The role of transient receptor potential vanilloid 1 (TRPV1) receptors in dextran sulfate-induced colitis in mice

Bidirectional regulation of the brain-gut-microbiota axis following traumatic brain injury

JOURNAL/nrgr/04.03/01300535-202508000-00002/figure1/v/2024-09-30T120553Z/r/image-tiff Traumatic brain injury is a prevalent disorder of the central nervous system. In addition to primary brain parenchymal damage, the enduring biological consequences of traumatic brain injury pose long-term risks for patients with traumatic brain injury; however, the underlying pathogenesis remains unclear, and effective intervention methods are lacking. Intestinal dysfunction is a significant consequence of traumatic brain injury. Being the most densely innervated peripheral tissue in the body, the gut possesses multiple pathways for the establishment of a bidirectional "brain-gut axis" with the central nervous system. The gut harbors a vast microbial community, and alterations of the gut niche contribute to the progression of traumatic brain injury and its unfavorable prognosis through neuronal, hormonal, and immune pathways. A comprehensive understanding of microbiota-mediated peripheral neuroimmunomodulation mechanisms is needed to enhance treatment strategies for traumatic brain injury and its associated complications. We comprehensively reviewed alterations in the gut microecological environment following traumatic brain injury, with a specific focus on the complex biological processes of peripheral nerves, immunity, and microbes triggered by traumatic brain injury, encompassing autonomic dysfunction, neuroendocrine disturbances, peripheral immunosuppression, increased intestinal barrier permeability, compromised responses of sensory nerves to microorganisms, and potential effector nuclei in the central nervous system influenced by gut microbiota. Additionally, we reviewed the mechanisms underlying secondary biological injury and the dynamic pathological responses that occur following injury to enhance our current understanding of how peripheral pathways impact the outcome of patients with traumatic brain injury. This review aimed to propose a conceptual model for future risk assessment of central nervous system-related diseases while elucidating novel insights into the bidirectional effects of the "brain-gut-microbiota axis."

Propolis alleviates ulcerative colitis injury by inhibiting the protein kinase C - transient receptor potential cation channel subfamily V member 1 - calcitonin gene-related peptide/substance P (PKC-TRPV1-CGRP/SP) signaling axis

This study investigated the protective effect of water-soluble propolis (WSP) on colonic tissues in ulcerative colitis (UC) and the role of the protein kinase C - transient receptor potential cation channel subfamily V member 1 - calcitonin gene-related peptide/substance P (PKC-TRPV1-CGRP/SP) signaling pathway. Male SD rats were divided into a control group, a UC model group, various WSP groups (Low-WSP, Medium-WSP, and High-WSP) with UC, and a salazosulfapyridine (SASP) positive control group with UC. After UC was established, the WSP and SASP groups were treated with WSP or SASP, respectively, for 7 d. Each day, body weight measurements were obtained, and the disease activity index (DAI) was recorded by observing fecal characteristics and blood in the stool. After the experiment, hematoxylin and eosin (HE) colonic tissue staining was performed to observe pathological changes, western blotting and immunohistochemistry were performed to detect PKC, TRPV1, CGRP, and SP expression in colonic tissues, and laser confocal microscopy was performed to observe the fluorescence colocalization of PKC/TRPV1, TRPV1/CGRP, and TRPV1/SP. HE staining showed significant colonic tissue structure disruption and inflammatory infiltration in the UC group. Western blotting and immunohistochemistry showed that the expression of PKC, TRPV1, CGRP, and SP in the colonic tissues of the UC group increased significantly compared with that of the control group. Compared with the UC group, the expression of PKC, TRPV1, CGRP, and SP in colonic tissues was significantly reduced in the High-WSP, Medium-WSP, and SASP groups. Immunofluorescence showed the colocalized expression of PKC/TRPV1, TRPV1/CGRP, and TRPV1/SP proteins in the colon tissue of the UC group was significantly reduced after WSP and SASP interventions compared with that of the control group. The results suggest that the mechanism of UC alleviation by propolis may inhibit the PKC-TRPV1-CGRP/SP signaling pathway and the release of inflammatory mediators, thus alleviating inflammation.

Ion channel regulation of gut immunity

Mounting evidence indicates that gastrointestinal (GI) homeostasis hinges on communications among many cellular networks including the intestinal epithelium, the immune system, and both intrinsic and extrinsic nerves innervating the gut. The GI tract, especially the colon, is the home base for gut microbiome which dynamically regulates immune function. The gut's immune system also provides an effective defense against harmful pathogens entering the GI tract while maintaining immune homeostasis to avoid exaggerated immune reaction to innocuous food and commensal antigens which are important causes of inflammatory disorders such as coeliac disease and inflammatory bowel diseases (IBD). Various ion channels have been detected in multiple cell types throughout the GI tract. By regulating membrane properties and intracellular biochemical signaling, ion channels play a critical role in synchronized signaling among diverse cellular components in the gut that orchestrates the GI immune response. This work focuses on the role of ion channels in immune cells, non-immune resident cells, and neuroimmune interactions in the gut at the steady state and pathological conditions. Understanding the cellular and molecular basis of ion channel signaling in these immune-related pathways and initial testing of pharmacological intervention will facilitate the development of ion channel-based therapeutic approaches for the treatment of intestinal inflammation.

A proteomic and RNA-seq transcriptomic dataset of capsaicin-aggravated mouse chronic colitis model

An inappropriate diet is a risk factor for inflammatory bowel disease (IBD). It is established that the consumption of spicy food containing capsaicin is strongly associated with the recurrence and worsening of IBD symptoms. Moreover, capsaicin can induce neutrophil accumulation in the lamina propria, contributing to disease deterioration. To uncover the potential signaling pathway involved in capsaicin-induced relapse and the effects of capsaicin on neutrophil activation, we performed proteomic analyses of intestinal tissues from chronic colitis mice following capsaicin administration and transcriptomic analyses of dHL-60 cells after capsaicin stimulation. Collectively, these multiomic analyses identified proteins and genes that may be involved in disease flares, thereby providing new insights for future research.

Lycium barbarum Polysaccharides and Capsaicin Inhibit Oxidative Stress, Inflammatory Responses, and Pain Signaling in Rats with Dextran Sulfate Sodium-Induced Colitis

Ulcerative colitis (UC) is an inflammatory disease with chronic relapsing symptoms. This study investigated the effects of Lycium barbarum polysaccharides (LBP) and capsaicin (CAP) in dextran sulfate sodium (DSS)-induced UC rats. Rats were divided into normal, DSS-induced UC, and UC treated with 100 mg LBP/kg bw, 12 mg CAP/kg bw, or 50 mg LBP/kg bw and 6 mg CAP/kg bw. Rats were fed LBP or CAP orally by gavage for 4 weeks, and UC model was established by feeding 5% DSS in drinking water for 6 days during week 3. Oral CAP and mixture significantly reduced disease activity index. Oral LBP significantly decreased serum malondialdehyde, interleukin (IL)-6, colonic tumor necrosis factor (TNF)-α levels, and protein expression of transient receptor potential cation channel V1 (TRPV1) and transient receptor potential ankyrin 1 (TRPA1), but increased serum catalase activity. Oral CAP significantly suppressed serum IL-6, colonic TRPV1 and TRPA1 protein expression, but elevated IL-10 levels, serum superoxide dismutase and catalase activities. The mixture of LBP and CAP significantly reduced serum IL-6, colonic TNF-α and TRPA1 protein. In conclusion, administration of LBP and/or CAP attenuate DSS-induced UC symptoms through inhibiting oxidative stress, proinflammatory cytokines, and protein expression of TRPV1 and TRPA1.

