The Expression of Epac2 and GluA3 in an Alzheimer's Disease Experimental Model and Postmortem Patient Samples

From postsynaptic neurons to astrocytes: the link between glutamate metabolism, Alzheimer's disease and Parkinson's disease

Glutamate is not only the main excitatory neurotransmitter of the human central nervous system, but also a potent neurotoxin. Therefore, maintaining low-dose, non-toxic extracellular glutamate concentrations between synapses to ensure the reliability of synaptic transmission is essential for maintaining normal physiological functions of neurons. More and more studies have confirmed that the specific pathogenesis of central nervous system diseases (such as Alzheimer's disease) caused by neuronal damage or death due to abnormal inter-synaptic glutamate concentration may be related to the abnormal function of excitatory amino acid transporter proteins and glutamine synthetase on astrocytes, and that the abnormal expression and function of the above two proteins may be related to the transcription, translation, and even modification of both by the process of transcription, translation, and even modification of astrocytes. oxidative stress, and inflammatory responses occurring in astrocytes during their transcription, translation and even modification. Therefore, in this review, we mainly discuss the relationship between glutamate metabolism (from postsynaptic neurons to astrocytes), Alzheimer's disease and Parkinson's disease in recent years.

Amyloid-β-Driven Synaptic Deficits Are Mediated by Synaptic Removal of GluA3-Containing AMPA Receptors

The detrimental effects of oligomeric amyloid-β (Aβ) on synapses are considered the leading cause for cognitive deficits in Alzheimer's disease. However, through which mechanism Aβ oligomers impair synaptic structure and function remains unknown. Here, we used electrophysiology and amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) imaging on mouse and rat neurons to demonstrate that GluA3 expression in neurons lacking GluA3 is sufficient to resensitize their synapses to the damaging effects of Aβ, indicating that GluA3-containing AMPARs at synapses are necessary and sufficient for Aβ to induce synaptic deficits. We found that Aβ oligomers trigger the endocytosis of GluA3 and promote its translocation toward endolysosomal compartments for degradation. Mechanistically, these Aβ-driven effects critically depend on the PDZ-binding motif of GluA3. A single point mutation in the GluA3 PDZ-binding motif prevented Aβ-driven effects and rendered synapses fully resistant to the effects of Aβ. Correspondingly, proteomics on synaptosome fractions from APP/PS1-transgenic mice revealed a selective reduction of GluA3 at an early age. These findings support a model where the endocytosis and lysosomal degradation of GluA3-containing AMPARs are a critical early step in the cascade of events through which Aβ accumulation causes a loss of synapses.

Novel crosstalk mechanisms between GluA3 and Epac2 in synaptic plasticity and memory in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease which accounts for the most cases of dementia worldwide. Impaired memory, including acquisition, consolidation, and retrieval, is one of the hallmarks in AD. At the cellular level, dysregulated synaptic plasticity partly due to reduced long-term potentiation (LTP) and enhanced long-term depression (LTD) underlies the memory deficits in AD. GluA3 containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are one of key receptors involved in rapid neurotransmission and synaptic plasticity. Recent studies revealed a novel form of GluA3 involved in neuronal plasticity that is dependent on cyclic adenosine monophosphate (cAMP), rather than N-methyl-d-aspartate (NMDA). However, this cAMP-dependent GluA3 pathway is specifically and significantly impaired by amyloid beta (Aβ), a pathological marker of AD. cAMP is a key second messenger that plays an important role in modulating memory and synaptic plasticity. We previously reported that exchange protein directly activated by cAMP 2 (Epac2), acting as a main cAMP effector, plays a specific and time-limited role in memory retrieval. From electrophysiological perspective, Epac2 facilities the maintenance of LTP, a cellular event closely associated with memory retrieval. Additionally, Epac2 was found to be involved in the GluA3-mediated plasticity. In this review, we comprehensively summarize current knowledge regarding the specific roles of GluA3 and Epac2 in synaptic plasticity and memory, and their potential association with AD.