The mechanism of action of the antiepileptic and antinociceptive drugs of the gabapentinoid family has remained poorly understood. Gabapentin (GBP) binds to an exofacial epitope of the ␣2␦-1 and ␣2␦-2 auxiliary subunits of voltage-gated calcium channels, but acute inhibition of calcium currents by GBP is either very minor or absent. We formulated the hypothesis that GBP impairs the ability of ␣2␦ subunits to enhance voltage-gated Ca 2؉ channel plasma membrane density by means of an effect on trafficking. Our results conclusively demonstrate that GBP inhibits calcium currents, mimicking a lack of ␣2␦ only when applied chronically, but not acutely, both in heterologous expression systems and in dorsal rootganglion neurons. GBP acts primarily at an intracellular location, requiring uptake, because the effect of chronically applied GBP is blocked by an inhibitor of the system-L neutral amino acid transporters and enhanced by coexpression of a transporter. However, it is mediated by ␣2␦ subunits, being prevented by mutations in either ␣2␦-1 or ␣2␦-2 that abolish GBP binding, and is not observed for ␣2␦-3, which does not bind GBP. Furthermore, the trafficking of ␣2␦-2 and CaV2 channels is disrupted both by GBP and by the mutation in ␣2␦-2, which prevents GBP binding, and we find that GBP reduces cell-surface expression of ␣2␦-2 and CaV2.1 subunits. Our evidence indicates that GBP may act chronically by displacing an endogenous ligand that is normally a positive modulator of ␣2␦ subunit function, thereby impairing the trafficking function of the ␣2␦ subunits to which it binds. V oltage-gated Ca 2ϩ channels (VGCCs) are heteromeric complexes. The Ca V 1 and Ca V 2 subfamilies are made up of a pore-forming ␣1 subunit, associated with a membraneanchored, predominantly extracellular, ␣ 2 ␦ subunit (for review see ref. 1) and an intracellular ␤ subunit (for review see ref.2). Mammalian genes encoding four ␣ 2 ␦ subunits have been identified (for reviews see refs. 2 and 3). The topology of the ␣ 2 ␦ protein was first determined for ␣ 2 ␦-1 and is thought to generalize to all ␣ 2 ␦ subunits (for reviews see refs. 1 and 4). They are type I transmembrane proteins, the exofacial ␣ 2 subunit being disulfide-bonded to a transmembrane ␦ subunit, formed by posttranslational cleavage of the ␣ 2 ␦ preprotein (5).The mechanism of action of the antiepileptic and antinociceptive drugs of the gabapentinoid family has remained poorly understood. Gabapentin (GBP) itself was originally developed as an analog of ␥-amino-butyric acid (GABA), but is now believed to have no effect on GABA receptors or transporters (for review see ref. 6). The first key to understanding the mechanism of action of GBP came from purification of the GBP-binding protein from porcine brain (7), which was identified as the ␣ 2 ␦-1 auxiliary subunit of VGCCs. It is now known that GBP binds to an exofacial epitope present in both the ␣ 2 ␦-1 and ␣ 2 ␦-2 subunits (for reviews see refs. 1 and 8). However, although it was originally reported that GBP application results in a...

