Editors' ChoiceNeuroscience

Extracellular miRNAs Mediate Pain

Sci. Signal.  08 Apr 2014:
Vol. 7, Issue 320, pp. ec90
DOI: 10.1126/scisignal.2005342

MicroRNAs (miRNAs) are short RNA sequences that regulate gene expression by binding to intracellular target transcripts. However, miRNAs can be detected in the circulation and in cerebrospinal fluid and are recognized by Toll-like receptor 7 (TLR7), and extracellular application of the miRNA let-7b triggers TLR7-mediated apoptosis of cortical neurons. Because TLR7 is also present on dorsal root ganglion (DRG) neurons associated with pain, Park et al. explored whether DRG neurons responded to let-7b. Electrophysiological analysis of dissociated mouse DRG neurons from wild-type mice, but not those from TLR7-knockout mice, showed that small-diameter neurons, which are typically nociceptive, produced an inward current in response to let-7b, a response that was abolished by mutation of the GUUGUGU motif in let-7b. Pharmacological inhibition of the calcium channel transient receptor potential A1 (TRPA1) blocked the response to let-7b, and DRG neurons from TRPA1-knockout mice also failed to respond. Pharmacological or genetic inhibition of various intracellular signaling components did not prevent let-7b–mediated inward currents, suggesting that TRPA1 and TLR7 may interact directly to produce the response. Indeed, in human embryonic kidney 293 (HEK293) cells coexpressing TRPA1 and TLR7, let-7b bound to the cells expressing TLR7; let-7b, TRPA1, and TLR7 colocalized; and TRPA1 and TLR7 coimmunoprecipitated, an interaction enhanced by exposure of the cells to let-7b. TLR7-deficient neurons exhibited reduced TRPA1 single-channel activity and reduced abundance of TRPA1 at the cell surface, suggesting that TLR7 may affect both the function of the channel and its delivery to the cell surface. Formalin stimulates TRPA1-mediated currents in nociceptive DRG neurons, a response that was attenuated by a let-7b inhibitor. Cultured DRG neurons exposed to formalin or capsaicin (two pain-producing stimuli) or depolarized with KCl released let-7b into the medium. Injection of let-7b into the footpad of mice produced behaviors associated with pain, which were absent in TLR7-knockout mice, reduced in TRPA1-knockout mice, and dose-dependently reduced by pretreatment with a TRPA1 antagonist. Injection of a high concentration of let-7b produced allodynia (pain in response to non-noxious stimuli) that lasted for several days. Pretreatment with a let-7b inhibitor prevented formalin-induced pain. Thus, release of let-7b appears to mediate inflammatory pain by binding TLR7 and activating TRPA1.

C.-K. Park, Z.-Z. Xu, T. Berta, Q. Han, G.-C. Chen, X.-J. Liu, R.-R. Ji, Extracellular microRNAs activate nociceptor neurons to elicit pain via TLR7 and TRPA1. Neuron 82, 47–54 (2014). [Abstract]