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. 2015 Jun 30;40(7):449–451. doi: 10.1093/chemse/bjv035

Honing in on the ATP Release Channel in Taste Cells

Kathryn F Medler 1,✉
PMCID: PMC4635639  PMID: 26126730

Abstract

Studies over the last 8 years have identified 3 potential channels that appear to release ATP from Type II cells in response to taste stimuli. These studies have taken different methodological approaches but have all provided data supporting their candidate channel as the ATP release channel. These potential channels include Pannexin 1, Connexins (30 and/or 43), and most recently, the Calhm1 channel. Two papers in this issue of Chemical Senses provide compelling new evidence that Pannexin 1 is not the ATP release channel. Tordoff et al. did a thorough behavioral analysis of the Pannexin1 knock out mouse and found that these animals have the same behavioral responses as wild type mice for 7 different taste stimuli that were tested. Vandenbeuch et al. presented an equally thorough analysis of the gustatory nerve responses in the Pannexin1 knock out mouse and found no differences compared with controls. Thus when the role of Pannexin 1 is analyzed at the systems level, it is not required for normal taste perception. Further studies are needed to determine the role of this hemichannel in taste cells.

Key words: behavior, chorda tympani, glossopharyngeal nerves, Pannexin 1

Understanding how taste receptor cells convert chemical signals from potential food taste items into an electrical signal that the brain can understand has been, and continues to be, a very complicated process. Some things are known: a subset of taste cells, the Type III cells, express the proteins that form conventional chemical synapses and anatomical studies have demonstrated that chemical synapses are present (Murray 1973; Royer and Kinnamon 1988). Conversely, the Type II cells do not have conventional synapses and yet release ATP as their primary neurotransmitter (Royer and Kinnamon 1988; Finger et al. 2005; Clapp et al. 2006). This ATP release is required for normal taste perception (Finger et al. 2005). So how is the ATP released? What is the channel involved? Answering this question has been the focus of studies from multiple labs which have generated conflicting results and to date, it is still not clear what channel(s) are responsible for releasing ATP from Type II cells in response to taste stimuli. However, 2 studies in this issue of Chemical Senses, Tordoff et al., and Vandenbeuch et al., provide compelling evidence for which channel it is not.

What is known about the signaling processes in Type II taste cells? These cells express G-protein coupled receptors that associate with G proteins which activate phospholipase Cβ2 (PLCβ2) (Miyoshi et al. 2001; Chandrashekar et al. 2006; Kim et al. 2006). When PLC is turned on, it cleaves phosphatidylinositol 4,5-bisphosphate to form diacylglycerol (DAG) and inositol trisphosphate (IP3). The role for DAG in this pathway is currently unknown but IP3 diffuses into the cytosol to bind to the IP3R3 receptor found on the endoplasmic reticulum (Clapp et al. 2001; Miura et al. 2007). Activation of the IP3R3 receptor generates a calcium release from internal stores which activates the transient receptor potential M subtype channel (TRPM5) (Perez et al. 2002; Hofmann et al. 2003; Liu and Liman 2003; Huang and Roper 2010). This channel is a monovalent selective TRP channel that primarily allows sodium entry into the taste cell to cause a depolarization (Hofmann et al. 2003; Zhang et al. 2007; Guinamard et al. 2011). This depolarization can result in the firing of an action potential but what happens next is not clear. There are no voltage-gated calcium channels and nor is there vesicular release of neurotransmitter as seen in Type III cells. What channel opens to allow ATP to be released from the cell? Several candidate channels have been identified.

The first potential candidate channel identified was Pannexin 1 (Panx1) by Huang et al. in 2007. Pannexins have homology with the invertebrate innexins which form gap junctions in those organisms. However, pannexins are thought to exist primarily in vertebrate systems as transmembrane channels which allow the passage of small molecules between the cell and the extracellular space. Specifically, pannexins have been shown to release ATP from cells (Bao et al. 2004). These characteristics made pannexins a good candidate to be the ATP release channel in taste cells. In 2007, the Roper lab published a study in which they showed Panx 1 is expressed in most Type II taste cells and that low concentrations of carbenoxolone which is a relatively specific inhibitor of pannexins, inhibited taste-evoked ATP release from taste cells (Huang et al. 2007).

But Panx1 wasn’t the only potential channel identified; both connexins 30 and 43 are also expressed in taste cells and could form hemichannels to release ATP (Romanov et al. 2007, 2008). Romanov et al. (2007) provided evidence that ATP release is through a hemichannel that is calcium independent and voltage dependent. They concluded that the hemichannels were likely pannexins or connexins. In the following year, the same lab published a study concluding that it was likely connexin hemichannels based on pharmacological effects and the kinetics of the responses they observed (Romanov et al. 2008). Further, Romanov et al. (2012) reported that deletion of Panx1 does not prevent ATP release from taste buds but they did not determine if there were any deficits in the animals’ ability to detect taste qualities. Thus, their data support a role for connexins 30 and 43 to form the hemichannel that releases ATP from taste buds.

