Metabolically derived potential on the outer membrane of mitochondria: a computational model

New insights into the structure and dynamics of the TOM complex in mitochondria

To date, there is no general physical model of the mechanism by which unfolded polypeptide chains with different properties are imported into the mitochondria. At the molecular level, it is still unclear how transit polypeptides approach, are captured by the protein translocation machinery in the outer mitochondrial membrane, and how they subsequently cross the entropic barrier of a protein translocation pore to enter the intermembrane space. This deficiency has been due to the lack of detailed structural and dynamic information about the membrane pores. In this review, we focus on the recently determined sub-nanometer cryo-EM structures and our current knowledge of the dynamics of the mitochondrial two-pore outer membrane protein translocation machinery (TOM core complex), which provide a starting point for addressing the above questions. Of particular interest are recent discoveries showing that the TOM core complex can act as a mechanosensor, where the pores close as a result of interaction with membrane-proximal structures. We highlight unusual and new correlations between the structural elements of the TOM complexes and their dynamic behavior in the membrane environment.

VDAC as a voltage-dependent mitochondrial gatekeeper under physiological conditions

Mitochondria, composed of two membranes, play a key role in energy production in eukaryotic cells. The main function of the inner membrane is oxidative phosphorylation, while the mitochondrial outer membrane (MOM) seems to control the energy flux and exchange of various charged metabolites between mitochondria and the cytosol. Metabolites cross MOM via the various isoforms of voltage-dependent anion channel (VDAC). In turn, VDACs interact with some enzymes, other proteins and molecules, including drugs. This work aimed to analyze various literature experimental data related to targeting mitochondrial VDACs and VDAC-kinase complexes on the basis of the hypothesis of generation of the outer membrane potential (OMP) and OMP-dependent reprogramming of cell energy metabolism. Our previous model of the VDAC-hexokinase-linked generation of OMP was further complemented in this study with an additional regulation of the MOM permeability by the OMP-dependent docking of cytosolic proteins like tubulin to VDACs. Computational analysis of the model suggests that OMP changes might be involved in the mechanisms of apoptosis promotion through the so-called transient hyperpolarization of mitochondria. The high concordance of the performed computational estimations with many published experimental data allows concluding that OMP generation under physiological conditions is highly probable and VDAC might function as an OMP-dependent gatekeeper of mitochondria, controlling cell life and death. The proposed model of OMP generation allows understanding in more detail the mechanisms of cancer death resistance and anticancer action of various drugs and treatments influencing VDAC voltage-gating properties, VDAC content, mitochondrial hexokinase activity and VDAC-kinase interactions in MOM.

Spatiotemporal stop-and-go dynamics of the mitochondrial TOM core complex correlates with channel activity

Single-molecule studies can reveal phenomena that remain hidden in ensemble measurements. Here we show the correlation between lateral protein diffusion and channel activity of the general protein import pore of mitochondria (TOM-CC) in membranes resting on ultrathin hydrogel films. Using electrode-free optical recordings of ion flux, we find that TOM-CC switches reversibly between three states of ion permeability associated with protein diffusion. While freely diffusing TOM-CC molecules are predominantly in a high permeability state, non-mobile molecules are mostly in an intermediate or low permeability state. We explain this behavior by the mechanical binding of the two protruding Tom22 subunits to the hydrogel and a concomitant combinatorial opening and closing of the two β-barrel pores of TOM-CC. TOM-CC could thus represent a β-barrel membrane protein complex to exhibit membrane state-dependent mechanosensitive properties, mediated by its two Tom22 subunits.

Energy Metabolic Plasticity of Colorectal Cancer Cells as a Determinant of Tumor Growth and Metastasis

Metabolic plasticity is the ability of the cell to adjust its metabolism to changes in environmental conditions. Increased metabolic plasticity is a defining characteristic of cancer cells, which gives them the advantage of survival and a higher proliferative capacity. Here we review some functional features of metabolic plasticity of colorectal cancer cells (CRC). Metabolic plasticity is characterized by changes in adenine nucleotide transport across the outer mitochondrial membrane. Voltage-dependent anion channel (VDAC) is the main protein involved in the transport of adenine nucleotides, and its regulation is impaired in CRC cells. Apparent affinity for ADP is a functional parameter that characterizes VDAC permeability and provides an integrated assessment of cell metabolic state. VDAC permeability can be adjusted via its interactions with other proteins, such as hexokinase and tubulin. Also, the redox conditions inside a cancer cell may alter VDAC function, resulting in enhanced metabolic plasticity. In addition, a cancer cell shows reprogrammed energy transfer circuits such as adenylate kinase (AK) and creatine kinase (CK) pathway. Knowledge of the mechanism of metabolic plasticity will improve our understanding of colorectal carcinogenesis.

