The DLK-dependent axonal injury response promotes microgliosis
Jacob Gluski1, Eileen Nguyen2, Josette Wlaschin1, Hanna Silberberg1, James Thompson2, Alexander Chesler2*, Claire Le Pichon1*
1NICHD 2NCCIH; National Institutes of Health, Bethesda MD 20892 USA
The response of sensory neurons of dorsal root ganglia (DRG) to sciatic nerve damage is known to promote neuropathic pain. One striking result of this injury is a microglial response in the dorsal horn of the spinal cord in the region innervated by the central terminals of the injured DRG neurons. Importantly, microglia have been implicated as major contributors to the development of neuropathic pain by altering spinal cord dorsal horn circuit function3. However, the molecular program linking peripheral neuron injury to central microgliosis remains poorly defined.
In this study, we investigate the role of Dual Leucine Zipper Kinase (DLK; Map3k12) in the pain behavior and microgliosis resulting from sciatic nerve injury in wild type and DLK conditional knockout mice (DLKcKO). Microglia in the whole spinal cord were visualized using microglial immunolabeling and tissue clearing (iDisco4). Strikingly, we observe that following nerve injury, DLKcKO mice exhibit no microgliosis, suggesting DLK is required for the microglial response, and also that the pain hypersensitivity is much reduced.
A previous study3 reported that the cytokine Colony stimulating factor 1, Csf1, deriving from DRG neurons, was responsible for mediating the microgliosis, since it was prevented in mice in which Csf1 was deleted from sensory neurons. We thus examined whether Csf1 expression in the DRG might be regulated by DLK, whose activation results in a transcriptional response, and found that Csf1 mRNA failed to be induced in DLKcKO mice as well as in mice dosed with a DLK inhibitor. Our data thus provide an explanation for how nerve injury upregulates Csf1, placing DLK upstream of neuronal Csf1 expression and providing a direct link between nerve injury and the recruitment of microglia to the spinal cord.
To delve further into the dynamics of this neuron-to-microglia relationship, we examine the time course of Csf1 expression in DRG neurons and of spinal cord microgliosis induction following nerve injury. We also test the effect of pharmacologically inhibiting DLK during different time windows following nerve injury.
3Guan Z et al., Nat Neurosci 2016, PMID 26642091
4Renier N et al., Cell 2014, PMID 25417164
Funding: NIH Intramural