Zinc is a voltage-dependent blocker of native and heterologously expressed epithelial Na+ channels

Perspectives on Scaffold Designs with Roles in Liver Cell Asymmetry and Medical and Industrial Applications by Using a New Type of Specialized 3D Bioprinter

Cellular asymmetry is an important element of efficiency in the compartmentalization of intracellular chemical reactions that ensure efficient tissue function. Improving the current 3D printing methods by using cellular asymmetry is essential in producing complex tissues and organs such as the liver. The use of cell spots containing at least two cells and basement membrane-like bio support materials allows cells to be tethered at two points on the basement membrane and with another cell in order to maintain cell asymmetry. Our model is a new type of 3D bioprinter that uses oriented multicellular complexes with cellular asymmetry. This novel approach is necessary to replace the sequential and slow processes of organogenesis with rapid methods of growth and 3D organ printing. The use of the extracellular matrix in the process of bioprinting with cells allows one to preserve the cellular asymmetry in the 3D printing process and thus preserve the compartmentalization of biological processes and metabolic efficiency.

Assessment of proarrhythmogenic risk for chloroquine and hydroxychloroquine using the CiPA concept

Chloroquine and hydroxychloroquine have been proposed recently as therapy for SARS-CoV-2-infected patients, but during 3 months of extensive use concerns were raised related to their clinical effectiveness and arrhythmogenic risk. Therefore, we estimated for these compounds several proarrhythmogenic risk predictors according to the Comprehensive in vitro Proarrhythmia Assay (CiPA) paradigm. Experiments were performed with either CytoPatch™2 automated or manual patch-clamp setups on HEK293T cells stably or transiently transfected with hERG1, hNav1.5, hKir2.1, hKv7.1+hMinK, and on Pluricyte® cardiomyocytes (Ncardia), using physiological solutions. Dose-response plots of hERG1 inhibition fitted with Hill functions yielded IC50 values in the low micromolar range for both compounds. We found hyperpolarizing shifts of tens of mV, larger for chloroquine, in the voltage-dependent activation but not inactivation, as well as a voltage-dependent block of hERG current, larger at positive potentials. We also found inhibitory effects on peak and late I_Na and on I_K1, with IC50 of tens of μM and larger for chloroquine. The two compounds, tested on Pluricyte® cardiomyocytes using the β-escin-perforated method, inhibited I_Kr, I_CaL, I_{Na peak}, but had no effect on I_f. In current-clamp they caused action potential prolongation. Our data and those from literature for I_to were used to compute proarrhythmogenic risk predictors B_net (Mistry HB, 2018) and Q_net (Dutta S et al., 2017), with hERG1 blocking/unblocking rates estimated from time constants of fractional block. Although the two antimalarials are successfully used in autoimmune diseases, and chloroquine may be effective in atrial fibrillation, assays place these drugs in the intermediate proarrhythmogenic risk group.

Raman micro-spectroscopy as a viable tool to monitor and estimate the ionic transport in epithelial cells

The typical response to the lowering of plasma Na⁺ concentration and blood pressure in our body involves the release of aldosterone from the adrenal glands, which triggers the reabsorption of sodium in the kidney. Although the effects of aldosterone on this physiological mechanism were extensively studied in the past decades, there are still some aspects to be fully elucidated. In the present study, we propose for the first time a new approach based on Raman spectroscopy to monitor the ionic activity in aldosterone-treated A6 renal epithelial cells. This spectroscopic technique is capable of probing the cells through their thickness in a non-destructive and nimble way. The spectroscopic variations of the Raman bands associated to the O-H stretching of water were correlated to the variations of ionic concentration in the intracellular and extracellular fluids. The increase of Na⁺ concentration gradients was clearly visualized in the cytosol of aldosterone-treated cells. The enhancement of the Na⁺ current density induced by aldosterone was estimated from the variation of the ionic chemical potential across the intracellular space. In addition, the variation of the O-H Raman bands of water was used to quantify the cell thickness, which was not affected by aldosterone.

