Lipopolysaccharide from Porphyromonas gingivalis sensitizes capsaicin-sensitive nociceptors

Although odontogenic infections are often accompanied by pain, little is known about the potential mechanisms mediating this effect. In this study we tested the hypothesis that trigeminal nociceptive neurons are directly sensitized by lipopolysaccharide (LPS) isolated from an endodontic pathogen, Porphyromonas gingivalis. In vitro studies conducted with cultures of rat trigeminal neurons demonstrated that pretreatment with LPS produced a significant increase in the capsaicin-evoked release of calcitonin gene-related peptide (CGRP) when compared with vehicle pretreatment, thus showing sensitization of the capsaicin receptor, TRPV1, by LPS. Furthermore, confocal microscopic examination of human tooth pulp samples showed the colocalization of the LPS receptor (toll-like receptor 4, TLR4) with CGRP-containing nerve fibers. Collectively, these results suggest the direct sensitization of nociceptors by LPS at concentrations found in infected canal systems as one mechanism responsible for the pain associated with bacterial infections.     

LPS sensitizes TRPV1 via activation of TLR4 in trigeminal sensory neurons

Recent studies have demonstrated that the lipopolysaccharide (LPS) receptor (TLR4) is expressed in TRPV1 containing trigeminal sensory neurons. In this study, we evaluated whether LPS activates trigeminal neurons, and sensitizes TRPV1 responses via TLR4. To test this novel hypothesis, we first demonstrated that LPS binds to receptors in trigeminal neurons using competitive binding. Second, we demonstrated that LPS evoked a concentration-dependent increase in intracellular calcium accumulation (Ca(2+))(i) and inward currents. Third, LPS significantly sensitized TRPV1 to capsaicin measured by (Ca(2+))(i), release of calcitonin gene-related peptide, and inward currents. Importantly, a selective TLR4 antagonist blocked these effects. Analysis of these data, collectively, demonstrates that LPS is capable of directly activating trigeminal neurons, and sensitizing TRPV1 via a TLR4-mediated mechanism. These findings are consistent with the hypothesis that trigeminal neurons are capable of detecting pathogenic bacterial components leading to sensitization of TRPV1, possibly contributing to the inflammatory pain often observed in bacterial infections.     

Toll‐Like Receptor 4 Expression in Trigeminal Neurons Is Increased During Ligature‐Induced Periodontitis in Rats

Background: Periodontitis, activated by oral bacteria and orchestrated by innate immune response, is regulated by primary nociceptive neurons, which are generally considered to have small‐ to medium‐sized perikaryons. Bacterial byproducts (e.g., lipopolysaccharides) activate primary nociceptive neurons directly through Toll‐like receptors (TLRs). Therefore, this study aims to morphometrically characterize rat trigeminal neurons, which express TLR4, and to investigate the changes in the TLR4 expression in neurons during periodontal inflammation. Methods: Trigeminal neurons innervating gingivomucosa were identified by application of the retrograde tracer hydroxystilbamidine into the gingival sulcus of the maxillary molar in 14 rats. Periodontitis was induced by ligature around the same molar in seven rats. TLR4 expression was investigated by immunohistochemistry on paraffin sections of the trigeminal ganglia (TG). Semiquantitative method was used to identify the intensity of TLR4 expression. Results: In the control group without the ligatures, TLR4 was detected in 19% of the neurons in the maxillary region of TG and in 29% of neurons innervating gingivomucosa. Expression of TLR4 was more frequent and intensive in small‐ to medium‐sized neurons than in large‐sized neurons. One week after ligature‐induced periodontitis, the percentage of TLR4‐positive neurons in the maxillary region and among the neurons innervating inflamed gingivomucosa significantly increased statistically to 32% and 41%, respectively. Conclusions: TLR4 is predominantly, but not exclusively, expressed in smaller trigeminal nociceptive neurons in the rat. Experimental periodontitis upregulates TLR4 expression in the trigeminal neurons. The hypothesis that bacterial byproducts regulate the pathogenesis of periodontitis by activation of trigeminal nociceptors through TLR4 should be explored.     

