Introduction
TRPV1 is a Ca
2+ permeable nonspecific cation channel, located on peripheral sensory neurons, that serves as a molecular detector for heat, capsaicin, protons, and endovanilloids [
1‐
4]. Moreover, its role as a heat sensor (activation threshold of ~43°C), its influence from trophic/inflammatory agents (i.e. nerve growth factor, bradykinin, prostaglandins, etc.) and its vasodilatory effect on small vessels (by releasing CGRP) make TRPV1 an essential component of the pain pathway [
3,
5‐
11].
In peripheral nerves, insulin does not promote its typical metabolic effects of glucose and amino acid uptake [
12]. However, physiologic concentrations can act as neurotrophic factors, in combination with nerve growth factor (NGF), to stimulate neurite outgrowth and survival of both sensory and sympathetic neurons [
13‐
15]. Furthermore, it has been proposed that insulin and IGFs exert trophic influences on the same neurons that are responsive to NGF by sharing common signaling pathways [
13].
Though it is widely believed these neural disturbances are secondary to hyperglycemia [
16], this remains controversial. Results from the Diabetes Control and Complications Trial show that intensive glycemic control for 5 years reduced the incidence of neuropathy by 60% in Type I patients [
17]. However, the fact that strict glucose regulation does not completely prevent diabetic peripheral neuropathy suggests additional mechanisms as a result of insulin deficiency may be involved.
Experiments in humans show that non-diabetic, normoglycemic subjects have heat thresholds that correlate positively with insulin sensitivity [
18]. Furthermore, circulating levels of IGFs and NGF are reduced in diabetes and to a greater degree in patients with thermal sensory deficits characteristic of neuropathy [
19‐
21]. Growth factor replacement therapy improves neuronal regeneration [
22,
23] and insulin, under conditions of rapid infusion, can restore decreased neurogenic vasodilation [
24,
25]. Taken together, these studies suggest that insulin and trophic factor deficiencies may alter sensory nerve function [
26].
In this study we demonstrate that insulin and IGF-I enhance TRPV1-mediated membrane currents by enhancing receptor sensitivity and translocation from cytosol to plasma membrane in heterologous expression systems and cultured dorsal root ganglion (DRG) neurons in a PKC-dependent manner. Preliminary results of this study have been published elsewhere in an abstract form [
27].
Materials and Methods
Oocyte Electrophysiology
Oocytes were isolated from tricaine-anesthetized Xenopus laevis and separated from the follicular layer after incubation with collagenase (1–2 mg/ml). 3–7 days following cRNA injections (50–70 nl, 1 μg/μl), oocytes were stored at 18°C and used for experiments. Double electrode voltage clamp (-60 mV) was performed using a Warner amplifier (Warner Instruments, OC725C, Hamden, CT, USA) with 100% d.c. gain, digitized and stored on video tape. Experiments were performed at 21–23°C. Oocytes were placed in a Perspex chamber superfused (5–10 ml min-1) with Ca2+-free Ringer solution containing (in mM): 100 NaCl, 2.5 KCl, 5 HEPES, pH 7.35. Within figures, current traces were shown as initial response to agonist juxtaposed on currents recorded following incubation with insulin, IGF-I and various inhibitors. Time course experiments consisted of current recording during capsaicin application in the presence of either control (first 20 min) or insulin (last 15 min) containing solutions. In all experiments, fold increase was calculated as agonist induced current amplitude in the presence of trophic factors and inhibitors divided by the agonist induced initial current amplitude under control conditions before their bath application. I-V relationships were measured using 500-ms voltage ramps from -80 mV to +80 mV.
DRG Culture and Electrophysiology
Dorsal root ganglia were isolated from embryonic day 18 (E18) rats, triturated and cultured for 5–7 d in Neurobasal/B-27 (Life Technologies; Grand Island, NY) + 10% fetal bovine serum (FBS) on poly-D-lysine-coated glass coverslips. For perforated patch recording, the bath solution contained (in mM): 140 NaGlu, 2.5 KCl, 10 HEPES, 2 MgCl2, 1 EGTA, pH 7.35 and the pipette solution contained (in mM): 130 NaGlu, 10 NaCl, 2.5 KCl, 10 HEPES, 1 MgCl2, 0.2 EGTA, and amphotericin B (240 μg/ml). For cell-attached recording, the bath solution contained (in mM): 140 Kglu, 10 NaCl, 10 HEPES, 1 EGTA, 2 MgCl2, pH 7.35 and the pipette contained (in mM): 140 NaGlu, 2.5 KCl, 10 HEPES, 2 MgCl2, pH 7.35. Currents were recorded using a WPC 100 patch clamp amplifier (E. S. F. Electronic, Goettingan, Germany) or Axopatch 200B (Axon Instruments, Union City, CA). Data were digitized (VR-10B, Instrutech Corp.; Great Neck, NY) and stored on video tape.
