In response to spinal surgery, neurons secrete a large amount of
substance P into the epidural area.
Substance P is involved in macrophage differentiation and fibrotic disease. However, the specific roles and mechanisms of
substance P in epidural
fibrosis remain unclear. In this study, we established a mouse model of L1-L3
laminectomy and found that dorsal root ganglion neurons and the macrophages infiltrating into the
wound area released
sphingolipids. In vitro experiments revealed that type 1 macrophages secreted
substance P, which promoted differentiation of type 1 macrophages towards a type 2 phenotype. High-throughput
mRNA-seq analysis revealed that the
sphingolipid metabolic pathway may be involved in the regulation of type 2 macrophages by
substance P. Specifically,
sphingomyelin synthase 2, a component of the
sphingolipid metabolic pathway, promoted M2 differentiation in
substance P-treated macrophages, while treating the macrophages with
LY93, a
sphingomyelin synthase 2 inhibitor, suppressed M2 differentiation. In addition,
substance P promoted the formation of neutrophil extracellular traps, which further boosted M2 differentiation. Blocking
substance P with the neurokinin receptor 1 inhibitor RP67580 decreased the number of M2 macrophages in the
wound area after spinal surgery and alleviated epidural
fibrosis, as evidenced by decreased
fibronectin, α-smooth muscle actin, and
collagen I in the
scar tissue. These results demonstrated that
substance P promotes M2 macrophage differentiation in epidural
fibrosis via
sphingomyelin synthase 2 and neutrophil extracellular traps. These findings provide a novel strategy for the treatment of epidural
fibrosis.