Factors Specifically Expressed in Osteoblasts

Summary

The subtractive screening of cDNA libraries derived from the mouse osteoblastic cell line, MC3T3-El has yielded cDNA clones functioned in osteoblast differentiation. One of them, termed OB-cadherin, whichis identicalto cadherin-11, is involvedin classic cadherinfamily which is generally identified as Ca²⁺ dependent adhesion molecule. Two isoforms of the human OB-cadherin/adherin- 11 transcripts were isolated, and one encodes a typical primary structure of cadherin and the other is a variant form encoding a different cytoplasmic domain because of alternative splicing. Results of immunohistochemical analyses using fetal calvaria indicated that OB-cadherin/adherin-11 is expressed on osteoblasts and osteocytes. The high expression of the variant form was detected in osteosarcoma by Northern blot and RT-PCR analyses. The anomalous expression of the OB-cadherin/adherin-11 molecules, the variant form as well as the secretory form found at protein level, modulates cell adhesive properties.

Another gene OSF-2 included in the fasciclin I family is specifically expressed in periosteum in adult mice found by both RNA in-situ hybridization and immunohistochemistry, and termed periostin. Periostin is a secretedmolecule as 90 kD, and the purified protein of periostin from the supernatant of MC3T3-El cells functioned in cell attachment. The transcription of periostin gene was upregulated by TGF-b and interleukin 6. The recent development of zebra fish in biotechnology led us toclone up a full length of the zebra periostin gene. The homology between mouse and zebra periostin is high (about 65%), suggesting the common function in bone formation. RNA in-situ hybridization revealed that zebra periostin was expressed in somites and fins.