Silica nanoparticles inhibit the cation channel TRPV4 in airway epithelial cells
© The Author(s). 2017
Received: 5 May 2017
Accepted: 27 October 2017
Published: 3 November 2017
Silica nanoparticles (SiNPs) have numerous beneficial properties and are extensively used in cosmetics and food industries as anti-caking, densifying and hydrophobic agents. However, the increasing exposure levels experienced by the general population and the ability of SiNPs to penetrate cells and tissues have raised concerns about possible toxic effects of this material. Although SiNPs are known to affect the function of the airway epithelium, the molecular targets of these particles remain largely unknown. Given that SiNPs interact with the plasma membrane of epithelial cells we hypothesized that they may affect the function of Transient Receptor Potential Vanilloid 4 (TRPV4), a cation-permeable channel that regulates epithelial barrier function. The main aims of this study were to evaluate the effects of SiNPs on the activation of TRPV4 and to determine whether these alter the positive modulatory action of this channel on the ciliary beat frequency in airway epithelial cells.
Using fluorometric measurements of intracellular Ca2+ concentration ([Ca2+]i) we found that SiNPs inhibit activation of TRPV4 by the synthetic agonist GSK1016790A in cultured human airway epithelial cells 16HBE and in primary cultured mouse tracheobronchial epithelial cells. Inhibition of TRPV4 by SiNPs was confirmed in intracellular Ca2+ imaging and whole-cell patch-clamp experiments performed in HEK293T cells over-expressing this channel. In addition to these effects, SiNPs were found to induce a significant increase in basal [Ca2+]i, but in a TRPV4-independent manner. SiNPs enhanced the activation of the capsaicin receptor TRPV1, demonstrating that these particles have a specific inhibitory action on TRPV4 activation. Finally, we found that SiNPs abrogate the increase in ciliary beat frequency induced by TRPV4 activation in mouse airway epithelial cells.
Our results show that SiNPs inhibit TRPV4 activation, and that this effect may impair the positive modulatory action of the stimulation of this channel on the ciliary function in airway epithelial cells. These findings unveil the cation channel TRPV4 as a primary molecular target of SiNPs.
Synthetic amorphous SiNPs are extensively used due to its interesting physico-chemical properties, low cost and relatively easy production. This material has many applications in industrial manufacturing, cosmetics, biotechnology, medicine, and food, pharmaceutical and chemical industries [1–4]. SiNPs are widely used in consumer products and as a consequence, human exposure to this nanomaterial has highly increased. However, there is very little information available about the risks associated to the exposure to this nanomaterial .
It is known that SiNPs can penetrate cells, interacting with the plasma membrane, intracellular structures and organelles, thereby posing potential health threats [6–8]. The toxicity generated by nanoparticles has been related to an increased generation of reactive oxygen species (ROS) [9, 10]. This results in oxidative stress, mitochondrial perturbation and the generation of inflammatory mediators leading to cell dysfunction and apoptosis [2, 3, 11–19].
One of the main entry pathways of nanoparticles into the body is the epithelium of the airways. In addition to its function in gas exchange, the respiratory epithelium protects the body against hazardous environmental substances and pathogens, constituting an active diffusion barrier, and supporting the mechanisms of mucociliary clearance and recruitment of inflammatory cells [20–22]. Several cellular responses to SiNPs in the airways have been reported. Rabiolli et al. demonstrated that SiNPs induce lung inflammation through the stimulation of IL-1β production by alveolar macrophages . Skuland et al. showed evidence of pro-inflammatory responses induced by amorphous SiNPs in lung epithelial cells . Delaval et al. reported that SiNPs pre-exposure in pneumonia induced by Pseudomonas aeruginosa increases lung permeability and enhance mortality , and Kasper et al. showed inflammatory and cytotoxic responses such as DNA damage, hypoxia and ER-stress induced by SiNPs in an alveolar-capillary co-culture model .
