Quantum dots, nanosized fluorescent semiconductor particles, are fast becoming a versatile tool for tracking movements of individual molecules in living systems thanks to their brightness, multiple colors, size, resistance to photobleaching, and commercial availability. Two recent papers highlight additional applications that cancer biologists could find useful.

In a paper published in Nature Medicine , a collaborative team lead by Rakesh Jain, Ph.D, and Dai Fukumura, M.D., Ph.D., both at Harvard Medical School, and Moungi Bawendi, Ph.D., of the Massachusetts Institute of Technology (MIT), detail the use of quantum dots to differentiate tumor cells from perivascular cells and the surrounding matrix and to study various processes that occur during tumor development. The researchers first prepared three different coated, cadmium-based quantum dots whose emission spectra are distinct from one another as well as from green fluorescent protein (GFP). The investigators also developed a transgenic mouse in which the perivascular cells expressed GFP. Using multi-photon fluorescence microscopy, the quantum dots were clearly distinguishable in vivo from perivascular cells expressing GFP.

In a second set of experiments, the researchers used the differently colored quantum dots to label silica nanospheres of various sizes to determine the optimal particle size for tumor penetration. This study showed clear differences in tumor uptake of 100-nanometer and 500-nanometer particles, suggesting that multicolored sets of quantum dots might find use in characterizing multiple parameters of nanoparticles being developed for drug delivery applications. In a third set of experiments, the investigators used their quantum dots to follow bone marrow-derived cells as they trafficked to tumors, a process involved in tumor-stimulated blood vessel growth. In a final note, the researchers added that the animals dosed with the coated quantum dots showed no toxic effects for up to one month. Toxicity is a concern when using cadmium-based quantum dots in vivo .

Meanwhile, a group led by Alice Ting, Ph.D., also of MIT, has shown that quantum dots can be targeted to label specific surface proteins in living cells, a technique that could prove useful for studying a variety of processes, such as how receptors are assembled on cell surfaces and how they behave after binding signaling molecules. While there are many techniques for labeling cell surface proteins in general, few are useful for labeling specific proteins in a way that does not interfere with a protein's ability to move freely within the membrane structure.

In a paper published in the Proceedings of the National Academy of Sciences , Dr. Ting's group used an enzyme from E. coli that adds a biotin molecule to a 15-amino acid sequence known as the acceptor protein (AP). This sequence can be genetically encoded at either end of a protein of interest and the resulting construct will produce AP-labeled protein that functions normally in living cultured cells. Adding the E. coli enzyme, biotin and ATP as an energy source to the cultured cells results in biotin being attached to the AP segment. Next, the researchers added quantum dots that have themselves been labeled with streptavidin, the natural ligand for biotin, to the cultured cells. Fluorescence microscopy clearly identified the resulting labeled cell surface proteins within two minutes of adding the quantum dots. Because of the inherent brightness of the quantum dots, the researchers were able to identify and track individual protein molecules as they moved within the cell membrane.

The work detailed in Nature Medicine , which was funded in part by the National Cancer Institute, appears in a paper titled “Quantum dots spectrally distinguish multiple species within the tumor milieu in vivo.” Researchers from the Northeastern University also participated in this study. This paper was posted online in advance of publication at the journal's website. An abstract is available there.
View abstract .

The work appearing in Proceedings of the National Academy of Sciences , which was funded in part by the National Institutes of Health, is detailed in a paper titled, “Targeting quantum dots to surface proteins in living cells with biotin ligase.” This paper is available free via PubMed Central.
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