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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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TRPA1 channels are activated by intracellular protons. (A) Channel activity from a TRPA1-expressing HEK-293 cell in cell-attached patch clamp (Vm = −80 mV) in response to the addition of 10 mM of acetic acid at pH 5.0 outside the patch. (B) Openings of single channels are shown on an expanded time scale from before, during, and after the application of acetic acid as indicated. Similar results were obtained in six out of six patches. (C) Channel activity in an inside-out patch from a HEK-293 cell transfected with TRPA1 (Vm = −80 mV) in response to cytoplasmic delivery of acetate anions or protons (pH 5.5; 1 mM polyP3 was added to all solutions to retain channel activity; Kim and Cavanaugh, 2007). Summary data represent the mean ± SEM. **, P < 0.01 (two-tailed Student’s t test).
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fig4: TRPA1 channels are activated by intracellular protons. (A) Channel activity from a TRPA1-expressing HEK-293 cell in cell-attached patch clamp (Vm = −80 mV) in response to the addition of 10 mM of acetic acid at pH 5.0 outside the patch. (B) Openings of single channels are shown on an expanded time scale from before, during, and after the application of acetic acid as indicated. Similar results were obtained in six out of six patches. (C) Channel activity in an inside-out patch from a HEK-293 cell transfected with TRPA1 (Vm = −80 mV) in response to cytoplasmic delivery of acetate anions or protons (pH 5.5; 1 mM polyP3 was added to all solutions to retain channel activity; Kim and Cavanaugh, 2007). Summary data represent the mean ± SEM. **, P < 0.01 (two-tailed Student’s t test).

Mentions: A prediction of the previous experiments is that it should be possible to activate TRPA1 channels without direct exposure of the extracellular surface of the cell to acids, simply by acidifying the cell cytosol. To test this prediction, we measured activity of TRPA1 channels in cell-attached patches, where the pipette solution was held at neutral pH (7.4) and acetic acid at pH 5.0 was applied outside the area of the patch. Under these conditions, we observed robust channel activity (Fig. 4, A and B; similar results were obtained in six out of six patches). The amplitude of the channels (−11.52 ± 0.65 pA; n = 6) at −80 mV was as expected based on the previously reported unitary conductance of 140 pS (Wang et al., 2008). No channel openings were observed in untransfected cells under the same conditions. Note that we omitted Ca2+ from the extracellular solution, so that activation of the channel could not be attributed secondarily to influx of Ca2+ through TRPA1 channels in the area outside the patch.


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)

TRPA1 channels are activated by intracellular protons. (A) Channel activity from a TRPA1-expressing HEK-293 cell in cell-attached patch clamp (Vm = −80 mV) in response to the addition of 10 mM of acetic acid at pH 5.0 outside the patch. (B) Openings of single channels are shown on an expanded time scale from before, during, and after the application of acetic acid as indicated. Similar results were obtained in six out of six patches. (C) Channel activity in an inside-out patch from a HEK-293 cell transfected with TRPA1 (Vm = −80 mV) in response to cytoplasmic delivery of acetate anions or protons (pH 5.5; 1 mM polyP3 was added to all solutions to retain channel activity; Kim and Cavanaugh, 2007). Summary data represent the mean ± SEM. **, P < 0.01 (two-tailed Student’s t test).
© Copyright Policy - openaccess
Related In: Results  -  Collection

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

fig4: TRPA1 channels are activated by intracellular protons. (A) Channel activity from a TRPA1-expressing HEK-293 cell in cell-attached patch clamp (Vm = −80 mV) in response to the addition of 10 mM of acetic acid at pH 5.0 outside the patch. (B) Openings of single channels are shown on an expanded time scale from before, during, and after the application of acetic acid as indicated. Similar results were obtained in six out of six patches. (C) Channel activity in an inside-out patch from a HEK-293 cell transfected with TRPA1 (Vm = −80 mV) in response to cytoplasmic delivery of acetate anions or protons (pH 5.5; 1 mM polyP3 was added to all solutions to retain channel activity; Kim and Cavanaugh, 2007). Summary data represent the mean ± SEM. **, P < 0.01 (two-tailed Student’s t test).
Mentions: A prediction of the previous experiments is that it should be possible to activate TRPA1 channels without direct exposure of the extracellular surface of the cell to acids, simply by acidifying the cell cytosol. To test this prediction, we measured activity of TRPA1 channels in cell-attached patches, where the pipette solution was held at neutral pH (7.4) and acetic acid at pH 5.0 was applied outside the area of the patch. Under these conditions, we observed robust channel activity (Fig. 4, A and B; similar results were obtained in six out of six patches). The amplitude of the channels (−11.52 ± 0.65 pA; n = 6) at −80 mV was as expected based on the previously reported unitary conductance of 140 pS (Wang et al., 2008). No channel openings were observed in untransfected cells under the same conditions. Note that we omitted Ca2+ from the extracellular solution, so that activation of the channel could not be attributed secondarily to influx of Ca2+ through TRPA1 channels in the area outside the patch.

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