doi: 10.1111/joim.12591 Sensory neuron regulation of gastrointestinal inflammation and bacterial host defence N. Y. Lai, K. Mills & I. M. Chiu From the Division of Immunology, Department of Microbiology and Immunobiology, Harvard Medical School, Boston, MA, USA Content List – Read more articles from the symposium: 13th Key Symposium - Bioelectronic Medicine: Technology targeting molecular mechanisms. Abstract. Lai NY, Mills K, Chiu IM (Harvard Medical School, Boston, MA, USA). Sensory neuron regulation of gastrointestinal inflammation and bacterial host defence (Key Symposium). J Intern Med 2017; 282:5–23. 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 pri- mary afferent neurons. Here, we examine how these distinct sensory neurons and their signal transducers participate in regulating gastrointesti- nal inflammation and host defence. Sensory neu- rons 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 com- mensal bacteria through specific metabolites, cell- wall components, and toxins. Here, we review recent work on the mechanisms of bacterial detec- tion by distinct subtypes of gut-innervating sen- sory 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 sen- sory neuropeptides including substance P, calci- tonin 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 enterotox- igenic Escherichia coli. Understanding how sensory neurons respond to gastrointestinal flora and communicate with immune cells to regulate host defence enhances our knowledge of host physiol- ogy and may form the basis for new approaches to treat gastrointestinal diseases. Keywords: gastrointestinal inflammation, host defence, neuro-immunology, pain, sensory neuron, vagus nerve. Introduction The peripheral sensory nervous system plays a critical role in regulating host physiology by mon- itoring the physical and chemical environment, relaying information to the central nervous system and initiating reflexes to maintain homeostasis. These features are important in coordinating gas- trointestinal functions, such as nutrient absorp- tion, gut motility, blood flow, and secretion. In addition to these well-established roles, sensory neurons play a crucial role in detecting danger and initiating protective responses. Nociceptive sensory neurons express molecular transducers at their nerve terminals to detect noxious and tissue- damaging stimuli including heat, cold and reactive chemicals. Similar to the surveillance capabilities of immune cells, sensory neurons also directly detect bacterial and fungal pathogens. The response kinetics of neurons is orders of magni- tude faster than immune cells (typically millisec- onds compared to hours). Therefore, the early response of the nervous system coordinates host defences to eliminate threats and mediate tissue repair processes. A growing body of evidence shows that sensory neurons communicate bidirectionally Key Symposium ª 2017 The Association for the Publication of the Journal of Internal Medicine 5
19
Embed
Sensory neuron regulation of gastrointestinal inflammation ...
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
doi: 10.1111/joim.12591
Sensory neuron regulation of gastrointestinal inflammationand bacterial host defenceN. Y. Lai, K. Mills & I. M. Chiu
From the Division of Immunology, Department of Microbiology and Immunobiology, Harvard Medical School, Boston, MA, USA
Content List – Read more articles from the symposium: 13th Key Symposium - Bioelectronic Medicine:Technology targeting molecular mechanisms.
Abstract. Lai NY, Mills K, Chiu IM (Harvard MedicalSchool, Boston, MA, USA). Sensory neuronregulation of gastrointestinal inflammation andbacterial host defence (Key Symposium). J InternMed 2017; 282: 5–23.
Sensory neurons in the gastrointestinal tract havemultifaceted roles in maintaining homeostasis,detecting danger and initiating protectiveresponses. The gastrointestinal tract is innervatedby three types of sensory neurons: dorsal rootganglia, nodose/jugular ganglia and intrinsic pri-mary afferent neurons. Here, we examine howthese distinct sensory neurons and their signaltransducers participate in regulating gastrointesti-nal inflammation and host defence. Sensory neu-rons are equipped with molecular sensors thatenable neuronal detection of diverse environmentalsignals including thermal and mechanical stimuli,inflammatory mediators and tissue damage.Emerging evidence shows that sensory neuronsparticipate in host–microbe interactions. Sensoryneurons are able to detect pathogenic and com-mensal bacteria through specific metabolites, cell-wall components, and toxins. Here, we review
recent work on the mechanisms of bacterial detec-tion by distinct subtypes of gut-innervating sen-sory neurons. Upon activation, sensory neuronscommunicate to the immune system to modulatetissue inflammation through antidromic signallingand efferent neural circuits. We discuss how thisneuro-immune regulation is orchestrated throughtransient receptor potential ion channels and sen-sory neuropeptides including substance P, calci-tonin gene-related peptide, vasoactive intestinalpeptide and pituitary adenylate cyclase-activatingpolypeptide. Recent studies also highlight a role forsensory neurons in regulating host defence againstenteric bacterial pathogens including Salmonellatyphimurium, Citrobacter rodentium and enterotox-igenic Escherichia coli. Understanding how sensoryneurons respond to gastrointestinal flora andcommunicate with immune cells to regulate hostdefence enhances our knowledge of host physiol-ogy and may form the basis for new approaches totreat gastrointestinal diseases.
The peripheral sensory nervous system plays acritical role in regulating host physiology by mon-itoring the physical and chemical environment,relaying information to the central nervous systemand initiating reflexes to maintain homeostasis.These features are important in coordinating gas-trointestinal functions, such as nutrient absorp-tion, gut motility, blood flow, and secretion. Inaddition to these well-established roles, sensoryneurons play a crucial role in detecting danger andinitiating protective responses. Nociceptive sensory
neurons express molecular transducers at theirnerve terminals to detect noxious and tissue-damaging stimuli including heat, cold and reactivechemicals. Similar to the surveillance capabilitiesof immune cells, sensory neurons also directlydetect bacterial and fungal pathogens. Theresponse kinetics of neurons is orders of magni-tude faster than immune cells (typically millisec-onds compared to hours). Therefore, the earlyresponse of the nervous system coordinates hostdefences to eliminate threats and mediate tissuerepair processes. A growing body of evidence showsthat sensory neurons communicate bidirectionally
Key Symposium
ª 2017 The Association for the Publication of the Journal of Internal Medicine 5
with immune cells via signalling mediators tomodulate inflammatory responses in ways thatmay be helpful or detrimental to the host [1, 2].
Here, we will review the role of gut-innervatingsensory neurons in regulating gastrointestinalinflammation and bacterial host defence. We willexamine how sensory neurons and their moleculartransducers and signalling mediators modulate gutimmune activation. We will also discuss mecha-nisms of bacterial detection by sensory neurons,and how these neurons contribute to host defenceagainst enteric bacterial pathogens. Understand-ing how sensory neurons shape host defence hasprofound implications for our knowledge of hostphysiology and may augment our ability to treatgastrointestinal diseases such as irritable bowelsyndrome (IBS), inflammatory bowel disease (IBD),gastrointestinal cancers, and microbial infections.
