enteroendocrine cells ͉ gastrointestinal chemosensation ͉ glucose sensor ͉ incretin G lucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are incretins, peptide hormones secreted from enteroendocrine L and K cells, respectively, that augment insulin secretion after oral intake of glucose (1). How carbohydrates in the gut lumen elicit the release of GLP-1 from L cells and GIP from K cells is unknown (2). Because i.v. glucose administration does not induce secretion of GLP-1 (3) it appears that glucose within the lumen of the gut acts on the luminal surface to stimulate secretion. Thus, we sought to determine what glucose-sensing mechanism in the gut lumen might underlie this L cell response.One mechanism for sensing glucose is by sweet taste receptors in taste receptor cells of the lingual epithelium (4). Sweet compounds bind to and activate specific G protein coupled receptors that couple through the G protein gustducin (5) to specific second messenger cascades (4, 6). Two type 1 taste G protein coupled receptors (T1Rs) heterodimerize to form the T1R2ϩT1R3 sweet taste receptor (7-11). Key elements of the taste transduction pathways are the ␣, ␤, and ␥ subunits of gustducin (␣-gustducin, G␤ 3 , and G␥ 13 ) (5, 12), phospholipase C␤2 (PLC␤2) (13), and transient receptor potential channel type M5 (14), a Ca 2ϩ -activated cation channel (15-17). ␣-Gustducin has been detected in brush cells of the stomach, duodenum, and pancreatic ducts in rat (18, 19), T1R2 and T1R3 are present in rodent gut and the enteroendocrine STC-1 cell line (20), and ␣-gustducin and GLP-1 are present in enteroendocrine cells of the human colon (21). However, the functional significance of expression of taste signaling elements in cells of the gastrointestinal (GI) tract had been unclear. Here, we present data that indicate that T1R3 and gustducin have a role in glucosemediated incretin release and may serve as the previously unknown gut lumen glucose sensor. ResultsWe examined L cells of the gut for the presence of taste receptors and elements of taste transduction pathways. In human duodenal biopsy sections ␣-gustducin was detected by immunofluorescence (

Elevated plasma levels of the sulfur-containing amino acid homocysteine increase the risk for atherosclerosis, stroke, and possibly Alzheimer's disease, but the underlying mechanisms are unknown. We now report that homocysteine induces apoptosis in rat hippocampal neurons. DNA strand breaks and associated activation of poly-ADP-ribose polymerase (PARP) and NAD depletion occur rapidly after exposure to homocysteine and precede mitochondrial dysfunction, oxidative stress, and caspase activation. The PARP inhibitor 3-aminobenzamide (3AB) protects neurons against homocysteine-induced NAD depletion, loss of mitochondrial transmembrane potential, and cell death, demonstrating a requirement for PARP activation and/or NAD depletion in homocysteine-induced apoptosis. Caspase inhibition accelerates the loss of mitochondrial potential and shifts the mode of cell death to necrosis; inhibition of PARP with 3AB attenuates this effect of caspase inhibition. Homocysteine markedly increases the vulnerability of hippocampal neurons to excitotoxic and oxidative injury in cell culture and in vivo, suggesting a mechanism by which homocysteine may contribute to the pathogenesis of neurodegenerative disorders.

SUMMARY N-methyl-D-aspartate (NMDA) receptors constitute a major subtype of glutamate receptors at extra-synaptic sites that link multiple intracellular catabolic processes responsible for irreversible neuronal death. Here, we report that cerebral ischemia recruits death-associated protein kinase 1 (DAPK1) into the NMDA receptor NR2B protein complex in the cortex of adult mice. DAPK1 directly binds with the NMDA receptor NR2B C-terminal tail consisting of amino acid 1292–1304 (NR2BCT). A constitutively active DAPK1 phosphorylates NR2B subunit at Ser-1303 and in turn enhances the NR1/NR2B receptor channel conductance. Genetic deletion of DAPK1 or administration of NR2BCT that uncouples an activated DAPK1 from an NMDA receptor NR2B subunit in vivo in mice blocks injurious Ca2+ influx through NMDA receptor channels at extrasynaptic sites and protects neurons against cerebral ischemic insults. Thus, DAPK1 physically and functionally interacts with the NMDA receptor NR2B subunit at extra-synaptic sites and this interaction acts as a central mediator for stroke damage.

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