Chemical Senses Vol. 30 No. suppl 1 © Oxford University
Press 2005; all rights reserved
Downstream Signaling Effectors for Umami Taste
1 Department of Biomedical Sciences, Division of Neuroscience, Colorado State University, Fort Collins, CO 80523, USA, 2 Rocky Mountain Taste and Smell Center, University of Colorado Health Sciences Center, Denver, CO 80262, USA and 3 Regis University, Denver, CO, USA
Correspondence to be sent to: Sue C. Kinnamon, e-mail: sue.kinnamon{at}colostate.edu
Key words: monosodium glutamate, umami, MSG, ribonucleotides, GMP, IMP, gustducin, Ca2+ signaling, TrpM5
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Introduction |
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 Top Introduction AcknowledgementsReferences |
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Monosodium glutamate (MSG) elicits a unique taste called umami. A characteristic feature of umami taste is its potentiation by 5'-ribonucleotides (primarily GMP and IMP), which also have an umami taste of their own. Based on recent molecular studies, two putative umami receptors have been identified: a truncated variant of the metabotropic glutamate receptor mGluR4 (taste-mGluR4) (Chaudhari et al., 1996
, 2000) and the heterodimer,
T1R1 + T1R3 (Li et al.,
2002;
Zhao et al., 2003). Both of
these receptors are expressed in taste cells and both receptors have been expressed in
heterologous cells, where they respond to glutamate at taste effective concentrations.
The T1R1 + T1R3 heterodimer is potentiated strongly by IMP, when MSG is presented
together with the nucleotide. Targeted gene deletion of T1R1 or T1R3 abolishes the
synergistic responses of MSG and IMP (Damak et
al., 2003;
Zhao et al., 2003),
suggesting that T1R1 + T1R3 is required for the nucleotide potentiation. Responses
to MSG alone vary, depending on the study. While
Zhao et al. (2003) found
that responses were completely abolished,
Damak et al. (2003) reported
only a slight reduction, even in the presence of amiloride to reduce the effects of
Na+. Thus, these results open the possibility for a role of additional
receptors, particularly for MSG.
Umami receptors are coupled to a signaling pathway involving activation of PLCß2,
production of IP3 and diacylglycerol, release of Ca2+ from intracellular
stores and activation of a transient receptor potential channel, TRPM5. Evidence for this
hypothesis comes primarily from targeted gene knockouts, showing that both PLCß2 and
TRPM5 are necessary for umami transduction (Zhang
et al., 2003). Several important questions remain, however,
including whether additional receptors contribute to umami taste, what G proteins couple
umami receptors to their downstream signaling effectors and how TRPM5 contributes to
transduction. In this paper, we review our studies of whole-cell recording and
Ca2+ imaging of rat fungiform taste buds in response to umami taste
stimuli. Collectively, these studies provide insights into umami receptors and their
downstream signaling effectors.
Whole cell recording from isolated taste cells showed that bath applied glutamate (1
mM MSG) elicits three types of responses: a depolarizing inward current with an increase
in conductance, mimicked by the ionotropic glutamate agonist NMDA; a hyperpolarizing
outward current with a decrease in conductance, mimicked by the metabotropic glutamate
agonist L-AP4; and a biphasic response, consisting of both types of responses (Lin and Kinnamon, 1999). These data are
consistent with earlier studies, showing that NMDA and/or L-AP4 elicit responses in
subsets of taste cells (Hayashi et al.,
1996;
Bigiani et al., 1997) or
activate channels isolated from taste cell membranes (Teeter et al., 1992). In a small subset of taste
cells, L-AP4 elicited a depolarizing inward current with an increase in conductance,
suggesting the presence of an additional metabotropic receptor for glutamate. Since NMDA
receptors have been localized to the basolateral membrane of taste cells (Caicedo et al., 2000), it is likely that they
represent neurotransmitter receptors rather than taste receptors.
To elucidate the role of GMP in umami signaling, MSG, GMP and MSG + GMP were
bath-applied to single rat fungiform taste cells during voltage-clamp recording or
Ca2+ imaging (Lin et
al., 2003). GMP elicited responses similar to those of MSG, although not
always in the same taste cells, indicating the likelihood of additional receptors for
GMP. Further, although most cells that responded to MSG or GMP usually responded to both
compounds, only a small subset (27%) exhibited synergy when nucleotides were
applied together with MSG. These data provide evidence that nucleotide receptors
independent of the T1R1 + T1R3 heterodimer are expressed in taste buds and may
contribute to umami taste. To determine whether responses resulted in changes in
intracellular Ca2+, fura-2 loaded taste cells were examined in response
to umami stimuli. All cells showed increases in intracellular Ca2+,
including cells showing synergism when MSG and GMP were presented together. These data
indicate that even hyperpolarizing responses involve increases in intracellular
Ca2+, suggesting release from intracellular stores. This was confirmed by
examining responses in Ca2+ free Tyrodes, showing that a portion of the
response was independent of intracellular Ca2+ (Lin et al., 2003). These data are consistent with
gene knockout data showing that PLCß2 is necessary for umami transduction.
