Turtles maintain mitochondrial integrity but reduce mitochondrial respiratory capacity in the heart after cold-acclimation and anoxia

Abstract

Mitochondria are important to cellular homeostasis, but can become a dangerous liability when cells recover from hypoxia. Anoxiatolerant freshwater turtles show reduced mitochondrial respiratory capacity and production of reactive oxygen species (ROS) after prolonged anoxia, but the mechanisms are unclear. Here, we investigated whether this mitochondrial suppression originates from downregulation of mitochondrial content or intrinsic activity by comparing heart mitochondria from (1) warm (25°C) normoxic, (2) cold-acclimated (4°C) normoxic and (3) cold-acclimated anoxic turtles. Transmission electron microscopy of heart ventricle revealed that these treatments did not affect mitochondrial volume density and morphology. Furthermore, neither enzyme activity, protein content nor supercomplex distribution of electron transport chain (ETC) enzymes changed significantly. Instead, our data imply that turtles inhibit mitochondrial respiration rate and ROS production by a cumulative effect of slight inhibition of ETC complexes. Together, these results show that maintaining mitochondrial integrity while inhibiting overall enzyme activities are important aspects of anoxia tolerance.

Cold-acclimation induces life stage-specific responses in the cardiac proteome of Western painted turtles (Chrysemys picta bellii): implications for anoxia tolerance

“…For adult turtles, observed changes in ETS component proteins induced by cold-acclimation may play a role in the metabolic preparations that enhance anoxia survival at low temperature. For example, the adultspecific decrease in several component proteins of complexes II, III, and V with coldacclimation would reduce the overall aerobic capacity of the heart and may contribute, at least in part, to the observed reduction in respiration rates of isolated mitochondria (Bundgaard et al, 2019; Galli et al, 2013) and the lower cellular ATP content (Stecyk et al, 2009) of cold-acclimated adult turtles.…”

“…Mitochondrial remodeling also plays a key role in managing metabolic depression (Bundgaard et al, 2020; St-Pierre et al, 2000; Stuart et al, 1998). For example, mitochondrial respiration rates are reduced in the brain and heart of cold-acclimated turtles without altering mitochondrial content or structure (Bundgaard et al, 2019; Pamenter et al, 2016). Rather, suppressing mitochondrial activity may be achieved by a non-specific decrease in substrate utilization across the electron transport system (ETS) (Bundgaard et al, 2019), or via targeted reductions in proton leak and ATP synthase activity (Galli et al, 2013).…”

“…For example, mitochondrial respiration rates are reduced in the brain and heart of cold-acclimated turtles without altering mitochondrial content or structure (Bundgaard et al, 2019; Pamenter et al, 2016). Rather, suppressing mitochondrial activity may be achieved by a non-specific decrease in substrate utilization across the electron transport system (ETS) (Bundgaard et al, 2019), or via targeted reductions in proton leak and ATP synthase activity (Galli et al, 2013). The capacity for turtles to evoke these responses is influenced by environmental temperature, such that cold-acclimation prior to anoxia exposure substantially decreases the onset of the metabolic insult and increases the survival time (Herbert and Jackson, 1985a; Herbert and Jackson, 1985b; Warren and Jackson, 2007).…”

Integrating the Hallmarks of Aging Throughout the Tree of Life: A Focus on Mitochondrial Dysfunction

“…Birds have a relatively long lifespan considering their high metabolic activity and small body size, and it is shown that birds have less ROS production and longer telomeres (Hickey et al, 2012). Similarly, it has been noted that hypoxia in the freshwater turtle Trachemys scripta elegans inhibits mitochondrial respiration and ROS production, preventing oxidative damage (Bundgaard et al, 2019). These examples illustrate that different species have different ways to maintain mitochondrial function in the face of physiological challenges, and thereby sustain a relatively long and healthy lifespan.…”

Use of in-gel peroxidase assay for cytochrome c to visualize mitochondrial complexes III and IV

“…IGA is applicable to eukaryotic organisms, e.g. turtle (Bundgaard et al, 2019), bovine, yeast (Ladig et al, 2011), potato (Eubel et al, 2004) and Drosophila (Elkholi et al, 2019). Using western blot analysis, it was reported that supercomplex formation was enhanced in genetically adipogenic differentiated human mesenchymal stem cells (Hofmann et al, 2012).…”