Scientists discovered ways to excite or mute neurons by shining light on them about 20 years ago. Optogenetics is a method that enables researchers to understand the roles played by particular neurons as well as how they interact with one another to construct circuits.
MIT and Harvard University researchers have now developed a method to achieve longer-term alterations in cell activity by building on that methodology. With this novel technique, they can utilize light exposure to modify the membranes’ electrical capacitance, which modifies the excitability of the neurons (how strongly or weakly they respond to electrical and physiological signals).
Neuron excitability changes have been associated with a variety of mental functions, such as learning and aging, as well as with some brain illnesses, such as Alzheimer’s disease.
According to Xiao Wang, the Thomas D. and Virginia Cabot Assistant Professor of Chemistry at MIT and a member of the Broad Institute of MIT and Harvard, “this new tool is designed to tune neuron excitability up and down in a light-controllable and long-term manner, which will enable scientists to directly establish the causality between the excitability of various neuron types and animal behaviors.” If neuron excitability can be adjusted, it will be possible to restore normal brain circuits in the future when our method is applied to disease models.
The paper’s senior authors are Jia Liu, an assistant professor at the Harvard School of Engineering and Applied Sciences, and Wang. It was published in Science Advances today.
The paper’s lead authors are Wenbo Wang, a graduate student at Harvard; Yiming Zhou, a postdoc at the Broad Institute; and Chanan Sessler, a graduate student in the Department of Chemistry at MIT.
manipulation of the membrane
By genetically altering neurons to express light-sensitive ion channels, optogenetics is a technique used by scientists to control neuron activity. Changes in the flow of ions through the channels suppress or increase neuron activity when those modified neurons are exposed to light.
“You can activate or shut down these ion channels using light, which will then stimulate or inhibit the neurons.” This enables a quick reaction in real time but necessitates constant illumination of the neurons if you want to regulate them, says Sessler.
The goal of the MIT and Harvard team’s modification of the method was to produce sustained changes in excitability rather than momentary activation or suppression of activity. To achieve this, they concentrated on changing the cell membrane’s capacitance, which is a crucial factor in determining the membrane’s capacity to conduct electricity.
Neurons become less excitable, or less likely to fire an action potential in response to input from other cells, as the capacitance of the cell membrane increases. More excitable neurons result from a decrease in capacitance.
“Conductivity and capacitance, two membrane characteristics, control how excitable neurons are.” While much research has concentrated on ion channel-mediated membrane conductivity, naturally occurring myelination processes imply that another useful method of adjusting neuron excitability throughout brain development, learning, and aging is to modulate membrane capacitance. “We therefore questioned whether we could modify neuron excitability by altering membrane capacitance,” Liu claims.
Liu and his colleagues demonstrated that they could change the excitability of neurons by causing them to construct conductive or insulating polymers in their membranes while Liu was a postdoc at Stanford University. In that investigation, which was released in 2020, Liu put the polymers together using an enzyme called peroxidase. Meanwhile, that method did not allow for precise control over where the polymers accumulated.The reaction’s requirement for hydrogen peroxide, which can harm cells, added to the danger.
Together with Wang’s MIT lab, Liu’s lab at Harvard developed a novel strategy to get around those restrictions. The scientists used a genetically modified light-sensitive protein that can catalyze the production of polymers in place of peroxidase.
The researchers developed miniSOG, a light-sensitive protein, to be expressed in neurons grown in lab dishes. The extremely reactive chemicals known as reactive oxygen species are produced by miniSOG when it is stimulated by blue light wavelengths. A conducting polymer known as PANI or an insulating polymer known as PDAB are both introduced to the cells at the same time.
The reactive oxygen species stimulate those building blocks to come together into either PDAB or PANI after several minutes of exposure to light.
The researchers discovered that neurons with conducting PANI polymers became less excitable, whereas neurons with insulating PDAB polymers grew more excitable using a method known as whole cell patch clamp. They also discovered that prolonged light exposure caused larger changes in excitability.
According to Zhou, the exact temporal control over the polymerization reaction offered by optogenetic polymerization enables the predicted step-by-step fine-tuning of membrane properties.
long-term modifications
As long as they could keep the neurons alive in their laboratory dish, the researchers demonstrated that the alterations in excitability persisted for up to three days. The technology is currently being modified so that it can be applied to slices of brain tissue and, eventually, they hope, to the brains of animals like mice or the worm C. elegans.
According to the researchers, these animal studies could shed light on how variations in cell excitability cause diseases like multiple sclerosis and Alzheimer’s disease.
Wenbo Wang explains, “If we have a specific 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 we see if that has the desired effect on behavior.” “Right now, we’re using it more as a model to study those diseases, but you may think about potential therapeutic applications,” the researcher said.
The Air Force Office of Scientific Research Young Investigator Program, the National Science Foundation through the Harvard University Materials Research Science and Engineering Center, the Stanley Center for Psychiatric Research at the Broad Institute, the Searle Scholars Program, and the Harvard Dean’s Competitive Fund for Promising Scholarship all provided funding for the study.
