Jasper H.N. Yik

Musculoskeletal changes following non-invasive knee injury using a novel mouse model of post-traumatic osteoarthritis

OBJECTIVE Post-traumatic osteoarthritis (PTOA) is a common consequence of traumatic joint injury, with 50% of anterior cruciate ligament (ACL) rupture patients developing PTOA within 10-20 years. Currently accepted mouse models of PTOA initiate symptoms using various methods, none of which faithfully mimic clinically-relevant injury conditions. In this study we characterize a novel non-invasive mouse model of PTOA that injures the ACL with a single load of tibial compression overload. We utilize this model to determine the time course of articular cartilage and subchondral bone changes following knee injury. DESIGN Mice were euthanized 1, 3, 7, 14, 28, or 56 days after non-invasive knee injury. Knees were scanned using micro-computed tomography (μCT) in order to quantify subchondral trabecular bone, subchondral bone plate, and non-native bone formation (heterotopic ossification). Development of osteoarthritis (OA) was graded using the osteoarthritis research society international (OARSI) scale on histological sections of injured and uninjured knees. RESULTS Following injury we observed a rapid loss of trabecular bone in injured knees compared to uninjured knees by 7 days post-injury, followed by a partial recovery of trabecular bone to a new steady state by 28 days post-injury. We also observed considerable non-native bone formation by 56 days post-injury. Grading of histological sections revealed deterioration of articular cartilage by 56 days post-injury, consistent with development of mild OA. CONCLUSIONS This study establishes a novel mouse model of PTOA, and describes the time course of musculoskeletal changes following knee injury, helping to establish the window of opportunity for preventative treatment.

Enhanced activity of transforming growth factor β1 (TGF-β1) bound to cartilage oligomeric matrix protein.

Background: Cartilage oligomeric matrix protein (COMP) is a cartilage protein with a repeated modular structure that binds to and assembles extracellular matrix proteins but is not known to bind growth factors. Results: TGF-β family growth factors bind to multiple sites on COMP. Binding enhances TGF-β-dependent gene expression. Conclusion: COMP enhances the cellular response to TGF-β. Significance: A new function of COMP is to modulate TGF-β activity. Cartilage oligomeric matrix protein (COMP) is an important non-collagenous cartilage protein that is essential for the structural integrity of the cartilage extracellular matrix. The repeated modular structure of COMP allows it to “bridge” and assemble multiple cartilage extracellular matrix components such as collagens, matrilins, and proteoglycans. With its modular structure, COMP also has the potential to act as a scaffold for growth factors, thereby affecting how and when the growth factors are presented to cell-surface receptors. However, it is not known whether COMP binds growth factors. We studied the binding interaction between COMP and TGF-β1 in vitro and determined the effect of COMP on TGF-β1-induced signal transduction in reporter cell lines and primary cells. Our results demonstrate that mature COMP protein binds to multiple TGF-β1 molecules and that the peak binding occurs at slightly acidic pH. These interactions were confirmed by dual polarization interferometry and visualized by rotary shadow electron microscopy. There is cation-independent binding of TGF-β1 to the C-terminal domain of COMP. In the presence of manganese, an additional TGF-β-binding site is present in the TSP3 repeats of COMP. Finally, we show that COMP-bound TGF-β1 causes increased TGF-β1-dependent transcription. We conclude that TGF-β1 binds to COMP and that TGF-β1 bound to COMP has enhanced bioactivity.

