Biophysics, Cell Biology
Dr. Zhuang is also a professor of chemistry and chemical biology and a professor of physics at Harvard University.
Single-Molecule and Superresolution Imaging of Biomolecular and Cellular Processes
Xiaowei Zhuang's first physics lessons began when she was 6 years old. Her father, a physics professor, taught her that if you put a block on the table, it doesn't fall to the floor because the table acts as a supporting force. Without the table, gravity makes the block fall. When her dad asked whether she could imagine anything else that exerts force on the block, Zhuang glanced around the room and replied, "Maybe the air does." Her dad was impressed.
Those early experiences nurtured Zhuang's passion for physics. Wanting to follow in the footsteps of her professor parents, she pursued a doctoral degree in physics from the University of California, Berkeley. Today, Zhuang is well known for using her physics knowledge to invent tools for biological research—in particular, techniques that help visualize biomolecular processes that cannot be seen with the naked eye. She is using these tools to answer intriguing questions in cell biology and neurobiology.
Zhuang is passionate about seeing individual molecular events unfold within living systems, but she wasn't always so interested in biology. Early in her research career, she thought of biology as being rather “messy,” unlike physics. That's partly because cells are packed with many different molecules, and it can be hard to see how they work in concert to keep the cell alive.
Biology's unknowns eventually won Zhuang over. As a postdoctoral researcher at Stanford University in the lab of Nobel laureate Steven Chu, she and colleagues explored single-molecule fluorescence tools—in particular, fluorescence resonance energy transfer (FRET)—to study processes such as protein and RNA folding.
Since joining Harvard University as a professor, Zhuang has continued to develop FRET to study more complex biomolecular assemblies, in particular the interactions between proteins and nucleic acids involved in gene replication and expression. In the past few years, Zhuang and her group have used FRET to study HIV reverse transcriptase—a viral enzyme that helps HIV copy its genetic information inside the host cell—and telomerase—a specialized cellular reverse transcriptase that extends the ends of linear chromosomes and gives them stability. More recently, the group has studied chromatin remodeling, the process that changes DNA packaging in the nucleus.
Unlike most scientists, Zhuang studies these processes by looking at molecules one at a time. That lets the group discover molecular intermediates, dynamics, and heterogeneities that cannot be seen by any other means. "If you study the average of many molecules, these important individualities often get lost," she says.
Ultimately, Zhuang would like to see biomolecular processes unfolding in their physiological context—namely, in cells. Along this line, her lab has developed fluorescence imaging methods that track viruses in live cells, allowing the group to follow the fate of individual virus particles, such as influenza, and to dissect the infection pathway.
However, live cells are packed with nanometer-sized molecules, such as proteins, DNA, and RNA. Traditional light microscopes, which are great for observing living cells, lack the resolving power to image the assembly and network of these molecules in fine detail. To overcome this challenge, Zhuang's group invented STORM (stochastic optical reconstruction microscopy). Dubbed a "super-resolution" imaging technique, STORM allows researchers to resolve 20-nanometer objects—a substantial improvement over the 200-nanometer resolution limit imposed by conventional light microscopy.
Spurring development of STORM was the Zhuang lab’s serendipitous discovery of a class of fluorescent molecules that can switch on and off and the group’s extensive experience in single-molecule imaging. In STORM, these so-called photoswitchable molecules are used to illuminate a small number of molecules throughout a specimen in any one snapshot. The spatially separated images of individual molecules allow their positions to be precisely determined. A fast camera is used to take many such snapshots, allowing the positions of numerous molecules to be determined, and software adds them together to compile the high-resolution photo. Since they introduced this technology in 2006, the group has further developed multicolor, three-dimensional, and live-cell STORM.
Using STORM to image live cells, Zhuang's group can see objects with a resolution of a few tens of nanometers. "[But] that's still a moving target," Zhuang says, adding that she will not be satisfied until she can achieve a resolution comparable to the size of a molecule (several nanometers) or better.
STORM has taken Zhuang's group to new areas of biology. For instance, her group has used STORM to study the molecular architecture of connections between neurons (synapses) in collaboration with other neuroscientists. Synapses are the fundamental functional units in the brain where chemical signals important for cognition are sent and received, but under a conventional light microscope, these connections appear as "unresolved" blobs. "You need to know the molecular organizations and interactions inside those tiny structures in order to understand their functional mechanisms," Zhuang says. "STORM is an ideal approach for this purpose."
Zhuang is also leading another collaborative effort to develop a STORM-based imaging platform for constructing a comprehensive wiring diagram of the mouse brain. A map of wires and connections will allow researchers to form new hypotheses about how the mind works. "This kind of wiring diagram of the brain really requires super-resolution, and we hope that the approach we developed can help with this effort."