Background
Artemin belongs to glial cell line-derived neurotrophic factor (GDNF) family that regulates the development and the function of the nervous system. Artemin binds to the GFR α3/RET receptor complex and then activates several intracellular signaling pathways [
1]. Artemin supports survival of sensory neurons
in vitro and
in vivo apparently by interacting with GFRα3, which is highly restricted in adult to neurons of the peripheral nervous system (sensory and sympathetic). GFRα3 is expressed by a subpopulation of nociceptive sensory neurons, some or all of which also express the Ret receptor tyrosine kinase, the transient receptor potential (TRP) ion channel proteins TRPV1 and TRPA1 [
2,
3]. The expression of these channels in GFR α3-positive neurons suggests that artemin signaling
via GFR α3/Ret binding could modulate neuron sensitivity.
TRPA1 is a member of branch A of the TRP family of cation channels[
4] and is expressed by a subset of small-sized DRG or trigeminal ganglia neurons in neonatal rats, adult rats and mice [
5‐
7]. TRPA1 has been reported to be activated by various kinds of noxious compounds through covalent modification of cysteines [
5,
8‐
13]. TRPA1 is also activated by an endogenous aldehyde, 4-hydroxynonenal, bradykinin, intracellular pH and CO2 [
8,
14‐
16]. Studies using knockout mice demonstrated that TRPA1 is an important component of the transduction machinery through which environmental irritants and endogenous proalgesic agents depolarize nociceptors to elicit inflammatory pain [
17,
18]. A recent report showing the nearly complete attenuation of formalin-induced pain behaviors by pharmacological blockade or genetic ablation indicated that TRPA1 is crucial in inflammatory pain [
19]. Taking the above into account, it is clear that this channel is importantly involved in pain transmission in the primary afferents and a potential target for analgesic development.
Recent reports suggested that artemin potentiates TRPV1 signaling and induces behavioral hyperalgesia. Overexpression of artemin increased the expression and sensitivity of TRPV1 and TRPA1 in trigeminal afferents signaling and induced behavioral hyperalgesia [
20,
21]. We have studied the modulation mechanism of TRPV1 and TRPA1 by inflammatory modulators and reported that trypsin/tryptase-PAR2 signaling or bradykinin-B2R signaling sensitizes TRPA1 channel through PLC and/or PKA pathways to induce inflammatory pain [
22‐
24]. In the present study, we hypothesized that a functional interaction of artemin and TRPA1 might contribute to the pathogenesis of inflammatory pain. We observed high co-expression of the TRPA1 with the GFR α3 receptor and found a significant enhancement of desensitization of TRPA1 activity induced by artemin in rat DRG neurons, which was also confirmed at the behavioral level.
Methods
Immunohistochemistry
All animal experimental procedures were approved by the Hyogo University of Health Sciences Committee on Animal Research (#2008-05, #2008-10) and were performed in accordance with the National Institutes of Health guidelines on animal
care. Adult male Sprague-Dawley (SD) rats (220-250 g; Japan Animals, Osaka, Japan) were perfused transcardially with 1% paraformaldehyde in 0.1 M phosphate buffer followed by 4% paraformaldehyde in 0.1 M phosphate buffer (PB, pH 7.4). The L4-5 DRGs were removed and processed for TRPA1 immunohistochemistry as described in our previous study [
23]. For double immunofluorescence, tyramide signal amplification (TSA; NEN) fluorescence procedures were used for TRPA1 (1:10,000) staining. The raised rabbit primary antibody for TRPA1 [
23] and biotin conjugated Griffonia simplicifolia Isolectin B4 (IB4, Sigma, St. Louis, MO) at 1:1000 combined with Alexa fluor 488 goat anti-rabbit IgG (1:500; Invitrogen/Molecular Probes, Inc., Carlsbad, CA) and strept-avidin conjugated with Alexa fluor 594 (1:500: Invitrogen/Molecular Probes), respectively, were used for double immunofluorescence staining [
22].