Comparative analysis with collagen type II distinguishes cartilage oligomeric matrix protein as a primary TGFβ-responsive gene

OBJECTIVE This study aims to investigate the regulation of expression of Cartilage oligomeric matrix protein (COMP), which is predominately expressed by chondrocytes and functions to organize the extracellular matrix. Mutations in COMP cause two skeletal dysplasias: pseudoachondroplasia and multiple epiphyseal dysplasia. The mechanism controlling COMP expression during chondrocyte differentiation is still poorly understood. DESIGN Primary human bone marrow-derived stem cells were induced to differentiate into chondrocyte by pellet cultures. We then compared the temporal expression of COMP with the well-characterized cartilage-specific Type II collagen (Col2a1), and their response to transforming growth factor (TGF)β and Sox trio (Sox5, 6, and 9) stimulation. RESULTS COMP and Col2a1 expression are differentially regulated by three distinct mechanisms. First, upregulation of COMP mRNA precedes Col2a1 by several days during chondrogenesis. Second, COMP expression is independent of high cell density but requires TGF-β1. Induction of COMP mRNA by TGF-β1 is detected within 2h in the absence of protein synthesis and is blocked by specific inhibitors of the TGFβ signaling pathway; and therefore, COMP is a primary TFGβ-response gene. Lastly, while Col2a1 expression is intimately controlled by the Sox trio, overexpression of Sox trio fails to activate the COMP promoter. CONCLUSION COMP and Col2a1 expression are regulated differently during chondrogenesis. COMP is a primary response gene of TGFβ and its fast induction during chondrogenesis suggests that COMP is suitable for rapidly accessing the chondrogenic potential of stem cells.

Cyclin-Dependent kinase 9 inhibition protects cartilage from the catabolic effects of proinflammatory cytokines

Cyclin‐dependent kinase 9 (CDK‐9) controls the activation of primary inflammatory response genes. The purpose of this study was to determine whether CDK‐9 inhibition protects cartilage from the catabolic effects of proinflammatory cytokines.

The oncogene LRF is a survival factor in chondrosarcoma and contributes to tumor malignancy and drug resistance.

Chondrosarcoma is a form of malignant skeletal tumor of cartilaginous origin. The non-malignant form of the disease is termed chondroma. Correctly distinguishing between the two forms is essential for making therapeutic decisions. However, due to their similar histological appearances and the lack of a reliable diagnostic marker, it is often difficult to distinguish benign tumors from low-grade chondrosarcoma. Therefore, it is necessary to search for a potential marker that has diagnostic and prognostic values in chondrosarcoma. In this study, we demonstrated by immunohistochemistry that elevated leukemia/lymphoma-related factor (LRF) expression was associated with increased malignancy in human chondrosarcoma tissue microarrays. Moreover, siRNA depletion of LRF drastically reduced proliferation of chondrosarcoma cell lines and effectively induced senescence in these cells. This could be attributed to the observation that LRF-depleted cells were arrested at the G(1) phase, and had increased p53 and p21 expression. Moreover, LRF depletion not only drastically reduces the cellular migration and invasion potentials of chondrosarcoma cells but also sensitized these cells to the apoptosis-inducing chemotherapeutic agent doxorubicin. We conclude that LRF is a survival factor in chondrosarcomas and its expression correlates with tumor malignancy and chemoresistance. Our data implicate the potential role of LRF as both a diagnostic marker and therapeutic target for chondrosarcomas.

In-vitro and in-vivo imaging of MMP activity in cartilage and joint injury

Non-destructive detection of cartilage-degrading activities represents an advance in osteoarthritis (OA) research, with implications in studies of OA pathogenesis, progression, and intervention strategies. Matrix metalloproteinases (MMPs) are principal cartilage degrading enzymes that contribute to OA pathogenesis. MMPSense750 is an in-vivo fluorimetric imaging probe with the potential to continuously and non-invasively trace real-time MMP activities, but its use in OA-related research has not been reported. Our objective is to detect and characterize the early degradation activities shortly after cartilage or joint injury with MMPSense750. We determined the appropriate concentration, assay time, and linear range using various concentrations of recombinant MMPs as standards. We then quantified MMP activity from cartilage explants subjected to either mechanical injury or inflammatory cytokine treatment in-vitro. Finally, we performed in-vivo MMP imaging of a mouse model of post-traumatic OA. Our in-vitro results showed that the optimal assay time was highly dependent on the MMP enzyme. In cartilage explant culture media, mechanical impact or cytokine treatment increased MMP activity. Injured knees of mice showed significantly higher fluorescent signal than uninjured knees. We conclude that MMPSense750 detects human MMP activities and can be used for in-vitro study with cartilage, as well as in-vivo studies of knee injury, and can offering real-time insight into the degradative processes that occurring within the joint before structural changes become evident radiographically.

