To understand the properties of networked nerve cells, researchers switch the cells on and off with light and observe the resulting behavior—a process known as optogenetics. To that end, researchers at the Karlsruhe Institute of Technology (KIT) in Germany and colleagues have found protein that facilitates optogenetic control of nerve cells—and could be used as a basis in studying diseases of the nervous system.
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With optogenetics, researchers use certain proteins—called channelrhodopsins—that form ion channels in the cell membrane. If light strikes the channels, they open and ions enter and render the cell active or inactive. In this way, a very fine tool is obtained to study functions in the network of nerve cells. So far, however, large amounts of light have been required and only closely limited areas in the network could be switched. The channelrhodopsin the researchers found reacts about 10,000 times more sensitively to light than other proteins used so far for switching off nerve cells.
“For the modification of the protein, we analyzed its structure on the computer,” explains Prof. Marcus Elstner, a theoretical chemist at KIT's Institute of Physical Chemistry. Elstner and his team modeled the proteins that consist of about 5000 atoms using the highest-performance computers in KIT’s Steinbuch Centre for Computing (SCC). Together with the protein environment (the cell membrane and cell water), about 100,000 atoms had to be considered for the computations that took several weeks. “It was found that ion conductivity of the channel is essentially based on three amino acids in the central region; i.e., on about 50 atoms in the channel only,” he explains. By exchanging the amino acids, scientists have now succeeded in increasing the sensitivity of the ion channel.
Within the framework of the study, researchers from Karlsruhe, Hamburg, and Berlin developed the ion channels further. Jonas Wietek and Nona Adeishvili, working in the team of Peter Hegemann at the Humboldt-Universität Berlin, have succeeded in identifying the selectivity filter of the channelrhodopsins and in modifying it such that negatively charged chloride ions are conducted. These chloride-conducting channels have been called ChlocC by the scientists. Hiroshi Watanabe from Elstner's team computed ion distribution in the protein and visualized the increased chloride distribution. Simon Wiegert from the team of Thomas Oertner of the Center for Molecular Neurobiology, Hamburg, demonstrated that ChlocC can be introduced into selected neurons for the inactivation of the latter, with very small light intensities similar to the processes taking place in the living organism.
With ChlocC, a novel optogenetic tool is now available that can be used in neuroscience to study the switching of neural networks together with the already known light-activated cation channels that mainly conduct sodium ions and protons. This fundamental knowledge might help better understand the mechanisms of diseases like epilepsy and Parkinson’s and could someday lead to new, more targeted therapies.
Full details of the work appear in the journal Science; for more information, please visit http://dx.doi.org/10.1126/science.1249375.
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