Channels in the mitochondrial outer membrane: Evidence from patch clamp studies

Patch clamp techniques were applied to outer mitochondrial membranes of giant mitochondria from mice kept on a cuprizone diet or to vesicles produced by fusing membranes derived from the outer membrane ofNeurospora mitochondria. In the negative range of potentials the conductances decreased with increases in the magnitude of voltage, suggesting the closing of channels. Experiments in which mitochondria were treated with the polyanion polymethacrylate maleate styrene (1:2:3) or succinic anhydride suggest that the channels correspond to VDAC. Although sometimes conductance also decreased with increasing potential over a narrow range of positive potentials, more commonly the conductances increased. Although this phenomenon may represent a detachment of the patch, the changes in conductance are reversible, suggesting that they correspond to the formation or the opening of channels.     

Modulation of a gated ion channel admittance in lipid bilayer membranes.

A future class of amperometric biosensors may utilize gated ion channels such as acetylcholine and glutamate receptors as chemical detection components. In this study, bilayer lipid membranes containing voltage-dependent anion channels (VDAC) were used to model an ion-channel-based biosensor which could continuously monitor AC amperometric changes resulting from induced changes in channel conductance. The in-phase and quadrature components of the induced alternating membrane current were monitored as a function of the applied DC offset voltage which was superimposed on the sinusoidal test voltage. The accuracy and sensitivity of the AC-measured VDAC response was dependent on the magnitude of the AC test voltage relative to the DC offset necessary for channel closure. The VDAC channel appears to be a suitable model protein for AC impedance-based biosensor fabrication.     

New insights into the mechanism of permeation through large channels

The mitochondrial channel, VDAC, regulates metabolite flux across the outer membrane. The open conformation has a higher conductance and anionic selectivity, whereas closed states prefer cations and exclude metabolites. In this study five mutations were introduced into mouse VDAC2 to neutralize the voltage sensor. Inserted into planar membranes, mutant channels lack voltage gating, have a lower conductance, demonstrate cationic selectivity, and, surprisingly, are still permeable to ATP. The estimated ATP flux through the mutant is comparable to that for wild-type VDAC2. The outer membranes of mitochondria containing the mutant are permeable to NADH and ADP/ATP. Both experiments support the counterintuitive conclusion that converting a channel from an anionic to a cationic preference does not substantially influence the flux of negatively charged metabolites. This finding supports our previous proposal that ATP translocation through VDAC is facilitated by a set of specific interactions between ATP and the channel wall.     

Does VDAC insert into membranes in random orientation?

It is widely accepted that voltage-dependent anion-selective channel (VDAC) inserts into planar lipid bilayers in a random orientation. This is in contrast to the well-documented oriented insertion of various channel-forming proteins. Because of the potential importance of this issue, we have examined the orientation of VDAC inserted in membranes. The time constants of the VDAC-current relaxation in response to applied positive and negative voltage pulses were used to characterize the channel orientation.We have found that VDAC channels can be separated into two groups according to differences in the time constant ratio. The difference in time constant ratio between the two main groups of VDAC channels was quantitative, and not qualitative as would be expected for opposite topologies. This finding allows us to hypothesize that both groups of VDAC channels possess a qualitatively similar asymmetry with respect to the localization of voltage-gated domains and, consequently, with respect to its entire molecular structure. The probability of having each type of VDAC channel conformation is predetermined by the protein structure in aqueous solution.A striking resemblance between asymmetry in voltage sensitivity at the single-channel and multi-channel levels was also demonstrated. The first inserted channel seems to direct subsequent insertions of channels with a similar conformation.     