Interferon-γ stimulates CD14, TLR2 and TLR4 mRNA expression in gingival fibroblasts increasing responsiveness to bacterial challenge

ObjectiveTo investigate the potential effects of IFN-03A5 on the responsiveness of human gingival fibroblasts to bacterial challenge.DesignmRNA and protein expression of CD14, TLR2 and TLR4 in human gingival fibroblasts was detected by quantitative polymerase chain reaction (Q-PCR) and flow cytometry. The effect of preincubation with IFN-03A5 on subsequent bacterial LPS-induced expression of IL-6 and IL-8 by gingival fibroblasts was determined by ELISA. Bacterial LPS-induced IκBα degradation in human gingival fibroblasts was investigated by western blot.ResultsHuman gingival fibroblasts express CD14, TLR2 and TLR4 mRNAs. IFN-03A5, but not IL-103B2, induced mRNA expression of all three receptors and the expression of membrane bound CD14 protein. Pre-incubation of fibroblasts with IFN-03A5 and subsequent stimulation with Escherichia coli LPS or Porphyromonas gingivalis LPS led to increased production of IL-6 and IL-8. LPS-induced pro-inflammatory cytokine production was abrogated by a blocking antibody to CD14. Both E. coli LPS and P. gingivalis LPS induced IκBα degradation in human gingival fibroblasts.ConclusionOur data indicate that IFN-03A5 primes human gingival fibroblasts, through the upregulation of CD14 expression, which results in increased responsiveness to bacterial LPS challenge, as determined by pro-inflammatory cytokine production.     

脂多糖信号受体CD14、TLR4在人牙髓组织和牙髓成纤维细胞中的表达

目的研究人牙髓组织和牙髓成纤维细胞中脂多糖（LPS）信号受体CD14、TLR4的表达特点,探讨牙髓炎症组织中LPS的信号转导途径.方法采用免疫组化染色法观察健康和炎症牙髓组织中CD14、TLR4的表达情况;应用直接免疫荧光标记法,采用流式细胞术检测体外培养人牙髓成纤维细胞在LPS刺激前后的CD14、TLR4阳性细胞率和细胞表面平均荧光强度.结果正常牙髓组织中未见CD14、TLR4阳性细胞;炎症牙髓组织中可见大量CD14、TLR4阳性细胞,CD14、TLR4阳性细胞率差异无统计学意义（P＞0.05）.牙髓成纤维细胞经LPS刺激后,TLR4平均荧光表达强度和TLR4阳性细胞率均显著增高（P＜0.05）,而CD14在LPS刺激前、后均无表达.结论炎症牙髓组织中CD14、TLR4的阳性表达,提示LPS可能通过CD14、TLR4信号受体在牙髓炎症组织中发挥作用,而牙髓成纤维细胞在LPS刺激后仅表达TLR4,表明LPS可能通过TLR4对牙髓成纤维细胞发挥作用.     

Porphyromonas gingivalis fimbriae induce unique dendritic cell subsets via Toll-like receptor 2

Backgound and Objective:  Dendritic cells (DCs) play a critical role in the activation of T cells as well as in shaping immune responses. We have reported previously that Porphyromonas gingivalis lipopolysaccharides ( Pg LPS) induced a CD14⁺CD16⁺ DC subset with a weak immuno-stimulatory activity. In contrast, Escherichia coli LPS ( Ec LPS) induced fully matured DCs with strong immunostimulatory activities. Since Pg LPS as well as Pg fimbriae have been indicated to work as Toll-like receptor (TLR) 2 ligands, we speculate that the TLR usage of bacterial antigens may be critical for DC maturation.
Material and Methods:  We investigated the effect of Pg fimbriae on the phenotype and function of human peripheral blood DCs in comparison with a TLR2 ligand, peptidoglycan, and a TLR4 ligand, Ec LPS.
Results:  Flow cytometry revealed that Pg fimbriae and peptidoglycan but not Ec LPS induced CD14 and CD16 expression on peripheral blood DCs (CD14⁻CD16⁻). A monoclonal antibody against TLR2 abrogated this induction, but an antibody against TLR4 had no effect. Dendritic cells stimulated with Pg fimbriae had a weaker capability to induce allogenic T cell proliferation and exhibited a weaker production of interleukin-8 and regulated upon activation, normal T cell expressed and secreted (RANTES) than DCs stimulated with Ec LPS.
Conclusion:  These results indicate that different TLR usage affects mature DC phenotype and function and is thus crucial to the regulation of immunity to the pathogen.     

