Novel Channels of the Outer Membrane of Mitochondria: Recent Discoveries Change Our View

Decoding mitochondria's role in immunity and cancer therapy

The functions of mitochondria, including energy production and biomolecule synthesis, have been known for a long time. Given the rising incidence of cancer, the role of mitochondria in cancer has become increasingly popular. Activated by components released by mitochondria, various pathways interact with each other to induce immune responses to protect organisms from attack. However, mitochondria play dual roles in the progression of cancer. Abnormalities in proteins, which are the elementary structures of mitochondria, are closely linked with oncogenesis. Both the aberrant accumulation of intermediates and mutations in enzymes result in the generation and progression of cancer. Therefore, targeting mitochondria to treat cancer may be a new strategy. Several drugs aimed at inhibiting mutated enzymes and accumulated intermediates have been tested clinically. Here, we discuss the current understanding of mitochondria in cancer and the interactions between mitochondrial functions, immune responses, and oncogenesis. Furthermore, we discuss mitochondria as hopeful targets for cancer therapy, providing insights into the progression of future therapeutic strategies.

Mitochondrial dysfunction in the pathogenesis of acute pancreatitis

The mechanism of cell damage during acute pancreatitis (AP) has not been fully elucidated, and there is still a lack of specific or effective treatments. Increasing evidence has implicated mitochondrial dysfunction as a key event in the pathophysiology of AP. Mitochondrial dysfunction is closely related to calcium (Ca2+) overload, intracellular adenosine triphosphate depletion, mitochondrial permeability transition pore openings, loss of mitochondrial membrane potential, mitophagy damage and inflammatory responses. Mitochondrial dysfunction is an early triggering event in the initiation and development of AP, and this organelle damage may precede the release of inflammatory cytokines, intracellular trypsin activation and vacuole formation of pancreatic acinar cells. This review provides further insight into the role of mitochondria in both physiological and pathophysiological aspects of AP, aiming to improve our understanding of the underlying mechanism which may lead to the development of therapeutic and preventive strategies for AP.

Methods of Measuring Mitochondrial Potassium Channels: A Critical Assessment

In this paper, the techniques used to study the function of mitochondrial potassium channels are critically reviewed. The majority of these techniques have been known for many years as a result of research on plasma membrane ion channels. Hence, in this review, we focus on the critical evaluation of techniques used in the studies of mitochondrial potassium channels, describing their advantages and limitations. Functional analysis of mitochondrial potassium channels in comparison to that of plasmalemmal channels presents additional experimental challenges. The reliability of functional studies of mitochondrial potassium channels is often affected by the need to isolate mitochondria and by functional properties of mitochondria such as respiration, metabolic activity, swelling capacity, or high electrical potential. Three types of techniques are critically evaluated: electrophysiological techniques, potassium flux measurements, and biochemical techniques related to potassium flux measurements. Finally, new possible approaches to the study of the function of mitochondrial potassium channels are presented. We hope that this review will assist researchers in selecting reliable methods for studying, e.g., the effects of drugs on mitochondrial potassium channel function. Additionally, this review should aid in the critical evaluation of the results reported in various articles on mitochondrial potassium channels.

The Diversity of the Mitochondrial Outer Membrane Protein Import Channels: Emerging Targets for Modulation

The functioning of mitochondria and their biogenesis are largely based on the proper function of the mitochondrial outer membrane channels, which selectively recognise and import proteins but also transport a wide range of other molecules, including metabolites, inorganic ions and nucleic acids. To date, nine channels have been identified in the mitochondrial outer membrane of which at least half represent the mitochondrial protein import apparatus. When compared to the mitochondrial inner membrane, the presented channels are mostly constitutively open and consequently may participate in transport of different molecules and contribute to relevant changes in the outer membrane permeability based on the channel conductance. In this review, we focus on the channel structure, properties and transported molecules as well as aspects important to their modulation. This information could be used for future studies of the cellular processes mediated by these channels, mitochondrial functioning and therapies for mitochondria-linked diseases.

