Upregulation of acid sensing ion channel 1a (ASIC1a) by hydrogen peroxide through the JNK pathway

Abstract

Oxidative stress is intimately tied to neurodegenerative diseases, including Parkinson’s disease and amyotrophic lateral sclerosis, and acute injuries, such as ischemic stroke and traumatic brain injury. Acid sensing ion channel 1a (ASIC1a), a proton-gated ion channel, has been shown to be involved in the pathogenesis of these diseases. However, whether oxidative stress affects the expression of ASIC1a remains elusive. In the current study, we examined the effect of hydrogen peroxide (H₂O₂), a major reactive oxygen species (ROS), on ASIC1a protein expression and channel function in NS20Y cells and primary cultured mouse cortical neurons. We found that treatment of the cells with H₂O₂ (20 µM) for 6 h or longer increased ASIC1a protein expression and ASIC currents without causing significant cell injury. H₂O₂ incubation activated mitogen-activated protein kinases (MAPKs) pathways, including the extracellular signal-regulated kinase1/2 (ERK1/2), c-Jun N-terminal kinase (JNK), and p38 pathways. We found that neither inhibition of the MEK/ERK pathway by U0126 nor inhibition of the p38 pathway by SB203580 affected H₂O₂-induced ASIC1a expression, whereas inhibition of the JNK pathway by SP600125 potently decreased ASIC1a expression and abolished the H₂O₂-mediated increase in ASIC1a expression and ASIC currents. Furthermore, we found that H₂O₂ pretreatment increased the sensitivity of ASIC currents to the ASIC1a inhibitor PcTx1, providing additional evidence that H₂O₂ increases the expression of functional ASIC1a channels. Together, our data demonstrate that H₂O₂ increases ASIC1a expression/activation through the JNK signaling pathway, which may provide insight into the pathogenesis of neurological disorders that involve both ROS and activation of ASIC1a.

Introduction

Acid-sensing ion channels (ASICs) are proton-gated, voltage-insensitive cationic channels that are widely expressed in the nervous system. Four ASIC genes, namely ASIC1, ASIC2, ASIC3 and ASIC4, and six ASIC isoforms, namely ASIC1a, ASIC1b, ASIC2a, ASIC2b, ASIC3, and ASIC4, have been identified [1, 2]. ASIC1a, 2a, and 2b are expressed in both the central and peripheral nervous systems, while ASIC3 and ASIC1b are predominantly expressed in the peripheral nervous system [1, 3–5]. ASIC1a, the predominant and functional ASIC subunit in the brain, has been demonstrated to be involved in important physiological processes, including synaptic transmission, learning and memory, olfactory function, and neurological disorders, including ischemic stroke, multiple sclerosis, epileptic seizure, and Parkinson’s disease [5–12].

Oxidative stress is the result of an imbalance between pro-oxidant and antioxidant homeostasis that leads to the generation of toxic reactive oxygen or nitrogen species (ROS/NOS), including superoxide anion (^.O₂^-), hydrogen peroxide (H₂O₂), hydroxyl (^.OH), nitric oxide (^.NO) and peroxynitrite (ONOO^-) [13]. ROS may damage all components of the cell, including proteins, lipids, and DNA [14]. The human brain is susceptible to oxidative stress, which has been shown to contribute to the pathogenesis of both acute and chronic neurological disorders, including ischemic stroke, PD, ALS, and diabetic encephalopathy [15, 16]. H₂O₂, one of the main ROS, plays an active role in the regulation of various physiological processes, including cell proliferation, differentiation, migration and apoptosis [17, 18]. Nevertheless, its overproduction results in oxidative stress, which can lead to extensive cellular damage. Exposure of cells to H₂O₂ induces activation of MAPKs, including ERK1/2, JNK, and p38 kinase [19].

The current study was designed to investigate the effect of H₂O₂ on ASIC1a expression and ASIC channel activation and the underlying signaling pathway in both neuronal cell lines and primary cultured mouse cortical neurons.

