A. van Caam

Ageing is associated with reduction of mechanically-induced activation of Smad2/3P signaling in articular cartilage

OBJECTIVE Mechanical signals control key cellular processes in articular cartilage. Previously we have shown that mechanical compression is an important ALK5/Smad2/3P activator in cartilage explants. However, age-related changes in the cartilage are known to affect tissue mechanosensitivity and also ALK5/Smad2/3P signaling. We have investigated whether ageing of cartilage is associated with an altered response to mechanical compression. DESIGN Articular cartilage explants of two different age groups (young-6-36 months old, aged-6 - 13 years old) were subjected to dynamic mechanical compression with 3 MPa (physiological) or 12 MPa (excessive) load. Subsequently, essential cartilage extracellular matrix (ECM) components and tissue growth factors gene expression was measured in young and aged cartilage by QPCR. Furthermore, the ability of young and aged cartilage, to activate the Smad2/3P signaling in response to compression was analyzed and compared. This was done by immunohistochemical (IH) Smad2P detection and Smad3-responsive gene expression analysis. RESULTS Aged cartilage showed a highly reduced capacity for mechanically-mediated activation of Smad2/3P signaling when compared to young cartilage. Compression of aged cartilage, induced collagen type II (Col2a1) and fibronectin (Fn1) expression to a far lesser extent than in young cartilage. Additionally, in aged cartilage no mechanically mediated up-regulation of bone morphogenetic protein 2 (Bmp2) and connective tissue growth factor (Ctgf) was observed. CONCLUSIONS We identified age-related changes in cellular responses to mechanical stimulation of articular cartilage. We propose that these changes might be associated with age-related alterations in cartilage functioning and can underlie mechanisms for development of age-related cartilage diseases like osteoarthritis (OA).

Osteoarthritis year in review 2016: biology

This review highlights a selection of literature in the area of osteoarthritis biology published between the 2015 and 2016 Osteoarthritis Research Society International (OARSI) World Congress. Highlights were selected from a pubmed search covering cartilage, bone, inflammation and pain. A personal selection was made based, amongst other things, on topics presented during the 2015 conference. This covers circadian rhythm, TGF-β signaling, autophagy, SIRT6, exercise, lubricin, TLRs, pain and NGF. Furthermore, in this review we have made an effort to connect these seemingly distant topics into one scheme of connections between them, revealing a theoretical big picture underneath.

IL37 dampens the IL1beta-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 IL1&bgr;, OA synovium-conditioned medium and TNF&agr; was investigated in human OA chondrocytes. Adenoviral overexpression of IL37 followed by IL1&bgr; 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 IL1&bgr;. After IL1&bgr; stimulation, increased IL1&bgr;, IL6 and IL8 expression was observed in OA chondrocytes. Elevated IL37 levels diminished the IL1&bgr;-induced IL1&bgr;, IL6 and IL8 gene levels and IL1&bgr; 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 IL1&bgr;, and IL37 itself reduced IL1&bgr;, 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.

A5.3 Elevated levels of BMP2 compensate for loss of TGF-BETA on proteoglycan level in articular cartilage during experimental osteoarthritis

Purpose We demonstrated earlier that in aging articular cartilage TGF-beta signalling via Smad2/3 is drastically reduced and loss of Smad2/3signalling predisposed cartilage for OA. We showed that TGF-beta inhibition reduced proteoglycan content in articular cartilage. In contrast, during OA we found elevated levels of BMP2 surrounding cartilage lesions. It is unclear what is the effect of this BMP2 presence on articular cartilage. Therefore, we investigated whether elevated BMP-2 could counteract the loss of TGF-beta signalling during OA. Methods We made a unique inducible transgenic mouse expressing human BMP2 under control of the Col2a1 promoter only when exposed to doxycycline (Col2a1-rtTA-BMP2). Functionality was tested by measuring human BMP2 mRNA on Q-PCR from articular cartilage, spleen and liver 72 hours after exposure to doxycycline food or standard diet. We induced OA in Col2a1-rtTA-BMP2 by destabilization of the medial meniscus (DMM-model) while treating them with doxycycline- versus standard diet. To study the effect of loss of TGF-beta activity during OA, we additionally intra-articularly injected an adenovirus overexpressing TGF-beta inhibitor LAP (Ad-LAP). Four weeks after DMM-induction knee joints were isolated for histology. OA was scored based on cartilage damage (adapted OARSI score, scale 0-30). Proteoglycan content was measured with digital image analysis in Safranin O stained articular cartilage of the medial tibia. Results Col2a1-rtTA-BMP2 mice with doxycycline food clearly had elevated expression of hBMP2 mRNA in articular cartilage, but not in spleen and liver thereby confirming functionality of the transgenic animals. Doxycycline exposure in Col2a1-rtTA-BMP2 up to 8 weeks did not result in any detectible alterations in healthy articular cartilage. When OA was induced OA score clearly increased (average of all DMM groups 16.9 versus 2.5 in non-DMM groups), but this was not significantly affected by elevated chondrocyte-specific BMP2. TGF-beta inhibition during DMM significantly reduced the proteoglycan content by 18% compared to DMM alone. BMP2 overexpression only did not affect the proteoglycan content during DMM. Nevertheless, the proteoglycan depletion by inhibition of TGF-beta during DMM was significantly and nearly completely counteracted by elevated chondrocyte-specific BMP2. Conclusions Our data show that in healthy articular cartilage and in OA cartilage in young animals elevated levels of BMP2 did not have any detectible effects on its own. However, when TGF-beta signalling was lost, a phenomenon occurring in aged individuals, this resulted in decreased levels of PG content in articular cartilage during OA. In this setting, elevated levels of BMP2 could compensate this loss of PG. Therefore the elevated levels of BMP2 near OA lesions could be a reparative response of the articular cartilage. Especially with ageing, when TGF-beta signalling is drastically reduced this compensatory mechanism could be of great importance as an attempt to restore damaged articular cartilage.