Intestinal Enteroids Model Guanylate Cyclase C-Dependent Secretion Induced by Heat-Stable Enterotoxins

Advancements in understanding bacterial enteritis pathogenesis through organoids

Bacterial enteritis has a substantial role in contributing to a large portion of the global disease burden and serves as a major cause of newborn mortality. Despite advancements gained from current animal and cell models in improving our understanding of pathogens, their widespread application is hindered by apparent drawbacks. Therefore, more precise models are imperatively required to develop more accurate studies on host-pathogen interactions and drug discovery. Since the emergence of intestinal organoids, massive studies utilizing organoids have been conducted to study the pathogenesis of bacterial enteritis, revealing new mechanisms and validating established ones. In this review, we focus on the advancements of several bacterial pathogenesis mechanisms observed in intestinal organoid/enteroid models, exploring the host response and bacterial effectors during the infection process. Finally, we address the features that warrant additional investigation or could be enhanced in existing organoid models in order to guide future research endeavors.

GUCY2C signaling limits dopaminergic neuron vulnerability to toxic insults

Mitochondrial dysfunction and reactive oxygen species (ROS) accumulation within the substantia nigra pars compacta (SNpc) are central drivers of dopaminergic (DA) neuron death in Parkinson's disease (PD). Guanylyl cyclases and their second messenger cyclic (c)GMP support mitochondrial function, protecting against ROS and promoting cell survival in several tissues. However, the role of the guanylyl cyclase-cGMP axis in defining the vulnerability of DA neurons in the SNpc in PD remains unclear, in part due to the challenge of manipulating cGMP levels selectively in midbrain DA neurons. In that context, guanylyl cyclase C (GUCY2C), a receptor primarily expressed by intestinal epithelial cells, was discovered recently in midbrain DA neurons. Here, we demonstrate that GUCY2C promotes mitochondrial function, reducing oxidative stress and protecting DA neurons from degeneration in the 1-methyl-4-phenyl- 1,2,3,6-tetrahydropyridine (MPTP) mouse model. GUCY2C is overexpressed in the SNpc in PD patients and in mice treated with MPTP, possibly reflecting a protective response to oxidative stress. Moreover, cGMP signaling protects against oxidative stress, mitochondrial impairment, and cell death in cultured DA neurons. These observations reveal a previously unexpected role for the GUCY2C-cGMP signaling axis in controlling mitochondrial dysfunction and toxicity in SNpc DA neurons, highlighting the therapeutic potential of targeting DA neuron GUCY2C to prevent neurodegeneration in PD.

Microbiota-derived butyrate dampens linaclotide stimulation of the guanylate cyclase C pathway in patient-derived colonoids

BACKGROUND & AIMS

Disorders of gut-brain interaction (DGBI) are complex conditions that result in decreased quality of life and a significant cost burden. Linaclotide, a guanylin cyclase C (GCC) receptor agonist, is approved as a DGBI treatment. However, its efficacy has been limited and variable across DGBI patients. Microbiota and metabolomic alterations are noted in DGBI patients, provoking the hypothesis that the microbiota may impact the GCC response to current therapeutics.

METHODS

Human-derived intestinal organoids were grown from pediatric DGBI, non-IBD colon biopsies (colonoids). Colonoids were treated with 250 nM linaclotide and assayed for cGMP to develop a model of GCC activity. Butyrate was administered to human colonoids overnight at a concentration of 1 mM. Colonoid lysates were analyzed for cGMP levels by ELISA. For the swelling assay, colonoids were photographed pre- and post-treatment and volume was measured using ImageJ. Principal coordinate analyses (PCoA) were performed on the Bray-Curtis dissimilarity and Jaccard distance to assess differences in the community composition of short-chain fatty acid (SCFA) producing microbial species in the intestinal microbiota from pediatric patients with IBS and healthy control samples.

KEY RESULTS

Linaclotide treatment induced a significant increase in [cGMP] and swelling of patient-derived colonoids, demonstrating a human in vitro model of linaclotide-induced GCC activation. Shotgun sequencing analysis of pediatric IBS patients and healthy controls showed differences in the composition of commensal SCFA-producing bacteria. Butyrate exposure significantly dampened linaclotide-induced cGMP levels and swelling in patient-derived colonoids.

CONCLUSIONS & INFERENCES

Patient-derived colonoids demonstrate that microbiota-derived butyrate can dampen human colonic responses to linaclotide. This study supports incorporation of microbiota and metabolomic assessment to improve precision medicine for DGBI patients.

