism may have a mechanistic impact on bone porosity and quality in NF1.

Network Analysis of Osteoarthritis Progression Using a Steiner Minimal Tree Algorithm

Purpose

To provide a comprehensive analysis of associated genes with osteoarthritis (OA). Here, we reported a network analysis of OA progression by using a Steiner minimal tree algorithm.

Methods

We collected the OA-related genes through screening the publications in MEDLINE. We performed functional analysis to analyze the associated biochemical pathways of the OA-related genes. Pathway crosstalk analysis was constructed to explore interactions of the enriched pathways. Steiner minimal tree algorithm was used to analyze molecular pathway networks. The average clustering coefficient was compared with the corresponding values of the Osteoarthritis-specific network. The new finding RNA was compared with former single-cell RNA-seq analysis results.

Results

A gene set with 177 members reported to be significantly associated with Osteoarthritis was collected from 187 studies. Functional enrichment analysis revealed a specific related-OA gene including skeletal system development, cytokine-mediated signaling pathway, inflammatory response, cartilage development, and extracellular matrix organization. We performed a pathway crosstalk analysis among the 72 significantly enriched pathways. A total of 151 of the 177 genes in the Osteoarthritis gene set were included in the human interactome network. There were 31 genes in the former single-cell RNA-seq analysis results. The CLU, ENO1, SRRM1, UBC, HMGB1, NR3C1, NOTCH2NL, and CBX5 have significantly increased expression in seven molecularly defined populations of OA cartilage.

Conclusion

The Steiner tree-based approach finds new biological molecules associated with OA genes.

Alterations in compositional and cellular properties of the subchondral bone are linked to cartilage degeneration in hip osteoarthritis

OBJECTIVE

The subchondral bone is an emerging regulator of osteoarthritis (OA). However, knowledge of how specific subchondral alterations relate to cartilage degeneration remains incomplete.

METHOD

Femoral heads were obtained from 44 patients with primary OA during total hip arthroplasty and from 30 non-OA controls during autopsy. A multiscale assessment of the central subchondral bone region comprising histomorphometry, quantitative backscattered electron imaging, nanoindentation, and osteocyte lacunocanalicular network characterization was employed.

RESULTS

In hip OA, thickening of the subchondral bone coincided with a higher number of osteoblasts (controls: 3.7 ± 4.5 mm-1, OA: 16.4 ± 10.2 mm-1, age-adjusted mean difference 10.5 mm-1 [95% CI 4.7 to 16.4], p < 0.001) but a similar number of osteoclasts compared to controls (p = 0.150). Furthermore, higher matrix mineralization heterogeneity (CaWidth, controls: 2.8 ± 0.2 wt%, OA: 3.1 ± 0.3 wt%, age-adjusted mean difference 0.2 wt% [95% CI 0.1 to 0.4], p = 0.011) and lower tissue hardness (controls: 0.69 ± 0.06 GPa, OA: 0.67 ± 0.06 GPa, age-adjusted mean difference -0.05 GPa [95% CI -0.09 to -0.01], p = 0.032) were detected. While no evidence of altered osteocytic perilacunar/canalicular remodeling in terms of fewer osteocyte canaliculi was found in OA, specimens with advanced cartilage degeneration showed a higher number of osteocyte canaliculi and larger lacunocanalicular network area compared to those with low-grade cartilage degeneration. Multiple linear regression models indicated that several subchondral bone properties, especially osteoblast and osteocyte parameters, were closely related to cartilage degeneration (R2 adjusted = 0.561, p < 0.001).

CONCLUSION

Subchondral bone properties in OA are affected at the compositional, mechanical, and cellular levels. Based on their strong interaction with cartilage degeneration, targeting osteoblasts/osteocytes may be a promising therapeutic OA approach.

DATA AND MATERIALS AVAILABILITY

All data are available in the main text or the supplementary materials.

Aging impairs the osteocytic regulation of collagen integrity and bone quality

Poor bone quality is a major factor in skeletal fragility in elderly individuals. The molecular mechanisms that establish and maintain bone quality, independent of bone mass, are unknown but are thought to be primarily determined by osteocytes. We hypothesize that the age-related decline in bone quality results from the suppression of osteocyte perilacunar/canalicular remodeling (PLR), which maintains bone material properties. We examined bones from young and aged mice with osteocyte-intrinsic repression of TGFβ signaling (TβRIIocy-/-) that suppresses PLR. The control aged bone displayed decreased TGFβ signaling and PLR, but aging did not worsen the existing PLR suppression in male TβRIIocy-/- bone. This relationship impacted the behavior of collagen material at the nanoscale and tissue scale in macromechanical tests. The effects of age on bone mass, density, and mineral material behavior were independent of osteocytic TGFβ. We determined that the decline in bone quality with age arises from the loss of osteocyte function and the loss of TGFβ-dependent maintenance of collagen integrity.

Does Chronic Pancreatitis in Growing Pigs Lead to Articular Cartilage Degradation and Alterations in Subchondral Bone?

Chronic pancreatitis (CP), a progressive inflammatory disease, poses diagnostic challenges due to its initially asymptomatic nature. While CP's impact on exocrine and endocrine functions is well-recognized, its potential influence on other body systems, particularly in young individuals, remains underexplored. This study investigates the hypothesis that CP in growing pigs leads to alterations in articular cartilage and subchondral bone, potentially contributing to osteoarthritis (OA) development. Utilizing a pig model of cerulein-induced CP, we examined the structural and compositional changes in subchondral bone, articular cartilage, and synovial fluid. Histological analyses, including Picrosirius Red and Safranin-O staining, were employed alongside immuno-histochemistry and Western blotting techniques. Our findings reveal significant changes in the subchondral bone, including reduced bone volume and alterations in collagen fiber composition. Articular cartilage in CP pigs exhibited decreased proteoglycan content and alterations in key proteins such as MMP-13 and TGF-β1, indicative of early cartilage degradation. These changes suggest a link between CP and musculoskeletal alterations, underscoring the need for further research into CP's systemic effects. Our study provides foundational insights into the relationship between CP and skeletal health, potentially guiding future pediatric healthcare strategies for early CP diagnosis and management.

Moderate mechanical stimulation antagonizes inflammation of annulus fibrosus cells through YAP-mediated suppression of NF-κB signaling

Intervertebral disc degeneration (IDD) is a leading cause of low back pain. The inflammatory responses caused by aberrant mechanical loading are one of the major factors leading to annulus fibrosus (AF) degeneration and IDD. Previous studies have suggested that moderate cyclic tensile strain (CTS) can regulate anti-inflammatory activities of AF cells (AFCs), and Yes-associated protein (YAP) as a mechanosensitive coactivator senses diverse types of biomechanical stimuli and translates them into biochemical signals controlling cell behaviors. However, it remains poorly understood whether and how YAP mediates the effect of mechanical stimuli on AFCs. In this study, we aimed to investigate the exact effects of different CTS on AFCs as well as the role of YAP signaling involving in it. Our results found that 5% CTS inhibited the inflammatory response and promoted cell growth through inhibiting the phosphorylation of YAP and nuclear localization of NF-κB, while 12% CTS had a significant proinflammatory effect with the inactivation of YAP activity and the activation of NF-κB signaling in AFCs. Furthermore, moderate mechanical stimulation may alleviate the inflammatory reaction of intervertebral discs through YAP-mediated suppression of NF-κB signaling in vivo. Therefore, moderate mechanical stimulation may serve as a promising therapeutic approach for the prevention and treatment of IDD.

Mice heterozygous for an osteogenesis imperfecta-linked MBTPS2 variant display a compromised subchondral osteocyte lacunocanalicular network associated with abnormal articular cartilage

Missense variants in the MBTPS2 gene, located on the X chromosome, have been associated with an X-linked recessive form of osteogenesis imperfecta (X-OI), an inherited bone dysplasia characterized by multiple and recurrent bone fractures, short stature, and various skeletal deformities in affected individuals. The role of site-2 protease, encoded by MBTPS2, and the molecular pathomechanism underlying the disease are to date elusive. This study is the first to report on the generation of two Mbtps2 mouse models, a knock-in mouse carrying one of the disease-causative MBTPS2 variants (N455S) and a Mbtps2 knock-out (ko) mouse. Because both loss-of-function variants lead to embryonic lethality in hemizygous male mutant mice, we performed a comprehensive skeletal analysis of heterozygous Mbtps2+/N455S and Mbtps2+/ko female mice. Both models displayed osteochondral abnormalities such as thinned subchondral bone, altered subchondral osteocyte interconnectivity as well as thickened articular cartilage with chondrocyte clustering, altogether resembling an early osteoarthritis (OA) phenotype. However, distant from the joints, no alterations in the bone mass and turnover could be detected in either of the mutant mice. Based on our findings we conclude that MBTPS2 haploinsufficiency results in early OA-like alterations in the articular cartilage and underlying subchondral bone, which likely precede the development of typical OI phenotype in bone. Our study provides first evidence for a potential role of site-2 protease for maintaining homeostasis of both bone and cartilage.

