Tissue- and bone cell-specific expression of bone sialoprotein is directed by a 9.0 kb promoter in transgenic mice

Bone sialoprotein (BSP) is a phosphorylated glycoprotein that is expressed almost exclusively in mineralizing connective tissues. In bone, expression of BSP correlates with the differentiation of osteoblasts and the onset of mineralization. To determine how the tissue- and differentiation-specific transcription of BSP is regulated, various lengths of promoter sequence were ligated to a luciferase reporter and stably transfected into a rat stromal bone marrow cell line, RBMC-D8 and undifferentiated C3H10T1/2 cells. Luciferase transcription of reporter constructs including 5.4 kb (mBSP5.4Luc) and 9.0 kb (mBSP9.0Luc) of the BSP promoter was strongly up-regulated in parallel with endogenous BSP mRNA in differentiating SBMCs, but not in C3H10T1/2 cells. In contrast, 0.1 kb and 1.4 kb BSP promoter constructs did not show selective expression. To determine tissue-specific expression in vivo, transgenic mice expressing reporter constructs for the 9.0 kb promoter and a 4.8 kb promoter lacking two upstream Cbfa1/Runx2 elements (mBSP9.0Luc and mBSP4.8Luc, respectively) were generated. Analysis of various tissues collected from 1-, 4-, 7-, 14-, and 42-day-old mice revealed extremely high levels of luciferase activity in calvaria, mandible, and tibia of the mBSP9.0Luc mice. In contrast, soft tissues showed negligible luciferase expression. Mice harboring the 4.8 kb transgene also showed selective luciferase expression but displayed a significantly lower activity in mineralized tissues. Northern hybridization of endogenous BSP mRNA and immunostaining of BSP in mBSP9.0Luc mice showed a temporo-spatial expression pattern consistent with the luciferase activity. These results indicate that regulatory elements within the 9.0 kb region of the promoter are required for strong, tissue- and differentiation-specific expression of BSP.