GUCY2C signaling limits dopaminergic neuron vulnerability to toxic insults

Mitochondrial dysfunction and reactive oxygen species (ROS) accumulation within the substantia nigra pars compacta (SNpc) are central drivers of dopaminergic (DA) neuron death in Parkinson's disease (PD). Guanylyl cyclases, and their second messengers cyclic (c)GMP, support mitochondrial function, protecting against ROS and promoting cell survival in a number of tissues. However, the role of the guanylyl cyclase-cGMP axis in defining the vulnerability of DA neurons in the SNpc in PD remains unclear, in part due to the challenge of manipulating cGMP levels selectively in midbrain DA neurons. In that context, guanylyl cyclase C (GUCY2C), a receptor primarily expressed by intestinal epithelial cells, was discovered recently in midbrain DA neurons. Here, we demonstrate that GUCY2C promotes mitochondrial function, reducing oxidative stress and protecting DA neurons from degeneration in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of neurodegeneration. GUCY2C is overexpressed in the SNpc in PD patients and in mice treated with MPTP, possibly reflecting a protective response to oxidative stress. Moreover, cGMP signaling protects against oxidative stress, mitochondrial impairment, and cell death in cultured DA neurons. These observations reveal a previously unexpected role for the GUCY2C-cGMP signaling axis in controlling mitochondrial dysfunction and toxicity in nigral DA neurons, highlighting the therapeutic potential of targeting DA neuron GUCY2C to prevent neurodegeneration in PD.

Human intestinal organoids as models to study enteric bacteria and viruses

Laboratory studies of host-microbe interactions have historically been carried out using transformed cell lines and animal models. Although much has been learned from these models, recent advances in the development of multicellular, physiologically active, human intestinal organoid (HIO) cultures are allowing unprecedented discoveries of host-microbe interactions. Here, we review recent literature using HIOs as models to investigate the pathogenesis of clinically important enteric bacteria and viruses and study commensal intestinal microbes. We also discuss limitations of current HIO culture systems and how technical advances and innovative engineering approaches are providing new directions to improve the model. The studies discussed here highlight the potential of HIOs for studying microbial pathogenesis, host-microbe interactions, and for preclinical development of therapeutics and vaccines.

Intestinal neuropod cell GUCY2C regulates visceral pain

Visceral pain (VP) is a global problem with complex etiologies and limited therapeutic options. Guanylyl cyclase C (GUCY2C), an intestinal receptor producing cyclic GMP(cGMP), which regulates luminal fluid secretion, has emerged as a therapeutic target for VP. Indeed, FDA-approved GUCY2C agonists ameliorate VP in patients with chronic constipation syndromes, although analgesic mechanisms remain obscure. Here, we revealed that intestinal GUCY2C was selectively enriched in neuropod cells, a type of enteroendocrine cell that synapses with submucosal neurons in mice and humans. GUCY2Chi neuropod cells associated with cocultured dorsal root ganglia neurons and induced hyperexcitability, reducing the rheobase and increasing the resulting number of evoked action potentials. Conversely, the GUCY2C agonist linaclotide eliminated neuronal hyperexcitability produced by GUCY2C-sufficient - but not GUCY2C-deficient - neuropod cells, an effect independent of bulk epithelial cells or extracellular cGMP. Genetic elimination of intestinal GUCY2C amplified nociceptive signaling in VP that was comparable with chemically induced VP but refractory to linaclotide. Importantly, eliminating GUCY2C selectively in neuropod cells also increased nociceptive signaling and VP that was refractory to linaclotide. In the context of loss of GUCY2C hormones in patients with VP, these observations suggest a specific role for neuropod GUCY2C signaling in the pathophysiology and treatment of these pain syndromes.

Guanylyl cyclase C as a diagnostic and therapeutic target in colorectal cancer

Colorectal cancer remains a major cause of mortality in the USA, despite advances in prevention and screening. Existing therapies focus primarily on generic treatment such as surgical intervention and chemotherapy, depending on disease severity. As personalized medicine and targeted molecular oncology continue to develop as promising treatment avenues, there has emerged a need for effective targets and biomarkers of colorectal cancer. The transmembrane receptor guanylyl cyclase C (GUCY2C) regulates intestinal homeostasis and has emerged as a tumor suppressor. Further, it is universally expressed in advanced metastatic colorectal tumors, as well as other cancer types that arise through intestinal metaplasia. In this context, GUCY2C satisfies many characteristics of a compelling target and biomarker for gastrointestinal malignancies.

