Mitochondrial Outer Membrane Channels

Deciphering mitochondrial dysfunction: Pathophysiological mechanisms in vascular cognitive impairment

Vascular cognitive impairment (VCI) encompasses a range of cognitive deficits arising from vascular pathology. The pathophysiological mechanisms underlying VCI remain incompletely understood; however, chronic cerebral hypoperfusion (CCH) is widely acknowledged as a principal pathological contributor. Mitochondria, crucial for cellular energy production and intracellular signaling, can lead to numerous neurological impairments when dysfunctional. Recent evidence indicates that mitochondrial dysfunction-marked by oxidative stress, disturbed calcium homeostasis, compromised mitophagy, and anomalies in mitochondrial dynamics-plays a pivotal role in VCI pathogenesis. This review offers a detailed examination of the latest insights into mitochondrial dysfunction within the VCI context, focusing on both the origins and consequences of compromised mitochondrial health. It aims to lay a robust scientific groundwork for guiding the development and refinement of mitochondrial-targeted interventions for VCI.

Concentration-Driven Evolution of Adaptive Artificial Ion Channels or Nanopores with Specific Anticancer Activities

In nature, ceramides are a class of sphingolipids possessing a unique ability to self-assemble into protein-permeable channels with intriguing concentration-dependent adaptive channel cavities. However, within the realm of artificial ion channels, this interesting phenomenon is scarcely represented. Herein, we report on a novel class of adaptive artificial channels, Pn-TPPs, based on PEGylated cholic acids bearing triphenylphosphonium (TPP) groups as anion binding motifs. Interestingly, the molecules self-assemble into chloride ion channels at low concentrations while transforming into small molecule-permeable nanopores at high concentrations. Moreover, the TPP groups endow the molecules with mitochondria-targeting properties, enabling them to selectively drill holes on the mitochondrial membrane of cancer cells and subsequently trigger the caspase 9 apoptotic pathway. The anticancer efficacies of Pn-TPPs correlate with their abilities to form nanopores. Significantly, the most active ensembles formed by P5-TPP exhibits impressive anticancer activity against human liver cancer cells, with an IC50 value of 3.8 μM. While demonstrating similar anticancer performance to doxorubicin, P5-TPP exhibits a selectivity index surpassing that of doxorubicin by a factor of 16.8.

Role of APR3 in cancer: apoptosis, autophagy, oxidative stress, and cancer therapy

APR3 (Apoptosis-related protein 3) is a gene that has recently been identified to be associated with apoptosis. The gene is located on human chromosome 2p22.3 and contains both transmembrane and EGF (epidermal growth factor)-like domains. Additionally, it has structural sites, including AP1, SP1, and MEF2D, that indicate NFAT (nuclear factor of activated T cells) and NF-κB (nuclear factor kappa-B) may be transcription factors for this gene. Functionally, APR3 participates in apoptosis due to the induction of mitochondrial damage to release mitochondrial cytochrome C. Concurrently, APR3 affects the cell cycle by altering the expression of Cyclin D1, which, in turn, affects the incidence and growth of malignancies and promotes cell differentiation. Previous reports indicate that APR3 is located in lysosomal membranes, where it contributes to lysosomal activity and participates in autophagy. While further research is required to determine the precise role and molecular mechanisms of APR3, earlier studies have laid the groundwork for APR3 research. There is growing evidence supporting the significance of APR3 in oncology. Therefore, this review aims to examine the current state of knowledge on the role of the newly discovered APR3 in tumorigenesis and to generate fresh insights and suggestions for future research.

