A role for transient receptor potential vanilloid 4 in tonicity-induced neurogenic inflammation.

BACKGROUND AND PURPOSE: Changes in extracellular fluid osmolarity, which occur after tissue damage and disease, cause inflammation and maintain chronic inflammatory states by unknown mechanisms. Here, we investigated whether the osmosensitive channel, transient receptor potential vanilloid 4 (TRPV4), mediates inflammation to hypotonic stimuli by a neurogenic mechanism. EXPERIMENTAL APPROACH: TRPV4 was localized in dorsal root ganglia (DRG) by immunofluorescence. The effects of TRPV4 agonists on release of pro-inflammatory neuropeptides from peripheral tissues and on inflammation were examined. KEY RESULTS: Immunoreactive TRPV4 was detected in DRG neurones innervating the mouse hindpaw, where it was co-expressed in some neurones with CGRP and substance P, mediators of neurogenic inflammation. Hypotonic solutions and 4alpha-phorbol 12,13-didecanoate, which activate TRPV4, stimulated neuropeptide release in urinary bladder and airways, sites of neurogenic inflammation. Intraplantar injection of hypotonic solutions and 4alpha-phorbol 12,13-didecanoate caused oedema and granulocyte recruitment. These effects were inhibited by a desensitizing dose of the neurotoxin capsaicin, antagonists of CGRP and substance P receptors, and TRPV4 gene knockdown or deletion. In contrast, antagonism of neuropeptide receptors and disruption of TRPV4 did not prevent this oedema. TRPV4 gene knockdown or deletion also markedly reduced oedema and granulocyte infiltration induced by intraplantar injection of formalin. CONCLUSIONS AND IMPLICATIONS: Activation of TRPV4 stimulates neuropeptide release from afferent nerves and induces neurogenic inflammation. This mechanism may mediate the generation and maintenance of inflammation after injury and during diseases, in which there are changes in extracellular osmolarity. Antagonism of TRPV4 may offer a therapeutic approach for inflammatory hyperalgesia and chronic inflammation.

Protease-activated receptor-4: a novel mechanism of inflammatory pain modulation.

BACKGROUND AND PURPOSE: Protease-activated receptor-4 (PAR(4)), the most recently discovered member of the PARs family, is activated by thrombin, trypsin and cathepsin G, but can also be selectively activated by small synthetic peptides (PAR(4)-activating peptide, PAR(4)-AP). PAR(4) is considered a potent mediator of platelet activation and inflammation. As both PAR(1) and PAR(2) have been implicated in the modulation of nociceptive mechanisms, we investigated the expression of PAR(4) in sensory neurons and the effects of its selective activation on nociception. EXPERIMENTAL APPROACH AND KEY RESULTS: We demonstrated the expression of PAR(4) in sensory neurons isolated from rat dorsal root ganglia by reverse transcription-polymerase chain reaction and immunofluorescence. We found that PAR(4) colocalized with calcitonin gene-related peptide and substance P. We also showed that a selective PAR(4)-AP was able to inhibit calcium mobilization evoked by KCl and capsaicin in rat sensory neurons. Moreover, the intraplantar injection of a PAR(4)-AP significantly increased nociceptive threshold in response to thermal and mechanical noxious stimuli, while a PAR(4) inactive control peptide had no effect. The anti-nociceptive effects of the PAR(4)-AP were dose-dependent and occurred at doses below the threshold needed to cause inflammation. Finally, co-injection of the PAR(4)-AP with carrageenan significantly reduced the carrageenan-induced inflammatory hyperalgesia and allodynia, but had no effect on inflammatory parameters such as oedema and granulocyte infiltration. CONCLUSIONS AND IMPLICATIONS: Taken together, these results identified PAR(4) as a novel potential endogenous analgesic factor, which can modulate nociceptive responses in normal and inflammatory conditions.

N-type calcium channel blockers: novel therapeutics for the treatment of pain.

