Painful neurotrophins and their role in visceral pain

How Tongxie-Yaofang Regulates Intestinal Synaptic Plasticity by Activating Enteric Glial Cells and NGF/TrkA Pathway in Diarrhea-Predominant Irritable Bowel Syndrome Rats

Purpose

Diarrhea-predominant irritable bowel syndrome (D-IBS) is a frequent functional gastrointestinal disease that affects health and quality of life owing to its high incidence and recurrence rate. Tongxie-Yaofang (TXYF) is a traditional Chinese medicine prescribed for D-IBS. However, the therapeutic mechanism of TXYF has not been fully elucidated. This study aimed to investigate the effects of TXYF on visceral hypersensitivity in stress-induced D-IBS rats and the underlying mechanisms.

Methods

Electromyographic (EMG) activity of the external oblique muscles and the abdominal withdrawal reflex (AWR) score captured by Barostat were used to quantify the effect of TXYF on visceral sensitivity. Transmission electron microscopy (TEM) was used to observe the ultrastructure of the enteric nervous system (ENS). For molecular detection, the colonic expression of enteric glial cell's (EGC's) activation markers, glial fibrillary acidic protein (GFAP) and calcium-binding protein S100β, NGF, TrkA, synaptic plasticity-related factors, synaptophysin (SYN) and postsynaptic density-95 (PSD-95), glutamate, glutamate receptors α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAR), and N-methyl-D-aspartate receptor (NMDAR) were detected by immunohistochemistry, enzyme-linked immunosorbent assay, and real-time PCR. An ex vivo experiment was conducted to measure the EGC-induced NGF release.

Results

TXYF decreased the EMG activity and AWR scores in rats with D-IBS. Under TEM, TXYF improved the dense and irregular nerve arrangement, narrowed the synaptic cleft, and decreased the number of synaptic vesicles in D-IBS rats. In addition, TXYF decreased the expression of GFAP, S100β, SYN, and PSD-95; down-regulated the levels of NGF, TrkA, and glutamate; and reduced the mRNA expression of AMPAR1, NMDAR1, and NMDAR2B. In an ex vivo experiment, TXYF decreased NGF release in D-IBS rats, and this trend disappeared under EGC inhibition.

Conclusion

TXYF alleviated visceral hypersensitivity in D-IBS rats possibly by improving synaptic plasticity through inhibiting the activity of EGCs and the NGF/TrkA signaling pathway in the colon.

miR-1306-3p directly activates P2X3 receptors in primary sensory neurons to induce visceral pain in rats

ABSTRACT

Mounting evidence indicates that microRNAs (miRNAs) play critical roles in various pathophysiological conditions and diseases, but the physiological roles of extracellular miRNAs on the disease-related ion channels remain largely unknown. Here, we showed that miR-1306-3p evoked action potentials and induced inward currents of the acutely isolated rat dorsal root ganglion (DRG) neurons. The miR-1306-3p-induced effects were significantly inhibited by A317491, a potent inhibitor of the P2X3 receptor (P2X3R), or disappeared after the knockdown of P2X3Rs in DRG neurons. We further identified R180, K315, and R52 as the miR-1306-3p interaction sites on the extracellular domain of P2X3Rs, which were distinct from the orthosteric ATP-binding sites. Intrathecal injection of miR-1306-3p produced visceral pain but not somatic pain in normal control rats. Conversely, intrathecal application of a miR-1306-3p antagomir and A317491 significantly alleviated visceral pain in a rat model of chronic visceral pain. Together, our findings suggest that miR-1306-3p might function as an endogenous ligand to activate P2X3Rs, eventually leading to chronic visceral pain.

NUCB2/Nesfatin-1 Regulation of Chronic Visceral Hyperalgesia

Objective

We previously described that different concentration Nucleobindin-2 (NUCB2)/Nesfatin-1 gradients differently regulated visceral hypersensitivity in irritable bowel syndrome. Therefore, this study is aimed at evaluating the effect of NUCB2/Nesfatin-1 on model rats with chronic visceral hyperalgesia.

Methods

Neonatal and mature Sprague-Dawley rats were randomly divided into the healthy control and chronic visceral hyperalgesia model groups. The model was built by combining maternal separation with the acetic acid enema. The models were identified by the distension volume threshold to reach abdominal withdraw reflex (AWR) score = 3, histological staining, and myeloperoxidase (MPO) detection. The visceral sensitivity to chronic visceral hyperalgesia was then evaluated.

