Methylglyoxal Evokes Pain by Stimulating TRPA1

Abstract

Diabetic neuropathy is a severe complication of long-standing diabetes and one of the major etiologies of neuropathic pain. Diabetes is associated with an increased formation of reactive oxygen species and the electrophilic dicarbonyl compound methylglyoxal (MG). Here we show that MG stimulates heterologously expressed TRPA1 in CHO cells and natively expressed TRPA1 in MDCK cells and DRG neurons. MG evokes [Ca2+]i-responses in TRPA1 expressing DRG neurons but is without effect in neurons cultured from Trpa1−/− mice. Consistent with a direct, intracellular action, we show that methylglyoxal is significantly more potent as a TRPA1 agonist when applied to the intracellular face of excised membrane patches than to intact cells. Local intraplantar administration of MG evokes a pain response in Trpa1+/+ but not in Trpa1−/− mice. Furthermore, persistently increased MG levels achieved by two weeks pharmacological inhibition of glyoxalase-1 (GLO-1), the rate-limiting enzyme responsible for detoxification of MG, evokes a progressive and marked thermal (cold and heat) and mechanical hypersensitivity in wildtype but not in Trpa1−/− mice. Our results thus demonstrate that TRPA1 is required both for the acute pain response evoked by topical MG and for the long-lasting pronociceptive effects associated with elevated MG in vivo. In contrast to our observations in DRG neurons, MG evokes indistinguishable [Ca2+]i-responses in pancreatic β-cells cultured from Trpa1+/+ and Trpa1−/− mice. In vivo, the TRPA1 antagonist HC030031 impairs glucose clearance in the glucose tolerance test both in Trpa1+/+ and Trpa1−/− mice, indicating a non-TRPA1 mediated effect and suggesting that results obtained with this compound should be interpreted with caution. Our results show that TRPA1 is the principal target for MG in sensory neurons but not in pancreatic β-cells and that activation of TRPA1 by MG produces a painful neuropathy with the behavioral hallmarks of diabetic neuropathy.

Streptozotocin Stimulates the Ion Channel TRPA1 Directly

“…It is possible that STZ, methylglyoxal, H 2 O 2 , nitroxyl (HNO) (16 -18, 25), and other agents that stimulate TRPA1 through disulfide formation produce a sustained mode of TRPA1 activation. Persistent TRPA1 channel activity may contribute to the development of long lasting sensory abnormalities seen in STZ-treated diabetic (14) and nondiabetic (43) animals, but also in otherwise healthy mice with elevated methylglyoxal levels (16,19). Formation of dityrosine and oxidation of dihydrorhodamine, as well as undetectable superoxide levels, suggest that peroxynitrite is the most probable active principle for STZ-induced TRPA1 stimulation.…”

Therapeutic opportunities for targeting cold pain pathways

“…The channel mediates, among others, the reaction to the pungent components of mustard, horseradish, and wasabi. TRPA1 is also activated by formaldehyde, bacterial endotoxins and pro-inflammatory mediators such as bradykinin, methylglyoxal, and prostaglandin E2 [50,[71][72][73][74][75][76][77][78][79]. TRPA1 expression in sensory neuronal populations overlaps little with TRPM8 expression, suggesting distinct physiological functions for the two channels.…”

The Role of Sodium Channels in Painful Diabetic and Idiopathic Neuropathy

“…Diabetic patients with painful neuropathy have higher plasma levels of methylglyoxal, which is known to determine different ion channel modifications and pain-like behaviors in animal models [82][83][84][85]. Methylglyoxal was found to exert opposite effects on DRG neuron Na v 1.7 and Na v 1.8, possibly explaining negative and positive symptoms in diabetic neuropathy.…”