Multiple sensory nervous systems innervate the gastrointestinaltract
The human gut has an estimated 200–600 millionneuronal cell bodies and is also the most denselyinnervated peripheral organ of the body [3]. Itcontains nerve endings that originate from bothextrinsic and intrinsic sources (Fig. 1). Extrinsicinnervation comprises spinal and vagal sensoryafferents whose cell bodies are housed in the dorsalroot ganglia (DRGs) and nodose/jugular ganglia,respectively. The endings of spinal afferents termi-nate in thedorsalhornof the spinal cord,whilst vagalafferents project to the nucleus of the solitary tract inthe brainstem [4]. Both types of extrinsic afferentsinnervate the muscular and mucosal layers withinthe gut. However, vagal innervation is densest in theproximal small intestine and decreases, but is stillpresent, in the colon [5]. The small intestine ismainly, but not exclusively innervated by thoraco-lumbar DRGs, while the large intestine is preferen-tially innervated by lumbosacral DRGs [6, 7]. Theseanatomically distinct neural systems can be furthercategorized by their neurochemical expression (e.g.ion channels, neuropeptides, transcription factors),conduction velocities and information transmitted[8]. Spinal afferents transmit noxious stimuli andconvey visceral sensations, including pain, thermaland mechanical sensation [9]. Vagal afferents areinvolved in homeostatic physiological processes (e.g.secretion, motility, nutrient sensation), nonpainfulvisceral sensations (e.g. satiety, nausea) and vomit-ing [10]. Subsets of vagal afferents encode differentgastrointestinal inputs. For example, it was recently
found that GPR65-expressing neurons innervatingthe intestinal villi detect nutrients and control gutmotility, whereas GLP1R-expressing neurons inner-vating the stomach and duodenum detect intestinalstretch [11].
Within the gut, intrinsic enteric neurons are spa-tially arranged into two continuous ganglionatednetworks encircling the digestive tube and extend-ing the length of the tract. The denser myentericplexus lies between the longitudinal and circularmuscular layers, and the sparser submucosalplexus lies within the submucosa [12]. The nerveprocesses of the myenteric and submucosalplexuses are interconnected, extend into all layersof the gut (muscularis externa, submucosa, epithe-lia) and also innervate structural elements includ-ing Peyer’s patches and the vasculature. Thesensory portion of the enteric nervous system,called intrinsic primary afferent neurons (IPANs),forms complete reflex circuits with enteric interneu-rons and motor neurons, and together it influencesall aspects of digestive function [3]. Unlike theextrinsic afferents, IPANs do not convey visceralsensations from the intestine to the brain [13].
Sensory neurons regulate gut inflammation
The concept that neurons may contribute to regu-lating inflammatory processes was first proposed in1874 by Goltz, who observed that stimulation of thesciatic nerve induced vasodilation [14]. In 1901,Bayliss identified sensory afferents from DRGs asthe main cellular mediators [15]. At the time, it wasproposed that stimulating sensory fibres that ter-minate near arterioles in the skin caused cuta-neous vasodilation through antidromic axonreflexes. Investigation over subsequent yearshelped strengthen the notion of dual transmissionby sensory neurons, namely afferent signals aresent from the periphery to the spinal cord, whilstefferent impulses propagate antidromically fromneural axons back into nerve terminals resulting inthe local release of vasoactive mediators in periph-eral tissues. Studies by Jancso and others [16–18]showed that this ‘neurogenic inflammation’ couldbe elicited by chemical irritants that activate sen-sory neurons. The components of the neurogenicinflammatory response include increased vasodila-tion, plasma extravasation and leucocyte recruit-ment [19]. Of note, capsaicin, the active ingredientof chilli peppers that induces a burning sensation,was found to activate neurons transiently, followedby a long-lasting desensitization [20]. Repeated
Sensory neurons in gut inflammation / N. Y. Lai et al.
6 ª 2017 The Association for the Publication of the Journal of Internal Medicine
Journal of Internal Medicine, 2017, 282; 5–23
applications of capsaicin prevented neurogenicinflammatory responses and also depleted cap-saicin-sensitive nerve fibres in tissues, thus givingrise to the concepts of sensory sensitization anddenervation respectively [20, 21]. The discoverythat capsaicin selectively activates a large, well-defined subset of DRG neurons was an importantmilestone in allowing future studies to characterizethe role of these neurons in inflammation [16].
Activation of vagal sensory afferents also elicits aregulatory feedback loop via an efferent arm,termed the cholinergic anti-inflammatory path-way, which induces release of neural mediatorsthat regulate T cell and innate immune responsesin the periphery. The mechanism of this potentanti-inflammatory reflex was discovered and char-acterized by Tracey and others [22–24]. In thisreflex, inflammatory signals sensed by vagal
afferents activate a neural reflex via the brainstemand vagal autonomic fibres to inhibit excessiveinflammation during sepsis and other inflamma-tory conditions. In the spleen, vagal efferentstrigger the release of acetylcholine from T cellsthat acts through nicotinic receptors on macro-phages to suppress pro-inflammatory cytokinerelease [25]. Given that the focus of this reviewis on direct sensory neuron crosstalk withimmune cells and microbes in the gut, the relatedcholinergic anti-inflammatory pathway is beyondour scope. For comprehensive discussions, read-ers are referred to in-depth reviews by Pavlov et al.[25], Olofsson et al. [26] and Tracey [27].
Role of sensory neurons in regulating gut inflammation
Studies depleting peripheral sensory fibresthrough physical denervation or chemical ablation
Long
itud
inal
Mye
nter
ic
ple
xus
Cir
cula
r m
uscl
e
Sub
muc
osa
l p
lexu
s
Lam
ina
pro
pri
a
Ep
ithe
lia
Ser
osa
IPANs
IPANs
mus
cle
DRGs
Nodose/jugularganglia
Lumbar
Sacral
IPANsCNS projection:None
Functional outcomes:Regulation of digestionRegulation of peristalsisRegulation of blood flowRegulation of fluid transportRegulation of secretion
Fig. 1 Distinct types of sensory neurons innervate the gastrointestinal tract and mediate different functional outcomes.Three sensory neuron types innervate the gastrointestinal tract and are able to respond to various environmental andinternal stimuli. The sensory neurons of the enteric nervous system, termed intrinsic primary afferent neurons (IPANs), havecell bodies in the myenteric and submucosal plexus layers of the gut. They form complete reflex circuits with entericinterneurons and motor neurons to regulate many aspects of digestive and gastrointestinal functions such as gut motility,blood flow, fluid transport, and secretion. The gut receives extrinsic innervation by sensory afferents from the dorsal rootganglia (DRGs) that mediate visceral pain, pressure, and neurogenic inflammation. The gut also receives extrinsicinnervation by sensory afferents from the nodose/jugular ganglia, mediating satiety, nausea, gut homeostasis, andinflammatory reflex circuits. Therefore, sensory information from the gut is differentially processed by distinct neuronaltypes, resulting in different gastrointestinal sensations and functional outcomes. CNS, central nervous system.
Sensory neurons in gut inflammation / N. Y. Lai et al.
