Migration mechanism of mesenchymal stem cells studied by QD/NSOM.

The migration of mesenchymal stem cells (MSCs) plays a key role in tumor-targeted delivery vehicles and tumor-related stroma formation. However, there so far has been no report on the distribution of cell surface molecules during the VEGF-induced migration of MSCs. Here, we have utilized near-field scanning optical microscopy (NSOM) combined with fluorescent quantum dot (QD)-based nano-technology to capture the functional relationship between CD44 and CD29 adhesion molecules on MSCs and the effect of their spatial rearrangements. Before VEGF-induced migration of MSCs, both CD44 and CD29 formed 200-220 nm nano-domains respectively, with little co-localization between the two types of domains. Surprisingly, the size of the CD44 nano-domain rapidly increased in size to 295 nm and apparently larger aggregates were formed following MSC treatment with VEGF for 10 min, while the area of co-localization increased to 0.327 µm2. Compared with CD44, CD29 was activated obviously later, for the fact that CD29 aggregation didn't appear until 30 min after VEGF treatment. Consistently, its co-localization area increased to 0.917 µm2. The CD44 and CD29 nano-domains further aggregated into larger nano-domains or even formed micro-domains on the membrane of activated MSCs. The aggregation and co-localization of these molecules promoted FAK formation and cytoskeleton rearrangement. All of the above changes induced by VEGF contributed to MSC migration. Taken together, our data of NSOM-based dual color fluorescent imaging demonstrated for the first time that CD44, together with CD29, involved in VEGF-induced migration of MSCs through the interaction between CD44 and its co-receptor of VEGFR-2.