CGRP upregulation in dorsal root ganglia and ileal mucosa during Clostridium difficile toxin A-induced enteritis

Constipation Caused by Anti-calcitonin Gene-Related Peptide Migraine Therapeutics Explained by Antagonism of Calcitonin Gene-Related Peptide's Motor-Stimulating and Prosecretory Function in the Intestine

The development of small-molecule calcitonin gene-related peptide (CGRP) receptor antagonists (gepants) and of monoclonal antibodies targeting the CGRP system has been a major advance in the management of migraine. In the randomized controlled trials before regulatory approval, the safety of these anti-CGRP migraine therapeutics was considered favorable and to stay within the expected profile. Post-approval real-world surveys reveal, however, constipation to be a major adverse event which may affect more than 50% of patients treated with erenumab (an antibody targeting the CGRP receptor), fremanezumab or galcanezumab (antibodies targeting CGRP). In this review article we address the question whether constipation caused by inhibition of CGRP signaling can be mechanistically deduced from the known pharmacological actions and pathophysiological implications of CGRP in the digestive tract. CGRP in the gut is expressed by two distinct neuronal populations: extrinsic primary afferent nerve fibers and distinct neurons of the intrinsic enteric nervous system. In particular, CGRP is a major messenger of enteric sensory neurons which in response to mucosal stimulation activate both ascending excitatory and descending inhibitory neuronal pathways that enable propulsive (peristaltic) motor activity to take place. In addition, CGRP is able to stimulate ion and water secretion into the intestinal lumen. The motor-stimulating and prosecretory actions of CGRP combine in accelerating intestinal transit, an activity profile that has been confirmed by the ability of CGRP to induce diarrhea in mice, dogs and humans. We therefore conclude that the constipation elicited by antibodies targeting CGRP or its receptor results from interference with the physiological function of CGRP in the small and large intestine in which it contributes to the maintenance of peristaltic motor activity, ion and water secretion and intestinal transit.

Two-Hour CGRP Infusion Causes Gastrointestinal Hyperactivity: Possible Relevance for CGRP Antibody Treatment

OBJECTIVE

The monoclonal antibodies against calcitonin gene-related peptide (CGRP) or its receptor are new antimigraine drugs from which many patients already benefit. Very few side effects have been reported from the antibody trials, including very few gastrointestinal (GI) side effects. The current data derive from a double-blind cross-over study of CGRP infusion for 2 hours. We present the GI side effects of the infusion and raise the question if underreporting of GI symptoms in CGRP antibody trials has occurred. We also discuss why constipation may be more likely with CGRP receptor blockade than with CGRP neutralizing antibodies.

METHODS

Thirty healthy volunteers were recruited to receive a 2-hour infusion of CGRP 1.5 µg/minutes on 2 different days. The participants were pretreated with sumatriptan tablets (2 × 50 mg) 1 day and with placebo the other day. During the infusion, the participants were asked about side effects including a detailed description about their GI symptoms. Clinical observations like flatulence, rumbling, and use of bedpan were also noted. After the infusion, the participants filled out a questionnaire about side effects at home until 12-hour after the infusion start. The study was conducted at the Danish Headache Center at Rigshospitalet Glostrup in the period February 2018 to July 2018.

RESULTS

On both study days 93% (27/29 participants) experienced symptoms from the GI system during the infusion. Rumbling, stomach pain, nausea, diarrhea, and an urge to defecate were the most commonly experienced GI side effects. There was no difference in symptoms between placebo and sumatriptan pretreatment.

CONCLUSION

We conclude that a 2-hour infusion of CGRP causes frequent and sometimes severe symptoms from the GI system. The symptoms are not antagonized by sumatriptan. More attention should be paid to constipation as a possible side effect of CGRP receptor antagonists.

