Redox-dependent loss of flavin by mitochondria complex I is different in brain and heart

Pathologies associated with tissue ischemia/reperfusion (I/R) in highly metabolizing organs such as the brain and heart are leading causes of death and disability in humans. Molecular mechanisms underlying mitochondrial dysfunction during acute injury in I/R are tissue-specific, but their details are not completely understood. A metabolic shift and accumulation of substrates of reverse electron transfer (RET) such as succinate are observed in tissue ischemia, making mitochondrial complex I of the respiratory chain (NADH:ubiquinone oxidoreductase) the most vulnerable enzyme to the following reperfusion. It has been shown that brain complex I is predisposed to losing its flavin mononucleotide (FMN) cofactor when maintained in the reduced state in conditions of RET both in vitro and in vivo. Here we investigated the process of redox-dependent dissociation of FMN from mitochondrial complex I in brain and heart mitochondria. In contrast to the brain enzyme, cardiac complex I does not lose FMN when reduced in RET conditions. We proposed that the different kinetics of FMN loss during RET is due to the presence of brain-specific long 50 kDa isoform of the NDUFV3 subunit of complex I, which is absent in the heart where only the canonical 10 kDa short isoform is found. Our simulation studies suggest that the long NDUFV3 isoform can reach toward the FMN binding pocket and affect the nucleotide affinity to the apoenzyme. For the first time, we demonstrated a potential functional role of tissue-specific isoforms of complex I, providing the distinct molecular mechanism of I/R-induced mitochondrial impairment in cardiac and cerebral tissues. By combining functional studies of intact complex I and molecular structure simulations, we defined the critical difference between the brain and heart enzyme and suggested insights into the redox-dependent inactivation mechanisms of complex I during I/R injury in both tissues.

Quantification of NADH:ubiquinone oxidoreductase (complex I) content in biological samples

Impairments in mitochondrial energy metabolism have been implicated in human genetic diseases associated with mitochondrial and nuclear DNA mutations, neurodegenerative and cardiovascular disorders, diabetes, and aging. Alteration in mitochondrial complex I structure and activity has been shown to play a key role in Parkinson's disease and ischemia/reperfusion tissue injury, but significant difficulty remains in assessing the content of this enzyme complex in a given sample. The present study introduces a new method utilizing native polyacrylamide gel electrophoresis in combination with flavin fluorescence scanning to measure the absolute content of complex I, as well as α-ketoglutarate dehydrogenase complex, in any preparation. We show that complex I content is 19 ± 1 pmol/mg of protein in the brain mitochondria, whereas varies up to 10-fold in different mouse tissues. Together with the measurements of NADH-dependent specific activity, our method also allows accurate determination of complex I catalytic turnover, which was calculated as 104 min-1 for NADH:ubiquinone reductase in mouse brain mitochondrial preparations. α-ketoglutarate dehydrogenase complex content was determined to be 65 ± 5 and 123 ± 9 pmol/mg protein for mouse brain and bovine heart mitochondria, respectively. Our approach can also be extended to cultured cells, and we demonstrated that about 90 × 103 complex I molecules are present in a single human embryonic kidney 293 cell. The ability to determine complex I content should provide a valuable tool to investigate the enzyme status in samples after in vivo treatment in mutant organisms, cells in culture, or human biopsies.

Cyclophilin D-dependent oligodendrocyte mitochondrial ion leak contributes to neonatal white matter injury

Postnatal failure of oligodendrocyte maturation has been proposed as a cellular mechanism of diffuse white matter injury (WMI) in premature infants. However, the molecular mechanisms for oligodendrocyte maturational failure remain unclear. In neonatal mice and cultured differentiating oligodendrocytes, sublethal intermittent hypoxic (IH) stress activated cyclophilin D-dependent mitochondrial proton leak and uncoupled mitochondrial respiration, leading to transient bioenergetic stress. This was associated with development of diffuse WMI: poor oligodendrocyte maturation, diffuse axonal hypomyelination, and permanent sensorimotor deficit. In normoxic mice and oligodendrocytes, exposure to a mitochondrial uncoupler recapitulated the phenotype of WMI, supporting the detrimental role of mitochondrial uncoupling in the pathogenesis of WMI. Compared with WT mice, cyclophilin D-knockout littermates did not develop bioenergetic stress in response to IH challenge and fully preserved oligodendrocyte maturation, axonal myelination, and neurofunction. Our study identified the cyclophilin D-dependent mitochondrial proton leak and uncoupling as a potentially novel subcellular mechanism for the maturational failure of oligodendrocytes and offers a potential therapeutic target for prevention of diffuse WMI in premature infants experiencing chronic IH stress.

