Bitter taste receptors in the domestic dog (Canis familiaris)

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Gibbs, Matthew (2021) Bitter taste receptors in the domestic dog (Canis familiaris). PhD thesis, University of Nottingham.

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Abstract

The sense of taste, or gustation, combines with olfactory and somatosensory cues to give the overall perception of flavour once a food is accepted into the mouth. The perception of bitter taste is thought to be primarily associated with rejection of a food, and protects animals from ingesting potentially toxic substances. Bitter taste is mediated through a group of G protein-coupled receptors known as the Taste type 2 receptors (Tas2rs) that are expressed in taste papillae on the tongue, and other surfaces in the oral cavity.

Bitter taste in the domestic dog is of interest for a number of reasons. Firstly, pet dogs are often fed a commercially prepared pet food. Any bitter taste from the raw materials used in the manufacture, or formed during the processing of the product has the potential to negatively impact on palatability. Secondly, bitter-tasting chemicals are sometimes used to deter pets from consuming substances that are harmful to them, such as automotive antifreeze and rodent poison.

In humans, bitter taste perception varies between individuals. Some of this variation is known to be related to gene sequence variation in some of the TAS2Rs. Whether this type of functional variation exists in dogs is not known. Dogs have been selectively bred, and now are one of the most phenotypically diverse of all species. If functional variation does exist in dog Tas2rs it may be associated with dog breed.

In order to understand the responses of dog Tas2rs and the impact of their variation, this project deorphanised dog bitter receptors, and characterised their receptive ranges. Equivalent human receptors were also tested so that, where possible, data for orthologous receptors from dog and human could be compared. A heterologous cell model was used, incorporating a novel Gα16/gust/o chimera. The performance of this model was compared to that of the frequently used Gα16/gust44 chimera. Receptor sensitivity and breadth of tuning varied, showing both similarities and differences between orthologous dog and human receptors when tested with a library of 48 bitter compounds.

Gene expression data from dog fungiform taste papillae were also generated and used to confirm the expression of putative bitter receptors in the dog. Papillae were selected from different areas of the anterior portion of the tongue. RNA-seq analysis showed that Tas2r expression varied depending on location, with papillae from the front of the tongue showing significantly less Tas2r expression. Generally expression levels were very low, and not all dog Tas2rs could be confirmed as expressed from these samples.

Variation in sequence and functionality of dog bitter receptors between different dogs and different dog breeds was assessed, with dTas2r1 being studied as an example. A database of dog genomes was used to assess the levels of variation between dogs. Gene variants found in dTas2r1 were tested for functional impacts on receptor function and expression in the heterologous cell model. Only subtle effects on receptor function were seen. However, several rare variants caused a loss of cell-surface expression, presumably due to retention of the receptor intracellularly.

In summary, this study showed that species differences in the sensitivity and tuning of dog Tas2rs were found, when compared with their human orthologues. Ligands were identified for 7 of the 16 putative dog Tas2rs and this information was used to study the impacts of naturally occurring sequence variants in dog Tas2r1. Further work on variants occurring in other dog Tas2rs will confirm if any high impact variants might influence dog bitter taste perception, as is the case for humans.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Holliday, Nicholas
Briddon, Stephen
Keywords: Bitterness (Taste); Dogs; G proteins; Chemoreceptors
Subjects: Q Science > QP Physiology
Faculties/Schools: UK Campuses > Faculty of Medicine and Health Sciences > School of Life Sciences
Item ID: 65278
Depositing User: Gibbs, Matthew
Date Deposited: 06 Oct 2023 11:01
Last Modified: 06 Oct 2023 11:01
URI: https://eprints.nottingham.ac.uk/id/eprint/65278

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