The mitochondrial permeability transition: Recent progress and open questions

Abstract

Major progress has been made in defining the basis of the mitochondrial permeability transition, a Ca²⁺ -dependent permeability increase of the inner membrane that has puzzled mitochondrial research for almost 70 years. Initially considered an artefact of limited biological interest by most, over the years the permeability transition has raised to the status of regulator of mitochondrial ion homeostasis and of druggable effector mechanism of cell death. The permeability transition is mediated by opening of channel(s) modulated by matrix cyclophilin D, the permeability transition pore(s) (PTP). The field has received new impulse (a) from the hypothesis that the PTP may originate from a Ca²⁺ -dependent conformational change of F-ATP synthase and (b) from the reevaluation of the long-standing hypothesis that it originates from the adenine nucleotide translocator (ANT). Here, we provide a synthetic account of the structure of ANT and F-ATP synthase to discuss potential and controversial mechanisms through which they may form high-conductance channels; and review some intriguing findings from the wealth of early studies of PTP modulation that still await an explanation. We hope that this review will stimulate new experiments addressing the many outstanding problems, and thus contribute to the eventual solution of the puzzle of the permeability transition.

Keywords: ATP synthase; adenine nucleotide translocator; calcium transport; channels; cyclophilin; cyclosporine; mitochondria; permeability transition.

Fig. 1

Schematic representation of the possible functions of ANT. (Left) The established function of ANT is to exchange adenine nucleotides, transporting matrix ATP to the cytosol and cytosolic ADP to the matrix in energised mitochondria. (Center) In response to fatty acids ANT mediates H⁺ currents, suggesting that nucleotide exchange and H⁺ transport are not mutually exclusive functions. (Right) In the presence of Ca²⁺ and CyPD, ANT may undergo a still undefined conformational change which allows the formation of a high‐conductance channel.

Fig. 2

Structure of the F‐ATP synthase monomer. (Upper panel) F‐ATP synthase structure is based on the atomic model of Spikes et al. [124]. (Lower panel) Density maps of F‐ATP synthase derived by cryo‐EM in the indicated studies, with fitted atomic models. Subunits b, e and g are green, cyan and red, respectively.

Fig. 3

Hypothetical models for PTP formation by F‐ATP synthase. (A) The ‘death finger’ model proposes the transition of the c‐ring into the PTP channel on a conformational change of the peripheral stalk that eventually perturbs subunit e, the final transducer of pore opening. The C‐terminus of subunit e makes contacts with lipids of the plug within the c‐ring from the IMS (left). In the presence of physiological, low Ca²⁺ concentrations with CyPD bound to OSCP subunit (middle), the peripheral stalk transmits mechanical force generated by Ca²⁺ binding to subunit e (red arrow), which may exert a pulling action dragging some lipids out of the plug. This condition, together with a secondary relaxation of the central stalk/c‐ring connections would accommodate a low‐conductance mode of channel opening mediating the passage of ions but not of larger solutes, representing the ‘flickering’ mode of the PTP (middle). As the matrix Ca²⁺ levels rise, the mechanical force exerted on subunit e becomes stronger allowing removal of the lipid plug from the c‐ring and displacement of F ₁ with formation of the high‐conductance PTP, which remains fully reversible if Ca²⁺ is removed [145]. (B) Entire F‐ATP synthase dimer (left), after removal of the F ₁ domains (centre) and following a 90° rotation to show the two F _O domains as viewed from the matrix side (right). The conformational change transmitted through the peripheral stalk (red arrows in the right panel) may affect both subunits g/e and the monomer–monomer interface, with possible channel formation at the point of contact of subunits j, which undergo a pivoting motion at their interface during catalysis [124].

Fig. 4

Sequence alignment of subunit e from various species. Accession number of sequences used for the alignment: H. sapiens (P56385), B. taurus (Q00361), S. scrofa (Q9MYT8), R. norvegicus (P29419), M. musculus (Q06185), D. rerio (A7YY99), D. willistoni (B4N947), D. melanogaster (O77134), D. sechellia (B4HEN9), S. cerevisiae (P81449) and Y. lipolytica (B5FVG3). The multiple sequence alignment was performed with the CLUSTALW program and analysed by jalview software.