Hip osteoarthritis (HOA) is one of the most disabling and common joint disorders with a large genetic component that is, however, still ill-defined. To date, genome-wide association studies (GWAS) in osteoarthritis (OA) and specifically in HOA have yielded only few loci, which is partly explained by heterogeneity in the OA definition. Therefore, we here focused on radiographically measured joint-space width (JSW), a proxy for cartilage thickness and an important underlying intermediate trait for HOA. In a GWAS of 6,523 individuals on hip-JSW, we identified the G allele of rs12982744 on chromosome 19p13.3 to be associated with a 5% larger JSW ( P = 4.8 × 10 −10 ). The association was replicated in 4,442 individuals from three United Kingdom cohorts with an overall meta-analysis P value of 1.1 × 10 −11 . The SNP was also strongly associated with a 12% reduced risk for HOA ( P = 1 × 10 −4 ). The SNP is located in the DOT1L gene, which is an evolutionarily conserved histone methyltransferase, recently identified as a potentially dedicated enzyme for Wnt target-gene activation in leukemia. Immunohistochemical staining of the DOT1L protein in mouse limbs supports a role for DOT1L in chondrogenic differentiation and adult articular cartilage. DOT1L is also expressed in OA articular chondrocytes. Silencing of Dot1l inhibited chondrogenesis in vitro. Dot1l knockdown reduces proteoglycan and collagen content, and mineralization during chondrogenesis. In the ATDC5 chondrogenesis model system, DOT1L interacts with TCF and Wnt signaling. These data are a further step to better understand the role of Wnt-signaling during chondrogenesis and cartilage homeostasis. DOT1L may represent a therapeutic target for OA.

Osteoarthritis is the most prevalent and crippling joint disease, and lacks curative treatment, as the underlying molecular basis is unclear. Here, we show that DOT1L, an enzyme involved in histone methylation, is a master protector of cartilage health. Loss of DOT1L disrupts the molecular signature of healthy chondrocytes in vitro and causes osteoarthritis in mice. Mechanistically, the protective function of DOT1L is attributable to inhibition of Wnt signalling, a pathway that when hyper-activated can lead to joint disease. Unexpectedly, DOT1L suppresses Wnt signalling by inhibiting the activity of sirtuin-1 (SIRT1), an important regulator of gene transcription. Inhibition of SIRT1 protects against osteoarthritis triggered by loss of DOT1L activity. Modulating the DOT1L network might therefore be a therapeutic approach to protect the cartilage against osteoarthritis.

ANK is a multipass transmembrane protein transporter thought to play a role in the export of intracellular inorganic pyrophosphate and so to contribute to the pathophysiology of chondrocalcinosis. As transforming growth factor-beta-1 (TGF-β1) was shown to favor calcium pyrophosphate dihydrate deposition, we investigated the contribution of ANK to the production of extracellular inorganic pyrophosphate (ePPi) by chondrocytes and the signaling pathways involved in the regulation of Ank expression by TGF-β1. Chondrocytes were exposed to 10 ng/mL of TGF-β1, and Ank expression was measured by quantitative polymerase chain reaction and Western blot. ePPi was quantified in cell supernatants. RNA silencing was used to define the respective roles of Ank and PC-1 in TGF-β1-induced ePPi generation. Finally, selective kinase inhibitors and dominant-negative/overexpression plasmid strategies were used to explore the contribution of several signaling pathways to Ank induction by TGF-β1. TGF-β1 strongly increased Ank expression at the mRNA and protein levels, as well as ePPi production. Using small interfering RNA technology, we showed that Ank contributed approximately 60% and PC-1 nearly 20% to TGF-β1-induced ePPi generation. Induction of Ank by TGF-β1 required activation of the extracellular signal-regulated kinase (ERK) pathway but not of p38-mitogen-activated protein kinase or of protein kinase A. In line with the general protein kinase C (PKC) inhibitor calphostin C, Gö6976 (a Ca 2+ -dependent PKC inhibitor) diminished TGF-β1-induced Ank expression by 60%, whereas a 10% inhibition was observed with rottlerin (a PKCδ inhibitor). These data suggest a regulatory role for calcium in TGF-β1-induced Ank expression. Finally, we demonstrated that the stimulatory effect of TGF-β1 on Ank expression was inhibited by the suppression of the Ras/Raf-1 pathway, while being enhanced by their constitutive activation. Transient overexpression of Smad 7, an inhibitory Smad, failed to affect the inducing effect of TGF-β1 on Ank mRNA level. These data show that TGF-β1 increases ePPi levels, mainly by the induction of the Ank gene, which requires activation of Ras, Raf-1, ERK, and Ca 2+ -dependent PKC pathways in chondrocytes.