For analysis, data were filtered at 2.5 kHz (-3db frequency with an 8-pole low-pass Bessel filter, Warner Instruments, LPF-8) and digitized at 5 kHz. Data analyses were performed on continuous stretches greater than 20 s from patches that contained one or two channels. Single channel current amplitude and Po were calculated from all point amplitude histograms fitted with Gaussian functions (Microcal Origin; Northampton, MA).
Ca2+ Imaging
Cells were grown on glass coverslips, then incubated with 5 μM Fluo-4AM (Molecular Probes; Eugene, OR) for 20 min at 37°C, washed with physiological buffer [(in mM): 140 NaCl, 5 HEPES, 2 CaCl2, 1 MgCl2, 2.5 KCl, pH 7.35], and treated with 500 nM capsaicin +/- 1 μM insulin immediately prior to analysis by confocal microscopy (Fluoview, Olympus; Mellville, NY). Data for each cell was quantified as the fluorescence after treatment (F) divided by initial fluorescence (Fo) at t = 0 (Fluoview software).
Transfection of Cells and Cell Culture
HEK 293 cells, which endogenously express IGF-I receptors [
29] were cultured in Dulbecco's modified Eagle's medium (high glucose) supplemented with 10% fetal bovine serum, 50 units/ml penicillin, and 50 μg/ml streptomycin (Invitrogen; Carlsbad, CA) and maintained under 95% air/ 5% CO
2 at 37°C. Cells were transiently transfected with wild type TRPV1, TRPV1eGFP, TRPV1 S502/S800A mutant (gifted by D. Julius, N. Kadei, M. Tominaga, respectively), or TRPV1-V5-His tagged plasmid using Lipofectamine2000 reagent (Invitrogen) according to manufacturer's protocol. TRPV1-V5-His tagged plasmid was constructed in pcDNA3.1 vector using Topo Cloning kit (Invitrogen). To determine the membrane translocation of TRPV1, confocal images (1 μm sections) of GFP-tagged TRPV1 fluorescence was obtained. The intensity of the brightest membrane fluorescence was selected and quantified using MCID imaging software (Imaging Research Inc; St. Catherines, ONT)
Determination of Surface TRPV1
After 36 h of TRPV1 transfection, the cells (grown on 100 mm tissue culture plate) were insulin-treated and biotinylated with membrane impermeable NHS-LC-biotin (1.5 mg/ml in PBS; Pierce, Rockford, IL) as per manufacturer's protocol. Labeled cells were lysed in RIPA buffer and immunoprecipitated overnight with 10 μl of goat anti-VR1 polyclonal antibody/ Protein A/G Plus-agarose (Santa Cruz Biotechnology, Santa Cruz, CA) following manufacturer's protocol. Immunoprecipitates were eluted with 2X SDS sample buffer, separated on 7.5% SDS-PAGE, and transferred to PVDF membrane. To detect total TRPV1, blots were probed with rabbit anti-VR1 polyclonal antibody (1:1000; Affinity Bioreagents, Golden, CO). Specific antibody binding was detected using HRP-conjugated anti-rabbit secondary antibody (1:20,000; Jackson Immunoresearch, West Grove, PA) and Super Signal reagent (Pierce). To detect surface TRPV1, blots were stripped and probed with neutravidin-HRP (1:20,000; Pierce). Chemiluminescence was captured in Hitachi CCD Bio Genetic Systems after exposing the blot to Super Signal reagent (Pierce). Data were quantitated using LabWorks analysis software (UVP Inc., Upland, CA) and surface TRPV1 was normalized using total TRPV1. TRPV1-V5-His transfected cells were biotinylated as above and purified using Ni-NTA agarose (Qiagen, Valencia, CA) as per manufacturer's protocol.
Immunohistochemistry
Cells were plated on glass coverslips coated with 40 μg ml-1 poly-L-lysine, treated with insulin/IGF/PDBu after 36 h of transfection, and fixed with 4% paraformaldehyde for 1 h. The cells were probed with rabbit anti-VR1 antibody (1:10,000; Affinity Bioreagents) followed by rhodamine red-X conjugated anti-rabbit secondary antibody (1:50; Jackson Immuno Research) following method described by Santa Cruz Biotechnology. Confocal images (1 μm sections) were captured using 570 nm laser. The brightest membrane fluorescence was selected from control and treated cells and quantified using MCID imaging software (Imaging Research Inc.).
Unless otherwise stated, all chemicals were obtained by Sigma (St. Louis, MO). Data are given as mean ± s.e.m. and statistical significance (set at P < .05) was evaluated using Student's t-test or one-way ANOVA.
