Cellular prion protein is expressed in a subset of neuroendocrine cells of the rat gastrointestinal tract

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.

Neuroimmunophysiology of the gut: advances and emerging concepts focusing on the epithelium

The epithelial lining of the gastrointestinal tract serves as the interface for digestion and absorption of nutrients and water and as a defensive barrier. The defensive functions of the intestinal epithelium are remarkable considering that the gut lumen is home to trillions of resident bacteria, fungi and protozoa (collectively, the intestinal microbiota) that must be prevented from translocation across the epithelial barrier. Imbalances in the relationship between the intestinal microbiota and the host lead to the manifestation of diseases that range from disorders of motility and sensation (IBS) and intestinal inflammation (IBD) to behavioural and metabolic disorders, including autism and obesity. The latest discoveries shed light on the sophisticated intracellular, intercellular and interkingdom signalling mechanisms of host defence that involve epithelial and enteroendocrine cells, the enteric nervous system and the immune system. Together, they maintain homeostasis by integrating luminal signals, including those derived from the microbiota, to regulate the physiology of the gastrointestinal tract in health and disease. Therapeutic strategies are being developed that target these signalling systems to improve the resilience of the gut and treat the symptoms of gastrointestinal disease.

Relevance of TNBS-colitis in rats: a methodological study with endoscopic, histologic and Transcriptomic [corrected] characterization and correlation to IBD

Background

Rectal instillation of trinitrobenzene sulphonic acid (TNBS) in ethanol is an established model for inflammatory bowel disease (IBD). We aimed to 1) set up a TNBS-colitis protocol resulting in an endoscopic and histologic picture resembling IBD, 2) study the correlation between endoscopic, histologic and gene expression alterations at different time points after colitis induction, and 3) compare rat and human IBD mucosal transcriptomic data to evaluate whether TNBS-colitis is an appropriate model of IBD.

Methodology/Principal Findings

Five female Sprague Daley rats received TNBS diluted in 50% ethanol (18 mg/0.6 ml) rectally. The rats underwent colonoscopy with biopsy at different time points. RNA was extracted from rat biopsies and microarray was performed. PCR and in situ hybridization (ISH) were done for validation of microarray results. Rat microarray profiles were compared to human IBD expression profiles (25 ulcerative colitis Endoscopic score demonstrated mild to moderate colitis after three and seven days, but declined after twelve days. Histologic changes corresponded with the endoscopic appearance. Over-represented Gene Ontology Biological Processes included: Cell Adhesion, Immune Response, Lipid Metabolic Process, and Tissue Regeneration. IL-1α, IL-1β, TLR2, TLR4, PRNP were all significantly up-regulated, while PPARγ was significantly down-regulated. Among genes with highest fold change (FC) were SPINK4, LBP, ADA, RETNLB and IL-1α. The highest concordance in differential expression between TNBS and IBD transcriptomes was three days after colitis induction. ISH and PCR results corresponded with the microarray data. The most concordantly expressed biologically relevant pathways included TNF signaling, Cell junction organization, and Interleukin-1 processing.

Conclusions/Significance

Endoscopy with biopsies in TNBS-colitis is useful to follow temporal changes of inflammation visually and histologically, and to acquire tissue for gene expression analyses. TNBS-colitis is an appropriate model to study specific biological processes in IBD.

Endogenous prion protein attenuates experimentally induced colitis

Although the cellular prion protein (PrP(C)) is expressed in the enteric nervous system and lamina propria, its function(s) in the gut is unknown. Because PrP(C) may exert a cytoprotective effect in response to various physiologic stressors, we hypothesized that PrP(C) expression levels might modulate the severity of experimental colitis. We evaluated the course of dextran sodium sulfate (DSS)-induced colitis in hemizygous Tga20 transgenic mice (approximately sevenfold overexpression of PrP(C)), Prnp(-/-) mice, and wild-type mice. On day 7, colon length, disease severity, and histologic activity indices were determined. Unlike DSS-treated wild-type and Prnp(-/-) animals, PrP(C) overexpressing mice were resistant to colitis induction, exhibited much milder histopathologic features, and did not exhibit weight loss or colonic shortening. In keeping with these results, pro-survival molecule expression and/or phosphorylation levels were elevated in DSS-treated Tga20 mice, whereas pro-inflammatory cytokine production and pSTAT3 levels were reduced. In contrast, DSS-treated Prnp(-/-) mice exhibited increased BAD protein expression and a cytokine expression profile predicted to favor inflammation and differentiation. PrP(C) expression from both the endogenous Prnp locus or the Tga20 transgene was increased in the colons of DSS-treated mice. Considered together, these findings demonstrate that PrP(C) has a previously unrecognized cytoprotective and/or anti-inflammatory function within the murine colon.

