Elsevier

Neuroscience Letters

Volume 648, 1 May 2017, Pages 53-58
Neuroscience Letters

Research article
A large increase of sour taste receptor cells in Skn-1-deficient mice does not alter the number of their sour taste signal-transmitting gustatory neurons

https://doi.org/10.1016/j.neulet.2017.03.046Get rights and content

Highlights

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    Population of sour TRCs in taste buds is markedly expanded in Skn-1a−/− mice.

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    However, gustatory neurons that innervate sour TRCs are not increased in Skn-1a−/− mice.

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    Change in ratio of TRC types does not influence the number of innervating gustatory neurons.

Abstract

The connections between taste receptor cells (TRCs) and innervating gustatory neurons are formed in a mutually dependent manner during development. To investigate whether a change in the ratio of cell types that compose taste buds influences the number of innervating gustatory neurons, we analyzed the proportion of gustatory neurons that transmit sour taste signals in adult Skn-1a−/− mice in which the number of sour TRCs is greatly increased. We generated polycystic kidney disease 1 like 3-wheat germ agglutinin (pkd1l3-WGA)/Skn-1a+/+ and pkd1l3-WGA/Skn-1a−/− mice by crossing Skn-1a−/− mice and pkd1l3-WGA transgenic mice, in which neural pathways of sour taste signals can be visualized. The number of WGA-positive cells in the circumvallate papillae is 3-fold higher in taste buds of pkd1l3-WGA/Skn-1a−/− mice relative to pkd1l3-WGA/Skn-1a+/+ mice. Intriguingly, the ratio of WGA-positive neurons to P2X2-expressing gustatory neurons in nodose/petrosal ganglia was similar between pkd1l3-WGA/Skn-1a+/+ and pkd1l3-WGA/Skn-1a−/− mice. In conclusion, an alteration in the ratio of cell types that compose taste buds does not influence the number of gustatory neurons that transmit sour taste signals.

Introduction

The sense of taste arises from the initial binding of taste substances to dedicated taste receptors expressed on the apical surface of taste receptor cells (TRCs). Different taste modalities are basically received by different subsets of TRCs [1]. Type II taste cells detect sweet, umami, and bitter tastes, while type III taste cells mainly sense sour taste [1]. Although the cell type characterization with molecular markers has not been reported, sodium taste is received by TRCs different from sweet, bitter, umami, and sour TRCs, suggesting that at least a subset of type I taste bud cells detect it [2]. And, a recent study suggested that types II and III taste cells are capable of detecting a high concentration of salt [3]. Gustatory signals generated in TRCs are transmitted to peripheral gustatory neurons whose cell bodies reside in the geniculate ganglion (GG) of the VIIth cranial nerve, petrosal ganglion (PG) of the IXth cranial nerve, or the nodose ganglion (NG) of the Xth cranial nerve, with a subsequent entry into the central nervous system.

In the peripheral taste system, the connections between the TRCs and innervating gustatory neurons are formed in a mutually dependent manner during development [4], [5], [6], [7]. When the nerve bundles are dissected, TRCs are eliminated until the regenerated nerve axon reaches the gustatory papilla [4]. On the other hand, when the expression level of brain-derived neurotrophic factor (BDNF) in taste buds was genetically manipulated, gustatory nerve innervation was drastically affected during the developmental stage. In mutant mice lacking BDNF, the gustatory nerve bundle did not reach the gustatory papilla and the morphology of the gustatory papillae was markedly altered [5], [6], [7]. Although gustatory neurons form strict connections with TRCs, the precise mechanism remains unknown.

The aim of this study is to investigate whether the change in the ratio of cell types that compose the taste buds influences the relationship between TRCs and gustatory neurons. Therefore, we generated polycystic kidney disease 1 like 3-wheat germ agglutinin (pkd1l3-WGA)/Skn-1a−/− mice by crossing Skn-1a−/− mice and pkd1l3-WGA transgenic mice. Skn-1a is one of the POU domain-containing transcription factors and was first found to play a role in the regulation of gene expression in epidermal keratinocytes [8], [9]. Skn-1a−/− mice have an unbalanced composition of taste bud cell types, among which sweet, umami, and bitter TRCs disappeared and a comparable number of sour TRCs emerged [10]. Pkd1l3 is expressed in posterior gustatory papillae, and exhibits a sour taste receptor-like properties by forming a heteromer with Pkd2l1 [11]. Neural pathways of taste signals from sour TRCs can be specifically labeled in pkd1l3-WGA transgenic mice by introducing the transsynaptic tracer WGA using the promoter/enhancer elements of pkd1l3 genes [12]. Thus, by analyzing the localization of WGA protein in taste buds and cranial sensory ganglia in pkd1l3-WGA/Skn-1a−/− mice, we determined whether the change in the ratio of cell types influences the relationship between TRCs and gustatory neurons.

Section snippets

Animals

Both pkd1l3-WGA transgenic and Skn-1a−/− mouse lines generated in our laboratory were used [10], [12]. Pkd1l3-WGA mice were crossed with Skn-1a−/− mice to generate pkd1l3-WGA/Skn-1a+/− mice. Crossing Pkd1l3-WGA/Skn-1a+/− mice generated Pkd1l3-WGA/Skn-1a−/− mice. All experiments used male animals aged greater than 3 months. All genetically modified animals were housed under standard conditions in a 12 h-light/12 h-dark cycle. All animal experiments were approved by the Animal Care and Use

Results

To investigate whether a change in the ratio of cell types that compose taste buds influences the number of innervating gustatory neurons, we generated pkd1l3-WGA/Skn1a−/− mice by crossing pkd1l3-WGA mice and Skn1a−/− mice. To examine the expression of WGA protein in the taste buds of CvP, we performed double fluorescent immunostaining using antibodies against WGA together with those against AADC, an enzyme of serotonin synthesis, which was used as a marker for sour TRCs. As demonstrated

Discussion

Neurotrophins such as BDNF and neurotrophin-4 (NT-4) are known to mediate normal development of peripheral gustatory systems. In mutant mice lacking either BDNF or NT-4 or both, a loss of geniculate ganglion neurons, a reduced number of gustatory papillae, and morphologically degenerating taste bud cells have been observed [5], [6], [7], [17], [18], [19]. Neurotrophins are thought to exert an influence on gustatory innervation via different mechanisms [20], [21], [22]. Although the role of

Conflict of interest

None.

Acknowledgements

We are grateful to Ms. Ayako Takahashi and Ms. Rie Kogure for supporting animal care. We thank Dr. Ichiro Matsumoto and Dr. Makoto Ohmoto for providing Skn-1-deficient mice. This work was supported in part by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Technologies for creating next-generation agriculture, forestry and fisheries,” and by a Grant-in-Aid for Young Research (A) (26712014 to MN) and Scientific Research (B) (

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    These authors contributed equally to this work.

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