Materials and methods

Chemicals and antibodies

H₂O₂ (Cat. No. 216763), SB203580 (Cat. No. S8037) and SP600125 (Cat. No. S5567) were purchased from Sigma-Aldrich (St. Louis, MO, USA). U0126 (Cat. No. 9903 S) was purchased from Cell Signaling Technology (Danvers, MA, USA). The ASIC1 antibody was a gift from Dr. Xiang-ming Zha (University of South Alabama, USA) [20]. Antibodies, including p38 MAPK (Cat. No. 8690), phospho-p38 (Cat. No. 4511), JNK (Cat. No. 9252), phospho-JNK (Cat. No. 9251), ERK1/2 (Cat. No. 4695), phospho-ERK1/2 (Cat. No. 9101), were purchased from Cell Signaling Technology. β-Actin (Cat. No. A5441) was purchased from Sigma-Aldrich.

Cell culture

NS20Y cells were purchased from Sigma-Aldrich and cultured in Dulbecco’s modified Eagle’s medium (Invitrogen, Carlsbad, CA, USA) as we described previously [21]. Cells were plated on  35-mm dishes coated with poly-L-ornithine and maintained at 37 °C in a humidified incubator with 5% CO₂. Mouse cortical neurons were isolated and cultured as described previously [22]. Pregnant Swiss mice were purchased from Charles River. Briefly, pregnant Swiss mice (embryonic day 16) were anesthetized with isoflurane and sacrificed by cervical dislocation. Fetal brains were quickly removed and placed in cold phosphate-buffered saline without Ca²⁺/Mg²⁺. Tissues were dissected, incubated with 0.05% trypsin-EDTA at 37 °C for 10 min, and triturated using fire-polished glass pipettes. Cortical neurons were counted and plated in poly-L-ornithine-coated  35-mm culture dishes (1 × 10⁶ cells/dish) or 24-well plates (2 × 10⁵ cells/well). Neurons were cultured in neurobasal medium (Invitrogen) supplemented with B-27 (Invitrogen) and glutamine and maintained in a humidified incubator with 5% CO₂ at 37 °C. The culture medium was changed every 3 days, and neurons were used for experiments 12–14 days after plating.

Western blot

Protein was extracted with M-PER™ Mammalian Protein Extraction Reagent (Cat. No. 78501, Thermo Fisher Scientific) and Halt™ Protease and Phosphatase Inhibitor Cocktail (Thermo Fisher Scientific). After centrifugation at 13,000 × g for 15 min at 4 °C, the lysates were collected and mixed with Laemmli sample buffer and then boiled for 10 min. The protein concentrations of the cell samples were measured using the Bio-Rad protein assay kit (BioRad, Hercules, CA, USA). The proteins were separated on 10% SDS-polyacrylamide gels and then transferred to PVDF membranes. After blocking, the blots were probed with primary antibodies followed by horseradish peroxidase-conjugated secondary antibodies. The signals were visualized using an ECL kit (Millipore, WBLUR0500).

Lactate dehydrogenase (LDH) assay

Cytotoxicity was measured by the LDH assay using a cytotoxicity detection kit (Cat. No. 11644793001, Roche Diagnostics) according to the manufacturer’s instructions, as described in our previous studies [22]. At the end of the experiments, 50 μL of culture medium was transferred from each well to a 96-well plate for the measurement of LDH release. To determine the maximal releasable amount of LDH, cells were incubated with Triton X-100 (final concentration of 0.5%) for 30 min at room temperature. Fifty microliters of mixed assay reagent from the cytotoxicity detection kit was added to each well and mixed in the dark for 30 min. The absorbance at 492 and 620 nm was measured with a spectrometer (SpectraMax Plus, Molecular Devices, Sunnyvale, CA, USA), and the absorbance at 492 nm was subtracted from that at 620 nm to calculate LDH release.

