Testing the reliability and selectivity of different bone-cell-specific Cre- expressing mouse models for studying bone cell metabolism

Abstract

The Cre/loxP system is a tool for targeted recombination of DNA. For applying Cre recombinase-mediated genome modifications, there is a requirement for reliable, high-fidelity, and specific transgenic expression of the Cre recombinase. This study focuses on the reliability of different bone cell specific Cre models in the Cre/loxP system. In this study, DMP1-Cre transgenic mouse which has a transgene driven by DMP1 promotor that allows Cre-expression only in late stage osteoblasts and osteocytes was used. Ctsk-Cre mouse with a driven by Ctsk promoter was used so that only osteoclasts would undergo Cre-mediated recombination. E2A-Cre mouse where the Cre recombinase is driven by a global promoter E2A was also included in this study as a control line to test the Cre reporter line Ai9. Dmp1-Cre, Ctsk-Cre and E2A-Cre mice were crossed to the fluorescent Cre-reporter line—Ai9, which harbors a floxed stop codon, followed by the fluorophoremTomato, inserted into the Rosa26 locus. This construct is expected to give red fluorescence when it recombines with Cre-expressing mouse cells and no fluorescence in non-recombinant mouse cells. Double positive (Ai9+/Cre+) offspring selected by PCR were perfused, and 5mu-m thick section of bone and soft tissues were examined for red fluorescent expression. Cre positive cells were quantitated using ‘ImageJ’ software program. The DMP1-vi Cre mouse results showed significant expression in the targeted osteocytes and osteoblasts. In addition, skeletal muscle tissue also showed significant Cre- expression. Ctsk-Cre mice showed significant expression in targeted osteoclasts. But brain tissue was positive in Cre-expression. Bone-Cre mouse models are expected to express Cre recombinase only in their respective bone cells and they have been used for gene deletion studies in bone cells. However, this study has revealed that the bone cell specific Cre mouse models DMP1-Cre and Ctsk-Cre have unexpected expression in muscle and brain respectively. In order to use these models for targeted gene deletion in bone cells, further testing and studies have to be conducted.