MMP13 promotes the osteogenic potential of BMP9 by enhancing Wnt/β-catenin signaling via HIF-1α upregulation in mouse embryonic fibroblasts

Bone morphogenetic protein 9 (BMP9) has been validated as one of the most potent osteoinduction factors, but its underlying mechanism remains unclear. As a member of the matrix metalloproteinase (MMP) family, MMP13 may be involved in regulating the lineage-specific differentiation of mouse embryonic fibroblasts (MEFs). The goal of this study was to determine whether MMP13 regulates the osteoinduction potential of BMP9 in MEFs, which are multipotent progenitor cells widely used for stem cell biology research. In vitro and in vivo experiments showed that BMP9-induced osteogenic markers and/or bone were enhanced by exogenous MMP13 in MEFs, but were reduced by MMP13 knockdown or inhibition. The expression of hypoxia inducible factor 1 alpha (HIF-1α) was induced by BMP9, which was enhanced by MMP13. The protein expression of β-catenin and phosphorylation level of glycogen synthase kinase-3 beta (GSK-3β) were increased by BMP9 in MEFs, as was the translocation of β-catenin from the cytoplasm to the nucleus; all these effects of BMP9 were enhanced by MMP13. Furthermore, the MMP13 effects of increasing BMP9-induced β-catenin protein expression and GSK-3β phosphorylation level were partially reversed by HIF-1α knockdown. These results suggest that MMP13 can enhance the osteoinduction potential of BMP9, which may be mediated, at least in part, through the HIF-1α/β-catenin axis. Our findings demonstrate a novel role of MMP13 in the lineage decision of progenitor cells and provide a promising strategy to speed up bone regeneration.

COL3A1, COL5A1 and COL6A2 serve as potential molecular biomarkers for osteoarthritis based on weighted gene co‑expression network analysis bioinformatics analysis

Osteoarthritis (OA) is a non-inflammatory degenerative joint disease, characterized by joint pain and stiffness. The prevalence of OA increases with age. However, the relationship between biomarkers [collagen type III α1 (COL3A1), COL5A1, COL6A2, COL12A1] and OA remains unclear. The OA subchondral bone dataset GSE51588 was downloaded from the GEO database, and the differentially expressed genes (DEGs) were screened. Weighted gene co-expression network analysis was performed, and a protein-protein interaction network was constructed and further analyzed using Cytoscape and STRING. Functional enrichment analysis was performed using Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, and then Gene Set Enrichment Analysis (GSEA) was used to formulate the molecular functions and pathways based on the results of GO and KEGG analyses. Comparative Toxicogenomics Database and TargetScan were used to identify the hub-gene-related diseases and the microRNAs that regulated the central hub genes. Immunohistochemical staining was performed to confirm the expression of related proteins in OA and non-OA tissue samples. A total of 1,679 DEGs were identified. GO analysis showed that the DEGs were primarily enriched in the process of 'immune system', 'extracellular region', 'secretory granule', 'collagen-containing extracellular matrix', 'ECM-receptor, glycosaminoglycan binding' and 'systemic lupus erythematosus'. The results of GSEA were similar to those of GO and KEGG enrichment terms for DEGs. A total of 25 important modules were generated, and two core gene clusters and seven core genes were obtained (COL6A2, COL5A2, COL12A1, COL5A1, COL6A1, LUM and COL3A1). Core genes were expressed differentially between OA subchondral bone and normal tissue samples. The expression levels of COL3A1, COL5A1 and COL6A2 in OA subchondral bone tissue were higher compared with those in normal tissues, but COL12A1 expression was not significantly increased; all stained markers were highly expressed in surrounding tissues of immunohistochemical staining. In conclusion, COL3A1, COL5A1 and COL6A2 may be potential molecular biomarkers for OA.

miR181a/b-1 controls osteocyte metabolism and mechanical properties independently of bone morphology

Bone derives its ability to resist fracture from bone mass and quality concurrently; however, many questions about the molecular mechanisms controlling bone quality remain unanswered, limiting the development of diagnostics and therapeutics. Despite the increasing evidence on the importance of miR181a/b-1 in bone homeostasis and disease, whether and how osteocyte-intrinsic miR181a/b-1 controls bone quality remains elusive. Osteocyte-intrinsic deletion of miR181a/b-1 in osteocytes in vivo resulted in compromised overall bone mechanical behavior in both sexes, although the parameters affected by miR181a/b-1 varied distinctly based on sex. Furthermore, impaired fracture resistance in both sexes was unexplained by cortical bone morphology, which was altered in female mice and intact in male mice with miR181a/b-1-deficient osteocytes. The role of miR181a/b-1 in the regulation of osteocyte metabolism was apparent in bioenergetic testing of miR181a/b-1-deficient OCY454 osteocyte-like cells and transcriptomic analysis of cortical bone from mice with osteocyte-intrinsic ablation of miR181a/b-1. Altogether, this study demonstrates the control of osteocyte bioenergetics and the sexually dimorphic regulation of cortical bone morphology and mechanical properties by miR181a/b-1, hinting at the role of osteocyte metabolism in the regulation of mechanical behavior.

Mitochondrial-targeting Mn3O4/UIO-TPP nanozyme scavenge ROS to restore mitochondrial function for osteoarthritis therapy

Excessive reactive oxygen species (ROS)-induced mitochondrial damage has impact on osteoarthritis (OA). Nanozyme mimics as natural enzyme alternatives to scavenge excessive ROS has offered a promising strategy for OA therapy. Herein, we reported a novel mitochondrial-targeting Mn3O4/UIO-TPP nanozyme using metal-organic frameworks with loaded Mn3O4 as the enzyme-like active core combining mitochondria-targeting triphenylphosphine (TPP) groups to serve as ROS scavengers for therapy of OA. With sequential catalysis of superoxide dismutase-like, catalase (CAT)-like, and hydroxyl radical (·OH) scavenging potentials, the nanozyme can target mitochondria by crossing subcellular barriers to effectively eliminate ROS to restore mitochondrial function and inhibit inflammation and chondrocyte apoptosis. It also has favorable biocompatibility and biosafety. Based on anterior cruciate ligament transection-induced OA joint models, this mitochondrial-targeting nanozyme effectively mitigated the inflammatory response with the Pelletier score reduction of 49.9% after 8-week therapy. This study offers a prospective approach to the design of nanomedicines for ROS-related diseases.

Regulation of the Osteocyte Secretome with Aging and Disease

As the most numerous and long-lived of all bone cells, osteocytes have essential functions in regulating skeletal health. Through the lacunar-canalicular system, secreted proteins from osteocytes can reach cells throughout the bone. Furthermore, the intimate connectivity between the lacunar-canalicular system and the bone vasculature allows for the transport of osteocyte-secreted factors into the circulation to reach the entire body. Local and endocrine osteocyte signaling regulates physiological processes such as bone remodeling, bone mechanoadaptation, and mineral homeostasis. However, these processes are disrupted by impaired osteocyte function induced by aging and disease. Dysfunctional osteocyte signaling is now associated with the pathogenesis of many disorders, including chronic kidney disease, cancer, diabetes mellitus, and periodontitis. In this review, we focus on the targeting of bone and extraskeletal tissues by the osteocyte secretome. In particular, we highlight the secreted osteocyte proteins, which are known to be dysregulated during aging and disease, and their roles during disease progression. We also discuss how therapeutic or genetic targeting of osteocyte-secreted proteins can improve both skeletal and systemic health.