ª 2017 The Association for the Publication of the Journal of Internal Medicine 7
Journal of Internal Medicine, 2017, 282; 5–23
have revealed a key role for nociceptive sensoryneurons in gut inflammation (Table 1). This rolehas mainly been examined in animal models of
colitis that involve colonic inflammation inducedby reactive haptens, dinitrobenzene or trinitroben-zene sulphonic acid (TNBS) or by feeding of dextran
Table 1 Sensory neuron regulation of gastrointestinal inflammation
References
Effect of sensory neuron inhibition
Physical denervation Increased pro-inflammatory cytokines in DSS-induced colitis [29, 30]
Exacerbation of DSS-induced colitis [30–32]
Chemical ablation Attenuation of TNBS- and DSS-induced colitis [37–40, 44, 57]
Exacerbation of TNBS- and DSS-induced colitis [41, 42, 57, 169]
Role of ion channels
TRPV1 Mediation of mechanical and thermal hyperalgesia [56]
Anti-ulcerative due to CGRP release [42, 57]
Antagonists improved TNBS- and DSS-induced colitis [40, 52–54]
Knockout mice protected from DSS-induced colitis [55]
TRPA1 Visceral hyperalgesia [63–65]
Agonists are protective in murine colitis [67]
Knockout mice protected against TNBS- and DSS-induced colitis [37]
Sensory neurons in gut inflammation / N. Y. Lai et al.
8 ª 2017 The Association for the Publication of the Journal of Internal Medicine
Journal of Internal Medicine, 2017, 282; 5–23
sulphate sodium (DSS) to injure intestinal epithe-lial cells and trigger inflammation through dissem-ination of enteric bacteria [28]. It remains to bedetermined whether neurons also play a role in T-cell transfer or transgenic preclinical models ofcolitis.
Physical denervationPhysical transection of the vagus nerve was oncecommonly used to prevent ulcers from excessivegastric acid; however, the development of moderndrugs has decreased the use of vagotomies inhumans. Regardless, recent studies in vago-tomized animals lend support to the idea that thevagus nerve confers protection against gastroin-testinal inflammation and colitis [29–32]. Vago-tomy increased expression of NF-jB, a mastertranscription factor controlling the expression ofmany pro-inflammatory genes [29], and led toaccelerated progression of inflammation in DSS-induced colitis, as indicated by higher colonic pro-inflammatory cytokine levels (TNF-a, IL-1b, IL-6,IFN-c) and macroscopic and histopathologicalscores [30, 31]. Although vagotomy had deleteriouseffects on acute experimental colitis, these effectsdiminished with longer rest intervals between sur-gical vagotomy and DSS treatment suggesting thatother compensatory anti-inflammatory processesmay occur post-vagotomy (e.g. regulatory T-cellinduction and IL-10 secretion) [31–33]. Thesefindings suggest an anti-inflammatory role forvagal neurons, although it is not clear whetherthis is mediated by vagal sensory or autonomicfibres, or both.
Chemical ablationRepeated or high-dose treatment of animals withcapsaicin, or the ultra-potent vanilloid analogresiniferatoxin, has been used to chemicallydesensitize or ablate nociceptive sensory neuronsin vivo [1]. These ligands selectively activate heat-sensitive transient receptor potential vanilloid 1(TRPV1) ion channels [34] leading to calcium-dependent mitochondrial damage, osmotic dysreg-ulation and eventually neuronal cell death [35, 36].Several studies showed that chemical ablation ofTRPV1+ neurons significantly attenuated colitisinduced by TNBS and DSS, as indicated byreduced weight loss, colonic histological damage,and neutrophil inflammation measured bymyeloperoxidase activity [37–40]. By contrast,other studies showed that sensory denervation bycapsaicin treatment exacerbated the severity ofcolonic damage upon DSS and TNBS treatment
[41–44]. These discrepant results may in part bedue to factors such as DSS dosages and time-points of denervation. For example, in one study,increased colonic damage was found at low dosesof DSS in sensory-denervated rats, but equivalentdamage was observed at higher doses [41]. Inanother study, sensory denervation resulted inincreased mucosal damage at early time-points,but not later when recovery processes may beoccurring. Therefore, sensory neurons may partic-ipate in both inflammatory and reparative pro-cesses [42]. Interpreting denervation studies isalso complicated by the fact that sensory neuronsrelease several neuropeptides and otherimmunomodulatory mediators, which may interactwith each other and have opposing effects [45].
Role of ion channels in regulating gut inflammation
Sensory neurons respond to diverse chemical andphysical stimuli and are equipped with molecularsensors to transduce these environmental signals.Transient receptor potential (TRP) ion channelsare a class of receptors involved in detection ofthermal and chemical stimuli, including acids andirritants. There are five families of TRP channelsand over 30 different members have been identi-fied in mammals [1]. Recent work using transgenicand pharmacological strategies to target TRPchannels has revealed their important roles ingastrointestinal inflammation and host defence(Table 1).
Transient receptor potential vanilloid 1 (TRPV1)The TRPV1 ion channel was initially identified asthe major receptor for capsaicin [34] and subse-quently found to mediate noxious heat sensitivityand inflammatory pain. In the nodose ganglia,TRPV1 is expressed by about 40–70% of sensoryneurons [46, 47], whilst 65–95% of DRG neuronsexpress it depending on the lumbar–sacral level [6,7, 48]. Its presence in enteric neurons is contro-versial with some studies finding TRPV1immunoreactivity, and others failing to do so [49].Additionally, there have been some reports ofTRPV1 expression in non-neural cell types, includ-ing intestinal epithelial cells and smooth musclecells; however, the existence of several splicevariants and false-positive antisera may contributeto disparate findings [1]. Nevertheless, TRPV1participates in regulating mucosal blood flow,bicarbonate secretion to counter excess acid, andmucus secretion to enhance epithelial barrierintegrity [10, 50, 51].
Sensory neurons in gut inflammation / N. Y. Lai et al.
ª 2017 The Association for the Publication of the Journal of Internal Medicine 9
Journal of Internal Medicine, 2017, 282; 5–23
During gastrointestinal inflammation, both sensi-tization and upregulation of TRPV1 channels con-tributed to heightened sensitivity to mechanicaland thermal pain [49]. Patients with IBD reportedincreased abdominal pain, including ‘burning’sensations, and correspondingly have increasedtissue expression of TRPV1+ immunoreactive fibres[1]. In murine studies of experimental colitis,TRPV1 antagonists reduced colon shrinkage, his-tological scores and weight loss in TNBS and DSSmodels [40, 52–54]. TRPV1�/� mice showeddecreased susceptibility to DSS treatment com-pared to wild-type mice [55], although conflictingresults have been reported [56]. The protectiveeffects observed during TRPV1 loss-of-functionsuggested that TRPV1 plays a role in augmentinginflammation. However, administration of cap-saicin ameliorated DSS- and TNBS-induced colitis[41, 57]. Goso et al. suggested that downstreameffects of TRPV1 channel activation, such asrelease of calcitonin gene-related peptide (CGRP),may contribute to anti-ulcerative effects [57]. Con-clusions from work using potent TRPV1 agonistsshould be interpreted with caution as these ligandsmay also desensitize neurons.
