Induction of an osteoarthritis-like phenotype and degradation of phosphorylated Smad3 by Smurf2 in transgenic mice.

Arthritis is the number one cause of disability in the US (1). It has been projected that by 2020, 59.4 million Americans will be affected (18.2%) (2). Osteoarthritis (OA), the most common form of arthritis, is a noninflammatory degenerative joint disease characterized by articular chondrocyte dysfunction, articular cartilage degradation, osteophyte formation, and subchondral sclerosis (3). There is limited understanding of the seminal molecular and/or cellular events in articular cartilage degeneration, and there are few therapeutic options for OA patients. Thus, understanding these events would have a tremendous impact on the development of more-effective therapeutic paradigms. Biochemical, genetic, and mechanical factors contribute to OA progression (4). In the early stages of the disease, the production of catabolic cytokines by the synovium induces transient articular chondrocyte proliferation and increased matrix synthesis (type II collagen, aggrecan) in an attempt to initiate repair (5,6). However, the chronic production of these cytokines, including interleukin-1 (IL-1), tumor necrosis factor α (TNFα), IL-17, and IL-18, as well as prostaglandin E2, leads to the enhanced synthesis of collagenases (matrix metalloproteinase 1 [MMP-1], MMP-8, MMP-9, and MMP-13) and aggrecanases (ADAMTS4 and ADAMTS5), which drive matrix degradation and progressively erode the articular surface (4,6-10). Coupled with the progressive degradation of the matrix is the dysregulation of articular chondrocytes, denoted by the inappropriate expression of genetic and morphologic markers of endochondral ossification, including type X collagen and alkaline phosphatase, and increased apoptosis (5,7,11,12). It has been hypothesized that this altered articular chondrocyte phenotype is an initiating step in the disease process. Since inappropriate maturation of articular chondrocytes may be a central mechanism in the process of articular cartilage degeneration, it is essential to investigate the impact of signaling pathways that regulate chondrogenesis and chondrocyte maturation. For example, mice with reduced transforming growth factor β (TGFβ) signaling due to overexpression of a dominant-negative form of the TGFβ receptor type II (TGFβRII) have accelerated chondrocyte maturation in the growth plate and articular chondrocyte hypertrophy coupled with an OA-like form of cartilage erosion (13). A similar growth plate and articular cartilage phenotype is present in mice lacking Smad3, a critical TGFβ signaling molecule (14). Since the TGFβ pathway inhibits chondrocyte maturation (15-17), loss of TGFβ signaling in these mice results in inappropriate articular chondrocyte maturational progression, leading to defects in the cartilage matrix. Thus, loss of TGFβ signaling in chondrocytes is likely a contributing mechanism in the progression of OA. While these studies indicate that loss of TGFβ signaling can lead to OA, it is not clear how this reduced signaling situation arises during normal pathogenesis. Given the role of the E3 ubiquitin ligase Smurf2 as an endogenous negative regulator of TGFβ signaling, we hypothesized that up-regulation of Smurf2 reduces TGFβ signaling and induces articular chondrocyte hypertrophy and cartilage degradation. In this study, we found that Smurf2 is up-regulated in cartilage from humans with knee OA. Consistent with this, we also found that overexpression of Smurf2 in mouse cartilage leads to the development of a severe cartilage arthropathy characterized by hypertrophy of articular chondrocytes, progressive degradation of the articular cartilage, up-regulation of type X collagen and MMP-13 by articular chondrocytes, and development of osteophytes and subchondral sclerosis. This phenotype was associated with inhibition of TGFβ signaling and induction of phosphorylated Smad3 (pSmad3) degradation. These findings, which suggest that inappropriate articular chondrocyte maturation is a seminal event that precedes joint degeneration during disease, implicate Smurf2 as a candidate etiologic mechanism in the development of OA.