Zinc inactivates melastatin transient receptor potential 2 channels via the outer pore

Zinc ion (Zn(2+)) is an endogenous allosteric modulator that regulates the activity of a wide variety of ion channels in a reversible and concentration-dependent fashion. Here we used patch clamp recording to study the effects of Zn(2+) on the melastatin transient receptor potential 2 (TRPM2) channel. Zn(2+) inhibited the human (h) TRPM2 channel currents, and the steady-state inhibition was largely not reversed upon washout and concentration-independent in the range of 30-1000 μM, suggesting that Zn(2+) induces channel inactivation. Zn(2+) inactivated the channels fully when they conducted inward currents, but only by half when they passed outward currents, indicating profound influence of the permeant ion on Zn(2+) inactivation. Alanine substitution scanning mutagenesis of 20 Zn(2+)-interacting candidate residues in the outer pore region of the hTRPM2 channel showed that mutation of Lys(952) in the extracellular end of the fifth transmembrane segment and Asp(1002) in the large turret strongly attenuated or abolished Zn(2+) inactivation, and mutation of several other residues dramatically changed the inactivation kinetics. The mouse (m) TRPM2 channels were also inactivated by Zn(2+), but the kinetics were remarkably slower. Reciprocal mutation of His(995) in the hTRPM2 channel and the equivalent Gln(992) in the mTRPM2 channel completely swapped the kinetics, but no such opposing effects resulted from exchanging another pair of species-specific residues, Arg(961)/Ser(958). We conclude from these results that Zn(2+) inactivates the TRPM2 channels and that residues in the outer pore are critical determinants of the inactivation.

Epithelial barrier resistance is increased by the divalent cation zinc in cultured MDCKII epithelial monolayers

Topical zinc applications promote wound healing and epithelialization. "Leaky" MDCKII epithelia exposed to apical ZnCl₂ (10 mM) showed a time-dependent increase (t (0.5) 22.2 ± 2.7 min) of transepithelial resistance (R (t)) from 82.3 ± 2.4 Ω cm² to 1,551 ± 225.6 Ω cm²; the increase was dose-dependent, being observed at 3 mM but not at 1 mM. Basal Zn²+ applications also increased epithelial resistance (at 10 mM to 323 ± 225.6 Ω cm²). The linear current-voltage relationship in control epithelia changed after apical 10 mM ZnCl₂ to show rectification. Voltage deflections resulting from inward currents showed time-dependent relaxation (basal potential difference (p.d.)-positive), with outward currents being time-independent. Cation selectivity was tested after apical ZnCl₂ elevated resistance; both the NaCl:mannitol (basal replacement) dilution p.d. and the choline:Na bi-ionic p.d. decreased (P(Na)/P(Cl) from 4.9 to 2.3 and P(Na)/P(choline) from 3.8 to 2.1, respectively). Transepithelial paracellular basal to apical ⁴⁵Ca fluxes increased approximately twofold when driven by a basal positive Na:NMDG bi-ionic p.d., but with basal 10 mM ZnCl₂, ⁴⁵Ca fluxes decreased approximately twofold. Neither ZO-1 nor occludin distribution was altered after ~2-h exposure to apical 10 mM ZnCl₂. However, claudin-2, though present at the tight junction, increased within the cell. Increased epithelial barrier resistance by Zn²+ is due to modification of the paracellular pathway, most probably by multiple mechanisms.

K+ channel openers restore verapamil-inhibited lung fluid resolution and transepithelial ion transport

Background

Lung epithelial Na⁺ channels (ENaC) are regulated by cell Ca²⁺ signal, which may contribute to calcium antagonist-induced noncardiogenic lung edema. Although K⁺ channel modulators regulate ENaC activity in normal lungs, the therapeutical relevance and the underlying mechanisms have not been completely explored. We hypothesized that K⁺ channel openers may restore calcium channel blocker-inhibited alveolar fluid clearance (AFC) by up-regulating both apical and basolateral ion transport.

