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.
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.