Mechanism of cGMP-gated channel block by intracellular polyamines

The role of polyamine metabolism in cellular function and physiology

Polyamines are molecules with multiple amino groups that are essential for cellular function. The major polyamines are putrescine, spermidine, spermine, and cadaverine. Polyamines are important for posttranscriptional regulation, autophagy, programmed cell death, proliferation, redox homeostasis, and ion channel function. Their levels are tightly controlled. High levels of polyamines are associated with proliferative pathologies such as cancer, whereas low polyamine levels are observed in aging, and elevated polyamine turnover enhances oxidative stress. Polyamine metabolism is implicated in several pathophysiological processes in the nervous, immune, and cardiovascular systems. Currently, manipulating polyamine levels is under investigation as a potential preventive treatment for several pathologies, including aging, ischemia/reperfusion injury, pulmonary hypertension, and cancer. Although polyamines have been implicated in many intracellular mechanisms, our understanding of these processes remains incomplete and is a topic of ongoing investigation. Here, we discuss the regulation and cellular functions of polyamines, their role in physiology and pathology, and emphasize the current gaps in knowledge and potential future research directions.

Blockade of TRPV channels by intracellular spermine

The Vanilloid thermoTRP (TRPV1-4) subfamily of TRP channels are involved in thermoregulation, osmoregulation, itch and pain perception, (neuro)inflammation and immune response, and tight control of channel activity is required for perception of noxious stimuli and pain. Here we report voltage-dependent modulation of each of human TRPV1, 3, and 4 by the endogenous intracellular polyamine spermine. As in inward rectifier K channels, currents are blocked in a strongly voltage-dependent manner, but, as in cyclic nucleotide-gated channels, the blockade is substantially reduced at more positive voltages, with maximal blockade in the vicinity of zero voltage. A kinetic model of inhibition suggests two independent spermine binding sites with different affinities as well as different degrees of polyamine permeability in TRPV1, 3, and 4. Given that block and relief occur over the physiological voltage range of action potentials, voltage-dependent polyamine block may be a potent modulator of TRPV-dependent excitability in multiple cell types.

Polyamine blockade and binding energetics in the MthK potassium channel

Polyamines such as spermidine and spermine are found in nearly all cells, at concentrations ranging up to 0.5 mM. These cations are endogenous regulators of cellular K⁺ efflux, binding tightly in the pores of inwardly rectifying K⁺ (K_ir) channels in a voltage-dependent manner. Although the voltage dependence of K_ir channel polyamine blockade is thought to arise at least partially from the energetically coupled movements of polyamine and K⁺ ions through the pore, the nature of physical interactions between these molecules is unclear. Here we analyze the polyamine-blocking mechanism in the model K⁺ channel MthK, using a combination of electrophysiology and computation. Spermidine (SPD³⁺) and spermine (SPM⁴⁺) each blocked current through MthK channels in a voltage-dependent manner, and blockade by these polyamines was described by a three-state kinetic scheme over a wide range of polyamine concentrations. In the context of the scheme, both SPD³⁺ and SPM⁴⁺ access a blocking site with similar effective gating valences (0.84 ± 0.03 e₀ for SPD³⁺ and 0.99 ± 0.04 e₀ for SPM⁴⁺), whereas SPM⁴⁺ binds in the blocked state with an ∼20-fold higher affinity than SPD³⁺ (K_d = 28.1 ± 3.1 µM for SPD³⁺ and 1.28 ± 0.20 µM for SPM⁴⁺), consistent with a free energy difference of 1.8 kcal/mol. Molecular simulations of the MthK pore in complex with either SPD³⁺ or SPM⁴⁺ are consistent with the leading amine interacting with the hydroxyl groups of T59, at the selectivity filter threshold, with access to this site governed by outward movement of K⁺ ions. These coupled movements can account for a large fraction of the voltage dependence of blockade. In contrast, differences in binding energetics between SPD³⁺ and SPM⁴⁺ may arise from distinct electrostatic interactions between the polyamines and carboxylate oxygens on the side chains of E92 and E96, located in the pore-lining helix.

Modulation of Ion Transport Across Plant Membranes by Polyamines: Understanding Specific Modes of Action Under Stress

This work critically discusses the direct and indirect effects of natural polyamines and their catabolites such as reactive oxygen species and γ-aminobutyric acid on the activity of key plant ion-transporting proteins such as plasma membrane H+ and Ca2+ ATPases and K+-selective and cation channels in the plasma membrane and tonoplast, in the context of their involvement in stress responses. Docking analysis predicts a distinct binding for putrescine and longer polyamines within the pore of the vacuolar TPC1/SV channel, one of the key determinants of the cell ionic homeostasis and signaling under stress conditions, and an additional site for spermine, which overlaps with the cytosolic regulatory Ca2+-binding site. Several unresolved problems are summarized, including the correct estimates of the subcellular levels of polyamines and their catabolites, their unexplored effects on nucleotide-gated and glutamate receptor channels of cell membranes and Ca2+-permeable and K+-selective channels in the membranes of plant mitochondria and chloroplasts, and pleiotropic mechanisms of polyamines' action on H+ and Ca2+ pumps.

Polyamine-mediated channel block of ionotropic glutamate receptors and its regulation by auxiliary proteins

Most excitatory neurotransmission in the mammalian brain is mediated by a family of plasma membrane-bound signaling proteins called ionotropic glutamate receptors (iGluRs). iGluRs assemble at central synapses as tetramers, forming a central ion-channel pore whose primary function is to rapidly transport Na⁺ and Ca²⁺ in response to binding the neurotransmitter l-glutamic acid. The pore of iGluRs is also accessible to bulkier cytoplasmic cations, such as the polyamines spermine, spermidine, and putrescine, which are drawn into the permeation pathway, but get stuck and block the movement of other ions. The degree of this polyamine-mediated channel block is highly regulated by processes that control the free cytoplasmic polyamine concentration, the membrane potential, or the iGluR subunit composition. Recently, an additional regulation by auxiliary proteins, most notably transmembrane AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor regulatory proteins (TARPs), cornichons, and neuropilin and tolloid-like proteins (NETOs), has been identified. Here, I review what we have learned of polyamine block of iGluRs and its regulation by auxiliary subunits. TARPs, cornichons, and NETOs attenuate the channel block by enabling polyamines to exit the pore. As a result, polyamine permeation occurs at more negative and physiologically relevant membrane potentials. The structural basis for enhanced polyamine transport remains unresolved, although alterations in both channel architecture and charge-screening mechanisms have been proposed. That auxiliary subunits can attenuate the polyamine block reveals an unappreciated impact of polyamine permeation in shaping the signaling properties of neuronal AMPA- and kainate-type iGluRs. Moreover, enhanced polyamine transport through iGluRs may have a role in regulating cellular polyamine levels.

