Uptake and distribution of carboxylated quantum dots in human mesenchymal stem cells: cell growing density matters

Abstract

Human mesenchymal stem cells (MSCs) have drawn much attention in the feld of regenerative medicine for their immunomodulatory and anti-infammatory efects. MSCs possess specifc tumor-oriented migration and incorporation highlighting the potential for MSCs to be used as an ideal carrier for anticancer agents. Bone marrow is the main source of MSCs for clinical applications. MSCs tracking in vivo is a critical component of the safety and efcacy evaluation of therapeutic cell products; therefore, cells must be labeled with contrast agents to enable visualization of the MSCs migration in vivo. Due to their unique properties, quantum dots (QDs) are emerging as optimal tools in long-term MSC optical imaging applications. The aim of this study was to investigate the uptake dynamics, cytotoxity, subcellular and extracellular distribution of non-targeted carboxylated quantum dots in human bone marrow MSCs at diferent cell growing densities. QDs had no negative impact on MSC viability throughout the experiment and accumulated in all observed cells efciently; however, in some MSCs QDs induced formation of lipid droplets. At low cell growing densities QDs distribute within MSCs cytoplasm already after 1 h of incubation reaching saturation after 6 h. After 24 h QDs localize mainly in the perinuclear region of the cells in endosomes. Interestingly, in more confuent culture QDs localize mostly outside MSCs. QDs abundantly mark MSC long flopodia-like structures attaching neighboring cells. At high cell density cultivation, we for the frst time demonstrated that carboxylated QDs localize in human bone marrow MSC extracellular matrix. Moreover, we observed that average photoluminescence lifetime of QDs distributed in extracellular matrix are longer than lifetimes of QDs entrapped in endocytic vesicles; thus, for the frst time showing the possibility to identify and distinguish localization of QDs in various extracellular and intracellular structures using fuorescence-lifetime imaging microscopy without additional staining assays. Carboxylated QDs can be used as nonspecifc and efective dye for staining of human bone marrow MSCs and their specifc extracellular structures. These results are promising in fundamental stem cell biology as well as in cellular therapy, anticancer drug delivery and tissue engineering.