Blast Overpressure-Induced Neuroinflammation and Axonal Injury in the Spinal Cord of Ferrets

Abstract

Background: Blast-induced spinal cord injuries (bSCI) account for 75% of all combat-related spinal trauma and are associated with long-term functional impairments. However, limited studies have evaluated the neuropathological outcomes in the spinal cord following blast exposure. Objectives In this study, we aimed to determine the acute and sub-acute neuropathological changes in the spinal cord of ferrets after blast exposure. Methods: An advanced blast simulator was used to expose ferrets to tightly coupled repeated blasts. The Catwalk XT system was used to detect gait performances in ferrets at 24 h and 1 month post-blast exposure. After euthanasia, the cervical spinal cord samples were collected at 24 h or 1 month post-blast. A quantitative real-time polymerase chain reaction was performed to evaluate changes in the gene expression of multiple Toll-like Receptors (TLR), Cyclooxygenase (COX-1 and COX-2) enzymes and cytokines. Western blotting was performed to investigate markers of axonal injury (Phosphorylated-Tau, pTau; Phosphorylated Neurofilament Heavy Chain, pNFH; and Neurofilament Light Chain present in degenerating neurons, NFL-degen) and neuroinflammation (Glial Fibrillary Acidic Protein, GFAP; and Ionized Calcium Binding Adaptor Molecule, Iba-1). Results: Blast exposure significantly affected the gait performances in ferrets, especially at 24 h post-blast. Multiple TLRs, COX-2, Interleukin-1-beta (IL-1β), Interleukin-6 (IL-6), and Tumor Necrosis Factor-α (TNF-α) were significantly upregulated in the spinal cord at 24 h after blast exposure. Although only TLR3 was significantly upregulated at 1 month, non-significant increases in TLR1 and TLR2 were observed in the spinal cord at 1 month post-blast. Phosphorylation of Tau at serine (Ser396 and Ser404) and threonine (Thr205) increased in the spinal cord at 24 h and 1 month post-blast exposure. The increased expression of pNFH and NFL-degen proteins was evident at both time points. The expression of GFAP, but not Iba-1, significantly increased at 24 h and 1 month following blast exposure. Conclusions: Our results indicate that blast exposure causes acute and sub-acute neuroinflammation and associated axonal injury in the cervical spinal cord. These data further suggest that inhibition of TLRs and/or COX-2 enzyme might offer protection against blast-induced injuries to the spinal cord.