Transient intra- and extra-cellular thermometry as probes of thermogenesis

Temperature is a fundamental thermodynamic property affecting every biochemical reaction in cellular milieu. Thermometry in tissues has proven useful in understanding thermoregulatory neuronal circuits and cancer metabolism. In contrast, intracellular temperature changes are relatively less explored. Theoretical temperature changes in intracellular organelles are widely debated, due to lack of understanding of intracellular thermal resistances. There is thus a need for thermometry techniques that can probe within a cell. Such intracellular thermometry can inform theory, and more importantly, provide insight into the role of temperature as a physiological parameter in intracellular studies. In this work, we describe an intracellular thermometry technique developed using silicon-based microelectromechanical techniques. We fabricated a 5 μm wide micro-thermocouple probe that has a calibration accuracy of 1% and a time constant of 32 μs. Through this probe, we measured transient temperature changes during stimulated mitochondrial proton uncoupling in neurons of Aplysia californica. We find that a transient proton motive force dissipation is more dominant than steady-state substrate oxidation in stimulated thermogenesis. Our measurements demonstrate the utility of transient intracellular thermometry in better understanding the thermochemistry of stimulated mitochondrial metabolism. Using insights from intracellular thermometry, we theoretically examine the validity of thermal conductivity approximation and find that the thermal interfacial resistances might dominate in the sub-cellular region. We develop a generalized thermal resistance network model to analyze cellular-level temperature changes. We find that intracellular temperature changes could be useful to probe stimulated transient biochemical reactions that can produce higher intracellular temperatures, which may not occur endogenously. On the other hand, to probe endogenously thermogenic reactions, we find extracellular thermometry to be better suited, especially at length-scales > 1 cm, such as tissues or organs. To this end, we develop a wireless temperature measurement technique using magnetostriction based sensors that can potentially measure temperatures at tissues length-scales remotely. We identify material properties that influence temperature sensitivity and demonstrate a 5-fold improvement through optimal selection. We further develop techniques that reduce instrument complexity and discuss ways to miniaturize wireless sensors. Overall, this work advances intra- and extra-cellular thermometry techniques that potentially provide unprecedented insight into thermogenesis in cells