Photothermal Nanoparticles Extend Range of Optogenetics

Targeted gold nanoparticles allow light to activate neurons, a finding that could enable the use of optogenetic techniques without genetic manipulation.

“This is effectively optogenetics without genetics,” said professor Dr. Francisco Bezanilla of the University of Chicago. “Many optogenetic experimental designs can now be applied to completely normal tissues or animals, greatly extending the scope of these research tools and possibly allowing for new therapies involving neuronal photostimulation.”

Optogenetics, the use of light to control neural activity, is a powerful technique that has seen widespread use in neuroscience research. It involves genetically engineered neurons that express a light-responsive protein originally discovered in algae. This allows scientists to stimulate individual neurons, as well as neural networks, with precise flashes of light.

Animation courtesy of Francisco Bezanilla/University of Chicago.

However, since optogenetics is reliant on genetic modification, its use is primarily limited to relatively few model organisms.

Bezanilla and colleagues previously demonstrated that normal, non-genetically modified neurons can be activated by heat generated by IR pulses. But this method lacks specificity and can damage cells.

To improve the technique, they used 20-nm gold particles that, when stimulated with green light, absorb and convert light energy into heat.

The researchers tested two kinds of nanoparticles: ones coupled with a synthetic molecule based on Ts1, a scorpion neurotoxin, which binds to sodium channels in neurons without blocking them, and ones coupled with antibodies that bind to ion channels.

Neurons treated with these nanoparticles in culture were readily activated by photothermal stimulation. Untreated neurons were unresponsive.

Targeted neurons could be stimulated repeatedly with no evidence of cell damage. Some individual neurons, targeted with millisecond pulses of light, produced more than 3,000 action potentials over the span of 30 minutes with no reduction in efficacy.

In addition to cultured cells, nanoparticles were tested on complex brain tissue using thin slices of mouse hippocampus. In these experiments, the researchers were able to activate groups of neurons and then observe the resulting patterns of neural activity.

Treated neurons could still be stimulated even after being continuously washed for 30 minutes, indicating that the nanoparticles were tightly bound to the cell surface. Excess nanoparticles wash away, minimizing potentially harmful elevated temperatures.

“With differently-shaped nanoparticles it can work in near-infrared, as well as in visible wavelengths, which has many practical advantages in living animals,” Bezanilla said. “Thus far, most optogenetic tools have been limited to visible wavelengths.”

That nanoparticles can be coupled to different antibodies and retain efficacy suggests flexibility for future applications, including human therapeutic development.

In retinal diseases such as age-related macular degeneration, for example, photoreceptor cells that absorb light signals are damaged or dead. However, the retinal nerve cells that carry visual information to the brain often remain intact and healthy. Nanoparticles targeted to these cells could potentially absorb light and directly stimulate the neurons, bypassing defective photoreceptors, the researchers said.

“While much additional research must be done to determine the feasibility of this nanoparticle approach as a vision restoration therapy, our results encourage further effort aimed at achieving this critical clinical objective,” said professor Dr. David Pepperberg of the University of Illinois at Chicago.

Although no harmful effects were observed, the researcher said that toxicity is a possibility. However, many live-animal tests and human clinical trials have already been completed using formulations of gold nanoparticles without serious side effects. The researchers are now testing the efficacy of the technique in animal models to verify its potential for therapeutic use.

The research was published in Neuron (doi: 10.1016/j.neuron.2015.02.033).

For more information, visit www.uchospitals.edu.