BACKGROUND: Trafficking of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) to excitatory synapses is critical to their synaptic functions. Previously, we have shown induction of neuronal pentraxin 1 (NP1) and its colocalization with AMPAR subunit GluR1 in hypoxic-ischemic (HI) brain injury. However, the role of NP1 in mediating GluR1 surface expression, trafficking, and clustering at synapses in HI neuronal death is unclear. METHODS AND RESULTS: Primary hippocampal neurons, isolated from wild-type (WT) and NP1-knockout (C57BL/6 background) mice at DIV 12 to 14 were exposed to 2 to 8 hours of oxygen glucose deprivation (OGD)-in vitro conditions that mimic human stroke. OGD exposure resulted in time-dependent induction of NP1 (∼4-fold), enhanced redistribution of AMAP GluR1 receptors at excitatory synapses, and increased neuronal death. We observed a significant increase in surface and synaptic GluR1 clusters that colocalized with PSD-95 on dendrites with a simultaneous decrease in internalized GluR1. Surface cross-linking with BS(3) showed enhanced membrane insertions of GluR1, and increased phosphorylation at Ser-845 further supported enhanced surface availability of GluR1 after OGD. NP1 protein colocalized with GluR1 and PSD-95, and OGD significantly increased their synaptic coclustering. Most strikingly, the genetic deletion of NP1 resulted in decreases in surface GluR1 cluster density, synaptic localization, phospho-GluR1 (Ser-845) levels, and neuronal death after OGD compared with WT neurons. AMPA (50 µmol/L) induced NP1 and significant cell death in WT but not in NP1-/- neurons. CONCLUSIONS: Our results indicate that NP1 plays a key role in synaptic clustering of GluR1, suggesting that targeting NP1 might be a practical approach to preventing ischemic brain damage.