Ion channels in synaptic vesicles from Torpedo electric organ

An electric generator using living Torpedo electric organs controlled by fluid pressure-based alternative nervous systems

Direct electric power generation using biological functions have become a research focus due to their low cost and cleanliness. Unlike major approaches using glucose fuels or microbial fuel cells (MFCs), we present a generation method with intrinsically high energy conversion efficiency and generation with arbitrary timing using living electric organs of Torpedo (electric rays) which are serially integrated electrocytes converting ATP into electric energy. We developed alternative nervous systems using fluid pressure to stimulate electrocytes by a neurotransmitter, acetylcholine (Ach), and demonstrated electric generation. Maximum voltage and current were 1.5 V and 0.64 mA, respectively, with a duration time of a few seconds. We also demonstrated energy accumulation in a capacitor. The current was far larger than that using general cells other than electrocytes (~pA level). The generation ability was confirmed against repetitive cycles and also after preservation for 1 day. This is the first step toward ATP-based energy harvesting devices.

Observations of calcium dynamics in cortical secretory vesicles

Calcium (Ca(2+)) dynamics were evaluated in fluorescently labeled sea urchin secretory vesicles using confocal microscopy. 71% of the vesicles examined exhibited one or more transient increases in the fluorescence signal that was damped in time. The detection of transient increases in signal was dependent upon the affinity of the fluorescence indicator; the free Ca(2+) concentration in the secretory vesicles was estimated to be in the range of ∼10 to 100 μM. Non-linear stochastic analysis revealed the presence of extra variance in the Ca(2+) dependent fluorescence signal. This noise process increased linearly with the amplitude of the Ca(2+) signal. Both the magnitude and spatial properties of this noise process were dependent upon the activity of vesicle p-type (Ca(v)2.1) Ca(2+) channels. Blocking the p-type Ca(2+) channels with ω-agatoxin decreased signal variance, and altered the spatial noise pattern within the vesicle. These fluorescence signal properties are consistent with vesicle Ca(2+) dynamics and not simply due to obvious physical properties such as gross movement artifacts or pH driven changes in Ca(2+) indicator fluorescence. The results suggest that the free Ca(2+) content of cortical secretory vesicles is dynamic; this property may modulate the exocytotic fusion process.

Cellular functions of transient receptor potential channels

Transient Receptor Potential channels are polymodal cellular sensors involved in a wide variety of cellular processes, mainly by increasing cellular Ca(2+). In this review we focus on the roles of these channels in: (i) cell death (ii) proliferation and differentiation and (iii) transmitter release. Cell death: Ca(2+) influx participates in apoptotic and necrotic cell death. The Ca(2+) permeability and high sensitivity of part of these channels to oxidative/metabolic stress make them important participants in cell death. Several examples are given. Transient Receptor Potential Melastatin 2 is activated by H(2)O(2), inducing cell death through an increase in cellular Ca(2+) and activation of Poly ADP-Ribose Polymerase. Exposure of cultured cortical neurons to oxygen-glucose deprivation, in vitro, causes cell death via cation influx, mediated by Transient Receptor Potential Melastatin 7. Metabolic stress constitutively activates the Ca(2+) permeable Transient Receptor Potential channels of Drosophila photoreceptor in the dark, potentially leading to retinal degeneration. Similar sensitivity to metabolic stress characterizes several mammalian Transient Receptor Potential Canonical channels. Proliferation and differentiation: The rise in cytosolic Ca(2+) induces cell growth, differentiation and proliferation via activation of several transcription factors. Activating a variety of store operated and Transient Receptor Potential channels cause a rise in cytosolic Ca(2+), making these channels components involved in proliferation and differentiation. Transmitter release: Transient Receptor Potential Melastatin 7 channels reside in synaptic vesicles and regulate neurotransmitter release by a mechanism that is not entirely clear. All the above features of Transient Receptor Potential channels make them crucial components in important, sometimes conflicting, cellular processes that still need to be explored.

Electrophysiological characterization of ATPases in native synaptic vesicles and synaptic plasma membranes

Vesicular V-ATPase (V-type H+-ATPase) and the plasma membrane-bound Na+/K+-ATPase are essential for the cycling of neurotransmitters at the synapse, but direct functional studies on their action in native surroundings are limited due to the poor accessibility via standard electrophysiological equipment. We performed SSM (solid supported membrane)-based electrophysiological analyses of synaptic vesicles and plasma membranes prepared from rat brains by sucrose-gradient fractionation. Acidification experiments revealed V-ATPase activity in fractions containing the vesicles but not in the plasma membrane fractions. For the SSM-based electrical measurements, the ATPases were activated by ATP concentration jumps. In vesicles, ATP-induced currents were inhibited by the V-ATPase-specific inhibitor BafA1 (bafilomycin A1) and by DIDS (4,4'-di-isothiocyanostilbene-2,2'-disulfonate). In plasma membranes, the currents were inhibited by the Na+/K+-ATPase inhibitor digitoxigenin. The distribution of the V-ATPase- and Na+/K+-ATPase-specific currents correlated with the distribution of vesicles and plasma membranes in the sucrose gradient. V-ATPase-specific currents depended on ATP with a K0.5 of 51+/-7 microM and were inhibited by ADP in a negatively co-operative manner with an IC50 of 1.2+/-0.6 microM. Activation of V-ATPase had stimulating effects on the chloride conductance in the vesicles. Low micromolar concentrations of DIDS fully inhibited the V-ATPase activity, whereas the chloride conductance was only partially affected. In contrast, NPPB [5-nitro-2-(3-phenylpropylamino)-benzoic acid] inhibited the chloride conductance but not the V-ATPase. The results presented describe electrical characteristics of synaptic V-ATPase and Na+/K+-ATPase in their native surroundings, and demonstrate the feasibility of the method for electrophysiological studies of transport proteins in native intracellular compartments and plasma membranes.

