Modulation of specific intestinal epithelial progenitors by enteric neurons

GLP-2 regulation of intestinal lipid handling

Lipid handling in the intestine is important for maintaining energy homeostasis and overall health. Mishandling of lipids in the intestine contributes to dyslipidemia and atherosclerotic cardiovascular diseases. Despite advances in this field over the past few decades, significant gaps remain. The gut hormone glucagon-like peptide-2 (GLP-2) has been shown to play pleotropic roles in the regulation of lipid handling in the intestine. Of note, GLP-2 exhibits unique actions on post-prandial lipid absorption and post-absorptive release of intestinally stored lipids. This review aims to summarize current knowledge in how GLP-2 regulates lipid processing in the intestine. Elucidating the mechanisms of GLP-2 regulation of intestinal lipid handling not only improves our understanding of GLP-2 biology, but also provides insights into how lipids are processed in the intestine, which offers opportunities for developing novel strategies towards prevention and treatment of dyslipidemia and atherosclerotic cardiovascular diseases.

RET is a sex-biased regulator of intestinal tumorigenesis

Ret is implicated in colorectal cancer (CRC) as both a proto-oncogene and a tumor suppressor. We asked whether RET signaling regulates tumorigenesis in an Apc-deficient preclinical model of CRC. We observed a sex-biased phenotype: Apc Min/+ Ret+/- females had significantly greater tumor burden than Apc Min/+Ret+/- males, a phenomenon not seen in Apc Min/+ mice, which had equal distributions by sex. Dysfunctional RET signaling was associated with gene expression changes in diverse tumor signaling pathways in tumors and normal-appearing colon. Sex-biased gene expression differences mirroring tumor phenotypes were seen in 26 genes, including the Apc tumor suppressor gene. Ret and Tlr4 expression were significantly correlated in tumor samples from female but not male Apc Min/+ Ret+/- mice. Antibiotics resulted in reduction of tumor burden, inverting the sex-biased phenotype such that microbiota-depleted Apc Min/+ Ret+/- males had significantly more tumors than female littermates. Reconstitution of the microbiome rescued the sex-biased phenotype. Our findings suggest that RET represents a sexually dimorphic microbiome-mediated "switch" for regulation of tumorigenesis.

A mixed-effects stochastic model reveals clonal dominance in gene therapy safety studies

BACKGROUND

Mathematical models of haematopoiesis can provide insights on abnormal cell expansions (clonal dominance), and in turn can guide safety monitoring in gene therapy clinical applications. Clonal tracking is a recent high-throughput technology that can be used to quantify cells arising from a single haematopoietic stem cell ancestor after a gene therapy treatment. Thus, clonal tracking data can be used to calibrate the stochastic differential equations describing clonal population dynamics and hierarchical relationships in vivo.

RESULTS

In this work we propose a random-effects stochastic framework that allows to investigate the presence of events of clonal dominance from high-dimensional clonal tracking data. Our framework is based on the combination between stochastic reaction networks and mixed-effects generalized linear models. Starting from the Kramers-Moyal approximated Master equation, the dynamics of cells duplication, death and differentiation at clonal level, can be described by a local linear approximation. The parameters of this formulation, which are inferred using a maximum likelihood approach, are assumed to be shared across the clones and are not sufficient to describe situation in which clones exhibit heterogeneity in their fitness that can lead to clonal dominance. In order to overcome this limitation, we extend the base model by introducing random-effects for the clonal parameters. This extended formulation is calibrated to the clonal data using a tailor-made expectation-maximization algorithm. We also provide the companion  package RestoreNet, publicly available for download at https://cran.r-project.org/package=RestoreNet .

CONCLUSIONS

Simulation studies show that our proposed method outperforms the state-of-the-art. The application of our method in two in-vivo studies unveils the dynamics of clonal dominance. Our tool can provide statistical support to biologists in gene therapy safety analyses.

Intestinal adaptation and rehabilitation

Massive intestinal resection is a regrettably necessary but life-saving intervention for progressive or fulminant necrotizing enterocolitis (NEC). However, the resultant short bowel syndrome (SBS) poses its own array of challenges and complications. Within hours of such an abrupt loss of intestinal length, the intestine begins to adapt. Our ability to understand this process of intestinal adaptation has proven critical in our ability to clinically treat the challenging problem of short bowel syndrome. This review first highlights key data relating to intestinal adaptation including structural and functional changes, biochemical regulation, and other factors affecting the magnitude of intestinal adaptation responses. We then focus on intestinal rehabilitation as it relates to strategies to enhance intestinal adaptation while meeting nutritional needs and preventing complications of parenteral nutrition.

Long-acting agonists of human and rodent GLP-2 receptors for studies of the physiology and pharmacological potential of the GLP-2 system

BACKGROUND AND PURPOSE

Glucagon-like peptide-2 (GLP-2) is secreted postprandially from enteroendocrine Lcells and has anabolic action on gut and bone. Short-acting teduglutide is the only approved GLP-2 analog for the treatment of short-bowel syndrome (SBS). To improve the therapeutic effect, we created a series of lipidated GLP-2R agonists.

EXPERIMENTAL APPROACH

Six GLP-2 analogs were studied in vitro for cAMP accumulation, β-arrestin 1 and 2 recruitment, affinity, and internalization. The trophic actions on intestine and bone were examined in vivo in rodents.

KEY RESULTS

Lipidations at lysines introduced at position 12, 16, and 20 of hGLP-2(1-33) were well-tolerated with less than 2.2-fold impaired potency and full efficacy at the hGLP-2R in cAMP accumulation. In contrast, N- and C-terminal (His1 and Lys30) lipidations impaired potency by 4.2- and 45-fold and lowered efficacy to 77% and 85% of hGLP-2, respectively. All variants were similarly active on the rat and mouse GLP-2Rs and the three most active variants displayed increased selectivity for hGLP-2R over hGLP-1R activation, compared to native hGLP-2. Impact on arrestin recruitment and receptor internalization followed that of Gαs-coupling, except for lipidation in position 20, where internalization was more impaired, suggesting desensitization protection. A highly active variant (C16 at position 20) with low internalization and a half-life of 9.5 h in rats showed improved gut and bone tropism with increased weight of small intestine in mice and decreased CTX levels in rats.

CONCLUSION AND IMPLICATION

We present novel hGLP-2 agonists suitable for in vivo studies of the GLP-2 system to uncover its pharmacological potential.

The enteric nervous system

Of all the organ systems in the body, the gastrointestinal tract is the most complicated in terms of the numbers of structures involved, each with different functions, and the numbers and types of signaling molecules utilized. The digestion of food and absorption of nutrients, electrolytes, and water occurs in a hostile luminal environment that contains a large and diverse microbiota. At the core of regulatory control of the digestive and defensive functions of the gastrointestinal tract is the enteric nervous system (ENS), a complex system of neurons and glia in the gut wall. In this review, we discuss 1) the intrinsic neural control of gut functions involved in digestion and 2) how the ENS interacts with the immune system, gut microbiota, and epithelium to maintain mucosal defense and barrier function. We highlight developments that have revolutionized our understanding of the physiology and pathophysiology of enteric neural control. These include a new understanding of the molecular architecture of the ENS, the organization and function of enteric motor circuits, and the roles of enteric glia. We explore the transduction of luminal stimuli by enteroendocrine cells, the regulation of intestinal barrier function by enteric neurons and glia, local immune control by the ENS, and the role of the gut microbiota in regulating the structure and function of the ENS. Multifunctional enteric neurons work together with enteric glial cells, macrophages, interstitial cells, and enteroendocrine cells integrating an array of signals to initiate outputs that are precisely regulated in space and time to control digestion and intestinal homeostasis.