Calmodulin Modulates the Gating Properties of Voltage-Dependent Anion Channel from Rat Brain Mitochondria

Calmodulin (CaM) is a key signaling protein that plays a decisive role in mitochondrial Ca2+ homeostasis and signaling and modulates the mitochondrial membrane properties. We propose that voltage-dependent anion channel 1 (VDAC1), one of the most abundant outer mitochondrial membrane (OMM) proteins, could be its possible target or site of action. VDAC1 is known to play a crucial role in the mitochondrial Ca2+ signaling mechanism. Bilayer electrophysiology experiments show that CaM significantly reduces VDAC1's conductivity and modulates its gating as well as permeability properties. Also, spectrofluorimetric analysis indicates the possibility of binding CaM with VDAC1. Theoretical analysis of fluorescence data shows that the aforementioned protein-protein interaction is not linear, but rather it is a complex nonlinear process. In VDAC1, CaM binding site has been predicted using various bioinformatics tools. It is proposed that CaM could interact with VDAC1's outer-loop region and regulate its gating properties. Our findings suggest that VDAC1-CaM interaction could play a crucial role in the transport of ions and metabolites through the OMM and the regulation of the mitochondrial Ca2+ signaling mechanism through alteration of VDAC1's gating and conductive properties.

The Homeostasis of Cartilage Matrix Remodeling and the Regulation of Volume-Sensitive Ion Channel

Degenerative joint diseases of the hips and knees are common and are accompanied by severe pain and movement disorders. At the microscopic level, the main characteristics of osteoarthritis are the continuous destruction and degeneration of cartilage, increased cartilage extracellular matrix catabolism, decreased anabolism, increased synovial fluid, and decreased osmotic pressure. Cell volume stability is mainly regulated by ion channels, many of which are expressed in chondrocytes. These ion channels are closely related to pain regulation, volume regulation, the inflammatory response, cell proliferation, apoptosis, and cell differentiation. In this review, we focus on the important role of volume control-related ion channels in cartilage matrix remodeling and summarize current views. In addition, the potential mechanism of the volume-sensitive anion channel LRRC8A in the early occurrence of osteoarthritis is discussed.

Mitochondrial ion channels in cardiac function

Mitochondria have been recognized as key organelles in cardiac physiology and are potential targets for clinical interventions to improve cardiac function. Mitochondrial dysfunction has been accepted as a major contributor to the development of heart failure. The main function of mitochondria is to meet the high energy demands of the heart by oxidative metabolism. Ionic homeostasis in mitochondria directly regulates oxidative metabolism, and any disruption in ionic homeostasis causes mitochondrial dysfunction and eventually contractile failure. The mitochondrial ionic homeostasis is closely coupled with inner mitochondrial membrane potential. To regulate and maintain ionic homeostasis, mitochondrial membranes are equipped with ion transporting proteins. Ion transport mechanisms involving several different ion channels and transporters are highly efficient and dynamic, thus helping to maintain the ionic homeostasis of ions as well as their salts present in the mitochondrial matrix. In recent years, several novel proteins have been identified on the mitochondrial membranes and these proteins are actively being pursued in research for roles in the organ as well as organelle physiology. In this article, the role of mitochondrial ion channels in cardiac function is reviewed. In recent times, the major focus of the mitochondrial ion channel field is to establish molecular identities as well as assigning specific functions to them. Given the diversity of mitochondrial ion channels and their unique roles in cardiac function, they present novel and viable therapeutic targets for cardiac diseases.

Porins as helpers in mitochondrial protein translocation

Mitochondria import the vast majority of their proteins via dedicated protein machineries. The translocase of the outer membrane (TOM complex) forms the main entry site for precursor proteins that are produced on cytosolic ribosomes. Subsequently, different protein sorting machineries transfer the incoming preproteins to the mitochondrial outer and inner membranes, the intermembrane space, and the matrix. In this review, we highlight the recently discovered role of porin, also termed voltage-dependent anion channel (VDAC), in mitochondrial protein biogenesis. Porin forms the major channel for metabolites and ions in the outer membrane of mitochondria. Two different functions of porin in protein translocation have been reported. First, it controls the formation of the TOM complex by modulating the integration of the central receptor Tom22 into the mature translocase. Second, porin promotes the transport of carrier proteins toward the carrier translocase (TIM22 complex), which inserts these preproteins into the inner membrane. Therefore, porin acts as a coupling factor to spatially coordinate outer and inner membrane transport steps. Thus, porin links metabolite transport to protein import, which are both essential for mitochondrial function and biogenesis.

