Amyloid ß, α-synuclein and the c subunit of the ATP synthase: Can these peptides reveal an amyloidogenic pathway of the permeability transition pore?

Mitochondrial Permeability Transition (PT) is a phenomenon of increased permeability of the inner mitochondrial membrane in response to high levels of Ca2+ and/or reactive oxygen species (ROS) in the matrix. PT occurs upon the opening of a pore, namely the permeability transition pore (PTP), which dissipates the membrane potential uncoupling the respiratory chain. mPT activation and PTP formation can occur through multiple molecular pathways. The specific focus of this review is to discuss the possible molecular mechanisms of PTP that involve the participation of mitochondrially targeted amyloid peptides Aß, α-synuclein and c subunit of the ATP synthase (ATPase). As activators of PTP, amyloid peptides are uniquely different from other activators because they are capable of forming channels in lipid bilayers. This property rises the possibility that in this permeabilization pathway the formation of the channel involves the direct participation of peptides, making it uniquely different from other PTP induction mechanisms. In this pathway, a critical step of PTP activation involves the import of amyloidogenic peptides from the cytosol into the matrix. In the matrix these peptides, which would fold into α-helical structure in native conditions, interact with cyclophilin D (CypD) and upon stimulation by elevated ROS and/or the Ca2+ spontaneously misfold into ß-sheet ion conducting pores, causing PTP opening.

Insoluble tau aggregates induce neuronal death through modification of membrane ion conductance, activation of voltage-gated calcium channels and NADPH oxidase.

Most neurodegenerative disorders are associated with aggregation and accumulation of misfolded proteins. One of these proteins, tau, is involved in a number of pathologies including Alzheimer's disease and frontotemporal dementia. Aggregation and phosphorylation of tau have been shown to be a trigger for abnormal signal transduction and disruption of cellular homeostasis. Here, we have studied the effect of extracellular tau at different stages of aggregation in cortical co-cultures of neurons and astrocytes, to understand how this process affects tau pathogenicity. We found that the species formed after prolonged in vitro aggregation of tau (longer than 1 day) are able to stimulate reactive oxygen species (ROS) production through the activation of NADPH oxidase without decreasing the level of the endogenous antioxidant glutathione. The same late insoluble aggregates of tau induced calcium signals in neurons and a gradual increase in the ionic current of artificial membranes. Both tau-induced calcium signals and ROS production in NADPH oxidase were reduced in the presence of the inhibitor of voltage-gated calcium channels (VGCC) nifedipine. This suggests that insoluble aggregates of tau incorporate into the membrane and modify ionic currents, changing plasma membrane potential and activating VGCCs, which induces a calcium influx that triggers ROS production in NADPH oxidase. The combination of all these effects likely leads to toxicity, as only the same insoluble tau aggregates which demonstrated membrane-active properties produced neuronal cell death.

The very low number of calcium-induced permeability transition pores in the single mitochondrion.

Mitochondrial permeability transition (PT) is a phenomenon of stress-induced increase in nonspecific permeability of the mitochondrial inner membrane that leads to disruption of oxidative phosphorylation and cell death. Quantitative measurement of the membrane permeability increase during PT is critically important for understanding the PT's impact on mitochondrial function. The elementary unit of PT is a PT pore (PTP), a single channel presumably formed by either ATP synthase or adenine nucleotide translocator (ANT). It is not known how many channels are open in a single mitochondrion during PT, which makes it difficult to quantitatively estimate the overall degree of membrane permeability. Here, we used wide-field microscopy to record mitochondrial swelling and quantitatively measure rates of single-mitochondrion volume increase during PT-induced high-amplitude swelling. PT was quantified by calculating the rates of water flux responsible for measured volume changes. The total water flux through the mitochondrial membrane of a single mitochondrion during PT was in the range of (2.5 ± 0.4) × 10-17 kg/s for swelling in 2 mM Ca2+ and (1.1 ± 0.2) × 10-17 kg/s for swelling in 200 µM Ca2+. Under these experimental conditions, a single PTP channel with ionic conductance of 1.5 nS could allow passage of water at the rate of 0.65 × 10-17 kg/s. Thus, we estimate the integral ionic conductance of the whole mitochondrion during PT to be 5.9 ± 0.9 nS for 2 mM concentration of Ca2+ and 2.6 ± 0.4 nS for 200 µM of Ca2+. The number of PTPs per mitochondrion ranged from one to nine. Due to the uncertainties in PTP structure and model parameters, PTP count results may be slightly underestimated. However, taking into account that each mitochondrion has ∼15,000 copies of ATP synthases and ANTs, our data imply that PTP activation is a rare event that occurs only in a small subpopulation of these proteins.

ATP Synthase C-Subunit-Deficient Mitochondria Have a Small Cyclosporine A-Sensitive Channel, but Lack the Permeability Transition Pore.

Permeability transition (PT) is an increase in mitochondrial inner membrane permeability that can lead to a disruption of mitochondrial function and cell death. PT is responsible for tissue damage in stroke and myocardial infarction. It is caused by the opening of a large conductance (∼1.5 nS) channel, the mitochondrial PT pore (mPTP). We directly tested the role of the c-subunit of ATP synthase in mPTP formation by measuring channel activity in c-subunit knockout mitochondria. We found that the classic mPTP conductance was lacking in c-subunit knockout mitochondria, but channels sensitive to the PT inhibitor cyclosporine A could be recorded. These channels had a significantly lower conductance compared with the cyclosporine A-sensitive channels detected in parental cells and were sensitive to the ATP/ADP translocase inhibitor bongkrekic acid. We propose that, in the absence of the c-subunit, mPTP cannot be formed, and a distinct cyclosporine A-sensitive low-conductance channel emerges.