Osteoarthritis (OA), the most prevalent aging-related joint disease, is characterized by insufficient extracellular matrix synthesis and articular cartilage degradation, mediated by several proteinases, including Adamts-5. miR-140 is one of a very limited number of noncoding microRNAs (miRNAs) specifically expressed in cartilage; however, its role in development and/or tissue maintenance is largely uncharacterized. To examine miR-140 function in tissue development and homeostasis, we generated a mouse line through a targeted deletion of miR-140. miR-140−/− mice manifested a mild skeletal phenotype with a short stature, although the structure of the articular joint cartilage appeared grossly normal in 1-mo-old miR-140−/− mice. Interestingly, miR-140−/− mice showed age-related OA-like changes characterized by proteoglycan loss and fibrillation of articular cartilage. Conversely, transgenic (TG) mice overexpressing miR-140 in cartilage were resistant to antigen-induced arthritis. OA-like changes in miR-140-deficient mice can be attributed, in part, to elevated Adamts-5 expression, regulated directly by miR-140. We show that miR-140 regulates cartilage development and homeostasis, and its loss contributes to the development of age-related OA-like changes.

Osteoarthritis (OA) is a chronic and highly prevalent degenerative joint disease. Approximately 40 million Americans are currently affected, and this number is predicted to increase to 60 million within the next 20 years as a result of population aging and an increase in life expectancy (1,2). Current treatment is limited to pain management, and disease-modifying therapies are not available in the late phase of the disease process, at which point joint replacement surgery is often indicated. OA has been associated with age-related loss of the homeostatic balance between degradation and repair mechanisms. Cartilage cellularity in OA is reduced by chondrocyte death, and remaining chondrocytes are activated by cytokines and growth factors to a catabolic and abnormal differentiation that leads to degradation

Mohawk (Mkx) is a member of the Three Amino acid Loop Extension superclass of atypical homeobox genes that is expressed in developing tendons. To investigate the in vivo functions of Mkx, we generated Mkx −/− mice. These mice had hypoplastic tendons throughout the body. Despite the reduction in tendon mass, the cell number in tail tendon fiber bundles was similar between wild-type and Mkx −/− mice. We also observed small collagen fibril diameters and a down-regulation of type I collagen in Mkx −/− tendons. These data indicate that Mkx plays a critical role in tendon differentiation by regulating type I collagen production in tendon cells.T endons are dense, fibrous connective tissues that connect muscle to bone, transmitting the forces that allow for body movement (1). Tendon damage from overuse or degeneration due to aging is a common clinical problem because damaged tendon tissue heals very slowly and rarely recovers completely (2). The establishment of new therapies, such as regenerative medicine, for injured tendons has been delayed by a limited understanding of tendon biology (1, 3).Tendons are composed primarily of collagen fibrils that cross-link to each other to form fibers (4). A small number of tendon cells reside between parallel chains of these fibrils and synthesize the specific ECM that contains collagens and proteoglycans (4, 5). The elasticity of tendons is provided by the large amount of collagen, predominantly type I collagen and small amounts of other collagens, including types III, IV, V, and VI (4, 6-9). The proteoglycans found in tendons, including decorin, fibromodulin, biglycan, and lumican, act to lubricate and organize collagen fiber bundles (4, 5). Targeted disruption of these proteoglycans in mice leads to abnormal collagen fibrils in tendons (3, 10-13). Tendon disruptions have also been described in patients with defects in collagen production, such as Ehlers-Danlos Syndrome, in which the type I collagen gene is mutated (14). These studies indicate that the ability of tendon cells to produce ECM is important for tendon formation.Recently, it was reported that Scleraxis (Scx), a basic helix-loophelix (bHLH) transcription factor expressed in the tendon progenitors and cells of all tendon tissues (15, 16), is essential for tendon differentiation. Scx knockout mice show severe disruption of force-transmitting tendons, although ligaments, which are tissues connecting bone to bone that closely resemble tendons in their components, and short-range anchoring tendons are not affected (17). It was also reported that Scx positively regulates the expression of type I collagen, a main ECM component of tendons (18). However, the type I collagen does not completely disappear from the tendons of Scx knockout mice (17), suggesting the presence of other regulatory factors for type I collagen. The tendon differentiation mechanisms remain largely unknown, with Scx being the only known transcription factor regulating tendon differentiation.Mohawk (Mkx; also known as Irxl1) is the sole member of a newly c...

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The transcription factor, Sry-related High Mobility Group (HMG) box containing gene 9 (Sox9), plays a critical role in cartilage development by initiating chondrogenesis and preventing the subsequent maturation process called chondrocyte hypertrophy. This suppression mechanism by Sox9 on late-stage chondrogenesis partially results from the inhibition of Runt-related transcription factor 2 (Runx2), the main activator of hypertrophic chondrocyte differentiation. However, the precise mechanism by which Sox9 regulates late chondrogenesis is poorly understood.In the present study, the transcriptional repressor vertebrate homolog of Drosophila bagpipe (Bapx1) was found to be a direct target of Sox9 for repression of Runx2 expression in chondrocytes. We identified a critical Sox9 responsive region in the Bapx1 promoter via a luciferase reporter assay. Analysis by chromatin immunoprecipitation and electrophoretic mobility shift assays indicated that Sox9 physically bound to this region of the Bapx1 promoter. Consistent with the notion that Bapx1 and Sox9 act as negative regulators of chondrocyte hypertrophy by regulating Runx2 expression, transient knockdown of Sox9 or Bapx1 expression by shRNA in chondrocytes increased Runx2 expression, as well as expression of the late chondrogenesis marker, Col10a1. Furthermore, while over-expression of Sox9 decreased Runx2 and Col10a1 expressions, simultaneous transient knockdown of Bapx1 diminished that Sox9 over-expressing effect.Our findings reveal that the molecular pathway modulated by Bapx1 links two major regulators in chondrogenesis, Sox9 and Runx2, to coordinate skeletal formation.