1. Mitochondrial-Produced Reactive Oxygen Species Matthew Zimmerman, PhD Assistant Professor Cellular & Integrative Physiology University of Nebraska Medical Center mczimmerman@unmc.edu

2. Lecture Outline 1.Sources of mitochondrial-produced reactive oxygen species (ROS) 2.Complex I and Complex III – Primary sources of ROS in mitochondria 3.Mitochondrial-localized antioxidants 4.Methods to measure mitochondrial- produced ROS 5.Diseases associated with mitochondrial-produced ROS  Amyotrophic lateral sclerosis (ALS; aka Lou Gehrig’s disease) Murphy MP. (2009) Biochem J. 417:1-13)

3. What are reactive oxygen species (ROS) and free radicals? ROS: species of oxygen, produced by all aerobic cells, that are in a more reactive state than molecular oxygen Free radicals: an atom or group of atoms possessing one or more unpaired electrons ROS best known for role in host defense mechanisms Often considered toxic byproducts of cellular metabolism More recently, ROS recognized as key signaling molecules O2O2 O2-O2- HO 2  H2O2H2O2 OH  H2OH2O e-e- e-e- e-e- e-e- superoxide hydrogen peroxide hydroxyl radical

4. Sources of Reactive Oxygen Species Mitochondria NADPH oxidase Hypoxanthine/xanthine oxidase Lipoxygenase Nitric oxide synthases Turrens JF. (2003) J Physiol. 552.2:335-344 NADPH oxidase

5. Sources of Mitochondrial-Produced Reactive Oxygen Species  One-electron reduction of oxygen is thermodynamically favorable for many mitochondrial oxidoreductases due to the moderate redox potential of the superoxide/dioxygen couple (E 1/2 = -0.16 V) 1.Cytochrome b5 reductase: Outer mitochondrial membrane localization Oxidizes cytoplasmic NAD(P)H Reduces cytochrome b5 in outer membrane May produce O 2  - (~ 300 nmol/min/mg protein) Upregulated in schizophrenic patients 2.Monoamine oxidase (MAO): Outer mitochondrial membrane localization Critical in turnover of monoamine neurotransmitters Catalyze the oxidative deamination of biogenic amines aldehyde and release of H 2 O 2 May be involved in ischemia, aging, Parkinson’s disease Bortolato M et al. Adv Drug Deliv Rev. 2008

6. Sources of Mitochondrial-Produced Reactive Oxygen Species 3.Dihyroorotate dehydrogenase (DHOH): Located at the outer surface of inner membrane In the process of pyrimidine nucleotide synthesis, DHOH converts dihydroorotate to orotate Electron receptor is coenzyme Q and in absence of coenzyme Q produces H 2 O 2 (in vitro) Role in producing ROS in vivo remains unclear and controversial 4.Dehydrogenase of  -glycerophosphate: Located at the outer surface of inner membrane Uses coenzyme Q as electron receptor and catalyzes oxidation of glycerol-3-phosphate to dihydroxyacetone Studies in mice and drosophila suggest it produces H 2 O 2 5.Aconitase: Localized in matrix Catalyzes conversion of citrate to isocitrate (tricarboxylic acid (TCA) cycle) Inactivated by O 2  - and, in turn, produces OH  most likely via Fe 2+ release

7. 6.  -Ketoglutarate dehydrogenase complex: Located on the matrix side of inner membrane Uses NAD+ as electron acceptor and catalyzes oxidation of  -ketoglutarate to succinyl-CoA Similar to other sources, limited supply of electron acceptor promotes production of ROS 7.Succinate dehydrogenase (SDH; aka Complex II): Located at the inner surface of inner membrane Flavoprotein that oxidizes succinate to furmarate using coenzyme Q as electron receptor Isolated SDH can produce ROS (again in absence of electron receptor) Mutations in SDH subunits results in an increase in mitochondrial-localized ROS, particularly superoxide Sources of Mitochondrial-Produced Reactive Oxygen Species Modified from Turrens JF, 2003