Bone-cartilage crosstalk informed by aging mouse bone transcriptomics and human osteoarthritis genome-wide association studies

Subchondral bone participates in crosstalk with articular cartilage to maintain joint homeostasis, and disruption of either tissue results in overall joint degeneration. Among the subchondral bone changes observed in osteoarthritis (OA), subchondral bone plate (SBP) thickening has a time-dependent relationship with cartilage degeneration and has recently been shown to be regulated by osteocytes. Here, we evaluate the effect of age on SBP thickness and cartilage degeneration in aging mice. We find that SBP thickness significantly increases by 18-months of age, corresponding temporally with increased cartilage degeneration. To identify factors in subchondral bone that may participate in bone cartilage crosstalk or OA, we leveraged mouse transcriptomic data from one joint tissue compartment - osteocyte-enriched bone - to search for enrichment with human OA in UK Biobank and Arthritis Research UK Osteoarthritis Genetics (arcOGEN) GWAS using the mouse2human (M2H, www.mouse2human.org) strategy. Genes differentially expressed in aging mouse bone are significantly enriched for human OA, showing joint site-specific (knee vs. hip) relationships, exhibit temporal associations with age, and unique gene clusters are implicated in each type of OA. Application of M2H identifies genes with known and unknown functions in osteocytes and OA development that are clinically associated with human OA. Altogether, this work prioritizes genes with a potential role in bone/cartilage crosstalk for further mechanistic study based on their association with human OA in GWAS.

Exercise improves subchondral bone microenvironment through regulating bone-cartilage crosstalk

Articular cartilage degeneration has been proved to cause a variety of joint diseases, among which osteoarthritis is the most typical. Osteoarthritis is characterized by articular cartilage degeneration and persistent pain, which affects the quality of life of patients as well as brings a heavy burden to society. The occurrence and development of osteoarthritis is related to the disorder of the subchondral bone microenvironment. Appropriate exercise can improve the subchondral bone microenvironment, thus playing an essential role in preventing and treating osteoarthritis. However, the exact mechanism whereby exercise improves the subchondral bone microenvironment remains unclear. There is biomechanical interaction as well as biochemical crosstalk between bone and cartilage. And the crosstalk between bone and cartilage is the key to bone-cartilage homeostasis maintenance. From the perspective of biomechanical and biochemical crosstalk between bone and cartilage, this paper reviews the effects of exercise-mediated bone-cartilage crosstalk on the subchondral bone microenvironment, aiming to provide a theoretical basis for the prevention and treatment of degenerative bone diseases.

Farnesyl diphosphate synthase promotes cell proliferation by regulating gene expression and alternative splicing profiles in HeLa cells

Farnesyl diphosphate synthase (FDPS), an essential enzyme involved in the mevalonate pathway, is implicated in various diseases, including multiple types of cancer. As an RNA-binding protein (RBP), FDPS is also involved in transcriptional and post-transcriptional regulation. However, to the best of our knowledge, transcriptome-wide targets of FDPS still remain unknown. In the present study, FDPS expression patterns in pan-cancer were analyzed. In addition, it was investigated how FDPS overexpression (FDPS-OE) regulates the transcriptome in HeLa cells. FDPS-OE increased the proliferation rate in HeLa cells by MTT assay. Using transcriptome-wide high throughput sequencing and bioinformatics analysis, it was found that FDPS upregulated the expression levels of genes enriched in cell proliferation and extracellular matrix organization, including the laminin subunit γ2, interferon-induced proteins with tetratricopeptide repeats 2 and matrix metallopeptidase 19 genes. According to alternative splicing (AS) analysis, FDPS modulated the splicing patterns of the bone morphogenic protein 1, semaphorin 4D, annexin A2 and sirtuin 2 genes, which are enriched in the cell cycle and DNA repair, and are related to cell proliferation. To corroborate the FDPS-regulated transcriptome findings, FDPS was overexpressed in human osteosarcoma cells. Differentially expressed genes and regulated AS genes in the cells were both validated by reverse transcription-quantitative PCR. The results suggested that, as an emerging RBP, FDPS may serve an important role in transcriptome profiles by altering gene expression and regulating AS. FDPS also affected the cell proliferation rate. These findings broaden the understanding of the molecular functions of FDPS, and the potential of FDPS as a target in therapy should be investigated.

Healthy and Osteoarthritis-Affected Joints Facing the Cellular Crosstalk

Osteoarthritis (OA) is a chronic, progressive, severely debilitating, and multifactorial joint disease that is recognized as the most common type of arthritis. During the last decade, it shows an incremental global rise in prevalence and incidence. The interaction between etiologic factors that mediate joint degradation has been explored in numerous studies. However, the underlying processes that induce OA remain obscure, largely due to the variety and complexity of these mechanisms. During synovial joint dysfunction, the osteochondral unit undergoes cellular phenotypic and functional alterations. At the cellular level, the synovial membrane is influenced by cartilage and subchondral bone cleavage fragments and extracellular matrix (ECM) degradation products from apoptotic and necrotic cells. These "foreign bodies" serve as danger-associated molecular patterns (DAMPs) that trigger innate immunity, eliciting and sustaining low-grade inflammation in the synovium. In this review, we explore the cellular and molecular communication networks established between the major joint compartments-the synovial membrane, cartilage, and subchondral bone of normal and OA-affected joints.

Targeted postnatal knockout of Sclerostin using a bone-targeted adeno-associated viral vector increases bone anabolism and decreases canalicular density

PURPOSE

The creation of murine gene knockout models to study bone gene functions often requires the resource intensive crossbreeding of Cre transgenic and gene-floxed strains. The developmental versus postnatal roles of genes can be difficult to discern in such models. For example, embryonic deletion of the Sclerostin (Sost) gene establishes a high-bone mass phenotype in neonatal mice that may impact on future bone growth. To generate a postnatal skeletal knockout of Sost in adult mice, this study used a single injection of a bone-targeted recombinant adeno-associated virus (rAAV) vector.

METHODS

8-week-old Sost^flox/flox mice were injected with saline (control) or a single injection containing 5 × 10¹¹ vg AAV8-Sp7-Cre vector. Ai9 fluorescent Cre reporter mice were dosed in parallel to confirm targeting efficiency. After 6 weeks, detailed bone analysis was performed via microCT, biomechanical testing, and bone histology on vertebral and long bone specimens.

RESULTS

The AAV8-Sp7-Cre vector induced widespread persistent recombination in the bone compartment. Regional microCT analyses revealed significant increases in bone with vector treatment. In the L3 vertebrae, Sost^flox/flox:AAV-Cre showed a 22 % increase in bone volume and 21 % in trabecular bone fraction compared to controls; this translated to a 17 % increase in compressive strength. In the tibiae, Sost^flox/flox:AAV-Cre led to small but statistically significant increases in cortical bone volume and thickness. These were consistent with a 25 % increase in mineral apposition rate, but this did not translate into increased four-point bending strength. Ploton silver nitrate stain on histological sections revealed an unexpected increase in canalicular density associated with Sost ablation.

CONCLUSION

This report demonstrates a proof-of-concept that the AAV8-Sp7-Cre vector can efficiently produce postnatal skeletal knockout mice using gene-floxed strains. This technology has the potential for broad utility in the bone field with existing conditional lines. These data also confirm an important postnatal role for Sost in regulating bone homeostasis, consistent with prior studies using neutralizing Sclerostin antibodies, and highlights a novel role of Sost in canalicular remodeling.

Osteocyte Remodeling of the Lacunar-Canalicular System: What's in a Name?

PURPOSE OF REVIEW

Osteocytes directly modify the bone surrounding the expansive lacunar-canalicular system (LCS) through both resorption and deposition. The existence of this phenomenon is now widely accepted, but is referred to as "osteocyte osteolysis," "LCS remodeling," and "perilacunar remodeling," among other names. The uncertainty in naming this physiological process reflects the many persistent questions about why and how osteocytes interact with local bone matrix. The goal of this review is to examine the purpose and nature of LCS remodeling and its impacts on multiscale bone quality.

RECENT FINDINGS

While LCS remodeling is clearly important for systemic calcium mobilization, this process may have additional potential drivers and may impact the ability of bone to resist fracture. There is abundant evidence that the osteocyte can resorb and replace bone mineral and does so outside of extreme challenges to mineral homeostasis. The impacts of the osteocyte on organic matrix are less certain, especially regarding whether osteocytes produce osteoid. Though multiple lines of evidence point towards osteocyte production of organic matrix, definitive work is needed. Recent high-resolution imaging studies demonstrate that LCS remodeling influences local material properties. The role of LCS remodeling in the maintenance and deterioration of bone matrix quality in aging and disease are active areas of research. In this review, we highlight current progress in understanding why and how the osteocyte removes and replaces bone tissue and the consequences of these activities to bone quality. We posit that answering these questions is essential for evaluating whether, how, when, and why LCS remodeling may be manipulated for therapeutic benefit in managing bone fragility.