Transient receptor potential ankyrin-repeat 1(TRPA1)The TRPA1 ion channel recognizes damaging reac-tive chemicals. Electrophilic compounds, such asmustard oils and allicin, induce gating of TRPA1 bycovalent modification of cytoplasmic cysteine resi-dues [58, 59]. In nodose ganglia and DRGs, TRPA1is expressed in a smaller proportion (20–30%) ofsensory neurons than TRPV1, although it mostlyoverlaps with TRPV1 in unmyelinated, small-diameter C-fibres [46, 58]. TRPA1+ neurons in thegastrointestinal tract often express neuropeptides[e.g. substance P (SP), CGRP], and TRPA1 ligandsinduce neuropeptide release upon activation [37,60], implying that TRPA1 participates in neuro-genic inflammation [59]. In addition to extrinsicafferents, TRPA1 transcripts and proteins areexpressed by enteric neurons [61], as well as non-neuronal enterochromaffin cells in the gut [62].
Evidence suggests that TRPA1 has a role in induc-ing or maintaining hyperalgesia and pain sensationduring colitis. Intracolonic administration of TNBSinduced inward currents in a TRPA1-dependentmanner, sensitized colonic sensory neurons, andelicited increased nocifensive visceromotorresponses to colorectal distension [63, 64]. Theseresponses were absent in TRPA1�/� mice or
inhibited in antagonist-treated mice [63–65]. Asidefrom pain, the role of TRPA1 in mediating colitis-associated inflammation is less clear. Functionalevidence from clinical studies is lacking, althoughone study found upregulation of TRPA1 mRNA andimmunoreactivity in inflamed colons of patientswith IBD [66]. Studies of TRPA1�/� mice showedmixed results in that colitis inflammationdecreased or was similar to that of wild-type miceafter TNBS or DSS treatment [37, 64]. By contrast,other studies showed that TRPA1 was upregulatedin murine colitis and its activation exerted protec-tive effects [66, 67]. Together, these findings sug-gest that TRPA1 is robustly activated by a widevariety of inflammatory compounds and may play arole in mediating pain and inflammation in thegastrointestinal tract.
Transient receptor potential melastatin 8 (TRPM8)The TRPM8 ion channel is activated by cold tem-peratures or agents such as menthol and icilin,which elicit a cooling sensation [68]. Peppermintoil, containing a high menthol content, reducedcalcium influx and caused long-lasting relaxationof the gastrointestinal smooth muscles [61]. About10% of DRG neurons respond to cold temperaturesor icillin [69, 70]. Expression in nodose ganglia hasnot been definitively confirmed; however, it isexpressed in trigeminal ganglia, particularly intongue-innervating regions [46, 71]. About a thirdof TRPM8-expressing and cold-sensitive neuronsoverlap with TRPV1-expressing neurons [69];however, its coexpression with TRPA1 is less clear[46, 69].
Recent studies indicate that TRPM8 has an anti-inflammatory role in colitis, in part due to neu-ropeptide release. TRPM8 expression is upregu-lated in human- and murine-inflamed colonsamples [72]. Systemic activation of TRPM8, usingthe selective agonist icilin, attenuated the severityof TNBS-induced colitis, as measured by decreasedhistological damage scores, bowel thickness,myeloperoxidase activity, and pro-inflammatorycytokines. In the same study, Ramachandran et al.showed no difference in DSS-induced inflamma-tion in TRPM8�/� mice, compared to wild-typemice, although colonic levels of CGRP wereincreased [72]. To confirm a link between TRPM8and neuropeptide release, the authors showed thaticilin blocked calcium currents and CGRP secre-tion from colon tissues ex vivo. Later findingsconfirmed that the anti-inflammatory effects ofTRPM8 were linked to induction of CGRP release
Sensory neurons in gut inflammation / N. Y. Lai et al.
10 ª 2017 The Association for the Publication of the Journal of Internal Medicine
Journal of Internal Medicine, 2017, 282; 5–23
[73] and, furthermore, showed that TRPM8�/�mice were hypersusceptible to DSS-induced colitis.The number of CD11c+ dendritic cells (DCs) foundin close proximity to CGRP+ colonic nerve fibreswas increased following DSS treatment inTRPM8�/� mice relative to wild-type animals.Additionally, absence of CGRP signalling asdemonstrated by mice deficient in RAMP1, themain receptor for CGRP, resulted in increasedsusceptibility to colitis, whilst RAMP1�/� DCsdisplayed a hyperinflammatory phenotype. Finally,treatment of TRPM8�/� mice with CGRP amelio-rated excessive DC activation and colitis. Theselines of evidence indicate that TRPM8+ nerve fibrespromote an anti-inflammatory tissue environmentthrough local release of CGRP and influence onmucosal innate immune cells.
Role of neuropeptides in regulating gut inflammation
Activation of sensory neurons induces calcium-dependent release of dense core vesicles containingneuropeptides from nerve terminals. Neuropep-tides are a family of peptides that have localneuroendocrine and signaling function. In somecases, larger precursor peptides are processed intomultiple neuropeptides that exert effects throughone or more neuropeptide receptors expressed byneurons, immune cells, and stromal cells in the gut[74, 75].
Substance P (SP)Substance P, a member of the tachykinin neu-ropeptide family, was discovered in in vitro extractsas the ability to stimulate intestinal contractilityand decrease blood pressure [76]. The Tac1 geneencodes preprotachykinin-1, which can be furtherprocessed into four alternatively spliced tachykininneuropeptides: SP, neurokinin A, neurokinin K,and neurokinin gamma [77]. Their physiologicalreceptors include NK1R, NK2R, and NK3R (encodedby Tacr1, Tacr2, and Tacr3, respectively), which areG protein-coupled receptors expressed by neuronaland non-neuronal cell types. In the gut, the majorsources of SP are intrinsic enteric neurons, whilstextrinsic neurons contribute to a lesser extent [42,78–80]. Tachykinin immunoreactivity wasobserved in neuronal bodies of the myenteric andsubmucosal plexuses, as well as nerve processesinnervating smooth muscle, submucosal arteries,and mucosa. NK1R is expressed by enteric neu-rons, interstitial cells of Cajal, and epithelial cells.NK2R is expressed by muscle and epithelial cells,and expression is higher in the ileum relative to the
duodenum and colon. Finally, NK3R is predomi-nantly expressed by enteric neurons and mediatesneuro-neuronal transmission. On immune cells,NK1R and NK2R have been localized to laminapropria T lymphocytes, macrophages, and mastcells; and their expression increases during inflam-mation [78]. Assessment in patients with IBD hasled to inconsistent results, with some studiesshowing increased SP content in tissues, andothers showing no alterations or a decrease in SP[81].
Substance P induces immune cells to secrete pro-inflammatory cytokines (e.g. IL-1b, IL-6, IL-8, TNF-a) via an NF-jB-dependent pathway in target cells[78]. Studies point to a deleterious role for SP andNK1R activation in DSS- and TNBS-induced colitis.Tac1�/� and NK1R antagonist-treated miceshowed decreased weight loss, myeloid peroxidaseactivity, histopathological scores, and pro-inflam-matory cytokine production compared to wild-typemice [37, 38, 82, 83]. A small number of studiesshowed no difference or more severe colitis afterblockade of NK1R signalling [84, 85]. Although SPenhances tissue inflammation, it may also con-tribute to tissue recovery. Stimulation of NK1R incolonic fibroblasts increased collagen synthesisand fibrogenesis [86] and promoted the survivalof colonocytes via anti-apoptotic Akt signallingmechanisms [87]. Thus, although NK1R antago-nists may have potential anti-inflammatory bene-fits in IBD, they may also counteract tissue repairprocesses [77].
