Volume 6, Issue 44 May 2009
Switching On the Lights
Richard Kramer is also a faculty member of Berkeley's Helen Wills Neuroscience Research Institute. photo credit: courtesy Richard Kramer
For those born with sight, blindness is a terrifying prospect. Unfortunately, hundreds of thousands of Americans are diagnosed with diseases such as retinitis pigmentosa and age-related macular degeneration every year. These diseases damage the photoreceptors on the outermost layer of the retina, destroying the light-collecting ability of the eye and leaving patients functionally blind.
Richard Kramer, a Berkeley professor of neurobiology, is working toward an organic way to restore sight to damaged retinas. Kramer has developed a molecular switch that responds to light. Once added to a cell's existing proteins, this device can repurpose ordinary neurons into light-sensitive tissues.
"If we can get the surviving layers of the retina to respond to light directly, we might be able to restore some sight," Kramer says.
Kramer's research into photoswitches, and how the retina transmits image data to the brain, have major implications for fields ranging from neuroscience to anesthesia, drug discovery to physiology.
Kramer has engineered a photoswitch that can turn cell signals on and off in response to light. Here, the switch arm straightens to plug a potassium channel under UV light, and bends to open the channel under green light. In this way, the switch can make ordinary neurons sensitive to light. image credit: Richard Kramer and Dirk Trauner
His photoswitch is a marvel of microscale engineering. It's made of azobenzene, a chemical that is normally stiff and straight. But when exposed to the right wavelength of light, azobenzene bends in the middle, and pulls its two ends closer together.
One end of the switch can be anchored to either an open ion channel or a protein receptor. The other end is affixed to a ligand specific to the channel or receptor. When the switch arm is fully extended, the ligand fits into the channel or receptor like a key in a lock. Exposing a cell containing this switch to light causes it to send an electrical signal or trigger a biochemical reaction.
Because the bent form of azobenzene is less stable, the molecule will relax on its own over time. It will also straighten on command under green light. Kramer is developing ways to tweak the switch to better suit different applications. For example, an arm that recovers its linear form rapidly would be most useful for vision.
The switch could be added to body cells in several ways. It binds to ion channels and receptors on its own, so all that's needed is a means to bring the chemical into contact with cells. Kramer is working with colleagues at UCSF on a polymer that could release small amounts of azobenzene over time. Placing it in the eye could supply the retina with steady doses of the switch.
The eye detects light with the aid of two types of photoreceptors: rods that are very sensitive to light and cones that can sense colors. This cross-section of the retina (red) shows synaptic vesicles (green) containing neurotransmitter in the terminals of several cones. image credit: courtesy Richard Kramer
"If you put this switch into neurons, you've installed a noninvasive way to control that part of the nervous system," Kramer says. That includes pain neurons. Kramer is now working on pairing the switch with an analgesic. One wavelength of light could turn those neurons off, while another would reawaken them. "Imagine leaving the dentist's office and not being numb for the next hour," Kramer says.
Even broader applications include neuroscience and drug research. At present, studying connections between neurons involves inserting electrodes into each individual cell. This painstaking technique is impossible to perform on large numbers of cells simultaneously. Kramer's photoswitch should eliminate this bottleneck, providing a means to stimulate many cells at once and understand neural circuits. Photoswitches can also be used to trigger electrical signals in cells used for drug assays. This is important for biotech efforts to assay vast molecular libraries for their effects on ion channels and other cell functions.
"Light is noninvasive. And you can target these switches to the right cells in several ways: genetically or chemically, but also by focusing the light to make sure only particular cells are being affected," Kramer says.
Synaptic vesicles (yellow circles) bound to the synaptic ribbon (black bar) in darkness and in light. Superimposed images from hundreds of ribbons show that vesicles are depleted from the base of the ribbon in darkness. image credit: courtesy Richard Kramer
Kramer's other research asks how photoreceptors pass visual information along to the brain. Photoreceptor neurons hand off information to the next layer of retinal cells via a mysterious structure known as a ribbon synapse.
Until recently, scientists thought the ribbon synapse was responsible for speeding the delivery of neurotransmitter to the cell membrane. Kramer has demonstrated that it does just the opposite. "It stores up the vesicles like a squirrel stashing nuts for the winter, then delivers them to the synapse when they are needed," he says. In light, the synapse releases a steady dribble of vesicles. But every time the lights go off, it dumps its vesicle stash all at once.
"What's being accentuated right at the very first synapse in the visual system is the change of light intensity that happens between light and dark. It's enhancing the representation of changing stimuli," Kramer says, and it's keeping our vision keen.