The Oncogene LRF Stimulates Proliferation of Mesenchymal Stem Cells and Inhibits Their Chondrogenic Differentiation

Objective: The oncogene leukemia/lymphoma-related factor (LRF) enhances chondrosarcoma proliferation and malignancy. This study aimed to investigate the roles of LRF in chondrogenic differentiation of primary human bone marrow–derived mesenchymal stem cells (BMSCs). Design: LRF was overexpressed in BMSC by lentiviral transduction. Chondrogenic differentiation of BMSC was induced by high-density pellet culture. Western blotting and real-time polymerase chain reaction were used to investigate changes in protein and mRNA levels, respectively, during chondrogenesis. Safranin-O staining, immunohistochemistry, and glycoaminoglycan contents were used to assess cartilage matrix deposition. BMSC proliferation was determined by mitochondrial dehydrogenase activity and cell counting. Cell cycle profiling was performed by flow cytometry. Results: LRF overexpression effectively inhibited protein and mRNA expression of chondrocyte markers and cartilage matrix deposition during chondrogenesis of BMSC. Endogenous LRF expression was constitutively high in undifferentiated BMSC but remained low in primary articular chondrocytes. Endogenous LRF protein was downregulated in a time-dependent manner during chondrogenesis. BMSCs overexpressing LRF had higher proliferation rates and cell population in the S phase. LRF suppressed p53 expression during chondrogenesis and this might prevent differentiating chondrocytes from entering a quiescent state. Conclusion: Our data showed that LRF is important for stimulating stem cell proliferation and cell cycle progression. It is known that LRF is highly expressed in the mouse limb buds prior to overt chondrogenesis; thus, LRF might function to prevent premature chondrogenic differentiation of stem cells.

c-Maf Transcription Factor Regulates ADAMTS-12 Expression in Human Chondrogenic Cells

Objective: ADAMTS (a disintegrin and metalloproteinase with thrombospondin type-1 motif) zinc metalloproteinases are important during the synthesis and breakdown of cartilage extracellular matrix. ADAMTS-12 is up-regulated during in vitro chondrogenesis and embryonic limb development; however, the regulation of ADAMTS-12 expression in cartilage remains unknown. The transcription factor c-Maf is a member of Maf family of basic ZIP (bZIP) transcription factors. Expression of c-Maf is highest in hypertrophic chondrocytes during embryonic development and postnatal growth. We hypothesize that c-Maf and ADAMTS-12 are co-expressed during chondrocyte differentiation and that c-Maf regulates ADAMTS-12 expression during chondrogenesis. Design: Promoter analysis and species alignments identified potential c-Maf binding sites in the ADAMTS-12 promoter. c-Maf and ADAMTS-12 co-expression was monitored during chondrogenesis of stem cell pellet cultures. Luciferase expression driven by ADAMTS-12 promoter segments was measured in the presence and absence of c-Maf, and synthetic oligonucleotides were used to confirm specific binding of c-Maf to ADAMTS-12 promoter sequences. Results: In vitro chondrogenesis from human mesenchymal stem cells revealed co-expression of ADAMTS-12 and c-Maf during differentiation. Truncation and point mutations of the ADAMTS-12 promoter evaluated in reporter assays localized the response to the proximal 315 bp of the ADAMTS-12 promoter, which contained a predicted c-Maf recognition element (MARE) at position -61. Electorphoretic mobility shift assay confirmed that c-Maf directly interacted with the MARE at position -61. Conclusions: These data suggest that c-Maf is involved in chondrocyte differentiation and hypertrophy, at least in part, through the regulation of ADAMTS-12 expression at a newly identified MARE in its proximal promoter.

