Electrochemical Microphysiometry Detects Cellular Glutamate Uptake

Abstract

Glutamate is the principal excitatory amino acid in the vertebrate nervous system and is responsible for learning and memory. Understanding of these complex biological processes can be gained through experimentally accessible systems of glutamate detection. In this work, the microclinical analyzer (μCA) was used with a sensitive and stable glutamate sensor and model neuronal cells for quantitative glutamate detection under physiological relevant shear. Glutamate was detected by immobilized glutamate oxidase on a screen-printed electrode array. The sensor's linear range spanned glutamate's physiological to pathophysiological concentration range, and the biologically relevant sub-second to month temporal range. After 11 hours of use, the sensor retained 91 ± 1% of its signal, and it was able to be stored for a month without a significant decrease. When model neuronal cells were integrated into the μCA bioreactor and exposed to glutamate, they initially took up 210 ± 100 μmoles of glutamate, which increased to 390 ± 50 μmoles during their second exposure. These data suggest that the neurotransmitter uptake systems were functional and may be upregulated. The dynamic and durable μCA platform offers an experimentally accessible system of glutamate detection that can be used to monitor glutamate metabolism and signaling. Glutamate is one of the 20 canonical amino acids that together provide the structural and enzymatic foundation of proteins. Alone, glutamate plays a very different role and is itself an excitatory signaling molecule widely distributed throughout the central nervous system. In fact, glutamate is the most prevalent neurotransmitter and its function is essential for proper neurocognition, learning, and memory. Overactivation of glutamate receptors causes excitotoxicity, a pathological process whereby neurons are damaged or killed, which may result in neurodegeneration.1,2 Because of its central role in metabolic and cognitive processes, a strong effort has been put toward developing methods to detect glutamate. [2][3][4][5] Accurate detection of glutamate can be accomplished using many techniques, including spectrometry, spectroscopy, and electrochemistry. The benefits of spectrometry and spectroscopy include sensitivity and selectivity. However, mass spectrometry requires chromatographic separation or vacuum preparation, which decreases the temporal resolution of the system. 4 Although spectroscopic techniques, such as the iGluSNFR, 2 offer impressive temporal resolution, they require optical transparency thereby limiting the scope of samples that can be analyzed. In contrast, electrochemical sensors require almost no sample manipulation and can be placed directly in the area of interest, allowing, for example, detection of glutamate and dopamine signaling in the brain.6-8 Electrochemical sensors are also versatile: They can be made on the nanoscale, 1 have been 3D-printed, 9 and can be inexpensive.10,11 The highly translatable nature of electrochemical techniques has already been demonstrated with the a...

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