The dimeric form of Ca2+-ATPase is involved in Ca2+ transport in the sarcoplasmic reticulum

To identify the functional unit of Ca2+-ATPase in the sarcoplasmic reticulum, we assessed Ca2+-transport activities occurring on sarcoplasmic reticulum membranes with different combinations of active and inactive Ca2+-ATPase molecules. We prepared heterodimers, consisting of a native Ca2+-ATPase molecule and a Ca2+-ATPase molecule inactivated by FITC labelling, by fusing vesicles loaded with each type of Ca2+-ATPase. The heterodimers exhibited neither Ca2+ transport nor ATP hydrolysis, suggesting that Ca2+ transport by the Ca2+-ATPase requires an interaction between functional Ca2+-ATPase monomers. This finding implies that the functional unit of the Ca2+-ATPase is a dimer.

The heterogeneity of ion channels in chromaffin granule membranes

Chromaffin granules are involved in catecholamine synthesis and traffic in the adrenal glands. The transporting membrane proteins of chromaffin granules play an important role in the ion homeostasis of these organelles. In this study, we characterized components of the electrogenic (86)Rb(+) flux observed in isolated chromaffin granules. In order to study single channel activity, chromaffin granules from the bovine adrenal medulla were incorporated into planar lipid bilayers. Four types of cationic channel were found, each with a different conductance. The unitary conductances of the potassium channels are 360 +/- 10 pS, 220 +/- 8 pS, 152 +/- 8 pS and 13 +/- 3 pS in a gradient of 450/150 mM KCl, pH 7.0. A multiconductance potassium channel with a conductivity of 110 +/- 8 pS and 31 +/- 4 pS was also found. With the exception of the 13 pS conductance channel, all are activated by depolarizing voltages. One type of chloride channel was also found. It has a unitary conductance of about 250 pS in a gradient of 500/150 mM KCl, pH 7.0.

Exocytotic catecholamine release is not associated with cation flux through channels in the vesicle membrane but Na+ influx through the fusion pore

Release of charged neurotransmitter molecules through a narrow fusion pore requires charge compensation by other ions. It has been proposed that this may occur by ion flow from the cytosol through channels in the vesicle membrane, which would generate a net outward current. This hypothesis was tested in chromaffin cells using cell-attached patch amperometry that simultaneously measured catecholamine release from single vesicles and ionic current across the patch membrane. No detectable current was associated with catecholamine release indicating that <2% of cations, if any, enter the vesicle through its membrane. Instead, we show that flux of catecholamines through the fusion pore, measured as an amperometric foot signal, decreases when the extracellular cation concentration is reduced. The results reveal that the rate of transmitter release through the fusion pore is coupled to net Na+ influx through the fusion pore, as predicted by electrodiffusion theory applied to fusion-pore permeation, and suggest a prefusion rather than postfusion role for vesicular cation channels.

ATP dependence of the non-specific ion channel in Torpedo synaptic vesicles

Synaptic vesicles of Torpedo electromotor neurons contain a high amount of ATP. The concentration of total ATP is around 120 mM, whereas the free [ATP] is about 5-6 mM. We examined the effect of intravesicular ATP on the non-specific ion channel in Torpedo-fused synaptic vesicles. It was found that this channel is closed when the ATP concentration is above 2 mM, but it is very frequently open at lower ATP concentrations. Unmasking this ion channel at a low ATP concentration may be significant for post-fusion control of transmitter release by the 'kiss and run' mechanism in normal conditions, while during metabolic stress it may underlie dissipation of important gradients across the vesicle membrane.

Hydrogen ions control synaptic vesicle ion channel activity in Torpedo electromotor neurones

During exocytosis the synaptic vesicle fuses with the surface membrane and undergoes a pH jump. When the synaptic vesicle is inside the presynaptic nerve terminal its internal pH is about 5.5 and after fusion, the inside of the vesicle comes in contact with the extracellular medium with a pH of about 7.25. We examined the effect of such pH jump on the opening of the non-specific ion channel in the synaptic vesicle membrane, in the context of the post-fusion hypothesis of transmitter release control. The vesicles were isolated from Torpedo ocellata electromotor neurones. The pH dependence of the opening of the non-specific ion channel was examined using the fused vesicle-attached configuration of the patch clamp technique. The rate of opening depends on both pH and voltage. Increasing the pH from 5.5 to 7.25 activated dramatically the non-specific ion channel of the vesicle membrane. The single channel conductance did not change significantly with the alteration in the pH, and neither did the mean channel open time. These results support the hypothesis that during partial fusion of the vesicle with the surface membrane, ion channels in the vesicle membrane open, admit ions and thus help in the ion exchange process mechanism, leading to the release of the transmitter from the intravesicular ion exchange matrix. This process may have also a pathophysiological significance in conditions of altered pH.

