Hippocampal endoplasmic reticulum stress drives a pro-inflammatory microglial bias and neuroinflammation in aged mice with perioperative neurocognitive disorder

BACKGROUND: Perioperative neurocognitive disorder (PND) is a frequent postoperative complication in older individuals and is commonly linked to microglial activation. Endoplasmic reticulum (ER) stress has been implicated in neuroinflammation and postoperative cognitive decline; however, whether hippocampal ER stress acts upstream to bias microglia toward a pro-inflammatory M1 phenotype, rather than serving only as a trigger of cytokine cascades, remains insufficiently defined in PND. To determine whether hippocampal ER stress promotes an M1-like microglial shift and amplifies neuroinflammation after anesthesia and surgery. METHODS: Eighteen-month-old male C57BL/6 mice underwent aseptic tibial fracture surgery under isoflurane anesthesia. Tauroursodeoxycholic acid（TUDCA）, a pharmacological ER stress suppressor, was delivered bilaterally into dorsal hippocampal CA1 as a perioperative mechanistic probe. ER ultrastructure in hippocampal cells was examined using transmission electron microscopy (TEM). ER stress related proteins (GRP78, p-PERK, p-eIF2ɑ, p-IRE1ɑ, ATF4, and ATF6) in hippocampus were quantified by Western blotting. Microglial phenotypic signatures were assessed by Western blotting (CD86, iNOS, CD206, ARG-1) and double immunofluorescence (CD86/Iba-1 and CD206/Iba-1). Open-field testing was performed to control for locomotion/anxiety, followed by memory assessments using novel object recognition and Morris water maze starting on postoperative day 3. RESULTS: Anesthesia and surgery induced postoperative cognitive deficits, accompanied by heightened hippocampal inflammatory signaling and a shift toward a pro-inflammatory microglial signature characterized by increased CD86 and iNOS with concomitant reductions in CD206 and ARG-1. In parallel, hippocampal ER stress activation was evident, including ER ultrastructural disruption and increased GRP78 with engagement of PERK/eIF2ɑ/ATF4-, IRE1ɑ-, and ATF6-related pathways. hippocampus-targeted delivery of TUDCA attenuated ER stress activation, mitigated the pro-inflammatory microglial shift (decreasing CD86/iNOS and restoring CD206/ARG-1), reduced inflammatory readouts, and improved postoperative memory performance. CONCLUSIONS: These findings support a mechanistic link between hippocampal ER stress and the pro-inflammatory microglia bias linked to postoperative neuroinflammation and cognitive impairment in aged mice.