Pore dilation occurs in TRPA1 but not in TRPM8 channels

Abundantly expressed in sensory neurons, TRPV1, TRPA1 and TRPM8 are involved in sensory function, pain and neurogenic inflammation [1]. The function of these ion channels has been attributed to their ability to pass certain ion species across the plasma membrane. Once activated, TRPV1, TRPA1 and TRPM8 are permeable to small cations such as Ca²⁺, K⁺, Na⁺; hence, channel activation simultaneously depolarizes the plasma membrane and raises intracellular Ca²⁺, which subsequently triggers a variety of physiological processes. By analogy to voltage-gated K⁺ channels, it is assumed that ion selectivity of TRP channels should be an invariant signature to the respective channel. However, this notion has been challenged recently. When activated, TRPV1 exhibits time and agonist-dependent changes in ion selectivity [2]. In fact, TRPV1 undergoes pore dilation and allows permeation of large organic cations, including spermine (202.3 Da), NMDG (195.2 Da), Yo-Pro (376 Da), gentamycin (477.6 Da) and QX-314 [3‐7]. Here we explored whether TRPA1 and TRPM8 undergo pore dilation by examining Yo-Pro uptake and changes in ion selectivity upon channel activation.

Yo-Pro is a divalent cation impermeable to the plasma membrane. However, under certain conditions, it can enter cells, bind nucleic acids and emit fluorescence. Hence the uptake of Yo-Pro has been used previously as an indicator of pore dilation [2, 8, 9]. In HEK293-F cells transiently expressing rat TRPA1, allyl isothiocyanate (AITC) evoked robust increases in intracellular Ca²⁺ (Fig. 1A). Concomitantly, AITC also induced Yo-Pro uptake in a concentration-dependent manner (Fig. 1B). At higher concentrations of AITC (100 or 300 μM), the increase in fluorescence was immediately noticeable and continued to increase for about 50 min. In addition, AITC also induced Ca²⁺ influx and Yo-Pro uptake in cells expressing human TRPA1 and mouse TRPA1, but not in untransfected cells (data not shown). In cells expressing human TRPM8, menthol activated TRPM8 as indicated by the concentration-dependent Ca²⁺ influx, but failed to induce Yo-Pro uptake (Fig. 1C and 1D). Other TRPM8 agonists (e.g., icilin) also evoked Ca²⁺ influx but failed to induce Yo-Pro uptake (data not shown). Hence, Yo-Pro uptake occurs upon activation of TRPA1, but not TRPM8.

In addition to AITC, TRPA1 can be activated by many other electrophilic agonists (e.g., cinnamaldehyde or CA, 4-hydroxynonenal or 4-HNE), and non-reactive agonists (e.g., URB597, farnesyl thiosalicylic acid or FTS) [10‐14]. We investigated whether the Yo-Pro uptake is limited to AITC. CA, 4-HNE, FTS and URB597 all evoked Ca²⁺ influx and Yo-Pro uptake in a concentration dependent-manner (Fig. 2A and 2B). In the Ca²⁺ assay, the EC₅₀ was 6.5 ± 0.35 μM for AITC, 6.8 ± 1.5 μM for CA, 4.4 ± 0.6 μM for 4-HNE, 33.2 ± 8.1 μM for FTS and 85.6 ± 10.4 μM for URB597 (n = 4–8). Compared to AITC, the maximal signals were 104% for CA, 88% for 4-HNE, 107% for FTS and 82% for URB597. In the Yo-Pro uptake assay, the EC₅₀ was 16.0 ± 3.8 μM for AITC, 5.9 ± 0.7 μM for CA, 7.1 ± 0.2 μM for 4-HNE, 41.8 ± 10.7 μM for FTS and 85.4 ± 19.8 μM for URB597 (n = 4–8). Compared to AITC, the maximal signals were 98% for CA, 82% for 4-HNE, 117% for FTS and 84% for URB597, respectively. Hence, TRPA1 activation by different agonists all induced Yo-Pro uptake.

Several small molecule inhibitors of TRPA1 have been described recently, including AP18, HC-030031 and ruthenium red (RR) [15, 16]. We tested whether these antagonists blocked Yo-Pro uptake. AP18 attenuated 30 μM AITC-induced Yo-Pro uptake in a concentration-dependent manner, with an IC₅₀ of 10.3 ± 0.8 μM (Fig. 2C and 2D). Likewise, HC-030031 and RR also completely blocked Yo-Pro uptake (IC₅₀: 9.4 ± 0.6 μM for HC-030031 and 10.0 ± 1.6 μM for RR). Taken together, these data show that agonist-evoked Yo-Pro uptake is related to TRPA1 channel activities.

Next, we investigated whether TRPA1 undergoes changes in ion selectivity upon channel activation. Currents were recorded under the outside-out patch configuration using NMDG⁺ as the sole external cation and Na⁺ as the major internal cation. Patch membrane potential was held at -80 mV, and a ramp voltage from -140 mV to 0 mV (500 ms duration) was applied every 3 seconds. Before addition of AITC, a small basal current was present, consistent with previous reports [17, 18]. The reversal potential (E_rev) of basal currents was -95.3 ± 4.8 mV (n = 5). Compared to activation of TRPV1 by capsaicin, activation of TRPA1 by AITC was relatively slow, probably due to the covalent reaction that is needed to activate TRPA1. Addition of AITC (100 μM) elicited gradual activation of TRPA1 and rightward shift in reversal potential (Fig. 3A). The shift in E_rev occurred as early as 6 s following addition of AITC, and continued to increase with nearly maximum shift at ~15 s. Addition of 10 μM RR nearly completely blocked AITC-evoked NMDG⁺ and Na⁺ currents (Fig. 3A inset), indicating the observed currents were mediated by TRPA1 channels.