Anti-CGRP antibodies block CGRP-induced diarrhea in mice

The multifunctional neuropeptide calcitonin gene-related peptide (CGRP) and its receptor are expressed throughout the gastrointestinal tract. Previous studies have shown that CGRP has roles in intestinal motility, water secretion, and inflammation. Furthermore, animal studies have demonstrated CGRP involvement in diarrhea secondary to C. difficile and food allergies. Diarrhea thus provides a convenient bioassay of CGRP activity in the GI system. In this proof of principle study, we report that prophylactic administration of an anti-CGRP antibody is able to block CGRP-induced diarrhea in mice. As a control, the CGRP-receptor antagonist olcegepant also attenuated the diarrhea response to CGRP. This preclinical study indicates that anti-CGRP antibodies may provide a new preventative therapy for gastrointestinal disorders involving CGRP.

Local injection of dsRNA targeting calcitonin receptor-like receptor (CLR) ameliorates Clostridium difficile toxin A-induced ileitis

Enteritis caused by Clostridium difficile toxin (Tx) is a nosocomial disease of increasing clinical concern, but the local mediators of C. difficile TxA inflammation are unknown. The potent vasodilator calcitonin gene-related peptide mediates neurogenic inflammation via the calcitonin receptor-like receptor (CLR). Here we examined the ileum-specific effects of reducing CLR on TxA ileitis by local preinjection of double-stranded RNAs. Treatment with CLR dsRNA for 7 d decreased CLR immunoreactivity, whereas treatment with non-CLR dsRNA did not. Subsequent injection of TxA in the same location increased CLR in rats treated with non-CLR dsRNA but not in rats treated with CLR dsRNA, documenting that local injection of dsRNA is effective in preventing the increase in CLR immunoreactivity in response to local TxA. After non-CLR dsRNA pretreatment, TxA induced robust intestinal secretion, myeloperoxidase activity, and histopathologic indications of inflammation including epithelial damage, congestion, neutrophil infiltration, loss of mucin from goblet cells, and increase in mast cell numbers. After CLR dsRNA pretreatment, TxA-induced changes in intestinal secretion and histopathologic inflammation were improved, including normal mucin staining and fewer resident mast cells. Loss of CLR prevented TxA-mediated activation of NF-κB and concomitant increases in pERK1/2 and TNF-α mRNA. Locally produced CLR plays a proinflammatory role in TxA ileitis via MAPK signaling and TNF-α. The results reported here strongly suggest that a local injection of dsRNA targeting CLR could be an effective local therapeutic approach at the inflammation site in the treatment of a growing, clinically relevant hospital-acquired disease, C. difficile infection.

Intestinal epithelial stem/progenitor cells are controlled by mucosal afferent nerves

Background

The maintenance of the intestinal epithelium is of great importance for the survival of the organism. A possible nervous control of epithelial cell renewal was studied in rats and mice.

Methods

Mucosal afferent nerves were stimulated by exposing the intestinal mucosa to capsaicin (1.6 mM), which stimulates intestinal external axons. Epithelial cell renewal was investigated in the jejunum by measuring intestinal thymidine kinase (TK) activity, intestinal ³H-thymidine incorporation into DNA, and the number of crypt cells labeled with BrdU. The influence of the external gut innervation was minimized by severing the periarterial nerves.

Principal Findings

Luminal capsaicin increased all the studied variables, an effect nervously mediated to judge from inhibitory effects on TK activity or ³H-thymidine incorporation into DNA by exposing the mucosa to lidocaine (a local anesthetic) or by giving four different neurotransmitter receptor antagonists i.v. (muscarinic, nicotinic, neurokinin1 (NK1) or calcitonin gene related peptide (CGRP) receptors). After degeneration of the intestinal external nerves capsaicin did not increase TK activity, suggesting the involvement of an axon reflex. Intra-arterial infusion of Substance P (SP) or CGRP increased intestinal TK activity, a response abolished by muscarinic receptor blockade. Immunohistochemistry suggested presence of M3 and M5 muscarinic receptors on the intestinal stem/progenitor cells. We propose that the stem/progenitor cells are controlled by cholinergic nerves, which, in turn, are influenced by mucosal afferent neuron(s) releasing acetylcholine and/or SP and/or CGRP. In mice lacking the capsaicin receptor, thymidine incorporation into DNA and number of crypt cells labeled with BrdU was lower than in wild type animals suggesting that nerves are important also in the absence of luminal capsaicin, a conclusion also supported by the observation that atropine lowered thymidine incorporation into DNA by 60% in control rat segments.

