Melatonin inhibits the up-regulation of N-type calcium channel in neuropathic pain by activating the MT2 receptor in rats

The aim of the study was to clarify the effect of melatonin on neuropathic pain by N-type calcium channel (Cav2.2) inhibition in dorsal root ganglion (DRG) neurons after spared nerve injury (SNI) surgery. Immunofluorescence was used to identify the co-expression of Cav2.2 and the MT2 receptor and detect the changes in Cav2.2 expression in DRG neurons. Western-blot was also performed to detect the expression of Cav2.2 in DRG neurons. The action potential and current of Cav2.2 channels in DRG neurons were detected using whole-cell patch clamp analysis. Behavioral studies were conducted using thermal stimulation and acetone after melatonin was injected intraperitoneally. The results revealed that Cav2.2 and the MT2 receptor were co-expressed in medium and small sized DRG neurons, and the intensity of Cav2.2 increased after SNI. Injection of melatonin activated the MT2 receptor and relieved nociceptive pain through decreased the Cav2.2 expression and current in DRG neurons. Melatonin can significantly decrease the increase in Cav2.2 current density and excitability after SNI. In addition, the Cav2.2 activation curve shifted to the left after SNI, but there was no change in inactivation. 10 μM melatonin significantly inhibited the excitability of DRG neurons and Cav2.2 current, the inactivation curve of Cav2.2 current shifted significantly to the left. However, the MT2 receptor antagonist 4-P-PDOT reversed the inhibition of melatonin on Cav2.2 current. We conclude that melatonin inhibits the increased Cav2.2 expression and current; on the other hand, it reduces the excitability of DRG neurons after SNI surgery via the MT2 receptor pathway.

Characterization of Convergent Suppression by UCL-2077 (3-(Triphenylmethylaminomethyl)pyridine), Known to Inhibit Slow Afterhyperpolarization, of erg-Mediated Potassium Currents and Intermediate-Conductance Calcium-Activated Potassium Channels

UCL-2077 (triphenylmethylaminomethyl)pyridine) was previously reported to suppress slow afterhyperpolarization in neurons. However, the information with respect to the effects of UCL-2077 on ionic currents is quite scarce. The addition of UCL-2077 decreased the amplitude of erg-mediated K+ current (IK(erg)) together with an increased deactivation rate of the current in pituitary GH3 cells. The IC50 and KD values of UCL-2077-induced inhibition of IK(erg) were 4.7 and 5.1 μM, respectively. UCL-2077 (10 μM) distinctly shifted the midpoint in the activation curve of IK(erg) to less hyperpolarizing potentials by 17 mV. Its presence decreased the degree of voltage hysteresis for IK(erg) elicitation by long-lasting triangular ramp pulse. It also diminished the probability of the opening of intermediate-conductance Ca2+-activated K+ channels. In cell-attached current recordings, UCL-2077 raised the frequency of action currents. When KCNH2 mRNA was knocked down, a UCL-2077-mediated increase in AC firing was attenuated. Collectively, the actions elaborated herein conceivably contribute to the perturbating effects of this compound on electrical behaviors of excitable cells.

Multiple Modulation of Acid-Sensing Ion Channel 1a by the Alkaloid Daurisoline

Acid-sensing ion channels (ASICs) are proton-gated sodium-selective channels that are expressed in the peripheral and central nervous systems. ASIC1a is one of the most intensively studied isoforms due to its importance and wide representation in organisms, but it is still largely unexplored as a target for therapy. In this study, we demonstrated response of the ASIC1a to acidification in the presence of the daurisoline (DAU) ligand. DAU alone did not activate the channel, but in combination with protons, it produced the second peak component of the ASIC1a current. This second peak differs from the sustained component (which is induced by RF-amide peptides), as the second (DAU-induced) peak is completely desensitized, with the same kinetics as the main peak. The co-application of DAU and mambalgin-2 indicated that their binding sites do not overlap. Additionally, we found an asymmetry in the pH activation curve of the channel, which was well-described by a mathematical model based on the multiplied probabilities of protons binding with a pool of high-cooperative sites and a single proton binding with a non-cooperative site. In this model, DAU targeted the pool of high-cooperative sites and, when applied with protons, acted as an inhibitor of ASIC1a activation. Moreover, DAU’s occupation of the same binding site most probably reverses the channel from steady-state desensitization in the pH 6.9–7.3 range. DAU features disclose new opportunities in studies of ASIC structure and function.

