At the annual Whitehead Symposium, attendees were treated to eye-catching images and videos captured by the latest microscopy techniques.
Pat McCaffrey
It’s been 400 years since Zacharias Janssen invented the compound microscope in the Netherlands, and scientists are still relying on this laboratory workhorse to look inside cells. But Janssen would have been surprised to see the kaleidoscopic images presented by local bioimaging luminaries at the 24th annual Whitehead Symposium, held last week at MIT. Microscopes today, with the help of all manner of fluorescent probes to light up individual proteins, enable researchers to “see” individual molecules.
Several hundred people turned out to ooh and aah over beautiful, and edifying, pictures presented by the six speakers, who covered the gamut of techniques from electron to light microscopy. They had a variety of samples under their microscopes, from frozen or fixed cells to living cells in culture dishes and even in live animals.
Xiaowei Zhuang, professor of physics at Harvard, unveiled a new imaging technique that she calls STORM (stochastic optical reconstruction microscopy), with the aim of improving the ability of light microscopy to visualize the locations of individual proteins in living cells. While light microscopy is the technique of choice because it doesn’t kill or disrupt the cells, it suffers from low resolution, compared to X-ray or electron microscopy imaging. Researchers who want to image several different cell proteins at once tag each protein with a fluorescent probe, one of the most common being green fluorescent protein. But when researchers shine a light on the tagged proteins, the proteins all light up at once. With the light signals overlapping each other, it’s difficult to clearly distinguish one protein from another.
Zhuang and her team showed that, using STORM, they could routinely resolve individual fluorescent molecules that are only 20 nanometers apart; other current techniques have only been able to resolve molecules that are about 10 times farther apart. The researchers achieved this by designing a special fluorescent probe that can be selectively turned on and off. By adjusting the intensity of the light shining on the sample, the researchers lit up only a few of the probes at a time to avoid overlapping signals. With this technique, they could, for example, distinguish between two probes just 135 base pairs apart on a long piece of immobilized DNA.
Zhuang’s group is now working on applying the technique to living cells. They must first find a way to target the fluorescent probes to specific proteins inside cells and to acquire images more quickly. (For pictures and videos from the Zhuang group, click here)
Live video
Zooming out to get a wider view, Ulrich von Andrian of the CBR Institute for Biomedical Research and Harvard Medical School showed movies he made using in-vivo microscopy to visualize the immune response in living animals. Working on mice, von Andrian pulled back a flap of skin and pointed his lens at a lymph node, an immune way station where roaming T cells first encounter foreign invaders. He showed T cells in the blood migrating into a lymph node, becoming activated, and then going back out into the circulation to fight infection. He even caught T cells in the act of killing other cells and being stopped by regulatory immune cells. (Click to see pictures and videos from the von Andrian group)
Out-of-towners sharing the podium included Karel Svoboda, who recently started a job at the newly opened Howard Hughes Medical Institute’s Janelia Farm in Ashburn, VA. He showed striking two-photon microscopy images of connections being broken and formed between brain cells in living mice after they’d had their whiskers trimmed, a well-known stimulus for rewiring neural circuits in the sensory cortex. The nervous system is notoriously difficult to image using in-vivo microscopy.
Seeing is believing?
Doug Koshland, from the Carnegie Institution of Washington, in Baltimore, MD, limped in on a broken leg to deliver some words of warning about the allure of cell imaging. “Images are incredibly powerful in providing new insights and encouraging new research dimensions,” he said. “But make sure you are open minded about interpreting the images.” Pretty pictures are both a blessing and a curse, he added, citing an example in his own field, where images of condensed chromosomes from yeast and mammals looked very different, leading researchers to think that the mechanism of condensation was not conserved between species. But Koshland subsequently showed condensation was mediated by a set of proteins conserved across species.
Asked by an audience member what image he most wanted to see, Koshland said he’d like to follow the sugar backbone of the DNA molecule along the length of the chromosome to see what kind of twists and turns it takes. With microscopy moving rapidly into smaller and smaller realms, that may be a journey he’ll get to take one day.