However, little is known about the influence of SiNPs on specific molecular targets and cell signaling events, especially at the level of the plasma membrane. In this study we hypothesized that SiNPs may affect the function of TRPV4, a Ca2+-permeable cation channel expressed in airway epithelial cells. This channel plays a role in the transduction of physical and chemical stimuli into Ca2+ signals that regulate ciliary beat frequency and mucociliary transport [26, 27]. Moreover, TRPV4 contributes to the barrier integrity in the lung and to the regulation of endothelial and epithelial permeability [28, 29], and has been implicated in the modulation of the respiratory function and proposed as target for the treatment of respiratory diseases such chronic obstructive pulmonary disease and asthma [30–33].
We used intracellular Ca2+ imaging and patch-clamp to evaluate the effects of SiNPs on TRPV4 activation. We found that SiNPs inhibit the activation of native TRPV4 channels in human and mouse airway epithelial cells, as well as recombinant TRPV4 in the heterologous expression system HEK293T. Furthermore, SiNPs inhibited the TRPV4-mediated increase in ciliary beat frequency in mouse airway epithelial cells. TRPV4 emerges therefore as a defined molecular target of SiNPs, with possible deleterious consequences for epithelial barrier function.
SM30 Ludox® SiNPs were purchased from Sigma-Aldrich (Bornem, Belgium) as the commercial source of 30% wt suspension in H2O. For the biological experiments the nanoparticle suspension was diluted to the desired concentrations in Krebs solution containing (in mM): 150 NaCl, 6 KCl, 1 MgCl2, 1.5 CaCl2, 10 glucose, 10 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) and titrated to pH 7.4 with NaOH.
Dynamic Light Scattering (DLS) and Zeta potential
The stock suspension of SiNPs particles was diluted in water to 30 μg/ml. DLS and Zeta potential measurements were performed with a Brookhaven 90 Plus/ZetaPlus instrument (Brookhaven Instruments Ltd, Redditch, UK). DLS measurements were performed using a NanoParticle Size Distribution Analyser (scattering angle 90 u, wavelength 659 nm, power 15 mW). Correlation functions were analyzed using the Clementine package (maximum entropy method) for Igor Pro 6.02A (WaveMetrics, Portland, OR, USA).
Zeta potential measurements were done by applying electrophoretic light scattering. A primary and reference beam (659 nm, 35 mW), modulated optics and a dip-in electrode system were used. The frequency shift of scattered light (relative to the reference beam) from a charged particle moving in an electric field is related to the electrophoretic mobility of the particle. The Smoluchowski limit was used to calculate the Zeta potential from the electrophoretic mobility.
Transmission Electron Microscopy (TEM)
Suspensions (5 μl of stock suspension and 30 μg/ml) of the SiNPs particles were applied on formvar-coated cupper mesh grids (drop on grid). After drying overnight (25 °C in the dark), the particles were characterized by TEM (JEOL JEM-1200 EX-II, Tokyo, Japan).
We used the Endosafe-PTS LAL assay for FDA-licensed endotoxin detection. The cartridges contained four channels to which LAL reagent and a chromogenic substrate were applied. Two of these channels contained also an endotoxin spike that served as positive control. The sensitivity of the assay was 0.05 EU/ml.
Human bronchial epithelial cell line, 16HBE, were grown in Dulbecco's modified Eagle's medium: nutrient mixture F-12 (DMEM/F-12) containing 5% (v/v) fetal calf serum (FCS), 2 mM L-glutamine, 2 U/ml penicillin and 2 mg/ml streptomycin at 37 °C in a humidity-controlled incubator with 5% CO2 and were seeded on 18-mm glass cover slips coated with poly-L-lysine (0.1 mg/ml).