Loading and recycling of synaptic vesicles in the Torpedo electric organ and the vertebrate neuromuscular junction

In vertebrate motor nerve terminals and in the electromotor nerve terminals of Torpedo there are two major pools of synaptic vesicles: readily releasable and reserve. The electromotor terminals differ in that the reserve vesicles are twice the diameter of the readily releasable vesicles. The vesicles contain high concentrations of ACh and ATP. Part of the ACh is brought into the vesicle by the vesicular ACh transporter, VAChT, which exchanges two protons for each ACh, but a fraction of the ACh seems to be accumulated by different, unexplored mechanisms. Most of the vesicles in the terminals do not exchange ACh or ATP with the axoplasm, although ACh and ATP are free in the vesicle interior. The VAChT is controlled by a multifaceted regulatory complex, which includes the proteoglycans that characterize the cholinergic vesicles. The drug (-)-vesamicol binds to a site on the complex and blocks ACh exchange. Only 10-20% of the vesicles are in the readily releasable pool, which therefore is turned over fairly rapidly by spontaneous quantal release. The turnover can be followed by the incorporation of false transmitters into the recycling vesicles, and by the rate of uptake of FM dyes, which have some selectivity for the two recycling pathways. The amount of ACh loaded into recycling vesicles in the readily releasable pool decreases during stimulation. The ACh content of the vesicles can be varied over eight-fold range without changing vesicle size.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Rat brain p64H1, expression of a new member of the p64 chloride channel protein family in endoplasmic reticulum

Many plasma membrane Cl- channels have been cloned, including the cystic fibrosis transmembrane conductance regulator and several members of the voltage-gated ClC family. In contrast, very little is known about the molecular identity of intracellular Cl- channels. We used a polymerase chain reaction-based approach to identify candidate genes in mammalian brain and cloned the cDNA corresponding to rat brain p64H1. This encoded a microsomal membrane protein of predicted Mr 28,635 homologous to the putative intracellular bovine kidney Cl- channel p64. In situ mRNA hybridization histochemistry showed marked expression in hippocampus and cerebellum, and in vitro expression revealed a large cytoplasmic domain, one membrane-spanning segment, and a small nonglycosylated N-terminal luminal domain. The predicted protein contained consensus phosphorylation sites for protein kinase C and protein kinase A, and protein kinase C-mediated phosphorylation increased the Mr of p64H1 to approximately 43,000, characteristic of the native protein in Western blots. Recombinant p64H1 was immunolocalized to the endoplasmic reticulum of human embryonic kidney 293 and HT-4 cells, and incorporation of human embryonic kidney 293 endoplasmic reticulum vesicles into planar lipid bilayers gave rise to intermediate conductance, outwardly rectifying anion channels. Although p64H1 is the first intracellular Cl- channel component or regulator to be identified in brain, Northern blotting revealed transcripts in many other rat tissues. This suggests that p64H1 may contribute widely to intracellular Cl- transport.

A mechanism for discharge of charged excitatory neurotransmitter

Excitatory neurotransmitter is charged, so that emptying of a transmitter-containing vesicle (discharge) would seem to require considerable energy. Even if the energy problem is surmounted and discharge thereby made possible, there is still a problem of making the discharge fast enough (considerably less than 1 ms). Proposed here is a mechanism wherein discharge of charged transmitter is accompanied by the influx of cocharged ions or coefflux of counter-charged particles (ion interchange). It is shown theoretically that ion interchange obviates the necessity for a separate energy source and can provide the observed rapid vesicle discharge.

Nerve growth factor induced modification of presynaptic elements in adult visual cortex in vivo

Nerve growth factor (NGF) has been shown to play important roles in neuronal survival, growth and differentiation. Recently, we have found that intracortical infusion of NGF into adult cat visual cortex can recreate ocular dominance plasticity, suggesting that NGF is also involved in activity-dependent modification of synaptic connectivity in the adult brain. To further explore the mechanisms of NGF-induced plasticity in adult visual cortex, we studied two presynaptic markers: GAP-43 and synaptophysin. Immunocytochemical staining showed that NGF-treatment of adult visual cortex selectively increased the level of the phosphorylated form of GAP-43, while the total level of GAP-43 was not changed. These results demonstrate that NGF-treatment stimulates phosphorylation processes of GAP-43 in vivo. In addition, NGF-treatment of adult visual cortex increased the level of synaptophysin immunoreactivity. Since the phosphorylated form of GAP-43 is known to be enriched in the membrane skeleton of growth cones and of developing synapses, and the phosphorylation of GAP-43 has been linked with events that underlie synaptic plasticity, and since synaptophysin is a major component of presynaptic vesicles, our results suggest that NGF-treatment of adult visual cortex modulates presynaptic terminals, possibly by inducing axonal sprouting and formation of new synapses, and that these changes may play a role in the NGF-induced functional plasticity.