Highly selective Ca(v)2.2 voltage-gated calcium channel (VGCC) inhibitors have emerged as a new class of therapeutics for the treatment of chronic and neuropathic pain. Cone snail venoms provided the first drug in class with FDA approval granted in 2005 to Prialt (omega-conotoxin MVIIA, Elan) for the treatment of neuropathic pain. Since this pioneering work, major efforts underway to develop alternative small molecule inhibitors of Ca(v)2.2 calcium channel have met with varied success. This review focuses on the properties of the Ca(v)2.2 calcium channel in different pain states, the action of omega-conotoxins GVIA, MVIIA and CVID, describing their structure-activity relationships and potential as leads for the design of improved Ca(v)2.2 calcium channel therapeutics, and finally the development of small molecules for the treatment of chronic pain.

Determinants of the differential gating properties of Cav3.1 and Cav3.3 T-type channels: a role of domain IV?

We have investigated the channel structural determinants that underlie the difference in gating properties of Cav3.1 and Cav3.3 T-type channels, by creating a series of chimeric channel constructs in which the major transmembrane domains were swapped. The chimeras were then expressed in tsA-201 cells and subjected to whole cell patch clamp analysis. Our data reveal that domains I and IV are major determinants of the half-activation potential. Substitution of domain IV was the most important determinant of activation time constant and time constant for recovery from inactivation, with domains I and II mediating a smaller role. In contrast, the carboxy terminal region did not appear to be involved. Determinants of the time constant for inactivation could not be localized to a specific transmembrane domain, but the concomitant substitution of domains I+IV was able to partially confer the inactivation kinetics among the two wild type channels. Our data indicate that the domain IV region mediates an important role in T-type channel activation, whereas multiple channel structural determinants appear to control T-type channel inactivation.

Temperature dependence of T-type calcium channel gating.

T-type calcium channel isoforms are expressed in a multitude of tissues and have a key role in a variety of physiological processes. To fully appreciate the physiological role of distinct channel isoforms it is essential to determine their kinetic properties under physiologically relevant conditions. We therefore characterized the gating behavior of expressed rat voltage-dependent calcium channels (Ca(v)) 3.1, Ca(v)3.2, and Ca(v)3.3, as well as human Ca(v)3.3 at 21 degrees C and 37 degrees C in saline that approximates physiological conditions. Exposure to 37 degrees C caused significant increases in the rates of activation, inactivation, and recovery from inactivation, increased the current amplitudes, and induced a hyperpolarizing shift of half-activation for Ca(v)3.1 and Ca(v)3.2. At 37 degrees C the half-inactivation showed a hyperpolarizing shift for Ca(v)3.1 and Ca(v)3.2 and human Ca(v)3.3, but not rat Ca(v)3.3. The observed changes in the kinetics were significant but not identical for the three isoforms, showing that the ability of T-type channels to conduct calcium varies with both channel isoform and temperature.

The Drosophila cacts2 mutation reduces presynaptic Ca2+ entry and defines an important element in Cav2.1 channel inactivation.

Voltage-gated Ca2+ channels in nerve terminals open in response to action potentials and admit Ca2+, the trigger for neurotransmitter release. The cacophony gene encodes the primary presynaptic voltage-gated Ca2+ channel in Drosophila motor-nerve terminals. The cac(ts2) mutant allele of cacophony is associated with paralysis and reduced neurotransmission at non-permissive temperatures but the basis for the neurotransmission deficit has not been established. The cac(ts2) mutation occurs in the cytoplasmic carboxyl tail of the alpha1-subunit, not within the pore-forming trans-membrane domains, making it difficult to predict the mutation's impact. We applied a Ca2+-imaging technique at motor-nerve terminals of mutant larvae to test the hypothesis that the neurotransmission deficit is a result of impaired Ca2+ entry. Presynaptic Ca2+ signals evoked by single and multiple action potentials showed a temperature-dependent reduction. The amplitude of the reduction was sufficient to account for the neurotransmission deficit, indicating that the site of the cac(ts2) mutation plays a role in Ca2+ channel activity. As the mutation occurs in a motif conserved in mammalian high-voltage-activated Ca2+ channels, we used a heterologous expression system to probe the effect of this mutation on channel function. The mutation was introduced into rat Ca(v)2.1 channels expressed in human embryonic kidney cells. Patch-clamp analysis of mutant channels at the physiological temperature of 37 degrees C showed much faster inactivation rates than for wild-type channels, demonstrating that the integrity of this motif is critical for normal Ca(v)2.1 channel inactivation.