Electrophysiology

ASIC currents were recorded using a patch-clamp technique as described in our previous studies [11]. The pipette solution contained (in mM) 140 CsF, 1 CaCl₂, 2 MgCl₂, 11 EGTA, 2 tetraethylammonium chloride, 10 HEPES and 4 MgATP (pH 7.3 adjusted with CsOH, 290–300 mOsm). The extracellular fluid (ECF) contained (in mM) 140 NaCl, 5.4 KCl, 2 CaCl₂, 1 MgCl₂, 10 glucose, and 10 HEPES (pH 7.4 or pH 6.0 adjusted with NaOH/HCl, 320–330 mOsm). A multibarrel perfusion system (SF-77 Warner Instruments, Hamden, CT, USA) was used to obtain rapid exchange of ECFs. Currents were recorded with an Axopatch 200B amplifier, filtered at 2 kHz, and digitized at 5 kHz using Digidata 1332 A. ASIC currents were induced by rapid perfusion of the cells with ECF, pH 6.0, for 4 s. The interval between acid applications was 90 s to allow complete recovery of the ASIC currents from desensitization. Unless otherwise stated, cells were clamped at a holding potential of −60 mV. The pipettes had a resistance of 3–5 MΩ when filled with the pipette solution.

Ethics approval and consent to participate

Mouse cortical neurons were cultured in accordance with approved animal protocols and the guidelines of the Institutional Animal Care and Use Committee of Morehouse School of Medicine.

Statistical analysis

All data are expressed as the mean ± SEM. Where applicable, groups were compared using one-way ANOVA followed by Bonferroni’s test or Student’s t test as appropriate using GraphPad Prism 8.  P < 0.05 was considered statistically significant.

Results

H₂O₂ increases ASIC1a protein expression in NS20Y cells

We first examined the effect of H₂O₂, a major ROS, on the expression of ASIC1a in NS20Y cells. High concentrations of H₂O₂ (e.g., >100 µM) have been well demonstrated to cause acute cell injury/death [19, 23–25]; however, these effects do not mimic the pathological conditions of most chronic neurodegenerative diseases, in which the concentrations of H₂O₂ are normally within the range of 10–60 µM [18, 26]. To determine the optimal concentration that is pathologically relevant but nontoxic or less toxic to cells for this study, we first investigated the concentration-dependent harmful effects of H₂O₂ on NS20Y cells. We found that exposure of NS20Y cells to high concentrations of H₂O₂ (≥ 40 μM) for 24 h significantly increased cell injury, as demonstrated by increased LDH release. In contrast, exposure to H₂O₂ at a concentration of 20 μM or lower did not cause significant morphological changes and increased LDH release (Fig. 1a–b, n = 7–8,  P< 0.001 compared with the control). We examined the effect of H₂O₂ at low concentrations (10 and 20 μM) on ASIC1a protein expression. As shown in Fig. 1c, exposure of NS20Y cells to 20 μM H₂O₂ for 24 h significantly increased ASIC1a expression. The relative protein expression was increased 1.74 ± 0.14-fold of the control level (Fig. 1d, n = 9, P < 0.001 compared with the control). The expression of ASIC1a was not significantly changed by 10 μM H₂O₂ (Fig. 1d, n = 12). We also examined whether high concentrations of H₂O₂ (≥ 40 μM) affect ASIC1a expression despite causing toxicity. More than half of the cells were killed by treatment with 40 µM H₂O₂, and almost all the cells were killed by treatment with 80 µM H₂O₂ (Supplementary Fig. 1a). These morphological data indicating cell death are consistent with the results of the LDH assay, as shown in Fig. 1b (~55% and ~96% LDH release in 40 and 80 µM H₂O₂-treated cells). Since more than half of the cells were killed by 40 μM H₂O₂, we were  only able to collect less than 50% of the cells for Western blot analysis. Our data showed that compared with the control, 40 µM H₂O₂ increased the expression of ASIC1a; however, there was no significant difference between the effects of 20 and 40 µM H₂O₂ (Supplementary Fig. 1b-c). One possible explanation could be that the injury to some cells induced by 40 µM H₂O₂ impaired the ability of the cells to synthesize new proteins, which may have, to some extent, limited the further increase in ASIC1a expression. Because of the massive cell death caused by 80 µM H₂O₂, we were unable to collect samples to perform reliable protein analysis. Furthermore, we determined the time-dependent effect of H₂O₂ on ASIC1a expression. We found that exposure of NS20Y cells to H₂O₂ (20 μM) for 6 h, but not 3 h, significantly increased ASIC1a expression (Fig. 1e). Relative ASIC1a expression was increased to 1.50 ± 0.08-fold of the control level by H₂O₂ treatment for 6 h (Fig. 1f, n = 9, P < 0.001 compared with the control).

