Properties of the Ca(2+)-activated Cl- current of Xenopus oocytes

The "www" of Xenopus laevis Oocytes: The Why, When, What of Xenopus laevis Oocytes in Membrane Transporters Research

After 50 years, the heterologous expression of proteins in Xenopus laevis oocytes is still essential in many research fields. New approaches and revised protocols, but also classical methods, such as the two-electrode voltage clamp, are applied in studying membrane transporters. New and old methods for investigating the activity and the expression of Solute Carriers (SLC) are reviewed, and the kinds of experiment that are still useful to perform with this kind of cell are reported. Xenopus laevis oocytes at the full-grown stage have a highly efficient biosynthetic apparatus that correctly targets functional proteins at the defined compartment. This small protein factory can produce, fold, and localize almost any kind of wild-type or recombinant protein; some tricks are required to obtain high expression and to verify the functionality. The methodologies examined here are mainly related to research in the field of membrane transporters. This work is certainly not exhaustive; it has been carried out to be helpful to researchers who want to quickly find suggestions and detailed indications when investigating the functionality and expression of the different members of the solute carrier families.

Cobalt oxide nanoparticles can enter inside the cells by crossing plasma membranes

The ability of nanoparticles (NPs) to be promptly uptaken by the cells makes them both dangerous and useful to human health. It was recently postulated that some NPs might cross the plasma membrane also by a non-endocytotic pathway gaining access to the cytoplasm. To this aim, after having filled mature Xenopus oocytes with Calcein, whose fluorescence is strongly quenched by divalent metal ions, we have exposed them to different cobalt NPs quantifying quenching as evidence of the increase of the concentration of Co(2+) released by the NPs that entered into the cytoplasm. We demonstrated that cobalt oxide NPs, but not cobalt nor cobalt oxide NPs that were surrounded by a protein corona, can indeed cross plasma membranes.

Interactions between permeation and gating in the TMEM16B/anoctamin2 calcium-activated chloride channel

Extracellular anions more permeant than Cl⁻ modulate TMEM16B gating to promote channel opening, whereas less permeant anions favor channel closure.

At least two members of the TMEM16/anoctamin family, TMEM16A (also known as anoctamin1) and TMEM16B (also known as anoctamin2), encode Ca²⁺-activated Cl⁻ channels (CaCCs), which are found in various cell types and mediate numerous physiological functions. Here, we used whole-cell and excised inside-out patch-clamp to investigate the relationship between anion permeation and gating, two processes typically viewed as independent, in TMEM16B expressed in HEK 293T cells. The permeability ratio sequence determined by substituting Cl⁻ with other anions (P_X/P_Cl) was SCN⁻ > I⁻ > NO₃⁻ > Br⁻ > Cl⁻ > F⁻ > gluconate. When external Cl⁻ was substituted with other anions, TMEM16B activation and deactivation kinetics at 0.5 µM Ca²⁺ were modified according to the sequence of permeability ratios, with anions more permeant than Cl⁻ slowing both activation and deactivation and anions less permeant than Cl⁻ accelerating them. Moreover, replacement of external Cl⁻ with gluconate, or sucrose, shifted the voltage dependence of steady-state activation (G-V relation) to more positive potentials, whereas substitution of extracellular or intracellular Cl⁻ with SCN⁻ shifted G-V to more negative potentials. Dose–response relationships for Ca²⁺ in the presence of different extracellular anions indicated that the apparent affinity for Ca²⁺ at +100 mV increased with increasing permeability ratio. The apparent affinity for Ca²⁺ in the presence of intracellular SCN⁻ also increased compared with that in Cl⁻. Our results provide the first evidence that TMEM16B gating is modulated by permeant anions and provide the basis for future studies aimed at identifying the molecular determinants of TMEM16B ion selectivity and gating.