Species barriers for chronic wasting disease by in vitro conversion of prion protein

Chronic wasting disease (CWD) is a transmissible spongiform encephalopathy that can affect North American cervids (deer, elk, and moose). Using a novel in vitro conversion system based on incubation of prions with normal brain homogenates, we now report that PrP(CWD) of elk can readily induce the conversion of normal cervid PrP (PrP(C)) molecules to a protease-resistant form, but is less efficient in converting the PrP(C) of other species, such as human, bovine, hamster, and mouse. However, when substrate brain homogenates are partially denatured by acidic conditions (pH 3.5), PrP(CWD)-induced conversion can be greatly enhanced in all species. Our results demonstrate that PrP(C) from cervids (including moose) can be efficiently converted to a protease-resistant form by incubation with elk CWD prions, presumably due to sequence and structural similarities between these species. Moreover, partial denaturation of substrate PrP(C) can apparently overcome the structural barriers between more distant species.

The neurochemical nature of PrPc-containing cells in the rat brain

The cellular prion protein (PrP(C)) is a membrane-bound glycoprotein abundantly expressed in neurons and glial cells within the CNS. The scrapie prion protein (PrP(Sc)) is a conformationally altered isoform of PrP(C) that is responsible for prion diseases, also termed transmissible spongiform encephalopathies (TSE), a group of neurodegenerative diseases that affect a wide variety of mammal species, including humans. The presence of the cellular isoform of PrP is necessary for the establishment and further evolution of prion diseases and the physiological conditions where PrP(C) is present seems to modulate the alterations in TSE. In this work, the presence of PrP(C) in GABAergic, glutamatergic, nitrergic, cholinergic, serotoninergic and orexinergic populations of cells within the rat brain is examined. Our observations show that PrP(C) is widely expressed in a subset of neurons that contain markers of inhibitory populations of cells throughout the rat brain. The presence of PrP(C) in other cells types containing important neurotransmitters for the overall brain function is congruent with the imbalances reported for some of them in TSE. Within the cerebral cortex, PrP(C) is scarcely located in a subset of cells expressing the laminin receptor precursor (LRP) to such a low extent that suggests that other LRP-independent mechanisms actively participate during the pathogenic process. Taken together, our data demonstrate that investigation of the chemical partners of PrP(C) within cells gives a rational basis for the interpretation of the histopathological alterations in TSE and might help analyze some pathogenic mechanisms of PrP(Sc).

Immunohistochemical characterization of cell types expressing the cellular prion protein in the small intestine of cattle and mice

The gastrointestinal tract is thought to be the main site of entry for the pathological isoform of the prion protein (PrP(Sc)). Prion diseases are believed to result from a conformational change of the cellular prion protein (PrP(c)) to PrP(Sc). Therefore, PrP(c) expression is a prerequisite for the infection and spread of the disease to the central nervous system. However, the distribution of PrP(c) in the gut is still a matter of controversy. We therefore investigated the localization of PrP(c) in the bovine and murine small intestine. In cattle, most PrP(c) positive epithelial cells were detected in the duodenum, while a few positive cells were found in the jejunum. PrP(c) was expressed in serotonin producing cells. In bovine Peyer's patches, PrP(c) was distributed in extrafollicular areas, but not in the germinal centre of the jejunum and ileum. PrP(c) was expressed in myeloid lineage cells such as myeloid dendritic cells and macrophages. In mice, PrP(c) was expressed in some epithelial cells throughout the small intestine as well as in cells such as follicular dendritic cell in the germinal centre of Peyer's patches. In this study, we demonstrate that there are a number of differences in the localization of PrP(c) between the murine and bovine small intestines.

Lymphoid follicles of the ileal Peyer's patch of lambs express low levels of PrP, as demonstrated by quantitative real-time RT-PCR on microdissected tissue compartments, in situ hybridization and immunohistochemistry

The expression level of normal cellular prion protein (PrPC) is thought to influence the transmission of transmissible spongiform encephalopathies (TSEs) from the peripheral entry site to the site of pathological changes in the central nervous system. In many TSEs, the clinical disease is preceded by a period in which the agent accumulates in lymphoid organs, particularly in association with follicular dendritic cells of lymphoid follicles. As the probable route of entry of the TSE agent is via the gut, the expression profile of PrP was examined in well-developed gut-associated lymphoid tissue of lambs, the ileal Peyer's patch, by laser microdissection and real-time RT-PCR. Lymphoid follicles were found to have very low levels of expression, whilst highest levels were detected in the outer submucosa and the muscular layer. These findings were supported by in situ hybridization and immunohistochemistry, which showed specific labelling in nerve cells in ganglia of the submucosal (Meissner's) and myenteric (Auerbach's) plexi of the enteric nervous system. Based on the assumption that potential sites for conversion to the scrapie-related prion protein (PrPSc) should display high levels of expression of PrPC, this study suggests that the accumulation of PrPSc in the lymphoid follicles of the Peyer's patch is not preceded by PrP conversion in the same tissue compartment.