Collective Dynamics of Ion Channels on Bilayer Lipid Membranes

Ion channels self-organize on cellular and organelle membranes as clusters and mutually modulate their gating behavior. It has been reported that the efficient information transfer is achieved by cooperative clustering of ion channels. To address the origin and nature of collective dynamics in ion channel clusters, a statistical mechanical model, namely, the Zimm-Bragg-type model in two dimensions with unequal weight distribution in channel-channel interactions, has been proposed. Nearest neighbor interaction along with next-nearest neighbor interaction has been considered, assuming symmetric spatial organization. The multichannel bilayer electrophysiology recordings of the voltage-dependent anion channel (VDAC) from rat brain mitochondria have been analyzed in order to test and further extend the model. The model successfully describes the multichannel gating behavior and self-organization of the VDAC cluster.

Recent progress in the use of mitochondrial membrane permeability transition pore in mitochondrial dysfunction-related disease therapies

Mitochondria have various cellular functions, including ATP synthesis, calcium homeostasis, cell senescence, and death. Mitochondrial dysfunction has been identified in a variety of disorders correlated with human health. Among the many underlying mechanisms of mitochondrial dysfunction, the opening up of the mitochondrial permeability transition pore (mPTP) is one that has drawn increasing interest in recent years. It plays an important role in apoptosis and necrosis; however, the molecular structure and function of the mPTP have still not been fully elucidated. In recent years, the abnormal opening up of the mPTP has been implicated in the development and pathogenesis of diverse diseases including ischemia/reperfusion injury (IRI), neurodegenerative disorders, tumors, and chronic obstructive pulmonary disease (COPD). This review provides a systematic introduction to the possible molecular makeup of the mPTP and summarizes the mitochondrial dysfunction-correlated diseases and highlights possible underlying mechanisms. Since the mPTP is an important target in mitochondrial dysfunction, this review also summarizes potential treatments, which may be used to inhibit pore opening up via the molecules composing mPTP complexes, thus suppressing the progression of mitochondrial dysfunction-related diseases.

Mechanisms and pathways of mitochondrial outer membrane protein biogenesis

Outer membrane proteins integrate mitochondria into the cellular environment. They warrant exchange of small molecules like metabolites and ions, transport proteins into mitochondria, form contact sites to other cellular organelles for lipid exchange, constitute a signaling platform for apoptosis and inflammation and mediate organelle fusion and fission. The outer membrane contains two types of integral membrane proteins. Proteins with a transmembrane β-barrel structure and proteins with a single or multiple α-helical membrane spans. All outer membrane proteins are produced on cytosolic ribosomes and imported into the target organelle. Precursors of β-barrel and α-helical proteins are transported into the outer membrane via distinct import routes. The translocase of the outer membrane (TOM complex) transports β-barrel precursors across the outer membrane and the sorting and assembly machinery (SAM complex) inserts them into the target membrane. The mitochondrial import (MIM) complex constitutes the major integration site for α-helical embedded proteins. The import of some MIM-substrates involves TOM receptors, while others are imported in a TOM-independent manner. Remarkably, TOM, SAM and MIM complexes dynamically interact to import a large set of different proteins and to coordinate their assembly into protein complexes. Thus, protein import into the mitochondrial outer membrane involves a dynamic platform of protein translocases.

The MyMOMA domain of MYO19 encodes for distinct Miro-dependent and Miro-independent mechanisms of interaction with mitochondrial membranes

MYO19 interacts with mitochondria through a C-terminal membrane association domain (MyMOMA). Specific mechanisms for localization of MYO19 to mitochondria are poorly understood. Using promiscuous biotinylation data in combination with existing affinity-capture databases, we have identified a number of putative MYO19-interacting proteins. We chose to explore the interaction between MYO19 and the mitochondrial GTPase Miro2 by expressing mchr-Miro2 in combination with GFP-tagged fragments of the MyMOMA domain and assaying for recruitment of MYO19-GFP to mitochondria. Coexpression of MYO19898-970 -GFP with mchr-Miro2 enhanced MYO19898-970 -GFP localization to mitochondria. Mislocalizing Miro2 to filopodial tips or the cytosolic face of the nuclear envelope did not recruit MYO19898-970 -GFP to either location. To address the kinetics of the Miro2/MYO19 interaction, we used FRAP analysis and permeabilization-activated reduction in fluorescence analysis. MyMOMA constructs containing a putative membrane-insertion motif but lacking the Miro2-interacting region displayed slow exchange kinetics. MYO19898-970 -GFP, which does not include the membrane-insertion motif, displayed rapid exchange kinetics, suggesting that MYO19 interacting with Miro2 has higher mobility than MYO19 inserted into the mitochondrial outer membrane. Mutation of well-conserved, charged residues within MYO19 or within the switch I and II regions of Miro2 abolished the enhancement of MYO19898-970 -GFP localization in cells ectopically expressing mchr-Miro2. Additionally, expressing mutant versions of Miro2 thought to represent particular nucleotide states indicated that the enhancement of MYO19898-970 -GFP localization is dependent on Miro2 nucleotide state. Taken together, these data suggest that membrane-inserted MYO19 is part of a larger complex, and that Miro2 plays a role in integration of actin- and microtubule-based mitochondrial activities.