Enterotoxigenic Escherichia coli enterotoxins regulate epithelial to immune relay of IL-33 and IL-1Ra cytokines

Enterotoxigenic Escherichia coli (ETEC) remain a major cause of diarrheal mortality and morbidity in children in low-resource settings. Few studies have explored the consequences of simultaneous intoxication with heat-stable (ST) and heat-labile (LT) enterotoxins despite the increased prevalence of wild ETEC isolates expressing both toxins. We therefore used a combination of tissue culture and murine models to explore the impact of simultaneous ST+LT intoxication of epithelial and myeloid cell responses. We report that LT induces sustained IL-33 and IL-1Ra responses in T84 intestinal epithelial cells via cAMP-production and protein kinase A activation. We demonstrate that combined ST+LT intoxication hastens epithelial transcriptional responses induced more slowly by LT alone. ST- and LT-mediated luminal fluid accumulation in vivo correlates with significant increases in IL-33 and IL-1Ra in small intestinal mucosal scrapings. Additionally, IL-33 receptor (IL-33R)-deficient mice are less susceptible to ST-mediated secretion. In the immune compartment, IL-33 is sensed by myeloid cells, and LT suppresses IL-33-induced TNFα secretion from macrophages but amplifies IL-33-mediated induction of IL-6 from bone marrow-derived dendritic cells. In conclusion, our studies suggest that enterotoxin-induced IL-33 and IL-1Ra modulate intestinal inflammation and IL-1 receptor signaling in the intestinal mucosa in response to ETEC enterotoxins.

Targeting gastrointestinal cancers with chimeric antigen receptor (CAR)-T cell therapy

The immune system is capable of remarkably potent and specific efficacy against infectious diseases. For decades, investigators sought to leverage those characteristics to create immune-based therapies (immunotherapy) that might be far more effective and less toxic than conventional chemotherapy and radiation therapy for cancer. Those studies revealed many factors and mechanisms underlying the success or failure of cancer immunotherapy, leading to synthetic biology approaches, including CAR-T cell therapy. In this approach, patient T cells are genetically modified to express a chimeric antigen receptor (CAR) that converts T cells of any specificity into tumor-specific T cells that can be expanded to large numbers and readministered to the patient to eliminate cancer cells, including bulky metastatic disease. This approach has been most successful against hematologic cancers, resulting in five FDA approvals to date. Here, we discuss some of the most promising attempts to apply this technology to cancers of the gastrointestinal tract.

Diarrheal pathogens trigger rapid evolution of the guanylate cyclase-C signaling axis in bats

The pathogenesis of infectious diarrheal diseases is largely attributed to enterotoxins that cause dehydration by disrupting intestinal water absorption. We investigated patterns of genetic variation in mammalian guanylate cyclase-C (GC-C), an intestinal receptor targeted by bacterially encoded heat-stable enterotoxins (STa), to determine how host species adapt in response to diarrheal infections. Our phylogenetic and functional analysis of GC-C supports long-standing evolutionary conflict with diarrheal bacteria in primates and bats, with highly variable susceptibility to STa across species. In bats, we further show that GC-C diversification has sparked compensatory mutations in the endogenous uroguanylin ligand, suggesting an unusual scenario of pathogen-driven evolution of an entire signaling axis. Together, these findings suggest that conflicts with diarrheal pathogens have had far-reaching impacts on the evolution of mammalian gut physiology.

Porcine small intestinal organoids as a model to explore ETEC-host interactions in the gut

Small intestinal organoids, or enteroids, represent a valuable model to study host–pathogen interactions at the intestinal epithelial surface. Much research has been done on murine and human enteroids, however only a handful studies evaluated the development of enteroids in other species. Porcine enteroid cultures have been described, but little is known about their functional responses to specific pathogens or their associated virulence factors. Here, we report that porcine enteroids respond in a similar manner as in vivo gut tissues to enterotoxins derived from enterotoxigenic Escherichia coli, an enteric pathogen causing postweaning diarrhoea in piglets. Upon enterotoxin stimulation, these enteroids not only display a dysregulated electrolyte and water balance as shown by their swelling, but also secrete inflammation markers. Porcine enteroids grown as a 2D-monolayer supported the adhesion of an F4⁺ ETEC strain. Hence, these enteroids closely mimic in vivo intestinal epithelial responses to gut pathogens and are a promising model to study host–pathogen interactions in the pig gut. Insights obtained with this model might accelerate the design of veterinary therapeutics aimed at improving gut health.

