Taste bud cells for sweet, umami, and bitter transmit sensory signals without a synapse. A study by Ma et al. (2018) finds a key ATP-permeable pore-forming subunit required for rapid neurotransmission from the tongue to secondary taste neurons

Oka, Yuki

English

Caltech Authors - Main

Opening a “Wide” Window onto Taste Signal Transmission
Yuki Oka1,*
1California Institute of Technology, Pasadena, California, USA
Abstract
Taste bud cells for sweet, umami, and bitter transmit sensory signals without a synapse. A study by 
Ma et al. (2018) finds a key ATP-permeable pore-forming subunit required for rapid 
neurotransmission from the tongue to secondary taste neurons.
Peripheral sensory neurons generally transmit sensory information to higher-order neurons 
via synaptic vesicle release. By contrast, some types of mature taste bud cells (TBCs) are 
known to lack synaptic structures, yet they can transmit taste signals. For more than two 
decades, sensory biologists have struggled to unveil how synapse-less TBCs communicate 
with downstream neurons. On the mammalian tongue, each taste bud contains several 
dozens of TBCs that are classified into receptor cells and support cells (sometimes referred 
to as type I cells). Individual taste receptor cells are uniquely tuned to mediate one of five 
basic tastes (Yarmolinsky et al., 2009). Notably, TBCs lack axons and are innervated by the 
afferent nerve fibers of secondary taste neurons (Figure 1, top).
Interestingly, acid-sensing TBCs (type III cells) have neuronal properties with visible 
synaptic structures (Kataoka et al., 2008), whereas sweet-, umami-, and bitter-sensing cells 
(type II cells) do not possess the synaptic machinery (Clapp et al., 2004). How are sweet, 
umami, and bitter signals transmitted from taste buds? An important clue came from a study 
on neurotransmitter molecules. Finger et al. (2005) found that TBCs use ATP as a critical 
transmitter to communicate with gustatory nerve fibers. Knocking out purinergic receptors 
P2X2 and P2X3 completely eliminated physiological responses to all tastants. Since P2X2 
and P2X3 are the major purinergic receptors in secondary taste neurons, this study 
implicated that sweet, umami, and bitter TBCs somehow secrete ATP without synaptic 
vesicles.
ATP can be released from cells via vesicular as well as non-vesicular routes, including 
certain types of ATP-permeable channels (Kinnamon, 2013). If such channels are 
responsible for neurotransmission in TBCs, they need to meet at least three criteria. First, the 
channels obviously have to be expressed in TBCs. Second, knocking out the gene should 
eliminate ATP release from TBCs without affecting cell excitability and viability. Third, 
such genetic manipulation should abolish physiological and behavioral responses to sweet, 
umami, and bitter taste stimuli. In the past 10 years, multiple channels have been proposed 
*Correspondence: yoka@caltech.edu. 
HHS Public Access
Author manuscript
Neuron. Author manuscript; available in PMC 2018 July 18.
Published in final edited form as:
Neuron. 2018 May 02; 98(3): 456–458. doi:10.1016/j.neuron.2018.04.020.
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as ATP-conducting molecules, including pannexin 1 (Huang et al., 2007) and connexins 
(Romanov et al., 2007). However, their roles in neurotransmission are still elusive.
In 2013, Taruno et al. (2013) discovered that calcium homeostasis modulator 1 (CALHM1 
or FAM26C), the pore-forming subunit of a voltage-gated, non-selective cation channel, is 
selectively expressed in type II TBCs. This large-pore channel seems to meet all the criteria 
mentioned above for ATP-conducting molecules in taste buds. Animals lacking the 
CALHM1 gene (CALHM1−/− mice) showed no taste-induced ATP release from taste 
tissues. Moreover, these mice lost behavioral preference for sweet, umami, and bitter 
without their responses to salty or sour tastes being affected. Importantly, type II TBCs 
retained normal voltage-gated Na+ currents as well as calcium responses upon taste 
stimulation in the absence of CALMH1. Data from this study led to the conclusion that 
CALHM1 is a key component in forming an ATP-permeable channel in type II TBCs.
Despite the compelling evidence that CALHM1 is involved in ATP release from TBCs, a 
critical issue remained unsolved. The kinetics of ATP release through CALHM1 monomeric 
channels is unrealistically slow for biological taste responses (τ > 500 msec). Generally, 
taste cells generate action potentials upon taste stimulation, with the latency of ATP release 
being ~10 msec. This obvious temporal discrepancy suggested that an additional 
component(s) is required for rapid ATP release from TBCs.
In this issue of Neuron, Ma et al. (2018) identified a key channel subunit that, together with 
CALHM1, forms the endogenous ATP-permeable channel complex in Type II TBCs. For a 
potential partner of CALHM1, the authors focused on the members of the CALHM family, 
CALHM2 (FAM26B) and CALHM3 (FAM26A). Using electrophysiological recording in 
heterologous expression systems, they found that co-expression of CALHM1 with 
CALHM3, but not with CALHM2, drastically enhances the activation kinetics of outward 
currents (presumably induced by ATP release) compared to expression of CALHM1 alone. 
In fact, cells transfected with CALHM1+3 exhibited gating kinetics similar to the kinetics in 
intact TBCs. Given these results, the authors hypothesized that CALHM1 and CALHM3 
form a complex to mediate rapid taste neurotransmission.
To test this idea, Ma et al. (2018) performed a series of biochemical experiments. First, they 
showed that CALHM1 and CALHM3 physically and specifically interact with each other by 
co-immunoprecipitation in culture cells. Second, the authors used various CALHM1/3 
concatemers to determine that these two subunits likely form a hexametric channel complex. 
Third, they employed a single-molecule photobleaching technique and showed that 
bleaching steps of individual, fluorescently tagged subunits (as an estimation of the number 
of subunits) decreased in the presence of the partner subunit. This suggests that CALHM1 
and CALHM3 assemble into a single channel complex. These biochemical studies also 