Characteristics of seasonal spermatogenesis in the soft-shelled turtle

Spermatogenesis in reptiles is a seasonally dependent physiological process that is not temporally associated with male mating behavior. Characteristics of seasonal spermatogenesis in reptiles, however, remain largely unknown. In this review, there is a coverage of the characteristics of soft-shelled turtle, Pelodiscus sinensis, during seasonal spermatogenesis that provides insights into spermatogenesis of testudines. The seminiferous epithelium of P. sinensis are undergoing spermatogenesis during the summer and fall, but are quiescent throughout the rest of the year; germ cells progress through spermatogenic stages in a temporal rather than a spatial pattern. While apoptotic germ cells mainly appear in the non-spermatogenic phase, these are seldom present during active spermatogenesis. It is inferred that apoptosis may be one of the reasons for germ cell loss during the resting phase of spermatogenesis. During the period when spermatogenesis is occurring, Sertoli cells become very narrow and are in contact with several round/elongated spermatids. Many residual spermatozoa can be internalized and degraded within Sertoli cells by entosis during the non-spermatogenic phase, which precedes the next reproductive cycle in P. sinensis. In the late spermatogenic phase, round-shaped mitochondria of spermatids become elongated and swollen, subsequently forming a crescent-like shape and develop into "onion-like" shaped mitochondria. As spermiogenesis progresses, the endoplasmic reticulum of spermatids is transferred into a specialized structure called the "Chrysanthemum flower center", which may be a source of autophagosomal membranes. The information provided in this review will help improve understanding of characteristics of seasonal spermatogenesis, which will hopefully promote interest in the study of reptilian species.

Cannabidiol directly targets mitochondria and disturbs calcium homeostasis in acute lymphoblastic leukemia

Anticancer properties of non-psychoactive cannabinoid cannabidiol (CBD) have been demonstrated on tumors of different histogenesis. Different molecular targets for CBD were proposed, including cannabinoid receptors and some plasma membrane ion channels. Here we have shown that cell lines derived from acute lymphoblastic leukemia of T lineage (T-ALL), but not resting healthy T cells, are highly sensitive to CBD treatment. CBD effect does not depend on cannabinoid receptors or plasma membrane Ca²⁺-permeable channels. Instead, CBD directly targets mitochondria and alters their capacity to handle Ca²⁺. At lethal concentrations, CBD causes mitochondrial Ca²⁺ overload, stable mitochondrial transition pore formation and cell death. Our results suggest that CBD is an attractive candidate to be included into chemotherapeutic protocols for T-ALL treatment.

Remodelling of mitochondria during spermiogenesis of Chinese soft-shelled turtle (Pelodiscus sinensis)

Mitochondria are vital cellular organelles that have the ability to change their shape under different conditions, such as in response to stress, disease, changes in metabolic rate, energy requirements and apoptosis. In the present study, we observed remodelling of mitochondria during spermiogenesis and its relationship with mitochondria-associated granules (MAG). At the beginning of spermiogenesis, mitochondria are characterised by their round shape. As spermiogenesis progresses, the round-shaped mitochondria change into elongated and then swollen mitochondria, subsequently forming a crescent-like shape and finally developing into onion-like shaped mitochondria. We also noted changes in mitochondrial size, location and patterns of cristae at different stages of spermiogenesis. Significant differences (P<0.0001) were found in the size of the different-shaped mitochondria. In early spermatids transitioning to the granular nucleus stage, the size of the mitochondria decreased, but increased subsequently during spermiogenesis. Changes in size and morphological variations were achieved through marked mitochondrial fusion. We also observed a non-membranous structure (MAG) closely associated with mitochondria that may stimulate or control fusion during mitochondrial remodelling. The end product of this sophisticated remodelling process in turtle spermatozoa is an onion-like mitochondrion. The acquisition of this kind of mitochondrial configuration is one strategy for long-term sperm storage in turtles.