Mitochondrial Metal Ion Transport in Cell Metabolism and Disease

Mitochondria are vital to life and provide biological energy for other organelles and cell physiological processes. On the mitochondrial double layer membrane, there are a variety of channels and transporters to transport different metal ions, such as Ca²⁺, K⁺, Na⁺, Mg²⁺, Zn²⁺ and Fe²⁺/Fe³⁺. Emerging evidence in recent years has shown that the metal ion transport is essential for mitochondrial function and cellular metabolism, including oxidative phosphorylation (OXPHOS), ATP production, mitochondrial integrity, mitochondrial volume, enzyme activity, signal transduction, proliferation and apoptosis. The homeostasis of mitochondrial metal ions plays an important role in maintaining mitochondria and cell functions and regulating multiple diseases. In particular, channels and transporters for transporting mitochondrial metal ions are very critical, which can be used as potential targets to treat neurodegeneration, cardiovascular diseases, cancer, diabetes and other metabolic diseases. This review summarizes the current research on several types of mitochondrial metal ion channels/transporters and their functions in cell metabolism and diseases, providing strong evidence and therapeutic strategies for further insights into related diseases.

Electrical control of the cell energy metabolism at the level of mitochondrial outer membrane

Energy, generated by the mitochondrial oxidative phosphorylation system, is transferred to the cytosol across the mitochondrial outer membrane (MOM), through the voltage-dependent anion channels (VDACs). The role of the VDAC's voltage-gating process to control the transfer of ATP, creatine phosphate and other negatively charged metabolites across MOM might be crucial for the cell energy metabolism regulation. However, it depends on the probability of the outer membrane potential (OMP) generation by a currently undefined mechanism that has usually been considered doubtful, based on the assumption that VDACs always stay in the electrically open state. Nevertheless, computational analysis of various possible metabolically-dependent mechanisms of OMP generation suggests that MOM is not a "coarse sieve", but in fact it functions as an electrical gatekeeper of cell energy metabolism, due to a probable OMP-dependent VDAC's gating. OMP generation could also be involved in the control of cell death resistance and mechanisms of various diseases.

Mitochondrial VDAC2 and cell homeostasis: highlighting hidden structural features and unique functionalities

Voltage-dependent anion channels (VDACs) are the gateway to mitochondrial processes, interlinking the cytosolic and mitochondrial compartments. The mitochondrion acts as a storehouse for cytochrome c, the effector of apoptosis, and hence VDACs become intricately involved in the apoptotic pathway. Isoform 1 of VDAC is abundant in the outer mitochondrial membrane of many cell types, while isoform 2 is the preferred channel in specialized cells including brain and some cancer cells. The primary role of VDACs is metabolite flux. The pro- and anti-apoptotic role of VDAC1 and VDAC2, respectively, are secondary, and are influenced by external factors and interacting proteins. Herein, we focus on the less-studied VDAC2, and shed light on its unique functions and features. VDAC2, along with sharing many of its functions with VDAC1, such as metabolite and Ca²⁺ transport, also has many delineating functions. VDAC2 is closely engaged in the gametogenesis and steroidogenesis pathways and in protection from oxidative stress as well as in neurodegenerative diseases like Alzheimer's and epilepsy. A closer examination of the functional pathways of VDACs indicates that the unique functions of VDAC2 are a result of the different interactome of this isoform. We couple functional differences to the structural and biophysical evidence obtained for the VDACs, and present a testament of why the two VDAC isoforms with >90% sequence similarity, are functionally diverse. Based on these differences, we suggest that the VDAC isoforms now be considered as paralogs. An in-depth understanding of VDAC2 will help us to design better biomolecule targets for cancer and neurodegenerative diseases.