In situ hybridization histochemistry (ISHH)
Adult male SD rats weighing 200-250 g were killed by decapitation under deep ether anesthesia. The bilateral L4, L5 DRGs were dissected out, rapidly frozen in powdered dry ice, and cut on a cryostat at a 5 μm thickness. Sections were thaw mounted onto MAS-coated glass slides (Matsunami, Osaka, Japan) and fixed in 4% formaldehyde in 0.1 M PB (pH 7.4) for 20 min. After washing with PB, the sections were treated with 10 μg/ml protease K in 50 mM Tris/5 mM EDTA (pH 8.0) for 3 min at room temperature, postfixed in the same fixative, acetylated with acetic anhydride in 0.1 M triethanolamine, rinsed with PB, and dehydrated through an ascending ethanol series. For dual ISHH of TRPA1 and GDNF family receptors, we used a DIG-labeled probe for TRPA1 (GenBank accession number AY496961, nucleotides 302 - 769) and a
35S-labeled RNA probe for Ret (GenBank accession number U22513, nucleotides 13-319), GFR α1 (GenBank accession number U97142, nucleotides 652 - 955), or GFR α2 (GenBank accession number U97143, nucleotides 362 - 702), or GFR α3 (GenBank accession number AF184920, nucleotides 164 - 604) in the same sections, respectively. The details of the dual ISHH procedure have been described in our previous study [
6].
Mammalian cell culture
For primary culture of DRG neurons, DRGs were collected from the adult SD rats (100-200 g) using sterile techniques, and placed in ice-cold Earle's balanced salt solution (EBSS, Sigma). Adhering fat and connective tissue were removed and each DRG was placed immediately in a medium consisting of 2 ml of EBSS and 1.25 mg/ml of collagenase P (Sigma) and kept at 37°C for 60 min with occasional agitation. After dissociation of the DRG cells, this cell suspension was centrifuged for 5 min at 1000 rpm and the cell pellet was re-suspended in EBSS supplemented with 10% fetal bovine serum (FBS), 2 mM glutamax, penicillin, streptomycin and vitamin solution. Recombinant rat nerve growth factor (100 ng/ml, Sigma) was added to the medium.
Electrophysiology
Whole-cell patch-clamp recordings were carried out at 1 day after dissociation of the DRG neurons. Voltage-clamp experiments were performed at -60 mV holding potential, and recordings were sampled at 5 kHz and filtered at 2 kHz. Current densities (pA/pF) and normalized currents (the third currents were normalized to the second currents evoked by an agonist) were measured. The current magnitude was quantified by peak current amplitude in all experiments. A normalized current was obtained by normalizing the third application-induced current to the second one, and just in case the magnitude of the second current by agonist was larger than a half of the first current to evaluate the results under less effect of the desensitization by the first current. In experiments with DRG neurons, after AITC (300 μM) application, capsaicin (10 μM) was applied at the end of recording to identify whether the AITC-induced current was mediated by TRPA1 channels. Data were obtained just in case the DRG neuron was sensitive to both AITC and capsaicin application, since an AITC-activated current in capsaicin-sensitive DRG neurons is certainly a TRPA1-mediated event [
18]. The standard bath solution contained 140 mM NaCl, 5 mM KCl, 2 mM MgCl
2, 2 mM CaCl
2, 10 mM HEPES and 10 mM glucose, pH 7.4 (adjusted with NaOH). In some experiments, artemin (100 ng/ml) was included in the bath solutions. The pipette solution contained 140 mM KCl, 2 mM MgCl
2, 0.5 mM CaCl
2, 5 mM MgATP, 5 mM EGTA and 5 mM HEPES, pH 7.2 (adjusted with Tris-base). The concentration-response curves for the effect of artemin on AITC-induced current densities (pA/pF) were fit by the Hill equation using Origin 8.1 (OriginLab Corporation, Northampton, MA, USA). All patch-clamp experiments were performed at room temperature (~25°C; RT). The solutions containing drugs were applied to the chamber (180 μl) by a gravity system at a flow rate of 4-7 ml/min.
