Nano to micro -- fluorescence measurements of electric fields in molecules and genetically specified neurons.

Our central nervous system is based on the generation and propagation of electrical signals along the neuronal pathways. These variations of the membrane potential are arranged by the concerted action of ion channels in the neuronal membrane. Therefore, the exact measurement of the electric field in the central nervous system is the focus of intensive investigation. While electrophysiological methods provide exact measurements on the single-cell or single-molecule level with high temporal resolution, they are limited in their spatial resolution ranging from a few single cells to a single molecule. To thoroughly understand how the voltage-dependent ion channels sense the membrane potential and are precisely gated by it, the electric field within the protein has to be investigated. Likewise, the propagation of electrical impulses in a network of neurons involves a large number of cells, which have to be monitored simultaneously. For these endeavors, optical methods have proven to be useful due to their scalability, temporal and spatial resolution. Here, we will summarize the properties of the optical probes that we used to determine the electrical field strength within voltage-sensitive ion channels and discuss the hybrid approach to detect membrane potential changes in genetically specified neurons in terms of design, limitations and future developments.

Bilayer reconstitution of voltage-dependent ion channels using a microfabricated silicon chip.

Painted bilayers containing reconstituted ion channels serve as a well defined model system for electrophysiological investigations of channel structure and function. Horizontally oriented bilayers with easy solution access to both sides were obtained by painting a phospholipid:decane mixture across a cylindrical pore etched into a 200-microm thick silicon wafer. Silanization of the SiO(2) layer produced a hydrophobic surface that promoted the adhesion of the lipid mixture. Standard lithographic techniques and anisotropic deep-reactive ion etching were used to create pores with diameters from 50 to 200 microm. The cylindrical structure of the pore in the partition and the surface treatment resulted in stable bilayers. These were used to reconstitute Maxi K channels in the 100- and 200-microm diameter pores. The electrophysiological characteristics of bilayers suspended in microchips were comparable with that of other bilayer preparations. The horizontal orientation and good voltage clamping properties make the microchip bilayer method an excellent system to study the electrical properties of reconstituted membrane proteins simultaneously with optical probes.

Periodic perturbations in Shaker K+ channel gating kinetics by deletions in the S3-S4 linker.

Upon depolarization positive charges contained in the transmembrane segment S4 of voltage-dependent channels are displaced from the cytoplasmic to the external milieu. This charge movement leads to channel opening. In Shaker K+ channels four positively charged arginines in the S4 domain are transferred from the internal to the external side of the channel during activation. The distance traveled by the S4 segment during activation is unknown, but large movements should be constrained by the S3-S4 linker. Constructing deletion mutants, we show that the activation time constant and the midpoint of the voltage activation curve of the Shaker K+ channel macroscopic currents becomes a periodic function of the S3-S4 linker length for linkers shorter than 7 aa residues. The periodicity is that typical of alpha-helices. Moreover, a linker containing only 3 aa is enough to recover the wild-type phenotype. The deletion method revealed the importance of the S3-S4 linker in determining the channel gating kinetics and indicated that the alpha-helical nature of S4 extends toward its N terminus. These results support the notion that a small displacement of the S4 segment suffices to displace the four gating charges involved in channel opening.

Histidine scanning mutagenesis of basic residues of the S4 segment of the shaker k+ channel.

The voltage sensor of the Shaker potassium channel is comprised mostly of positively charged residues in the putative fourth transmembrane segment, S4 (Aggarwal, S.K., and R. MacKinnon. 1996. Neuron. 16:1169-1177; Seoh, S.-A., D. Sigg, D.M. Papazian, and F. Bezanilla. 1996. Neuron. 16:1159-1167). Movement of the voltage sensor in response to a change in the membrane potential was examined indirectly by measuring how the accessibilities of residues in and around the sensor change with voltage. Each basic residue in the S4 segment was individually replaced with a histidine. If the histidine tag is part of the voltage sensor, then the gating charge displaced by the voltage sensor will include the histidine charge. Accessibility of the histidine to the bulk solution was therefore monitored as pH-dependent changes in the gating currents evoked by membrane potential pulses. Histidine scanning mutagenesis has several advantages over other similar techniques. Since histidine accessibility is detected by labeling with solution protons, very confined local environments can be resolved and labeling introduces minimal interference of voltage sensor motion. After histidine replacement of either residue K374 or R377, there was no titration of the gating currents with internal or external pH, indicating that these residues do not move in the transmembrane electric field or that they are always inaccessible. Histidine replacement of residues R365, R368, and R371, on the other hand, showed that each of these residues traverses entirely from internal exposure at hyperpolarized potentials to external exposure at depolarized potentials. This translocation enables the histidine to transport protons across the membrane in the presence of a pH gradient. In the case of 371H, depolarization drives the histidine to a position that forms a proton pore. Kinetic models of titrateable voltage sensors that account for proton transport and conduction are presented. Finally, the results presented here are incorporated into existing information to propose a model of voltage sensor movement and structure.