Conclusion

Enteric nerves are of importance in maintaining the intestinal epithelial barrier.

The enterotoxicity of Clostridium difficile toxins

The major virulence factors of Clostridium difficile infection (CDI) are two large exotoxins A (TcdA) and B (TcdB). However, our understanding of the specific roles of these toxins in CDI is still evolving. It is now accepted that both toxins are enterotoxic and proinflammatory in the human intestine. Both purified TcdA and TcdB are capable of inducing the pathophysiology of CDI, although most studies have focused on TcdA. C. difficile toxins exert a wide array of biological activities by acting directly on intestinal epithelial cells. Alternatively, the toxins may target immune cells and neurons once the intestinal epithelial barrier is disrupted. The toxins may also act indirectly by stimulating cells to produce chemokines, proinflammatory cytokines, neuropeptides and other neuroimmune signals. This review considers the mechanisms of TcdA- and TcdB-induced enterotoxicity, and recent developments in this field.

Infectious diarrhea: Cellular and molecular mechanisms

Diarrhea caused by enteric infections is a major factor in morbidity and mortality worldwide. An estimated 2-4 billion episodes of infectious diarrhea occur each year and are especially prevalent in infants. This review highlights the cellular and molecular mechanisms underlying diarrhea associated with the three classes of infectious agents, i.e., bacteria, viruses and parasites. Several bacterial pathogens have been chosen as model organisms, including Vibrio cholerae as a classical example of secretory diarrhea, Clostridium difficile and Shigella species as agents of inflammatory diarrhea and selected strains of pathogenic Escherichia coli (E. coli) to discuss the recent advances in alteration of epithelial ion absorption. Many of the recent studies addressing epithelial ion transport and barrier function have been carried out using viruses and parasites. Here, we focus on the rapidly developing field of viral diarrhea including rotavirus, norovirus and astrovirus infections. Finally we discuss Giardia lamblia and Entamoeba histolytica as examples of parasitic diarrhea. Parasites have a greater complexity than the other pathogens and are capable of creating molecules similar to those produced by the host, such as serotonin and PGE(2). The underlying mechanisms of infectious diarrhea discussed include alterations in ion transport and tight junctions as well as the virulence factors, which alter these processes either through direct effects or indirectly through inflammation and neurotransmitters.

Yeast, beef and pork extracts counteract Clostridium difficile toxin A enterotoxicity

Clostridium difficile is responsible for a large proportion of nosocomial cases of antibiotic-associated diarrhoea and pseudomembranous colitis. The present study provides evidence that yeast, beef and pork extracts, ingredients commonly used to grow bacteria, can counteract C. difficile toxin A enterotoxicity in vitro and in vivo. In model intestinal epithelial cells the individual extracts could prevent the toxin A-induced decrease in epithelial barrier function and partially prevented actin disaggregation and cell rounding. Mice with ad libitum access to individual extracts for 1 week had almost complete reduction in toxin A-induced fluid secretion in intestinal loops. Concomitantly, the toxin A-induced expression of the essential proinflammatory mediator Cox-2 was normalized. Moreover this protective effect was also seen when mice received only two doses of extract by intragastric gavage within 1 week. These results show that yeast, beef and pork extracts have the potential to counteract the intestinal pathogenesis triggered by C. difficile toxin A.