Human embryonic kidney cells, HEK293T, were grown in Dulbecco's modified Eagle's medium (DMEM) containing 10% (v/v) fetal calf serum (FCS), 2 mM L-glutamine, 2 U/ml penicillin, 1% non-essential amino acids (Invitrogen, Erembodegem - Aalst, Belgium) and 2 mg/ml streptomycin at 37 °C in a humidity-controlled incubator with 10% CO2 and were seeded on 18-mm glass cover slips coated with poly-L-lysine (0.1 mg/ml). For intracellular Ca2+ imaging and patch-clamp experiments, HEK293T cells were transiently transfected with mouse TRPV4 in the CAGGSM2/Ires/GFP/R1R2 vector, using Mirus TransIT-293 (Mirus Corporation; Madison, WI, USA). In all experiments, transfected cells were identified by green fluorescent protein (GFP) expression.
C57Bl/6J male mice from 8-12 weeks old were used for the experiments. The animals were maintained under standard conditions with a maximum of four animals per cage on a 12-h light/12-h dark cycle and with food and water ad libitum.
Culture of mouse tracheal epithelial cells
Mouse tracheal epithelial cells (mTEC) were isolated following the protocol described by Lam et al. , and seeded on 18-mm glass cover slips coated with collagen solution containing 50 μg/ml collagen (type I solution from rat tail, Sigma-Aldrich). Cells were grown for 2-3 days in the appropriate proliferation medium and maintained at 37 °C in a humidity-controlled incubator with 5% CO2.
Intracellular Ca2+ imaging experiments
Ca2+-imaging experiments were conducted with the ratiometric fluorescent indicator Fura-2 acetoxymethyl (AM) ester. Cells were incubated with 2 μM Fura-2 AM for 30 min at 37 °C. Bath solutions were perfused by gravity via a multi-barreled pipette tip with a single outlet of 0.8 mm inner diameter. This system allows full exchange of the medium bathing the recorded cell in less than 2-4 s. For recording in control condition cells were rinsed with Krebs solution. The [Ca2+]i was monitored through the ratio of fluorescence measured upon alternating illumination at 340 and 380 nm using an MT-10 illumination system and the xcellence pro software (Olympus, Planegg, Germany). All experiments conducted in the native 16HBE and mTEC cells were performed at 35 °C. Experiments in HEK293T cells were performed at 25 °C because at 35 °C the TRPV4-transfected cells were heavily overloaded with Ca2+ in basal condition.
where ∆[Ca2+] Max is the maximal amplitude of the response to SiNPs, [SiNPs] is the concentration of SiNPs, EC 50 is the effective concentration and H is the Hill coefficient.
where ∆[Ca2+] Max is the amplitude of the response in the absence of SiNPs, ∆[Ca2+] Inf is the amplitude of the response the presence of saturating concentrations of SiNPs, [SiNPs] is the concentration of SiNPs, IC 50 is the effective inhibitory concentration and H is the Hill coefficient.
Whole-cell voltage-clamp recordings were performed at 35 °C with standard patch pipettes (2-3 MΩ resistance) pulled using a DMZ-Universal puller (Zeitz Instruments, Augsburg, Germany). The pipette solution contained (in mM): 2 ATPNa2, 5 EGTA, 10 HEPES, 1 MgCl2, 135 CsCl2 (292 mOsm/kg; pH 7.2, adjusted with CsOH). For perforated patch experiments, 250 μg/ml of Amphotericin B was added to the pipette solution and data were collected after the access resistance reached stable values of ~15 MΩ.
