Identifying the components of the elusive mitochondrial permeability transition pore.

The mitochondrial permeability transition pore (MPTP) describes an inducible activity that regulates solute exchange between mitochondrial matrix contents and the surrounding cytoplasm, which acutely leads to loss of mitochondrial inner membrane potential but eventually organelle swelling and rupture. Mitochondrial rupture due to prolonged MPTP engagement, which is often the result of ischemic cellular injury due to elevated intracellular Ca levels and reactive oxygen species, underlies regulated necrotic cell death. An understanding of the molecular constituents that generate the MPTP, as well as the other proteins that can affect it, is of profound disease relevance. However, the molecular identity of the MPTP has been the subject of a protracted scientific debate for nearly three decades. In PNAS, Alavian et al. make a strong case that the c-subunit of the F1FO ATP synthase forms the inner mitochondrial membrane pore of the MPTP (1). The MPTP is a highly evolutionarily conserved pore within the inner membrane of the mitochondrion that is permeable to molecules less than 1.5 kDa in size. MPTP opening results in loss of inner membrane potential, the proton gradient and ATP production, eventually leading to mitochondrial dysfunction and cell death (2). The existence of the MPTP was first proposed in the mid20th century with the observations that high levels of Ca could lead to mitochondrial swelling and dysfunction (3, 4). It was not until 1988 when the first pharmacological inhibitor, cyclosporine A (CsA), was shown to inhibit MPTP opening (5). CsA is an immunosuppressant that binds to cyclophilin proteins and inhibits their activity. Two years later, the adenine nucleotide translocator (ANT) within the inner mitochondrial membrane was hypothesized to be the pore-forming component of the MPTP (Fig. 1A). Not only could ANT form porelike properties in reconstituted membranes, but the ANT binding agents bongkrekic acid and atractyloside, which are effectors of ADP/ATP translocation across the ANT, can either block or induce mitochondrial swelling, respectively (6). The next suggested component of the MPTP was the voltage-dependent anion channel (VDAC), which is highly abundant within the outer mitochondrial membrane, where it enables conductance of most solutes into the intermembrane space (7). In 1997, the specific cyclophilin that regulated the MPTP was identified as cyclophilin D (CypD), the only known mitochondrial localized cyclophilin protein, which functions as a peptidylprolyl isomerase (8). With these proposed components, the model predicted that the MPTP was generated as one contiguous pore spanning the intermembrane space by apposition of VDAC within the outer membrane and ANT within the inner membrane, regulated by CypD within the mitochondrial matrix (Fig. 1A) (9). This model would go untested until 2004, when Kokoszka et al. determined through genetic ablation of two genes encoding ANT isoforms that it was not required for MPTP opening (10). However, mitochondria from these null mice held twofold more Ca than WT control mitochondria, suggesting that although ANT was not the pore itself, it was still an important regulator. This report largely disproved the long-standing model of the MPTP shown in Fig. 1A, although 1 y later deletion of the gene encoding CypD (Ppif) in mice supported the basic tenet of the entire field that the MPTP was dependent on cyclophilin protein function (11, 12); hence, this aspect of the original model was proven correct. Indeed, mitochondria lacking CypD are desensitized to MPTP opening and mice lacking the gene encoding CypD are desensitized to various MPTP-dependent pathologies, such as ischemia reperfusion injuries across multiple tissues, as well as degenerative disorders associated with Ca overload such as muscular dystrophy (11–13). Another suggested inner membrane pore-forming component of the MPTP was the mitochondrial phosphate carrier (PiC). PiC was initially hypothesized to be a component of the MPTP due to its ability to form a pore and bind to CypD and due to the fact that phosphate greatly influences MPTP opening (14). However, similar to ANT, PiC has also recently NEW MODEL OLD MODEL VDAC OMM