Species-specific expression of type II TGF-β receptor isoforms by articular chondrocytes: Effect of proteoglycan depletion and aging

Recently, a new isoform of the type II transforming growth factor beta receptor (TGF-βRLII) was identified. This isoform (TGF-βRII₂) contains an insertion of 25 amino acids in the extracellular domain of the receptor. Using RT-PCR the authors demonstrated that both TGF-βRII₁ and TGF-βRII₂ are expressed by chondrocytes in murine and human articular cartilage. Bovine articular chondrocytes expressed TGF-βRII₁ mRNA but did not express detectable levels of TGF-βRII₂ mRNA, suggesting that the new isoform does not play an important role in normal bovine cartilage physiology. Because TGF-β responses seem to be age related and differential TGF-β responses have been described between normal cartilage and cartilage undergoing repair the authors studied if the relative mRNA expression between these isoforms is altered during cartilage repair and aging. No differences in the relative mRNA expression of the two isoforms of the type II TGF-β receptor could be demonstrated in murine cartilage during aging or during the repair phase after mild PG depletion indicating that it is unlikely that age-related TGF-β responses and differential TGF-β responses between normal cartilage and cartilage undergoing repair are the result of differences in the relative expression of the two TGF-βRII isoforms.