Histological study on regional specificity of the mucosal nerve network in the rat large intestine

Our previous studies and others have revealed detailed characteristics of the mucosal nerve network in the small intestine, but much remains unknown about the corresponding network in the large intestine. We herein investigated regional differences in the expression of neurochemical markers, the nerve network structure, and the cells in contact with nerve fibers by histological analysis using both immunohistochemistry and serial block-face scanning electron microscopy (SBF-SEM). Immunohistochemistry revealed that immunopositive structures for protein gene product 9.5, vasoactive intestinal peptide (VIP), calretinin and vesicular acetylcholine transporter were more prevalent in the lamina propria of the ascending colon than the cecum and descending colon (DC). There was no significant difference in the frequency of most neurochemical markers between the cecum and DC, but the frequencies of VIP⁺ structures were higher in the cecum than in the DC. SBF-SEM analysis showed that the nerve network structure was more developed on the luminal side of the DC than the cecum. The cells that nerve fibers abundantly contacted were subepithelial and lamina propria fibroblast-like cells and macrophages. In addition, nerve fibers in the cecum were in more frequent contact with immune cells such as macrophages and plasma cells than nerve fibers in the DC. Thus, the present histological analysis suggested that the mucosal nerve network in the large intestine possessed both regional universality and various specificities, and revealed the intimate relationship between the nerve network and immune cells, especially in the cecum.

Role of Wnt signaling in the maintenance and regeneration of the intestinal epithelium

The intestinal epithelium plays a key role in digestion and protection against external pathogens. This tissue presents a high cellular turnover with the epithelium being completely renewed every 5days, driven by intestinal stem cells (ISCs) residing in the crypt bases. To sustain this dynamic renewal of the intestinal epithelium, the maintenance, proliferation, and differentiation of ISCs must be precisely controlled. One of the central pathways supporting ISC maintenance and dynamics is the Wnt pathway. In this chapter, we examine the role of Wnt signaling in intestinal epithelial homeostasis and tissue regeneration, including mechanisms regulating ISC identity and fine-tuning of Wnt pathway activation. We extensively discuss the contribution of the stem cell niche in maintaining Wnt signaling in the intestinal crypts that support ISC functions. The integration of these findings highlights the complex interplay of multiple niche signals and cellular components sustaining ISC behavior and maintenance, which together supports the immense plasticity of the intestinal epithelium.

The Molecular Determinants of Glucagon-like Peptide Secretion by the Intestinal L cell

The intestinal L cell secretes a diversity of biologically active hormones, most notably the glucagon-like peptides, GLP-1 and GLP-2. The highly successful introduction of GLP-1-based drugs into the clinic for the treatment of patients with type 2 diabetes and obesity, and of a GLP-2 analog for patients with short bowel syndrome, has led to the suggestion that stimulation of the endogenous secretion of these peptides may serve as a novel therapeutic approach in these conditions. Situated in the intestinal epithelium, the L cell demonstrates complex relationships with not only circulating, paracrine, and neural regulators, but also ingested nutrients and other factors in the lumen, most notably the microbiota. The integrated input from these numerous secretagogues results in a variety of temporal patterns in L cell secretion, ranging from minutes to 24 hours. This review combines the findings of traditional, physiological studies with those using newer molecular approaches to describe what is known and what remains to be elucidated after 5 decades of research on the intestinal L cell and its secreted peptides, GLP-1 and GLP-2.

Building gut from scratch - progress and update of intestinal tissue engineering

Short bowel syndrome (SBS), a condition defined by insufficient absorptive intestinal epithelium, is a rare disease, with an estimated prevalence up to 0.4 in 10,000 people. However, it has substantial morbidity and mortality for affected patients. The mainstay of treatment in SBS is supportive, in the form of intravenous parenteral nutrition, with the aim of achieving intestinal autonomy. The lack of a definitive curative therapy has led to attempts to harness innate developmental and regenerative mechanisms to engineer neo-intestine as an alternative approach to addressing this unmet clinical need. Exciting advances have been made in the field of intestinal tissue engineering (ITE) over the past decade, making a review in this field timely. In this Review, we discuss the latest advances in the components required to engineer intestinal grafts and summarize the progress of ITE. We also explore some key factors to consider and challenges to overcome when transitioning tissue-engineered intestine towards clinical translation, and provide the future outlook of ITE in therapeutic applications and beyond.

Glucagon-Like Peptide-2 Stimulates S-Phase Entry of Intestinal Lgr5+ Stem Cells

BACKGROUND & AIMS

Leucine-rich repeat-containing G-protein-coupled receptor-5 (Lgr5)+/olfactomedin-4 (Olfm4)+ intestinal stem cells (ISCs) in the crypt base are crucial for homeostatic maintenance of the epithelium. The gut hormone, glucagon-like peptide-2^1-33 (GLP-2), stimulates intestinal proliferation and growth; however, the actions of GLP-2 on the Lgr5+ ISCs remain unclear. The aim of this study was to determine whether and how GLP-2 regulates Lgr5+ ISC cell-cycle dynamics and numbers.

METHODS

Lgr5-Enhanced green-fluorescent protein - internal ribosome entry site - Cre recombinase - estrogen receptor T2 (eGFP-IRES-creERT2) mice were acutely administered human Glycine² (Gly2)-GLP-2, or the GLP-2-receptor antagonist, GLP-2^3-33. Intestinal epithelial insulin-like growth factor-1-receptor knockout and control mice were treated chronically with human Gly2 (hGly2)-GLP-2. Cell-cycle parameters were determined by 5-Ethynyl-2'-deoxyuridine (EdU), bromodeoxyuridine, antibody #Ki67, and phospho-histone 3 labeling and cell-cycle gene expression.

RESULTS

Acute hGly2-GLP-2 treatment increased the proportion of eGFP+EdU+/OLFM4+EdU+ cells by 11% to 22% (P < .05), without affecting other cell-cycle markers. hGly2-GLP-2 treatment also increased the ratio of eGFP+ cells in early to late S-phase by 97% (P < .001), and increased the proportion of eGFP+ cells entering S-phase by 218% (P < .001). hGly2-GLP-2 treatment induced jejunal expression of genes involved in cell-cycle regulation (P < .05), and increased expression of Mcm3 in the Lgr5-expressing cells by 122% (P < .05). Conversely, GLP-2^3-33 reduced the proportion of eGFP+EdU+ cells by 27% (P < .05), as well as the expression of jejunal cell-cycle genes (P < .05). Finally, chronic hGly2-GLP-2 treatment increased the number of OLFM4+ cells/crypt (P < .05), in an intestinal epithelial insulin-like growth factor-1-receptor-dependent manner.

CONCLUSIONS

These findings expand the actions of GLP-2 to encompass acute stimulation of Lgr5+ ISC S-phase entry through the GLP-2R, and chronic induction of Lgr5+ ISC expansion through downstream intestinal insulin-like growth factor-1 signaling.

Novel agonist- and antagonist-based radioligands for the GLP-2 receptor - useful tools for studies of basic GLP-2R pharmacology

Background

Glucagon‐like peptide‐2 (GLP‐2) is a pro‐glucagon‐derived hormone secreted from intestinal enteroendocrine L cells with actions on gut and bones. GLP‐2(1–33) is cleaved by DPP‐4, forming GLP‐2(3–33), having low intrinsic activity and competitive antagonism properties at GLP‐2 receptors. We created radioligands based on these two molecules.