Endogenous transport systems in the Xenopus laevis oocyte plasma membrane

Oocytes of the South African clawed frog Xenopus laevis are widely used as a heterologous expression system for the characterization of transport systems such as passive and active membrane transporters, receptors and a whole plethora of other membrane proteins originally derived from animal or plant tissues. The large size of the oocytes and the high degree of expression of exogenous mRNA or cDNA makes them an optimal tool, when compared with other expression systems such as yeast, Escherichia coli or eukaryotic cell lines, for the expression and functional characterization of membrane proteins. This easy to handle expression system is becoming increasingly attractive for pharmacological research. Commercially available automated systems that microinject mRNA into the oocytes and perform electrophysiological measurements fully automatically allow for a mass screening of new computer designed drugs to target membrane transport proteins. Yet, the oocytes possess a large variety of endogenous membrane transporters and it is absolutely mandatory to distinguish the endogenous transporters from the heterologous, expressed transport systems. Here, we review briefly the endogenous membrane transport systems of the oocytes.

Kinetic relationship between the voltage sensor and the activation gate in spHCN channels

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are activated by membrane hyperpolarizations that cause an inward movement of the positive charges in the fourth transmembrane domain (S4), which triggers channel opening. The mechanism of how the motion of S4 charges triggers channel opening is unknown. Here, we used voltage clamp fluorometry (VCF) to detect S4 conformational changes and to correlate these to the different activation steps in spHCN channels. We show that S4 undergoes two distinct conformational changes during voltage activation. Analysis of the fluorescence signals suggests that the N-terminal region of S4 undergoes conformational changes during a previously characterized mode shift in HCN channel voltage dependence, while a more C-terminal region undergoes an additional conformational change during gating charge movements. We fit our fluorescence and ionic current data to a previously proposed 10-state allosteric model for HCN channels. Our results are not compatible with a fast S4 motion and rate-limiting channel opening. Instead, our data and modeling suggest that spHCN channels open after only two S4s have moved and that S4 motion is rate limiting during voltage activation of spHCN channels.

Functional expression of type 1 rat GABA transporter in microinjected Xenopus laevis oocytes

In this chapter we describe technical aspects and experimental potential of the two electrodes voltage clamp (TEVC) electrophysiological approach applied to the Xenopus oocyte-expression system. This technique is addressed to the study of a particular class of expressed proteins, those responsible to drive ion fluxes through the plasma membrane. In fact the voltage-clamp technique provides the most direct and sensitive measurement of the functional properties of ion channels and electrogenic transporters, allowing specific ion currents to be recorded under well-defined voltage conditions and temporal control. Besides the study of the physiological properties of specific ion channels as well as their pharmacological modulation, further applications of the TEVC on oocytes include the possibility to introduce single point mutations in the channel construct and to infer to possible structural aspects and functional involvements of single amino acidic residues. To achieve these results these technique should be strictly tied to basic molecular biology techniques. Recent advance of this technique in drug discovery procedures have been briefly enlightened.

Differential ion current activation by human 5-HT(1A) receptors in Xenopus oocytes: evidence for agonist-directed trafficking of receptor signalling

The subject of the present study was the functional and pharmacological characterization of human 5-HT(1A) receptor regulation of ion channels in Xenopus oocytes. Activation of the heterologously expressed human 5-HT(1A) receptor induced two distinct currents in Xenopus oocytes, consisting of a smooth inward current (I(smooth)) and an oscillatory calcium-activated chloride current, I(Cl(Ca)). 5-HT(1A) receptor coupling to both ionic responses as well as to co-expressed inward rectifier potassium (GIRK) channels was pharmacologically characterized using 5-HT(1A) receptor agonists. The relative order of efficacy for activation of GIRK current was 5-HT approximately F 13714 approximately L 694,247 approximately LY 228,729>flesinoxan approximately (+/-)8-OH-DPAT. In contrast, flesinoxan and (+/-)8-OH-DPAT typically failed to activate I(Cl(Ca)). The other ligands behaved as full or partial agonists, exhibiting an efficacy rank order of 5-HT approximately L 694,247>F 13714 approximately LY 228,729. The pharmacological profile of I(smooth) activation was completely distinct: flesinoxan and F 13714 were inactive and rather exhibited an inhibition of this current. I(smooth) was activated by the other agonists with an efficacy order of L 694,247>5-HT approximately LY 228,729>(+/-)8-OH-DPAT. Moreover, activation of I(smooth) was not affected by application of pertussis toxin or the non-hydrolyzable GDP-analogue, guanosine-5'-O-(2-thio)-diphosphate (GDP betaS), suggesting a GTP binding protein-independent pathway. Together, these results suggest the existence of distinct and agonist-specific signalling states of this receptor.

