Fluorescent Nanodiamonds for Labeling and Tracking Human Cancer Stem Cells

Cells within tumor population exhibit functional and phenotypic heterogeneities, and there exists a minor population of tumor cells, termed cancer stem cells (CSCs), having the capacity of self-renewal and differentiation. These CSCs are thought to drive tumor growth, tumor recurrence, drug resistance and metastasis. Therefore, how to use markers to label and track CSCs has been an important issue in biomedical research. Although organic dyes have been commonly used for cell labeling due to their advantages of strong fluorescence emission and easiness of conjugation with biomolecules, but there are some restrictions, such as photobleaching, which prevent long-term observations. This problem can be readily overcome by using fluorescent nanodiamonds (FNDs), which have not only high biocompatibility but also excellent photostability. Here, we proposed to use the FND labeling method to investigate the cell differentiation and growth of primary mammospheres, with the results to be compared with those of the fluorescent markers, CFSE (carboxyfluorescentsuccinimidy ester) and EdU (5-ethynyl-2-deoxyuridine).
The formation of primary mammospheres is a measure of stem cell activity, and thus cannot be used to assess stem cell self-renewal. However, primary mammospheres can be harvested, digested to single cells, and passaged for the assessment of their self-renewal (secondary generation). In this study, we found that mammospheres derived from ASB-145-1R cells can be passaged to satisfy the true criteria of self-renewal. The fluorescence of FND, but not of CFSE and EdU, can be detected in the second generation of the mammospheres by using confocal microscopy and flow cytometry. Results of long-term labeling and tracking of breast CSCs show that the number of cells retaining FNDs in the mammospheres is higher than those of CFSE- and EdU-labeled cells. It is concluded that the FND labeling is a valuable and effective tool for long-term tracking of CSCs.