A conducting state with properties of a slow inactivated state in a shaker K(+) channel mutant.

In Shaker K(+) channel, the amino terminus deletion Delta6-46 removes fast inactivation (N-type) unmasking a slow inactivation process. In Shaker Delta6-46 (Sh-IR) background, two additional mutations (T449V-I470C) remove slow inactivation, producing a noninactivating channel. However, despite the fact that Sh-IR-T449V-I470C mutant channels remain conductive, prolonged depolarizations (1 min, 0 mV) produce a shift of the QV curve by about -30 mV, suggesting that the channels still undergo the conformational changes typical of slow inactivation. For depolarizations longer than 50 ms, the tail currents measured during repolarization to -90 mV display a slow component that increases in amplitude as the duration of the depolarizing pulse increases. We found that the slow development of the QV shift had a counterpart in the amplitude of the slow component of the ionic tail current that is not present in Sh-IR. During long depolarizations, the time course of both the increase in the slow component of the tail current and the change in voltage dependence of the charge movement could be well fitted by exponential functions with identical time constant of 459 ms. Single channel recordings revealed that after prolonged depolarizations, the channels remain conductive for long periods after membrane repolarization. Nonstationary autocovariance analysis performed on macroscopic current in the T449V-I470C mutant confirmed that a novel open state appears with increasing prepulse depolarization time. These observations suggest that in the mutant studied, a new open state becomes progressively populated during long depolarizations (>50 ms). An appealing interpretation of these results is that the new open state of the mutant channel corresponds to a slow inactivated state of Sh-IR that became conductive.

Seasonal variation in conduction velocity of action potentials in squid giant axons.

To determine whether the electrical properties of the squid giant axon are seasonally acclimated, action potentials, recorded at different temperatures, were compared between giant axons isolated from Loligo pealei caught in May, from relatively cold waters (approximately 10 degrees-12 degrees C), and in August, from relatively warm waters (approximately 20 degrees C). Parameters relating to the duration of the action potential (e.g., maximum rate of rise, maximum rate of fall, and duration at half-peak) did not change seasonally. The relationship between conduction velocity and temperature remained constant between seasons as well, in spite of the fact that May axons were significantly larger than August axons. When normalized to the fiber diameter, mean May conduction velocities were 83% of the August values at all temperatures tested, and analysis of the rise time of the action potential foot suggested that a change in the axoplasmic resistivity was responsible for this difference. Direct measurements of axoplasmic resistance further supported this hypothesis. Thus seasonal changes in the giant axon's size and resistivity are not consistent with compensatory thermal acclimation, but instead serve to maintain a constant relationship between conduction velocity and temperature.

The voltage sensor in voltage-dependent ion channels.

In voltage-dependent Na, K, or Ca channels, the probability of opening is modified by the membrane potential. This is achieved through a voltage sensor that detects the voltage and transfers its energy to the pore to control its gate. We present here the theoretical basis of the energy coupling between the electric field and the voltage, which allows the interpretation of the gating charge that moves in one channel. Movement of the gating charge constitutes the gating current. The properties are described, along with macroscopic data and gating current noise analysis, in relation to the operation of the voltage sensor and the opening of the channel. Structural details of the voltage sensor operation were resolved initially by locating the residues that make up the voltage sensor using mutagenesis experiments and determining the number of charges per channel. The changes in conformation are then analyzed based on the differential exposure of cysteine or histidine-substituted residues. Site-directed fluorescence labeling is then analyzed as another powerful indicator of conformational changes that allows time and voltage correlation of local changes seen by the fluorophores with the global change seen by the electrophysiology of gating currents and ionic currents. Finally, we describe the novel results on lanthanide-based resonance energy transfer that show small distance changes between residues in the channel molecule. All of the electrophysiological and the structural information are finally summarized in a physical model of a voltage-dependent channel in which a change in membrane potential causes rotation of the S4 segment that changes the exposure of the basic residues from an internally connected aqueous crevice at hyperpolarized potentials to an externally connected aqueous crevice at depolarized potentials.

Three distinct and sequential steps in the release of sodium ions by the Na+/K+-ATPase.