Patch clamping VDAC in liposomes containing whole mitochondrial membranes

Summary Whole mitochondrial membranes isolated fromNeurospora crassa were reconstituted into liposomes and patch clamped. Clear activity characteristic of the mitochondrial channel VDAC was found, namely: open state conductance of 650 pS (in 150mm KCl, 1mm CaCl₂, 20mm HEPES, pH 7.2), voltage-dependent closure at both positive and negative potentials, change in conductance upon channel closure of about 450 pS in response to negative and positive potentials, and increased voltage dependence in the presence of König's polyanion. This is the first clear demonstration of VDAC single channels using the patch-clamp technique, even though others used this method before to study whole mitochondrial membranes and liposomes containing mitochondrial proteins. We also found one other channel with a conductance change of about 120 pS.     

Voltage-activated complexation of α-synuclein with three diverse β-barrel channels: VDAC,MspA, and α-hemolysin

Voltage-activated complexation is the process by which a transmembrane potential drives complex formation between a membrane-embedded channel and a soluble or membrane-peripheral target protein. Metabolite and calcium flux across the mitochondrial outer membrane was shown to be regulated by voltage-activated complexation of the voltage-dependent anion channel (VDAC) and either dimeric tubulin or α-synuclein (αSyn). However, the roles played by VDAC's characteristic attributes—its anion selectivity and voltage gating behavior—have remained unclear. Here, we compare in vitro measurements of voltage-activated complexation of αSyn with three well-characterized β-barrel channels—VDAC, MspA, and α-hemolysin—that differ widely in their organism of origin, structure, geometry, charge density distribution, and voltage gating behavior. The voltage dependences of the complexation dynamics for the different channels are observed to differ quantitatively but have similar qualitative features. In each case, energy landscape modeling describes the complexation dynamics in a manner consistent with the known properties of the individual channels, while voltage gating does not appear to play a role. The reaction free energy landscapes thus calculated reveal a non-trivial dependence of the αSyn/channel complex stability on the surface density of αSyn.     

Ionic conductances of squid giant fiber lobe neurons

The cell bodies of the neurons in the giant fiber lobe (GFL) of the squid stellate ganglion give rise to axons that fuse and thereby form the third-order giant axon, whose initial portion functions as the postsynaptic element of the squid giant synapse. We have developed a preparation of dissociated, cultured cells from this lobe and have studied the voltage-dependent conductances using patch-clamp techniques. This system offers a unique opportunity for comparing the properties and regional differentiation of ionic channels in somatic and axonal membranes within the same cell. Some of these cells contain a small inward Na current which resembles that found in axon with respect to tetrodotoxin sensitivity, voltage dependence, and inactivation. More prominent is a macroscopic inward current, carried by Ca2+, which is likely to be the result of at least two kinetically distinct types of channels. These Ca channels differ in their closing kinetics, voltage range and time course of activation, and the extent to which their conductance inactivates. The dominant current in these GFL neurons is outward and is carried by K+. It can be accounted for by a single type of voltage-dependent channel. This conductance resembles the K conductance of the axon, except that it partially inactivates during relatively short depolarizations. Ensemble fluctuation analysis of K currents obtained from excised outside-out patches is consistent with a single type of K channel and yields estimates for the single channel conductance of approximately 13 pS, independently of membrane potential. A preliminary analysis of single channel data supports the conclusion that there is a single type of voltage-dependent, inactivating K channel in the GFL neurons.     

Fluoxetine (Prozac) interaction with the mitochondrial voltage-dependent anion channel and protection against apoptotic cell death

Fluoxetine (Prozac) is a potent antidepressant compound inhibiting serotonin reuptake, but also Na+, K+ and Ca2+ channels and reported to both trigger and prevent apoptosis. Recently, fluoxetine was found to increase the voltage sensitivity of the mitochondrial voltage-dependent anion channel (VDAC). VDAC which functions in transporting metabolites across the mitochondria also plays a crucial role in apoptosis. Here, we demonstrate that fluoxetine interacted with VDAC and decreased its conductance. Fluoxetine inhibited the opening of the mitochondrial permeability transition pore, the release of cytochrome c, and protected against staurosporine-induced apoptotic cell death. These findings may explain some of the reported fluoxetine side effects.     