Ion channels from synaptic vesicle membrane fragments reconstituted into lipid bilayers

Cholinergic synaptic vesicles were isolated from the electric organ of Torpedo californica. Vesicle membrane proteins were reconstituted into planar lipid bilayers by the nystatin/ergosterol fusion technique. After fusion, a variety of ion channels were observed. Here we identify four channels and describe two of them in detail. The two channels share a conductance of 13 pS. The first is anion selective and strongly voltage dependent, with a 50% open probability at membrane potentials of -15 mV. The second channel is slightly cation selective and voltage independent. It has a high open probability and a subconductance state. A third channel has a conductance of 4-7 pS, similar to the subconductance state of the second channel. This channel is fairly nonselective and has gating kinetics different from those of the cation channel. Finally, an approximately 10-pS, slightly cation selective channel was also observed. The data indicate that there are one or two copies of each of the above channels in every synaptic vesicle, for a total of six channels per vesicle. These observations confirm the existence of ion channels in synaptic vesicle membranes. It is hypothesized that these channels are involved in vesicle recycling and filling.

The non-specific ion channel in Torpedo ocellata fused synaptic vesicles

1. Synaptic vesicles were isolated and fused into large structures with a diameter of more than 20 microns to characterize their ionic channels. The 'cell'-attached and inside-out configurations of the patch clamp technique were used. 2. Two types of ion channels were most frequently observed: a low conductance chloride channel and a high conductance non-specific channel. 3. The non-specific channel has a main conducting state and a substate. The main conducting state has a slope conductance of 246 +/- 15 pS (+/- S.E.M., n = 15), in the presence of different combinations of KCl and potassium glutamate. 4. From the reversal potentials of the current-voltage (I-V) relation, it was concluded that this channel conducts both Cl- and K+. 5. The non-specific channel is highly voltage dependent: under steady-state voltages it has a high open probability near 0 mV and does not inactivate; when the membrane is hyperpolarized (pipette side more positive), the open probability decreases dramatically. 6. Voltage pulses showed that upon hyperpolarization (from holding potentials between -20 and + 20 mV), the channels deactivated; when the membrane was stepped back to the holding potential, the channels reactivated rapidly. 7. In a number of experiments, when the pipette side was made more negative than the bath, the open probability also decreased. 8. Frequently, a substate with a conductance of about 44 +/- 4% (+/- S.E.M., n = 3) of the main state was detected. 9. We speculate that this non-specific ion channel may have different roles at the various stages of the life cycle of the synaptic vesicle. When the synaptic vesicle is an intracellular structure, it might help its transmitter-concentrating capacity by dissipating the polarization. After fusion with the surface membrane, it might constitute an additional conductance pathway, taking part in frequency modulation of synaptic transmission.

Evaluation of the evidence for ion channels in synaptic vesicles

Synaptic vesicles (SVs) have been the focus of much research for many years, however only recently have ion channels from SV membranes been reported. There is now convincing evidence that SVs contain ion channels. This conclusion is based on direct experimental results from several different laboratories using the patch clamp or planar lipid bilayer technique on SVs and neurosecretory granules (NSG). Some limitations of patch clamping and of fusing synamptic vesicles to a bilayer are described and the advantages of the nystatin/ergosterol fusion method are presented. Six different channels appear to exist in SV (or NSG) membranes. Two large channels (250 and 154 pS) have been observed in SVs isolated from mammalian brain, two channels (180 and 13 pS) from Torpedo electric organ, and two channels (130 and 30-40 pS) from NSG. The three larger channels from each set (250, 180 and 130 pS7) are novel in that they have a subconductance state. The 154 pS channel has been identified as synaptophysin but the identity and function of the other channels is unknown. Although some of the channels are gated by voltage, only the 130 pS channel is modulated by Ca2+. Further knowledge of what regulates these channels is mandatory if we are to determine the physiological significance of these channels.

Synaptic vesicle proteins and regulated exocytosis

The recent identification of novel proteins associated with the membranes of synaptic vesicles has ignited the field of molecular neurobiology to probe the function of these molecules. Evidence is mounting that the vesicle proteins vamp (synaptobrevin), rab3A, synaptophysin, synaptotagmin (p65) and SV2 play an important role in regulated exocytosis, by regulating neurotransmitter uptake, vesicle targeting and fusion with the presynaptic plasma membrane.

Evidence for a K+ channel in bovine chromaffin granule membranes: single-channel properties and possible bioenergetic significance

A K+ channel was incorporated into voltage-clamped planar lipid bilayers from bovine chromaffin granules and resealed granule membranes ("ghosts"). It was not incorporated from plasma membrane-rich fractions from the adrenal medulla. The channel had a conductance of approximately 400 pS in symmetric 450 mM KCl, with the permeability sequence K+ > Rb+ > Cs+ > Na+ > Li+, and was insensitive to both Ca2+ and charybdotoxin. It exhibited complex gating kinetics, consistent with the presence of multiple open and closed states, and its gating was voltage-dependent. The channels appeared to incorporate into bilayers with the same orientation, and were blocked from one side (the side of vesicle addition) by 0.2-1 mM TEA+. The block was slightly voltage-dependent. Acidification of resealed granule membranes in response to external ATP (which activated the vacuolar-type ATPase) was significantly reduced in the presence of 1 mM intralumenal TEACl (with 9 mM KCl), and parallel measurements with the potential-sensitive dye Oxonol V showed that such vesicles tended to develop higher internal-positive membrane potentials than control vesicles containing only 10 mM KCl. 1 mM TEA+ had no effect on proton-pumping activity when applied externally, and did not directly affect either the proton-pumping or ATP hydrolytic activity of the partially-purified ATPase. These results suggest that chromaffin granule membranes contain a TEA(+)-sensitive K+ channel which may have a role in regulating the vesicle membrane potential.