All auxiliary ␣2␦ subunits of voltage-gated Ca 2؉ (CaV) channels contain an extracellular Von Willebrand factor-A (VWA) domain that, in ␣2␦-1 and -2, has a perfect metal-ion-dependent adhesion site (MIDAS). Modeling of the ␣2␦-2 VWA domain shows it to be highly likely to bind a divalent cation. Mutating the three key MIDAS residues responsible for divalent cation binding resulted in a MIDAS mutant ␣2␦-2 subunit that was still processed and trafficked normally when it was expressed alone. However, unlike WT ␣2␦-2, the MIDAS mutant ␣2␦-2 subunit did not enhance and, in some cases, further diminished Ca V1.2, -2.1, and -2.2 currents coexpressed with ␤1b by using either Ba 2؉ or Na ؉ as a permeant ion. Furthermore, expression of the MIDAS mutant ␣2␦-2 reduced surface expression and strongly increased the perinuclear retention of Ca V␣1 subunits at the earliest time at which expression was observed in both Cos-7 and NG108 -15 cells. Despite the presence of endogenous ␣2␦ subunits, heterologous expression of ␣2␦-2 in differentiated NG108 -15 cells further enhanced the endogenous high-threshold Ca 2؉ currents, whereas this enhancement was prevented by the MIDAS mutations. Our results indicate that ␣2␦ subunits normally interact with the CaV␣1 subunit early in their maturation, before the appearance of functional plasma membrane channels, and an intact MIDAS motif in the ␣2␦ subunit is required to promote trafficking of the ␣1 subunit to the plasma membrane by an integrin-like switch. This finding provides evidence for a primary role of a VWA domain in intracellular trafficking of a multimeric complex, in contrast to the more usual roles in binding extracellular ligands in other exofacial VWA domains.integrin ͉ neuron ͉ motif ͉ expression V oltage-gated Ca 2ϩ (Ca V ) channels are composed of a poreforming ␣1 subunit that determines the main biophysical properties of the channel. For the Ca V 1 and -2 subfamilies, this subunit is associated with an intracellular ␤ subunit (for review, see refs. 1 and 2) and a membrane-anchored, predominantly extracellular ␣ 2 ␦ subunit (for review, see ref.3). Mammalian genes encoding 10 ␣1, 4 ␤, and 4 ␣ 2 ␦ subunits have been identified (for reviews, see refs. 2 and 4). The topology of the ␣ 2 ␦ protein has been determined in detail only for ␣ 2 ␦-1 but is thought to generalize to all 4 ␣ 2 ␦ subunits (for review, see ref.3). All ␣ 2 ␦ subunits have predicted N-terminal signal sequences, indicating that the N terminus is extracellular. In early studies of ␣ 2 ␦-1 purified from skeletal and cardiac muscle, it was determined that the ␣ 2 subunit is disulfide-bonded to a transmembrane ␦ subunit, and both subunits are the products of a single gene, encoding the ␣ 2 ␦ protein, that is posttranslationally cleaved into ␣ 2 and ␦ (5).Subsequent to the identification of ␣ 2 ␦ subunits as stoichiometric components of skeletal muscle Ca 2ϩ channels, ␣ 2 ␦ subunits have also been shown to be associated with native cardiac (L-type) (6) and neuronal N-and P͞Q-type channels (7,8). In coexpression stud...

Phosphatidylinositol 3-kinase (PI3K) has been shown to enhance native voltage-dependent calcium channel (Ca(v)) currents both in myocytes and in neurons; however, the mechanism(s) responsible for this regulation were not known. Here we show that PI3K promotes the translocation of GFP-tagged Ca(v) channels to the plasma membrane in both COS-7 cells and neurons. We show that the effect of PI3K is mediated by Akt/PKB and specifically requires Ca(v)beta(2) subunits. The mutations S574A and S574E in Ca(v)beta(2a) prevented and mimicked, respectively, the effect of PI3K/Akt-PKB, indicating that phosphorylation of Ser574 on Ca(v)beta(2a) is necessary and sufficient to promote Ca(v) channel trafficking.

