Mast Cell-Microglia Crosstalk Drives Aberrant Synaptic Pruning Through PAR-2/MAPK/NF-κB Signaling in Neonatal Hypoxic-Ischemic Encephalopathy.

AIMS: Neonatal hypoxic-ischemic encephalopathy (HIE) causes severe neurodevelopmental impairment, but the upstream immune mechanisms that initiate this process remain unclear. We investigated whether mast cell (MC)-derived tryptase contributes to aberrant microglia-mediated synaptic pruning through PAR-2/MAPK/NF-κB signaling after neonatal hypoxic-ischemic (HI) injury.

METHODS

A postnatal Day-7 rat HI model was established using the Rice-Vannucci method. MC abundance, c-Kit and tryptase expression, PAR-2/MAPK/NF-κB signaling, and synaptic integrity were assessed using histological, immunofluorescence, biochemical, ultrastructural, and three-dimensional (3D) reconstruction analyses. Microglia-targeted F2rl1 silencing and pharmacological inhibition with FSLLRY-NH2 or APC366 were used to examine PAR-2 and tryptase-related signaling. Complementary oxygen-glucose deprivation/reperfusion (OGD/R) experiments were performed in BV-2 microglia. Cognitive outcomes were assessed using the Morris water maze and Y maze.

RESULTS

HI increased hippocampal MC abundance and tryptase expression, and these changes were associated with acute neurological deficits. HI insult also increased complement associated synaptic labeling, PAR-2/MAPK/NF-κB signaling, CD68 expression, and engulfment of PSD95 positive synaptic material. Microglia-targeted F2rl1 silencing attenuated pathway activation and synaptic engulfment. FSLLRY-NH2 and APC366 similarly reduced HI associated signaling and microglial synaptic engulfment, while APC366 preserved dendritic spine density and synaptic ultrastructure and improved long-term spatial learning and memory. In BV-2 microglial cells, exogenous tryptase enhanced OGD/R associated CD68 expression and MAPK/NF-κB phosphorylation, which were attenuated by F-NH2.

CONCLUSION

MC-derived tryptase is an upstream contributor to pathological microglial synaptic pruning and cognitive impairment after neonatal HI, potentially involving the PAR-2/MAPK/NF-κB axis.