Volume 1, Issue 7
Reengineering Cells To See
Dirk Trauner has a grand vision. The UC Berkeley chemist hopes that the success he and his colleagues have had making blind cells "see" could someday lead to a cure for human blindness.
Before joining the UC Berkeley faculty, Dirk Trauner was a postdoctoral fellow at the Sloan-Kettering Institute for Cancer Research.
"We've borrowed one of nature's molecular machines, reengineered it, and put it back into a cell to add new functionality," he says. "If you could re-educate a neuron to be light sensitive, you might be able to create an artificial retina."
Trauner and Richard H. Kramer, a professor of molecular and cell biology, are taking the first steps toward that futuristic goal. The researchers have already inserted a biological switch into a rat brain cell so that it can be turned on or off in response to different frequencies of light.
Their technique, Trauner explains, could eventually become the basis of a gene therapy for macular degeneration, the leading cause of legal blindness in people over 55. Macular degeneration refers to the degradation and death of the nerve cells in the retina that collect the light that the brain uses to construct a picture of the world. In macular degeneration and other similar diseases, the nerve cells downstream from the photoreceptors are still alive though, making them an interesting site for a possible treatment.
"Many of the living neurons that project the signals into your brain also turn out to be on the surface of the eye," Trauner says. "That means they can be easily reached by painlessly injecting the eye with a harmless virus that would carry the therapeutic gene into the correct cells."
The idea behind the gene therapy is that by tricking cells into responding to light, they would fill in for the photoreceptors knocked out by disease. The researchers' targets are the tiny electrical gates in a cell's membrane that govern such basic function as signaling, regulation, and excitability.
Working with molecular and cellular biology professor Ehud Isacoff, an expert in the dynamics of ion channels, the researchers focused on potassium channels. These channels are proteins that function like pores, selectively letting potassium pass in and out of the cell. The channels are triggered by changes in voltage in a mechanism familiar to electrical engineers.
Potassium channels normally open in response to a voltage difference between the inside and outside of a nerve cell, letting potassium ions (K+) flow out to equalize the voltage and turn the cell off. This channel has been broken, then re-engineered to open when hit with ultraviolet light and close when hit with green light. The opening and closing is achieved with a molecule that kinks and unkinks in response to different wavelengths of light. This photoswitch can be used to selectively silence nerve cells or to give the gift of "sight" to normally sightless organisms or cells. (courtesy the researchers)
"A voltage-gated ion channel is nature's version of a field-effect transistor," Trauner says. "It's a device that changes its conductance based on the voltage applied. However, we wanted to make its conductance change in response to light."
To add the new functionality, the researchers mutated a gene for the ion channel so that it remains open and added an extra molecule that would act as a hook for a photosensitive molecule. They then introduced the mutated gene into cells from a rat's hippocampus, a sliver of the rodent's brain. Next, they washed the hippocampus cells with a photosensitive chemical, called an azobenzene compound. The azobenzene molecules bind to the hook on the potassium channel and function like a stopper in a sink drain. Depending on the wavelength of light hitting it, the stopper either plugs the pore or is yanked out of it.
A computer-generated visualization of a potassium ion channel structure. (courtesy the researchers)
Based on the promising in vitro results with the rat hippocampus, Trauner and his colleagues will next apply the technique to mice that they've genetically engineered for blindness comparable to the kind caused by macular degeneration.
"We'd like to demonstrate that the blind mice can become sensitive to light," he says. 'We're not even dreaming yet of color vision or real spatial resolution."
In the shorter term, the light-sensitive ion channels could also be used in more basic research on the brain. Scientists might use lasers to selectively activate or silence certain neurons to gain insight into the precise function of various neuronal circuits.
The development of synthetic ion channels could have other applications as well. For example, an ion channel might be synthesized that's sensitive to heavy metals or various pathogens in the air. The channel could then be used as an ultra-sensitive environmental sensor.
"We're taking a basic building block of life and souping it up to do something it did not evolve to do," Trauner says. "This is true synthetic biology."