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Comparative Study
. 2009 Apr 15;29(15):4808-19.
doi: 10.1523/JNEUROSCI.5380-08.2009.

TRPA1 modulates mechanotransduction in cutaneous sensory neurons

Kelvin Y Kwan  1 , Joshua M Glazer, David P Corey, Frank L Rice, Cheryl L Stucky
Affiliations
Comparative Study

TRPA1 modulates mechanotransduction in cutaneous sensory neurons

Kelvin Y Kwan et al. J Neurosci. .

Abstract

Transient receptor potential ankyrin 1 (TRPA1) is expressed by nociceptive neurons of the dorsal root ganglia (DRGs) and trigeminal ganglia, but its roles in cold and mechanotransduction are controversial. To determine the contribution of TRPA1 to cold and mechanotransduction in cutaneous primary afferent terminals, we used the ex vivo skin-nerve preparation from Trpa1(+/+), Trpa1(+/-), and Trpa1(-/-) adult mouse littermates. Cutaneous fibers from TRPA1-deficient mice showed no deficits in acute cold sensitivity, but they displayed striking deficits in mechanical response properties. C-fiber nociceptors from Trpa1(-/-) mice exhibited action potential firing rates 50% lower than those in wild-type C-fibers across a wide range of force intensities. Adelta-fiber mechanonociceptors also had reduced firing, but only at high intensity forces (>100 mN). Surprisingly, the firing rates of low-threshold Abeta and D-hair mechanoreceptive fibers were also altered. TRPA1 protein and mRNA expression was assessed in DRG neurons and cutaneous innervation by using Trpa1 in situ hybridization, an antibody for TRPA1, and an antibody for placental alkaline phosphatase (PLAP) in mice in which PLAP was substituted for Trpa1. DRG neurons of all sizes expressed Trpa1 mRNA or PLAP immunoreactivity. TRPA1 or PLAP immunolabeling was detected not only on many thin-caliber axons and intraepidermal endings but also on many large-caliber axons as well as lanceolate and Meissner endings. Epidermal and hair follicle keratinocytes also express TRPA1 message and protein. We propose that TRPA1 modulates mechanotransduction via a cell-autonomous mechanism in nociceptor terminals and possibly through a modulatory role in keratinocytes, which may interact with sensory terminals to modify their mechanical firing properties.