The Na+/K+ pump, a P-type ion-motive ATPase, exports three sodium ions and then imports two potassium ions in each transport cycle. Ions on one side of the membrane bind to sites within the protein and become temporarily occluded (trapped within the protein) before being released to the other side, but details of these occlusion and de-occlusion transitions remain obscure for all P-type ATPases. If it is deprived of potassium ions, the Na+/K+ pump is restricted to sodium translocation steps, at least one involving charge movement through the membrane's electric fields. Changes in membrane potential alter the rate of such electrogenic reactions and so shift the distribution of enzyme conformations. Here we use high-speed voltage jumps to initiate this redistribution and show that the resulting pre-steady-state charge movements relax in three identifiable phases, apparently reflecting de-occlusion and release of the three sodium ions. Reciprocal relationships among the sizes of these three charge components show that the three sodium ions are de-occluded and released to the extracellular solution one at a time, in a strict order.

Modulation of the Shaker K(+) channel gating kinetics by the S3-S4 linker.

In Shaker K(+) channels depolarization displaces outwardly the positively charged residues of the S4 segment. The amount of this displacement is unknown, but large movements of the S4 segment should be constrained by the length and flexibility of the S3-S4 linker. To investigate the role of the S3-S4 linker in the ShakerH4Delta(6-46) (ShakerDelta) K(+) channel activation, we constructed S3-S4 linker deletion mutants. Using macropatches of Xenopus oocytes, we tested three constructs: a deletion mutant with no linker (0 aa linker), a mutant containing a linker 5 amino acids in length, and a 10 amino acid linker mutant. Each of the three mutants tested yielded robust K(+) currents. The half-activation voltage was shifted to the right along the voltage axis, and the shift was +45 mV in the case of the 0 aa linker channel. In the 0 aa linker, mutant deactivation kinetics were sixfold slower than in ShakerDelta. The apparent number of gating charges was 12.6+/-0.6 e(o) in ShakerDelta, 12.7+/-0.5 in 10 aa linker, and 12.3+/-0.9 in 5 aa linker channels, but it was only 5.6+/-0.3 e(o) in the 0 aa linker mutant channel. The maximum probability of opening (P(o)(max)) as measured using noise analysis was not altered by the linker deletions. Activation kinetics were most affected by linker deletions; at 0 mV, the 5 and 0 aa linker channels' activation time constants were 89x and 45x slower than that of the ShakerDelta K(+) channel, respectively. The initial lag of ionic currents when the prepulse was varied from -130 to -60 mV was 0.5, 14, and 2 ms for the 10, 5, and 0 aa linker mutant channels, respectively. These results suggest that: (a) the S4 segment moves only a short distance during activation since an S3-S4 linker consisting of only 5 amino acid residues allows for the total charge displacement to occur, and (b) the length of the S3-S4 linker plays an important role in setting ShakerDelta channel activation and deactivation kinetics.

Deletion of the S3-S4 linker in the Shaker potassium channel reveals two quenching groups near the outside of S4.

When attached outside the voltage-sensing S4 segment of the Shaker potassium channel, the fluorescent probe tetramethylrhodamine (TMRM) undergoes voltage-dependent fluorescence changes (DeltaF) due to differential interaction with a pH-titratable external protein-lined vestibule (Cha, A., and F. Bezanilla. 1998. J. Gen. Physiol. 112:391-408.). We attached TMRM at the same sites [corresponding to M356C and A359C in the wild-type (wt) channel] in a deletion mutant of Shaker where all but the five amino acids closest to S4 had been removed from the S3-S4 linker. In the deletion mutant, the maximal DeltaF/F seen was diminished 10-fold, and the DeltaF at M356C became pH independent, suggesting that the protein-lined vestibule is made up in large part by the S3-S4 linker. The residual DeltaF showed that the probe still interacted with two putative quenching groups near the S4 segment. One group was detected by M356C-TMRM (located outside of S3 in the deletion mutant) and reported on deactivation gating charge movement when applying hyperpolarizing voltage steps from a holding potential of 0 mV. During activating voltage steps from a holding potential of -90 mV, the fluorescence lagged considerably behind the movement of gating charge over a range of potentials. Another putative quenching group was seen by probes attached closer to the S4 and caused a DeltaF at extreme hyperpolarizations (more negative than -90 mV) only. A signal from the interaction with this group in the wt S3-S4 linker channel (at L361C) correlated with gating charge moving in the hyperpolarized part of the Q-V curve. Probe attached at A359C in the deletion mutant and at L361C in wt channel showed a biphasic DeltaF as the probe oscillated between the two groups, revealing that there is a transient state of the voltage sensor in between, where the probe has maximal fluorescence. We conclude that the voltage sensor undergoes two distinct conformational changes as seen from probes attached outside the S4 segment.