Recombinant production and structure-function study of ts1 toxin from the Brazilian scorpion Tityus serrulatus

This work introduces an effective bacterial system for the production of β-toxin Ts1, the main component of the Brazilian scorpion Tityus serrulatus venom. Recombinant toxin and its 15N-labeled analogue are obtained by direct expression of the synthetic gene in Escherichia coli, with subsequent folding from the inclusion bodies. NMR spectroscopy data assert that the recombinant toxin is structured in aqueous solution and is composed of a significant fraction of β-structure. Moreover, the formation of a stable Ts1 disulfide-bond isomer of a disordered structure is observed during folding; recombinant Ts1 blocks Na+ current through NaV1.5 channels, without affecting the processes of activation and inactivation. Simultaneously, the effect upon NaV1.4 channels is associated with a shift of the activation curve toward the more negative membrane potentials.

Expression of JAK2-V617F Kinase in Myeloid Progenitors and Proerythroblasts Induces Differential Patterns of Hypersensitivity in Activation of Key Signaling Nodes upon Incubation with Low-Dose, Physiologic EPO Concentrations

Introduction: Chronic myeloproliferative neoplasms (MPNs), including Polycythemia vera (PV), Essential Thrombocythemia (ET) and Primary Myelofibrosis (PMF) are a spectrum of clonal hematological disorders. An acquired somatic mutation of the JAK2-gene (JAK2V617F) drives clonal expansion of hematopoietic progenitor cells in 90-95% in cases of PV and in approximately 40-50% in ET and PMF patients. This mutation induces an aberrant cellular signaling affecting the activation of several downstream protein cascades of cytokine receptors and substrates of the JAK2 protein. This constitutive activation leads to deregulated proliferation and differentiation of one or several myeloid lineages. Studies have shown that progenitor cells derived from JAK2V617F-positive patients are characterized by hypersensitivity to cytokines, e.g. erythropoietin (EPO), leading to an increased proliferation and disturbed differentiation under low –dose cytokine treatment. Methods: Murine 32D progenitor cells and I-11 proerythroblasts were transduced with retroviral vectors expressing cDNAs of the erythropoietin receptor and JAK2WT/JAK2V617F. Stably transduced cell lines were treated with increasing concentrations of EPO. To study the kinetics within the EPO signaling network, cell lines were starved for 4h and then treated with increasing doses of EPO (1 – 10 IU EPO for 32D, 0.5 – 3 IU EPO for I-11) for 1h (32D cells) and for 30min (I-11 cells). Phosphorylation of STAT3/STAT5, JAK2, Erk1/2, Akt and PLCg1, respectively were detected by immunoblot analysis. Densitometric analysis of immunoblot-signals (phospho-signals adjusted to total protein signals) was performed to quantitatively determine differences in phosphorylation of the main molecular pathways of EPO signaling. Results: Treatment with low physiologic doses of EPO resulted in an enhanced cell proliferation in JAK2V617F expressing cells compared to JAK2WT. However, this effect leveled off upon EPO concentrations >0.75 IU/ml. To investigate the molecular mechanisms of this hypersensitive status, we quantitatively monitored dose- and time-dependent phosphorylation of signaling proteins, which play a major role in the EPO signaling network, including STAT3/STAT5, Erk1/2, Akt and PLCg1. In unstimulated JAK2V617F mutated 32D and I-11 cells, we identified constitutive activation of these key signaling regulators. Moreover, we could demonstrate that in JAK2V617F expressing cells EPO dependent activation of these key signaling molecules is significantly more sensitive to low EPO concentrations as compared to the situation in JAK2WT expressing cells. Thus, we detected a higher peak of phosphorylation/activation of STAT3/STAT4, Erk1/2, Akt and PLCg1 in low-dose treated JAK2V617F cells. In addition, in JAK2V617F positive cells, three patterns of signaling kinetics were observed: 1. Left shift of activation curve and higher maximum of activation which is overcome by high EPO concentrations (e.g. phospho-JAK2); 2. Left shift of activation curve and higher maximum of activation which cannot be overcome by high EPO concentrations (e.g. phospho-STAT5); 3. Pattern exhibiting minor differences only (e.g. phospho-ERK1/2). We hypothesize that this is due to differential activation of feedback and feedforward loops. Quantitative and qualitative modelling is currently being performed to identify the molecular mechanisms involved. Along this line, we identified the docking protein Grb2-associated-binding protein1 (Gab1) as a member of EPO-dependent proteins. Gab1 plays a major role in co-activation of MAPK- and PI3K-pathway. Thus, for the first time we demonstrate constitutive activation of Gab1 in JAK2V617F mutated cells. Conclusions: We here demonstrate that hypersensitivity in proliferation of JAK2V617F mutated progenitor cells and proerythroblasts is molecularly corroborated by differential sensitivity of the pro-proliferative signaling network. We identified at least four different signaling pathways, which show higher sensitivity for activation in JAK2V617F mutated cells compared wild type cells. Hypersensitivity to low-dose EPO treatment on a molecular level may explain some of the biological features observed in JAK2V617F positive MPNs and may offer novel targets for therapeutic intervention. Disclosures No relevant conflicts of interest to declare.