Projection of non-peptidergic afferents to mouse tooth pulp

A large proportion of pulpal nociceptors are known to contain neuropeptides such as CGRP. However, the projection of non-peptidergic nociceptors to tooth pulp is controversial. Recently, the non- peptidergic subset of nociceptors has been implicated in mechanical pain in the skin. Since mechanical irritation of pulpal nociceptors is critical for evoking tooth pain under pathophysiological conditions, we investigated whether the non-peptidergic afferents project to tooth pulp as potential mechanotransducing afferents. For clear visualization of the non-peptidergic afferents, we took advantage of a recently generated knock-in mouse model in which an axonal tracer, farnesylated green fluorescence protein (GFP), is expressed from the locus of a sensory neuron-specific gene, Mrgprd. In the trigeminal ganglia (TG), we demonstrated that GFP is exclusively expressed in afferents binding to isolectin B4 (IB4), a neurochemical marker of non-peptidergic nociceptors, but is rarely co-localized with CGRP. Retrograde labeling of pulpal afferents demonstrated that a low proportion of pulpal afferents was co-localized with GFP. Immunohistochemical detection of the axonal tracer revealed that GFP-positive afferent terminals were densely projected into the tooth pulp. These results provide convincing evidence that non-peptidergic nociceptors are projected into the tooth pulp and suggest a potential role for these afferents in tooth pain.     

CD14、TLR4在人炎症牙髓组织中的表达

目的 探讨CD14、TLR4在人牙髓组织中的表达,为阐明CD14、TLB4在LPS引起的炎症反应中的作用机制提供实验依据.方法 通过免疫组织化学染色和流式细胞术观察正常、深龋和牙髓炎牙髓组织中CD14、TLB4表达情况.结果 免疫组化染色观察正常组牙髓组织中未见CD14、TLR4阳性表达细胞;深龋组中可见CD14、TLB4表达阳性细胞,多为中性粒细胞阳性染色;慢性牙髓炎组可见大量CD14、TLR4表达阳性细胞,多为单核细胞、淋巴细胞阳性染色.流式细胞术检测到CD14、TLB4阳性细胞率随牙髓组织炎症的加重而增大;深龋组和牙髓炎组中TLB4的平均荧光强度比CD14高.结论 在牙髓炎症发展过程中,CD14、TLB4阳性细胞表达率随牙髓组织炎症的加重而增大,与炎症程度呈正相关.同时检测到在深龋组和牙髓炎组中TLR4的平均荧光强度比CD14高,可以推测LPS主要通过TLR4进行信号转导.     

TLR4 mediates LPS-induced VEGF expression in odontoblasts

Lipopolysaccharide (LPS) from gram-negative bacteria cell walls such as Prevotella intermedia and Escherichia coli induce vascular endothelial growth factor (VEGF) expression in odontoblasts, but not in undifferentiated dental pulp cells. CD14 and TLR4 are responsible for LPS signaling in macrophages, but their expression levels and function in dental pulp cells are unknown. We showed here that murine odontoblast-like cells (MDPC-23) express CD14 and TLR4 by immunohistochemistry and flow cytometry. In contrast, undifferentiated dental pulp cells (OD-21) presented low or no expression of these two receptors. MDPC-23 cells showed CD14 and TLR4 up-regulation upon exposure to LPS, as determined by real time PCR. Dominant negative murine TLR4 (DN-mTLR4) transfected MDPC-23 cells did not show upregulated VEGF expression in response to LPS stimulation. These results demonstrate that odontoblast-like cells express CD14 and TLR4, and that LPS-induced VEGF expression is mediated, at least in part, by TLR4 signaling.     