IntroductionThe aim of this research was to study molecular changes in the articular cartilage and subchondral bone of the tibial plateau from mice deficient in frizzled-related protein (Frzb) compared to wild-type mice by transcriptome analysis.MethodsGene-expression analysis of the articular cartilage and subchondral bone of three wild-type and three Frzb-/- mice was performed by microarray. Data from three wild-type and two Frzb-/- samples could be used for pathway analysis of differentially expressed genes and were explored with PANTHER, DAVID and GSEA bioinformatics tools. Activation of the wingless-type (WNT) pathway was analysed using Western blot. The effects of Frzb gain and loss of function on chondrogenesis and cell proliferation was examined using ATDC5 micro-masses and mouse ribcage chondrocytes.ResultsExtracellular matrix-associated integrin and cadherin pathways, as well as WNT pathway genes were up-regulated in Frzb-/- samples. Several WNT receptors, target genes and other antagonists were up-regulated, but no difference in active β-catenin was found. Analysis of ATDC5 cell micro-masses overexpressing FRZB indicated an up-regulation of aggrecan and Col2a1, and down-regulation of molecules related to damage and repair in cartilage, Col3a1 and Col5a1. Silencing of Frzb resulted in down-regulation of aggrecan and Col2a1. Pathways associated with cell cycle were down-regulated in this transcriptome analysis. Ribcage chondrocytes derived from Frzb-/- mice showed decreased proliferation compared to wild-type cells.ConclusionsOur analysis provides evidence for tight regulation of WNT signalling, shifts in extracellular matrix components and effects on cell proliferation and differentiation in the articular cartilage - subchondral bone unit in Frzb-/- mice. These data further support an important role for FRZB in joint homeostasis and highlight the complex biology of WNT signaling in the joint.

These results demonstrate that FGF23 sustains differentiation of OA chondrocytes in a Klotho-independent manner.

Objectives To identify genetic associations with hip osteoarthritis (HOA), we performed a meta-analysis of genome-wide association studies (GWAS) of HOA. Methods The GWAS meta-analysis included approximately 2.5 million imputed HapMap single nucleotide polymorphisms (SNPs). HOA cases and controls defined radiographically and by total hip replacement were selected from the Osteoporotic Fractures in Men (MrOS) Study and the Study of Osteoporotic Fractures (SOF) (654 cases and 4697 controls, combined). Replication of genome-wide significant SNP associations (P-value ≤ 5x10−8) was examined in five studies (3243 cases and 6891 controls, combined). Functional studies were performed using in vitro models of chondrogenesis and osteogenesis. Results The A allele of rs788748, located 65 kb upstream of the IGFBP3 gene, was associated with lower HOA odds at the genome-wide significance level in the discovery stage (OR = 0.71, P-value = 2x10−8). The association replicated in five studies (OR = 0.92, P-value = 0.020), but the joint analysis of discovery and replication results was not genome-wide significant (P-value = 1x10−6). In separate study populations, the rs788748 A allele was also associated with lower circulating IGFBP3 protein levels (P-value = 4x10−13), suggesting that this SNP or a variant in linkage disequilibrium (LD) could be an IGFBP3 regulatory variant. Results from functional studies were consistent with association results. Chondrocyte hypertrophy, a deleterious event in OA pathogenesis, was largely prevented upon IGFBP3 knockdown in chondrocytes. Furthermore, IGFBP3 overexpression induced cartilage catabolism and osteogenic differentiation. Conclusions Results from GWAS and functional studies provided suggestive links between IGFBP3 and HOA.

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BackgroundEffective treatments for fibrotic diseases such as idiopathic pulmonary fibrosis are largely lacking. Transforming growth factor beta (TGFβ) plays a central role in the pathophysiology of fibrosis. We hypothesized that bone morphogenetic proteins (BMP), another family within the TGFβ superfamily of growth factors, modulate fibrogenesis driven by TGFβ. We therefore studied the role of endogenous BMP signaling in bleomycin induced lung fibrosis.MethodsLung fibrosis was induced in wild-type or noggin haploinsufficient (Nog+/LacZ) mice by intratracheal instillation of bleomycin, or phosphate buffered saline as a control. Invasive pulmonary function tests were performed using the flexiVent® SCIREQ system. The mice were sacrificed and lung tissue was collected for analysis using histopathology, collagen quantification, immunohistochemistry and gene expression analysis.ResultsNog+/LacZ mice are a known model of increased BMP signaling and were partially protected from bleomycin-induced lung fibrosis with reduced Ashcroft score, reduced collagen content and preservation of pulmonary compliance. In bleomycin-induced lung fibrosis, TGFβ and BMP signaling followed an inverse course, with dynamic activation of TGFβ signaling and repression of BMP signaling activity.ConclusionsUpon bleomycin exposure, active BMP signaling is decreased. Derepression of BMP signaling in Nog+/LacZ mice protects against bleomycin-induced pulmonary fibrosis. Modulating the balance between BMP and TGFβ, in particular increasing endogenous BMP signals, may therefore be a therapeutic target in fibrotic lung disease.