Chemical Senses Vol. 30 No. suppl 1 © Oxford University
Press 2005; all rights reserved
Regional Expression Patterns of T1r Family in the Mouse Tongue
1 National Food Research Institute, 2 Bio-oriented Technology Research Advancement Institution, 3 Asahi Breweries Ltd and 4 Kyushu University
Correspondence to be sent to: Yuko Kusakabe, e-mail: ykusa{at}nfri.affrc.go.jp
Key words: taste receptor, circumvallate papillae, fungiform papillae, umami, sweet, gustducin
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Introduction |
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 Top Introduction Results and discussionAcknowledgementsReferences |
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The T1r family is one of the receptor family belong to class C type of G protein coupled receptors, and comprised of three taste bud-specific receptors, T1r1, T1r2 and T1r3 (Nelson et al., 2001
). T1r1 and
T1r2 are known to have distinctive patterns of regional expression, respectively
(Hoon et al., 1999).
T1r1 is expressed in taste buds in the fungiform papillae, but is rare in the
taste buds of circumvallate papillae. In contrast, T1r2 is rarely expressed in
fungiform papillae but is expressed in all taste buds of the circumvallate papillae.
T1r3 is strongly expressed in both fungiform and circumvallate papillae
(Kitagawa et al., 2001;
Nelson et al., 2001) and
forms an amino-acid (umami) receptor and a sweet receptor in combination with
T1r1 and T1r2, respectively (Nelson et al., 2001;
Li et al., 2002). These
expression patterns suggest that taste cells in circumvallate papillae receive the sweet
taste substances through the heterodimer of T1r2 and T1r3
(T1r2/T1r3), and those in fungiform papillae receive the umami
substances through the heterodimer of T1r1 and T1r3
(T1r1/T1r3). However, physiological studies in mice showed that taste
receptor cells in the anterior as well as in the posterior parts of the tongue were
sensitive to both sweet and umami substances (Ninomiya et al., 1993). This contradiction may
suggest an inadequacy of information about the expression of T1r family. We compared T1r receptors (T1rs) expression pattern between circumvallate and fungiform papillae in mice using double-colored in situ hybridization. Furthermore, we examined the expression patterns of T1rs and the taste cell specific G protein, gustducin, in order to compare taste signal transductions in circumvallate and fungiform papillae.
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Results and discussion |
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TopIntroductionResults and discussionAcknowledgementsReferences |
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Comparison of T1rs expression in circumvallate and fungiform papillae
The expression patterns of T1rs were examined by double-colored in
situ hybridization using all combinations of T1rs cRNA probes to obtain
further information about the regional expression patterns of T1rs. In
circumvallate papillae, we found that T1r1 was expressed in larger numbers of
taste cells than T1r3, and that the majority of T1r3-positive cells
were included among the T1r1-positive cells (Figure
1). This result raised the possibility
that T1r3-positive cells not only mediate sweet taste through
T1r2/T1r3 as reported previously, but also madiate umami taste through
T1r1/T1r3 in the circumvallate papillae. This novel finding is
consistent with the nerve recording data (Ninomiya
et al., 1993), but is not consistent with the data of T1r3
knock out mouse (Damak et al.,
2003). Therefore, we could not determine the role of T1r1 in the
circumvallate papillae. We also found that the signal intensity of T1r1 was
lower than those of T1r2 and T1r3, suggesting that T1r1
expression in circumvallate papillae was weaker than T1r2 and T1r3.
Taking this result and the data of T1r3 knock out mouse into consideration,
T1r1/T1r3 might be related to the restricted part of the umami taste signal
transduction, and other receptors such as mGluR1 and mGluR4 might play central roles in
this transduction. In fungiform papillae, we found that T1r2 was expressed in
restricted taste cells (Figure
1). Double-colored in situ
hybridization showed that about half of T1r3-positive cells expressed
T1r2, and T1r2-positive cells were included among the
T1r3-positive cells (Figure
3). We also observed that the
expression of T1r1 and T1r2 partly overlapped each other. Because
almost all T1r2-positive cells were observed to express T1r3,
T1r1 and T1r2-positive cells were considered to express T1r3.
These data suggest that sweet and umami taste signal transduction in fungiform papillae
are mediated by T1r1/T1r3 and T1r2/T1r3,
respectively, corresponding to the nerve recording data in which chorda tympani (CT)
nerves innervating the fungiform papillae respond to both sweet and umami stimuli
(Ninomiya et al., 1993).
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Expression patterns of T1rs with gustducin in circumvallate papillae and fungiform papillae
Gustducin, a taste cell-specific G protein, has been considered to play roles
in bitter, sweet and umami taste signal transduction, based on behavioral and
electrophysiological studies using gustducin-null mutant mice (Wong et al., 1996;
Ruiz et al., 2003). However,
histological studies in taste buds of circumvallate papillae indicated that
gustducin and the sweet receptor T1r2/T1r3 were expressed separately
(Nelson et al., 2001), while
bitter taste receptors, T2rs were co-expressed with gustducin
(Adler et al., 2000).
Moreover, there is little information about the co-expression pattern of T1r1
and gustducin. Accordingly, it was hard to understand how gustducin was
associated with the sweet and umami taste receptors. We considered that the confusion
might be due to differences in the taste cells tested in behavioral and histological
experiments. The data of the two bottle choice tests using gustducin-null mutant
mice (Wong et al., 1996;
Ruiz et al., 2003) reflect
the total responses of all taste cells on the tongue. On the other hand, the results from
histological experiments using sections of circumvallate papillae show the expression
profile of taste cells in the circumvallate papillae only (Adler et al., 2000;
Nelson et al., 2001).
Therefore, we hypothesized that the role of gustducin in the fungiform papillae
were different from that in the circumvallate papilla, and then compared the expression
patterns of gustducin with T1rs in the fungiform papillae.
In fungiform papillae, T1r2-positive cells were included among the
gustducin-positive cells (Figure
2). As mentioned above,
T1r2-positive cells were also included among T1r3-positive cells,
therefore, every T1r2- positive cell co-expressed T1r3 and
gustducin in the fungiform papillae (Figure
3). Moreover, a part of
T1r1-positive cells co-expressed both T1r3 and gustducin
(Figures
2 and
3). These results raise the
possibility that taste cells in fungiform papillae respond to sweet and/or umami
substances through T1r2/T1r3 and/or T1r1/T1r3, and then
transduce the signals by gustducin. This speculation is in agreement with the
physiological and behavioral data of gustducin-null mouse (Wong et al., 1996;
Ruiz et al., 2003).
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To summarize, we observed different expression patterns of T1rs and gustducin in circumvallate and fungiform papillae. These findings suggested for the first time that gustducin might be involved in different taste signal transductions in the circumvallate papilla and the fungiform papillae, and might play a role in sweet and umami taste signal transduction in fungiform papillae, from the view point of gene expression.
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Acknowledgements |
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TopIntroductionResults and discussionAcknowledgementsReferences |
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This work was supported by Bio-Oriented Technology Research Advancement Institution and Japan Society for the Promotion of Science. We thank Ms Hashimoto, Ms Okano, Ms Kato and Ms Tagami for technical help, and thank Mr Matsushita for providing the experimental mice.
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References |
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TopIntroductionResults and discussionAcknowledgementsReferences |
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