Piezo1, the new actor in cell volume regulation

All animal cells control their volume through a complex set of mechanisms, both to counteract osmotic perturbations of the environment and to enable numerous vital biological processes, such as proliferation, apoptosis, and migration. The ability of cells to adjust their volume depends on the activity of ion channels and transporters which, by moving K+, Na+, and Cl- ions across the plasma membrane, generate the osmotic gradient that drives water in and out of the cell. In 2010, Patapoutian's group identified a small family of evolutionarily conserved, Ca2+-permeable mechanosensitive channels, Piezo1 and Piezo2, as essential components of the mechanically activated current that mediates mechanotransduction in vertebrates. Piezo1 is expressed in several tissues and its opening is promoted by a wide range of mechanical stimuli, including membrane stretch/deformation and osmotic stress. Piezo1-mediated Ca2+ influx is used by the cell to convert mechanical forces into cytosolic Ca2+ signals that control diverse cellular functions such as migration and cell death, both dependent on changes in cell volume and shape. The crucial role of Piezo1 in the regulation of cell volume was first demonstrated in erythrocytes, which need to reduce their volume to pass through narrow capillaries. In HEK293 cells, increased expression of Piezo1 was found to enhance the regulatory volume decrease (RVD), the process whereby the cell re-establishes its original volume after osmotic shock-induced swelling, and it does so through Ca2+-dependent modulation of the volume-regulated anion channels. More recently we reported that Piezo1 controls the RVD in glioblastoma cells via the modulation of Ca2+-activated K+ channels. To date, however, the mechanisms through which this mechanosensitive channel controls cell volume and maintains its homeostasis have been poorly investigated and are still far from being understood. The present review aims to provide a broad overview of the literature discussing the recent advances on this topic.

A Multi-Scale Approach to Model K+ Permeation Through the KcsA Channel

K+ channels allow a very efficient passage of K+ ions through the membrane while excluding Na+ ions, and these properties are essential for life. The 3D structure of the KcsA K+ channel, solved more than 20 years ago, allows to address many relevant aspects of K+ permeation and selectivity mechanisms at the molecular level. Recent crystallographic data and molecular dynamics (MD) studies suggest that no water is normally present inside the selectivity filter (SF), which can instead accommodate four adjacent K+ ions. Using a multi-scale approach, whereby information taken from a low-level simulation approach is used to feed a high-level model, we studied the mechanism of K+ permeation through KcsA channels. More specifically, we used MD to find stable ion configurations under physiological conditions. They were characterized by two adjacent K+ ions occupying the more central positions of the SF (sites S2 and S3), while the other two K+ ions could be found at the external and internal entrances to the SF. Sites S1 and S4 were instead not occupied by K+. A continuum Bikerman–Poisson–Boltzmann model that takes into account the volume of the ions and their dehydration when entering the SF fully confirmed the MD results, showing peaks of K+ occupancy at S2, S3, and the external and internal entrances, with S1 and S4 sites being virtually never occupied by K+. Inspired by the newly found ion configuration in the SF at equilibrium, we developed a simple kinetic permeation model which, fed with kinetic rate constants assessed from molecular meta-dynamics, reproduced the main permeation properties of the KcsA channel found experimentally, including sublinear current-voltage and saturating conductance-concentration relationships. This good agreement with the experimental data also implies that the ion configuration in the SF we identified at equilibrium would also be a key configuration during permeation.

Trigeminal ganglion neuron subtype-specific alterations of Ca(V)2.1 calcium current and excitability in a Cacna1a mouse model of migraine