Experimental approach

The methionine in position 10 of GLP‐2(1–33) and GLP‐2(3–33) was substituted with tyrosine (M10Y) enabling oxidative iodination, creating [¹²⁵I]‐hGLP‐2(1–33,M10Y) and [¹²⁵I]‐hGLP‐2(3–33,M10Y). Both were characterized by competition binding, on‐and‐off‐rate determination and receptor activation. Receptor expression was determined by target‐tissue autoradiography and immunohistochemistry.

Key results

Both M10Y‐substituted peptides induced cAMP production via the GLP‐2 receptor comparable to the wildtype peptides. GLP‐2(3–33,M10Y) maintained the antagonistic properties of GLP‐2(3–33). However, hGLP‐2(1–33,M10Y) had lower arrestin recruitment than hGLP‐2(1–33). High affinities for the hGLP‐2 receptor were observed using [¹²⁵I]‐hGLP‐2(1–33,M10Y) and [¹²⁵I]‐hGLP‐2(3–33,M10Y) with K_D values of 59.3 and 40.6 nM. The latter (with antagonistic properties) had higher B_max and faster on and off rates compared to the former (full agonist). Both bound the hGLP‐1 receptor with low affinity (K_i of 130 and 330 nM, respectively). Autoradiography in wildtype mice revealed strong labelling of subepithelial myofibroblasts, confirmed by immunohistochemistry using a GLP‐2 receptor specific antibody that in turn was confirmed in GLP‐2 receptor knock‐out mice.

Conclusion and implications

Two new radioligands with different binding kinetics, one a full agonist and the other a weak partial agonist with antagonistic properties were developed and subepithelial myofibroblasts identified as a major site for GLP‐2 receptor expression.

Regenerative medicine for childhood gastrointestinal diseases

Several paediatric gastrointestinal diseases result in life-shortening organ failure. For many of these conditions, current therapeutic options are suboptimal and may not offer a cure. Regenerative medicine is an inter-disciplinary field involving biologists, engineers, and clinicians that aims to produce cell and tissue-based therapies to overcome organ failure. Exciting advances in stem cell biology, materials science, and bioengineering bring engineered gastrointestinal cell and tissue therapies to the verge of clinical trial. In this review, we summarise the requirements for bioengineered therapies, the possible sources of the various cellular and non-cellular components, and the progress towards clinical translation of oesophageal and intestinal tissue engineering to date.

Reciprocal signals between nerve and epithelium: how do neurons talk with epithelial cells?

Most epithelium tissues continuously undergo self-renewal through proliferation and differentiation of epithelial stem cells (known as homeostasis), within a specialized stem cell niche. In highly innervated epithelium, peripheral nerves compose perineural niche and support stem cell homeostasis by releasing a variety of neurotransmitters, hormones, and growth factors and supplying trophic factors to the stem cells. Emerging evidence has shown that both sensory and motor nerves can regulate the fate of epithelial stem cells, thus influencing epithelium homeostasis. Understanding the mechanism of crosstalk between epithelial stem cells and neurons will reveal the important role of the perineural niche in physiological and pathological conditions. Herein, we review recent discoveries of the perineural niche in epithelium mainly in tissue homeostasis, with a limited touch in wound repair and pathogenesis.

Modeling Intestinal Stem Cell Function with Organoids

Intestinal epithelial cells (IECs) are crucial for the digestive process and nutrient absorption. The intestinal epithelium is composed of the different cell types of the small intestine (mainly, enterocytes, goblet cells, Paneth cells, enteroendocrine cells, and tuft cells). The small intestine is characterized by the presence of crypt-villus units that are in a state of homeostatic cell turnover. Organoid technology enables an efficient expansion of intestinal epithelial tissue in vitro. Thus, organoids hold great promise for use in medical research and in the development of new treatments. At present, the cholinergic system involved in IECs and intestinal stem cells (ISCs) are attracting a great deal of attention. Thus, understanding the biological processes triggered by epithelial cholinergic activation by acetylcholine (ACh), which is produced and released from neuronal and/or non-neuronal tissue, is of key importance. Cholinergic signaling via ACh receptors plays a pivotal role in IEC growth and differentiation. Here, we discuss current views on neuronal innervation and non-neuronal control of the small intestinal crypts and their impact on ISC proliferation, differentiation, and maintenance. Since technology using intestinal organoid culture systems is advancing, we also outline an organoid-based organ replacement approach for intestinal diseases.

BMP Antagonists Secreted by Mesenchymal Stromal Cells Improve Colonic Organoid Formation: Application for the Treatment of Radiation-induced Injury

Radiation therapy is crucial in the therapeutic arsenal to cure cancers; however, non-neoplastic tissues around an abdominopelvic tumor can be damaged by ionizing radiation. In particular, the radio-induced death of highly proliferative stem/progenitor cells of the colonic mucosa could induce severe ulcers. The importance of sequelae for patients with gastrointestinal complications after radiotherapy and the absence of satisfactory management has opened the field to the testing of innovative treatments. The aim of this study was to use adult epithelial cells from the colon, to reduce colonic injuries in an animal model reproducing radiation damage observed in patients. We demonstrated that transplanted in vitro-amplified epithelial cells from colonic organoids (ECO) of C57/Bl6 mice expressing green fluorescent protein implant, proliferate, and differentiate in irradiated mucosa and reduce ulcer size. To improve the therapeutic benefit of ECO-based treatment with clinical translatability, we performed co-injection of ECO with mesenchymal stromal cells (MSCs), cells involved in niche function and widely used in clinical trials. We observed in vivo an improvement of the therapeutic benefit and in vitro analysis highlighted that co-culture of MSCs with ECO increases the number, proliferation, and size of colonic organoids. We also demonstrated, using gene expression analysis and siRNA inhibition, the involvement of bone morphogenetic protein antagonists in MSC-induced organoid formation. This study provides evidence of the potential of ECO to limit late radiation effects on the colon and opens perspectives on combined strategies to improve their amplification abilities and therapeutic effects.

The Antiresorptive Effect of GIP, But Not GLP-2, Is Preserved in Patients With Hypoparathyroidism-A Randomized Crossover Study

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-2 (GLP-2) are gut hormones secreted postprandially. In healthy humans, both hormones decrease bone resorption accompanied by a rapid reduction in parathyroid hormone (PTH). The aim of this study was to investigate whether the changes in bone turnover after meal intake and after GIP- and GLP-2 injections, respectively, are mediated via a reduction in PTH secretion. This was tested in female patients with hypoparathyroidism given a standardized liquid mixed-meal test (n = 7) followed by a peptide injection test (n = 4) using a randomized crossover design. We observed that the meal- and GIP- but not the GLP-2-induced changes in bone turnover markers were preserved in the patients with hypoparathyroidism. To understand the underlying mechanisms, we examined the expression of the GIP receptor (GIPR) and the GLP-2 receptor (GLP-2R) in human osteoblasts and osteoclasts as well as in parathyroid tissue. The GIPR was expressed in both human osteoclasts and osteoblasts, whereas the GLP-2R was absent or only weakly expressed in osteoclasts. Furthermore, both GIPR and GLP-2R were expressed in parathyroid tissue. Our findings suggest that the GIP-induced effect on bone turnover may be mediated directly via GIPR expressed in osteoblasts and osteoclasts and that this may occur independent of PTH. In contrast, the effect of GLP-2 on bone turnover seems to depend on changes in PTH and may be mediated through GLP-2R in the parathyroid gland. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Muscarinic receptor M3 contributes to intestinal stem cell maintenance via EphB/ephrin-B signaling

Acetylcholine (ACh) signaling through activation of nicotinic and muscarinic ACh receptors regulates expression of specific genes that mediate and sustain proliferation, differentiation, and homeostasis in the intestinal crypts. This signaling plays a pivotal role in the regulation of intestinal stem cell function, but the details have not been clarified. Here, we performed experiments using type 3 muscarinic acetylcholine receptor (M3) knockout mice and their intestinal organoids and report that endogenous ACh affects the size of the intestinal stem niche via M3 signaling. RNA sequencing of crypts identified up-regulation of the EphB/ephrin-B signaling pathway. Furthermore, using an MEK inhibitor (U0126), we found that mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) signaling, which is downstream of EphB/ephrin-B signaling, is activated in M3-deficient crypts. Collectively, M3, EphB/ephrin-B, and the MAPK/ERK signaling cascade work together to maintain the homeostasis of intestinal epithelial cell growth and differentiation following modifications of the cholinergic intestinal niche.