Chemical suppressors of mlo-mediated powdery mildew resistance

Loss-of-function of barley mildew locus o (Mlo) confers durable broad-spectrum penetration resistance to the barley powdery mildew pathogen, Blumeria graminis f. sp. hordei (Bgh). Given the importance of mlo mutants in agriculture, surprisingly few molecular components have been identified to be required for this type of resistance in barley. With the aim to identify novel cellular factors contributing to mlo-based resistance, we devised a pharmacological inhibitor screen. Of the 41 rationally chosen compounds tested, five caused a partial suppression of mlo resistance in barley, indicated by increased levels of Bgh host cell entry. These chemicals comprise brefeldin A (BFA), 2',3'-dideoxyadenosine (DDA), 2-deoxy-d-glucose, spermidine, and 1-aminobenzotriazole. Further inhibitor analysis corroborated a key role for both anterograde and retrograde endomembrane trafficking in mlo resistance. In addition, all four ribonucleosides, some ribonucleoside derivatives, two of the five nucleobases (guanine and uracil), some guanine derivatives as well as various polyamines partially suppress mlo resistance in barley via yet unknown mechanisms. Most of the chemicals identified to be effective in partially relieving mlo resistance in barley also to some extent compromised powdery mildew resistance in an Arabidopsis mlo2 mlo6 double mutant. In summary, our study identified novel suppressors of mlo resistance that may serve as valuable probes to unravel further the molecular processes underlying this unusual type of disease resistance.

Stargazin and cornichon-3 relieve polyamine block of AMPA receptors by enhancing blocker permeation

Polyamine block of AMPA-type ionotropic glutamate receptors is attenuated by auxiliary proteins stargazin and cornichon-3. Brown et al. show that relief from block is due to enhanced blocker permeation and present a modified model of permeant channel block to account for their experimental findings.

Most ligand- and voltage-gated ion channels assemble as signaling complexes consisting of pore-forming and auxiliary subunits. In the mammalian brain, AMPA-type ionotropic glutamate receptors (AMPARs) coassemble with several families of auxiliary subunits that regulate channel gating as well as ion channel block and permeation. Previous work has shown that auxiliary proteins stargazin (or γ2) and cornichon-3 (CNIH-3) attenuate the cytoplasmic polyamine channel block of AMPARs, although the underlying mechanism has yet to be established. Here, we show that γ2 and CNIH-3 relieve channel block by enhancing the rate of blocker permeation. Surprisingly, the relative permeability of the polyamine spermine (Spm) through the pore of the AMPAR-γ2 or -CNIH-3 complexes is considerably more than AMPARs expressed alone. Spm permeability is comparable to that of Na⁺ for the GluA2-γ2 complex and four times greater than Na⁺ with GluA2 + CNIH-3. A modified model of permeant channel block fully accounts for both the voltage- and time-dependent nature of Spm block. Estimates of block rate constants reveal that auxiliary subunits do not attenuate block by shifting the location of the block site within the membrane electric field, and they do not affect the blocker’s ability to reach it. Instead, γ2 and CNIH-3 relieve channel block by facilitating the blocker’s exit rates from the open channel. From a physiological perspective, the relief of channel block exerted by γ2 and CNIH-3 ensures that there is unfettered signaling by AMPARs at glutamatergic synapses. Moreover, the pronounced ability of AMPARs to transport polyamines may have an unexpected role in regulating cellular polyamine levels.

Divergence of Ca(2+) selectivity and equilibrium Ca(2+) blockade in a Ca(2+) release-activated Ca(2+) channel

The Ca²⁺ selectivity of CRAC channels depends on the kinetics of ion entry and exit as well as the steady-state Ca²⁺ binding affinity.

Prevailing models postulate that high Ca²⁺ selectivity of Ca²⁺ release-activated Ca²⁺ (CRAC) channels arises from tight Ca²⁺ binding to a high affinity site within the pore, thereby blocking monovalent ion flux. Here, we examined the contribution of high affinity Ca²⁺ binding for Ca²⁺ selectivity in recombinant Orai3 channels, which function as highly Ca²⁺-selective channels when gated by the endoplasmic reticulum Ca²⁺ sensor STIM1 or as poorly Ca²⁺-selective channels when activated by the small molecule 2-aminoethoxydiphenyl borate (2-APB). Extracellular Ca²⁺ blocked Na⁺ currents in both gating modes with a similar inhibition constant (K_i; ∼25 µM). Thus, equilibrium binding as set by the K_i of Ca²⁺ blockade cannot explain the differing Ca²⁺ selectivity of the two gating modes. Unlike STIM1-gated channels, Ca²⁺ blockade in 2-APB–gated channels depended on the extracellular Na⁺ concentration and exhibited an anomalously steep voltage dependence, consistent with enhanced Na⁺ pore occupancy. Moreover, the second-order rate constants of Ca²⁺ blockade were eightfold faster in 2-APB–gated channels than in STIM1-gated channels. A four-barrier, three–binding site Eyring model indicated that lowering the entry and exit energy barriers for Ca²⁺ and Na⁺ to simulate the faster rate constants of 2-APB–gated channels qualitatively reproduces their low Ca²⁺ selectivity, suggesting that ion entry and exit rates strongly affect Ca²⁺ selectivity. Noise analysis indicated that the unitary Na⁺ conductance of 2-APB–gated channels is fourfold larger than that of STIM1-gated channels, but both modes of gating show a high open probability (P_o; ∼0.7). The increase in current noise during channel activation was consistent with stepwise recruitment of closed channels to a high P_o state in both cases, suggesting that the underlying gating mechanisms are operationally similar in the two gating modes. These results suggest that both high affinity Ca²⁺ binding and kinetic factors contribute to high Ca²⁺ selectivity in CRAC channels.

