TGF-β/Smad3 Signals Repress Chondrocyte Hypertrophic Differentiation and Are Required for Maintaining Articular Cartilage

TLDR

Endochondral ossification proceeds from mesenchymal condensation to cartilage, which undergoes proliferation, hypertrophic differentiation, calcification, apoptosis, and bone replacement, but articular cartilage uniquely stops before terminal hypertrophic differentiation. The study investigates whether TGF‑β/Smad3 signaling suppresses terminal hypertrophic differentiation of chondrocytes and is required to preserve articular cartilage. Loss of Smad3 signaling causes osteoarthritis‑like degeneration, with articular cartilage loss, osteophyte formation, reduced proteoglycans, and increased type X collagen–positive chondrocytes, while in vitro TGF‑β1 inhibition of chondrocyte differentiation is blunted in Smad3‑deficient tissues, demonstrating that TGF‑β/Smad3 signaling is essential to repress terminal differentiation and prevent osteoarthritis.

Abstract

Endochondral ossification begins from the condensation and differentiation of mesenchymal cells into cartilage. The cartilage then goes through a program of cell proliferation, hypertrophic differentiation, calcification, apoptosis, and eventually is replaced by bone. Unlike most cartilage, articular cartilage is arrested before terminal hypertrophic differentiation. In this study, we showed that TGF-β/Smad3 signals inhibit terminal hypertrophic differentiation of chondrocyte and are essential for maintaining articular cartilage. Mutant mice homozygous for a targeted disruption of Smad3 exon 8 (Smad3ex8/ex8) developed degenerative joint disease resembling human osteoarthritis, as characterized by progressive loss of articular cartilage, formation of large osteophytes, decreased production of proteoglycans, and abnormally increased number of type X collagen–expressing chondrocytes in synovial joints. Enhanced terminal differentiation of epiphyseal growth plate chondrocytes was also observed in mutant mice shortly after weaning. In an in vitro embryonic metatarsal rudiment culture system, we found that TGF-β1 significantly inhibits chondrocyte differentiation of wild-type metatarsal rudiments. However, this inhibition is diminished in metatarsal bones isolated from Smad3ex8/ex8 mice. These data suggest that TGF-β/Smad3 signals are essential for repressing articular chondrocyte differentiation. Without these inhibition signals, chondrocytes break quiescent state and undergo abnormal terminal differentiation, ultimately leading to osteoarthritis.