title: Transsynaptic Tracing from Taste Receptor Cells Reveals Local Taste Receptor Gene Expression in Gustatory Ganglia and Brain.
abstract: Taste perception begins in the oral cavity by interactions of taste stimuli with specific receptors. Specific subsets of taste receptor cells (TRCs) are activated upon tastant stimulation and transmit taste signals to afferent nerve fibers and ultimately to the brain. How specific TRCs impinge on the innervating nerves and how the activation of a subset of TRCs leads to the discrimination of tastants of different qualities and intensities is incompletely understood. To investigate the organization of taste circuits, we used gene targeting to express the transsynaptic tracer barley lectin (BL) in the gustatory system of mice. Because TRCs are not synaptically connected with the afferent nerve fibers, we first analyzed tracer production and transfer within the taste buds (TBs). Surprisingly, we found that BL is laterally transferred across all cell types in TBs of mice expressing the tracer under control of the endogenous Tas1r1 and Tas2r131 promotor, respectively. Furthermore, although we detected the BL tracer in both ganglia and brain, we also found local low-level Tas1r1 and Tas2r131 gene, and thus tracer expression in these tissues. Finally, we identified the Tas1r1 and Tas2r131-expressing cells in the peripheral and CNS using a binary genetic approach. Together, our data demonstrate that genetic transsynaptic tracing from bitter and umami receptor cells does not selectively label taste-specific neuronal circuits and reveal local taste receptor gene expression in the gustatory ganglia and the brain.
  
  
  CONCLUSIONS
  Previous papers described the organization of taste pathways in mice expressing a transsynaptic tracer from transgenes in bitter or sweet/umami-sensing taste receptor cells. However, reported results differ dramatically regarding the numbers of synapses crossed and the reduction of signal intensity after each transfer step. Nevertheless, all groups claimed this approach appropriate for quality-specific visualization of taste pathways. In the present study, we demonstrate that genetic transsynaptic tracing originating from umami and bitter taste receptor cells does not selectively label taste quality-specific neuronal circuits due to lateral transfer of the tracer in the taste bud and taste receptor expression in sensory ganglia and brain. Moreover, we visualized for the first time taste receptor-expressing cells in the PNS and CNS.
authors.affiliation: Department of Molecular Genetics, German Institute of Human Nutrition Potsdam-Rehbruecke (DIfE), 14558 Nuthetal, Germany, and Institute for Neural Signal Transduction, Center for Molecular Neurobiology Hamburg (ZMNH), 20251 Hamburg, Germany.
authors.forename: Anja
authors.initials: A
authors.lastname: Voigt
authors.affiliation: Department of Molecular Genetics, German Institute of Human Nutrition Potsdam-Rehbruecke (DIfE), 14558 Nuthetal, Germany, and.
authors.forename: Juliane
authors.initials: J
authors.lastname: Bojahr
authors.affiliation: Department of Molecular Genetics, German Institute of Human Nutrition Potsdam-Rehbruecke (DIfE), 14558 Nuthetal, Germany, and.
authors.forename: Masataka
authors.initials: M
authors.lastname: Narukawa
authors.affiliation: Department of Molecular Genetics, German Institute of Human Nutrition Potsdam-Rehbruecke (DIfE), 14558 Nuthetal, Germany, and.
authors.forename: Sandra
authors.initials: S
authors.lastname: Hübner
authors.affiliation: Institute for Neural Signal Transduction, Center for Molecular Neurobiology Hamburg (ZMNH), 20251 Hamburg, Germany meyerhof@dife.de ulrich.boehm@uks.eu.
authors.forename: Ulrich
authors.initials: U
authors.lastname: Boehm
authors.affiliation: Department of Molecular Genetics, German Institute of Human Nutrition Potsdam-Rehbruecke (DIfE), 14558 Nuthetal, Germany, and meyerhof@dife.de ulrich.boehm@uks.eu.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Meyerhof
mesh_terms: D000818:Animals; D001142:Aromatic-L-Amino-Acid Decarboxylases / Q000378:metabolism; D001921:Brain / Q000166:cytology* / Q000378:metabolism; D005724:Ganglia / Q000166:cytology*; D005786:Gene Expression Regulation / Q000187:drug effects / Q000235:genetics*; D037102:Lectins / Q000378:metabolism; D008164:Luminescent Proteins / Q000235:genetics / Q000378:metabolism; D051379:Mice; D008822:Mice, Transgenic; D054799:Phospholipase C beta / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D043562:Receptors, G-Protein-Coupled / Q000235:genetics / Q000378:metabolism*; D011984:Sensory Receptor Cells / Q000378:metabolism*; D013650:Taste Buds / Q000166:cytology; D014909:Wheat Germ Agglutinins / Q000378:metabolism
chemical_list: D037102:Lectins; D008164:Luminescent Proteins; D012333:RNA, Messenger; D043562:Receptors, G-Protein-Coupled; C578844:Tas2r131 protein, mouse; D014909:Wheat Germ Agglutinins; C431926:taste receptors, type 1; D054799:Phospholipase C beta; D001142:Aromatic-L-Amino-Acid Decarboxylases
keywords: bitter; gene-targeting; genetic labeling; genetic transsynaptic tracing; knock-in; umami