Elimination and restoration of voltage dependence in the mitochondrial channel,VDAC, by graded modification with succinic anhydride

Summary The major permeability pathways of the outer mitochondrial membrane are the voltage-gated channels called VDAC. It is known that the conductance of these channels decreases as the transmembrane voltage is increased in the positive or negative direction. These channels are known to display a preference for anions over cations of similar size and valence. It was proposed (Doring & Colombini, 1985b) that a set of positive charges lining the channel may be responsible for both voltage dependence and selectivity. A prediction of this proposal is that progressive replacement of the positive charges with negative charges should at first diminish, and then restore, voltage dependence. At the same time, the channel's preference for anions over cations should diminish then reverse. Succinic anhydride was used to perform these experiments as it replaces positively charged amino groups with negatively charged carboxyl groups. When channels, which had been inserted into phospholipid membranes, were treated with moderate amounts of the anhydride, they lost their voltage dependence and preference for anions. With further succinylation, voltage dependence was regenerated while the channels became cation selective. The voltage needed to close one-half of the channels increased in those treatments in which voltage dependence was diminished. As voltage dependence was restored, the voltage needed to close half of the channels decreased. The energy difference between the open and closed state in the absence of an applied field changed little with succinylation, indicating that the procedure did not cause large changes in VDAC's structure but specifically altered those charges responsible for voltage gating and selectivity.     

The calcium-activated potassium channels of turtle hair cells

A major factor determining the electrical resonant frequency of turtle cochlear hair cells is the time course of the Ca-activated K current (Art, J. J., and R. Fettiplace. 1987. Journal of Physiology. 385:207- 242). We have examined the notion that this time course is dictated by the K channel kinetics by recording single Ca-activated K channels in inside-out patches from isolated cells. A hair cell's resonant frequency was estimated from its known correlation with the dimensions of the hair bundle. All cells possess BK channels with a similar unit conductance of approximately 320 pS but with different mean open times of 0.25-12 ms. The time constant of relaxation of the average single- channel current at -50 mV in 4 microM Ca varied between cells from 0.4 to 13 ms and was correlated with the hair bundle height. The magnitude and voltage dependence of the time constant agree with the expected behavior of the macroscopic K(Ca) current, whose speed may thus be limited by the channel kinetics. All BK channels had similar sensitivities to Ca which produced half-maximal activation for a concentration of approximately 2 microM at +50 mV and 12 microM at -50 mV. We estimate from the voltage dependence of the whole-cell K(Ca) current that the BK channels may be fully activated at -35 mV by a rise in intracellular Ca to 50 microM. BK channels were occasionally observed to switch between slow and fast gating modes which raises the possibility that the range of kinetics of BK channels observed in different hair cells reflects a common channel protein whose kinetics are regulated by an unidentified intracellular factor. Membrane patches also contained 30 pS SK channels which were approximately 5 times more Ca-sensitive than BK channels at -50 mV. The SK channels may underlie the inhibitory synaptic potential produced in hair cells by efferent stimulation.     

Voltage gating in VDAC is markedly inhibited by micromolar quantities of aluminum

Summary The mitochondrial outer membrane contains voltagegated channels called VDAC that are responsible for the flux of metabolic substrates and metal ions across this membrane. The addition of micromolar quantities of aluminum chloride to phospholipid membranes containing VDAC channels greatly inhibits the voltage dependence of the channels' permeability. The channels remain in their high conducting (open) state even at high membrane potentials. An analysis of the change in the voltage-dependence parameters revealed that the steepness of the voltage dependence decreased while the voltage needed to close half the channels increased. The energy difference between the open and closed states in the absence of an applied potential did not change. Therefore, the results are consistent with aluminum neutralizing the voltage sensor of the channel. pH shift experiments showed that positively charged aluminum species in solution were not involved. The active form was identified as being either (or both) the aluminum hydroxide or the tetrahydroxoaluminate form. Both of these could reasonably be expected to neutralize a positively charged voltage sensor. Aluminum had no detectable effect of either single-channel conductance or selectivity, indicating that the sensor is probably not located in the channel proper and is distinct from the selectivity filter.     