Patch clamp studies of single intact secretory granules

The membrane of secretory granules is involved in the molecular events that cause exocytotic fusion. Several of the proteins that have been purified from the membrane of secretory granules form ion channels when they are reconstituted in lipid bilayers and, therefore, have been thought to form part of the molecular structure of the exocytotic fusion pore. We have used the patch clamp technique to study ion conductances in single isolated secretory granules from beige mouse mast cells. We found that the membrane of the intact granule had a conductance of < 50 pS. No abrupt changes in current corresponding to the opening and closing of ion channels were observed, even under conditions where exocytotic fusion occurred. However, mechanical tension or a large voltage pulse caused the breakdown of the granule membrane resulting in the abrupt opening of a pore with an ion conductance of about 1 nS that fluctuated rapidly and could expand to an immeasurably large conductance or close completely. Surprisingly, the behavior of these pores resembled the pattern of conductance changes of exocytotic fusion pores observed in degranulating beige mast cells. This similarity supports the view that the earliest fusion pore is formed upon the breakdown of a bilayer such as that formed during hemifusion.

Activation and inactivation of the bursting potassium channel from fused Torpedo synaptosomes

1. The voltage dependence of the bursting potassium channel in fused synaptosomes from Torpedo electric organ was studied in vitro, using the inside-out and the cell-attached configurations of the patch clamp technique. 2. The patch of membrane was held at various holding potentials (-140 to -50 mV) and then stepped to test potentials (-50 to +40 mV) for periods ranging from 5 to 300 ms. Each potential step was repeated 200-600 times. After subtraction of the capacitative transients and the leakage currents, an ensemble-averaged current was obtained. This ensemble current showed a marked activation upon depolarization, followed by an inactivation. 3. The activation of the bursting potassium channel is markedly dependent on the voltage step. Activation was detected at voltages positive to -50 mV. The peak of the ensemble current increases with the degree of depolarization, while the time to the peak decreases. With progressively larger depolarization, there is a shortening in the delay between the onset of the voltage step and the opening of the bursting potassium channels. 4. The inactivation phase of the ensemble current could be described adequately in most of the experiments, as a single exponential decay to a steady-state inactivation level. The time constant of inactivation was not markedly voltage dependent. 5. Single channel analysis of the inactivation reveals that it is due to a reduction in the number of channel openings and not due to changes in single channel current amplitude or channel mean open time along the pulse. 6. The holding potential has a marked effect on the peak amplitude of the ensemble current, indicating that hyperpolarization removes inactivation and depolarization induces it. The peak amplitude vs. voltage relation was fitted by the Boltzmann equation. The half-maximal inactivation was -105.2 +/- 5.8 mV (mean +/- S.E.M.), suggesting that at the resting potential a substantial fraction of the bursting potassium channels is in an inactivated state. 7. Two-pulse experiments show that the recovery from inactivation is a slow process which lasts well over 1 s. 8. High-frequency stimulation (20-66.7 Hz) by 5 ms pulses produces a progressive decline in the peak ensemble current amplitude. The decline is larger at higher stimulation frequencies.(ABSTRACT TRUNCATED AT 400 WORDS)

Channels in mitochondrial membranes: knowns, unknowns, and prospects for the future

Rapid diffusion of hydrophilic molecules across the outer membrane of mitochondria has been related to the presence of a protein of 29 to 37 kDa, called voltage-dependent anion channel (VDAC), able to generate large aqueous pores when integrated in planar lipid bilayers. Functional properties of VDAC from different origins appear highly conserved in artificial membranes: at low transmembrane potentials, the channel is in a highly conducting state, but a raise of the potential (both positive and negative) reduces drastically the current and changes the ionic selectivity from slightly anionic to cationic. It has thus been suggested that VDAC is not a mere molecular sieve but that it may control mitochondrial physiology by restricting the access of metabolites of different valence in response to voltage and/or by interacting with a soluble protein of the intermembrane space. The latest application of the patch clamp and tip-dip techniques, however, has indicated both a different electric behavior of the outer membrane and that other proteins may play a role in the permeation of molecules. Biochemical studies, use of site-directed mutants, and electron microscopy of two-dimensional crystal arrays of VDAC have contributed to propose a monomeric beta barrel as the structural model of the channel. An important insight into the physiology of the inner membrane of mammalian mitochondria has come from the direct observation of the membrane with the patch clamp. A slightly anionic, voltage-dependent conductance of 107 pS and one of 9.7 pS, K(+)-selective and ATP-sensitive, are the best characterized at the single channel level. Under certain conditions, however, the inner membrane can also show unselective nS peak transitions, possibly arising from a cooperative assembly of multiple substrates.