Polyamines control of cation transport across plant membranes: implications for ion homeostasis and abiotic stress signaling

Polyamines are unique polycationic metabolites, controlling a variety of vital functions in plants, including growth and stress responses. Over the last two decades a bulk of data was accumulated providing explicit evidence that polyamines play an essential role in regulating plant membrane transport. The most straightforward example is a blockage of the two major vacuolar cation channels, namely slow (SV) and fast (FV) activating ones, by the micromolar concentrations of polyamines. This effect is direct and fully reversible, with a potency descending in a sequence Spm(4+) > Spd(3+) > Put(2+). On the contrary, effects of polyamines on the plasma membrane (PM) cation and K(+)-selective channels are hardly dependent on polyamine species, display a relatively low affinity, and are likely to be indirect. Polyamines also affect vacuolar and PM H(+) pumps and Ca(2+) pump of the PM. On the other hand, catabolization of polyamines generates H2O2 and other reactive oxygen species (ROS), including hydroxyl radicals. Export of polyamines to the apoplast and their oxidation there by available amine oxidases results in the induction of a novel ion conductance and confers Ca(2+) influx across the PM. This mechanism, initially established for plant responses to pathogen attack (including a hypersensitive response), has been recently shown to mediate plant responses to a variety of abiotic stresses. In this review we summarize the effects of polyamines and their catabolites on cation transport in plants and discuss the implications of these effects for ion homeostasis, signaling, and plant adaptive responses to environment.

Interactions of natural polyamines with mammalian proteins

The ubiquitously expressed natural polyamines putrescine, spermidine, and spermine are small, flexible cationic compounds that exert pleiotropic actions on various regulatory systems and, accordingly, are essentially involved in diverse life functions. These roles of polyamines result from their capability to interact with negatively charged regions of all major classes of biomolecules, which might act in response by changing their structures and functions. The present review deals with polyamine-protein interactions, thereby focusing on mammalian proteins. We discuss the various modes in which polyamines can interact with proteins, describe major types of affected functions illustrated by representative examples of involved proteins, and support information with respective structural evidence from elucidated three-dimensional structures. A specific focus is put on polyamine interactions at protein surfaces that can modulate the aggregation of proteins to organized structural networks as well as to toxic aggregates and, moreover, can play a role in important transient protein-protein interactions.

Voltage profile along the permeation pathway of an open channel

For ion channels, the transmembrane potential plays a critical role by acting as a driving force for permeant ions. At the microscopic level, the transmembrane potential is thought to decay nonlinearly across the ion permeation pathway because of the irregular three-dimensional shape of the channel's pore. By taking advantage of the current structural and functional understanding of cyclic nucleotide-gated channels, in this study we experimentally explore the transmembrane potential's distribution across the open pore. As a readout for the voltage drop, we engineered cysteine residues along the selectivity filter and scanned the sensitivity of their modification rates by Ag(+) to the transmembrane potential. The experimental data, which indicate that the majority of the electric field drops across the selectivity filter, are in good agreement with continuum electrostatic calculations using a homology model of an open CNG channel. By focusing the transmembrane potential across the selectivity filter, the electromotive driving force is coupled with the movement of permeant ions in the filter, maximizing the efficiency of this process.

Intrinsic versus extrinsic voltage sensitivity of blocker interaction with an ion channel pore

Many physiological and synthetic agents act by occluding the ion conduction pore of ion channels. A hallmark of charged blockers is that their apparent affinity for the pore usually varies with membrane voltage. Two models have been proposed to explain this voltage sensitivity. One model assumes that the charged blocker itself directly senses the transmembrane electric field, i.e., that blocker binding is intrinsically voltage dependent. In the alternative model, the blocker does not directly interact with the electric field; instead, blocker binding acquires voltage dependence solely through the concurrent movement of permeant ions across the field. This latter model may better explain voltage dependence of channel block by large organic compounds that are too bulky to fit into the narrow (usually ion-selective) part of the pore where the electric field is steep. To date, no systematic investigation has been performed to distinguish between these voltage-dependent mechanisms of channel block. The most fundamental characteristic of the extrinsic mechanism, i.e., that block can be rendered voltage independent, remains to be established and formally analyzed for the case of organic blockers. Here, we observe that the voltage dependence of block of a cyclic nucleotide-gated channel by a series of intracellular quaternary ammonium blockers, which are too bulky to traverse the narrow ion selectivity filter, gradually vanishes with extreme depolarization, a predicted feature of the extrinsic voltage dependence model. In contrast, the voltage dependence of block by an amine blocker, which has a smaller "diameter" and can therefore penetrate into the selectivity filter, follows a Boltzmann function, a predicted feature of the intrinsic voltage dependence model. Additionally, a blocker generates (at least) two blocked states, which, if related serially, may preclude meaningful application of a commonly used approach for investigating channel gating, namely, inferring the properties of the activation gate from the kinetics of channel block.

Mutations reveal voltage gating of CNGA1 channels in saturating cGMP

Activity of cyclic nucleotide-gated (CNG) cation channels underlies signal transduction in vertebrate visual receptors. These highly specialized receptor channels open when they bind cyclic GMP (cGMP). Here, we find that certain mutations restricted to the region around the ion selectivity filter render the channels essentially fully voltage gated, in such a manner that the channels remain mostly closed at physiological voltages, even in the presence of saturating concentrations of cGMP. This voltage-dependent gating resembles the selectivity filter-based mechanism seen in KcsA K(+) channels, not the S4-based mechanism of voltage-gated K(+) channels. Mutations that render CNG channels gated by voltage loosen the attachment of the selectivity filter to its surrounding structure, thereby shifting the channel's gating equilibrium toward closed conformations. Significant pore opening in mutant channels occurs only when positive voltages drive the pore from a low-probability open conformation toward a second open conformation to increase the channels' open probability. Thus, the structure surrounding the selectivity filter has evolved to (nearly completely) suppress the expression of inherent voltage-dependent gating of CNGA1, ensuring that the binding of cGMP by itself suffices to open the channels at physiological voltages.