Extracellular Mg(2+) modulates slow gating transitions and the opening of Drosophila ether-à-Go-Go potassium channels.

We have characterized the effects of prepulse hyperpolarization and extracellular Mg(2+) on the ionic and gating currents of the Drosophila ether-à-go-go K(+) channel (eag). Hyperpolarizing prepulses significantly slowed channel opening elicited by a subsequent depolarization, revealing rate-limiting transitions for activation of the ionic currents. Extracellular Mg(2+) dramatically slowed activation of eag ionic currents evoked with or without prepulse hyperpolarization and regulated the kinetics of channel opening from a nearby closed state(s). These results suggest that Mg(2+) modulates voltage-dependent gating and pore opening in eag channels. To investigate the mechanism of this modulation, eag gating currents were recorded using the cut-open oocyte voltage clamp. Prepulse hyperpolarization and extracellular Mg(2+) slowed the time course of ON gating currents. These kinetic changes resembled the results at the ionic current level, but were much smaller in magnitude, suggesting that prepulse hyperpolarization and Mg(2+) modulate gating transitions that occur slowly and/or move relatively little gating charge. To determine whether quantitatively different effects on ionic and gating currents could be obtained from a sequential activation pathway, computer simulations were performed. Simulations using a sequential model for activation reproduced the key features of eag ionic and gating currents and their modulation by prepulse hyperpolarization and extracellular Mg(2+). We have also identified mutations in the S3-S4 loop that modify or eliminate the regulation of eag gating by prepulse hyperpolarization and Mg(2+), indicating an important role for this region in the voltage-dependent activation of eag.

Voltage sensors in domains III and IV, but not I and II, are immobilized by Na+ channel fast inactivation.

Using site-directed fluorescent labeling, we examined conformational changes in the S4 segment of each domain of the human skeletal muscle sodium channel (hSkM1). The fluorescence signals from S4 segments in domains I and II follow activation and are unaffected as fast inactivation settles. In contrast, the fluorescence signals from S4 segments in domains III and IV show kinetic components during activation and deactivation that correlate with fast inactivation and charge immobilization. These results indicate that in hSkM1, the S4 segments in domains III and IV are responsible for voltage-sensitive conformational changes linked to fast inactivation and are immobilized by fast inactivation, while the S4 segments in domains I and II are unaffected by fast inactivation.

Kramers' diffusion theory applied to gating kinetics of voltage-dependent ion channels.

Kramers' diffusion theory of reaction rates in the condensed phase is considered as an alternative to the traditional discrete-state Markov (DSM) model in describing ion channel gating current kinetics. Diffusion theory can be expected to be particularly relevant in describing high-frequency (>100 kHz) events in channel activation. The generalized voltage sensor of a voltage-dependent ion channel is treated as a Brownian motion particle undergoing spatial diffusion along a one-dimensional energy landscape. Two classes of energy landscapes are considered. The first class contains large barriers, which give rise to gating currents with two distinct time scales: the usual low-frequency decay, which can modeled with a DSM scheme, and a high-frequency component arising from intrastate relaxation. Large depolarizations reduce potential barriers to such a degree that activation rates are diffusion limited, causing the two time scales to merge. Landscapes of the second class are either featureless or contain barriers that are small compared to kT; these are termed "drift landscapes." These landscapes require a larger friction coefficient to generate slow gating kinetics. The high-frequency component that appears with barrier models is not present in pure drift motion. The presence of a high-frequency component can be tested experimentally with large-bandwidth recordings of gating currents. Topics such as frequency domain analysis, spatial dependence of the friction coefficient, methods for determining the adequacy of a DSM model, and the development of physical models of gating are explored.

Atomic scale movement of the voltage-sensing region in a potassium channel measured via spectroscopy.

Voltage-gated ion channels are transmembrane proteins that are essential for nerve impulses and regulate ion flow across cell membranes in response to changes in membrane potential. They are made up of four homologous domains or subunits, each of which contains six transmembrane segments. Studies of potassium channels have shown that the second (S2) and fourth (S4) segments contain several charged residues, which sense changes in voltage and form part of the voltage sensor. Although these regions clearly undergo conformational changes in response to voltage, little is known about the nature of these changes because voltage-dependent distance changes have not been measured. Here we use lanthanide-based resonance energy transfer to measure distances between Shaker potassium channel subunits at specific residues. Voltage-dependent distance changes of up to 3.2 A were measured at several sites near the S4 segment. These movements directly correlated with electrical measurements of the voltage sensor, establishing the link between physical changes and electrical charge movement. Measured distance changes suggest that the region associated with the S4 segment undergoes a rotation and possible tilt, rather than a large transmembrane movement, in response to voltage. These results demonstrate the first in situ measurement of atomic scale movement in a trans-membrane protein.