8. Complex I: A Primary Source of Mitochondrial- Produced Reactive Oxygen Species Complex I (aka NADH-ubiquinone oxidoreductase; NADH dehydrogenase) Major entry point for electrons into the electron transport chain (ETC) Flavin mononucleotide (FMN) accepts electrons from NADH FMN passes electrons to chain of FeS centers (n=7) and finally to CoQ Produces O 2  - from the reaction of oxygen with the fully reduced FMN (dependent on NADH/NAD + ratio) Inhibition of respiratory chain or increased levels of NADH increases NADH/NAD + ratio and, in turn produces O 2  -

9. Complex I: A Primary Source of Mitochondrial- Produced Reactive Oxygen Species Reverse Electron Transfer (RET) production of superoxide Electrons are transferred against redox potential gradient (reduced CoQ NAD+) Occurs during low ATP production resulting in a high protonmotive force (  p) and reduced CoQ (succinate or  -glycerophosphate supply electrons to reduce CoQ) Rate of RET-dependent superoxide production may be the highest that can occur in mitochondria RET: high  p and high CoQH 2 /CoQ Modified from Murphy MP. (2009)

10. Complex I: A Primary Source of Mitochondrial- Produced Reactive Oxygen Species Increasing Complex I-produced superoxide experimentally: Rotenone- induced inhibition of Complex I Rotenone binds to the CoQ-binding site Electrons in Complex I “leak” from either FMN or FeS centers to oxygen producing superoxide Modified from Liu Y. et al. (2002). J Neurochem. 780-7.

11. Complex III: A Primary Source of Mitochondrial- Produced Reactive Oxygen Species Oxidizes CoQ using cytochrome c as electron acceptor Reduced CoQ (QH 2 ) transfers one electron to FeS protein (ISP, aka Rieske protein) and eventually cytochrome c The resulting semiquinone (Q -  ) transfers electrons to cytochrome b, then to the Q i site which results in the reduction of another CoQ molecule (Q-cycle) The semiquinone (Q -  ) is unstable and can donate electron to oxygen forming superoxide Complex III (aka ubiquinone:cytochrome c reductase) Modified from Turrens JF, 2003

12. Complex III: A Primary Source of Mitochondrial- Produced Reactive Oxygen Species Increasing Complex III-produced superoxide experimentally: Antimycin- induced inhibition of Complex III Antimycin blocks the transfer of electrons to the Q i -site, which results in the accumulation of the unstable semiquinone The unstable semiquinone can transfer electrons to oxygen producing superoxide Andreyev A.U., et al. (2005). Biochemistry (Moscow). 70:200-14.

13. Mitochondrial-localized antioxidants 1.Manganese superoxide dismutase (MnSOD, SOD2): Catalyzes dismutation of superoxide producing hydrogen peroxide and oxygen Located exclusively in matrix of mitochondria Nuclear-encoded protein with a mitochondrial-target sequence Homozygous knockout mice only live for few days Large percentage of tumor cells have low MnSOD activity O 2  - + O 2  - + 2H + H 2 O 2 + O 2 MnSOD 2. Copper/Zinc superoxide dismutase (CuZnSOD, SOD1) Catalyzes same reaction as MnSOD Primarily found in cytoplasm, but also present in mitochondria Precise mitochondrial localization is unclear – most evidence indicates intermembrane space Mechanism of transport into mitochondria is also unclear Mutant SOD1, associated with amyotrophic lateral sclerosis (ALS), appears to accumulate in mitochondria Zhang DX. (2006). Am J Physiol. 292:H2023-31)

14. Mitochondrial-localized antioxidants 3.Glutathione ~ 10% glutathione levels in cells is in mitochondria Can be transported into mitochondria via specialized GSH-transporters Oxidized glutathione (GSSG) can be reduced back to GSH by glutathione reductase localized in the matrix 4.Glutathione peroxidase (GPx1) Uses GSH for the reduction of hydrogen peroxide to water Found in mitochondrial matrix and intermembrane space 5.Phospholipid glutathione peroxidase (PhGPx; GPx4) Reduces lipid hydroperoxides and hydrogen peroxide GPx4 long form expressed in mitochondria Knockout mice are embryonic lethal 6.Cytochrome C Present in intermembrane space Can scavenge superoxide The reduced cytochrome c is recycled by cytochrome c oxidase Biological significance of cytochrome c as a superoxide scavenger in vivo remains to be fully elucidated