The gut, its microbiome, and the brain: connections and communications

Modern research on gastrointestinal behavior has revealed it to be a highly complex bidirectional process in which the gut sends signals to the brain, via spinal and vagal visceral afferent pathways, and receives sympathetic and parasympathetic inputs. Concomitantly, the enteric nervous system within the bowel, which contains intrinsic primary afferent neurons, interneurons, and motor neurons, also senses the enteric environment and controls the detailed patterns of intestinal motility and secretion. The vast microbiome that is resident within the enteric lumen is yet another contributor, not only to gut behavior, but to the bidirectional signaling process, so that the existence of a microbiota-gut-brain "connectome" has become apparent. The interaction between the microbiota, the bowel, and the brain now appears to be neither a top-down nor a bottom-up process. Instead, it is an ongoing, tripartite conversation, the outline of which is beginning to emerge and is the subject of this Review. We emphasize aspects of the exponentially increasing knowledge of the microbiota-gut-brain "connectome" and focus attention on the roles that serotonin, Toll-like receptors, and macrophages play in signaling as exemplars of potentially generalizable mechanisms.

5,6-dihydroxy-8Z,11Z,14Z,17Z-eicosatetraenoic acid accelerates the healing of colitis by inhibiting transient receptor potential vanilloid 4-mediated signaling

5,6-dihydroxy-8Z,11Z,14Z,17Z-eicosatetraenoic acid (5,6-DiHETE) is an eicosapentaenoic acid-derived lipid metabolite, which we previously detected in inflamed mouse colon. In this study, we investigated the pathophysiological roles of 5,6-DiHETE in murine colitis and its underlying mechanisms of action, focusing on the effects on transient receptor potential vanilloid (TRPV) channel activity. Oral administration of dextran sodium sulfate (DSS, 2%, for 4 days) caused colon inflammation, which peaked on day 7 and gradually declined by day 18. 5,6-DiHETE concentration in colon tissue was significantly increased during the healing phase of colitis (days 9 to 18). In vitro study showed that pretreatment with 5,6-DiHETE (0.1-1 μM, 30 minutes) significantly inhibited endothelial barrier disruption induced by a TRPV4 agonist (GSK1016790A, 50 nM). Intracellular Ca²⁺ imaging also showed that pretreatment with 5,6-DiHETE (1 μM, 10 minutes) reduced GSK1016790A-induced intracellular Ca²⁺ increase in HEK293T cells overexpressing TRPV4. In vivo, intraperitoneal administration of 5,6-DiHETE (50 µg kg^-1  day^-1 ) during the healing phase accelerated the recovery from DSS-induced colitis. Pathological studies showed that the administration of 5,6-DiHETE inhibited edema formation and leukocyte infiltration in inflamed colon tissue. In conclusion, we identified 5,6-DiHETE as a novel endogenous TRPV4 antagonist, and we also demonstrated that its administration promotes the healing of colitis by inhibiting inflammatory responses.

Gain of Function of Ion Channel TRPV1 Exacerbates Experimental Colitis by Promoting Dendritic Cell Activation

Dysregulated mucosal immunity plays an essential role in the pathophysiology of inflammatory bowel disease (IBD). Transient receptor potential vanilloid 1 (TRPV1) is a Ca²⁺-permeable ion channel that is implicated in modulating immune responses. However, its role in the pathogenesis of intestinal inflammation remains elusive. Here, we found that TRPV1 gain of function significantly increased the susceptibility of mice to experimental colitis, and that was associated with excessive recruitment of dendritic cells and enhanced Th17 immune responses in the lamina propria of colon. TRPV1 gain of function promoted dendritic cell activation and cytokine production upon inflammatory stimuli, and consequently enhanced dendritic cell-mediated Th17 cell differentiation. Further mechanistic studies showed that TRPV1 gain of function in dendritic cells enhanced activation of calcineurin/nuclear factor of activated T cells (NFATc2) signaling induced by inflammatory stimuli. Moreover, in patients with IBD, TRPV1 expression was increased in lamina propria cells of inflamed colon compared with healthy controls. Our findings identify an important role for TRPV1 in modulating dendritic cell activation and sustaining Th17 responses to inflammatory stimuli, which suggest that TRPV1 might be a potential therapeutic target in controlling mucosal immunity and IBD.

Neural control of gut homeostasis

The gut-brain axis is a coordinated communication system that not only maintains homeostasis, but significantly influences higher cognitive functions and emotions, as well as neurological and behavioral disorders. Among the large populations of sensory and motor neurons that innervate the gut, insights into the function of primary afferent nociceptors, whose cell bodies reside in the dorsal root ganglia and nodose ganglia, have revealed their multiple crosstalk with several cell types within the gut wall, including epithelial, vascular, and immune cells. These bidirectional communications have immunoregulatory functions, control host response to pathogens, and modulate sensations associated with gastrointestinal disorders, through activation of immune cells and glia in the peripheral and central nervous system, respectively. Here, we will review the cellular and neurochemical basis of these interactions at the periphery, in dorsal root ganglia, and in the spinal cord. We will discuss the research gaps that should be addressed to get a better understanding of the multifunctional role of sensory neurons in maintaining gut homeostasis and regulating visceral sensitivity.

Inhibition of transient receptor potential vanilloid 1 (TRPV1) channel regulates chikungunya virus infection in macrophages

Chikungunya virus (CHIKV), a virus that induces pathogenic inflammatory host immune responses, is re-emerging worldwide, and there are currently no established antiviral control measures. Transient receptor potential vanilloid 1 (TRPV1), a non-selective Ca²⁺-permeable ion channel, has been found to regulate various host inflammatory responses including several viral infections. Immune responses to CHIKV infection in host macrophages have been reported recently. However, the possible involvement of TRPV1 during CHIKV infection in host macrophages has not been studied. Here, we investigated the possible role of TRPV1 in CHIKV infection of the macrophage cell line RAW 264.7. It was found that CHIKV infection upregulates TRPV1 expression in macrophages. To confirm this observation, the TRPV1-specific modulators 5'-iodoresiniferatoxin (5'-IRTX, a TRPV1 antagonist) and resiniferatoxin (RTX, a TRPV1 agonist) were used. Our results indicated that TRPV1 inhibition leads to a reduction in CHIKV infection, whereas TRPV1 activation significantly enhances CHIKV infection. Using a plaque assay and a time-of-addition assay, it was observed that functional modulation of TRPV1 affects the early stages of the viral lifecycle in RAW 264.7 cells. Moreover, CHIKV infection was found to induce of pNF-κB (p65) expression and nuclear localization. However, both activation and inhibition of TRPV1 were found to enhance the expression and nuclear localization of pNF-κB (p65) and production of pro-inflammatory TNF and IL-6 during CHIKV infection. In addition, it was demonstrated by Ca²⁺ imaging that TRPV1 regulates Ca²⁺ influx during CHIKV infection. Hence, the current findings highlight a potentially important regulatory role of TRPV1 during CHIKV infection in macrophages. This study might also have broad implications in the context of other viral infections as well.

