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

Inorganic polyphosphate and ion transport across biological membranes

Inorganic polyphosphate (polyP) is widely recognized for playing important roles and processes involved in energy and phosphate storage, regulation of gene expression, and calcium signaling. The less well-known role of polyP is as a direct mediator of ion transport across biological membranes. Here, we will briefly summarize current knowledge of the molecular mechanisms of how polyP can be involved in membrane ion transport. We discuss three types of mechanisms that might involve polyP: (1) formation of non-protein channel complex that includes calcium, polyP, and polyhydroxybutyrate (PHB); (2) modulation of the channel activity of PHBlated protein channels; and (3) direct effects of polyP on the function of the voltage-gated ion channels in the process that do not involve PHB.

Hydralazine inhibits neuroinflammation and oxidative stress in APP/PS1 mice via TLR4/NF-κB and Nrf2 pathways

Alzheimer's disease (AD) is a common chronic progressive neurodegenerative disorder, and curative treatment has not been developed. The objective of this study was to investigate the potential effects of hydralazine (Hyd, a hypertension treatment drug) on the development process of AD and its mechanisms. We treated 6-month-old male APP/PS1 mice with Hyd for 5 weeks, measured changes in behavior and pathological status, and analyzed differences in gene expression by RNA sequencing. The results demonstrated that Hyd improved cognitive deficits and decreased amyloid beta protein deposition in the cortex and hippocampus, while RNA sequencing analysis suggested that the regulation of neuroinflammation and energy metabolism might play pivotal roles for Hyd's beneficial effects. Therefore, we further investigated inflammatory response, redox state, and mitochondrial function, as well as the expression of toll-like receptor 4 (TLR4)/nuclear factor Kappa B (NF-κB)-dependent neuroinflammation gene and nuclear factor erythroid 2-related factor 2 (Nrf2)-mediated antioxidant gene in AD mice. The results showed that Hyd reduced the damage of neuroinflammation and oxidative stress, improved mitochondrial dysfunction, downregulated pro-inflammation gene expression, and upregulated antioxidant gene expression. The results in lipopolysaccharide (LPS)-induced BV2 cell model demonstrated that Hyd suppressed pro-inflammatory response via TLR4/NF-κB signaling pathway. In addition, by silencing the Nrf2 gene expression, it was found that Hyd can reduce LPS-induced reactive oxygen species production by activating the Nrf2 signaling pathway. Therefore, administration of Hyd in the early stage of AD might be beneficial in delaying the pathological development of AD via inhibiting neuroinflammation and oxidative stress.

Cyclophilin D in Mitochondrial Dysfunction: A Key Player in Neurodegeneration?

Mitochondrial dysfunction plays a pivotal role in numerous complex diseases. Understanding the molecular mechanisms by which the "powerhouse of the cell" turns into the "factory of death" is an exciting yet challenging task that can unveil new therapeutic targets. The mitochondrial matrix protein CyPD is a peptidylprolyl cis-trans isomerase involved in the regulation of the permeability transition pore (mPTP). The mPTP is a multi-conductance channel in the inner mitochondrial membrane whose dysregulated opening can ultimately lead to cell death and whose involvement in pathology has been extensively documented over the past few decades. Moreover, several mPTP-independent CyPD interactions have been identified, indicating that CyPD could be involved in the fine regulation of several biochemical pathways. To further enrich the picture, CyPD undergoes several post-translational modifications that regulate both its activity and interaction with its clients. Here, we will dissect what is currently known about CyPD and critically review the most recent literature about its involvement in neurodegenerative disorders, focusing on Alzheimer's Disease and Parkinson's Disease, supporting the notion that CyPD could serve as a promising therapeutic target for the treatment of such conditions. Notably, significant efforts have been made to develop CyPD-specific inhibitors, which hold promise for the treatment of such complex disorders.

Identity, structure, and function of the mitochondrial permeability transition pore: controversies, consensus, recent advances, and future directions

The mitochondrial permeability transition (mPT) describes a Ca2+-dependent and cyclophilin D (CypD)-facilitated increase of inner mitochondrial membrane permeability that allows diffusion of molecules up to 1.5 kDa in size. It is mediated by a non-selective channel, the mitochondrial permeability transition pore (mPTP). Sustained mPTP opening causes mitochondrial swelling, which ruptures the outer mitochondrial membrane leading to subsequent apoptotic and necrotic cell death, and is implicated in a range of pathologies. However, transient mPTP opening at various sub-conductance states may contribute several physiological roles such as alterations in mitochondrial bioenergetics and rapid Ca2+ efflux. Since its discovery decades ago, intensive efforts have been made to identify the exact pore-forming structure of the mPT. Both the adenine nucleotide translocase (ANT) and, more recently, the mitochondrial F1FO (F)-ATP synthase dimers, monomers or c-subunit ring alone have been implicated. Here we share the insights of several key investigators with different perspectives who have pioneered mPT research. We critically assess proposed models for the molecular identity of the mPTP and the mechanisms underlying its opposing roles in the life and death of cells. We provide in-depth insights into current controversies, seeking to achieve a degree of consensus that will stimulate future innovative research into the nature and role of the mPTP.