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Figures

Figure 1.
Figure 1.
Cold activation in Trpa1 mutant fibers. a, Typical response of wild-type (top) and TRPA1-deficient (bottom) cutaneous C-fiber to a cold ramp (32 to 2°C, over 20 s). Action potential waveform is shown at the right of the trace. b, Percentage of C-fibers responding to cold ramp in wild-type versus Trpa1−/− mice. c, Average cold-evoked action potential firing rate in C-fibers from wild-type and Trpa1−/− mice. d, Typical response of AM-fiber from wild-type (top) and Trpa1−/− mouse to cold ramp (32 to 2°C, over 20 s). e, Percentage of AM-fibers responding to a cold ramp in wild-type and Trpa1−/− mice. f, Average cold-evoked action potential firing rate in AM-fibers from wild-type and Trpa1−/− mice. Error bars indicate SEM.
Figure 2.
Figure 2.
Mechanically sensitive afferents are present in normal proportions. a, Percentage of all Aβ-, Aδ-, and C-fibers encountered in Trpa1−/− and wild-type preparations that were mechanically sensitive or mechanically insensitive. Fibers were identified using an electrical search protocol. Mechanically insensitive neurons did not respond to von Frey filaments up to 147 mN or a glass rod stimulus. b, Percentage of Aβ-fibers that were classified as SA or RA based on responses to sustained force (left). Percentage of Aδ-fibers that were classified as D-hair receptors or AM-fibers (right).
Figure 3.
Figure 3.
Nociceptors are less responsive to mechanical force. a, Example of response of a C-fiber from a wild-type (top) and Trpa1−/− (bottom) mouse to sustained mechanical force (40, 100, 150 mN; 10 s each). b, Average action potential firing rate per second to increasing sustained force (each stimulus 10 s) in wild-type (+/+), heterozygote (+/−), and mutant (−/−) C-fibers. C-fibers from Trpa1−/− mice responded with significantly fewer action potentials at all force intensities compared with Trpa1+/+ C-fibers (***p < 0.0001 between genotypes; mixed-model ANOVA). Furthermore, as force increased, the difference increased proportionally between Trpa1−/− and wild-type C-fibers, and this was evident by a significant interaction between force and genotype (p = 0.0074). C-fibers from Trpa1+/− heterozygotes also fired significantly fewer action potentials to force than wild-type C-fibers (**p = 0.0078 between genotypes; mixed-model ANOVA). c, Example of an AM-fiber from a Trpa1+/+ (top) and Trpa1−/− (bottom) mouse to sustained mechanical force. d, Average action potential firing rate to increasing force in AM-fibers. AM-fibers from Trpa1−/− mice responded with significantly fewer action potentials than those from wild-type mice at intense forces >100 mN as evident by a significant interaction between force and genotype (**p = 0062; mixed-model ANOVA). Error bars indicate SEM.
Figure 4.
Figure 4.
Mechanical firing is differentially altered in slowly versus rapidly adapting mechanoreceptors from Trpa1 mutants. a, Average action potentials per second in response to sustained force in all slowly adapting Aβ-fibers from Trpa1−/− and wild-type mice. SA Aβ-fibers from Trpa1−/− mice responded with significantly fewer action potentials at all force intensities compared with Trpa1+/+ SA Aβ-fibers (**p = 0.0003 between genotypes; mixed-model ANOVA). SA Aβ-fibers were divided into “high threshold” [von Frey threshold (VFT), ≥4 mN] (b) and “low threshold” (VFT, <4 mN) (c). b, High-threshold SA Aβ-fibers from Trpa1−/− mice responded with significantly fewer action potentials than those from wild-type mice, but the difference was present only at forces >100 mN as evident by a significant interaction between force and genotype (**p = 0.0074; mixed-model ANOVA). c, Low-threshold SA Aβ-fibers from Trpa1−/− mice responded with significantly fewer action potentials at all force intensities than these fibers from wild-type mice (***p < 0.0001 between genotypes; mixed-model ANOVA). d, Average action potentials per second in RA Aβ-fibers. RA Aβ-fibers from Trpa1−/− mice responded with significantly more action potentials than those in wild-type mice over all force intensities (*p = 0.012 between genotypes; mixed-model ANOVA). e, Average action potentials per second in D-hair fibers. D-hair fibers from Trpa1−/− mice responded to force with significantly more action potentials than those in wild-type mice at forces <100 mN (**p = 0.0011 between genotypes). Error bars indicate SEM.
Figure 5.
Figure 5.
Adaptation properties of slowly adapting fibers are altered. Average action potentials per second during a 10 s sustained, intense (150 mN) mechanical force in subclasses of slowly adapting fibers. Both C-fiber (a) and AM-fiber nociceptors (b) showed significantly reduced firing initially and during the entire duration of sustained force. SA Aβ-fibers (all) (c), including low-threshold (d) and high-threshold (e) subtypes, exhibited normal firing during the first second of force, but more rapid adaptation during the remaining stimulus duration. High-threshold SA Aβ-fibers showed almost complete adaptation by 2 s (e). Error bars indicate SEM.
Figure 6.
Figure 6.
Correlation of Trpa1 transcript with PLAP reporter. In situ hybridization using a Trpa1 mRNA probe and PLAP immunostaining were sequentially performed on L2–L6 DRG sections. a, The in situ hybridization probe for Trpa1 mRNA, which encodes the last two transmembrane domains and a portion of the C terminus, is represented by a green line. Only fluorescent DRG cell bodies were counted and analyzed. b, In confocal images (three panels), Trpa1 mRNA was present but no PLAP immunoreactivity PLAP-IR) was observed in small, medium, and large DRG cell bodies (small, medium, and large green arrows, respectively) from Trpa1+/+ animals. Total DRG neurons analyzed, n = 488. c, In confocal images (three panels) of Trpa1+/− heterozygote animals containing both Trpa1 and the PLAP reporter, the vast majority of fluorescent neurons displayed both Trpa1 message and PLAP-IR. Total DRG neurons analyzed, n = 539. Small, medium, and large neurons are double labeled (small, medium, and large yellow arrows). d, In confocal images (three panels), small, medium, and large DRG neurons (small, medium, and large red arrows, respectively) from Trpa1−/− animals showed prominent PLAP-IR but no detectable Trpa1 in situ signal. Total DRG neurons analyzed, n = 465. e, In epifluorescence images (three panels), neurofilament immunoreactivity (NF-IR) is