Transient Receptor Potential Channels and Inflammatory Bowel Disease

The transient receptor potential (TRP) cation channels are present in abundance across the gastrointestinal (GI) tract, serving as detectors for a variety of stimuli and secondary transducers for G-protein coupled receptors. The activation of TRP channels triggers neurogenic inflammation with related neuropeptides and initiates immune reactions by extra-neuronally regulating immune cells, contributing to the GI homeostasis. However, under pathological conditions, such as inflammatory bowel disease (IBD), TRP channels are involved in intestinal inflammation. An increasing number of human and animal studies have indicated that TRP channels are correlated to the visceral hypersensitivity (VHS) and immune pathogenesis in IBD, leading to an exacerbation or amelioration of the VHS or intestinal inflammation. Thus, TRP channels are a promising target for novel therapeutic methods for IBD. In this review, we comprehensively summarize the functions of TRP channels, especially their potential roles in immunity and IBD. Additionally, we discuss the contradictory findings of prior studies and offer new insights with regard to future research.

TRPV1 in experimental autoimmune prostatitis

BACKGROUND

Chronic prostatitis/chronic pelvic pain syndrome (CP/CPPS) is a disorder that is characterized by persistent pelvic pain in men of any age. Although several studies suggest that the transient receptor potential vanilloid 1 (TRPV1) channel is involved in various pathways of chronic pain, the TRPV1 channel has not been implicated in chronic pelvic pain associated with CP/CPPS.

METHODS

Male C57BL/6J (B6) and TRPV1 knockout (TRPV1 KO) mice (5-7 weeks old) were used to study the development of pelvic allodynia in a murine model of CP/CPPS called experimental autoimmune prostatitis (EAP). The prostate lobes, dorsal root ganglia (DRG), and spinal cord were excised at day 20. The prostate lobes were assessed for inflammation, TRPV1 expression, and mast cell activity. DRG and spinal cord, between the L6-S4 regions, were analyzed to determine the levels of phosphorylated ERK1/2 (p-ERK 1/2). To examine the therapeutic potential of TRPV1, B6 mice with EAP received intraurethral infusion of a TRPV1 antagonist at day 20 (repeated every 2 days) and pelvic pain was evaluated at days 20, 25, 30, and 35.

RESULTS

Our data showed that B6 mice with EAP developed pelvic tactile allodynia at days 7, 14, and 20. In contrast, TRPV1 KO mice with EAP do not develop pelvic tactile allodynia at any time point. Although we observed no change in the levels of TRPV1 protein expression in the prostate from B6 mice with EAP, there was evidence of significant inflammation and elevated mast cell activation. Interestingly, the prostate from TRPV1 KO mice with EAP showed a lack of mast cell activation despite evidence of prostate inflammation. Next, we observed a significant increase of p-ERK1/2 in the DRG and spinal cord from B6 mice with EAP; however, p-ERK1/2 expression was unaltered in TRPV1 KO mice with EAP. Finally, we confirmed that intraurethral administration of a TRPV1 antagonist peptide reduced pelvic tactile allodynia in B6 mice with EAP after day 20.

CONCLUSIONS

We demonstrated that in a murine model of CP/CPPS, the TRPV1 channel is key to persistent pelvic tactile allodynia and blocking TRPV1 in the prostate may be a promising strategy to quell chronic pelvic pain.

Eosinophils in Inflammatory Bowel Disease

Clinical investigations in inflammatory bowel disease (IBD) patients have provided increasing evidence that eosinophils contribute to chronic intestinal inflammation. Accumulation of eosinophils in the gastrointestinal tract correlates with the variations of eosinophil regulatory molecules; however, their role in gastrointestinal dysfunction in IBD has not been fully elucidated. This review will describe the development and characterization of gastrointestinal eosinophils, mechanisms of eosinophil recruitment to the gastrointestinal tract. Moreover, the eosinophil-induced changes to the enteric nervous system associated with disease severity and gastrointestinal dysfunction will be analyzed with suggestive molecular pathways for enteric neuronal injury. Current and potential therapeutic interventions targeting eosinophils will be discussed.

Role of TRPV1 and TRPA1 Ion Channels in Inflammatory Bowel Diseases: Potential Therapeutic Targets?

Inflammatory bowel diseases (IBD) have long been recognized to be accompanied by pain resulting in high morbidity. Transient receptor potential vanilloid 1 (TRPV1) and ankyrin 1 (TRPA1) ion channels located predominantly on the capsaicin-sensitive sensory neurons play a complex role in hyperalgesia and neurogenic inflammation. This review provides an overview of their expression and role in intestinal inflammation, in particular colitis, that appears to be virtually inconsistent based on the thorough investigations of the last twenty years. However, preclinical results with pharmacological interventions, as well as scarcely available human studies, more convincingly point out the potential therapeutic value of TRPV1 and TRPA1 antagonists in colitis and visceral hypersensitivity providing future therapeutical perspectives through a complex, unique mechanism of action for drug development in IBD.

TRPA1 and substance P mediate stress induced duodenal lesions in water immersion restraint stress rat model

BACKGROUND/AIMS

Transient receptor potential ankyrin 1 (TRPA1) and substance P (SP), both expression in sensory neurons, have important roles in stress-induced duodenal lesions. The possible contribution of TRPA1 and SP to stress-induced duodenal lesions was explored by using the water immersion restraint stress (WIRS) rat model.

MATERIALS AND METHODS

Western blotting, Real-time polymerase chain reaction (RT-PCR), and immunohistochemistry assay were used to evaluate the changes of TRPA1and SP expression in the dorsal root ganglia (DRG, T8-11), the corresponding segment of the spinal cord (T8-11), and the duodenum in a duodenal lesions rat model. The SP concentrations of duodenal mucosa were investigated using an enzyme-linked immunosorbent assay (ELISA). Duodenal lesions were assessed according to histopathological changes. TRPA1 specific antagonist HC-030031 was intrathecally or intraperitoneally performed to suppress the expression of both TRPA1 and SP for evaluating the roles of TRPA1 and SP in duodenal lesions.

RESULTS

In contrast to the control group, TRPA1 and substance P in the DRG (T8-11) and duodenum were up-regulated, and concentrations of SP in the duodenal mucosa were increased after WIRS (p<0.05), which are closely associated with duodenal lesions. SP concentrations in the duodenal mucosa were decreased and duodenal lesions were alleviated by pretreatment with TRPA1 antagonist HC-030031. We identified a protective role for HC-030031 in WIRS-induced duodenal lesions. Furthermore, we demonstrated that WIRS increased the concentrations of SP in the duodenal mucosa in a TRPA1-dependent manner. However, WIRS caused no significant changes of TRPA1 and SP in the spinal cord (T8-11) compared with the control group (p>0.05).

CONCLUSION

Our study indicates that TRPA1 antagonist HC-030031 alleviates duodenal lesions. TRPA1 is activated and sensitized, therefore concomitant neuropeptide SP is released, which exerts a critical role in inducing and maintaining duodenal lesions following WIRS in rats. This provides evidence that neuroimmune interactions may control duodenal injury. TRPA1 may be a potential drug target to inhibit the development of duodenal lesions by stress-induced in patients.

