Separating friend from foe: Inhibition of TGF-β-induced detrimental SMAD1/5/9 phosphorylation while maintaining protective SMAD2/3 signaling in OA chondrocytes

OBJECTIVE

Transforming growth factor-β (TGF-β) signaling via SMAD2/3 is crucial to control cartilage homeostasis. However, TGF-β can also have detrimental effects by signaling via SMAD1/5/9 and thereby contribute to diseases like osteoarthritis (OA). In this study, we aimed to block TGF-β-induced SMAD1/5/9 signaling in primary human OA chondrocytes, while maintaining functional SMAD2/3 signaling.

DESIGN

Human OA chondrocytes were pre-incubated with different concentrations of ALK4/5/7 kinase inhibitor SB-505124 before stimulation with TGF-β. Changes in SMAD C-terminal phosphorylation were analyzed using Western blot and response genes were measured with quantitative Polymerase Chain Reaction. To further explore the consequences of our ability to separate pathways, we investigated TGF-β-induced chondrocyte hypertrophy.

RESULTS

Pre-incubation with 0.5 µM SB-505124, maintained ±50% of C-terminal SMAD2/3 phosphorylation and induction of JUNB and SERPINE1, but blocked SMAD1/5/9-C phosphorylation and expression of ID1 and ID3. Furthermore, TGF-β, in levels comparable to those in the synovial fluid of OA patients, resulted in regulation of hypertrophic and dedifferentiation markers in OA chondrocytes; i.e. an increase in COL10, RUNX2, COL1A1, and VEGF and a decrease in ACAN expression. Interestingly, in a subgroup of OA chondrocyte donors, blocking only SMAD1/5/9 caused stronger inhibition on TGF-β-induced RUNX2 than blocking both SMAD pathways.

CONCLUSION

Our findings indicate that using low dose of SB-505124 we maintained functional SMAD2/3 signaling that blocks RUNX2 expression in a subgroup of OA patients. We are the first to show that SMAD2/3 and SMAD1/5/9 pathways can be separately modulated using low and high doses of SB-505124 and thereby split TGF-β's detrimental from protective function in chondrocytes.

An improved diagnostic tool to predict cartilage formation in an osteoarthritic joint environment

Osteoarthritis (OA) is a degenerative joint disease with progressive articular cartilage loss. Due to the chondrogenic potential of human mesenchymal stromal cells (MSCs), MSC-based therapies are promising treatment strategies for cartilage loss. However, the local joint microenvironment has a great impact on the success of cartilage formation by MSCs. This local joint environment is different between patients and therefore the outcome of MSC therapies is uncertain. We previously developed gene promoter-based reporter assays as a novel tool to predict the effect of a patient's OA joint microenvironment on the success of MSC-based cartilage formation. Here we describe an improved version of this molecular tool with increased prediction accuracy. For this, we generated fourteen stable cell lines using transcription factor (TF) binding elements (AP1, ARE, CRE, GRE, ISRE, NFAT5, NFκB, PPRE, SBE, SIE, SOX9, SRE, SRF, TCF/LEF) to drive luciferase reporter gene expression, and evaluated the cell lines for their responsiveness to an osteoarthritic microenvironment by stimulation with OA synovium-conditioned medium (OAs-cm; n=31). To study the effect of this OA microenvironment on MSC-based cartilage formation, MSCs were cultured in a three-dimensional pellet culture model while stimulated with OAs-cm. Cartilage formation was assessed histologically and by quantifying sulfated glycosaminoglycan (sGAG) production. Six TF reporters correlated significantly with the effect of OAs-cm on cartilage formation. We validated the predictive value of these TF reporters with an independent cohort of OAs-cm (n=22) and compared the prediction accuracy between our previous and the current new tool. Furthermore, we investigated which combination of reporters could predict the effect of the OA microenvironment on cartilage repair with the highest accuracy. A combination between the TF (NFκB) and the promoter-based (IL6) reporter proved to reach a more accurate prediction compared to the tools separately. These developments are an important step towards a diagnostic tool that can be used for personalized cartilage repair strategies for OA patients.