Voltage-dependent channels found in the membrane fraction of corn mitochondria

Transmembrane channels have been found in the membrane fraction of corn (Zea mays W64AN) mitochondria that exhibit a remarkable resemblance to the voltage dependent anion-selective channels (VDAC) located in the outer membrane of animal (Rattus norvegicus), protist (Paramecium aurelia), and fungal (Neurospora crassa) mitochondria. The channels in corn were demonstrated to be essentially identical to VDAC channels in three characteristic properties: (a) single channel conductance magnitude, (b) weak anion selectivity, and (c) nature of voltage dependence. These findings led us to conclude that the channels present in corn mitochondria are VDAC channels. This discovery may have repercussions concerning the regulation and function of higher plant mitochondria, and the causation of higher plant excitability.     

Low conductance sodium channels in canine cardiac Purkinje cells.

Low conductance sodium (Na) channels have been observed in nerve, skeletal muscle, and cardiac cells. In cardiac tissues the higher amplitude, more commonly observed Na channel was first investigated in detail by Cachelin et al. (Cachelin, A.B., J.E. de Peyer, S. Kokubun, and H. Reuter, 1983, J. Physiol. (Lond.), 340:389-402). They also reported low amplitude Na channel events. We have studied this low conductance Na channel in single canine cardiac Purkinje cells using cell-attached patches. Patch pipette solutions contained either 140 or 280 mM NaCl, and cells were bathed in a solution of 150 mM KCl to bring their resting potential close to zero. In 140 mM Na+, during steps to -50 mV, the lower and higher openings had amplitudes of 0.57 +/- 0.2 and 1.2 +/- 0.2 pA (means +/- SD of Gaussian fits). In 280 mM Na+ at -50 mV, amplitudes were 0.72 +/- 0.2 and 1.55 +/- 0.2 pA. Over a substantial voltage range, the lower events had amplitudes of about one-third that of the higher events. The frequency of the low conductance openings varied in different patches from zero to 22% of total openings. Histograms of open durations and latencies at several voltages suggested no difference in kinetics between the two channel events. The behavior of the low conductance channels was more consistent with a second population of channels rather than a second open state.     

Regulation of Metabolite Flux through Voltage-Gating of VDAC Channels

The mitochondrial outer membrane channel, VDAC, is thought to serve as the major permeability pathway for metabolite flux between the cytoplasm and mitochondria. The permeability of VDAC to citrate, succinate, and phosphate was studied in channels reconstituted into planar phospholipid membranes. All ions showed large changes in permeability depending on whether the channel was in the open or in the low conductance, ``closed' state, with the closed state always more cation selective. This was especially true for the divalent and trivalent anions. Additionally, the anion flux when the voltage was zero was shown to decrease to 5–11% of the open state flux depending on the anion studied. These results give the first rigorous examination of the ability of metabolites to permeate through VDAC channels and indicate that these channels can control the flux of these ions through the outer membrane. This lends more evidence to the growing body of experiments that suggest that the outer mitochondrial membrane has a much more important role in controlling mitochondrial activity than has been thought historically. Received: 4 November 1996/Revised: 8 January 1997     

The sensor regions of VDAC are translocated from within the membrane to the surface during the gating processes.