Comparative study of PrPc expression in rat, monkey, and cow gastrointestinal tract

The gastrointestinal tract (GIT) appears to be the main site of entry for the pathological isoform of prions (PrP(sc)). To understand how the PrP(sc) internalization process occurs, it is important to characterize the cell types that express normal prion protein (PrP(c)) along the GIT. To do so, we studied the distribution of PrP(c) in the rat, monkey, and cow GIT. Using Western blot analysis, we found that PrP(c) is expressed in all digestive regions of the three species. Immunoreactivity for PrP(c) was found throughout the GIT in epithelial cells sharing the neuroendocrine (NE) phenotype. Immunostained cells appeared scattered throughout the epithelium of fundic and pyloric glands as well as in intestinal villi and crypts.

Characterization of PrPc-Immunoreactive Cells in Monkey (Macaca fascicularis) Gastrointestinal Tract

The gastrointestinal tract (GIT) is one of the most likely entry sites for the pathological isoform of prions (PrP(sc)). To understand how PrP(sc) crosses the digestive mucosa, it is crucial to characterize the cells expressing normal prion protein (PrP(c)). By means of double immunofluorescence applied to sections of the monkey GIT, we demonstrated that, in the stomach, PrP(c) immunostaining occurs in subsets of histamine, somatostatin (Som), ghrelin (Ghr), gastrin (G), and serotonin (5HT) cells. In the small and large bowels, PrP(c) cells were found in subpopulations of cells immunolabeled for 5HT, Som, G, and peptide YY (PYY).

Expression of PrP(C) in the rat brain and characterization of a subset of cortical neurons

The cellular prion protein (PrP(C)) is a membrane-bound glycoprotein mainly present in the CNS. The scrapie prion protein (PrP(Sc)) is an isoform of PrP(C), and it is responsible for transmissible spongiform encephalopathies (TSEs), a group of neurodegenerative diseases affecting both humans and animals. The presence of the cellular form is necessary for the establishment and further evolution of prion diseases. Here, we map the regional distribution of PrP(C) in the rat brain and study the chemical nature of these immunopositive neurons. Our observations are congruent with retrograde transport of prions, as shown by the ubiquitous distribution of PrP(C) throughout the rat brain, but especially in the damaged areas that send projections to primarily affected nuclei in fatal familial insomnia. On the other hand, the presence of the cellular isoform in a subset of GABAergic neurons containing calcium-binding proteins suggests that PrP(C) plays a role in the metabolism of calcium. The lack of immunostaining in neurons ensheathed by perineuronal nets indicates that prions do not directly interact with components of these nets. The destruction of these nets is more likely to be the consequence of a factor needed for prions during the early stages of TSEs. This would cause destruction of these nets and death of the surrounded neurons. Our results support the view that destruction of this extracellular matrix is caused by the pathogenic effect of prions and not a primary event in TSEs.

Effects on the serotoninergic system in sub-acute transmissible spongiform encephalopathies: current data, hypotheses, suggestions for experimentation

Sub-acute transmissible spongiform encephalopathies (TSEs), or prion diseases, are affections in which little is known of their etiology. The predominant theory stipulates that an abnormal protease-resistant prion protein (PrPres) would be infectious by directly inducing its defective conformation to the normal native protein (PrPC). The function of PrPC remains unknown. The preferred localization of PrPC at the level of the synapses supposes a function in neuronal transmission. Several neurotransmitter systems (acetylcholine, GABA, dopamine, etc.) are damaged in TSEs, mainly the serotonin (5-HT) system. At a hypothetical level, PrPC would play a trophic and functional role by regulating the capture of amino acid precursors of neurotransmitters and the functions of neuroreceptors, in particular regarding tryptophan and 5-HT receptors. By comparison with the modes of action of Ras proteins and of the envelope glycoprotein of jaagsiekte sheep retrovirus, the adaptation of an oncogenic model is suggested for the mode of action of PrPres. The sequence of events could be the following: capture of PrPres and forming of an abnormal receptor, chronic disturbance of transduction pathways, more particularly of the phosphatidylinositol-3 kinase (PI-3K)/protein kinase B (Akt)/glycogen synthetase kinase 3 (GSK 3)/Wnt-beta catenin pathway, deregulation of the PrP gene and infrequent and transitory forming of abnormal RNA messengers and, finally, the forming of abnormal proteins and the deterioration of the serotoninergic system. The involvement of endogenous nucleic acids is supposed. The infectious agent of TSEs could be an ancestral form of retrovirus, such as a retrotransposon using the prion protein as an envelope glycoprotein. Pharmacological tests, by comparison with a rare disease of unknown etiology in cattle, bovine spastic paresis, are suggested with the amino acid precursors of neuromediators (tryptophan, tyrosine, glutamic acid, etc.) and with lithium, neuroprotector and regulator of the serotonergic system.