PI 3-kinase- and ERK-MAPK-dependent mechanisms underlie Glucagon-Like Peptide-1-mediated activation of Sprague Dawley colonic myenteric neurons

Glucagon-Like Peptide-1 Receptor Agonists as Potential Myelination-Inducible and Anti-Demyelinating Remedies

Glucagon-like peptide-1 receptor agonists (GLP-1RAs) were developed as insulinotropic and anti-hyperglycemic agents for the treatment of type 2 diabetes, but their neurotrophic and neuroprotective activities have been receiving increasing attention. Myelin plays a key role in the functional maintenance of the central and peripheral nervous systems, and recent in vivo and in vitro studies have shed light on the beneficial effects of GLP-1RAs on the formation and protection of myelin. In this article, we describe the potential efficacy of GLP-1RAs for the induction of axonal regeneration and remyelination following nerve lesions and the prevention and alleviation of demyelinating disorders, particularly multiple sclerosis.

Mechanisms underlying the prokinetic effects of endogenous glucagon-like peptide-1 in the rat proximal colon

Glucagon-like peptide-1 (GLP-1), a well-known insulin secretagogue, is released from enteroendocrine L cells both luminally and basolaterally to exert different effects. Basolaterally released GLP-1 increases epithelial ion transport by activating CGRP-containing enteric afferent neurons. Although bath-applied GLP-1 reduced the contractility of colonic segments, GLP-1-induced stimulation of afferent neurons could also accelerate peristaltic contractions. Here, the roles of endogenous GLP-1 in regulating colonic peristalsis were investigated using isolated colonic segments. Isolated segments of rat proximal colon were placed in an organ bath, serosally perfused with oxygenated physiological salt solution, and luminally perfused with degassed 0.9% saline. Colonic wall motion was recorded using a video camera and converted into spatiotemporal maps. Intraluminal administration of GLP-1 (100 nM) stimulating the secretion of GLP-1 from L cells increased the frequency of oro-aboral propagating peristaltic contractions. The acceleratory effect of GLP-1 was blocked by luminally applied exendin-3 (9-39) (100 nM), a GLP-1 receptor antagonist. GLP-1-induced acceleration of peristaltic contractions was also prevented by bath-applied BIBN4069 (1 μM), a CGRP receptor antagonist. In colonic segments that had been exposed to bath-applied capsaicin (100 nM) that desensitizes extrinsic afferents, GLP-1 was still capable of exerting its prokinetic effect. Stimulation of endogenous GLP-1 secretion with a luminally applied cocktail of short-chain fatty acids (1 mM) increased the frequency of peristaltic waves in an exendin-3 (9-39)-sensitive manner. Thus, GLP-1 activates CGRP-expressing intrinsic afferents to accelerate peristalsis in the proximal colon. Short-chain fatty acids appear to stimulate endogenous GLP-1 secretion from L cells resulting in the acceleration of colonic peristalsis.NEW & NOTEWORTHY Glucagon-like peptide-1 (GLP-1) activates CGRP-containing intrinsic afferent neurons resulting in the acceleration of colonic peristalsis. Short-chain fatty acids stimulate the secretion of endogenous GLP-1 from L cells that accelerates colonic peristalsis. Thus, besides the well-known humoral insulinotropic action, GLP-1 exerts a local action via the activation of the enteric nervous system to accelerate colonic motility. Such a prokinetic action of GLP-1 could underlie the mechanisms causing diarrhea in patients with type-2 diabetes treated with GLP-1 analogs.

