SP phenotype and stem cell-specific molecule Bcrp1

Abstract

Side population (SP) cells are highly enriched for hematopoietic stem cells and are defined by their ability to efficiently efflux the fluorescent DNA-binding dye Hoechst 33342. To identify Hoechst-dye efflux molecules, we developed a retrovirus-mediated expression cloning strategy; a combination of retrovirus-mediated gene transfer and enrichment by cell sorting. In this experiment, quail fibroblast QT6 cells were infected with libraries of retrovirally expressed cDNA derived from murine Lin-SP bone marrow cells. These infected QT6 cells were selected for the SP phenotype by cell sorting. Consequently we isolated the Bcrp1 gene, also known as Abcg2. Enforced expression of the Bcrp1 cDNA directly conferred the SP phenotype to bone marrow cells and murine fibroblast cells. Bcrp1 mRNA was expressed at high levels in primitive murine hematopoietic stem cells, and was sharply downregulated with differentiation. These results show that expression of the Bcrp1 gene is an important determination of the SP phenotype. Since SP phenotype is found a common charactor of stem cells, it will be important to fully characterize the function of Bcrp1 in the stem cell differentiation and to define the underlying molecular mechanism.