NMDA receptors mediate calcium accumulation in myelin during chemical ischaemia.

Central nervous system myelin is a specialized structure produced by oligodendrocytes that ensheaths axons, allowing rapid and efficient saltatory conduction of action potentials. Many disorders promote damage to and eventual loss of the myelin sheath, which often results in significant neurological morbidity. However, little is known about the fundamental mechanisms that initiate myelin damage, with the assumption being that its fate follows that of the parent oligodendrocyte. Here we show that NMDA (N-methyl-d-aspartate) glutamate receptors mediate Ca2+ accumulation in central myelin in response to chemical ischaemia in vitro. Using two-photon microscopy, we imaged fluorescence of the Ca2+ indicator X-rhod-1 loaded into oligodendrocytes and the cytoplasmic compartment of the myelin sheath in adult rat optic nerves. The AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)/kainate receptor antagonist NBQX completely blocked the ischaemic Ca2+ increase in oligodendroglial cell bodies, but only modestly reduced the Ca2+ increase in myelin. In contrast, the Ca2+ increase in myelin was abolished by broad-spectrum NMDA receptor antagonists (MK-801, 7-chlorokynurenic acid, d-AP5), but not by more selective blockers of NR2A and NR2B subunit-containing receptors (NVP-AAM077 and ifenprodil). In vitro ischaemia causes ultrastructural damage to both axon cylinders and myelin. NMDA receptor antagonism greatly reduced the damage to myelin. NR1, NR2 and NR3 subunits were detected in myelin by immunohistochemistry and immunoprecipitation, indicating that all necessary subunits are present for the formation of functional NMDA receptors. Our data show that the mature myelin sheath can respond independently to injurious stimuli. Given that axons are known to release glutamate, our finding that the Ca2+ increase was mediated in large part by activation of myelinic NMDA receptors suggests a new mechanism of axo-myelinic signalling. Such a mechanism may represent a potentially important therapeutic target in disorders in which demyelination is a prominent feature, such as multiple sclerosis, neurotrauma, infections (for example, HIV encephalomyelopathy) and aspects of ischaemic brain injury.

Protection of ischemic rat spinal cord white matter: Dual action of KB-R7943 on Na+/Ca2+ exchange and L-type Ca2+ channels.

The effect of the Na+/Ca(2+)-exchange inhibitor KB-R7943 was investigated in spinal cord dorsal column ischemia in vitro. Oxygen/glucose deprivation at 37 degrees C for 1 h causes severe injury even in the absence of external Ca2+. KB-R7943 was very protective in the presence and absence of external Ca2+ implicating mechanisms in addition to extracellular Ca2+ influx through Na+/Ca(2+)-exchange, such as activation of ryanodine receptors by L-type Ca2+ channels. Indeed, blockade of L-type Ca2+ by nimodipine confers a certain degree of protection of dorsal column against ischemia; combined application of nimodipine and KB-R7943 was not additive suggesting that KB-R7943 may also act on Ca2+ channels. KB-R7943 reduced inward Ba2+ current with IC50 = 7 microM in tsA-201 cells expressing Ca(v)1.2. Moreover, nifedipine and KB-R7943 both reduced depolarization-induced [Ca2+]i increases in forebrain neurons and effects were not additive. Nimodipine or KB-R7943 also reduced ischemic axoplasmic Ca2+ increase, which persisted in 0Ca2+/EGTA perfusate in dorsal column during ischemia. While KB-R7943 cannot be considered to be a specific Na+/Ca2+ exchange inhibitor, its profile makes it a very useful neuroprotectant in dorsal columns by: reducing Ca2+ import through reverse Na+/Ca2+ exchange; reducing influx through L-type Ca2+ channels, and indirectly inhibiting Ca2+ release from the ER through activation of ryanodine receptors.