[Changes of guanylate cyclase C in colon tissues of rats with intestinal injury associated with severe acute pancreatitis]

OBJECTIVE

To explore the dynamic changes of guanylate cyclase C (GC-C) in the colon tissues of rats with intestinal injury associated with severe acute pancreatitis (SAP).

OBJECTIVE

Thirty-six SD rats were randomized equally into two groups to receive either sham operation or retrograde pumping of 5% sodium taurocholate (0.1 mL/100 g) into the pancreaticobiliary duct following laparotomy to induce SAP. At 12, 24, and 48 h after modeling, 6 rats from each group were euthanized and the colon tissues were collected for Western blotting, immunohistochemistry and RT-PCR to determine the changes in GC-C expression, and the lowest GC-C expression was deemed to indicate the most serious intestinal injury and the time window for intervention. Another 18 SD rats were randomized into 3 groups for sham operation, SAP modeling or intragastric administration of linaclotide (a GC-C agonist) solution once daily at the dose of 10 μg/kg. At 12 h after modeling, the pathological changes in the pancreas and colon were observed with HE staining; the serum level of AMY, DAO, D-Lac and TNF-α were measured with ELISA, and the expressions of GC-C and claudin-1 were detected using Western blotting, immunohistochemical and transmission electron microscopy.

OBJECTIVE

The expression of GC-C was significantly reduced in the colon of rats in SAP group, and its lowest expression occurred at 12 h after modeling (P < 0.05) followed by gradual increase over time. Claudin-1 showed a similar trend in the colon. Compared with the sham-operated rats, the rats in SAP and Linaclotide groups showed significantly increased pathological scores of the colon tissues (P < 0.05) and serum levels of AMY, DAO, D-Lac and TNF-α and decreased expressions of GC-C and claudin-1 in the colon (P < 0.05). Compared with those in SAP group, the rats in linaclotide group had significantly lower colonic histopathological scores, lower serum levels of AMY, DAO, D-Lac and TNF-α, and higher expression levels of GC-C and claudin-1 in the colon tissue.

OBJECTIVE

In rats with SAP-related intestinal injury, the expression of GC-C in the colon tissue decreases to the lowest level at 12 h after SAP onset followed by gradual increase. activating GC-C can increase the expression levels of GC-C and claudin-1 and alleviate intestinal injury, suggesting the role of GC-C in maintaining intestinal barrier integrity by regulating the expression of tight junction proteins.

Heat-stable enterotoxin inhibits intestinal stem cell expansion to disrupt the intestinal integrity by downregulating the Wnt/β-catenin pathway

Enterotoxigenic Escherichia coli causes severe infectious diarrhea with high morbidity and mortality in newborn and weanling pigs mainly through the production of heat-stable enterotoxins (STs). However, the precise regulatory mechanisms involved in ST-induced intestinal epithelium injury remain unclear. Consequently, we conducted the experiments in vivo (mice), ex vivo (mouse and porcine enteroids), and in vitro (MODE-K and IPEC-J2 cells) to explore the effect of STp (one type of STa) on the integrity of the intestinal epithelium. The results showed that acute STp exposure led to small intestinal edema, disrupted intestinal integrity, induced crypt cell expansion into spheroids, and downregulated Wnt/β-catenin activity in the mice. Following a similar trend, the enteroid-budding efficiency and the expression of Active β-catenin, β-catenin, Lgr5, PCNA, and KRT20 were significantly decreased after STp treatment, as determined ex vivo. In addition, STp inhibited cell proliferation, induced cell apoptosis, destroyed cell barriers, and reduced Wnt/β-catenin activity by downregulating its membrane receptor Frizzled7 (FZD7). In contrast, Wnt/β-catenin reactivation protected the IPEC-J2 cells from STp-induced injury. Taking these findings together, we conclude that STp inhibits intestinal stem cell expansion to disrupt the integrity of the intestinal mucosa through the downregulation of the Wnt/β-catenin signaling pathway.

