SAN FRANCISCO, Sept. 15, 2009 – A hybrid protein has been created that causes mouse cells to move in response to laser light. Such cells can be trained to follow a light beam or stop on command.

This is the first time researchers have been able to import a light-controlled “on-off switch” from plants into a mammalian cell to instantly control a variety of cell functions, the researchers said. As such, it offers both a powerful new tool in cancer and cardiovascular research, and the potential to ultimately control complex processes such as nerve growth.

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Focusing a red laser at the periphery of a fibroblast cell induces the cell to grow outward toward the illuminated point via the phytochrome remote control system. The laser point is slowly moved outward as the protrusion grows to “extrude” the surface of the cell about 30 µm from the cell body. (Image: Anselm Levskaya)

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“This is a powerful tool for cell biology and cancer research,” said Wendell Lim, a professor in the University of California, San Francisco (UCSF) department of cellular and molecular pharmacology and a Howard Hughes Medical Institute researcher.

“If you have a controllable ‘light switch’ that is generic enough to use in multiple cell functions, it gives you the ability to control where and when a cell moves, using a simple beam of light, and control what it does when it gets there.”

Researchers built the switch using a light-sensitive protein from Arabidopsis thaliana, a plant in the mustard family. Their approach hinges on a specific type of phytochrome – proteins that plants use to regulate seed germination, shade avoidance and other processes in response to light. In plant cells, the protein changes shape when exposed to red light but returns to its normal state in infrared light.

They set up their system in such a way that shining red light on the hybrid phytochrome/regulatory protein would cause it to migrate to the cell’s outer membrane, where it could alter the structure of the cell’s cytoskeleton.

That alteration would, in turn, cause the cell to change shape or move. In contrast, infrared light would quickly stop the migration of the hybrid protein to the outer membrane, so by toggling between the two wavelengths of light, scientists could turn cell movement on and off. Using a laser beam, which can be directed at a precise location within the cell, the scientists can sculpt cell shape down to a resolution of 1 µm – about 1/10,000th of an inch.

The research appears in the Sept. 13 advanced online publication of the journal Nature, reported alongside a paper on similar research led by Klaus Hahn, PhD, and his colleagues at the University of North Carolina, Chapel Hill (See: Light Controls Cell Movement).

Together, the papers are the first to demonstrate that plant light switches can be imported into mammalian cells to control complex regulatory processes. The UCSF research is unique in developing a generic plug-and-play switch, based on protein recruitment, which can be wired to control diverse processes in many types of cells and organisms, the researchers said.

The findings could have various therapeutic applications down the road, such as the ability to guide nerve cells to reconnect across a broken spinal pathway in a spinal cord injury, according to Lim, one of three senior authors on the paper and the director of the Cell Propulsion Laboratory, a National Institutes of Health Nanomedicine Development Center at UCSF and UC Berkeley.

More immediately, the findings offer a new approach for scientific research into the complex regulatory processes involved in diseases like cancer and inflammation, he said.

Many cell processes are governed by where and when proteins appear in the cell, Lim said. When those processes are based on an extremely complex network of signals, such as in diseases like cancer, he added, it’s helpful to have an on-off switch to insert into that process.

The research was carried out by Anselm Levskaya, a graduate student in both Lim’s laboratory and the laboratory of Chris Voigt, PhD, a synthetic biologist and assistant professor of pharmaceutical chemistry in the UCSF School of Pharmacy, who was also a senior author on the paper.

Levskaya initially looked to plants for proteins that might serve as the light sensor. Plants are known to rely upon phytochromes, or light-sensing signaling proteins, to control a variety of processes, such as a plant’s growth toward sunlight and seed germination.

He proposed that these phytochromes could be genetically engineered into mammalian cells and tied to a specific function, in this case, cell movement.

Levskaya identified a pair of interacting proteins from plants, known as the PhyB-PIF interaction, that could be turned on and off like a switch, and then imported that cellular signaling system into live mouse cells in a cellular pathway that controls cell motion. The resulting cells can be pulled by an external beam of dilute red light, or pushed away by an external infrared beam.

“We’ve been able to use similar light sensors to program bacteria and yeast cells to follow a chain of if-then commands,” Voigt said. “What’s remarkable here is the ability to, first, do this in mammalian cells and, secondly, find a method to turn them off again after they’ve performed the function we selected.”

The reversible aspect of Levskaya’s work is significant, Voigt said. While many methods are aimed at disrupting cellular pathways, most are fairly simple and work only in one direction: they shut a process down, or prevent two proteins from interacting, but they are limited to that one action.

This approach, by contrast, enables researchers to control precisely when the disruption occurs and for how long, then stop it at will.

“It’s tremendously exciting that we can control cell shape and movement with a novel and noninvasive input such as light,” Lim said. “If you’re trying to understand complex processes like metastasis or development, it’s a great advantage to have a dial – a light-controlled knob – you can turn to activate just one place.”

While these studies were done in cells grown in the laboratory, Lim is now collaborating with other researchers to use the light-triggered switch in live animals, opening up even more possibilities for study.

He is equally enthusiastic about using the same strategy to design light-programmable systems to control other cellular processes that involve recruiting proteins to new locations or to new partners. Linking the phytochrome to a variety of signaling proteins could therefore be a powerful way to manipulate cell behavior.

“Because that is a generic cell mechanism, you could in theory use the light-controlled switch to manipulate a range of functions, such as cell division or turning genes on and off – like a universal remote,” he said.

For more information, visit: www.ucsf.edu