Inhibition of transiently expressed low- and high-voltage-activated calcium channels by trivalent metal cations.

Calcium channels are important regulators of neuronal excitability and contribute to transmitter release, calcium dependent gene expression, and oscillatory behavior in many cell types. Under physiological conditions, native low-voltage (T-type)- and high-voltage-activated (HVA) currents are potently inhibited by trivalent cations. However, the presence of multiple calcium channel isoforms has hampered our ability to unequivocally assess the effects of trivalent cations on channel activity. Here, we describe the actions of nine trivalent metal ions on transiently expressed alpha1G (Cav3.1) T-type calcium channels cloned from human brain. In 2 mM external barium solution, yttrium most potently inhibited alpha1G current (IC50 = 28 nM), followed by erbium > gadolinium ~ cerium > holmium > ytterbium > neodymium > lanthanum >> scandium. With the exception of scandium, blocking affinity was loosely correlated with decreasing ionic radius. A detailed characterization of yttrium block revealed a 25-fold decrease in blocking affinity when the external concentration of charge carrier was increased from 2 mM to 20 mM. In 20 mM barium, yttrium also effectively inhibited various types of cloned HVA channels indicating that this ion is a nonselective blocker. For all calcium channels examined, yttrium preferentially inhibited inward over outward current, but block was otherwise voltage independent. In addition to peak current inhibition, P/Q- and L-type channels underwent a unique speeding of the macroscopic time course of inactivation. Whereas peak current block of alpha1A channels was highly sensitive to the external charge carrier concentration, the inactivation effects mediated by yttrium were not, suggesting that the two effects are due to distinct mechanisms. Moreover, the speeding effect was greatly attenuated by manipulations that slowed the inactivation kinetics of the channels. Thus, our evidence suggests that yttrium effects are mediated by two distinct events: peak current block likely occurring by occlusion of the pore, and kinetic speeding arising from yttrium interactions with the channel that alter the state of the inactivation gate.

Voltage-dependent calcium channels--beyond dihydropyridine antagonists.

The blockade of L-type calcium channels by dihydropyridines, phenylalkylamines and benzothiazepines has been well described and forms the basis of a multibillion dollar market for the treatment of cardiovascular disease and migraine. More recently, neuron-specific calcium channels have become the subject of intense interest regarding their potential as therapeutic targets for the treatment of chronic and neuropathic pain. A number of recently described agents that selectively target neuronal calcium channels have been described and appear promising for a variety of pain conditions.

Interactions between presynaptic Ca2+ channels, cytoplasmic messengers and proteins of the synaptic vesicle release complex.

Influx of Ca(2+) through presynaptic voltage-gated Ca(2+) channels is a key step in rapid neurotransmitter release. The amount of Ca(2+) entering through these channels is modulated by a plethora of intracellular messenger molecules, including betagamma-subunits of G proteins, and protein kinases. In addition, Ca(2+) channels bind physically to proteins of the vesicle-release machinery in a Ca(2+)-dependent manner, which can, in turn, regulate the activity of Ca(2+) channels. Recent evidence suggests that second messengers and presynaptic vesicle-release proteins do not regulate Ca(2+) channel activity as independent entities, but that there is extensive crosstalk between these two mechanisms. The complex interactions between second messengers, vesicle-release proteins and voltage-gated Ca(2+) channels might provide multiple avenues for fine-tuning Ca(2+) entry into the presynaptic terminal and, consequently, neurotransmission.

Calcium channel beta subunits differentially regulate the inhibition of N-type channels by individual Gbeta isoforms.