In contrast, TRPM8 showed no shift in E_rev following addition of 500 μM menthol, despite a clear increase in current (Fig. 3B). The time-dependent changes in E_rev for TRPA1 and TRPM8 following their activation are shown in Fig. 4A. The shift in E_rev for TRPA1 was not due to an increase in anion selectivity, as removal of Cl^- in the bath solution caused a similar shift in E_rev from -96 mV to -42 mV. From E_rev values, permeability ratios (P_NMDG/P_Na) before and 60 s after agonist addition were derived. As shown in Fig. 4B, P_NMDG/P_Na increased ~4.4-fold for TRPA1 from 0.05 ± 0.003 to 0.22 ± 0.013 (n = 4, P < 0.05, paired t-test), comparable to the ~5.5 fold increase reported for TRPV1 [2]. In contrast, P_NMDG/P_Na did not change significantly for TRPM8. It is interesting that the shift in E_rev occurred much earlier than the increase in TRPA1 currents (Fig. 4C), indicating that pore dilation occurs well before maximal channel activation.

The accuracy of E_rev measurement could be compromised by small current amplitudes, especially for basal currents and currents immediately following AITC application. However, the E_rev of basal currents was consistent across patches (-95.3 ± 4.8 mV, n = 5), and E_rev shifts consistently occurred in TRPA1, but not in TRPM8. In addition, even for relatively large TRPA1 currents, significant shifts in E_rev occurred. For example, the shift in E_rev was 31.6 mV between 6 s and 15 s pulses, and 14.2 mV between 9 s and 15 s pulses (Fig. 3A and 4A). Taken together, these data suggests that the dynamic change in E_rev results from TRPA1 channel activity. Another concern in extrapolating the change in E_rev to the change in ion selectivity is that ion accumulation can occur during prolonged activation, particularly when large currents are conducted under whole cell configuration. However, the ion accumulation should not significantly compromise our TRPA1 experiments using outside-out patch configuration, in which extracellular and intracellular ionic conditions were well controlled. In addition, reversal potentials changed within seconds of AITC application when currents were small, but reached a steady state (from 15 to 60 s) when currents were relatively large (Fig. 3A and 4A). Furthermore, TRPM8 conducted currents with similar amplitudes but without significant shifts in reversal potential. Consistent with the electrophysiology data, the large divalent cation Yo-Pro did not cross the membrane when the channel was closed or blocked by antagonists, but permeated the membrane freely when the channel was open (Fig. 1B and 2C). Collectively, our data suggest that TRPA1, but not TRPM8, undergoes pore dilation.

Pore dilation has been previously described for the ATP-gated P2X and TRPV1 [2, 8, 9]. For P2X channels, the mechanism underlying pore dilation remains controversial. Several alternative mechanisms have been proposed including a direct change in ion selectivity, formation of channel multimers, and recruitment of a downstream, nonselective pore [19, 20]. For TRPV1, pore dilation most likely arises from a change in ion selectivity, as indicated by the dynamic change in ion selectivity during agonist stimulation, and the effects of mutations and chemical modification of certain residues within the selectivity filter [2]. In the current study, we did not elucidate the biophysical mechanism underlying pore dilation of TRPA1. However, there were several notable observations. First, the permeability to NMDG⁺ increased almost immediately upon channel activation. Second, the outside-out patch configuration should largely disrupt cytoskeletal structures and washout cytosolic factors. Third, the AITC evoked- NMDG⁺ and Na⁺ conductance was sensitive to blockade by ruthenium red. Finally, under identical conditions, TRPM8 conducted large currents, but did not exhibit Yo-Pro uptake or a significant change in NMDG⁺ permeability. Thus, TRPA1 pore dilation most likely represents a direct change in ion selectivity. Nonetheless, our present study does not completely rule out the involvement of other proteins.

TRPV1, TRPA1 and TRPM8 are major TRP channels involved in somatosensation. Within dorsal root ganglia, TRPV1 and TRPA1 are co-expressed and interact functionally in one population of sensory neurons, while TRPM8 is expressed largely in a separate neuronal population. Interestingly, pore dilation occurs in TRPA1, TRPV1 but not TRPM8, suggesting that this property is not ubiquitous, but rather specific to subtypes of channels within a subpopulation of neurons. The change in cation permeability, in turn, may alter channel function, affect a host of downstream processes (e.g., neurotransmitter release, cellular toxicity) and contribute to pain hypersensitivity [21]. Recently, it was reported that TRPV1-mediated pore dilation could be utilized to deliver QX-314 (a membrane-impermeant sodium channel blocker) specifically to TRPV1-positive sensory neurons, achieving analgesic effects without motor deficits associated with local anesthetics [5]. However, this strategy of targeting TRPV1-positive neurons could be compromised by several factors, including the broad expression pattern of TRPV1, its role in regulating body temperature, and its involvement in hippocampal synaptic plasticity [22, 23]. By analogy to TRPV1, the pore dilation of TRPA1 could be exploited to mediate entry of QX-314 specifically into TRPA1-positive neurons. Given the restrictive expression of TRPA1 in sensory neurons, this strategy may offer analgesic efficacy without unwanted side effects.

In conclusion, the present study demonstrates that pore dilution occurs in TRPA1 but not in TRPM8 channels. This finding raises many interesting questions: What is the exact biophysical mechanism underlying pore dilation of TRPA1? What are the physiological, pathological and therapeutic implications? Why does pore dilation not occur in TRPM8? What are the pore behaviors of other TRP channels? Answers to these questions will certainly extend our understanding of this family of ion channels.