Hypersensitivity of myelinated A-fibers via toll-like receptor 5 promotes mechanical allodynia in tenascin-X-deficient mice associated with Ehlers–Danlos syndrome

Abstract

Abstract Deficiency of an extracellular matrix glycoprotein tenascin-X (TNX) leads to a human heritable disorder Ehlers–Danlos syndrome, and TNX-deficient patients complain of chronic joint pain, myalgia, paresthesia, and axonal polyneuropathy. We previously reported that TNX-deficient ( Tnxb −/ − ) mice exhibit mechanical allodynia and hypersensitivity to myelinated A-fibers. Here, we investigated the pain response of Tnxb −/ − mice using pharmacological silencing of A-fibers with co-injection of N -(2,6-Dimethylphenylcarbamoylmethyl) triethylammonium bromide (QX-314), a membrane-impermeable lidocaine analog, plus flagellin, a toll-like receptor 5 (TLR5) ligand. Intraplantar co-injection of QX-314 and flagellin significantly increased the paw withdrawal threshold to transcutaneous sine wave stimuli at frequencies of 250 Hz (Aδ fiber responses) and 2000 Hz (Aβ fiber responses), but not 5 Hz (C fiber responses) in wild-type mice. The QX-314 plus flagellin-induced silencing of Aδ- and Aβ-fibers was also observed in Tnxb −/ − mice. Co-injection of QX-314 and flagellin significantly inhibited the mechanical allodynia and neuronal activation of the spinal dorsal horn in Tnxb −/ − mice. Interestingly, QX-314 alone inhibited the mechanical allodynia in Tnxb −/ − mice, and it increased the paw withdrawal threshold to stimuli at frequencies of 250 Hz and 2000 Hz in Tnxb −/ − mice, but not in wild-type mice. The inhibition of mechanical allodynia induced by QX-314 alone was blocked by intraplantar injection of a TLR5 antagonist TH1020 in Tnxb −/ − mice. These results suggest that mechanical allodynia due to TNX deficiency is caused by the hypersensitivity of Aδ- and Aβ-fibers, and it is induced by constitutive activation of TLR5.

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