Amphiphilic blockers punch through a mutant CLC-0 pore

Intracellularly applied amphiphilic molecules, such as p-chlorophenoxy acetate (CPA) and octanoate, block various pore-open mutants of CLC-0. The voltage-dependent block of a particular pore-open mutant, E166G, was found to be multiphasic. In symmetrical 140 mM Cl(-), the apparent affinity of the blocker in this mutant increased with a negative membrane potential but, paradoxically, decreased when the negative membrane potential was greater than -80 mV, a phenomenon similar to the blocker "punch-through" shown in many blocker studies of cation channels. To provide further evidence of the punch-through of CPA and octanoate, we studied the dissociation rate of the blocker from the pore by measuring the time constant of relief from the block under various voltage and ionic conditions. Consistent with the voltage dependence of the effect on the steady-state current, the rate of CPA dissociation from the E166G pore reached a minimum at -80 mV in symmetrical 140 mM Cl(-), and the direction of current recovery suggested that the bound CPA in the pore can dissociate into both intracellular and extracellular solutions. Moreover, the CPA dissociation depends upon the Cl(-) reversal potential with a minimal dissociation rate at a voltage 80 mV more negative than the Cl(-) reversal potential. That the shift of the CPA-dissociation rate follows the Cl(-) gradient across the membrane argues that these blockers can indeed punch through the channel pore. Furthermore, a minimal CPA-dissociation rate at a voltage 80 mV more negative than the Cl(-) reversal potential suggests that the outward blocker movement through the CLC-0 pore is more difficult than the inward movement.

Orai1 mutations alter ion permeation and Ca2+-dependent fast inactivation of CRAC channels: evidence for coupling of permeation and gating

Ca(2+) entry through store-operated Ca(2+) release-activated Ca(2+) (CRAC) channels is an essential trigger for lymphocyte activation and proliferation. The recent identification of Orai1 as a key CRAC channel pore subunit paves the way for understanding the molecular basis of Ca(2+) selectivity, ion permeation, and regulation of CRAC channels. Previous Orai1 mutagenesis studies have indicated that a set of conserved acidic amino acids in trans membrane domains I and III and in the I-II loop (E106, E190, D110, D112, D114) are essential for the CRAC channel's high Ca(2+) selectivity. To further dissect the contribution of Orai1 domains important for ion permeation and channel gating, we examined the role of these conserved acidic residues on pore geometry, properties of Ca(2+) block, and channel regulation by Ca(2+). We find that alteration of the acidic residues lowers Ca(2+) selectivity and results in striking increases in Cs(+) permeation. This is likely the result of enlargement of the unusually narrow pore of the CRAC channel, thus relieving steric hindrance for Cs(+) permeation. Ca(2+) binding to the selectivity filter appears to be primarily affected by changes in the apparent on-rate, consistent with a rate-limiting barrier for Ca(2+) binding. Unexpectedly, the mutations diminish Ca(2+)-mediated fast inactivation, a key mode of CRAC channel regulation. The decrease in fast inactivation in the mutant channels correlates with the decrease in Ca(2+) selectivity, increase in Cs(+) permeability, and enlargement of the pore. We propose that the structural elements involved in ion permeation overlap with those involved in the gating of CRAC channels.

Effect of transjunctional KCl gradients on the spermine inhibition of connexin40 gap junctions

Spermine inhibits rat connexin40 (Cx40) gap junctions. Glutamate residues at positions 9 and 13 and a basic amino acid (HKH) motif at positions 15-17 on the amino terminal domain are essential for this inhibitory activity. Questions remain as to whether spermine occludes the channel within the ion permeation pathway. To examine this question, cis or trans [KCl] was systematically lowered and the equilibrium dissociation constants (K(d)) and kinetics of unilateral spermine block on wild-type Cx40 gap junctions were determined. Asymmetric reductions in the trans [KCl] produced noticeable asymmetric shifts in the V(1/2) and G(min) values that progressively resembled G(j)-V(j) relationships observed in heterotypic connexin gap junction combinations. As cis or trans [KCl] was reduced by 25%, 50%, or 75% relative to the spermine-containing side, the transjunctional voltage (V(j))-dependent K(d) values increased or decreased, respectively. The spermine on-rates and off-rates, calculated from the junctional current decay and recovery time constants, were similarly affected. Hill coefficients for the spermine dose-response curves were approximately 0.58, indicative of negative cooperativity and possible multiple spermine inhibitory sites. The equivalent "electrical distance" (delta) ranged from 0.61 at 25% cis [KCl] to 1.4 at 25% trans [KCl], with a Hill coefficient of 1.0. Symmetrical reductions in [KCl] resulted in intermediate decreases in the spermine K(d)s, indicative of a minor electrostatic effect and a more significant effect of the transjunctional KCl electrodiffusion potential on the spermine association and dissociation rates. These data are consistent with a single spermine molecule being sufficient to occlude the Cx40 gap junction channel within the KCl permeation pathway.

Vibrational spectroscopy studies on linear polyamines

Vibrational spectroscopy [both Raman and INS (inelastic neutron scattering)], coupled to quantum mechanical calculations, was used in order to perform a thorough structural analysis of linear polyamines and polynuclear polyamine metal chelates [e.g. with Pt(II) and Pd(II)] with potential anticancer activity. The complementarity of the Raman and INS spectroscopies was exploited in order to gain a better knowledge of the conformational behaviour of these systems. Moreover, the conjugation of the experimental spectroscopic data to the theoretical results allows us to obtain valuable information on the structural preferences of this kind of system, which may lead to the establishment of SARs (structure–activity relationships) ruling their biological activity. Some of the most significant results obtained by the ‘Molecular Physical-Chemistry’ Research Group of the University of Coimbra (Portugal) are reviewed here.