Microbiome-encoded bile acid metabolism modulates colonic transit times

Gut motility is regulated by the microbiome via mechanisms that include bile acid metabolism. To localize the effects of microbiome-generated bile acids, we colonized gnotobiotic mice with different synthetic gut bacterial communities that were metabolically phenotyped using a functional in vitro screen. Using two different marker-based assays of gut transit, we inferred that bile acids exert effects on colonic transit. We validated this using an intra-colonic bile acid infusion assay and determined that these effects were dependent upon signaling via the bile acid receptor, TGR5. The intra-colonic bile acid infusion experiments further revealed sex-biased bile acid-specific effects on colonic transit, with lithocholic acid having the largest pro-motility effect. Transcriptional responses of the enteric nervous system (ENS) were stereotypic, regional, and observed in response to different microbiota, their associated bile acid profiles, and even to a single diet ingredient, evidencing exquisite sensitivity of the ENS to environmental perturbations.

The Role of Dipeptidyl Peptidase 4 as a Therapeutic Target and Serum Biomarker in Inflammatory Bowel Disease: A Systematic Review

BACKGROUND

The roles dipeptidyl peptidase 4 (DPP4), aminopeptidase N (APN), and their substrates in autoimmune diseases are being increasingly recognized. However, their significance in inflammatory bowel diseases (IBD) is not entirely understood. This systematic review aims to discuss the pathophysiological processes related to these ectopeptidases while comparing findings from preclinical and clinical settings.

METHODS

This review was conducted according to the PRISMA guidelines. We performed a literature search in PubMed, SCOPUS, and Web of Science to identify all reports from inception until February 2020. The search included validated animal models of intestinal inflammation and studies in IBD patients. Quality assessment was performed using SYRCLE's risk of bias tool and CASP qualitative and cohort checklists.

RESULTS

From the 45 included studies, 36 were performed in animal models and 12 in humans (3 reports included both). Overall, the methodological quality of preclinical studies was acceptable. In animal models, DPP4 and APN inhibition significantly improved intestinal inflammation.Glucagon-like peptide (GLP)-1 and GLP-2 analogs and GLP-2-relase-inducing drugs also showed significant benefits in recovery from inflammatory damage. A nonsignificant trend toward disease remission with the GLP-2 analog teduglutide was observed in the sole interventional human study. All human studies reported an inverse correlation between soluble DPP4/CD26 levels and disease severity, in accordance with the proposal of DPP4 as a biomarker for IBD.

CONCLUSIONS

The use of DPP4 inhibitors and analogs of its substrates has clear benefits in the treatment of experimentally induced intestinal inflammation. Further research is warranted to validate their potential diagnostic and therapeutic applications in IBD patients.

Intestinal crypt-derived enteroid coculture in presence of peristaltic longitudinal muscle myenteric plexus

The role of enteric neurons in driving intestinal peristalsis has been known for over a century. However, in recent decades, scientists have begun to unravel additional complex interactions between this nerve plexus and other cell populations in the intestine. Investigations into these potential interactions are complicated by a paucity of tractable models of these cellular relationships. Here, we describe a novel technique for ex vivo coculture of enteroids, so called “mini-guts,” in juxtaposition to the longitudinal muscle myenteric plexus (LMMP). Key to this system, we developed a LMMP culture media that: (i) allows the LMMP to maintain ex vivo peristalsis for 2 weeks along with proliferation of neurons, glia, smooth muscle and fibroblast cells, and (ii) supports the proliferation and differentiation of the intestinal stem cells into enteroids complete with epithelial enterocytes, Paneth cells, goblet cells, and enteroendocrine cells. Importantly, this technique identifies a culture condition that supports both the metabolic needs of intestinal epithelium as well as neuronal elements, demonstrating the feasibility of maintaining these two populations in a single culture system. This sets the stage for experiments to better define the regulatory interactions of these two important intestinal cell populations.

Intraluminal nutrients acutely strengthen rat intestinal MRP2 barrier function by a glucagon-like peptide-2-mediated mechanism

AIM

MRP2 is an intestinal ABC transporter that prevents the absorption of dietary xenobiotics. The aims of this work were: (1) to evaluate whether a short-term regulation of intestinal MRP2 barrier function takes place in vivo after luminal incorporation of nutrients and (2) to explore the underlying mechanism.

METHODS

MRP2 activity and localization were assessed in an in vivo rat model with preserved irrigation and innervation. Nutrients were administered into distal jejunum. After 30-minutes treatments, MRP2 activity was assessed in proximal jejunum by quantifying the transport of the model substrate 2,4-dinitrophenyl-S-glutathione. MRP2 localization was determined by quantitative confocal microscopy. Participation of extracellular mediators was evaluated using selective inhibitors and by immunoneutralization. Intracellular pathways were explored in differentiated Caco-2 cells.

RESULTS

Oleic acid, administered intraluminally at dietary levels, acutely stimulated MRP2 insertion into brush border membrane. This was associated with increased efflux activity and, consequently, enhanced barrier function. Immunoneutralization of the gut hormone glucagon-like peptide-2 (GLP-2) prevented oleic acid effect on MRP2, demonstrating the participation of this trophic factor as a main mediator. Further experiments using selective inhibitors demonstrated that extracellular adenosine synthesis and its subsequent binding to enterocytic A2B adenosine receptor (A2BAR) take place downstream GLP-2. Finally, studies in intestinal Caco-2 cells revealed the participation of A2BAR/cAMP/PKA intracellular pathway, ultimately leading to increased MRP2 localization in apical domains.

CONCLUSION

These findings reveal an on-demand, acute regulation of MRP2-associated barrier function, constituting a novel physiological mechanism of protection against the absorption of dietary xenobiotics in response to food intake.

Functional circuits and signal processing in the enteric nervous system

The enteric nervous system (ENS) is an extensive network comprising millions of neurons and glial cells contained within the wall of the gastrointestinal tract. The major functions of the ENS that have been most studied include the regulation of local gut motility, secretion, and blood flow. Other areas that have been gaining increased attention include its interaction with the immune system, with the gut microbiota and its involvement in the gut-brain axis, and neuro-epithelial interactions. Thus, the enteric circuitry plays a central role in intestinal homeostasis, and this becomes particularly evident when there are faults in its wiring such as in neurodevelopmental or neurodegenerative disorders. In this review, we first focus on the current knowledge on the cellular composition of enteric circuits. We then further discuss how enteric circuits detect and process external information, how these signals may be modulated by physiological and pathophysiological factors, and finally, how outputs are generated for integrated gut function.