Calcitonin gene-related peptide (CGRP)Calcitonin gene-related peptide is a sensory neu-ropeptide with highly potent, long-lasting vasodila-tory activity in the femtomolar range and is 10- to1000-fold more potent than other classic vasodila-tors (e.g. acetylcholine) [88]. There are two isoformsof this neuropeptide, CGRPa and CGRPb, whichdiffer by only three amino acids in humans, andare encoded by the genes Calca and Calcb, respec-tively. CGRP immunoreactivity is found in alllayers of the gut and concentrated around submu-cosal blood vessels. CGRP is largely coexpressedwith SP; however, in contrast to SP fibres, CGRPfibres innervate Peyer’s patches [89]. Additionally,capsaicin-induced denervation and co-labellingexperiments indicated that the majority (50–80%)of CGRP expression in small and large intestines isfrom extrinsic and DRG origins [90]. Enteric neu-rons represent a minor contribution to the totalCGRP intestinal expression, and these neurons
Sensory neurons in gut inflammation / N. Y. Lai et al.
ª 2017 The Association for the Publication of the Journal of Internal Medicine 11
Journal of Internal Medicine, 2017, 282; 5–23
preferentially express the CGRPb isoform [90, 91].The receptor for CGRP consists of a heterodimerbetween calcitonin receptor-like receptor (CLR) andreceptor activity-modifying protein 1 (RAMP1).CGRP also promiscuously binds other heterodi-mers, for example CLR/RAMP3 complexes that arethe high-affinity receptors for adrenomedullin.CGRP is a neuropeptide that exemplifies neuro-immune bidirectional crosstalk in that manyinnate and adaptive immune cells modify theirfunction in response to CGRP, whilst someimmune cells (e.g. monocytes, T cells, B cells)release CGRP or regulate sensory neuron expres-sion of CGRP [92]. CGRP plays important roles inlymphocyte maturation, proliferation, migration,antigen presentation, and cytokine production (foran extensive review, see Ref. [93]).
During colonic inflammation, CGRP immunoreac-tivity and release increase in distal portions of thecolon [38]. Exogenous administration of CGRPshowed anti-inflammatory effects, leading to ame-lioration of ulcerative lesions, and attenuation ofcolonic weight increase [44, 94]. Blockade of CGRPsignalling by specific antagonists in mice increasedmacroscopic damage, ulcers, and myeloperoxidaseactivity [45, 85, 94]. CGRP�/� mice developedsevere TNBS-induced colitis, although this waslater attributed to a combination of the deleteriouseffects of SP and the lack of CGRP-mediated tissueprotection [45]; the authors argued that CGRPcounters the pro-inflammatory effects of SP, but isnot required for colonic protection in wild-typemice [45]. CGRP/SP double-knockout mice wereequally protected against TNBS-induced colitiscompared to SP�/� mice, suggesting that CGRPdoes not provide additional protection if tissue-damaging effects of SP are absent. Currently, thecellular and molecular mechanisms of CGRP reg-ulation of gastrointestinal inflammation have notbeen finely mapped.
Vasoactive intestinal peptide (VIP)Vasoactive intestinal peptide (VIP) was discoveredas a vasodilatory polypeptide in porcine intestine[95]. VIP acts on smooth muscle and intestinalepithelial cells to influence gut motility, fluidabsorption, and electrolyte and mucus secretion[96–98]. It exerts immunological effects throughaction on its main receptors, VPAC1 and VPAC2(encoded by the genes Vipr1 and Vipr2, respec-tively). The receptors are expressed on severalinnate and adaptive immune cell types, includingT cells, macrophages, DCs, neutrophils, and innate
lymphoid cells [99–102]. VIP+ nerves are found inall layers of the gut and also innervate Peyer’spatches [103]. In TNBS-induced colitis, the VIPcontent in nerve fibres decreased in the submucosaand increased in the mucosa [104].
The role of VIP in gastrointestinal inflammation isunclear due to contrasting results. Several studiesshowed that VIP administration conferred protec-tion in mouse models of colitis with improvement ofsurvival, weight loss, diarrhea, and tissuehistopathology [105–107]. The authors of thesestudies proposed different mechanisms by whichVIP exerted its beneficial effects, including promot-ing anti-inflammatory cytokines (IL-10, IL-4, IL-13), decreasing pro-inflammatory cytokines (TNF-a, IL-1b, IL-6), and ameliorating bacteria-induceddisruption of intestinal epithelial tight junctions.VIP also restored homeostatic immune cell traffick-ing to mesenteric lymph nodes and Peyer’s patches[105]. However, protection disappeared when VIPwas administered at higher doses [108]. In contrastto studies in which a protective role of exogenousVIP was demonstrated, VIP�/� mice appeared tobe resistant to DSS- and TNBS-induced colitis,exhibiting modest weight loss, lower levels of pro-inflammatory cytokines, and no difference inhistopathology scores compared to wild-type mice[106, 109]. The discrepancy may in part be due tothe fact that VIP binds to two receptors, VPAC1 andVPAC2, which have distinct affinities for VIP andare expressed at varying levels on different immunecell types. VPAC1 is expressed on resting T cells andmacrophages. On the other hand, VPAC2 expres-sion is constitutively low on T cells, but is upreg-ulated upon activation, whereas VPAC1 isdownregulated [110]. The relative importance ofthese two receptors was investigated in vivo.VPAC1�/� mice were resistant to DSS-inducedcolitis, whereas colitis in VPAC2�/�mice appearedrapidly with greater weight loss and pro-inflamma-tory cytokine production, and more severehistopathology compared to wild-type mice [111].The intriguing findings of this study highlight theneed to utilize receptor-specific agonists or antag-onists to target distinct neuropeptide signallingpathways during gastrointestinal inflammation.
Pituitary adenylate cyclase-activating polypeptide(PACAP)The neuropeptide PACAP was originally extractedfrom ovine hypothalamus and found to potentlystimulate adenylate cyclase to produce cyclic AMPin rat pituitary cultures [112]. The Adcyap1 gene
Sensory neurons in gut inflammation / N. Y. Lai et al.
12 ª 2017 The Association for the Publication of the Journal of Internal Medicine
Journal of Internal Medicine, 2017, 282; 5–23
encodes a protein that is cleaved into two peptides,27 and 38 amino acids long, that are both biologi-cally active. PACAPhas70%aminoacidhomology toVIP, and biochemical studies indicate that it bindspromiscuously to receptors for VIP (VPAC1 andVPAC2), as well as its receptor PAC1 (encoded byAdcyap1r1). A higher proportion (70%) of nodose/jugular neurons express PACAP compared to DRGneurons (30%) [113, 114]. In enteric neurons,PACAP immunoreactivity is denser in myentericneurons than in submucosal neurons [115]. PACAPis highly coexpressed with VIP and SP, and CGRP toa lesser extent, in nodose ganglia [114]. Similar toVIP, PACAP induces relaxation of smooth musclesanddecreases gutmotility [116]. PACAPalso acts onintestinal epithelial cells and enterochromaffin cellsto induce gastrointestinal secretion of histamine,bicarbonate, and chloride [116–118]. Unlike VPACreceptors, PAC1may bemore limited in the immunesystem and is mainly expressed by macrophagesand neutrophils [100, 119].
