Two-point calibration protocol for the Förster Resonance Energy Transfer indicator Pyronic in neurons

Significance: Pyruvate is a nodal intermediate in cellular metabolism, positioned at the crossroads between glycolysis and fermentative metabolism. It is exchanged between the intracellular and extracellular compartments through the proton-coupled monocarboxylate transporters and between the cytosol and mitochondria through the mitochondrial pyruvate carrier, where it serves as a primary carbon source for respiration.

Aim: Our goal is to present a detailed protocol for quantifying cytosolic pyruvate concentration in neurons at single-cell resolution using a minimally invasive, two-point calibration approach with the Förster Resonance Energy Transfer (FRET)-based genetically encoded fluorescent indicator Pyronic.

Approach: This protocol is based on a noninvasive pharmacological two-point calibration approach, where Pyronic's dynamic range ( $\Delta R_{\mathrm{MAX}}$ ) is established by withdrawing all extracellular substrates to deplete intracellular pyruvate ( $R_{\mathrm{MIN}}$ ) and by inducing Pyronic saturation ( $R_{\mathrm{MAX}}$ ) through the combination of inhibition of pyruvate export, stimulation of its production, and blockade of its mitochondrial consumption. The protocol also incorporates the previously published $K_D$ values for Pyronic obtained from in vitro experiments. This procedure does not require the use of detergents to permeabilize the cells.

Results: Implementing this protocol enables the measurement of absolute cytosolic pyruvate concentrations. This quantitative parameter facilitates comparisons of pyruvate metabolism across different cells, samples, and experimental batches, thereby enabling the comparison between a plethora of experimental conditions.

Conclusion: The FRET-based fluorescent indicator Pyronic can be reliably calibrated using a minimally invasive, pharmacology-based two-point calibration protocol in neurons, thus providing a robust and quantitative method to study pyruvate metabolism under various physiological and pathological scenarios.

Pyronic; calibration; fluorescence; metabolism; neurons; pyruvate; quantification.