APC mutations in human colon lead to decreased neuroendocrine maturation of ALDH+ stem cells that alters GLP-2 and SST feedback signaling: Clue to a link between WNT and retinoic acid signalling in colon cancer development

APC mutations drive human colorectal cancer (CRC) development. A major contributing factor is colonic stem cell (SC) overpopulation. But, the mechanism has not been fully identified. A possible mechanism is the dysregulation of neuroendocrine cell (NEC) maturation by APC mutations because SCs and NECs both reside together in the colonic crypt SC niche where SCs mature into NECs. So, we hypothesized that sequential inactivation of APC alleles in human colonic crypts leads to progressively delayed maturation of SCs into NECs and overpopulation of SCs. Accordingly, we used quantitative immunohistochemical mapping to measure indices and proportions of SCs and NECs in human colon tissues (normal, adenomatous, malignant), which have different APC-zygosity states. In normal crypts, many cells staining for the colonic SC marker ALDH1 co-stained for chromogranin-A (CGA) and other NEC markers. In contrast, in APC-mutant tissues from familial adenomatous polyposis (FAP) patients, the proportion of ALDH+ SCs progressively increased while NECs markedly decreased. To explain how these cell populations change in FAP tissues, we used mathematical modelling to identify kinetic mechanisms. Computational analyses indicated that APC mutations lead to: 1) decreased maturation of ALDH+ SCs into progenitor NECs (not progenitor NECs into mature NECs); 2) diminished feedback signaling by mature NECs. Biological experiments using human CRC cell lines to test model predictions showed that mature GLP-2R+ and SSTR1+ NECs produce, via their signaling peptides, opposing effects on rates of NEC maturation via feedback regulation of progenitor NECs. However, decrease in this feedback signaling wouldn't explain the delayed maturation because both progenitor and mature NECs are depleted in CRCs. So the mechanism for delayed maturation must explain how APC mutation causes the ALDH+ SCs to remain immature. Given that ALDH is a key component of the retinoic acid (RA) signaling pathway, that other components of the RA pathway are selectively expressed in ALDH+ SCs, and that exogenous RA ligands can induce ALDH+ cancer SCs to mature into NECs, RA signaling must be attenuated in ALDH+ SCs in CRC. Thus, attenuation of RA signaling explains why ALDH+ SCs remain immature in APC mutant tissues. Since APC mutation causes increased WNT signaling in FAP and we found that sequential inactivation of APC in FAP patient tissues leads to progressively delayed maturation of colonic ALDH+ SCs, the hypothesis is developed that human CRC evolves due to an imbalance between WNT and RA signaling.

The Enteric Nervous System and Its Emerging Role as a Therapeutic Target

The gastrointestinal (GI) tract is innervated by the enteric nervous system (ENS), an extensive neuronal network that traverses along its walls. Due to local reflex circuits, the ENS is capable of functioning with and without input from the central nervous system. The functions of the ENS range from the propulsion of food to nutrient handling, blood flow regulation, and immunological defense. Records of it first being studied emerged in the early 19th century when the submucosal and myenteric plexuses were discovered. This was followed by extensive research and further delineation of its development, anatomy, and function during the next two centuries. The morbidity and mortality associated with the underdevelopment, infection, or inflammation of the ENS highlight its importance and the need for us to completely understand its normal function. This review will provide a general overview of the ENS to date and connect specific GI diseases including short bowel syndrome with neuronal pathophysiology and current therapies. Exciting opportunities in which the ENS could be used as a therapeutic target for common GI diseases will also be highlighted, as the further unlocking of such mechanisms could open the door to more therapy-related advances and ultimately change our treatment approach.

Glucose transporters in the small intestine in health and disease

Absorption of monosaccharides is mainly mediated by Na+-D-glucose cotransporter SGLT1 and the facititative transporters GLUT2 and GLUT5. SGLT1 and GLUT2 are relevant for absorption of D-glucose and D-galactose while GLUT5 is relevant for D-fructose absorption. SGLT1 and GLUT5 are constantly localized in the brush border membrane (BBM) of enterocytes, whereas GLUT2 is localized in the basolateral membrane (BLM) or the BBM plus BLM at low and high luminal D-glucose concentrations, respectively. At high luminal D-glucose, the abundance SGLT1 in the BBM is increased. Hence, D-glucose absorption at low luminal glucose is mediated via SGLT1 in the BBM and GLUT2 in the BLM whereas high-capacity D-glucose absorption at high luminal glucose is mediated by SGLT1 plus GLUT2 in the BBM and GLUT2 in the BLM. The review describes functions and regulations of SGLT1, GLUT2, and GLUT5 in the small intestine including diurnal variations and carbohydrate-dependent regulations. Also, the roles of SGLT1 and GLUT2 for secretion of enterohormones are discussed. Furthermore, diseases are described that are caused by malfunctions of small intestinal monosaccharide transporters, such as glucose-galactose malabsorption, Fanconi syndrome, and fructose intolerance. Moreover, it is reported how diabetes, small intestinal inflammation, parental nutrition, bariatric surgery, and metformin treatment affect expression of monosaccharide transporters in the small intestine. Finally, food components that decrease D-glucose absorption and drugs in development that inhibit or downregulate SGLT1 in the small intestine are compiled. Models for regulations and combined functions of glucose transporters, and for interplay between D-fructose transport and metabolism, are discussed.

Plasticity of Paneth cells and their ability to regulate intestinal stem cells

Paneth cells (PCs) are located at the bottom of small intestinal crypts and play an important role in maintaining the stability of the intestinal tract. Previous studies reported on how PCs shape the intestinal microbiota or the response to the immune system. Recent studies have determined that PCs play an important role in the regulation of the homeostasis of intestinal epithelial cells. PCs can regulate the function and homeostasis of intestinal stem cells through several mechanisms. On the one hand, under pathological conditions, PCs can be dedifferentiated into stem cells to promote the repair of intestinal tissues. On the other hand, PCs can regulate stem cell proliferation by secreting a variety of hormones (such as wnt3a) or metabolic intermediates. In addition, we summarise key signalling pathways that affect PC differentiation and mutual effect with intestinal stem cells. In this review, we introduce the diverse functions of PCs in the intestine.

Plasticity of Paneth cells and their ability to regulate intestinal stem cells

Paneth cells (PCs) are located at the bottom of small intestinal crypts and play an important role in maintaining the stability of the intestinal tract. Previous studies reported on how PCs shape the intestinal microbiota or the response to the immune system. Recent studies have determined that PCs play an important role in the regulation of the homeostasis of intestinal epithelial cells. PCs can regulate the function and homeostasis of intestinal stem cells through several mechanisms. On the one hand, under pathological conditions, PCs can be dedifferentiated into stem cells to promote the repair of intestinal tissues. On the other hand, PCs can regulate stem cell proliferation by secreting a variety of hormones (such as wnt3a) or metabolic intermediates. In addition, we summarise key signalling pathways that affect PC differentiation and mutual effect with intestinal stem cells. In this review, we introduce the diverse functions of PCs in the intestine.

