If you're watching the complex processes in a living cell, it is easy to miss
something important-especially if you are watching changes that take a long
time to unfold and require high-spatial-resolution imaging. But new research*
makes it possible to scrutinize activities that occur over hours or even days
inside cells, potentially solving many of the mysteries associated with molecular-scale
events occurring in these tiny living things.
A joint research team, working at the National
Institute of Standards and Technology (NIST) and the National Institute
of Allergy and Infectious Diseases (NIAID), has discovered a method of using
nanoparticles to illuminate the cellular interior to reveal these slow processes.
Nanoparticles, thousands of times smaller than a cell, have a variety of applications.
One type of nanoparticle called a quantum dot glows when exposed to light. These
semiconductor particles can be coated with organic materials, which are tailored
to be attracted to specific proteins within the part of a cell a scientist wishes
"Quantum dots last longer than many organic dyes and fluorescent proteins
that we previously used to illuminate the interiors of cells," says biophysicist
Jeeseong Hwang, who led the team on the NIST side. "They also have the
advantage of monitoring changes in cellular processes while most high-resolution
techniques like electron microscopy only provide images of cellular processes
frozen at one moment. Using quantum dots, we can now elucidate cellular processes
involving the dynamic motions of proteins."
For their recent study, the team focused primarily on characterizing quantum
dot properties, contrasting them with other imaging techniques. In one example,
they employed quantum dots designed to target a specific type of human red blood
cell protein that forms part of a network structure in the cell's inner
membrane. When these proteins cluster together in a healthy cell, the network
provides mechanical flexibility to the cell so it can squeeze through narrow
capillaries and other tight spaces. But when the cell gets infected with the
malaria parasite, the structure of the network protein changes.
"Because the clustering mechanism is not well understood, we decided
to examine it with the dots," says NIAID biophysist Fuyuki Tokumasu. "We
thought if we could develop a technique to visualize the clustering, we could
learn something about the progress of a malaria infection, which has several
distinct developmental stages."
The team's efforts revealed that as the membrane proteins bunch up, the
quantum dots attached to them are induced to cluster themselves and glow more
brightly, permitting scientists to watch as the clustering of proteins progresses.
More broadly, the team found that when quantum dots attach themselves to other
nanomaterials, the dots' optical properties change in unique ways in each
case. They also found evidence that quantum dot optical properties are altered
as the nanoscale environment changes, offering greater possibility of using
quantum dots to sense the local biochemical environment inside cells.
"Some concerns remain over toxicity and other properties," Hwang
says, "but altogether, our findings indicate that quantum dots could be
a valuable tool to investigate dynamic cellular processes."
* H. Kang, F. Tokumasu, M. Clarke, Z. Zhou, J. Tang, T. Nguyen and J. Hwang.
Probing dynamic fluorescence properties of single and clustered quantum dots
towards quantitative biomedical imaging of cells. WIREs Nanomedicine and Nanobiotechnology.
Early view online at http://wires.wiley.com/WileyCDA/WiresIssue/wisId-WNAN.html?pageType=early.