Redox-Dependent Loss of Flavin by Mitochondrial Complex I in Brain Ischemia/Reperfusion Injury

Aims: Brain ischemia/reperfusion (I/R) is associated with impairment of mitochondrial function. However, the mechanisms of mitochondrial failure are not fully understood. This work was undertaken to determine the mechanisms and time course of mitochondrial energy dysfunction after reperfusion following neonatal brain hypoxia-ischemia (HI) in mice. Results: HI/reperfusion decreased the activity of mitochondrial complex I, which was recovered after 30 min of reperfusion and then declined again after 1 h. Decreased complex I activity occurred in parallel with a loss in the content of noncovalently bound membrane flavin mononucleotide (FMN). FMN dissociation from the enzyme is caused by succinate-supported reverse electron transfer. Administration of FMN precursor riboflavin before HI/reperfusion was associated with decreased infarct volume, attenuation of neurological deficit, and preserved complex I activity compared with vehicle-treated mice. In vitro, the rate of FMN release during oxidation of succinate was not affected by the oxygen level and amount of endogenously produced reactive oxygen species. Innovation: Our data suggest that dissociation of FMN from mitochondrial complex I may represent a novel mechanism of enzyme inhibition defining respiratory chain failure in I/R. Strategies preventing FMN release during HI and reperfusion may limit the extent of energy failure and cerebral HI injury. The proposed mechanism of acute I/R-induced complex I impairment is distinct from the generally accepted mechanism of oxidative stress-mediated I/R injury. Conclusion: Our study is the first to highlight a critical role of mitochondrial complex I-FMN dissociation in the development of HI-reperfusion injury of the neonatal brain. Antioxid. Redox Signal. 31, 608-622.

Attenuation of oxidative damage by targeting mitochondrial complex I in neonatal hypoxic-ischemic brain injury

BACKGROUND

Establishing sustained reoxygenation/reperfusion ensures not only the recovery, but may initiate a reperfusion injury in which oxidative stress plays a major role. This study offers the mechanism and this mechanism-specific therapeutic strategy against excessive release of reactive oxygen species (ROS) associated with reperfusion-driven recovery of mitochondrial metabolism.

AIMS AND METHODS

In neonatal mice subjected to cerebral hypoxia-ischaemia (HI) and reperfusion, we examined conformational changes and activity of mitochondrial complex I with and without post-HI administration of S-nitrosating agent, MitoSNO. Assessment of mitochondrial ROS production, oxidative brain damage, neuropathological and neurofunctional outcomes were used to define neuroprotective strength of MitoSNO. A specificity of reperfusion-driven mitochondrial ROS production to conformational changes in complex I was examined in-vitro.

RESULTS

HI deactivated complex I, changing its conformation from active form (A) into the catalytically dormant, de-active form (D). Reperfusion rapidly converted the D-form into the A-form and increased ROS generation. Administration of MitoSNO at the onset of reperfusion, decelerated D→A transition of complex I, attenuated oxidative stress, and significantly improved neurological recovery. In cultured neurons, after simulated ischaemia-reperfusion injury, MitoSNO significantly reduced ROS generation and neuronal mortality. In isolated mitochondria subjected to anoxia-reoxygenation, MitoSNO restricted ROS release during D→A transitions.

CONCLUSION

Rapid D→A conformation in response to reperfusion reactivates complex I. This is essential not only for metabolic recovery, but also contributes to excessive release of mitochondrial ROS and reperfusion injury. We propose that the initiation of reperfusion should be followed by pharmacologically-controlled gradual reactivation of complex I.

Mild intermittent hypoxemia in neonatal mice causes permanent neurofunctional deficit and white matter hypomyelination

Very low birth weight (VLBW) premature infants experience numerous, often self-limited non-bradycardic episodes of intermittent hypoxemia (IH). We hypothesized that these episodes of IH affect postnatal white matter (WM) development causing hypomyelination and neurological handicap in the absence of cellular degeneration. Based on clinical data from ten VLBW neonates; a severity, daily duration and frequency of non-bradycardic IH episodes were reproduced in neonatal mice. Changes in heart rate and cerebral blood flow during IH were recorded. A short-term and long-term neurofunctional performance, cerebral content of myelin basic protein (MBP), 2'3' cyclic-nucleotide 3-phosphodiesterase (CNPase), electron microscopy of axonal myelination and the extent of cellular degeneration were examined. Neonatal mice exposed to IH exhibited no signs of cellular degeneration, yet demonstrated significantly poorer olfactory discrimination, wire holding, beam and bridge crossing, and walking-initiation tests performance compared to controls. In adulthood, IH-mice demonstrated no alteration in navigational memory. However, sensorimotor performance on rota-rod, wire-holding and beam tests was significantly worse compared to naive littermates. Both short- and long-term neurofunctional deficits were coupled with decreased MBP, CNPase content and poorer axonal myelination compared to controls. In neonatal mice mild, non-ischemic IH stress, mimicking that in VLBW preterm infants, replicates a key phenotype of non-cystic WM injury: permanent hypomyelination and sensorimotor deficits. Because this phenotype has developed in the absence of cellular degeneration, our data suggest that cellular mechanisms of WM injury induced by mild IH differ from that of cystic periventricular leukomalacia where the loss of myelin-producing cells and axons is the major mechanism of injury.