| The use of quantum dots has been proven effective for detection of single molecules. Integrating quantum dots with single particle tracking technique, the integrin dynamics in the cell membrane was elucidated during human mesenchymal stem cell (hMSC) differentiation into the osteogenic lineage. There is a major shift in the integrin lateral mobility from the undifferentiated hMSC to different stages of hMSC osteodifferentiation. The integrins in the undifferentiated hMSCs were found to be essentially immobile or diffuse slowly. Following a 3-day biochemical differentiation, the integrin diffusion became no longer distinguishable with that measured on the human osteoblasts. A longer osteogenic culture did not change this shift. A quantitative model was applied to analyze the integrin diffusion data. The major immobility constraint was found to be imposed by the direct interaction between integrins and microfilaments. A non-invasive electrical stimulus was used to manipulate the integrin diffusion. Apparently, the electrical stimulus caused the integrins to diffuse more slowly. This phenomenon suggested the integrins may be recruited into forming addition focal adhesions in response to the electrical stimulus. Finally, when the membrane mechanical properties of hMSCs were studied with laser optical tweezers technique, much longer tethers were extracted from the hMSC plasma membrane than those from fully differentiated osteoblasts, suggesting that different membrane mechanics may be attributed to the differential integrin dynamics. In osteoblasts, the membrane is tightly bound to the cytoskeleton, and integrins are confined in the domains that are defined by the microfilament network. In hMSCs, however, the mechanical properties of the cell membrane can be attributed to its weak attachment to the cytoskeleton (e.g., actins), and slow integrin diffusion or lateral immobilization are likely due to direct association of integrins with microfilaments. It is plausible that the cellular mechanics is largely determined by the association between the membrane and cytoskeleton, and not directly influenced by distribution or redistribution of the cell surface molecules. The cell surface receptor dynamics can be used not only as a biophysical marker for stem cell differentiation, but also potentially influence the differentiation at the early stages of stem cell differentiation. |
