<em>In Situ</em> Ca<sup>2+</sup> Imaging of the Enteric Nervous System

Reflex behaviors of the intestine are controlled by the enteric nervous system (ENS). The ENS is an integrative network of neurons and glia in two ...

Michigan State University

Nasal trigeminal chemosensitivity in mice and rats is mediated in part by solitary chemoreceptor cells (SCCs) in the nasal epithelium (Finger et al., 2003). Many nasal SCCs express the G-protein alpha-gustducin as well as other elements of the bitter-taste signaling cascade including phospholipase Cbeta2, TRPM5 and T2R bitter-taste receptors. While some populations of sensory cells are replaced throughout life (taste and olfaction), others are not (hair cells and carotid body chemoreceptors). These experiments were designed to test whether new SCCs are generated within the epithelium of adult mice. Wild type C57/B6 mice were injected with the thymidine analog 5-bromo-2'-deoxyuridine (BrdU) to label dividing cells. At various times after injection (1-40 days), the mice were perfused with 4% paraformaldehyde and prepared for dual-label immunocytochemistry. Double labeled cells were detected as early as 3 days post BrdU injection and remained for as long as 12 days post-injection suggesting that SCCs do undergo turnover like the surrounding nasal epithelium. No BrdU labeled cells were detected after 24 days suggesting relatively rapid replacement of the SCCs.

Solitary chemoreceptor cell proliferation in adult nasal epithelium.

Nasal trigeminal chemosensitivity in mice and rats is mediated in part by epithelial solitary chemoreceptor (chemosensory) cells (SCCs), but the exact role of these cells in chemoreception is unclear. Histological evidence suggests that SCCs express elements of the bitter taste transduction pathway including T2R (bitter taste) receptors, the G protein alpha-gustducin, PLCbeta2, and TRPM5, leading to speculation that SCCs are the receptor cells that mediate trigeminal nerve responses to bitter taste receptor ligands. To test this hypothesis, we used calcium imaging to determine whether SCCs respond to classic bitter-tasting or trigeminal stimulants. SCCs from the anterior nasal cavity were isolated from transgenic mice in which green fluorescent protein (GFP) expression was driven by either TRPM5 or gustducin. Isolated cells were exposed to a variety of test stimuli to determine which substances caused an increase in intracellular Ca2+ ([Ca2+]i). GFP-positive cells respond with increased [Ca2+]i to the bitter receptor ligand denatonium and this response is blocked by the PLC inhibitor U73122. In addition, GFP+ cells respond to the neuromodulators adenosine 5'-triphosphate and acetylcholine but only very rarely to other bitter-tasting or trigeminal stimuli. Our results demonstrate that TRPM5- and gustducin-expressing nasal SCCs respond to the T2R agonist denatonium via a PLC-coupled transduction cascade typical of T2Rs in the taste system.

Nasal solitary chemoreceptor cell responses to bitter and trigeminal stimulants in vitro.

Enteric glia are intimately associated with neurons in the enteric nervous system (ENS) and display morphologic and molecular similarities to central nervous system (CNS) astrocytes. Enteric glia express neurotransmitter receptors, suggesting that, like astrocytes, they are active participants in neuronal communication. In the ENS, the purine adenosine triphosphate (ATP) is co-released with the neurotransmitters noradrenaline and acetylcholine. Enteric glia express purinergic receptors and respond to ATP in vitro, suggesting that enteric glia participate in functional gastrointestinal responses to nerve signaling. We investigated whether enteric glia are activated by ATP released from enteric neurons.

Purinergic neuron-to-glia signaling in the enteric nervous system.