A third candidate channel, the calcium homeostasis modulator CALHM1, was recently identified as the ATP release channel in Type II cells (Taruno et al. 2013). This channel is voltage-gated and can release ATP from cells. In this study, CALHM1-KO mice were severely impaired in their ability to detect sweet, bitter, and umami and CALHM1 expression was primarily found in Type II cells (Taruno et al. 2013). Behavioral studies also demonstrated that CALHM1-KO and T1R3-KO mice have similar deficits in sugar intake (Sclafani et al. 2014) and that CALHM1-KO mice are impaired in their ability to detect salt (Tordoff et al. 2014), further supporting a role for CALHM1 in taste transduction. A mark in favor of CALHM1 is the behavioral taste deficits associated with the lack of CALHM1 expression.

Thus 3 candidate ATP release channels have been evaluated in taste cells using different methods. Multiple studies have presented data suggesting that these channels are required for ATP release from taste cells. Of the 3, most work has focused on Panx1. Panx 1 is a known ATP release channel in other cell types and low doses of the pannexin inhibitor carbenoxolone inhibits taste evoked ATP release. However, deletion of Panx 1 does not affect ATP release from taste cells, introducing a potential confound.

Two studies in this issue of Chemical Senses have now provided convincing evidence that Panx 1 is not obligatory for taste-evoked ATP release. Tordoff et al. subjected Panx 1-KO mice to a thorough behavioral analysis to identify any deficits in their ability to detect taste stimuli. Both brief access tests and longer term tests were used to analyze their ability to detect 7 different taste stimuli and no differences from wild type were found. Licking rates and preference scores were not different between the KO and wild type mice. Vandenbeuch et al. took a different approach but reached the same conclusion. In this study, they analyzed the gustatory nerve recordings in the Panx 1-KO mouse for both the chorda tympani and gloospharyngeal nerves for 6 different taste stimuli. There were no differences in the responses to any of the stimuli tested when the Panx 1 -KO and wild type mice were compared. They also found robust ATP release in response to a bitter mix in the Panx 1-KO mice that was comparable to wild type, in agreement with the findings of the earlier study by Romanov et al. (2012). Vandenbeuch et al also behaviorally tested the artificial sweetener SC45647 and found no difference in preference between the wild type and KO mice, which adds further support for the findings in the Tordoff et al. study. Clearly, when the impact of Panx 1 on taste is evaluated at the systems level, it does not significantly affect the ability to perceive taste stimuli.

So what is Panx 1 doing in taste cells? It is widely expressed in taste cells but it is not required for transduction of taste stimuli to occur. Both studies address this question and suggest several potential roles for Panx1 in taste cell function. Panx1 is widely expressed in taste cells and has a role in apoptosis in other cell types, so perhaps it has a similar function in taste cells (Huang et al. 2007; Chekeni et al. 2010). While Panx1 is not required for the ATP release that is needed to transmit taste signals to the gustatory nerves, it may still release ATP from taste cells for other reasons, such as a means for cell to cell communication between taste cells. It is also possible that multiple channels release ATP onto the gustatory nerves and when one is knocked out the other channels can compensate for the loss of that particular channel. If that is occurring, then the cells are able to compensate quite well when Panx 1 is missing but not as well when CALHM1 is gone. Clearly, further studies are required to determine what role Panx 1 is playing in the taste bud.

Channel Pannexins Connexins CALHM1
Evidence for
ATP release channel in other cell types (Bao et al. 2004; Koval et al. 2014) Proteins are expressed in taste cells (Romanov et al. 2007, 2008) Calhm1 can release ATP from cells (Taruno et al. 2013)
Channel is widely expressed in taste cells (Huang et al. 2007) Connexin mimetic peptide inhibited ATP release and outward currents (Romanov et al. 2007) Channel is expressed in taste cells (Taruno et al. 2013)
Low concentrations of carbenoxolone inhibits ATP release from taste cells (Huang et al. 2007, Murata et al. 2010) The kinetics of ATP release in taste cells are comparable to the kinetics of connexin hemichannels (Romanov et al. 2008) Calhm1-KO mice have taste deficits (Taruno et al. 2013; Tordoff et al. 2014)
Taste-evoked ATP release is lost in Calhm1-KO mice (Taruno et al. 2013)
Evidence against
Taste cells from Panx1-KO mice still release ATP (Romanov et al. 2012; Vandenbeuch et al. this issue) No evidence to demonstrate that connexins form hemichannels in taste cells. Not a complete taste loss in the absence of Calhm1—suggesting multiple channels may be involved (Taruno et al. 2013)
Panx1-KO mice detect taste stimuli like WT mice (Tordoff et al. this issue; Vandenbeuch et al. this issue)
Nerve recordings from Panx1-KO mice are not different from wild type mice (Vandenbeuch et al. this issue)
Predicted channel kinetics do not match the currents produced in taste cells (Romanov et al. 2008)
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Funding

Supported by NSF 1256950 and NIH RO1GM098609 to KM.

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