The N-terminus of VDAC: Structure, mutational analysis, and a potential role in regulating barrel shape

A novel feature of the voltage-dependent anion channel (VDAC, mitochondrial porin), is the barrel, comprising an odd number of β-strands and closed by parallel strands. Recent research has focused on the N-terminal segment, which in the available structures, resides in the lumen and is not part of the barrel. In this review, the structural data obtained from vertebrate VDAC are integrated with those from VDAC in artificial bilayers, emphasizing the array of native and tagged versions of VDAC used. The data are discussed with respect to a recent gating model (Zachariae et al. (2012) Structure 20:1-10), in which the N-terminus acts not as a gate on a stable barrel, but rather stabilizes the barrel, preventing its shift into a partially collapsed, low-conductance, closed state. Additionally, the role of the N-terminus in VDAC oligomerization, apoptosis through interactions with hexokinase and its interaction with ATP are discussed briefly.

VDAC electronics: 1. VDAC-hexo(gluco)kinase generator of the mitochondrial outer membrane potential

The simplest mechanism of the generation of the mitochondrial outer membrane potential (OMP) by the VDAC (voltage-dependent anion channel)-hexokinase complex (VHC), suggested earlier, and by the VDAC-glucokinase complex (VGC), was computationally analyzed. Even at less than 4% of VDACs bound to hexokinase, the calculated OMP is high enough to trigger the electrical closure of VDACs beyond the complexes at threshold concentrations of glucose. These results confirmed our previous hypothesis that the Warburg effect is caused by the electrical closure of VDACs, leading to global restriction of the outer membrane permeability coupled to aerobic glycolysis. The model showed that the inhibition of the conductance and/or an increase in the voltage sensitivity of a relatively small fraction of VDACs by factors like tubulin potentiate the electrical closure of the remaining free VDACs. The extrusion of calcium ions from the mitochondrial intermembrane space by the generated OMP, positive inside, might increase cancer cell resistance to death. Within the VGC model, the known effect of induction of ATP release from mitochondria by accumulated glucose-6-phosphate in pancreatic beta cells might result not only of the known effect of GK dissociation from the VDAC-GK complex, but also of a decrease in the free energy of glucokinase reaction, leading to the OMP decrease and VDAC opening. We suggest that the VDAC-mediated electrical control of the mitochondrial outer membrane permeability, dependent on metabolic conditions, is a fundamental physiological mechanism of global regulation of mitochondrial functions and of cell death.

Computational modeling of mitochondrial energy transduction

Mitochondria are the power plant of the heart, burning fat and sugars to supply the muscle with the adenosine triphosphate (ATP) free energy that drives contraction and relaxation during each heart beat. This function was first captured in a mathematical model in 1967. Today, interest in such a model has been rekindled by ongoing in silico integrative physiology efforts such as the Cardiac Physiome project. Here, the status of the field of computational modeling of mitochondrial ATP synthetic function is reviewed.

Yeast mitochondrial interactosome model: metabolon membrane proteins complex involved in the channeling of ADP/ATP

The existence of a mitochondrial interactosome (MI) has been currently well established in mammalian cells but the exact composition of this super-complex is not precisely known, and its organization seems to be different from that in yeast. One major difference is the absence of mitochondrial creatine kinase (MtCK) in yeast, unlike that described in the organization model of MI, especially in cardiac, skeletal muscle and brain cells. The aim of this review is to provide a detailed description of different partner proteins involved in the synergistic ADP/ATP transport across the mitochondrial membranes in the yeast Saccharomyces cerevisiae and to propose a new mitochondrial interactosome model. The ADP/ATP (Aacp) and inorganic phosphate (PiC) carriers as well as the VDAC (or mitochondrial porin) catalyze the import and export of ADP, ATP and Pi across the mitochondrial membranes. Aacp and PiC, which appear to be associated with the ATP synthase, consist of two nanomotors (F₀, F₁) under specific conditions and form ATP synthasome. Identification and characterization of such a complex were described for the first time by Pedersen and co-workers in 2003.

Origami in outer membrane mimetics: correlating the first detailed images of refolded VDAC with over 20 years of biochemical data

Mitochondrial porin forms an aqueous pore in the outer membrane, through which selective passage of small metabolites and ions occurs, thereby regulating both mitochondrial function and cellular respiration. Investigations of the structure and function of porin have been performed with whole mitochondria, membrane vesicles, artificial membranes, and in detergent solutions, resulting in numerous models of porin structure. The mechanisms by which this protein functions are undoubtedly linked to its structure, which remained elusive until 2008, with reports of 3 high-resolution structures of this voltage-dependent, anion-selective channel (VDAC). The barrel structure is relatively simple yet unique: it is arranged as 19 anti-parallel β-strands, with β-strands 1 and 19 aligned parallel to each other to close the barrel. The N-terminal helical component is located within the lumen of the channel, although its precise structure and location in the lumen varies. With the basic barrel structure in hand, the data obtained in attempts to model the structure and understand porin over the past 20 years can be re-evaluated. Herein, using the mammalian VDAC structures as templates, the amassed electrophysiological and biochemical information has been reassessed with respect to the functional mechanisms of VDAC activity, with a focus on voltage-dependent gating.

