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Inhibition of mechanical allodynia in neuropathic pain by TLR5-mediated A-fiber blockade

Abstract

Mechanical allodynia, induced by normally innocuous low-threshold mechanical stimulation, represents a cardinal feature of neuropathic pain. Blockade or ablation of high-threshold, small-diameter unmyelinated group C nerve fibers (C-fibers) has limited effects on mechanical allodynia1,2,3,4. Although large, myelinated group A fibers, in particular Aβ-fibers, have previously been implicated in mechanical allodynia5,6,7, an A-fiber–selective pharmacological blocker is still lacking. Here we report a new method for targeted silencing of A-fibers in neuropathic pain. We found that Toll-like receptor 5 (TLR5) is co-expressed with neurofilament-200 in large-diameter A-fiber neurons in the dorsal root ganglion (DRG). Activation of TLR5 with its ligand flagellin results in neuronal entry of the membrane-impermeable lidocaine derivative QX-314, leading to TLR5-dependent blockade of sodium currents, predominantly in A-fiber neurons of mouse DRGs. Intraplantar co-application of flagellin and QX-314 (flagellin/QX-314) dose-dependently suppresses mechanical allodynia after chemotherapy, nerve injury, and diabetic neuropathy, but this blockade is abrogated in Tlr5-deficient mice. In vivo electrophysiology demonstrated that co-application of flagellin/QX-314 selectively suppressed Aβ-fiber conduction in naive and chemotherapy-treated mice. TLR5-mediated Aβ-fiber blockade, but not capsaicin-mediated C-fiber blockade, also reduced chemotherapy-induced ongoing pain without impairing motor function. Finally, flagellin/QX-314 co-application suppressed sodium currents in large-diameter human DRG neurons. Thus, our findings provide a new tool for targeted silencing of Aβ-fibers and neuropathic pain treatment.

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Figure 1: TLR5 is co-localized with the A-fiber marker NF200 in DRG neurons, skin nerve fibers, and spinal cord axonal terminals in mice.
Figure 2: Co-application of flagellin and QX-314 blocks sodium currents in large-diameter A-fiber neurons of mouse and human DRGs.
Figure 3: Co-application of flagellin and QX-314 selectively inhibits Aβ-fiber conduction in sciatic nerves of naive and chemotherapy-treated mice.
Figure 4: A-fiber blockade by co-application of flagellin/QX-314 inhibits mechanical allodynia and ongoing pain in different neuropathic pain conditions.

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Acknowledgements

This study is supported by US National Institutes of Health (NIH) R01 grants NS67686 (R.-R.J.), NS87988 (R.-R.J.), NS89479 (R.-R.J.), DE17794 (R.-R.J.), and DE22743 (R.-R.J.); NIH R21 grants NS82985 (Z.-Z.X.) and NS91779 (Z.-Z.X.); NIH R01 grant DE19440 (F.W.); a Korea government grant 2012R1A3A2048834 (S.B.O.); and a Korea National Research Foundation grant 2013R1A6A3A04065858 (Y.H.K.).

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Z.-Z.X. developed the behavioral part of the project and designed and performed immunohistochemical and behavioral experiments. Y.H.K. recorded action potentials in mouse DRG neurons and compound potentials in intact mice, and recorded sodium currents in human DRG neurons. S.B. initially tested the idea of flagellin/QX-314 blockade of sodium currents in mouse DRG neurons and examined dye (phallodin-rhodamine) uptake in DRGs. Y.Z. performed in situ hybridization under the guidance of F.W. T.B. performed the PCR experiments. R.-R.J. conceived and supervised the project. S.B.O. and F.W. discussed the project; and R.-R.J., Z.-Z.X., S.B.O., and F.W. wrote the paper.

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Correspondence to Ru-Rong Ji.

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Xu, ZZ., Kim, Y., Bang, S. et al. Inhibition of mechanical allodynia in neuropathic pain by TLR5-mediated A-fiber blockade. Nat Med 21, 1326–1331 (2015). https://doi.org/10.1038/nm.3978

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