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 1

Localization of CASP6 in central terminals of primary afferents in the superficial dorsal horn of the spinal cord.

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Localization of CASP6 in central terminals of primary afferents in the s...
(A) CASP6 immunoreactivity in the spinal cord dorsal horn. Scale bar: 100 μm. (B) Confocal images of double immunofluorescence showing colocalization of CASP6 and CGRP in axonal terminals of the superficial dorsal horn. Scale bar: 20 μm. (C) Ablation of the TRPV1-expressing primary afferents with RTX (10 mg/kg, i.p., daily for 3 days) abolishes CASP6 immunostaining but not IB4 staining in the superficial dorsal horn. Scale bar: 100 μm. (D) Absence of CASP6 immunostaining in the dorsal horn of Casp6–/– mice. Scale bar: 100 μm. (E) CASP6 immunostaining in DRGs of WT and Casp6–/– mice. Scale bar: 50 μm.