Journal of Cellular Biochemistry 93:761–773 (2004) The Tyrosine Phosphatase, OST-PTP, is Expressed in Mesenchymal Progenitor Cells Early During Skeletogenesis in the Mouse Laurie A. Yunker, 1,2 Anne Undersander, 3 Jane B. Lian, 4 Gary S. Stein, 4 Cathy S. Carlson, 1,3 and Laura J. Mauro 1,2 * 1 Graduate Program in Molecular Veterinary Biosciences, University of Minnesota, St. Paul, Minnesota 55108 2 Department of Animal Science, University of Minnesota, St. Paul, Minnesota 55108 3 Department of Veterinary Diagnostic Medicine, University of Minnesota, St. Paul, Minnesota 55108 4 Department of Cell Biology and the Cancer Center, University of Massachusetts Medical School, Worcester, Massachusetts 01655 Abstract Osteotesticular protein tyrosine phosphatase (OST-PTP; OST), is a signaling molecule which catalyzes the removal of phosphates from tyrosine residues. It is known to be highly regulated in bone cells and has been shown to be important for the in vitro progression from a preosteoblast to a mature, mineralizing cell. However, the in vivo expression of this phosphatase during skeletogenesis has not been examined. Using Northern analysis and in situ hybridization (ISH), we have observed that this gene is strongly expressed early during the formation of the mouse skeleton. By 12.5 days post- coitum (dpc), expression of OST mRNA transcripts increases and is localized within the mesenchyme of craniofacial bones, ribs, limbs, and Meckel’s cartilage. Following initiation of chondrogenesis, OST mRNA becomes restricted to the perichondrium of all endochondral elements. With ossification, this gene is also expressed by cells, presumably osteo- blasts, at the chondro-osseous border and along cortical and trabecular bone surfaces. Unlike other bone markers examined such as Osterix and type II collagen, OST transcripts do not appear to be expressed by chondrocytes of epiphyseal cartilage or by non-hypertrophic or hypertrophic chondrocytes. Because the temporal expression patterns of OST and Runx2 were similar suggesting a potential interrelationship in bone regulation and function, OST expression was examined in transgenic mice lacking a functional Runx2/Cbfa1 protein (Runx2/Cbfa1 delta C (DC)) and possessing a cartilaginous skeleton. Interestingly, the OST gene was expressed with localization similar in wild-type, homozygous, and heterozygous embryos. These studies suggest that the expression of the OST gene may be important during skeletogenesis, potentially from commitment of mesenchymal cells to the ossification of new bones. Early in embryogenesis, regulation of OST expression may be independent of Runx2/Cbfa1. J. Cell. Biochem. 93: 761 – 773, 2004. ß 2004 Wiley-Liss, Inc. Key words: tyrosine phosphatase; embryogenesis; in situ hybridization; perichondrium; Runx2 The skeleton serves many functions in the vertebrate: it gives an organism the ability to move, provides support, maintains a storage site for minerals, and serves a critical role in hematopoiesis [Karaplis, 2002]. Accordingly, the specialized bone tissue which comprises the skeleton must offer rigidity, strength, and elasticity. During embryogenesis, the develop- ment of this complex tissue is accomplished through either endochondral or intramembra- neous ossification and these two types of bone formation provide the classification for the hundreds of individual bones of the skeleton [Olsen et al., 2000]. The multistep process of endochondral ossification begins with the ß 2004 Wiley-Liss, Inc. Part of this work was presented as an Abstract at the 2002 American Society for Bone and Mineral Research meeting, San Antonio, TX [Yunker et al., 2002]. Grant sponsor: National Institutes of Health (to LJM, CSC, GSS, JBL); Grant numbers: AR44226, RR14099, P0148818; Grant sponsor: USDA Minnesota Agricultural Experiment Station (to LJM); Grant number: MIN-16-016. *Correspondence to: Laura J. Mauro, PhD, University of Minnesota, 495 Animal Science/Veterinary Medicine Bldg, 1988 Fitch Ave, St. Paul, MN 55108. E-mail: mauro002@umn.edu Received 30 April 2004; Accepted 3 May 2004 DOI 10.1002/jcb.20183