15. Mitochondrial-localized antioxidants 6.Peroxiredoxins (Prx) Reduce hydrogen peroxide and lipid hydroperoxides Prx3 highly expressed in heart, adrenal gland, liver and brain mitochondria Prx5 highly expressed testis 7.Thioredoxin (Trx) system Trx2 recycles Prx by reducing the disulfide Oxidazed Trx2 is then recycled by thioredoxin reductase (TrxR), which uses NADPH as the source of reducing equivalents Echtay KS. (2007) Free Rad Biol Med. 43:1351-71

16. Smith R.A.J., et al. (2008) Ann NY Acad Sci. 1147:105-111 Exogenous mitochondrial-targeted antioxidants Antioxidant compounds covalently attached to a lipophilic triphenylphosphonium cation target mitochondria Such compounds include: SOD mimetic M40403 (MitoSOD) and tempol (MitoTempol); peroxidase mimetic ebselen (MitoPrx); coenzyme Q (MitoQ); tocopherol (MitoE)

17. Methods for measuring mitochondrial-produced ROS 1.MitoSOX Red fluorescence Mitochondrial-targeted superoxide sensitive fluorogenic probe (Invitrogen/Molecular Probes) MitoSOX is dihydroethidum (DHE; aka hydroethidine) linked to a triphenylphosphonium group Like DHE, fluorescence can be detected using 405, 488, 510 nm excitation However, only the 405 nm excitation detects the 2-hydroxyethium fluorescent product which is specifically dependent on superoxide Dikalov S. et al. (2007). Hypertension

18. Methods for measuring mitochondrial-produced ROS 2. Electron paramagnetic resonance (EPR) on isolated mitochondria Mitochondria can be isolated from tissue or cultured cells Isolated mitochondria incubated with EPR spin trap or spin probe Amount of spin trap/probe radical are detected using EPR Important issues to consider: Purity and integrity of mitochondria preparation Depending on spin trap/probe selected use specific antioxidants (e.g. SOD) to selectively measure a particular ROS Mariappan N. et al. Free Rad Biol Med. (2009). 46:462-70.

19. Methods for measuring mitochondrial-produced ROS 3. Amplex Red to detect hydrogen peroxide efflux from isolated mitochondria Isolated mitochondria incubated with Amplex Red in the presence of HRP Measure levels of fluorescent product, resorufin Important issues to consider: Purity and integrity of mitochondria preparation If using this method to indirectly measure superoxide, remember not all superoxide is converted to hydrogen peroxide (nitric oxide in mitochondria) Numerous matrix peroxidases will consume hydrogen peroxide

20. Fatal neurodegenerative disease that specifically targets motor neurons in the spinal cord, brain stem, and cortex Most common adult motor neuron disease; 5,600 cases diagnosed each year in U.S. Disease onset usually begins with weakness in arms and legs and quickly progresses to total paralysis Patients generally die of respiratory failure 2-5 years after the first symptoms appear ALS often referred to as Lou Gehrig’s disease Amyotrophic Lateral Sclerosis (ALS) From alsa.org

21. 20-25% of familial ALS cases are associated with mutations in a cellular antioxidant enzyme called CuZnSOD (SOD1) Mutant CuZnSOD-induced neuronal toxicity is believed to involve a toxic gain of function; not a loss of SOD activity Many familial ALS mutant CuZnSOD proteins retain SOD activity CuZnSOD knockout mice do not develop motor neuron disease Overexpressing wild-type CuZnSOD in animal or cell culture models of ALS does not provide protection Mutant CuZnSOD expression is ubiquitous, although only motor neurons appear to be affected From Valentine JS, 2005. Annu. Rev. Biochem 1. Control 2. Wild-type CuZnSOD 3. Mutant CuZnSOD #1 4. Mutant CuZnSOD #2 5. Mutant CuZnSOD #3 6. Mutant CuZnSOD #4 Amyotrophic Lateral Sclerosis (ALS)

22. Expression of different CuZnSOD mutants in cultured neurons decreases cell survival

23. Mutant CuZnSOD increases superoxide levels in mitochondria

24. Overexpressing MnSOD attenuates mutant CuZnSOD-mediated increase in mitochondrial superoxide

25. Overexpression of MnSOD (SOD2) inhibits mutant CuZnSOD-mediated neuronal toxicity

26. Intramuscular injection of AdSOD2 results in retrograde transport and SOD2 expression in spinal cord motor neurons Control AdSOD2