Regulation of CRAC channel activity by recruitment of silent channels to a high open-probability gating mode

CRAC (calcium release-activated Ca²⁺) channels attain an extremely high selectivity for Ca²⁺ from the blockade of monovalent cation permeation by Ca²⁺ within the pore. In this study we have exploited the blockade by Ca²⁺ to examine the size of the CRAC channel pore, its unitary conductance for monovalent cations, and channel gating properties. The permeation of a series of methylammonium compounds under divalent cation-free conditions indicates a minimum pore diameter of 3.9 Å. Extracellular Ca²⁺ blocks monovalent flux in a manner consistent with a single intrapore site having an effective K_i of 20 μM at −110 mV. Block increases with hyperpolarization, but declines below −100 mV, most likely due to permeation of Ca²⁺. Analysis of monovalent current noise induced by increasing levels of block by extracellular Ca²⁺ indicates an open probability (P_o) of ∼0.8. By extrapolating the variance/mean current ratio to the condition of full blockade (P_o = 0), we estimate a unitary conductance of ∼0.7 pS for Na⁺, or three to fourfold higher than previous estimates. Removal of extracellular Ca²⁺ causes the monovalent current to decline over tens of seconds, a process termed depotentiation. The declining current appears to result from a reduction in the number of active channels without a change in their high open probability. Similarly, low concentrations of 2-APB that enhance I_CRAC increase the number of active channels while open probability remains constant. We conclude that the slow regulation of whole-cell CRAC current by store depletion, extracellular Ca²⁺, and 2-APB involves the stepwise recruitment of silent channels to a high open-probability gating mode.

Transverse Acoustic Modes of Biogenic and α,ω-Polyamines: A Study by Inelastic Neutron Scattering and Raman Spectroscopies Coupled to DFT Calculations

A complete analysis of the transverse acoustic modes (TAMs) for the homologous series of alpha,omega-diamines (H2N(CH2)nNH2) (n = 2-10, n = 12) as well as for the biogenic polyamines spermidine and spermine was undertaken, by Raman and inelastic neutron scattering (INS) spectroscopies combined with density functional theory (DFT) calculations. A complete assignment of the whole set of TAMs was carried out, for both the undeuterated and N-deuterated species. 1,2-Diaminoethane was found to display exceptional behavior, probably due to the formation of dimers in the solid state. An n-even/n-odd dependence of the low frequency INS pattern was observed for these polyamines. The very good accordance between their INS experimental TAMs and the ones previously reported for the corresponding n-alkanes suggest a close conformational similarity between these systems.

Ring of negative charge in BK channels facilitates block by intracellular Mg2+ and polyamines through electrostatics

Intracellular Mg2+ and natural polyamines block outward currents in BK channels in a highly voltage-dependent manner. Here we investigate the contribution of the ring of eight negatively charged residues (4 x E321/E324) at the entrance to the inner vestibule of BK channels to this block. Channels with or without (E321N/E324N) the ring of negative charge were expressed in oocytes and unitary currents were recorded from inside-out patches over a range of intracellular Mg2+ and polyamine concentrations. Removing the ring of charge greatly decreased the block, increasing K(B)(ap) (0 mV) for Mg2+ block from 48.3 +/- 3.0 to 143 +/- 8 mM, and for spermine block from 8.0 +/- 1.0 to 721 +/- 9 mM (150 mM symmetrical KCl). Polyamines with fewer amine groups blocked less: putrescine < spermidine < spermine. An equation that combined an empirical Hill function for block together with a Boltzmann function for the voltage dependence of K(B)(ap) described the voltage and concentration dependence of the block for channels with and without the ring of charge. The Hill coefficients for these descriptions were <1 for both Mg2+ and spermine block, and were unchanged by removing the ring of charge. When KCl(i) was increased from 150 mM to 3 M, the ring of charge no longer facilitated block, Mg2+ block was reduced, spermine block became negligible, and the Hill coefficients became approximately 1.0. BK channels in cell-attached oocyte patches displayed inward rectification, which was reduced for channels without the ring of charge. Taken together, these observations suggest that the ring of negative charge facilitates block through a preferential electrostatic attraction of Mg2+ and polyamine over K+. This preferential attraction of multivalent blockers over monovalent K+ would decrease the K+ available at the inner vestibule to carry outward current in the presence of Mg2+ or polyamines, while increasing the concentration of blocker available to enter and block the conduction pathway.

The polyamine binding site in inward rectifier K+ channels

Strongly inwardly rectifying potassium channels exhibit potent and steeply voltage-dependent block by intracellular polyamines. To locate the polyamine binding site, we have examined the effects of polyamine blockade on the rate of MTSEA modification of cysteine residues strategically substituted in the pore of a strongly rectifying Kir channel (Kir6.2[N160D]). Spermine only protected cysteines substituted at a deep location in the pore, between the “rectification controller” residue (N160D in Kir6.2, D172 in Kir2.1) and the selectivity filter, against MTSEA modification. In contrast, blockade with a longer synthetic polyamine (CGC-11179) also protected cysteines substituted at sites closer to the cytoplasmic entrance of the channel. Modification of a cysteine at the entrance to the inner cavity (169C) was unaffected by either spermine or CGC-11179, and spermine was clearly “locked” into the inner cavity (i.e., exhibited a dramatically slower exit rate) following modification of this residue. These data provide physical constraints on the spermine binding site, demonstrating that spermine stably binds at a deep site beyond the “rectification controller” residue, near the extracellular entrance to the channel.

