SENSING NEURONAL ACTIVITY WITH LIGHT
For years,
neuroscientists have been trying to develop tools that would allow them to
clearly view the brain's circuitry in action -- from the first moment a neuron
fires to the resulting behavior in a whole organism. To get this complete
picture, neuroscientists are working to develop a range of new tools to study
the brain. Researchers at Caltech have developed one such tool that provides a
new way of mapping neural networks in a living organism.
Hunt Morgan Professor
of Biology, to test Archer1 as a sensor in a living organism -- the tiny
nematode worm C. elegans. "There are a few reasons why we used
the worms here: they are powerful organisms for quick genetic engineering and
their tissues are nearly transparent, making it easy to see the fluorescent
protein in a living animal," she says.
After incorporating
Archer1 into neurons that were a part of the worm's olfactory system -- a
primary source of sensory information for C. elegans -- the
researchers exposed the worm to an odorant. When the odorant was present, a
baseline fluorescent signal was seen, and when the odorant was removed, the
researchers could see the circuit of neurons light up, meaning that these
particular neurons are repressed in the presence of the stimulus and active in
the absence of the stimulus. The experiment was the first time that an Arch
variant had been used to observe an active circuit in a living organism.
Gradinaru next hopes
to use tools like Archer1 to better understand the complex neuronal networks of
mammals, using microbial opsins as sensing and actuating tools in
optogenetically modified rodents.
"For the future
work it's useful that this tool is bifunctional. Although Archer1 acts as a
voltage sensor under red light, with green light, it's an inhibitor," she
says. "And so now a long-term goal for our optogenetics experiments is to
combine the tools with behavior-controlling properties and the tools with
voltage-sensing properties. This would allow us to obtain all-optical access to
neuronal circuits. But I think there is still a lot of work ahead."
One goal for the
future, Gradinaru says, is to make Archer1 even brighter. Although the
protein's fluorescence can be seen through the nearly transparent tissues of
the nematode worm, opaque organs such as the mammalian brain are still a
challenge. More work, she says, will need to be done before Archer1 could be
used to detect voltage changes in the neurons of living, behaving mammals.
And that will require
further collaborations with protein engineers and biochemists like Arnold.
"As
neuroscientists we often encounter experimental barriers, which open the
potential for new methods. We then collaborate to generate tools through
chemistry or instrumentation, then we validate them and suggest optimizations,
and it just keeps going," she says. "There are a few things that we'd
like to be better, and through these many iterations and hard work it can
happen."
The work published in
both papers was supported with grants from the National Institutes of Health
(NIH), including an NIH/National Institute of Neurological Disorders and Stroke
New Innovator Award to Gradinaru; Beckman Institute funding for the BIONIC
center; grants from the U.S. Army Research Office as well as a Caltech Biology
Division Training Grant and startup funds from Caltech's President and Provost,
and the Division of Biology and Biological Engineering; and other financial
support from the Shurl and Kay Curci Foundation and the Life Sciences Research
Foundation.
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