Cellular bases for interactions between immunocytes and enteroendocrine cells in the intestinal mucosal barrier of rhesus macaques

Intestinal Enteroendocrine Cells: Present and Future Druggable Targets

Enteroendocrine cells are specialized secretory lineage cells in the small and large intestines that secrete hormones and peptides in response to luminal contents. The various hormones and peptides can act upon neighboring cells and as part of the endocrine system, circulate systemically via immune cells and the enteric nervous system. Locally, enteroendocrine cells have a major role in gastrointestinal motility, nutrient sensing, and glucose metabolism. Targeting the intestinal enteroendocrine cells or mimicking hormone secretion has been an important field of study in obesity and other metabolic diseases. Studies on the importance of these cells in inflammatory and auto-immune diseases have only recently been reported. The rapid global increase in metabolic and inflammatory diseases suggests that increased understanding and novel therapies are needed. This review will focus on the association between enteroendocrine changes and metabolic and inflammatory disease progression and conclude with the future of enteroendocrine cells as potential druggable targets.

A Focus on Enterochromaffin Cells among the Enteroendocrine Cells: Localization, Morphology, and Role

The intestinal epithelium plays a key role in managing the relationship with the environment, the internal and external inputs, and their changes. One percent of the gut epithelium is represented by the enteroendocrine cells. Among the enteroendocrine cells, a group of specific cells characterized by the presence of yellow granules, the enterochromaffin cells, has been identified. These granules contain many secretion products. Studies showed that these cells are involved in gastrointestinal inflammatory conditions and hyperalgesia; their number increases in these conditions both in affected and not-affected zones of the gut. Moreover, they are involved in the preservation and modulation of the intestinal function and motility, and they sense metabolic-nutritional alterations. Sometimes, they are confused or mixed with other enteroendocrine cells, and it is difficult to define their activity. However, it is known that they change their functions during diseases; they increased in number, but their involvement is related mainly to some secretion products (serotonin, melatonin, substance P). The mechanisms linked to these alterations are not well investigated. Herein, we provide an up-to-date highlight of the main findings about these cells, from their discovery to today. We emphasized their origin, morphology, and their link with diet to better evaluate their role for preventing or treating metabolic disorders considering that these diseases are currently a public health burden.

Enteroendocrine cells-sensory sentinels of the intestinal environment and orchestrators of mucosal immunity

The intestinal epithelium must balance efficient absorption of nutrients with partitioning commensals and pathogens from the bodies' largest immune system. If this crucial barrier fails, inappropriate immune responses can result in inflammatory bowel disease or chronic infection. Enteroendocrine cells represent 1% of this epithelium and have classically been studied for their detection of nutrients and release of peptide hormones to mediate digestion. Intriguingly, enteroendocrine cells are the key sensors of microbial metabolites, can release cytokines in response to pathogen associated molecules and peptide hormone receptors are expressed on numerous intestinal immune cells; thus enteroendocrine cells are uniquely equipped to be crucial and novel orchestrators of intestinal inflammation. In this review, we introduce enteroendocrine chemosensory roles, summarize studies correlating enteroendocrine perturbations with intestinal inflammation and describe the mechanistic interactions by which enteroendocrine and mucosal immune cells interact during disease; highlighting this immunoendocrine axis as a key aspect of innate immunity.

Alteration of Polymeric Immunoglobulin Receptor and Neonatal Fc Receptor Expression in the Gut Mucosa of Immunodeficiency Virus-Infected Rhesus Macaques