Discussion
From these results, we propose that insulin and its related growth factors provide both trophic and sensory support to peripheral nerve endings. Insulin and IGF-I can directly influence nociceptive ion channel function through phosphorylation and receptor translocation. In the presence of these modulators, TRPV1 is more responsive to painful stimuli (capsaicin, pH and heat) by increasing sensitivity and lowering thresholds. Based on this evidence, we conclude that insulin/IGF levels maintain TRPV1 function and their deficiency or resistance leads to deficits in inflammatory thermal sensation. The role of TRPV1 in animal models of diabetes has been suggested [
27,
35‐
37]. The intriguing aspect is that insulin deficiency results in thermal hyperalgesia in streptozotocin induced diabetes. Sensitization of TRPV1 by phosphorylation could account for the hyperalgesic phenotype seen in this animal model of diabetes [
35]. The exact mechanism of sensitization of TRPV1 is not clear, but could be due to over compensation by other tropic factors, such as NGF in response to insulin deficiency or elevated PKC activity in diabetes [
38].
Using multiple methods, we elucidate molecular mechanism(s) by which insulin/IGF-I potentiate TRPV1 current (Fig.
3 and
4). These neurotrophic factors, operating through RTKs, trigger a signaling cascade leading to PI(3)K and PKC activation [
27,
28,
35]. PKC, an enzyme known to sensitize TRPV1 through phosphorylation [
5,
8,
31], increases channel activity and receptor translocation to the cell surface [
39]. Activation of a similar pathway to insulin by NGF via PI(3)K has been shown to robustly potentiate TRPV1 current [
40]. Since insulin/IGF-I levels and PKC activity are altered in diabetes, we speculate that abnormalities in TRPV1 function may contribute to neuropathy in diabetes.
In DRG sensory neurons, diminished growth factor levels are some of the earliest changes noticed with diabetic neuropathy [
11,
20,
21]. One explanation for diabetic neuropathy states that it is, in essence, a microvascular disturbance [
41]. Peripheral C fibers (which express TRPV1) release vasoactive substances like CGRP, causing small vessel dilation to increase cutaneous circulation and nerve terminal viability. Activation of TRPV1 has been shown to induce and enhance the release of CGRP (7). This neural component of microcirculatory control is decreased in diabetic neuropathy, and the consequent reduction in local blood flow contributes to peripheral vascular complications [
41,
42]. With regards to growth factor deficiency, insulin has a vasodilatory effect that is dependent on CGRP release, which is compromised in diabetes [
43,
44]. Our findings illustrates insulin/IGFs can cause vasodilation via their influence on TRPV1.
A concept is emerging where signals emanating from IR and/or IGFR can activate kinases with the potential to control ion channel phosphorylation, subcellular localization and overall expression. Our work elucidates the mechanism insulin/IGF-I use for TRPV1 sensitization (i.e. RTK→PI(3)K→PKC), but the transduction pathway regulating expression has not been identified. Previously, it was suggested that insulin exerts influences on the same neurons that are responsive to NGF by sharing common pathways [
13]. Recent reports demonstrate that NGF and glial cell line-derived neurotrophic factor (GDNF) upregulate TRPV1 expression on DRG neurons using transduction mechanisms common to insulin/IGF-I (i.e. MAPKs, PI(3)K, Ras, etc.) [
11,
40,
45,
46]. Taken together, these studies set precedence for growth factor influence on nociceptor levels and implicate signaling cascades, which may be compromised by the absence of insulin/IGF.
Insulin potentiates both whole cell and single channel currents mediated by NMDA receptors in
Xenopus oocytes in a PKC-dependent manner [
47]. Moreover, this effect was found to be due to membrane translocation involving both PI(3)K and PKC [
33,
34,
38]. In DRG neurons insulin increased capsaicin induced cobalt uptake [
48]. Along these same lines, insulin can potentiate currents in HEK293 cells through recruitment of GABA
A receptors to postsynaptic domains [
32]. In cultured cerebellar granular cells, IGF-I potentiates kinate receptors through a PI(3)K dependent mechanism [
49].
Furthermore, vanilloid receptors (VRs) are present throughout the body, widely believed to have functions other than temperature sensation. TRPV1 expressed in the central terminals of the sensory neurons robustly modify synaptic transmission [
50,
51]. TRPV1 could be detected using RT-PCR technique throughout the neuroaxis [
52] and identification of specific ligands such as NADA in certain brain regions confirms a role in the CNS [
53]. The nature of the receptors involved in this response and their role in the CNS are not clearly understood, but suggestive of a direct or indirect role in modifying neurotransmitter release [
54]. TRPV1 is also located in vasculature, bronchi and urinary bladder [
7,
55,
56]. Modulation of these receptors by lack of insulin and IGF-I may contribute to CNS disturbances, cardiovascular, respiratory and urinary complications resulting from diabetes.
Lastly, these findings emphasize the importance of maintaining proper insulin levels and suggest a potential benefit of IGF-I administration in the treatment and prevention of diabetic complications. We propose that insulin and IGF-I therapy, partially working through TRPV1, can improve complications associated with diabetes mellitus.