Mitochondrial temperature homeostasis resists external metabolic stresses
Based on studies with a fluorescent reporter dye, Mito Thermo Yellow, and the genetically encoded gTEMP ratiometric fluorescent temperature indicator targeted to mitochondria, the temperature of active mitochondria in four mammalian and one insect cell-line was estimated to be up to 15 {degrees}C above that of the external environment to which the cells were exposed. High mitochondrial temperature was maintained in the face of a variety of metabolic stresses, including substrate starvation or modification, decreased ATP demand due to inhibition of cytosolic protein synthesis, inhibition of the mitochondrial adenine nucleotide transporter and, if an auxiliary pathway for electron transfer was available via the alternative oxidase, even respiratory poisons acting downstream of OXPHOS complex I. We propose that the high temperature of active mitochondria is an inescapable consequence of the biochemistry of oxidative phosphorylation and is homeostatically maintained as a primary feature of mitochondrial metabolism. IMPACT STATEMENTMitochondria are up to 15 {degrees}C hotter than their external environment in living cells. In response to diverse metabolic stresses, mitochondrial temperature re-adjusts to this value whenever possible.