The amyloid-β (Aβ) peptide is a destroyer of synapses, and its attack on neurotransmission is blamed for devastating memory loss experienced in Alzheimer disease. Now, two new studies suggest that Aβ’s initial assault is aimed squarely at synaptic glutamate receptors. The related papers, both collaborations between the labs of Paul Greengard of Rockefeller University and Gunnar Gouras from the Weill Medical College of Cornell University in New York, show that Aβ peptides, whether added to cultures or produced by neurons, decrease the number and activity of synaptic NMDA and AMPA glutamate receptors.
The findings, published June 17 in Nature Neuroscience, and in press in Neurobiology of Disease last April (with corrected proof available online), provide a mechanism for how Aβ could perturb synaptic function, plasticity and memory, and show that the deleterious effects of Aβ can start early in the life of neurons.
The NMDA receptor is well-known to regulate synapse density and memory formation, and the Nature Neuroscience report describes the fate of this complex in cultured cortical neurons dosed with Aβ1-42 peptide. First author Eric Snyder and colleagues show that one hour of Aβ exposure lowered the number of NMDA receptor subunits on the cell surface, due to increased endocytosis. The imbalance in NMDA trade resulted in loss of NMDA subunits from synapses, and the researchers showed that excess endocytosis required the α-7 nicotinic acid receptor, (known to bind Aβ), as well as the activation of the protein phosphatase 2B (PP2B). Downstream of PP2B, the phosphotyrosine phosphatase STEP was activated, which is known to dephosphorylate tyrosines on the NMDA NR2B subunit in a region that controls receptor endocytosis.
Lowering synaptic NMDA receptor levels had a functional impact, revealed when Snyder et al. used patch-clamping to measure NMDA-evoked currents—these were reduced in neurons treated with Aβ. Interestingly, the authors found that this reduction only occurred in those neurons in which Aβ alone induced an inward current. Associated with this Aβ-driven current, there was reduced phosphorylation of CREB, a transcription factor important for synapse survival and memory formation.
Several of the biochemical aberrations observed after treatment of neuronal cultures with Aβ, like lower STEP and CREB phosphorylation, are also seen in human Alzheimer brain or in mouse models of the disease. When the researchers checked NMDA receptor levels in neurons from Tg2576 mice that express mutated human APP, they found them to be half that in wild-type neurons. To prove the decrease was due to Aβ, they treated cultures with the γ-secretase inhibitor DAPT, and showed that the treated neurons recovered their full complement of NMDA receptors.
In the Neurobiology of Disease paper, first author Claudia Almeida of the Gouras lab and her collaborators draw a similar picture of Aβ’s injurious effects on the GluR1 subunit of the AMPA-selective glutamate receptor. Almeida et al. cultured embryonic neurons from the same Tg2576 transgenic Aβ-expressing mice to look for early synaptic changes. The cultured neurons produce and accumulate intracellular Aβ progressively, and the new work reveals early decreases in the presynaptic protein synaptophysin, and the postsynaptic proteins PSD95 and GluR1. Decreases in these proteins were associated with altered synaptic function, as glutamate-stimulated expression of the immediate early gene Zif268 was impaired, and the neurons displayed lower numbers of functional synapses. The remaining synapses were enlarged, reminiscent of the alterations observed in AD brains.
Synaptic changes were detected early, after less than 2 weeks in culture, and accumulation of Aβ was required for the changes, since, again, treatment of cultures with the γ-secretase inhibitor DAPT prevented the loss of surface expression of AMPA receptor subunits. The results parallel those seen with NMDA receptor, and in fact were obtained with the same cells. To complete the circle, Almeida et al. finally showed that addition of Aβ to the same cortical neurons used in the Nature Neuroscience paper caused a 32 percent reduction of cell-surface AMPA receptors.
How Aβ elicits these losses in glutamate receptors is unclear, but the earliest change detected in the neuronal cultures from AD mice was a decrease in the postsynaptic protein PSD95, implicated by Bill Klein and colleagues in Aβ toxicity (see Lacor et al., 2004). This protein may play a central role in synapse dysfunction, since cell surface expression of both the NMDA and AMPA receptors depends on it. Interestingly, the STEP-sensitive tyrosine phosphorylation of NR2B implicated in NMDA receptor trafficking by Snyder et al. was previously shown to regulate the binding of PSD95 to the NMDA receptor (Lavezzari et al., 2003).
All told, these two papers, by tracing a direct route from Aβ peptide production to synaptic alterations via the downregulation of glutamate receptors, suggest that prolonged depression of NMDA- and AMPA-mediated neurotransmission could kick off the pathological changes seen in AD brain. An important question that remains is: Where does the process start? Is it intracellular or extracellular Aβ that matters, and what are the earliest events? The cultured neurons described in these papers should provide a useful system to answer these questions.—Pat McCaffrey
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