Dramatic changes in mitochondrial substrate use at critically high temperatures: a comparative study usingDrosophila

Abstract

Ectotherm thermal tolerance is critical to species distribution, but at present the physiological underpinnings of heat tolerance remain poorly understood. Mitochondrial function is perturbed at critically high temperatures in some ectotherms, including insects, suggesting that heat tolerance of these animals is linked to failure of oxidative phosphorylation (OXPHOS) and/or ATP production. To test this hypothesis we measured mitochondrial oxygen consumption rates in six Drosophila species with different heat tolerance using high-resolution respirometry. Using a substrate-uncoupler-inhibitor titration protocol we examined specific steps of the electron transport system to study how temperatures below, bracketing and above organismal heat limits affected mitochondrial function and substrate oxidation. At benign temperatures (19 and 30°C), complex I-supported respiration (CI-OXPHOS) was the most significant contributor to maximal OXPHOS. At higher temperatures (34, 38, 42 and 46°C), CI-OXPHOS decreased considerably, ultimately to very low levels at 42 and 46°C. The enzymatic catalytic capacity of complex I was intact across all temperatures and accordingly the decreased CI-OXPHOS is unlikely to be caused directly by hyperthermic denaturation/inactivation of complex I. Despite the reduction in CI-OXPHOS, maximal OXPHOS capacities were maintained in all species, through oxidation of alternative substrates; proline, succinate and, particularly, glycerol-3-phosphate, suggesting important mitochondrial flexibility at temperatures exceeding the organismal heat limit. Interestingly, this failure of CI-OXPHOS and compensatory oxidation of alternative substrates occurred at temperatures that tended to correlate with species heat tolerance, such that heat-tolerant species could defend “normal” mitochondrial function at higher temperatures than sensitive species. Future studies should investigate why CI-OXPHOS is perturbed and how this potentially affects ATP production rates.

Flexible Thermal Sensitivity of Mitochondrial Oxygen Consumption and Substrate Oxidation in Flying Insect Species

“…Therefore, mitochondrial thermal sensitivity has been put forward to tentatively explain organismal failure at high temperatures and is believed to be paramount for thermal adaptation of several organisms ( Fangue et al, 2009 ; Iftikar et al, 2014 ; Chung et al, 2018 ; Harada et al, 2019 ; Pichaud et al, 2019 ; Hraoui et al, 2020 ; Hraoui et al, 2021 ). However, the study of insects has often been overlooked mainly due to technical reasons which resulted in a gap of knowledge concerning mitochondrial functions and the implication of mitochondrial adaptation in insects at high temperature [but see ( Davison and Bowler, 1971 ; Kashmerry and Bowler, 1977 ; Chamberlin, 2004 ; Pichaud et al, 2010 ; Pichaud et al, 2011 ; Pichaud et al, 2012 ; Pichaud et al, 2013 ; Jørgensen et al, 2021 )]. The population structure and geographic distribution of insects are thus linked to the plasticity of mitochondria and their capacity to buffer daily temperature fluctuations encountered during summer.…”

“…Although mitochondria have been suggested as a putative site of dysfunction resulting in organismal failure at high temperature, no specific mechanism has been identified so far due to their complexity and their involvement in a wide range of metabolic and signalling processes. In a recent study using a comparative model of six different Drosophila species with different thermal tolerance, we showed that heat tolerance (CT max ) was correlated with failure of mitochondrial oxygen consumption at the level of complex I and increased utilization of alternative oxidative substrates ( Jørgensen et al, 2021 ). Specifically, the mitochondrial oxygen consumption supported by complex I substrates was drastically reduced at temperatures near CT max of the specific species, and glycerol-3-phosphate (G3P) oxidation by the mitochondrial glycerol-3-phosphate dehydrogenase (mtG3PDH) was augmented ( Jørgensen et al, 2021 ).…”

“…In a recent study using a comparative model of six different Drosophila species with different thermal tolerance, we showed that heat tolerance (CT max ) was correlated with failure of mitochondrial oxygen consumption at the level of complex I and increased utilization of alternative oxidative substrates ( Jørgensen et al, 2021 ). Specifically, the mitochondrial oxygen consumption supported by complex I substrates was drastically reduced at temperatures near CT max of the specific species, and glycerol-3-phosphate (G3P) oxidation by the mitochondrial glycerol-3-phosphate dehydrogenase (mtG3PDH) was augmented ( Jørgensen et al, 2021 ). This suggests that mitochondria are extremely flexible in the context of temperature changes and have the ability to switch from a “standard” metabolic pathway to a more heat-tolerant pathway, potentially to sustain higher energetic turnover and maintain homeostasis when temperature increases.…”

“…This suggests that mitochondria are extremely flexible in the context of temperature changes and have the ability to switch from a “standard” metabolic pathway to a more heat-tolerant pathway, potentially to sustain higher energetic turnover and maintain homeostasis when temperature increases. Although this mitochondrial flexibility driven by temperature has been demonstrated in different Drosophila species ( Jørgensen et al, 2021 ), it is important to use a wider phylogenetic comparison of insect orders to assess the fine regulations of such complex responses and then be able to generalize to a wide range of animals ( González-Tokman et al, 2020 ).…”

Mitonuclear Interactions Produce Diverging Responses to Mild Stress in Drosophila Larvae

“…We note that the mean summer temperatures in Barcelona are 5–8°C greater than Oregon, imposing greater metabolic demands on BAR mitochondria (as the Q 10 suggests a doubling in metabolic rate for every 10°C rise in temperature). While the Oregon strain was collected in 1925, hence has had nearly a century to adapt to lab conditions, differences in heat tolerance at the species level can persist for decades in the lab, and correspond to differences in complex I and substrate use ( Jørgensen et al, 2019 , 2021 ). It is possible that BAR flies could have a respiratory architecture adapted to higher temperatures.…”