Relative to VIP, less is known about the effects ofPACAP on gastrointestinal inflammation.PACAP�/� mice developed more severe DSS-induced colitis with increased mortality, colonicpathology, colon length reduction, and pro-inflam-matory cytokine production (IL-1b, IL-6, IFN-c)[120, 121]. Exogenous administration of PACAPto PACAP�/� mice at an early time-point duringDSS treatment improved survival and colitis dis-ease [121]. Given the complexity of VIP and PACAPsignalling with multiple receptors, the specific rolesfor PACAP or PAC1 have not been fully defined inthe gastrointestinal tract.
Sensory neuron regulation of bacterial host defence
The human gastrointestinal tract is inhabited bytrillions of microbes that vastly outnumber ourown cells [122]. Commensal microbes not onlyproduce metabolites and vitamins that are utilizedby mammalian hosts, but they also enhance pro-tection of epithelial barriers, regulate gastrointesti-nal functions, and modulate cognitive processesthrough the gut–brain axis [123]. Whilst intestinalepithelial cells and immune cells have well-recog-nized roles in gastrointestinal surveillance andinitiation of protective responses during breach ofthe intestinal barrier, recent work indicates thatsensory neurons also participate in these pro-cesses [124]. Sensory neurons may thus play anunderappreciated role in bacterial detection and
mediation of host defence in the gastrointestinaltract.
Sensory neuron detection of bacterial products
Recent studies have shown that sensory neuronsrespond directly to several bacterial products,including cell-wall components, toxins, andmetabolites (Fig. 2). Bacterial activation of DRGneurons, vagal afferent neurons or IPANs couldhave various physiological outcomes on regulatingpain, satiety, neuroendocrine control, and gutmotility.
Bacterial detection by DRG afferent neuronsBacterial infections of the gastrointestinal tract areoften accompanied by pain, an unpleasant sensa-tion mediated by nociceptive DRG neurons inner-vating the spinal cord. It was generally thoughtthat inflammatory mediators produced by immunecells during bacterial infection induced nociceptoractivation and pain; however, it has recently beenshown that bacterial pathogens can directly inducethese effects.
Citrobacter rodentium is a Gram-negative murinebacterial pathogen that causes similar gastroin-testinal infections as enterotoxigenic Escherichiacoli (ETEC) in humans. C. rodentium infectionincreased the hyperexcitability of mouse colonicDRG neurons and resulted in enhanced pain tocolorectal distension [125]. The ability of individualbacterial products to activate colon-innervatingDRG neurons was further investigated [126]. Bac-terial lipopolysaccharide (LPS) enhanced theexcitability and firing rate of DRG neurons andincreased their production of pro-inflammatoryTNF-a and IL-1b transcripts and cytokines [126].
Bacteria may also inhibit neural activity and pain.Oral administration of Lactobacillus strainsreduced the firing rate of lumbar DRG neuronsand reduced nociceptive muscle responses to col-orectal distension as measured by electromyogra-phy [127, 128]. It was also found that oral gavage ofLactobacillus acidophilus induced the expression ofl-opioid and cannabinoid receptors in intestinalepithelial cells and produced analgesic effects[129]. These findings demonstrate that sensoryneurons change their membrane properties inresponse to bacterial products, although the speci-fic molecular mechanisms involved are not yet fullydefined.
Sensory neurons in gut inflammation / N. Y. Lai et al.
ª 2017 The Association for the Publication of the Journal of Internal Medicine 13
Journal of Internal Medicine, 2017, 282; 5–23
Toll-like receptors (TLRs) are expressed by DRGneurons [130], and TLR ligands induce neuronalactivation and pain [131]. E. coli-derived LPS sen-sitized TRPV1 channels and enhanced inwardcalcium currents via a TLR4-dependent mecha-nism in trigeminal neurons [132]. Additionally, thelipid A moiety of LPS was found to directly gateTRPA1 channels independently of TLR4 andinduced neuronal excitability, calcium influx, andneurogenic inflammation [133]. In Staphylococcusaureus infections, binding of bacterial N-formy-lated peptides to formyl peptide receptor 1 and
pores formed by a-haemolysin induced calciumflux, depolarization, and firing of action potentialsin DRG neurons [134]. This neuronal activationwas independent of tissue swelling and innateimmune components. Conversely, mycolactone, ametabolite from Mycobacterium ulcerans, the cau-sative agent of painless skin ulcers, signalledthrough angiotensin-2 receptors to activateTRAAK-mediated potassium channels, hyperpolar-ized neurons, and produced analgesia [135].Although these recent studies indicate that DRGneurons respond to bacterial products directly by
Fig. 2 Sensory neurondetection of bacterial products.Sensory neurons of the dorsalroot ganglia (a) and jugular/nodose ganglia (b) and intrinsicprimary afferent neurons of theenteric nervous system (c) havebeen found to respond directlyto pathogenic and commensalbacteria. Bacterial cell-wallcomponents, metabolites andtoxins from differentcommensal and pathogenicbacteria interact with neuronalreceptors or ion channels toinduce changes in ion flux,signalling, and neuronalexcitability. TLR4, Toll-likereceptor 4; TRPA1, transientreceptor potential ankyrin-like1; TLR5, Toll-like receptor 5;FPR, formyl peptide receptor;TRAAK, TWIK-relatedarachidonic acid-activated K+channel; IPAN, intrinsicprimary afferent neuron; PSA,polysaccharide A; IKCa,calcium-activated potassiumcurrent.
Sensory neurons in gut inflammation / N. Y. Lai et al.
14 ª 2017 The Association for the Publication of the Journal of Internal Medicine
Journal of Internal Medicine, 2017, 282; 5–23
increasing or decreasing their activity, the func-tional outcomes on host defence remain mostlyunderexplored.
Bacterial detection by vagal afferent neuronsVagal sensory afferent fibres are an importantneural link between the gut and brain and mayserve as an early signalling pathway of infection bygastrointestinal pathogens [123]. Oral inoculationof enteric pathogens Campylobacter jejuni or Sal-monella typhimurium led to activation of neurons inthe vagal sensory ganglia and the nucleus solitarytract in the brain as measured by c-Fos expression[136]. Other studies demonstrated that theseeffects were mediated by capsaicin-sensitive vagalafferents because vagotomy partially prevented c-Fos induction in the hypothalamus [137].
Within the gut, vagal sensory nerve fibres extend upinto the villi and are found in close proximity tointestinal epithelial cells. It is still unclear whetherenteric pathogens breach the intestinal epithelialbarrier to activate underlying vagal processesdirectly, or whether epithelial cells transduce bac-terial signals and secrete paracrine mediators thatsubsequently alter vagal activity [138]. Direct appli-cation of LPS has been shown to elicit currents innodose neurons in culture [139]. Moreover, it wasdemonstrated that Lactobacillus rhamonusincreased the intrinsic firing rate of vagal afferentsto gut distension within minutes of exposure of thebacterium to the gut lumen [138]. Using CSFE-labelled bacteria, the authors of the study found noevidence of bacterial translocation across theepithelial layer into deeper mucosal and submu-cosal layers. Thus, the mechanisms of neuronalactivation by bacteria in vivo have yet to be deter-mined.