Differential regulation of Ca(2+)-dependent Cl- currents by FP prostanoid receptor isoforms in Xenopus oocytes

The FP(A) and FP(B) prostanoid receptor isoforms are G-protein-coupled receptors that are activated by prostaglandin F(2alpha) (PGF(2alpha)). Differences in their carboxyl termini prompted us to examine the intracellular calcium (Ca(2+)) signaling of these receptor isoforms using the Xenopus oocyte expression system. Protein expression was determined by immunofluorescence microscopy and whole cell binding with [3H]PGF(2alpha). Positive immunolabeling was observed on the outer membranes of oocytes expressing FLAG-tagged FP receptor isoforms, but not on control (water-injected) oocytes. Intracellular signaling was examined using a two-electrode voltage clamp. Specific whole-cell binding was also detected for both receptor isoforms. Bath application of 10 microM PGF(2alpha) to FP(A)-expressing oocytes produced a chloride (Cl-) current response similar to that of an injection of inositol 1,4,5-trisphosphate (InsP(3)) (5.76+/-0.6 microA, peak current; N=23) that returned to control levels within 25 min. In FP(B)-expressing oocytes the activation of the Cl- current was delayed or completely absent (1.38+/-0.2 microA, peak current; N=18). Control oocytes were not responsive to the application of PGF(2alpha) (0.87+/-0.1 microA, peak current; N=10). Activation of Cl- currents for both FP receptor isoforms was dependent upon intracellular Ca(2+) stores as a 30-min pretreatment with thapsigargin (1 microM; N=5) blocked the PGF(2alpha) induction of the Cl- current. These data indicate that the FP prostanoid receptor isoforms differ in their ability to activate Ca(2+)-dependent Cl- channels when expressed in Xenopus oocytes. The difference appears to be in the ability of the two FP prostanoid receptor isoforms to mobilize intracellular calcium.

Evidence that the TRP-1 protein is unlikely to account for store-operated Ca2+ inflow in Xenopus laevis oocytes

The role of the TRP-1 protein, an animal cell homologue of the Drosophila transient receptor potential Ca2+ channel, in store-operated Ca2+ inflow in Xenopus laevis oocytes was investigated. A strategy involving RT-PCR and 3' and 5' rapid amplification of cDNA ends (RACE) was used to confirm and extend previous knowledge of the nucleotide and predicted amino acid sequences of Xenopus TRP-1 (xTRP-1). The predicted amino acid sequence was used to prepare an anti-TRP-l polyclonal antibody which detected the endogenous oocyte xTRP-1 protein and the human TRPC-1 protein expressed in Xenopus oocytes. Ca2+ inflow (measured using fura-2) initiated by 3-deoxy-3-fluoroinositol 1,4,5-trisphosphate (InsP3F) or lysophosphatidic acid (LPA) was completely inhibited by low concentrations of lanthanides (IC50 = 0.5 microM), indicating that InsP3F and LPA principally activate store-operated Ca2+ channels (SOCs). Antisense cRNA or antisense oligodeoxynucleotides, based on different regions of the xTRP-1 cDNA sequence, when injected into Xenopus oocytes, did not inhibit InsP3F-, LPA- or thapsigargin-stimulated Ca2+ inflow. Oocytes expressing the hTRPC-1 protein, which is 96% similar to xTRP-1, exhibited no detectable enhancement of either basal or InsP3F-stimulated Ca2+ inflow and only a very small enhancement of LPA-stimulated Ca2+ in-flow compared with control oocytes. It is concluded that the endogenous xTRP-1 protein is unlikely to be responsible for Ca2+ inflow through the previously-characterised Ca2+ -specific SOCs which are found in Xenopus oocytes. It is considered that xTRP-1 is likely to be a receptor-activated non-selective cation channel such as the channel activated by maitotoxin.