The motion of the sensor regions in a mitochondrial voltage-gated channel called VDAC were probed by attaching biotin at specific locations and determining its ability to bind to added streptavidin. Site-directed mutagenesis was used to introduce single cysteine residues into Neurospora crassa VDAC (naturally lacks cysteine). These were chemically biotinylated and reconstituted into planar phospholipid membranes. In the 19 sites examined, only two types of results were observed upon streptavidin addition: in type 1, channel conductance was reduced, but voltage gating could proceed; in type 2, channels were locked in a closed state. The result at type 1 sites is interpreted as streptavidin binding to sites in static regions close to the channel opening. The binding sterically interferes with ion flow. The result at type 2 sites indicates that these are located on a mobile domain and coincide with the previously identified sensor regions. The findings are consistent with closure resulting from the movement of a domain from within the transmembrane regions to the membrane surface. No single site was accessible to streptavidin from both membrane surfaces, indicating that the motion is limited. From the streptavidin-induced reduction in conductance at type 1 sites, structural information was obtained about the location of these sites.     

Spike latency and jitter of neuronal membrane patches with stochastic Hodgkin-Huxley channels

Ion channel stochasticity can influence the voltage dynamics of neuronal membrane, with stronger effects for smaller patches of membrane because of the correspondingly smaller number of channels. We examine this question with respect to first spike statistics in response to a periodic input of membrane patches including stochastic Hodgkin-Huxley channels, comparing these responses to spontaneous firing. Without noise, firing threshold of the model depends on frequency—a sinusoidal stimulus is subthreshold for low and high frequencies and suprathreshold for intermediate frequencies. When channel noise is added, a stimulus in the lower range of subthreshold frequencies can influence spike output, while high subthreshold frequencies remain subthreshold. Both input frequency and channel noise strength influence spike timing. Specifically, spike latency and jitter have distinct minima as a function of input frequency, showing a resonance like behavior. With either no input, or low frequency subthreshold input, or input in the low or high suprathreshold frequency range, channel noise reduces latency and jitter, with the strongest impact for the lowest input frequencies. In contrast, for an intermediate range of suprathreshold frequencies, where an optimal input gives a minimum latency, the noise effect reverses, and spike latency and jitter increase with channel noise. Thus, a resonant minimum of the spike response as a function of frequency becomes more pronounced with less noise. Spike latency and jitter also depend on the initial phase of the input, resulting in minimal latencies at an optimal phase, and depend on the membrane time constant, with a longer time constant broadening frequency tuning for minimal latency and jitter. Taken together, these results suggest how stochasticity of ion channels may influence spike timing and thus coding for neurons with functionally localized concentrations of channels, such as in “hot spots” of dendrites, spines or axons.     

The light-sensitive conductance of hyperpolarizing invertebrate photoreceptors: a patch-clamp study

Tight-seal recording was employed to investigate membrane currents in hyperpolarizing ciliary photoreceptors enzymatically isolated from the eyes of the file clam (Lima scabra) and the bay scallop (Pecten irradians). These two organisms are unusual in that their double retinas also possess a layer of depolarizing rhabdomeric cells. Ciliary photoreceptors from Lima have a rounded soma, 15-20 microns diam, and display a prominent bundle of fine processes up to 30 microns long. The cell body of scallop cells is similar in size, but the ciliary appendages are modified, forming small spherical structures that protrude from the cell. In both species light stimulation at a voltage near the resting potential gives rise to a graded outward current several hundred pA in amplitude, accompanied by an increase in membrane conductance. The reversal potential of the photocurrent is approximately -80 mV, and shifts in the positive direction by approximately 39 mV when the concentration of extracellular K is increased from 10 to 50 mM, consistent with the notion that light activates K-selective channels. The light-activated conductance increases with depolarization in the physiological range of membrane voltages (-30 to -70 mV). Such outward rectification is greatly reduced after removal of divalent cations from the superfusate. In Pecten, cell- attached recordings were also obtained; in some patches outwardly directed single-channel currents could be activated by light but not by voltage. The unitary conductance of these channels was approximately 26 pS. Solitary ciliary cells also gave evidence of the post stimulus rebound, which is presumably responsible for initiating the "off" discharge of action potentials at the termination of a light stimulus: in patches containing only voltage-dependent channels, light stimulation suppressed depolarization-induced activity, and was followed by a strong burst of openings, directly related to the intensity of the preceding photostimulation.     