Mutation of the pore glutamate affects both cytoplasmic and external dequalinium block in the rat olfactory CNGA2 channel

Dequalinium has recently been reported to block CNGA1 and CNGA2 channels expressed in Xenopus laevis. Using the inside-out configuration of the patch-clamp technique, we examined the effects of dequalinium on rat olfactory CNGA2 channels expressed in human embryonic kidney (HEK293) cells and studied aspects of its molecular mechanism of action. We found that cytoplasmic dequalinium blocked wild-type (WT) CNGA2 channels in a voltage-dependent manner with an IC(50) of approximately 1.3 muM at a V(m) of + 60 mV, and an effective fractional charge, zdelta, of +0.8 (z=2, delta=+0.4), suggesting that cytoplasmic dequalinium interacts with a binding site that is about two fifths of the way along the membrane electric field (from the intracellular side). Neutralizing the negatively charged pore lining glutamate acid residue (E342Q) still allows effective channel block by cytoplasmic dequalinium with an IC(50) of approximately 2.2 muM at a V(m) of +60 mV but now having a zdelta of +0.1 (delta=+0.05), indicating a profoundly decreased level of voltage-dependence. In addition, by comparing the extent of block under different levels of channel activation, we show that the block by cytoplasmic dequalinium displayed clear state-dependence in WT channels by interacting predominantly with the closed channel, whereas the block in E342Q channels was state-independent. Application of dequalinium to the external membrane surface also blocked currents through WT channels and the E342Q mutation significantly increased the IC(50) for external block approximately fivefold. These results confirm dequalinium as a potent, voltage-dependent and state-dependent blocker of cyclic-nucleotide-gated channels, and show that neutralization of the E342 residue profoundly affects the block by both cytoplasmic and external application of dequalinium.

Revisiting voltage-dependent relief of block in ion channels: a mechanism independent of punchthrough

Voltage dependence of block was investigated in a simple model for permeation in a multiion pore. Internal blocker could bind to three states of the open channel that differed in the locations and number of permeant ion bound; blocker dissociation occurs exclusively to the internal solution, and the blocker does not itself enter the electric field. By changing the relative stability of blocker binding to these three states, block displayed voltage dependence with relief of block at high potentials. Similar patterns of block could also be generated in more detailed models of ion permeation. These results illustrate that the observation of relief of block at high potentials is not a sufficient criterion for establishing that a blocker is permeant in a channel that has a complex permeation cycle.

Spectroscopic and Theoretical Studies on Solid 1,2-Ethylenediamine Dihydrochloride Salt

A structural study of [H3N(CH2)2NH3)]2+.2Cl-, the smallest element of the homologous series of the alpha,omega-diamine dihydrochlorides, was carried out by means of Raman and FTIR spectroscopy coupled to ab initio molecular orbital (MO) calculations. As a primary concern, an adequate molecular model for the representation of these solid amine salts was chosen. Thus, several models, varying in the number and position of the counterions as well as in the number of diamine units, were considered. It was found that the best molecular system (i.e., that yielding the best compromise between accuracy and computational requirements) consists of one ethylenediamine cation surrounded by six chloride ions in an arrangement based on the crystal structure reported in the literature for [H3N(CH2)2NH3)]2+.2Cl-. This conclusion will hopefully allow for a better understanding of the conformational preferences, in the solid state, of these biologically relevant linear polyamines.

Effects of polyamines on the muscarinic receptor-operated cation current in guinea-pig ileal smooth muscle myocytes

The effects of extracellular and intracellular polyamines (PAs), spermine and putrescine, on the cation current (mI(CAT)) evoked either by activating muscarinic receptors with carbachol or by intracellularly applied GTPgammaS (in the absence of carbachol) were studied using patch-clamp recording techniques in single guinea-pig ileal myocytes. Extracellular spermine and putrescine rapidly and reversibly inhibited mI(CAT) in a concentration- and voltage-dependent manner with the IC(50) values at -40 mV of about 1 and 5 mM, respectively. Membrane depolarization relieved the blocking action of PAs although cation conductance activation curve remained N-shaped. The inhibition was similar for both carbachol- and GTPgammaS-evoked currents, suggesting that the cation channel rather than the muscarinic receptor was the primary site of the PA action. In outside-out membrane patches, both cation channel unitary conductance and open probability were reduced. In perforated-patch experiments used to retain cytoplasmic PAs sustained 100 microM carbachol-induced mI(CAT) was significantly smaller (478 +/- 76 pA, n = 7) compared to that recorded using conventional whole-cell configuration with nominally PA-free pipette solution (1314 +/- 76 pA, n = 12), but comparable in size to mI(CAT) with 0.3 mM spermine in the pipette solution (509 +/- 41 pA, n = 19). Intracellular putrescine inhibited mI(CAT) less potently compared to spermine. In conclusion, these results show a novel role of intestinal PAs in mI(CAT) inhibition, which can contribute to their well-known suppressing effect on the gastrointestinal smooth muscle excitability and contractility.

Potassium-dependent slow inactivation of Kir1.1 (ROMK) channels

The ROMK (Kir1.1) family of epithelial K channels can be inactivated by a combination of low internal pH and low external K, such that alkalization does not reopen the channels unless external K is elevated. Previous work suggested that this inactivation results from an allosteric interaction between an inner pH gate and an outer K sensor, and could be described by a simple three-state kinetic model. In the present study, we report that a sustained depolarization slowly inactivated (half-time = 10-15 min) ROMK channels that had been engineered for increased affinity to internal polyamines. Furthermore, this inactivation occurred at external [K] < or =1 mM in ROMK mutants whose inner pH gate was constitutively open (ROMK2-K61M mutation). Both pH and voltage inactivation depended on external K in a manner reminiscent of C-type inactivation, but having a much slower time course. Replacement of ROMK extracellular loop residues by Kir2.1 homologous residues attenuated or abolished this inactivation. These results are consistent with the hypothesis that there are (at least) two separate closure processes in these channels: an inner pH-regulated gate, and an outer (inactivation) gate, where the latter is modulated by both voltage and external [K].

