Novel Method to Study Mouse Bone Marrow Endothelial Cells in Vivo and in Vitro

Abstract 617

Self-renewal, differentiation, and proliferation of hematopoietic stem cells (HSCs) and leukemia stem cells (LSCs) are maintained in a complex microenvironment of the adult bone marrow (BM). BM endothelial cells (ECs) have been proposed to be a key component of HSC and LSC niche. However, in contrast to the well-developed culture system of human ECs, current work of murine BM endothelial cells is hindered by a lack of mouse bone marrow endothelial cell primary culture and suitable assay methods to clearly define murine BMEC functionality in vivo and in vitro, which limits genetic and mechanistic studies by using mouse models. To establish an in vivo approach to study the EC function in adult mice, a strain of Tie2-CreER transgenic mice was generated to allow conditional and inducible manipulation of BMECs by Cre recombinase expression under the Tie2 promoter. In vivo lineage tracing was achieved in a Tie2-CreER/TD-tomato or -EGFP reporter mouse strain. Upon a four day Tamoxifen injection regimen, TD-tomato or EGFP reporter was readily visualized in bone marrow vasculature that colocalizes with CD31+ ECs as determined by immunostaining. FACS analysis of Tie2-CreER/EGFP reporter mice showed that the EGFP+ cells in the BM were exclusively in the CD45- VEGFR2+ and CD31+ cell fraction, with no EGFP+ cells being detectable in the CD45+ hematopoietic lineages or osteoblast/stroma cell fractions, suggesting that the Tie2-driven CreER expression is limited to the endothelial lineage in the adult BM. Next, we developed an in vitro method to culture and assay the mouse BMECs functionally. An in vitro selection process allowed us to establish a primary BM cell culture condition that permitted functional expansion and maintenance of mouse BMECs in long-term tissue culture, yielding homogenous CD45- cells expressing endothelial markers CD31, CD34 and VEGFR2. These cells formed capillary-like structures in 2-demensional and 3-demensional tubes/capillaries, and showed TD-tomato reporter color when derived from the Tamoxifen induced Tie2-CreER/TD-tomato mouse BM. They showed expected adhesion and migration activities and morphology of ECs. Lineage chasing assays using isolated CD45+ and CD45- BM cells from the Tie2-CreER/Td-tomato mice demonstrated that the BMECs in our culture system, bearing the Tie2-promoter driven TD-tomato color and CD31+ marker, were exclusively derived from CD45- non-hematopoietic lineage. Taken together, we have established a faithful assay method for studying murine BM EC functions in vivo and in vitro, allowing the tracking and genetic manipulation of adult BM ECs in mice and in culture. The method can be useful for delineating molecular and cellular mechanisms of BMEC regulation and EC-mediated BM niche function, and may have value in testing anti-angiogenic activities of anticancer drugs in animal models.