Apical Nonspecific Cation Channels in Everted Collecting Tubules of Potassium-Adapted Ambystoma

We observed intermediate conductance channels in approximately 20% of successful patch-clamp seals made on collecting tubules dissected from Ambystoma adapted to 50 mm potassium. These channels were rarely observed in collecting tubules taken from animals which were maintained in tap water. Potassium-adaptation either leads to an increase in the number of channels present or activates quiescent channels. In cell-attached patches the conductance averaged 30.3 ± 2.4 (9) pS. Since replacement of the chloride in the patch pipette with gluconate did not change the conductance, the channel carries cations, not anions. Notably, channel activity was observed at both positive and negative pipette voltages. When the pipette was voltage clamped at 0 mV or positive voltages, the current was directed inward, consistent with the movement of sodium into the cell. The pipette voltage at which the polarity of the current reversed (movement of potassium into the pipette) was −29.6 ± 6.5(9) mV. Open probability at 0 mV pipette voltage was 0.08 ± 0.03 and was unaffected when the apical membrane was exposed to either 2 × 10⁻⁶ or 2 × 10⁻⁵ m of amiloride. Exposure of the basolateral surface of the tubule to a saline containing 15 mm potassium caused a significant increase (P less than 0.001) in the open probability of these channels to 0.139 ± 0.002 without affecting the conductance of the apical channel. These data illustrate the presence of an intermediate conductance, poorly selective, amiloride-insensitive cation channel in native vertebrate collecting tubule. We postulate that, at least in amphibia, this channel may be used to secrete potassium. Received: 14 January 2000/Revised: 16 June 2000     

Single voltage-dependent K+ and Cl- channels in cultured rat astrocytes

The kinetic reactions of a voltage-dependent K+ channel, which constituted about 14% of all the recorded K+ channels in the membrane of cultured rat astrocytes were studied in detail. A scheme of one open and three closed states is necessary to describe the kinetic reactions of this channel. The channel contributes little to the resting membrane potential. Its steady state open probability (Po) is 0.06 at -70 mV. When the cell is depolarized to O mV, Po approaches 1. This represents a 17-fold increase. Such channels could contribute to the potassium clearance by enhancing the effect of "spatial buffering." Additionally, single anion-selective channels with very high conductances were found in inside-out patches in approximately 15% of all recorded channels in the membrane of rat astrocytes. Channel openings are characterized by more than one conductance level; the main level showed a mean conductance of 400 pS. These channels are divided into two groups. Approximately 90% of the recorded chloride channels showed a strong voltage dependency of their current fluctuations. Within a relatively small potential range (+/- 15 mV) the channels have a high probability of being in the active state. After a voltage jump to varying testing potentials in the range of +/- 20 to +/- 50 mV the channels continued to be in the active state for some time and then closed to a shut state. If the testing potential persisted, the channels were not able to leave this shut state.(ABSTRACT TRUNCATED AT 250 WORDS)     

A soluble mitochondrial protein increases the voltage dependence of the mitochondrial channel,VDAC

A soluble protein isolated from mitochondria has been found to modulate the voltage-dependent properties of the mitochondrial outer membrane channel, VDAC. This protein, called the VDAC modulator, was first found inNeurospora crassa and then discovered in species from other eukaryotic kingdoms. The modulator-containing fraction (at a crude protein concentration of 20 µg/ml) increases the voltage dependence of VDAC channels over 2–3-fold. At higher protein concentrations (50–100 µg/ml), some channels seem to remain in a closed state or be blocked while others display the higher voltage dependence and are able to close at low membrane potentials. By increasing the steepness of the voltage-dependent properties of VDAC channels, this modulator may serve as an amplifierin vivo to increase the sensitivity of the channels in response to changes in the cell's microenvironment, and consequently, regulate the metabolic flux across the outer mitochondrial membrane by controlling the gating of VDAC channels.