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A TRPA1-dependent mechanism for the pungent sensation of weak acids.

Wang YY, Chang RB, Allgood SD, Silver WL, Liman ER - J. Gen. Physiol. (2011)

Bottom Line: Our results show that heterologously expressed TRPA1 currents can be induced by a series of weak organic acids, including acetic, propionic, formic, and lactic acid, but not by strong acids.Importantly, responses of trigeminal neurons to weak acids were highly overrepresented in the subpopulation of TRPA1-expressing neurons and were severely reduced in neurons from TRPA1 knockout mice.We conclude that TRPA1 is a general sensor for weak acids that produce intracellular acidification and suggest that it functions within the pain pathway to mediate sensitivity to cellular acidosis.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biological Sciences, Section of Neurobiology, University of Southern California, Los Angeles, CA 90089, USA.

ABSTRACT
Acetic acid produces an irritating sensation that can be attributed to activation of nociceptors within the trigeminal ganglion that innervate the nasal or oral cavities. These sensory neurons sense a diverse array of noxious agents in the environment, allowing animals to actively avoid tissue damage. Although receptor mechanisms have been identified for many noxious chemicals, the mechanisms by which animals detect weak acids, such as acetic acid, are less well understood. Weak acids are only partially dissociated at neutral pH and, as such, some can cross the cell membrane, acidifying the cell cytosol. The nociceptor ion channel TRPA1 is activated by CO(2), through gating of the channel by intracellular protons, making it a candidate to more generally mediate sensory responses to weak acids. To test this possibility, we measured responses to weak acids from heterologously expressed TRPA1 channels and trigeminal neurons with patch clamp recording and Ca(2+) microfluorometry. Our results show that heterologously expressed TRPA1 currents can be induced by a series of weak organic acids, including acetic, propionic, formic, and lactic acid, but not by strong acids. Notably, the degree of channel activation was predicted by the degree of intracellular acidification produced by each acid, suggesting that intracellular protons are the proximate stimulus that gates the channel. Responses to weak acids produced a Ca(2+)-independent inactivation that precluded further activation by weak acids or reactive chemicals, whereas preactivation by reactive electrophiles sensitized TRPA1 channels to weak acids. Importantly, responses of trigeminal neurons to weak acids were highly overrepresented in the subpopulation of TRPA1-expressing neurons and were severely reduced in neurons from TRPA1 knockout mice. We conclude that TRPA1 is a general sensor for weak acids that produce intracellular acidification and suggest that it functions within the pain pathway to mediate sensitivity to cellular acidosis.

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Acid and reactive compounds act on different sites of TRPA1. Whole cell currents evoked in HEK-293 cells transfected with wild-type TRPA1 or cysteine mutant C622S. (A and B) 100 µM Cin strongly activated wild-type TRPA1 but only weakly activated the TRPA1 mutant C622S. The subsequent addition of 2 mM Ca2+ induced no further activation and promoted rapid inactivation of both wild-type and C622S currents. In contrast, 10 mM of acetic acid, pH 5.0, strongly activated both wild-type TRPA1- and C622S-mutant channels. (C) Average peak current amplitude measured at +80 mV from experiments as in A and B. Comparison between wild type and C622S was with the two-tailed Student’s t test. Data are represented by the mean ± SEM. **, P < 0.01.
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fig5: Acid and reactive compounds act on different sites of TRPA1. Whole cell currents evoked in HEK-293 cells transfected with wild-type TRPA1 or cysteine mutant C622S. (A and B) 100 µM Cin strongly activated wild-type TRPA1 but only weakly activated the TRPA1 mutant C622S. The subsequent addition of 2 mM Ca2+ induced no further activation and promoted rapid inactivation of both wild-type and C622S currents. In contrast, 10 mM of acetic acid, pH 5.0, strongly activated both wild-type TRPA1- and C622S-mutant channels. (C) Average peak current amplitude measured at +80 mV from experiments as in A and B. Comparison between wild type and C622S was with the two-tailed Student’s t test. Data are represented by the mean ± SEM. **, P < 0.01.

Mentions: Previous work has shown that the activation of TRPA1 by reactive compounds is mediated by intracellular N-terminal cysteines (Hinman et al., 2006; Macpherson et al., 2007). To determine whether intracellular protons and reactive compounds act on the same site of TRPA1, we tested responses to acids of TRPA1-mutant channels that were unresponsive to reactive compounds because of a mutation in a critical cysteine residue, C622S. As shown in Fig. 5, this mutation strongly attenuated activation of TRPA1 channels by Cin. Interestingly, responses to acetic acid were preserved in the mutant, suggesting that activation by weak acids is mechanistically distinct from activation by reactive compounds.


A TRPA1-dependent mechanism for the pungent sensation of weak acids.

