Summary: Researchers report that they have identified the first natural light-gated potassium channel rhodopsins.
Source: Baylor College of Medicine
A key approach to understanding the brain is to observe the behavioral effects of turning on specific populations of neurons. One of the most popular approaches to controlling neuronal activity in model systems is termed optogenetics and depends on the expression of microbial light-gated channels in the neurons of interest.
These channels function as light-sensitive switches that turn neurons on with a flash of light and have been available since 2005. A critical way to confirm the function of neuronal populations would be to repeat the experiment, but this time by turning off or silencing the same neuronal subpopulations. However, the neuroscientific community has lacked a quick and effective way to turn off or silence neurons—until now.
Researchers from the University of Texas Health Science Center at Houston McGovern Medical School, Baylor College of Medicine, Rice University and the University of Guelph, Ontario, Canada have reported a new class of light-triggered channels that promise to pave the way for to pave fast and efficient optical neural silencing.
Published in nature neurosciencedescribe researchers how they identified the first natural light-gated potassium (potassium) channel rhodopsins (KCRs).
“A light-activated potassium channel has long been sought as a neuron silencer because potassium conduction naturally and universally hyperpolarizes neuron membranes, terminating action potentials and returning depolarized neurons to their resting membrane potential,” said study lead author Dr. John Spudich, Robert A Welch Distinguished Chair in Chemistry at McGovern Medical School.
By systematically screening uncharacterized opsins (proteins that bind to light-responsive chemicals) for their electrophysiological properties, researchers searched for a channel rhodopsin with an elusive potassium selectivity using a patch-clamp photocurrent screen of opsin-encoding genes without known function expressed in HEK293 cells utilized.
“Our screening strategy involves emphasizing opsins from organisms that differ in their metabolism and habitats from previously studied opsin-containing organisms and are therefore more likely to have evolved different opsin functions adapted to different selection pressures during their evolution,” says said.
“This strategy led us to two opsin-encoding genes from the sequenced genome of Hypohochytrium catenoides, a non-photosynthetic, heterotrophic fungal-like protist that is both phylogenetically and physiologically distant from algae and contains the closely related sodium-selective CCRs.”
“We found that the two H. catenoides channel rhodopsins – we named HcKCR1 and HcKCR2 for H. catenoides potassium channel rhodopsins 1 and 2 – were highly selective for potassium over sodium, in contrast to all other known channel rhodopsins .” said Dr. Elena Govorunova, associate professor in the Spudich laboratory and first author.
“In particular, the permeability ratio (PK/PNa) of 23 makes HcKCR1 a powerful hyperpolarizing tool for suppressing excitable neurons that fire upon illumination.”
The laboratory of Dr. Mingshan Xue at Baylor and the Cain Foundation Laboratories, Jan and Dan Duncan Neurological Research Institute at Texas Children’s Hospital then tested these new tools in neurons.
“When my student Yueyang Gou expressed HcKCR1 in mouse neurons and applied a flash of light, the neurons became electrically still. This channel overcomes many limitations of previous inhibitors and will be a crucial tool to help us understand brain functions,” said Xue, a Baylor faculty member and co-author of this work.
PhD student Xiaoyu Lu at Baylor and Rice University’s St Pierre lab then demonstrated that silencing can also be achieved using two-photon excitation, a popular technique to target single neurons in vivo with high spatiotemporal resolution.
“The two-photon control of KCRs could allow neuroscientists to decipher which neurons are crucial for certain behaviors and when their activity is important,” said Dr. François St-Pierre, Assistant Professor of Neuroscience at Baylor and McNair Fellow and co-author of this work.
“This work is a wonderful example of how multi-agency collaborations in Houston produce innovative research. Houston is emerging as a premier location for the development and application of cutting-edge molecular neurotechnology,” said St-Pierre.
In the future, the group will evaluate the ability of KCRs to silence neurons in vivo and further investigate their biophysical mechanisms to develop even better variants. In the long term, they also hope that KCRs could be used to treat potassium channelopathies such as epilepsy, Parkinson’s disease and long QT syndrome, as well as other cardiac arrhythmias.
About this news from neuroscientific research
Author: Graciela Gutierrez
Source: Baylor College of Medicine
Contact: Graciela Gutierrez – Baylor College of Medicine
Picture: The image is in the public domain
Original research: Closed access.
“Discovery of long-sought light-gated potassium channels: natural potassium channel rhodopsins” by John Spudich et al. nature neuroscience
Discovery of long-sought light-gated potassium channels: natural potassium channel rhodopsins
We report light-gated channels in a fungus-like protist that are highly selective for K+ about well+.
These microbial rhodopsin channels, called potassium channelrhodopsins, enable robust inhibition of mouse cortical neurons with millisecond precision.
In addition, potassium channelrhodopsins reveal a previously unknown mechanism of potassium selectivity.