Wendell Lim is using light as a type of remote control to manipulate cells and tease out the daunting complexity of muscle, heart tissue, and particularly brain tissue.

The handiest light-activated molecule so far is channelrhodopsin-2 (ChR2), which was originally found in the light-sensitive “eye spot” of Chlamydomonas reinhardtii, a type of green algae. In Chlamydomonas, upon exposure to blue light, ChR2 opens and allows positively charged ions into the cell. That triggers a sequence of changes that influence the cell's cilia-based propulsion and, thereby, its motion and feeding behavior. When transferred into neurons, ChR2 becomes a light-activated trigger that makes the modified neurons fire more easily. Deisseroth's group used motor neurons that they genetically modified to bear ChR2 receptors to make the mouse march in response to light.

If ChR2 is a blue activator, halorhodopsin (NpHR) is a yellow silencer. It was discovered in Natronobacterium pharaonis, a bacterium isolated from a high-alkaline, high-salt lake in Egypt. In the bacterium, the light-driven NpHR channels pump chloride ions into the cell, a flow that ultimately helps drive the synthesis of ATP, the cell's biochemical fuel. Transferred into neurons, however, these channels respond to yellow light by hyperpolarizing the cells, effectively silencing them.

ChR2 and NpHR make for a powerful duo. They enable researchers to rig neurons and other cell types, including muscle cells and perhaps even insulin-making pancreas cells and immune system cells (see Web Extra, “No Neuron Left Unturned,”), with light-controlled on and off switches. From a third light-activated gate, VChR1, Deisseroth's group developed a tool that responds to light on the red side of the spectrum. VChR1, a channelrhodopsin in Volvox algae, is a cell excitor like ChR2.

For neuroscientists like HHMI investigator Massimo Scanziani of the University of California, San Diego, the real experimental power of these switches becomes clear when they are inserted into specific types of neurons.

To achieve this, Scanziani appends the gene for, say, ChR2 or NpHR, to stretches of DNA known as “cell selective promoters,” each of which becomes operative in only one neuron type. So, even though the gene-insertion step might occur in all neuron types, only one of those types will actually express the ChR2 or NpHR switches.

		No Neuron Left Unturned Practitioners in the nascent field of optogenetics are getting the hang of genetically modifying cells to make them responsive to and controllable by light. Read More

“This gives you immense sensitivity,” says Scanziani, who uses the technique to probe the function of specific cells in the cortex of animal brains, the region associated with sensation and thought. In mice and rats, for example, Scanziani studies how sensory inputs, such as the contrast between different elements of a visual scene, are processed by neuronal circuitry in the visual cortex. “You now can manipulate a circuit and understand what the heck it does in the brain,” Scanziani says.

Photo: George Nikitin / AP ©HHMI

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