Interaction mechanisms between polyamines and IRK1 inward rectifier K+ channels

Rectification of macroscopic current through inward-rectifier K⁺ (Kir) channels reflects strong voltage dependence of channel block by intracellular cations such as polyamines. The voltage dependence results primarily from the movement of K⁺ ions across the transmembrane electric field, which accompanies the binding–unbinding of a blocker. Residues D172, E224, and E299 in IRK1 are critical for high-affinity binding of blockers. D172 appears to be located somewhat internal to the narrow K⁺ selectivity filter, whereas E224 and E299 form a ring at a more intracellular site. Using a series of alkyl-bis-amines of varying length as calibration, we investigated how the acidic residues in IRK1 interact with amine groups in the natural polyamines (putrescine, spermidine, and spermine) that cause rectification in cells. To block the pore, the leading amine of bis-amines of increasing length penetrates ever deeper into the pore toward D172, while the trailing amine in every bis-amine binds near a more intracellular site and interacts with E224 and E299. The leading amine in nonamethylene-bis-amine (bis-C9) makes the closest approach to D172, displacing the maximal number of K⁺ ions and exhibiting the strongest voltage dependence. Cells do not synthesize bis-amines longer than putrescine (bis-C4) but generate the polyamines spermidine and spermine by attaching an amino-propyl group to one or both ends of putrescine. Voltage dependence of channel block by the tetra-amine spermine is comparable to that of block by the bis-amines bis-C9 (shorter) or bis-C12 (equally long), but spermine binds to IRK1 with much higher affinity than either bis-amine does. Thus, counterintuitively, the multiple amines in spermine primarily confer the high affinity but not the strong voltage dependence of channel block. Tetravalent spermine achieves a stronger interaction with the pore by effectively behaving like a pair of tethered divalent cations, two amine groups in its leading half interacting primarily with D172, whereas the other two in the trailing half interact primarily with E224 and E299. Thus, nature has optimized not only the blocker but also, in a complementary manner, the channel for producing rapid, high-affinity, and strongly voltage-dependent channel block, giving rise to exceedingly sharp rectification.

Amino terminal glutamate residues confer spermine sensitivity and affect voltage gating and channel conductance of rat connexin40 gap junctions

Connexin40 (Cx40) contains a specific binding site for spermine (affinity approximately 100 microm) whereas connexin43 (Cx43) is unaffected by identical concentrations of intracellular spermine. Replacement of two unique glutamate residues, E9 and E13, from the cytoplasmic amino terminal domain of Cx40 with the corresponding lysine residues from Cx43 eliminated the block by 2 mm spermine, reduced the transjunctional voltage (V(j)) gating sensitivity, and reduced the unitary conductance of this Cx40E9,13K gap junction channel protein. The single point mutations, Cx40E9K and Cx40E13K, predominantly affected the residual conductance state (G(min)) and V(j) gating properties, respectively. Heterotypic pairing of Cx40E9,13K with wild-type Cx40 in murine neuro2A (N2A) cells produced a strongly rectifying gap junction reminiscent of the inward rectification properties of the Kir (e.g. Kir2.x) family of potassium channels. The reciprocal Cx43K9,13E mutant protein exhibited reduced V(j) sensitivity, but displayed much less rectification in heterotypic pairings with wtCx43, negligible changes in the unitary channel conductance, and remained insensitive to spermine block. These data indicate that the connexin40 amino terminus may form a critical cytoplasmic pore-forming domain that serves as the receptor for V(j)-dependent closure and block by intracellular polyamines. Functional reciprocity between Cx40 and Cx43 gap junctions involves other amino acid residues in addition to the E or K 9 and 13 loci located on the amino terminal domain of these two connexins.

Mechanism of Rectification in Inward-Rectifier K+ Channels

Inward rectifiers are a class of K+ channels that can conduct much larger inward currents at membrane voltages negative to the K+ equilibrium potential than outward currents at voltages positive to it, even when K+ concentrations on both sides of the membrane are made equal. This conduction property, called inward rectification, enables inward rectifiers to perform many important physiological tasks. Rectification is not an inherent property of the channel protein itself, but reflects strong voltage dependence of channel block by intracellular cations such as Mg2+ and polyamines. This voltage dependence results primarily from the movement of K+ ions across the transmembrane electric field along the pore, which is energetically coupled to the blocker binding and unbinding. This mutual displacement mechanism between several K+ ions and a blocker explains the signature feature of inward rectifier K+ channels, namely, that at a given concentration of intracellular K+, their macroscopic conductance depends on the difference between membrane voltage and the K+ equilibrium potential rather than on membrane voltage itself.

Polyvalent cations as permeant probes of MIC and TRPM7 pores

Recent studies in Jurkat T cells and in rat basophilic leukemia cells revealed an Mg(2+)-inhibited cation (MIC) channel that has electrophysiological properties similar to TRPM7 Eyring rate model expressed exogenously in mammalian cells. Here we compare the characteristics of several polyvalent cations and Mg(2+) to block monovalent MIC current from the outside. Putrescine, spermidine, spermine, PhTX-343 (a derivative of the naturally occurring polyamine toxin philanthotoxin), and Mg(2+) each blocked in a dose- and voltage-dependent manner, indicating a blocking site within the electric field of the ion channel. Spermine and the relatively bulky PhTX-343 exhibited voltage dependence steeper than that expected for the number of charges on the molecule. Polyamines and Mg(2+) are permeant blockers, as judged by relief of block at strongly negative membrane potentials. Intracellular dialysis with spermine (300 microM) had no effect, indicating an asymmetrical pore. At the single-channel level, spermine and Mg(2+) induced flickery block of 40-pS single channels. I/V characteristics and polyamine block are similar in expressed TRPM7 and in native MIC currents, consistent with the conclusion that native MIC channels are composed of TRPM7 subunits. An Eyring rate model is developed to account for I/V characteristics and block of MIC channels by polyvalent cations from the outside.

Voltage-dependent blockade of connexin40 gap junctions by spermine

The effects of spermine and spermidine, endogenous polyamines that block many forms of ion channels, were investigated in homotypic connexin (Cx)-40 gap junctions expressed in N2A cells. Spermine blocked up to 95% of I(j) through homotypic Cx40 gap junctions in a concentration- and transjunctional voltage (V(j))-dependent manner. V(j) was varied from 5 to 50 mV in 5-mV steps and the dissociation constants (K(m)) were determined from spermine concentrations ranging from 10 micro M to 2 mM. The K(m) values ranged from 4.9 mM to 107 micro M for 8.6 < or = V(j) < or = 37.7 mV, within the physiological range of intracellular spermine for V(j) > or = 20 mV. The K(m) values for spermidine were > or = 5 mM. Estimates of the electrical distance (delta) for spermine (z = +4) and spermidine (z = +3) were 0.96 and 0.76 respectively. Cx40 single channel conductance was 129 pS in the presence of 2-mM spermine and channel open probability was significantly reduced in a V(j)-dependent manner. Similar concentrations of spermine did not block I(j) through homotypic Cx43 gap junctions, indicating that spermine selectively blocks Cx40 gap junctions. This is contrary to our previous findings that large tetraalkylammonium ions, also known to block several forms of ion channels, block junctional currents (I(j)) through homotypic connexin Cx40 and Cx43 gap junctions.