Familial hemiplegic migraine type-1 (FHM1), a monogenic subtype of migraine with aura, is caused by gain-of-function mutations in Ca(V)2.1 (P/Q-type) calcium channels. The consequences of FHM1 mutations on the trigeminovascular pathway that generates migraine headache remain largely unexplored. Here we studied the calcium currents and excitability properties of two subpopulations of small-diameter trigeminal ganglion (TG) neurons from adult wild-type (WT) and R192Q FHM1 knockin (KI) mice: capsaicin-sensitive neurons without T-type calcium currents (CS) and capsaicin-insensitive neurons characterized by the expression of T-type calcium currents (CI-T). Small TG neurons retrogradely labelled from the dura are mostly CS neurons, while CI-T neurons were not present in the labelled population. CS and CI-T neurons express Ca(V)2.1 channels with different activation properties, and the Ca(V)2.1 channels are differently affected by the FHM1 mutation in the two TG neuron subtypes. In CI-T neurons from FHM1 KI mice there was a larger P/Q-type current density following mild depolarizations, a larger action potential (AP)-evoked calcium current and a longer AP duration when compared to CI-T neurons from WT mice. In striking contrast, the P/Q-type current density, voltage dependence and kinetics were not altered by the FHM1 mutation in CS neurons. The excitability properties of mutant CS neurons were also unaltered. Congruently, the FHM1 mutation did not alter depolarization-evoked CGRP release from the dura mater, while CGRP release from the trigeminal ganglion was larger in KI compared to WT mice. Our findings suggest that the facilitation of peripheral mechanisms of CGRP action, such as dural vasodilatation and nociceptor sensitization at the meninges, does not contribute to the generation of headache in FHM1.

Histamine activates a background, arachidonic acid-sensitive K channel in embryonic chick dorsal root ganglion neurons

Histamine has been proposed to be an important modulator of developing neurons, but its mechanism of action remains unclear. In embryonic chick dorsal root ganglion neurons we found that histamine activates, through the pyrilamine-sensitive H1 receptor, a K-selective, background channel. The K channel activated by histamine was also activated by arachidonic acid in a dose-dependent way, with a KD of 4 microM and a slope of 2.5, had a unitary conductance of about 150 pS (symmetrical 140 KCl) and a moderate voltage dependence. The channel was insensitive to the classical K channel blockers tetraethylammonium, charybdotoxin, 4-aminopyridine, but inhibited by millimolar Ba2+. Channel activity could also be increased by lowering the intracellular pH from 7.2 to 5.5, or by applying negative pressure pulses through the patch pipette. Experiments aimed at delineating the metabotropic pathway leading to K channel activation by histamine indicated the involvement of a pertussis toxin-insensitive G protein, and a quinacrine-sensitive cytosolic phospholipase A2. The histamine-induced K channel activation was observed only with elevated internal Ca2+ (achieved using 0.5 microM ionomycin or elevated external KCl). An increase in the histamine-induced phosphoinositide hydrolysis was also observed upon internal Ca2+ elevation, showing the presence of a Ca2+ dependent step upstream to inositol 1,4,5-triphosphate production. In view of the functional importance of K conductances during cell differentiation, we propose that histamine activation of this K channel may have a significant role during normal development of embryonic chick neurons.

A fast transient outward monovalent current in rat saphenous myocytes passing through Ca2+ channels

Transient outward currents in rat saphenous arterial myocytes were studied using the perforated configuration of the patch-clamp method. When myocytes were bathed in a Na-gluconate solution containing TEA to block large-conductance Ca2+-activated K+ (BK) currents, depolarizing pulses positive to +20 mV from a holding potential of -100 mV induced fast transient outward currents. The activation and inactivation time constants of the current were voltage dependent, and at +40 mV were 3.6 +/- 0.8 ms and 23.9 +/- 6.4 ms (n = 4), respectively. The steady-state inactivation of the transient outward current was steeply voltage dependent (z = 1.7), with 50% of the current inactivated at -55 mV. The current was insensitive to the A-type K+ channel blocker 4-AP (1-5 mM), and was modulated by external Ca, decreasing to approximately 0.85 of control values upon raising Ca2+ from 1 to 10 mM, and increasing approximately 3-fold upon lowering it to 0.1 mM. Transient outward currents were also recorded following replacement of internal K+ with either Na+ or Cs+, raising the possibility that the current was carried by monovalent ions passing through voltage-gated Ca2+ channels. This hypothesis was supported by the finding that the transient outward current had the same inactivation rate as the inward Ba2+ current, and that both currents were effectively blocked by the L-type Ca2+ channel blocker, nifedipine and enhanced by the agonist BAYK8644.

