Transient heat release during induced mitochondrial proton uncoupling

Abstract

Non-shivering thermogenesis through mitochondrial proton uncoupling is one of the dominant thermoregulatory mechanisms crucial for normal cellular functions. The metabolic pathway for intracellular temperature rise has widely been considered as steady-state substrate oxidation. Here, we show that a transient proton motive force (pmf) dissipation is more dominant than steady-state substrate oxidation in stimulated thermogenesis. Using transient intracellular thermometry during stimulated proton uncoupling in neurons of Aplysia californica , we observe temperature spikes of ~7.5 K that decay over two time scales: a rapid decay of ~4.8 K over ~1 s followed by a slower decay over ~17 s. The rapid decay correlates well in time with transient electrical heating from proton transport across the mitochondrial inner membrane. Beyond ~33 s, we do not observe any heating from intracellular sources, including substrate oxidation and pmf dissipation. Our measurements demonstrate the utility of transient thermometry in better understanding the thermochemistry of mitochondrial metabolism.

Molecular Regulations and Functions of the Transient Receptor Potential Channels of the Islets of Langerhans and Insulinoma Cells

“…It is noteworthy that β-cells have other TRP channels including TRPM4, TRPM5, TRPV1, TRPV2, and TRPV4 that are temperature sensitive. Although there is much skepticism, it has been demonstrated in other systems that temperatures inside the cells can increase dramatically, but it remains unclear whether such increases in the temperature have any signaling functions [61].…”

Pitfalls in Monitoring Mitochondrial Temperature Using Charged Thermosensitive Fluorophores

“…The presence of mitochondrial cardiolipins involved in phase transition, the abundance of mitochondrial heat shock proteins possibly acting as thermostabilizers of protein structures, and the presence of thermo-protectant solutes (reviewed in [30]) further support the concept that mitochondria sustain and operate at elevated temperatures. Finally, using different biochemical approaches, not based on charged fluorophores, large temperature differences with surrounding cytosol were observed when uncoupling mitochondria from HeLa cells [6,31] or from aplasia neurons [32]. Nevertheless, until the unmasking of the exact mitochondrial distribution of these nanothermometers, caution is needed before considering a global mitochondrial temperature.…”

“…Finally, based on theoretical considerations, some concerns have been raised against the possibility of maintaining a temperature gradient between mitochondrial subcompartments, and the cytosol [33,34], discussed in [16,35]. However, their validity appears undermined by recently proposed models of mitochondrial thermogenesis as a special case of the thermal diffusion equation and by the experimental data demonstrating the occurrence of intracellular temperature differentials [6,8,31,32,36,37]. Finally, the internal mitochondrial membrane is mainly composed of highly packed proteins, with a quite low lipid content resulting in a distinct mitochondria-specific protein/lipid ratio [38].…”

The challenge of intracellular temperature

“…Two more recent studies using microfabricated thermocouples also report a temperature increase in individual cells, one in relatively longer (Yang et al 2017) and the other on shorter time scales (Rajagopal et al 2019). The approach by Yang et al was to build their high-performance micro-thermocouple arrays, over which adherent cells were cultured, within a double-stabilized tent (Yang et al 2017).…”

“…The authors detected frequent fluctuations of about 60 mK during each measurement for over 30 h, as well as a continuous elevation up to 285 mK in one of the detection areas, while other areas remained stable in a measurement for about 40 h long. Rajagopal et al (2019) employed a biocompatible microscale thermocouple probe to detect temperature increase in the vicinity of mitochondria in Aplysia californica neurons. By stimulating targeted cells using proton uncoupler, the authors observed rapid temperature increases of about 7.5 K that can be fit by a biexponential curve.…”