Acetylcholine transport, storage, and release

ACh is released from cholinergic nerve terminals under both resting and stimulated conditions. Stimulated release is mediated by exocytosis of synaptic vesicle contents. The structure and function of cholinergic vesicles are becoming known. The concentration of ACh in vesicles is about 100-fold greater than the concentration in the cytoplasm. The AChT exhibits the lowest binding specificity among known ACh-binding proteins. It is driven by efflux of protons pumped into the vesicle by the V-type ATPase. A potent pharmacology of the AChT based on the allosteric VR has been developed. It has promise for clinical applications that include in vivo evaluation of the density of cholinergic innervation in organs based on PET and SPECT. The microscopic kinetics model that has been developed and the very low transport specificity of the vesicular AChT-VR suggest that the transporter has a channel-like or multidrug resistance protein-like structure. The AChT-VR has been shown to be tightly associated with proteoglycan, which is an unexpected macromolecular relationship. Vesamicol and its analogs block evoked release of ACh from cholinergic nerve terminals after a lag period that depends on the rate of release. Recycling quanta of ACh that are sensitive to vesamicol have been identified electrophysiologically, and they constitute a functional correlate of the biochemically identified VP2 synaptic vesicles. The concept of transmitter mobilization, including the observation that the most recently synthesized ACh is the first to be released, has been greatly clarified because of the availability of vesamicol. Differences among different cholinergic nerve terminal types in the sensitivity to vesamicol, the relative amounts of readily and less releasable ACh, and other aspects of the intracellular metabolism of ACh probably are more apparent than real. They easily could arise from differences in the relative rates of competing or sequential steps in the complicated intraterminal metabolism of ACh rather than from fundamental differences among the terminals. Nonquantal release of ACh from motor nerve terminals arises at least in part from the movement of cytoplasmic ACh through the AChT located in the cytoplasmic membrane, and it is blocked by vesamicol. Possibly, the proteoglycan component of the AChT-VR produces long-term residence of the macromolecular complex in the cytoplasmic membrane through interaction with the synaptic matrix. The preponderance of evidence suggests that a significant fraction of what previously, heretofore, had been considered to be nonquantal release from the motor neuron actually is quantal release from the neuron at sites not detected electrophysiologically.(ABSTRACT TRUNCATED AT 400 WORDS)

Ion channels on synaptic vesicle membranes studied by planar lipid bilayer method

An anion selective channel and three types of cation selective channels were found in planar lipid bilayers incorporating synaptic vesicles from rat brains. In asymmetric KCl solutions (cis: 300 mM/trans: 150 mM), the anion selective channel showed a single-channel conductance of 94 pS and was inactivated by negative voltages and by 4-acetoamido-4'-isothiocyanostilbene-2,2'-disulfonic acid disodium salt (SITS). In the same solution, single-channel conductances of three types of cation selective channels were 250 pS (Type 1), 248 pS (Type 2), and 213 pS (Type 3), respectively. These channels resembled one another in single-channel conductances but were different in gating behaviors. Type 1 channel, which was most frequently observed, had a remarkable subconducting state (175 pS). Type 2 channel had a flickering state that increased as the potential became more positive, and a long inactive state that increased as the potentials were more negative. Type 3 channel, which was also sensitive to the potentials, had the open-channel probability increased as the potential became more positive.

Calcium-independent K(+)-selective channel from chromaffin granule membranes

Intact adrenal chromaffin granules and purified granule membrane ghosts were allowed to fuse with acidic phospholipid planar bilayer membranes in the presence of Ca2+ (1 mM). From both preparations, we were able to detect a large conductance potassium channel (ca. 160 pS in symmetrical 400 mM K+), which was highly selective for K+ over Na+ (PK/PNa = 11) as estimated from the reversal potential of the channel current. Channel activity was unaffected by charybdotoxin, a blocker of the [Ca2+]-activated K+ channel of large conductance. Furthermore, this channel proved quite different from the previously described channels from other types of secretory vesicle preparations, not only in its selectivity and conductance, but also in its insensitivity to both calcium and potential across the bilayer. We conclude that the chromaffin granule membrane contains a K(+)-selective channel with large conductance. We suggest that the role of this channel may include ion movement during granule assembly or recycling, and do not rule out events leading to exocytosis.

Giant sarcoplasmic reticulum vesicles: a study of membrane morphogenesis

Rabbit sarcoplasmic reticulum vesicles were fused into giant proteoliposomes in a medium of 0.1 M KCl, 10 mM Tris-maleate, pH 7.0, 10 micrograms ml-1 antipain, 10 micrograms ml-1 leupeptin, 25 IU per ml Trasylol, 3 mM NaN3, 3.75% PEG 1500 and 3% DMSO by brief exposure to 37 degrees C, followed by incubation for 4 h at 25 degrees C. Approximately 5-10% of the sarcoplasmic reticulum elements underwent fusion, forming single-walled spherical vesicles of 1-25 microns diameter, in which the polarity of the native membrane was preserved. The Ca(2+)-stimulated ATPase activity remained essentially unchanged after fusion. On exposure to decavanadate in a Ca(2+)-free medium the spherical vesicles assumed a corrugated appearance with the formation of long ridges separated by deep furrows that eventually pinched off longitudinally and separated into numerous long crystalline tubules of uniform (approximately 0.1 microns) diameter. The vanadate-induced transformation of giant vesicles into tubules implies that the geometry of the sarcoplasmic reticulum membrane is determined by the conformation of the Ca(2+)-ATPase.

SV2, a brain synaptic vesicle protein homologous to bacterial transporters

Synaptic vesicle protein 2 (SV2) is a membrane glycoprotein specifically localized to secretory vesicles in neurons and endocrine cells. As a first step toward understanding the function of SV2 in neural secretion, a rat brain complementary DNA (cDNA) that encodes SV2 was isolated and characterized. Analyses of this cDNA predict that SV2 contains 12 transmembrane domains. The NH2-terminal half of the protein shows significant amino acid sequence identity to a family of bacterial proteins that transport sugars, citrate, and drugs. Expression of the SV2 cDNA in COS cells yielded a high level of SV2-like immunoreactivity distributed in a reticular and punctate pattern, which suggests localization to intracellular membranes. Its localization to vesicles, predicted membrane topology, and sequence identity to known transporters suggest that SV2 is a synaptic vesicle-specific transporter.

