Controllable Ion Channel Expression through Inducible Transient Transfection

Transfection, the delivery of foreign nucleic acids into a cell, is a powerful tool in protein research. Through this method, ion channels can be ...

The Hebrew University of Jerusalem (HUJI)

Ivermectin (IVM), a widely used antiparasitic agent in human and veterinary medicine, was recently shown to augment macroscopic currents through rat P2X(4) receptor channels. In the present study, the effects of IVM on the human P2X(4) (hP2X(4)) receptor channel stably transfected in HEK293 cells were investigated by recording membrane currents using the patch clamp technique. In whole-cell recordings, IVM (< or =10 microM) applied from outside the cell (but not from inside) increased the maximum current activated by ATP, and slowed the rate of current deactivation. These two phenomena likely result from the binding of IVM to separate sites. A higher affinity site (EC(50) 0.25 microM) increased the maximal current activated by saturating concentrations of ATP without significantly changing the rate of current deactivation or the EC(50) and Hill slope of the ATP concentration-response relationship. A lower affinity site (EC(50) 2 microM) slowed the rate of current deactivation, and increased the apparent affinity for ATP. In cell-attached patch recordings, P2X(4) receptor channels exhibited complex kinetics, with multiple components in both the open and shut distributions. IVM (0.3 microM) increased the number of openings per burst, without significantly changing the mean open or mean shut time within a burst. At higher concentrations (1.5 microM) of IVM, two additional open time components of long duration were observed that gave rise to long-lasting bursts of channel activity. Together, the results suggest that the binding of IVM to the higher affinity site increases current amplitude by reducing channel desensitization, whereas the binding of IVM to the lower affinity site slows the deactivation of the current predominantly by stabilizing the open conformation of the channel.

Mechanism of ivermectin facilitation of human P2X4 receptor channels.

The AMPA-type glutamate receptors mediate the majority of the fast excitatory synaptic transmission and critically contribute to synaptic plasticity in the brain, hence the existence of numerous trafficking proteins dedicated to regulation of their synaptic delivery and turnover. Stargazin (also termed gamma2) is a member of a recently identified protein family termed transmembrane AMPA receptor regulatory proteins (TARPs). TARPs physically associate with AMPA receptors and participate in their surface delivery and anchoring at the postsynaptic membrane. Here, we report that next to its trafficking roles, stargazin may also act as a positive allosteric modulator of AMPA receptor ion channel function. Coexpression of stargazin with AMPA receptor subunits, either in Xenopus oocytes or in human embryonic kidney 293 cells, significantly reduced receptor desensitization in response to glutamate. Receptor deactivation rates were also slowed, and the recovery from desensitization was accelerated. Structurally, based on the data showing a tight correlation between desensitization and the stability of the AMPA receptor intradimer interface, we propose that binding of stargazin may stabilize the receptor conformation. Functionally, our data suggest that AMPA receptors complexed with stargazin (and possibly also with other TARPs) at the postsynaptic membrane are significantly more responsive to synaptically released glutamate compared with AMPA receptors lacking stargazin/TARP interaction. The putative existence of such two states of synaptic AMPA receptors, with and without stargazin/TARP binding, may provide a novel mechanism for regulation of excitatory synaptic strength during development and/or in synaptic plasticity in the adult brain.

Stargazin reduces desensitization and slows deactivation of the AMPA-type glutamate receptors.

Airway ciliated cells express an ATP-gated P2X receptor channel of unknown subunit composition (P2X(cilia)) which is modulated by Na+ and by long exposures to ATP. P2X(cilia) was investigated by recording currents from freshly dissociated rabbit airway ciliated cells with the patch-clamp technique in the whole-cell configuration. During the initial continuous exposure to extracellular ATP, P2X(cilia) currents gradually increase in magnitude (priming), yet the permeability to N-methyl-D-glucamine (NMDG) does not change, indicating that priming does not arise from a progressive change in pore diameter. Na+, which readily permeates P2X(cilia) receptor channels, was found to inhibit the channel extracellular to the electric field. The rank order of permeability to various monovalent cations is: Li+, Na+, K+, Rb+, Cs+, NMDG+ and TEA+, with a relative permeability of 1.35, 1.0, 0.99, 0.91, 0.79, 0.19 and 0.10, respectively. The rank order for the alkali cations follows an Eisenman series XI for a high-strength field site. Ca2+ has been estimated to be 7-fold more permeant than Na+. The rise in [Ca2+]i in ciliated cells, induced by the activation of P2X(cilia), is largely inhibited by either Brilliant Blue G or KN-62, indicating that P2X7 may be a part of P2X(cilia). P2X(cilia) is augmented by Zn2+ and by ivermectin, and P2X4 receptor protein is detected by immunolabelling at the basal half of the cilia, strongly suggesting that P2X4 is a component of P2X(cilia) receptor channels. Taken together, these results suggest that P2X(cilia) is either assembled from P2X4 and P2X7 subunits, or formed from modified P2X4 subunits.

