The reaction patterns of chondrocytes in osteoarthritis can be summarized in five categories: (1) proliferation and cell death (apoptosis); changes in (2) synthetic activity and (3) degradation; (4) phenotypic modulation of the articular chondrocytes; and (5) formation of osteophytes. In osteoarthritis, the primary responses are reinitiation of synthesis of cartilage macromolecules, the initiation of synthesis of types IIA and III procollagens as markers of a more primitive phenotype, and synthesis of active proteolytic enzymes. Reversion to a fibroblast-like phenotype, known as 'dedifferentiation', does not appear to be an important component. Proliferation plays a role in forming characteristic chondrocyte clusters near the surface, while apoptosis probably occurs primarily in the calcified cartilage.

Recent work has established that IL-1β plays a central role in the inflammation and connective tissue destruction observed in both rheumatoid arthritis and osteoarthritis. These processes result from the ability of this inflammatory cytokine to activate expression of genes for neutral proteases, such as the matrix metalloproteinases. While IL-1β activates matrix metalloproteinase genes within several hours, it also activates immediate early genes, which are required for the later expression of matrix metalloproteinases and other arthritis-perpetuating genes, are also activated. To identify putative immediate early genes involved in IL-1β-mediated arthritic disease, a chondrocytic cell line (SW1353) was stimulated with this cytokine for 2 hours, total RNA was isolated, and expressed genes were identified by microarray analysis. This analysis identified alterations in the expression of multiple transcription factors, cytokines, growth factors and their receptors, adhesion molecules, proteases, and signaling intermediates that may contribute to inflammation and cartilage destruction in arthritis. Interestingly, confirmation of the expression of activating protein-1 family members by reverse transcriptase polymerase chain reaction revealed a preferential increase in junB, a known transcriptional antagonist of c-jun. The failure to observe induction of early growth response gene-1, which was detected by reverse transcriptase polymerase chain reaction to be substantially and transiently induced by 1 hour of IL-1 treatment, may be explained by the known instability of the message after early induction. However, this analysis has identified numerous IL-1β-responsive genes that warrant further investigation as mediators of disease in arthritis.

Bone-resorbing osteoclasts are formed from hemopoietic cells of the monocyte–macrophage lineage under the control of bone-forming osteoblasts. We have cloned an osteoblast-derived factor essential for osteoclastogenesis, the receptor activator of NF-κB ligand (RANKL). Synovial fibroblasts and activated T lymphocytes from patients with rheumatoid arthritis also express RANKL, which appears to trigger bone destruction in rheumatoid arthritis as well. Recent studies have shown that T lymphocytes produce cytokines other than RANKL such as IL-17, granulocyte–macrophage colony-stimulating factor and IFN-γ, which have powerful regulatory effects on osteoclastogenesis. The possible roles of RANKL and other cytokines produced by T lymphocytes in bone destruction are described.

The extracellular framework and two-thirds of the dry mass of adult articular cartilage are polymeric collagen. Type II collagen is the principal molecular component in mammals, but collagens III, VI, IX, X, XI, XII and XIV all contribute to the mature matrix. In developing cartilage, the core fibrillar network is a cross-linked copolymer of collagens II, IX and XI. The functions of collagens IX and XI in this heteropolymer are not yet fully defined but, evidently, they are critically important since mutations in COLIX and COLXI genes result in chondrodysplasia phenotypes that feature precocious osteoarthritis. Collagens XII and XIV are thought also to be bound to fibril surfaces but not covalently attached. Collagen VI polymerizes into its own type of filamentous network that has multiple adhesion domains for cells and other matrix components. Collagen X is normally restricted to the thin layer of calcified cartilage that interfaces articular cartilage with bone.

Bone loss is one of the major health problems for astronauts during long-term spaceflight and for patients during prolonged bed rest or paralysis. Growing evidence suggests that osteocytes, the most abundant cells in the mineralized bone matrix, play a key role in sensing mechanical forces applied to the skeleton and in transducing them into subcellular biochemical signals to modulate bone homeostasis. However, the precise molecular mechanisms underlying both mechanosensation and mechanotransduction in osteocytes under the real microgravity (µG) condition are poorly understood. To unravel the mechanisms by which osteocyte, sense and responds to mechanical unloading, we exposed murine osteocytic cell line, Ocy454, seeded on a highly porous polystyrene 3D scaffold, to 2, 4, or 6 days of µG on board the International Space Station (ISS) and compared their gene expression with cells at 1G on Earth. Bioinformatics analysis of cells exposed to µG revealed several pathways differentially regulated upon exposure to microgravity.

While many proteases in articular cartilage have been described, current studies indicate that members of two families of metalloproteases – MMPs and the ADAMTSs – are responsible for the degradation of the major components of this tissue. Collagenases (MMPs) make the first cleavage in triple-helical collagen, allowing its further degradation by other proteases. Aggrecanases (ADAMTSs), in conjunction with other MMPs, degrade aggrecan, a component of the proteoglycan aggregate. Anti-neoepitope antibodies that recognize the cleavage products of collagen and aggrecan generated by these enzymes are now available and are being used to detect the sites of action and to quantitate degradation products.

Cartilage contains numerous noncollagenous proteins in its extracellular matrix, including proteoglycans. At least 40 such molecules have been identified, differing greatly in structure, distribution, and function. Some are present in only selected cartilages or cartilage zones, some vary in their presence with a person's development and age, and others are more universal in their expression. Some may not even be made by the chondrocytes, but may arise by absorption from the synovial fluid. In many cases, the molecules' function is unclear, but the importance of others is illustrated by their involvement in genetic disorders. This review provides a selective survey of these molecules and discusses their structure, function, and involvement in inherited and arthritic disorders.

Background Bone morphogenetic proteins (BMPs) and transforming growth factor-βs (TGF-βs) are important regulators of bone repair and regeneration. BMP-2 and TGF-β1 have been shown to inhibit gap junctional intercellular communication (GJIC) in MC3T3-E1 cells. Connexin 43 (Cx43) has been shown to mediate GJIC in osteoblasts and it is the predominant gap junctional protein expressed in these murine osteoblast-like cells. We examined the expression, phosphorylation, and subcellular localization of Cx43 after treatment with BMP-2 or TGF-β1 to investigate a possible mechanism for the inhibition of GJIC. Results Northern blot analysis revealed no detectable change in the expression of Cx43 mRNA. Western blot analysis demonstrated no significant change in the expression of total Cx43 protein. However, significantly higher ratios of unphosphorylated vs. phosphorylated forms of Cx43 were detected after BMP-2 or TGF-β1 treatment. Immunofluorescence and cell protein fractionation revealed no detectable change in the localization of Cx43 between the cytosol and plasma membrane. Conclusions BMP-2 and TGF-β1 do not alter expression of Cx43 at the mRNA or protein level. BMP-2 and TGF-β1 may inhibit GJIC by decreasing the phosphorylated form of Cx43 in MC3T3-E1 cells.

Macrophages, osteoclasts, dendritic cells, and microglia are highly specialized cells that belong to the mononuclear phagocyte system. Functional and phenotypic heterogeneity within the mononuclear phagocyte system may reveal differentiation plasticity of a common progenitor, but developmental pathways leading to such diversity are still unclear.