Phosphatidic acid modulation of Kv channel voltage sensor function

Membrane phospholipids can function as potent regulators of ion channel function. This study uncovers and investigates the effect of phosphatidic acid on Kv channel gating. Using the method of reconstitution into planar lipid bilayers, in which protein and lipid components are defined and controlled, we characterize two effects of phosphatidic acid. The first is a non-specific electrostatic influence on activation mediated by electric charge density on the extracellular and intracellular membrane surfaces. The second is specific to the presence of a primary phosphate group, acts only through the intracellular membrane leaflet and depends on the presence of a particular arginine residue in the voltage sensor. Intracellular phosphatidic acid accounts for a nearly 50 mV shift in the midpoint of the activation curve in a direction consistent with stabilization of the voltage sensor's closed conformation. These findings support a novel mechanism of voltage sensor regulation by the signaling lipid phosphatidic acid.

Saikosaponin a Enhances Transient Inactivating Potassium Current in Rat Hippocampal CA1 Neurons

Saikosaponin a (SSa), a main constituent of the Chinese herbBupleurum chinenseDC., has been demonstrated to have antiepileptic activity. Recent studies have shown that SSa could inhibit NMDA receptor current and persistent sodium current. However, the effects of SSa on potassium (K+) currents remain unclear. In this study, we tested the effect of SSa on 4AP-induced epileptiform discharges and K+currents in CA1 neurons of rat hippocampal slices. We found that SSa significantly inhibited epileptiform discharges frequency and duration in hippocampal CA1 neurons in the 4AP seizure model in a dose-dependent manner with anIC50of 0.7 μM. SSa effectively increased the amplitude ofITotalandIA, significantly negative-shifted the activation curve, and positive-shifted steady-state curve ofIA. However, SSa induced no significant changes in the amplitude and activation curve ofIK. In addition, SSa significantly increased the amplitude of 4AP-sensitive K+current, while there was no significant change in the amplitude of TEA-sensitive K+current. Together, our data indicate that SSa inhibits epileptiform discharges induced by 4AP in a dose-dependent manner and that SSa exerts selectively enhancing effects onIA. These increases inIAmay contribute to the anticonvulsant mechanisms of SSa.

Distinct subunit contributions to the activation of M-type potassium channels by PI(4,5)P2