An update on guanylyl cyclase C in the diagnosis, chemoprevention, and treatment of colorectal cancer

Introduction: Colorectal cancer remains the second leading cause of cancer death in the United States, underscoring the need for novel therapies. Despite the successes of new targeted agents for other cancers, colorectal cancer suffers from a relative scarcity of actionable biomarkers. In this context, the intestinal receptor, guanylyl cyclase C (GUCY2C), has emerged as a promising target.Areas covered: GUCY2C regulates a tumor-suppressive signaling axis that is silenced through loss of its endogenous ligands at the earliest stages of tumorigenesis. A body of literature supports a cancer chemoprevention strategy involving reactivation of GUCY2C through FDA-approved cGMP-elevating agents such as linaclotide, plecanatide, and sildenafil. Its limited expression in extra-intestinal tissues, and retention on the surface of cancer cells, also positions GUCY2C as a target for immunotherapies to treat metastatic disease, including vaccines, chimeric antigen receptor T-cells, and antibody-drug conjugates. Likewise, GUCY2C mRNA identifies metastatic cells, enhancing colorectal cancer detection, and staging. Pre-clinical and clinical programs exploring these GUCY2C-targeting strategies will be reviewed.Expert opinion: Recent mechanistic insights characterizing GUCY2C ligand loss early in tumorigenesis, coupled with results from the first clinical trials testing GUCY2C-targeting strategies, continue to elevate GUCY2C as an ideal target for prevention, detection, and therapy.

Stem cell-derived enteroid cultures as a tool for dissecting host-parasite interactions in the small intestinal epithelium

Toxoplasma gondii and Cryptosporidium spp. can cause devastating pathological effects in humans and livestock, and in particular to young or immunocompromised individuals. The current treatment plans for these enteric parasites are limited due to long drug courses, severe side effects or simply a lack of efficacy. The study of the early interactions between the parasites and the site of infection in the small intestinal epithelium has been thwarted by the lack of accessible, physiologically relevant and species-specific models. Increasingly, 3D stem cell-derived enteroid models are being refined and developed into sophisticated models of infectious disease. In this review, we shall illustrate the use of enteroids to spearhead research into enteric parasitic infections, bridging the gap between cell line cultures and in vivo experiments.

Human Intestinal Enteroids With Inducible Neurogenin-3 Expression as a Novel Model of Gut Hormone Secretion

BACKGROUND & AIMS

Enteroendocrine cells (EECs) are specialized epithelial cells that produce molecules vital for intestinal homeostasis, but because of their limited numbers, in-depth functional studies have remained challenging. Human intestinal enteroids (HIEs) that are derived from intestinal crypt stem cells are biologically relevant in an in vitro model of the intestinal epithelium. HIEs contain all intestinal epithelial cell types; however, similar to the intestine, HIEs spontaneously produce few EECs, which limits their study.

METHODS

To increase the number of EECs in HIEs, we used lentivirus transduction to stably engineer jejunal HIEs with doxycycline-inducible expression of neurogenin-3 (NGN3), a transcription factor that drives EEC differentiation (tetNGN3-HIEs). We examined the impact of NGN3 induction on EECs by quantifying the increase in the enterochromaffin cells and other EEC subtypes. We functionally assessed secretion of serotonin and EEC hormones in response to norepinephrine and rotavirus infection.

RESULTS

Treating tetNGN3-HIEs with doxycycline induced a dose-dependent increase of chromogranin A (ChgA)-positive and serotonin-positive cells, showing increased enterochromaffin cell differentiation. Despite increased ChgA-positive cells, other differentiated cell types of the epithelium remained largely unchanged by gene expression and immunostaining. RNA sequencing of doxycycline-induced tetNGN3-HIEs identified increased expression of key hormones and enzymes associated with several other EEC subtypes. Doxycycline-induced tetNGN3-HIEs secreted serotonin, monocyte chemoattractant protein-1, glucose-dependent insulinotropic peptide, peptide YY, and ghrelin in response to norepinephrine and rotavirus infection, further supporting the presence of multiple EEC types.

CONCLUSIONS

We have combined HIEs and inducible-NGN3 expression to establish a flexible in vitro model system for functional studies of EECs in enteroids and advance the molecular and physiological investigation of EECs.

