Mast cell tetrahydrobiopterin contributes to itch in mice

<jats:title>Abstract</jats:title><jats:p><jats:styled-content style="fixed-case">GTP</jats:styled-content> cyclohydrolase (<jats:styled-content style="fixed-case">GCH</jats:styled-content>1) governs de novo synthesis of the enzyme cofactor, tetrahydrobiopterin (<jats:styled-content style="fixed-case">BH</jats:styled-content>4), which is essential for biogenic amine production, bioactive lipid metabolism and redox coupling of nitric oxide synthases. Overproduction of <jats:styled-content style="fixed-case">BH</jats:styled-content>4 via upregulation of <jats:styled-content style="fixed-case">GCH</jats:styled-content>1 in sensory neurons is associated with nociceptive hypersensitivity in rodents, and neuron‐specific <jats:styled-content style="fixed-case">GCH</jats:styled-content>1 deletion normalizes nociception. The translational relevance is revealed by protective polymorphisms of <jats:styled-content style="fixed-case">GCH</jats:styled-content>1 in humans, which are associated with a reduced chronic pain. Because myeloid cells constitute a major non‐neuronal source of <jats:styled-content style="fixed-case">BH</jats:styled-content>4 that may contribute to <jats:styled-content style="fixed-case">BH</jats:styled-content>4‐dependent phenotypes, we studied here the contribution of myeloid‐derived <jats:styled-content style="fixed-case">BH</jats:styled-content>4 to pain and itch in lysozyme M Cre‐mediated <jats:styled-content style="fixed-case">GCH</jats:styled-content>1 knockout (LysM‐<jats:styled-content style="fixed-case">GCH</jats:styled-content>1<jats:sup>−/−</jats:sup>) and overexpressing mice (LysM‐<jats:styled-content style="fixed-case">GCH</jats:styled-content>1‐<jats:styled-content style="fixed-case">HA</jats:styled-content>). Unexpectedly, knockout or overexpression in myeloid cells had no effect on nociceptive behaviour, but LysM‐driven <jats:styled-content style="fixed-case">GCH</jats:styled-content>1 knockout reduced, and its overexpression increased the scratching response in Compound 48/80 and hydroxychloroquine‐evoked itch models, which involve histamine and non‐histamine dependent signalling pathways. Mechanistically, <jats:styled-content style="fixed-case">GCH</jats:styled-content>1 overexpression increased <jats:styled-content style="fixed-case">BH</jats:styled-content>4, nitric oxide and hydrogen peroxide, and these changes were associated with increased release of histamine and serotonin and degranulation of mast cells. LysM‐driven <jats:styled-content style="fixed-case">GCH</jats:styled-content>1 knockout had opposite effects, and pharmacologic inhibition of <jats:styled-content style="fixed-case">GCH</jats:styled-content>1 provided even stronger itch suppression. Inversely, intradermal <jats:styled-content style="fixed-case">BH</jats:styled-content>4 provoked scratching behaviour in vivo and <jats:styled-content style="fixed-case">BH</jats:styled-content>4 evoked an influx of calcium in sensory neurons. Together, these loss‐ and gain‐of‐function experiments suggest that itch in mice is contributed by <jats:styled-content style="fixed-case">BH</jats:styled-content>4 release plus <jats:styled-content style="fixed-case">BH</jats:styled-content>4‐driven mediator release from myeloid immune cells, which leads to activation of itch‐responsive sensory neurons.</jats:p>