Virus-mediated Jaws expression (green) in the mouse hippocampus. Figure from Chuong et al.,1 Nature Publishing Group.

Optogenetics is a powerful approach to manipulate and understand the function of different neurons in the brain. Channelrhodopsin is widely used to activate neurons, but inhibitory tools are less prominent despite their value in showing that the manipulated neurons are required for a certain function or behavior. Ed Boyden and his colleagues at the Massachusetts Institute of Technology now report a red light–activated chloride pump that effectively inhibits neurons both in vitro and in vivo.

Ideally, optogenetic activators and inhibitors should be activated by red light because it can penetrate tissues more deeply and can even pass through the intact skull. Boyden and his colleagues, led by graduate student Amy Chuong, screened a collection of archaeal halorhodopsins for molecules that are red shifted compared to typical halorhodopsins and that express well in neurons. They found a molecule with the desired properties in the 'Shark' strain of Halobacterium salinarum. They inserted two mutations to increase the light-induced photocurrents and added trafficking sequences to improve its expression in neurons. Aptly, the researchers named the new optogenetic inhibitor Jaws.

Upon stimulation with red light, Jaws leads to a threefold-higher light-induced current in cultured primary neurons compared to previously identified optogenetic inhibitors. This current is mediated by the influx of chloride ions and nearly completely abolishes neural activity in Jaws-expressing neurons. A similarly high inhibition of spontaneous or stimulus-induced neural activity can be achieved when Jaws is expressed in different areas of the mouse cortex and illuminated with red or yellow light. Jaws “has been engineered to be powerful, but we have also validated it to show that it results in powerful silencing in a wide variety of brain regions,” says Boyden.

In addition to applications in basic neuroscience research, Jaws has potential for visual therapeutics. Boyden and his collaborators demonstrated this potential by expressing Jaws in cone photoreceptors of a mouse model for retinal degeneration. Healthy cones hyperpolarize in response to light, which can be mimicked by the light-induced chloride currents in Jaws-expressing cells. In the retinal degeneration mouse model, Jaws expression in cone cells restored light-induced activity in downstream ganglion cells.

A complication with optogenetic inhibitors derived from halorhodopsins or archaerhodopsins is their behavior after light is switched off: a rebound burst of action potentials is sometimes observed. “Optogenetic tools are not purely mediating some abstract kind of electricity,” says Boyden. Ions are moved across the cell membrane, which can have effects on the cell. To overcome this rebound problem, he and his colleagues have found that gradually decreasing the stimulating light intensity instead of abruptly switching off the light reduces the rebound effect. He thinks that a powerful optogenetic tool can often be made even more powerful by adapting the optical stimulation protocols. In his opinion, “molecular engineering and optical engineering can often go best hand in hand.”

Jaws joins an expanding arsenal of tools that allow the optical manipulation and monitoring of neurons. In particular, potentially noninvasive red-shifted tools such as Jaws will be invaluable in our quest to understand how neural circuits function and how they mediate behavior.