GLP-2, EGF, and the Intestinal Epithelial IGF-1 Receptor Interactions in the Regulation of Crypt Cell Proliferation

Glucagon-like peptide-2 (GLP-2) is an intestinotrophic hormone that promotes intestinal growth and proliferation through downstream mediators, including epidermal growth factor (EGF) and insulin-like growth factor-1 (IGF-1). EGF synergistically enhances the proliferative actions of IGF-1 in intestinal cell lines, and both of these factors are known to be essential for the trophic effects of GLP-2 in vivo. However, whether EGF and IGF-1 interact to mediate the proliferative actions of GLP-2 in vivo remains unknown. Normal and knockout (KO) mice lacking the intestinal epithelial IGF-1 receptor (IE-IGF-1R) were therefore treated chronically with EGF and/or long-acting human hGly2GLP-2, followed by determination of intestinal growth parameters. Intestines from control and IE-IGF-1R KO mice were also used to generate organoids (which lack the GLP-2 receptor) and were treated with EGF and/or IGF-1. Combination treatment with EGF and hGly2GLP-2 increased small intestinal weight and crypt-villus height in C57Bl/6 mice in an additive manner, whereas only hGly2GLP-2 treatment increased crypt cell proliferation. However, although combination treatment also increased small intestinal weight and crypt-villus height in IE-IGF-1R KO mice, the proliferative responses to hGly2GLP-2 alone or with EGF were diminished in these animals. Finally, IGF-1 treatment of organoids undergoing EGF withdrawal was not additive to the effect of EGF replacement on proliferation, but could restore normal proliferation in the absence of EGF. Together, these findings demonstrate that the intestinal proliferative effects of hGly2GLP-2 are augmented by exogenous EGF in a manner that is partially dependent upon IE-IGF-1R signaling.

Requirement for the intestinal epithelial insulin-like growth factor-1 receptor in the intestinal responses to glucagon-like peptide-2 and dietary fat

The intestinal hormone, glucagon-like peptide-2 (GLP-2), enhances the enterocyte chylomicron production. However, GLP-2 is known to require the intestinal-epithelial insulin-like growth factor-1 receptor (IE-IGF-1R) for its other actions to increase intestinal growth and barrier function. The role of the IE-IGF-1R in enterocyte lipid handling was thus tested in the GLP-2 signaling pathway, as well as in response to a Western diet (WD). IE-IGF-1R knockout (KO) and control mice were treated for 11 days with h(GLY² )GLP-2 or fed a WD for 18 weeks followed by a duodenal fat tolerance test with C¹⁴ -labeled triolein. Human Caco-2BBE cells were treated with an IGF-1R antagonist or signaling inhibitors to determine triglyceride-associated protein expression. The IE-IGF-1R was required for GLP-2-induced increases in CD36 and FATP-4 in chow-fed mice, and for expression in vitro; FATP-4 also required PI3K/Akt. Although WD-fed IE-IGF-1R KO mice demonstrated normal CD36 expression, the protein was incorrectly localized 2h post-duodenal fat administration. IE-IGF-1R KO also prevented the WD-induced increase in MTP and decrease in APOC3, increased jejunal mucosal C¹⁴ -fat accumulation, and elevated plasma triglyceride and C¹⁴ -fat levels. Collectively, these studies elucidate new roles for the IE-IGF-1R in enterocyte lipid handling, under basal conditions and in response to GLP-2 and WD-feeding.

Pharmacological Characterization of Apraglutide, a Novel Long-Acting Peptidic Glucagon-Like Peptide-2 Agonist, for the Treatment of Short Bowel Syndrome

Glucagon-like peptide-2 (GLP-2) agonists have therapeutic potential in clinical indications in which the integrity or absorptive function of the intestinal mucosa is compromised, such as in short bowel syndrome (SBS). Native hGLP-2, a 33-amino acid peptide secreted from the small intestine, contributes to nutritional absorption but has a very short half-life because of enzymatic cleavage and renal clearance and thus is of limited therapeutic value. The GLP-2 analog teduglutide (Revestive/Gattex; Shire Inc.) has been approved for use in SBS since 2012 but has a once-daily injection regimen. Pharmacokinetic (PK) and pharmacodynamic studies confirm that apraglutide, a novel GLP-2 analog, has very low clearance, long elimination half-life, and high plasma protein binding compared with GLP-2 analogs teduglutide and glepaglutide. Apraglutide and teduglutide retain potency and selectivity at the GLP-2 receptor comparable to native hGLP-2, whereas glepaglutide was less potent and less selective. In rat intravenous PK studies, hGLP-2, teduglutide, glepaglutide, and apraglutide had clearances of 25, 9.9, 2.8, and 0.27 ml/kg per minute, respectively, and elimination half-lives of 6.4, 19, 16, and 159 minutes, respectively. The unique PK profile of apraglutide administered via intravenous and subcutaneous routes was confirmed in monkey and minipig and translated into significantly greater in vivo pharmacodynamic activity, measured as small intestinal growth in rats. Apraglutide showed greater intestinotrophic activity than the other peptides when administered at less-frequent dosing intervals because of its prolonged half-life. We postulate that apraglutide offers several advantages over existing GLP-2 analogs and is an excellent candidate for the treatment of gastrointestinal diseases, such as SBS. SIGNIFICANCE STATEMENT: Apraglutide is a potent and selective GLP-2 agonist with an extremely low clearance and prolonged elimination half-life, which differentiates it from teduglutide (the only approved GLP-2 agonist). The enhanced pharmacokinetics of apraglutide will benefit patients by enabling a reduced dosing frequency and removing the need for daily injections.

Prox1-positive cells monitor and sustain the murine intestinal epithelial cholinergic niche

The enteric neurotransmitter acetylcholine governs important intestinal epithelial secretory and immune functions through its actions on epithelial muscarinic Gq-coupled receptors such as M3R. Its role in the regulation of intestinal stem cell function and differentiation, however, has not been clarified. Here, we find that nonselective muscarinic receptor antagonism in mice as well as epithelial-specific ablation of M3R induces a selective expansion of DCLK1-positive tuft cells, suggesting a model of feedback inhibition. Cholinergic blockade reduces Lgr5-positive intestinal stem cell tracing and cell number. In contrast, Prox1-positive endocrine cells appear as primary sensors of cholinergic blockade inducing the expansion of tuft cells, which adopt an enteroendocrine phenotype and contribute to increased mucosal levels of acetylcholine. This compensatory mechanism is lost with acute irradiation injury, resulting in a paucity of tuft cells and acetylcholine production. Thus, enteroendocrine tuft cells appear essential to maintain epithelial homeostasis following modifications of the cholinergic intestinal niche.

Intestinal Fructose and Glucose Metabolism in Health and Disease

The worldwide epidemics of obesity and diabetes have been linked to increased sugar consumption in humans. Here, we review fructose and glucose metabolism, as well as potential molecular mechanisms by which excessive sugar consumption is associated to metabolic diseases and insulin resistance in humans. To this end, we focus on understanding molecular and cellular mechanisms of fructose and glucose transport and sensing in the intestine, the intracellular signaling effects of dietary sugar metabolism, and its impact on glucose homeostasis in health and disease. Finally, the peripheral and central effects of dietary sugars on the gut-brain axis will be reviewed.

Recent Developments in Therapeutic Peptides for the Glucagon-like Peptide 1 and 2 Receptors

Glucagon-like peptide 1 (GLP-1) and glucagon-like peptide 2 (GLP-2) are proglucagon derived peptides that are released from gut endocrine cells in response to nutrient intake. These molecules are rapidly inactivated by the action of dipeptidyl peptidase IV (DPP-4) which limits their use as therapeutic agents. The recent emergence of three-dimensional structures of GPCRs such as GLP-1R and glucagon receptor has helped to drive the rational design of innovative peptide molecules that hold promise as novel peptide therapeutics. One emerging area is the discovery of multifunctional molecules that act at two or more pharmacological systems to enhance therapeutic efficacy. In addition, drug discovery efforts are also focusing on strategies to improve patient convenience through alternative routes of peptide delivery. These novel strategies highlight the broad utility of peptide-based therapeutics in human disease settings where unmet needs still exist.

