Osteoarthritis is a chronic degenerative disorder of the joint and represents one of the most common diseases worldwide. Its prevalence and severity are increasing owing to aging of the population, but treatment options remain largely limited to painkillers and anti-inflammatory drugs, which only provide symptomatic relief. In the late stages of the disease, surgical interventions are often necessary to partially restore joint function. Although the focus of osteoarthritis research has been originally on the articular cartilage, novel findings are now pointing to osteoarthritis as a disease of the whole joint, in which failure of different joint components can occur. In this Review, we summarize recent progress in the field, including data from novel ‘omics’ technologies and from a number of preclinical and clinical trials. We describe different in vitro and in vivo systems that can be used to study molecules, pathways and cells that are involved in osteoarthritis. We illustrate that a comprehensive and multisystem approach is necessary to understand the complexity and heterogeneity of the disease and to better guide the development of novel therapeutic strategies for osteoarthritis.

Using the DMM model, we demonstrated that FRZB and SFRP1 differentially modulate joint homeostasis in two distinct compartments of the joint. These data highlight the fine-tuning of Wnt signaling in joint homeostasis and disease, show differential regulation of the cascade in cartilage and subchondral bone, and provide further evidence for a role of endogenous Wnt modulators as key players in OA.

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.

To avoid malformation and disease, tissue development and homoeostasis are co-ordinated precisely in time and space. Secreted Frizzled-related protein 3 (sFRP3), encoded by the Frizzled-related protein gene (FRZB), acts as an antagonist of Wnt signalling in bone development by delaying the maturation of proliferative chondrocytes into hypertrophic chondrocytes. A disintegrin and metalloprotease 17 (ADAM17) is a transmembrane protease that is essential for developmental processes and promotes cartilage maturation into bone. sFRP3 is chondroprotective and is expressed in chondrocytes of healthy articular cartilage. Upon damage to cartilage, sFRP3 is down-regulated. Rare variants of sFRP3 are associated with osteoarthritis. The present study demonstrates a novel function of sFRP3 in suppression of the enzymatic activity of ADAM17 which results in the inhibition of ADAM17-meditated interleukin-6 receptor (IL-6R) shedding. By contrast, the rare double variant of sFRP3 failed to suppress ADAM17. The shed soluble IL-6R (sIL-6R) is linked to inflammation, cartilage degeneration and osteolysis. Accordingly, enhanced activity of ADAM17 in cartilage, caused by the expression of the rare double sFRP3 variant, provides an explanation for the genetic effect of sFRP3 variants in joint disease. The finding that sFRP3 interacts with the ADAM17 substrate IL-6R also suggests a new regulatory mechanism by which the substrate is protected against shedding.

Abnormal Wnt signaling is associated with bone mass disorders. Frizzled-related protein (FRZB, also known as secreted frizzled-related protein-3 (SFRP3)) is a Wnt modulator that contains an amino-terminal cysteine-rich domain (CRD) and a carboxy-terminal Netrin-like (NTN) motif. Frzb(-/-) mice show increased cortical thickness. However, the direct effect of FRZB on osteogenic differentiation and the involvement of the structural domains herein are not fully understood. In this study, we observed that stable overexpression of Frzb in MC3T3-E1 cells increased calcium deposition and osteoblast markers compared with control. Western blot analysis showed that the increased osteogenesis was associated with reduced canonical, but increased non-canonical Wnt signaling. On the contrary, loss of Frzb induced the opposite effects on osteogenesis and Wnt signaling. To translationally validate the positive effects of FRZB on primary human cells, we treated human periosteal and human bone marrow stromal cells with conditioned medium from MC3T3-E1 cells overexpressing Frzb and observed an increase in Alizarin red staining. We further studied the effect of the domains. FrzbNTN overexpression induced similar effects on osteogenesis as full-length Frzb, whereas FrzbCRD overexpressing cells mimicked loss of Frzb experiments. The CRD is considered as the Wnt binding domain, but the NTN domain also has important effects on bone biology. FRZB and other SFRPs or their specific domains may hold surprising potential as therapeutics for bone and joint disorders considering that excess of SFRPs has effects that are not expected under physiological, endogenous expression conditions.