MASS SPECTROMETRY-BASED MITOCHONDRIAL PROTEOMICS IN HUMAN OVARIAN CANCERS

The prominent characteristics of mitochondria are highly dynamic and regulatory, which have crucial roles in cell metabolism, biosynthetic, senescence, apoptosis, and signaling pathways. Mitochondrial dysfunction might lead to multiple serious diseases, including cancer. Therefore, identification of mitochondrial proteins in cancer could provide a global view of tumorigenesis and progression. Mass spectrometry-based quantitative mitochondrial proteomics fulfils this task by enabling systems-wide, accurate, and quantitative analysis of mitochondrial protein abundance, and mitochondrial protein posttranslational modifications (PTMs). Multiple quantitative proteomics techniques, including isotope-coded affinity tag, stable isotope labeling with amino acids in cell culture, isobaric tags for relative and absolute quantification, tandem mass tags, and label-free quantification, in combination with different PTM-peptide enrichment methods such as TiO₂ enrichment of tryptic phosphopeptides and antibody enrichment of other PTM-peptides, increase flexibility for researchers to study mitochondrial proteomes. This article reviews isolation and purification of mitochondria, quantitative mitochondrial proteomics, quantitative mitochondrial phosphoproteomics, mitochondrial protein-involved signaling pathway networks, mitochondrial phosphoprotein-involved signaling pathway networks, integration of mitochondrial proteomic and phosphoproteomic data with whole tissue proteomic and transcriptomic data and clinical information in ovarian cancers (OC) to in-depth understand its molecular mechanisms, and discover effective mitochondrial biomarkers and therapeutic targets for predictive, preventive, and personalized treatment of OC. This proof-of-principle model about OC mitochondrial proteomics is easily implementable to other cancer types. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Channels and transporters for inorganic ions in plant mitochondria: Prediction and facts

Mitochondria are crucial bioenergetic organelles for providing different metabolites, including ATP, to sustain cell growth both in animals and in plants. These organelles, delimited by two membranes (outer and inner mitochondrial membrane), maintain their function by an intensive communication with other organelles as well as with the cytosol. Transport of metabolites across the two membranes, but also that of inorganic ions, takes place through specific ion channels and transporters and plays a crucial role in ensuring an adequate ionic milieu within the mitochondria. In the present review we briefly summarize the current knowledge about plant mitochondrial ion channels and transporters in comparison to those of animal mitochondria and examine the possible molecular identity of the so far unidentified transport systems taking into account subcellular targeting predictions and data from literature.

Contribution of Mitochondrial Ion Channels to Chemo-Resistance in Cancer Cells

Mitochondrial ion channels are emerging oncological targets, as modulation of these ion-transporting proteins may impact on mitochondrial membrane potential, efficiency of oxidative phosphorylation and reactive oxygen production. In turn, these factors affect the release of cytochrome c, which is the point of no return during mitochondrial apoptosis. Many of the currently used chemotherapeutics induce programmed cell death causing damage to DNA and subsequent activation of p53-dependent pathways that finally leads to cytochrome c release from the mitochondrial inter-membrane space. The view is emerging, as summarized in the present review, that ion channels located in this organelle may account in several cases for the resistance that cancer cells can develop against classical chemotherapeutics, by preventing drug-induced apoptosis. Thus, pharmacological modulation of these channel activities might be beneficial to fight chemo-resistance of different types of cancer cells.

Mitochondrial porin links protein biogenesis to metabolism

In this report, we summarize recent findings about a role of the outer membrane metabolite channel VDAC/porin in protein import into mitochondria. Mitochondria fulfill key functions for cellular energy metabolism. Their biogenesis involves the import of about 1000 different proteins that are produced as precursors on cytosolic ribosomes. The translocase of the outer membrane (TOM complex) forms the entry gate for mitochondrial precursor proteins. Dedicated protein translocases sort the preproteins into the different mitochondrial subcompartments. While protein transport pathways are analyzed to some detail, only little is known about regulatory mechanisms that fine-tune protein import upon metabolic signaling. Recently, a dual role of the voltage-dependent anion channel (VDAC), also termed porin, in mitochondrial protein biogenesis was reported. First, VDAC/porin promotes as a coupling factor import of carrier proteins into the inner membrane. Second, VDAC/porin regulates the formation of the TOM complex. Thus, the major metabolite channel in the outer membrane VDAC/porin connects protein import to mitochondrial metabolism.

