Biologists studying health often concern themselves with preserving cells. However, it is sometimes advantageous to learn how to precisely kill them, without having their contents contaminate the rest of the body – and that's exactly what bioengineers at the University of California San Francisco have done. By linking a "suicide" enzyme together with a light-sensitive molecule, they were able to cause precise, tidy and effective cell death in fruit flies. The finding could have significant implications for the way in which researchers study disease, especially those of the neurodegenerative variety, such as Alzheimer's and Parkinson's.
Cell ablation is a method in which a researcher destroys cells in a biological organism to study the function of those cells. It can be accomplished through a variety of means including chemical, surgical or through the use of a laser. But, according to UCSF, all of these means lack pinpoint precision and can be a bit messy. Sometimes, for example, when a cell is ablated, it can leak its contents into the surrounding tissue, causing inflammation.
The UCSF method brings precision and ease to the process. To trigger cellular death, researchers there created a protein they have called Caspase-LOV which links together the enzyme known as caspase-3 with a molecule that is sensitive to light, called a light-oxygen-voltage-sensing domain (LOV), which was derived from oats. Caspase-3 is an enzyme that triggers a process known as apoptosis, a sort of suicide cells commit when they have reached the end of their lifespans. Apoptosis occurs naturally in the body every day. In fact, according to the book, Molecular Biology of the Cell, billions of cells die in our bone marrow and intestines every hour through apoptosis.
By linking an enzyme that causes cell death to a molecule that responds to light, the researchers found that they were able to perfectly control cell death in fruit flies based on light exposure. The fruit fly is a particularly useful subject to manipulate as it is often used in research thanks to the fact that is shares 75 percent of the genes that cause disease in humans, matures quickly, and is easy to study. Light is also able to penetrate fruit fly embryos, which enhances the research possibilities.
In their study, UCSF protein engineers and neurobiologists created fruit flies that expressed Caspase-LOV in motor neurons. When the neurons were then exposed to light, apoptosis was triggered and they died, impacting the flies' ability to move. The researchers also proved their finding by using the technique to ablate specific cells in the flies' retinas and sensory organs.
Because apoptosis is a process that has developed over the course of evolution, it works quite effectively at killing cells. Triggering the process with caspase-3 also results in a cleaner cell execution, as the contents of the cell simply shrink to the point that they can be absorbed by cells next to them or by immune-system cells.
"Caspases are like demolition experts," said James A. Wells, professor and chair of pharmaceutical chemistry at the UCSF School of Pharmacy, and co-senior author of the new study. "They know where to put explosives in a bridge to bring it down without having to use a nuclear weapon. One of the reasons for using caspases to kill is because they do so in a very clean way — unlike ablating cells with lasers or surgery, which is messy and not precise, this leads to a corpse-removal process which nature knows how to deal with, without any collateral damage to the tissue."
The researchers also found that controlling the duration of exposure to light of cells that expressed Caspase-LOV had a direct impact on how many of them died. They feel that this finding has a particular import in terms of studying neurodegeneration, as the condition can be advanced slowly based on light exposure, much in the same way diseases including Alzheimer's and Parkinson's advance in the human body.
"We're excited about this," said Wells. "There are already a couple of labs using it, and I think more will follow now that we've published these findings."
The work has just appeared in the Online Early Edition of Proceedings of the National Academy of Sciences (PNAS).