Identification of Transcription Factors Responsible for a Transforming Growth Factor-β-Driven Hypertrophy-like Phenotype in Human Osteoarthritic Chondrocytes

During osteoarthritis (OA), hypertrophy-like chondrocytes contribute to the disease process. TGF-β's signaling pathways can contribute to a hypertrophy(-like) phenotype in chondrocytes, especially at high doses of TGF-β. In this study, we examine which transcription factors (TFs) are activated and involved in TGF-β-dependent induction of a hypertrophy-like phenotype in human OA chondrocytes. We found that TGF-β, at levels found in synovial fluid in OA patients, induces hypertrophic differentiation, as characterized by increased expression of RUNX2, COL10A1, COL1A1, VEGFA and IHH. Using luciferase-based TF activity assays, we observed that the expression of these hypertrophy genes positively correlated to SMAD3:4, STAT3 and AP1 activity. Blocking these TFs using specific inhibitors for ALK-5-induced SMAD signaling (5 µM SB-505124), JAK-STAT signaling (1 µM Tofacitinib) and JNK signaling (10 µM SP-600125) led to the striking observation that only SB-505124 repressed the expression of hypertrophy factors in TGF-β-stimulated chondrocytes. Therefore, we conclude that ALK5 kinase activity is essential for TGF-β-induced expression of crucial hypertrophy factors in chondrocytes.

Prediction of the Effect of the Osteoarthritic Joint Microenvironment on Cartilage Repair

Osteoarthritis (OA) is characterized by progressive articular cartilage loss. Human mesenchymal stromal cells (MSCs) can be used for cartilage repair therapies based on their potential to differentiate into chondrocytes. However, the joint microenvironment is a major determinant of the success of MSC-based cartilage formation. Currently, there is no tool that is able to predict the effect of a patient's OA joint microenvironment on MSC-based cartilage formation. Our goal was to develop a molecular tool that can predict this effect before the start of cartilage repair therapies. Six different promoter reporters (hIL6, hIL8, hADAMTS5, hWISP1, hMMP13, and hADAM28) were generated and evaluated in an immortalized human articular chondrocyte for their responsiveness to an osteoarthritic microenvironment by stimulation with OA synovium-conditioned medium (OAs-cm) obtained from 32 different knee OA patients. To study the effect of this OA microenvironment on MSC-based cartilage formation, MSCs were cultured in a three-dimensional pellet culture model, while stimulated with OAs-cm. Cartilage formation was assessed histologically and by quantifying sulfated glycosaminoglycan (sGAG) production. We confirmed that OAs-cm of different patients had significantly different effects on sGAG production. In addition, significant correlations were obtained between the effect of the OAs-cm on cartilage formation and promoter reporter outcome. Furthermore, we validated the predictive value of measuring two promoter reporters with an independent cohort of OAs-cm and the effect of 87.5% of the OAs-cm on MSC-based cartilage formation could be predicted. Together, we developed a novel tool to predict the effect of the OA joint microenvironment on MSC-based cartilage formation. This is an important first step toward personalized cartilage repair strategies for OA patients.

Transcription Factors in Cartilage Homeostasis and Osteoarthritis

Osteoarthritis (OA) is the most common degenerative joint disease, and it is characterized by articular cartilage loss. In part, OA is caused by aberrant anabolic and catabolic activities of the chondrocyte, the only cell type present in cartilage. These chondrocyte activities depend on the intra- and extracellular signals that the cell receives and integrates into gene expression. The key proteins for this integration are transcription factors. A large number of transcription factors exist, and a better understanding of the transcription factors activated by the various signaling pathways active during OA can help us to better understand the complex etiology of OA. In addition, establishing such a profile can help to stratify patients in different subtypes, which can be a very useful approach towards personalized therapy. In this review, we discuss crucial transcription factors for extracellular matrix metabolism, chondrocyte hypertrophy, chondrocyte senescence, and autophagy in chondrocytes. In addition, we discuss how insight into these factors can be used for treatment purposes.

