Extracellular caspase-6 drives murine inflammatory pain via microglial TNF-α secretion
Temugin Berta, … , Yen-Chin Liu, Ru-Rong Ji
Temugin Berta, … , Yen-Chin Liu, Ru-Rong Ji
Published February 17, 2014
Citation Information: J Clin Invest. 2014;124(3):1173-1186. https://doi.org/10.1172/JCI72230.
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Research Article Neuroscience

Extracellular caspase-6 drives murine inflammatory pain via microglial TNF-α secretion

Abstract

Increasing evidence indicates that the pathogenesis of neuropathic pain is mediated through spinal cord microglia activation. The intracellular protease caspase-6 (CASP6) is known to regulate neuronal apoptosis and axonal degeneration; however, the contribution of microglia and CASP6 in modulating synaptic transmission and pain is unclear. Here, we found that CASP6 is expressed specifically in C-fiber axonal terminals in the superficial spinal cord dorsal horn. Animals exposed to intraplantar formalin or bradykinin injection exhibited CASP6 activation in the dorsal horn. Casp6-null mice had normal baseline pain, but impaired inflammatory pain responses. Furthermore, formalin-induced second-phase pain was suppressed by spinal injection of CASP6 inhibitor or CASP6-neutralizing antibody, as well as perisciatic nerve injection of CASP6 siRNA. Recombinant CASP6 (rCASP6) induced marked TNF-α release in microglial cultures, and most microglia within the spinal cord expressed Tnfa. Spinal injection of rCASP6 elicited TNF-α production and microglia-dependent pain hypersensitivity. Evaluation of excitatory postsynaptic currents (EPSCs) revealed that rCASP6 rapidly increased synaptic transmission in spinal cord slices via TNF-α release. Interestingly, the microglial inhibitor minocycline suppressed rCASP6 but not TNF-α–induced synaptic potentiation. Finally, rCASP6-activated microglial culture medium increased EPSCs in spinal cord slices via TNF-α. Together, these data suggest that CASP6 released from axonal terminals regulates microglial TNF-α secretion, synaptic plasticity, and inflammatory pain.

Authors

Temugin Berta, Chul-Kyu Park, Zhen-Zhong Xu, Ruo-Gang Xie, Tong Liu, Ning Lü, Yen-Chin Liu, Ru-Rong Ji

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Figure 7

Intrathecal injection of rCASP6 induces mechanical allodynia via microglial and TNF-α signaling.

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Intrathecal injection of rCASP6 induces mechanical allodynia via microgl...
(A) Intrathecal rCASP6 but not rCASP3 (5 U) elicits persistent mechanical allodynia. *P < 0.05 versus vehicle (PBS), n = 6–8 mice. (B) rCASP6-induced (i.t., 5 U) mechanical allodynia is abrogated in Tnfr double knockout (Tnfr1/2 DKO) mice. *P < 0.05, n = 7 mice. (C) rCASP6 (i.t., 5 U, 3 hours) increases TNF-α levels in spinal cord but not DRG tissues. *P < 0.05, n = 4 mice. (D) rCASP6-induced (i.t., 5 U) mechanical allodynia is reduced by minocycline pretreatment (i.t., 50 μg). *P < 0.05, n = 5 mice. (E) Spinal (i.t.) injection of rCASP6-stimulated microglia, but not control microglia, induces mechanical allodynia. *P < 0.05, n = 5–7 mice. (F) Transient reversal of mechanical allodynia following injection of rCASP6-stimulated microglia by i.t. TNF-α–neutralizing antibody (5 μg). *P < 0.05, n = 5–7 mice.