The direct inhibition of N- and P/Q-type calcium channels by G protein betagamma subunits is considered a key mechanism for regulating presynaptic calcium levels. We have recently reported that a number of features associated with this G protein inhibition are dependent on the G protein beta subunit isoform (Arnot, M. I., Stotz, S. C., Jarvis, S. E., Zamponi, G. W. (2000) J. Physiol. (Lond.) 527, 203-212; Cooper, C. B., Arnot, M. I., Feng, Z.-P., Jarvis, S. E., Hamid, J., Zamponi, G. W. (2000) J. Biol. Chem. 275, 40777-40781). Here, we have examined the abilities of different types of ancillary calcium channel beta subunits to modulate the inhibition of alpha(1B) N-type calcium channels by the five known different Gbeta subunit subtypes. Our data reveal that the degree of inhibition by a particular Gbeta subunit is strongly dependent on the specific calcium channel beta subunit, with N-type channels containing the beta(4) subunit being less susceptible to Gbetagamma-induced inhibition. The calcium channel beta(2a) subunit uniquely slows the kinetics of recovery from G protein inhibition, in addition to mediating a dramatic enhancement of the G protein-induced kinetic slowing. For Gbeta(3)-mediated inhibition, the latter effect is reduced following site-directed mutagenesis of two palmitoylation sites in the beta(2a) N-terminal region, suggesting that the unique membrane tethering of this subunit serves to modulate G protein inhibition of N-type calcium channels. Taken together, our data suggest that the nature of the calcium channel beta subunit present is an important determinant of G protein inhibition of N-type channels, thereby providing a possible mechanism by which the cellular/subcellular expression pattern of the four calcium channel beta subunits may regulate the G protein sensitivity of N-type channels expressed at different loci throughout the brain and possibly within a neuron.

Multiple structural elements contribute to voltage-dependent facilitation of neuronal alpha 1C (CaV1.2) L-type calcium channels.

Voltage- and frequency-dependent facilitation of calcium channel activity has been implicated in a number of key physiological processes. Various mechanisms have been proposed to mediate these regulations, including a switch between channel gating modes, voltage-dependent phosphorylation, and a voltage-dependent deinhibition of G-protein block. Studying such modulation on recombinant Ca channels expressed in oocytes, we previously reported that alpha(1C) L-type calcium channel contrast with non-L type Ca channels by its ability to exhibit facilitation by pre-depolarization (Voltage-dependent facilitation of a neuronal alpha(IC) L-type calcium channel, E. Bourinet et al., EMBO Journal, 1994; 13, 5032-5039). To further analyze this effect, we have investigated the molecular determinants which mediate the differences in voltage-dependent facilitation between "facilitable" alpha(1C) and "non facilitable" alpha(1E) calcium channels. We used a series of chimeras which combine the four transmembrane domains of the two channels. Results show that the four domains of alpha(1C) contribute to facilitation, with domain I being most critical. This domain is required but not sufficient alone to generate facilitation. The minimal requirement to observe the effect is the presence of domain I plus one of the three others. We conclude that similarly to activation gating, voltage-dependent facilitation of alpha(1C) is a complex process which involves multiple structural elements were domains I and III play the major role.

Identification of inactivation determinants in the domain IIS6 region of high voltage-activated calcium channels.

We have recently reported that transfer of the domain IIS6 region from rapidly inactivating R-type (alpha(1E)) calcium channels to slowly inactivating L-type (alpha(1C)) calcium channel confers rapid inactivation (Stotz, S. C., Hamid, J., Spaetgens, R. L., Jarvis, S. E., and Zamponi, G. W. (2000) J. Biol. Chem. 275, 24575-24582). Here we have identified individual amino acid residues in the IIS6 regions that are responsible for these effects. In this region, alpha(1C) and alpha(1E) channels differ in seven residues, and exchanging five of those residues individually or in combination did not significantly affect inactivation kinetics. By contrast, replacement of residues Phe-823 or Ile-829 of alpha(1C) with the corresponding alpha(1E) residues significantly accelerated inactivation rates and, when substituted concomitantly, approached the rapid inactivation kinetics of R-type channels. A systematic substitution of these residues with a series of other amino acids revealed that decreasing side chain size at position 823 accelerates inactivation, whereas a dependence of the inactivation kinetics on the degree of hydrophobicity could be observed at position 829. Although these point mutations facilitated rapid entry into the inactivated state of the channel, they had little to no effect on the rate of recovery from inactivation. This suggests that the development of and recovery from inactivation are governed by separate structural determinants. Finally, the effects of mutations that accelerated alpha(1C) inactivation could still be antagonized following coexpression of the rat beta(2a) subunit or by domain I-II linker substitutions that produce ultra slow inactivation of wild type channels, indicating that the inactivation kinetics seen with the mutants remain subject to regulation by the domain I-II linker. Overall, our results provide novel insights into a complex process underlying calcium channel inactivation.