Polymeric immunoglobulin receptors (pIgR) and neonatal Fc receptors (FcRn) are crucial immunoglobulin (Ig) receptors for the transcytosis of immunoglobulins, that is IgA, IgM and IgG, the levels of which in mucosal secretions were altered in both HIV- and SIV-infected individuals. To gain an insight into the changes of pIgR and FcRn expression after immunodeficiency virus (SHIV/SIV) infection, real-time RT-PCR methods were established and the mRNA levels of pIgR and FcRn in normal and SHIV/SIV-infected rhesus macaques were quantitatively examined. It was found that the levels of pIgR mRNA were within a range of 10(7) copies per million copies of GAPDH mRNA in the gut mucosa of rhesus macaques, which were up to 55 times higher than that in the oral mucosa, the highest among the non-gut tissues examined. Levels of FcRn mRNA were generally lower than that of pIgR, and the levels of FcRn mRNA in the gut mucosa were also lower than that in most non-gut tissues examined. Notably, the levels of pIgR mRNA in the duodenal mucosa were positively correlated with that of IL-17A in normal rhesus macaques. Both pIgR and FcRn mRNA levels were significantly reduced in the duodenal mucosa during acute SHIV infection and in the jejunum and caecum during chronic SHIV/SIV infection. These data expanded our knowledge on the expression of pIgR and FcRn in the gastrointestinal tract of rhesus macaques and demonstrated altered expression of pIgR and FcRn in SHIV/SIV, and by extension HIV infections, which might have contributed to HIV/AIDS pathogenesis.

Inflammation Meets Metabolic Disease: Gut Feeling Mediated by GLP-1

Chronic diseases, such as obesity and diabetes, cardiovascular, and inflammatory bowel diseases (IBD) share common features in their pathology. Metabolic disorders exhibit strong inflammatory underpinnings and vice versa, inflammation is associated with metabolic alterations. Next to cytokines and cellular stress pathways, such as the unfolded protein response (UPR), alterations in the enteroendocrine system are intersections of various pathologies. Enteroendocrine cells (EEC) have been studied extensively for their ability to regulate gastrointestinal motility, secretion, and insulin release by release of peptide hormones. In particular, the L-cell-derived incretin hormone glucagon-like peptide 1 (GLP-1) has gained enormous attention due to its insulinotropic action and relevance in the treatment of type 2 diabetes (T2D). Yet, accumulating data indicate a critical role for EEC and in particular for GLP-1 in metabolic adaptation and in orchestrating immune responses beyond blood glucose control. EEC sense the lamina propria and luminal environment, including the microbiota via receptors and transporters. Subsequently, mediating signals by secreting hormones and cytokines, EEC can be considered as integrators of metabolic and inflammatory signaling. This review focuses on L cell and GLP-1 functions in the context of metabolic and inflammatory diseases. The effects of incretin-based therapies on metabolism and immune system are discussed and the interrelation and common features of metabolic and immune-mediated disorders are highlighted. Moreover, it presents data on the impact of inflammation, in particular of IBD on EEC and discusses the potential role of the microbiota as link between nutrients, metabolism, immunity, and disease.

The intestinal immunoendocrine axis: novel cross-talk between enteroendocrine cells and the immune system during infection and inflammatory disease

The intestinal epithelium plays a crucial role in maintaining barrier function and immune homeostasis, a failure of which results in disease. This review focuses on the epithelial enteroendocrine cells and the crosstalk that exists with immune cells during inflammation.

The intestinal epithelium represents one of our most important interfaces with the external environment. It must remain tightly balanced to allow nutrient absorption, but maintain barrier function and immune homoeostasis, a failure of which results in chronic infection or debilitating inflammatory bowel disease (IBD). The intestinal epithelium mainly consists of absorptive enterocytes and secretory goblet and Paneth cells and has recently come to light as being an essential modulator of immunity as opposed to a simple passive barrier. Each epithelial sub-type can produce specific immune modulating factors, driving innate immunity to pathogens as well as preventing autoimmunity. The enteroendocrine cells comprise just 1% of this epithelium, but collectively form the bodies’ largest endocrine system. The mechanisms of enteroendocrine cell peptide secretion during feeding, metabolism and nutrient absorption are well studied; but their potential interactions with the enriched numbers of surrounding immune cells remain largely unexplored. This review focuses on alterations in enteroendocrine cell number and peptide secretion during inflammation and disease, highlighting the few in depth studies which have attempted to dissect the immune driven mechanisms that drive these phenomena. Moreover, the emerging potential of enteroendocrine cells acting as innate sensors of intestinal perturbation and secreting peptides to directly orchestrate immune cell function will be proposed. In summary, the data generated from these studies have begun to unravel a complex cross-talk between immune and enteroendocrine cells, highlighting the emerging immunoendocrine axis as a potential target for therapeutic strategies for infections and inflammatory disorders of the intestine.

