Apposition of enteric nerve fibers to plasma cells and immunoblasts in the mouse small bowel

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.

Neural pathways involved in infection-induced inflammation: recent insights and clinical implications

Although the immune and nervous systems have long been considered independent biological systems, they turn out to mingle and interact extensively. The present review summarizes recent insights into the neural pathways activated by and involved in infection-induced inflammation and discusses potential clinical applications. The simplest activation concerns a reflex action within C-fibers leading to neurogenic inflammation. Low concentrations of pro-inflammatory cytokines or bacterial fragments may also act on these afferent nerve fibers to signal the central nervous system and bring about early fever, hyperalgesia and sickness behavior. In the brain, the preoptic area and the paraventricular hypothalamus are part of a neuronal network mediating sympathetic activation underlying fever while brainstem circuits play a role in the reduction of food intake after systemic exposure to bacterial fragments. A vagally-mediated anti-inflammatory reflex mechanism has been proposed and, in turn, questioned because the major immune organs driving inflammation, such as the spleen, are not innervated by vagal efferent fibers. On the contrary, sympathetic nerves do innervate these organs and modulate immune cell responses, production of inflammatory mediators and bacterial dissemination. Noradrenaline, which is both released by these fibers and often administered during sepsis, along with adrenaline, may exert pro-inflammatory actions through the stimulation of β1 adrenergic receptors, as antagonists of this receptor have been shown to exert anti-inflammatory effects in experimental sepsis.

Purinergic signalling and immune cells

This review article provides a historical perspective on the role of purinergic signalling in the regulation of various subsets of immune cells from early discoveries to current understanding. It is now recognised that adenosine 5'-triphosphate (ATP) and other nucleotides are released from cells following stress or injury. They can act on virtually all subsets of immune cells through a spectrum of P2X ligand-gated ion channels and G protein-coupled P2Y receptors. Furthermore, ATP is rapidly degraded into adenosine by ectonucleotidases such as CD39 and CD73, and adenosine exerts additional regulatory effects through its own receptors. The resulting effect ranges from stimulation to tolerance depending on the amount and time courses of nucleotides released, and the balance between ATP and adenosine. This review identifies the various receptors involved in the different subsets of immune cells and their effects on the function of these cells.

Reprint of: Role of enteric neurotransmission in host defense and protection of the gastrointestinal tract

Host defense is a vital role played by the gastrointestinal tract. As host to an enormous and diverse microbiome, the gut has evolved an elaborate array of chemical and physicals barriers that allow the digestion and absorption of nutrients without compromising the mammalian host. The control of such barrier functions requires the integration of neural, humoral, paracrine and immune signaling, involving redundant and overlapping mechanisms to ensure, under most circumstances, the integrity of the gastrointestinal epithelial barrier. Here we focus on selected recent developments in the autonomic neural control of host defense functions used in the protection of the gut from luminal agents, and discuss how the microbiota may potentially play a role in enteric neurotransmission. Key recent findings include: the important role played by subepithelial enteric glia in modulating intestinal barrier function, identification of stress-induced mechanisms evoking barrier breakdown, neural regulation of epithelial cell proliferation, the role of afferent and efferent vagal pathways in regulating barrier function, direct evidence for bacterial communication to the enteric nervous system, and microbial sources of enteric neurotransmitters. We discuss these new and interesting developments in our understanding of the role of the autonomic nervous system in gastrointestinal host defense.