Pharmacological modulation of mitochondrial ion channels

The field of mitochondrial ion channels has undergone a rapid development during the last three decades, due to the molecular identification of some of the channels residing in the outer and inner membranes. Relevant information about the function of these channels in physiological and pathological settings was gained thanks to genetic models for a few, mitochondria-specific channels. However, many ion channels have multiple localizations within the cell, hampering a clear-cut determination of their function by pharmacological means. The present review summarizes our current knowledge about the ins and outs of mitochondrial ion channels, with special focus on the channels that have received much attention in recent years, namely, the voltage-dependent anion channels, the permeability transition pore (also called mitochondrial megachannel), the mitochondrial calcium uniporter and some of the inner membrane-located potassium channels. In addition, possible strategies to overcome the difficulties of specifically targeting mitochondrial channels versus their counterparts active in other membranes are discussed, as well as the possibilities of modulating channel function by small peptides that compete for binding with protein interacting partners. Altogether, these promising tools along with large-scale chemical screenings set up to identify new, specific channel modulators will hopefully allow us to pinpoint the actual function of most mitochondrial ion channels in the near future and to pharmacologically affect important pathologies in which they are involved, such as neurodegeneration, ischaemic damage and cancer. LINKED ARTICLES: This article is part of a themed section on Mitochondrial Pharmacology: Featured Mechanisms and Approaches for Therapy Translation. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.22/issuetoc.

Apoptosis induced by ursodeoxycholic acid in human melanoma cells through the mitochondrial pathway

Ursodeoxycholic acid (UDCA) is a type of hydrophilic bile acid extracted from animal bile with a wide range of biological functions. The present results demonstrated that UDCA could effectively inhibit the proliferation of two human melanoma cell line (M14 and A375) with time‑ and concentration‑dependence. Following exposure to various concentrations of UDCA, M14 cells exhibited typical morphological changes and weaker ability of colony forming. Flow cytometry analysis demonstrated that UDCA could induce a decrease of mitochondrial membrane potential and an increase in reactive oxygen species (ROS) levels in M14 cells. The cell cycle was arrested in the G2/M phase, which was confirmed by the decrease of cyclin‑dependent kinase 1 and cyclinB1 at the protein level. However, when M14 cells were treated with UDCA and Z‑VAD‑FMK (caspase inhibitor) synchronously, the apoptosis rate of the cells was reduced significantly. In addition, it was demonstrated that UDCA induced apoptosis of human melanoma M14 cells through the ROS‑triggered mitochondrial‑associated pathway, which was indicated by the increased expression of cleaved‑caspase‑3, cleaved‑caspase‑9, apoptotic protease activating factor‑1, cleaved‑poly (ADP‑ribose) polymerase 1 and the elevation of B cell lymphoma‑2 (Bcl‑2) associated X protein/Bcl‑2 ratio associated with apoptosis. Therefore, UDCA may be a potential drug for the treatment of human melanoma.

BANFF: ending of bilayer membranes by mphiphilic α-helices is ecessary for orm and unction of organelles 1

By binding to and inserting into the lipid bilayer, amphiphilic α-helices of proteins are involved in the curvature of biological membranes in all organisms. In particular, they are involved in establishing the complex membrane architecture of intracellular organelles like the endoplasmatic reticulum, Golgi apparatus, mitochondria, and chloroplasts. Thus, amphiphilic α-helices are essential for maintenance of cellular metabolism and fitness of organisms. Here we focus on the structure and function of membrane-intrinsic proteins, which are involved in membrane curvature by amphiphilic α-helices, in mitochondria and chloroplasts of the eukaryotic model organisms yeast and Arabidopsis thaliana. Further, we propose a model for transport of fatty acids and lipid compounds across the envelope of chloroplasts in which amphiphilic α-helices might play a role.