A novel twist on the young field of optogenetics may provide a new way to study living human brains as well as offering innovative therapeutic uses.
From time immemorial, philosophers, anatomists and scientists have pondered the inner workings of the brain. Efforts to look inside the black box consistently yielded far more questions than answers. After all, the alchemists of the 16th century no more found actual homunculi residing inside our heads than the anatomists of Descartes’ day found the gears of an intricate clock.
Galvanometers and electroencephalograms (EEGs) opened the way to exploring the brain’s electrical activity, but they mostly told us how much we didn’t understand about the brain’s workings. Subsequent study revealed thousands of types of neurons intricately organized and interconnected into a vast network of roughly 100 billion cells in the average adult. Individual neurons are activated based on the outputs of thousands of upstream cells and then contribute to the activation of thousands of downstream neurons. Even with the improved spatial and temporal resolutions offered by later technologies such as fMRI and MEG, the language of the brain continued to remain a mystery.
Then in the 2000s, a novel technique called optogenetics was developed, which has allowed scientists to study the behavior of neurons in ways they never could before. The technique involves splicing neurons with genes that code for opsins, organic molecules that activate in response to light. These neurons can then be triggered with a flash of light via an optical fiber, causing them to fire. This tool offers a high level of spatial and temporal resolution, as well as real-time behavioral feedback when used with other neuroimaging technologies.
However, optogenetics can only take us so far. When it comes to the idea of genetically modifying people’s brains, considerable ethical and safety issues remain. As a result, the technology has been limited to Petri dishes and animal experiments in the lab, but applications in human subjects may one day be possible thanks to a related method tentatively named “optocapacitance”.
The new procedure takes a different approach to the external activation of neurons, a process generally known as neuromodulation. In 2011, Dr. Mikhail Shapiro and collaborators, including Francisco Bezanilla, Lillian Eichelberger Cannon Professor of biochemistry and molecular biology at the University of Chicago, found the mechanism that activated neurons by changing their membrane capacitance. This was done using heat from pulses of infrared light. But infrared stimulation wasn’t good at targeting and the resultant heat easily damaged cells.
Building on this, Bezanilla and Dr. David Pepperberg, Searls-Schenk Professor of ophthalmology and visual sciences at the University of Illinois at Chicago, began working with gold nanoparticles in order to more accurately target the cells in vitro. A mere 20 nanometers in diameter, these nanoparticles absorbed the light pulses and converted them into very localized heat, eliciting the desired and very specific neuronal activation. But the nanoparticles, which are 300 times smaller than a human blood cell, didn’t stay in place and were quickly diffused in the neuron’s immediate environment. In order to better bond them to the target neurons, the team coupled them with synthetic molecules based on the scorpion toxin Ts1. These Ts1-coupled nanoparticles bonded to the cell’s sodium channels and could be stimulated repeatedly. Using millisecond pulses of light, individual neurons produced more than 3,000 action potentials over the course of a half hour, with no loss of efficacy or apparent damage.
While this method of using what are known as ligand-conjugated nanoparticles was effective, however, it didn’t allow for activating neurons that weren’t specifically responsive to Ts1. In order to develop a more generally useful method, Bezanilla, Pepperberg and their team switched to coupling the gold nanoparticles with antibodies that bonded to ion channels TRPV1 and P2X3. Similar to the Ts1 particles, these molecules continued to activate the cells when triggered with light, even after continuous washing over a significant period of time. This meant that nanoparticles could be coupled to different antibodies in order to target different cell types, even non-neuronal populations.
The study, which was co-authored with Joao L. Carvalho-de-Souza, Jeremy S. Treger, Bobo Dang and Stephen B.H. Kent, and published in the journal Neuron in February, has many potentially useful applications. I spoke with Bezanilla and his collaborators, who explained that optocapacitance has similar spatial and temporal resolution to optogenetics, while avoiding the need for genetic modification of the target cells. Another one of the more interesting differentiating aspects of optocapacitance that Bezanilla mentioned is that “it will likely work in any excitable cell, using an appropriate antibody to target the desired cell type.”
As a result, optocapacitance could be used for research on a wide range of cell types and organs, not just neurons, and could have considerable in-vivo research uses, as well as therapeutic applications. For instance, in macular degeneration and certain other retinal diseases, the photoreceptors have degenerated, preventing them from sending signals to the retina’s ganglion cells and on to the brain. Using optocapacitance it may be possible to circumvent the failed cells and stimulate the optical pathway through a different mechanism in order to restore sight.
“This technique should be applicable to any therapeutic approach that requires stimulation of specific neurons in the brain or peripheral nerve,” Bezanilla told me. But he also pointed out that “much research remains to be done prior to any application to human subjects. Without real testing in living animals, it is too early to speculate any more.”
I’d also speculate that this technology could one day improve the ways we integrate robotic and neural prosthetics with our bodies. Direct connections between various devices and our nervous system could become possible, providing improved sensory feedback and replacing control methods such as targeted muscle reinnervation (TMR).
The study team has coined the term optocapacitance because of the light-induced alteration of electrical capacitance of the cell membrane, which depolarizes the membrane, “activating sodium channels and producing an action potential.” Though these are still early days, optocapacitance is a technique that could well shine new light on the inner workings of the human brain and much more.