Neuropeptide Y Y1 receptor effects on pulpal nociceptors

Neuropeptide Y (NPY) is an important modulatory neuropeptide that regulates several physiological systems, including the activity of sensory neurons. We evaluated whether activation of the NPY Y1 receptor could modulate the activity of capsaicin-sensitive nociceptors in trigeminal ganglia and dental pulp. We tested this hypothesis by measuring capsaicin-stimulated calcitonin gene-related peptide release (CGRP) as a measure of nociceptor activity. Capsaicin-evoked CGRP release was inhibited by 50% (p < 0.05) in trigeminal ganglia and by 26% (p < 0.05) in dental pulp when tissues were pre-treated with [Leu(31),Pro(34)]NPY. The Y1 receptor was found to co-localize with the capsaicin receptor TRPV1 in trigeminal ganglia. These results demonstrate that activation of the Y1 receptor results in the inhibition of the activity of capsaicin-sensitive nociceptors in the trigeminal ganglia and dental pulp. These findings are relevant to the physiological modulation of dental nociceptors by endogenous NPY and demonstrate an important novel analgesic target for the treatment of dental pain.     

根尖周炎动物模型中CD14、TLR4的表达

目的研究大鼠根尖周炎炎症组织中脂多糖炎症信号受体CD14、Toll样受体4（Toll．1ikereceptor4，TLR4）的表达特点，探讨根尖周炎中脂多糖的信号转导途径。方法建立大鼠磨牙内毒素根尖周炎模型，采用免疫组化染色观察根尖周炎炎症组织中CD14、TLR4的表达情况，并计算CD14、TLR4的阳性细胞率。结果正常根尖周组织中未发现CD14和TLR4免疫阳性细胞，根尖周炎症组织中CD14和TLR4表达阳性，CD14、TLR4阳性细胞率差异无统计学意义（P〉0．05）。结论与正常根尖周组织相比，炎症根尖周组织中CD14和TLR4的表达显著增强，CD14和TLR4的表达量差异无统计学意义，提示脂多糖可能通过CD14、TLR4信号受体在根尖周炎症中发挥作用。     

Direct Effect of Endodontic Sealers on Trigeminal Neuronal Activity

Introduction

Endodontic sealers are selected on the basis of their antimicrobial properties and ability to provide a tight seal. Sealer extrusions, whether intentional or unintentional, are common during obturation procedures. Such events have been correlated with increased postoperative discomfort and persistent pain states. However, the mechanisms underlying this phenomenon are largely unknown. Thus, we sought to evaluate the effect of commonly used endodontic sealers on peripheral nociceptors. We hypothesized that endodontic sealers can directly activate trigeminal nociceptors in a concentration-dependent manner, resulting in release of calcitonin gene-related peptide (CGRP), a potent modulator of neurogenic inflammation.

Methods

Rat trigeminal sensory neurons were exposed in vitro to vehicle, zinc oxide-eugenol (ZOE)–based sealer, AH Plus, EndoSequence BC sealer, or RealSeal SE. Neuronal activation was measured by quantification of neuropeptide (CGRP) release. In addition, cultured neurons were also subjected to the set form of all 4 sealers. The concentration of CGRP released was quantified by using a radioimmunoassay. Data were analyzed by using one-way analysis of variance with Newman-Keuls multiple comparison post hoc test.

Results

Both ZOE-based sealer and AH Plus in their fresh form evoked greater CGRP release than the control groups. Conversely, EndoSequence BC and RealSeal sealers both reduced basal GCRP release at all concentrations tested. Evaluation of the set sealers revealed that only ZOE-based sealer evoked significant CGRP release compared with its control group.

Conclusions

Overall, our results suggest that sealers can directly activate trigeminal nociceptors, leading to a robust release of CGRP, and may therefore lead to pain and neurogenic inflammation. This direct activation along with the immunologic response may underlie the symptoms and flare-up occurrences often seen with sealer extrusions.     

