Metal ions from calcium to zinc help the human body stay healthy, performing key roles in signaling pathways of living cells. Yet, nailing down exactly what each ion does, and where, is challenging, since individual molecules and ions are too small to see in most situations. Now, a team of chemists has developed a new way to track the movement of zinc ions inside cells by creating a sensor that lights up in the presence of the metal. The sensor, described in a new PNAS Early Edition paper, let the researchers observe the decrease in zinc accumulation that happens in prostate cells when they become cancerous.
“What we were interested in was developing a reaction-based sensor that would turn on in the presence of zinc and produce a bright signal,” says Stephen Lippard, a bioinorganic chemist at the Massachusetts Institute of Technology who led the new work. “We picked prostate cells as an example to test the sensor.”
Most zinc in the body is tightly bound to proteins that require the element to function. But a small amount—called mobile zinc—exists in an exchangeable, chelatable form that’s free from protein. Mobile zinc is known to accumulate inside brain, pancreas, and prostate cells, but its functions in those cells aren’t fully understood.
In the past, Lippard’s team has developed other sensors that rely on the idea of fluorescence quenching. The sensors are created by starting with a molecule that, when excited with light, fluoresces brightly. A unit that binds zinc ions is then installed onto the molecule. If there’s no zinc, the zinc-binding unit prevents the molecule from fluorescing. But when zinc binds to the sensor, this so-called quenching process is eliminated and the light turns on.
Previous versions of this sensor, called Zinpyrs, were effective at lighting up in the presence of zinc, but had trouble detecting zinc at defined locations within the cell. Lippard, along with MIT postdoc Robert Radford and undergraduate Wen Chyan—the first author of the new paper—wanted to see how zinc moved not only between entire cells, but within the organelles of single cells. So they added another chemical group onto ZP1, a positively charged chemical that’s known to accumulate inside mitochondria.
In addition, they added an additional chemical group which prevent the sensor from fluorescing but is rapidly removed when zinc binds to the sensor. These groups decreased the background fluorescence of the experiments, making the sensor more sensitive. “With the new version of the molecule, zinc not only de-quenches the sensor to produce fluorescence, but also removes this added protective group,” Lippard explains.
Studies on the role of zinc in the prostate have suggested that levels of zinc decrease in cancerous prostate cells. So Lippard, Radford, and Chyan tested out the new sensor on prostate cells—comparing cancerous cells to healthy ones. They discovered that cancerous cells could still take in zinc, but the element no longer accumulates in the cells’ mitochondria. The observation not only shows the usefulness of the new sensor, but provides a new avenue of research to pursue related to prostate cancer.
“In this paper we just show that there is a statistical difference in zinc accumulation in mitochondria of cancer cells versus non-cancer cells,” says Lippard. “But we think that tracking mobile zinc may have applications for both early cancer detection and following cancer progression and treatment effectiveness.”
The approach that Lippard’s group used in the new paper, he says, could be adapted to study not only the movement of zinc in other cellular organelles and cell types, but the visualization of entirely different elements and molecules, such as copper and calcium. For now, though, his group is remaining focused on zinc, using the sensor to gain more insight into what mobile zinc does in the human body.
“Once we know where mobile zinc goes, the next steps would be to identify what it’s actually binding to and what signaling pathways it’s involved in,” he says.