Cellular Thermometry Considerations for Probing Biochemical Pathways

Abstract

Temperature is a fundamental thermodynamic property that can serve as a probe of biochemical reactions. Extracellular thermometry has previously been used to probe cancer metabolism and thermoregulation, with measured temperature changes of ~1-2 K in tissues, consistent with theoretical predictions. In contrast, previous intracellular thermometry studies remain disputed due to reports of >1 K intracellular temperature rises over 5 min or more that are inconsistent with theory. Thus, the origins of such anomalous temperature rises remain unclear. An improved quantitative understanding of intracellular thermometry is necessary to provide a clearer perspective for future measurements. Here, we develop a generalizable framework for modeling cellular heat diffusion over a range of subcellular-to-tissue length scales. Our model shows that local intracellular temperature changes reach measurable limits (> 0.1 K) only when exogenously stimulated. On the other hand, extracellular temperatures can be measurable (> 0.1 K) in tissues even from endogenous biochemical pathways. Using these insights, we provide a comprehensive approach to choosing an appropriate cellular thermometry technique by analyzing thermogenic reactions of different heat rates and time constants across length scales ranging from sub-cellular to tissues. Our work provides clarity on cellular heat diffusion modeling and on the required thermometry approach for probing thermogenic biochemical pathways.

Challenges for optical nanothermometry in biological environments

“…Regarding theoretical modelling, sources of inaccuracy are insufficient data on the validity of the thermodynamic equations that apply in the complex cell environment, and the accuracy of the thermal constants used for cell components (thermal conductivity, mainly). 27 However, as more thermometric materials and sensing strategies have been applied to the measurement of intracellular temperature (fluorescent and non-fluorescent, a good summary can be found in ref. 10), all providing consistent results, a higher reliability of experimental data is accepted.…”

Local Temperature Increments and Induced Cell Death in Intracellular Magnetic Hyperthermia

“… 30 , 43 , 44 Moreover, cell temperature gradients go beyond intracellular temperature measurements and extend to endogenous heat generation in general. 32 , 40 , 45 − 47 A detailed discussion of this controversy, which extends to many other nanoscale heat transfer phenomena, 48 is provided in Section I in the Supporting Information .…”

“…Up to now, distinct luminescent intracellular thermometers have been reported, including organic dyes, , oligonucleotides, fluorescent proteins, − polymers, , nanodiamonds, , quantum dots, and lanthanide-doped nanoparticles. − Intracellular thermometry measurements in living cells have shown spontaneous thermogenesis, − inhomogeneous temperature progressions, , and “hot organelles”, including the mitochondria, , the nucleus, and nucleoli . These reports, however, have been hotly debated, given the questions surrounding the observation of intracellular temperature gradients based on simple thermodynamic arguments that predicted intracellular temperature fluctuations of less than several millikelvins. ,, Moreover, cell temperature gradients go beyond intracellular temperature measurements and extend to endogenous heat generation in general. ,,− A detailed discussion of this controversy, which extends to many other nanoscale heat transfer phenomena, is provided in Section I in the Supporting Information.…”