实验性牙移动三叉神经节内降钙素基因相关肽改变

目的：观察实验性牙移动后,初级感觉神经元兴奋性递质降钙素基因相关肽（CGRP）的改变。分析正畸疼痛时,初级感觉神经元痛信号发生、传导及敏感化机制。方法：制备大鼠实验性牙移动动物模型,采用免疫荧光染色法和RT-PCR法分别观察不同时间点三叉神经节内CGRP免疫阳性结构的改变及CGRP mRNA表达的变化。结果：实验性牙移动后,实验侧三叉神经节（TG）内CGRP-ir节细胞以及CGRP mRNA的表达强于对侧。结论：实验性牙移动后,初级感觉神经元中痛感受兴奋性神经递质CGRP的合成、表达增加。表明实验性牙移动影响了初级感觉神经元痛信号的发生、传导,使之致敏。     

Immunohistochemical localization of Toll-like receptors 2 and 4 in gingival tissue from patients with periodontitis

The present study investigated the expression of Toll-like receptor (TLR) 2, TLR4, cluster of differentiation (CD) 14 and CD1a in human periodontitis gingiva using immunohistochemical methods. The specimens were classified according to the degree of inflammation into three groups (mild, moderate and severe). We established three zones in which to evaluate the ratios of TLR2-, TLR4-, CD14- and CD1a-positive cells to total cells in the connective tissues of each section. TLR2 and TLR4 were expressed in human periodontal tissues, and the ratio of TLR2-positive cells was highest overall in zone 1 (connective tissue subjacent to pocket epithelium) of the severe group and that of TLR4-positive cells was higher in the severe group than in the other groups. These results suggest that TLR2 and TLR4 participate in the innate immune response to stimulation by bacterial products in periodontal tissues. The ratio of CD14-positive cells was lowest overall in zone 1 of the severe group and that of CD1a was higher in the severe group than in the other groups. These results suggest that CD14 may be down-regulated during the development of inflammation and/or dendritic cells might infiltrate chronically inflamed gingival tissue.     

Immunohistochemical localization of Toll‐like receptors 2 and 4 in gingival tissue from patients with periodontitis

The present study investigated the expression of Toll‐like receptor (TLR) 2, TLR4, cluster of differentiation (CD) 14 and CD1a in human periodontitis gingiva using immunohistochemical methods. The specimens were classified according to the degree of inflammation into three groups (mild, moderate and severe). We established three zones in which to evaluate the ratios of TLR2‐, TLR4‐, CD14‐ and CD1a‐positive cells to total cells in the connective tissues of each section. TLR2 and TLR4 were expressed in human periodontal tissues, and the ratio of TLR2‐positive cells was highest overall in zone 1 (connective tissue subjacent to pocket epithelium) of the severe group and that of TLR4‐positive cells was higher in the severe group than in the other groups. These results suggest that TLR2 and TLR4 participate in the innate immune response to stimulation by bacterial products in periodontal tissues. The ratio of CD14‐positive cells was lowest overall in zone 1 of the severe group and that of CD1a was higher in the severe group than in the other groups. These results suggest that CD14 may be down‐regulated during the development of inflammation and/or dendritic cells might infiltrate chronically inflamed gingival tissue.     

CD16参与大鼠外周神经损伤及痛觉过敏的研究

目的:观察实验性大鼠神经损伤过程中CD16在三叉神经节的动态表达和分布,探讨三叉神经系统通过神经免疫反应参与外周神经损伤及痛觉过敏的潜在机制。方法:建立大鼠眶下神经慢性压榨损伤动物模型,应用免疫荧光标记观察CD16在三叉神经节的表达和分布,通过与CGRP的相关性分析,检测痛觉过敏过程中CD16的参与作用。结果:大鼠眶下神经压榨性损伤72 h后,三叉神经节初级感觉神经元CD16的表达显著性增强,5 d后达峰值,且神经元内CD16的表达与CGRP的表达具有显著相关性。结论:三叉神经系统初级感觉神经元细胞表达CD16,并参与外周神经损伤和痛觉过敏过程。     

