Mitogen- and stress-activated kinase 1 in primary sensory neurons contributes to formalin-induced tonic pain.

The nuclear mitogen- and stress-activated kinases (MSKs) play a critical role in the development and persistence of pain after tissue injury. Here, we ascertained the MSK isoform, the cells and mechanisms, which mediate MSKs' pronociceptive function. Nocifensive behaviour evoked by subcutaneous formalin injection into the paw was quantified in wild type (WT), MSK1 and MSK2 global knock out (MSK1-/- and MSK2-/-) mice, and a month after injecting adeno-associated viral vector carrying short-hairpin (sh) RNA directed towards the MSK1-encoding gene Rps6ka5 mRNA or scrambled shRNA into the sciatic nerve of WT mice. Rps6ka5 expression in nociceptors was ascertained by analysing publicly available single cell and single nucleus RNA sequencing datasets on primary sensory neurons and reverse transcription polymerase chain reaction on dorsal root ganglia (DRG). Mitogen- and stress-activated kinase 1 expression was verified by immunofluorescent staining on DRG sections. MSK1-/- but not MSK2-/- mice exhibited significantly attenuated evoked nocifensive behaviour specifically in the second but not the first phase of the formalin test. Downregulating Rps6ka5 in nociceptors by the viral vector tool attenuated formalin-induced pain behaviour to the same extent as observed in MSK1-/- animals. Rps6ka5 expression was found in DRG and various transcriptionally defined groups of nociceptive primary sensory neurons (nociceptors). Immunofluorescence confirmed the presence of MSK1 predominantly in peptidergic nociceptors. MSK1 constitutes the principal MSK isoform, which is critically important for regulating cellular components that enable the transient activation of a specific subpopulation of nociceptors by formalin.