No optical fibers, no headgear, no implants: This is the new optogenetics, systems that enable scientists to control cell behavior using simple flashes of visible light. Researchers today reported that they had successfully manipulated deep brain cells in mice using a light-based tool without an invasive surgical procedure.
The achievement is a first in mammals, according to the report, which was published today in journal Science. “It was exceptionally well executed,” says Polina Anikeeva, a professor of materials science and engineering at MIT who was not involved in the report.
Optogenetics involves genetically altering neurons to make them sensitive to light, and then shining a light—usually from silica optical fibers or light-emitting diodes—on a select group of them to make them turn on or off.
Optogenetics tools can be applied non-invasively as long as the target neurons are close to the skin’s surface, where an external visible light can penetrate easily. But to reach neurons deep in the brain or body, a device that emits light must be surgically implanted.
Researchers have come up with lots of different light source implant designs to reach these deeper areas, and tested them on mice, including wireless peppercorn-sized implants for the brain, stretchy implants for the body, and tethering optical fibers. Such demonstrations have proven highly successful.
But all involve surgery. That’s a major drawback when the target is deep brain tissue. The procedures can cause brain damage and affect mouse behavior, and can ultimately limit the application of optogenetics to humans.
In the advance described today, researchers at the National University of Singapore, along with Japan’s RIKEN Brain Science Institute, managed to emit visible light deep in the brain using only nanoparticles delivered via injection.
And here’s what’s cool: These nanoparticles, called upconversion nanoparticles, emit visible light by absorbing and converting near-infrared (NIR) light. The source for that NIR light can come from outside the skin—no implant necessary—because NIR light can penetrate tissue further than visible light.
The experiment goes like this: The scientists inject light-sensitizing genes into the mouse brain, then separately inject the specially designed nanoparticles, and then beam NIR light at the target area of the brain. The nanoparticles do their job of converting the NIR light to visible light, and that activates nearby neurons. (The wavelength of light and the type of genes introduced determines whether a neuron is inhibited or excited.)
The researchers tested the strategy in awake mice in a variety of ways, including an experiment in which they artificially replicated scary memories, causing the mice to freeze in fear. (The opposite—activating happy memories to protect mice from stress—can be done as well.)
The calculations to use upconversion nanoparticles in optogenetics was first described and patented in 2011 by Anikeeva (at MIT) and Karl Deisseroth at Stanford. Other scientists have since used to tool to manipulate brain cells in zebrafish larvae and a roundworm called Caenorhabditis elegans, both of which are transparent.
But no one had performed it successfully in a mammal, where the light would have to penetrate further through opaque tissue. To achieve that, the nanoparticles would have to be engineered to improve the efficiency of the conversion of the NIR light to visible light.
“I was the doubting partner in this,” says Thomas McHugh, a neuroscientist at RIKEN who collaborated on the experiments. “I was like: ‘There’s no way this is going to be efficient enough.’ But I was surprised at every junction.”
Indeed, the seemingly impossible task is what made Anikeeva and Deisseroth—the inventors of the technique—reluctant to pursue the line of work beyond patenting the calculations, Anikeeva says. “One of the reasons Karl and I haven’t pursued it [experimentally] is because we saw all of those challenges in terms of the chemistry and how much power it would need and that seemed quite daunting,” she says.
Xiaogang Liu, a chemist at the National University of Singapore, and his team pressed forward with the task. The key was constructing the nanoparticles with a ladder-like electronic energy structure of lanthanide—a family of heavy metals. The composition allows the lanthanide molecules to be excited and reactive to the wavelength of photons in the NIR spectrum, and then emit visible, high-energy photons as they return to their resting state.
With Liu’s chemistry, the nanoparticles converted NIR light to visible light with about 2.5% efficiency. It doesn’t sound like much, but it’s far more than previous attempts using older chemistries, and enough to modulate mouse cells. “This was a feat of chemical ingenuity and experience,” says Anikeeva.
The team injected incredibly high concentrations of lanthanide-doped nanoparticles, but how biocompatible they are, especially long term, is still unclear. The materials will have to be tested for toxicity in long-term experiments in mice, Anikeeva says. So far, the team has only tested the particles in mouse brains for about a month.
Equally unclear is whether the technique will be effective in larger brains, such as those of primates. “All of the challenges we saw in this small mouse would be amplified cubically” in a larger mammal, Anikeeva says. The distance the light must travel will increase, and so will the volume of cells that need to be stimulated, so a more efficient particle or even higher concentration of particles will be needed, she says.
Still, that’s something McHugh says his team would like to pursue, as it could lead to human therapies. Some research groups and companies are already testing optogenetics for the restoration of vision and hearing and treatment of pain and neurological disorders such as Parkinson’s.
In the short term, McHugh’s team aims to use the nanoparticle strategy to manipulate and better understand neural circuits in deep brain structures such as the hypothalamus in mice. “That’s a place where an optical fiber can cause some issues,” he says.