Bacterial detection by IPANsIntrinsic primary afferent neurons are the sensoryarm of the enteric nervous system. IPANs expressinnate immune receptors, including TLR2, TLR3,TLR4 and TLR7, that enable detection of patho-gen-associated molecular patterns [124]. Luminalapplication of L. rhamosus and Bacteroides fragilisto the epithelia produced responses in myentericIPANs within seconds and facilitated excitabilitywithin minutes [140]. The short latency of ortho-dromic action potentials suggested that IPANresponses were direct sensory action potentialsmediated by neural processes extending into themucosal epithelia. IPANs also received synapticinputs from local IPAN circuits as secondary
excitatory postsynaptic potentials were alsorecorded. It was found that the capsular polysac-charide A from B. fragilis was a critical mediator ofactivation of sensory neuronal responses [140]. Itis likely that other commensal or pathogenicbacterial strains could signal to IPANs via specificmolecules or metabolites.
Bacteria can also induce distinct effects on differ-ent intestinal sensory subtypes. Although Lacto-bacillus reuteri decreased firing in extrinsic DRGneurons to mediate analgesia [127], the samebacterium increased excitability in a subset ofcolonic myenteric IPANs [141]. IPANs were moreexcitable due to decreased hyperpolarizing potas-sium currents mediated through IKCa channels[141]. Another study showed that IKCa channelantagonists decreased muscle contraction andcolon motility [142]. These lines of evidence suggestthat L. reuteri-mediated enhancement of IPANexcitability may reduce colonic motility and thatlower muscle tension may be a contributing factorin probiotic alleviation of visceral pain [141]. Acti-vation of enteric neurons also resulted in neuralmodulation of the inflammatory response as exem-plified by enhanced TNF-a production in LPS-treated myenteric plexus preparations and primaryenteric cultures [143].
Sensory neuron regulation of gastrointestinal bacterial infections
The ability of neurons to directly respond tobacteria may be a beneficial mechanism for mam-malian hosts to combat pathogens. Indeed,increasing evidence indicates a role for sensoryneurons in regulating effective host defence againstbacterial pathogens during gastrointestinal infec-tions. Here, we focus on the specific entericpathogens Salmonella, C. rodentium, and ETEC(Table 2). Much remains to be determined regard-ing the role of neuronal sensing in successful hostdefence against bacterial pathogens.
SalmonellaIt has been shown that S. typhimurium translo-cates across the intestinal lumen by inducinguptake by M cells in Peyer’s patches, which arehighly innervated by peptidergic sensory neurons,as well as sympathetic and cholinergic neurons[144]. Neuronal input may regulate barrierdefences against the invasiveness of Salmonella.Total pharmacological blockade of neural activityincreased epithelial uptake of S. choleraesuis inporcine jejunum [145].
Sensory neurons in gut inflammation / N. Y. Lai et al.
ª 2017 The Association for the Publication of the Journal of Internal Medicine 15
Journal of Internal Medicine, 2017, 282; 5–23
Sensory neuron-derived neuropeptides and theirsignalling receptors may play an important role inregulating innate and adaptive immune responsesduring Salmonella infections. Expression of Tac1(tachykinin precursor for SP) and SP receptortranscripts increased within hours of oral infec-tion in Peyer’s patches, mesenteric lymph nodes,and spleen of infected animals [146, 147]. Macro-phages cocultured with Salmonella upregulatedSP receptors and exhibited increased binding ofSP in vitro [146]. SP polarized immune cells toadopt a more pro-inflammatory phenotype,including induction of IL-1, IL-6, and TNF-aexpressions by monocytes [148], and IL-12expression by macrophages [149]. Mice treatedwith an SP antagonist succumbed more rapidly toSalmonella infection and showed lower IL-12 andIFN-c expressions in mucosal tissues [146]. Thesestudies suggest that SP mediates protectionagainst infection by enhancing innate immune
cell function, in part through SP receptor upreg-ulation and production of IL-12 and IFN-c. Bycontrast, another study found improved survivalin receptor-deficient NK1R�/� mice when chal-lenged with Salmonella compared to wild-typemice [150]. SP may also suppress adaptiveimmune cell function. Immunization of NK1R�/�mice with a Salmonella-adjuvant vaccine showedincreased mucosal and systemic immunoglobulinIgA responses. Secretory IgA responses may havebeen boosted in NK1R�/� mice by CD4 T helpercells secreting IL-5 and IL-6 in Peyer’s patches[150]. Whilst this contradicts earlier studies show-ing that SP enhances IgA secretion [151–153], theauthors argued that there are compartmentalizedeffects due to SP+ fibres differentially innervatingB-cell and T-cell zones in lymphoid tissues [150].Taken together, these results indicate that SPinfluences both innate and adaptive responsesduring Salmonella infection.
Table 2 Neuropeptide modulation of gastrointestinal bacterial host defence
Neuropeptides Bacterial pathogen References
Substance P Salmonella typhimurium
Expression increased in gut-associated tissues [146, 147]
Influenced immune cell functions [146, 148–150]
Receptor knockout protected from infection [150]
Antagonists decreased survival [146]
Citrobacter rodentium
Expression not changed upon infection [162]
Enterotoxigenic Escherichia coli
Enhanced LPS-induced faecal output [168]
VIP Salmonella typhimurium
Protected mice from LPS-induced endotoxemia [154]
Inhibited S. typhimurium clearance by immune cells [156, 157, 173, 174]
Inhibited pro-inflammatory cytokine production [157]
Sensory neurons in gut inflammation / N. Y. Lai et al.
16 ª 2017 The Association for the Publication of the Journal of Internal Medicine
Journal of Internal Medicine, 2017, 282; 5–23
Vasoactive intestinal peptide and PACAP protectmice from endotoxic shock induced by lethalinjections of Salmonella-derived LPS though inhi-bition of TNF-a and IL-6 productions [154]. PAC1,the receptor for PACAP, was shown to inhibit LPS-induced neutrophil recruitment, as measured bymyeloperoxidase activity in the liver and intestine[119]. VIP also downregulated pro-inflammatorycytokines (TNF-a, IL-1, IL-12, IFN-c) and upregu-lated anti-inflammatory cytokines (IL-10, TGFb)[154, 155]. Although VIP-mediated immunosup-pression was beneficial during excessive inflam-mation induced by lethal endotoxemia, it may bedetrimental to host clearance of pathogens. VIPinhibited IFNc-mediated production of reactiveoxygen species and prevented killing of Salmonellaby cultured macrophages [156]. VIP also inhibitedTNF-a and IL-1b productions in S. typhimurium-infected macrophages [157].