An Ag-AgCl wire was used as reference electrode. The cover slips with cells were placed in the stage of an inverted microscope (Olympus IX70, Tokyo, Japan) and stabilized for a few minutes in Krebs solution, containing (in mM): 150 NaCl, 6 KCl, 1 MgCl2, 1.5 CaCl2, 10 glucose, and 10 HEPES and titrated to pH 7.4 with NaOH. The control bath solution was kept at room temperature and contained (in mM): 140 NaCl, 1.3 MgCl2, 2.4 CaCl2, 10 HEPES, 10 glucose and (311 mOsm/kg; pH 7.4, adjusted with NaOH). Bath solutions were perfused by gravity via a multi-barreled pipette. A bath solution in which all cations were isotonically substituted by NMDG+ (N-methyl-D-glucamine) was used to monitor the size of the leak currents during the patch-clamp recordings . Current signals were recorded using the patch-clamp technique by using an EPC-7 (LIST Electronics, Darmstadt, Germany) amplifier and the Clampex 9.0 software program (Axon instruments, Sunnyvale, CA, USA). Currents were acquired at 10 kHz, filtered at 2 kHz, and stored for off-line analysis on a personal computer. In order to minimize voltage errors, the series resistance was compensated by 30-50% and the capacitance artifact was reduced using the amplifier circuitry. Membrane TRPV4 currents were elicited by a 600 ms long voltage ramp from -100 mV to +100 mV every 5 s with a holding potential of 0 mV.
where ∆I Max is the current density increase in the absence of SiNPs, ∆I Inf is the current density increase in the presence of saturating concentrations of SiNPs, [SiNPs] is the concentration of SiNPs, IC 50 is the effective inhibitory concentration and H is the Hill coefficient.
Ciliary beat frequency (CBF) measurements
CBF was measured in primary cultures ciliated cells using with a high-speed digital imaging system as previously described . Briefly, phase-contrast images (512 × 512 pixels) were collected at 120–135 frames per second with a high speed CCD camera using a frame grabber (Infaimon, Barcelona, Spain) and recording software from Video Savant (IO Industries, London, ON, Canada). The ciliary beat frequency was determined from the frequency of variation in light intensity of the image as a result of repetitive motion of cilia.
All chemicals were purchased from Sigma-Aldrich (Bornem, Belgium).
Data are given as mean ± standard error of the mean. Comparisons tests are indicated in the text were appropriate. Statistical significance were taken at P < 0.05 or P < 0.01.
Characterization of the SM30 Ludox® SiNPs
Analysis of the SiNPs by DLS showed a single population of average size 10.2 nm (P10: 8.1 nm - P90: 11.8). The particles had a Zeta-potential of 20 ± 3 mV. TEM analysis of undiluted samples showed large aggregates, but in the diluted samples only a few aggregates could be found, and the particles appeared as spherical entities. No endotoxin contamination was detected in 30 μg/ml SiNPs dilutions.
Silica NPs inhibit TRPV4 activation in cultured human airway epithelial cells
SiNPs increase basal [Ca2+]i in a TRPV4-independent manner
An increase in basal [Ca2+]i does not inhibit a subsequent TRPV4 activation
SiNPs inhibit TRPV4 activation in mouse tracheal epithelial (mTEC) cells
SiNPs inhibit activation of recombinant TRPV4
Application of SiNPs induced a concentration-dependent increase of [Ca2+]i in cells with low basal [Ca2+]i (Fig. 5b, c), an effect reminiscent of that we observed in 16HBE cells (Fig. 1b-e). In contrast, SiNPs reduced [Ca2+]i in cells with high basal Ca2+ levels (Fig. 5b, c), which may be an indicative of an inhibitory effect of the SiNPs on the basal activity of TRPV4. In both groups SiNPs reduced the response to GSK1016790A with an IC 50 of 1.44 ± 0.06 μg/ml and a Hill coefficient of -2.0 ± 0.16 (Fig. 5d). Again, we found that application of SiNPs at high concentrations left ~30% of the response to the TRPV4 agonist.
Next, we determined whether SiNPs inhibit the activation of TRPV4 by another synthetic chemical agonist, 4α-phorbol 12,13-didecanoate (4αPDD). We found that SiNPs (300 μg/ml) strongly inhibited the response to 4αPDD (Fig. 5e, f), showing that their effect is not exclusive for channel activation with GSK1016790A.