Clostridium difficile toxins: mechanism of action and role in disease

As the leading cause of hospital-acquired diarrhea, Clostridium difficile colonizes the large bowel of patients undergoing antibiotic therapy and produces two toxins, which cause notable disease pathologies. These two toxins, TcdA and TcdB, are encoded on a pathogenicity locus along with negative and positive regulators of their expression. Following expression and release from the bacterium, TcdA and TcdB translocate to the cytosol of target cells and inactivate small GTP-binding proteins, which include Rho, Rac, and Cdc42. Inactivation of these substrates occurs through monoglucosylation of a single reactive threonine, which lies within the effector-binding loop and coordinates a divalent cation critical to binding GTP. By glucosylating small GTPases, TcdA and TcdB cause actin condensation and cell rounding, which is followed by death of the cell. TcdA elicits effects primarily within the intestinal epithelium, while TcdB has a broader cell tropism. Important advances in the study of these toxins have been made in the past 15 years, and these are detailed in this review. The domains, subdomains, and residues of these toxins important for receptor binding and enzymatic activity have been elegantly studied and are highlighted herein. Furthermore, there have been major advances in defining the role of these toxins in modulating the inflammatory events involving the disruption of cell junctions, neuronal activation, cytokine production, and infiltration by polymorphonuclear cells. Collectively, the present review provides a comprehensive update on TcdA and TcdB's mechanism of action as well as the role of these toxins in disease.

Neuropeptide neurotensin stimulates intestinal wound healing following chronic intestinal inflammation

Because neurotensin (NT) and its high-affinity receptor (NTR1) modulate immune responses, chloride secretion, and epithelial cell proliferation, we sought to investigate their role in the repair process that follows the development of mucosal injuries during a persistent inflammation. Colonic NT and NTR1, mRNA, and protein significantly increased only after dextran sodium sulfate (DSS)-induced inflammatory damage developed. Colitis-induced body weight loss, colonic myeloperoxidase activity, and histological damage were significantly enhanced by SR-48642 administration, a nonpeptide NTR1 antagonist, whereas continuous NT infusion ameliorated colitis outcome. To evaluate the NT and NTR1 role in tissue healing, mucosal inflammatory injury was established administering 3% DSS for 5 days. After DSS discontinuation, mice rapidly gained weight, ulcers were healed, and colonic NT, NTR1, and cyclooxygenase (COX)-2 mRNA levels were upregulated, whereas SR-48642 treatment caused a further body weight loss, ulcer enlargement, and a blunted colonic COX-2 mRNA upregulation. In a wound-healing model in vitro, NT-induced cell migration in the denuded area was inhibited by indomethacin but not by an antitransforming growth factor-beta neutralizing antibody. Furthermore, NT significantly increased COX-2 mRNA levels by 2.4-fold and stimulated PGE(2) release in HT-29 cells. These findings suggest that NT and NTR1 are part of the network activated after mucosal injuries and that NT stimulates epithelial restitution at least, in part, through a COX-2 dependent pathway.

Vascular Actions of Calcitonin Gene-Related Peptide and Adrenomedullin

This review summarizes the receptor-mediated vascular activities of calcitonin gene-related peptide (CGRP) and the structurally related peptide adrenomedullin (AM). CGRP is a 37-amino acid neuropeptide, primarily released from sensory nerves, whilst AM is produced by stimulated vascular cells, and amylin is secreted from the pancreas. They share vasodilator activity, albeit to varying extents depending on species and tissue. In particular, CGRP has potent activity in the cerebral circulation, which is possibly relevant to the pathology of migraine, whilst vascular sources of AM contribute to dysfunction in cardiovascular disease. Both peptides exhibit potent activity in microvascular beds. All three peptides can act on a family of CGRP receptors that consist of calcitonin receptor-like receptor (CL) linked to one of three receptor activity-modifying proteins (RAMPs) that are essential for functional activity. The association of CL with RAMP1 produces a CGRP receptor, with RAMP2 an AM receptor and with RAMP3 a CGRP/AM receptor. Evidence for the selective activity of the first nonpeptide CGRP antagonist BIBN4096BS for the CGRP receptor is presented. The cardiovascular activity of these peptides in a range of species and in human clinical conditions is detailed, and potential therapeutic applications based on use of antagonists and gene targeting of agonists are discussed.

