Glowing protein could serve as light source for optogenetics

Researchers at the Emory University School of Medicine (Atlanta, GA) and the Georgia Institute of Technology (Georgia Tech; also in Atlanta) have developed a tool for optogenetics that uses a glowing protein from coral as the light source instead of a fiber-optic cable.

Related: New optogenetics switch could increase understanding of cell signaling

Biomedical engineering student Jack Tung and neurosurgeon/neuroscientist Robert Gross, MD, Ph.D., have dubbed these tools "inhibitory luminopsins" because they inhibit neuronal activity both in response to light and to a chemical supplied from outside. A study has demonstrated luminopsins’ capabilities, showing that these tools enabled them to modulate neuronal firing, both in culture and in vivo, and modify the behavior of live animals.

Tung and Gross are now using inhibitory luminopsins to study ways to halt or prevent seizure activity in animals. "We think that this approach may be particularly useful for modeling treatments for generalized seizures and seizures that involve multiple areas of the brain," Tung says. "We’re also working on making luminopsins responsive to seizure activity: turning on the light only when it is needed, in a closed-loop, feedback-controlled fashion."

In conventional optogenetics, scientists use genetic engineering techniques to make neurons in animals produce light-sensitive proteins called opsins. These opsins, which come from various algae and salt-tolerant bacteria, respond to a particular wavelength of light by moving ions across the cell membrane—thus stimulating or silencing the neurons where they are expressed. However, fiber-optic cables, the usual mode of delivering the light needed to activate the opsins, have a limited reach into the brain, pose a risk of infection, and can limit animals’ movements.

To supply light locally and internally, the team took the enzyme luciferase from the soft coral Renilla, which glows in presence of its substrate luciferin, and fused it to an inhibitory opsin—creating a novel fusion protein termed an inhibitory luminopsin. An excitatory luminopsin has been reported by other scientists, but its properties were demonstrated only in cultured cells.

To show that luminopsins could modulate behavior in live animals, they injected a gene vector encoding their inhibitory luminopsin into the globus pallidus of rats, on just one side of the brain. Two weeks later, the rats were injected with luciferin. The globus pallidus is involved in motor control, and the presence of luciferin had the effect of disabling the globus pallidus. In response to the drug amphetamine, the animals rotated preferentially in one direction, which mimicked the behavior that results when the globus pallidus is damaged on one side.

Glowing protein could serve as light source for optogenetics
Bioluminescence from cells cultured in a multielectrode array, next to a fiber-optic cable. (Credit: Jack Tung)

The researchers also showed that luminopsins and luciferase together could suppress neural activity in the hippocampus, another region of the brain, in anesthetized rats.
 
The authors note that similar techniques have been used to stimulate selected neurons in animals with designer drugs, but conclude that luminopsins could be cleaner and easier to use, while offering scientists the choice of influencing neuronal activity with either light or an externally supplied chemical that is otherwise inert.
 
Full details of the work appear in the journal Scientific Reports; for more information, please visit http://dx.doi.org/10.1038/srep14366.

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