Optochemical sensor based systems for the analysis of cell metabolism and bioenergetics: an overview

Optochemical sensor based systems for analysis of cell metabolism are well established and continue to grow providing valuable and detailed information on cell bioenergetics, metabolic signature and ability to withstand stress conditions and drug treatments. The area is currently dominated by the Seahorse/Agilent XF (eXtracellular Flux) Analyzer and Luxcel/Agilent MitoXpress platforms, which provide measurement of Oxygen Consumption (OCR) and Extracellular Acidification (ECA) rates under different physiological conditions (metabolic substrates, drugs, stressors), by means of fluorescent and phosphorescent pH and O2 sensors or probes. The XF system uses solid-state sensors and specialized microplates, it is highly integrated, sensitive and fast, and user-friendly. However, the outdated sensor chemistry and basic fluorescence intensity readouts limit its performance (unstable sensor calibrations, cross-talk) and make it expensive. The MitoXpress platform, which uses soluble probes, standard microplates and plate readers, provides stable and accurate lifetime based sensing of O2 and pH with internal referencing and no cross-talk. However, its cost-efficient DIY (do it yourself) approach is less sensitive and less user-friendly than the XF. Several new dual pH/O2 sensing platforms are emerging, which can potentially improve the above systems and overcome their limitations. Examples include: i) a meso-substituted Pt-porphyrin Schiff base dye, which senses pH and O2 via phosphorescence intensity and lifetime changes; ii) the fully-referenced solid-state dual sensor based on a pH-sensitive fluorescent porphyrin and an O2-sensitive phosphorescent Pt-porphyrin. iii) a sensor with near-infrared fluorescent pH-indicator and phosphorescence O2 indicator dyes and phase measurements at multiple frequencies. In this paper, we discuss the merits and limitations of the different sensor systems for cell analysis.