Spatiotemporal Measurement of the Temperature Changes in Acidic Organelles with Fluorescent Nanothermometers

Abstract

Fluorescence thermometry has successfully measured the temperature increase in living cells that is not only induced by external heating, but also by intracellular thermogenesis due to chemical reactions. On the other hand, living cells respond to temperature changes in the form of varying intracellular Ca2+ concentrations, pH adjustment, and so on. The chemical reactions in living cells cause the local and temporary temperature changes, which have a potential to modulate the activity of biological systems in the surrounding cells in addition to themselves. The viewpoint of “Thermo-Chemical Signaling” should be essential for understanding how the living cells efficiently use the local temperature changes around and inside them. The dynamics of the intracellular environment involved in the biological activity, however, disrupts a reliable temperature measurement. The conventional fluorescent thermometers are also sensitive to other environmental parameters such as pH and/or ionic strength, therefore the accurate temperature measurements are complicated. Here, we newly designed fluorescent nanoparticles, termed “nanothermometers”, which were insensitive to pH and ionic strength. The fluorescence intensity of each nanothermometer changes and reaches a steady level at least within 17 msec. The nanothermometers were spontaneously uptaken into living HeLa cells via endocytosis, enclosed in acidic organelle, i.e., endosome/lysosome, and then transported along microtubules. Moreover, the majority of the transported nanothermometers were accelerated responding to microscopic heat pulse with IR laser. In this study, we demonstrate that the nanothermometers can measure the position of moving acidic organelle, endosome/lysosome, and its temperature change as “walking nanothermometers”. The methods to simultaneously measure both microscopic temperature and molecular activities within living cells should shed light on the unrevealed cellular processes related to the temperature-sensitive organelle transport.