Abnormal morphological features of osteocyte lacunae in adolescent idiopathic scoliosis: A large-scale assessment by ultra-high-resolution micro-computed tomography

AIM

Abnormal osteocyte lacunar morphology in adolescent idiopathic scoliosis (AIS) has been reported while the results were limited by the number of osteocyte lacunae being quantified. The present study aimed to validate previous findings through (a) comparing morphological features of osteocyte lacunae between AIS patients and controls in spine and ilium using a large-scale assessment, and (b) investigating whether there is an association between the acquired morphological features of osteocyte lacunae and disease severity in AIS.

METHOD

Trabecular bone tissue of the facet joint of human vertebrae on both concave and convex sides at the apex of the scoliotic curve were collected from 4 AIS and 5 congenital scoliosis (CS) patients, and also at the same anatomic site from 3 non-scoliosis (NS) subjects intraoperatively. Trabecular bone tissue from ilium was obtained from 12 AIS vs 9 NS subjects during surgery. Osteocyte lacunae were assessed using ultra-high-resolution micro-computed tomography. Clinical information such as age, body mass index (BMI) and radiological Cobb angle of the major curve were collected.

RESULTS

There was no significant difference between density of osteocyte lacuna and bone volume fraction (BV/TV) between groups. A total of 230,076 and 78,758 osteocyte lacunae from facet joints of apical vertebra of scoliotic curve and iliac bone were included in the analysis, respectively. In facet joint bone biopsies, lacunar stretch (Lc.St) was higher, and lacunar equancy (Lc.Eq), lacunar oblateness (Lc.Ob), and lacunar sphericity (Lc.Sr) were lower in AIS and CS groups when compared with NS group. CA side was associated with higher Lc.St when compared with CX side. In iliac bone biopsies, Lc.Ob was higher and lacunar surface area (Lc.S) was lower in AIS group than NS group. Median values of Lc.St, Lc.Eq and Lc.Sr were significantly associated with radiological Cobb angle with adjustment for age and BMI (R-squared: 0.576, 0.558 and 0.543, respectively).

CONCLUSIONS

This large-scale assessment of osteocyte lacunae confirms that AIS osteocyte lacunae are more oblate in iliac bone that is less influenced by asymmetric loading of the deformed spine than the vertebrae. Shape of osteocyte lacunae in iliac bone is associated with radiological Cobb angle of the major curve in AIS patients, suggesting the likelihood of systemic abnormal osteocyte morphology in AIS. Osteocyte lacunae from concave side of scoliotic curves were more stretched in both AIS and CS groups, which is likely secondary to asymmetric mechanical loading.

Temporomandibular Joint Osteoarthritis: Pathogenic Mechanisms Involving the Cartilage and Subchondral Bone, and Potential Therapeutic Strategies for Joint Regeneration

The temporomandibular joint (TMJ) is a specialized synovial joint that is crucial for the movement and function of the jaw. TMJ osteoarthritis (TMJ OA) is the result of disc dislocation, trauma, functional overburden, and developmental anomalies. TMJ OA affects all joint structures, including the articular cartilage, synovium, subchondral bone, capsule, ligaments, periarticular muscles, and sensory nerves that innervate the tissues. The present review aimed to illustrate the main pathomechanisms involving cartilage and bone changes in TMJ OA and some therapeutic options that have shown potential restorative properties regarding these joint structures in vivo. Chondrocyte loss, extracellular matrix (ECM) degradation, and subchondral bone remodeling are important factors in TMJ OA. The subchondral bone actively participates in TMJ OA through an abnormal bone remodeling initially characterized by a loss of bone mass, followed by reparative mechanisms that lead to stiffness and thickening of the condylar osteochondral interface. In recent years, such therapies as intraarticular platelet-rich plasma (PRP), hyaluronic acid (HA), and mesenchymal stem cell-based treatment (MSCs) have shown promising results with respect to the regeneration of joint structures or the protection against further damage in TMJ OA. Nevertheless, PRP and MSCs are more frequently associated with cartilage and/or bone repair than HA. According to recent findings, the latter could enhance the restorative potential of other therapies (PRP, MSCs) when used in combination, rather than repair TMJ structures by itself. TMJ OA is a complex disease in which degenerative changes in the cartilage and bone develop through intricate mechanisms. The regenerative potential of such therapies as PRP, MSCs, and HA regarding the cartilage and subchondral bone (alone or in various combinations) in TMJ OA remains a matter of further research, with studies sometimes obtaining discrepant results.

Subchondral bone structure and synovial fluid metabolism are altered in injured and contralateral limbs 7 days after non-invasive joint injury in skeletally-mature C57BL/6 mice

OBJECTIVE

Post-traumatic osteoarthritis (PTOA) commonly develops after ACL injury, but early changes to the joint soon after injury are insufficiently understood. The objectives of this study were (1) evaluate the response of subchondral bone tissue modulus to joint injury and (2) identify which bone structural, material, and metabolic outcomes are local (i.e., injured joint only) or systemic (i.e., injured and contralateral-to-injured).

DESIGN

Female C57Bl∖6N mice (19 weeks at injury) underwent tibial compression overload to simulate ACL injury (n = 8) or a small pre-load (n = 8). Synovial fluid was harvested at euthanasia 7 days later for metabolomic profiling. Bone outcomes included epiphyseal and SCB microarchitecture, SCB nanoindentation modulus, SCB formation rate, and osteoclast number density.

RESULTS

Injury decreased epiphyseal bone volume fraction ([-5.29, -1.38%], P = 0.0016) and decreased SCB thickness for injured vs sham-injured limbs ([2.2, 31.4 μm], P = 0.017)). Epiphyseal bone loss commonly occurred for contralateral-to-injured limbs. There was not sufficient evidence to conclude that SCB modulus changes with injury. Metabolomic analyses revealed dysregulated synovial fluid metabolism with joint injury but that many metabolic pathways are shared between injured and contralateral-to-injured limbs.

CONCLUSION

This study demonstrates rapid changes to bone structure and synovial fluid metabolism after injury with the potential for influencing the progression to PTOA. These changes are often evidenced in the contralateral-to-injured limb, indicating that systemic musculoskeletal responses to joint injury should not be overlooked.

Partial prevention of glucocorticoid-induced osteocyte deterioration in young male mice with osteocrin gene therapy

Glucocorticoid excess suppresses osteocyte remodeling of surrounding bone minerals, causes apoptosis of osteoblasts and osteocytes, and disrupts bone remodeling, eventually, leading to glucocorticoid-induced osteoporosis and bone fragility. Preventing apoptosis and preserving osteocyte morphology could be an effective means of preventing bone loss during glucocorticoid treatment. We hypothesized that osteocrin, which preserves osteocyte viability and morphology in Sp7-deficient mice, could prevent osteocyte death and dysfunction in a glucocorticoid excess model. We used adeno-associated virus (AAV8) to induce osteocrin overexpression in mice one week before implantation with prednisolone or placebo pellets. After 28 days, prednisolone caused the expected reduction in cortical bone thickness and osteocyte canalicular length in control AAV8-treated mice, and these effects were blunted in mice receiving AAV8-osteocrin. Glucocorticoid-induced changes in cortical porosity, trabecular bone mass, and gene expression were not prevented by osteocrin. These findings support a modest therapeutic potential for AAV8-osteocrin in preserving osteocyte morphology during disease.

Inhibition of PGE2 in Subchondral Bone Attenuates Osteoarthritis

Aberrant subchondral bone architecture is a crucial driver of the pathological progression of osteoarthritis, coupled with increased sensory innervation. The sensory PGE2/EP4 pathway is involved in the regulation of bone mass accrual by the induction of differentiation of mesenchymal stromal cells. This study aimed to clarify whether the sensory PGE2/EP4 pathway induces aberrant structural alteration of subchondral bone in osteoarthritis. Destabilization of the medial meniscus (DMM) using a mouse model was combined with three approaches: the treatment of celecoxib, capsaicin, and sensory nerve-specific prostaglandin E2 receptor 4 (EP4)-knockout mice. Cartilage degeneration, subchondral bone architecture, PGE2 levels, distribution of sensory nerves, the number of osteoprogenitors, and pain-related behavior in DMM mice were assessed. Serum and tissue PGE2 levels and subchondral bone architecture in a human sample were measured. Increased PGE2 is closely related to subchondral bone’s abnormal microstructure in humans and mice. Elevated PGE2 concentration in subchondral bone that is mainly derived from osteoblasts occurs in early-stage osteoarthritis, preceding articular cartilage degeneration in mice. The decreased PGE2 levels by the celecoxib or sensory denervation by capsaicin attenuate the aberrant alteration of subchondral bone architecture, joint degeneration, and pain. Selective EP4 receptor knockout of the sensory nerve attenuates the aberrant formation of subchondral bone and facilitates the prevention of cartilage degeneration in DMM mice. Excessive PGE2 in subchondral bone caused a pathological alteration to subchondral bone in osteoarthritis and maintaining the physiological level of PGE2 could potentially be used as an osteoarthritis treatment.

