Bioluminescent Genetically Encoded Glutamate Indicators for Molecular Imaging of Neuronal Activity

Genetically encoded optical sensors and advancements
in microscopy
instrumentation and techniques have revolutionized the scientific
toolbox available for probing complex biological processes such as
release of specific neurotransmitters. Most genetically encoded optical
sensors currently used are based on fluorescence and have been highly
successful tools for single-cell imaging in superficial brain regions.
However, there remains a need to develop new tools for reporting neuronal
activity in vivo within deeper structures without
the need for hardware such as lenses or fibers to be implanted within
the brain. Our approach to this problem is to replace the fluorescent
elements of the existing biosensors with bioluminescent elements.
This eliminates the need of external light sources to illuminate the
sensor, thus allowing deeper brain regions to be imaged noninvasively.
Here, we report the development of the first genetically encoded neurotransmitter
indicators based on bioluminescent light emission. These probes were
optimized by high-throughput screening of linker libraries. The selected
probes exhibit robust changes in light output in response to the extracellular
presence of the excitatory neurotransmitter glutamate. We expect this
new approach to neurotransmitter indicator design to enable the engineering
of specific bioluminescent probes for multiple additional neurotransmitters
in the future, ultimately allowing neuroscientists to monitor activity
associated with a specific neurotransmitter as it relates to behavior
in a variety of neuronal and psychiatric disorders, among many other
applications.