TRPM5 is presumed to be the target ion channel for umami transduction. TRPM5
knockouts are insensitive to umami stimuli, as well as to sweet and bitter stimuli,
(Zhang et al., 2003) and the
channel is co-expressed with PLCß2 signaling components (Perez et al., 2002, 2003). The properties of TRPM5
vary according to the heterologous expression system used. When expressed in oocytes,
TRPM5 behaves as a store-operated Ca2+ channel (Perez et al., 2002, 2003), but when expressed in
HEK293 cells TRPM5 behaves as a Ca2+-activated, monovalent-selective
cation channel (Hofmann et al.,
2003;
Liu and Liman, 2003;
Prawitt et al., 2003). Our
electrophysiological studies do not support a role of TRPM5 as a monovalent cation
channel in taste cells. In only a small subset of taste cells do umami stimuli activate G
protein-coupled pathways that result in an increase in membrane conductance (Lin and Kinnamon, 1999). In most cells responses
to umami stimuli, including synergistic responses to MSG and GMP, involve decreases in
membrane conductance and hyperpolarization, likely due to inhibition of a
Cl– channel (Lin and Kinnamon,
1999;
Lin et al., 2003). We
hypothesize that in taste cells, TRPM5 may associate with other TRP channels or
particular scaffolding proteins to confer Ca2+ permeability on the
channel. Thus, the resulting Ca2+ influx would contribute directly to
transmitter release without the need for membrane depolarization and voltage-gated
Ca2+ influx. Indeed, at least in vallate taste buds of mice, taste cells
expressing PLC signaling components generally lack voltage-dependent Ca2+
channels (Medler et al.,
2003). Further studies will be required to determine if all umami-responsive
taste cells lack voltage-gated Ca2+ channels.
What G proteins couple umami taste receptors to downstream signaling effectors? A
recent study shows that T1R3 and T1R1 are co-expressed with 
-gustducin in
fungiform taste buds (Kim et al.,
2003) and we have found that a similar relationship exists in palatal taste
buds (L. Stone, T. Finger and S. Kinnamon, unpublished data). Thus, gustducin may be the
-subunit activated by T1R1 + T1R3. We hypothesized that since gustducin
decreases intracellular cAMP levels, membrane permeant analogs of cAMP should antagonize
responses to umami stimuli. This was indeed the case; 8cpt-cAMP and 8-bromo cAMP
completely abolished the electrophysiological responses to MSG, GMP and the potentiated
response to MSG + GMP (Lin et al.,
2003). These data suggest that -gustducin is an essential component of
the umami signaling pathway. Finally, we have conducted two-bottle preference tests on
-gustducin knockout mice and have found, as predicted, that they are insensitive
to umami stimuli (Ruiz et al.,
2003). These data, coupled with those of Damak et al. (this
symposium) provide strong support that umami receptors couple to -gustducin.
The role of the decreased cAMP in transduction is unclear. Membrane permeant analogs
of cAMP do not activate or block any conductances in umami-sensitive taste cells
(Lin et al., 2003),
suggesting that cAMP may target upstream signaling effectors, possibly by altering their
phosphorylation state.
A model for the transduction of umami stimuli is illustrated in Figure
1. The heterodimer T1R1 + T1R3
binds both MSG and IMP/GMP, resulting in a synergistic response. Other receptors,
possibly taste-mGluR4 (Chaudhari et al.,
2000) and taste-mGluR1 (Gabriel, this symposium) likely bind GMP and/or MSG.
These receptors couple to a heterotrimeric G protein consisting of -gustducin and
its ß
partners. Gustducin activates a phosphodiesterase to decrease
intracellular cAMP, while its ß partners activate PLCß2 to produce IP3 and
diacylglycerol. IP3 elicits release of Ca2+ from intracellular stores,
which subsequently activates TRPM5, although its precise role is unclear. The increase in
intracellular Ca2+ presumably causes release of transmitter and
activation of gustatory afferents. What is still unclear is the mechanism involved in the
inhibition of chloride channels and whether these channels play any significant role in
the transduction process. Also unclear are the role of TRPM5 in the transduction cascade
and the precise target of the decreased cAMP. Further studies will be required to
elucidate these questions.
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Acknowledgements |
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TopIntroductionAcknowledgementsReferences |
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Supported by DC003013 [GenBank] and DC006021 [GenBank] .
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