Neuronal Ca2+ -activated Cl- channels--homing in on an elusive channel species

Ca2+ -activated Cl- channels control electrical excitability in various peripheral and central populations of neurons. Ca2+ influx through voltage-gated or ligand-operated channels, as well as Ca2+ release from intracellular stores, have been shown to induce substantial Cl- conductances that determine the response to synaptic input, spike rate, and the receptor current of various kinds of neurons. In some neurons, Ca2+ -activated Cl- channels are localized in the dendritic membrane, and their contribution to signal processing depends on the local Cl- equilibrium potential which may differ considerably from those at the membranes of somata and axons. In olfactory sensory neurons, the channels are expressed in ciliary processes of dendritic endings where they serve to amplify the odor-induced receptor current. Recent biophysical studies of signal transduction in olfactory sensory neurons have yielded some insight into the functional properties of Ca2+ -activated Cl- channels expressed in the chemosensory membrane of these cells. Ion selectivity, channel conductance, and Ca2+ sensitivity have been investigated, and the role of the channels in the generation of receptor currents is well understood. However, further investigation of neuronal Ca2+ -activated Cl- channels will require information about the molecular structure of the channel protein, the regulation of channel activity by cellular signaling pathways, as well as the distribution of channels in different compartments of the neuron. To understand the physiological role of these channels it is also important to know the Cl- equilibrium potential in cells or in distinct cell compartments that express Ca2+ -activated Cl- channels. The state of knowledge about most of these aspects is considerably more advanced in non-neuronal cells, in particular in epithelia and smooth muscle. This review, therefore, collects results both from neuronal and from non-neuronal cells with the intent of facilitating research into Ca2+ -activated Cl- channels and their physiological functions in neurons.

Hyperpolarization-activated inward potassium and calcium-sensitive chloride currents in beating pacemaker insect neurosecretory cells (dorsal unpaired median neurons)

Hyperpolarization-activated inward currents were studied in single adult cockroach Periplaneta americana pacemaker neurosecretory cells, identified as dorsal unpaired median neurons using the whole-cell patch-clamp technique. Under current clamp, injection of negative current produced a hyperpolarization of the cell membrane with a sag in the membrane potential toward the resting value. Under voltage clamp, the whole-cell current-voltage relationship exhibited an unexpected biphasic aspect. The global hyperpolarization-activated inward current could be dissociated by means of 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid and tetraethylammonium chloride sensitivity, ionic selectivity, voltage dependence and activation threshold as inward potassium and calcium-sensitive chloride currents. The inward potassium current was activated around -80 mV. The reversal potential followed the potassium equilibrium potential when the extracellular potassium concentration was raised. This current was not dependent on the external sodium concentration and was sensitive to 10 mM tetraethylammonium chloride or 5 mM barium chloride. The hyperpolarization-activated inward calcium-sensitive chloride current was activated in a range of potential 20 mV more positive than the potassium current. The estimated reversal potential (-71 mV) was very close to the equilibrium potential for chloride ions ( 73 mV). Intracellularly applied 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and external application of 1 mM zinc chloride, calcium-free saline or high concentrations of intracellular 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetate blocked the inward chloride current. Current-clamp experiments indicated that the inward potassium current accounted for inward rectification of dorsal unpaired median neurons. Our findings report, for the first time in pacemaker neurosecretory cells, the co-existence of two distinct hyperpolarization-activated inward currents which have specialized function in pacemaker activity.

Ca2+-dependence of the depolarization-inducible Na+ current of Xenopus oocytes

The role of Ca2+ on the depolarization-induced appearance of a Na+ current in Xenopus oocytes was studied. Oocytes were voltage-clamped and the induction of the Na+ current was tested under various conditions. In oocytes pre-injected with 400 pmol EGTA to increase the intracellular Ca2+ buffering power, the current was significantly reduced. Conversely, when intracellular Ca2+ was made to increase by injecting an analogue of inositol 1,4,5-trisphosphate (3-F InsP3), to cause Ca2+ release from internal stores, the induction of the Na+ current was potentiated. The depolarization-inducible Na+ channels of the Xenopus oocyte membrane appear, therefore, to be Ca2+ sensitive, as well as depolarization-activated.