27. Intramuscular injection of AdSOD2 delays motor dysfunction in ALS transgenic mice

28. Summary 1.Numerous sources of ROS production in mitochondria 2.Complex I and III have been the most studied and best characterized, and, to date, are generally considered the primary sources of mitochondrial-produced ROS 3.Collection of mitochondrial-localized antioxidants also play a significant role in the levels of ROS in mitochondria 4.Mitochondrial superoxide can be elevated experimentally with rotenone (Complex I inhibitor) or antimycin A (Complex III) inhibitor 5.Mitochondrial ROS can be reduced experimentally by using antioxidant compounds linked to a triphenylphosphonium group or by increasing expression of endogenous antioxidant proteins

30. Andreyev A.U., et al. (2005). Biochemistry (Moscow). 70:200-14.

31. superoxide not a strong oxidant, but precursor to other ROS and involved in propagation of oxidative chain rxns mitos are main cellular source of ROS other cellular sources include NADPH oxidase, cytochrome P 450 -dependent oxygenases, xanthine oxidase non-enzymatic production of superoxide occurs when 1 electron is transferred to molecular oxygen by reduced coenzymes (i.e. flavins, iron sulfur clusters) mitos contain numerous redox centers that “leak” electrons to oxygen 1-4% of all oxygen consume is incompletely reduced to superoxide, thus increase oxygen consumption (hyperoxic conditions) can increase superoxide however, hypoxic conditions can also increase superoxide ALS

32. Zhang and Gutterman

33. Mitochondrial sources of superoxide high reducing environment in mitochondria allows respiratory components, such as flavoproteins, iron-sulfur clusters, to transfer 1 electron to oxygen in fact most steps in ETC involve single-electron reductions steady state concentration of superoxide estimated to be 10 -10 M and h2o2 estimated to be 5x10 -10 M. superoxide can be produced on outer mito membrane, in matrix, and on both sides of inner membrane matrix superoxide removed by mnsod superoxide in intermembrane space may be carried to cytoplasm via voltage-dependent anion channels (han et al. 2003) Complex III main source in heart and lung Complex I main source in brain (important in aging and Parkinson’s) superoxide formation can increase both electron flow slows down and when concentration of oxygen increase superoxide production increases as repiratory chain becomes more reduced rotenone (cplx I) and antimycin (cplx III) may increase superoxide because upstream of site of inhibition carriers are fully reduced however not all inhibitors have this effect

34. Mitochondrial sources of superoxide high reducing environment in mitochondria allows respiratory components, such as flavoproteins, iron-sulfur clusters, to transfer 1 electron to oxygen in fact most steps in ETC involve single-electron reductions steady state concentration of superoxide estimated to be 10 -10 M and h2o2 estimated to be 5x10 -10 M. superoxide can be produced on outer mito membrane, in matrix, and on both sides of inner membrane matrix superoxide removed by mnsod superoxide in intermembrane space may be carried to cytoplasm via voltage-dependent anion channels (han et al. 2003) Complex III main source in heart and lung Complex I main source in brain (important in aging and Parkinson’s) superoxide formation can increase both electron flow slows down and when concentration of oxygen increase rotenone (cplx I) and antimycin (cplx III) may increase superoxide because upstream of site of inhibition carriers are fully reduced however not all inhibitors have this effect

35. Mitochondrial sources of superoxide calcium, nitric oxide and mito membrane potential all play a role in generating mito ROS UCP, mito uncoupling proteins (mito anion carriers) induce protein leak across inner membrane and suppress mito membrane potential, thus decreasing ROS NO may inhibit complex IV, thus modulating mito respiration and increase mito ROS reverse electron transport increases superoxide and may come from III or II and can be blocked by rotenone, thus indicating that it is due to electrons entering Complex I thought the CoQ binding site.

36. Pathological conditions increase mito ROS can induce apoptosis by increasing mito permeablity via increasing opening of transition pores cytochrome c is released and can activate caspase cascade cytochrome c release can further increase mito superoxide because cytochome c can reduce superoxide and because the ETC become more reduced because transfer of electrons between II and III slows down