Expression of the calcium channel Ca V 2.2 is markedly suppressed by coexpression with truncated constructs of Ca V 2.2. Furthermore, a two-domain construct of Ca V 2.1 mimicking an episodic ataxia-2 mutation strongly inhibited Ca V 2.1 currents. We have now determined the specificity of this effect, identified a potential mechanism, and have shown that such constructs also inhibit endogenous calcium currents when transfected into neuronal cell lines. Suppression of calcium channel expression requires interaction between truncated and full-length channels, because there is inter-subfamily specificity. Although there is marked cross-suppression within the Ca V 2 calcium channel family, there is no cross-suppression between Ca V 2 and Ca V 3 channels. The mechanism involves activation of a component of the unfolded protein response, the endoplasmic reticulum resident RNA-dependent kinase (PERK), because it is inhibited by expression of dominant-negative constructs of this kinase. Activation of PERK has been shown previously to cause translational arrest, which has the potential to result in a generalized effect on protein synthesis. In agreement with this, coexpression of the truncated domain I of Ca V 2.2, together with full-length Ca V 2.2, reduced the level not only of Ca V 2.2 protein but also the coexpressed ␣2␦-2. Thapsigargin, which globally activates the unfolded protein response, very markedly suppressed Ca V 2.2 currents and also reduced the expression level of both Ca V 2.2 and ␣2␦-2 protein. We propose that voltage-gated calcium channels represent a class of difficult-to-fold transmembrane proteins, in this case misfolding is induced by interaction with a truncated cognate Ca V channel. This may represent a mechanism of pathology in episodic ataxia-2.

The mechanism of action of gabapentin is still not well understood. It binds to the alpha(2)delta-1 and alpha(2)delta-2 subunits of voltage-gated calcium channels but has little acute effect on calcium currents in several systems. However, our recent results conclusively demonstrated that gabapentin inhibited calcium currents when applied chronically but not acutely, both in heterologous expression systems and in dorsal root ganglion neurons.(1) In that study we only examined a 40-hour time point of incubation with gabapentin, and here we have extended these results to include the effect of up to 6 and 20 hours incubation with gabapentin on calcium channel currents formed from Ca(V)2.1/beta(4)/alpha(2)delta-2 subunits. Gabapentin was significantly effective to inhibit the currents if included for 17-20 hours prior to recording, but it did not produce a significant inhibition if included for 3-6 hours. We previously concluded that gabapentin acts primarily at an intracellular location, requiring uptake into cells. However, this effect is mediated by alpha(2)delta subunits, being prevented by mutations in either alpha(2)delta-1 or alpha(2)delta-2 that abolish gabapentin binding.(1) Furthermore, we also showed that the trafficking of alpha(2)delta-2 and Ca(V)2 channels was disrupted by gabapentin. Here we have also extended that study, to show that the cell-surface expression of Ca(V)2.1 is not reduced by chronic gabapentin if it is co-expressed with alpha(2)delta-2 containing a point mutation (R282A) that prevents gabapentin binding.

Expression of the calcium channels CaV2.1 and CaV2.2 is markedly suppressed by co-expression with truncated constructs containing Domain I. This is the basis for the phenomenon of dominant negative suppression observed for many of the episodic ataxia type 2 mutations in CaV2.1 that predict truncated channels. The process of dominant negative suppression has been shown previously to stem from interaction between the full-length and truncated channels and to result in downstream consequences of the unfolded protein response and endoplasmic reticulum-associated protein degradation. We have now identified the specific domain that triggers this effect. For both CaV2.1 and CaV2.2, the minimum construct producing suppression was the cytoplasmic N terminus. Suppression was enhanced by tethering the N terminus to the membrane with a CAAX motif. The 11-amino acid motif (including Arg52 and Arg54) within the N terminus, which we have previously shown to be required for G protein modulation, is also essential for dominant negative suppression. Suppression is prevented by addition of an N-terminal tag (XFP) to the full-length and truncated constructs. We further show that suppression of CaV2.2 currents by the N terminus-CAAX construct is accompanied by a reduction in CaV2.2 protein level, and this is also prevented by mutation of Arg52 and Arg54 to Ala in the truncated construct. Taken together, our evidence indicates that both the extreme N terminus and the Arg52, Arg54 motif are involved in the processes underlying dominant negative suppression.

