RESEARCH ARTICLE Adjustments in the control of mitochondrial respiratory capacity to tolerate temperature fluctuations Katrina Y. Scott 1 , Rebecca Matthew 1 , Jennifer Woolcock 1 , Maise Silva 1,2 and He ́ lè ne Lemieux 1,3, * ABSTRACT As the world’s climate changes, life faces an evolving thermal environment. Mitochondrial oxidative phosphorylation (OXPHOS) is critical to ensure sufficient cellular energy production, and it is strongly influenced by temperature. The thermally induced changes to the regulation of specific steps within the OXPHOS process are poorly understood. In our study, we used the eurythermal species of planarian Dugesia tigrina to study the thermal sensitivity of the OXPHOS process at 10, 15, 20, 25 and 30°C. We conducted cold acclimation experiments where we measured the adjustment of specific steps in OXPHOS at two assay temperatures (10 and 20°C) following 4 weeks of acclimation under normal (22°C) or low (5°C) temperature conditions. At the low temperature, the contribution of the NADH pathway to the maximal OXPHOS capacity, in a combined pathway (NADH and succinate), was reduced. There was partial compensation by an increased contribution of the succinate pathway. As the temperature decreased, OXPHOS became more limited by the capacity of the phosphorylation system. Acclimation to the low temperature resulted in positive adjustments of the NADH pathway capacity due, at least in part, to an increase in complex I activity. The acclimation also resulted in a better match between OXPHOS and phosphorylation system capacities. Both of these adjustments following acclimation were specific to the low assay temperature. We conclude that there is substantial plasticity in the mitochondrial OXPHOS process following thermal acclimation in D. tigrina, and this probably contributes to the wide thermal range of the species. KEY WORDS: Mitochondrial respiration, Oxidative phosphorylation, Thermal sensitivity, Cold acclimation, Control, Regulation, Complex I, Phosphorylation system, Dugesia tigrina INTRODUCTION Temperature is an important variable to life processes, affecting the rate of all chemical reactions. For ectothermic organisms, which do not regulate their body temperature, adjusting their biochemical processes to temperature variations is critical for survival (Pörtner, 2002). The molecular response to temperature is complex, and not all enzymes and metabolic pathways are affected in the same order of magnitude. A comprehensive overview is needed to understand how temperature impacts different enzymes and transporters, and how unequal variations of specific steps modify not only the flow through the pathway but also the control and regulation of the pathway. Biologists are challenged with determining the metabolic adjustments essential to preserve function, homeostasis and survival of an organism encountering a thermal challenge. This is becoming increasingly important now, as we are threatened by global warming (Blier et al., 2014; Clark et al., 2013; Franklin et al., 2013). Mitochondrial oxidative phosphorylation (OXPHOS) is among the most important pathways for survival; it is responsible for a large part of the cellular energy production in eukaryotic organisms. OXPHOS provides the energy to sustain cellular functions and is linked to many other cellular activities and signalling pathways. The tight regulation of the OXPHOS process ensures balanced metabolism and controls the production of reactive oxygen species (ROS). Overproduction of ROS is associated with damage to proteins, lipids and DNA; however, in small quantities, ROS are known to be important signalling molecules playing a role in the regulation of biological processes (Finkel, 2011). Temperature, by changing OXPHOS efficiency and regulation, can affect cell signalling through modification of ROS production (Abele et al., 2002; Jarmuszkiewicz et al., 2015). Although thermal sensitivity of the electron transport system has been studied in many species, it is still not clear how temperature influences the process (Blier et al., 2014; Norin and Metcalfe, 2019). A recent study examined the thermal sensitivity of OXPHOS pathways in the mouse heart and identified two strong limiting factors at low temperature (Lemieux et al., 2017). The first is the phosphorylation system, the functional unit utilizing the protonmotive force to phosphorylate ADP into ATP (Gnaiger, 2009). The phosphorylation system includes three essential components: ATP synthase, adenine nucleotide translocase and the phosphate carrier (Gnaiger, 2009). When OXPHOS capacity is limited by the phosphorylation system, an increase in respiration is obtained after uncoupling the electron transfer from ATP synthesis (Gnaiger, 2009; Lemieux et al., 2017, 2011; Lemieux and Warren, 2012). The second is the NADH pathway through complex I; more specifically, pyruvate dehydrogenase complex (PDC) was identified as a component responsible for the broad thermal sensitivity of this pathway in the mouse heart (Lemieux et al., 2017). A separate study on the rat heart also showed a very strong effect of temperature on PDC activity compared with multiple other enzymes involved in the OXPHOS process (Lemieux et al., 2010a). In Drosophila, increased activity of PDC, due to an increase in calcium activation, was associated with a preference for a cold environment (Takeuchi et al., 2009). However, these studies did not address the adjustment of the phosphorylation system or PDC activity following thermal acclimation. It is unclear whether the controlling steps are modified to cope with temperature variation. Furthermore, there is definitive interspecies variability in steps controlling the OXPHOS pathway. The PDC does not seem to play such a major role in controlling OXPHOS at low temperature in a cold-adapted Atlantic wolffish (Anarhichas lupus) (Lemieux et al., Received 28 May 2019; Accepted 14 August 2019 1 Faculty Saint-Jean, University of Alberta, Edmonton, AB, Canada, T6C 4G9. 2 Faculdade de Tecnologia e Ciências, Salvador, Bahia, 41741-590, Brazil. 3 Department of Medicine, Women and Children’s Health Research Institute, University of Alberta, Edmonton, AB, Canada, T6G 2R7. *Author for correspondence (helene.lemieux@ualberta.ca) M.S., 0000-0002-6124-4020; H.L., 0000-0002-8864-6062 1 © 2019. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2019) 222, jeb207951. doi:10.1242/jeb.207951 Journal of Experimental Biology