[Development of fluorescent labeling methods for stem cells].

For successful development of cell-based therapy, both the disposition and differentiation of transplanted cells are directly related to therapeutic effects. In vivo imaging is an attractive tool to obtain real-time information on the disposition of target cells. In various types of imaging methods such as positron emission tomography(PET) and magnetic resonance imaging(MRI), fluorescence imaging is suitable for visualizing the disposition of cells because it can visualize single cells both in vitro and in vivo. For the trafficking of stem cells after transplantation, it is necessary to label living cells for long time periods without disturbing the function or differentiation of the labeled cells. Recently, we have developed quantum dots modified with polyamidoamine(PAMAM) dendrimers. These can more rapidly escape from endosomes and sustain their fluorescence intensity compared with unmodified quantum dots in primary cultured mesenchymal stem cells (MSCs). Fluorescence intensity was also sustained after intravenous injection of MSCs labeled with PAMAM dendrimer-modified quantum dots. To study the dynamics of MSCs in vivo, we constructed a piggyBack transposon vector that can integrate the target gene into the genome in mammalian cells, and established primary MSCs with long-term expression of EmGFP. In addition, we also developed a suction device stabilizing tissue for in vivo real time imaging. In this section, I present our recent findings on long-term fluorescent labeling of MSCs and in vivo visualizing of cell dynamics in a living mouse.