revealed one unexpected pharmacological property of CALHM channels. The application of 
carbenoxolone (CBX), a nonselective gap-junction blocker, has been reported to suppress 
ATP release from TBCs (Huang et al., 2007). Previous studies used this evidence to support 
the contribution of gap-junction proteins to taste neurotransmission. Remarkably, while 
CBX has no inhibitory effect on CALHM1 homomeric channels, the authors found that 
CALHM1+3 heteromeric channels are highly sensitive to CBX. These results, at least in 
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part, explain the inhibitory effect of the gap junction blocker on ATP release from intact 
TBCs.
So the next important question is whether CALHM1+3 indeed functions as the endogenous, 
voltage-gated ATP-release channel in vivo. Ma et al. (2018) first demonstrated that these two 
subunits are co-expressed in TBCs that also express TRPM5, a cation channel required for 
sweet, umami, and bitter tastes. Next, they found that voltage-gated nonselective outward 
currents were absent in type II TBCs isolated from CALHM3−/− mice. However, such 
genetic deletion did not alter taste-evoked calcium signaling in these cells. Finally, they 
examined the behavioral taste preference using these knockout mice. Consistent with their 
hypothesis, CALHM3−/− mice exhibited practically no electrophysiological responses and 
exhibited severely reduced behavioral preference toward GPCR-mediated tastes (i.e., sweet, 
umami, and bitter).
This study by Ma et al. (2018) elegantly demonstrated that CALHM1+3 channels form 
voltage-dependent cation channels in type II cells and are required for rapid ATP release. 
There are a few caveats, though, in their interpretations and conclusions. For instance, the 
ATP permeability of the channel complex has not been directly tested in this study. This 
leaves the possibility that effector molecules downstream of CALHM1+3 channels may 
contribute to ATP release. Another issue has to do with technical difficulties: obtaining 
workable antibodies for ion channels and sufficient amounts of protein from TBCs are 
notoriously challenging. Due to these limitations, the direct interaction between CALHM1 
and CALHM3 has been tested only in a heterologous overexpression system instead of in 
intact TBCs. Notably, the authors examined aversive salt responses (high salt) but not 
attractive salt responses (amiloride-sensitive low salt) in CALHM3−/− mice. Since these two 
aspects of salt taste are mediated by distinct pathways, attractive salt taste may also be 
affected in CALHM3−/− mice.
Identification of a critical component for ATP transmission in taste buds addresses a long-
standing question in the field. However, it also opens up a new avenue to tackle important 
problems to be solved in the future. Taste buds are densely packed with functionally distinct 
TBCs, each of which is innervated by secondary taste neurons in a quality-specific manner 
(Lee et al., 2017). How does a given nerve fiber selectively respond to ATP release from the 
target TBC, but not from adjacent cells? Possible mechanisms are: 1) ATP-permeable 
channels involving CALHM1+3 may be localized to the sites of taste nerve ending, and 2) 
the open probability of the channels may be locally regulated by additional cellular 
components. In either scenario, deciphering the underlying molecular basis would provide 
further insight for purinergic transmission in the taste system. Another remaining question is 
the interaction between CALHM and other ATP-permeable channels. For example, pannexin 
1 is highly expressed in TBCs and is capable of releasing ATP, whereas gene deletion has no 
effect on taste responses. It is feasible that these molecules may have a role for cell-to-cell 
communication between TBCs. This model would indicate that ATP is used for both 
neurotransmission and intra-taste-bud signaling via distinct ATP-release mechanisms. 
Further studies would clarify the contribution of each ATP-permeable channel to different 
aspects of physiological functions mediated by ATP. So far, two neurotransmission 
mechanisms have been found in the peripheral taste system (Figure 1). Acid-sensing TBCs 
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are “neuronal” cells with visible synaptic vesicles that secrete ATP upon stimulation. Sweet, 
umami, and bitter TBCs are “non-neuronal” cell types that release ATP via CALHM 
channels. How about salty? It seems that salt-sensing TBCs also use ATP as a 
neurotransmitter because knocking out of P2X2 and P2X3 eliminates salt responses (Finger 
et al., 2005). However, the route of ATP release is unclear. Understanding how salt-sensing 
cells transmit signals to secondary taste neurons would put the final piece in the puzzle of 
taste neurotransmission.
The important roles of purinergic signaling in neuromodulation are widely accepted at both 
peripheral and central levels. In this study, Ma et al. (2018) provide the first functional 
evidence of ion-channel-mediated fast purinergic neurotransmission. Whether this 
mechanism is unique to the taste system or more broadly used in other systems remains to 
be investigated.
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Figure 1. Purinergic Neurotransmission of Taste Bud Cells
Schematic diagram illustrating that taste bud cells (TBCs) are innervated by afferent nerve 
fibers from secondary taste neurons (top). Individual TBCs are tuned to mediate one of five 
basic tastes. Based on morphological characteristics, support cells are sometimes termed 
type I; sweet, umami, and bitter cells are type II; and sour cells are type III TBCs. Once 
TBCs receive taste stimulation, they generate action potentials leading to ATP release, which 
in turn activates P2X2 and P2X3 receptors expressed by afferent nerve fibers. Sour-sensing 
TBCs use conventional synaptic vesicles to secrete ATP (bottom left), whereas sweet, 
umami, and bitter TBCs release ATP via CALHM1/CALHM3 channels (bottom right).
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Opening a “Wide” Window onto Taste Signal Transmission

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Opening a “Wide” Window onto Taste Signal Transmission

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