Methods

Verapamil-induced depression of heterologously expressed human αβγ ENaC in Xenopus oocytes, apical and basolateral ion transport in monolayers of human lung epithelial cells (H441), and in vivo alveolar fluid clearance were measured, respectively, using the two-electrode voltage clamp, Ussing chamber, and BSA protein assays. Ca²⁺ signal in H441 cells was analyzed using Fluo 4AM.

Results

The rate of in vivo AFC was reduced significantly (40.6 ± 6.3% of control, P < 0.05, n = 12) in mice intratracheally administrated verapamil. K_Ca3.1 (1-EBIO) and K_ATP (minoxidil) channel openers significantly recovered AFC. In addition to short-circuit current (Isc) in intact H441 monolayers, both apical and basolateral Isc levels were reduced by verapamil in permeabilized monolayers. Moreover, verapamil significantly altered Ca²⁺ signal evoked by ionomycin in H441 cells. Depletion of cytosolic Ca²⁺ in αβγ ENaC-expressing oocytes completely abolished verapamil-induced inhibition. Intriguingly, K_V (pyrithione-Na), K _Ca3.1 (1-EBIO), and K_ATP (minoxidil) channel openers almost completely restored the verapamil-induced decrease in Isc levels by diversely up-regulating apical and basolateral Na⁺ and K⁺ transport pathways.

Conclusions

Our observations demonstrate that K⁺ channel openers are capable of rescuing reduced vectorial Na⁺ transport across lung epithelial cells with impaired Ca²⁺ signal.

Effect of divalent heavy metals on epithelial Na+ channels in A6 cells

To better understand how renal Na(+) reabsorption is altered by heavy metal poisoning, we examined the effects of several divalent heavy metal ions (Zn(2+), Ni(2+), Cu(2+), Pb(2+), Cd(2+), and Hg(2+)) on the activity of single epithelial Na(+) channels (ENaC) in a renal epithelial cell line (A6). None of the cations changed the single-channel conductance. However, ENaC activity [measured as the number of channels (N) x open probability (P(o))] was decreased by Cd(2+) and Hg(2+) and increased by Cu(2+), Zn(2+), and Ni(2+) but was not changed by Pb(2+). Of the cations that induced an increase in Na(+) channel function, Zn(2+) increased N, Ni(2+) increased P(o), and Cu(2+) increased both. The cysteine modification reagent [2-(trimethylammonium)ethyl]methanethiosulfonate bromide also increased N, whereas diethylpyrocarbonate, which covalently modifies histidine residues, affected neither P(o) nor N. Cu(2+) increased N and stimulated P(o) by reducing Na(+) self-inhibition. Furthermore, we observed that ENaC activity is slightly voltage dependent and that the voltage dependence of ENaC is insensitive to extracellular Na(+) concentration; however, apical application of Ni(2+) or diethylpyrocarbonate reduced the channel voltage dependence. Thus the voltage sensor of Xenopus ENaC is different from that of typical voltage-gated channels, since voltage appears to be sensed by histidine residues in the extracellular loops of ENaC, rather than by charged amino acids in a transmembrane domain.

