Imaging technique monitors cancer cell proliferation
Process will enable better screening of cancer drugs

By CZERNE M. REID

A team of cell biologists at the School of Medicine has developed a new imaging technique using biosensors that precisely monitor the timing of cell division.

Researchers tested the technique by observing and measuring the slowdown of cell division associated with an anti-cancer drug. They believe the discovery may allow them to screen for many more anti-cancer compounds in the future.

Tissues and organs form and grow through a highly regulated process of cell division known as mitosis. Normally, cells stop dividing once they start performing specialized functions. If the process is incorrectly regulated, however, cells divide too fast or too slowly. Accelerated cell division can result in cancers that proliferate rapidly unless anti-cancer agents intervene.

To measure cell division timing, the researchers incorporated fluorescent proteins, called biosensors, into the cell nuclei. When used with a specialized microscopy technique called total internal reflection fluorescence, the biosensor glows when the nuclear membrane breaks down, passes through the surrounding cellular material and is released into the cell membrane.

An imaging technique discovered at the medical school enables researchers to use biosensors to quantify the timing as a cell divides and proliferates. The finding will improve the efficiency of testing various compounds for anti-cancer properties. Photo: Courtesy of Joshua Jones

When genetic material is re-enclosed in the nuclear envelope of newly formed cells, the biosensor moves back into the reformed nucleus and there is no fluorescence. The effect is like a light switch being turned on and off, signaling the start and end of the cell division process, respectively.

The biosensor is a first example of new types of probes designed to observe and measure cellular processes in real time rather than just looking at before-and-after static snapshots, said Tobias Meyer, PhD, associate professor of molecular pharmacology, who led the research team.

“The biosensor will be useful for discovering genes involved in cell proliferation and cancer,” he said.

The technique, published in the February edition of the journal Nature Biotechnology, allows simultaneous monitoring of up to 100 cells. Previous methods allowed researchers to observe only a single cell at a time.

“The exciting thing is the ability to screen compound libraries to discover novel cancer therapies,” said Joshua Jones, a graduate student in molecular pharmacology and lead author of the study.

He added that the idea of screening hundreds of thousands of potential anti-cancer compounds was previously inconceivable when researchers had to rely on techniques that monitor only one cell at a time. The group is patenting the new imaging technology.

In one experiment testing this technique, the team used rat leukemia cells that contained biosensors. The cells were then exposed to a low dosage of the anti-cancer drug Taxol to observe how it affected cell division.

After being mounted onto the glass of a special microscope, cells were hit with laser light from below. The light was angled such that after it went through the lower side of the glass, the upper side reflected it downward instead of allowing it to pass through.

The light did not therefore pass through the cells on top of the glass, but still supplied enough energy to illuminate the fluorescent biosensors in their plasma membranes, allowing the researchers to quantify the timing of cell division.

This biosensor also can be used with conventional microscopy techniques and, although the resolution is not as great as with total internal reflection microscopy, these experiments allowed the researchers to observe defects in cell function as well as the timing of cell division events.

In each experiment, the researchers captured microscope images every two minutes then assembled them in sequence as movies, marking the onset of the various stages of cell division.

The next phase of this research, which is funded by the National Institutes of Health, will examine the use of biosensors to screen for new genes that promote cell proliferation.

The team is now developing ways to automate the cell-imaging process and the analysis of the massive body of data the technique generates. “It’s going to be tricky,” said Jones. “We’re probably going to have to get a computer that thinks like we do.”