Living cells twitch. They crawl, vibrate, expand, contract. They rush off to fight infections, heal wounds, or grow organs in developing embryos. How do they know how and where and when to move?
Each cell has an internal network of proteins that signal it to move or change shape in response to what’s going on inside and outside the cell, pharmacologist Klaus Hahn says. Hahn’s spent his career trying to figure out how that complicated network does its job — and how to use it to let scientists control where cells are going in a living organism. That means working on ways to reach inside the cell and turn the signaling proteins on and off.
Researchers figured out a way to turn proteins on with light years ago, Hahn says. “You take a protein and bind a photocleavable molecule to it — when you shine light on it, the group falls off and the protein turns on.” That technique had a lot of problems.Once the protein-silencing molecule fell off and the protein was turned on, it couldn’t be turned off again. The ultraviolet light used to break the bond with the amino acids was so strong that it hurt other parts of the cell. And getting these modified proteins into a cell without damaging it was hard to do.
There’s a different kind of protein that’s better at responding to light, called LOV (Light, Oxygen, or Voltage). Plants use it to sense light and turn their leaves toward the sun, Hahn says. Last year a postdoc in his lab, Yi Wu, changed the genetic code of a signaling protein to include instructions for making the LOV protein. They chose this signaling protein, Rac1, because it controls the first step in cell movement, in which the edge of a cell protrudes. Then that edge attaches to whatever it’s moving on, and the other end follows behind.
The resulting protein comes out wound up sort of like a yo-yo string, Hahn says: “When you shine light on the side of it, the string unwinds and the protein can do what it wants. When you turn the light off, it winds back up. The LOV protein blocks it from activating just by covering it up.”
Shine a blue light on one of these modified proteins near the edge of the cell, and it signals the cell to head for the light. In video clips from Hahn’s lab spliced together by the National Institutes of Health, a cell chases a circle of light across the screen (“like a horse trotting after a carrot on a stick,” the narrator says). In another video, a protrusion emerges from the side of a cell, ending in a sharp point where a beam of light shines: the cell gets bigger in response to the light.
The cells Hahn’s lab works with are fibroblasts: large connective tissue cells that help heal wounds and “like to move anyway,” Hahn says. Last fall, after he and Wu published a paper on moving cells with light, other researchers started using the technique in living organisms: first zebrafish, then fruit fly embryos.
At Johns Hopkins, Denise Montell found that activating the signaling protein in one cell in the fly embryo made nearby cells fall into line with it. “As if all the other cells said, ‘Aha! You’ve got more activity so we’re heading your way,’” she says. Developmental biologists study how a small number of cells in an embryo differentiate into brain and blood and other types of cells. They’ll be able to move groups of undifferentiated stem cells in a fruit fly embryo, for example, and figure out what those cells do by watching to see whether the embryo develops differently from how it normally does, Hahn says.
Cell movement is also a central part of many illnesses — cancer is usually the first one mentioned, because it’s most deadly when cells break away from a tumor and travel to other organs. Moving metastasized cells with light may be a possibility, Hahn says, but that could turn out to be impractical because the cells are so scattered. He’s more excited about the technique’s potential to help with nerve regeneration: surgeons may someday fix a severed spinal cord by guiding the cells back together with light. And then there’s the young field of tissue engineering, which still lacks good ways to assemble cells into muscle and bone and other kinds of tissue. “Imagine paint pots with different cell types,” Hahn says. “We can take some from one pot, some from another, and paint them into the right places using light.”
Could the technique work on other proteins, or use multiple colors of light to change different proteins in one cell? Yes, Hahn says: stay tuned. A more complete version of his answer is in UNC’s Office of Technology Development, somewhere in the patenting process.
Meanwhile, his lab is using the LOV protein technique to study the movement of immune cells from the bloodstream into other tissues. “The immune cells travel through the bloodstream,” Hahn says, “and when they have to leave the blood, they attach to the endothelial cells lining the wall of the blood vessel. The walls somehow sense the cells are there and open up a porthole — not between the vessels, but right in the middle of one — to allow the immune cells to pass.” His lab will look at what triggers the immune cells to stop in the right place and what lets the endothelial cells know to let them out. They picked this type of cell movement to study because immune cells are part of the body’s inflammation response, which plays a role in all kinds of diseases from Alzheimer’s to diabetes to heart disease. Hahn also wants to make the LOV protein technique available commercially to other researchers. “I’ve found that that’s the best route to get something you make into a lot of people’s hands,” he says. “It’s a simple tool, and we made it that way so that people will be able to use it for a lot of different things.”