Senescent preosteoclast secretome promotes metabolic syndrome associated osteoarthritis through cyclooxygenase 2

Background

Metabolic syndrome-associated osteoarthritis (MetS-OA) is a distinct osteoarthritis phenotype defined by the coexistence of MetS or its individual components. Despite the high prevalence of MetS-OA, its pathogenic mechanisms are unclear. The aim of this study was to determine the role of cellular senescence in the development of MetS-OA.

Methods

Analysis of the human osteoarthritis initiative (OAI) dataset was conducted to investigate the MRI subchondral bone features of MetS-human OA participants. Joint phenotype and senescent cells were evaluated in two MetS-OA mouse models: high-fat diet (HFD)-challenged mice and STR/Ort mice. In addition, the molecular mechanisms by which preosteoclasts become senescent as well as how the senescent preosteoclasts impair subchondral bone microenvironment were characterized using in vitro preosteoclast culture system.

Results

Humans and mice with MetS are more likely to develop osteoarthritis-related subchondral bone alterations than those without MetS. MetS-OA mice exhibited a rapid increase in joint subchondral bone plate and trabecular thickness before articular cartilage degeneration. Subchondral preosteoclasts undergo senescence at the pre- or early-osteoarthritis stage and acquire a unique secretome to stimulate osteoblast differentiation and inhibit osteoclast differentiation. Antagonizing preosteoclast senescence markedly mitigates pathological subchondral alterations and osteoarthritis progression in MetS-OA mice. At the molecular level, preosteoclast secretome activates COX2-PGE2, resulting in stimulated differentiation of osteoblast progenitors for subchondral bone formation. Administration of a selective COX2 inhibitor attenuated subchondral bone alteration and osteoarthritis progression in MetS-OA mice. Longitudinal analyses of the human Osteoarthritis Initiative (OAI) cohort dataset also revealed that COX2 inhibitor use, relative to non-selective nonsteroidal antiinflammatory drug use, is associated with less progression of osteoarthritis and subchondral bone marrow lesion worsening in participants with MetS-OA.

Conclusions

Our findings suggest a central role of a senescent preosteoclast secretome-COX2/PGE2 axis in the pathogenesis of MetS-OA, in which selective COX2 inhibitors may have disease-modifying potential.

Funding

This work was supported by the National Institutes of Health grant R01AG068226 and R01AG072090 to MW, R01AR079620 to SD, and P01AG066603 to XC.

Mmp13 deletion in mesenchymal cells increases bone mass and may attenuate the cortical bone loss caused by estrogen deficiency

The protective effect of estrogens against cortical bone loss is mediated via direct actions on mesenchymal cells, but functional evidence for the mediators of these effects has only recently begun to emerge. We report that the matrix metalloproteinase 13 (MMP13) is the highest up-regulated gene in mesenchymal cells from mice lacking the estrogen receptor alpha (ERα). In sham-operated female mice with conditional Mmp13 deletion in Prrx1 expressing cells (Mmp13^ΔPrrx1), the femur and tibia length was lower as compared to control littermates (Mmp13f.^/f). Additionally, in the sham-operated female Mmp13^ΔPrrx1 mice cortical thickness and trabecular bone volume in the femur and tibia were higher and osteoclast number at the endocortical surfaces was lower, whereas bone formation rate was unaffected. Notably, the decrease of cortical thickness caused by ovariectomy (OVX) in the femur and tibia of Mmp13f.^/f mice was attenuated in the Mmp13^ΔPrrx1 mice; but the decrease of trabecular bone caused by OVX was not affected. These results reveal that mesenchymal cell-derived MMP13 may regulate osteoclast number and/or activity, bone resorption, and bone mass. And increased production of mesenchymal cell-derived factors may be important mediators of the adverse effect of estrogen deficiency on cortical, but not trabecular, bone.

Identification of mechanics-responsive osteocyte signature in osteoarthritis subchondral bone

Aims

Osteoarthritis (OA) is a common degenerative joint disease. The osteocyte transcriptome is highly relevant to osteocyte biology. This study aimed to explore the osteocyte transcriptome in subchondral bone affected by OA.

Methods

Gene expression profiles of OA subchondral bone were used to identify disease-relevant genes and signalling pathways. RNA-sequencing data of a bone loading model were used to identify the loading-responsive gene set. Weighted gene co-expression network analysis (WGCNA) was employed to develop the osteocyte mechanics-responsive gene signature.

Results

A group of 77 persistent genes that are highly relevant to extracellular matrix (ECM) biology and bone remodelling signalling were identified in OA subchondral lesions. A loading responsive gene set, including 446 principal genes, was highly enriched in OA medial tibial plateaus compared to lateral tibial plateaus. Of this gene set, a total of 223 genes were identified as the main contributors that were strongly associated with osteocyte functions and signalling pathways, such as ECM modelling, axon guidance, Hippo, Wnt, and transforming growth factor beta (TGF-β) signalling pathways. We limited the loading-responsive genes obtained via the osteocyte transcriptome signature to identify a subgroup of genes that are highly relevant to osteocytes, as the mechanics-responsive osteocyte signature in OA. Based on WGCNA, we found that this signature was highly co-expressed and identified three clusters, including early, late, and persistently responsive genes.

Conclusion

In this study, we identified the mechanics-responsive osteocyte signature in OA-lesioned subchondral bone.

Cite this article: Bone Joint Res 2022;11(6):362–370.

Elevated levels of active Transforming Growth Factor β1 in the subchondral bone relate spatially to cartilage loss and impaired bone quality in human knee osteoarthritis

OBJECTIVE

The association between the spatially distributed level of active TGFβ1 in human subchondral bone, and the characteristic structural and cellular parameters of human knee OA, was assessed.

DESIGN

Paired subchondral bone samples from 35 OA arthroplasty patients, (15 men and 20 women, aged 69 ± 9 years) were obtained from beneath macroscopically present (CA+) or denuded cartilage (CA-) to determine the concentration of active TGFβ1 (ELISA) and its relationship to bone quality (synchrotron micro-CT), cellularity, and vascularization (histology).

RESULTS

Bone samples beneath (CA-) regions had significantly increased concentrations of active TGFβ1 protein (mean difference: 26.4; 95% CI: [3.2, 49.7]), when compared to bone in CA + regions. Trabecular Bone below (CA-) regions had increased bone volume (median difference: 4.3; 96.49% CI: [-1.7, 17.8]), increased trabecular number (1.5 [0.006, 2.6], decreased trabecular separation (-0.05 [-0.1,-0.005]), and increased bone mineral density (394.5 [65.7, 723.3]) comparing to (CA+) regions. Further, (CA-) bone regions showed increased osteocyte density (0.012 [0.006, 0.018]), with larger osteocyte lacunae (39.8 [7.8, 71.7]) that were less spherical (-0.02 [-0.04, -0.003]), and increased bone matrix vascularity (12.4 [0.3, 24.5]) compared to (CA+). In addition, increased levels of active TGFβ1 related to increased bone volume (0.04 [-0.11, 0.9]), while increased OARSI grade associated with lacunar volume (-44.1 [-71.1, -17.2]), and orientation (2.7 [0.8, 4.6]).

CONCLUSION

Increased concentration of active TGFβ1 in the subchondral bone of human knee OA associates spatially with impaired bone quality and disease severity, suggesting that TGFβ1 is a potential therapeutic target to prevent or reduce human OA disease progression.

Melatonin: A novel candidate for the treatment of osteoarthritis

Osteoarthritis (OA), characterized by cartilage erosion, synovium inflammation, and subchondral bone remodeling, is a common joint degenerative disease worldwide. OA pathogenesis is regulated by multiple predisposing factors, including imbalanced matrix metabolism, aberrant inflammatory response, and excessive oxidative stress. Moreover, melatonin has been implicated in development of several degenerative disorders owing to its potent biological functions. With regards to OA, melatonin reportedly promotes synthesis of cartilage matrix, inhibition of chondrocyte apoptosis, attenuation of inflammatory response, and suppression of matrix degradation by regulating the TGF-β, MAPK, or NF-κB signaling pathways. Notably, melatonin has been associated with amelioration of oxidative damage by restoring the OA-impaired intracellular antioxidant defense system in articular cartilage. Findings from preliminary application of melatonin or melatonin-loaded biomaterials in animal models have affirmed its potential anti-arthritic effects. Herein, we summarize the anti-arthritic effects of melatonin on OA cartilage and demonstrate that melatonin has potential therapeutic efficacy in treating OA.

