Date of Award

2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

First Advisor

Eugene R. Delay

Second Advisor

Gary M. Mawe

Abstract

The sense of taste is one of the most important factors in regulating ingestive decisions. This is central to a number of disease conditions including but not limited to obesity, diabetes, anorexia, hypertension, coronary artery diseases and malnutrition. The detection of the molecules eliciting taste qualities in food is mediated by the coordinated actions of distinct types of taste sensory cells (TSCs) housed in taste buds within specialized papillae throughout the oral cavity. Taste receptors in the taste sensory cells that detect food molecules are the key players in selecting dietary nutrients. One such example is L-amino acids, a critical part of one's diet.

L-glutamate is the prototypical umami compound and is known to increase palatability of food. A unique characteristic of umami taste is the response potentiation of glutamate by 5' ribonucleotide monophosphates, such as inosine 5' monophosphate (IMP), which is also capable of eliciting an umami taste. Candidate receptors for umami taste include a heterodimer T1r1+T1r3, brain variants of mGluR1 and mGluR4, and the truncated variants of mGluR1 and mGluR4. Studies using heterogeneous expression of T1r1+T1r3 suggest it is an umami and a broadly tuned L-amino acid receptor. While much attention is devoted to understanding glutamate transduction, the detection mechanisms for other L-amino acids by TSCs are less well understood. Here calcium imaging of isolated TSCs and taste cell clusters from the circumvallate and foliate papillae of C57BL/6J and T1r3 knockout mice was performed to determine if other receptors are involved in the detection of L-amino acids and IMP. Ratiometric imaging with Fura-2 was used to study calcium responses to IMP and four L-amino acids (monopotassium L-glutamate, L-serine, L-arginine, and L-glutamine) with and without IMP. The results of these experiments showed that the response patterns elicited by L-amino acids varied significantly across TSCs. Only a small subset of cells responded to all stimuli. Interestingly, L-amino acids other than glutamate elicited synergistic responses in a subset of TSCs. Additionally IMP alone elicited a response in a large number of TSCs. Our data indicate that synergistic and non-synergistic responses to L-amino acids and IMP are mediated by multiple receptors or possibly a receptor complex.

Next the roles of mGluR1 and mGluR4 in the detection of the IMP and L-amino acids were investigated. Selective agonists for mGluR1, (RS)-3, 5-dihydroxyphenylglycine (DHPG; a group I mGluR agonist), and mGluR4, L-(+)-2-amino-4-phosphonobutyric acid (L-AP4; a group III mGluR4 agonist) elicited responses in TSCs. In addition, TSCs responsive to these agonists were also responsive to L-amino acids and IMP. More importantly, selective antagonists against different mGluRs such as (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; a group I mGluR antagonist), and (RS)-α-methylserine-O-phosphate (MSOP; a group III mGluR antagonist) significantly suppressed L-amino acid- and IMP-mediated responses in TSCs of T1r3 knockout mice. Collectively, these data provide evidence for the involvement of taste and the brain variants of mGluR1 and mGluR4 in L-amino acid and IMP taste responses in mice, and support the hypothesis that multiple receptors contribute to the IMP and L-amino acid tastes.

Language

en

Number of Pages

231 p.