The ATP synthase and the permeability transition in mitochondria

The permeability transition in human mitochondria refers to the opening of a non-specific channel, known as the permeability transition pore (PTP) in the inner membrane. Opening can be triggered by calcium ions, leading to swelling of the organelle, disruption of the inner membrane and ATP synthesis, followed by cell death [1][2]. These events have been linked to pathways leading to cell death, and to human diseases including cardiac ischemia and muscle dystrophy [3]. The opening of the pore can be induced artificially by compounds such as thapsigargin [4], a non-competitive inhibitor of the Ca2+-ATPase in the sarcoplasmic and endoplasmic reticula, and by ionophores for divalent cations such as ionomycin [5], and ferutinin [6]. It can also be inhibited by drugs such as cyclosporin A, mediated via its binding to the prolyl cis-trans isomerase, cyclophilin D, in the mitochondrial matrix[7][8]. Many proposals have been made about the protein constituents of the PTP itself, including the ADP/ATP translocase, an abundant component of the inner membranes of mitochondria, and the voltage dependent anion channel found in the outer membranes of the organelle, but none of these proposals has been established definitively [9][10]. A further proposition, that another component of the inner mitochondrial membrane, the AAA-protease, SPG7, participates in formation of the pore, has been disputed [11][12].

Recently, it has been proposed that the PTP is associated with the ATP synthase complex [13]. Some proposals suggest that the ring of c-subunits that constitute the membrane domain of the enzyme’s rotor provides the pore [14][15][16]. Others suggest that the pore is associated with membrane subunits of the enzyme found in the region of interface between monomers in the dimeric ATP synthase complexes found in mitochondria [13][17].
 
  • We have tested the proposal that the PTP is associated with the c-subunit by producing a clonal human cell line where all three genes encoding subunit c have been disrupted. The cells are devoid of subunit c, and yet the characteristic properties of the PTP persist. Therefore, the c8-ring of human ATP synthase does not provide the PTP[18].
     
  • We have also examined ρ0 human cells for the presence of the PTP. These cells have no mitochondrial DNA, and therefore the ATP6 and ATP8 membrane subunits of the ATP synthase are absent from their mitochondria. The PTP persists in these cells also. Therefore, the ATP6 and ATP8 subunits do not provide the PTP[18].
     
  • We are continuing to examine the possible involvement of other subunits of the ATP synthase in the PTP.

References

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  3. Rasola A & Bernardi P (2007) The mitochondrial permeability transition pore and its involvement in cell death and in disease pathogenesis. Apoptosis 12, 815-33
  4. Korge P & Weiss JN (1999) Thapsigargin directly induces the mitochondrial permeability transition. Eur J Biochem 265, 273-80
  5. Morgan AJ & Jacob R (1994) Ionomycin enhances Ca2+ influx by stimulating store-regulated cation entry and not by a direct action at the plasma membrane. Biochem J 300 ( Pt 3), 665-72
  6. Abramov AY & Duchen MR (2003) Actions of ionomycin, 4-BrA23187 and a novel electrogenic Ca2+ ionophore on mitochondria in intact cells. Cell Calcium 33, 101-12
  7. Crompton M, Ellinger H & Costi A (1988) Inhibition by cyclosporin A of a Ca2+-dependent pore in heart mitochondria activated by inorganic phosphate and oxidative stress. Biochem J 255, 357-60
  8. Elrod JW & Molkentin JD (2013) Physiologic functions of cyclophilin D and the mitochondrial permeability transition pore. Circ J 77, 1111-22
  9. Kokoszka JE, Waymire KG, Levy SE, Sligh JE, Cai J, Jones DP, MacGregor GR & Wallace DC (2004) The ADP/ATP translocator is not essential for the mitochondrial permeability transition pore. Nature 427, 461-5
  10. Baines CP, Kaiser RA, Sheiko T, Craigen WJ & Molkentin JD (2007) Voltage-dependent anion channels are dispensable for mitochondrial-dependent cell death. Nat Cell Biol 9, 550-5
  11. Shanmughapriya S, Rajan S, Hoffman NE, Higgins AM, Tomar D, Nemani N, Hines KJ, Smith DJ, Eguchi A, Vallem S, Shaikh F, Cheung M, Leonard NJ, Stolakis RS, Wolfers MP, Ibetti J, J Chuprun K, Jog NR, Houser SR, Koch WJ, Elrod JW & Madesh M (2015) SPG7 Is an Essential and Conserved Component of the Mitochondrial Permeability Transition Pore. Mol Cell 60, 47-62
  12. König T, Tröder SE, Bakka K, Korwitz A, Richter-Dennerlein R, Lampe PA, Patron M, Mühlmeister M, Guerrero-Castillo S, Brandt U, Decker T, Lauria I, Paggio A, Rizzuto R, Rugarli EI, De Stefani D & Langer T (2016) The m-AAA Protease Associated with Neurodegeneration Limits MCU Activity in Mitochondria. Mol Cell 64, 148-162
  13. Giorgio V, von Stockum S, Antoniel M, Fabbro A, Fogolari F, Forte M, Glick GD, Petronilli V, Zoratti M, Szabó I, Lippe G & Bernardi P (2013) Dimers of mitochondrial ATP synthase form the permeability transition pore. Proc Natl Acad Sci U S A 110, 5887-92
  14. Strauss M, Hofhaus G, Schröder RR & Kühlbrandt W (2008) Dimer ribbons of ATP synthase shape the inner mitochondrial membrane. EMBO J 27, 1154-60
  15. Azarashvili T, Odinokova I, Bakunts A, Ternovsky V, Krestinina O, Tyynelä J & Saris N-ELeo (2014) Potential role of subunit c of F0F1-ATPase and subunit c of storage body in the mitochondrial permeability transition. Effect of the phosphorylation status of subunit c on pore opening. Cell Calcium 55, 69-77
  16. Alavian KN, Beutner G, Lazrove E, Sacchetti S, Park H-A, Licznerski P, Li H, Nabili P, Hockensmith K, Graham M, Porter GA & Jonas EA (2014) An uncoupling channel within the c-subunit ring of the F1FO ATP synthase is the mitochondrial permeability transition pore. Proc Natl Acad Sci U S A 111, 10580-5
  17. Carraro M, Giorgio V, Šileikytė J, Sartori G, Forte M, Lippe G, Zoratti M, Szabó I & Bernardi P (2014) Channel formation by yeast F-ATP synthase and the role of dimerization in the mitochondrial permeability transition. J Biol Chem 289, 15980-5
  18. He J, Ford HC, Carroll J, Ding S, Fearnley IM & Walker JE (2017) Persistence of the mitochondrial permeability transition in the absence of subunit c of human ATP synthase. Proc Natl Acad Sci U S A