Behavioral studies
Sixteen and 10 male adult SD rats (200-250 g) were used for the AITC-induced nocifensive behavioral analyses and formalin test, respectively. After adaptation, 50 μl artemin (10 μg/ml, in PBS) or PBS was injected intraplantarly into the rat left hind paw. Five minutes after these injections, rats received intradermal injection of 50 μl of AITC (3% in liquid paraffin, Wako Pure Chemical Industries, Ltd., Osaka, Japan) or formalin (3% in saline) to the same area of artemin-injected plantar surface. The rats were placed in a wire mesh cage immediately after the injection. The numbers of hind paw lifts and durations of flinches per 5 min interval during the initial 30 min post-injection of AITC and 60 min post-infection of formalin period were measured for the AITC-induced nocifensive behavior and formalin test, respectively. The total number of lifts and durations of flinches during the entire initial 30 min for AITC and 60 min for formalin post-injection were also calculated.
Statistical analysis
All results are expressed as mean ± SEM. An unpaired t-test or ANOVA followed by Fisher's PLSD was used to compare the calcium imaging data and electrophysiological data between the groups. Two-way repeated ANOVA followed by Fisher's PLSD was applied to the behavioral data. A difference was accepted as significant if the probability was less than 5% (p < 0.05).
Discussion
In recent years, the role of artemin, a GDNF family member, in mediating neuropathic and inflammatory pain has been received much attention. Artemin is known to be one of the survival factors for sensory and sympathetic neurons
in vitro and
in vivo. The long-term intrathecal or systemic administration of artemin prevented many of the nerve injury-induced changes in the histochemistry of nociceptor neurons, and produced dose- and time-related reversal of nerve injury-induced pain behaviors [
26‐
28]. On the other hand, overexpression of artemin up-regulated expression of TRPV1 and TRPA1 channels and subsequently led to an increase of neuronal activity and hyperalgesia [
2,
20]. Acute application of artemin induced a significant potentiation of TRPV1 function and produced acute thermal hyperalgesia [
21]. In the present study, in contrast to the potentiation of TRPV1 function by artemin, we observed that a short-term application of artemin significantly suppressed TRPA1 channels activity and the TRPA1-mediated pain behaviors.
Artemin binds to GFR α3/Ret to induce extracellular signals [
1]. We found TRPA1 was highly coexpressed with GFR α3 and RET (Figure
1). This finding is consistent with a previous report that indicates nearly all GFR α3-positive neurons express TRPV1 immunoreactivity, and most of these neurons are also TRPA1 positive [
2]. The high co-localization of TRPA1 and GFR α3 provides a possible histological prerequisite of the functional interaction between these two molecules.
We found in the present study that a short term application of artemin significant reduced the AITC-induced TRPA1 current. This inhibitory regulation was examined in two ways. First, artemin was delivered between two applications of AITC and then the normalized current (the next AITC-current versus the first one) was collected (Figure
2). This approach allowed us to confirm that the tested neuron was indeed a TRPA1-expressing one, but tachyphylaxis of AITC-induced current may have interrupted the analysis and made it difficult to explain the reduced normalized current solely as an inhibition of TRPA1 channels, rather a cooperative effect of an artemin-induced inhibition and the tachyphylaxis may have taken place. Therefore, we also performed an experiment with sensory neurons pretreated by artemin and then applied AITC to evaluate the current density. Both of these results clearly indicated inhibitions of TRPA1 channel function by acute application of artemin.
Acute application of artemin has been reported to induce potentiation of another TRP channel, TRPV1, and produce acute thermal hyperalgesia [
21], TRPV1 and TRPA1 are structurally endowed with the same TRP domain but have distinctly different intracellular loops. AITC activates TRPA1 by covalently modifying cysteine residues located in the N terminus of the channel, different from a classical lock-and-key binding mechanism for capsaicin-TRPV1. Thus it is not a wonder that the regulatory mechanism of TRPV1 and TRPA1 may differ. For example, PKC has been reported to potentiate TRPV1 [
22,
29,
30] but did not have any effect on TRPA1[
23,
24]. On the other hand, Elitt et al have reported that overexpression of artemin up-regulated expression of TRPV1 and TRPA1 channels and subsequently led to greater neuronal activity and hyperalgesia [
2,
20]. These reports suggested a post-transcriptional regulation of artemin (which might contribute as a neurotrophic factor) on TRPV1 and TRPA1 channels, are not discordant with our findings.
