Evidence for Neuromodulation of Enteropathogen Invasion in the Intestinal Mucosa

Influence of bacterial components on the developmental programming of enteric neurons

BACKGROUND

Intestinal bacteria have been increasingly shown to be involved in early postnatal development. Previous work has shown that intestinal bacteria are necessary for the structural development and intrinsic function of the enteric nervous system in early postnatal life. Furthermore, colonization with a limited number of bacteria appears to be sufficient for the formation of a normal enteric nervous system. We tested the hypothesis that common bacterial components could influence the programming of developing enteric neurons.

METHODS

The developmental programming of enteric neurons was studied by isolating enteric neural crest-derived cells from the fetal gut of C57Bl/6 mice at embryonic day 15.5. After the establishment of the cell line, cultured enteric neuronal precursors were exposed to increasing concentrations of a panel of bacterial components including lipopolysaccharide, flagellin, and components of peptidoglycan.

KEY RESULT

Exposure to bacterial components consistently affected proportions of enteric neuronal precursors that developed into nitrergic neurons. Furthermore, flagellin and D-gamma-Glu-mDAP were found to promote the development of serotonergic neurons. Proportions of dopaminergic neurons remained unchanged. Proliferation of neuronal precursor cells was significantly increased upon exposure to lipopolysaccharide and flagellin, while no significant changes were observed in the proportion of apoptotic neuronal precursors compared to baseline with exposure to any bacterial component.

CONCLUSIONS AND INTERFACES

These findings suggest that bacterial components may influence the development of enteric neurons.

Modulation of the Interaction of Enteric Bacteria with Intestinal Mucosa by Stress-Related Catecholamines

Stress associated with parturition, transport or mixing has long been correlated with enhanced faecal excretion of diarrhoeal zoonotic pathogens in animals such as Salmonella enterica and Escherichia coli. It may also predispose humans to infection and/or be associated with more severe outcomes. One possible explanation for this phenomenon is the ability of enteric bacterial pathogens to sense and respond to host stress-related catecholamines. This article reviews evidence of the ability of catecholamine hormones to modulate interactions between Gram-negative diarrhoeal pathogens and intestinal mucosa, as well as the molecular mechanisms that may be at work.

Enteric neuronal density contributes to the severity of intestinal inflammation

BACKGROUND & AIMS

Enteric neurons have been reported to be increased in inflamed regions of the bowel in patients with inflammatory bowel disease or intestinal neurogangliomatosis. It is impossible to determine whether this hyperinnervation predates intestinal inflammation, results from it, or contributes to its severity in humans, so we studied this process in mice.

METHODS

To determine whether the density of enteric neurons determines the severity of inflammation, we studied transgenic mice that have greater than normal (NSE-noggin mice, which overexpress noggin under the control of the neuron-specific enolase promoter) or fewer than normal (Hand2(+/-) mice) numbers of neurons in the enteric nervous system. Colitis was induced with trinitrobenzene sulfonic acid or dextran sulfate sodium, and the intensity of the resulting inflammation in Hand2(+/-) and NSE-noggin mice was compared with that of wild-type littermates.

RESULTS

Severity of each form of colitis (based on survival, symptom, and histologic scores; intestinal expression of genes that encode proinflammatory molecules; and levels of neutrophil elastase and p50 nuclear factor κB) were significantly reduced in Hand2(+/-) mice and significantly increased in NSE-noggin animals. Neither mouse differed from wild-type in the severity of delayed-type hypersensitivity (edema, T-cell and neutrophil infiltration, or expression of interleukin-1β, interferon-γ, or tumor necrosis factor-α) induced in the ears using 2,4-dinitro-1-fluorobenzene. Transgene effects on inflammation were therefore restricted to the gastrointestinal tract.

CONCLUSIONS

The severity of intestinal inflammation is associated with the density of the enteric innervation in mice. Abnormalities in development of the enteric nervous system might therefore contribute to the pathogenesis of inflammatory bowel disease.

Stress at the intestinal surface: catecholamines and mucosa-bacteria interactions

Psychological stress has profound effects on gastrointestinal function, and investigations over the past few decades have examined the mechanisms by which neural and hormonal stress mediators act to modulate gut motility, epithelial barrier function and inflammatory states. With its cellular diversity and large commensal bacterial population, the intestinal mucosa and its overlying mucous environment constitute a highly interactive environment for eukaryotic host cells and prokaryotic bacteria. The elaboration of stress mediators, particularly norepinephrine, at this interface influences host cells engaged in mucosal protection and the bacteria which populate the mucosal surface and gut lumen. This review will address growing evidence that norepinephrine and, in some cases, other mediators of the adaptation to stress modulate mucosal interactions with enteric bacteria. Stress-mediated changes in this delicate interplay may shift the microbial colonization patterns on the mucosal surface and alter the susceptibility of the host to infection. Moreover, changes in host-microbe interactions in the digestive tract may also influence ongoing neural activity in stress-responsive brain areas.

Antibiotic-associated diarrhea: epidemiology, trends and treatment

A common complication of antibiotic use is the development of gastrointestinal disease. This complication ranges from mild diarrhea to pseudomembranous colitis. Outbreaks of antibiotic-associated diarrhea (AAD) may also occur in healthcare settings, usually caused by Clostridium difficile. AAD typically occurs in 5-35% of patients taking antibiotics and varies depending upon the specific type of antibiotic, the health of the host and exposure to pathogens. The pathogenesis of AAD may be mediated through the disruption of the normal microbiota resulting in pathogen overgrowth or metabolic imbalances. The key to addressing AAD is prompt diagnosis followed by effective treatment and institution of control measures. Areas of active research include the search for other etiologies and more effective treatments.

Catecholamines and sympathomimetic drugs decrease early Salmonella Typhimurium uptake into porcine Peyer's patches

Peyer's patches of the small intestine serve as inductive sites for mucosal immunity as well as targets for invasive enteropathogens, including Salmonella. Because they are innervated by catecholamine-containing enteric nerves, the hypothesis that the endogenous catecholamines dopamine and norepinephrine or sympathomimetic drugs alter Salmonella Typhimurium uptake into Peyer's patches was tested. Porcine jejunal Peyer's patch explants were mounted in Ussing chambers and inoculated with a porcine field isolate of Salmonella Typhimurium DT104. Salmonella recovery from gentamicin-treated tissues increased significantly between 30 and 90 min of bacterial exposure to the mucosal surface. Addition of the neuronal conduction blocker saxitoxin (0.1 micromol L(-1)) or dopamine (30 micromol L(-1)) to the contraluminal aspect of explants decreased bacterial recovery after 60 min of Salmonella exposure. The effects of dopamine were mimicked by cocaine and methamphetamine (30 micromol L(-1)), which act on catecholaminergic nerve terminals to increase synaptic neurotransmitter concentrations. These results suggest that enteric catecholaminergic nerves modulate Salmonella colonization of Peyer's patches at the earliest stages of infection, in part by altering epithelial uptake of bacteria.