Susceptibility to Aβo and TBOA of LTD and Extrasynaptic NMDAR-Dependent Tonic Current in the Aged Rat Hippocampus

Bioactive human Alzheimer brain soluble Aβ: pathophysiology and therapeutic opportunities

The accumulation of amyloid-β protein (Aβ) plays an early role in the pathogenesis of Alzheimer's disease (AD). The precise mechanism of how Aβ accumulation leads to synaptic dysfunction and cognitive impairment remains unclear but is likely due to small soluble oligomers of Aβ (oAβ). Most studies have used chemical synthetic or cell-secreted Aβ oligomers to study their pathogenic mechanisms, but the Aβ derived from human AD brain tissue is less well characterized. Here we review updated knowledge on the extraction and characterization of bioactive human AD brain oAβ and the mechanisms by which they cause hippocampal synaptic dysfunction. Human AD brain-derived oAβ can impair hippocampal long-term potentiation (LTP) and enhance long-term depression (LTD). Many studies suggest that oAβ may directly disrupt neuronal NMDA receptors, AMPA receptors and metabotropic glutamate receptors (mGluRs). oAβ also impairs astrocytic synaptic functions, including glutamate uptake, D-serine release, and NMDA receptor function. We also discuss oAβ-induced neuronal hyperexcitation. These results may suggest a multi-target approach for the treatment of AD, including both oAβ neutralization and reversal of glutamate-mediated excitotoxicity.

Differential Effects of Human P301L Tau Expression in Young versus Aged Mice

The greatest risk factor for developing Alzheimer’s disease (AD) is increasing age. Understanding the changes that occur in aging that make an aged brain more susceptible to developing AD could result in novel therapeutic targets. In order to better understand these changes, the current study utilized mice harboring a regulatable mutant P301L human tau transgene (rTg(TauP301L)4510), in which P301L tau expression can be turned off or on by the addition or removal of doxycycline in the drinking water. This regulatable expression allowed for assessment of aging independent of prolonged mutant tau expression. Our results suggest that P301L expression in aged mice enhances memory deficits in the Morris water maze task. These behavioral changes may be due to enhanced late-stage tau pathology, as evidenced by immunoblotting and exacerbated hippocampal dysregulation of glutamate release and uptake measured by the microelectrode array technique. We additionally observed changes in proteins important for the regulation of glutamate and tau phosphorylation that may mediate these age-related changes. Thus, age and P301L tau interact to exacerbate tau-induced detrimental alterations in aged animals.

Long-term depression links amyloid-β to the pathological hyperphosphorylation of tau

In Alzheimer's disease, soluble oligomers of the amyloid-β peptide (Aβo) trigger a cascade of events that includes abnormal hyperphosphorylation of the protein tau, which is essential for pathogenesis. However, the mechanistic link between these two key pathological proteins remains unclear. Using hippocampal slices, we show here that an Aβo-mediated increase in glutamate release probability causes enhancement of synaptically evoked N-methyl-d-aspartate subtype glutamate receptor (NMDAR)-dependent long-term depression (LTD). We also find that elevated glutamate release probability is required for Aβo-induced pathological hyperphosphorylation of tau, which is likewise NMDAR dependent. Finally, we show that chronic, repeated chemical or optogenetic induction of NMDAR-dependent LTD alone is sufficient to cause tau hyperphosphorylation without Aβo. Together, these results support a possible causal chain in which Aβo increases glutamate release probability, thus leading to enhanced LTD induction, which in turn drives hyperphosphorylation of tau. Our data identify a mechanistic pathway linking the two critical pathogenic proteins of AD.