Suppression of miR-155 attenuates neuropathic pain by inducing an M1 to M2 switch in microglia

Introduction
The polarization state of microglia affects the progress of neuropathic pain. MiR-155 modulates polarization of microglia, while its role in neuropathic pain has not been well studied.

Material and methods
We separately used lipopolysaccharide (LPS) and interleukin 4 (IL-4) for constructing an M1/M2 polarization model in BV-2 cells. The levels of CD86, iNOS, CD206, Arg and miR-155 were measured by western blot or qRT-PCR, as needed. Subsequently, BV-2 cells were transfected with miR-155 mimics or inhibitor to explore the effects of miR-155 on polarization states. We also constructed a neuropathic pain model by applying spinal nerve ligation (SNL) in Wistar rats with miR-155 agomir or antagomir injection. The withdrawal threshold was measured by Von Frey fibre needle. The levels of interleukin 1b (IL-1b), tumour necrosis factor a (TNF-a) and the proportion of M1-polarized microglia in primary microglia from rats were measured by ELISA and flow cytometry.

Results
LPS induced M1 polarization in BV-2 cells with increasing of CD86, iNOS and miR-155, while IL-4 induced M2 polarization in BV-2 cells with increasing of CD206, Arg and decreasing of miR-155. MiR-155 mimics upregulated CD86 and downregulated CD206, whereas miR-155 inhibitor downregulated CD86 and upregulated CD206. MiR-155 antagomir increased the withdrawal threshold, decreased the production of IL-1b, TNF-a and the proportion of M1-polarized microglia in primary microglia.

Conclusions
Results demonstrate that suppression of miR-155 attenuates neuropathic pain by inducing an M1 to M2 switch in microglia. Our findings provide a new perspective to understand the function of miR-155 in microglia.