Theoretical evaluation of a possible nature of the outer membrane potential of mitochondria

A possibility of generation of the outer membrane potential in mitochondria has been suggested earlier in the literature, but the potential has not been directly measured yet. Even its nature, metabolic impact and a possible range of magnitudes are not clear, and require further theoretical and experimental analysis. Here, using simple mathematical model, we evaluated a possible contribution of the Donnan and metabolically derived potentials to the outer membrane potential, concluding that the superposition of both is most probable; exclusively Donnan origin of the potential is doubtful because unrealistically high concentrations of charged macromolecules are needed for maintaining its relatively high levels. Regardless of the mechanism(s) of generation, the maximal possible potential seems to be less than 30 mV because significant osmotic gradients, created at higher values, increase the probability of the outer membrane rupture. New experimental approaches for direct or indirect determination of true value of the outer membrane potential are suggested here to avoid a possible interference of the surface electrical potential of the inner membrane, which may change as a result of the extrusion of matrix protons under energization of mitochondria.

Theoretical evaluation of voltage inducement on internal membranes of biological cells exposed to electric fields

Several reports have recently been published on effects of very short and intense electric pulses on cellular organelles; in a number of cases, the cell plasma membrane appeared to be affected less than certain organelle membranes, whereas with longer and less intense pulses the opposite is the case. The effects are the consequence of the voltages induced on the membranes, and in this article we investigate the conditions under which the induced voltage on an organelle membrane could exceed its counterpart on the cell membrane. This would provide a possible explanation of the observed effects of very short pulses. Frequency-domain analysis yields an insight into the dependence of the voltage inducement on the electric and geometric parameters characterizing the cell and its vicinity. We show that at sufficiently high field frequencies, for a range of parameter values the voltage induced on the organelle membrane can indeed exceed the voltage induced on the cell membrane. Particularly, this can occur if the organelle interior is electrically more conductive than the cytosol, or if the organelle membrane has a lower dielectric permittivity than the cell membrane, and we discuss the plausibility of these conditions. Time-domain analysis is then used to determine the courses of the voltage induced on the membranes by pulses with risetimes and durations in the nanosecond range. The particularly high resting voltage in mitochondria, to which the induced voltage superimposes, could contribute to the explanation why these organelles are the primary target of many observed effects.

A possible restriction of ferro- and ferricyanide oxidoreductase activities of rat liver mitochondria by the outer membrane

In this work, various ferro-ferricyanide oxidoreductase activities of rat liver mitochondria were studied to find conditions under which the outer membrane might restrict the flux of these highly charged non-biological anions. When the isotonic low ionic strength medium was supplemented with 25mM KCl, a several-fold increase in the succinate-ferricyanide reductase activity of mitochondria and in the rate of external NADH oxidation in the presence of ferrocyanide was observed. Mitochondrial respiration with 5mM ferrocyanide was almost completely inhibited after consumption of 3.8-18.5% of the dissolved oxygen, depending on the medium and the presence of 2,4-dinitrophenol. These and other experimental data together with mathematical modeling of the redox-state equilibrium suggest that the measured activities might be restricted by two factors: first, the permeability of the outer mitochondrial membrane and second, a strong influence of the ionic strength of incubation media on the intermembrane space redox reactions.

Oxidative stress in the brain: novel cellular targets that govern survival during neurodegenerative disease