Pore properties and pharmacological features of the P2X receptor channel in airway ciliated cells.

A prominent feature of ionotropic glutamate receptors from the AMPA and kainate subtypes is their profound desensitization in response to glutamate-a process thought to protect the neuron from overexcitation. In AMPA receptors, it is well established that desensitization results from rearrangements of the interface formed between agonist-binding domains of adjacent subunits; however, it is unclear how this mechanism applies to kainate receptors. Here we show that stabilization of the binding domain dimer by the generation of intermolecular disulfide bonds apparently blocked desensitization of the kainate receptor GluR6. This result establishes a common desensitization mechanism in both AMPA and kainate receptors. Surprisingly, however, surface expression of these nondesensitizing mutants was drastically reduced and did not depend on channel activity. Therefore, in addition to its role at the synapse, we now propose an intracellular role for desensitization in controlling maturation and trafficking of glutamate receptors.

Block of kainate receptor desensitization uncovers a key trafficking checkpoint.

Patch-clamp recording has revolutionized the study of ion channels, transporters, and the electrical activity of small cells. Vital to this method is formation of a tight seal between glass recording pipette and cell membrane. To better understand seal formation and improve practical application of this technique, we examine the effects of divalent ions, protons, ionic strength, and membrane proteins on adhesion of membrane to glass and on seal resistance using both patch-clamp recording and atomic force microscopy. We find that H(+), Ca(2+), and Mg(2+) increase adhesion force between glass and membrane (lipid and cellular), decrease the time required to form a tight seal, and increase seal resistance. In the absence of H(+) (10(-10) M) and divalent cations (<10(-8) M), adhesion forces are greatly reduced and tight seals are not formed. H(+) (10(-7) M) promotes seal formation in the absence of divalent cations. A positive correlation between adhesion force and seal formation indicates that high resistance seals are associated with increased adhesion between membrane and glass. A similar ionic dependence of the adhesion of lipid membranes and cell membranes to glass indicates that lipid membranes without proteins are sufficient for the action of ions on adhesion.

Ionic requirements for membrane-glass adhesion and giga seal formation in patch-clamp recording.

Toxins have evolved to target regions of membrane ion channels that underlie ligand binding, gating, or ion permeation, and have thus served as invaluable tools for probing channel structure and function. Here, we describe a peptide toxin from the Earth Tiger tarantula that selectively and irreversibly activates the capsaicin- and heat-sensitive channel, TRPV1. This high-avidity interaction derives from a unique tandem repeat structure of the toxin that endows it with an antibody-like bivalency. The "double-knot" toxin traps TRPV1 in the open state by interacting with residues in the presumptive pore-forming region of the channel, highlighting the importance of conformational changes in the outer pore region of TRP channels during activation.

A bivalent tarantula toxin activates the capsaicin receptor, TRPV1, by targeting the outer pore domain.

Direct interaction of α9β1 integrin with nerve growth factor (NGF) has been previously reported to induce pro-proliferative and pro-survival activities of non-neuronal cells. We investigated participation of p75(NTR) in α9β1 integrin-dependent cellular response to NGF stimulation. Using selective transfection of glioma cell lines with these receptors, we showed a strong, cation-independent association of α9 integrin subunit with p75(NTR) on the cellular membrane by selective immunoprecipitation experiments. The presence of the α9/p75(NTR) complex increases NGF-dependent cell adhesion, proliferation and migration. Other integrin subunits including β1 were not found in complex with p75(NTR). FRET analysis indicated that p75(NTR) and α9 integrin subunit are not closely associated through their cytoplasmic domains, most probably because of the molecular interference with other cytoplasmic proteins such as paxillin. Interaction of α9β1 integrin with another ligand, VCAM-1 was not modulated by the p75(NTR). α9/p75(NTR) complex elevated NGF-dependent activation of MAPK Erk1/2 arty for integrin that may create active complexes with other types of receptors belonging to the TNF superfamily.

Association of p75(NTR) and α9β1 integrin modulates NGF-dependent cellular responses.

The receptor channel TRPV1 (Transient Receptor Potential Vanilloid 1) is expressed by primary afferent sensory neurons of the pain pathway, where it functions as a sensor of noxious heat and various chemicals, including eicosanoids, capsaicin, protons and peptide toxins. Comprised of four identical subunits that organize into a non-selective cationic permeable channel, this receptor has a variety of binding sites responsible for detecting their respective agonists. Although its physiological role as a chemosensor has been described in detail, the stoichiometry of TRPV1 activation by its different ligands remains unknown. Here, we combined the use of concatemeric constructs harboring mutated binding sites with patch-clamp recordings in order to determine the stoichiometry for TRPV1 activation through the vanilloid binding site and the outer-pore domain by capsaicin and protons, respectively. We show that, while a single capsaicin-bound subunit was sufficient to achieve a maximal open-channel lifetime, all four proton-binding sites were required. Thus, our results demonstrate a distinct stoichiometry of TRPV1 activation through two of its different agonist-binding domains.