Changes in urinary bladder cytokine mRNA and protein after cyclophosphamide-induced cystitis

Cyclophosphamide (CYP)-induced cystitis alters micturition function and produces reorganization of the micturition reflex. This reorganization may involve cytokine expression in the urinary bladder. These studies have determined candidate cytokines in the bladder that may contribute to the reorganization process. An RNase protection assay was used to measure changes in rat bladder cytokine mRNA [interferon-gamma (IFN)-gamma, interleukin-1alpha/beta (IL-1alpha/beta), IL-2, IL-3, IL-4, IL-5, IL-6, IL-10, and tumor necrosis factor-alpha/beta (TNF-alpha/beta)] after acute (4 h), intermediate (48 h), or chronic (10 day) cystitis. The correlation between bladder cytokine mRNA and protein expression was also determined by immunoassay. Although at each time point after cystitis significant changes in bladder cytokine mRNA were observed, the magnitude differed (acute > intermediate > chronic). Acute cystitis demonstrated the most robust changes (P </= 0.005; IL-1beta, 330-fold increase; IL-2, 20-fold increase; IL-4, 8-fold increase; IL-6, 80-fold increase) in cytokine mRNA expression and TNF-alpha or TNF-beta mRNA were only increased (2-10-fold) after acute cystitis. More modest increases in cytokine mRNA expression were observed after 48-h or 10-day cystitis. Cytokine protein expression generally paralleled that of mRNA. Increased cytokine expression after CYP-induced cystitis, alone or in combination with other inflammatory mediators or growth factors, may contribute to altered lower urinary tract function after cystitis.

Deletion of neutral endopeptidase exacerbates intestinal inflammation induced by Clostridium difficile toxin A

Toxin A (TxA) of Clostridium difficile induces acute inflammation of the intestine initiated by release of substance P (SP) and activation of the neurokinin-1 receptor. However, the mechanisms that terminate this response are unknown. We determined whether the SP-degrading enzyme neutral endopeptidase (NEP, EC 3.4.24.11) terminates TxA-induced enteritis. We used both genetic deletion and pharmacological inhibition of NEP to test this hypothesis. In wild-type mice, instillation of TxA (0.5-5 microg) into ileal loops for 3 h dose dependently increased ileal fluid secretion, stimulated granulocyte transmigration determined by myeloperoxidase activity, and caused histological damage characterized by depletion of enterocytes, edema, and neutrophil accumulation. Deletion of NEP reduced the threshold secretory and inflammatory dose of TxA and exacerbated the inflammatory responses by more than twofold. This exacerbated inflammation was prevented by pretreatment with recombinant NEP. Conversely, pretreatment of wild-type mice with the NEP inhibitor phosphoramidon exacerbated enteritis. Thus NEP terminates enteritis induced by C. difficile TxA, underlying the importance of SP degradation in limiting neurogenic inflammation.

Endogenous corticosteroids modulate Clostridium difficile toxin A-induced enteritis in rats

We examined the role of glucocorticoids in acute inflammatory diarrhea mediated by Clostridium difficile toxin A. Toxin A (5 microg) or buffer was injected in rat ileal loops, and intestinal responses were measured after 30 min to 4 h. Ileal toxin A administration increased plasma glucocorticoids after 1 h, at which time the toxin-stimulated secretion was not significant. Administration of the glucocorticoid analog dexamethasone inhibited toxin A-induced intestinal secretion and inflammation and downregulated toxin A-mediated increase of macrophage inflammatory protein-2. Adrenalectomy followed by replacement with glucocorticoids at various doses suggested that intestinal responses to toxin A were related to circulating levels of glucocorticoids. Administration of the glucocorticoid receptor antagonist RU-486 enhanced toxin A-mediated intestinal secretion and inflammation. We conclude that C. difficile toxin A causes increased secretion of endogenous glucocorticoids, which diminish the intestinal secretory and inflammatory effects of toxin A.

