Free d-aspartate triggers NMDA receptor-dependent cell death in primary cortical neurons and perturbs JNK activation, Tau phosphorylation, and protein SUMOylation in the cerebral cortex of mice lacking d-aspartate oxidase activity

Abstract In mammals, free d -aspartate (D-Asp) is abundant in the embryonic brain, while levels remain very low during adulthood as a result of the postnatal expression and activity of the catabolizing enzyme d -aspartate oxidase (DDO). Previous studies have shown that long-lasting exposure to nonphysiological, higher D-Asp concentrations in Ddo knockout ( Ddo −/− ) mice elicits a precocious decay of synaptic plasticity and cognitive functions, along with a dramatic age-dependent expression of active caspase 3, associated with increased cell death in different brain regions, including hippocampus, prefrontal cortex, and substantia nigra pars compacta . Here, we investigate the yet unclear molecular and cellular events associated with the exposure of abnormally high D-Asp concentrations in cortical primary neurons and in the brain of Ddo −/− mice. For the first time, our in vitro findings document that D-Asp induces in a time-, dose-, and NMDA receptor-dependent manner alterations in JNK and Tau phosphorylation levels, associated with pronounced cell death in primary cortical neurons. Moreover, observations obtained in Ddo −/− animals confirmed that high in vivo levels of D-Asp altered cortical JNK signaling, Tau phosphorylation and enhanced protein SUMOylation, indicating a robust indirect role of DDO activity in regulating these biochemical NMDA receptor-related processes. Finally, no gross modifications in D-Asp concentrations and DDO mRNA expression were detected in the cortex of patients with Alzheimer's disease when compared to age-matched healthy controls.