Fig. 1. Effect of H₂O₂ on cell injury and ASIC1a protein expression in NS20Y cells.

a Representative phase-contrast images of NS20Y cells treated with or without H₂O₂ (20 μM) for 24 h. b LDH assay showing the concentration-dependent cytotoxic effect of H₂O₂ on NS20Y cells (n = 8, P < 0.001 versus the control, one-way ANOVA followed by Bonferroni’s post hoc test). c Representative blots and (d) quantification of ASIC1a protein expression in the presence or absence of H₂O₂ (10 and 20 μM) for 24 h in NS20Y cells (n = 9–12, P < 0.001 versus the control, unpaired Student’s t test). e Representative blots and (f) quantification of ASIC1a protein expression in the presence or absence of H₂O₂ (20 μM) for 3 or 6 h in NS20Y cells (n = 9, P < 0.001 versus the control, unpaired Student’s t test).

In addition to total protein expression, we also examined whether the surface expression of ASIC1a protein was affected by H₂O₂ using a surface biotinylation assay. We found that the surface component of ASIC1a proteins was also increased by H₂O₂ (20 μM, 24 h), suggesting an increase in the number of ASIC1a channels on the plasma membrane (Supplementary Fig. 2).

H₂O₂ enhances ASIC currents

To further confirm whether increased ASIC1a protein expression by H₂O₂ results in an increase in the number of functional ASIC channels on the plasma membrane, we used whole-cell patch-clamp recording to determine the change in ASIC currents. As shown in Fig. 2a, pretreatment of NS20Y cells with 20 μM H₂O₂ for 24 h dramatically increased the amplitude of ASIC currents. Considering that variations in cell size may cause differences in whole-cell currents, we also used the current density to compare the difference in ASIC currents. H₂O₂ treatment increased the ASIC current density from −20.38 ± 3.16 to −34.92 ± 4.08 pA/pF (Fig. 2b, n = 21, P = 0.008 compared with the control), suggesting an increase in the number of functional channels, which is consistent with the increase in protein expression. Next, we examined whether H₂O₂ has a direct modulatory effect on ASIC channels. After establishment of a stable whole-cell current, we applied 20 μM H₂O₂ in both normal pH ECF (pH 7.4) and acidic pH ECF (pH 6.0) and acutely perfused cells with H₂O₂ for 5 min. Our data showed that there was no significant change in the amplitude of ASIC currents (Fig. 2c-d, n = 7, P = 0.328).

Fig. 2. Effect of H₂O₂ on ASIC currents in NS20Y cells.

a Representative whole-cell recordings and (b) summary of the data showing increased ASIC currents in NS20Y cells after treatment with H₂O₂ (20 μM) for 24 h (n = 21 cells, P = 0.008 versus the control, unpaired Student’s t test). c Representative recordings of ASIC currents and (d) summary of the data showing the lack of effect of acute perfusion of NS20Y cells with H₂O₂ (20 μM, 5 min) on ASIC currents (n = 7 cells, paired Student’s t test).