The Transient Receptor Potential Vanilloid 1 Is Associated with Active Inflammation in Ulcerative Colitis

The transient receptor potential vanilloid 1 (TRPV1) may play a role in the pathogenesis of ulcerative colitis (UC). The aim of the study was to determine the gene and protein expression of TRPV1 in UC patients and noninflamed controls. Gene expression was performed by RT-PCR, and protein expression was performed by immunohistochemistry. The gene expression of TRPV1 was significantly increased in the remission UC group compared to active UC patients (P = 0.002), and an upregulation of the TRPV1 gene was associated with clinical outcomes such as age at diagnosis (<40 years) (P = 0.02) and clinical disease course characterized by relapsing and continuous activity (P = 0.07). TRPV1 immunoreactive cells were conspicuously higher in all intestinal layers from active UC patients compared with noninflamed control tissue. These findings suggest that TRPV1 might be involved in UC pathogenesis.

Oral administration of curcumin attenuates visceral hyperalgesia through inhibiting phosphorylation of TRPV1 in rat model of ulcerative colitis

Background Curcumin has been reported to have anti-inflammatory and anti-nociceptive effects. The present study was designed to explore the potential therapeutic effects of curcumin on visceral hyperalgesia and inflammation in a rat model of ulcerative colitis. We observed the effects of orally administered curcumin on the disease activity index, histological change in colon, colorectal distension-induced abdominal withdrawal reflex, the expression of transient receptor potential vanilloid 1 (TRPV1) and phosphorylated TRPV1 in dextran sulfate sodium-induced colitis rats. In addition, a HEK293 cell line stably expressing human TRPV1 (hTRPV1) was used to examine the effects of curcumin on the change in membrane expression of TRPV1 induced by phorbol myristate acetate (a protein kinase C activator). Results Repeated oral administration of curcumin inhibited the increase in abdominal withdrawal reflex score induced by dextran sulfate sodium without affecting dextran sulfate sodium-induced histological change of colon and the disease activity index. A significant increase in colonic expression of TRPV1 and pTRPV1 was observed in dextran sulfate sodium-treated rats and this was reversed by oral administration of curcumin. TRPV1 expression in L6-S1 dorsal root ganglion was increased in the small- to medium-sized isolectin B4-positive non-peptidergic and calcitonin gene-related peptide-positive peptidergic neurons in dextran sulfate sodium-treated rats and oral administration of curcumin mitigated such changes. In the HEK293 cell line stably expressing hTRPV1, curcumin (1, 3 µm) inhibited phorbol myristate acetate-induced upregulation of membrane TRPV1. Conclusion Oral administration of curcumin alleviates visceral hyperalgesia in dextran sulfate sodium-induced colitis rats. The anti-hyperalgesic effect is partially through downregulating the colonic expression and phosphorylation of TRPV1 on the afferent fibers projected from peptidergic and non-peptidergic nociceptive neurons of dorsal root ganglion.

Sensory neuron regulation of gastrointestinal inflammation and bacterial host defence

Sensory neurons in the gastrointestinal tract have multifaceted roles in maintaining homeostasis, detecting danger and initiating protective responses. The gastrointestinal tract is innervated by three types of sensory neurons: dorsal root ganglia, nodose/jugular ganglia and intrinsic primary afferent neurons. Here, we examine how these distinct sensory neurons and their signal transducers participate in regulating gastrointestinal inflammation and host defence. Sensory neurons are equipped with molecular sensors that enable neuronal detection of diverse environmental signals including thermal and mechanical stimuli, inflammatory mediators and tissue damage. Emerging evidence shows that sensory neurons participate in host-microbe interactions. Sensory neurons are able to detect pathogenic and commensal bacteria through specific metabolites, cell-wall components, and toxins. Here, we review recent work on the mechanisms of bacterial detection by distinct subtypes of gut-innervating sensory neurons. Upon activation, sensory neurons communicate to the immune system to modulate tissue inflammation through antidromic signalling and efferent neural circuits. We discuss how this neuro-immune regulation is orchestrated through transient receptor potential ion channels and sensory neuropeptides including substance P, calcitonin gene-related peptide, vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide. Recent studies also highlight a role for sensory neurons in regulating host defence against enteric bacterial pathogens including Salmonella typhimurium, Citrobacter rodentium and enterotoxigenic Escherichia coli. Understanding how sensory neurons respond to gastrointestinal flora and communicate with immune cells to regulate host defence enhances our knowledge of host physiology and may form the basis for new approaches to treat gastrointestinal diseases.

Systemic desensitization through TRPA1 channels by capsazepine and mustard oil - a novel strategy against inflammation and pain

We demonstrate a novel dual strategy against inflammation and pain through body-wide desensitization of nociceptors via TRPA1. Attenuation of experimental colitis by capsazepine (CPZ) has long been attributed to its antagonistic action on TRPV1 and associated inhibition of neurogenic inflammation. In contrast, we found that CPZ exerts its anti-inflammatory effects via profound desensitization of TRPA1. Micromolar CPZ induced calcium influx in isolated dorsal root ganglion (DRG) neurons from wild-type (WT) but not TRPA1-deficient mice. CPZ-induced calcium transients in human TRPA1-expressing HEK293t cells were blocked by the selective TRPA1 antagonists HC 030031 and A967079 and involved three cysteine residues in the N-terminal domain. Intriguingly, both colonic enemas and drinking water with CPZ led to profound systemic hypoalgesia in WT and TRPV1^−/− but not TRPA1^−/− mice. These findings may guide the development of a novel class of disease-modifying drugs with anti-inflammatory and anti-nociceptive effects.

The Neuromodulation of the Intestinal Immune System and Its Relevance in Inflammatory Bowel Disease

One of the main tasks of the immune system is to discriminate and appropriately react to “danger” or “non-danger” signals. This is crucial in the gastrointestinal tract, where the immune system is confronted with a myriad of food antigens and symbiotic microflora that are in constant contact with the mucosa, in addition to any potential pathogens. This large number of antigens and commensal microflora, which are essential for providing vital nutrients, must be tolerated by the intestinal immune system to prevent aberrant inflammation. Hence, the balance between immune activation versus tolerance should be tightly regulated to maintain intestinal homeostasis and to prevent immune activation indiscriminately against all luminal antigens. Loss of this delicate equilibrium can lead to chronic activation of the intestinal immune response resulting in intestinal disorders, such as inflammatory bowel diseases (IBD). In order to maintain homeostasis, the immune system has evolved diverse regulatory strategies including additional non-immunological actors able to control the immune response. Accumulating evidence strongly indicates a bidirectional link between the two systems in which the brain modulates the immune response via the detection of circulating cytokines and via direct afferent input from sensory fibers and from enteric neurons. In the current review, we will highlight the most recent findings regarding the cross-talk between the nervous system and the mucosal immune system and will discuss the potential use of these neuronal circuits and neuromediators as novel therapeutic tools to reestablish immune tolerance and treat intestinal chronic inflammation.

TRP channel functions in the gastrointestinal tract

Transient receptor potential (TRP) channels are predominantly distributed in both somatic and visceral sensory nervous systems and play a crucial role in sensory transduction. As the largest visceral organ system, the gastrointestinal (GI) tract frequently accommodates external inputs, which stimulate sensory nerves to initiate and coordinate sensory and motor functions in order to digest and absorb nutrients. Meanwhile, the sensory nerves in the GI tract are also able to detect potential tissue damage by responding to noxious irritants. This nocifensive function is mediated through specific ion channels and receptors expressed in a subpopulation of spinal and vagal afferent nerve called nociceptor. In the last 18 years, our understanding of TRP channel expression and function in GI sensory nervous system has been continuously improved. In this review, we focus on the expressions and functions of TRPV1, TRPA1, and TRPM8 in primary extrinsic afferent nerves innervated in the esophagus, stomach, intestine, and colon and briefly discuss their potential roles in relevant GI disorders.