Despite our present knowledge of some of the cellular pathways that modulate central nervous system injury, complete therapeutic prevention or reversal of acute or chronic neuronal injury has not been achieved. The cellular mechanisms that precipitate these diseases are more involved than initially believed. As a result, identification of novel therapeutic targets for the treatment of cellular injury would be extremely beneficial to reduce or eliminate disability from nervous system disorders. Current studies have begun to focus on pathways of oxidative stress that involve a variety of cellular pathways. Here we discuss novel pathways that involve the generation of reactive oxygen species and oxidative stress, apoptotic injury that leads to nuclear degradation in both neuronal and vascular populations, and the early loss of cellular membrane asymmetry that mitigates inflammation and vascular occlusion. Current work has identified exciting pathways, such as the Wnt pathway and the serine-threonine kinase Akt, as central modulators that oversee cellular apoptosis and their downstream substrates that include Forkhead transcription factors, glycogen synthase kinase-3beta, mitochondrial dysfunction, Bad, and Bcl-x(L). Other closely integrated pathways control microglial activation, release of inflammatory cytokines, and caspase and calpain activation. New therapeutic avenues that are just open to exploration, such as with brain temperature regulation, nicotinamide adenine dinucleotide modulation, metabotropic glutamate system modulation, and erythropoietin targeted expression, may provide both attractive and viable alternatives to treat a variety of disorders that include stroke, Alzheimer's disease, and traumatic brain injury.

Parkinson's disease: the first common neurological disease due to auto-intoxication?

Parkinson's disease may be a disease of autointoxication. N-methylated pyridines (e.g. MPP+) are well-established dopaminergic toxins, and the xenobiotic enzyme nicotinamide N-methyltransferase (NNMT) can convert pyridines such as 4-phenylpyridine into MPP+, using S-adenosyl methionine (SAM) as the methyl donor. NNMT has recently been shown to be present in the human brain, a necessity for neurotoxicity, because charged compounds cannot cross the blood-brain barrier. Moreover, it is present in increased concentration in parkinsonian brain. This increase may be part genetic predisposition, and part induction, by excessive exposure to its substrates (particularly nicotinamide) or stress. Elevated enzymic activity would increase MPP+-like compounds such as N-methyl nicotinamide at the same time as decreasing intraneuronal nicotinamide, a neuroprotectant at several levels, creating multiple hits, because Complex 1 would be poisoned and be starved of its major substrate NADH. Developing xenobiotic enzyme inhibitors of NNMT for individuals, or dietary modification for the whole population, could be an important change in thinking on primary and secondary prevention.

Proapoptotic triterpene electrophiles (avicins) form channels in membranes: cholesterol dependence

Avicins, a family of triterpenoid saponins from Acacia victoriae, can regulate the innate stress response in human cells. Their ability to induce apoptosis in transformed cells makes them potential anticancer agents. We report that avicins can form channels in membranes. The conductance reached a steady state after each addition, indicating a dynamic equilibrium between avicin in solution and in the membrane. The high power dependence (up to 10) of the membrane conductance on the avicin concentration indicates the formation of multimeric channels, consistent with the estimated pore radius of 1.1 nm. This radius is too small to allow protein flux across the mitochondrial outer membrane, a process known to initiate apoptosis. Channel formation is lost when avicin's amphipathic side chain is removed, implicating this as the channel-forming region. A small difference in this side chain results in strong cholesterol dependence of channel formation in avicin G that is not found in avicin D. In neutral membranes, avicin channels are nonselective, but negatively-charged lipids confer cation selectivity (5:1, K(+):Cl(-)), indicating that phospholipids form part of the permeation pathway. Avicin channels in the mitochondrial outer membrane may favor apoptosis by altering the potential across this membrane and the intermembrane space pH.

Random walk analysis of restricted metabolite diffusion in skeletal myofibril systems

The purpose of this work was the development of a basal mathematical model for the diffusion of low-molecular metabolites in a skeletal muscle cell. A three-dimension diffusion of low-molecular particles was simulated by a Monte-Carlo method (random walks of diffusing molecules). The model takes into account the following structural elements: (i) a regular lattice of actin and myosin filaments inside a myofibril; (ii) the membranes of sarcoplasmic reticulum and mitochondria surrounding the myofibrils; (iii) a set of myofibrils inside a skeletal muscle cell. We simulated diffusion of particles in the bulk of intracellular water phase and their reflections from the rigid surfaces of intracellular structures. The model allowed to calculate the apparent coefficients of particle diffusion in the axial and radial directions, Dparallel(app) and Dperpendicular(app), respectively. In accordance with experimental data from literature, the coefficient Dparallel(app) was independent of time. The coefficient of radial diffusion Dperpendicular(app) decreased with time to steady state values similar to that determined by the NMR diffusion spectroscopy methods. The interactions of diffusing particles with thin and thick filaments of myofibrils could explain the decrease in the Dperpendicular(app) value by a factor of 20%. The collisions of particles with myofilaments began to reveal themselves as a gradual decrease in the Dperpendicular(app) value at early stages of diffusion (t1/2 approximately equal to 0.05 microsec). The contribution of particle reflections from the membranes of sarcoplasmic reticulum and mitochondria to the retardation of the radial diffusion was about of 20-30%, depending on porosity of a membranous shield around the myofibril. For conventional sizes of a membranous shield (diameter 2 microm), the interactions of particles with the shield caused a decrease in the Dperpendicular(app) value with a half-time t1/2 approximately equal to 0.5 msec. This time is essentially lower by a factor about of 100 than that found in published NMR measurements. When we considered diffusion of particles inside a cell compartment confined to impermeable membranous shield, the reflection of particles from this shield led the drastic decrease in the radial diffusion coefficient (Dperpendicular(app) --> porportional to when t --> porportional to). This pattern of the Dperpendicular(app)(t) time-course might be expected in the NMR measurements on skeletal muscle tissue where a sarcolemma represents an impermeable shield for ATP and PCr molecules.

