ABSTRACT: To shed light on the early processes of immune response to peripheral nerve injury, we first used genome-wide transcriptional profiling and bioinformatics (Ingenuity, NextBio) pathway analyses of the proximal (P; regenerating) and distal (D; degenerating) nerve stumps at day 1 in the sciatic nerve axotomy model in rats. We identified a number of specific immunomodulatory genes and pathways that were regulated shortly post-injury in both the P and D segments, including all members of the interleukin (IL), chemokine, tumor necrosis factor (TNF), matrix metalloproteinase (MMP), toll-like receptor (TLR), tissue inhibitor of metalloproteinase (TIMP), ion channel and myosin families. Immunomodulatory calcium-binding S100A8 and S100A9 were the top up-regulated genes in both the D and P segments. In cultured Schwann cells stimulated with the purified S100A8/A9 heterodimer we recorded a high level of similarity of the activated genes and pathways with that of the injured nerve, especially in the activation of the chemokine and cytokine gene networks that support agranulocyte and granulocyte chemotaxis, adhesion, transmigration and rolling signaling pathways. We also confirmed activation of multiple cell death related gene networks supporting TNFR1, natural killer cell receptor and death receptor apoptosis signaling in the D stump, and the gluconeogenesis/glycolysis and cytoskeletal motility signaling in the P stump, corroborated by activation of ERK, PI3K and JNK kinase pathways. As compared to the D segment, multiple additional pathways were more efficiently upregulated in the P stump, including the IL-6 and -17, MMP-9, calcium, activated agranulocyte, leukocyte rolling and glutathione-mediated detoxification signaling pathways. These data suggest that shortly after nerve injury, upregulation of S100A8/A9 is responsible for the expression and release of chemokines and cytokines by Schwann cells, necessary to generate the initial chemotactic gradient and guide the hematogenous immune cells into the injury site. Gene expression profiling of total RNAs extracted from injured and non-injured rat sciatic nerves, and primary rat Schwann cells stimulated with S100A8/A9 proteins