Determinants of G protein inhibition of presynaptic calcium channels.

The modulation of presynaptic calcium (Ca) channels by heterotrimeric G proteins is a key factor for the regulation of neurotransmission. Over the past 20 yr, a significant understanding of the molecular events underlying this regulation has been acquired. It is now widely accepted that binding of G protein betagamma dimers directly to the cytoplasmic region linking domains I and II of the Ca channel alpha1 subunit results in a stabilization of the closed conformation of the channel, thereby inhibiting current activity. The extent of the inhibition is dependent on the Gbeta subunit isoform, and is antagonized by both strong membrane depolarizations and protein kinase C-dependent phosphorylation of the channel. Finally, the inhibition is critically modulated by regulator of G protein signaling proteins, and by proteins forming the presynaptic vesicle release complex. Thus, the regulation of the activities of presynaptic Ca channels is becoming increasingly complex, a feature that may contribute to the overall fine-tuning of Ca entry into presynaptic nerve termini, and thus, neurotransmission.

Interaction of SNX482 with domains III and IV inhibits activation gating of alpha(1E) (Ca(V)2.3) calcium channels.

We have investigated the action of SNX482, a toxin isolated from the venom of the tarantula Hysterocrates gigas, on voltage-dependent calcium channels expressed in tsa-201 cells. Upon application of 200 nM SNX482, R-type alpha(1E) calcium channels underwent rapid and complete inhibition, which was only poorly reversible upon washout. However, upon application of strong membrane depolarizations, rapid and complete recovery from inhibition was obtained. Tail current analysis revealed that SNX482 mediated an approximately 70 mV depolarizing shift in half-activation potential, suggesting that the toxin inhibits alpha(1E) calcium channels by preventing their activation. Experiments involving chimeric channels combining structural features of alpha(1E) and alpha(1C) subunits indicated that the presence of the domain III and IV of alpha(1E) is a prerequisite for a strong gating inhibition. In contrast, L-type alpha(1C) channels underwent incomplete inhibition at saturating concentrations of SNX482 that was paralleled by a small shift in half-activation potential and which could be rapidly reversed, suggesting a less pronounced effect of the toxin on L-type calcium channel gating. We conclude that SNX482 does not exhibit unequivocal specificity for R-type channels, but highly effectively antagonizes their activation.

Distinct molecular determinants govern syntaxin 1A-mediated inactivation and G-protein inhibition of N-type calcium channels.

We have reported recently that syntaxin 1A mediates two effects on N-type channels transiently expressed in tsA-201 cells: a hyperpolarizing shift in the steady-state inactivation curve as well as a tonic inhibition of the channel by G-protein betagamma subunits (Jarvis et al., 2000). Here we have examined some of the molecular determinants and factors that modulate the action of syntaxin 1A on N-type calcium channels. With the additional coexpression of SNAP25, the syntaxin 1A-induced G-protein modulation of the channel became reduced in magnitude by approximately 50% but nonetheless remained significantly higher than the low levels of background inhibition seen with N-type channels alone. In contrast, coexpression of nSec-1 did not reduce the syntaxin 1A-mediated G-protein inhibition; however, interestingly, nSec-1 was able to induce tonic G-protein inhibition even in the absence of syntaxin 1A. Both SNAP25 and nSec-1 blocked the negative shift in half-inactivation potential that was induced by syntaxin 1A. Activation of protein kinase C via phorbol esters or site-directed mutagenesis of three putative PKC consensus sites in the syntaxin 1A binding region of the channel (S802, S896, S898) to glutamic acid (to mimic a permanently phosphorylated state) did not affect the syntaxin 1A-mediated G-protein modulation of the channel. However, in the S896E and S898E mutants, or after PKC-dependent phosphorylation of the wild-type channels, the susceptibility of the channel to undergo shifts in half-inactivation potential was removed. Thus, separate molecular determinants govern the ability of syntaxin 1A to affect N-type channel gating and its modulation by G-proteins.