Linaclotide improves gastrointestinal transit in cystic fibrosis mice by inhibiting sodium/hydrogen exchanger 3

Gastrointestinal dysfunction in cystic fibrosis (CF) is a prominent source of pain among patients with CF. Linaclotide, a guanylate cyclase C (GCC) receptor agonist, is a US Food and Drug Administration-approved drug prescribed for chronic constipation but has not been widely used in CF, as the cystic fibrosis transmembrane conductance regulator (CFTR) is the main mechanism of action. However, anecdotal clinical evidence suggests that linaclotide may be effective for treating some gastrointestinal symptoms in CF. The goal of this study was to determine the effectiveness and mechanism of linaclotide in treating CF gastrointestinal disorders using CF mouse models. Intestinal transit, chloride secretion, and intestinal lumen fluidity were assessed in wild-type and CF mouse models in response to linaclotide. CFTR and sodium/hydrogen exchanger 3 (NHE3) response to linaclotide was also evaluated. Linaclotide treatment improved intestinal transit in mice carrying either F508del or null Cftr mutations but did not induce detectable Cl^- secretion. Linaclotide increased fluid retention and fluidity of CF intestinal contents, suggesting inhibition of fluid absorption. Targeted inhibition of sodium absorption by the NHE3 inhibitor tenapanor produced improvements in gastrointestinal transit similar to those produced by linaclotide treatment, suggesting that inhibition of fluid absorption by linaclotide contributes to improved gastrointestinal transit in CF. Our results demonstrate that linaclotide improves gastrointestinal transit in CF mouse models by increasing luminal fluidity through inhibiting NHE3-mediated sodium absorption. Further studies are necessary to assess whether linaclotide could improve CF intestinal pathologies in patients. GCC signaling and NHE3 inhibition may be therapeutic targets for CF intestinal manifestations. NEW & NOTEWORTHY Linaclotide's primary mechanism of action in alleviating chronic constipation is through cystic fibrosis transmembrane conductance regulator (CFTR), negating its use in patients with cystic fibrosis (CF). For the first time, our findings suggest that in the absence of CFTR, linaclotide can improve fluidity of the intestinal lumen through the inhibition of sodium/hydrogen exchanger 3. These findings suggest that linaclotide could improve CF intestinal pathologies in patients.

The Guanylate Cyclase C-cGMP Signaling Axis Opposes Intestinal Epithelial Injury and Neoplasia

Guanylate cyclase C (GUCY2C) is a transmembrane receptor expressed on the luminal aspect of the intestinal epithelium. Its ligands include bacterial heat-stable enterotoxins responsible for traveler's diarrhea, the endogenous peptide hormones uroguanylin and guanylin, and the synthetic agents, linaclotide, plecanatide, and dolcanatide. Ligand-activated GUCY2C catalyzes the synthesis of intracellular cyclic GMP (cGMP), initiating signaling cascades underlying homeostasis of the intestinal epithelium. Mouse models of GUCY2C ablation, and recently, human populations harboring GUCY2C mutations, have revealed the diverse contributions of this signaling axis to epithelial health, including regulating fluid secretion, microbiome composition, intestinal barrier integrity, epithelial renewal, cell cycle progression, responses to DNA damage, epithelial-mesenchymal cross-talk, cell migration, and cellular metabolic status. Because of these wide-ranging roles, dysregulation of the GUCY2C-cGMP signaling axis has been implicated in the pathogenesis of bowel transit disorders, inflammatory bowel disease, and colorectal cancer. This review explores the current understanding of cGMP signaling in the intestinal epithelium and mechanisms by which it opposes intestinal injury. Particular focus will be applied to its emerging role in tumor suppression. In colorectal tumors, endogenous GUCY2C ligand expression is lost by a yet undefined mechanism conserved in mice and humans. Further, reconstitution of GUCY2C signaling through genetic or oral ligand replacement opposes tumorigenesis in mice. Taken together, these findings suggest an intriguing hypothesis that colorectal cancer arises in a microenvironment of functional GUCY2C inactivation, which can be repaired by oral ligand replacement. Hence, the GUCY2C signaling axis represents a novel therapeutic target for preventing colorectal cancer.

Guanylate cyclase-C as a therapeutic target in gastrointestinal disorders

Functional gastrointestinal disorders (FGIDs) and IBDs are two of the most prevalent disorders of the GI tract and consume a significant proportion of healthcare resources. Recent studies have shown that membrane-bound guanylate cyclase-C (GC-C) receptors lining the GI tract may serve as novel therapeutic targets in the treatment of FGIDs and IBDs. GC-C receptor activation by its endogenous paracrine hormones uroguanylin and guanylin, and the resulting intracellular production of its downstream effector cyclic GMP, occurs in a pH-dependent manner and modulates key physiological functions. These include fluid and electrolyte homeostasis, maintenance of the intestinal barrier, anti-inflammatory activity and regulation of epithelial regeneration. Studies of the GC-C paracrine signalling axis have revealed the therapeutic potential of these receptors in treating GI disorders, including chronic idiopathic constipation and irritable bowel syndrome-constipation. This review focuses on the evolving understanding of GC-C function in health and disease, and strategies for translating these principles into new treatments for FGIDs and IBDs.

