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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC San Francisco Electronic Theses and Dissertations (2011)

The unique material properties of skeletal matrices are critical for musculoskeletal tissue function. For example, bone matrix material properties (BMMPs) - including elastic modulus, hardness, and fracture toughness - reflect the ability of bone matrix to resist deformation and catastrophic failure. However, the mechanisms by which these properties are established remain unclear. Many reports support a role for TGF-&beta and Smad3 in regulating the skeletal matrix. Thus, we hypothesized that Smad3 is essential for maintaining the extracellular matrix in bone, cartilage, and tendon.

Previously, our lab determined that TGF-&beta regulates BMMPs in development by signaling through its intracellular effector, Smad3. Here, we showed that BMMPs could also be regulated by pharmacologically disrupting TGF-&beta signaling. To further dissect the mechanism of BMMP regulation, we identified the downstream targets of TGF-&beta signaling. In vitro studies showed that TGF-&beta inhibits osteoblast differentiation by Smad3 repression of Runx2 function. Using nanoindentation, we found that TGF-&beta regulation of BMMPs through Smad3 is also Runx2-dependent. We further show that osteopontin, a matrix protein regulated by both TGF-&beta and Runx2, is essential for normal bone matrix material properties.

Our results establish a mechanism through which TGF-&beta regulates BMMPs to maintain healthy bone. This suggests a paradigm in which matrix material properties are established by growth factors through their regulation of lineage-specific transcription factors. Indeed, our further studies in cartilage and tendon indicate an essential role for TGF-&beta/Smad3 signaling in regulating cell lineage-specific transcription factor expression and activity as well as matrix structure and composition. We anticipate that our research will lead to the identification of novel diagnostic strategies and therapeutics for the prevention or treatment of skeletal diseases, as well as aid in the development of improved biomaterials for tissue regeneration.

    Cover page: TGF-beta/Smad3 Regulation of Bone, Cartilage, and Tendon Matrix Composition and Material Properties
    • Article
    • Peer Reviewed
    UC San Francisco Previously Published Works (2012)

    Objective

    To identify mechanisms by which Smad3 maintains articular cartilage and prevents osteoarthritis.

    Methods

    A combination of in vivo and in vitro approaches was used to test the hypothesis that Smad3 represses Runx2-inducible gene expression to prevent articular cartilage degeneration. Col2-Cre;Smad3(fl/fl) mice allowed study of the chondrocyte-intrinsic role of Smad3 independently of its role in the perichondrium or other tissues. Primary articular cartilage chondrocytes from Smad3(fl/fl) mice and ATDC5 chondroprogenitor cells were used to evaluate Smad3 and Runx2 regulation of matrix metalloproteinase 13 (MMP-13) messenger RNA (mRNA) and protein expression.

    Results

    Chondrocyte-specific reduction of Smad3 caused progressive articular cartilage degeneration due to imbalanced cartilage matrix synthesis and degradation. In addition to reduced type II collagen mRNA expression, articular cartilage from Col2-Cre;Smad3(fl/fl) mice was severely deficient in type II collagen and aggrecan protein due to excessive MMP-13-mediated proteolysis of these key cartilage matrix constituents. Normally, transforming growth factor β (TGFβ) signals through Smad3 to confer a rapid and dynamic repression of Runx2-inducible MMP-13 expression. However, we found that in the absence of Smad3, TGFβ signals through p38 and Runx2 to induce MMP-13 expression.

    Conclusion

    Our findings elucidate a mechanism by which Smad3 mutations in humans and mice cause cartilage degeneration and osteoarthritis. Specifically, Smad3 maintains the balance between cartilage matrix synthesis and degradation by inducing type II collagen expression and repressing Runx2-inducible MMP-13 expression. Selective activation of TGFβ signaling through Smad3, rather than p38, may help to restore the balance between matrix synthesis and proteolysis that is lost in osteoarthritis.
      Cover page: Chondrocyte‐intrinsic Smad3 represses Runx2‐inducible matrix metalloproteinase 13 expression to maintain articular cartilage and prevent osteoarthritis
      • Article
      • Peer Reviewed
      UC San Francisco Previously Published Works (2009)
      During development, growth factors and hormones cooperate to establish the unique sizes, shapes and material properties of individual bones. Among these, TGF-beta has been shown to developmentally regulate bone mass and bone matrix properties. However, the mechanisms that control postnatal skeletal integrity in a dynamic biological and mechanical environment are distinct from those that regulate bone development. In addition, despite advances in understanding the roles of TGF-beta signaling in osteoblasts and osteoclasts, the net effects of altered postnatal TGF-beta signaling on bone remain unclear. To examine the role of TGF-beta in the maintenance of the postnatal skeleton, we evaluated the effects of pharmacological inhibition of the TGF-beta type I receptor (TbetaRI) kinase on bone mass, architecture and material properties. Inhibition of TbetaRI function increased bone mass and multiple aspects of bone quality, including trabecular bone architecture and macro-mechanical behavior of vertebral bone. TbetaRI inhibitors achieved these effects by increasing osteoblast differentiation and bone formation, while reducing osteoclast differentiation and bone resorption. Furthermore, they induced the expression of Runx2 and EphB4, which promote osteoblast differentiation, and ephrinB2, which antagonizes osteoclast differentiation. Through these anabolic and anti-catabolic effects, TbetaRI inhibitors coordinate changes in multiple bone parameters, including bone mass, architecture, matrix mineral concentration and material properties, that collectively increase bone fracture resistance. Therefore, TbetaRI inhibitors may be effective in treating conditions of skeletal fragility.
        Cover page: Pharmacologic inhibition of the TGF-beta type I receptor kinase has anabolic and anti-catabolic effects on bone.Creative Commons 'BY' version 4.0 license
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