We also examined whether H₂O₂ treatment (20 μM, 24 h), in addition to altering peak amplitude, affects the kinetics of ASIC currents in NS20Y cells. The activation and desensitization kinetics of ASIC currents were well fitted by a single exponential function (Supplementary Fig. 3). No significant difference was found in the time constant of activation (τ_act) between the control (108.51 ± 20.58 ms) and H₂O₂ treatment (91.18 ± 10.79 ms) groups (Supplementary Fig. 3a-b). Similarly, no significant difference was found in the time constant of desensitization (τ_des) between the control (1651.96 ± 96.03 ms) and H₂O₂ treatment (1587.67 ± 40.48 ms) groups (Supplementary Fig. 3c-d)._.

Activation of MAPK pathways by H₂O₂

Activation of the MAPK signaling pathways by H₂O₂, including the ERK, JNK, and p38 pathways, has been previously noted in several cell types, and the activation of these pathways is involved in H₂O₂-mediated multiple downstream processes, including changes in gene expression [27, 28]. To explore whether MAPK pathways are involved in H₂O₂-induced changes in ASIC1a expression, we first examined the activation of MAPK pathways by H₂O₂ in NS20Y cells. We found that incubation with 20 µM H₂O₂ dramatically activated the JNK, p38, and ERK1/2 pathways in NS20Y cells within 30–60 min (Fig. 3a-f, n = 4, P = 0.004, P < 0.001 and P = 0.016 compared with the control).

Fig. 3. Effect of H₂O₂ on the activation of MAPK signaling pathways in NS20Y cells.

a, c, and e Representative blots showing increased phosphorylation of JNK, p38, and ERK1/2 at 30 and 60 min following H₂O₂ (20 μM) treatment in NS20Y cells. b, d, and f Quantification analysis of the phosphorylation of JNK, p38, and ERK1/2 following H₂O₂ (20 μM) treatment in NS20Y cells (n = 4, P = 0.004, P < 0.001 and P = 0.016 versus the control, one-way ANOVA followed by Bonferroni’s post hoc test).

The JNK signaling pathway is involved in the H₂O₂-mediated increases in ASIC1a protein expression and ASIC currents

To explore which specific MAPK pathway is involved in H₂O₂-mediated changes in ASIC1a protein expression, we examined the effect of ERK, p38, and JNK pathway inhibitors on ASIC1a expression. We found that neither inhibition of the MEK/ERK pathway by U0126 (10 μM) nor inhibition of the p38 MAPK pathway by SB203580 (10 μM) affected H₂O₂-induced changes in ASIC1a expression (Fig. 4a–d, n = 8–9, P < 0.001 and  P = 0.014 compared with the control). In contrast, inhibition of the JNK pathway by SP600125 (10 μM) dramatically decreased basal ASIC1a expression and completely abolished the H₂O₂-induced increase in ASIC1a expression (Fig. 4e-f, n = 9, P = 0.001, P < 0.001 compared with the control; P < 0.001 compared with H₂O₂ alone). Relative ASIC1a expression was decreased to 0.36 ± 0.08 and 0.30 ± 0.07 by SP600125 in the absence or presence of H₂O₂, respectively, and there was no significant difference between these levels. Furthermore, we examined whether the increase in surface expression of ASIC1a by H₂O₂ can also be affected by SP600125. Our data showed that, similar to total protein expression, the increased surface expression of ASIC1a by H₂O₂ was inhibited by SP600125 (Supplementary Fig. 4).

Fig. 4. Effect of MAPK signaling pathway blockade on the H₂O₂-mediated increase in ASIC1a expression.

