Osteoarthritis is a leading cause of disability in the industrialized world but little is known about mechanisms of cartilage destruction associated with osteoarthritis. Recent advances in the genetic manipulation of mice have lead to new animal models and new concepts that are relevant to understanding osteoarthritis in humans. The long-term objective of my laboratory is to understand the factors that mediate the development, persistence, and repair of articular cartilage and to identify specific targets for prevention and treatment strategies for osteoarthritis. TGF-? is a multifunctional peptide that has been shown to regulate cellular differentiation and tissue-specific gene expression. Previously, we generated transgenic mice that express a dominant-negative mutation of the TGF-? type II receptor (Tgfbr2) in articular cartilage. Altered responsiveness to TGF-? resulted in a progressive skeletal disease that resembled osteoarthritis in humans. We recently identified several down-stream targets of TGF-? that regulate post-translational processing of the major extracellular matrix proteins in articular cartilage. One in particular, 3-Prime-Phoshoadenosine 5-Prime-Phosphosulfate Synthase 2 (Papss2), has been associated with Spondyloepimetaphyseal Dysplasias in humans and is required for proper sulfation of proteoglycans in cartilage. In addition to TGF-?, the transcription factor Sox9 is also associated with the maintenance of mature articular cartilage. Our preliminary studies indicate that TGF-? enhances the level of Sox9 protein in chondrocytes independently of changes in mRNA. The preliminary data suggest that treatment with TGF-? results in sumoylation of Sox9. Sumoylation has been shown to regulate protein stability, activity and cellular localization. We hypothesize that TGF-? maintains the differentiated chondrocyte phenotype in permanent cartilages, like articular cartilage, by regulating Sox9 levels and activity via protein sumoylation. We propose to test this model with the following specific aims: 1a) to determine which sites on Sox9 are sumoylated in response to TGF-? and determine the role of sumoylation in TGF-?-mediated Sox9 levels, localization, and activity;1b) to determine the mechanism of TGF-?-mediated sumoylation of Sox9;2) to determine the mechanism of TGF-? mediated expression of Papss2 and 3) to determine if Papss2 activity can alleviate cartilage degeneration when TGF-? signaling is disrupted and determine if activation of TGF-?'s chondroprotective signals can restore biochemical and biomechanical properties to OA cartilage. These studies will identify mechanisms of chondroprotection that can be used as targets for therapies in osteoarthritis.
Osteoarthritis (OA) is the most common form of arthritis and the primary cause of disability in the US. We propose TGF- acting through Sox9 acts as a chondroprotective factor by regulating the expression of enzymes that globally regulate the biochemical properties of the extracellular matrix. These studies will enhance our current knowledge of how cartilage is maintained and provide new therapeutic targets for cartilage regeneration in OA.