Effects of SiNPs on the capsaicin receptor TRPV1
SiNPs inhibit TRPV4-mediated increase of ciliary beat frequency in airway epithelial cells
Despite the current advances in the characterization of the toxicological properties of SiNPs, little is known about how this material interacts with specific cellular components. Under the plausible assumption that SiNPs interact primarily with the plasma membrane of epithelial cells, in this study we evaluated the effects on TRPV4, a cation-permeable channel that is highly enriched in these cells.
In essence, we found that SiNPs inhibit intracellular Ca2+ signals triggered by activation of native TRPV4 channels in human and mouse airway epithelial cells. TRPV4 inhibition by SiNPs was confirmed with Ca2+ imaging and direct measurements of TRPV4 currents in the heterologous expression system HEK293T. In sharp contrast to these results, we show that SiNPs enhanced the activation of TRPV1, demonstrating that these particles have a specific inhibitory action on TRPV4 channels. Finally, we found that SiNPs abrogate the increase in ciliary beat frequency induced by TRPV4 activation in mouse airway epithelial cells.
The comparison of the data obtained in mTEC and HEK293T cells transfected with mouse TRPV4 indicate that SiNPs have very similar effects on the responses to GSK1016790A, with IC 50 values around 1 μg/ml. In contrast, SiNPs appeared to be much less effective in 16HBE cells, with a 100-fold higher IC 50 value. This may indicate that human TRPV4 is less sensitive than the mouse isoform. Nevertheless, the SiNPs concentrations required to inhibit TRPV4 mediated responses in the human-derived cells (100 - 3000 μg/ml) are in the same range or lower than those used in cytotoxicity and cytokine release in vitro experiments performed in previous studies (25 - 6000 μg/ml [18, 23, 24]. Moreover, we observed the inhibitory effect on TRPV4 in a matter of minutes, which represents a time scale 3- to 150-fold shorter than that of those previous reports. This strongly suggests TRPV4 as a primary and sensitive target of SiNPs.
As for the mechanism underlying the effects of SiNPs, it may be speculated that these particles somehow disrupt the binding site of GSK1016790A. SiNPs are roughly the same size of the whole channel protein, and more than twice the size of the length of the channel’s transmembrane segments. So, it is unlikely that these nanoparticles interact directly with a relatively small binding pocket for GSK1016790A, unless this would be located on the channel’s outer interface. However, to the best of our knowledge, the binding site for GSK1016790A is not yet known. On the other hand, we gained some insight into this issue from the result that SiNPs also strongly inhibit TRPV4 activation by 4αPDD, a compound that was reported to interact with an internal pocket of the channel formed between transmembrane segments 3 and 4 . Notably, SiNPs also altered the response of TRPV1 to capsaicin, which was reported to bind to an occluded region of this channel . Thus, according to our reasoning above, SiNPs seem not to act on TRPV4 and TRPV1 activation mechanisms by competitive inhibition. At this point we may just speculate that SiNPs induce mechanical perturbations in the plasma membrane that may disrupt activation of TRPV4 and enhance activation of TRPV1.
An interesting observation was that the SiNPs failed to completely inhibit TRPV4 activation. This could result from the presence of two channel populations with distinct sensitivities to SiNPs. This might be the case for the HEK293T cells, in which an endogenous human TRPV4 channel population may co-exist with the transfected mouse TRPV4 channels. However, we observed the lack of complete inhibition also in native 16HBE and mTEC, for which there is no evidence for separate populations of TRPV4. Although further studies are required to address this point, we may also consider that if SiNPs inhibit channel activation by inducing mechanical perturbations in the plasma membrane, these might not be sufficient to completely silence channel activity.