The pain receptor TRPV1 displays agonist-dependent activation stoichiometry.

Vanilloids are pain evoking molecules that serve as ligands of the "heat and capsaicin receptor" TRPV1. Binding of either endogenous or exogenous vanilloids evokes channel and subsequent neuronal activation, leading to pain sensation. Despite its pivotal physiological role, the molecular basis of TRPV1 activation and deactivation is not fully understood. The highly conserved tyrosine in position 511 (Tyr(511)) of the rat TRPV1 (rTRPV1) was the first residue to be identified as a necessary participant in the vanilloid-mediated response. rTRPV1 cryo-EM structures implicated rotation of this residue in the vanilloids bound state. Therefore, we hypothesize that the rTRPV1 Tyr(511) residue entraps vanilloids in their binding site, prolonging channel activity. To test our hypothesis, we generated an array of rTRPV1 mutants, containing the whole spectrum of Tyr(511) substitutions, and tested their response to both exo- and endovanilloids. Our data show that only substitutions of Tyr(511) to aromatic amino acids were able to mimic, albeit partially, the vanilloid-evoked activation pattern of the wt receptor. Although these substitutions reduced the channel sensitivity to vanilloids, a maximal open-channel lifetime could be achieved. Moreover, whereas their current activation rate remains intact, receptors with Tyr(511) substitutions exhibited a faster current deactivation. Our findings therefore suggest that the duration of channel activity evoked by vanilloids is regulated by the interaction between Tyr(511) and the agonist. To conclude, we suggest that Tyr(511)-mediated anchoring of vanilloids in their binding pocket is pivotal for TRPV1 activation and subsequent pain sensation.

Tyrosine Residue in the TRPV1 Vanilloid Binding Pocket Regulates Deactivation Kinetics.

Peripheral neuronal activation by inflammatory mediators is a multifaceted physiological response that involves a multitude of regulated cellular functions. One key pathway that has been shown to be involved in inflammatory pain is Gq/GPCR, whose activation by inflammatory mediators is followed by the regulated response of the cation channel transient receptor potential vanilloid 1 (TRPV1). However, the mechanism that underlies TRPV1 activation downstream of the Gq/GPCR pathway has yet to be fully defined. In this study, we employ pharmacological and molecular biology tools to dissect this activation mechanism via perforated-patch recordings and calcium imaging of both neurons and a heterologous system. We showed that TRPV1 activity downstream of Gq/GPCR activation only produced a subdued current, which was noticeably different from the robust current that is typical of TRPV1 activation by exogenous stimuli. Moreover, we specifically demonstrated that 2 pathways downstream of Gq/GPCR signaling, namely endo-vanilloids production by lipoxygenases and channel phosphorylation by PKC, converge on TRPV1 to evoke a tightly regulated response. Of importance, we show that only when both pathways are acting on TRPV1 is the inflammatory-mediated response achieved. We propose that the requirement of multiple signaling events allows subdued TRPV1 activation to evoke regulated neuronal response during inflammation.-Kumar R., Hazan, A., Geron, M., Steinberg, R., Livni, L., Matzner, H., Priel, A. Activation of transient receptor potential vanilloid 1 by lipoxygenase metabolites depends on PKC phosphorylation.

Activation of transient receptor potential vanilloid 1 by lipoxygenase metabolites depends on PKC phosphorylation.

Peptide and protein toxins are essential tools to dissect and probe the biology of their target receptors. Venoms target vital physiological processes to evoke pain. Snake venoms contain various factors with the ability to evoke, enhance and sustain pain sensation. While a number of venom-derived toxins were shown to directly target TRPV1 channels expressed on somatosensory nerve terminals to evoke pain response, such toxins were yet to be identified in snake venoms.

Avi Priel

The Institute for Drug Research (IDR),
School of Pharmacy,
Faculty of Medicine,
The Institute for Drug Research (IDR), School of Pharmacy, Faculty of Medicine

The Hebrew University of Jerusalem (HUJI)

Controllable Ion Channel Expression through Inducible Transient Transfection

Avi Priel

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}The Institute for Drug Research (IDR),School of Pharmacy,Faculty of Medicine,The Institute for Drug Research (IDR), School of Pharmacy, Faculty of Medicine

The Hebrew University of Jerusalem (HUJI)

Controllable Ion Channel Expression through Inducible Transient Transfection

The Institute for Drug Research (IDR),
School of Pharmacy,
Faculty of Medicine,
The Institute for Drug Research (IDR), School of Pharmacy, Faculty of Medicine