Syntaxin 1A supports voltage-dependent inhibition of alpha1B Ca2+ channels by Gbetagamma in chick sensory neurons.

N-type Ca(2+) channels are modulated by a variety of G-protein-coupled pathways. Some pathways produce a transient, voltage-dependent (VD) inhibition of N channel function and involve direct binding of G-protein subunits; others require the activation of intermediate enzymes and produce a longer-lasting, voltage-independent (VI) form of inhibition. The ratio of VD:VI inhibition differs significantly among cell types, suggesting that the two forms of inhibition play unique physiological roles in the nervous system. In this study, we explored mechanisms capable of altering the balance of VD and VI inhibition in chick dorsal root ganglion neurons. We report that (1) VD:VI inhibition is critically dependent on the Gbetagamma concentration, with VI inhibition dominant at low Gbetagamma concentrations, and (2) syntaxin-1A (but not syntaxin-1B) shifts the ratio in favor of VD inhibition by potentiating the VD effects of Gbetagamma. Variations in expression levels of G-proteins and/or syntaxin provide the means to alter over a wide range both the extent and the rate of Ca(2+) influx through N channels.

Residue Gly1326 of the N-type calcium channel alpha 1B subunit controls reversibility of omega-conotoxin GVIA and MVIIA block.

We recently reported that amino acid residues contained within a putative EF hand motif in the domain III S5-H5 region of the alpha(1B) subunit affected the relative barium:calcium permeability of N-type calcium channels (Feng, Z. P., Hamid, J., Doering, C., Jarvis, S. E., Bosey, G. M., Bourinet, E., Snutch, T. P., and Zamponi, G. W. (2001) J. Biol. Chem. 276, 5726-5730). Since this region partially overlaps with residues previously implicated in block of the channel by omega-conotoxin GVIA, we assessed the effects of mutations in the putative EF hand domain on channel block by omega-conotoxin GVIA and the structurally related omega-conotoxin MVIIA. Both of the toxins irreversibly block the activity of wild type alpha(1B) N-type channels. We find that in addition to previously identified amino acid residues, residues in positions 1326 and 1332 are important determinants of omega-conotoxin GVIA blockade. Substitution of residue Glu(1332) to arginine slows the time course of development of block. Point mutations in position Gly(1326) to either arginine, glutamic acid, or proline dramatically decrease the time constant for development of the block. Additionally, in the G1326P mutant channel activity was almost completely recovered following washout. A qualitatively similar result was obtained with omega-conotoxin MVIIA, suggesting that common molecular determinants underlie block by these two toxins. Taken together the data suggest that residue Gly(1326) may form a barrier, which controls the access of peptide toxins to their blocking site within the outer vestibule of the channel pore and also stabilizes the toxin-channel interaction.

Structural determinants of fast inactivation of high voltage-activated Ca(2+) channels.

The fast inactivation of voltage-dependent Ca(2+) channels is a key mechanism that contributes to the precise control of Ca(2+) entry into excitable cells. Recent advances have revealed that multiple structural elements contribute to the intrinsic inactivation properties of the alpha(1) subunit, including its cytoplasmic and transmembrane regions. Another major determinant of Ca(2+) channel inactivation is the association with one of four types of ancillary beta subunits that differentially modulate the intrinsic inactivation properties of the alpha(1) subunit. This could occur partly via interactions with the N-terminal region of the alpha(1) subunit and through lipid modification of the beta subunit. However, the latest findings suggest a mechanism in which fast Ca(2+) channel inactivation could occur through physical occlusion of the pore of the channel in a manner reminiscent of Na(+) and K(+) channel inactivation.