Toll样受体2和4信号通路在炎症治疗中的作用和意义

脂多糖（LPS）在细菌破坏细胞的过程中起着重要的作用。Toll样受体（TLR）2对LPS的识别是通过与TLR1和TLR6构成异源二聚体来完成的,TLR2识别LPs后介导的细胞内免疫反应遵循髓样分化因子（MyD）88依赖性通路。MyD88的死亡结构域募集下游的白细胞介素-1受体相关激酶1和4,肿瘤坏死因子受体相关因子6和转化生长因子-B1活化激酶等信号分子,促使核因子-KB、激活蛋白1和P38促丝裂原激活蛋白激酶活化,继而导致促炎症细胞因子相关基因转录。MyD88非依赖性通路分别募集和激活下游分子受体相互作用蛋白1或肿瘤坏死因子受体相关因子3,通过核因子-κB、激活蛋白1和干扰素调节因子3,诱导Ⅰ型干扰素的产生。CD14和MyD2是LPS与TLR4结合的关键蛋白,控制CD14或MyD2可阻止LPs和TLR4的结合,将炎症反应阻断在信号转导的上游。TLR2和TLR4对LPS的识别是引发炎症反应的关键,限制细胞对TLR2和TLR4的表达是进行炎症控制最直接有效的方法。调控TLR2和TLR4信号通路,有望给予牙周炎、炎症性肠炎、心血管疾病及和自身免疫性疾病等更有效和更安全的临床治疗。     

Trigeminal nociceptors express prostaglandin receptors

Orofacial inflammation is associated with prostaglandin release and the sensitization of nociceptive receptors such as the transient receptor potential subtype V(1) (TRPV(1)). We hypothesized that certain PGE(2) receptor subtypes (EP1-EP4) are co-expressed with TRPV(1) in trigeminal nociceptors and sensitize responses to a TRPV(1) agonist, capsaicin. Accordingly, combined in situ hybridization was performed with immunohistochemistry on rat trigeminal ganglia. We next evaluated the effects of specific EP2 and EP3 agonists (butaprost and sulprostone) in cultured trigeminal ganglia neurons. The results showed that EP2 and EP3 are expressed in trigeminal neurons (58% and 53% of total neurons, respectively) and are co-expressed in TRPV(1)-positive neurons (64% and 67 % of TRPV(1)-positive neurons, respectively). Moreover, most of the cells expressing EP2 or EP3 mRNA were of small to medium diameter (< 30 microm). The application of butaprost and sulprostone triggered neuropeptide exocytosis, and butaprost sensitized capsaicin responses. Analysis of these data, collectively, supports the hypothesis that prostaglandins regulate trigeminal TRPV(1) nociceptors via activation of the EP2 and EP3 receptors.     

Peripheral and Central Mechanisms of Trigeminal Neuropathic and Inflammatory Pain

While physiological pain (nociceptive pain) has a protective role in warning of potential tissue damage in response to a variety of noxious stimuli, pathological pain (neuropathic and inflammatory pain) serves no such meaningful purpose. Injury/inflammation in the peripheral tissue that innervates the trigeminal nerve may also alter the properties of trigeminal somatic sensory pathways, causing behavioral hypersensitivity (e.g., pathological pain) and induce pain abnormality caused by noxious stimulation (hyperalgesia) or normally innocuous stimulation (allodynia). These hypersensitivities to nociception are caused by changes in the excitability of trigeminal ganglion neurons (peripheral sensitization), which alter sensory information processing in spinal trigeminal spinal subnucleus caudalis (SpVc)/upper cervical spinal cord (C_1–2) neurons (central sensitization). More is being learned about the activation of peripheral and central glia that play an important role in creating and maintaining pathological pain. This review therefore focuses on the possible sites for sensitization of nociceptive signaling through pain pathways that contribute to trigeminal pathological pain and also discuss potential therapeutic targets in neuron-glial interactions for preventing trigeminal neuropathic and inflammatory pain.