As you are reading these words, certain regions of your
brain are displaying a flurry of millisecond-fast electrical activity.
Visualizing and measuring this electrical activity is crucial to understand how
the brain enables us to see, move, behave or read these words. However,
technological limitations are delaying neuroscientists from achieving their
goal of improving the understanding of how the brain works.
Scientists at Baylor College of Medicine and collaborating
institutions report in the journal Cell a new sensor that allows
neuroscientists to image brain activity without missing signals, for an
extended time and deeper in the brain than previously possible. This work is
paving the way to new discoveries on how the brain functions in awake, active
animals both those that are healthy and those with neurological conditions.
The holy grail of neuroscience
'Not only is the brain's electrical activity very fast,
it also involves a variety of cell types that have different roles in brain
computations,' said corresponding author, Dr. François St-Pierre,
assistant professor of neuroscience and a McNair scholar at Baylor. He also is
an adjunct assistant professor of electrical and computer sciences at Rice
University. 'It has been challenging to figure out how to noninvasively
observe the millisecond-fast electrical activity in individual neurons of
specific cell types in animals carrying on an activity. To be able to do this
has been the holy grail of neuroimaging.'
There are existing technologies to measure electrical
activity in the brain. 'For example, electrodes can record very fast
activity, but they cannot tell what type of cells they are listening to,'
St-Pierre said.
Researchers also are using fluorescent proteins that respond
to calcium changes associated with electrical activity. These changes in
fluorescence can be followed using a 2-photon microscope. 'This kind of
sensor is excellent to determine which neurons are active and which are not.
However, they are very slow. They measure voltage changes indirectly, thereby
missing a lot of key signals.'
The goal of St-Pierre and his colleagues was to combine the
best of these methodologies -- to develop a sensor that can monitor activity in
specific cell types while capturing fast brain signals. 'We have achieved
this with a new generation of engineered fluorescent proteins called
genetically-encoded voltage indicators or GEVIs,' St-Pierre said.
Co-first authors -- Zhuohe (Harry) Liu, Xiaoyu (Helen) Lu
and Yueyang (Eric) Gou -- created and used an automated system that provided a
better and more efficient way to engineer and optimize fluorescent voltage
indicators for two-photon microscopy, the standard method for noninvasive
deep-tissue imaging in neuroscience. 'Using this system, we tested
thousands of indicator variants and identified JEDI-2P, which is faster,
brighter and more sensitive and photostable than its predecessors,' said
Liu, a graduate student in Electrical and Computer Engineering at Rice who is
working in the St-Pierre lab.
'With JEDI-2P, we solved three important drawbacks of
previous methods,' said Lu, a graduate student from the Systems, Synthetic
and Physical Biology (SSPB) program at Rice who is working in the St-Pierre
lab. 'First, it allows us to follow electrical activity in a living animal
for as long as 40 minutes instead of at most a few minutes. Second, we can
image spikes of electrical activity with a temporal resolution of about one
millisecond, and third we can image individual cells deeper in the brain
because our indicator is bright and produces large signals in response to brain
activity.'
Until now, researchers were limited to observe the surface
of the brain, 'but most of brain activity is obviously not confined to the
first 50 microns below the brain surface,' St-Pierre said. 'Our
methodology allows researchers to non-invasively monitor voltage signals in
deep layers of the cortex for the first time,' said Gou, a previous member
of the St-Pierre lab who is now in the Neuroscience Graduate Program at Baylor.
Baylor co-authors Dr. Andreas Tolias, professor of
neuroscience, and Dr. Jacob Reimer, assistant professor of neuroscience,
demonstrated that JEDI-2P can report electrical activity in mice using imaging
equipment available in many neuroimaging labs. Co-author Stéphane Dieudonné
(École Normale Supérieure, France) showed deep and ultrafast detection of brain
electrical signals in mice by monitoring JEDI-2P fluorescence with a rapid
microscopy method called ULoVE.
The labs of co-authors Drs. Katrin Franke (Group leader,
University of Tübingen, Germany) and Tom Clandinin (Stanford University) showed
how JEDI-2P could also be applied to image electrical activity in the retina
and in flies, respectively. Taken together, this international collaborative
effort demonstrated that the new technology could be readily deployed by
neuroscience groups working on different animal models and using various
microscopy techniques.
'In 2014, I gave a presentation at the Society of
Neuroscience meeting about the first version of this indicator and people were
rolling their eyes. They thought that rapid voltage imaging with fluorescent
indicators would never be possible in awake animals because of the tremendous
technical challenge of imaging millisecond-timescale activity,' St-Pierre
said. 'Eight years later, we have achieved this goal. And there is still
room to evolve the indicator -- it won't be the last JEDI!'
Other contributors to this work include Vincent Villette,
Kevin L. Colbert, Shujuan Lai, Sihui Guan, Michelle A. Land, Jihwan Lee, Tensae
Assefa, Daniel R. Zollinger, Maria M. Korympidou, Anna L. Vlasits, Michelle M.
Pang, Sharon Su, Changjia Cai, Emmanouil Froudarakis, Na Zhou, Saumil S. Patel,
Cameron L. Smith, Annick Ayon, Pierre Bizouard, Jonathan Bradley and Andrea
Giovannucci. The authors are affiliated with one or more of the following
institutions: Baylor College of Medicine, Rice University, École Normale Supérieure,
University of Tübingen, Stanford University, University of North Carolina at
Chapel Hill, North Carolina State University and the Foundation for Research
and Technology Hellas-Greece.
Resource: Science Daily