By Kathryn Ippolito, MPFI Postbac 2019-2020
Our brains help us process everything around us, organizing thoughts and behaviors appropriate for a wide range of situations. To do so, it stores information in cells called neurons, which communicate through a series of lightning-fast electrical and chemical signals, known as action potentials. In order for us to speak, see, or remember the route we take to drive home, different groups of neurons must interact in reliable patterns forming a circuit. Scientists are interested in circuits because they are the key to understanding everything the brain does. From there, it’s possible to see how certain injuries or diseases change brain function, and how doctors can help treat those changes.
One of the difficulties of studying circuits is that so many neurons are active in the brain at once that it becomes impossible to trace which ones connect, and what triggered them in the first place. In the past, scientists have tried to sort this out by using electrical stimulation—jumpstarting neuronal activity or canceling it out completely. While electricity immediately generates a very strong response, it acts upon a wide area of brain tissue. Alternatively, manipulating individual neurons by editing their genetic code or using targeted drugs can provide data about how the brain acts over hours or days, but not in an instant. In 2005, a revolutionary technique called optogenetics was demonstrated to activate particular cells in milliseconds.
The method of optogenetics almost sounds like science fiction. A laser is directed through a microscopic fiber optic cable to a particular region of the brain where just one type of cell will become active when illuminated. The briefest pulse of light can induce an action potential, allowing rapid patterns of activity to be controlled with high precision. This is possible because of proteins called opsins that are naturally sensitive to light, which were first described as the mechanism that helps move algae towards sunlight. Researchers can use several methods to copy the DNA blueprint for an opsin into target neurons. The most effective of these is a two-step method. First, a line of research animals is bred such that one type of neuron will express Cre, an enzyme that acts like a key to access a piece of DNA being held in an inactive state. Then, a non-infectious virus carrying the inactivated opsin DNA is injected into the area of the brain being studied, allowing the new DNA to enter all the cells. Cre will recognize tags placed around the opsin DNA and flip it into an active state so that it can be translated into protein.
Two of the most widely used types of opsins are Channelrhodopsin 2 (ChR2) and a halorhodopsin (NpHR). Both are simple ion channels that work quickly and are easy to put into cells of interest. ChR2 is activated by blue light and effectively turns neurons on while at the opposite end of the spectrum, NpHR is activated by red light and shuts neurons off. In the past fifteen years, many more opsins have been discovered in organisms from those as small as microbes to those as complex as primates. Using different colors and patterns of light, scientists can produce a wide range of responses that only affect neurons expressing one of these special proteins. Having opsins with different functions that are activated by different means enables scientists to simultaneously influence different types of cells and chart their interactions.
Currently, MPFI researchers are using optogenetics to study questions such as, “What properties of neurons allow us to respond to different kinds of stimuli” (Fitzpatrick Lab), “How does the brain remember so many details?” (Wang Lab), and “What does motivation look like at the molecular level?” (Stern Lab). The unmatched level of control provided by optogenetics helps scientists find increasingly specific answers about the brain’s diverse functions. As time goes on, optogenetics will become widely accessible, even easier to use, and optimized to be faster and more precise. Please feel free to visit the Science section of our website to learn more about how MPFI labs combine cutting edge techniques with brilliant research design to understand the brain in new ways.