Capturing Metabolism-Dependent Solvent Polarity Fluctuations in a Trafficking Lysosome

Abstract

The eukaryotic cell compartmentalization acts as a master regulator of metabolism by arranging specific sets of molecular components in spatially confined, membrane-enclosed, intracellular structures (e.g. organelles, endosomes, vesicles). Here, peculiar physicochemical properties of the local environment do emerge and participate to the regulation of molecular processes. In spite of the huge amount of available environmental probes, experiments on sub-cellular structures are severely challenged by their restless 3D-movement. Classical optical microscopy tools fail to subtract the 3D evolution of the system while preserving the temporal resolution required to probe the physicochemical properties of the local environment. This bottleneck is tackled by focusing an excitation light-beam in a periodic orbit around the structure of interest. The recorded signal is used as feedback to localize the structure position with unprecedented temporal resolution: microseconds along the orbit, milliseconds between orbits. The lysosome is proposed as paradigmatic intracellular target. 6-acetyl-2-dimethylaminonaphthalene (ACDAN) is used as a probe of the physicochemical properties of the intra-lysosomal environment. Generalized Polarization (GP) analysis of ACDAN emission affords a quantitative view on intra-lysosomal solvent polarity. Thus, Raster Image Correlation Spectroscopy (RICS) analysis of the GP signal reveals that intra-lysosomal solvent polarity is fluctuating during natural organelle 3D trafficking, with characteristic dissipation times in the micro-to-millisecond range. In brief, it is found that: i) polarity fluctuations are a prerogative of living cells (i.e. they disappear upon cell fixation); ii) polarity fluctuations are selectively abolished by lysosomal basification; iii) polarity fluctuations depend on metabolic energy (ATP). Collectively, reported results suggest that intra-lysosomal solvent polarity fluctuates according to the ongoing organelle metabolism. The strategy proposed sheds light onto the local environment of a trafficking organelle and paves the way to new investigations of the natural, yet elusive, biochemistry of living matter at the subcellular level.