The 3.6 kb DNA fragment from the rat Col1a1 gene promoter drives the expression of genes in both osteoblast and osteoclast lineage cells

The type I collagen promoter has been used to develop transgenic constructs that are able to mark different stages of osteoblastic differentiation. The pOBCo13.6 promoter is active in early mesenchymal progenitors, including preosteoblasts and osteoblasts, while the pOBCo12.3 promoter is more restricted, showing expression in mature osteoblasts and osteocytes. Transgenic mouse lines have been created that express various GFP reporters under the control of both promoters. These transgenic mice permit the tracking of osteoblastic lineage progression in vitro. They also represent a system to test lineage progression in vivo after the transplantation of progenitors. A parabiosis system was used in which pOBCo13.6GFP transgenic mice were surgically joined with mice bearing a Co12.3 Delta TK transgene. The Co12.3 Delta TK transgenic mouse bears a herpes thymidine kinase gene driven by the pOBCo12.3 promoter, and upon treatment with gancyclovir (GCV) displays extensive destruction of the bone lining cells. After a common circulation was established, parabiotic pairs were treated with GCV for 15 days. Histological analysis of their bones showed the clear presence of GFP positive cells in the Co12.3 Delta TK parabionts, around trabecular bone and on the endosteal and periosteal surfaces. Stromal cell cultures from these Co12.3 Delta TK parabionts did not display mineralized colonies coexpressing GFP. In contrast, scattered GFP positive clusters that contained large cells with morphology similar to osteoclast like cells (OCLs) were observed. These cells were also TRAP positive. They were readily detected in Co12.3 Delta TK mice treated with GCV and transplanted with purified hematopoietic stem cells (HSCs) isolated from pOBCo13.6GFP mice. OCLs were also generated in vitro from osteoclast progenitor cells obtained from pOBCo13.6GFP mice that were defined by the B220(-) CD3(-) CDIIb(-) c-fms(+) phenotype. Molecular analysis showed that OCLs did not express type I collagen indicating that the Co13.6 promoter contains elements that are active during osteoclastogenesis and are not strictly related to collagen transcription. In summary, we demonstrate that pOBCo13.6 unexpectedly directs the expression of transgenes in the osteoclast lineage and this effect must be considered when utilizing this promoter to study of mesenchymal progenitor cells. (c) 2006 Elsevier Inc. All rights reserved.