Citrobacter rodentiumCitrobacter rodentium is a noninvasive, attachingand effacing Gram-negative murine pathogen thatadheres to and disrupts colonic epithelial cells,thereby inducing mucosal inflammation and colitis[158]. Animal models of C. rodentium infectionhave been used to model human inflammatorybowel disorders. Neurobehavioural changes occurfollowing C. rodentium infection, including changesin anxiety parameters in open-field tests, andincrease in risk assessment behaviours, whichwere correlated with increased vagal sensory acti-vation as measured by c-Fos expression [159].Vagal afferents likely serve as an early neuralpathway transmitting gastrointestinal signals fromthe gut to the brain. Modification of behaviour mayserve a protective role in allowing the host to avoiddanger and recuperate [159]. Although plasmalevels of IFN-c, TNF-a, and IL-12 did not differpost-infection, it is possible that gastrointestinalrelease of inflammatory mediators by immune orepithelial cells activated cytokine receptors onvagal sensory neurons [160, 161]. It is also possi-ble that vagal sensory neurons are able to directlydetect C. rodentium and signal to brain circuitrythat regulates anxiety-like behaviour.
Sensory neuropeptides may also play a role in hostdefence against C. rodentium. VIP immunoreactiv-ity in inflamed colon tissues was increased uponC. rodentium infection in one study [158]. However,another study showed that immunoreactivity ofseveral neuropeptides (VIP, SP, CGRP) in gastroin-testinal tissues of C. rodentium-infected mice was
not different compared with sham-infected controls[162]. Exogenous administration of VIP did notimpact bacterial attachment to epithelial cells;however, VIP prevented C. rodentium-inducedredistribution of tight junction proteins (e.g. ZO-1,occludin) in amyosin light-chain kinase-dependentmanner [158]. Body weight loss, colonic epithelialdamage, and paracellular permeability were alsoreduced, indicating that VIP ameliorates C. roden-tium-induced gastrointestinal inflammation.
ETECEnterotoxigenic Escherichia coli is a major bacterialpathogen that causes diarrhoea in humans. ETECproduces high levels of enterotoxins that stimulateexcessive fluid secretion by intestinal epithelialcells. Recent studies indicate that sensory neu-ropeptides may be utilized as an alternative toantibiotics in treating diarrhoeal diseases. Exoge-nously administered VIP to newly weaned pigletsinfected with ETEC reduced the incidence of diar-rhoea and improved growth rates [163]. VIP treat-ment also reversed ETEC-induced increases ininflammatory mediators including IL-2, IL-12,IFN-c, and TNF-a [163]. Supporting the hypothesisthat VIP may be protective against enterotoxin-induced diarrhoeal disease, one study showed thatheat-labile and heat-stable enterotoxins fromE. coli induced increases in jejunal fluid secretionthat were dose-dependently reversed by VIP treat-ment [164]. Others have shown that vagotomy andintraluminal capsaicin administration inhibitETEC-induced fluid secretion, suggesting thatTRPV1-expressing vagal sensory neurons play arole in regulating epithelial function [165, 166].Systemic administration of LPS derived from E. coliincreased nitric oxide synthase enzymatic activityin the small intestine and accelerated intestinaltransit [167]. LPS-induced increase in fecal outputwas found to be mediated by capsaicin-sensitivevagal neurons and could be blocked by treatmentwith SP receptor antagonists or nitric oxide syn-thase inhibitors [168]. Therefore, sensory neuronsmay play an important role in regulating inflam-matory responses, gut motility, and intestinalsecretions during bacterial infection.
Summary and future directions
The studies highlighted above show the complexroles that sensory neurons and their mediatorsplay during gastrointestinal inflammation andbacterial host defence. On one hand, sensoryneurons detect bacterial products and
Sensory neurons in gut inflammation / N. Y. Lai et al.
ª 2017 The Association for the Publication of the Journal of Internal Medicine 17
Journal of Internal Medicine, 2017, 282; 5–23
inflammatory inputs and transmit informationthrough neural circuits to the central nervoussystem; on the other hand, they can also potentlydrive or suppress innate and adaptive immuneresponses in the gastrointestinal tract throughneurogenic mechanisms such as the release ofneuropeptides. Because many distinct subsets ofpeptidergic and nonpeptidergic neurons innervatethe gastrointestinal tract with distinct sensorymodalities and anatomical distributions, the roleof the sensory nervous system in gastrointestinalhost defence will be an important field of futurestudy. It is possible that distinct species of com-mensal and pathogenic bacteria stimulate or mod-ulate neuronal excitability of different sensorysubtypes, thus regulating pain, gut motility, andinflammation. These bacteria–neuron molecularinteractions remain to be defined. Furthermore,evidence indicates differential roles for distinct TRPchannels and neuropeptides expressed by sensoryneurons in regulating immune cell function. There-fore, defining how distinct subtypes of sensoryneurons communicate with innate and adaptivebranches of the immune system may be critical inallowing us to understand their roles in homeosta-sis, inflammation, and host defence. Eventually,targeting these specific neuron–microbe and neu-ron–immune interactions may lead to new treat-ments for gastrointestinal diseases including IBS/IBD and bacterial infections.
Conflict of interest statement
No conflicts of interest to declare.
Acknowledgement
Funding support was provided by HarvardDigestiveDisease Center, NIH/NCCID grant DP2AT009499,and NIH/NIAID grant K22AI114810-02.
References
1 Holzer P. Transient receptor potential (TRP) channels as
drug targets for diseases of the digestive system. Pharmacol
Ther 2011; 131: 142–70.
2 McMahon SB, La Russa F, Bennett DL. Crosstalk between
the nociceptive and immune systems in host defence and
disease. Nat Rev Neurosci 2015; 16: 389–402.
3 Furness JB, Callaghan BP, Rivera LR, Cho HJ. The enteric
nervous system and gastrointestinal innervation: integrated
local and central control.Adv ExpMedBiol 2014;817: 39–71.
4 Forsythe P, Bienenstock J, Kunze WA. Vagal pathways for
microbiome-brain-gut axis communication. Adv Exp Med
Biol 2014; 817: 115–33.
5 Wang FB, Powley TL. Topographic inventories of vagal
afferents in gastrointestinal muscle. J Comp Neurol 2000;
421: 302–24.
6 Tan LL, Bornstein JC, Anderson CR. Distinct chemical
classes of medium-sized transient receptor potential chan-
nel vanilloid 1-immunoreactive dorsal root ganglion neurons
innervate the adult mouse jejunum and colon. Neuroscience
2008; 156: 334–43.
7 Robinson DR, McNaughton PA, Evans ML, Hicks GA.
Characterization of the primary spinal afferent innervation
of the mouse colon using retrograde labelling. Neurogas-
troenterol Motil 2004; 16: 113–24.
8 Le Pichon CE, Chesler AT. The functional and anatomical
dissection of somatosensory subpopulations using mouse
genetics. Front Neuroanat 2014; 8: 21.
9 Holzer P, Michl T, Danzer M, Jocic M, Schicho R, Lippe IT.
Surveillance of the gastrointestinal mucosa by sensory
neurons. J Physiol Pharmacol 2001; 52: 505–21.
10 Holzer P. Sensory neurone responses to mucosal noxae in
the upper gut: relevance to mucosal integrity and gastroin-