VDAC: the channel at the interface between mitochondria and the cytosol

The mitochondrial outer membrane is not just a barrier but a site of regulation of mitochondrial function. The VDAC family of proteins are the major pathways for metabolite flux through the outer membrane. These can be regulated in a variety of ways and the integration of these regulatory inputs allows mitochondrial metabolism to be adjusted to changing cellular conditions. This includes total blockage of the flux of anionic metabolites leading to permeabilization of the outer membrane to small proteins followed by apoptotic cell death.

Targeting WNT, protein kinase B, and mitochondrial membrane integrity to foster cellular survival in the nervous system

Targeting essential cellular pathways that determine neuronal and vascular survival can foster a successful therapeutic platform for the treatment of a wide variety of degenerative disorders in the central nervous system. In particular, oxidative cellular injury can precipitate several nervous system disorders that may either be acute in nature, such as during cerebral ischemia, or more progressive and chronic, such as during Alzheimer disease. Apoptotic injury in the brain proceeds through two distinct pathways that ultimately result in the early externalization of membrane phosphatidylserine (PS) residues and the late induction of genomic DNA fragmentation. Degradation of DNA may acutely impact cellular survival, while the exposure of membrane PS residues can lead to microglial phagocytosis of viable cells, cellular inflammation, and thrombosis in the vascular system. Through either independent or common pathways, the Wingless/Wnt pathway and the serine-threonine kinase Akt serve central roles in the maintenance of cellular integrity and the prevention of the phagocytic disposal of cells "tagged" by PS exposure. By selectively governing the activity of specific downstream substrates that include GSK-3beta, Bad, and beta-catenin, Wnt and Akt serve to foster neuronal and vascular survival and block the induction of programmed cell death. Novel to Akt is its capacity to protect cells from phagocytosis through the direct modulation of membrane PS exposure. Intimately linked to the activation of Wnt signaling and Akt is the maintenance of mitochondrial membrane potential and the regulation of Bcl-xL, mitochondrial energy metabolism, and cytochrome c release that can lead to specific cysteine protease activation.

Membrane potential, adenylate levels and Mg2+ are interconnected via adenylate kinase equilibrium in plant cells

Concentrations of adenylate species and free magnesium (Mg(2+)) within cells are mediated by the equilibrium governed by adenylate kinase (AK), the enzyme abundant in plants in chloroplast stroma and intermembrane spaces of chloroplasts and mitochondria. Ratios of free and Mg-bound adenylates (linked to the values of [Mg(2+)] established under AK equilibrium) can be rationalized in terms of the overall dependence of concentrations of Mg(2+) and free and Mg-bound adenylates, as well as electric potential values across the inner membranes of mitochondria and chloroplasts. The potential across the inner mitochondrial membrane, by driving adenylate translocators, equilibrates free adenylates across the inner membrane according to the Nernst equation and contributes to the ATP(total)/ADP(total) ratio in the cytosol. The ratio affects the exchange of free adenylates with chloroplasts and this, in turn, influences the value of potential across the inner chloroplast membrane. From measurements of subcellular ATP(total)/ADP(total) ratios, we suggest a method of estimating the values of potential across inner membranes of mitochondria and chloroplasts in vivo, which allows a comparison of the operation of these organelles under different physiological conditions. We discuss also how the equilibration of adenylates by AK drives adenylate transport across membranes, and establishes [Mg(2+)] in the cytosol and chloroplast stroma, maintaining the rates of photosynthesis and respiration. This provides a tool for metabolomic research, by which the determined concentrations of adenylate species could be used for computation of essential metabolic parameters in the cell and in subcellular compartments.