a Representative blots and (b) quantification of ASIC1a protein expression in the presence or absence of H₂O₂ (20 μM) for 24 h in NS20Y cells and in the presence or absence of the MEK/ERK pathway inhibitor U0126 (10 μM) (n = 9,  P < 0.001 versus the group not treated with H₂O₂ or U0126 treatment, one-way ANOVA followed by Bonferroni’s post hoc test). U0126 or vehicle (0.1% DMSO) was added 1 h prior to and during H₂O₂ treatment. c Representative blots and d quantification of ASIC1a protein expression in NS20Y cells in the presence or absence of H₂O₂ (20 μM) for 24 h and in the presence or absence of the p38 MAPK inhibitor SB203580 (10 μM) (n = 8, P = 0.014 versus the group not treated with H₂O₂ or SB203580 treatment). SB203580 or vehicle (0.1% DMSO) was added 1 h prior to and during H₂O₂ treatment. e Representative blots and (f) quantification of ASIC1a protein expression in NS20Y cells in the presence or absence of H₂O₂ (20 μM) for 24 h and in the presence or absence of the JNK inhibitor SP600125 (10 μM) (n = 9, P < 0.001 versus the group not treated with H₂O₂ or SP600125 treatment, P < 0.001 versus H₂O₂ alone, one-way ANOVA followed by Bonferroni’s post hoc test). SP600125 or vehicle (0.1% DMSO) was added 1 h prior to and during H₂O₂ treatment.

Since the surface proteins of ion channels more accurately represent functional channels, we further determined whether a decrease in surface ASIC1a protein expression by a JNK inhibitor results in decreased ASIC current density. We found that incubation with SP600125 for 24 h dramatically decreased the ASIC current density in NS20Y cells and completely abolished the H₂O₂-mediated increase in ASIC currents (Fig. 5a-b, n = 20–23, P = 0.04 and P = 0.004 compared with the control; P < 0.001 compared with H₂O₂ alone). The finding that incubation with SP600125 alone decreased the ASIC current raised the question of whether SP600125 directly regulates the gating process of ASIC1a. Before the recording of ASIC currents, we washed the cultures 3 times with ECF. It is likely that the residual SP600125 left in the dish had a negligible effect. However, we cannot absolutely exclude the possibility that the residual SP directly gated ASIC1a. To test this possibility, we performed an additional experiment to examine whether brief application of SP600125 has a direct effect on ASIC currents. We perfused NS20Y cells with 10 µM SP600125 for 5 min but did not observe significant inhibition of ASIC currents (Supplementary Fig. 5a-b). These data suggest that SP600125 does not have a direct effect on the gating process of ASIC and that the change in ASIC currents induced by long-term SP600125 treatment is likely due to the change in ASIC1a protein expression.

Fig. 5. Effect of JNK pathway blockade on H₂O₂-mediated changes in ASIC currents.

a Representative whole-cell recordings of ASIC currents in the presence or absence of H₂O₂ (20 μM) for 24 h in NS20Y cells and in the presence or absence of JNK inhibitor SP600125 (SP, 10 μM). b Summary of the data showing the alteration of ASIC current density by H₂O₂ in the presence or absence of SP600125 (n = 20–23 cells, P = 0.04 and P = 0.004 versus vehicle control, P < 0.001 versus H₂O₂ alone, one-way ANOVA). SP600125 or vehicle (0.1% DMSO) was added 1 h prior to and during H₂O₂  treatment.

H₂O₂ increases ASIC1a protein expression and ASIC currents in primary cultured mouse cortical neurons

We then examined whether H₂O₂ has a similar effect on ASIC1a expression in primary cultured mouse cortical neurons. Similar to the findings in NS20Y cells, incubation with higher concentrations of H₂O₂ (40 and 80 µM) caused significant neuronal injury, whereas lower concentrations of H₂O₂ (less than 20 μM) did not cause neuronal injury (n = 8, P < 0.001 compared with the control). We found that 20 μM H₂O₂ caused increases in the expression of ASIC1a and ASIC currents in cultured mouse cortical neurons (Fig. 6c and e), which is consistent with the results in NS20Y cells. Relative ASIC1a protein expression was increased 1.48-fold compared with the control level (Fig. 6d, n = 12, P < 0.001 compared with the control), and the density of ASIC currents was increased 1.41-fold compared with the control level (Fig. 6f, n = 26–29, P = 0.028 compared with the control).