The roles of glucagon-like peptide-2 and the intestinal epithelial insulin-like growth factor-1 receptor in regulating microvillus length

Microvilli are tiny projections on the apical end of enterocytes, aiding in the digestion and absorption of nutrients. One of their key features is uniform length, but how this is regulated is poorly understood. Glucagon-like peptide-2 (GLP-2) has been shown to increase microvillus length but, the requirement of its downstream mediator, the intestinal epithelial insulin-like growth factor-1 receptor (IE-IGF-1R), and the microvillus proteins acted upon by GLP-2, remain unknown. Using IE-IGF-1R knockout (KO) mice, treated with either long-acting human (h) (GLY²)GLP-2 or vehicle for 11d, it was found that the h(GLY²)GLP-2-induced increase in microvillus length required the IE-IGF-1R. Furthermore, IE-IGF-1R KO alone resulted in a significant decrease in microvillus length. Examination of the brush border membrane proteome as well as of whole jejunal mucosa demonstrated that villin was increased with h(GLY²)GLP-2 treatment in an IE-IGF-1R-dependent manner. Under both basal conditions and with h(GLY²)GLP-2 treatment of the IE-IGF-1R KO mice, changes in villin, IRTKS-1, harmonin, β-actin, and myosin-1a did not explain the decrease in microvillus length, in either the brush border or jejunal mucosa of KO animals. Collectively, these studies define a new role for the IE-IGF-1R within the microvillus, in both the signaling cascade induced by GLP-2, as well as endogenously.

Localization of Glucagon-Like Peptide-2 Receptor Expression in the Mouse

Glucagon-like peptide-2 (GLP-2), secreted from enteroendocrine cells, attenuates gut motility, enhances barrier function, and augments nutrient absorption, actions mediated by a single GLP-2 receptor (GLP-2R). Despite extensive analyses, the precise distribution and cellular localization of GLP-2R expression remains controversial, confounded by the lack of suitable GLP-2R antisera. Here, we reassessed murine Glp2r expression using regular and real-time quantitative PCR (qPCR), in situ hybridization (ISH), and a Glp2rLacZ reporter mouse. Glp2r mRNA expression was detected from the stomach to the rectum and most abundant in the jejunum. Glp2r transcripts were also detected in cerebral cortex, mesenteric lymph nodes, gallbladder, urinary bladder, and mesenteric fat. Surprisingly, Glp2r mRNA was found in testis by qPCR at levels similar to jejunum. However, the testis Glp2r transcripts, detected by different primer pairs and qPCR, lacked 5' mRNA coding sequences, and only a minute proportion of them corresponded to full-length Glp2r mRNA. Within the gut, Glp2r-driven LacZ expression was localized to enteric neurons and lamina propria stromal cells, findings confirmed by ISH analysis of the endogenous Glp2r mRNA. Unexpectedly, vascular Glp2rLacZ expression was localized to mesenteric veins and not arteries. Moreover, mesenteric fat Glp2rLacZ expression was detected within blood vessels and not adipocytes. Reporter LacZ expression was not detected in all tissues expressing an endogenous Glp2r transcript, such as gallbladder, urinary bladder, and mesenteric lymph nodes. Collectively, these findings extend our understanding of the cellular domains of Glp2r expression and highlight limitations inherent in application of commonly used technologies to infer analysis of gene expression.

T Cell Recruitment to the Intestinal Stem Cell Compartment Drives Immune-Mediated Intestinal Damage after Allogeneic Transplantation

The key sites within the gastrointestinal (GI) tract where T cells mediate effector responses and the impact of these responses on intestinal stem cells (ISCs) remain unclear. Using experimental bone marrow transplantation to model immune-mediated GI damage and 3D imaging to analyze T cell localization, we found that the ISC compartment is the primary intestinal site targeted by T cells after transplantation. Recruitment to the crypt base region resulted in direct T cell engagement with the stem cell compartment and loss of crypt base columnar ISCs, which expressed both MHC classes I and II. Vasculature expressing the adhesion molecule MAdCAM-1 clustered near the crypt base, preferentially regulating crypt compartment invasion and ISC reduction without affecting T cell migration to villi. These findings indicate that allogeneic T cells rapidly access the stem cell niche after transplantation, and this targeted recruitment to the stem cell compartment results in ISC loss during immune-mediated GI damage.

Unexpected Roles for the Second Brain: Enteric Nervous System as Master Regulator of Bowel Function

At the most fundamental level, the bowel facilitates absorption of small molecules, regulates fluid and electrolyte flux, and eliminates waste. To successfully coordinate this complex array of functions, the bowel relies on the enteric nervous system (ENS), an intricate network of more than 500 million neurons and supporting glia that are organized into distinct layers or plexi within the bowel wall. Neuron and glial diversity, as well as neurotransmitter and receptor expression in the ENS, resembles that of the central nervous system. The most carefully studied ENS functions include control of bowel motility, epithelial secretion, and blood flow, but the ENS also interacts with enteroendocrine cells, influences epithelial proliferation and repair, modulates the intestinal immune system, and mediates extrinsic nerve input. Here, we review the many different cell types that communicate with the ENS, integrating data about ENS function into a broader view of human health and disease. In particular, we focus on exciting new literature highlighting relationships between the ENS and its lesser-known interacting partners.

Neuropods

Enteroendocrine cells (EECs) are sensory cells of the gastrointestinal tract. Most EECs reside in the mucosal lining of the stomach or intestine and sense food in the gut lumen. Food signals stimulate the release of hormones into the paracellular space where they either act locally or are taken up into the blood and circulate to distant organs. It recently was recognized that many EECs possess basal processes known as neuropods that not only contain hormones but also connect to nerves. This review describes how neuropods contribute to EEC function beyond typical hormonal actions. For example, gastrointestinal hormones not only act on distant organs, but, through neuropods, some act locally to stimulate other mucosal cells such as intestinal stem cells, enterocytes, or other EECs. With the recent discovery that EECs communicate directly with enteric nerves, EECs not only have the ability to sense food and bacteria in the gastrointestinal tract, but can communicate these signals directly to the nervous system.

Glucagon-Like Peptide-2 and the Enteric Nervous System Are Components of Cell-Cell Communication Pathway Regulating Intestinal Na+/Glucose Co-transport

The Na⁺/glucose cotransporter 1, SGLT1 is the major route for transport of dietary glucose from the lumen of the intestine into absorptive enterocytes. Sensing of dietary sugars and artificial sweeteners by the sweet taste receptor, T1R2-T1R3, expressed in the enteroendocrine L-cell regulates SGLT1 expression in neighboring absorptive enterocytes. However, the mechanism by which sugar sensing by the enteroendocrine cell is communicated to the absorptive enterocytes is not known. Here, we show that glucagon-like peptide-2 (GLP-2) secreted from the enteroendocrine cell in response to luminal sugars regulates SGLT1 mRNA and protein expression in absorptive enterocytes, via the enteric neurons. Glucose and artificial sweeteners induced secretion of GLP-2 from mouse small intestine, which was inhibited by the sweet-taste receptor inhibitor, gurmarin. In wild type mice there was an increase in sugar-induced SGLT1 mRNA and protein abundance that was not observed in GLP-2 receptor knockout mice. GLP-2 receptor is expressed in enteric neurons, and not in absorptive enterocytes ruling out a paracrine effect of GLP-2. Electric field stimulation of the intestine resulted in upregulation of SGLT1 expression that was abolished by the nerve blocking agent tetrodotoxin. We conclude that GLP-2 and the enteric nervous system are components of the enteroendocrine-absorptive enterocyte communication pathway regulating intestinal glucose transport.