TRPV1 sensitization mediates postinflammatory visceral pain following acute colitis

Quiescent phases of inflammatory bowel disease (IBD) are often accompanied by chronic abdominal pain. Although the transient receptor potential vanilloid 1 (TRPV1) ion channel has been postulated as an important mediator of visceral hypersensitivity, its functional role in postinflammatory pain remains elusive. This study aimed at establishing the role of TRPV1 in the peripheral sensitization underlying chronic visceral pain in the context of colitis. Wild-type and TRPV1-deficient mice were separated into three groups (control, acute colitis, and recovery), and experimental colitis was induced by oral administration of dextran sulfate sodium (DSS). Recovery mice showed increased chemically and mechanically evoked visceral hypersensitivity 5 wk post-DSS discontinuation, at which point inflammation had completely resolved. Significant changes in nonevoked pain-related behaviors could also be observed in these animals, indicative of persistent discomfort. These behavioral changes correlated with elevated colonic levels of substance P (SP) and TRPV1 in recovery mice, thus leading to the hypothesis that SP could sensitize TRPV1 function. In vitro experiments revealed that prolonged exposure to SP could indeed sensitize capsaicin-evoked currents in both cultured neurons and TRPV1-transfected human embryonic kidney (HEK) cells, a mechanism that involved TRPV1 ubiquitination and subsequent accumulation at the plasma membrane. Importantly, although TRPV1-deficient animals experienced similar disease severity and pain as wild-type mice in the acute phase of colitis, TRPV1 deletion prevented the development of postinflammatory visceral hypersensitivity and pain-associated behaviors. Collectively, our results suggest that chronic exposure of colon-innervating primary afferents to SP could sensitize TRPV1 and thus participate in the establishment of persistent abdominal pain following acute inflammation.

TRPM8 on mucosal sensory nerves regulates colitogenic responses by innate immune cells via CGRP

TRPM8 is the molecular sensor for cold; however, the physiological role of TRPM8+ neurons at mucosal surfaces is unclear. Here we evaluated the distribution and peptidergic properties of TRPM8+ fibers in naive and inflamed colons, as well as their role in mucosal inflammation. We found that Trpm8(-/-) mice were hypersusceptible to dextran sodium sulfate (DSS)-induced colitis, and that Trpm8(-/-) CD11c+ DCs (dendritic cells) showed hyperinflammatory responses to toll-like receptor (TLR) stimulation. This was phenocopied in calcitonin gene-related peptide (CGRP) receptor-deficient mice, but not in substance P receptor-deficient mice, suggesting a functional link between TRPM8 and CGRP. The DSS phenotype of CGRP receptor-deficient mice could be adoptively transferred to wild-type (WT) mice, suggesting that CGRP suppresses the colitogenic activity of bone marrow-derived cells. TRPM8+ mucosal fibers expressed CGRP in human and mouse colon. Furthermore, neuronal CGRP contents were increased in colons from naive and DSS-treated Trpm8(-/-) mice, suggesting deficient CGRP release in the absence of TRPM8 triggering. Finally, treatment of Trpm8(-/-) mice with CGRP reversed their hyperinflammatory phenotype. These results suggest that TRPM8 signaling in mucosal sensory neurons is indispensable for the regulation of innate inflammatory responses via the neuropeptide CGRP.

Role of transient receptor potential channels in intestinal inflammation and visceral pain: novel targets in inflammatory bowel diseases

Transient receptor potential (TRP) channels are a large group of ion channels that are prevalent in mammalian tissues. They are widely distributed in the central and peripheral nervous systems, and in nonneuronal cells, where they are implicated in sensing temperature, noxious substances, and pain. TRPs play an important role in immune response and nociception and, therefore, may be involved in the pathogenesis of inflammatory bowel diseases, whose major symptoms include chronic inflammatory state and abdominal pain. In this review, we summarize what is known on TRP channels in inflammatory bowel disease and visceral pain; we focus in particular on TRPV1, TRPV4, TRPA1, and TRPM. We also analyze scientific reports that evidence potential use of TRP regulators in future inflammatory bowel disease treatment.

Thermal spring water drinking attenuates dextran-sulfate-sodium-induced colitis in mice

INTRODUCTION

The present study investigates the effect of oral consumption of hydrogen sulfide-containing Harkány thermal spring water, as well as sodium hydrogen sulfide (NaHS) solution on experimental colitis.

METHODS

Colitis was induced by 2% dextran sulfate sodium (DSS) in the drinking water of C57BL/6 mice for 7 days. Some animal groups drank Harkány thermal spring water or water supplemented with 21.68 mg/L NaHS. General signs of colitis, myeloperoxidase (MPO) enzyme activity of colon samples, histological features of colitis and function of the enteric nervous system were assessed.

RESULTS

Oral administration of Harkány thermal spring water significantly attenuated general signs of colitis, MPO enzyme activity of colon samples and detrimental effect of colitis on the function of the enteric nervous system, but not histological signs of colitis. These findings could be reproduced using NaHS solution with additional significantly diminished histological damage.

CONCLUSIONS

We conclude that oral treatment with Harkány thermal spring water relieves various aspects of DSS-evoked colitis in mice. This effect is most likely to be mediated by hydrogen sulfide content of the Harkány water. Our data might promote complementary utilization of sulfurous thermal spring water in the therapy of inflammatory bowel disease.

N-Acylethanolamine-hydrolyzing acid amidase inhibition increases colon N-palmitoylethanolamine levels and counteracts murine colitis

N-Palmitoylethanolamine or palmitoylethanolamide (PEA) is an anti-inflammatory compound that was recently shown to exert peroxisome proliferator-activated receptor-α-dependent beneficial effects on colon inflammation. The actions of PEA are terminated following hydrolysis by 2 enzymes: fatty acid amide hydrolase (FAAH), and the less-studied N-acylethanolamine-hydrolyzing acid amidase (NAAA). This study aims to investigate the effects of inhibiting the enzymes responsible for PEA hydrolysis in colon inflammation in order to propose a potential therapeutic target for inflammatory bowel diseases (IBDs). Two murine models of IBD were used to assess the effects of NAAA inhibition, FAAH inhibition, and PEA on macroscopic signs of colon inflammation, macrophage/neutrophil infiltration, and the expression of proinflammatory mediators in the colon, as well as on the colitis-related systemic inflammation. NAAA inhibition increases PEA levels in the colon and reduces colon inflammation and systemic inflammation, similarly to PEA. FAAH inhibition, however, does not increase PEA levels in the colon and does not affect the macroscopic signs of colon inflammation or immune cell infiltration. This is the first report of an anti-inflammatory effect of a systemically administered NAAA inhibitor. Because NAAA is the enzyme responsible for the control of PEA levels in the colon, we put forth this enzyme as a potential therapeutic target in chronic inflammation in general and IBD in particular.

