Transient Receptor Potential Vanilloid 1 Modulates Central Inflammation in Multiple Sclerosis

Abstract

Introduction: Disease course of multiple sclerosis (MS) is negatively influenced by proinflammatory molecules released by activated T and B lymphocytes and local immune cells. The endovanilloid system plays different physiological functions, and preclinical data suggest that transient receptor potential vanilloid type 1 (TRPV1) could modulate neuroinflammation in this disorder. Methods: The effect of TRPV1 activation on the release of two main proinflammatory cytokines, tumor necrosis factor (TNF) and interleukin (IL)-6, was explored in activated microglial cells. Furthermore, in a group of 132 MS patients, the association between the cerebrospinal fluid (CSF) levels of TNF and IL-6 and a single nucleotide polymorphisms (SNP) influencing TRPV1 protein expression and function (rs222747) was assessed. Results: In in vitro experiments, TRPV1 stimulation by capsaicin significantly reduced TNF and IL-6 release by activated microglial cells. Moreover, the anti-inflammatory effect of TRPV1 activation was confirmed by another TRPV1 agonist, the resiniferatoxin (RTX), whose effects were significantly inhibited by the TRPV1 antagonist, 5-iodoresiniferatoxin (5-IRTX). Vice versa, BV2 pre-treatment with 5-IRTX increased the inflammatory response induced by LPS. Moreover, in MS patients, a significant association emerged between TRPV1 SNP rs222747 and CSF TNF levels. In particular, the presence of a G allele, known to result in increased TRPV1 protein expression and function, was associated to lower CSF levels of TNF. Conclusions: Our results indicate that TRPV1 influences central inflammation in MS by regulating cytokine release by activated microglial cells. The modulation of the endovanilloid system may represent a useful approach to contrast neuroinflammation in MS.

Keywords: IL-6; TNF; capsaicin; endocannabinoids; microglia; neuroinflammation.

Figure 1

Activated TRPV1 in microglia controls TNF and IL-6 release. TRPV1 stimulation reduces TNF and IL-6 release by BV2 cells under inflammatory challenge. The levels (pg/ml) of TNF (A,B) and IL-6 (C,D) were measured in BV2 culture media harvested after 6 and 24 h of LPS treatment, respectively. One-way ANOVA p < 0.001, followed by Tukey HSD: ^***p < 0.001 for comparisons of LPS, LPS+CAP 10 μM, LPS+CAP 25 μM to Ctrl; ^###p < 0.001 for comparisons of LPS+CAP 25 μM to LPS, ^§§§p < 0.001 for comparison of LPS+CAP 25 μM LPS+ CAP 10 μM.

Figure 2

Pharmacological agonism/antagonism of TRPV1 elicits opposite responses of BV2 cells to LPS. BV2 cells were pre-treated with the TRPV1 agonist RTX (1 nM), the TRPV1 antagonist 5-IRTX (1 μM) or a combination of both before the addition of LPS. RTX alone significantly attenuated the amount of TNF released by the cells, while 5-IRTX significantly increased the TNF release induced by LPS and counteracted the anti-inflammatory effect of RTX. One-way ANOVA p < 0.001, followed by Tukey HSD: ^***p < 0.001 for comparisons of LPS, 5-IRTX+LPS, RTX+5-IRTX+LPS to Ctrl, ^*p < 0.05 for comparisons of RTX+LPS to Ctrl, ^###p < 0.001 for comparisons of RTX+LPS to LPS, ^#p < 0.05 for comparison of 5-IRTX+LPS to LPS, ^§§§p < 0.001 for comparison of 5-IRTX+LPS and RTX+5-IRTX+LPS to RTX+LPS.

Figure 3

TRPV1 SNP rs222747 modulates central TNF levels in MS. TNF (A) and IL-6 (B) concentrations are shown in logarithmic scale. Higher CSF levels of TNF are observed in the TRPV1 rs222747 “CC” group. Mann–Whitney: ^*p = 0.01.