The upper respiratory tract is continually assaulted with harmful dusts and xenobiotics carried on the incoming airstream. Detection of such irritants by the trigeminal nerve evokes protective reflexes, including sneezing, apnea, and local neurogenic inflammation of the mucosa. Although free intra-epithelial nerve endings can detect certain lipophilic irritants (e.g., mints, ammonia), the epithelium also houses a population of trigeminally innervated solitary chemosensory cells (SCCs) that express T2R bitter taste receptors along with their downstream signaling components. These SCCs have been postulated to enhance the chemoresponsive capabilities of the trigeminal irritant-detection system. Here we show that transduction by the intranasal solitary chemosensory cells is necessary to evoke trigeminally mediated reflex reactions to some irritants including acyl-homoserine lactone bacterial quorum-sensing molecules, which activate the downstream signaling effectors associated with bitter taste transduction. Isolated nasal chemosensory cells respond to the classic bitter ligand denatonium as well as to the bacterial signals by increasing intracellular Ca(2+). Furthermore, these same substances evoke changes in respiration indicative of trigeminal activation. Genetic ablation of either G alpha-gustducin or TrpM5, essential elements of the T2R transduction cascade, eliminates the trigeminal response. Because acyl-homoserine lactones serve as quorum-sensing molecules for gram-negative pathogenic bacteria, detection of these substances by airway chemoreceptors offers a means by which the airway epithelium may trigger an epithelial inflammatory response before the bacteria reach population densities capable of forming destructive biofilms.

Nasal chemosensory cells use bitter taste signaling to detect irritants and bacterial signals.

Astrocytes respond to synaptic activity in the CNS. Astrocytic responses are synapse specific and precisely regulate synaptic activity. Glia in the peripheral nervous system also respond to neuronal activity, but it is unknown whether glial responses are synapse specific. We addressed this issue by examining the activation of enteric glia by distinct neuronal subpopulations in the enteric nervous system. Enteric glia are unique peripheral glia that surround enteric neurons and respond to neuronally released ATP with increases in intracellular calcium ([Ca2+]i). Autonomic control of colonic function is mediated by intrinsic (enteric) and extrinsic (sympathetic, parasympathetic, primary afferent) neural pathways. Here we test the hypothesis that a defined population of neurons activates enteric glia using a variety of techniques to ablate or stimulate components of the autonomic innervation of the colon. Our findings demonstrate that, in the male guinea pig colon, activation of intrinsic neurons does not stimulate glial [Ca2+]i responses and fast enteric neurotransmission is not necessary to initiate glial responses. However, ablating extrinsic innervation significantly reduces glial responses to neuronal activation. Activation of primary afferent fibers does not activate glial [Ca2+]i responses. Selectively ablating sympathetic fibers reduces glial activation to a similar extent as total extrinsic denervation. Neuronal activation of glia follows the same frequency dependence as sympathetic neurotransmitter release, but the only sympathetic neurotransmitter that activates glial [Ca2+]i responses is ATP, suggesting that sympathetic fibers release ATP to activate enteric glia. Therefore, enteric glia discern activity in adjacent synaptic pathways and selectively respond to sympathetic activation.

Enteric glia are targets of the sympathetic innervation of the myenteric plexus in the guinea pig distal colon.

Extracellular nucleotides and adenosine are biologically active molecules that bind members of the P2 and P1 receptor families, respectively. In the digestive system, these receptors modulate various functions, including salivary, gastric, and intestinal epithelial secretion and enteric neurotransmission. The availability of P1 and P2 ligands is modulated by ectonucleotidases, enzymes that hydrolyze extracellular nucleotides into nucleosides. Nucleoside triphosphate diphosphohydrolases (NTPDases) and ecto-5'-nucleotidase are the dominant ectonucleotidases at physiological pH. While there is some information about the localization of ecto-5'-nucleotidase and NTPDase1 and -2, the distribution of NTPDase3 in the digestive system is unknown. We examined the localization of these ectonucleotidases, with a focus on NTPDase3, in the gastrointestinal tract and salivary glands. NTPDase1, -2, and -3 are responsible for ecto-ATPase activity in these tissues. Semiquantitative RT-PCR, immunohistochemistry, and in situ enzyme activity revealed the presence of NTPDase3 in some epithelial cells in serous acini of salivary glands and mucous acini and duct cells of sublingual salivary glands, in cells from the stratified esophageal and forestomach epithelia, and in some enteroendocrine cells of the gastric antrum. Interestingly, NTPDase2 and ecto-5'-nucleotidase are coexpressed with NTPDase3 in salivary gland cells and stratified epithelia. In the colon, neurons express NTPDase3 and glial cells express NTPDase2. Ca(2+) imaging experiments demonstrate that NTPDases regulate P2 receptor ligand availability in the enteric nervous system. In summary, the specific localization of NTPDase3 in the digestive system suggests functional roles of the enzyme, in association with NTPDase2 and ecto-5'-nucleotidase, in epithelial functions such as secretion and in enteric neurotransmission.

Ectonucleotidases in the digestive system: focus on NTPDase3 localization.