A concomitant finding in our experiments was that SiNPs induce an increase in basal [Ca2+]i in the human-derived cells. However, our data demonstrates that TRPV4 channels are not involved in this effect (e.g., lack of inhibitory effect of the TRPV4 blocker HC067047). This is different from what was previously suggested by Gilardino et al., who found that the unspecific TRPV channel blocker ruthenium red inhibited intracellular Ca2+ responses to SiNPs . A possible cause for this is that these authors used particles of 50 nm in diameter, which represents about a 170-fold larger volume than that of the ones we used here. Considering that TRPV4 can be activated by mechanical stress at the plasma membrane [39, 46], it is conceivable that only the larger particles may induce TRPV4 activation. On the other hand, Gilardino et al.  did find TRPV4-independent responses to SiNPs, which could be triggered via mechanisms similar to those underlying the responses we found in 16HBE cells and in HEK293T cells displaying low basal Ca2+ concentration. Of note, for some yet unclear reasons mTEC did not display Ca2+ responses upon SiNPs application.
Other features of our results are also qualitatively comparable to those obtained by Gilardino et al. . For instance, the intracellular Ca2+ responses to SiNPs occurred after a significant delay and showed a transient initial phase (Fig. 2b). The mechanisms underlying these responses remain fully unknown, but could be related to Ca2+ release from intracellular stores. However, the fact that we found SiNPs to increase inward and outward basal currents in HEK293T cells is more consistent with enhanced activities of Ca2+-permeable channels in the plasma membrane (e.g., the ubiquitously expressed TRPM7 channels) . These mechanisms should be addressed in future studies because they may bare relevance for the toxic effects of SiNPs (Ca2+ overload) in airway epithelial cells.
Our results show that SiNPs inhibit TRPV4 activation, and that this effect may impair the positive modulatory action of the stimulation of this channel on the ciliary function in airway epithelial cells. It has been proposed that inhibition of TRPV4 could have therapeutic benefits in several respiratory conditions, such as chronic heart failure, hypoxia-induced pulmonary hypertension, acute lung injury, chronic obstructive pulmonary disease and cough [48–51]. However, TRPV4 activity has been shown to underlie protective responses in airway epithelial cells, including the increase in CBF [26, 43]. In addition, TRPV4 function was reported to be important for essential functions in other cells that are direct targets of polluting SiNPs. These include the enhancement of barrier function is skin keratynocytes [52, 53], endothelium-dependent vasorelaxation in pulmonary arteries , and ATP release from oesophageal keratynocytes . Thus, inhibition of TRPV4 by SiNPs is expected to have complex effects on airway pathophysiology and rather certain detrimental effects on several epithelial cell functions. Our findings unveil TRPV4 and TRPV1 as defined molecular targets of SiNPs, and prompt for further exploration of the role of these channels in the cellular effects of other types of particulate matter.
The authors would like to thank Prof. Bernd Nilius and the members of the LICR laboratory for helpful discussions and to Melissa Benoit for the maintenance of the cell cultures.
Availability of data and materials
All data generated or analysed during this study are included in this published article
Y.A.A. held a Postdoctoral Mandate of the KU Leuven and is currently a Postdoctoral Fellow of the Fund for Scientific Research Flanders (FWO). Research was supported by grants from the Research Foundation Flanders FWO (G076714), the Research Council of the KU Leuven (Grants GOA/14/011 and PF-TRPLe), The Spanish Ministry of Economy and Competitiveness (SAF2015-69762R and María de Maeztu Programme for Units of Excellence in R&D MDM-2014-0370), and the FEDER Funds.
AS, KD and YAA performed Ca2+ imaging experiments; AS, JLA and JAC performed the patch-clamp experiments; CJ performed the CBF measurements; SMC performed the characterization of the particles; MAV, PHH and KT supervised the project; AS and KT wrote the manuscript. All authors edited the manuscript, and have given approval to its final version.
Experiments were performed in accordance with the guidelines established by the European Communities Council Directive (86/609/ECC) and by the Ethics Committee of the KU Leuven, Leuven (021/2012).
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