Comparative electrophysiological study of reconstituted giant vesicle preparations of the rabbit skeletal muscle sarcoplasmic reticulum K+ channel

Sarcoplasmic reticulum (SR) membranes isolated from rabbit skeletal muscle were reconstituted into two types of giant vesicles: (1) Giant proteoliposomes prepared by freeze-thawing of a mixture of SR vesicles and sonicated phospholipid vesicles without the use of detergent. (2) Giant SR vesicles prepared by fusion of SR vesicles using poly(ethylene glycol) (PEG) as a fusogen and without the addition of exogenous lipid. These giant vesicles were patch-clamped and properties of the single voltage-dependent potassium channel in the excised patch were studied. Single-channel conductance in a symmetrical solution of 0.1 M KCl and 1 mM CaCl2 was 140.0 +/- 10 pS (n = 5) for freeze-thawed vesicles and 136.4 +/- 15 pS (n = 7) for PEG vesicles. Both types of vesicles exhibited a sub-conductance state having 55% of the fully open state conductance. The voltage-dependence of open-channel probability could be expressed in terms of thermodynamic parameters of delta Gi = 0.95 kcal/mol and z = -0.77 for freeze-thawed vesicles and delta Gi = 0.92 kcal/mol and z = -0.87 for PEG vesicles. These values correlated well with previous data obtained by fusion of native SR vesicles with a planar lipid membrane. Channel orientation was found to be conserved in both types of vesicles used in the present study.

Ultrastructure of afferent axon endings in a neuroma

Injured sensory axons with endings trapped in a nerve-end neuroma become a source of abnormal impulse discharge and neuropathic pain. We have examined the ultrastructure of such endings anterogradely transported WGA-HRP and freeze-fracture replication, with emphasis on the postinjury period during which the abnormal neural discharge is maximal. Most axons ended in a terminal swelling, depleted of myelin but surrounded by Schwann cell processes. These 'neuroma endbulbs' were richly packed with membrane-bound organelles, and had a smoothly undulating surface with (in neuromas of several weeks standing) a moderate number of short filopodia. Massive sprouting did not occur until several months postinjury. Both p- and e-faces of endbulb axolemma had larger intramembranous particles, on average, than corresponding internodal membrane of control axons. This change, interpreted as indicating remodelling of axolemmal channel (and perhaps receptor) content, may be related to the abnormal electrical behavior of neuroma afferents.

A bursting potassium channel in isolated cholinergic synaptosomes of Torpedo electric organ

1. Pinched-off cholinergic nerve terminals (synaptosomes) prepared from the electric organ of Torpedo ocelata were fused into large structures (greater than 20 microns) using dimethyl sulphoxide and polyethylene glycol 1500, as previously described for synaptic vesicles from the same organ. 2. The giant fused synaptosomes were easily amenable to the patch clamp technique and 293 seals with a resistance greater than 4 G omega were obtained in the 'cell-attached' configuration. In a large fraction of the experiments, an 'inside-out' patch configuration was achieved. 3. Several types of unitary ionic currents were observed. This study describes the most frequently observed single-channel activity which was found in 247 out of the 293 membrane patches (84.3%). 4. The single-channel current-voltage relation was linear between -60 and 20 mV and showed a slope conductance of 23.8 +/- 1.3 pS when the pipette contained 350-390 mM-Na+ and the bath facing the inside of the synaptosomal membrane contained 390 mM-K+. 5. From extrapolated reversal potential measurements, it was concluded that this channel has a large selectivity for K+ over Na+ (70.4 +/- 11.5, mean +/- S.E.M.). Chloride ions are not transported significantly through this potassium channel. 6. This potassium channel has a low probability of opening. The probability of being in the open state increases upon depolarization and reaches about 1% when the inside of the patch is 20 mV positive compared to the pipette side. 7. The mean channel open time increases with depolarization; thus the product current x time (= charge) also increases upon depolarization, showing properties of an outward rectifier. 8. The potassium channel in the giant synaptosome membrane has a bursting behaviour. Open-time distribution, closed-time distribution and a Poisson analysis indicate that the minimal kinetic scheme requires one open state and three closed states.

Na(+)-Ca2+ exchange activity in synaptic plasma membranes derived from the electric organ of Torpedo ocellata