Large-conductance calcium-activated potassium channels in neonatal rat intracardiac ganglion neurons

The properties of single Ca2+-activated K+ (BK) channels in neonatal rat intracardiac neurons were investigated using the patch-clamp recording technique. In symmetrical 140 mM K+, the single-channel slope conductance was linear in the voltage range -60/+60 mV, and was 207+/-19 pS. Na+ ions were not measurably permeant through the open channel. Channel activity increased with the cytoplasmic free Ca2+ concentration ([Ca2+]i) with a Hill plot giving a half-saturating [Ca2+] (K0.5) of 1.35 microM and slope of approximately equals 3. The BK channel was inhibited reversibly by external tetraethylammonium (TEA) ions, charybdotoxin, and quinine and was resistant to block by 4-aminopyridine and apamin. Ionomycin (1-10 microM) increased BK channel activity in the cell-attached recording configuration. The resting activity was consistent with a [Ca2+]i <100 nM and the increased channel activity evoked by ionomycin was consistent with a rise in [Ca2+]i to > or =0.3 microM. TEA (0.2-1 mM) increased the action potential duration approximately equals 1.5-fold and reduced the amplitude and duration of the afterhyperpolarization (AHP) by 26%. Charybdotoxin (100 nM) did not significantly alter the action potential duration or AHP amplitude but reduced the AHP duration by approximately equals 40%. Taken together, these data indicate that BK channel activation contributes to the action potential and AHP duration in rat intracardiac neurons.

Verapamil block of large-conductance Ca-activated K channels in rat aortic myocytes

The effects of verapamil on the large conductance Ca-activated K (BK) channel from rat aortic smooth muscle cells were examined at the single channel level. Micromolar concentrations of verapamil produced a reversible flickering block of the BK channel activity. Kinetic analysis showed that verapamil decreased markedly the time constants of the open states, without any significant change in the time constants of the closed states. The appearance of an additional closed state-specifically, a nonconducting, open-blocked state--was also observed, whose time constant would reflect the mean residence time of verapamil on the channel. These observations are indicative of a state-dependent, open-channel block mechanism. Dedicated kinetic (group) analysis confirmed the state-dependent block exerted by verapamil. D600 (gallopamil), the methoxy derivative of verapamil, was also tested and found to exert a similar type of block, but with a higher affinity than verapamil. The permanently charged and membrane impermeant verapamil analogue D890 was used to address other important features of verapamil block, such as the sidedness of action and the location of the binding site on the channel protein. D890 induced a flickering block of BK channels similar to that observed with verapamil only when applied to the internal side of the membrane, indicating that D890 binds to a site accessible from the cytoplasmic side. Finally, the voltage dependence of D890 block was assessed. The experimental data fitted with a Langmuir equation incorporating the Woodhull model for charged blockers confirms that the D890-binding site is accessed from the internal mouth of the BK channel, and locates it approximately 40% of the membrane voltage drop along the permeation pathway.

Characterization of the large-conductance Ca-activated K channel in myocytes of rat saphenous artery

We used the patch-clamp method to characterize the BK channel in freshly isolated myocytes from the saphenous branch of the rat femoral artery. Single-channel recordings revealed that the BK channel had a conductance of 187 pS in symmetrical 150 mM KCl, was blocked by external tetraethylammonium (TEA) with a KD(TEA) of approx. 300 microM at +40 mV, and by submicromolar charybdotoxin (CTX). The sensitivity of the BK channel to Ca was especially high (KD(ca) approx. 0.1 microM at +60 mV) compared to skeletal muscle and neuronal tissues. We also investigated the macroscopic K current, which under certain conditions is essentially sustained by BK channels. This conclusion is based on the findings that the macroscopic current activated upon depolarization follows a single exponential time course and is virtually fully blocked by 100 nM CTX and 5 mM external TEA. We made use of this occurrence to assess the voltage and Ca dependence of the macroscopic BK current. In intact myocytes, the BK channel showed a strong and voltage-dependent reduction of the outward current (62% at +40 mV), most likely due to block by intracellular Ba and polyamines. The results obtained from macroscopic and unitary current indicate that approx. 2.5% of the BK channels are active under physiological conditions, sustaining approx. 20 pA of outward current. Given the high input resistance of these cells, few BK channels are required to open in order to cause a significant membrane hyperpolarization, and thus function to limit the contraction resulting from acute increases in intravascular pressure, or in response to hypertensive pathologies.

