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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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Responses of TG neurons to weak acids are TRPA1 depedent. (A) Elevation of intracellular Ca2+ in sensory neurons isolated from the TG of wild-type mice in response to weak acids and other agonists. Traces illustrate the different types of responses observed: cells responsive to PA, Cin, and Cap (but not to 10 mM HEPES, pH 6.5) are shown with green, blue, and cyan traces; a cell responsive to Cap, but not to Cin or PA, is shown in red; and a cell that responded to none of the agonists is shown in black. (B) TG neurons from TRPA1 knockout mice showed only reduced sensitivity to PA and Cin but retained normal responses to Cap (orange and red traces). Venn diagrams show the aggregate results from all experiments. Numbers represent a total count of the responsive cells to each agonist, and overlap and size of each circle are drawn to scale. (C) Summary data from experiments as in A and B showing the magnitude of the PA response as a function of the Cin response in the same cell. Note that the PA response was positively correlated with the magnitude of the Cin response in cells from wild-type mice (r = 0.52 and P < 0.0001). (D) The percentage of different populations of TG neurons in cultures from wild-type and TRPA1 knockout mice.
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fig9: Responses of TG neurons to weak acids are TRPA1 depedent. (A) Elevation of intracellular Ca2+ in sensory neurons isolated from the TG of wild-type mice in response to weak acids and other agonists. Traces illustrate the different types of responses observed: cells responsive to PA, Cin, and Cap (but not to 10 mM HEPES, pH 6.5) are shown with green, blue, and cyan traces; a cell responsive to Cap, but not to Cin or PA, is shown in red; and a cell that responded to none of the agonists is shown in black. (B) TG neurons from TRPA1 knockout mice showed only reduced sensitivity to PA and Cin but retained normal responses to Cap (orange and red traces). Venn diagrams show the aggregate results from all experiments. Numbers represent a total count of the responsive cells to each agonist, and overlap and size of each circle are drawn to scale. (C) Summary data from experiments as in A and B showing the magnitude of the PA response as a function of the Cin response in the same cell. Note that the PA response was positively correlated with the magnitude of the Cin response in cells from wild-type mice (r = 0.52 and P < 0.0001). (D) The percentage of different populations of TG neurons in cultures from wild-type and TRPA1 knockout mice.

Mentions: To determine whether there is a specific population of nociceptors that is sensitive to carboxylic acids, we measured agonist responses in cultured trigeminal sensory neurons using ratiometric Ca2+ imaging. Each culture was tested sequentially with 10 mM of HEPES-buffered solution, pH 6.5, 100 mM PA, pH 6.5, 100 µM Cin, 1 µM Cap, and 150 mM KCl, in that order, with a recovery period of at least 5 min between exposure to each stimulus. As shown in Fig. 9 A, a subset of cells (14%; 36/263) responded to PA but not to HEPES-buffered solution adjusted with HCl to the same pH (6.5). Interestingly, the response of these cells to PA showed a biphasic time course (Fig. 9 A), reminiscent of the nerve response to PA and the responses of TRPA1-expressing HEK cells to PA (Figs. 1 B and 8 A).


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)

Responses of TG neurons to weak acids are TRPA1 depedent. (A) Elevation of intracellular Ca2+ in sensory neurons isolated from the TG of wild-type mice in response to weak acids and other agonists. Traces illustrate the different types of responses observed: cells responsive to PA, Cin, and Cap (but not to 10 mM HEPES, pH 6.5) are shown with green, blue, and cyan traces; a cell responsive to Cap, but not to Cin or PA, is shown in red; and a cell that responded to none of the agonists is shown in black. (B) TG neurons from TRPA1 knockout mice showed only reduced sensitivity to PA and Cin but retained normal responses to Cap (orange and red traces). Venn diagrams show the aggregate results from all experiments. Numbers represent a total count of the responsive cells to each agonist, and overlap and size of each circle are drawn to scale. (C) Summary data from experiments as in A and B showing the magnitude of the PA response as a function of the Cin response in the same cell. Note that the PA response was positively correlated with the magnitude of the Cin response in cells from wild-type mice (r = 0.52 and P < 0.0001). (D) The percentage of different populations of TG neurons in cultures from wild-type and TRPA1 knockout mice.
© Copyright Policy - openaccess
Related In: Results  -  Collection

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

fig9: Responses of TG neurons to weak acids are TRPA1 depedent. (A) Elevation of intracellular Ca2+ in sensory neurons isolated from the TG of wild-type mice in response to weak acids and other agonists. Traces illustrate the different types of responses observed: cells responsive to PA, Cin, and Cap (but not to 10 mM HEPES, pH 6.5) are shown with green, blue, and cyan traces; a cell responsive to Cap, but not to Cin or PA, is shown in red; and a cell that responded to none of the agonists is shown in black. (B) TG neurons from TRPA1 knockout mice showed only reduced sensitivity to PA and Cin but retained normal responses to Cap (orange and red traces). Venn diagrams show the aggregate results from all experiments. Numbers represent a total count of the responsive cells to each agonist, and overlap and size of each circle are drawn to scale. (C) Summary data from experiments as in A and B showing the magnitude of the PA response as a function of the Cin response in the same cell. Note that the PA response was positively correlated with the magnitude of the Cin response in cells from wild-type mice (r = 0.52 and P < 0.0001). (D) The percentage of different populations of TG neurons in cultures from wild-type and TRPA1 knockout mice.
Mentions: To determine whether there is a specific population of nociceptors that is sensitive to carboxylic acids, we measured agonist responses in cultured trigeminal sensory neurons using ratiometric Ca2+ imaging. Each culture was tested sequentially with 10 mM of HEPES-buffered solution, pH 6.5, 100 mM PA, pH 6.5, 100 µM Cin, 1 µM Cap, and 150 mM KCl, in that order, with a recovery period of at least 5 min between exposure to each stimulus. As shown in Fig. 9 A, a subset of cells (14%; 36/263) responded to PA but not to HEPES-buffered solution adjusted with HCl to the same pH (6.5). Interestingly, the response of these cells to PA showed a biphasic time course (Fig. 9 A), reminiscent of the nerve response to PA and the responses of TRPA1-expressing HEK cells to PA (Figs. 1 B and 8 A).

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