Associated changes in the transcription levels of IL-17A and tight junction-associated genes in the duodenal mucosa of rhesus macaques repeatedly exposed to simian/human immunodeficiency virus

BACKGROUND

Mucosal barrier dysfunction might play a key role in HIV/AIDS, yet the early effects of HIV-1 on intestinal mucosal barrier, especially tight junctions (TJ) have not been well addressed.

AIMS

To investigate the effects of acute HIV-1 infection on the expression of intestinal IL-17A and TJ-associated genes using an NHP-AIDS model.

METHODS

TaqMan probe real-time RT-PCR methods were established and claudin-1, claudin-3, occludin and zonula occluden-1 (ZO-1) mRNA levels in the duodenal biopsies of rhesus macaques collected before and after rectal exposures to SHIV-SF162P4 were examined and compared with that of IL-17A, IL-6, TGF-β, RORγt, T-bet, Foxp-3 and GATA-3.

RESULTS

The mRNA levels of TJ-associated genes were statistically significantly reduced soon after viral exposures and the mRNA levels of claudin-1, occludin and ZO-1 in viral positive tissues (from Group I) were lower than that in viral negative tissues (from Group II) after viral exposure. IL-17A mRNA levels were also decreased and positively correlated with the mRNA levels of the TJ-associated genes after viral exposure or infection, although the levels of IL-6, TGF-β and RORγt mRNA showed no statistical difference. The levels of GATA-3 mRNA in tissues collected before viral exposure were statistically different between Group I and Group II animals. The balance between T-bet and GATA-3 mRNA levels in Group II was markedly altered and statistically significantly different from that in Group I.

CONCLUSIONS

Acute SHIV, and by extension HIV infection could affect the expression of TJ-associated genes, probably through IL-17A and other immune alterations.

Quantification of endocrine cells and ultrastructural study of insulin granules in the large intestine of opossum Didelphis aurita (Wied-Neuwied, 1826)

This study aimed to investigate the distribution of argyrophil, argentaffin, and insulin-immunoreactive endocrine cells in the large intestine of opossums (Didelphis aurita) and to describe the ultrastructure of the secretory granules of insulin-immunoreactive endocrine cells. Fragments of the large intestine of 10 male specimens of D. aurita were collected, processed, and subjected to staining, immunohistochemistry, and transmission electron microscopy. The argyrophil, the argentaffin, and the insulin-immunoreactive endocrine cells were sparsely distributed in the intestinal glands of the mucous layer, among other cell types of the epithelium in all regions studied. Proportionally, the argyrophil, the argentaffin, and the insulin-immunoreactive endocrine cells represented 62.75%, 36.26%, and 0.99% of the total determined endocrine cells of the large intestine, respectively. Quantitatively, there was no difference between the argyrophil and the argentaffin endocrine cells, whereas insulin-immunoreactive endocrine cells were less numerous. The insulin-immunoreactive endocrine cells were elongated or pyramidal, with rounded nuclei of irregularly contoured, and large amounts of secretory granules distributed throughout the cytoplasm. The granules have different sizes and electron densities and are classified as immature and mature, with the mature granules in predominant form in the overall granular population. In general, the granule is shown with an external electron-lucent halo and electron-dense core. The ultrastructure pattern in the granules of the insulin-immunoreactive endocrine cells was similar to that of the B cells of pancreatic islets in rats.

Enteroendocrine cells in gastrointestinal pathophysiology

Enteroendocrine cells in the gastrointestinal tract play an important role in the regulation of appetite and digestive responses through the secretion of peptides. Their involvement in gastrointestinal diseases has been acknowledged, but relatively few studies have sought to clearly define their role in the pathogenesis or as therapeutic targets. Recent, but still limited, work has identified new roles for EEC in GI diseases.