TGFβ/BMP Signaling Pathway in Cartilage Homeostasis

Cartilage homeostasis is governed by articular chondrocytes via their ability to modulate extracellular matrix production and degradation. In turn, chondrocyte activity is regulated by growth factors such as those of the transforming growth factor β (TGFβ) family. Members of this family include the TGFβs, bone morphogenetic proteins (BMPs), and growth and differentiation factors (GDFs). Signaling by this protein family uniquely activates SMAD-dependent signaling and transcription but also activates SMAD-independent signaling via MAPKs such as ERK and TAK1. This review will address the pivotal role of the TGFβ family in cartilage biology by listing several TGFβ family members and describing their signaling and importance for cartilage maintenance. In addition, it is discussed how (pathological) processes such as aging, mechanical stress, and inflammation contribute to altered TGFβ family signaling, leading to disturbed cartilage metabolism and disease.

IL37 dampens the IL1β-induced catabolic status of human OA chondrocytes

Objective

A crucial feature of OA is cartilage degradation. This process is mediated by pro-inflammatory cytokines, among other factors, via induction of matrix-degrading enzymes. Interleukin 37 (IL37) is an anti-inflammatory cytokine and is efficient in blocking the production of pro-inflammatory cytokines during innate immune responses. We hypothesize that IL37 is therapeutic in treating the inflammatory cytokine cascade in human OA chondrocytes and can act as a counter-regulatory cytokine to reduce cartilage degradation in OA.

Methods

Human OA cartilage was obtained from patients undergoing total knee or hip arthroplasty. Immunohistochemistry was applied to study IL37 protein expression in cartilage biopsies from OA patients. Induction of IL37 expression by IL1β, OA synovium-conditioned medium and TNFα was investigated in human OA chondrocytes. Adenoviral overexpression of IL37 followed by IL1β stimulation was performed to investigate the anti-inflammatory potential of IL37.

Results

IL37 expression was detected in cartilage biopsies of OA patients and induced by IL1β. After IL1β stimulation, increased IL1β, IL6 and IL8 expression was observed in OA chondrocytes. Elevated IL37 levels diminished the IL1β-induced IL1β , IL6 and IL8 gene levels and IL1β and IL8 protein levels. In addition to the reduction in pro-inflammatory cytokine expression, IL37 reduced MMP1 , MMP3 , MMP13 and disintegrin and metalloproteinase with thrombospondin motifs 5 gene levels and MMP3 and MMP13 protein levels.

Conclusion

IL37 is induced by IL1β, and IL37 itself reduced IL1β, IL6 and IL8 production, indicating that IL37 is able to induce a counter-regulatory anti-inflammatory feedback loop in chondrocytes. In addition, IL37 dampens catabolic enzyme expression. This supports IL37 as a potential therapeutic target in OA.

Reduced Euchromatin histone methyltransferase 1 causes developmental delay, hypotonia, and cranial abnormalities associated with increased bone gene expression in Kleefstra syndrome mice

Haploinsufficiency of Euchromatin histone methyltransferase 1 (EHMT1), a chromatin modifying enzyme, is the cause of Kleefstra syndrome (KS). KS is an intellectual disability (ID) syndrome, with general developmental delay, hypotonia, and craniofacial dysmorphisms as additional core features. Recent studies have been focused on the role of EHMT1 in learning and memory, linked to the ID phenotype of KS patients. In this study we used the Ehmt1(+/-) mouse model, and investigated whether the core features of KS were mimicked in these mice. When comparing Ehmt1(+/-) mice to wildtype littermates we observed delayed postnatal growth, eye opening, ear opening, and upper incisor eruption, indicating a delayed postnatal development. Furthermore, tests for muscular strength and motor coordination showed features of hypotonia in young Ehmt1(+/-) mice. Lastly, we found that Ehmt1(+/-) mice showed brachycephalic crania, a shorter or bent nose, and hypertelorism, reminiscent of the craniofacial dysmorphisms seen in KS. In addition, gene expression analysis revealed a significant upregulation of the mRNA levels of Runx2 and several other bone tissue related genes in P28 Ehmt1(+/-) mice. Runx2 immunostaining also appeared to be increased. The mRNA upregulation was associated with decreased histone H3 lysine 9 dimethylation (H3K9me2) levels, the epigenetic mark deposited by Ehmt1, in the promoter region of these genes. Together, Ehmt1(+/-) mice indeed recapitulate KS core features and can be used as an animal model for Kleefstra syndrome. The increased expression of bone developmental genes in the Ehmt1(+/-) mice likely contributes to their cranial dysmorphisms and might be explained by diminished Ehmt1-induced H3K9 dimethylation.