TGF-beta regulates Ankexpression
- Lillian Shum1
© Biomed Central Ltd 2002
Received: 12 June 2002
Accepted: 11 July 2002
Published: 11 July 2002
The Ank gene encodes a multipass transmembrane protein that is thought to function as a transporter that regulates the balance of inorganic pyrophosphate between intracellular and extracellular chondrocyte compartments (see additional information). Mice with progressive ankylosis carry a G?T substitution in the Ank gene, which results in a truncated and defective protein. These animals develop arthritis-like disease characterized by articular cartilage erosion, ectopic joint calcification, osteophyte formation and joint immobility and fusion. Mutations in the human homolog are associated with autosomal dominant craniometaphyseal dysplasia. The transforming growth factor-? (TGF-?) superfamily of growth and differentiation factors includes the bone morphogenetic proteins (BMP). Members of this superfamily have diverse and pleiotrophic functions. Interestingly, a genetically engineered mouse model expressing a dominant-negative form of TGF-? receptor in skeletal tissues exhibits an osteoarthritis-like phenotype. Furthermore, TGF-? has been shown to regulate inorganic pyrophosphate levels in vitro. Therefore, this investigation tested the hypothesis that Ank is expressed in a restrictive pattern during cartilage hypertrophic maturation and is regulated by TGF-? signaling.
In both E15.5 and E17.5 mouse embryos, Ank mRNA expression was localized to sites of endochondral and intramembraneous ossification. In particular, Ank was detected in hypertrophic chondrocytes, and inner lining cells of the perichondrium and periosteum at the bone collar. During postnatal development, Ank was detected in hypertrophic chondrocytes at the transition zone between the articular cartilage and the secondary ossification center. Ank expression overlapped in part with that of type X collagen. Ank was also detected in the perichondrium and periosteum at the metaphysis. Using an ex vivo culture system for E15.5 mouse metatarsal, 10ng/ml TGF-?1 promoted Ank expression by 3.64-fold after 5 days in culture, as determined by RT-PCR. This data was corroborated by morphological observations of Ank expression in cultured metatarsal. In addition to the in vivo pattern of expression, Ank expression was observed in prehypertrophic chondrocytes in TGF-?-treated cultures, suggesting that TGF-? can induced ectopic Ank expression. Taken together, these results suggest that Ank is expressed, and may be significantly involved, in cartilage hypertrophy. Ank expression is regulated by TGF-?.
The most significant finding was the demonstration that TGF-? regulates Ank expression. This regulation includes both the promotion of Ank mRNA levels at sites of normal Ank expression, and also the induction of ectopic Ank expression. Therefore, this finding supports a mechanistic relationship among TGF-? signaling, Ank expression pattern, and arthritis and other joint calcification disorders. The Ank expression pattern during embryonic and early postnatal development is consistent with the phenotype of diseases and disorders associated with defective Ank protein. Although it may be possible to regulate pyrophosphate levels by intervention with small molecules, this study suggests that TGF-? may be an earlier and additional therapeutic target.
Polymerase chain reaction (PCR) cloning, sequencing, in situ hybridization, metatarsal culture, reverse-transcription-polymerase chain reaction (RT-PCR)
Ryan LM: The%20ank%20gene%20story Arthritis Res 2001, 3(2):77-79