Cross-talk between neural and immune receptors provides a potential mechanism of homeostatic regulation in the gut mucosa

The relationship between elements of the immune system and the nervous system in the presence of bacteria has been addressed recently. In particular, the sensory vanilloid receptor 1 (transient receptor potential cation channel subfamily V member 1 (TRPV1)) and the neuropeptide calcitonin gene-related peptide (CGRP) have been found to modulate cytokine response to lipopolysaccharide (LPS) independently of adaptive immunity. In this review we discuss mucosal homeostasis in the gastrointestinal tract where bacterial concentration is high. We propose that the Gram-negative bacterial receptor Toll-like receptor 4 (TLR4) can activate TRPV1 via intracellular signaling, and thereby induce the subsequent release of anti-inflammatory CGRP to maintain mucosal homeostasis.

Upregulation of the transient receptor potential ankyrin 1 ion channel in the inflamed human and mouse colon and its protective roles

Transient Receptor Potential Ankyrin 1 (TRPA1) channels are localized on sensory nerves and several non-neural cells, but data on their functional significance are contradictory. We analysed the presence and alterations of TRPA1 in comparison with TRP Vanilloid 1 (TRPV1) at mRNA and protein levels in human and mouse intact and inflamed colons. The role of TRPA1 in a colitis model was investigated using gene-deficient mice. TRPA1 and TRPV1 expressions were investigated in human colon biopsies of healthy subjects and patients with inflammatory bowel diseases (IBD: ulcerative colitis, Crohn's disease) with quantitative PCR and immunohistochemistry. Mouse colitis was induced by oral 2% dextran-sulphate (DSS) for 10 days. For investigating the functions of TRPA1, Disease Activity Index (weight loss, stool consistency, blood content) was determined in C57BL/6-based Trpa1-deficient (knockout: KO) and wildtype (WT) mice. Sensory neuropeptides, their receptors, and inflammatory cytokines/chemokines were determined with qPCR or Luminex. In human and mouse colons TRPA1 and TRPV1 are located on epithelial cells, macrophages, enteric ganglia. Significant upregulation of TRPA1 mRNA was detected in inflamed samples. In Trpa1 KO mice, Disease Activity Index was significantly higher compared to WTs. It could be explained by the greater levels of substance P, neurokinins A and B, neurokinin 1 receptor, pituitary adenylate-cyclase activating polypeptide, vasoactive intestinal polypeptide, and also interleukin-1beta, macrophage chemoattractant protein-1, monokine induced by gamma interferon-1, tumor necrosis factor-alpha and B-lymphocyte chemoattractant in the distal colon. TRPA1 is upregulated in colitis and its activation exerts protective roles by decreasing the expressions of several proinflammatory neuropeptides, cytokines and chemokines.

TRPV1 Antagonism: From Research to Clinic

The capsaicin receptor, TRPV1, has been one of the most extensively studied molecules in sensory research. Its contribution to the sensation of pain in numerous pre-clinical inflammatory and neuropathic paradigms has been well-established and expression analysis suggests a potential role clinically in pain and bladder conditions. The field has now reached an exciting point in time with the development of a number of high quality TRPV1 antagonist drug candidates and the release of clinical data. What has become apparent from this work is that inhibition of TRPV1 function brings with it the potential liabilities of increased body temperature and altered thermal perception. However, there is cause for optimism because it appears that not all antagonists have the same properties and compounds can be identified that lack significant on-target side-effects whilst retaining efficacy, at least pre-clinically. What is perhaps now more critical to address is the question of how effective the analgesia provided by a TRPV1 antagonist will be. Although tantalizing clinical data showing effects on experimentally-induced pain or pain following molar extraction have been reported, no clear efficacy in a chronic pain condition has yet been demonstrated making it difficult to perform an accurate risk-benefit analysis for TRPV1 antagonists. Here we provide an overview of some of the most advanced clinical candidates and discuss the approaches being taken to avoid the now well established on-target effects of TRPV1 antagonists.

5-Aminosalicylic Acid Inhibits Acute Clostridium difficile Toxin A-Induced Colitis in Rats

We tested the hypothesis that 5-aminosalicylic acid (5-ASA) inhibits toxin A-induced generation of colonic leukotriene B₄ (LTB₄) and toxin A colitis in rats. Isolated colonic segments in anesthetized rats were treated intraluminally with toxin A for 3 hours with or without 30 minutes of pretreatment with either 5-ASA or sulfapyridine and then colonic tissue levels of LTB₄ were measured and inflammation was assessed. Separately, sulfasalazine was administered to rats in their drinking water for 5 days, isolated colonic segments were then prepared, toxin A was administered, and inflammation was assessed as before. Pretreatment with 5-ASA inhibited toxin A-induced increased tissue LTB₄ concentration in the colon. Sulfasalazine and 5-ASA but not sulfapyridine significantly inhibited toxin A colitis. However, pretreatment with 5-ASA did not protect against direct TRPV1-mediated colitis caused by capsaicin. Toxin A stimulated the release of substance P (SP), and this effect was also inhibited by sulfasalazine and 5-ASA but not by sulfapyridine. Thus, toxin A stimulates colonic LTB₄ resulting in activation of TRPV1, release of SP, and colitis. Inhibition of 5-LO by 5-ASA disrupts this pathway and supports the concept that LTB₄ activation of TRPV1 plays a role in toxin A colitis.

The neurochemical changes in the innervation of human colonic mesenteric and submucosal blood vessels in ulcerative colitis and Crohn's disease

BACKGROUND

Neurogenic inflammation involves vasodilation, oedema and sensory nerve hypersensitivity. Extrinsic sensory nerves to the intestinal wall mediate these effects and functional subsets of these extrinsic nerves can be characterized by immunohistochemical profiles. In this study such profiles were examined in samples from patients with inflammatory bowel disease (IBD), in particular ulcerative colitis (UC) and Crohn's disease (CD).

METHODS

Healthy margins from cancer patients were compared to specimens from IBD patients. All nerve fibres were labelled by PGP 9.5. Double and triple labelling with TH, NPY, SP, SOM, NOS, VIP, VAChT, CGRP, TRPv1 were performed. Perivascular nerve fibres in the mesentery, and submucosa, were examined. The percentage of all labelled nerve fibres was calculated with a transect method.

KEY RESULTS

Total number of varicosities on mesenteric vessels increased in IBD but decreased around submucosal vessels. The percentage of nerve fibres around submucosal arteries labelled by SP increased from 11% in controls to 20% (UC) and 24% (CD) and mesenteric artery nerve fibres were unchanged. Nerve fibres labelled by SOM were markedly reduced surrounding submucosal arteries, from 22% to 1% (UC) and 2% (CD), but not perivascular mesenteric nerve fibres. 87 to 93% of SP immunoreactive nerve fibres were also reactive for TRvP1. TRPv1 labelling without SP was 12%in controls and increased to 40% in CD submucosal specimens.

CONCLUSIONS & INFERENCES

There is an increase in SP and TRPv1, and a reduction in SOM immunoreactive nerve fibres in IBD. Changes in the perivascular functional nerve subclasses may underlie the hyperaemia, and ulceration, characteristic of IBD. Furthermore, pain may relate to underlying neural changes.

Role of neurokinin 1 receptors in dextran sulfate-induced colitis: studies with gene-deleted mice and the selective receptor antagonist netupitant

OBJECTIVE AND DESIGN

The function of the neurokinin 1 (NK1) receptor was investigated in the DSS-induced mouse colitis model using NK1 receptor-deficient mice and the selective antagonist netupitant.