Microbes and microbial toxins: paradigms for microbial-mucosal interactions II. The integrated response of the intestine to Clostridium difficile toxins

Clostridium difficile, the major etiologic factor of antibiotic-associated diarrhea and colitis, mediates its effects by releasing two large protein exotoxins, toxins A and B. A major toxin effect is related to the disassembly of actin microfilaments, leading to impairment of tight junctions in human colonocytes. The mechanism of actin disaggregation involves monoglucosylation of the signaling proteins Rho A, Rac, and Cdc 42, which control stress fiber formation directly by toxins A and B. An important aspect of C. difficile infection is the acute necroinflammatory changes seen in patients with pseudomembranous colitis. The early mechanism of toxin-mediated inflammation involves toxin effects on cellular mitochondria, release of reactive oxygen species, and activation of mitogen-activated protein kinases and the transcription factor nuclear factor-kappaB. Injection of toxin A into animal intestine triggers secretion of fluid and intestinal inflammation characterized by epithelial cell destruction and neutrophil activation. A critical feature of C. difficile enterotoxicity is communication between enterocytes and lamina propria nerves, macrophages, and mast cells mediated via release of neuropeptides and proinflammatory cytokines.

Neurotensin is a proinflammatory neuropeptide in colonic inflammation

The neuropeptide neurotensin mediates several intestinal functions, including chloride secretion, motility, and cellular growth. However, whether this peptide participates in intestinal inflammation is not known. Toxin A, an enterotoxin from Clostridium difficile, mediates pseudomembranous colitis in humans. In animal models, toxin A causes an acute inflammatory response characterized by activation of sensory neurons and intestinal nerves and immune cells of the lamina propria. Here we show that neurotensin and its receptor are elevated in the rat colonic mucosa following toxin A administration. Pretreatment of rats with the neurotensin receptor antagonist SR-48, 692 inhibits toxin A-induced changes in colonic secretion, mucosal permeability, and histologic damage. Exposure of colonic explants to toxin A or neurotensin causes mast cell degranulation, which is inhibited by SR-48,692. Because substance P was previously shown to mediate mast cell activation, we examined whether substance P is involved in neurotensin-induced mast cell degranulation. Our results show that neurotensin-induced mast cell degranulation in colonic explants is inhibited by the substance P (neurokinin-1) receptor antagonist CP-96,345, indicating that colonic mast activation in response to neurotensin involves release of substance P. We conclude that neurotensin plays a key role in the pathogenesis of C. difficile-induced colonic inflammation and mast cell activation.

Clostridium difficile Toxin A Stimulates Macrophage- Inflammatory Protein-2 Production in Rat Intestinal Epithelial Cells

Neutrophil infiltration of the colonic mucosa is a hallmark of Clostridium difficile toxin A-mediated enterocolitis. Macrophage-inflammatory protein-2 (MIP-2) is a potent neutrophil chemoattractant secreted by rat macrophages and epithelial cells in response to inflammatory stimuli. In this work, we report that administration of toxin A into rat ileal loops increased mucosal levels of MIP-2 before the onset of fluid secretion and mucosal neutrophil infiltration. Administration of rabbit anti-MIP-2 IgG, but not control IgG, reduced toxin A-mediated secretion (by 58%), mucosal permeability (by 80%), and myeloperoxidase activity (by 85%). Immunohistochemical analysis demonstrated increased MIP-2 expression in intestinal epithelial and lamina propria cells 1 h after toxin A administration. Intestinal epithelial cells purified from toxin A-exposed ileal loops also showed increased MIP-2 mRNA expression and MIP-2 protein release that was inhibited by pretreatment of rats with the transcriptional inhibitor actinomycin D. These results indicate that C. difficile toxin A induces MIP-2 release from intestinal epithelial cells and that MIP-2 contributes to neutrophil mucosal influx during toxin A enteritis.