SLC6A14, an amino acid transporter, modifies the primary CF defect in fluid secretion

The severity of intestinal disease associated with Cystic Fibrosis (CF) is variable in the patient population and this variability is partially conferred by the influence of modifier genes. Genome-wide association studies have identified SLC6A14, an electrogenic amino acid transporter, as a genetic modifier of CF-associated meconium ileus. The purpose of the current work was to determine the biological role of Slc6a14, by disrupting its expression in CF mice bearing the major mutation, F508del. We found that disruption of Slc6a14 worsened the intestinal fluid secretion defect, characteristic of these mice. In vitro studies of mouse intestinal organoids revealed that exacerbation of the primary defect was associated with reduced arginine uptake across the apical membrane, with aberrant nitric oxide and cyclic GMP-mediated regulation of the major CF-causing mutant protein. Together, these studies highlight the role of this apical transporter in modifying cellular nitric oxide levels, residual function of the major CF mutant and potentially, its promise as a therapeutic target.

A G542X cystic fibrosis mouse model for examining nonsense mutation directed therapies

Nonsense mutations are present in 10% of patients with CF, produce a premature termination codon in CFTR mRNA causing early termination of translation, and lead to lack of CFTR function. There are no currently available animal models which contain a nonsense mutation in the endogenous Cftr locus that can be utilized to test nonsense mutation therapies. In this study, we create a CF mouse model carrying the G542X nonsense mutation in Cftr using CRISPR/Cas9 gene editing. The G542X mouse model has reduced Cftr mRNA levels, demonstrates absence of CFTR function, and displays characteristic manifestations of CF mice such as reduced growth and intestinal obstruction. Importantly, CFTR restoration is observed in G542X intestinal organoids treated with G418, an aminoglycoside with translational readthrough capabilities. The G542X mouse model provides an invaluable resource for the identification of potential therapies of CF nonsense mutations as well as the assessment of in vivo effectiveness of these potential therapies targeting nonsense mutations.

Linaclotide activates guanylate cyclase-C/cGMP/protein kinase-II-dependent trafficking of CFTR in the intestine

The transmembrane receptor guanylyl cyclase‐C (GC‐C), expressed on enterocytes along the intestine, is the molecular target of the GC‐C agonist peptide linaclotide, an FDA‐approved drug for treatment of adult patients with Irritable Bowel Syndrome with Constipation and Chronic Idiopathic Constipation. Polarized human colonic intestinal cells (T84, CaCo‐2BBe) rat and human intestinal tissues were employed to examine cellular signaling and cystic fibrosis transmembrane conductance regulator (CFTR)‐trafficking pathways activated by linaclotide using confocal microscopy, in vivo surface biotinylation, and protein kinase‐II (PKG‐II) activity assays. Expression and activity of GC‐C/cGMP pathway components were determined by PCR, western blot, and cGMP assays. Fluid secretion as a marker of CFTR cell surface translocation was determined using in vivo rat intestinal loops. Linaclotide treatment (30 min) induced robust fluid secretion and translocation of CFTR from subapical compartments to the cell surface in rat intestinal loops. Similarly, linaclotide treatment (30 min) of T84 and CaCo‐2BBe cells increased cell surface CFTR levels. Linaclotide‐induced activation of the GC‐C/cGMP/PKGII signaling pathway resulted in elevated intracellular cGMP and pVASP^ser239 phosphorylation. Inhibition or silencing of PKGII significantly attenuated linaclotide‐induced CFTR trafficking to the apical membrane. Inhibition of protein kinase‐A (PKA) also attenuated linaclotide‐induced CFTR cell surface trafficking, implying cGMP‐dependent cross‐activation of PKA pathway. Together, these findings support linaclotide‐induced activation of the GC‐C/cGMP/PKG‐II/CFTR pathway as the major pathway of linaclotide‐mediated intestinal fluid secretion, and that linaclotide‐dependent CFTR activation and recruitment/trafficking of CFTR from subapical vesicles to the cell surface is an important step in this process.