A Pore Forming Toxin-like Protein Derived from Chinese Red Belly Toad Bombina maxima Triggers the Pyroptosis of Hippomal Neural Cells and Impairs the Cognitive Ability of Mice

Toxin-like proteins and peptides of skin secretions from amphibians play important physiological and pathological roles in amphibians. βγ-CAT is a Chinese red-belly toad-derived pore-forming toxin-like protein complex that consists of aerolysin domain, crystalline domain, and trefoil factor domain and induces various toxic effects via its membrane perforation process, including membrane binding, oligomerization, and endocytosis. Here, we observed the death of mouse hippocampal neuronal cells induced by βγ-CAT at a concentration of 5 nM. Subsequent studies showed that the death of hippocampal neuronal cells was accompanied by the activation of Gasdermin E and caspase-1, suggesting that βγ-CAT induces the pyroptosis of hippocampal neuronal cells. Further molecular mechanism studies revealed that the pyroptosis induced by βγ-CAT is dependent on the oligomerization and endocytosis of βγ-CAT. It is well known that the damage of hippocampal neuronal cells leads to the cognitive attenuation of animals. The impaired cognitive ability of mice was observed after intraperitoneal injection with 10 μg/kg βγ-CAT in a water maze assay. Taken together, these findings reveal a previously unknown toxicological function of a vertebrate-derived pore-forming toxin-like protein in the nerve system, which triggers the pyroptosis of hippocampal neuronal cells, ultimately leading to hippocampal cognitive attenuation.

Small-molecule inhibitors of cyclophilin D as potential therapeutics in mitochondria-related diseases

Cyclophilin D (CypD) is a key regulator of mitochondrial permeability transition pore (mPTP) opening. This pathophysiological phenomenon is associated with the development of several human diseases, including ischemia-reperfusion injury and neurodegeneration. Blocking mPTP opening through CypD inhibition could be a novel and promising therapeutic approach for these conditions. While numerous CypD inhibitors have been discovered to date, none have been introduced into clinical practice, mostly owing to their high toxicity, unfavorable pharmacokinetics, and low selectivity for CypD over other cyclophilins. This review summarizes current knowledge of CypD inhibitors, with a particular focus on small-molecule compounds with regard to their in vitro activity, their selectivity for CypD, and their binding mode within the enzyme's active site. Finally, approaches for improving the molecular design of CypD inhibitors are discussed.

Inorganic Polyphosphate in Mitochondrial Energy Metabolism and Pathology

In this chapter, the current understanding of the potential roles played by polyphosphate in mitochondrial function with a specific focus on energy metabolism and mitochondrial pathologies caused by stress is summarized. Here we will discuss details of the possible ion transporting mechanisms of mitochondria that might involve polyP and their role in mitochondrial physiology and pathology are discussed.

Formation of High-Conductive C Subunit Channels upon Interaction with Cyclophilin D

The c subunit of the ATP synthase is an inner mitochondrial membrane (IMM) protein. Besides its role as the main component of the rotor of the ATP synthase, c subunit from mammalian mitochondria exhibits ion channel activity. In particular, c subunit may be involved in one of the pathways leading to the formation of the permeability transition pore (PTP) during mitochondrial permeability transition (PT), a phenomenon consisting of the permeabilization of the IMM due to high levels of calcium. Our previous study on the synthetic c subunit showed that high concentrations of calcium induce misfolding into cross-β oligomers that form low-conductance channels in model lipid bilayers of about 400 pS. Here, we studied the effect of cyclophilin D (CypD), a mitochondrial chaperone and major regulator of PTP, on the electrophysiological activity of the c subunit to evaluate its role in the functional properties of c subunit. Our study shows that in presence of CypD, c subunit exhibits a larger conductance, up to 4 nS, that could be related to its potential role in mitochondrial toxicity. Further, our results suggest that CypD is necessary for the formation of c subunit induced PTP but may not be an integral part of the pore.

C subunit of the ATP synthase is an amyloidogenic calcium dependent channel-forming peptide with possible implications in mitochondrial permeability transition

The c subunit is an inner mitochondrial membrane (IMM) protein encoded by three nuclear genes. Best known as an integral part of the F₀ complex of the ATP synthase, the c subunit is also present in other cytoplasmic compartments in ceroid lipofuscinoses. Under physiological conditions, this 75 residue-long peptide folds into an α-helical hairpin and forms oligomers spanning the lipid bilayer. In addition to its physiological role, the c subunit has been proposed as a key participant in stress-induced IMM permeabilization by the mechanism of calcium-induced permeability transition. However, the molecular mechanism of the c subunit participation in IMM permeabilization is not completely understood. Here we used fluorescence spectroscopy, atomic force microscopy and black lipid membrane methods to gain insights into the structural and functional properties of unmodified c subunit protein that might make it relevant to mitochondrial toxicity. We discovered that c subunit is an amyloidogenic peptide that can spontaneously fold into β-sheets and self-assemble into fibrils and oligomers in a Ca²⁺-dependent manner. C subunit oligomers exhibited ion channel activity in lipid membranes. We propose that the toxic effects of c subunit might be linked to its amyloidogenic properties and are driven by mechanisms similar to those of neurodegenerative polypeptides such as Aβ and α-synuclein.