Block of the ryanodine receptor channel by neomycin is relieved at high holding potentials

In this study we have investigated the actions of the aminoglycoside antibiotic neomycin on K+ conductance in the purified sheep cardiac sarcoplasmic reticulum (SR) calcium-release channel (RyR). Neomycin induces a concentration- and voltage-dependent partial block from both the cytosolic and luminal faces of the channel. Blocking parameters for cytosolic and luminal block are markedly different. Neomycin has a greater affinity for the luminal site of interaction than the cytosolic site: zero-voltage dissociation constants (Kb(0)) are respectively 210.20 +/- 22.80 and 589.70 +/- 184.00 nM for luminal and cytosolic block. However, neomycin also exhibits voltage-dependent relief of block at holding potentials >+60 mV when applied to the cytosolic face and a similar phenomenon may occur with luminal neomycin at high negative holding potentials. These observations indicate that, under appropriate conditions, neomycin is capable of passing through the RyR channel.

Structural similarities between glutamate receptor channels and K(+) channels examined by scanning mutagenesis

The pores of glutamate receptors and K(+) channels share sequence homology, suggesting a conserved secondary structure. Scanning mutagenesis with substitution of alanine and tryptophan in GluR6 channels was performed based on the structure of KcsA. Our assay used disruption of voltage-dependent polyamine block to test for changes in the packing of pore-forming regions. Alanine scanning from D567 to R603 revealed reduced rectification resulting from channel block in two regions. A periodic pattern from F575 to M589 aligned with the pore helix in KcsA, whereas a cluster of sensitive positions around Q590, a site regulated by RNA editing, mapped to the selectivity filter in KcsA. Tryptophan scanning from D567 to R603 revealed similar patterns, but with a complete disruption of spermine block for 7 out of the 37 positions and a pM dissociation constant for Q590W. Molecular modeling with KcsA coordinates showed that GluR6 pore helix mutants disrupting polyamine block pack against M1 and M2, and are not exposed in the ion channel pore. In the selectivity filter, tryptophan creates an aromatic cage consistent with the pM dissociation constant for Q590W. A scan with glutamate substitution was used to map the cytoplasmic entrance to the pore based on charge neutralization experiments, which established that E594 was uniquely required for high affinity polyamine block. In E594Q mutants, introduction of glutamate at positions S593-L600 restored polyamine block at positions corresponding to surface-exposed residues in KcsA. Our results reinforce proposals that the pore region of glutamate receptors contains a helix and pore loop analogous to that found in K(+) channels. At the cytoplasmic entrance of the channel, a negatively charged amino acid, located in an extended loop with solvent-exposed side chains, is required for high affinity polyamine block and probably attracts cations via a through space electrostatic mechanism.

A pore-lining glutamic acid in the rat olfactory cyclic nucleotide-gated channel controls external spermine block

Spermine is a potent, voltage-dependent blocker of the olfactory cyclic nucleotide-gated channel from both the intracellular and extracellular sides. However, its sites of action are unknown. This study investigated the external spermine binding site in the rat CNCalpha3 subunit. Neutralization of a glutamic acid residue (E342Q) in the P-loop region eliminated voltage-dependence of block by externally applied spermine. The charge-conservative E342D mutation had little effect on spermine block. Thus, E342 forms the binding site for externally applied spermine. However, spermine remained a potent voltage-independent blocker of the E342Q mutant channel, suggesting that the mutation either created a novel binding site outside the membrane electrical field or that it dramatically changed the properties of the existing pore site.

Pore block versus intrinsic gating in the mechanism of inward rectification in strongly rectifying IRK1 channels

The IRK1 channel is inhibited by intracellular cations such as Mg(2+) and polyamines in a voltage-dependent manner, which renders its I-V curve strongly inwardly rectifying. However, even in excised patches exhaustively perfused with a commonly used artificial intracellular solution nominally free of Mg(2+) and polyamines, the macroscopic I-V curve of the channels displays modest rectification. This observation forms the basis of a hypothesis, alternative to the pore-blocking hypothesis, that inward rectification reflects the enhancement of intrinsic channel gating by intracellular cations. We find, however, that residual rectification is caused primarily by the commonly used pH buffer HEPES and/or some accompanying impurity. Therefore, inward rectification in the strong rectifier IRK1, as in the weak rectifier ROMK1, can be accounted for by voltage-dependent block of its ion conduction pore by intracellular cations.

Mechanism of IRK1 channel block by intracellular polyamines

Intracellular polyamines inhibit the strongly rectifying IRK1 potassium channel by a mechanism different from that of a typical ionic pore blocker such as tetraethylammonium. As in other K(+) channels, in the presence of intracellular TEA, the IRK1 channel current decreases with increasing membrane voltage and eventually approaches zero. However, in the presence of intracellular polyamines, the channel current varies with membrane voltage in a complex manner: when membrane voltage is increased, the current decreases in two phases separated by a hump. Furthermore, contrary to the expectation for a nonpermeant ionic pore blocker, a significant residual IRK1 current persists at very positive membrane voltages; the amplitude of the residual current decreases with increasing polyamine concentration. This complex blocking behavior of polyamines can be accounted for by a minimal model whereby intracellular polyamines inhibit the IRK1 channel by inducing two blocked channel states. In each of the blocked states, a polyamine is bound with characteristic affinity and probability of traversing the pore. The proposal that polyamines traverse the pore at finite rates is supported by the observation that philanthotoxin-343 (spermine with a bulky chemical group attached to one end) acts as a nonpermeant ionic blocker in the IRK1 channel.