Acidosis potentiates endothelium-dependent vasorelaxation and gap junction communication in the superior mesenteric artery

Extracellular pH is an important physiological determinant of vascular tone that is normally maintained within 7.35-7.45. Any change outside this range leads to severe pathological repercussions. We investigated the unknown effects of extracellular acidosis on relaxation in the superior mesenteric artery (SMA) of goat. SMA rings were employed to maintain isometric contractions at extracellular pH (pH_o) 7.4 and 6.8. We analyzed the effect of acidosis (pH_o 6.8) compared to physiological pH (pH_o 7.4) on three signaling mediators of endothelium-dependent hyperpolarization: nitric oxide (NO), prostaglandin I₂ (PGI₂), and myoendothelial gap junctions (MEGJ). NO and cyclic guanosine monophosphate (cGMP) levels were compared between normal and acidic pH. Quantitative real-time PCR (qPCR) studies determined the change in expression of vascular connexin (Cx), Cx37, Cx40, and Cx43. Under acidosis, acetyl choline-induced relaxation was augmented in an endothelium-dependent manner via eNOS-NO-cGMP signaling. Conversely, at normal pH, acetyl choline-induced vasorelaxation was mediated primarily via COX-PGI₂ pathway. The functional activity of MEGJ was increased under acidosis as evident from increased sensitivity of connexin blockers and upregulated gene and protein expression of connexins. In conclusion, acetyl choline-induced augmented vasorelaxation under acidosis is mediated by NOS-NO-cGMP, with a partial role of MEGJ as EDH mediators in the SMA. Present data suggest a novel role of connexin as therapeutic targets to attenuate the detrimental effect of acidosis on vascular tone.

Engineered Human Gastrointestinal Cultures to Study the Microbiome and Infectious Diseases

New models to study the intestine are key to understanding intestinal diseases and developing novel treatments. Intestinal organ-like culture systems (organoids and enteroids) have substantially advanced the study of the human gastrointestinal tract. Stem cell-derived cultures produce self-organizing structures that contain the multiple differentiated intestinal epithelial cell types including enterocytes, goblet, Paneth, and enteroendocrine cells. Understanding host-microbial interactions is one area in which these cultures are expediting major advancements. This review discusses how organoid and enteroid cultures are biologically and physiologically relevant systems to investigate the effects of commensal organisms and study the pathogenesis of human infectious diseases. These cultures can be established from many donors and they retain the genetic and biologic properties of the donors, which can lead to the discovery of host-specific factors that affect susceptibility to infection and result in personalized approaches to treat individuals. The continued development of these cultures to incorporate more facets of the gastrointestinal tract, including neurons, immune cells, and the microbiome, will unravel new mechanisms regulating host-microbial interactions with the long-term goal of translating findings into novel preventive or therapeutic treatments for gastrointestinal infections.

Randomized clinical trial: efficacy and safety of plecanatide in the treatment of chronic idiopathic constipation

BACKGROUND

Plecanatide, with the exception of a single amino acid replacement, is identical to human uroguanylin and is approved in the United States for adults with chronic idiopathic constipation (CIC). This double-blind, placebo-controlled, phase III study evaluated the efficacy and safety of plecanatide versus placebo in CIC.

METHODS

Adults meeting modified Rome III CIC criteria were randomized to plecanatide 3 mg (n = 443), 6 mg (n = 449), or placebo (n = 445). Patients recorded bowel movement (BM) characteristics [including spontaneous BMs (SBMs) and complete SBMs (CSBMs)] and rated CIC symptoms in daily electronic diaries. The primary endpoint was the percentage of durable overall CSBM responders (weekly responders for ⩾9 of 12 treatment weeks, including ⩾3 of the last 4 weeks). Weekly responders had ⩾3 CSBMs/week and an increase of ⩾1 CSBM from baseline for the same week.

RESULTS

A significantly greater percentage of durable overall CSBM responders resulted with each plecanatide dose compared with placebo (3 mg = 20.1%; 6 mg = 20.0%; placebo = 12.8%; p = 0.004 each dose). Over the 12 weeks, plecanatide significantly improved stool consistency and stool frequency. Significant increases in mean weekly SBMs and CSBMs began in week 1 and were maintained through week 12 in plecanatide-treated patients. Adverse events were mostly mild/moderate, with diarrhea being the most common (3 mg = 3.2%; 6 mg = 4.5%; placebo = 1.3%).

CONCLUSIONS

Plecanatide resulted in a significantly greater percentage of durable overall CSBM responders and improved stool frequency and secondary endpoints. Plecanatide was well tolerated; the most common AE, diarrhea, occurred in a small number of patients.[ClinicalTrials.gov identifier: NCT02122471].