The voltage-dependent anion channel as a biological transistor: theoretical considerations

The voltage-dependent anion channel (VDAC) is a porin of the mitochondrial outer membrane with a bell-shaped permeability-voltage characteristic. This porin restricts the flow of negatively charged metabolites at certain non-zero voltages, and thus might regulate their flux across the mitochondrial outer membrane. Here, we have developed a mathematical model illustrating the possibility of interaction between two steady-state fluxes of negatively charged metabolites circulating across the VDAC in a membrane. The fluxes interact by contributing to generation of the membrane electrical potential with subsequent closure of the VDAC. The model predicts that the VDAC might function as a single-molecule biological transistor and amplifier, because according to the obtained calculations a small change in the flux of one pair of different negatively charged metabolites causes a significant modulation of a more powerful flux of another pair of negatively charged metabolites circulating across the same membrane with the VDAC. Such transistor-like behavior of the VDAC in the mitochondrial outer membrane might be an important principle of the cell energy metabolism regulation under some physiological conditions.

Nicotinamide: necessary nutrient emerges as a novel cytoprotectant for the brain

Although usually identified as an essential cellular nutrient for cellular growth and maintenance, nicotinamide is under development as a novel cytoprotectant for acute and chronic neurodegenerative disorders. Here, we outline support for the premise that nicotinamide both prevents and reverses neuronal and vascular cell injury. Nicotinamide fosters DNA integrity and maintains phosphatidylserine membrane asymmetry to prevent cellular inflammation, cellular phagocytosis and vascular thrombosis. The downstream cellular and molecular cascades are considered vital for the cytoprotection offered by nicotinamide. These pathways encompass the modulation of Akt, the forkhead transcription factor FKHRL1, mitochondrial membrane potential, caspase activities and cellular energy metabolism, but remain independent of intracellular pH and mitogen-activated protein kinases. As both a therapeutic agent and an investigational tool, nicotinamide offers new therapeutic strategies for degenerative disorders of the CNS.

Model of the outer membrane potential generation by the inner membrane of mitochondria

Voltage-dependent anion channels in the outer mitochondrial membrane are strongly regulated by electrical potential. In this work, one of the possible mechanisms of the outer membrane potential generation is proposed. We suggest that the inner membrane potential may be divided on two resistances in series, the resistance of the contact sites between the inner and outer membranes and the resistance of the voltage-dependent anion channels localized beyond the contacts in the outer membrane. The main principle of the proposed mechanism is illustrated by simplified electric and kinetic models. Computational behavior of the kinetic model shows a restriction of the steady-state metabolite flux through the mitochondrial membranes at relatively high concentration of the external ADP. The flux restriction was caused by a decrease of the voltage across the contact sites and by an increase in the outer membrane potential (up to +60 mV) leading to the closure of the voltage-dependent anion channels localized beyond the contact sites. This mechanism suggests that the outer membrane potential may arrest ATP release through the outer membrane beyond the contact sites, thus tightly coordinating mitochondrial metabolism and aerobic glycolysis in tumor and normal proliferating cells.

Bcl-xL promotes the open configuration of the voltage-dependent anion channel and metabolite passage through the outer mitochondrial membrane

The diffusion of metabolites across the outer mitochondrial membrane is essential for coupled cellular respiration. The outer membrane of mitochondria isolated from growth factor-deprived cells is impaired in its ability to exchange metabolic anions. When added to mitochondria, recombinant Bcl-x(L) restores metabolite exchange across the outer membrane without inducing the loss of cytochrome c from the intermembrane space. Restoration of outer membrane permeability to anionic metabolites does not occur directly through Bcl-x(L) ion channels. Instead, recombinant Bcl-x(L) maintains the outer mitochondrial membrane channel, VDAC, in an open configuration. Consistent with these findings, when ADP-induced oxidative phosphorylation is limited by exogenous beta-NADH, recombinant Bcl-x(L) can sustain outer mitochondrial membrane permeability to ADP. beta-NADH limits respiration by promoting the closed configuration of VDAC. Together these results demonstrate that following an apoptotic signal, Bcl-x(L) can maintain metabolite exchange across the outer mitochondrial membrane by inhibiting VDAC closure.