At the Crux of Joint Crosstalk: TGFβ Signaling in the Synovial Joint

PURPOSE OF REVIEW

The effect of the transforming growth factor beta (TGFβ) signaling pathway on joint homeostasis is tissue-specific, non-linear, and context-dependent, representing a unique complexity in targeting TGFβ signaling in joint disease. Here we discuss the variety of mechanisms that TGFβ signaling employs in the synovial joint to maintain healthy joint crosstalk and the ways in which aberrant TGFβ signaling can result in joint degeneration.

RECENT FINDINGS

Osteoarthritis (OA) epitomizes a condition of disordered joint crosstalk in which multiple joint tissues degenerate leading to overall joint deterioration. Synovial joint tissues, such as subchondral bone, articular cartilage, and synovium, as well as mesenchymal stem cells, each demonstrate aberrant TGFβ signaling during joint disease, whether by excessive or suppressed signaling, imbalance of canonical and non-canonical signaling, a perturbed mechanical microenvironment, or a distorted response to TGFβ signaling during aging. The synovial joint relies upon a sophisticated alliance among each joint tissue to maintain joint homeostasis. The TGFβ signaling pathway is a key regulator of the health of individual joint tissues, and the subsequent interaction among these different joint tissues, also known as joint crosstalk. Dissecting the sophisticated function of TGFβ signaling in the synovial joint is key to therapeutically interrogating the pathway to optimize overall joint health.

Study of the Efficacy of Artificial Intelligence Algorithm-Based Analysis of the Functional and Anatomical Improvement in Polynucleotide Treatment in Knee Osteoarthritis Patients: A Prospective Case Series

Knee osteoarthritis (OA) is one of the most common degenerative diseases in old age. Recent studies have suggested new treatment approaches dealing with subchondral remodeling, which is a typical feature of OA progression. However, diagnostic tools or therapeutic approaches related to such a process are still being researched. The automated artificial intelligence (AI) algorithm-based texture analysis is a new method used for OA-progression detection. We designed a prospective case series study to examine the efficacy of the AI algorithm-based texture analysis in detecting the restoration of the subchondral remodeling process, which is expected to follow therapeutic intervention. In this study, we used polynucleotide (PN) filler injections as the therapeutic modality and the treatment outcome was verified by symptom improvement, as well as by the induction of subchondral microstructural changes. We used AI algorithm-based texture analysis to observe these changes in the subchondral bone with the bone structure value (BSV). A total of 51 participants diagnosed with knee OA were enrolled in this study. Intra-articular PN filler (HP cell Vitaran J) injections were administered once a week and five times in total. Knee X-rays and texture analyses with BSVs were performed during the screening visit and the last visit three months after screening. The Visual Analogue Scale (VAS) and Korean-Western Ontario MacMaster (K-WOMAC) measurements were used at the screening visit, the fifth intra-articular injection visit, and the last visit. The VAS and K-WOMAC scores decreased after PN treatment and lasted for three months after the final injection. The BSV changed in the middle and deep layers of tibial bone after PN injection. This result could imply that there were microstructural changes in the subchondral bone after PN treatment, and that this change could be detected using the AI algorithm-based texture analysis. In conclusion, the AI- algorithm-based texture analysis could be a promising tool for detecting and assessing the therapeutic outcome in knee OA.

Osteoarthritis Pathophysiology: Therapeutic Target Discovery may Require a Multifaceted Approach

Molecular understanding of osteoarthritis (OA) has greatly increased through careful analysis of tissue samples, preclinical models, and large-scale agnostic "-omic" studies. There is broad acceptance that systemic and biomechanical signals affect multiple tissues of the joint, each of which could potentially be targeted to improve patient outcomes. In this review six experts in different aspects of OA pathogenesis provide their independent view on what they believe to be good tractable approaches to OA target discovery. We conclude that molecular discovery has been high but future transformative studies require a multidisciplinary holistic approach to develop therapeutic strategies with high clinical efficacy.

Prioritization of Genes Relevant to Bone Fragility Through the Unbiased Integration of Aging Mouse Bone Transcriptomics and Human GWAS Analyses

Identifying new genetic determinants of bone mineral density (BMD) and fracture promises to yield improved diagnostics and therapies for bone fragility. However, prioritizing candidate genes from genome-wide screens can be challenging. To overcome this challenge, we prioritized mouse genes that are differentially expressed in aging mouse bone based on whether their human homolog is associated with human BMD and/or fracture. Unbiased RNA-seq analysis of young and old male C57BL/6 mouse cortical bone identified 1499, 1685, and 5525 differentially expressed genes (DEGs) in 1, 2, and 2.5-year-old bone, relative to 2-month-old bone, respectively. Gene-based scores for heel ultrasound bone mineral density (eBMD) and fracture were estimated using published genome-wide association studies (GWAS) results of these traits in the UK Biobank. Enrichment analysis showed that mouse bone DEG sets for all three age groups, relative to young bone, are significantly enriched for eBMD, but only the oldest two DEG sets are enriched for fracture. Using gene-based scores, this approach prioritizes among thousands of DEGs by a factor of 5- to 100-fold, yielding 10 and 21 genes significantly associated with fracture in the two oldest groups of mouse DEGs. Though these genes were not the most differentially expressed, they included Sost, Lrp5, and others with well-established functions in bone. Several others have, as yet, unknown roles in the skeleton. Therefore, this study accelerates identification of new genetic determinants of bone fragility by prioritizing a clinically relevant and experimentally tractable number of candidate genes for functional analysis. Finally, we provide a website (www.mouse2human.org) to enable other researchers to easily apply our strategy. © 2022 American Society for Bone and Mineral Research (ASBMR).

Altered canalicular remodeling associated with femur fracture in mice

We previously showed that femur fracture in mice caused a reduction in bone volume at distant skeletal sites within 2 weeks post-fracture. Osteocytes also have the ability to remodel their surrounding bone matrix through perilacunar/canalicular remodeling (PLR). If PLR is altered systemically following fracture, this could affect bone mechanical properties and increase fracture risk at all skeletal sites. In this study, we investigated whether lacunar-canalicular microstructure and the rate of PLR are altered in the contralateral limb following femoral fracture in mice. We hypothesized that femoral fracture would accelerate PLR by 2 weeks postfracture, followed by partial recovery by 4 weeks. We used histological evaluation and high-resolution microcomputed tomography to quantify the morphology of the lacunar-canalicular network at the contralateral tibia, and we used quantitative real-time polymerase chain reaction (RT-PCR) and RNA-seq to measure the expression of PLR-associated genes in the contralateral femur. We found that at both 2 and 4 weeks postfracture, canalicular width was significantly increased by 18.6% and 16.6%, respectively, in fractured mice relative to unfractured controls. At 3 days and 4 weeks post-fracture, we observed downregulation of PLR-associated genes; RNA-seq analysis at 3 days post-fracture showed a deceleration of bone formation and mineralization in the contralateral limb. These data demonstrate notable canalicular changes following fracture that could affect bone mechanical properties. These findings expand our understanding of systemic effects of fracture and how biological and structural changes at distant skeletal sites may contribute to increased fracture risk following an acute injury.

Subchondral bone plate thickness is associated with micromechanical and microstructural changes in the bovine patella osteochondral junction with different levels of cartilage degeneration

The influence of joint degeneration on the biomechanical properties of calcified cartilage and subchondral bone plate at the osteochondral junction is relatively unknown. Common experimental difficulties include accessibility to and visualization of the osteochondral junction, application of mechanical testing at the appropriate length scale, and availability of tissue that provides a consistent range of degenerative changes. This study addresses these challenges. A well-established bovine patella model of early joint degeneration was employed, in which micromechanical testing of fully hydrated osteochondral sections was carried out in conjunction with high-resolution imaging using differential interference contrast (DIC) optical light microscopy. A total of forty-two bovine patellae with different grades of tissue health ranging from healthy to mild, moderate, and severe cartilage degeneration, were selected. From the distal-lateral region of each patella, two adjacent osteochondral sections were obtained for the mechanical testing and the DIC imaging, respectively. Mechanical testing was carried out using a robotic micro-force acquisition system, applying compression tests over an array (area: 200 μm × 1000 μm, step size: 50 μm) across the osteochondral junction to obtain a stiffness map. Morphometric analysis was performed for the DIC images of fully hydrated cryo-sections. The levels of cartilage degeneration, DIC images, and the stiffness maps were used to associate the mechanical properties onto the specific tissue regions of cartilage, calcified cartilage, and subchondral bone plate. The results showed that there were up to 20% and 24% decreases (p < 0.05) in the stiffness of calcified cartilage and subchondral bone plate, respectively, in the severely degenerated group compared to the healthy group. Furthermore, there were increases (p < 0.05) in the number of tidemarks, bone spicules at the cement line, and the mean thickness of the subchondral bone plate with increasing levels of degeneration. The decreasing stiffness in the subchondral bone plate coupled with the presence of bone spicules may be indicative of a subchondral remodeling process involving new bone formation. Moreover, the mean thickness of the subchondral bone plate was found to be the strongest indicator of mechanical and associated structural changes in the osteochondral joint tissues.

