Optical measurement of synaptic glutamate spillover and reuptake by linker optimized glutamate-sensitive fluorescent reporters

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 105; no. 11; pp. 4411 - 4416
• Main Authors: Hires, Samuel Andrew, Zhu, Yongling, Tsien, Roger Y
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 18.03.2008; National Acad Sciences
• Subjects: Amino Acid Sequence; Astrocytes; Bacterial proteins; Biological Sciences; Cell Line; Dendrites; Fluorescence Resonance Energy Transfer; Genes, Reporter - genetics; Glutamic Acid - metabolism; HEK293 cells; Humans; Imaging; Measurement; Molecular Sequence Data; Neurons; Neuroscience; Neurosciences; Receptors; Sensitivity and Specificity; Sensors; Synapses; Synaptic Transmission; Truncation
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0712008105

Genetically encoded sensors of glutamate concentration are based on FRET between cyan and yellow fluorescent proteins bracketing a bacterial glutamate-binding protein. Such sensors have yet to find quantitative applications in neurons, because of poor response amplitude in physiological buffers or when expressed on the neuronal cell surface. We have improved our glutamate-sensing fluorescent reporter (GluSnFR) by systematic optimization of linker sequences and glutamate affinities. Using SuperGluSnFR, which exhibits a 6.2-fold increase in response magnitude over the original GluSnFR, we demonstrate quantitative optical measurements of the time course of synaptic glutamate release, spillover, and reuptake in cultured hippocampal neurons with centisecond temporal and spine-sized spatial resolution. During burst firing, functionally significant spillover persists for hundreds of milliseconds. These glutamate levels appear sufficient to prime NMDA receptors, potentially affecting dendritic spike initiation and computation. Stimulation frequency-dependent modulation of spillover suggests a mechanism for nonsynaptic neuronal communication.