Role of enteric neurotransmission in host defense and protection of the gastrointestinal tract

Host defense is a vital role played by the gastrointestinal tract. As host to an enormous and diverse microbiome, the gut has evolved an elaborate array of chemical and physicals barriers that allow the digestion and absorption of nutrients without compromising the mammalian host. The control of such barrier functions requires the integration of neural, humoral, paracrine and immune signaling, involving redundant and overlapping mechanisms to ensure, under most circumstances, the integrity of the gastrointestinal epithelial barrier. Here we focus on selected recent developments in the autonomic neural control of host defense functions used in the protection of the gut from luminal agents, and discuss how the microbiota may potentially play a role in enteric neurotransmission. Key recent findings include: the important role played by subepithelial enteric glia in modulating intestinal barrier function, identification of stress-induced mechanisms evoking barrier breakdown, neural regulation of epithelial cell proliferation, the role of afferent and efferent vagal pathways in regulating barrier function, direct evidence for bacterial communication to the enteric nervous system, and microbial sources of enteric neurotransmitters. We discuss these new and interesting developments in our understanding of the role of the autonomic nervous system in gastrointestinal host defense.

Neuronal and nonneuronal cholinergic structures in the mouse gastrointestinal tract and spleen

Accumulating evidence demonstrates that acetylcholine can directly modulate immune function in peripheral tissues including the spleen and gastrointestinal tract. However, the anatomical relationships between the peripheral cholinergic system and immune cells located in these lymphoid tissues remain unclear due to inherent technical difficulties with currently available neuroanatomical methods. In this study, mice with specific expression of the tdTomato fluorescent protein in choline acetyltransferase (ChAT)-expressing cells were used to label preganglionic and postganglionic cholinergic neurons and their projections to lymphoid tissues. Notably, our anatomical observations revealed an abundant innervation in the intestinal lamina propria of the entire gastrointestinal tract principally originating from cholinergic enteric neurons. The aforementioned innervation frequently approached macrophages, plasma cells, and lymphocytes located in the lamina propria and, to a lesser extent, lymphocytes in the interfollicular areas of Peyer's patches. In addition to the above innervation, we observed labeled epithelial cells in the gallbladder and lower intestines, as well as Microfold cells and T-cells within Peyer's patches. In contrast, we found only a sparse innervation in the spleen consisting of neuronal fibers of spinal origin present around arterioles and in lymphocyte-containing areas of the white pulp. Lastly, a small population of ChAT-expressing lymphocytes was identified in the spleen including both T- and B-cells. In summary, this study describes the variety of cholinergic neuronal and nonneuronal cells in a position to modulate gastrointestinal and splenic immunity in the mouse.

Sympathetic modulation of immunity: relevance to disease

Optimal host defense against pathogens requires cross-talk between the nervous and immune systems. This paper reviews sympathetic-immune interaction, one major communication pathway, and its importance for health and disease. Sympathetic innervation of primary and secondary immune organs is described, as well as evidence for neurotransmission with cells of the immune system as targets. Most research thus far has focused on neural-immune modulation in secondary lymphoid organs, has revealed complex sympathetic modulation resulting in both potentiation and inhibition of immune functions. SNS-immune interaction may enhance immune readiness during disease- or injury-induced 'fight' responses. Research also indicate that dysregulation of the SNS can significantly affect the progression of immune-mediated diseases. However, a better understanding of neural-immune interactions is needed to develop strategies for treatment of immune-mediated diseases that are designed to return homeostasis and restore normal functioning neural-immune networks.

Morphological relationships between peptidergic nerve fibers and immunoglobulin A-producing lymphocytes in the mouse intestine

Immunoglobulin A (IgA) lymphocytes are present close to the nerve fibers in the lamina propria of the small intestine, and the administration of lipopolysaccharides (LPSs) increases the number of these cells and IgA secretion to the lumen. In the present study, we demonstrated that the nerve fibers immunoreactive for vasoactive intestinal peptide (VIP), neuropeptide Y (NPY), and calcitonin gene-related peptide (CGRP) were close to the IgA lymphocytes in the mouse ileum lamina propria. Three hours after intraperitoneal administration of LPSs, IgA lymphocytes close to VIP nerve fibers, those close to basement membrane, and those close to both VIP nerve fibers and basement membrane were increased in number. Further, all IgA lymphocytes seen in the ileum lamina propria expressed the receptors for VIP, VIPR1, and VIPR2. Electron microscopy revealed that varicosities were in close apposition to the lymphocyte plasma membrane. The present study suggests that VIP/NPY/CGRP neurons in the submucosal plexus have a close anatomical relationship to IgA lymphocytes, playing a role in the secretion of IgA and intestinal fluid in response to stimulation by lipopolysaccharides, pathogens, or toxins.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Non-synaptic transmission at autonomic neuroeffector junctions