Synaptic plasma membranes obtained by hypo-osmotic treatment of purified Torpedo ocellata synaptosomes, contain an electrogenic Na(+)-Ca2+ exchange system. The dependence of the initial reaction rate on [Ca2+] reveals a single binding site for Ca2+ with an average apparent Km of 13.66 (S.D. = 12.07) microM [Ca2+] and maximal reaction velocity of Vmax = 11.33 (S.D. = 5.93) nmol/mg protein per s. The dependence of the initial rate of the Na+ gradient dependent Ca2+ influx on the internal [Na+] exhibits a sigmoidal curve which reaches half-maximal reaction rate at 170.8 (S.D. = 19.9) mM [Na+]. Addition of ATP gamma S does not change the K0.5 to Na+. The average Hill coefficient is 3.09 (S.D. = 0.86) indicating that 3-4 Na+ ions are exchanged for each Ca2+. Na+ gradient dependent Ca2+ uptake in Torpedo SPMs takes place also in the absence of K+ suggesting that K+ co-transport is not obligatory. The temperature dependence of the initial and steady-state rates of Na+ gradient dependent Ca2+ influx reveal that maximal reaction velocities of the Torpedo exchanger are attained between 15 and 20 degrees C. The energy of activation between 0 and 20 degrees C is 20,826 cal/mol. In comparison, rat brain synaptic plasma membrane Na(+)-Ca2+ exchanger reaches maximal reaction rates between 30 and 40 degrees C. Reconstitution of Torpedo or rat brain Na(+)-Ca2+ exchangers into a membrane composed of either Torpedo or brain phospholipids, does not alter the temperature dependence of the native Torpedo or rat brain Na(+)-Ca2+ exchangers; inspite of considerable differences in the composition of the fatty acyl chains that are esterified to brain and Torpedo phospholipid head groups and differences in membrane fluidity that were detected. An ATP-dependent Ca2+ pump, which is insensitive to FCCP, is also present in the same synaptic membrane.

Further investigation on the high-conductance ion channel of the inner membrane of mitochondria

By use of the patch-clamp technique, the inner membrane of mouse liver and heart mitochondria is shown to contain a highly conductive (around 100 pS in symmetrical 150 mM KCl) and voltage-dependent ion channel. This channel closely resembles that previously found in cuprizone-treated mouse liver inner mitochondrial membrane. The paper discusses the electrical properties of the channel and its possible physiological function. The reconstitution in giant liposomes of a partially purified ox heart inner membrane fraction containing the channel and the use of various inhibitors are also presented.

Anion channel blockers cause apparent inhibition of exocytosis by reacting with agonist or secretory product, not with cell

Agents that act as anion channel blockers (ACBs) and do not permeate cells appear to inhibit exocytosis in platelets, parathyroid cells, and neutrophils. Based in large part on these observations, anion influx through plasma membrane channels has been considered a factor controlling cellular secretion, but there have been no direct anion influx measurements in cells or granules to support this concept. We have found that ACBs inhibit only thrombin-induced platelet secretion, not secretion induced by ADP, collagen, or A23187. ACBs inhibit thrombin esterolytic activity, binding of thrombin to platelets, and thrombin-stimulated platelet production of malondialdehyde in proportion to the degree of inhibition of thrombin-induced platelet secretion. Thus inhibition of platelet secretion by ACBs is due to inactivation of the stimulatory agonist, thrombin, and not to interference with cellular secretion per se. We have also found that previously reported inhibition of secretion of parathyroid cells and neutrophils by ACBs can be explained by the ability of ACBs to interfere with detection of the cellular secretory products that were measured to assess exocytosis. Our measurements of parathyroid hormone and beta-glucuronidase in the presence of ACBs were reduced to the same degree as the reported reduction in apparent cellular secretion produced by these agents. We conclude that plasma membrane anion channels of the type that can be blocked by ACBs such as 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid, suramin, and probenecid do not participate in cellular secretory processes. Whether other types of anion channels exist that are not affected by these ACBs and whether there are mechanisms of anion flux during secretion not dependent on channels remain open questions.

Synaptobrevin: an integral membrane protein of 18,000 daltons present in small synaptic vesicles of rat brain

A protein with an apparent mol. wt of 18,000 daltons (synaptobrevin) was identified in synaptic vesicles from rat brain. Some of its properties were studied using monoclonal and polyclonal antibodies. Synaptobrevin is an integral membrane protein with an isoelectric point of approximately 6.6. During subcellular fractionation, synaptobrevin followed the distribution of small synaptic vesicles, with the highest enrichment in the purified vesicle fraction. Immunogold electron microscopy of subcellular particles revealed that synaptobrevin is localized in nerve endings where it is concentrated in the membranes of virtually all small synaptic vesicles. No significant labeling was observed on the membranes of peptide-containing large dense core vesicles. In agreement with these results, synaptobrevin immunoreactivity has a widespread distribution in nerve terminal-containing regions of the central and peripheral nervous system as shown by light microscopy immunocytochemistry. Outside the nervous system, synaptobrevin immunoreactivity was found in endocrine cells and cell lines (endocrine pancreas, adrenal medulla, PC12 cells, insulinoma cells) but not in other cell types, for example smooth muscle, skeletal muscle and exocrine pancreas. Thus, the distribution of synaptobrevin is similar to that of synaptophysin, a well-characterized membrane protein of small vesicles in neurons and endocrine cells.

Unusual calcium-activated potassium channels of bovine parathyroid cells

Parathyroid cells have unusual responses to an increase in the external calcium concentration--a reduction in secretion and a depolarization. In an attempt to explain this behavior, we have studied voltage-clamped inside-out patches from bovine parathyroid cells. We found a potassium-selective channel that requires internal calcium to open and has a 175-pS conductance. This channel differs from calcium-activated potassium channels that have been found in other cells in that it closes when the internal calcium concentration is increased above about 160 nM. This channel can account for the depolarization that occurs in parathyroid cells in high-calcium solutions. It may also account for the reduced secretion in high-calcium solutions.