Optogenetics Tool could provide long-term control of brain processes

Researchers at MIT and Harvard University are using optogenetics to develop an optically controlled method to facilitate long-term changes in neuronal activity. The team is using light exposure to change the electrical capacitance of neuron cell membranes to modulate the neurons’ excitability.

Changes in neuron excitability have been linked to many processes in the brain, including learning and aging, and have been observed in some brain disorders, including Alzheimer’s disease. According to MIT professor Xiao Wang, the research team designed its newly developed tool to tune neuron excitability up and down, as well as in a light-controllable and long-term manner. This will enable scientists to directly establish the causality between the excitability of various neuron types and animal behaviors, Wang said.

The capacitance of a cell membrane is a key determinant of the membrane’s ability to conduct electricity. When the capacitance of the membrane is increased, neurons become less excitable — that is, less likely to fire an action potential in response to input from other cells. When the capacitance is decreased, neurons grow more excitable.

MIT and Harvard University researchers have devised a way to achieve long-term changes in neuron activity. Their strategy uses light exposure to change the electrical capacitance of neuron cells’ membranes, which alters their excitability (i.e., how strongly they respond to electrical signals). Courtesy of MIT News, with iStockphoto.

In a 2020 study, Harvard professor Jia Liu and colleagues showed that cell membrane capacitance could be modulated by inducing neurons to assemble either conductive or insulating polymers in their membranes. “The excitability of neurons is governed by two membrane properties: conductivity and capacitance,” Liu said.

“While many studies have focused on membrane conductivity executed by ion channels, naturally occurring myelination processes suggest that modulating membrane capacitance is another effective way of tuning neuron excitability during brain development, learning, and aging. So, we wondered if we could tune neuron excitability by changing membrane capacitance,” she said.

Poor spatiotemporal control of the polymer deposition, as well as cytotoxicity caused by the use of hydrogen peroxide in the process to assemble the polymers hindered the initial technique.

Now, the scientists have addressed these issues via an optogenetic method to assemble conductive and insulating polymers on the neuronal plasma membrane. The optogenetic polymerization and assembly of electrically functional polymers allows the membrane’s capacitance to be precisely modulated in a light-controlled, step-by-step manner similar to conventional optogenetic stimulations. It provides cell type-specific control over neuron excitability. Cytotoxicity can be limited by controlling the light exposure.

“The advantage of optogenetic polymerization is the precise temporal control over polymerization reaction, which allows the predictable, stepwise fine-tuning of membrane properties,” researcher Yiming Zhou said.

Using lab-grown neurons, the researchers engineered cells to express a light-sensitive protein called miniSOG, which, when activated by blue light, produced highly reactive molecules. The researchers exposed the engineered cells to building blocks of either a conducting polymer called photopolymerized polyaniline (PANI), or an insulating polymer called poly(3,3′-diaminobenzidine) (PDAB). After several minutes of light exposure, the reactive molecules in the cells caused the building blocks to assemble into either PANI or PDAB.

The researchers used whole-cell patch-clamp measurement to characterize the electrophysiological properties of the same neurons before and after the optogenetic polymerization. They demonstrated that optogenetic polymerization can be used to achieve step-by-step modulation of both neuron membrane capacitance and neuronal excitability.

The changes in excitability lasted for up to three days, which was as long as the researchers could keep the neurons alive in a lab dish.

The team is now adapting its optogenetic polymerization technique, initially for use in slices of brain tissue and then, potentially, in the brains of mice or C. elegans. Animal studies could provide insight into how changes in neuron excitability affect disorders such as multiple sclerosis and Alzheimer’s disease, the researchers said.

“If we have a certain neuron population that we know has higher or lower excitability in a specific disease, then we can potentially modulate that population by transducing mice with one of these photosensitizing proteins that’s only expressed in that neuron type, and then see if that has the desired effect on behavior,” researcher Wenbo Wang said. “In the near future, we’re using it more as a model to investigate those diseases, but you could imagine potential therapeutic applications.”

Continued development of genetically targetable photosensitizers could enable polymer assembly with subcellular specificity and lower diffuse cytotoxicity, the researchers said. They believe that developments in biocompatible, in vivo polymer assembly will further allow the optogenetic polymerization technique to be applied to in vivo manipulation of neuronal circuits and animal behaviors.

“Future application of our approach in disease models will tell whether fine-tuning neuron excitability could help reset abnormal brain circuits to normal,” professor Xiao Wang said.