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In the intestine, changes of sugar concentration generated in the lumen during digestion induce adaptive responses of glucose transporters in the epithelium. A close matching between the intestinal expression of glucose transporters and the composition and amount of the diet has been provided by several experiments. Functional evidence has demonstrated that the regulation of glucose transporters into enterocytes is induced by the sensing of sugar of the enteroendocrine cells through activation of sweet taste receptors (T1R2 and T1R3) and their associated elements of G-protein-linked signaling pathways (e.g. α-gustducin, phospholipase C β type 2 and transient receptor potential channel M5), which are signaling molecules also involved in the perception of sweet substances in the taste receptor cells (TRCs) of the tongue. Considering this phenotypical similarity between the intestinal cells and TRCs, we evaluated whether the TRCs themselves possess proteins of the glucose transport mechanism. Therefore, we investigated the expression of the typical intestinal glucose transporters (i.e. GLUT2, GLUT5 and SGLT1) in rat circumvallate papillae, using immunohistochemistry, double-labeling immunofluorescence, immunoelectron microscopy and reverse transcriptase-polymerase chain reaction analysis. The results showed that GLUT2, GLUT5 and SGLT1 are expressed in TRCs; their immunoreactivity was also observed in cells that displayed staining for α-gustducin and T1R3 receptor. The immunoelectron microscopic results confirmed that GLUT2, GLUT5 and SGLT1 were predominantly expressed in cells with ultrastructural characteristics of chemoreceptor cells. The presence of glucose transporters in TRCs adds a further link between chemosensory information and cellular responses to sweet stimuli that may have important roles in glucose homeostasis, contributing to a better understanding of the pathways implicated in glucose metabolism., (© 2011 The Authors. Journal of Anatomy © 2011 Anatomical Society of Great Britain and Ireland.)

The chemical composition of the luminal content is now accepted to have a profound influence on the performance of chemosensory receptors. Gustatory and intestinal chemoreceptors have in common their expression of molecules involved in taste sensing and signal transduction pathways. The recent finding that enterocytes of the duodenal epithelium are capable of expressing luminal pancreatic amylase suggests that taste cells of the gustatory epithelium might, in the same way, express salivary amylase in the oral cavity. Therefore, we investigated amylase expression in rat circumvallate papillae by using analyses involving immunohistochemistry, Western blot, and reverse transcription with the polymerase chain reaction. In addition, we used double-labeling confocal laser microscopy to compare amylase immunolabeling with that of the following markers: protein gene product 9.5 (PGP 9.5) and chromogranin A (CgA) for endocrine cells, alpha-gustducin and phospholipase C beta 2 (PLC beta 2) as taste-signaling molecules, and cystic fibrosis transmembrane regulator (CFTR) and Clara-cell-specific secretory protein of 10-kDa (CC10) as secretory markers. The results showed that amylase was present in some taste bud cells; its immunoreactivity was observed in subsets of cells that expressed CgA, alpha-gustducin, PLC beta 2, CFTR, or CC10. PGP 9.5 immunoreactivity was never colocalized with amylase. The data suggest that amylase-positive cells constitute an additional subset of taste receptor cells also associated with chemoreceptorial and/or secretory molecules, confirming the occurrence of various pathways in taste buds.

Taste receptor cells (TRCs) are the sensory cells of taste transduction and are organized into taste buds embedded in the epithelium of the tongue, palate, pharynx, and larynx. Several studies have demonstrated that TRCs involved in sweet as well as bitter and umami responses express alpha-gustducin, an alpha-subunit of the G-protein complex. It has been further demonstrated that this typical taste protein is a potent marker of chemosensory cells located in several tissues, including gastric and pancreatic mucosa and the respiratory apparatus. We recently observed that alpha-gustducin and phospholipase C beta 2-immunoreactive cells were colocalized in the airways with cystic fibrosis transmembrane regulator (CFTR) and Clara cell-specific secretory protein of 10 (CC10) and 26 kDa (CC26). This finding suggests that TRCs might themselves express secretory markers. To test this hypothesis, we investigated the expression of CFTR, CC10, and CC26 in rat circumvallate papillae using reverse transcriptase-polymerase chain reaction analysis, immunohistochemistry, and confocal laser microscopy. The results showed that secretory markers such as CFTR, CC10, and CC26 are present in taste cells of rat circumvallate papillae, and their immunoreactivity is expressed, to a different extent, in subsets of taste cells that express alpha-gustducin. The presence of CFTR, CC10, and CC26 in taste bud cells and their coexpression pattern with alpha-gustducin confirms and extends our previous findings in airway epithelium, lending further credence to the notion that chemoreception and secretion may be related processes.

