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 4

CASP6 contributes to the development of inflammatory pain induced by intraplantar bradykinin, carrageenan, and CFA.

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CASP6 contributes to the development of inflammatory pain induced by int...
(A) Bradykinin-induced spontaneous pain in the second phase (5–30 minutes), but not the first phase (0–5 minutes), is reduced in Casp6–/– mice. *P < 0.05, n = 7–8 mice. (B and C) Carrageenan-induced mechanical allodynia (B) and heat hyperalgesia (C) are reduced in Casp6–/– mice. *P < 0.05, compared with WT mice, n = 5 mice. (D) Carrageenan-induced paw edema (measured as paw volume) is unaltered in Casp6–/– mice; n = 5 mice. (E and F) CFA-induced mechanical allodynia (E) but not heat hyperalgesia (F) is partially and transiently reduced in Casp6–/– mice. *P < 0.05, compared with WT mice, n = 5 mice. (G and H) Reversal of CFA-induced mechanical allodynia and heat hyperalgesia by CASP6-neutralizing antibody (1 μg, i.t.) in WT mice. *P < 0.05, n = 5 mice. Note that the CASP6 antibody does not affect inflammatory pain in Casp6–/– mice.