The Intestinal Stem Cell Niche: Homeostasis and Adaptations

The intestinal epithelium is a rapidly renewing cellular compartment. This constant regeneration is a hallmark of intestinal homeostasis and requires a tightly regulated balance between intestinal stem cell (ISC) proliferation and differentiation. Since intestinal epithelial cells directly contact pathogenic environmental factors that continuously challenge their integrity, ISCs must also actively divide to facilitate regeneration and repair. Understanding niche adaptations that maintain ISC activity during homeostatic renewal and injury-induced intestinal regeneration is therefore a major and ongoing focus for stem cell biology. Here, we review recent concepts and propose an active interconversion of the ISC niche between homeostasis and injury-adaptive states that is superimposed upon an equally dynamic equilibrium between active and reserve ISC populations.

T1R2 receptor-mediated glucose sensing in the upper intestine potentiates glucose absorption through activation of local regulatory pathways

Objective

Beyond the taste buds, sweet taste receptors (STRs; T1R2/T1R3) are also expressed on enteroendocrine cells, where they regulate gut peptide secretion but their regulatory function within the intestine is largely unknown.

Methods

Using T1R2-knock out (KO) mice we evaluated the role of STRs in the regulation of glucose absorption in vivo and in intact intestinal preparations ex vivo.

Results

STR signaling enhances the rate of intestinal glucose absorption specifically in response to the ingestion of a glucose-rich meal. These effects were mediated specifically by the regulation of GLUT2 transporter trafficking to the apical membrane of enterocytes. GLUT2 translocation and glucose transport was dependent and specific to glucagon-like peptide 2 (GLP-2) secretion and subsequent intestinal neuronal activation. Finally, high-sucrose feeding in wild-type mice induced rapid downregulation of STRs in the gut, leading to reduced glucose absorption.

Conclusions

Our studies demonstrate that STRs have evolved to modulate glucose absorption via the regulation of its transport and to prevent the development of exacerbated hyperglycemia due to the ingestion of high levels of sugars.

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The intestinal T1R2 receptor enhances glucose absorption in vivo and ex vivo.

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Pharmacological inhibition of STRs reduces glucose flux in human intestinal preparations.

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T1R2 regulates glucose absorption dependent on GLUT2 activity in enterocytes.

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GLP-2 mediates the effects of T1R2 signaling through activation of enteric neurons.

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High sucrose diet rapidly downregulates STRs leading to reduced glucose absorption.

The Intestinotrophic Effects of Glucagon-Like Peptide-2 in Relation to Intestinal Neoplasia

CONTEXT

Glucagon-like peptide-2 (GLP-2) is a gastrointestinal hormone with intestinotrophic and antiapoptotic effects. The hormone's therapeutic potential in intestinal diseases and relation to intestinal neoplasia has raised great interest among researchers. This article reviews and discusses published experimental and clinical studies concerning the growth-stimulating and antiapoptotic effects of GLP-2 in relation to intestinal neoplasia.

EVIDENCE ACQUISITION

The data used in this narrative review were collected through literature research in PubMed using English keywords. All studies to date examining GLP-2's relation to intestinal neoplasms have been reviewed in this article, as the studies on the matter are sparse.

EVIDENCE SYNTHESIS

GLP-2 has been found to stimulate intestinal growth through secondary mediators and through the involvement of Akt phosphorylation. Studies on rodents have shown that exogenously administered GLP-2 increases the growth and incidence of adenomas in the colon, suggesting that GLP-2 may play an important role in the progression of intestinal tumors. Clinical studies have found that exogenous GLP-2 treatment is well tolerated for up to 30 months, but the tolerability for even longer periods of treatment has not been examined.

CONCLUSION

Exogenous GLP-2 is currently available as teduglutide for the treatment of short bowel syndrome. However, the association between exogenous GLP-2 treatment and intestinal neoplasia in humans has not been fully identified. This leads to a cause for concern regarding the later risk of the development or progression of intestinal tumors with long-term GLP-2 treatment. Therefore, further research regarding GLP-2's potential relation to intestinal cancers is needed.

Cholinergic Submucosal Neurons Display Increased Excitability Following in Vivo Cholera Toxin Exposure in Mouse Ileum

Cholera-induced hypersecretion causes dehydration and death if untreated. Cholera toxin (CT) partly acts via the enteric nervous system (ENS) and induces long-lasting changes to enteric neuronal excitability following initial exposure, but the specific circuitry involved remains unclear. We examined this by first incubating CT or saline (control) in mouse ileal loops in vivo for 3.5 h and then assessed neuronal excitability in vitro using Ca²⁺ imaging and immunolabeling for the activity-dependent markers cFos and pCREB. Mice from a C57BL6 background, including Wnt1-Cre;R26R-GCaMP3 mice which express the fluorescent Ca²⁺ indicator GCaMP3 in its ENS, were used. Ca²⁺-imaging using this mouse model is a robust, high-throughput method which allowed us to examine the activity of numerous enteric neurons simultaneously and post-hoc immunohistochemistry enabled the neurochemical identification of the active neurons. Together, this provided novel insight into the CT-affected circuitry that was previously impossible to attain at such an accelerated pace. Ussing chamber measurements of electrogenic ion secretion showed that CT-treated preparations had higher basal secretion than controls. Recordings of Ca²⁺ activity from the submucous plexus showed that increased numbers of neurons were spontaneously active in CT-incubated tissue (control: 4/149; CT: 32/160; Fisher's exact test, P < 0.0001) and that cholinergic neurons were more responsive to electrical (single pulse and train of 20 pulses) or nicotinic (1,1-dimethyl-4-phenylpiperazinium (DMPP; 10 μM) stimulation. Expression of the neuronal activity marker, pCREB, was also increased in the CT-treated submucous plexus neurons. c-Fos expression and spontaneous fast excitatory postsynaptic potentials (EPSPs), recorded by intracellular electrodes, were increased by CT exposure in a small subset of myenteric neurons. However, the effect of CT on the myenteric plexus is less clear as spontaneous Ca²⁺ activity and electrical- or nicotinic-evoked Ca²⁺ responses were reduced. Thus, in a model where CT exposure evokes hypersecretion, we observed sustained activation of cholinergic secretomotor neuron activity in the submucous plexus, pointing to involvement of these neurons in the overall response to CT.

Hirschsprung disease - integrating basic science and clinical medicine to improve outcomes

Hirschsprung disease is defined by the absence of enteric neurons at the end of the bowel. The enteric nervous system (ENS) is the intrinsic nervous system of the bowel and regulates most aspects of bowel function. When the ENS is missing, there are no neurally mediated propulsive motility patterns, and the bowel remains contracted, causing functional obstruction. Symptoms of Hirschsprung disease include constipation, vomiting, abdominal distension and growth failure. Untreated disease usually causes death in childhood because bloodstream bacterial infections occur in the context of bowel inflammation (enterocolitis) or bowel perforation. Current treatment is surgical resection of the bowel to remove or bypass regions where the ENS is missing, but many children have problems after surgery. Although the anatomy of Hirschsprung disease is simple, many clinical features remain enigmatic, and diagnosis and management remain challenging. For example, the age of presentation and the type of symptoms that occur vary dramatically among patients, even though every affected child has missing neurons in the distal bowel at birth. In this Review, basic science discoveries are linked to clinical manifestations of Hirschsprung disease, including partial penetrance, enterocolitis and genetics. Insights into disease mechanisms that might lead to new prevention, diagnostic and treatment strategies are described.