Result

Rats in the model group responded more strongly to pulling stimulation than healthy controls; the distension volume threshold causing AWR3 response in model rats was lower than the control group before NUCB2/Nesfatin-1 intervention. After intervention, the distension volume threshold was significantly lower in the NUCB2/Nesfatin-1 central intervention group than in the NUCB2/Nesfatin-1 peripheral intervention group, and the peak value of external oblique muscle electrical activity was significantly higher. Additionally, compared with the male intervention group, in the female intervention group, the volume threshold was significantly lower and the peak value was higher.

Conclusion

NUCB2/Nesfatin-1 could regulate visceral sensitivity in chronic visceral hyperalgesia model rats; its regulatory effect correlated with the type of NUCB2/Nesfatin-1 intervention approaches (central or peripheral) and sex (male or female).

Oral squamous cell carcinoma-released brain-derived neurotrophic factor contributes to oral cancer pain by peripheral tropomyosin receptor kinase B activation

ABSTRACT

Oral cancer pain is debilitating and understanding mechanisms for it is critical to develop novel treatment strategies treatment strategies. Brain-derived neurotrophic factor (BDNF) signaling is elevated in oral tumor biopsies and is involved with tumor progression. Whether BDNF signaling in oral tumors contributes to cancer-induced pain is not known. The current study evaluates a novel peripheral role of BDNF-tropomyosin receptor kinase B (TrkB) signaling in oral cancer pain. Using human oral squamous cell carcinoma (OSCC) cells and an orthotopic mouse tongue cancer pain model, we found that BDNF levels were upregulated in superfusates and lysates of tumor tongues and that BDNF was expressed by OSCC cells themselves. Moreover, neutralization of BDNF or inhibition of TrkB activity by ANA12, within the tumor-bearing tongue reversed tumor-induced pain-like behaviors in a sex-dependent manner. Oral squamous cell carcinoma conditioned media also produced pain-like behaviors in naïve male mice that was reversed by local injection of ANA12. On a physiological level, using single-fiber tongue-nerve electrophysiology, we found that acutely blocking TrkB receptors reversed tumor-induced mechanical sensitivity of A-slow high threshold mechanoreceptors. Furthermore, single-cell reverse transcription polymerase chain reaction data of retrogradely labeled lingual neurons demonstrated expression of full-form TrkB and truncated TrkB in distinct neuronal subtypes. Last but not the least, intra-TG siRNA for TrkB also reversed tumor-induced orofacial pain behaviors. Our data suggest that TrkB activities on lingual sensory afferents are partly controlled by local release of OSCC-derived BDNF, thereby contributing to oral cancer pain. This is a novel finding and the first demonstration of a peripheral role for BDNF signaling in oral cancer pain.

Targeting Enteric Neurons and Plexitis for the Management of Inflammatory Bowel Disease

Ulcerative colitis (UC) and Crohn's disease (CD) are pathological conditions with an unknown aetiology that are characterised by severe inflammation of the intestinal tract and collectively referred to as inflammatory bowel disease (IBD). Current treatments are mostly ineffective due to their limited efficacy or toxicity, necessitating surgical resection of the affected bowel. The management of IBD is hindered by a lack of prognostic markers for clinical inflammatory relapse. Intestinal inflammation associates with the infiltration of immune cells (leukocytes) into, or surrounding the neuronal ganglia of the enteric nervous system (ENS) termed plexitis or ganglionitis. Histological observation of plexitis in unaffected intestinal regions is emerging as a vital predictive marker for IBD relapses. Plexitis associates with alterations to the structure, cellular composition, molecular expression and electrophysiological function of enteric neurons. Moreover, plexitis often occurs before the onset of gross clinical inflammation, which may indicate that plexitis can contribute to the progression of intestinal inflammation. In this review, the bilateral relationships between the ENS and inflammation are discussed. These include the effects and mechanisms of inflammation-induced enteric neuronal loss and plasticity. Additionally, the role of enteric neurons in preventing antigenic/pathogenic insult and immunomodulation is explored. While all current treatments target the inflammatory pathology of IBD, interventions that protect the ENS may offer an alternative avenue for therapeutic intervention.

Driving effect of BDNF in the spinal dorsal horn on neuropathic pain

Neuropathic pain (NP) is caused by direct or indirect damage to the nervous system and is a common symptom of many diseases. The mechanisms underlying the onset and persistence of NP are unclear. Therefore, research concerning these mechanisms has become an important focus in the medical field. Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophic factor family of signaling molecules. BDNF is an important regulator of neuronal development, synaptic transmission, and cellular and synaptic plasticity, which are essential for nerve maintenance and repair. However, BDNF is upregulated in the spinal dorsal horn and can promote NP by activating glial cells, reducing inhibitory functions and enhancing excitement after nociceptive stimulation. This review considers the relationship between NP and BDNF signaling in the spinal dorsal horn and discusses potentially related pathological mechanisms.