Wang YY, Chang RB, Allgood SD, Silver WL, Liman ER - J. Gen. Physiol. (2011)

Acid and reactive compounds act on different sites of TRPA1. Whole cell currents evoked in HEK-293 cells transfected with wild-type TRPA1 or cysteine mutant C622S. (A and B) 100 µM Cin strongly activated wild-type TRPA1 but only weakly activated the TRPA1 mutant C622S. The subsequent addition of 2 mM Ca2+ induced no further activation and promoted rapid inactivation of both wild-type and C622S currents. In contrast, 10 mM of acetic acid, pH 5.0, strongly activated both wild-type TRPA1- and C622S-mutant channels. (C) Average peak current amplitude measured at +80 mV from experiments as in A and B. Comparison between wild type and C622S was with the two-tailed Student’s t test. Data are represented by the mean ± SEM. **, P < 0.01.
© Copyright Policy - openaccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC3105510&req=5

fig5: Acid and reactive compounds act on different sites of TRPA1. Whole cell currents evoked in HEK-293 cells transfected with wild-type TRPA1 or cysteine mutant C622S. (A and B) 100 µM Cin strongly activated wild-type TRPA1 but only weakly activated the TRPA1 mutant C622S. The subsequent addition of 2 mM Ca2+ induced no further activation and promoted rapid inactivation of both wild-type and C622S currents. In contrast, 10 mM of acetic acid, pH 5.0, strongly activated both wild-type TRPA1- and C622S-mutant channels. (C) Average peak current amplitude measured at +80 mV from experiments as in A and B. Comparison between wild type and C622S was with the two-tailed Student’s t test. Data are represented by the mean ± SEM. **, P < 0.01.
Mentions: Previous work has shown that the activation of TRPA1 by reactive compounds is mediated by intracellular N-terminal cysteines (Hinman et al., 2006; Macpherson et al., 2007). To determine whether intracellular protons and reactive compounds act on the same site of TRPA1, we tested responses to acids of TRPA1-mutant channels that were unresponsive to reactive compounds because of a mutation in a critical cysteine residue, C622S. As shown in Fig. 5, this mutation strongly attenuated activation of TRPA1 channels by Cin. Interestingly, responses to acetic acid were preserved in the mutant, suggesting that activation by weak acids is mechanistically distinct from activation by reactive compounds.

Bottom Line: Our results show that heterologously expressed TRPA1 currents can be induced by a series of weak organic acids, including acetic, propionic, formic, and lactic acid, but not by strong acids.Importantly, responses of trigeminal neurons to weak acids were highly overrepresented in the subpopulation of TRPA1-expressing neurons and were severely reduced in neurons from TRPA1 knockout mice.We conclude that TRPA1 is a general sensor for weak acids that produce intracellular acidification and suggest that it functions within the pain pathway to mediate sensitivity to cellular acidosis.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Biological Sciences, Section of Neurobiology, University of Southern California, Los Angeles, CA 90089, USA.

ABSTRACT
Acetic acid produces an irritating sensation that can be attributed to activation of nociceptors within the trigeminal ganglion that innervate the nasal or oral cavities. These sensory neurons sense a diverse array of noxious agents in the environment, allowing animals to actively avoid tissue damage. Although receptor mechanisms have been identified for many noxious chemicals, the mechanisms by which animals detect weak acids, such as acetic acid, are less well understood. Weak acids are only partially dissociated at neutral pH and, as such, some can cross the cell membrane, acidifying the cell cytosol. The nociceptor ion channel TRPA1 is activated by CO(2), through gating of the channel by intracellular protons, making it a candidate to more generally mediate sensory responses to weak acids. To test this possibility, we measured responses to weak acids from heterologously expressed TRPA1 channels and trigeminal neurons with patch clamp recording and Ca(2+) microfluorometry. Our results show that heterologously expressed TRPA1 currents can be induced by a series of weak organic acids, including acetic, propionic, formic, and lactic acid, but not by strong acids. Notably, the degree of channel activation was predicted by the degree of intracellular acidification produced by each acid, suggesting that intracellular protons are the proximate stimulus that gates the channel. Responses to weak acids produced a Ca(2+)-independent inactivation that precluded further activation by weak acids or reactive chemicals, whereas preactivation by reactive electrophiles sensitized TRPA1 channels to weak acids. Importantly, responses of trigeminal neurons to weak acids were highly overrepresented in the subpopulation of TRPA1-expressing neurons and were severely reduced in neurons from TRPA1 knockout mice. We conclude that TRPA1 is a general sensor for weak acids that produce intracellular acidification and suggest that it functions within the pain pathway to mediate sensitivity to cellular acidosis.

Show MeSH
Related in: MedlinePlus