present without PLAP-IR on small, medium, and large DRG neurons in Trpa1+/+ animals (small, medium, and large green arrows, respectively). f, In epifluorescence images (three panels), NF-IR is present on small, medium, and large neurons that are also PLAP positive in Trpa1+/− animals (small, medium, and large yellow arrows, respectively). Small PLAP-labeled neurons are also present that lack NF (small red arrow), as well as large PLAP-negative neurons that are NF-positive (green arrow). g, Percentages of DRG neurons containing Trpa1 mRNA and PLAP immunoreactivity from each genotype of mice (n = 4 mice for each genotype). h, Size distribution of PLAP-positive neurons in Trpa1−/− and Trpa1+/− DRGs. The open bars are proportions of all neurons in each size range. The solid bars are only those neurons with PLAP immunofluorescence rated from 3 to 5 on a scale of 1–5. Since no differences in PLAP-positive neurons in Trpa1−/− and Trpa1+/− mice were observed, the genotypes were combined for the histogram. i, Size distribution of neurons expressing Trpa1 mRNA in Trpa1+/+ and Trpa1+/− DRGs. The open bars are proportions of all neurons in each size range. The solid bars are only those neurons with Trpa1 mRNA levels rated from 3 to 5 on a scale of 1–5. Since no differences in Trpa1 mRNA-expressing neurons were observed in Trpa1+/+ and Trpa1+/− mice, the genotypes were combined. Scale bar, 50 μm.
Figure 7.
Figure 7.
TRPA1 is normally present in keratinocytes of glabrous skin as well as on some intraepidermal endings and Meissner endings. Immunostaining of glabrous (nonhairy) hindpaw skin with PGP9.5, PLAP, and TRPA1 antibodies. Ep, Epidermis; DP, dermal papillae. a, Double-label confocal images (three panels) of skin from Trpa1+/+ animals lacking the PLAP gene and stained for PGP9.5 and PLAP. Intraepidermal endings (small green arrows) and axons located just beneath the epidermis (medium green arrows) have PGP9.5 immunoreactivity (PGP9.5-IR) but no PLAP-IR. Some background fluorescence is present the epidermis (center panel). b, Double-label confocal images (three panels) of skin from Trpa1−/− animals having the PLAP gene and stained with PGP9.5 and PLAP antibodies. Sensory endings in the epidermis are only definitive for PGP9.5-IR (small green arrows). Some axons located just beneath the epidermis have only PGP9.5-IR (medium green arrows), whereas others are immunoreactive for both PGP9.5 and PLAP (medium yellow arrows). Nerves slightly deeper in the dermis have axons that are PLAP-positive and -negative (large yellow-green arrows). Keratinocytes are intensely labeled for PLAP in all live epidermal layers: stratum basalis (SB), stratum spinosum (SS), and stratum granulosum (SG). No PLAP is detectable in the dead keratinocytes of stratum corneum (SC). c, In an epifluorescence image, TRPA1 antibody does not label keratinocytes or innervation in this location in Trpa1−/− mice. d, In an epifluorescence image, TRPA1-IR is present in live and dead layers of keratinocytes in Trpa1+/+ mice. However, TRPA1-IR is lacking in SB. Occasional sensory endings in the epidermis are clearly TRPA1-immunoreactive (2× magnification inset). e, In an epifluorescence image, sensory endings in Meissner corpuscles (white arrows), located in dermal papillae, have TRPA1-IR that is more intense than the TRPA1-IR in the keratinocytes of the adjacent epidermis. f, Real-time PCR traces of total RNA from three different samples of glabrous skin obtained from Trpa1+/+ or Trpa1−/− animals. Trpa1 mRNA levels were normalized using cytokeratin 14 (Krt 14) transcript levels to show the presence of Trpa1 transcript in wild-type but not in mutant animals. g, In an epifluorescence image, the TRPA1 antibody fails to label axons in nerves deep in the dermis of Trpa1−/− animals. h, In an epifluorescence image, TRPA1 antibody labels small- (small arrows) and large-caliber axons (large arrows) in nerves deep in the dermis of Trpa1+/+ animals. Scale bar, 25 μm.
Figure 8.
Figure 8.
TRPA1 is normally expressed in epidermal and hair follicle keratinocytes, in some intraepidermal endings, and in lanceolate endings in hairy skin. a, b, Epifluorescent images of TRPA1 immunoreactivity of hairy skin taken from the dorsal medial portion of the hindpaw in Trpa1−/− (a) and Trpa1+/+ mice (b). Ep, Epidermis; H, hair; HF, hair follicle; SbG, sebaceous glands; TRPA1 immunolabeling is present in keratinocytes throughout the epidermis and the hair follicles only in the wild-type mice. Autofluorescence is present in sebaceous glands and in hair shafts in the Trpa1−/− mice, whereas the fluorescence is much higher after TRPA1 labeling in Trpa1+/+ mice. The solid line square in b indicates the location of the enlarged images shown in c and d. The broken line square indicates the location of the enlarged images shown in e–g. Hair follicles have autofluorescence. c, Confocal images of PGP9.5 immunoreactivity (PGP9.5-IR) (left and right panels) reveal endings in the epidermis, at the epidermal border and deeper in the dermis (small, medium, and large green arrows) in Trpa1+/+ animals. PLAP immunolabeling is lacking in the innervation and keratinocytes (center panel). The broken line indicates the border of the epidermis and hair follicle where a basement membrane will be located. d, Confocal images of PGP9.5-IR (left and right panels) reveal endings in the epidermis and at the epidermal border (small and medium arrows) in Trpa1−/− animals. Intense PLAP-immunolabeling is present in keratinocytes and the sebaceous gland (center and right panels). However, some PLAP-IR aligns uniquely with PGP9.5-labeled endings in the epidermis (yellow arrows), whereas other innervation is labeled only for PGP9.5 (green arrows). e, In epifluorescence images, both PGP9.5-IR and PLAP-IR (three panels) are detected in Trpa1−/− mice on a lanceolate ending (yellow arrows) located parallel to the basement membrane of a hair follicle. Keratinocytes in the hair follicle also express PLAP-IR and label with anti-NF. f, In epifluorescent images, lanceolate endings, which also normally express NF-IR (left and right panels, green arrows), lack PLAP-IR (center and right panels) in Trpa1+/+ mice. The sebaceous glands have autofluorescence. g, In an epifluorescence image, lanceolate endings (white arrows) and hair follicle keratinocytes in Trpa1+/+ mice label with TRPA1 antibodies. Scale bar, 100 μm.
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