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Spinal injection of TNF-α-activated astrocytes produces persistent pain symptom mechanical allodynia by releasing monocyte chemoattractant protein-1.

Sensory Plasticity Laboratory, Department of Anesthesiology, Brigham and Women's Hospital and Harvard Medical School, Pain Research Center, Boston, Massachusetts 02115, USA.
Glia (Impact Factor: 6.03). 11/2010; 58(15):1871-80. DOI: 10.1002/glia.21056
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ABSTRACT Accumulating evidence suggests that spinal astrocytes play an important role in the genesis of persistent pain, by increasing the activity of spinal cord nociceptive neurons, i.e., central sensitization. However, direct evidence of whether activation of astrocytes is sufficient to induce chronic pain symptoms is lacking. We investigated whether and how spinal injection of activated astrocytes could produce mechanical allodynia, a cardinal feature of chronic pain, in naïve mice. Spinal (intrathecal) injection of astrocytes, which were prepared from cerebral cortexes of neonatal mice and briefly stimulated by tumor necrosis factor-alpha (TNF-α), induced a substantial decrease in paw withdrawal thresholds, indicating the development of mechanical allodynia. This allodynia was prevented when the astrocyte cultures were pretreated with a peptide inhibitor of c-Jun N-terminal kinase (JNK), D-JNKI-1. Of note a short exposure of astrocytes to TNF-α for 15 min dramatically increased the expression and release of the chemokine monocyte chemoattractant protein-1 (MCP-1), even 3 h after TNF-α withdrawal, in a JNK-dependent manner. In parallel, intrathecal administration of TNF-α induced MCP-1 expression in spinal cord astrocytes. In particular, mechanical allodynia induced by TNF-α-activated astrocytes was reversed by a MCP-1 neutralizing antibody. Finally, pretreatment of astrocytes with MCP-1 siRNA attenuated astrocytes-induced mechanical allodynia. Taken together, our results suggest that activated astrocytes are sufficient to produce persistent pain symptom in naïve mice by releasing MCP-1.

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Available from: Ru-Rong Ji, Aug 03, 2015
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    • "Our previous data have shown that intrathecal injection of TNF-a–activated astrocyte-induced mechanical allodynia was attenuated by both the CCL2 neutralizing antibody and CCL2 siRNA [11]. In vivo study also showed that intrathecal TNF-a markedly increased CCL2 expression in spinal cord astrocytes [11]. Intrathecal injection of CCL2 has been shown to induce chronic pain symptoms such as heat hyperalgesia [12] and mechanical allodynia [43]. "
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    ABSTRACT: The proinflammatory cytokines TNF- and IL-1 have been strongly implicated in the pathogenesis of neuropathic pain, but the intracellular signaling of these cytokines in glial cells are not fully understood. Tumor necrosis factor receptor associated factor 6 (TRAF6) plays a key role in signal transduction in the TNF receptor superfamily and the interleukin-1 receptor superfamily. In this study, we investigated the role of TRAF6 in neuropathic pain in mice following spinal nerve ligation (SNL). SNL induced persistent TRAF6 upregulation in the spinal cord. Interestingly, TRAF6 was mainly colocalized with the astrocytic marker GFAP on SNL day 10 and partially expressed in microglia on SNL day 3. In cultured astrocytes, TRAF6 was up-regulated after exposure to TNF-α or IL-1β. TNF-α or IL-1β also increased CCL2 expression, which was suppressed by both siRNA and shRNA targeting TRAF6. TRAF6 siRNA treatment also inhibited the phosphorylation of c-Jun N-terminal kinase (JNK) in astrocytes induced by TNF-α or IL-1β. JNK inhibitor D-NKI-1 dose-dependently decreased IL-1-induced CCL2 expression. Moreover, spinal injection of TRAF6 siRNA decreased intrathecal TNF-- or IL-1-induced allodynia and hyperalgesia. Spinal TRAF6 inhibition via TRAF6 siRNA, shRNA lentivirus, or antisense oligodeoxynucleotides partially reversed SNL-induced neuropathic pain and spinal CCL2 expression. Finally, intrathecal injection of TNF-α-activated astrocytes induced mechanical allodynia, which was attenuated by pretreatment of astrocytes with TRAF6 siRNA. Taken together, the results suggest that TRAF6, upregulated in spinal cord astrocytes in the late phase after nerve injury, maintains neuropathic pain by integrating TNF- and IL-1 signaling and activating the JNK/CCL2 pathway in astrocytes.
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    • "Numerous studies have indicated that astrocytes make major contributions to pain-related behavior following peripheral nerve injury and inflammation, and this extensive body of work is covered comprehensively by several recent reviews (Ellis and Bennett, 2013; Ji et al., 2013; Mika et al., 2013). Importantly, spinal injection of astrocytes that had been activated by TNFα was shown to be sufficient to produce mechanical hypersensitivity in uninjured animals (Gao et al., 2010). Far fewer studies have been made of astroglial contributions to neuropathic pain caused by SCI, but the findings thus far show interesting similarities to what has been described in peripheral neuropathic pain models. "
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    • "Our previous and present studies clearly demonstrated that there may be different pathways contributing to extra-territorial tactile allodynia/hyperalgesia via astrocytes or microglia. Although the priority of these different pathways remains to be established, intrathecal injection of astrocytes, which were prepared from cerebral cortexes of neonatal mice and briefly stimulated by TNF-α, induced mechanical allodynia in the paw (Gao et al., 2010). Therefore, there is a possibility that sTNF-α released from microglia following P2X 7 activation may activate astrocytes and induce the release of IL-1β in this MNT model. "
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