Probes Light Up Synaptic Structures that Store Memories

Microscopic probes that light up synapses in living neurons could give scientists insight into how these structures change when new memories are formed.

With this imaging technique, synapses — the gaps between one neuron and the next — appear as bright spots along dendrites — the branches of a neuron that transmit electrochemical signals. As the brain processes new information, those bright spots change, indicating visually how synaptic structures in the brain have been altered by the new data.

To see this information processing in action, Don Arnold and Richard Roberts of the University of Southern California engineered microscopic probes, called FingRs, that light up synapses in living neurons in real time by attaching a green fluorescent protein (GFP) onto synaptic proteins — all without affecting the neuron’s ability to function. The fluorescent markers allow scientists to see live excitatory and inhibitory synapses for the first time — and, importantly, how they change as new memories form.

“When you make a memory or learn something, there's a physical change in the brain. It turns out that the thing that gets changed is the distribution of synaptic connections,” said Arnold, associate professor of molecular and computational biology at the USC Dornsife College of Letters, Arts and Sciences.

Scientists at the University of Southern California have engineered microscopic probes that light up synapses in a living neuron in real time by attaching fluorescent markers onto synaptic proteins — all without affecting the neuron’s ability to function. Pictured here, a living neuron in culture. Green dots indicate excitatory synapses, and red dots indicate inhibitory synapses. Courtesy of Don Arnold.

The probes behave similarly to antibodies but bind more tightly, and they are optimized to work inside the cell — something that ordinary antibodies cannot do. To make these probes, the team used a technique known as “mRNA display,” which was developed by Roberts and Nobel laureate Jack Szostak.

“Using mRNA display, we can search through more than a trillion different potential proteins simultaneously to find the one protein that binds the target the best,” said Roberts, professor of chemistry and chemical engineering with joint appointments at USC Dornsife and the USC Viterbi School of Engineering.

Because FingRs are proteins, the genes encoding them can be put into brain cells in living animals, causing the cells themselves to manufacture the probes.

The design of FingRs also includes a regulation system that cuts off the amount of FingR-GFP that is generated after 100 percent of the target protein is labeled, effectively eliminating background fluorescence — generating a sharper, clearer picture.

These probes can be put in the brains of living mice and then imaged through cranial windows using two-photon microscopy.

The new research could offer crucial insight for scientists responding to President Obama's Brain Research Through Advancing Innovative Neurotechnologies (BRAIN) Initiative, which was announced in April (See: Optics Community Hails Obama’s Brain Mapping Initiative). Modeled after the Human Genome Project, the objective of the $100 million undertaking is to fast-track research that maps out exactly how the brain works to uncover ways to treat, prevent and cure disorders such as Alzheimer’s, Parkinson’s, schizophrenia, autism, epilepsy and traumatic brain injury.

The study appeared in Neuron (doi: 10.1016/j.neuron.2013.04.017).

For more information, visit: www.usc.edu