1. In this study we have investigated the action of bradykinin (Bk) on cultured neonatal rat dorsal root ganglion (DRG) cells, with the aim of elucidating whether the neuronal response to Bk is influenced by association with non-neuronal satellite cells. 2. Bradykinin (100 nM) evoked an inward current (I(Bk)) in 51 of 58 voltage clamped DRG neurones (holding potential (V(h)) = -80 mV) that were in contact with non-neuronal satellite cells. 3. Bradykinin failed to evoke an inward current in isolated DRG neurones (V(h) = -80 mV) that were not in contact with non-neuronal satellite cells (n = 41). 4. The lack of neuronal response to Bk was not influenced by time in culture. Bradykinin failed to evoke a response in isolated neurones through 1-5 days in culture. By contrast neurones in contact with satellite cells responded to Bk throughout the same time period. 5. Failure of isolated neurones to respond to Bk was not due to the replating procedure or to selective subcellular distribution of receptors/ion channels to the processes rather than the somata of neurones. 6. Using Indo-1 AM microfluorimetry Bk (100 nM) was demonstrated to evoke an intracellular Ca(2+) increase (Ca(Bk)) in DRG neurones in contact with non-neuronal satellite cells and in isolated neurones. 7. These data suggest that the inward current response to Bk requires contact between DRG neurones and non-neuronal satellite cells. This implies an indirect mechanism of action for Bk via the non-neuronal cells, which may perform a nociceptive role. However, Bk can also act directly on the neurones, since it evokes Ca(Bk) in isolated neurones. The relationship between Ca(Bk) and the Bk-induced inward current is unknown at present.

1. We have studied the effect of bradykinin (Bk) on fibroblast-like satellite (FLS) cells isolated from cultures of neonatal rat dorsal root ganglia (DRG). In voltage-clamped FLS cellsBk evoked an inward current response that was concentration dependent with a half-maximal concentration of 2 nM.3. In indo-1 AM-loaded FLS cells Bk evoked a rise in intracellular Ca 2+ that was concentration dependent with a half-maximal concentration of 1 nM.4. The FLS cells still produced an inward current in response to Bk in the absence of extracellular Ca 2+ but the response was inhibited if the intracellular concentration of EGTA was increased from 0.5 to 5 mM, which suggests that the inward current was dependent on the release and subsequent rise of intracellular Ca 2+ .5. The reversal potential of the Bk-induced inward current was consistent with the current being due to an increase in Cl _ conductance and shifted in a Nernstian manner when the intracellular Cl _ concentration was reduced.6. The inward current response to Bk was blocked by the B 2 receptor antagonist HOE-140, which indicates that the response was due to activation of B 2 receptors.7. The data suggest that Bk evokes a rise in intracellular Ca 2+ and activation of a Ca 2+-activated Cl _ conductance in the FLS cells and raise the possibility that FLS cells contribute to the proinflammatory effects of Bk in DRG.Journal of Physiology (2001), 530.3, pp. 395-403 11490 395 conditions. Preliminary data have already been published (England et al. 1995). METHODS Cell cultureCell cultures were obtained following enzymatic dispersal of neonatal rat dorsal root ganglia (DRG), as described previously (Wood et al. 1988). Briefly, 1-to 3-day-old Sprague-Dawley rat pups were killed by cervical dislocation followed by decapitation, and the DRG were removed. These were collected into Ham's F14 medium containing penicillin (100 i.u. ml), streptomycin (100 µg ml ) and L-glutamine (2 mM), and supplemented with 10 % fetal calf serum. Ganglia were then transferred to F14 medium containing 0.125 % collagenase, and were incubated at 37 o C and 3 % CO 2 in air, for a period of 40 min. The partially digested ganglia were washed in enzyme-free F14 medium, and triturated in fresh medium to which had been added 50 ng m l nerve growth factor (Promega or Alamone). Cells were cultured on 35 mm Petri dishes (Nunc).After approximately 3 days in vitro the glial cells began to undergo more rapid cell division, and approached confluence at around 5-7 days after dissociation. It was therefore necessary to replate the cells in order to obtain single cells for recording. This was achieved by resuspension of the cells and gentle trituration with a fire-polished wide-bore Pasteur pipette, approximately 4 h prior to electrophysiological measurements. This procedure virtually eliminated neurones from the cultures since the dishes were not treated with polyornithine or laminin, which aids adherence of cell soma to the culture dish. The attachment and growth of the glial cells was unaffected by thi...