SUBJECTS

Colitis was induced by oral administration of 20 mg/ml DSS solution for 7 days in C57BL/6 and Tacr1 KO animals (n = 5-7).

TREATMENT

During the induction, one-half of the C57BL/6 and Tacr1 KO group received one daily dose of 6 mg/kg netupitant, administered intraperitoneally, the other half of the group received saline, respectively.

METHODS

Disease activity index (DAI), on the basis of stool consistency, blood and weight loss, was determined over 7 days. Histological evaluation, myeloperoxidase (MPO) measurement, cytokine concentrations and receptor expression analysis were performed on the colon samples.

RESULTS

NK1 receptors are up-regulated in the colon in response to DSS treatment. DSS increased DAI, histopathological scores, BLC, sICAM-1, IFN-γ, IL-16 and JE in wildtype mice, which were significantly reduced in NK1 receptor-deficient ones. NK1 receptor antagonism with netupitant significantly diminished DAI, inflammatory histopathological alterations, BLC, IFN-γ, IL-13 and IL-16 in wildtype mice, but not in the NK1-deficient ones. MPO was similarly elevated and netupitant significantly decreased its activity in both groups.

CONCLUSIONS

NK1 receptor antagonism could be beneficial for colitis via inhibiting different inflammatory mechanisms.

Lack of transient receptor potential vanilloid 1 channel modulates the development of neurogenic bladder dysfunction induced by cross-sensitization in afferent pathways

Background

Bladder pain of unknown etiology has been associated with co-morbid conditions and functional abnormalities in neighboring pelvic organs. Mechanisms underlying pain co-morbidities include cross-sensitization, which occurs predominantly via convergent neural pathways connecting distinct pelvic organs. Our previous results showed that colonic inflammation caused detrusor instability via activation of transient receptor potential vanilloid 1 (TRPV1) signaling pathways, therefore, we aimed to determine whether neurogenic bladder dysfunction can develop in the absence of TRPV1 receptors.

Methods

Adult male C57BL/6 wild-type (WT) and TRPV1^−/− (knockout) mice were used in this study. Colonic inflammation was induced by intracolonic trinitrobenzene sulfonic acid (TNBS). The effects of transient colitis on abdominal sensitivity and function of the urinary bladder were evaluated by cystometry, contractility and relaxation of detrusor smooth muscle (DSM) in vitro to various stimuli, gene and protein expression of voltage-gated sodium channels in bladder sensory neurons, and pelvic responses to mechanical stimulation.

Results

Knockout of TRPV1 gene did not eliminate the development of cross-sensitization between the colon and urinary bladder. However, TRPV1^−/− mice had prolonged intermicturition interval and increased number of non-voiding contractions at baseline followed by reduced urodynamic responses during active colitis. Contractility of DSM was up-regulated in response to KCl in TRPV1^−/− mice with inflamed colon. Application of Rho-kinase inhibitor caused relaxation of DSM in WT but not in TRPV1^−/− mice during colonic inflammation. TRPV1^−/− mice demonstrated blunted effects of TNBS-induced colitis on expression and function of voltage-gated sodium channels in bladder sensory neurons, and delayed development of abdominal hypersensitivity upon colon-bladder cross-talk in genetically modified animals.

Conclusions

The lack of TRPV1 receptors does not eliminate the development of cross-sensitization in the pelvis. However, the function of the urinary bladder significantly differs between WT and TRPV^−/− mice especially upon development of colon-bladder cross-sensitization induced by transient colitis. Our results suggest that TRPV1 pathways may participate in the development of chronic pelvic pain co-morbidities in humans.

Vanilloid receptor-1 regulates neurogenic inflammation in colon and protects mice from colon cancer

Neuroinflammation driven by the vanilloid-type ion channel receptor transient receptor potential vanilloid type 1 (TRPV-1) is suspected to play a role in the pathophysiology of inflammatory bowel disease. Because inflammatory bowel disease is known to elevate the risk of colon cancer, we examined postulated roles for TRPV-1-driven neuroinflammation in promoting colitis-associated and spontaneous colon cancer development. Using a well-established model of colitis-associated cancer (CAC), we found that mice genetically deficient in TRPV-1 showed a higher incidence and number of tumors in the distal colon. In like manner, genetic deficiency of TRPV-1 in the APC(Min/+) model of spontaneous colon cancer accentuated the number of colonic adenomas formed. Mechanistic analyses in the CAC model revealed an increased infiltration of inflammatory cells into the tumors along with elevated expression of interleukin (IL)-6 and IL-11 and activation of the STAT3 and NF-κB signaling pathways. Notably, TPRV-1-deficient mice exhibited a defect in expression of the anti-inflammatory neuropeptides, vasoactive intestinal peptide (VIP), and pituitary adenylate cyclase-activating peptide (PACAP) which contributed to the generation of a local proinflammatory environment. Together, our findings argue that by limiting neuroinflammatory processes, TRPV-1 exerts a protective role that restricts the initiation and progression of colon cancer.

The proximodistal aggravation of colitis depends on substance P released from TRPV1-expressing sensory neurons

BACKGROUND

Transient receptor potential vanilloid type-1 (TRPV1)-expressing sensory neurons release neuropeptides such as substance P (SP) and calcitonin gene-related peptide (CGRP), which play a crucial role in the pathomechanism of experimental colitis. We investigated whether innervation density and neuropeptide release were responsible for the proximodistal aggravation of murine dextran-sulfate-sodium-salt (DSS) colitis.

METHODS

Whole mount TRPV1/CGRP immunostained mouse colon preparations were semiquantitatively analyzed. TRPV1 activation by capsaicin and acidic solution (pH 5.1) induced colonic CGRP/SP release, measured by EIA. Single cell quantitative PCR was employed to measure TRPV1 expression levels in DiI-labeled colonic dorsal root ganglion (DRG) neurons. The proximodistal gradient of DSS colitis severity was investigated in WT, CGRP(-/-), SP(-/-), and resiniferatoxin (RTX)-desensitized mice, employing mouse endoscopy, histology, and body weight measurement.

RESULTS

TRPV1/CGRP-positive nerve fiber density was increased in the distal colon wall. CGRP/SP release induced by TRPV1 activation from the distal colon was greater than that from the proximal colon. This gradient further increased in colitis. TRPV1 gene expression increased in colonic DRGs projecting to the distal, compared to that in colonic DRGs projecting to the proximal colon, and was further enhanced during colitis. In contrast to WT and CGRP(-/-) mice, SP(-/-) and RTX-desensitized mice showed amelioration of DSS colitis accompanied by a loss of the proximodistal gradient of inflammation.

CONCLUSIONS

The spatial correlation among increased colonic innervation density, TRPV1 receptor expression, stimulated SP release, and colitis severity suggested that TRPV1/SP-expressing sensory neurons should be considered as a therapeutic target in human ulcerative colitis.

Opposite effects of substance P and calcitonin gene-related peptide in oxazolone colitis

BACKGROUND

Extrinsic sensory neurons play a crucial role in aberrant immune responses in colitis. The activation of peptidergic sensory nerve fibres is accompanied by a release of the neuropeptides calcitonin gene-related peptide (CGRP) and substance P (SP). SP levels increase whilst CGRP levels decrease in colon specimens from patients with inflammatory bowel disease; thus suggesting the pro- and anti-inflammatory roles, respectively, of these neuropeptides.