Inflammatory bowel diseases (IBDs) are chronic relapsing and remitting conditions associated with long-term gut dysfunction resulting from alterations to the enteric nervous system and a loss of enteric neurons. The mechanisms underlying inflammation-induced enteric neuron death are unknown. Here using in vivo models of experimental colitis we report that inflammation causes enteric neuron death by activating a neuronal signaling complex composed of P2X7 receptors (P2X7Rs), pannexin-1 (Panx1) channels, the Asc adaptor protein and caspases. Inhibition of P2X7R, Panx1, Asc or caspase activity prevented inflammation-induced neuron cell death. Preservation of enteric neurons by inhibiting Panx1 in vivo prevented the onset of inflammation-induced colonic motor dysfunction. Panx1 expression was reduced in Crohn's disease but not ulcerative colitis. We conclude that activation of neuronal Panx1 underlies neuron death and the subsequent development of abnormal gut motility in IBD. Targeting Panx1 represents a new neuroprotective strategy to ameliorate the progression of IBD-associated dysmotility.

Activation of neuronal P2X7 receptor-pannexin-1 mediates death of enteric neurons during colitis.

The barrier function of the epithelium lining the intestine is essential for health by preventing the free passage of colonic bacteria into the mucosa. Epithelia treated with interferon (IFN)-γ display increased bacteria transcytosis. Much is known of how IFNγ affects the tight junction and paracellular permeability, yet its role in modifying transcellular traffic of commensal bacteria remains poorly understood. Using immunoblotting, ELISA and immunolocalization, IFNγ was found to activate extracellular regulated kinase (ERK)1/2 in the human colon-like T84 epithelial cell line. Pharmacological inhibition of MEK/ERK1/2 signalling with U0126 significantly inhibited IFNγ-induced increases in the transcytosis of non-invasive Escherichia coli (strain HB101). IFNγ treatment enhanced epithelial internalization of E. coli, some of which subsequently escaped the enterocyte. Molecular analyses revealed that ERK1/2 inhibition prevented activation of the ADP-ribosylation factor (ARF)-6, a protein associated with endocytosis, and that siRNA knock-down of ARF6 expression reduced IFNγ-induced E. coli internalization into T84 cells. None of these interventions affected the drop in transepithelial resistance caused by IFNγ. Thus, increased transcellular passage may be a major component of IFNγ-induced increases in epithelial permeability, and ERK1/2 and ARF6 are presented as important molecules in IFNγ-evoked transcytosis of bacteria across gut epithelia.

Interferon-gamma signals via an ERK1/2-ARF6 pathway to promote bacterial internalization by gut epithelia.

Enteric glia are a unique class of peripheral glial cells within the gastrointestinal tract. Major populations of enteric glia are found in enteric ganglia in the myenteric and submucosal plexuses of the enteric nervous system (ENS); these cells are also found outside of the ENS, within the circular muscle and in the lamina propria of the mucosa. These different populations of cells probably represent unique classes of glial cells with differing functions. In the past few years, enteric glia have been found to be involved in almost every gut function including motility, mucosal secretion and host defence. Subepithelial glia seem to have a trophic and supporting relationship with intestinal epithelial cells, but the necessity of these roles in the maintenance of normal epithelial functions remains to be shown. Likewise, glia within enteric ganglia are activated by synaptic stimulation, suggesting an active role in synaptic transmission, but the precise role of glial activation in normal enteric network activity is unclear. Excitingly, enteric glia can also give rise to new neurons, but seemingly only under limited circumstances. In this Review, we discuss the current body of evidence supporting functional roles of enteric glia and identify key gaps in our understanding of the physiology of these unique cells.

Novel functional roles for enteric glia in the gastrointestinal tract.

In the enteric nervous system, neurotransmitters initiate changes in calcium (Ca(2+) responses) in glia, but it is not clear how this process affects intestinal function. We investigated whether Ca(2+)-mediated responses in enteric glia are required to maintain gastrointestinal function.

Brian D. Gulbransen

Department of Physiology

Michigan State University

In Situ Ca²⁺ Imaging of the Enteric Nervous System

Brian D. Gulbransen

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Physiology

Michigan State University

In Situ Ca2+ Imaging of the Enteric Nervous System

Department of Physiology

In Situ Ca²⁺ Imaging of the Enteric Nervous System