TRPA1 is known to be expressed on sensory neurons and acted as an important component of pain. If artemin can suppress the TRPA1activation, pain sensation that is caused through the TRPA1 channel may also be suppressed by the artemin activation. Topical application of AITC has been reported to excite sensory nerve fibers, thereby producing acute pain [
5,
17,
31]. Consistent with our electrophysiological data, artemin produced a significantly persistent suppression of the duration of paw flinch and the number of paw lifts induced by intraplantar injection of AITC (Figure
3). A recent study indicates that TRPA1 is the principal site of formalin's pain-producing action in rodents [
19]. The formalin test is a widely used model of continuous pain resulting from formalin-induced tissue injury. Subcutaneous injection of formalin into the rat hindpaw produces a biphasic pain response that consists of an early, acute phase (Phase I) and a late, tonic phase (Phase II) that is manifested behaviorally as lifting, flinching or licking of the affected paw, and these behaviors are robust and readily quantifiable [
32,
33]. Phase I is thought to result from direct activation of primary afferent sensory neurons, whereas Phase II has been proposed reflect the combined effects of afferent input and central sensitization in the dorsal horn. In our present study, we found artemin treatment significantly suppressed the formalin-induced pain behaviors at both two phases (Figure
3). Direct inhibition of TRPA1 activity by local injection of artemin in the plantar nerve terminal may result the suppression of behaviors in Phases I, then the reduced input of primary afferent may contribute the reduction of pain behaviors in Phase II. These findings suggest that a short-term application of artemin inhibits the TRPA1 channel's activity and the sequential pain behaviors.
The mechanism of artemin-induced inhibition of TRPA1 activity is not clear. Two potential mechanisms are suggested to be involved in this regulation. One would be that artemin increases EC
50 by suppressing the binding affinity of AITC to TRPA1. The other mechanism would be that artemin lowers the channel density of TRPA1 on cell membranes through promoting receptor internalization or by inhibiting membrane insertion of TRPA1 channels. In the present study, the concentration-response curves of AITC resulting from the effect of artemin (Figure
3C) revealed that artemin lowered the maximal response of AITC without conspicuously altering EC
50, indicating that artemin may suppress AITC response in DRG neurons principally by lowering the channel density. Distinct constitutive and regulated vesicular trafficking mechanisms have critical roles not only in controlling the surface expression of TRP channels but also their activation in response to stimuli [
34]. TRPA1 may cycle between the plasma membrane and intracellular compartments, and the balance between membrane insertion and retrieval determines its surface abundance and activity [
35]. A recent report indicated that TRPA1 channel desensitization in sensory neurons was regulated by internalization of itself [
36]. Artemin-induced GFR α3/RET activation results in stimulation of multiple signal transduction pathways, including the MAP kinase/Erk and PI3 kinase/Akt pathways. A potential mechanism of inhibition of TRPA1 by artemin in the present study may be the internalization of the channels, which is regulated by the downstream of GFR α3/RET intracellular signals. On the other hand, it has been reported that TRPA1 is functionally inactivated in sensory neuron by extracellular Ca
2 influx [
37]. Although activation of GFR α3/RET signals results in intracellular Ca
2 mobilization, however, it is not a likely mechanism for the AITC-evoked current decrease observed in our experiment. The reason simply comes from that cytosolic-free Ca
2 is tightly chelated with the 5 mM EGTA included in the pipette solution. Anyway, detailed mechanisms of artemin regulation on TRPA1 need to be further studied.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
NY carried out the electrophysiological and behavioral studies, performed the statistical analysis, and participated in drafting the manuscript. KK carried out the histochemistry studies. LY, GN, SW and SY participated in the electrophysiological studies. SW also participated in drafting the manuscript. KN supervised the project and edited the manuscript. YD conceived of the project, designed and coordinated the studies, and drafted the manuscript. All authors contributed to data interpretation, have read and approved the final manuscript.