BH4 Oxidation‐Derived H2O2 Activates ERK1/2 Signaling via B‐Raf in Rat Dorsal Root Ganglion Neurons

TL;DR

This study shows that hydrogen peroxide from BH4 oxidation activates a pain-related signaling pathway in sensory neurons, offering a new target for pain treatment without reducing BH4 levels.

Contribution

The study identifies H2O2 as the mediator of BH4-induced ERK1/2 activation via B-Raf in sensory neurons, revealing a novel pain signaling pathway.

Findings

BH4 exposure increases pERK1/2 levels in a dose- and time-dependent manner in DRG neurons.

H2O2, not BH4 itself, activates ERK1/2 via B-Raf and MEK1/2 in sensory neurons.

Pharmacological interference can block H2O2-induced ERK1/2 activation.

Abstract

Elevated Tetrahydrobiopterin (BH4) levels are linked to various pain conditions. Pharmacological or genetic reduction of BH4 levels has analgesic effects in rodent models of neuropathic and inflammatory pain, but also affects neurological and cardiovascular functions. Little is known about the downstream mechanisms of BH4 that could be targeted to attenuate BH4‐induced pain hypersensitivity without lowering BH4 levels. In this study, we exposed ex vivo‐cultured rat dorsal root ganglion (DRG) neurons to BH4 and analyzed the activity of the sensory neuron‐sensitizing kinase ERK1/2 via high‐content imaging. We show that BH4 exposure leads to increased pERK1/2 levels in a dose‐ and time‐dependent manner. Interestingly, we found that H2O2, as a by‐product of BH4 oxidation and not BH4 itself, induces increased pERK1/2 levels via MEK1/2 and B‐Raf (but not A‐Raf or C‐Raf) and that this can be blocked by pharmacological interference. In conclusion, elevated BH4 levels, as observed in various pain conditions, may drive sensory neuron sensitization via oxidation‐derived H2O2 and the B‐Raf‐MEK1/2‐ERK1/2 axis, which presents a novel pathway that could be targeted to attenuate BH4‐induced pain hypersensitivity without the necessity to reduce BH4 levels.

Tetrahydrobiopterin (BH4) is linked to various pain conditions, but targeting its synthesis in vivo is prone to side effects. We now characterized a so far unknown mechanism of BH4‐induced signaling in sensory neurons. In rat dorsal root ganglion (DRG) neurons, we identified neither BH4 nor its oxidation product, BH2, but the byproduct H2O2 to induce increased pERK1/2 levels via B‐Raf and MEK1/2 in sensory, including nociceptive, neurons. Since ERK1/2 activation via phosphorylation is known to cause sensitization, this H2O2‐induced B‐Raf‐MEK1/2‐ERK1/2 axis presents a novel pathway that could be targeted to attenuate BH4‐induced pain hypersensitivity.

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