The expression of molecules involved in the transductory cascade of the sense of taste (TRs, alpha-gustducin, PLCbeta2, IP3R3) has been described in lingual taste buds or in solitary chemoreceptor cells located in different organs. At the laryngeal inlet, immunocytochemical staining at the light and electron microscope levels revealed that alpha-gustducin and PLCbeta2 are mainly localized in chemosensory clusters (CCs), which are multicellular organizations differing from taste buds, being generally composed of two or three chemoreceptor cells. Compared with lingual taste buds, CCs are lower in height and smaller in diameter. In laryngeal CCs, immunocytochemistry using the two antibodies identified a similar cell type which appears rather unlike the alpha-gustducin-immunoreactive (IR) and PLCbeta2-IR cells visible in lingual taste buds. The laryngeal IR cells are shorter than the lingual ones, with poorly developed basal processes and their apical process is shorter and thicker. Some cells show a flask-like shape due to the presence of a large body and the absence of basal processes. CCs lack pores and their delimitation from the surrounding epithelium is poorly evident. The demonstration of the existence of CCs strengthens the hypothesis of a phylogenetic link between gustatory and solitary chemosensory cells., (Copyright 2004 Oxford University Press)

Summary: The liver is one of the most complex organs in the body, which responds to hepatocellular damage with inflammatory, regenerative and repair processes designed to restore functional liver tissue mass. Rat LRRP Ba1-651, a liver regeneration related protein induced during partial hepatectomy, is classified as a member of the aldehyde dehydrogenase (ALDh) 4A1 superfamily. During a BLAST protein search, this protein basically showed three structural and functional domains: an intermediate filament-like protein, a Delta-1-pyrroline-5-carboxylate dehydrogenase (P5CDh) and an atrial natriuretic factor (ANF) receptor. We suggest that all amniotic mammals possess a Ba1-651 ortholog to that of rats. The ANF receptor domain of rat LRRP Ba1-651, which domain is part of the receptor family ligand binding region, shows a very high sequence homology (almost identity) to the extracellular amino-terminal domains of the mammalian sweet taste receptor T1R2. This receptor belongs to the type C family of G protein coupled receptors (GPCRs) and is characterized by the presence of large extracellular amino-terminal domains, a nine cysteine domain of family 3 GPCR and a 7tm_3 transmembrane type domain. We suggest that rat LRRP Ba1-651 protein is a liver P5CDh-ANF that is activated by changes in the concentration of sweet molecules. If the sugar concentration in the organ increases due to liver damage or the intake of carbohydrate-rich or protein-rich foods, the P5CDh-ANF enzyme is activated to help in P5C catabolism. The hormone insulin probably plays a key role in the regulation of this enzyme. In the model that we propose, the P5CDh-ANF enzyme is activated by a conformational change in protein structure in the P5C docking site due to sugars binding in the AFN receptor region of the LRRP Ba1-651 protein. Our research could be a further understanding of the biological significance of this P5CDh-ANF enzyme, with important potential applications in the treatment of HPII and liver diseases and in liver transplantation. Further studies of our P5CDh-ANF enzyme are needed to clarify its features and functions, and which substances are involved in its induction. These might use liver cell lines or purified LRRP Ba1-651 protein with sweet molecules in vitro. Other experiments may help to localize LRRP Bal-651 in the organ and to link its abnormal presence or absence to certain tumors like hepatocellular carcinoma. [Copyright &y& Elsevier]

The presence of solitary chemosensory cells was studied in rat vallate papillae during the first week of post-natal life by α-gustducin immunocytochemistry. In 1- to 3-day-old rats, isolated α-gustducin-immunoreactive cells were found within the epithelium of the vallate papilla. These cells, mainly located in the basal layer, were scattered among keratocytes and wrapped in α-gustducin-negative epithelial cells in a glia-like fashion. The α-gustducin-immunoreactive cells were usually round and some of them gave rise to short, large processes directed towards the lumen of the oral cavity or the basal lamina. Rarely, some cells showed an evident bipolar shape. Small taste buds containing either α-gustducin-immunoreactive or α-gustducinnegative cells appeared in the vallate papillae of 4-day-old rats in which isolated, bipolar-shaped α-gustducin-immunoreactive cells were also found. After the first week of post-natal life, the taste buds appeared basically similar to those of adult animals. In conclusion, the present study demonstrates that the presence of epithelial cells with characteristics of solitary chemosensory cells precedes the development of the taste buds. [ABSTRACT FROM AUTHOR]