Objective To characterize mesenchymal stem cells (MSCs) from human synovial membrane (SM). Methods Cell populations were enzymatically released from the SM obtained from knee joints of adult human donors and were expanded in monolayer with serial passages at confluence. Cell clones were obtained by limiting dilution. At different passages, SM-derived cells were subjected to in vitro assays to investigate their multilineage potential. Upon treatments, phenotypes of cell cultures were analyzed by histo- and immunohistochemistry and by semiquantitative reverse transcription–polymerase chain reaction for the expression of lineage-related marker genes. Results SM-derived cells could be expanded extensively in monolayer, with limited senescence. Under appropriate culture conditions, SM-derived cells were induced to differentiate to the chondrocyte, osteocyte, and adipocyte lineages. Sporadic myogenesis was also observed. Five independent cell clones displayed multilineage potential. Interestingly, only 1 clone was myogenic. Donor age, cell passaging, and cryopreservation did not affect the multilineage potential of SM-derived cells. In contrast, normal dermal fibroblasts under the same culture conditions did not display this potential. Conclusion Our study demonstrates that human multipotent MSCs can be isolated from the SM of knee joints. These cells have the ability to proliferate extensively in culture, and they maintain their multilineage differentiation potential in vitro, establishing their progenitor cell nature. SM-derived MSCs may play a role in the regenerative response during arthritic diseases and are promising candidates for developing novel cell-based therapeutic approaches for postnatal skeletal tissue repair.

Activation of transforming growth factor β receptors causes the phosphorylation and nuclear translocation of Smad proteins, which then participate in the regulation of expression of target genes. We describe a novel Smad-interacting protein, SIP1, which was identified using the yeast two-hybrid system. Although SIP1 interacts with the MH2 domain of receptor-regulated Smads in yeast andin vitro, its interaction with full-length Smads in mammalian cells requires receptor-mediated Smad activation. SIP1 is a new member of the δEF1/Zfh-1 family of two-handed zinc finger/homeodomain proteins. Like δEF1, SIP1 binds to 5′-CACCT sequences in different promoters, including the Xenopus brachyury promoter. Overexpression of either full-length SIP1 or its C-terminal zinc finger cluster, which bind to the Xbra2promoter in vitro, prevented expression of the endogenousXbra gene in early Xenopus embryos. Therefore, SIP1, like δEF1, is likely to be a transcriptional repressor, which may be involved in the regulation of at least one immediate response gene for activin-dependent signal transduction pathways. The identification of this Smad-interacting protein opens new routes to investigate the mechanisms by which transforming growth factor β members exert their effects on expression of target genes in responsive cells and in the vertebrate embryo.

Objective: Ligands and antagonists of the WNT pathway are linked to osteoporosis and osteoarthritis. In particular, polymorphisms in the FRZB gene, a secreted WNT antagonist, have been associated with osteoarthritis. The aim of this study was to examine cartilage and bone in Frzb^−/− mice.Methods: The Frzb gene in mice was inactivated using a Cre/loxP strategy. Three models of osteoarthritis were used: collagenase, papain, and methylated bovine serum albumin induced. Bone biology was studied using density measurements and microfocal computed tomography. Bone stiffness and mechanical loading–induced bone adaptation were studied by compression of the ulnae.Results: Targeted deletion of the Frzb gene in mice increased articular cartilage loss during arthritis triggered by instability, enzymatic injury, or inflammation. Cartilage damage in Frzb^−/− mice was associated with increased WNT signaling and matrix metalloproteinase 3 (MMP‐3) expression and activity. Frzb^−/− mice had increased cortical bone thickness and density, resulting in stiffer bones, as demonstrated by stress–strain relationship analyses. Moreover, Frzb^−/− mice had an increased periosteal anabolic response to mechanical loading as compared with wild‐type mice.Conclusion: The genetic association between osteoarthritis and FRZB polymorphisms is corroborated by increased cartilage proteoglycan loss in 3 different models of arthritis in Frzb^−/− mice. Loss of Frzb may contribute to cartilage damage by increasing the expression and activity of MMPs, in a WNT‐dependent and WNT‐independent manner. FRZB deficiency also resulted in thicker cortical bone, with increased stiffness and higher cortical appositional bone formation after loading. This may contribute to the development of osteoarthritis by producing increased strain on the articular cartilage during normal locomotion but may protect against osteoporotic fractures.

Objectives Osteoarthritis (OA) is the most prevalent form of arthritis and accounts for substantial morbidity and disability, particularly in older people. It is characterised by changes in joint structure, including degeneration of the articular cartilage, and its aetiology is multifactorial with a strong postulated genetic component. Methods A meta-analysis was performed of four genome-wide association (GWA) studies of 2371 cases of knee OA and 35 909 controls in Caucasian populations. Replication of the top hits was attempted with data from 10 additional replication datasets. Results With a cumulative sample size of 6709 cases and 44 439 controls, one genome-wide significant locus was identified on chromosome 7q22 for knee OA (rs4730250, p=9.2×10⁻⁹), thereby confirming its role as a susceptibility locus for OA. Conclusion The associated signal is located within a large (500 kb) linkage disequilibrium block that contains six genes: PRKAR2B (protein kinase, cAMP-dependent, regulatory, type II, β), HPB1 (HMG-box transcription factor 1), COG5 (component of oligomeric golgi complex 5), GPR22 (G protein-coupled receptor 22), DUS4L (dihydrouridine synthase 4-like) and BCAP29 (B cell receptor-associated protein 29). Gene expression analyses of the (six) genes in primary cells derived from different joint tissues confirmed expression of all the genes in the joint environment.