Cellular distribution of glutamate transporters in the gastrointestinal tract of mice. An immunohistochemical and in situ hybridization approach

Abstract

L-Glutamate transport by intestinal epithelial cells is an initial step of the entire glutamate metabolism pathway in the gut mucosa. The present study examined the cellular distribution of glutamate transporters in the digestive tract of adult mice using immunohistochemistry and in situ hybridization technique. Expression of EAAC1 mRNA was more intense in the ileum, where the epithelium in crypts and the basal half of intestinal villi showed high levels of transcripts, suggesting an essential role of EAAC1 in differentiating or premature epithelial cells. Electron-microscopically, EAAC1 immunoreactivity was predominantly localized in the striated border of enterocytes. Immunoreactivity for GLT-1 was found in the lateral membrane of epithelial cells at the bottom of gastric glands and at the intestinal crypts, and also in the lateral membrane of secretory cells at the duodenal gland. GLAST immunoreactivity was restricted to the fundic and pyloric glands, and was especially intense in the neck portion of both glands. However, in situ hybridization analysis failed to confirm the expression of GLT-1 and GLAST at the mRNA level, possibly due to limited sensitivity. The strong and specific luminal localization of EAAC1 in the intestinal epithelium suggests that EAAC1 is a predominant transporter of glutamate, at least in the lower part of the small intestine.Glutamate is one of important excitatory neurotransmitters in the central nervous system, and also in the peripheral nervous system such as the enteric nervous system. Neurons and effector cells express a variety of glutamate receptor subtypes (more than 20) for this potent neurotransmitter. For glutamate to fulfill its diverse functions, the existence of a glutamate uptake system with high accumulative power is of critical importance, because the high-affinity glutamate transport prevents the extracellular glutamate concentration from reaching neurotoxic levels in the synaptic clefts. cDNAs encoding various mammalian glutamate transporter isoforms have been cloned and characterized. Grouping of protein products with similar functional characteristics has identified five subtypes of the excitatory amino acid transporter (EAAT) family: GLAST (EAAT1), GLT-1 (EAAT2), EAAC1 (EAAT3), EAAT4 and EAAT5. The expression of GLT-1, EAAT4, and EAAT5 is mainly restricted to the brain and retina, whereas the expression of GLAST and EAAC1 transcripts has also been reported in non-neuronal tissues, including the kidney, heart, lung, liver, placenta, and small intestine (2, 3, 7, 13, 14,17), thus suggesting the important roles of GLAST and EAAC1 in the uptake of nutrients and metabolites in non-neuronal organs.Ingested glutamate is extensively metabolized to provide whole-animal energy and support N homeo-

Gene expression of nutrient transporters and digestive enzymes in the yolk sac membrane and small intestine of the developing embryonic chick

“…The EAAT3 transporter provides intestinal enterocytes with glutamate, the amino acid that fuels cellular processes and is the main form of energy for enterocytes (Iwanaga et al, 2005). The increase in expression levels of EAAT3 from E15 to 21 is consistent with the increase in the small intestine reported by Gilbert et al (2007) from E18 to day of hatch.…”

Expression of digestive enzymes and nutrient transporters in Eimeria-challenged broilers

“…The expression of b 0,+ AT was also downregulated in the duodenum of E. acervulina-challenged layers (Su et al, 2014) and in the jejunum of E. maxima challenged broilers (Paris and Wong, 2013). The anionic amino acid transporter EAAT3, which is also present at the brush border membrane, plays an important role in enterocyte metabolism as it transports glutamate, the main energy source of intestinal epithelial cells (Iwanaga et al, 2005;Kanai et al, 2013). Downregulation of EAAT3 was also observed in the jejunum of E. maxima-challenged broilers and the duodenum of E. acervulina-challenged layers (Paris and Wong, 2013;Su et al, 2014).…”

The Neurogenesis-Controlling Factor, Pax6, Inhibits Proliferation and Promotes Maturation in Murine Astrocytes

“…The sections were washed with PBS three times, incubated with Alexa Fluor 488 or 555 (Invitrogen, Carlsbad, CA) for 1 h at room temperature, washed, and mounted on slide glass. Antibodies against GLAST and fatty-acidbinding protein 7 (FABP7) were kind gifts from Dr. Masahiko Watanabe (Hokkaido University, Sapporo, Japan) (Iwanaga et al, 2005) and Dr. Yuji Owada (Yamaguchi University, Yamaguchi, Japan) (Owada et al, 2006), respectively. BrdU labeling analyses.…”