Upregulation of TLR9 may contribute to activation of microglia and painful diabetic neuropathy via the p38 MAPK pathway in rats.

Painful diabetic neuropathy is a common chronic complication of diabetes, and the underlying mechanism remains largely elusive. A rat model of painful diabetic neuropathy was established via streptozotocin (STZ) injection and assessed as increased heat and mechanical hypersensitivity. An upregulation of TLR9 was observed in the spinal cords of rats injected with STZ and rat microglia (primary microglia and immortalized microglia HAPI) treated with high glucose. To investigate the role of TLR9 in high glucose-induced microglia activation, short hairpin RNAs targeting TLR9 were used in vitro to knock down TLR9 in HAPI cells. TLR9 interference suppressed the high glucose-induced expression and secretion of inflammatory cytokines (TNF-α, IL-1β, and IL-6), IBA-1 expression and the chemotaxis of HAPI microglia. Similar results were obtained when HAPI microglia were incubated with a p38 inhibitor (SB203580). P38 and ERK were downstream of TLR9 because TLR9 ablation markedly inhibited the phosphorylation of p38 and ERK. TLR9 was also knocked down in vivo via the injection of shTLR9 lentiviral vector into the rat spinal cord. Relief of STZ-induced heat and mechanical hypersensitivity was observed in rats with TLR9 interference, and TLR9 knockdown prevented STZ-induced inflammatory cytokine secretion and microglial and MAPK signaling activation. Our study revealed the participation of TLR9 in microglial activation and diabetes-induced hyperalgesia likely via the MAPK pathway. The targeting of TLR9 may be an effective strategy for the treatment of painful diabetic neuropathy.