A novel cell-cell signaling by microglial transmembrane TNFα with implications for neuropathic pain

Neuropathic pain is accompanied by neuroimmune activation in dorsal horn of spinal cord. We have observed that in animal models this activation is characterized by an increased expression of transmembrane tumor necrosis factor α (mTNFα) without the release of soluble tumor necrosis factor α (sTNFα). Herein we report that the pain-related neurotransmitter peptide substance P (SP) increases the expression of mTNFα without the release of sTNFα from primary microglial cells. We modeled this interaction using an immortalized microglial cell line; exposure of these cells to SP also resulted in the increased expression of mTNFα but without any increase in the expression of the TNF-cleaving enzyme (TACE) and no release of sTNFα. In order to evaluate the biological function of uncleaved mTNFα, we transfected COS-7 cells with a mutant full-length TNFα construct resistant to cleavage by TACE. Coculture of COS-7 cells expressing the mutant TNFα with microglial cells led to microglial cell activation indicated by increased OX42 immunoreactivity and release of macrophage chemoattractant peptide 1 (CCL2) by direct cell-cell contact. These results suggest a novel pathway through which the release of SP by primary afferents activates microglial expression of mTNFα, establishing a feed-forward loop that may contribute to the establishment of chronic pain.