Cytosolic GAPDH as a redox-dependent regulator of energy metabolism

BACKGROUND

Plant cytosolic NAD-dependent glyceraldehyde-3-phosphate dehydrogenase (GapC) displays redox-dependent changes in its subcellular localizations and activity. Apart from its fundamental role in glycolysis, it also exhibits moonlighting properties. Since the exceptional redox-sensitivity of GapC has been suggested to play a crucial role in its various functions, we here studied its redox-dependent subcellular localization and the influence of the redox-state on GapC protein interactions.

RESULTS

In mesophyll protoplasts from Arabidopsis thaliana, colocalization of GapC with mitochondria was more pronounced under reducing conditions than upon oxidative stress. In accordance, reduced GapC showed an increased affinity to the mitochondrial voltage-dependent anion-selective channel (VDAC) compared to the oxidized one. On the other hand, nuclear localization of GapC was increased under oxidizing conditions. The essential role of the catalytic cysteine for nuclear translocation was shown by using the corresponding cysteine mutants. Furthermore, interaction of GapC with the thioredoxin Trx-h3 as a candidate to revert the redox-modifications, occurred in the nucleus of oxidized protoplasts. In a yeast complementation assay, we could demonstrate that the plant-specific non-phosphorylating glyceraldehyde 3-P dehydrogenase (GapN) can substitute for glucose 6-P dehydrogenase to generate NADPH for re-reduction of the Trx system and ROS defense.

CONCLUSIONS

The preferred association of reduced, glycolytically active GapC with VDAC suggests a substrate-channeling metabolon at the mitochondrial surface for efficient energy generation. Increased occurrence of oxidized GapC in the nucleus points to a function in signal transduction and gene expression. Furthermore, the interaction of GapC with Trx-h3 in the nucleus indicates reversal of the oxidative cysteine modification after re-establishment of cellular homeostasis. Both, energy metabolism and signal transfer for long-term adjustment and protection from redox-imbalances are mediated by the various functions of GapC. The molecular properties of GapC as a redox-switch are key to its multiple roles in orchestrating energy metabolism.

Anion Channels of Mitochondria

Mitochondria are the "power house" of a cell continuously generating ATP to ensure its proper functioning. The constant production of ATP via oxidative phosphorylation demands a large electrochemical force that drives protons across the highly selective and low-permeable mitochondrial inner membrane. Besides the conventional role of generating ATP, mitochondria also play an active role in calcium signaling, generation of reactive oxygen species (ROS), stress responses, and regulation of cell-death pathways. Deficiencies in these functions result in several pathological disorders like aging, cancer, diabetes, neurodegenerative and cardiovascular diseases. A plethora of ion channels and transporters are present in the mitochondrial inner and outer membranes which work in concert to preserve the ionic equilibrium of a cell for the maintenance of cell integrity, in physiological as well as pathophysiological conditions. For, e.g., mitochondrial cation channels KATP and BKCa play a significant role in cardioprotection from ischemia-reperfusion injury. In addition to the cation channels, mitochondrial anion channels are equally essential, as they aid in maintaining electro-neutrality by regulating the cell volume and pH. This chapter focusses on the information on molecular identity, structure, function, and physiological relevance of mitochondrial chloride channels such as voltage dependent anion channels (VDACs), uncharacterized mitochondrial inner membrane anion channels (IMACs), chloride intracellular channels (CLIC) and the aspects of forthcoming chloride channels.

