Extracellular caspase-6 drives murine inflammatory pain via microglial TNF-α secretion
Temugin Berta, Chul-Kyu Park, Zhen-Zhong Xu, Ruo-Gang Xie, Tong Liu, Ning Lü, Yen-Chin Liu, Ru-Rong Ji
Temugin Berta, Chul-Kyu Park, Zhen-Zhong Xu, Ruo-Gang Xie, Tong Liu, Ning Lü, Yen-Chin Liu, Ru-Rong Ji
View: Text | PDF
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

×

Figure 9

Endogenous CASP6 is essential for inducing spinal cord synaptic plasticity in spinal cord slices and LTP in spinal cord of anesthetized mice.

Options: View larger image (or click on image) Download as PowerPoint
Endogenous CASP6 is essential for inducing spinal cord synaptic plastici...
(A) Traces of sEPSCs in lamina IIo neurons of spinal cord slices of WT and Casp6–/– mice before and after CFA inflammation (1 day). (B) Frequency of sEPSCs in WT and Casp6–/– mice before and after CFA inflammation and the effects of the CASP6 inhibitor ZVEID. Note the impairment of CFA-induced sEPSC but not the basal sEPSC increases in Casp6–/– mice. Also note the inhibition of sEPSCs after CFA inflammation by ZVEID in WT but not Casp6–/– mice. *P < 0.05, n = 5–6 neurons. (C) Tetanic stimulation (100 Hz, 1 second, 4 trains, 10 second interval) induces LTP of C-fiber–evoked field potentials in the dorsal horn of anesthetized WT mice but not in Casp6–/– mice. *P < 0.05 (2-way ANOVA, n = 5 mice). (D) Reversal of LTP of C-fiber–evoked field potentials in the dorsal horn of anesthetized mice by the CASP6 inhibitor (10 μg, i.t.), administered 2 hours after LTP induction. *P < 0.05 (2-way ANOVA, n = 5 mice). (E) Traces of sEPSCs before and after capsaicin (CAP) treatment (0.5 μM) and the effects of ZVEID (10 μg/ml). (F) Frequency and amplitude of sEPSCs recorded in E. *P < 0.05, n = 5–6 neurons. (G) Traces of eEPSCs following dorsal root stimulation and the effects of the CASP6 inhibitor in WT and Casp6–/– mice. (H) Amplitude of eEPSCs. Note that the CASP6 inhibitor loses its effects in Casp6–/– mice. *P < 0.05, n = 5–15 neurons.