Ephrin-B2 promotes nociceptive plasticity and hyperalgesic priming through EphB2-MNK-eIF4E signaling in both mice and humans
Ephrin-B-EphB signaling promotes pain through signaling between dorsal root ganglion (DRG) neurons and spinal cord neurons in the dorsal horn, and through signaling between peripheral cells and EphB receptors expressed by DRG neurons. Previous findings link ephrin-B expression in painful peripheral tissues in patients to chronic pain, suggesting the clinical significance of this signaling, but the direct effects of ephrins on DRG neurons have not been widely studied. We hypothesized that ephrin-B2 would promote nociceptor plasticity and hyperalgesic priming through MNK-eIF4E signaling, a critical mechanism for nociceptive plasticity induced by growth factors, cytokines and nerve injury. Our work demonstrates that ephrin-B2-EphB2 signaling drives activation of MNK-eIF4E in DRG neurons to cause an enhanced response to inflammatory mediator signaling in both mice and humans and hyperalgesic priming in two models in mice. Both male and female mice developed dose-dependent mechanical hypersensitivity in response to ephrin-B2, and both sexes showed hyperalgesic priming when challenged with PGE2 injection into the same hindpaw. Acute nociceptive behaviors and hyperalgesic priming were blocked in mice lacking MNK1 (Mknk1 knockout mice) and by the MNK inhibitor eFT508. Similar effects on hyperalgesic priming were seen in a dural injection model. We generated a sensory neuron specific knockout of EphB2 using Pirt-Cre mice and found that these mice lacked responses to ephrin-B2 injection. We used Ca2+-imaging to determine direct effects of ephrin-B2 on DRG neurons and found that ephrin-B2 treatment enhanced Ca2+ transients in response to PGE2 which were absent in DRG neurons from MNK1-/- and EphB2-PirtCre mice. In experiments on human DRG neurons we found that ephrin-B2 increased eIF4E phosphorylation and enhanced Ca2+ responses to PGE2 treatment, both of which were blocked by eFT508 treatment. We conclude that ephrin-B2 acts directly on mouse and human sensory neurons to induce nociceptor plasticity via MNK-eIF4E signaling. The findings offer insight into how ephrin-B signaling promotes pain, and suggests treatment avenues for prevention or reversal of chronic pain associated with EphB activation in sensory neurons.