Binding and selectivity in L-type calcium channels: a mean spherical approximation

L-type calcium channels are Ca(2+) binding proteins of great biological importance. They generate an essential intracellular signal of living cells by allowing Ca(2+) ions to move across the lipid membrane into the cell, thereby selecting an ion that is in low extracellular abundance. Their mechanism of selection involves four carboxylate groups, containing eight oxygen ions, that belong to the side chains of the "EEEE" locus of the channel protein, a setting similar to that found in many Ca(2+)-chelating molecules. This study examines the hypothesis that selectivity in this locus is determined by mutual electrostatic screening and volume exclusion between ions and carboxylate oxygens of finite diameters. In this model, the eight half-charged oxygens of the tethered carboxylate groups of the protein are confined to a subvolume of the pore (the "filter"), but interact spontaneously with their mobile counterions as ions interact in concentrated bulk solutions. The mean spherical approximation (MSA) is used to predict ion-specific excess chemical potentials in the filter and baths. The theory is calibrated using a single experimental observation, concerning the apparent dissociation constant of Ca(2+) in the presence of a physiological concentration of NaCl. When ions are assigned their independently known crystal diameters and the carboxylate oxygens are constrained, e.g., to a volume of 0.375 nm(3) in an environment with an effective dielectric coefficient of 63.5, the hypothesized selectivity filter produces the shape of the calcium binding curves observed in experiment, and it predicts Ba(2+)/Ca(2+) and Na(+)/Li(+) competition, and Cl(-) exclusion as observed. The selectivities for Na(+), Ca(2+), Ba(2+), other alkali metal ions, and Cl(-) thus can be predicted by volume exclusion and electrostatic screening alone. Spontaneous coordination of ions and carboxylates can produce a wide range of Ca(2+) selectivities, depending on the volume density of carboxylate groups and the permittivity in the locus. A specific three-dimensional structure of atoms at the binding site is not needed to explain Ca(2+) selectivity.

Bimodal kinetics of a chloride channel from human fibroblasts

Excised patches were used to study the kinetics of a Cl channel newly identified in cultured human fibroblasts (L132). The conductance of ca. 70 pS in 150 mm symmetrical Cl, and the marked outward rectification ascribe this channel to the ICOR family. Long single-channel recordings (>30 min) revealed that the channel spontaneously switches from a kinetic mode characterized by high voltage dependence (with activity increasing with depolarization; mode 1), into a second mode (mode 2) insensitive to voltage, and characterized by a high activity in the voltage range +/-120 mV. On patch excision the channel always appeared in mode 1, which was maintained for a variable time (5-20 min). In most instances the channels then switched into mode 2, and never were seen to switch back, in spite of the eight patches that cumulatively dwelled in this mode 2.33-fold as compared to mode 1. Stability plots of long recordings showed that the channel was kinetically stable in both modes, allowing standard analysis of steady-state kinetics to be performed. Open and closed time distributions of mode 1 and mode 2 revealed that the apparent number of kinetic states of the channel was the same in the two modes. The transition from mode 1 into mode 2 was not instantaneous, but required a variable time in the range 5-60 sec. During the transition the channel mean open time was intermediate between mode 1 and mode 2. The intermediate duration in the stability plot however is not to be interpreted as if the channel, during the transition, rapidly switches between mode 1 and mode 2, but represents a distinct kinetic feature of the transitional channel.