Flavopiridol Protects Bone Tissue by Attenuating RANKL Induced Osteoclast Formation

Bone resorption and homeostasis is carried out by osteoclasts, whose differentiation and activity are regulated by the RANK/RANKL axis. Our previous studies using a mouse model of joint injury show that joint trauma induces local inflammation followed by bone remodeling. The transcription factor cyclin-dependent kinase 9 (CDK9) is the major regulator of inflammation, as CDK9 inhibitor flavopiridol effectively suppress injury-induced inflammatory response. The objective of this study was to investigate the underlying mechanism through which flavopiridol regulates bone resorption. The effects of CDK9 inhibition, by the specific-inhibitor flavopiridol, on bone resorption were determined in vivo using two distinct and clinically relevant bone remodeling models. The first model involved titanium particle-induced acute osteolysis, and the second model was ovariectomy-induced chronic osteoporosis. The effects and mechanism of CDK9 inhibition on osteoclastogenesis were examined using in vitro culture of bone marrow macrophages (BMMs). Our results indicated that flavopiridol potently suppressed bone resorption in both in vivo bone-remodeling models. In addition, CDK9 inhibition suppressed in vitro osteoclastogenesis of BMM and reduced their expression of osteoclast-specific genes. Finally, we determined that flavopiridol suppressed RANKL signaling pathway via inhibition of p65 phosphorylation and nuclear translocation of NF-κB. Summary, CDK9 is a potential therapeutic target to prevent osteolysis and osteoporosis by flavopiridol treatment.

Inhibition of Early Response Genes Prevents Changes in Global Joint Metabolomic Profiles in Mouse Post-Traumatic Osteoarthritis

Osteoarthritis (OA) is the most common degenerative joint disease, and joint injury increases the risk of OA by 10-fold. Although the injury event itself damages joint tissues, a substantial amount of secondary damage is mediated by the cellular responses to the injury. Cellular responses include the production and activation of proteases (MMPs, ADAMTSs, Cathepsins), the production of inflammatory cytokines, and we hypothesize, changes to the joint metabolome. The trajectory of cellular responses is driven by the transcriptional activation of early response genes, which requires Cdk9-dependent RNA Polymerase II phosphorylation. Flavopiridol is a potent and selective inhibitor of Cdk9 kinase activity, which prevents the transcriptional activation of early response genes. To model post-traumatic osteoarthritis, we subjected mice to non-invasive ACL-rupture joint injury. Following injury, mice were treated with flavopiridol to inhibit Cdk9-dependent transcriptional activation, or vehicle control. Global joint metabolomics were analyzed 1 hour after injury. We found that injury induced metabolomic changes, including increases in Vitamin D3 metabolism and others. Importantly, we found that inhibition of primary response gene activation at the time of injury largely prevented the global changes in the metabolomics profiles. Cluster analysis of joint metabolomes identified groups of injury-induced and drug-responsive metabolites, which may offer novel targets for cell-mediated secondary joint damage. Metabolomic profiling provides an instantaneous snapshot of biochemical activity representing cellular responses, and these data demonstrate the potential for inhibition of early response genes to alter the trajectory of cell-mediated degenerative changes following joint injury. Significance Statement Joint injury is an excellent predictor of future osteoarthritis. It is increasingly apparent that the acute cellular responses to injury contribute to the initiation and pathogenesis of OA. Although changes to the joint transcriptome have been extensively studied in the context of joint injury, little is known about changes to small-molecule metabolites. Here we use a non-invasive ACL rupture model of joint injury in mice to identify injury-induced changes to the global metabolomic profiles. In one experimental group we prevented the activation of primary response gene transcription using the Cdk9 inhibitor flavopiridol. Through this comparison, we identified two sets of metabolites that change acutely after joint injury: those that require transcription of primary response genes, and those that do not.