Regulation of transforming growth factor-β1-stimulation of Runx2 acetylation for matrix metalloproteinase 13 expression in osteoblastic cells

Transforming growth factor beta 1 (TGF-β1) functions as a coupling factor between bone development and resorption. Matrix metalloproteinase 13 (MMP13) is important in bone remodeling, and skeletal dysplasia is caused by a deficiency in MMP13 expre-ssion. Runx2, a transcription factor is essential for bone development, and MMP13 is one of its target genes. TGF-β1 promoted Runx2 phosphorylation, which was necessary for MMP13 production in osteoblastic cells, as we previously shown. Since the phosphorylation of some proteins causes them to be degraded by the ubiquitin/proteasome pathway, we hypothesized that TGF-β1 might stabilize the phosphorylated Runx2 protein for its activity by other post-translational modification (PTM). This study demonstrated that TGF-β1-stimulated Runx2 acetylation in rat osteoblastic cells. p300, a histone acetyltransferase interacted with Runx2, and it promoted Runx2 acetylation upon TGF-β1-treatment in these cells. Knockdown of p300 decreased the TGF-β1-stimulated Runx2 acetylation and MMP13 expression in rat osteoblastic cells. TGF-β1-treatment stimulated the acetylated Runx2 bound at the MMP13 promoter, and knockdown of p300 reduced this effect in these cells. Overall, our studies identified the transcriptional regulation of MMP13 by TGF-β1 via Runx2 acetylation in rat osteoblastic cells, and these findings contribute to the knowledge of events presiding bone metabolism.

New imaging tools for mouse models of osteoarthritis

Osteoarthritis (OA) is a chronic degenerative disease characterized by a disruption of articular joint cartilage homeostasis. Mice are the most commonly used models to study OA. Despite recent reviews, there is still a lack of knowledge about the new development in imaging techniques. Two types of modalities are complementary: those that assess structural changes in joint tissues and those that assess metabolism and disease activity. Micro MRI is the most important imaging tool for OA research. Automated methodologies for assessing periarticular bone morphology with micro-CT have been developed allowing quantitative assessment of tibial surface that may be representative of the whole OA joint changes. Phase-contrast X-ray imaging provides in a single examination a high image precision with good differentiation between all anatomical elements of the knee joint (soft tissue and bone). Positron emission tomography, photoacoustic imaging, and fluorescence reflectance imaging provide molecular and functional data. To conclude, innovative imaging technologies could be an alternative to conventional histology with greater resolution and more efficiency in both morphological analysis and metabolism follow-up. There is a logic of permanent adjustment between innovations, 3R rule, and scientific perspectives. New imaging associated with artificial intelligence may add to human clinical practice allowing not only diagnosis but also prediction of disease progression to personalized medicine.

Associations of osteoclastogenesis and nerve growth in subchondral bone marrow lesions with clinical symptoms in knee osteoarthritis

Background/objective

Subchondral bone marrow lesions (BMLs) are common magnetic resonance imaging (MRI) features in joints affected by osteoarthritis (OA), however, their clinical impacts and mechanisms remain controversial. Thus, we aimed to investigate subchondral BMLs in knee OA patients who underwent total knee arthroplasty (TKA), then evaluate the associations of osteoclastogenesis and nerve growth in subchondral BMLs with clinical symptoms.

Methods

Total 70 patients with primary symptomatic knee OA were involved, then separated into three groups based on MRI (without BMLs group, n ​= ​14; BMLs without cyst group, n ​= ​37; BMLs with cyst group, n ​= ​19). Volume of BMLs and cyst-like lesions was calculated via the OsiriX system. The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) questionnaire was used to assess clinical symptoms. Histology and immunohistochemistry were deployed to assess subchondral osteoclastogenesis and nerve distribution. Pearson's correlation coefficient was used to evaluate the associations between volume of BMLs and joint symptoms, and to assess the associations of osteoclastogenesis and nerve growth in subchondral BMLs with joint symptoms.

Results

In BMLs combined with cyst group, patients exhibited increased osteoclastogenesis and nerve distribution in subchondral bone, as shown by increased expression of tartrate resistant acid phosphatase (TRAP) and protein gene product 9.5 (PGP9.5). Volume of subchondral cyst-like component was associated with joint pain (p ​< ​0.05). Subchondral osteoclastogenesis and nerve distribution were positively associated with joint pain in BMLs with cyst group (p ​< ​0.05).

Conclusion

The subchondral cyst-like lesion was an independent factor for inducing pain in OA patients; osteoclastogenesis and nerve growth in subchondral cyst-like lesions could account for this joint pain.

The translational potential of this article

Our results indicated that the increased osteoclastogenesis and nerve growth in subchondral cyst-like lesions could account for the pain of OA joints. These findings may provide valuable basis for the treatment of OA.

Osteocyte Apoptosis Contributes to Cold Exposure-induced Bone Loss

Emerging evidence indicates that bone mass is regulated by systemic energy balance. Temperature variations have profound effects on energy metabolism in animals, which will affect bone remodeling. But the mechanism remains unclear. 2-month-old C57BL/6J male mice were exposed to cold (4°C) and normal (23°C) temperatures for 28 days and the effects of cold exposure on bone mass was investigated. Micro-computed tomography results showed that bone volume fraction was significantly reduced after 14 days of exposure to cold temperature, and it was recovered after 28 days. Ploton silver staining and immunohistochemical results further revealed that exposure to cold decreased canalicular length, number of E11-and MMP13-positive osteocytes after 14 days, but they returned to the baseline levels after 28 days, different from the normal temperature control group. In addition, change of Caspase-3 indicated that exposure to cold temperature augmented apoptosis of osteocytes. In vitro results confirmed the positive effect of brown adipocytes on osteocyte‘s dendrites and E11 expression. In conclusion, our findings indicate that cold exposure can influence bone mass in a time-dependent manner, with bone mass decreasing and recovering at 2 and 4 weeks respectively. The change of bone mass may be caused by the apoptosis osteocytes. Brown adipocyte tissue could influence bone remodeling through affecting osteocyte.

Cellular alterations and crosstalk in the osteochondral joint in osteoarthritis and promising therapeutic strategies

Osteoarthritis (OA) is a joint disorder involving cartilage degeneration and subchondral bone sclerosis. The bone-cartilage interface is implicated in OA pathogenesis due to its susceptibility to mechanical and biological factors. The crosstalk between cartilage and the underlying subchondral bone is elevated in OA due to multiple factors, such as increased vascularization, porosity, microcracks and fissures. Changes in the osteochondral joint are traceable to alterations in chondrocytes and bone cells (osteoblasts, osteocytes and osteoclasts). The phenotypes of these cells can change with the progression of OA. Aberrant intercellular communications among bone cell-bone cell and bone cell-chondrocyte are of great importance and might be the factors promoting OA development. An appreciation of cellular phenotypic changes in OA and the mechanisms by which these cells communicate would be expected to lead to the development of targeted drugs with fewer side effects.