Mechanisms of verapamil inhibition of action potential firing in rat intracardiac ganglion neurons

The effects of verapamil and related phenylalkylamines on neuronal excitability were investigated in isolated neurons of rat intracardiac ganglia using whole-cell perforated patch-clamp recording. Verapamil (>/=10 microM) inhibits tonic firing observed in response to depolarizing current pulses at 22 degrees C. The inhibition of discharge activity is not due to block of voltage-dependent Ca2+ channels because firing is not affected by 100 microM Cd2+. The K+ channel inhibitors charybdotoxin (100 nM), 4-aminopyridine (0.5 mM), apamin (30-100 nM), and tetraethylammonium ions (1 mM) also have no effect on firing behavior at 22 degrees C. Verapamil does not antagonize the acetylcholine-induced inhibition of the muscarine-sensitive K+ current (M-current) in rat intracardiac neurons. Verapamil inhibits the delayed outwardly rectifying K+ current with an IC50 value of 11 microM, which is approximately 7-fold more potent than its inhibition of high voltage-activated Ca2+ channel currents. These data suggest that verapamil inhibits tonic firing in rat intracardiac neurons primarily via inhibition of delayed outwardly rectifying K+ current. Verapamil inhibition of action potential firing in intracardiac neurons may contribute, in part, to verapamil-induced tachycardia.

Mechanism of verapamil block of a neuronal delayed rectifier K channel: active form of the blocker and location of its binding domain

1. The mechanism of verapamil block of the delayed rectifier K currents (I K(DR)) in chick dorsal root ganglion (DRG) neurons was investigated using the whole-cell patch clamp configuration. In particular we focused on the location of the blocking site, and the active form (neutral or charged) of verapamil using the permanently charged verapamil analogue D890. 2. Block by D890 displayed similar characteristics to that of verapamil, indicating the same state-dependent nature of block. In contrast with verapamil, D890 was effective only when applied internally, and its block was voltage dependent (136 mV/e-fold change of the on rate). Given that verapamil block is insensitive to voltage (Trequattrini et al., 1998), these observations indicate that verapamil reaches its binding site in the uncharged form, and accesses the binding domain from the cytoplasm. 3. In external K and saturating verapamil we recorded tail currents that did not decay monotonically but showed an initial increase (hook). As these currents can only be observed if verapamil unblock is significantly voltage dependent, it has been suggested (DeCoursey, 1995) that neutral drug is protonated upon binding. We tested this hypothesis by assessing the voltage dependence of the unblock rate from the hooked tail currents for verapamil and D890. 4. The voltage dependence of the off rate of D890, but not of verapamil, was well described by adopting the classical Woodhull (1973) model for ionic blockage of Na channels. The voltage dependence of verapamil off rate was consistent with a kinetic scheme where the bound drug can be protonated with rapid equilibrium, and both charged and neutral verapamil can unbind from the site, but with distinct kinetics and voltage dependencies.

Verapamil block of the delayed rectifier K current in chick embryo dorsal root ganglion neurons

We have used the patch-clamp method in the whole-cell configuration to investigate the mechanism of block of the delayed rectifier K current (IDRK) by verapamil in embryonic chick dorsal root ganglion (DRG) neurons. Verapamil induced a dose-dependent decay of the current, without altering its activation kinetics. This observation, together with the good description of IDRK time course at various blocker concentrations with the computer simulation of a three-state chain model (closed left and right arrow open left and right arrow open-blocked), indicates that verapamil acts as a state-dependent, open-channel blocker. To account for the double-exponential time course of recovery from block, this minimal kinetics scheme was expanded to include a closed-blocked state resulting from channel closure (at hyperpolarized voltages) with verapamil still bound to it. The apparent block and unblock rate constants assessed from verapamil-induced current decay in the presence of external Na were 0.95 +/- 0.05 ms-1mM-1 and 0.0037 +/- 0.0016 ms-1, respectively. When external Na was replaced by K, only the unblock rate constant changed, to 0.02 +/- 0.009 ms-1. Under these ionic conditions it was also observed that the recovery from block was modified from the double-exponential time course in the presence of external Na (tau1 = 160 ms; tau2 = 1600 ms), to a faster single-exponential recovery (tau = 100 ms). We tested the voltage dependence of block by applying stimulation protocols aimed at eliminating bias easily introduced by the shift of the gating equilibrium and by the coupling of channel activation and block. Under these experimental conditions the resulting block rate constant was not measurably voltage dependent.