Transport Across Chloroplast Membranes: Optimizing Photosynthesis for Adverse Environmental Conditions

Chloroplasts are central to solar light harvesting and photosynthesis. Optimal chloroplast functioning is vitally dependent on a very intensive traffic of metabolites and ions between the cytosol and stroma, and should be attuned for adverse environmental conditions. This is achieved by an orchestrated regulation of a variety of transport systems located at chloroplast membranes such as porines, solute channels, ion-specific cation and anion channels, and various primary and secondary active transport systems. In this review we describe the molecular nature and functional properties of the inner and outer envelope and thylakoid membrane channels and transporters. We then discuss how their orchestrated regulation affects thylakoid structure, electron transport and excitation energy transfer, proton-motive force partition, ion homeostasis, stromal pH regulation, and volume regulation. We link the activity of key cation and anion transport systems with stress-specific signaling processes in chloroplasts, and discuss how these signals interact with the signals generated in other organelles to optimize the cell performance, with a special emphasis on Ca(2+) and reactive oxygen species signaling.

Ion Channels in Native Chloroplast Membranes: Challenges and Potential for Direct Patch-Clamp Studies

Photosynthesis without any doubt depends on the activity of the chloroplast ion channels. The thylakoid ion channels participate in the fine partitioning of the light-generated proton-motive force (p.m.f.). By regulating, therefore, luminal pH, they affect the linear electron flow and non-photochemical quenching. Stromal ion homeostasis and signaling, on the other hand, depend on the activity of both thylakoid and envelope ion channels. Experimentally, intact chloroplasts and swollen thylakoids were proven to be suitable for direct measurements of the ion channels activity via conventional patch-clamp technique; yet, such studies became infrequent, although their potential is far from being exhausted. In this paper we wish to summarize existing challenges for direct patch-clamping of native chloroplast membranes as well as present available results on the activity of thylakoid Cl⁻ (ClC?) and divalent cation-permeable channels, along with their tentative roles in the p.m.f. partitioning, volume regulation, and stromal Ca²⁺ and Mg²⁺ dynamics. Patch-clamping of the intact envelope revealed both large-conductance porin-like channels, likely located in the outer envelope membrane and smaller conductance channels, more compatible with the inner envelope location. Possible equivalent model for the sandwich-like arrangement of the two envelope membranes within the patch electrode will be discussed, along with peculiar properties of the fast-activated cation channel in the context of the stromal pH control.

Mitochondrial Ion Channels in Cancer Transformation

Cancer transformation involves reprograming of mitochondrial function to avert cell death mechanisms, monopolize energy metabolism, accelerate mitotic proliferation, and promote metastasis. Mitochondrial ion channels have emerged as promising therapeutic targets because of their connection to metabolic and apoptotic functions. This mini review discusses how mitochondrial channels may be associated with cancer transformation and expands on the possible involvement of mitochondrial protein import complexes in pathophysiological process.

Regulation of ceramide channel formation and disassembly: Insights on the initiation of apoptosis

Sphingolipid research has surged in the past two decades and has produced a wide variety of evidence supporting the role of this class of molecules in mediating cellular growth, differentiation, senescence, and apoptosis. Ceramides are a subgroup of sphingolipids (SLs) that are directly involved in the process of initiation of apoptosis. We, and others, have recently shown that ceramides are capable of the formation of protein-permeable channels in mitochondrial outer membranes under physiological conditions. These pores are indeed good candidates for the pathway of release of pro-apoptotic proteins from the mitochondrial intermembrane space (IMS) into the cytosol to initiate intrinsic apoptosis. Here, we review recent findings on the regulation of ceramide channel formation and disassembly, highlighting possible implications on the initiation of the intrinsic apoptotic pathway.

