Sparking neurons help scientists understand brain signals

Ashley N. Paddock, ashley.paddock@photonics.com

Neurons that light up as they fire are providing researchers with a better picture of how signals travel within the brain.

The genetically altered neurons use a gene from a Dead Sea microorganism to produce a protein that fluoresces when exposed to the electrical signal in neurons, allowing the propagation of signals to be tracked through the cell. The research was led by Adam E. Cohen, the John L. Loeb associate professor of the natural sciences at Harvard University.

Cohen’s team created the neurons by infecting brain cells cultured in the lab with a genetically altered virus that contained the protein-producing gene. Once the cells were infected, they manufactured the protein themselves, enabling them to light up.

In a culture of neurons genetically modified to express a protein derived from a Dead Sea micro-organism, the fluorescence of the cells depends on the voltage across the cell membrane. An increase in voltage in the cell (pink) caused its fluorescence to stand out above the background of other quiescent cells. This technique enables researchers to observe electrical firing of neurons as flashes of light, detectable in a microscope. Courtesy of Daniel Hochbaum and Adam Cohen, Harvard University.

Neurons have an active membrane around the whole cell, and the inside of the cell normally is negatively charged relative to the outside. However, when a neuron fires, the voltage reverses briefly, traveling down the neuron and activating other neurons downstream. The genetically altered protein sits in the membrane of the neuron and lights up as the pulse passes through it.

“In the wild, these proteins convert sunlight into electricity, which provides energy for the host organism,” Cohen said. “We realized that we could run these proteins ‘in reverse’ to convert electricity into an optical signal which we could detect.”

Previously, the best way to measure electrical activity within a cell was to stick an electrode into it and record the results on a voltmeter. Unfortunately, this method could measure the voltage at only one point, and it killed the cells relatively quickly. The new technique makes it possible to study how the signal spreads, whether it moves through all neurons at the same speed, and how signals change if the cells are undergoing something akin to learning. It also could make it possible for scientists to study cells for much longer periods of time.

Besides studying how electrical signals move through the brain and around the body, the research could be helpful in the study of drug development. Many drugs target ion channels – proteins that play an important role in governing the activity of the heart and brain. If researchers want to test a compound designed to activate or deactivate an ion channel, they would have to test it with an electrode, then add the drug to see what happens – a process that could take a few hours for every data point. The ability to see how neurons are firing could radically speed up the testing process for drug efficacy to a few seconds.

The work was published online Nov. 27, 2011, in Nature Methods (doi: 10.1038/ nmeth.1782).

Cohen’s team is working on a screen to identify mutants of the protein that are even faster, brighter, more sensitive and less perturbative to the host. They also plan to work on the optics to build microscopes that can record the very dim and brief flashes of fluorescence that these proteins generate when a neuron fires.