Subunit-specific modulation of T-type calcium channels by zinc

Zinc (Zn2+) functions as a signalling molecule in the nervous system and modulates many ionic channels. In this study, we have explored the effects of Zn2+ on recombinant T-type calcium channels (CaV3.1, CaV3.2 and CaV3.3). Using tsA-201 cells, we demonstrate that CaV3.2 current (IC50, 0.8 microm) is significantly more sensitive to Zn2+ than are CaV3.1 and CaV3.3 currents (IC50, 80 microm and approximately 160 microm, respectively). This inhibition of CaV3 currents is associated with a shift to more negative membrane potentials of both steady-state inactivation for CaV3.1, CaV3.2 and CaV3.3 and steady-state activation for CaV3.1 and CaV3.3 currents. We also document changes in kinetics, especially a significant slowing of the inactivation kinetics for CaV3.1 and CaV3.3, but not for CaV3.2 currents. Notably, deactivation kinetics are significantly slowed for CaV3.3 current (approximately 100-fold), but not for CaV3.1 and CaV3.2 currents. Consequently, application of Zn2+ results in a significant increase in CaV3.3 current in action potential clamp experiments, while CaV3.1 and CaV3.2 currents are significantly reduced. In neuroblastoma NG 108-15 cells, the duration of CaV3.3-mediated action potentials is increased upon Zn2+ application, indicating further that Zn2+ behaves as a CaV3.3 channel opener. These results demonstrate that Zn2+ exhibits differential modulatory effects on T-type calcium channels, which may partly explain the complex features of Zn2+ modulation of the neuronal excitability in normal and disease states.

Extracellular zinc and ATP-gated P2X receptor calcium entry channels: New zinc receptors as physiological sensors and therapeutic targets

In this review, we focus on two attributes of P2X receptor channel function, one essential and one novel. First, we propose that P2X receptors are extracellular sensors as well as receptors and ion channels. In particular, the large extracellular domain (that comprises 70% of the molecular mass of the receptor channel protein) lends itself to be a cellular sensor. Moreover, its exquisite sensitivity to extracellular pH, ionic strength, and multiple ligands evokes the function of a sensor. Second, we propose that P2X receptors are extracellular zinc receptors as well as receptors for nucleotides. We provide novel data in multiple publications and illustrative data in this invited review to suggest that zinc triggers ATP-independent activation of P2X receptor channel function. In this light, P2X receptors are the cellular site of integration between autocrine and paracrine zinc signaling and autocrine and paracrine purinergic signaling. P2X receptors may sense changes in these ligands as well as in extracellular pH and ionic strength and transduce these sensations via calcium and/or sodium entry and changes in membrane potential.

Opposite effects of Ni2+ on Xenopus and rat ENaCs expressed in Xenopus oocytes

The epithelial Na+ channel (ENaC) is modulated by various extracellular factors, including Na+, organic or inorganic cations, and serine proteases. To identify the effect of the divalent Ni2+ cation on ENaCs, we compared the Na+ permeability and amiloride kinetics of Xenopus ENaCs (xENaCs) and rat ENaCs (rENaCs) heterologously expressed in Xenopus oocytes. We found that the channel cloned from the kidney of the clawed toad Xenopus laevis [wild-type (WT) xENaC] was stimulated by external Ni2+, whereas the divalent cation inhibited the channel cloned from the rat colon (WT rENaC). The kinetics of amiloride binding were determined using noise analysis of blocker-induced fluctuation in current adapted for the transoocyte voltage-clamp method, and Na+ conductance was assessed using the dual electrode voltage-clamp (TEVC) technique. The inhibitory effect of Ni2+ on amiloride binding is not species dependent, because Ni2+ decreased the affinity (mainly reducing the association rate constant) of the blocker in both species in competition with Na+. Importantly, using the TEVC method, we found a prominent difference in channel conductance at hyperpolarizing voltage pulses. In WT xENaCs, the initial ohmic current response was stimulated by Ni2+, whereas the secondary voltage-activated current component remained unaffected. In WT rENaCs, only a voltage-dependent block by Ni2+ was obtained. To further study the origin of the xENaC stimulation by Ni2+, and based on the rationale of the well-known high affinity of Ni2+ for histidine residues, we designed alpha-subunit mutants of xENaCs by substituting histidines that were expressed in oocytes, together with WT beta- and gamma-subunits. Changing His215 to Asp in one putative amiloride-binding domain (WYRFHY) in the extracellular loop between Na+ channel membrane segments M1 and M2 had no influence on the stimulatory effect of Ni2+, and neither did complete deletion of this segment. Next, we mutated His416 flanked by His411 and Cys417, a unique site for possible heavy metal ion chelation, and, with this quality, most proximal (approximately 100 amino acids upstream of the second putative amiloride binding site at the pore entrance), was found localized at M2. Replacing His416 with arginine, aspartate, tyrosine, and alanine clearly affected amiloride binding in all cases, as well as Na+ conductance, as expressed in the xENaC current-voltage relationship, especially with regard to aspartate and tyrosine. However, similarly to those obtained with the WYRFHY stretch, none of these mutations could either abolish the stimulating effect of Ni2+ or reverse it to an inhibitory type.