Markov chain Monte Carlo based analysis of post-translationally modified VDAC gating kinetics

The voltage-dependent anion channel (VDAC) is the main conduit for permeation of solutes (including nucleotides and metabolites) of up to 5 kDa across the mitochondrial outer membrane (MOM). Recent studies suggest that VDAC activity is regulated via post-translational modifications (PTMs). Yet the nature and effect of these modifications is not understood. Herein, single channel currents of wild-type, nitrosated, and phosphorylated VDAC are analyzed using a generalized continuous-time Markov chain Monte Carlo (MCMC) method. This developed method describes three distinct conducting states (open, half-open, and closed) of VDAC activity. Lipid bilayer experiments are also performed to record single VDAC activity under un-phosphorylated and phosphorylated conditions, and are analyzed using the developed stochastic search method. Experimental data show significant alteration in VDAC gating kinetics and conductance as a result of PTMs. The effect of PTMs on VDAC kinetics is captured in the parameters associated with the identified Markov model. Stationary distributions of the Markov model suggest that nitrosation of VDAC not only decreased its conductance but also significantly locked VDAC in a closed state. On the other hand, stationary distributions of the model associated with un-phosphorylated and phosphorylated VDAC suggest a reversal in channel conformation from relatively closed state to an open state. Model analyses of the nitrosated data suggest that faster reaction of nitric oxide with Cys-127 thiol group might be responsible for the biphasic effect of nitric oxide on basal VDAC conductance.

Evidence supporting the 19 β-strand model for Tom40 from cysteine scanning and protease site accessibility studies

Most proteins found in mitochondria are translated in the cytosol and enter the organelle via the TOM complex (translocase of the outer mitochondrial membrane). Tom40 is the pore forming component of the complex. Although the three-dimensional structure of Tom40 has not been determined, the structure of porin, a related protein, has been shown to be a β-barrel containing 19 membrane spanning β-strands and an N-terminal α-helical region. The evolutionary relationship between the two proteins has allowed modeling of Tom40 into a similar structure by several laboratories. However, it has been suggested that the 19-strand porin structure does not represent the native form of the protein. If true, modeling of Tom40 based on the porin structure would also be invalid. We have used substituted cysteine accessibility mapping to identify several potential β-strands in the Tom40 protein in isolated mitochondria. These data, together with protease accessibility studies, support the 19 β-strand model for Tom40 with the C-terminal end of the protein localized to the intermembrane space.

Bax channel triplet: co-operativity and voltage gating

Bax, despite being a cytosolic protein, has the distinct ability to form channels in the mitochondrial outer membrane, which are capable of releasing proteins that initiate the execution phase of apoptosis. When studied in a planar phospholipid membrane system, full-length activated Bax can form conducting entities consistent with linearly organized three-channel units displaying steep voltage-gating (n=14) that rivals that of channels in excitable membranes. In addition, the channels display strong positive co-operativity possibly arising from the charge distribution of the voltage sensors. On the basis of functional behaviour, one of the channels in this functional triplet is oriented in the opposite direction to the others often resulting in conflicts between the effects of the electric field and the positive co-operativity of adjacent channels. The closure of the first channel occurs at positive potentials and this permits the second to close, but at negative potentials. The closure of the second channel in turn permits closure of the third, but at positive potentials. Positive co-operativity manifests itself in a number of ways including the second and the third channels opening virtually simultaneously. This extraordinary behaviour must have important, although as yet undefined, physiological roles.

Increased mitochondrial activity in renal proximal tubule cells from young spontaneously hypertensive rats

Renal proximal tubule cells from spontaneously hypertensive rats (SHR), compared with normotensive Wistar-Kyoto rats (WKY), have increased oxidative stress. The contribution of mitochondrial oxidative phosphorylation to the subsequent hypertensive phenotype remains unclear. We found that renal proximal tubule cells from SHR, relative to WKY, had significantly higher basal oxygen consumption rates, ATP synthesis-linked oxygen consumption rates, and maximum and reserve respiration. These bioenergetic parameters indicated increased mitochondrial function in renal proximal tubule cells from SHR compared with WKY. Pyruvate dehydrogenase complex activity was consistently higher in both renal proximal tubule cells and cortical homogenates from SHR than WKY. Treatment for 6 days with dichloroacetate, an inhibitor of pyruvate dehydrogenase kinase, significantly increased renal pyruvate dehydrogenase complex activity and systolic blood pressure in 3-week old WKY and SHR. Therefore, mitochondrial oxidative phosphorylation is higher in renal proximal tubule cells from SHR compared with WKY. Thus the pyruvate dehydrogenase complex is a determinant of increased mitochondrial metabolism that could be a causal contributor to the hypertension in SHR.