A team of scientists from Baylor College of Medicine and Texas Children’s Hospital found a way to improve a light-gated chloride channel that is used for optogenetic inhibition while investigating why GtACR2, which was expected to suppress neuronal activity was actually stimulating it. Their findings were published last week in eLife.
Corresponding author Mingshan Xue, assistant professor of neuroscience and of molecular and human genetics at Baylor, and his colleagues initially set out to study the effect of inhibiting the activity of specific neurons in the visual cortex of mice. They used an optogenetic approach by which they genetically introduced into specific neurons a light-sensitive protein (light-gated chloride channel GtACR2.
“When we used the light-gated channel GtACR2, we expected to silence the cell body, so no matter which signal it received, the cell body would not send a signal down the axon. But we found that even though the cell body was indeed silenced, signals still ran through the axon and neurotransmitters were released,” said first author Jessica Messier.
When activated, channel GtACR2 opens a door on the cells through which negatively charged chloride ions flow, from where their concentration is higher toward where it is lower. The flow of chloride ions from inside the neuron toward the outside triggers a ‘fire’ signal, while the opposite, flow of negative ions from the outside to the inside of the cells, results in a ‘no fire’ signal. Usually, chloride concentration is higher outside of the cell than in the inside, so when channel GtACR2 opens, ions flow toward the inside of the cell, which results in a ‘no fire’ signal. That’s why chloride channels usually inhibit neuronal activity.
“However, we found that, in the particular neurons we were studying, the chloride ion concentration inside the cell body is lower than the concentration outside of the cell, but inside the axon it is the opposite, the concentration of chloride ions is higher inside than on the outside,” Xue explained. “This difference in chloride ion concentration between the cell body and the axon of the same cell explained why channel GtACR2 triggered a ‘no fire’ response in the cell body and a ‘fire’ response in the axon.”
To minimize the ‘fire’ signal running through the axon, the researchers modified channel GtACR2 so it would be mostly expressed in the cell body, and not in the axons.
“Relocating channel GtACR2 to the cell body significantly reduced the signals running through the axon, but there is still room for improvement,” Xue said. “This approach also enhanced the inhibitory effect in the cell body and resulted in increased inhibition of the activity of the neuron. A take-home message for us is that this light-gated inhibitory channel can be used, but it’s important to take into consideration the effects it can have in different parts of the cell.”