Fig. 6. H₂O₂ increases ASIC1a protein expression and ASIC currents in primary cultured mouse cortical neurons.

a Representative phase-contrast images showing primary cultured cortical neurons treated with or without H₂O₂ (20 μM) for 24 h. b LDH assay showing the concentration-dependent cytotoxic effect of H₂O₂ (n = 8, P < 0.001 versus the control, one-way ANOVA followed by Bonferroni’s post hoc test). c Representative blots and (d) quantification of ASIC1a protein expression in the presence or absence of H₂O₂ (20 μM) for 24 h in primary cultured cortical neurons (n = 12, P < 0.001 versus the control, unpaired Student’s t test). e Representative whole-cell recordings and (f) summary of the data showing an increase in ASIC currents in NS20Y cells after treatment with H₂O₂ (20 μM) for 24 h (n = 26–29 cells, P = 0.028 versus the control, unpaired Student’s t test).

In addition to ASIC1a, the predominant ASIC subunit, the ASIC2 gene is widely expressed in the brain. We further examined whether H₂O₂ affects the expression of ASIC2 in primary cultured cortical neurons. Our data showed that H₂O₂ treatment for 24 h did not significantly change the expression of ASIC2 (Supplementary Fig. 6a-b). We also performed a similar experiment in NS20Y cells; however, we did not observe a noticeable band between 60 and 80 kDa, suggesting that there was no expression of the ASIC2 protein in NS20Y cells (Supplementary Fig. 6c).

H₂O₂ increases the sensitivity of ASIC currents to PcTx1 blockade

To further determine whether the increase in ASIC currents by H₂O₂ is due to increased expression of ASIC1a channels, we compared the sensitivity of the ASIC currents to PcTx1 blockade with and without H₂O₂ treatment. PcTx1 has been shown to block ASIC1a-containing channels, including homomeric ASIC1a and heteromeric ASIC1a/2b [29, 30]. We found that there was a significant increase in the sensitivity of ASIC currents to PcTx1 blockade after H₂O₂ treatment for 24 h (Fig. 7a). A total of 34.20% ± 5.95% of the ASIC currents were inhibited by PcTx1 in control cells, whereas 56.27% ± 5.61% of the ASIC currents were inhibited in H₂O₂-treated cells (Fig. 7b). These data suggest that there are more functional ASIC1a channels distributed on the plasma membrane in the presence of H₂O₂ than in the absence of H₂O₂, providing additional evidence for our conclusion that H₂O₂ upregulates functional ASIC1a expression.

Fig. 7. H₂O₂ increases the sensitivity of ASIC currents to the inhibitory effect of PcTx1 in primary cultured mouse cortical neurons.

a Representative current traces and b summary of the data showing the responses of ASIC currents to the inhibitor PcTx1 (20 nM) in cortical neurons in the presence or absence of H₂O₂ pretreatment for 24 h (n = 14–15 cells, P = 0.012, unpaired Student’s t test). The black traces represent the currents before application of PcTx1, and the red traces represent the remaining currents after inhibition by PcTx1.

Discussion

In the current study, we explored the modulatory effect of ASIC1a by H₂O₂, a major ROS. We showed that H₂O₂ at a concentration of 20 µM significantly increases ASIC1a expression and ASIC currents in both NS20Y cells and primary cultured cortical neurons. Furthermore, we demonstrated that H₂O₂ activates MAPK pathways, including the ERK1/2, JNK, and p38 pathways and that inhibition of the JNK pathway, but not the ERK1/2 or p38 pathway, potently inhibits H₂O₂-induced changes in ASIC1a expression. Previous studies by us and others have shown that reducing and oxidizing reagents, including dithiothreitol, 5,5’-dithio-bis-(2-nitrobenzoic acid) and nitric oxide, acutely modulate ASIC currents [31, 32]. In contrast, we showed in the current study that H₂O₂ does not have a rapid modulatory effect on ASIC currents. Rather, it has a slow effect on ASIC1a protein expression, which indirectly affects channel function.