The emerging role of the BDNF-TrkB signaling pathway in the modulation of pain perception

The brain derived neurotrophic factor (BDNF) is a crucial neuromodulator in pain transmission both in peripheral and central nervous system (CNS). Despite evidence of a pro-nociceptive role of BDNF, recent studies have reported contrasting results, including anti-nociceptive and anti-inflammatory activities. Moreover, BDNF polymorphisms can interfere with BDNF role in pain perception. In Val66Met carriers, the Met allele may have a dual role, with anti-nociceptive actions in normal condition and pro-nociceptive effects during chronic pain. In order to elucidate the main effects of BDNF in nociception, we reviewed the main characteristics of this neurotrophin, focusing on its involvement in pain.

In vitro affinity optimization of an anti-BDNF monoclonal antibody translates to improved potency in targeting chronic pain states in vivo

The role of brain-derived neurotrophic factor (BDNF) signaling in chronic pain has been well documented. Given the important central role of BDNF in long term plasticity and memory, we sought to engineer a high affinity, peripherally-restricted monoclonal antibody against BDNF to modulate pain. BDNF shares 100% sequence homology across human and rodents; thus, we selected chickens as an alternative immune host for initial antibody generation. Here, we describe the affinity optimization of complementarity-determining region-grafted, chicken-derived R3bH01, an anti-BDNF antibody specifically blocking the TrkB receptor interaction. Antibody optimization led to the identification of B30, which has a > 300-fold improvement in affinity based on BIAcore, an 800-fold improvement in potency in a cell-based pERK assay and demonstrates exquisite selectivity over related neurotrophins. Affinity improvements measured in vitro translated to in vivo pharmacological activity, with B30 demonstrating a 30-fold improvement in potency over parental R3bH01 in a peripheral nerve injury model. We further demonstrate that peripheral BDNF plays a role in maintaining the plasticity of sensory neurons following nerve damage, with B30 reversing neuron hyperexcitability associated with heat and mechanical stimuli in a dose-dependent fashion. In summary, our data demonstrate that effective sequestration of BDNF via a high affinity neutralizing antibody has potential utility in modulating the pathophysiological mechanisms that drive chronic pain states.

Microglia P2X4R-BDNF signalling contributes to central sensitization in a recurrent nitroglycerin-induced chronic migraine model

Background

According to our previous study, microglia P2X4 receptors (P2X4Rs) play a pivotal role in the central sensitization of chronic migraine (CM). However, the molecular mechanism that underlies the crosstalk between microglia P2X4Rs and neurons of the trigeminal nucleus caudalis (TNC) is not fully understood. Therefore, the aim of this study is to examine the exact P2X4Rs signalling pathway in the development of central sensitization in a CM animal model.

Methods

We used an animal model with recurrent intermittent administration of nitroglycerin (NTG), which closely mimics CM. NTG-induced basal mechanical and thermal hypersensitivity were evaluated using a von Frey filament test and an increasing-temperature hot plate apparatus (IITC). We detected P2X4Rs, brain-derived neurotrophic factor (BDNF) and phosphorylated p38 mitogen-activated protein kinase (p-p38-MAPK) expression profiles in the TNC. We investigated the effects of a P2X4R inhibitor (5-BDBD) and an agonist (IVM) on NTG-induced hyperalgesia and neurochemical changes as well as on the expression of p-p38-MAPK and BDNF. We also detected the effects of a tropomyosin-related kinase B (TrkB) inhibitor (ANA-12) on the CM animal model in vivo. Then, we evaluated the effect of 5-BDBD and SB203580 (a p38-MAPK inhibitors) on the release and synthesis of BDNF in BV2 microglia cells treated with 50 μM adenosine triphosphate (ATP).

Results

Chronic intermittent administration of NTG resulted in chronic mechanical and thermal hyperalgesia, accompanied by the upregulation of P2X4Rs and BDNF expression. 5-BDBD or ANA-12 prevented hyperalgesia induced by NTG, which was associated with a significant inhibition of the NTG-induced increase in phosphorylated extracellular regulated protein kinases (p-ERK) and calcitonin gene related peptide (CGRP) release in the TNC. Repeated administration of IVM produced sustained hyperalgesia and significantly increased the levels of p-ERK and CGRP release in the TNC. Activating P2X4Rs with ATP triggered BDNF release and increased BDNF synthesis in BV2 microglia, and these results were then reduced by 5-BDBD or SB203580.

Conclusions

Our results indicated that the P2X4R contributes to the central sensitization of CM by releasing BDNF and promoting TNC neuronal hyper-excitability. Blocking microglia P2X4R-BDNF signalling may have an effect on the prevention of migraine chronification.