J. Neurochem. (2010) 115, 1520–1529.
Abstract
Soluble amyloid‐β peptide (Aβ) oligomers, known to accumulate in Alzheimer’s disease brains, target excitatory post‐synaptic terminals. This is thought to trigger synapse deterioration, a mechanism possibly underlying memory loss in early stage Alzheimer’s disease. A major unknown is the identity of the receptor(s) targeted by oligomers at synapses. Because oligomers have been shown to interfere with N‐methyl‐d‐aspartate receptor (NMDAR) function and trafficking, we hypothesized that NMDARs might be required for oligomer binding to synapses. An amplicon vector was used to knock‐down NMDARs in mature hippocampal neurons in culture, yielding 90% reduction in dendritic NMDAR expression and blocking neuronal oxidative stress induced by Aβ oligomers, a pathological response that has been shown to be mediated by NMDARs. Remarkably, NMDAR knock‐down abolished oligomer binding to dendrites, indicating that NMDARs are required for synaptic targeting of oligomers. Nevertheless, oligomers do not appear to bind directly to NMDARs as indicated by the fact that both oligomer‐attacked and non‐attacked neurons exhibit similar surface levels of NMDARs. Furthermore, pre‐treatment of neurons with insulin down‐regulates oligomer‐binding sites in the absence of a parallel reduction in surface levels of NMDARs. Establishing that NMDARs are key components of the synaptic oligomer binding complex may illuminate the development of novel approaches to prevent synapse failure triggered by Aβ oligomers.
Highlights-An inhibitor of kynurenine-3-monoxygenase was administered to pregnant female rats-Adult offspring at postnatal day 60 (P60) showed alterations in synaptic plasticity-Expression of GluN2A, sonic hedgehog and other developmental proteins were changed-Most earlier protein changes had disappeared with no changes in RNA or learning tasks-Prenatal kynurenine pathway inhibition produces persistent synaptic and protein changes 3 Abstract During early brain development, NMDA receptors are involved in cell migration, neuritogenesis, axon guidance and synapse formation, but the mechanisms which regulate NMDA receptor density and function remain unclear. The kynurenine pathway of tryptophan metabolism includes an agonist (quinolinic acid) and an antagonist (kynurenic acid) at NMDA receptors and we have previously shown that inhibition of the pathway using the kynurenine-3-monoxygenase inhibitor Ro61-8048 in late gestation produces rapid changes in protein expression in the embryos and effects on synaptic transmission lasting until postnatal day 21 (P21). The present study sought to determine whether any of these effects are maintained into adulthood. After prenatal injections of Ro61-8048 the litter was allowed to develop to P60 when some offspring were euthanised and the brains removed for examination. Analysis of protein expression by western blotting revealed significantly reduced expression of the GluN2A subunit (32%) and the morphogenetic protein sonic hedgehog (31%), with a 29% increase in expression of doublecortin, a protein associated with neurogenesis. No changes were seen in mRNA abundance using qRT-PCR. Neuronal excitability was normal in the CA1 region of hippocampal slices but paired-pulse stimulation revealed less inhibition at short interpulse intervals. The amount of long-term potentiation was decreased by 49% in treated pups and recovery after low frequency stimulation was delayed. The results not only strengthen the view that basal, constitutive kynurenine metabolism is involved in normal brain development, but also show that changes induced prenatally can affect the brains of adult offspring and those changes are quite different from those seen previously at weaning (P21). Those changes may be mediated by altered expression of NMDAR subunits and sonic hedgehog.
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