Structural implications of fluorescence quenching in the Shaker K+ channel.

When attached to specific sites near the S4 segment of the nonconducting (W434F) Shaker potassium channel, the fluorescent probe tetramethylrhodamine maleimide undergoes voltage-dependent changes in intensity that correlate with the movement of the voltage sensor (Mannuzzu, L.M., M.M. Moronne, and E.Y. Isacoff. 1996. Science. 271:213-216; Cha, A., and F. Bezanilla. 1997. Neuron. 19:1127-1140). The characteristics of this voltage-dependent fluorescence quenching are different in a conducting version of the channel with a different pore substitution (T449Y). Blocking the pore of the T449Y construct with either tetraethylammonium or agitoxin removes a fluorescence component that correlates with the voltage dependence but not the kinetics of ionic activation. This pore-mediated modulation of the fluorescence quenching near the S4 segment suggests that the fluorophore is affected by the state of the external pore. In addition, this modulation may reflect conformational changes associated with channel opening that are prevented by tetraethylammonium or agitoxin. Studies of pH titration, collisional quenchers, and anisotropy indicate that fluorophores attached to residues near the S4 segment are constrained by a nearby region of protein. The mechanism of fluorescence quenching near the S4 segment does not involve either reorientation of the fluorophore or a voltage-dependent excitation shift and is different from the quenching mechanism observed at a site near the S2 segment. Taken together, these results suggest that the extracellular portion of the S4 segment resides in an aqueous protein vestibule and is influenced by the state of the external pore.

Voltage gating of Shaker K+ channels. The effect of temperature on ionic and gating currents.

Ionic (Ii) and gating currents (Ig) from noninactivating Shaker H4 K+ channels were recorded with the cut-open oocyte voltage clamp and macropatch techniques. Steady state and kinetic properties were studied in the temperature range 2-22 degreesC. The time course of Ii elicited by large depolarizations consists of an initial delay followed by an exponential rise with two kinetic components. The main Ii component is highly temperature dependent (Q10 > 4) and mildly voltage dependent, having a valence times the fraction of electric field (z) of 0.2-0.3 eo. The Ig On response obtained between -60 and 20 mV consists of a rising phase followed by a decay with fast and slow kinetic components. The main Ig component of decay is highly temperature dependent (Q10 > 4) and has a z between 1.6 and 2.8 eo in the voltage range from -60 to -10 mV, and approximately 0.45 eo at more depolarized potentials. After a pulse to 0 mV, a variable recovery period at -50 mV reactivates the gating charge with a high temperature dependence (Q10 > 4). In contrast, the reactivation occurring between -90 and -50 mV has a Q10 = 1.2. Fluctuation analysis of ionic currents reveals that the open probability decreases 20% between 18 and 8 degreesC and the unitary conductance has a low temperature dependence with a Q10 of 1.44. Plots of conductance and gating charge displacement are displaced to the left along the voltage axis when the temperature is decreased. The temperature data suggests that activation consists of a series of early steps with low enthalpic and negative entropic changes, followed by at least one step with high enthalpic and positive entropic changes, leading to final transition to the open state, which has a negative entropic change.

Cut-open oocyte voltage-clamp technique.

In this article, we described the use and limitations of the COVG technique. The advantages of the present method as follows: 1. High-frequency response and low noise recording (24-microseconds time constant and 1.2-nA rms at 5 kHz). This allows the accurate description of small gating currents and fast activation or deactivation of ionic currents to be adequately resolved. Currents up to 20-30 microA can be adequately clamped. 2. Stable recording conditions lasting for several hours. Even though this technique has internal access, rundown is minimized compared to excised patches. 3. Access to the cell interior. The intracellular medium can be exchanged with various solutions. This was of invaluable help in recording K+ gating currents, an experimental condition that required the complete blockade of all ionic currents. This advantage will make the present method suitable for the study of channel modulation by second messengers and drugs. In addition, channel selectivity properties can be defined by ion substitution experiments. 4. Channel localization. A study of the membrane compartmentalization of channels can be easily obtained with this method, since it allows the voltage clamping of different regions of the oocyte surface.