Quantum dot imaging could benefit embryonic stem cell therapy
Oct. 15, 2007
(Nanowerk Spotlight) A quantum dot (QD), also called a nanocrystal,
is a semiconductor nanostructure that can be as small as 2 to 10 nm.
The usefulness of quantum dots comes from their peak emission
frequency's extreme sensitivity - quantum mechanical in nature - to
both the dot's size and composition. QDs have been touted as possible
replacements for organic dyes in the imaging of biological systems,
due to their excellent fluorescent properties, good chemical
stability, broad excitation ranges and high photobleaching
thresholds. By contrast, conventional organic dyes cannot be easily
synthesized to emit different colors and have narrow excitation
spectra and broad emission spectra that often cross into the red
wavelengths, making it difficult to use these dyes for multiplexing.
QDs hold increasing potential for cellular imaging both in vitro and
in vivo. Researchers have now used QDs for in vivo imaging of
embryonic stem cells in mice. This opens up the possibility of using
QDs for fast and accurate imaging applications in stem cell therapy.
Stem cell therapy is the process of injecting stem cells into an
organism in the hope that they will differentiate and replace damaged
tissue or grow new organs. This technology holds great promise for
treatment of a wide range of intractable conditions such as
Parkinson's disease, diabetes, or degenerative joint diseases. There
are two types of cells used in stem cell therapy, adult stem cells
and embryonic stem (ES) cells. ES cells are the ultimate source for
use in cell-based therapy because they posses a virtually unlimited
capacity for self-renewal and because they possess the ability to
differentiate into all other cell types found in the body, from brain
cell to toe nail. For stem cell therapy, it is important to develop
methods to monitor cell survival and location after transplantation.
"In stem cell therapy, monitoring of cell survival and location after
transplantation is important for determining their efficacy" Dr.
Joseph C. Wu explains to Nanowerk. "Because the absorption and
scattering of light in biological tissue can be considerable, any
optical signal transmitted from deep tissues to the surface tends to
diminish in strength. With QDs' many advantages over traditional
organic dyes, QDs may provide an excellent tool for imaging stem cell
therapy."
Fluorescent images of embryonic mouse stem cells labeled with QDs on
day 1 post labeling. (Copyright: BioMed Central)
Wu, an Assistant Professor of Medicine & Radiology at Stanford
University School of Medicine, together with collaborators from the
University's Molecular Imaging Program, conducted a study in which
the scientists used the peptide-based reagent QTracker to label mouse
ES cells with QDs and evaluate the utility of QDs for imaging stem
cell therapy." The results have been published in a recent free
access paper in the July issue of BMC Biotechnology ("Quantum dot
imaging for embryonic stem cells")
Wu and his colleagues successfully labeled murine embryonic stem
cells with six different quantum dots and demonstrated the ES cell
viability, proliferation, and differentiation were not adversely
affected by QDs. They showed that QD 525, QD 565, QD605, QD 655, QD
705, and QD 800 labeled ES cells can be detected in vivo using a
single excitation wavelength (465 nm). This finding makes QDs an
attractive choice for regenerative therapy.
1 million ES cells labeled with QD 525, 565, 605, 655, 705, and 800
were subcutaneously injected on the back of the athymic nude mice
right after labeling and the image was taken with a single excitation
light source right after injection. (Copyright: BioMed Central)
"We also successfully imaged labeled ES cells with good contrast with
one single excitation wavelength in vivo" says Wu. "This versatility
makes them good candidates for tumor targeting, lymph node and
vascular mapping, and cell trafficking in small animal imaging."
Toxicity of QDs of course is a key factor in determining whether it
will be a feasible probe for both cellular and clinical use. The
Stanford scientists carefully examined QDs' effect on ES cells and
found that QD labeling had no detectable effect on ES cell growth.
Next they tested its effect on cellular development and
differentiation and again found no effects of the labeling.
Wu points out that another advantage of QDs is their ability to do
multiplex imaging of different QDs at the same time. "However" he
says, "in our study, ES cells labeled with different QDs were only
capable of being imaged up to day 2 after subcutaneous implantation.
A likely cause for this could be the loss of signal due to rapid cell
division. Another possible cause could be serum instability of the
QDs."
While this study appears to be the first successful demonstration of
in vivo multiplex imaging of mouse ES cells labeled QDs, the use of
QDs in stem cells is only beginning to be explored.
Due to their many advantages over conventional organic dyes, QDs
serve as good candidates to monitor cell survival and location after
transplantation in stem cell therapy However, the poor retention of
QDs in targets cells may be a problem for long-term tracking. Wu says
that upon further improvements – such as near-infrared QDs, better
serum stability, and improved cell retention – QDs will have greater
potential for tracking of stem cells within deep tissues.
By Michael Berger, Copyright 2007 Nanowerk LLC
http://www.nanowerk
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StemCells subscribers may also be interested in these sites:
Children's Neurobiological Solutions
http://www.CNSfoundation.org/
Cord Blood Registry
http://www.CordBlood.com/at.cgi?a=150123
The CNS Healing Group
http://groups.yahoo.com/group/CNS_Healing
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