Oxidative stress is defined as an imbalance between pro-oxidant and antioxidant homeostasis that leads to the generation of toxic reactive oxygen or nitrogen species [13]. In the current study, we only focused on the effect of H₂O₂, a major ROS, on ASIC1a expression. Future studies need to be performed to explore the effects of other types of reactive oxygen or nitrogen species, including nitric oxide (^.NO) and peroxynitrite (ONOO^-) on ASIC. In addition, the potential effects of antioxidants such as vitamin E and glutathione on ASIC1a expression and channel function should be determined.

The concentrations of H₂O₂ in blood and plasma have been measured, but the exact values remain controversial due to the great variability of the results published in the literature [18, 26]. The majority of studies have suggested that the normal values of H₂O₂ in human blood range from 1 to 5 µM [26]. However, in chronic disease conditions, the concentration of H₂O₂ can reach 10–60 µM [18, 26]. For example, the level of plasma H₂O₂ can reach ~35 µM in rats fed a high-fat diet [33] and ~60 µM in patients with type 1 and type 2 diabetes [34]. Following acute brain ischemia, the H₂O₂ concentration can reach ~100 µM during reperfusion [35]. Higher concentrations of H₂O₂ (100 µM-2 mM) have been used widely in studies of acute cell toxicity [19, 23–25]. Interestingly, non-physiological high concentrations of H₂O₂ (1–10 mM) have been reported to increase the formation of intersubunit disulfide bonds, which reduces the surface expression of ASIC1a [36]. In the current study, we used 20 µM H₂O₂, which is a pathologically relevant concentration that occurs in chronic diseases.

ASIC1a has been implicated in the pathogenesis of PD, ischemic stroke, and ALS, and inhibition of ASIC1a channels alleviates these neurological diseases [10, 11, 37]. Intriguingly, oxidative stress has been intimately linked to the pathogenesis of these disorders. The brain, particularly, the substantia nigra and the striatum, which are critical in the development of PD, is susceptible to oxidative stress. Although the etiology of PD remains elusive, postmortem analyses of PD patients’ brains have strongly suggested the involvement of oxidative stress and mitochondrial dysfunction [38]. A previous study showed that pharmacological inhibition of ASIC1a has beneficial effects in a mouse model of PD [37], suggesting that enhanced activation of ASIC1a contributes to the pathogenesis of PD. The current finding that H₂O₂ upregulates ASIC1a expression may provide additional mechanistic insight into the pathogenesis of PD. A number of studies have provided evidence that increased oxidative stress occurs in diabetes or diabetic conditions in addition to PD and causes worse stroke outcomes [39–41]. ASIC1a has been shown to play a critical role in brain injury associated with cerebral ischemia [11], and whether increased expression of ASIC1a by H₂O₂ contributes to worse stroke outcomes in diabetic patients remains to be determined.

It is well established that exposure of cells to H₂O₂ induces activation of multiple MAP kinases, including ERK, JNK, and p38 MAPK [19]. In the current study, we demonstrated that inhibition of the JNK pathway, but not the ERK or p38 pathway, abolished the H₂O₂-induced increase in ASIC1a expression. Interestingly, we found that JNK inhibition by SP600125 dramatically reduced basal ASIC1a expression, suggesting that the JNK pathway may play a key role in regulating ASIC1a expression under physiological conditions. In this regard, it would be interesting to explore the detailed molecular mechanisms underlying the modulatory effect of the JNK pathway on ASIC expression in the future.

In conclusion, in the present study, we showed evidence that H₂O₂, a key player in oxidative stress, upregulates ASIC1a protein expression and increases channel function through the JNK signaling pathway. These findings will have broad implications given that oxidative stress and ASIC1a play important roles in several neurodegenerative diseases.

Author contributions

BMW performed the electrophysiology experiments. JB and LZ performed the Western blot experiments. TY performed the neuronal culture. ZGX and TDL designed the experiment, interpreted the data and wrote the paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version of this article (10.1038/s41401-020-00559-3) contains supplementary material, which is available to authorized users.