Low-threshold voltage-gated M-type potassium channels (M channels) are tetraheteromers, commonly of two Kv7.2 and two Kv7.3 subunits. Though gated by voltage, the channels have an absolute requirement for binding of the membrane phospholipid phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2) to open. We have investigated the quantitative relation between the concentration of a water-soluble PI(4,5)P2 analog, dioctanoyl-PI(4,5)P2 (DiC8-PI(4,5)P2), and channel open probability (Popen) by fast application of increasing concentrations of DiC8-PI(4,5)P2 to the inside face of membrane patches excised from Chinese hamster ovary cells expressing M channels as heteromeric Kv7.2/7.3 subunits. The rationale for the experiments is that this will mimic the effect of changes in membrane PI(4,5)P2 concentration. Single-channel conductances from channel current–voltage relations in cell-attached mode were 9.2 ± 0.1 pS with a 2.5-mM pipette [K+]. Plots of Popen against DiC8-PI(4,5)P2 concentration were best fitted using a two-component concentration–Popen relationship with high and low affinity, half-maximal effective concentration (EC50) values of 1.3 ± 0.14 and 75.5 ± 2.5 µM, respectively, and Hill slopes of 1.4 ± 0.06. In contrast, homomeric channels from cells expressing only Kv7.2 or Kv7.3 constructs yielded single-component curves with EC50 values of 76.2 ± 19.9 or 3.6 ± 1.0 µM, respectively. When wild-type (WT) Kv7.2 was coexpressed with a mutated Kv7.3 subunit with >100-fold reduced sensitivity to PI(4,5)P2, the high-affinity component of the activation curve was lost. Fitting the data for WT and mutant channels to an activation mechanism with independent PI(4,5)P2 binding to two Kv7.2 and two Kv7.3 subunits suggests that the two components of the M-channel activation curve correspond to the interaction of PI(4,5)P2 with the Kv7.3 and Kv7.2 subunits, respectively, that channels can open when only the two Kv7.3 subunits have bound DiC8-PI(4,5)P2, and that maximum channel opening requires binding to all four subunits.

Serotonin regulates voltage-dependent currents in type Ie(A) and Ii interneurons of Hermissenda

Serotonin (5-HT) has both direct and modulatory actions on central neurons contributing to behavioral arousal and cellular-synaptic plasticity in diverse species. In Hermissenda, 5-HT produces changes in intrinsic excitability of different types of identified interneurons in the circumesophageal nervous system. Using whole cell patch-clamp techniques we have examined membrane conductance changes produced by 5-HT that contribute to intrinsic excitability in two identified classes of interneurons, types Ii and IeA. Whole cell currents were examined before and after 5-HT application to the isolated nervous system. A 4-aminopyridine-sensitive transient outward K+ current [ IK(A)], a tetraethylammonium-sensitive delayed rectifier K+ current [ IK(V)], an inward rectifier K+ current [ IK(IR)], and a hyperpolarization-activated current ( Ih) were characterized. 5-HT decreased the amplitude of IK(A) and IK(V) in both type Ii and IeA interneurons. However, differences in 5-HT's effects on the activation-inactivation kinetics were observed in different types of interneurons. 5-HT produced a depolarizing shift in the activation curve of IK(V) and a hyperpolarizing shift in the inactivation curve of IK(A) in type Ii interneurons. In contrast, 5-HT produced a depolarizing shift in the activation curve and a hyperpolarizing shift in the inactivation curve of both IK(V) and IK(A) in type IeA interneurons. In addition, 5-HT decreased the amplitude of IK(IR) in type Ii interneurons and increased the amplitude of Ih in type IeA interneurons. These results indicate that 5-HT-dependent changes in IK(A), IK(V), IK(IR), and Ih contribute to multiple mechanisms that synergistically support modulation of increased intrinsic excitability associated with different functional classes of identified type I interneurons.

The pathway and spatial scale for MscS inactivation

The mechanosensitive channel of small conductance (MscS) is a bacterial tension-driven osmolyte release valve with homologues in many walled eukaryotic organisms. When stimulated by steps of tension in excised patches, Escherichia coli MscS exhibits transient opening followed by reversible adaptation and then complete inactivation. Here, we study properties of the inactivation transition, which renders MscS nonconductive and tension insensitive. Using special pressure protocols we demonstrate that adaptation and inactivation are sequential processes with opposite tension dependencies. In contrast to many eukaryotic channels, which inactivate from the open state, MscS inactivates primarily from the closed state because full openings by preconditioning pulses do not influence the degree of inactivation, and saturating tensions keeping channels open prevent inactivation. The easily opened A98S mutant lacks inactivation completely, whereas the L111S mutant with a right-shifted activation curve inactivates silently before reaching the threshold for opening. This suggests that opening and inactivation are two independent tension-driven pathways, both starting from the closed state. Analysis of tension dependencies for inactivation and recovery rates estimated the in-plane expansion (ΔA) associated with inactivation as 8.5 nm2, which is about half of the area change for opening. Given that the interhelical contact between the outer TM1–TM2 pairs and the core TM3s is the force-transmitting path from the periphery to the gate, the determined ΔA now can be used as a constraining parameter for the models of the inactivated state in which the lipid-facing TM1–TM2 pairs are displaced and uncoupled from the gate.