Non-synaptic transmission is characteristic of autonomic neuroeffector junctions. The structure of the autonomic neuromuscular junction is described. The essential features are that: the terminal portions of autonomic nerve fibers are varicose and mobile, transmitters being released 'en passage' from varying distances from the effector cells; while there is no structural post-junctional specialization on effector cells, receptors for neurotransmitters accumulate on cell membranes at close junctions; muscle effectors are bundles rather than single smooth muscle cells, that are connected by gap junctions which allow electrotonic spread of activity between cells. A multiplicity of transmitters are utilized by autonomic nerves, and cotransmission occurs often involving synergistic actions of the cotransmitters, although pre- and post-junctional neuromodulation of neurotransmitter release also take place. It is suggested that autonomic neural control of immune, epithelial and endothelial cells also involves non-synaptic transmission.

Caspase inhibition increases survival of neural stem cells in the gastrointestinal tract

Neural stem cell (NSC) transplantation is a promising tool for the restoration of the enteric nervous system in a variety of motility disorders. Post-transplant survival represents a critical limiting factor for successful repopulation. The aim of this study was to determine the role of both immunological as well as non-immune-mediated mechanisms on post-transplant survival of NSC in the gut. Mouse CNS-derived NSC (CNS-NSC) were transplanted into the pylorus of recipient mice with and without the addition of a caspase-1 inhibitor (Ac-YVAD-cmk) in the injection media. In a separate experiment, CNS-NSC were transplanted in the pylorus of mice that were immunosuppressed by administration of cyclosporin A (CsA). Apoptosis and proliferation of the implanted cells was assessed 1 and 7 days post-transplantation. Survival was assessed 1 week post-transplantation. The degree of immunoresponse was also measured. The addition of a caspase-1 inhibitor significantly reduced apoptosis, increased proliferation and enhanced survival of CNS-NSC. CsA-treatment did not result in improved survival. Our results indicate that caspase-1 inhibition, but not immunosuppression, improves survival of CNS-NSC in the gut. Pre-treatment with a caspase-1 inhibitor may be a practical method to enhance the ability of transplanted CNS-NSC to survive in their new environment.

Morphological analysis of the relation between immunoglobulin A production in the small intestine and the enteric nervous system

To investigate the relation between the enteric nervous system and immunocytes, fecal immunoglobulin (Ig) A and the distribution of IgA-positive cells close to nerve fibers in the lamina propria around the crypts of the small intestine were examined after intraperitoneal injection of lipopolysaccharides (LPS). In mice, IgA in the feces 1 h (LPS1) and 6 h (LPS6) after LPS injection increased compared to that in the control group. In the ileum lamina propria of the LPS1 group, the number of IgA-positive cells close to the epithelial basement membrane was increased. In the LPS6 group, on the other hand, there was a significant increase in the number of IgA-positive cells close to both the basement membrane and the nerve fibers. Our data suggest that LPS induced an increase in the number of IgA-positive cells migrating to the nerve fibers, and that migration to the nerve fibers is as important as migration to the basement membrane for IgA production.

Motility of the small intestine

Motility of the small intestine is controlled by myogenic, neural, and hormonal mechanisms and is modulated by external influences such as meals, central nervous system activation, and immune factors. Small-bowel dysmotility is recognized in a number of diseases, but its precise role in symptom generation remains unclear in many instances. We review publications that in the year under review added to the basic understanding of small-intestinal motility as well as its alteration in disease.