Exendin-4 Promotes Schwann Cell Survival/Migration and Myelination In Vitro

Besides its insulinotropic actions on pancreatic β cells, neuroprotective activities of glucagon-like peptide-1 (GLP-1) have attracted attention. The efficacy of a GLP-1 receptor (GLP-1R) agonist exendin-4 (Ex-4) for functional repair after sciatic nerve injury and amelioration of diabetic peripheral neuropathy (DPN) has been reported; however, the underlying mechanisms remain unclear. In this study, the bioactivities of Ex-4 on immortalized adult rat Schwann cells IFRS1 and adult rat dorsal root ganglion (DRG) neuron–IFRS1 co-culture system were investigated. Localization of GLP-1R in both DRG neurons and IFRS1 cells were confirmed using knockout-validated monoclonal Mab7F38 antibody. Treatment with 100 nM Ex-4 significantly enhanced survival/proliferation and migration of IFRS1 cells, as well as stimulated the movement of IFRS1 cells toward neurites emerging from DRG neuron cell bodies in the co-culture with the upregulation of myelin protein 22 and myelin protein zero. Because Ex-4 induced phosphorylation of serine/threonine-specific protein kinase AKT in these cells and its effects on DRG neurons and IFRS1 cells were attenuated by phosphatidyl inositol-3′-phosphate-kinase (PI3K) inhibitor LY294002, Ex-4 might act on both cells to activate PI3K/AKT signaling pathway, thereby promoting myelination in the co-culture. These findings imply the potential efficacy of Ex-4 toward DPN and other peripheral nerve lesions.

Glucagon-Like Peptide-1 Secreting L-Cells Coupled to Sensory Nerves Translate Microbial Signals to the Host Rat Nervous System

An intact gut epithelium preserves the immunological exclusion of “non-self” entities in the external environment of the gut lumen. Nonetheless, information flows continuously across this interface, with the host immune, endocrine, and neural systems all involved in monitoring the luminal environment of the gut. Both pathogenic and commensal gastrointestinal (GI) bacteria can modulate centrally-regulated behaviors and brain neurochemistry and, although the vagus nerve has been implicated in the microbiota-gut-brain signaling axis, the cellular and molecular machinery that facilitates this communication is unclear. Studies were carried out in healthy Sprague–Dawley rats to understand cross-barrier communication in the absence of disease. A novel colonic-nerve electrophysiological technique was used to examine gut-to-brain vagal signaling by bacterial products. Calcium imaging and immunofluorescent labeling were used to explore the activation of colonic submucosal neurons by bacterial products. The findings demonstrate that the neuromodulatory molecule, glucagon-like peptide-1 (GLP-1), secreted by colonic enteroendocrine L-cells in response to the bacterial metabolite, indole, stimulated colonic vagal afferent activity. At a local level indole modified the sensitivity of submucosal neurons to GLP-1. These findings elucidate a cellular mechanism by which sensory L-cells act as cross-barrier signal transducers between microbial products in the gut lumen and the host peripheral nervous system.

The Glucagon-like peptide-1 receptor agonist, exendin-4, ameliorated gastrointestinal dysfunction in the Wistar Kyoto rat model of Irritable Bowel Syndrome

BACKGROUND

Glucagon-like peptide-1 (GLP-1) is beneficial in relieving pain-related symptoms of Irritable bowel syndrome (IBS), a prevalent, multi-factorial functional bowel disorder characterized by diarrhea and/or constipation, abdominal bloating, and pain. Activation of myenteric neurons has been implicated in the inhibitory effects of GLP-1 on gastrointestinal motility; however, the mechanisms of action underlying this are not clear.

METHODS

A rat model of IBS was used to examine physiological changes evoked by intraperitoneal administration of a GLP-1 receptor agonist, exendin-4. Behavioral and physiological analysis of stress-sensitive Wister Kyoto (WKY) rats was used to determine if administration of exendin-4, in the presence or absence of neutralizing interleukin-6 receptor monoclonal antibodies, modified IBS-like symptoms. Immunofluorescence, calcium imaging, and Western blotting techniques were used to investigate the potential role of enteric neural plexi and tight junction protein expression in this effect.

KEY RESULTS

Consistent with the expression of GLP-1 and interleukin-6 receptors in both submucosal and myenteric ganglia, exendin-4 and interleukin-6 stimulated calcium responses in these neurons. In vivo administration of exendin-4 normalized stress-induced defecation and visceral pain sensitivity in WKY rats. No additional changes were noted in rats co-treated with exendin-4 and anti-interleukin-6 receptor antibodies. Mucosal expression of occludin, a tight junction protein, was decreased by exendin-4. Centrally regulated anxiety-like behaviors were not modified.

CONCLUSIONS AND INFERENCES

These data suggest that intraperitoneal injection of exendin-4 improves bowel dysfunction in WKY rats without impacting on centrally regulated anxiety-like behaviors. Modulation of enteric neuronal function and tight junction expression appear to underlie the functional benefits of this intervention.