Extracellular Zn2+ activates epithelial Na+ channels by eliminating Na+ self-inhibition

Inhibition of epithelial Na(+) channel (ENaC) activity by high concentrations of extracellular Na(+) is referred to as Na(+) self-inhibition. We investigated the effects of external Zn(2+) on whole cell Na(+) currents and on the Na(+) self-inhibition response in Xenopus oocytes expressing mouse alphabetagamma ENaC. Na(+) self-inhibition was examined by analyzing inward current decay from a peak current to a steady-state current following a fast switching of a low Na(+) (1 mm) bath solution to a high Na(+) (110 mm) solution. Our results indicate that external Zn(2+) rapidly and reversibly activates ENaC in a dose-dependent manner with an estimated EC(50) of 2 microm. External Zn(2+) in the high Na(+) bath also prevents or reverses Na(+) self-inhibition with similar affinity. Zn(2+) activation is dependent on extracellular Na(+) concentration and is absent in ENaCs containing gammaH239 mutations that eliminate Na(+) self-inhibition and in alphaS580Cbetagamma following covalent modification by a sulfhydryl-reactive reagent that locks the channels in a fully open state. In contrast, external Ni(2+) inhibition of ENaC currents appears to be additive to Na(+) self-inhibition when Ni(2+) is present in the high Na(+) bath. Pretreatment of oocytes with Ni(2+) in a low Na(+) bath also prevents the current decay following a switch to a high Na(+) bath but rendered the currents below the control steady-state level measured in the absence of Ni(2+) pretreatment. Our results suggest that external Zn(2+) activates ENaC by relieving the channel from Na(+) self-inhibition, and that external Ni(2+) mimics or masks Na(+) self-inhibition.

External Ni2 + and ENaC in A6 cells: Na+ current stimulation by competition at a binding site for amiloride and Na+

In cultured A6 monolayers from distal Xenopus kidney, external Ni2+ stimulated active Na+ uptake via the epithelial Na+ channel, ENaC. Transepithelial capacitance measurements ruled out exocytosis of ENaC-containing vesicles underlying the Ni2+ effect. Na+ current noise analysis was performed using the neutral Na(+) -channel blocker 6-chloro-3,5-diamino-pyrazine-2-carboxamide (CDPC) and amiloride. The analysis of CDPC-induced noise in terms of a three-state channel model revealed that Ni2+ elicits an increase in the number of open channels as well as in the spontaneous open probability. While Ni2+ had no influence on CDPC-blocker kinetics, the macroscopic and microscopic blocking kinetics of amiloride were affected. Ni2+ turned out to compete with amiloride for a putative binding site but not with CDPC. Moreover, external Na(+)--known to compete with amiloride and so producing the "self-inhibition" phenomenon--and Ni2+ exerted mutually exclusive analogous effects on amiloride kinetics. Na+ current kinetics revealed that Ni2+ prevents ENaC to be downregulated by self-inhibition. Co2+ behaved similarly to Ni2+, whereas Zn2+ did not. Attempts to disclose the chemical nature of the site reacting with Ni2+ suggested cysteine but not histidine as reaction partner.