Polyphosphate coacervation prevents Aβ-induced NMDA receptor sensitization and neuronal death

In Alzheimer's disease (AD), amyloid-β (Aβ) accumulation leads to synaptic dysfunction and N-methyl-d-aspartate (NMDA) receptor-dependent calcium dysregulation, contributing to neuronal death. Current AD therapies often target Aβ clearance but struggle to fully mitigate downstream neurotoxicity. Inorganic polyphosphate (polyP), a naturally occurring polymer released by activated platelets, has emerged as a potential neuroprotective agent, but its precise mechanism against Aβ-induced excitotoxicity and its therapeutic applicability remain underexplored. This study investigates polyP's protective mechanism against Aβ-induced NMDA receptor sensitization.

Researchers exposed primary rat neuronal cultures and PC12 pheochromocytoma cells to the neurotoxic Aβ fragment Aβ(25-35) at 10 µM, following a 2-5 day pre-incubation to induce its toxic conformation. Co-incubation with sodium polyphosphate (Na-polyP) at 50 µg/mL was used to assess its protective effects. Key endpoints included neuronal apoptosis, NMDA receptor-mediated calcium influx, and the mechanistic basis of polyP action, including experiments with calcium-chelating polyP coacervates formed with serotonin. Additionally, the release kinetics of three polyP-based brain-targeted formulations (nanogels, nanoparticles, micelles) were characterized.

Pre-incubated Aβ(25-35) at 10 µM induced significant apoptotic neuronal death within 3 days. However, coincubation with Na-polyP at 50 µg/mL completely abolished this Aβ-induced neurotoxicity. Mechanistically, Na-polyP significantly attenuated glutamate-evoked NMDA receptor-dependent calcium influx, indicating a direct impact on excitotoxicity.

Mechanistic analyses demonstrated that Na-polyP forms calcium-chelating coacervates, a process notably promoted by serotonin at physiological Ca²⁺ concentrations. This coacervate formation is identified as a key mechanism underlying protection against Aβ-induced NMDA receptor sensitization and subsequent neuronal death. Furthermore, characterization of polyP nanogels, nanoparticles, and micelle-based formulations revealed controlled release profiles, suggesting their potential for targeted brain delivery. These findings highlight a novel pathway for neuroprotection against Aβ toxicity.