Electrical engineering in the nervous system
Our thoughts and sensations originate as electrochemical signals that are passed along complex networks of nerves. They begin when a nerve generates an electrical charge that runs along the cell to the other end, causing the release of tiny neurotransmitters. These molecules jump across a gap called a synapse and open channels in the membranes of neighboring cells. The channels allow the entry of charged particles, generating an impulse in the next neuron that can be passed further through the nervous system. Whether a signal is amplified, dampened, or transmitted at all depends on the opening and closing of the channels. This process has been hard to study at "high resolution" because it has been difficult for researchers to control the behavior of specific channels in individual cells. Inés Ibañez-Tallon's lab at the MDC has now developed a new method of precisely controlling individual channels, which will permit a much more detailed study of the behavior of the nervous system as it responds to sensations such as pain. The work appears in the March 7 edition of Nature Methods.
Creating artificial toxins tethered to the surface of nerve cells has given Ines and her colleagues new tools to control and study the activity of channel proteins in specific neurons. Under the microscope, a fluorescent marker reveals where the tether is active in cells.
PhD students Sebastian Auer, Annika Stürzebecher and other members of the lab take advantage of certain natural toxins that are able to block such ion channels in nerve cells. For example MVIIA, found in the venom of cone snails that live in tropical oceans, inhibits a channel that normally allows the passage of charged calcium atoms. This has made it of interest to the pharmaceutical industry because it deadens pain signals; scientists have estimated that it might be 100 to 1000 times as potent as morphine.
Researchers have used MVIIA and other toxins to lower the receptivity of nerve cells and study the transmission of pain stimuli and other types of signals. But these molecules do not allow very precise control because they have a global effect on many cells and only act for a limited amount of time.
The lab wanted a better tool that could be applied to selected nerves in a network. This would allow them to study the roles of specific cells, like interrupting a single connection on the electronics board of a computer to study its functions.
Making such a tool required rebuilding the MVIIA toxin so that it could be attached to the surface of a nerve. There it could act as a doorkeeper to lock up a specific type of channel that transmits sensations such as pain.
The scientists built a new version of MVIIA with some additional features that could be added to the genome of neurons in cell cultures as well as to living mice. The new molecule had a tether that would attach it to the membranes of nerve cells, near the target channels. It also had a fluorescent reporter module that would allow the scientists to study its location and activity under the microscope. They also made similar versions of additional toxins derived from poisonous spiders and other organisms, which affect other types of channels.
In the next step, Sebastian, Annika and their colleagues introduced the genes for the new molecules into the brains of mice. These genes contained additional control modules that would only make them active in cells in specific regions of the brain at a timepoint that the scientists chose. They discovered that the modified toxins were just as effective at blocking signals as their natural counterparts, but could be activated in precise neural circuits for long periods of time.
"The approach of 'tethered' toxins that our group developed has already been used effectively in other model organisms such as the fruit fly and zebrafish," Inés says, "but this is the first time this has been done in a living mammal. Since the mouse nervous system is much more similar to that of humans, it should provide deep insights into how specific neural circuits contribute to the transmission of pain and other sensations. And unlike natural toxins, these molecules remain active over an animal's lifetime. This gives us a unique chance to study the role of specific channels in the transmission of sensations like pain, in precise locations of the nervous system, over the long term."
- Russ Hodge
Highlight Reference:
Silencing neurotransmission with membrane-tethered toxins. Auer S, Stürzebecher AS, Jüttner R, Santos-Torres J, Hanack C, Frahm S, Liehl B, Ibañez-Tallon I. Nat Methods. 2010 Mar;7(3):229-36. Epub 2010 Feb 7.