METHODS

Oxazolone (4-ethoxymethylene-2-phenyl-2-oxazolin-5-one) colitis was induced in wild-type (WT), SP and CGRP knockout ((-/-)) mice. CGRP(-/-) mice were treated with the neurokinin 1-receptor antagonist CP-96345 (CP). The permeability of the mouse colon was evaluated by Evans Blue uptake. Cytokines produced by colonic lamina propria mononuclear cells were measured by ELISA.

RESULTS

Colons of WT, CGRP(-/-) and SP(-/-) mice showed similar tissue architecture and permeability. SP(-/-) mice were protected against oxazolone colitis, whereas CGRP(-/-) showed increased susceptibility to colitis compared to WT mice. SP(-/-) and CP-treated CGRP(-/-) mice showed no significant body weight loss during the period of sickness in contrast to untreated CGRP(-/-) and WT mice. Decreased production of IL-4, IL-5, and IL-13 by colonic lamina propria mononuclear cells of the protected SP(-/-) mice confirms the crucial role of these cytokines in oxazolone colitis.

CONCLUSION

We demonstrate that the neuropeptides CGRP and SP exert opposing effects in oxazolone colitis and provide further evidence for a prominent neuroimmune association in the gut.

Factors influencing the development of animal models of dextran sulphate sodium-induced colitis

The animal models of dextran sulphate sodium (DSS)-induced colitis have demonstrated several correlations with human ulcerative colitis (UC) since the first report of DSS-induced colitis in hamsters in 1985. These animal models have similarities to human UC in etiology, pathology, pathogenesis and therapeutic response, and are deemed suitable for investigating the pathogenesis and therapeutic options of UC and UC-related dysplasia-adenocarcinoma sequence. Although induction of colitis with DSS is relatively cheap and simple, the development of this model is influenced by many factors, such as DSS concentration, administration duration, DSS molecular weight and animal species. These factors are important for successful development of DSS-induced colitis. In this paper we summarize factors influencing the development of animal models of DSS-induced colitis.

TRPV1 involvement in inflammatory tissue fibrosis in mice

We examined whether absence or blocking of transient receptor potential vanilloid subtype 1 (TRPV1) affects the level of inflammation and fibrosis/scarring during healing of injured tissue using an alkali burn model of cornea in mice. A cornea burn was produced with 1 N NaOH instilled into one eye of TRPV1-/- (KO) (n = 88) or TRPV1+/+ (n = 94) mice. Examinations of the corneal surface and eye globe size suggested that the loss of TRPV1 suppressed inflammation and fibrosis/scarring after alkali burn, and this was confirmed by histology, IHC, and gene expression analysis. The loss of TRPV1 inhibited inflammatory cell invasion and myofibroblast generation in association with reduction of expression of proinflammatory and profibrogenic components. Experiments of bone marrow transplantation between either genotype of mice showed that KO corneal tissue resident cells, but not KO bone marrow-derived cells, are responsible for KO-type wound healing with reduced inflammation and fibrosis. The absence of TRPV1 attenuated expression of transforming growth factor β 1 (TGFβ1) and other proinflammatory gene expression in cultured ocular fibroblasts, but did not affect TGFβ1 expression in macrophages. Loss of TRPV1 inhibited myofibroblast transdifferentiation in cultured fibroblasts. Systemic TRPV1 antagonists reproduced the KO type of healing. In conclusion, absence or blocking of TRPV1 suppressed inflammation and fibrosis/scarring during healing of alkali-burned mouse cornea. TRPV1 is a potential drug target for improving the outcome of inflammatory/fibrogenic wound healing.

TRPA1 and substance P mediate colitis in mice

BACKGROUND & AIMS

The neuropeptides calcitonin gene-related peptide (CGRP) and substance P, and calcium channels, which control their release from extrinsic sensory neurons, have important roles in experimental colitis. We investigated the mechanisms of colitis in 2 different models, the involvement of the irritant receptor transient receptor potential of the ankyrin type-1 (TRPA1), and the effects of CGRP and substance P.

METHODS

We used calcium-imaging, patch-clamp, and neuropeptide-release assays to evaluate the effects of 2,4,6-trinitrobenzene-sulfonic-acid (TNBS) and dextran-sulfate-sodium-salt on neurons. Colitis was induced in wild-type, knockout, and desensitized mice.

RESULTS

TNBS induced TRPA1-dependent release of colonic substance P and CGRP, influx of Ca2+, and sustained ionic inward currents in colonic sensory neurons and transfected HEK293t cells. Analysis of mutant forms of TRPA1 revealed that TNBS bound covalently to cysteine (and lysine) residues in the cytoplasmic N-terminus. A stable sulfinic acid transformation of the cysteine-SH group, shown by mass spectrometry, might contribute to sustained sensitization of TRPA1. Mice with colitis had increased colonic neuropeptide release, mediated by TRPA1. Endogenous products of inflammatory lipid peroxidation also induced TRPA1-dependent release of colonic neuropeptides; levels of 4-hydroxy-trans-2-nonenal increased in each model of colitis. Colitis induction by TNBS or dextran-sulfate-sodium-salt was inhibited or reduced in TRPA1-/- mice and by 2-(1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydro-7H-purin-7-yl)-N-(4-isopro-pylphenyl)-acetamide, a pharmacologic inhibitor of TRPA1. Substance P had a proinflammatory effect that was dominant over CGRP, based on studies of knockout mice. Ablation of extrinsic sensory neurons prevented or attenuated TNBS-induced release of neuropeptides and both forms of colitis.

CONCLUSIONS

Neuroimmune interactions control intestinal inflammation. Activation and sensitization of TRPA1 and release of substance P induce and maintain colitis in mice.

Transient receptor potential (TRP) channels as drug targets for diseases of the digestive system

Approximately 20 of the 30 mammalian transient receptor potential (TRP) channel subunits are expressed by specific neurons and cells within the alimentary canal. They subserve important roles in taste, chemesthesis, mechanosensation, pain and hyperalgesia and contribute to the regulation of gastrointestinal motility, absorptive and secretory processes, blood flow, and mucosal homeostasis. In a cellular perspective, TRP channels operate either as primary detectors of chemical and physical stimuli, as secondary transducers of ionotropic or metabotropic receptors, or as ion transport channels. The polymodal sensory function of TRPA1, TRPM5, TRPM8, TRPP2, TRPV1, TRPV3 and TRPV4 enables the digestive system to survey its physical and chemical environment, which is relevant to all processes of digestion. TRPV5 and TRPV6 as well as TRPM6 and TRPM7 contribute to the absorption of Ca²⁺ and Mg²⁺, respectively. TRPM7 participates in intestinal pacemaker activity, and TRPC4 transduces muscarinic acetylcholine receptor activation to smooth muscle contraction. Changes in TRP channel expression or function are associated with a variety of diseases/disorders of the digestive system, notably gastro-esophageal reflux disease, inflammatory bowel disease, pain and hyperalgesia in heartburn, functional dyspepsia and irritable bowel syndrome, cholera, hypomagnesemia with secondary hypocalcemia, infantile hypertrophic pyloric stenosis, esophageal, gastrointestinal and pancreatic cancer, and polycystic liver disease. These implications identify TRP channels as promising drug targets for the management of a number of gastrointestinal pathologies. As a result, major efforts are put into the development of selective TRP channel agonists and antagonists and the assessment of their therapeutic potential.