Spatiotemporally controlled calcitonin delivery: Long-term and targeted therapy of skeletal diseases

Bone is a connective tissue that support the entire body and protect the internal organs. However, there are great challenges on curing intractable skeletal diseases such as hypercalcemia, osteoporosis and osteoarthritis. To address these issues, calcitonin (CT) therapy is an effective treatment alternative to regulate calcium metabolism and suppress inflammation response, which are closely related to skeletal diseases. Traditional calcitonin formulation requires frequent administration due to the low bioavailability resulting from the short half-life and abundant calcitonin receptors distributed through the whole body. Therefore, long-term and targeted calcitonin delivery systems (LCDS and TCDS) have been widely explored as the popular strategies to overcome the intrinsic limitations of calcitonin and improve the functions of calcium management and inflammation inhibition in recent years. In this review, we first explain the physiological effects of calcitonin on bone remodeling: (i) inhibitory effects on osteoclasts and (ii) facilitated effects on osteoblasts. Then we summarized four strategies for spatiotemporally controlled delivery of calcitonin: micro-/nanomedicine (e.g. inorganic micro-/nanomedicine, polymeric micro-/nanomedicine and supramolecular assemblies), hydrogels (especially thermosensitive hydrogels), prodrug (PEGylation and targeting design) and hybrid biomaterials. Subsequently, we discussed the application of LCDS and TCDS in treating hypercalcemia, osteoporosis, and arthritis. Understanding and analyzing these advanced calcitonin delivery applications are essential for future development of calcitonin therapies toward skeletal diseases with superior efficacy in clinic.

Targeting Dysregulation of Metalloproteinase Activity in Osteoarthritis

Metalloproteinases were first identified as collagen cleaving enzymes and are now appreciated to play important roles in a wide variety of biological processes. The aberrant activity and dysregulation of the metalloproteinase family are linked to numerous diseases including cardiovascular and pulmonary diseases, chronic wounds, cancer, fibrosis and arthritis. Osteoarthritis (OA) is the most prevalent age-related joint disorder that causes pain and disability, but there are no disease-modifying drugs available. The hallmark of OA is loss of articular cartilage and elevated activities of matrix-degrading metalloproteinases are responsible. These enzymes do not exist in isolation and their activity is tightly regulated by a number of processes, such as transcription, proteolytic activation, interaction with their inhibitors, cell surface and extracellular matrix molecules, and endocytic clearance from the extracellular milieu. Here, we describe the functions and roles of metalloproteinase family in OA pathogenesis. We highlight recent studies that have illustrated novel mechanisms regulating their extracellular activity and impairment of such regulations that lead to the development of OA. We also discuss how to stop or slow down the degenerative processes by targeting aberrant metalloproteinase activity, which may in future become therapeutic interventions for the disease.

Amelioration of post-traumatic osteoarthritis via nanoparticle depots delivering small interfering RNA to damaged cartilage

The progression of osteoarthritis is associated with inflammation triggered by the enzymatic degradation of extracellular matrix in injured cartilage. Here we show that a locally injected depot of nanoparticles functionalized with an antibody targeting type II collagen and carrying small interfering RNA targeting the matrix metalloproteinase 13 gene (Mmp13), which breaks down type II collagen, substantially reduced the expression of MMP13 and protected cartilage integrity and overall joint structure in acute and severe mouse models of post-traumatic osteoarthritis. MMP13 inhibition suppressed clusters of genes associated with tissue restructuring, angiogenesis, innate immune responses and proteolysis. We also show that intra-articular injections of the nanoparticles led to greater reductions in disease progression than either a single injection or weekly injections of the steroid methylprednisolone. Sustained drug retention by targeting collagen in the damaged extracellular matrix of osteoarthritic cartilage may also be an effective strategy for the treatment of osteoarthritis with other disease-modifying drugs.

Treadmill exercise influences the microRNA profiles in the bone tissues of mice

As an important regulator involved in cell activity, microRNAs (miRNAs) are important in the process of exercise influencing bone metabolism. The present study aimed to detect and select differentially expressed miRNAs in the bone tissues of mice trained on a treadmill, predict the target genes of these differentially expressed miRNAs and lay a foundation for exploring the effect of treadmill training on bone metabolism through miRNAs. In this experiment, after the mice were trained on a treadmill for 8 weeks, the mechanical properties of mouse femur bone were assessed, and the alkaline phosphatase (ALP) activity and osteocalcin (OCN) protein levels of the bone were assayed. miRNA microarray and reverse transcription-quantitative (RT-q)PCR were performed to select and validate differentially expressed miRNAs in the bone, and the target genes of these miRNAs were predicted with bioinformatics methods. In addition, the differentially expressed miRNAs in the bone tissues were compared with those in mechanically strained osteocytes in vitro. Treadmill training improved the mechanical properties of the femur bones of mice, and elevated the ALP activity and OCN protein level in the bone. In addition, 122 differentially expressed miRNAs were detected in the bone, of which nine were validated via RT-qPCR. Among the target genes of these differentially expressed miRNAs, certain candidates were involved in bone metabolism. A total of eight miRNAs were differentially expressed in both bone tissue and osteocytes, exhibiting the same expression trends, and various target genes of these eight miRNAs were also involved in bone metabolism. Treadmill training resulted in altered miRNA expression profiles in the bones of mice (mainly in osteocytes) and the differentially expressed miRNAs may serve important roles in regulating bone metabolism and osteogenic differentiation.

Osteocyte Dysfunction in Joint Homeostasis and Osteoarthritis

Structural disturbances of the subchondral bone are a hallmark of osteoarthritis (OA), including sclerotic changes, cystic lesions, and osteophyte formation. Osteocytes act as mechanosensory units for the micro-cracks in response to mechanical loading. Once stimulated, osteocytes initiate the reparative process by recruiting bone-resorbing cells and bone-forming cells to maintain bone homeostasis. Osteocyte-expressed sclerostin is known as a negative regulator of bone formation through Wnt signaling and the RANKL pathway. In this review, we will summarize current understandings of osteocytes at the crossroad of allometry and mechanobiology to exploit the relationship between osteocyte morphology and function in the context of joint aging and osteoarthritis. We also aimed to summarize the osteocyte dysfunction and its link with structural and functional disturbances of the osteoarthritic subchondral bone at the molecular level. Compared with normal bones, the osteoarthritic subchondral bone is characterized by a higher bone volume fraction, a larger trabecular bone number in the load-bearing region, and an increase in thickness of pre-existing trabeculae. This may relate to the aberrant expressions of sclerostin, periostin, dentin matrix protein 1, matrix extracellular phosphoglycoprotein, insulin-like growth factor 1, and transforming growth factor-beta, among others. The number of osteocyte lacunae embedded in OA bone is also significantly higher, yet the volume of individual lacuna is relatively smaller, which could suggest abnormal metabolism in association with allometry. The remarkably lower percentage of sclerostin-positive osteocytes, together with clustering of Runx-2 positive pre-osteoblasts, may suggest altered regulation of osteoblast differentiation and osteoblast-osteocyte transformation affected by both signaling molecules and the extracellular matrix. Aberrant osteocyte morphology and function, along with anomalies in molecular signaling mechanisms, might explain in part, if not all, the pre-osteoblast clustering and the uncoupled bone remodeling in OA subchondral bone.

Hox11 expression characterizes developing zeugopod synovial joints and is coupled to postnatal articular cartilage morphogenesis into functional zones in mice

Previous studies on mouse embryo limbs have established that interzone mesenchymal progenitor cells emerging at each prescribed joint site give rise to joint tissues over fetal time. These incipient tissues undergo structural maturation and morphogenesis postnatally, but underlying mechanisms of regulation remain unknown. Hox11 genes dictate overall zeugopod musculoskeletal patterning and skeletal element identities during development. Here we asked where these master regulators are expressed in developing limb joints and whether they are maintained during postnatal zeugopod joint morphogenesis. We found that Hoxa11 was predominantly expressed and restricted to incipient wrist and ankle joints in E13.5 mouse embryos, and became apparent in medial and central regions of knees by E14.5, though remaining continuously dormant in elbow joints. Closer examination revealed that Hoxa11 initially characterized interzone and neighboring cells and was then restricted to nascent articular cartilage, intra joint ligaments and structures such as meniscal horns over prenatal time. Postnatally, articular cartilage progresses from a nondescript cell-rich, matrix-poor tissue to a highly structured, thick, zonal and mechanically competent tissue with chondrocyte columns over time, most evident at sites such as the tibial plateau. Indeed, Hox11 expression (primarily Hoxa11) was intimately coupled to such morphogenetic processes and, in particular, to the topographical rearrangement of chondrocytes into columns within the intermediate and deep zones of tibial plateau that normally endures maximal mechanical loads. Revealingly, these expression patterns were maintained even at 6 months of age. In sum, our data indicate that Hox11 genes remain engaged well beyond embryonic synovial joint patterning and are specifically tied to postnatal articular cartilage morphogenesis into a zonal and resilient tissue. The data demonstrate that Hox11 genes characterize adult, terminally differentiated, articular chondrocytes and maintain region-specificity established in the embryo.