Volume 1, Issue 5
Neurobiology's Lighter Side
What happens when you touch a hot pan on the stove? You probably yell and yank your hand away. Between the sizzle and the scream though, an amazingly fast and complex cascade of cellular communication occurs inside your body. To study the electrical intricacies of the nervous system, neurobiologist Ehud Isacoff is developing new optical methods that enable scientists to watch the cellular symphony unfold at the nanoscale.
Ehud Isacoff is also Chair of the Graduate Group in Biophysics.
Understanding the dynamic structure of neuronal proteins could lead to new treatments for diseases like cystic fibrosis, epilepsy, and some forms of paralysis, Isacoff says. The first step though is to take a long, hard look at the dynamics of ion channels, the tiny electrical gates in a cell's membrane that govern cellular transport and signaling.
"An ion channel operates like a transistor with extreme speed, at an extremely small scale, and with tremendous reliability," says Isacoff, professor of Molecular and Cellular Biology. "The question is how it works mechanistically."
Ion channels are proteins that function as pores to selectively let ions like sodium and potassium pass in and out of a cell. As the ion channels rapidly open and close, the voltage of the cell changes and a voltage wave is produced. The ion channels themselves are triggered by a change in voltage as well, enabling electrical impulses to propagate from cell to cell through the nervous system.
To truly understand the molecular dynamics of an ion channel, you have to watch the protein move. The problem though is that traditional protein visualization techniques like x-ray crystallography can only produce high-resolution "still lifes" of a particular structural state. Furthermore, crystallography requires the protein to be removed from the cell and purified.
Isacoff and his colleagues have developed a method to study ion channel structure and protein motion within the cell in real time. He calls the technique "in situ optical biochemistry."
"It allows us to get a dynamic measure of the functioning protein within its natural physiological environment," says Isacoff, who is also a faculty scientist at Lawrence Berkeley National Laboratory.
First, the researchers attach fluorescent molecules to specific points in the ion channel's amino acid sequence. The fluorophore's brightness and wavelength change based on the local chemistry. As the protein structure changes, the fluorescent molecule moves as well. Even a very small motion affects the chemistry around the fluorophore, altering its color or brightness.
"The fluorescent lightbulbs in different locations illuminate who moves when and in what sequence," he says. "And that can be detected by a simple camera."
At the same time, a functional measurement of the ion channel is taken. That way, the visual "signature" of the transition as indicated by the fluorescence can be matched to particular function.
Once the technique is perfected, Isacoff hopes it can be used as the basis for a biochemical "device" that scientists could deliver to specific parts of an animal's nervous system. The strategically-placed fluorophores would illuminate the pathways of neural impulses, possibly even revealing how and where certain sensory inputs are processed in the brain.
"By using these kinds of optical systems, the hope is that you can follow voltage, which is one of the key signals in the nervous system," Isacoff says.
The fluorescent molecule rhodamine is attached to a fruit fly potassium channel. It becomes brighter when the channel protein changes its structure in response to a change in membrane voltage. Someday, Isacoff's research on ion channels could aid engineers in developing advanced biosensors, "artificial noses" that detect the most miniscule amount of pathogens or contaminants in the air.
The next step, he explains, is using optics as a tool not just to measure protein activity, but actually control it. For example, ion channels genetically engineered to be light sensitive could be switched on and off with a beam of a certain wavelength. That way, cells in neuronal circuits could be easily knocked in and out of service, helping scientists deduce their function.
"What you'd like to do is turn on or off a protein in a particular cell in a reversible manner without altering its ability to function otherwise," Isacoff says. "That would revolutionize biology. And an ideal way to do that would be with light.