Neuroprotective Potential of Mild Uncoupling in Mitochondria. Pros and Cons

There has been an explosion of interest in the use of uncouplers of oxidative phosphorylation in mitochondria in the treatment of several pathologies, including neurological ones. In this review, we analyzed all the mechanisms associated with mitochondrial uncoupling and the metabolic and signaling cascades triggered by uncouplers. We provide a full set of positive and negative effects that should be taken into account when using uncouplers in experiments and clinical practice.

Mitochondrial responses to prolonged anoxia in brain of red-eared slider turtles

Mitochondria are central to aerobic energy production and play a key role in neuronal signalling. During anoxia, however, the mitochondria of most vertebrates initiate deleterious cell death cascades. Nonetheless, a handful of vertebrate species, including some freshwater turtles, are remarkably tolerant of low oxygen environments and survive months of anoxia without apparent damage to brain tissue. This tolerance suggests that mitochondria in the brains of such species are adapted to withstand prolonged anoxia, but little is known about potential neuroprotective responses. In this study, we address such mechanisms by comparing mitochondrial function between brain tissues isolated from cold-acclimated red-eared slider turtles ( Trachemys scripta elegans ) exposed to two weeks of either normoxia or anoxia. We found that brain mitochondria from anoxia-acclimated turtles exhibited a unique phenotype of remodelling relative to normoxic controls, including: (i) decreased citrate synthase and F 1 F O -ATPase activity but maintained protein content, (ii) markedly reduced aerobic capacity, and (iii) mild uncoupling of the mitochondrial proton gradient. These data suggest that turtle brain mitochondria respond to low oxygen stress with a unique suite of changes tailored towards neuroprotection.

Possible Mechanisms of Fullerene C60Antioxidant Action

Novel mechanism of antioxidant activity of buckminsterfullerene C60based on protons absorbing and mild uncoupling of mitochondrial respiration and phosphorylation was postulated. In the present study we confirm this hypothesis using computer modeling based on Density Functional Theory. Fullerene's geroprotective activity is sufficiently higher than those of the most powerful reactive oxygen species scavengers. We propose here that C60has an ability to acquire positive charge by absorbing inside several protons and this complex could penetrate into mitochondria. Such a process allows for mild uncoupling of respiration and phosphorylation. This, in turn, leads to the decrease in ROS production.

Mitochondrial (‘mild’) uncoupling and ROS production: physiologically relevant or not?

During the last decade, the possibility that ‘mild’ uncoupling could be protective against oxidative damage by diminishing ROS (reactive oxygen species) production has attracted much interest. In the present paper, we briefly examine the evidence for this possibility. It is only ROS production from succinate under reverse electron-flow conditions that is sensitive to membrane potential fluctuations, and so only this type of ROS production could be affected; however, the conditions under which succinate-supported ROS production is observed include succinate concentrations that are supraphysiological. Any decrease in membrane potential, even ‘mild uncoupling’, must necessarily lead to large increases in respiration, i.e. it must be markedly thermogenic. Mitochondria within cells are normally ATP-producing and thus already have a diminished membrane potential, and treatment of cells, organs or animals with small amounts of artificial uncoupler does not seem to have beneficial effects that are explainable via reduced ROS production. Although it has been suggested that members of the uncoupling protein family (UCP1, UCP2 and UCP3) may mediate a mild uncoupling, present evidence does not unequivocally support such an effect, e.g. the absence of the truly uncoupling protein UCP1 is not associated with increased oxidative damage. Thus present evidence does not support mild uncoupling as a physiologically relevant alleviator of oxidative damage.

Mitochondrial phospholipase A2 activated by reactive oxygen species in heart mitochondria induces mild uncoupling

Homeostasis of reactive oxygen species (ROS) in cardiomyocytes is critical for elucidation of normal heart physiology and pathology. Mitochondrial phospholipases A2 (mt-PLA2) have been previously suggested to be activated by ROS. Therefore, we have attempted to elucidate physiological role of such activation. We have found that function of a specific i-isoform of mitochondrial phospholipase A2 (mt-iPLA2) is activated by tert-butylhydroperoxide in isolated rat heart mitochondria. Isoform specificity was judged from the inhibition by bromoenol lactone (BEL), a specific iPLA2 inhibitor. Concomitant uncoupling has been caused by free fatty acids, since it was inhibited by bovine serum albumin. The uncoupling was manifested as a respiration burst accompanied by a slight decrease in mitochondrial inner membrane potential. Since this uncoupling was sensitive to carboxyatractyloside and purine nucleotide di- and triphosphates, we conclude that it originated from the onset of fatty acid cycling mediated by the adenine nucleotide translocase (major contribution) and mitochondrial uncoupling protein(s) (minor contribution), respectively. Such a mild uncoupling may provide a feedback downregulation of oxidative stress, since it can further attenuate mitochondrial production of ROS. In conclusion, ROS-induced function of cardiac mt-iPLA2 may stand on a pro-survival side of ischemia-reperfusion injury.