Up-regulation of thrombospondin-4 (TSP4) or voltage-gated calcium channel subunit alpha-2-delta-1 (Caαδ) proteins in the spinal cord contributes to neuropathic pain development through an unidentified mechanism. We have previously shown that TSP4 interacts with Caαδ to promote excitatory synaptogenesis and the development of chronic pain states. However， the TSP4 determinants responsible for these changes are not known. Here， we tested the hypothesis that the Caαδ-binding domains of TSP4 are synaptogenic and pro-nociceptive. We mapped the major Caαδ-binding domains of TSP4 within the coiled-coil and epidermal growth factor (EGF)-like domains in vitro. Intrathecal injection of TSP4 fragment proteins ... More
Up-regulation of thrombospondin-4 (TSP4) or voltage-gated calcium channel subunit alpha-2-delta-1 (Caαδ) proteins in the spinal cord contributes to neuropathic pain development through an unidentified mechanism. We have previously shown that TSP4 interacts with Caαδ to promote excitatory synaptogenesis and the development of chronic pain states. However， the TSP4 determinants responsible for these changes are not known. Here， we tested the hypothesis that the Caαδ-binding domains of TSP4 are synaptogenic and pro-nociceptive. We mapped the major Caαδ-binding domains of TSP4 within the coiled-coil and epidermal growth factor (EGF)-like domains in vitro. Intrathecal injection of TSP4 fragment proteins containing the EGF-like domain (EGF-LIKE) into naïve rodents was sufficient for inducing behavioral hypersensitivity similar to that produced by an equal molar dose of full-length TSP4. Gabapentin， a drug that binds to Caαδ， blocked EGF-LIKE-induced behavioral hypersensitivity in a dose-dependent manner， supporting the notion that EGF-LIKE interacts with Caαδ and thereby mediates behavioral hypersensitivity. This notion was further supported by our findings that a peptide within EGF-LIKE (EGFD355-369) could block TSP4- or Cavα2δ1-induced behavioral hypersensitivity after intrathecal injections. Furthermore， only TSP4 proteins that contained EGF-LIKE could promote excitatory synaptogenesis between sensory and spinal cord neurons， which could be blocked by peptide EGFD355-369. Together， these findings indicate that EGF-LIKE is the molecular determinant that mediates aberrant excitatory synaptogenesis and chronic pain development. Blocking interactions between EGF-LIKE and Caαδ could be an alternative approach in designing target-specific pain medications.
